Cellulosic substrates for removal of pollutants from aqueous systems: A review. 1. Metals

Abstract

Recent years have seen explosive growth in research concerning the use of cellulosic materials, either in their as-recieved state or as modified products, for the removal of heavy metal ions from dilute aqueous solutions. Despite highly promising reports of progress in this area, important questions remain. For instance, it has not been clearly established whether knowledge about the composition and structure of the bioadsorbent raw material is equally important to its availability at its point of use. Various physical and chemical modifications of biomass have been shown to boost the ability of the cellulose-based material to bind various metal ions. Systems of data analysis and mechanistic models are described. There is a continuing need to explain the mechanisms of these approaches and to determine the most effective treatments. Finally, the article probes areas where more research is urgently needed. For example, life cycle analysis studies are needed, comparing the use of renewable biosorbents vs. conventional means of removing toxic metal ions from water.

Full Article

CELLULOSIC SUBSTRATES FOR REMOVAL OF POLLUTANTS FROM AQUEOUS SYSTEMS: A REVIEW. 1. METALS

Martin A. Hubbe,^a * Syed Hadi Hasan, ^b and Joel J. Ducoste ^c

Recent years have seen explosive growth in research concerning the use of cellulosic materials, either in their as-recieved state or as modified products, for the removal of heavy metal ions from dilute aqueous solutions. Despite highly promising reports of progress in this area, important questions remain. For instance, it has not been clearly established whether knowledge about the composition and structure of the bioadsorbent raw material is equally important to its availability at its point of use. Various physical and chemical modifications of biomass have been shown to boost the ability of the cellulose-based material to bind various metal ions. Systems of data analysis and mechanistic models are described. There is a continuing need to explain the mechanisms of these approaches and to determine the most effective treatments. Finally, the article probes areas where more research is urgently needed. For example, life cycle analysis studies are needed, comparing the use of renewable biosorbents vs. conventional means of removing toxic metal ions from water.

Keywords: Cellulose; Remediation; Pollutants; Heavy metals; Adsorption; Biosorbents

Contact information: a: Department of Forest Biomaterials, North Carolina State University, Campus Box 8005, Raleigh, NC 27695-8005; b: Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi – 221005, U. P., India; c: Department of Civil, Construction and Environmental Engineering, Campus Box 7908, Raleigh, North Carolina 27695-7908;

* Corresponding author: hubbe@ncsu.edu

INTRODUCTION

This article reviews publications in which lignocellulosic materials have been used, either “as-received” or in modified form, to remove various heavy metals from dilute aqueous solution. There have been an impressive number of relevant publications in this field. The preparation of the present article was made easier by the existence of earlier reviews, some of which are listed in Table 1. As shown, certain reviews have dealt with the biosorption of metal ions in general, while others have focused on specific ionic species or classes of biomass. Some of the articles have reviewed chemical or thermochemical modifications of cellulosic raw materials to render them more effective for the collection and binding of various metal ions. Readers interested in certain metals, certain types of sorbents, or certain aspects of metal bioadsorption are encouraged to scan the columns of Table A (see Appendix), as well as chapters in Wase and Forster (1998). In addition, a book by Cooney (1998) describes engineering principles and strategies for implementation of absorbent-based water treatment systems. Kurniawann et al. (2006a) and Owlad et al. 2009) reviewed systems other than biosorption for removal of metals.

Table 1. Selective List of Relevant Review Articles and Chapters

Table 2 displays the main organization of the present article. An attempt was made to gather metal sorption data from many individual studies, bearing in mind that conditions of sample preparation, treatment, and testing verried greatly among the published studies. A second main goal of this review article is to provide a fairly complete overview of several mathematical formulas that have been employed to fit metal adsorption data. By using Table 2, readers can select topics of highest interest within the article.

Table 2. Organization of the Present Article

Metals in soluble form have raised increasing concerns in recent years. Toxic effects of various metals have been described in detail by Chang (1996), and more recently by Babula et al. (2008) for less common metals. Metal-induced neurological disorders in particular are covered in a book edited by Zatta (2003). Progress has been achieved recently in understanding the attributes of metal ions that contribute to their toxicity (Yoon et al. 2008). Most metal ions become harmful when their concentration exceeds a certain threshold, which depends on the sensitivity of the consuming organism. At the same time, a majority of the same metal species can be considered as essential nutrients, and serious adverse health effects would result if they were completely eliminated from an environment or from a drinking water/food supply system. The most dangerous metals are those that tend to bioaccumulate, building up in the fatty tissues of animals in a food chain (Luoma 2008; Chojnacka 2009, 2010). Chromium(VI) is of particular concern in this regard, since the chromate ion (CrO₄^2-) is easily transportable across cell membranes. The species is readily reduced to the Cr(III) form, which tends to form insoluble complexes that cannot easily be expelled by the affected organism (Cabtingan et al. 2001; Srinath et al. 2002; Aravindhan et al. 2004b; Deng et al. 2006). Metal speciation and the analysis of metal ion species in water have been reviewed by Ali and About-Enien (2006).

Many of the published studies considered in this review article may have been motivated by a desire to find profitable uses of specific waste streams or under-utilized materials produced during industrial operations. When considered separately, almost every such study can be considered successful. However, there has been a need to answer some practical questions, such as those that follow:

• Are cellulosic materials universally effective at removing hazardous metal ions from aqueous solutions?
• Are there rules of thumb that can lead to the selection of suitable biomass for use in sorption of metal ions from solution?
• What are the most useful mathematical expressions that can be used to fit adsorption isotherm data?
• What mechanisms governing metal uptake have been well established? Where are there opportunities for progress in useful theories?
• Can the biosorption of metal ions be improved by mechanical treatments of the cellulosic material?
• What kinds of chemical extractions, derivatizations, or grafting can greatly improve the efficiency of metal ion uptake?
• Should one attempt to regenerate or incinerate cellulose-based biosorbent materials after they have been used to remove metals from water and thereby change the material’s life-cycle?

Guide to the Tabulation of Data

As a first step in attempting to answer questions such as those listed above, an extensive literature search was performed, and information reported in the various articles are collected in Table A, which due to its size is placed in the Appendix to this article. Because Table A will be mentioned frequently during subsequent discussions, a description of its organization is provided here. Columns in Table A indicate the type of biomass, the type of modification (if any), the studied metal species, the adsorption capacity (listed both on a mass basis and a molar basis per unit mass), an abbreviated summary of key findings, and the author-year information, which can be used to find the full citation in the “Literature Cited” section. Going down the table, the entries are organized according to biomass type (first column) and then alphabetically by author name within each category. An exception is made when considering studies in which the biomass was so profoundly modified that the nature of the original biomass was judged to be unimportant in comparison. Thus, the various kinds of chemical modifications, as well as production of activated carbon products from cellulose-derived resources, are given unique groups with no regard for the biomass type that was used as the starting material. Starting at the top of the table, the biomass types are organized as follows: Wood: (hardwood, softwood, unspecified), wood fibers, bark, foliage, cones, nut shells; Crop residuals: husk, stalks; Food residuals: sugar cane bagasse, sugar beet pulp, other, seeds, fruit stone, fruit peel, tea leaves; straw and grasses; weeds and plants; Aquatic plants: fresh water, seaweed, loofa; Microbiota, etc.: algae, bacterial biomass, yeast; Fungal biomass, Lignin-related: isolated lignin, lignite and humic matter, peat moss, sludge and biogas residuals; Chemically modified: alkali-treated, oxidized, with adsorbed materials, derivatized (succinylated, citric acid-treated, carboxymethylated, aminated, other), grafted; Activated carbons; and Ash.

Criteria for Success

A wide range of criteria have been considered by different authors when judging the relative success of methods to remove heavy metal ions from water. Most authors list adsorptive capacity of the biosorbent among their top concerns. It has been pointed out, however, that one of the most advantageous applications of cellulose-derived sorbents is in the treatment of very dilute solutions and in the reduction of aqueous metal concentrations to very low levels (Gaballah and Kilbertus 1998; Gupta et al. 2000; Amuda et al. 2007; Demirbas 2008). None of the reviewed works expressed the opinion that adsorbtion was not rapid enough for any envisioned usage, though the speed of uptake is mentioned by many authors. Rather, much attention has been paid to modeling the kinetics of metal uptake (see, for instance Table A), and the obtained rate expressions have been used in modeling water treatment systems based on both packed-bed operations and batch treatment (Ho and McKay 1999a; Ho et al. 2000b).

Far less attention has been paid to a number of other criteria that might be used to judge the success of a metal remediation strategy. One such criterion is the stability of partially or fully saturated biosorbent. A question remains as to whether the bioadsorbent will continue to hold onto adsorbed metal ions during long-term storage. Another issue that has received relatively little attention is the practical handling of the biomass, including its efficient collection from an aqueous mixture for proper disposal or regeneration without discharging into a surrounding waterbody (Kapoor and Viraraghavan 1998b). Some powdered biomass tends to become soft when placed into water. Its low density and fine particle size can make it difficult to separate from treated wastewater, and fixed bed reactors filled with biomass powders have a tendency to clog (Kapoor and Viraraghavan 1998b).

Using the cellulose-derived material as a support for a primary adsorbent

Another way to define successful use of cellulose-based matter in removal of heavy metals involves the concept of “support”. In other words, the biomass may serve as a backbone structure upon which the main adsorbent material is attached. Zhu et al. (2009b) demonstrated such a concept in their use of zero-valent-iron (ZVI) nanoparticles supported on activated carbon. The combination was found to be effective for the removal of arsenic from water. The ZVI nanoparticles act as a strong reducing agent, having the potential to change the valence state of such metals as arsenic and chromium to less toxic forms. The cited article is a prime example of how it is possible to address such problems without needing to release a strong reducing agent directly into aqueous streams or groundwater, which would create an additional contribution to the pollutant load.

Life-cycle issues

It is important, for both environmental and economic reasons, to consider in detail what happens to an absorbent material after it has been employed to remove heavy metals from water. As evidenced by numerous entries in Table A, most cellulose-based sorbents can be “regenerated” by treatment with acid solution (see, e.g., Chang et al. 1997), though some studies also evaluated the feasibility of using an alkaline solution or brine. In each case, the idea is to displace the metal ions back into a relatively concentrated solution, which either can be disposed of or further processed as a source of valuable metals or inorganic compounds (Cui and Zhang 2008). Another approach is to incinerate the metal-containing biomass, so that the metal content can be concentrated in the ash (Gaballah and Kilbertus 1998).

Relatively little attention has been paid by researchers to landfilling as an alternative fate for used sorbent material. Unlike the options considered in the previous paragraph, landfilling does not require the use of either chemical treatment or incinera-tion of the contaminated sorbent. Treatments with acid or brine can have environmental consequences, even if the pH is subsequently neutralized. Energy may be required to dry sorbent material before it can be incinerated. Thus, as a potential end-of-use strategy for metal-containing biosorbent material, landfilling should be an option in future life-cycle analyses. Issues that need to be considered include the degree to which typical biosorbents will hold onto their metal content during long-term storage and the likely concentrations of metal ions in leachate from such operations. Considering the case where material in a landfill is subjected to rainfall, research results suggest that typical biosorbents will release relatively low concentrations of metals (Gaballah and Kilbertus 1998; Gupta et al. 2000; Amuda et al. 2007; Demirbas 2008). None of the cited studies, however, addressed what might happen as the biomaterial breaks down in the soil.

BIOMASS TYPES AND KEY FACTORS

Based on the reviewed literature, it appears that almost every possible category of biomass material has been evaluated for the uptake of heavy metal ions. As indicated in Table A, multiple respresentatives from many different classes of cellulose-derived materials have been evaluated and judged to be successful as biosorbents. Individual studies have generally tended to be narrow in scope, considering relatively few sorbents, relatively few heavy metal ions, and a limited range of aqueous conditions. Taken together, however, a voluminous collection of scientific work has been published, most of it within the last 20 years. In addition, effects of a great many chemical and thermo-chemical modifications of cellulosic materials have been used in an attempt to achieve higher adsorption capacities. The take-away message is that there is a large selection of suitable sorbent materials with which one can remove heavy metals from water.

Evaluation of First Hypothesis: The Type of Biomass is Important

The first question to consider is whether there are clear differences in metal uptake, depending on the type of untreated biomass support. Figures 1A and 1B display the amounts of lead and chromate ions that were taken up by unit mass of different classes of cellose-based materials under the conditions specified in the cited works. Each plotted “X” symbol in the figure corresponds to the results of an individual study. In general, the data taken from studies considered in this review show that the adsorbed amounts varied over very wide ranges, even within each class of sorbent. For instance in the case of “Wood, sawdust” (as represented by the left-most column), the results for Cr(VI) sorption (Fig. 1B) spanned a factor of about 300). Subsequent sections of this article will describe a variety of reasons that each might account for part of these differences. Note that the rectangular “boxes” in these figures indicate the 25%, 50%, and 75% levels based on the relative frequency of articles reporting different values. The “stems” in the diagram extend upwards and downwards to the highest and lowest reported values of metal sorption in each case.

Figure 1A. Graphical summary of reported amounts of lead ion, Pb(II), adsorbed by diffent classes of cellulose-based matter according to the conditions specified in articles cited in Table A .

Figure 1B. Graphical summary of reported amounts of adsorbed Cr(VI) taken up by diffent classes of cellulose-based sorbents according to the conditions specified in articles cited in Table A. Note that Cr(VI) can be present as the CrO₄^2- (cromate) anion, depending on the pH.

Visual inspection of Fig. 1A and B suggests that certain biomass classes were somewhat more promising in terms of achieving relatively high levels of sorption of specific metal ions. Though there were important differences attributable to the metal species (especially when comparing adsorption of the chromate ion vs. cationic metal species), such differences appeared to be dominated by effects attributable to differences among the substrate samples, even when the nominal material was similar. As a class, average adsorptive capacities reported for “Wood” did not appear to be as high as those reported for such classes as “Bark, cones, leaves, and nuts,” “Crop/food residuals,” “Aquatic plants,” “Lignin-related,” and “Pyrolysis products.”

Figure 2 addresses a related question: How did different metal ions generally compare against each other, with respect to their tendency to be taking up by a given class of cellulosic material? To make this comparison, seven kinds of metal ions were compared with respect to their sorption by the “crop/food residuals” types of biomass. Though later sections of this article will refer to studies providing evidence of significant “metal selectivity,” such effects are not apparent when one looks at the assemblage of data plotted in Fig. 2.

One of the most fascinating aspects of these results is the finding that the chromate ion (data plotted farthest to the right) compared very well with the other ions, even though the chromate ion (CrO₄^2-) has a negative charge, which is the same net charge as that of typical cellulosic surfaces.

Figure 2. Graphical summary of reported amounts of seven types of metal ions taken up by one class of cellulose-based sorbents (“crop/food residuals) according to the conditions specified in articles cited in Table A

Because the proportions of cellulose, hemicellulose, and lignins can differ to a great extent, not only among the biomass categories mentioned above, but also from species to species, it makes sense to compare the metal sorption efficiency of biomass samples showing large differences in compostion. Various authors have proposed that lignin-rich samples, such as composts, would be expected to have a high affinity for metals ions due to an expected high level of carboxylation (Harman et al. 2007). Indeed, various studies support this hypothesis (Srivastva et al. 1996; Lalvani et al. 1997; Dizhbite 1999; Crist et al. 2002, 2003; Acemioğlu et al. 2003; Babel and Kurniawan 2003; Basso et al. 2004; Demirbas 2004, 2005; Sciban and Klasnja 2004a,b; Celik and Demirbas 2005; Mohan et al. 2006; Harman et al. 2007; Guo et al. 2008; Quintana et al. 2008; Wu et al. 2008; Harmita et al. 2009). By contrast, there has been a notable lack of attention paid to hemicelluloses and extractive components of biomass in this regard; this is surprising, since these components of biomass are known generally to be rich in carboxylic acid groups (Sjöström 1993).

Cost and Availability

A possible lesson that can be drawn from Table A is that almost any biomass-derived product can be used for metal ions removal from solution. It is difficult, however, to claim that any one type of source material is consistently superior to others, though large differences have been observed between different types of biosorbents. That being the case, it is worth questioning whether it is sometimes adequate to make one’s selection based only on cost and local availability. For instance, if one were able to obtain peanut shells, fungal biomass, and pine sawdust from local sources, what factors other than performance ought to guide one’s choice?

In principle, transportation costs and associated usage of energy can be minimized by using locally-collected biomass as the basis for a biosorbent system. But the overall cost and energy expenditure will also depend on the performance of the material. One needs to consider that a higher-performing biosorbent may achieve one’s objectives for metal removal with much less biosorbent material, thus reducing labor costs and operational costs. There may be savings related to safe disposal or regeneration of the spent material.

The ideal biosorbent should be very cheap, an unwanted byproduct that currently has to be hauled away and landfilled or burnt in heaps. On the other hand, the ideal biosorbent should have a huge appetite for a broad range of metal ions, binding them quickly, tightly, and dependably. One approach is just to test various readily available materials in “as-received” form. However, as will be shown in the course of this review, other investigators have employed a more proactive option, treating the biomass in various ways to improve its performance. Though the detailed prices of various biosorbents, both in their as-received and treated forms, lie beyond the scope of the present review article, it is expected that some of the collected data will permit subsequent investigators to make judicious choices among existing biosorbents and to develop additional variations to further enhance performance of biosorbents for different applications.

Cost

The words “low cost” have been used frequently, especially in review articles, by authors summarizing the main motivations prompting the use of biosorbent technology (Scheider et al. 1995; Gupta et al. 1998; Bailey et al. 1999; Brown et al. 2000; Gérente et al. 2000; Kumar et al. 2000; Marchetti et al. 2000a; Wartelle and Marshall 2000; Yu et al. 2000; Reddad et al. 2002b; Babel and Kurniawan 2003; Fiol et al. 2003; Ulmanu et al. 2003; Krishnani et al. 2004; Chuah et al. 2005; Horsfall and Spiff 2005b; Karthikeyan et al. 2005; Agarwal et al. 2006; Ali and Gupta 2006; Kumar 2006; Kumar and Bandyopadhyay 2006a; Kurniawan et al. 2006b; Lodeiro et al. 2006; Mohan and Pittman 2006; Parab et al. 2006a; Pino et al. 2006; Sarin and Pant 2006; Singh et al. 2006; Upendra and Manas 2006; Abdel-Ghani et al. 2007; Dubey and Krishna 2007; Garg et al. 2007; Ghodbane et al. 2007; Nouri et al. 2007; Soleimani and Kghazchi 2007; Zafar et al. 2007; Ahmady-Asbchin et al. 2008; Arief et al. 2008; Chakravarty et al. 2008; Demirbas 2008; Farinella et al. 2008; Igwe et al. 2008; Sud et al. 2008; Anandkumar and Mandal 2009; Gadd 2009; Rai 2009; Shukla et al. 2009; Wang and Chen 2009; Parab et al. 2010; Zahra 2010). Though Gupta et al. (2000) claimed that the operating costs involved in the usage of biosorbents for metal removal can be low relative to various alternative pollution abatement measures, there has been insufficient attention to operating costs, including the costs of transporting the sorbent material to the point of use, as well as costs associated with transportation to a site of final disposal, regeneration, or other beneficial use.

Gadd (2009) raised the following challenge to those attempting to make distinctions, other than cost, among alternative cellulose-derived sorbents: Because the composition of biomass does not vary a great deal between different species, it would seem pointless to spend a lot of effort testing many different representatives within a given class of biomass.

As an alternative, it was suggested that researchers should focus on biomass types that have distinct chemical differences from other types. Fungal biomass was mentioned as a key example, since it contains chitin within its cell walls (Ahluwalia and Goyal 2005b). The amino groups within chitin may have the potential to bind certain metal ion species in a different way from other kinds of biomass. Details about the use of fungal biomass for metal remediation have been reviewed (Sag 2001; Bishnoi and Garima 2005).

Local availability of large quantities

When the attributes of a needed commodity include “very low cost,” and often “bulky,” it can be a great advantage to minimize transportation costs. One promising low transportation strategy is to position one’s bioremediation facility adjacent to a business that produces a suitable lignocellulosic waste stream. For example, substantial quantities of microbial biomass are produced during industrial-scale fermentation processes (Ahluwalia and Goyal 2005b). Likewise, there may be opportunities to accumulate such bypoducts as bark, sawdust, carcoal, or ash adjacent to a facility that produces wood products or paper pulp as a primary product.

Performace Factors

Having already noted that cellulosic biomass is mainly composed of cellulose, hemicellulosics, lignin, and some extractives – the next challenge is to try to account for the huge ranges of metal ion adsorptive capabilities, such as those that are indicated in Fig. 1. Indeed, while certain cellulose-based sorbents represented in the figure were not very effective on a unit mass basis, others can equal or exceed the sorption capability of commercial ion exchange resins (e.g. Chang and Hong 1994; Ariff et al. 1999; Chamarthy et al. 2001; Saliba et al. 2002b; Choi and Yun 2004; Bishnoi and Garima 2005; Cochrane et al. 2006; Arshad et al. 2007; Ziagova et al. 2007).

Surface area

Cellulosic materials of biological origin tend to be organized with systems of interconnecting pores, thus providing a relatively high surface area per unit mass. However in such cases, there may be questions about: (a) whether parts of that surface area are inaccessible to the metal ions in question, and (b) whether parts of that surface area are lacking in potential binding sites for the metal ions. The importance of maximizing the accessible surface area has been demonstrated by studies that considered effects of particle size of the sorbent (Bai and Abraham 2001). However, there has been a lack of detailed study to compare surface area parameters vs. metal uptake under well-defined conditions. Future research might use, for instance, the BET nitrogen adsorption method (Faur-Brasquet et al. 2002; Budinova et al. 2006; Demiral et al. 2008; Hanafiah and Ngah 2009) to quantify the surface area of sorbent materials. A solvent-replacement method and freeze-drying could be used to minimize collapse of submicroscopic pores as water is removed (Stone and Scallan 1996).

A shown in Figs. 1 and 2, the typical data for adsorption of specified metal ions on nominally similar biomass samples range over three orders of magnitude. Including all the factors to be considered in this article, such a wide spread of data would appear to be best explained by differences in accessible surface area. Indeed, the cited work of Stone and Scallan (1996) showed that the drying of cellulosic material has the potential to decrease the apparent surface area by at least a factor of 100. Stone and Scallan revealed that about half of the mesopores that collapsed during drying of cellulosic pulp failed to re-open when the same fibers were rehydrated under the conditions of testing. Future studies related to metal remediation could address this point by scrupulously avoiding inadvertent drying of fresh biomass material at any point before the start of experimentation. This approach has the potential to show whether or not “un-dried biomass,” in contrast to more typically available samples of unknown drying history, might offer substantially higher metal uptake capacity.

Agitation

Another possibility that researchers have studied is whether the uptake of metal ions may be limited by the rate of diffusion of metal ions to surfaces. Some researchers have found that the amount of metals adsorbed increased with increasing agitation during batch testing (Ahalya et al. 2005; Basci et al. 2003). Though both of the cited studies observed higher adsorbed amounts associated with higher rates of agitation, the mechanism is not completely clear. Agitation can be expected to facilitate convective transport of metal ions to sorbent surfaces, and several studies indeed showed positive effects of agitation on the rate or extent of metal sorption (Basci et al. 2003; Ahalya et al. 2005; Shen and Duvnjak 2005b,c; Malkoc 2006; Martinez-Garcia et al. 2006b; Chaves et al. 2009; Garg et al. 2009). Alternatively, the associated pressure pulses acting on suspended particles in a mixture might also create a pump-like action, creating intermittent flow into and out of pore spaces within cellulosic materials in suspension. Such questions have not been adequately resolved and will require further research.

Ion exchange capacity

The issue of “binding sites,” as mentioned previously, can be addressed by considering the ion exchange capacities of candidate sorbent materials (Gadd 2009). Strictly speaking, ion exchange capacity can be measured by determining how much of one type of ion desorbs from a unit mass of a given sorbent when the system is saturated with a specified metal ion. The desorbed ion is usually either the proton, sodium, or an alkaline earth ion, such as calcium. The term “ion exchange” usually implies that the researchers are considering non-specific, electrostatic mechanisms of metal binding. Though such approaches often can be used as a first approximation, later sections of this article will consider alternative approaches that can help to explain deviations from an anticipated 1:1 stoichiometry between the ion exchange capacity of a sorbent and the adsorption capacities of different metal ions, even those having the same valence.

The major proportion of the ion exchange capacity of a biosorbent material usually can be attributed to surface-bound carboxylic acid groups. In principle, the content of carboxylic acid groups can be estimated by titrating a mixture of the sorbent material in water between two levels of pH, such as 3 and 9, and comparing the result with a blank determination (Gill 1989; Herrington and Petzold 1992; TAPPI 1993; Lindgren et al. 2002). An alternative method is required if one needs to determine carboxylic acid groups in the presence of other sources of acidity (e.g., Chai et al. 2003). The importance of carboxyl groups in sorbing metal ions has been demonstrated in many studies (Maranon and Sastre 1992b; Gloaguen and Morvan 1997; Jia and Thomas 2000; Kadirvelu et al. 2000; Merdy et al. 2002; Tiemann et al. 2002; Chubar et al. 2003; Davis et al. 2003; Pagnanellil et al. 2003; Karunasagar et al. 2005; Leyva-Ramos et al. 2005; Southichak et al. 2009b; Gurgel et al. 2008; Lodeiro et al. 2008; Bakir et al. 2009; Iqbal et al. 2009b; Jaramillo et al. 2009; Martin-Lara et al. 2008, 2009).

MODIFICATION OF BIOSORBENTS

Once a decision has been made to use a certain type of biosorbent , perhaps due to favorable performance of the as-received material, the next decision may involve whether and how to modify that material to improve its efficiency. The following discussion will start with gross mechanical and thermal treatments, then proceed to chemical treatments. Of the latter, relatively superficial “rinsing” strategies will be considered first, then more pervasive treatments such as oxidation, polymer adsorption, and formation of chemical derivatives or graft polymers at the surface of cellulosic materials. Activated carbon, which is often prepared from cellulosic biomass, will be considered last.

Size Reduction

In principle, a more finely ground sample of a given cellulose-based material is expected to adsorb more metal ions from solution, under specified conditions, compared to coarser particles. Indeed, this statement has been proven in a few studies (Ajmal et al. 1998; Blazquez et al. 2005). A further question is whether the effort and expense of size reduction can be justified. Here the answer is less clear. The study by Ajmal et al. (1998) observed an increase in metal sorption capacity by only a factor of about 2 when the particle size of sawdust was decreased from 500 µm to 100 µm. In addition to the high energy requirements, especially if one aims to achieve particles much smaller than 1 mm, one can anticipate increased problems with the handling of very fine material, including greater difficulties in later separation from the water phase, clogging of filters, and even dust and fire hazards if and when the material is dried. Sawdust actually represents a favorable case, since the energy to reduce the particle size has already been expended, perhaps in the production of lumber. Unfortunately, there have been few studies dealing systematically to determine under what circumstances one can justify the energy and time needed to reduce the particle size of a selected biomass sample in preparation for its use as a metal sorbent.

Living vs. Dead Biomass

One of the likely consequences of mechanically or chemically treating a biomass sample (see next sections) is its conversion from living organisms into dead biomass. Several investigators have investigated whether a change from living to dead has a significant effect on the ability of the material to adsorb metals ions (Kapoor et al. 1999; Srinath et al. 2002; Chen et al. 2005; Methta and Guar 2005; Yan and Viraraghavan 2003). Mehta and Guar (2005) reviewed relevant studies and concluded that dead biomass samples typically outperform living biomass for the uptake of heavy metals. Only in some cases did the investigators find substantially higher uptake when utilizing live cells (Yan and Viraraghavan 2003; Chen et al. 2005). The ability of some cells to accumulate metals internally by active biological processes has been suggested (Chen et al. 2005), and interest in the use of living cells for biosorption has been predicted to increase (Wang and Chen 2009). Other researchers have found cases where metal sorption was actually increased after such processes as autoclaving (Kapoor et al. 1999; Srinath et al. 2002; Deepa et al. 2006). The explanation for the latter findings may be that killing the cells opens internal surfaces, making the material more accessible. Kapoor and Viraraghavan (1998a) found mixed results; pretreatment of live Aspergillus niger biomass with various reagants yielded the best results for uptake of lead, cadmium, and copper, but the best uptake of nickel ions was observed with dead cells.

As noted by Ahluawalia and Goyal (2005b), non-living biosorbants have many potential advantages, including insensitivity to growth conditions or toxins, easier handling, easier storage, and easier disposal. Also, as noted by Ozdemir et al. (2004) and Hasan et al. (2007), the growth of living cells may be inhibited in the presence of significant concentrations of the specific metal that one would like to remove from an aqueous system. Kurek and Majewska (2004) found that autoclaving of different strains of fungal biomass tended to minimize differences among them in terms of metal sorbency.

Heat Treatment

Rocha et al. (2006) observed that the drying conditions used to prepare algal biomass may impact its sorptive capacity. In general, drying tended to shrink the material and resulted in the closing of pores. Various authors have reported increases in metal sorption capacity in the case of heat-inactivated biomass samples (Kacar et al. 2002; Bayramoglu et al. 2003; Gurisik et al. 2004; Arica et al. 2005; Tunali et al. 2005).

Drying of biomass also has been reported by many researchers as having a positive effect on metal uptake (Rocha et al. 2006), and many of the articles cited in this work specified the use of dried biomass. However, there has been almost no systematic study of this important issue.

Alkaline Treatment

Treatment with alkaline solution has been shown to enhance metal uptake in a few cases (Azab and Peterson 1989; Luef et al. 1991; Fourest and Roux 1992; Addour et al. 1999; Kapoor et al. 1999; Mameri et al. 1999; Kumar et al. 2000; Reddad et al. 2002d; Spanelova et al. 2003; Min et al. 2004; Tuanli et al. 2005; Sciban et al. 2006b; Southichak et al. 2006a; Afkhami et al. 2007; Nasir et al. 2007; Gupta and Rastogi 2008b; Argun et al. 2009). The mechanism has not been confirmed in detail. It seems likely that saponification of various ester groups may be involved, increasing the number of carboxylate groups on the treated surfaces (Reddad et al. 2002e; Xuan et al. 2006; Li et al. 2007, 2008).

Oxidation

One kind of treatment that has been consistently shown to increase the ability of cellulose-derived substrates to adsorb cationic metal species is oxidation (Maekawa and Koshihima 1984; Jia and Thomas 2000; Rangel-Mendez and Streat 2002; Chen and Zeng 2003; El-Hendawy 2003; Park et al. 2003; Park and Kim 2004; Babel and Kurniawan 2004; Saito and Isogai 2005; de Mesquita et al. 2006; Kikuchi et al. 2006; Argun et al. 2008a; Chavez-Guerrero et al. 2008; Baccar et al. 2009; Berenquer et al. 2009; El-Hendawy 2009; Foglarova et al. 2009; Han et al. 2009; Jaramillo et al. 2009; Klasson et al. 2009; Shukla et al. 2009). As already has been noted, oxidation of cellulose-derived material may result in increased numbers of carboxyl groups, which can dissociate to their negatively charged carboxylate form as the pH is increased in a range of 3 to 6.

In contrast to oxidation, chemical reduction may be useful for metal ion uptake, but only in isolated circumstances. Harry et al. (2008) observed enhanced adsorption on carbon cloth following electrochemical reduction, especially in the case of Cr(VI). Reduction would be expected to render the biomass-derived surfaces less negative in charge, thus favoring the adsorption of a negative species. Consistent with this finding, Aggarwal et al. (1999) found that oxidation of activated carbon samples suppressed the adsorption of the Cr(VI) chromate anion, though it favored the adsorption of the Cr(III) cation. However, other authors observed higher adsorption onto substrates that had been oxidized, even for the adsorption of Cr(VI) (Babel and Kurniawan 2004). Redox interactions during the adsorption of Cr(VI) and certain arsenic and mercury ions will be discussed in a later section.

Polymer Adsorption

Although polymer adsorption can be considered as a gentle treatment in the sense that no covalent reactions need to take place with the substrate, the effects on metal adsorption capacity can be profound. There has been a notable lack of research attention paid to adsorption of carboxyl-containing species onto biosorbents, for purposes of enhancing metal uptake.

Surprisingly, increases in adsorption of metal cations have been observed when using positively charged polyelectrolytes such as polyamines (Deng and Ting 2005b,c). Such systems are especially effective for adsorption of the chromate anion (Deng et al. 2006; Fang et al. 2007). Analogously, a cationic surfactant has been used to enhance the uptake of chromate ions onto fungal biomass (Mungasavalli et al. 2007). Chitosan, a positively charged polymer of natural origin, was likewise found to be effective for adsorption of Cr(VI) (Nomanbhay and Palanisamy 2005). The chitosan was loaded onto a charcoal support in the cited work. Tschabalala et al. (2004) observed, similarly, that a cationized cellulosic support was effective for removing phosphate, a negatively charged material, from aqueous solution. Gérente et al. (2007) have provided context from some of these findings in their review of research related to the use of chitosan for removal of heavy metals from aqueous solution.

Chemical Derivatization

Chemical derivatization of cellulosic material can be defined as the covalent attachment of various functional groups. This approach makes it possible for technologists to select chemical functionalities that may be expected to enhance metal uptake. From a scientific standpoint, derivatization also can be considered as a way to evaluate different hypotheses regarding which chemical groups, some of which may be present naturally, are likely to contribute to observed metal-binding effects.

Carboxylic acid derivatives

Earlier it was noted that carboxylic acid groups contribute directly to the ion exchange capacity of sorbent materials. Accordingly, many authors have reported favorable effects on metal uptake when using biosorbents that have been derivatized to increase their carboxylic acid content. Xie et al. (1996) describe the use of chloroacetic acid for this purpose. Other authors have achieved similar effects by reacting the cellulosic material with succinic anhydride (Marchetti et al. 2000a; Nada and Hassan 2006; Karnitz et al. 2007; Gurgel et al. 2008; Parab et al. 2008; Belhalfaoui et al. 2009; Chandlia et al. 2009; Garg et al. 2009). In general, such approaches have been shown to increase the adsorption capacity of a biosorbent for the target metal(s).

Multifunctonal carboxylic acid derivative

Esterification of a cellulose-based polymer with 1,2,3-propanetricarboxylic acid was found to yield strong binding of a wide range of heavy metals (Sugur and Babaoglu 2005). By such a reaction, there is potential to create adjacent carboxylic acid sites at the sorbent surface. Ideally, if one were to use just the right monomer, there is a theoretical possibility to approach the strong metal binding capabilities of a chelating agent. However, one needs to keep in mind that the most effective chelating agents require three to six carboxylic acid groups in a specific arrangement that is best suited for coordinating with a given type of metal ion (Lawrance 2010).

Phosphate derivative

Alternatively, it has been shown that a phosphate group can be attached to the surface of activated carbon (Nada et al. 2002a; Puziy et al. 2002). The product was found to be stable and offering a good ion-exchange capability and ability to bind heavy metal ions.

Sulfur-containing derivatives

When the goal is to adsorb such metals as Hg, Ag, and As, it may make sense to prepare sulfur-containing derivatives. Such an approach has been demonstrated in several cases (Tashihiro and Shimura 1982; Igwe et al. 2008). Thus, Aoki et al. (1999a) derivatized cellulose with a variety of groups, including isothiouronium and mercapto groups and achieved higher adsorption of Ag(I) and Hg(II). Macias-Garcia et al. (2004) and Marshall et al. (2007) introduced sulfur groups – presumably sulfonate – by use of SO₂ gas during propration of activated carbon. Kim et al. (1999) observed a three-fold increase in the adsorption of lead after derivatizing algal biomass with xanthate groups.

Grafting

The term “grafting” will be used to represent a polymeric or oligomeric group that is attached to the cellulosic surface, usually by a covalent reaction involving the hydroxyl groups. Researchers have demonstrated the potential to create high-performing biosorbents by such treatments (Kubota and Shigehisa 1995; Yu et al. 2007), and the field has been reviewed by O’Connell et al. (2008). In particular, researchers have attached acrylamide-related chains to cellulosic surfaces (Aoki et al. 1996b; Raji and Anirudhan 1998; Bicak et al. 1999; Marchetti et al. 2000b; Shibi and Anirudhan 2002, 2005, 2006; Guclu et al. 2003; Choi et al. 2004; Unnithan et al. 2004; Chauhan et al. 2005a,b, 2006; Deng and Ting 2005a; Hashem 2006; Hashem et al. 2006a; Nada and El-Wakil 2006; Nada et al. 2007a; Sharma and Chauhan 2009; Sokker et al. 2009). The major increases in metal-binding capability that have been observed by many of these authors are consistent with the chelating effects that can be achieved by multifunctional carboxylic acids, as in the case of substrates that have been grafted with acrylic acid chains. In principle, grafting technologies enable the technologist to attach a wide range of highly specific functional groups to the substrate surface. As such, the approach can be compared to the use of the underlying biomaterial as a kind of support, rather than necessarily being a significant individual contributor to metal uptake.

Pyrolizing to Produce Activated Carbon Products

Very strong heating in the relative absence of oxygen is known to convert cellulosic materials into carbon. Careful control of the pyrolysis conditions, including the temperature and the composition of the surrounding gases, make it possible to achieve a very high accessible surface area per unit mass, in addition to providing significant control regarding the chemical sites at the carbon surface (Dias et al. 2007; Chen et al. 2008). For instance, the pore structure often can be enhanced by the use of steam during preparation of the activated carbon (Budinova et al. 2006). Many authors have demonstrated the potential to remove metal ions from aqueous systems by the use of such products, and these are listed in the appropriate section of Table A. Because of the generally hydrophobic nature of many activated carbon products, some authors regard such products as being especially suitable for removal of organic pollutants from aqueous systems, whereas removal of metal ions might be considered as a secondary benefit (O’Connell et al. 2008).

Numerous strategies have been used to render activated carbon products more effective in the uptake of one or more kinds of metal ions, and these are likewise indicated in Table A (see the “Activated Carbon” section indicated in the first column, near to the back of the table, and see the “Modifications” as indicated in the second column). Many of these treatments have already been discussed in preceding paragraphs. In general, these modifications can be achieved by the following strategies: First, the gaseous conditions can be adjusted, e.g. by the addition of steam, controlled amounts of oxygen, and by controlling the temperature, etc. Second, the mixture fed into the pyrolysis operation can be treated, for instance, with such materials as phosphoric acid to achieve a higher proportion of carboxylic acid groups or phosphorous-containing groups on the resulting carbon surface. Finally, the resulting powder can be post-treated, e.g. with nitric acid, to oxidize the surface, thus increasing the number of carboxyl groups (see, e.g. Lyubchik et al. 2004).

Ash

Ash often is available as an under-utilized waste product of the combustion of lignocellulosic materials. Though the properties of the ash itself are seldom a prime consideration in its genesis, it still makes sense to consider its possible beneficial applications. Chonjacka and Michalak (2009) found, for instance, that ash from wood was three times as effective for the removal of metal ions, compared to bone ash. Other studies that have considered the use of ash for metal removal can be cited (Rao et al. 2002; Banarjee et al. 2004; Chaves et al. 2009; Chu and Hashim 2002; Gupta et al. 1998, 1999, 2003; Gupta and Ali 2000, 2004; Pehlivan et al. 2006; Srivastava et al. 2006a,b,c, 2007, 2008a, 2009a,b).

MECHANISMS OF INTERACTIONS WITH METAL SPECIES

Ion-Exchange

“Ion exchange” refers to a class of mechanisms in which adsorbing metal ions take the place of other species already associated with the sorbent surface. For instance, these entitities may be metal ions such as Na⁺, Ca²⁺, or the proton, etc. Let’s suppose that a given biosorbent material has been prepared by equilibration in either NaOH solution or NaCl brine. In such cases, its reasonable to expect that acidic sites on the substrate will be mainly associated with Na ions. If the Cu²⁺ ion, for instance, is then introduced to the system, it may have a higher affinity for acidic sites in comparison to a monovalent cation. The resulting competition will lead to a net desorption of Na⁺ from surface sites and a net uptake of Cu²⁺.

Recent studies reviewed by Sag (2001) with fungal biomass in general and seaweed in particular have indicated a dominant role of ion exchange in metal binding. The classical ion-exchange concept, based on exchange-equilibrium constants and separation factors, can be applied. For a generalized ion-exchange reaction for dissolved species A being exchanged for a bound species B, with the underlined character representing the bound species,

 (1)

where the equilibrium constant K_AB and the separation factor γ_AB are given as follows, for the case of ideal behaviour of the exchanging species (1:1 ion exchange, activity = 1) in both of the phases:

 (2)

 (3)

For a binary ion-exchange system, the value of the equilibrium constant K_AB can be determined from the slope of the plot of q_A/q_B versus C_A/C_B. Biosorbents can also be prepared in different ionic forms, and the sorption analysis is often reduced to considering a series of simple binary ion-exchange systems. By eliminating q_B through substitutions, the following expression is obtained:

 (4)

As q_A / Q represents the fraction of the binding sites occupied by A, this equation may be used to evaluate the decrease of the equilibrium uptake of the species A by the biosorbent caused by the presence of species B. Using simple dimensionless concentration fractions as variables, Eq. (4) can be re-written as follows:

 (5)

This equation is the most generalized description of the ion-exchange sorption equilibrium for binary systems (Kratochvil and Volesky 1998). Modeling multi-metal ion exchange in biosorption has been applied to the brown alga Sargassum fluitans, which contains the carboxyl groups of alginate and the sulfate groups of fucoidan. An ion-exchange-based two-site model has been developed (Schiewer and Volesky 1995) and extended to describe multi-site and multi-ion system behavior (Schiewer and Volesky 1996).

Substantial published evidence supports the validity of the concept as just described, when it is applied to cellulosic-derived substrates. Desorption of displaced ionic species has been demonstrated during the uptake of a charge-equivalent amount of the heavy metal of interest. For instance calcium or magnesium ions may be released, depending on how the sorbent material has been prepared (Akthar et al. 1995; Diniz and Volesky 2005).

As an example, alkali-treated A. niger biomass was used to sequester silver ions from dilute as well as concentrated solutions (Akthar et al. 1995). The bound Ag⁺ was fully desorbed by dilute HNO₃, and the biosorbent was regenerated by washing with Ca²⁺/Mg^2÷ solution. The binding of Ag⁺ was attended by a stoichiometric release of Ca²⁺ and Mg²⁺ ions together (the sum of the two released being equal to Ag⁺ bound in molar terms, as expected on the basis of the univalency of the Ag⁺ ion). This equivalence established the mechanism of Ag⁺ sorption as being quantitatively due to exchange with (Ca²⁺ + Mg²⁺) ions of the biosorbent.

Biosorption of the lanthanides, lanthanum (La³⁺), europium (Eu³⁺), and ytterbium (Yb³⁺) from single component and multi-component batch systems using Sargassum polycystum Ca-loaded biomass was studied (Diniz and Volesky 2005). The ion exchange sorption mechanism was confirmed by the release of calcium ions from the biomass that matched the total number of metal and protons removed from the solution.

In other cases, the adsorption of metal ions is accompanied by dissociation of a stoichiometrically equivalent amount of protons. Metal (Pb, Cu, Zn, Cd, Ca) uptake by kraft lignin was also found to occur by displacement of protons or bound metals (Crist et al. 2002), as shown by the following equations, where X stands for a mono-valant bonding site on the substrate. Square brackets indicate concentrations in solution, while terms in parentheses represent the sorbate on the sorbent after ion exchange.

 (6)

 (7)

 (8)

 (9)

Crist et al. (2003) also conducted a detailed study with a kraft pine lignin powder. This material has acid functions that can act as ion exchange sites, which showed that uptake of divalent toxic metals was accompanied by a release of protons or existing metals from the lignin. A demonstrated stoichiometry of one mole Ca displaced for one mole of metal (Sr or Cd) sorbed was fully consistent with a chemical reaction of ion exchange and difficult to explain by frequently used adsorption models.

Adsorption of Hg(II), Cr(III), Cu(II), Cd(II), Ni(II), Ca(II), Sr(II), Zn(II), Co(II), Mn(II), Mg(II), K(I), and Na(I) by activated carbon made from pecan shells (Dastgheib and Rockstraw 2002a), showed that the Slips and Freundlich equations (see later) were satisfactory for explaining the experimental data. The ratio of equivalent metal ions adsorbed to protons released was calculated for the studied metal ions over a range of concentrations. In most cases, particularly at low concentrations, this ratio approached one, confirming that ion exchange of one proton with one equivalent metal ion was the dominant reaction mechanism.

Other studies have shown correlations between the number of weak-acid sites on the substrate in comparison to the amount of metal ions that can be adsorbed. The concept of ion exchange is also supported by the fact that a sufficiently high concentration of salt, acid, or base can cause a reversal of the process; this type of phenomenon will be discussed later in this article with respect to the regeneration of sorptive materials that have been used at least once in the sorption of heavy metals.

Figure 3 contrasts the ion exchange concept, as just described, with a site-specific concept. Ion exchange is mainly concerned with the stoichiometry of displacement of bound ions by dissolved ions, as displayed in the left-hand frame of the figure. For instance, adsorption of a trivalent metal ion is shown to occur from the displacement of three mono-valent species. By contrast, a complexation model (see later discussion) focuses on the underlying interactions of ions with surface sites. Thus, the right frame of Fig. 1 envisions the adsorption of a hydrated divalent ion, the simultaneous release of some of the waters of hydration, and the formation of a bi-dentate complex having moderate stability. Concepts related to such complexation are considered near the end of this article.

Fig. 3. Pictorial comparison of ion exchange concept (left frame) vs. metal adsorption involving chemical complexation (right frame)

To further understand implications of the foregoing observations, the next sections deal with mathematical, as well as theoretical fits between adsorption character-istics and solution concentrations, i.e. sorption isotherms.

SORPTION ISOTHERMS

When any sorption system reaches a state of equilibrium, there is a defined distribution of sorbate molecules at the solid-liquid interface and also in the bulk at a particular temperature. This provides an idea of the capacity of the sorbent for the sorbate. The maximum possible accumulation of the sorbate at the solid surface is a function of its concentration at a constant temperature, and it can be expressed by the following generalized relationship,

q_e = f(C_e) (10)

where q_e is the amount of sorbate sorbed at equilibrium (mg/g), C_e is the equilibrium concentration of the sorbate (mg/L), and “f” can be equated to the phrase “is a function of”. This type of relation is termed a ‘sorption isotherm’, which represents equilibrium between the concentration of a solute in solution and its concentration on the sorbent, at a given temperature. For assessing the maximum sorption capacity of a given biosorbent, the derivation of sorption isotherms is the most appropriate method. Further, the study of sorption isotherms is useful not only to evaluate to what extent a sorption system can be improved, but also to help predict conditions for working in open reactors and estimate optimal operating conditions.

The mathematical modeling of sorption is a very powerful tool for understanding the sorption process and essential for process design and optimization (Esposito et al. 2002). Several equilibrium-based models have been used to describe the metal transfer between the solution and solid phase during the sorption process (Vijayaraghavan et al. 2006a).

Langmuir Sorption Isotherm

The Langmuir model, which is one of the most widely used (see Table A), was initially proposed for the adsorption of a gas on the surface of a solid. Nevertheless, it has been extended to include the sorption of solute at a solid–liquid interface. The Langmuir model suggests that the sorption occurs on the surface of the solid that is made up of elementary sites, each of which can adsorb one sorbate molecule, i.e. monolayer sorption. It was also assumed that every sorption site is equivalent and the ability of sorbate to get bound there is independent of whether or not the neighbouring sites are occupied (Langmuir 1918).

The Langmuir model is given as follows (Langmuir 1918):

 (11)

Equation (11) can be linearized as follows:

C_e/ q_e = 1/ Q^ob + C_e/ Q^o (12)

where q_e (mg/g) and C_e (mg/L) are the sorbed metal ions on the sorbent and the metal ion concentration in the solution at equilibrium, respectively, b (L/mg) is the constant related to the affinity of binding sites, i.e. the affinity of sorbent for the sorbed species. Q^o (mg/g) is known as the Langmuir constant, which represents the monolayer sorption capacity, i.e. a practical limiting sorption capacity when the surface is fully covered with metal ions. Q^o assists in the comparison of sorption performances. In general, for good sorbents, high values of Q^o and low values of b are required (Kratochvil and Volesky 1998). Equation 11 also can be linearized to other forms. The final linearized form will be a function of the data distribution.

The affinity between adsorbate and adsorbent can be predicted using the Langmuir parameter b from the dimensionless separation factor R_L,

R_L = 1/ (1 + bC_o) (13)

where C_o is the initial metal ion concentration and b is the Langmuir isotherm constant. The adsorption process as a function of R_L may be described as follows: When R_L is greater than one, then the sorption reaction is unfavourable, and it is linear when R_L is equal to one. When R_L is between zero and one, the reaction is favourable, while the reaction is supposed to be irreversible when R_L is equal to zero. This can be summarized as follows:

R_L > 1 unfavorable

R_L = 1 linear

0 < R_L < 1 favorable

R_L = 0 irreversible

Freundlich Sorption Isotherm

The Freundlich isotherm model, which is also very widely used, describes the sorption of solute from liquid to solid surface and assumes that the stronger binding sites are occupied first and that the binding strength decreases with an increasing degree of site occupation. The Freundlich model proposes a monolayer sorption with a heterogeneous energetic distribution of active sites, and/or interactions between sorbed species, i.e. multilayer sorption (Freundlich 1907).

The Freundlich model can expressed by the following empirical equation:

 (14)

Equation (14) can be expressed in logarithmic terms to obtain the following form,

 (15)

where K_F (mg^1-n/g Lⁿ) and n (dimensionless) represent the Freundlich constants characteristic of the system. K_F is indicative of the relative sorption capacity, whereas n is the measure of the nature and strength of the sorption process and the distribution of active sites. If (n) < 1, then the bond energies increase with the surface density. If (n) > 1, the bond energies decreases with the surface density. When n = 1, all surface sites are equivalent. Alternatively, it has been shown using mathematical calculation that n values between 1 and 10 represent beneficial sorption. These parameters are empirical constants, and they depend on several factors (Bajpai et al. 2004; Febrianto et al. 2009).

Dubinin-Radushkevich (D-R) Sorption Isotherm

The Dubinin–Radushkevich isotherm model (Dubinin and Radushkevich 1947) is postulated within a sorption space close to the sorbent surface to evaluate the sorption free energy and to help determine the nature of bonding, i.e. either physisorption or chemisorption.

The D–R isotherm can be presented as follows,

 (16)

where q_m is the amount of sorbate sorbed (mmol/g), X_m is the maximum sorption capacity of the sorbate retained (mmol/g), K_DR is the activity coefficient constant related to the sorption free energy of the transfer of the solute from the bulk solution to the solid sorbent (mol² kJ²), and F is the Polanyi potential, which is given by the equation,

 (17)

where R is the universal gas constant (0.0834 kJ/mol/K) and T is the absolute temperature in Kelvin. C_e was defined earlier.

Assuming that the surface of the sorbent is heterogeneous and when choosing an approximation to a Langmuir isotherm model as a local isotherm for all sites that are energetically equivalent, the quantity K_DR, which is related to the mean free energy (E) of the transfer of 1 mol of solute from infinity to the surface of the sorbent, can be expressed by the equation:

 (18)

If the magnitude of E is between 8 and 16 kJ/mol, then the sorption process is supposed to proceed via chemisorption, while for values of E < 8 kJ/mol, the sorption process is of physical nature (Basar 2006; Hasany and Chaudhary 2001; Saeed et al. 1996; Tunali et al. 2006; Vijayaraghavan et al. 2006a).

Temkin Isotherm

The Temkin isotherm equation (Temkin and Pyzhev 1940) contains a factor that explicitly takes into account adsorbing species–adsorbate interactions. It assumes that the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbate–adsorbate repulsions and that adsorption involves a uniform distribution of maximum binding energy. In addition, it assumes that the fall in the heat of sorption is linear rather than logarithmic, as implied in the Freundlich equation. The Temkin isotherm has commonly been written in the following form,

 (19)

Equation 19 can also be represented as follows:

(20)

where, T is the absolute temperature in Kelvin, and R is the universal gas constant, 8.314 J/mol/K. The constant b_Te is related to the heat of adsorption (J/mol), and a_Te is the equilibrium binding constant (L/g) corresponding to the maximum binding energy.

Flory-Huggins Isotherm

Another two-parameter isotherm is the Flory-Huggins model, which can be represented as follows (Padmesh et al. 2006):

 (21)

The equilibrium constant, K_FH has been used to compute the Gibbs free energy (ΔG):

G = –RT lnK_FH (22)

where θ = (1−C_f /C_o) is the degree of surface coverage, K_FH is the Flory-Huggins equilibrium constant, and n_FH is the Flory-Huggins exponent.

Redlich–Peterson Isotherm

The R–P isotherm can be described as follows (Padmesh et al. 2006),

 (23)

where K_RP is a first R–P isotherm constant (l/g), a_RP is a second R–P isotherm constant (L/mg), β is an exponent, the value of which lies between 0 and 1, and C_e is the equilibrium liquid phase concentration (mg/L).

If β = 1, then the Langmuir will be the preferable isotherm, while if β = 0, the Freundlich isotherm will be preferred. Although the two parameters in the Langmuir and Freundlich equations can be graphically determined, Redlich–Peterson constants are not computed by graphing, because there are three unknown parameters. However, the values of the three parameters in the equation can be obtained using non-linear regression analysis.

Toth Isotherm

The Toth isotherm is derived from the potential theory, and it is applicable for heterogeneous adsorption (Toth 1971). This model assumes a quasi-Gaussian energy distribution, where most sites have adsorption energies lower than the peak or maximum adsorption energy. The Toth isotherm is represented as follows (Padmesh et al. 2006),

 (24)

where q_max is the maximum dye sorption (mg dye/g biomass), b_T is the Toth model constant, and n_T is the Toth model exponent.

Determining Isotherm Parameters

By linearization

The simplest approach to determining isotherm constants for two-parameter isotherms is to transform the isotherm variables so that the equation is converted to a linear form and then to apply linear regression (Ho et al. 2002). Although a linear analysis is not possible for a three-parameter isotherm, a trial and error procedure has previously been applied to a pseudo-linear form of the Redlich-Peterson isotherm to obtain values for the isotherm constants (McKay et al. 1984), and this involves varying the isotherm parameter, K_RP, to obtain the maximum value of the correlation coefficient for the regression.

By non-linear regression

Due to the inherent bias resulting from linearization, alternative isotherm parameter sets can be determined by non-linear regression (Ho et al. 2002). This provides a mathematically rigorous method for determining isotherm parameters using the original form of the isotherm equation (Seidel and Gelbin 1988; Seidel-Morgenstern and Guiochon 1993; Malek and Farooq 1996; Khan et al. 1996). Most commonly, algorithms based on the Levenberg-Marquardt or Gauss-Newton methods (Edgar and Himmelblau 1989; Hanna and Sandall 1995) are used.

The optimization procedure requires the selection of an error function in order to evaluate the fit of the isotherm to the experimental equilibrium data. The choice of error function can affect the parameters derived. Error functions based primarily on absolute deviation bias the fit towards high concentration data, and this weighting increases when the square of the deviation is used to penalize extreme errors.

This bias can be offset partly by dividing the deviation by the measured value in order to emphasize the significance of fractional deviations. In the cited study (Ho et al. 2002), five non-linear error functions were examined and in each case a set of isotherm parameters were determined by minimizing the respective error function across the concentration range studied. The error functions employed were as follows:

The Sum of the Squares of the Errors (ERRSQ):

 (25)

A Composite Fractional Error Function (HYBRD):

 (26)

A Derivative of Marquardt’s Percent Standard Deviation (MPSD):

 (27)

The Average Relative Error (ARE):

 (28)

The Sum of the Absolute Errors (EABS):

 (29)

As each of the error criteria is likely to produce a different set of isotherm parameters, an overall optimum parameter set is difficult to identify directly. Hence, in order to try to make a meaningful comparison between the parameter sets, a procedure of normalizing and combining the error results was adopted, producing a so-called ‘sum of the normalized errors’ for each parameter set for each isotherm.

The calculation method for the ‘sum of the normalized errors’ was as follows:

(a) select one isotherm and one error function and determine the isotherm parameters that minimize that error function for that isotherm to produce the isotherm parameter set for that error function;

(b) determine the values for all the other error functions for that isotherm parameter set;

(c) calculate all other parameter sets and all their associated error function values for that isotherm;

(d) select each error measure in turn and ratio the value of that error measure for a given parameter set to the largest value of that error from all the parameter sets for that isotherm; and

(e) sum all these normalised errors for each parameter set.

The parameter set thus providing the smallest normalised error sum can be considered to be optimal for that isotherm, provided that:

• There is no bias in the data sampling – i.e. the experimental data are evenly distributed, providing an approximately equal number of points in each concentration range; and
• There is no bias in the type of error methods selected.

APPLYING ISOTHERM EQUATIONS TO METAL SORPTION DATA

The following subsections describe a number of cases in which authors have provided justification for different types of isotherm models for the analysis of metal sorption onto cellulosic materials under different experimental conditions.

Langmuir Isotherm

As has been noted, the use of the Langmuir adsorption isotherm implies an assumption of uniform, non-interacting adsorption sites. When one considers the impure nature of typical biomass-derived sorbents, it is remarkable how large a proportion of the publications considered in this review reported that good fits were achieved by means of the Langmuir equation (see Table A). Possible ways to explain the goodness of fit, in so many of the listed cases, are as follows:

• Many studies tend to be dominated by effects due to one kind of chemical group, e.g. a certain kind of carboxylate group present on that type of modified biomass.
• In addition, it is likely that in many cases the adsorption experiments were performed at sufficiently high ionic strength such that the adsorption of a metal ion at one site did not have an appreciable influence on the adsorption of the next metal ion at an adjacent site. The likely range of influence can be roughly estimated based on the Debye-Hückel reciprocal length parameter (Hiemenz and Rajagopalan 1997).

Multifunctional Langmuir Adsorption Models

Aksu et al. (1997, 1999) studied adsorption onto Chlorella vulgaris for Fe(III), Cr(VI) and Cu(II) as single-component systems, as well as Fe(III)–Cr(VI) and Cu(II)– Cr(VI) binaries. They concluded that single-component isotherms could be modeled by either the Freundlich or Langmuir isotherms. The binary Freundlich equation proposed by Fritz and Schlünder was appropriate for fitting the data of both binary systems, while the extended Langmuir equation was used successfully for only the Fe(III)–Cr(VI) system.

The simultaneous biosorption of copper(II) and chromium(VI) to C. vulgaris from binary metal mixtures was investigated by Aksu et al. (1999) in a single-staged batch reactor as a function of V_o/X_o ratio (volume of wastewater containing heavy metal mixture/quantity of biosorbent) at different orders of second metal ion addition and at pH values of 2.0 and 4.0 chosen as the optimum biosorption pH values for chromium(VI) and copper(II), respectively. The sorption phenomenon was expressed by a competitive, multi-component Freundlich adsorption isotherm, which was then used for calculating each residual or adsorbed metal ion concentration at equilibrium (C_eq,i or C_ad,eq,i) at a constant V_o/X_o ratio for a given combination of heavy metals in a single-staged batch reactor. In the cited study, the non-competitive Freundlich isotherm model (Eq. 14) was used for describing the short-term and mono-component adsorption of heavy metal ions by algal cells. However, for binary mixtures, an empirical extension of the Freundlich model has been proposed where the coefficients relating to isotherms could be determined from mono-component isotherm data, except for the biosorption competition coefficients, which had to be determined experimentally. The Freundlich models for the first and the second components restricted to binary mixtures are given by Eqs. (30) and (31),

 (30)

 (31)

where K_FI, K_FII, n_I, and n_II are derived from the corresponding individual Freundlich isotherm equations and the six other parameters (noting that xI, yI, zI, xII, yII, and zII are the competitive Freundlich adsorption constants of the first and second metal ions, respectively, for the binary system) are the competition coefficients for two metal ion species.

The biosorption of heavy metal ion mixtures by the biomass in a batch reactor can be considered as a single-staged equilibrium operation. Consideration of the single-stage equilibrium operation would depend on two basic constraints, that of equilibrium (shown in Eqs. (30) and (31)) and that of a mass balance. The mass balance for the first component in the mixture is given by,

 (32)

 (33)

 (34)

where C_0I is the initial concentration of the first component (mg L^-1); C_eqI is the residual concentration of the first component at equilibrium (mg L^-1); q_0I is the amount of the first component adsorbed per unit weight of algae at the beginning (mg g^-1); q_eqI is the amount of the first component adsorbed per unit weight of algae at equilibrium (mg g^-1); V₀ is the volume of solution containing heavy metal ion mixture in the batch reactor (l); and X₀ is the amount of biosorbent in the batch reactor (g)

Equation (34) belongs to a straight line for the first metal ion, and the line passes through points (C_0I; q_0I) and (C_eqI; q_eqI) with (-V₀:X₀) slope. This is the operating line for this stage at a known concentration of the second metal ion. As q_eqI and C_eqI values are known from experimental data for the first metal ion in the mixture, the single-staged batch operation can be shown in a figure on the same coordinates by drawing the operation line and equilibrium curve for the first metal ion at a known combination and pH value. V₀:X₀ for a desired purification or C_eqI and q_eqI values at a given V₀:X₀ and a second metal ion concentration can be determined. The initial metal ion concentration of the first metal ion must be equal to,

 (35)

where C_ad,eqI is the adsorbed concentration of the first component at equilibrium (mg L^-1). The value of C_ad,eqI can be calculated easily from Eq. (35). If a calculation is required, then Eq. (34) can be rearranged as:

 (36)

The amount of the first metal adsorbed per unit weight of biomass at the beginning of the biosorption (q_0I) is equal to 0.0, so Eq. (36) can be rewritten:

 (37)

As K_FI, n_I, K_FII, and n_II can be found from experimental data, Eq. (37) also provides the V₀:X₀ ratio for desired purification or C_eqI and C_eqII (or indirectly C_ad,eqI and C_ad,eqII) at a given V₀:X₀ ratio for a given heavy metal mixture at a known combination. Equation (37) can also be rewritten for the second metal ion in the same manner.

Thus the copper(II)-chromium(VI) multi-ion system was defined with the multi-component Freundlich adsorption isotherm and used to model the adsorption of a binary system to C. vulgaris in a single-staged batch reactor as a function of V₀:X₀ ratio and second metal ion concentration at pH 2.0 and 4.0. The pH of the biosorption medium, the order of addition of the metal ions, and the amount of biosorbent (V₀:X₀ ratio) strongly affected the equilibrium uptake of the first metal ion by the algae. The individual Freundlich constants evaluated from the non-competitive isotherms were used to find the competitive Freundlich constants in a competitive Freundlich model describing multicomponent adsorption equilibrium. These constants were used in Eq. (37) to calculate the residual concentration of the first metal ion at a known second metal ion concentration and the V₀:X₀ ratio in a single-staged equilibrium operation.

The equilibrium isotherms for the first metal ion at the known second metal ion concentrations with the operation line with V₀:X₀ slope were also developed to predict the residual concentrations of the first metal ion. It was considered that these two methods may be used successfully to estimate the residual concentrations of the first and second metal ions in a mixture at equilibrium. C. vulgaris biomass offers a practical approach for removing mixtures of copper(II) and chromium(VI) ions from waste waters containing mainly these two components. Using low V₀:X₀ ratios, high purification yields can be obtained for the first metal ion at its optimum pH value and at low second metal ion concentrations or for desired purifications of the first metal ion, V₀:X₀ ratios, pH, and second metal ion concentrations. Parameters can be chosen according to Eq. (37) by using individual and competitive Freundlich constants in a single-staged batch reactor up to 150 mg/L initial metal ion concentration for each metal ion. Multi-staged reactors can also be designed and operated by estimating a sufficient amount of algae for a known volume of waste water (choosing V₀:X₀) with a known heavy metal ions combination, especially if required purification cannot be provided in a one-staged reactor when studying higher metal ion concentrations.

In the work of Leyva et al. (2001), single and simultaneous Cd(II) and Zn(II) adsorption isotherms from aqueous solution onto activated carbon were determined experimentally. Single isotherms for these ions were fitted to Langmuir isotherms, while the simultaneous adsorption isotherms was fitted to the bisolute Langmuir isotherm modified with an interaction factor. Experimental data for single adsorption isotherms for Zn(II) and Cd(II) onto C (carbon) were fitted to the Langmuir isotherm (Eq. 11).

The constants for this isotherm were obtained by a least-squares method based on the optimization algorithm of Rosenbrock-Newton. The average percent deviation was calculated, and a reasonable fit to the experimental data was obtained based on application of the Langmuir isotherm. The Freundlich isotherm (Eq. 15) was also tested, but produced a weaker goodness of fit compared to the Langmuir isotherm. The max-imum molar uptake of Zn(II) averaged 1.6 times that of Cd(II). This result was explained by the author as probably being related to the electrostatic attraction between the very heterogeneous surface of the activated carbon and the metal ions in solution. Another possible explanation for the relatively high Zn(II) selectivity was related to the ability of both Cd(II) and Zn(II) to be adsorbed at one class of surface sites, while Zn(II) was exclusively adsorbed on other class of surface sites. Thus the single adsorption isotherm of Zn(II) can be represented by a dual-site Langmuir isotherm, known as the bi-Langmuir isotherm. However, a Scatchard plot (q/C vs. q, presented in the paper) of the single solute adsorption data of Zn(II) did not suggest that Zn(II) was adsorbed on two kinds of sites. Thus, it was said that the single-site Langmuir isotherm was appropriate to represent the single adsorption of both metal ions (Leyva et al. 2001).

The simultaneous adsorption of Cd(II) and Zn(II) was also studied, as these ions usually occur together in industrial wastewaters. In multicomponent systems, the adsorption isotherm of a certain solute also depends on the concentration and characteristics of the other solutes in the aqueous solution. The solute of interest may be in competition with other solutes for the same active adsorption sites. The experimental data for simultaneous Cd(II) and Zn(II) adsorption were interpreted with the bisolute Langmuir isotherm. The competitive Langmuir isotherms for Cd(II) and Zn(II) are represented as follows:

 (38)

 (39)

The constants of these two isotherms are from the single-solute Langmuir isotherms. The experimental molar uptake values of Cd(II) and Zn(II) were compared to the molar uptake values of Cd(II) and Zn(II) predicted with the bisolute Langmuir isotherm, and it was found that the bisolute Langmuir isotherm overestimated the molar uptake of Zn(II) with an average percent deviation of 94.0%. However, it underestimated the molar uptake of Cd(II), and the average percent deviation was 33.36%. Thus, the binary adsorption data were not properly described with the bisolute Langmuir isotherm. The literature reports that the bisolute Langmuir model provides a reasonable fit to the multicomponent adsorption data when the q_m,i values for each metal evaluated from single-solute Langmuir isotherm are similar to each other. As previously noted in this study, q_m,Zn is approximately 1.6 times greater than q_m,Cd. Jain and Snoeyink (1973) assumed that some adsorption occurs without competition, because not all sites were available to all solutes. Consequently, the bisolute Langmuir isotherm was modified for systems in which the q_m,i values of components were different, proposing the following isotherm for the solute with the higher q_m,i, which in the cited study was Zn(II):

 (40)

The isotherm for the solute with the lower q_m,i is the same as that represented in Eq. (38), and the constants are from the single-solute Langmuir isotherms. In Eq. (40), the difference between the maximum molar uptakes is the number of sites with noncompetitive adsorption. The modified bisolute Langmuir model was applied to the experimental data for simultaneous Cd(II) and Zn(II) adsorption and it overpredicted the molar uptake of Zn(II), with an average deviation of 112.7%. Thus the modified bisolute Langmuir isotherm failed to predict the molar uptake of Zn(II), and its prediction had a higher average percent deviation than that obtained with the bisolute Langmuir isotherm.

Ho and McKay (1999b) modified the bisolute Langmuir isotherm with an interaction factor, η, and obtained an excellent fit of the adsorption data of Cu(II) and Ni(II) onto peat. The model proposed by these authors can be represented as follows,

 (41)

 (42)

where η_i,j is the interaction factor of metal i for the adsorption of metal j. This interaction factor is specific to each metal ion in a given system and depends upon the other metal ions present. In the cited study, the best interaction factor was obtained by fitting the isotherm models to the experimental data with a least-squares method that employs an optimization algorithm.

The values of the interaction factors were calculated and found to be as follows: η_Cd,Cd = 1.02; η_Zn,Cd = 3.29; η_Cd,Zn = 0.089; and η_Zn,Zn = 1.26. The experimental molar uptake and the molar uptake predicted with the bisolute Langmuir isotherm that had been modified with the interaction factor were compared, and it was found that the bisolute Langmuir isotherm interpreted the experimental data for both ions reasonably well. The average percent deviation was 18.8% for Cd(II) and 31.0% for Zn(II), which are the lowest values for all the models tested in the cited work. Thus, the experimental data for simultaneous Cd(II) and Zn(II) adsorption onto carbon correlated well with the bisolute Langmuir isotherm modified with an interaction factor. The simultaneous adsorption isotherms for Cd(II) and Zn(II) were always reduced compared to the single adsorption isotherms for these ions. The Zn(II) adsorption isotherm was affected more by the presence of the other ion than Cd(II). The adsorption isotherm for a given ion is always reduced by the presence of the other because the two ions compete for some of the same active sites.

The adsorption behavior of Cu(II) and Mn (II) cations in the presence of other metal ions that display strong or intermediate affinities for adsorption sites was system-atically investigated, taking into consideration the following factors: (1) metal ion site competition; (2) charge accumulation near the carbon surface; and (3) speciation of the metal ions. Two multicomponent adsorption models were proposed, and the results were then compared to two models presented in the literature (Dastgheib and Rockstraw 2002b).

For modeling multicomponent systems comprised of species whose single–solute isotherms obey the Freundlich Isotherm, a multicomponent Freundlich equation was used. The first equation of this type was proposed for binary systems by Fritz and Schlünder (1974, 1981), as provided in Eq. (43),

 (43)

where q₁ and C₁ are the concentrations of solute 1 in the solid and liquid phases, respectively; C₂ is the concentration of the solute 2 in liquid phase; K₁ and n₁ are Freundlich equation constants in the single solute 1 system; and β₁₁, α₁₂ and β₂₁ are constants that are determined from the least squares analysis of the binary data.

The second multicomponent Freundlich equation, which was proposed by Sheindorf et al. (1981), was derived under the assumption that: (1) each component in a single system obeys the Freundlich model and (2) for each component in multicomponent system, the adsorption energies of different sites are distributed exponentially, with the distribution function being identical to that for the single-component system. The proposed binary system equation is shown in Eq. (44),

 (44)

where η₁₂ is the interaction parameter (with other parameters are defined in the same manner as Eq. (43)).

Thus the proposed equation, written for solute 1 in a binary system of solutes 1 and 2, can be shown as,

 (45)

where q₁ and C₁ are the concentrations of solute 1 in the solid and liquid phases, respectively; C₂ is the concentration of solute 2 in the liquid phase; K₁, n₁, K₂, and n₂ are the single component Freundlich constants, and a₁₂ , b₁₂, and n₁₂ are interaction constants obtained from a least squares analysis of the binary data. The term inside the bracket on the right hand side of Eq. (45) represents the overall competition and interaction factor, and has a value of less than or equal to unity (when C₂ → 0, this term is equal to 1). The term a₁₂K₂ can be condensed to a single term, and was considered as one constant.

It was found in most cases that the value of 0.5 for n₁₂ in Eq. (45) gave acceptable results. Using this assumption, Eq. (45) was reduced to a form that has only two interaction constants, as shown by Eq. (46),

 (46)

The general case of the proposed multicomponent Freundlich model is Eq. (47),

 (47)

where q_i and C_i represent concentrations of solute i in the solid and liquid phases, respectively; C_j represents the concentration of other solutes in liquid phase; K_i, n_i, K_j, and n_j are the single-component Freundlich constants; a_ij, b_ij, and n_ij are binary interaction constants obtained from a_ii = b_ii = 0; and m is the number of solutes.

Least squares analysis was used to find the constants of Eqs. (43 through 46). In each case, the objective function, as defined in Eq. (48), was minimized

 (48)

In Eq. (48), q_exp,i is the experimental value of the metal ion uptake in binary system at data point I, q_cal,i is the calculated value of metal ion uptake (from the selected model), and m is the number of data points.

To evaluate and compare the performance of each model, average relative error (ARE%) and root mean squares error (RMSE%) were calculated for each binary system. The large calculated values of RMSE% corresponding to Eq. (44) demonstrated that this model did not predict the metal ion adsorption isotherms in binary systems particularly well. Equation (43) and the proposed model in Eq. (45) were both found to be good models for predicting adsorption isotherms of metal ions in binary systems, as demonstrated by low RMSE% of different binaries.

Samples of dead biomass from the marine brown algae Fucus ceranoides, Fucus vesiculosus, and Fucus serratus were studied for their ability to remove cadmium from aqueous solutions by Herrero et al. (2006). A non-ideal competitive adsorption isotherm model (NICCA) (Kinniburgh et al. 1999), which described very well the competition between protons and metal ions, in contrast to a simpler discrete competitive Langmuir model, was applied. This model is a semi-empirical, thermodynamically consistent model, which implicitly accounts for a variable degree of heterogeneity of the sorbent. The basic NICCA equation for the overall binding of species i in the competitive situation is,

 (49)

where _i is the coverage fraction of the species i, K_i is the median value of the affinity distribution for species i, p is the width of the distribution (usually interpreted as a generic or intrinsic heterogeneity seen by all ions), and n_i is an ion-specific non-ideality term. Strictly speaking, c_i should be the local concentration of species i at the binding site, i.e., the bulk concentration (or activity) corrected for the double layer effect (for instance, the concentrations in the Donnan phase). In this work, the bulk concentrations was used instead, and therefore, the metal binding constants calculated were conditional parameters (referred to 0.05 M ionic strength). The following normalization condition was used to calculate the amount of species i bound, q_i,

 (50)

where q_max,H is the maximum binding capacity for protons, which can be calculated from the equivalence point of the acid-base titrations in absence of heavy metal. The ratio n_i/n_H was interpreted by Kinniburgh et al. (1999) in terms of stoichiometry and cooperativity. When this ratio is less than one, then the maximum binding of species i is lower than the total amount of sites (defined as the amount of titratable protons), which would be a consequence of some degree of multi-dentism. On the other hand, a value of n_i/n_H greater than one would reflect some degree of cooperativity. Finally, if n_i/n_H=1, it can be demonstrated that the maximum proton/metal exchange ratio is one, and the NICCA isotherm reduces to the generalized (multicomponent) Langmuir-Freundlich isotherm (GLF):

 (51)

If only the proton binding is considered (i.e., absence of competing ions) in Eqs. (50) and (51), then the LF isotherm is recovered,

 (52)

where now the heterogeneity parameter m_H describes the combined effect of n_H and p (m_H=n_Hp). In the case of a homogeneous system (no chemical heterogeneity), m_H=1, and then the Langmuir isotherm is obtained. For instance, the ideal Langmuir competitive isotherm for the binding of Cd²⁺ (assuming a 1:1 stoichiometry) would be,

 (53)

with q_Cd = _Cdq_max,H.

It was found that the fit of the NICCA model to the cadmium binding data (discarding the data at pH 6 and lower metal concentrations) was satisfactory.

Other Models

In addition to the relatively well known isotherm approaches summarized on previous pages, one of the goals of the present review is to provide some guidance on alternative equations that have been demonstrated in at least one study involving metal removal but not widely used as those discussed earlier. Such approaches may have potential to become more widely used in the future.

Incorporation of Donnan relationships

Schiewer and Volesky (1997a) used biomass of the brown alga Sargassum for the biosorption of Cd²⁺ ions. This work provided a mathematical model for predicting the equilibrium of proton and metal ion binding as a function of metal ion concentration, pH, and ionic strength. Since the presence of sodium significantly influenced Cd binding, it is recommended to use models that incorporate ionic strength effects. Although swelling of the biomass particle was observed, a simple Donnan model that assumed a rigid particle already yielded a good prediction of the experimental data. A combined Donnan-Biosorption Isotherm equation was derived that allowed for direct calculation of cation binding without interactions. Three versions of the Donnan model were considered: one that assumes a rigid particle (DORI), one (DOSWa) that accounts for swelling by a linear correlation (Eq 54), and one (DOSWb) that accounts for swelling by a more complex relation (Eq 55).

Since the swelling of sorbent increased with the number of free sites C, the following simple linear relationship between the specific particle volume and C was assumed,

 (L/g) (54)

where Y_v is a constant that has to be determined from the experimental data. For C approaching zero (i.e., all sites are occupied), electrostatic effects and therefore the volume are irrelevant (i.e., it does not matter that the value calculated for V_m approaches zero). Equation 54 expresses that the charge density per volume is constant, independent of the degree of site occupation.

Since, the swelling not only increased significantly with C but, additionally, it decreased with ^Mq (the metal ion binding (mequiv/g)), the following swelling correlation was considered:

 L/g (55)

Schiewer and Wong (2000) investigated the binding of protons and metal ions by three brown seaweeds Sargassum hemiphyllum, Colpomenia sinuosa, and Petalonia fascia as well as the marine green alga Ulva fascia as a function of metal concentration, pH, and ionic strength. Differences in overall biosorption behavior were explained as a result of different numbers of binding sites, affinities for metal complexation, and charge density. These relationships were predicted using the Donnan model combined with an ion exchange biosorption isotherm for covalent binding of metals (Cu and Ni) and protons.

The concentration factor [H]_p/[H] was modeled according to the Donnan equilibrium, whereby the concentration of any ion was assumed to be homogeneous throughout the biomass particle, and the negative charge of the biomass was balanced by counter-ions such as protons (H), sodium (Na), or divalent metal ions (M), whose concentration factor was determined by:

 (56)

One main factor determining K is the ionic strength. K decreases with increasing ionic strength, approaching a value 1.0.

A pH-sensitive isotherm equation was derived, which allows for the calculation of the amount of metal and protons bound covalently.

 (mequiv/g) (57)

 (mequiv/g) (58)

The Donnan model with particle swelling (DOSW) was represented by the following equation:

 (L/g) (59)

where Y_V is a ftting parameter and C is the number of free carboxyl groups (i.e., assumed not covalently bind to any cation). The following equations were derived by combining the Donnan model Eq. (56) with the isotherm equations (60) and (61):

 (mequiv/g) (60)

 (mequiv/g) (61)

The Donnan model was successfully used to account for the ionic strength effects in pH titrations and in metal binding. In metal binding experiments at high ionic strength swelling of the biomass particles was observed. The model fit improved when compared to the Donnan model for rigid particles when particle swelling proportional to the number of free binding sites was assumed.

Sundman et al. (2008) characterized the interactions between Ca²⁺, Cu²⁺, and two different fibre materials—a fully bleached softwood kraft pulp, and a chemically modified fully bleached softwood kraft fibre material—aiming for a better understanding of the interactions between water suspended cellulose fibres and metal ions. The study was conducted as a function of pH (2 to 7), both in the absence and presence of an excess of Na⁺ ions (0 to 100 mM NaCl). For both fibre materials, adsorption data collected in the absence of Na⁺ were fully explained by the non-specific Donnan ion-exchange model. However, in the presence of an excess of NaCl, the data clearly indicated that higher amounts of divalent metal ions adsorbed in comparison to the prediction of the Donnan model. Therefore, to model these data, specific metal ion–fibre surface complexes were assumed to form, in addition to the Donnan ion-exchange. It was found that the Donnan ion exchange model satisfactorily described Ca²⁺ and Cu²⁺ ion adsorption by both fibre materials when no excess of Na⁺ ions was present in the fibre suspensions. On the other hand, in an excess of ionic medium, the Donnan model underestimated the Ca²⁺– and Cu²⁺-ion uptake in all experiments. The deviation was greatest for the native low-charged fibre material and at the highest ionic medium.

Lumped parameter isotherm model

Schiewer and Wong (1999) used a lumped parameter isotherm model, where they emphasized the need to incorporate pH effects into the isotherm model. Since pH is one of the key parameters in biosorption, it is desirable to use isotherm equations that can accommodate pH as one model variable. Their model also incorporates ion exchange constants, reflecting that the biosorbent is initially saturated with some ions that have to be released when the metal ion is consumed. Ion exchange constants do not, however, take into account that the degree of binding site occupation may change. Therefore, multicomponent isotherm models have to be proposed that can account for ion exchange and pH effects. The model needs to be based on a 1:2 binding stoichiometry, whereby one divalent metal ion M binds to two binding sites B. Two different approaches have been proposed, i.e., to use ion exchange constants that assume the formation of B₂M complexes or a multicomponent isotherm assuming the formation of BM_0.5 complexes.

These isotherm models can be represented as:

 for B+M = BM (62)

 for 2B+M = 2BM_0.5 (63)

 for 2B+M B₂M (64)

The main advantage of the BM_0.5 and B₂M models is that they adequately represent the possible occurrence of divalent metals occupying two binding sites. Therefore, these stoichiometric assumptions are better suited to model the exchange between metals and protons. The BM_0.5 and B₂M models yield slightly lower deviations from the experimental data in comparison to the BM_b model, but all have similar magnitudes. The cited authors found that for Cu binding, the B₂M model was better than the BM_0.5 stoichiometry, while the reverse was true for Ni. This improved performance for BM_0.5 for Ni was due to different isotherm shapes, whereby the BM_0.5 model showed rather gradual changes of ion binding with metal concentration. In either case, the stoichiometric assumption that yielded the better fit displayed a slope in the exchange plot much closer to 1.0 compared to the other model. It can be concluded that the slope in the log/log plot for metal proton ion exchange is the best indicator of the appropriateness of the stoichiometric assumptions.

The B₂M model can be advantageous at very low metal concentrations, where the BM_0.5 model sometimes tends to overpredict the metal binding. Since however both stoichiometric assumptions typically fit equally well and since the BM_0.5 model offers the additional advantage of being much simpler to use (no iterations are required), it is recommended to use the BM_0.5 model. An exception is when utilizing the assumption that binding sites must be a suitable distance from each other in order to form a stable complex. Thus, the lesser affinity in Ulva in the cited work may be caused by a lack of suitably spaced sites (i.e., the individual carboxyl sites may be too far apart to allow bidentate binding).

Other multicomponent fits

Srivastava et al. (2006) studied the competitive adsorption of Cd(II) and Zn(II) ions onto bagasse fly ash (BFA) from binary systems and used different isotherm models to study the equilibrium of systems.

Various monocomponent isotherm equations such as those of Freundlich (Eq. 14), Langmuir (Eq. 11), and Redlich–Peterson (R–P) (Eq. 23) were used to describe the equilibrium characteristics of adsorption; the Redlich–Peterson (R–P) and the Freundlich models represented the single ion equilibrium adsorption data better than the Langmuir model. Equilibrium isotherms for the binary adsorption of Cd(II) and Zn(II) ions on BFA had been analyzed by non-modified Langmuir, modified Langmuir, extended-Langmuir, Sheindorf–Rebuhn–Sheintuch (SRS), non-modified R–P, and modified R–P adsorption models. These multicomponent isotherm equations that have been used are presented as follows:

Non-modified competitive Langmuir model

The extension of the basic Langmuir model for component i in a N-component system to competitive adsorption can be formulated as follows:

 (65)

where q_m,i and K_L,i are derived from the corresponding individual Langmuir isotherm equations.

Modified competitive Langmuir isotherm

Individual adsorption constants may not define exactly the multi-component adsorption behavior of metal ion mixtures. For that reason, better accuracy may be achieved by using modified isotherms related to the individual isotherm parameters and the correction factors. An interaction term, η_L,i, which is a characteristic of each species and depends on the concentrations of the other components, has been added in the competitive Langmuir model. The modified competitive Langmuir isotherm is given as,

 (66)

where q_m,i and K_L,i are derived from the corresponding individual Langmuir isotherm equations, and η_L,i values are estimated from competitive adsorption data. For binary mixtures, this equation can be rewritten as the first and the second component, respectively, and the two equations can be solved simultaneously to obtain the multicomponent Langmuir adsorption constants for each component.

Extended Langmuir isotherm

Assuming that the surface sites are uniform, and that all the adsorbate molecules (ions) in the solution compete for the same surface sites, the extended Langmuir equation for multicomponent systems can be written as

 (67)

Sheindorf–Rebuhn–Sheintuch (SRS) model

A Freundlich-type multi-component adsorption isotherm known as the Sheindorf–Rebuhn–Sheintuch (SRS) equation was derived by Sheindorf et al. (1981), to represent experimental data. A general SRS equation for the adsorption isotherm for component i in a N-component system is given as:

 (68)

The pre-exponential coefficient K_F,i and the exponent n_i are determined from the mono-component systems. The competition coefficients a_ij describe the inhibition to the adsorption of component i by component j, and can be determined from the thermodynamic data, or more likely, from the experimental data of multicomponent systems. The SRS equation assumes that (I) each component individually obeys the Freundlich isotherm; (II) that for each component in a multicomponent adsorption system, there exists an exponential distribution of site adsorption energies,

 (69)

where a_i and b_i are constants; and (III) the coverage by each adsorbate molecule (or ion) at each energy level Q is given by the multicomponent Langmuir isotherm equation:

 (70)

where,

Integration of N_i(Q)θ_i (Q) over energy levels in the range of – ∞ to + ∞ yields Eq. (71), and the competition coefficients are defined as a_ij = K_0j/K_0i and thus a_ji = 1/a_ij. The SRS equation was successfully applied to a multicomponent equilibrium adsorption of different types of contaminants,

 (71)

where

 and a_ij = b_0j / b_0i

Non-modified competitive Redlich–Peterson model

The competitive non-modified R–P model related to the individual isotherm parameters only is given as follows,

 (72)

where K_R,i, a_R,I, and β_i are the R–P parameters derived from the corresponding individual R–P isotherm equations. The competitive non-modified R–P model is rearranged to the following modified competitive R–P Model to take the characteristics of each species into account,

 (73)

where values of _R,i are estimated from competitive adsorption data.

Marquardt’s percent standard deviation (MPSD) was used to test the adequacy and accuracy of various isotherm model fits with the experimental data. Based on a linearly regression analysis, Srivastava et al. (2006) showed that R² was closer to unity for the R–P and the Freundlich models compared to the Langmuir model. Thus it was concluded that equilibrium adsorption data of single component adsorption, i.e. Cd(II) and Zn(II) ion, could be represented more appropriately by the R–P and the Freundlich models in the studied concentration range and at lower concentrations, since the Langmuir isotherm did not adequately represent the equilibrium sorption. The single-component Langmuir constants are Q^o (monolayer saturation at equilibrium) and b (corresponding to the concentration where the amount of metal ion bound to adsorbent is equal to Q^o/2 and which indicates the affinity of the metal ions to bind with adsorbent). The results of this study showed that the amount of Zn(II) ions per unit weight of BFA for the complete monolayer surface coverage was higher than that of Cd(II), and a large value of b implied strong bonding of Zn(II) ions to BFA. K_F and n, the single-component Freundlich constants (indicating the adsorption capacity and adsorption intensity, respectively) were also calculated, and the BFA displayed greater heterogeneity for Cd(II) than for Zn(II) ions. The value of n was found to be 1, which implied that both the Cd(II) and Zn(II) ions were favorably adsorbed by BFA at pH 6.0. The magnitude of K_F also showed higher uptake of Zn(II) than Cd(II) ions by BFA at pH 6.0. It was noted that the Redlich–Peterson constant b normally lies between 0 and 1, indicating a favorable adsorption. The experimental and predicted equilibrium uptake (q_e) evaluated from the single-component Langmuir, Freundlich, and Redlich–Peterson models for the individual adsorption of Cd(II) and Zn(II) onto BFA at pH 6.0 were also compared, and the MPSD values were calculated. Based on the lower MPSD values, the R–P and Freundlich models displayed better fit to the experimental adsorption data than the Langmuir model.

Studies comparing alternative multi-component models to fit data

The simultaneous adsorption data of Cd(II) and Zn(II) on the BFA was also fitted to multi-component isotherm models (Srivastava et al. 2006). The multi-component non-modified Langmuir model displayed a poor fit to the experimental data (MPSD = 101.6). All the modified Langmuir coefficients (η_L,i) estimated were much greater than 1.0, indicating that the non-modified multi-component Langmuir model related to the individual isotherm parameters could not be used to predict the binary-system adsorption. However, the use of the interaction term, η_L,i, in the modified Langmuir model (MPSD = 28.3) improved the fit of the non-modified Langmuir model. The use of the multi-component extended-Langmuir model in the cited study showed its inadequacy to represent the experimental data (MPSD values were large). The K_i values, reflecting the affinity between the adsorbent and the metals in the binary systems by using the BFA were found to be 0.04 L/mg for both Cd(II) and Zn(II). The overall total metal ions uptake (q_max) by BFA is 7.24 mg/g. These values were found to be considerably lower than the sum of the maximum total capacities of Cd(II) and Zn(II) ions resulting from the single component adsorption systems. For that reason, it was concluded that the adsorption sites of Cd(II), and Zn(II) in binary systems onto BFA may likely be partially overlapped. These lower metal ion uptake results also implied that there may be a variety of binding sites on the adsorbents showing partial specificity to the individual metal ions. The information obtained from the maximum capacities seems to violate the basic assumptions of the Langmuir model, i.e. that the entire adsorbent surface is homogeneous and that there is no lateral interaction between the adsorbate molecules. Consequently, the affinity of each binding site for the adsorbate molecules should be uniform. The use of interaction terms, η_R,i, for the modified R–P model (MPSD = 24.1) improved the fit of the non-modified R–P model (MPSD = 52.0); however, the SRS model (MPSD = 15.4) provided the best-fit to the binary adsorption data of Cd(II) and Zn(II) onto BFA. Thus, the SRS isotherm was found to best represent the binary system adsorption. This improved SRS performance was expected, as BFA has a heterogeneous surface, and the adsorption of the single metal ions had also been well represented by the Freundlich isotherm equation. It was evident that the modification of the Freundlich equation, as given by the SRS model, took into account the interactive effects of individual metal adsorbate ions between and among themselves and the adsorbent reasonably well. Therefore, the binary adsorption of metal ions onto BFA can be represented satisfactorily and adequately by the SRS model. The multicomponent SRS model is applicable to those systems where each component individually obeys the single-component Freundlich isotherm.

The isotherm coefficients can be determined from the mono-component isotherm except for the adsorption competition coefficients, a_ij, which have to be determined experimentally. The competition coefficients, a_ij, describe the inhibition to the adsorption of component i by component j. The two components for the cited study were found to obey the single-component Freundlich model individually. The competition coefficients a_ij and a_ji were estimated from the competitive adsorption data for Cd(II), Ni(II), and Zn(II) ions by using the MS EXCEL 2002 program. A comparison of the competition coefficients in the adsorption isotherm equation shows that the uptake of the strongly adsorbed Zn(II) was significantly inhibited by the presence of Cd(II) (a₂₁ = 2.70). Similarly, the uptake of Cd(II) by BFA was suppressed in the presence of Zn(II) ion in the solution (a₁₂ = 2.15).

Three-dimensional (3-D) adsorption isotherm surfaces were used to evaluate the performance of the binary metal ions adsorption system. It was found that the SRS model predictions for the simultaneous adsorption of Cd(II) and Zn(II) ions by BFA from aqueous solution were very satisfactory.

Srivastava et al. (2009a) analyzed the competitive adsorption of Cd(II), Ni(II), and Zn(II) ions onto rice husk ash (RHA) from ternary metal ion mixtures. Various isotherm equations such as those of Freundlich (Eq. 6), Langmuir (Eq. 2), and R-P (Eq. 14) were used to describe the monocomponent equilibrium characteristics of adsorption of individual ions onto RHA. The MPSD error values were the lowest for the Freundlich model, followed by the R-P and Langmuir models. Therefore, the equilibrium adsorption data of Cd(II), Ni(II), and Zn(II) ion adsorption on RHA could be represented appropriately by the Freundlich model within the studied concentration range. RHA has a heterogeneous surface. It is, therefore, expected that the Freundlich and R-P isotherm equations can better represent the equilibrium sorption data than the Langmuir isotherm model. The simultaneous sorption data of Cd(II), Ni(II), and Zn(II) from the ternary mixture onto RHA was fitted to the multicomponent isotherm models, viz., nonmodified, modified, and extended Langmuir models (Eq. 2); nonmodified and modified R-P models (Eq. 14); and the SRS model. On the basis of the MPSD error function, it was found that the simultaneous sorption phenomena of Cd(II), Ni(II), and Zn(II) ions on the RHA could be adequately represented by the SRS model.

The sorption of heavy metals (lead, copper, and cadmium) by a marine algal biomass Sargassum sp. was studied in single and multiple metal-ion systems (Sheng et al. 2007). The equilibrium data for the single metal ion system was studied with the help of the Langmuir adsorption isotherm model (Eq. 11). The effect of the presence of multiple metal ions on the biosorption performance was investigated, and the results were evaluated using the modified competitive Langmuir model and modified Jain-Snoeyink model.

The extension of the basic Langmuir model to account for competitive adsorption in multiple-metal systems can be formulated as follows,

 (74)

where the terms q_max,i (monolayer sorption capacity) and b_i (affinity of sorbent for the sorbed species) are derived from the corresponding individual Langmuir isotherm equations; q_e,i and C_e,i are, respectively, the uptake and final concentration when adsorption equilibrium is reached, and n is the number of metal ions in solutions. The Langmuir model assumes that each component is adsorbed onto the surface according to ideal solute behaviors, where there is no interaction or competition between molecules involved under homogeneous conditions. To account for nonideal systems using the Langmuir theory, Jain and Snoeyink introduced an additional term into Eq. 74 for binary metal systems,

 (75)

where q_max,1 > q_max,2. The additional term on the right-hand side of Eq. 75 (proportional to the quantity q_max,1 – q_max,2) is the Langmuir expression for the amount of solute 1 adsorbed on to the surface without competition. The second term on the right hand side represents the amount of solute 1 adsorbed onto the surface in competition with solute 2. The amount of solute 2 adsorbed onto the sorbent surface can be calculated from Eq 74.

All the model parameters in these competitive isotherms for multiple-metal systems may be derived from single-component isotherms. Indeed, better accuracy may be achieved by extracting additional coefficients from the multiple-metal isotherms. For instance, an interaction term η, which is a characteristic of each species and is dependent on the sorption properties of the sorbents, has been defined in the modified competitive isotherms.

The modified competitive Langmuir model takes the form

 (76)

For a binary system, the modified Jain-Snoeyink model becomes

 (77)

The root-mean-square error (RMSE) was used to check the adequacy of the model. The sorption data for single metal system at different pH values were well-modeled by the Langmuir isotherm. However, in case of binary and tertiary metal systems, the original competitive Langmuir model and the Jain-Snoeyink model failed to fit the experimental data adequately, with all the R² values being less than 0.70. Experimental data fitted both the modified competitive Langmuir model and the modified Jain-Snoeyink model well. It was evident that the modified models, with the introduction of the interaction coefficient (η), considerably improved the accuracy of the modeling. Furthermore, it was also shown that the interaction coefficient ηderived from the binary metal system could be successfully applied to the ternary metal system, thus indicating the possibility of predicting biosorption performance of such complex systems, based on the modeling parameters obtained from simpler experiments.

The lead (II) biosorption potential of Aspergillus parasiticus fungal biomass was investigated in a batch system (Akar et al. 2007b). Freundlich (Eq. 14), Langmuir (Eq. 11), and Dubinin–Radushkevich (D–R) isotherms (Eq. 16) were used for the biosorption isotherm modelling. Results indicated that the Langmuir, Freundlich, and D–R isotherm models are suitable for describing the lead (II) biosorption equilibrium by A. parasiticus in the studied concentration range with the regression coefficient (R²) values more than 0.97. The R_L (affinity between sorbent and sorbate using Langmuir constants) value for this study was 1.73 × 10^–2, indicating that the biosorption of lead (II) was favorable. The Freundlich constants K_F and n indicate the biosorption capacity of the biosorbent and a measure of the deviation from linearity of the biosorption, respectively. The adequate description of the experimental results with all of the isotherm models investigated in this study implied that the biosorption of lead (II) ions onto A. parasiticus biomass was complex, involving more than one mechanism. The biosorption process could be described by ion exchange as the dominant mechanism, in addition to complexation with groups at the surface of this biosorbent. The ion exchange mechanism was confirmed by the E value obtained from D-R isotherm model as well.

The biosorption of chromium(VI) from saline solutions onto dried Rhizopus arrhizus was studied as a function of pH, initial chromium(VI), and salt (NaCl) concentrations in a batch system by Aksu and Balibek (2007). The equilibrium sorption data were analysed by using Freundlich (Eq. 14), Langmuir (Eq. 11), Redlich–Peterson (Eq. 23), and Langmuir–Freundlich (Sips) models. The two- and three-parameter adsorption models, using non-linear regression technique and isotherm constants, were evaluated depending on salt concentration.

The Langmuir–Freundlich (Sips) model used in the cited study is another three-parameter empirical model for the representing equilibrium biosorption data (Eq. 78). This model suggests that the equilibrium data follow Freundlich isotherm at lower solute concentration, and thus, do not obey Henry’s law, but that they follow a Langmuir pattern at higher solute concentration,

 (78)

where A, B, and m are the Langmuir–Freundlich parameters. Values for m>>1 indicate heterogeneous adsorbents, while values closer to or even equal to 1.0 indicate a material with relatively homogenous binding sites. In this case, the Sips model is reduced to the Langmuir equation.

Thus the equilibrium data were fitted to these isotherm models, and the values of average percentage errors and linear regression coefficients were the criteria for the selection of the most suitable isotherm model. On the basis of lower average percentage errors (in the range 0.8 to 2.4) and higher linear regression coefficients (in the range 0.998 to 1.000), the three-parameter Langmuir–Freundlich (Sips) model best described the chromium(VI) sorption isotherm data compared to other models examined, which suggested the monolayer, homogeneous sorption in single as well as salt-added binary-systems. The relatively lower percentage errors also indicated that both the two-parameter Langmuir and three-parameter Redlich–Peterson models were also very suitable for describing the biosorption equilibrium of chromium(VI) by the fungal cells in all cases. The other two-parameter model of Freundlich exhibited a poor fit to the biosorption data of chromium(VI) with an average percentage error more than 8.3.

The value of n (Freundlich constant), which was significantly higher than unity, indicated that chromium(VI) ions were favorably adsorbed under all the experimental conditions examined. The values of n at different salt concentrations also indicated that decreased chromium(VI) biosorption intensity was affected by salt addition into biosorption medium. The magnitude of the constant K_F (Freundlich constant) showed a relatively easy uptake of chromium(VI) ions from aqueous solution, with high adsorptive capacity of biomass for chromium(VI) in both single and binary systems. The presence of salt at any initial concentration was found to reduce the K_F constant significantly. The salt added at different levels also affected the Langmuir constants (Q^o and b). Dried R. arrhizus exhibited the maximum biosorption capacity (Q^o) for single chromium(VI) biosorption. The addition of salt decreased the Q^o value of chromium(VI) biosorption to an insignificant extent. A high value of the other Langmuir parameter, b, indicated a high affinity of the biosorbent for the sorbate. The highest b value obtained for monometal conditions also decreased with the addition of salt, indicating its negative effect on chromium(VI) biosorption. Related biosorption parameters were also calculated according to the three-parameter isotherm of Redlich–Peterson using a non-linear regression method for chromium(VI) biosorption at different salt levels. The Redlich–Peterson constant, K_RP, indicated that the adsorption capacity of biosorbent also diminished with increasing salt concentration. It is noted that β normally lies between 0 and 1, indicating favorable biosorption. In the case considered, the value of β was 1.0 for 50 g/L salt containing medium and tended to unity for other salt concentrations studied, suggesting that the isotherms approached the Langmuir form. The corresponding Langmuir–Freundlich parameters of A, B, and m for different salt concentrations were also calculated. Constant A, indicating the biosorption capacity and affinity of biosorbent to chromium(VI) ions, also decreased with salt addition. The value of m, an indicator of heterogeneity index, which was calculated to be about 1.0 for all levels of salt, showed that the chromium(VI) sorption data obtained in the cited study tended towards the Langmuir form rather than the Freundlich form, and thus, the fungus had a homogeneous surface. The results showed that three-parameter models represented the biosorption isotherm data much better than two-parameter models for all cases, with low percentage error values.

Again all these parameters changed with respect to the level of salt, and the results could be used to predict the adsorption behavior of chromium(VI) in an aqueous solution at a specific salt concentration. When isotherm constants were plotted against the salt concentration, the functional relationship between isotherm constants and salt concentration were not linear for the entire range of salt concentration. The results showed that the Freundlich and Langmuir parameters decreased following a second-order polynomial function of salt concentration with high linear regression coefficients. An exponential relationship between the Redlich–Peterson parameters (a_RP and K_RP) of chromium(VI) and salt concentration was obtained (β is assumed as 1 for all cases) with the correlation coefficients of 0.935 and 0.981, respectively. The relationship between the Langmuir–Freundlich model constants of A and B of chromium(VI) and salt concentration also followed an exponential equation with a high linear regression coefficient, while other Langmuir–Freundlich parameter, m, varied linearly with salt concentration.

The sorption of hexavalent chromium by marine brown algae Cystoseira indica, which was chemically-modified by cross-linking with epichlorohydrin (CB1, CB2), oxidized by potassium permanganate (CB3), or only washed by distilled water (RB), was studied with variations in the parameters: contact time, pH, initial metal ion concentration, and solid/liquid ratio (Basha et al. 2008). Langmuir (Eq. 11), Freundlich (Eq. 14), and Dubinin–Radushkevich models (Eq. 16) were used to describe the equilibrium between the Cr(VI) sorbed on the four pretreated biomasses of C. indica (q) and Cr(VI) ions in the solution. It was found that the Dubinin–Radushkevich isotherm model was in good agreement with all the experimental data as compared to the Langmuir and Freundlich isotherm models. The magnitude of E (mean free energy, kJ/mol), as obtained in the cited work, was useful for estimating the type of sorption reaction. The E values obtained were around 15 kJ/mol, which is in the energy range of an ion-exchange reaction, i.e., 8 to 16 kJ/mol. This E value suggests that biosorption of Cr(VI) by C. indica may be classed as an ion exchange reaction.

Chakravarty et al. (2008) used newspaper pulp as an adsorbent for the removal of copper from aqueous medium. The experimental data were analyzed using Freundlich (Eq. 14), Langmuir (Eq. 11), Dubinin–Radushkevich (D–R) (Eq. 16), and Redlich-Peterson (R–P) (Eq. 23) isotherm models. Their results showed that adsorption data fit reasonably well to the Langmuir, Freundlich, and R–P isotherms, as was reflected by the high correlation coefficients (R²). The Freundlich constant n increased with increasing initial Cu concentration. The D–R isotherm showed a definite trend for the K_DR value. However, the R² in D–R isotherm decreased with increasing initial concentration of Cu. No definite trend was observed for the Langmuir constants b and Q^o. The calculated R_L value (affinity) for adsorption of Cu(II) on the newspaper pulp adsorbent were found to be in the range of 0 to 1 at all initial Cu(II) concentrations, which confirms the favorable uptake of Cu(II) in the sorption process.

El Nemir et al. (2008) used a new activated carbon developed from date palm seed wastes, generated in the jam industry, for removing toxic chromium from an aqueous solution. The equilibrium data were tested using several isotherm models, including the Langmuir (Eq. 11), Freundlich (Eq. 14), Redlich–Peterson (Eq. 23), Temkin (Eq. 19), Dubinin–Radushkevich (Eq. 16), Koble–Corrigan (79), and generalized isotherm (80) equations.

The Koble–Corrigan equation used in the cited study is another isotherm model that depends on the combination of the Langmuir and Freundlich equations in one non-linear equation for representing the equilibrium adsorption data. It is represented as follows,

 (79)

where a, b, and n are the Koble–Corrigan parameters, which were obtained by solving Eq. (79) using SPSS version 10.0 computer program.

The generalized isotherm equation was also tested for correlation of the equilibrium data. The linear form of the generalized isotherm is given by,

 (80)

where K_G is the saturation constant (mg L^-1); N_b the cooperative binding constant; Q^o the maximum adsorption capacity of the adsorbent (mg g⁻¹) (obtained from the Langmuir isotherm model); and q_e (mg g⁻¹) and C_e (mg L⁻¹) are the equilibrium chromium concentrations in the solid and liquid phases, respectively.

In the cited study, four different linear forms of Langmuir isotherm were used, which are presented as follows (El Nemir et al. 2008):

Form Linear equation

Langmuir-1  (81)

Langmuir-2  (82)

Langmuir-3  (83)

Langmuir-4  (84)

Parameters related to each isotherm were determined by using linear regression analysis, and R² was calculated. Their results showed that the linear forms 1 and 2 of the Langmuir isotherm were best fitted for the equilibrium data in comparison to 3 and 4. The experimental data were also found to fit well to the Freundlich model, with n_F > 1, indicating that adsorption of Cr⁶⁺ onto DSC is a favorable physical process. The Koble-Corrigan equation also displayed high R² values, which indicates that the Koble-Corrigan equation had a strong goodness of fit to the experimental data. The values of b were 0.01 and 0.02, indicating the combination between heterogeneous and homogeneous adsorption of Cr⁶⁺ on DSC. The three isotherm constants (A, B, and g) of the Redlich-Peterson isotherm model were also calculated using non-linear regression analysis. The correlation coefficients obtained were comparable to the Langmuir and Freundlich equations, indicating that the Redlich–Peterson isotherm can be representative of the data obtained from the adsorption of Cr⁶⁺ on DSC. According to the R² values, the Temkin isotherm can also characterize the equilibrium adsorption data. However, this isotherm appeared to be less suitable than both the Koble-Corrigan and Redlich-Peterson isotherm models. The results using the D-R isotherm indicated that the D-R model had a poorer fit to the experimental data compared to the Langmuir, Freundlich, Koble-Corrigan, and Temkin isotherm models. The values of E (mean free energy) were also calculated and were found to be in the range of ion-exchange mechanisms, indicating that the adsorption process of Cr⁶⁺ ion onto DSC was physisorption. The results also showed that the generalized adsorption isotherm represented the equilibrium data reasonably well. The results showed that most of the tested isotherm models fitted well to the experimental data obtained for the adsorption of Cr⁶⁺ on DSC. Only the Dubinin-Radushkevich isotherm showed less agreement with the experimental data obtained.

The removal of poisonous Pb (II) from wastewater by different low-cost abundant adsorbents, e.g. rice husks, maize cobs, and sawdust was investigated, and the equilibrium adsorption capacity of adsorbents used for lead were measured and extrapolated using linear Freundlich, Langmuir, and Temkin isotherms (Abdel-Ghani et al. 2007). The experimental data were found to best fit the Temkin isotherm model.

Biosorption of chromium using suspended and immobilized cells of Rhizopus arrhizus was studied by evaluating the physicochemical parameters of the solution such as initial chromium ion concentration in both batch and packed bed reactor. The Langmuir, Freundlich, and Redlich-Peterson adsorption isotherm models were used in the equilibrium modeling. The Freundlich and Redlich-Peterson adsorption isotherm models were found to fit accurately with the experimental data (Preetha and Viruthagiri 2007b).

A need for further comparative evaluation of different isotherm models

Based on the discussion in the foregoing section, it is clear that data from metal sorption studies can be well fitted to a large number of different isotherm models. These models consider various aspects of the problem, including surface characterisitcs of the sorbent, affinity between sorbate and sorbent, potentiality of the sorbent, and the nature of the sorption process. However, two of the earliest models, those of Langmuir and Freundlich, continue to dominate the attention of the majority of researchers. This situation suggests that there may be important opportunities for researchers to compare the goodness of fit of existing or newly generated data to a wider variety of the available isotherm models, as outlined in this article. A goal of such efforts can be to determine which of the more recently developed models offers sufficient benefits in terms of fitting accuracy and mechanistic insights to justify their more frequent usage, with some emphasis placed on making the methods available to engineers in a user-friendly format.

CHEMICAL FACTORS AFFECTING SORPTION

Chemical Complexation

The concept of chemical complexation presupposes that there will be a site-specific interaction between particular kinds of metal ions and functional groups at the sorbate surface (Fourest and Volesky 1996; Kim et al. 1998; Merdy et al. 2002; Vijayaraghavan and Yun 2008; Lawrance 2010). Based on such concepts, some authors (Paagnanelli et al. 2005b; Zhang et al. 2005; Lodiero et al. 2006; Valex et al. 2006) have advocated an approach in which chemical complexation, rather than ion exchange, is used to account for adsorbed amounts as a function of solution concentrations.

Metal specificity

Some of the most cogent evidence in support of a chemical complexation concept of metal ion sorption consists of a dependency of molar adsporption capacities on the identity of the tested metal ion. Such differences have been widely reported, and the following citations are representative (Qadeer et al. 1996; Puranik and Paknikar 1999; Schiewer and Wong 1999; Kogej and Pavko 2001; Iqbal et al. 2002; Pardo et al. 2003; Chauhan et al. 2005b; Kobya et al. 2005; Saito and Isogai 2005; Romera et al. 2006; Zhang and Banks 2006; Afkhami et al. 2007; Arslan and Pehlivan 2008; Chen and Wang 2008b; Krishnani et al. 2008b). According to theories of chemical complexation (Lawrance 2010), differences in the ability of a surface site to bind different metals are often attributed to matching the radius of the metal ion, as well as the symmetry (octahedral, etc.) of its valence electron orbitals, to the positions of the surface-bound atoms (e.g. carboxylate groups) at the site of adsorption.

Hard and soft ions

The concept of hard and soft ions (Pearson 1963) has been used effectively to explain why certain metal ions tend to have greater affinity for certain types of sorbent surfaces (Avery and Tobin 1993; Brady and Tobin 1995; Chen and Wang 2007b,c, 2010; Gadd 2009). To summarize, ions that are called “soft” are those that have relatively loosely held, polarizable outer electrons, so that greater contributions of covalent character can be expected in their interaction with surface sites. Examples include lead and mercury. By contrast, “hard” ions have more closely held, less polarizable outer electrons, so that their interactions are more simply dominated by electrostatic factors. Nickel is a prime example. In some cases the molar amounts of adsorbed ions have been shown to be related to the ionic radius of the metal (Chen and Wang 2007b,c). Avery and Tobin (1993) observed that soft ions can be expected to favor sites containing S and N atoms, whereas hard ions can be expected to favor sites with oxygen atoms.

Demonstration of Metal Ion Interaction with Functional Groups

A large number of studies have provided evidence in support of metal ions interaction with specific functional groups at the substrate surface. Many authors have used shifts in the maxima of infrared light adsorption as evidence for specific interactions (Ashkenazy et al. 1997; Guibaud et al. 2003; Ahluwalia and Goyal 2005a; Deng and Ting 2005a; Ahalya et al. 2006, 2007; Chen and Yang 2006; Murphy et al. 2007; Arief et al. 2008; Li et al. 2008; Bakir et al. 2009; Garcia-Reyes et al. 2009; Iqbal et al. 2009a; O’Connell et al. 2010). NMR spectra also have been used to substantiate the involvement of specific chemical sites in the binding of metal ions (Araujo et al. 2007). One question that has not been completely resolved by these investigations is whether the observed shifts in IR absorbance maxima might be an effect, rather than indicating a cause of metal ion binding. This lack of certainty regarding cause/effect is because anything that changes the electron density significantly near a functional group can be expected to have an impact on the energy content of the associated covalent bonds. It follows that the presence of a metal ion in a fixed position on a substrate surface will change the IR spectra associated with adjacent functional groups, whether or not they each have a positive effect on the bonding mechanism.

In particular, IR evidence has supported the participation of carboxylate groups in metal ion binding (Ahluwalia and Goyal 2005a; Gardea-Torresdey et al. 2002). Several studies have supported such a conclusion by selectively converting the existing carboxylic acid groups to another form and displaying a substantial decrease in metal uptake (Beveridge and Murray 1980; Fourest and Volesky 1996; Ashkenazy et al. 1997; Kapoor and Viraraghavan 1997; Tiemann et al. 1999; Romero-Gonzalez et al. 2001; Bai and Abraham 2002; Tiemann et al. 2002; Chubar et al. 2003; Meunier et al. 2003b; Sheng et al. 2004b; Southichak et al. 2006b; Suksabye et al. 2007; Iqbal et al. 2009b; Murphy et al. 2009a). Chen and Wang (2008a) and Park et al. (2008d) used X-ray methods to display evidence of oxygen atom participation in the binding of zinc to yeast cell biomass. Other authors have used the pH-dependency of adsorption as contributing evidence to support the importance of carboxylic acid groups in metal ion binding (Tiemann et al. 2000; Malik et al. 2002; Guo et al. 2008). A related approach, involving immersion calorimetry and shifts in pH, has been pioneered by Lopez-Ramon et al. (1999).

Effects of pH on Metal Ion Sorption

Although a great deal of attention has been paid to dissociation constants associated with different kinds of acidic groups at sorbent surfaces, inadequate attention has been paid to the simultaneous effects of pH on the ionic species of metal ions that are present in the bulk solution.

Close inspection of the next to last column in Table A indicates a common theme for the effects of pH on adsorption of metal cations. Authors of several studies have indicated that adsorption is favored by “increasing pH,” but only up to a certain limit. The trend with increasing pH (i.e., 3 to 6) is generally explained by increasing dissociation of carboxylic acid groups on the cellulosic substrate, which results in an increasing ion exchange capacity of the material. The climactic upper end of a typical curve of adsorptive capacity vs. pH is generally understood to entail an equilibrium between the soluble hydrated metal ion and a corresponding insoluble neutral hydroxide species (Chang et al. 1997; Leyva-Ramos et al. 1997; Schneider et al. 2001; Dastgheib and Rockstraw 2002a; Mohan and Pittman 2006; Sciban et al. 2006b; Sheng et al. 2007; Demirbas 2008). In addition, certain ions tend to form polynuclear species as they progressively interact with OH^– ions in solution (O’Connell et al. 2010).

Essentially the opposite pH dependency is often observed when evaluating the adsorption of Cr(VI) species onto biosorbent surfaces. As shown by many entries in Table A, the highest sorption of Cr from Cr(VI)-containing solutions is generally found within a pH range of about zero to 2 (see, for instance Gupta et al. 1999). Under such conditions the cellulosic carboxyl groups are expected to be fully protonated, thus minimizing any electrostatic barrier to sorption of negative chromate ions.

Redox Effects

Changes in the valence state of the metal

During the past decade significant progress has been achieved in understanding the adsorption of metal ions that are prone to changes in their oxidation state. Though chromium has received the greatest attention, valence changes also can play a role in the removal of platinum (Chen et al. 2007), mercury (Lloyd-Jones et al. 2004; El-Shafey 2010); copper (Chandran et al. 2002), gold, silver, and palladium (Cox et al. 2005) from aqueous solution. The following researchers have reported evidence for Cr(VI) reduction to Cr(III) accompanying its adsorption (Raji and Anirudhan 1997; Selomulya et al. 1999; Han et al. 2000; El-Shavey and Canepa 2003; Deng and Ting 2005c; Deng et al. 2006; El-Shafey 2005; Gao et al. 2005; Park et al. 2004b, 2005b,c, 2006a, 2007a,b, 2008d; Murphy et al. 2009b; Suksabye et al. 2007, 2009). In certain cases, reduction from Cr(VI) to Cr(V) was observed (Suksabye et al. 2009). The reduction of Cr(VI) to Cr(III) has been found to proceed under highly acidic conditions (i.e., pH between 1 and 3) (Candela et al. 1995). The reduction of metal ions by microbes has also been demonstrated as a mechanism of metal removal from water (Chandran et al. 2002).

Changes in the valence state of the substrate

Research has confirmed an expected balance between reduction and oxidation whenever there is a valence change of a metal during the course of its adsorption. In other words, if the metal is reduced, something else in the system must be oxidized. Several researchers have observed oxidation of surface groups of a cellulose-based sorbent material when reductively adsorbing various precious metals or hexavalent chromium (Cox et al. 2005; Park et al. 2005a,b, 2006c, 2007b, 2008b; Elangovan et al. 2008a; Murphy et al. 2009b). Oxidation of the substrate as part of the adsorption mechanism is consistent also with the form of the rate expression that governs the uptake of chromium (Park et al. 2005c). Further support for this type of mechanism was provided by Praghakran et al. (2009), who showed that the rate of reductive sorption of Cr(VI) was highly dependent on the nature of the substrate, with superior results being achieved using coffee dust. Studies involving the blocking of specific sites showed that both carboxyl goups and amino groups can participate in the reductive adsorption process of Cr(VI) (Park et al. 2005d). In the case of chromium (VI) adsorption, it can be considered highly fortuitous that oxidation of the surface often increases the number of carboxylate groups, which are well suited to the bonding of the positively charged Cr(III) ions that result from the process.

Figure 4 provides a schematic of some of the concepts just described. The question mark at the left of the figure represents an intial challenge in accounting for the adsorption of a negative ion onto a predominantly negative cellulose-based material. The rightward arrow represents a combined process in which (a) groups at the substrate surface (or in the bulk phase) are oxidized, (b) the chromium(VI) is reduced to Cr(III), and (c) the positively charged Cr(III) ion is simultaneously or sequentially bound to negative sites at the substrate surface.

A worthy question for future research is to explore whether intentional changes to the state of oxidation of a substrate can be used in order to induce subsequent changes in valence of metal ions, thus facilitating their collection. Such a proposal is consistent with work reported by Berenquer et al. (2009) who applied galvanic reduction and oxidation to activated carbon. Changes in the surface groups were observed, similar to what can be achieved by treatment with reducing agents or oxidizing agents. However, it is not yet known whether related approaches could facilitate the conversion of, say, Cr(VI) to the less toxic and more adsorbable Cr(III) form. Such observations raise questions as to whether surface sites also could be manipulated by selecting anaerobic vs. aerobic conditions of digestion (Joseph et al. 2009).

Reversibility of Metal Binding

In order to optimize conditions of regeneration or disposal, it is important to understand what circumstances can lead to the reversal of metal sorption onto cellulose-dried surfaces The most straightforward way to encourage metal ions to go back into solution is to replace those ions with a large excess of something else. Hydrochloric acid is the most commonly specified reagent, as indicated by the many entries in the next to last column of Table A. Regeneration, however, also has been achieved by strong base, or less commonly by a concentrated salt solution. Many studies have demonstrated restoration of a majority of the original sorption capacity after the restorative treatment. Increases in adsorption capacity, relative to the original capacity, have been observed in some cases (Chang et al. 1997).

Figure 5 illustrates one way that an ion-exchange material can be regenerated. The left-hand part of the figure depicts a metal ion bound in bi-dentate fashion to a pair of carboxylate groups at a surface. Addition of salt brine (or acid, NaOH, etc.) causes the target metal(s) to be released into the brine, while sodium ions are left weakly associated with the carboxylate groups on the sorbate.

CLOSING COMMENTS

Any review of such an active and multifaceted field as biosorption of metals is bound to leave out information or to oversimplify critical details. Fortunately, as noted in the Introduction, subtopics of the field have already been reviewed, often with a particular focus that may be of interest to readers (see Table 1).

The explosive growth in publications in this area, with expected faster pace of technological developments to come, may still be difficult to grasp. Combinations of biomass types, chemical or thermal modificationis, and target metals are often being studied in relative isolation and often without a strong theoretical justification for the combinations of factors that are compared. Some studies appear to just apply well-known methods to different species of biomass or different metal ions. The scene sometimes can be compared to a wild frontier, in which the first researcher to generate, say, a Langmuir isotherm, can lay claim to having found something new.

The field of study will hopefully continue to mature during the coming decades, and in our opinion two kinds of studies would be very valuable to lead the way into the future. On the one hand, there is a need for mechanistic studies that deal more cogently with the molecular mechanisms, including such issues as binding sites, valence states, coordination chemistry, and the speciation of metals in solution. On the other hand, there need to be a life cycle analysis, considering the wasteproducts of metal remediation operations.

A Need for Studies Related to Proteinacious Binding Sites

One apparent gap in research that can be expected to attract increasing attention during the coming decade concerns the use of engineered proteins for advanced adsorption technologies. A study by Kostal et al. (2005) points the way in this direction, showing how biotechnological and nanotechnological principles can be applied. A study by Vinopal et al. (2007) showed that certain peptide sequences have highly specific affinity for certain metal ions. In principle, the sorptive properties of enzymatic materials could be optimized by genetic manipulation. Once the optimized biomaterial has been created, the genes could be inserted into a suitable organism to produce suitable quantities for the needed application. It is possible that such proteinaceous biosorbent products could be supported on cellulosic scaffolds, facilitating their use in either batch-type or column-type treatment schemes.

A Need for Application of Nanotechnology Concepts

Given the major attention to nanotechnology during the past decade, there has been a notable lack of consideration of related concepts in the case of biosorption. In particular, the concept of self-assembly (Ninham and Lo Nostro 2010) should be considered relative to the manner in which adjacent hydrated metal cations arrange themselves in various instantaneous configurations at a substrate surface. Attention also should be focused on the nucleation of colloidal precipitates of metal hydroxides as the pH is raised in the neighborhood of a solubility limit (Chang et al. 1997; Leyva-Ramos et al. 1997; Schneider et al. 2001; Sciban et al. 2006b; Demirbas 2008). Presumably such precipitated metal hydroxides could either remain as a colloidal suspension or it may be adsorbed onto the biosorbent material. In the latter case, the mechanisms of adsorption may be entirely different from those governing adsorption of the cationic forms of the same metals.

Another aspect of nanotechnology involves accessibility. Metal-binding sites at the surfaces of biomass samples can be effective only if the metal ions are able to reach them. Mesopores in the biomass may have closed up irreversibily at some point due to an uncertain history of the drying of the material (Stone and Scallan 1966). Such effects have the potential to explain the huge ranges (typically three orders of magnitude or more) that have been reported by different researchers for the metal-uptake capacities of nominally similar biomass samples. Future studies need to address these issues science at the nanoscale level.

A Need for Life Cycle Analysis Research

An article by Chojnacka (2010) has helped to define the scope of some needed research related to the life cycle of bioremediation. For a healthy ecosystem it is necessary to account not only for biosorption (which mainly involves dead biomass), but also for bioaccumulation, which involves ingestion and retention within living organisms. The two phenomena can be expected to interact in complex ways, keeping in mind that some metals are prone to building up over time to toxic levels in living tissues.

As was noted in the introduction, there has been inadequate research towards the merits of landfilling or land application as an alternative strategy for dealing with spent cellulose-based sorbent material. Though it is reasonable to be concerned with leachate that contains metals, research is needed to determine whether the metal-binding capacity of the cellulosic material is sufficient such that the resulting concentrations of soluble species remain in a generally safe or beneficial range, especially in the case of metals that can be considered as essential minerals when they are present at low levels. By contrast, the most widely used regeneration schemes require major shifts in pH, leading to the requirement that the pH be adjusted prior to discharge of the water, which will then have a higher level of salinity. Another potentially beneficial strategy that deserves exploration is the incineration of spent biosorbent, making it possible to concentrate the collected metals in the resulting ash. Though there is potential to make use of biosorption as a fuel source, there is need to be concerned about the energy requirement for evaporation of water, as well as measures to reduce airborne particulates.

While there is clear evidence that bio-sorption could be a viable alternative for the remediation of contaminated aquifers, there are several decision factors that companies must consider when selecting an appropriate treatment technology. These factors include but are not limited to treatment cost, volume of contaminated material to be treated, treatment time, site location, the complexity of the target pollutant, and the surrounding water quality. Technologies that merely change the form of toxic wastes, such as in adsorption processes without proper removal, may lead to future problems and costs. Therefore, effective remediation treatment technologies must consider the entire fate of wastes over a longer period. It would be easy to say that these factors are independent. However, it’s more likely that the decision complexity is increased due to the interdependency of these factors. Further, companies that would like to select in-situ soil remediation options must weigh that approach against dig-and-haul operators, who remove contaminated soil to landfill sites at a possibly cheaper cost. Consequently, the decision to utilize bio-sorption processes, regardless of whether it is a proven technology, is a complex decision.

A Need for Non-Selective Sorption

The mining industry has provided early stimulus for research into metal removal technology. Perhaps as a consequence, many of the studies cited in this review dealing with simultaneous adsorption of more than one type of metal contain the implicit assumption that highly specific adsorption of certain classes of metal may be advantageous. However, it would appear that the multi-component nature of typical biomass samples renders them more suitable, in principle, for a significantly different application, the removal of a wide range of pollutants simultaneously. Biosorbents have the potential for use during polishing treatments of both freshwater and wastewater. One can expect that the public will become increasingly aware, of the importance of metal ion control in both tap water and in water discharged to the environment. By contrast, in cases where highly selective sorption of certain metal ions is required, perhaps as a preliminary step in the isolation of pure metals or compounds, it would make sense to employ relatively pure chelating agents. As one option, chelating functional groups can optionally be bound to cellulosic supports, depending on the needs of the process.

Another area that requires research attention is the use of cellulosic materials to remove not only heavy metals, but also organic toxins. It is reasonable to expect that various industrial outfalls and municipal wastewater streams may contain both heavy metals and organic contaminants. The subject of biosorption of organic materials such as dyes will be addressed in Part 2 of this series.

REFERENCES CITED

Abdel-Aal, S. E., Gad, Y. H., and Dessouki, A. M. (2006). “The use of wood pulp and radiation modified starch in wastewater treatment,” Journal of Applied Polymer Science 99(5), 2460-2469.

Abdel-Ghani, N. T., Hefny, M., and El-Chagbaby, G. A. F. (2007). “Removal of lead from aqueous solution using low cost abundantly available adsorbents,” Int. J. Environ. Sci. Tech. 4(1), 67-73.

Abdel-Ghani, N. T., Hefny, M. M., and El-Chaghaby, G. A. (2008). “Removal of metal ions from synthetic wastewater by adsorption onto Eucalyptus camaldulenis tree leaves,” J. Chilean Chem. Soc. 53(3), 1585-1587.

Abia, A. A., Horsfall, M., and Didi, O. (2003). “The use of chemically modified and unmodified cassava waste for the removal of Cd, Cu and Zn ions from aqueous solution,” Bioresour. Technol. 90, 345-348.

Abia, A. A., and Igwe, J. C. (2005). “Sorption kinetics and intraparticulate diffusivities of Cd, Pb and Zn ions on maize cob,” Afr. J. Biotech. 4(6), 509-512

Acar, F. N., and Malkoc, E. (2004). “Removal of chromium (VI) from aqueous solution by Fagus orientalis,” Bioresour. Technol. 94, 13-15.

Acemioğlu, B., Samil, A., Alma, M. H., and Gundogan, R. (2003). “Copper(II) removal from aqueous solution by organosolv lignin and its recovery,” J. Appl. Polym. Sci. 89(6), 1537-1541.

Acharya, J., Sahu, J. N., Mohanty, C. R., and Meikap, B. C. (2009). “Removal of lead(II) from wastewater by activated carbon developed from Tamarind wood by zinc chloride activation,” Chem. Eng. J. 149(1-3), 249-262.

Addour, L., Belhocine, D., Boudries, N., Comeau, Y., Pauss, A., and Mameri, N. (1999). “Zinc uptake by Streptomyces rimosus biomass using a packed-bed column,” J. Chem. Technol. Biotechnol. 74(11), 1089-1095.

Aderhold, D., Williams, C. J., and Edyvean, R. G. J. (1996). “The removal of heavy-metal ions by seaweeds and their derivatives,” Bioresource Technology 58(1), 1-6.

Afkhami, A., Madrakian, T., Karimi, Z., and Amini, A. (2007). “Effect of treatment of carbon cloth with sodium hydroxide solution on its adsorption capacity for the adsorption of some cations,” Colloids Surf. A – Physicochem. Eng. Aspects 304(1-3), 36-40.

Agarwal, G. S., Bhuptawat, H. K., and Chaudhari, S. (2006). “Biosorption of aqueous chromium (VI) by Tamarindus indica seeds,” Bioresour. Technol. 97, 949-956.

Aggarwal, D., Goyal, M., and Bansal, R. C. (1999). “Adsorption of chromium by activated carbon from aqueous solution,” Carbon 37, 1989-1997.

Agrawal, A., Kumar, V., and Pandey, B. D. (2006). “Remediation options for the treatment of electroplating and leather tanning effluent containing chromium – A review,” Mineral Processing Extrac. Metall. Rev. 27(2), 99-130.

Agrawal, A., Sahu, K. K., and Pandey, B. D. (2004). “Removal of zinc from aqueous solutions using sea nodule residue,” Colloids Surf. A 237, 133-140.

Ahalya N., Kanamadi R. D., and Ramachandra, T. V. (2005). “Biosorption of chromium (VI) from aqueous solutions by the husk of Bengal gram (Cicer arientinum),” Electron. J. Biotechnol. 8, 258-264.

Ahalya, N., Kanamadi, R. D., and Ramachandra, T. V. (2006). “Biosorption of iron(III) from aqueous solutions using the husk of Cicer arientinum,” Indian J. Chem. Technol. 13(2), 122-127.

Ahalya, N., Kanamadi, R. D., and Ramachandra, T. V. (2007). “Cr(VI) and Fe(III) removal using Cajanus cajan husk,” J. Environ. Biol. 28(4), 765-769.

Ahluwalia, S. S., and Goyal, D. (2005a). “Removal of heavy metals from waste tea leaves from aqueous solution,” Eng. Life Sci. 5, 158-162.

Ahluwalia, S. S., and Goyal, D. (2005b). “Microbial and plant derived biomass for removal of heavy metals from waste water,” Bioresour. Technol. 98, 2243-2257.

Ahmady-Asbchin, S., Andres, Y, Gerente, C., and Le Cloirec, P. (2008). “Biosorption of Cu(II) from aqueous solution by Fucus serratus: Surface characterization and sorption mechanisms,” Bioresour. Technol. 99(14), 6150-6155.

Ajmal, M., Khan, A. H., Ahmad, S., and Ahmad, A. (1998). “Role of sawdust in the removal of copper(II) from industrial wastes,” Water Res. 32(10), 3085-3091.

Ajmal, M., Rao, R. A. K., Ahmad, R., and Ahmad, J. (2000). “Adsorption studies on Citrus reticulata (fruit peel of orange): Removal and recovery of Ni(II) from electroplating wastewater,” Journal of Hazardous Materials 79(1-2), 117-131.

Ajmal, M., Rao, R. A. K., Ahmad, R., and Khan, M. A. (2006). “Adsorption studies on parthenium hysterophrous weed: Removal and recovery of Cd (II) from wastewater,” J. Hazard. Mater. B135, 242-248.

Ajmal, M., Rao, R. A. K., Anwar, S., Ahmad, J., and Ahmad, R. (2003). “Adsorption studies on rice husk: Removal and recovery of Cd(II) from wastewater,” Bioresour. Technol. 86(2), 147-149.

Ajmal, M., Rao, R. A. K., and Khan, M. A. (2005). “Adsorption of Cu from aqueous solution on Brassica cumpestris (mustard oil cake),” J. Hazard. Mater. B122, 177-183.

Akar, T., and Tunali, S. (2006). “Biosorption characteristics of Aspergillus flavus biomass for removal of Pb(II) and Cu(II) ions from aqueous solutions,” Bioresour. Technol. 97, 1780-1787.

Akar, T., Tunali, S., and Cabuk, A. (2007b). “Study on the characterization of lead (II) biosorption by fungus Aspergillus parasiticus,” Appl. Biochem. Biotech. 136(3), 389-405.

Akhtar, N., Iqbal, J., and Iqbal, M. (2004). “Removal and recovery of nickel (II) from aqueous solution by loofa sponge-immobilized biomass of Chlorella sorokiniana: Characterization studies,” J. Hazard. Mater. B108, 85-94.

Aksakal, O., Ucun, H., and Kaya, Y. (2008). “Removal of nickel from aqueous solutions by Nordmann fir (Abies nordmanniana (Stev.) Spach subsp. nordmanniana) cones,” Environ. Eng. Manag. J. 7(4), 359-363.

Aksu, Z. (2001). “Equilibrium and kinetic modeling of cadmium(II) biosorption by C. vulgaris in a batch system: Effect of temperature,” Separation and Purification Technology 21, 285-294.

Aksu, Z., and Açikel, U. (1999). “A single-staged bioseparation process for simultaneous removal of copper(II) and chromium(VI) by using C. vulgaris,” Process Biochem. 34(6-7), 589-599.

Aksu, Z., and Açikel, U. (2000). “Modeling of a single-staged bioseparation process for simultaneous removal of iron(III) and chromium(VI) by using Chlorella vulgaris,” Biochem. Eng. J. 4(3), 229-238.

Aksu, Z., Açikel, U., Kabasakal, E., and Tezer, S. (2002b). “Equilibrium modeling of individual and simultaneous biosorption of chromium(VI) and nickel(II) onto dried activated sludge,” Water Res. 36(12), 3063-3073.

Aksu, Z., Acikel, U., and Kutsal, T. (1997). “Application of multicomponent adsorption isotherms to simultaneous biosorption of iron(III) and chromium(VI) on C. vulgaris,” J. Chem. Technol. Biotechnol. 70(4), 368-378.

Aksu, Z., Acikel, U., and Kutsal, T. (1999). “Investigation of simultaneous biosorption of copper (II) and chromium (VI) on dried Chlorella vulgaris from binary metal mixtures: Application of multicomponent adsorption isotherms,” Sep. Sci. Technol. 34(3), 501-524.

Aksu, Z., and Akpinar, D. (2001). “Competitive biosorption of phenol and chromium(VI) from binary mixtures onto dried anaerobic activated sludge,” Biochem. Eng. J. 7(3), 183-193.

Aksu, Z., and Balibek, E. (2007). “Chromium(VI) biosorption by dried Rhizopus arrhizus: Effect of salt (NaCl) concentration on equilibrium and kinetic parameters,” J. Hazardous Mater.145(1-2), 210-220.

Aksu, Z., and Donmez, G. (2006). “Binary biosorption of cadmium(II) and nickel(II) onto dried Chlorella vulgaris: Co-ion effect on mono-component isotherm parameters,” Process Biochem. 41(4), 860-868.

Aksu, Z., Gönen, F., and Demircan, Z. (2002a). “Biosorption of chromium(VI) ions by Mowital (R) B30H resin immobilized activated sludge in a packed bed: Comparison with granular activated carbon,” Process Biochem. 38(2), 175-186.

Aksu, Z., and Isoglu, I. A. (2005). “Removal of copper(II) ions from aqueous solution by biosorption onto agricultural waste sugar beet pulp,” Process Biochem. 40(9), 3031-3044.

Aksu, Z., Sag, Y., and Kutsal, T. (1992). “The biosorption of Copper(II) by C. vulgaris and Z. ramigera,” Environ. Technol. 13, 579-586.

Akthar, N., Sastry, S., and Mohan, M. (1995). “Biosorption of silver ions by processed Aspergillus niger biomass,” Biotechnol. Lett. 17, 551-556.

Akthar, M., Sastry, K., and Mohan, P. (1996). “Mechanism of metal ion biosorption by fungal biomass,” Biometals 9, 21-28.

Alaerts, G. J., Jitjaturunt, V., and Kelderman, P. (1989). “Use of coconut shell-based activated carbon for chromium(VI) removal,” Water Sci. Technol. 21, 1701-1704.

Al-Asheh, S., and Duvnjak, Z. (1998). “Binary metal sorption by pine bark: Study of equilibria and mechanisms,” Sep. Sci. Technol. 33, 1303-1329.

Aldor, I., Fourest, E., and Volesky, B. (1995). “Desorption of cadmium from algal biosorbent,” Can. J. Chem. Eng. 73(4), 516-522.

Ali, I., and About-Enein, H. Y. (2006). Instrumental Methods in Metal Ion Speciation, CRC, Taylor & Francis Group, New York, Chromatographic Science Ser. Vol. 96,

Ali, I., and Gupta, V. K. (2006). “Advances in water treatment by adsorption technology,” Nature Protocols 1(6), 2661-2667.

Aloysius, R., Karim, M. I. A., and Ariff, A.B. (1999). “Cadmium biosorption by free and immobilized live biomass of Rhizopus oligosporus,” Pakistan Journal of Biological Sciences 2, 74-79.

Alpana, S. (2008). “Chromium adsorption on agro based waste materials: A review,” Res. J. Biotech. 207-209.

Altun, T., and Pehlivan, E. (2007). “Removal of copper(II) ions from aqueous solutions by walnut-, hazelnut- and almond-shells,” Clean – Soil Air Water 35(6), 601-606.

Altundogan, H. S. (2005). “Cr(VI) removal from aqueous solution by iron(III) hydroxide-loaded sugar beat pulp,” Process Biochem. 40, 1443-1452.

Altundogan, H. S., Arslan, N. E., and Tumen, F. (2007). “Copper removal from aqueous solutions by sugar beet pulp treated by NaOH and citric acid,” J. Hazard. Mater. 149(2), 432-439.

Alves Gurgel, L. V., and Gil, L. F. (2009). “Adsorption of Cu(II), Cd(II) and Pb(II) from aqueous single metal solutions by succinylated twice-mercerized sugarcane bagasse functionalized with triethylenetetramine,” Water Res. 43(18), 4479-4488.

Amin, M. N., Kaneco, S., Kitagawa, T., Begum, A., Katsumata, H., Suzuki, T., and Ohta, K. (2006). “Removal of arsenic in aqueous solutions by adsorption onto waste rice husk,” Indus. Eng. Chem. Res. 45(24), 8105-8110.

Amuda, O. S., Giwa, A. A., and Bello, I. A. (2007). “Removal of heavy metal from industrial wastewater using modified activated coconut shell carbon,” Biochem. Eng. J. 36(2), 174-181.

Anandkumar, J., and Mandal, B. (2009). “Removal of Cr(VI) from aqueous solution using Bael fruit (Aegle marmelos correa) shell as an adsorbent,” J. Hazard. Mater. 168(2-3), 633-640.

Anayurt, R. A., Sari, A., and Tuzen, M. (2009). “Equilibrium, thermodynamic and kinetic studies on biosorption of Pb(II) and Cd(II) from aqueous solution by macrofungus (Lactarius scrobiculatus) biomass,” Chem. Eng. J. 151(1-3), 255-261.

Andrès, Y., Texier, A.C., and Le Cloirec, P. (2003). “Rare earth elements removal by microbial biosorption: A review,” Environ. Technol. 24, 1367-1375.

Anirudhan, T. S., Rijith, S., and Suchithra, P. S. (2010). “Preparation and characterization of iron(iii) complex of an amino-functionalized polyacrylamide-grafted lignocellu-losics and its application as adsorbent for chromium(VI) removal from aqueous media,” J. Appl. Polymer Sci. 115(4), 2069-2083.

Annadurai, G., Juang, R. S., and Lee, D .L. (2003). “Adsorption of heavy metals from water using banana and orange peels,” Water Sci. Technol. 47(1), 185-190.

Anoop Krishnan, K., and Anirudhan, T. S. (2003). “Removal of cadmium (II) from aqueous solutions by steam-activated sulphurized carbon prepared from sugar-cane bagasse pith: Kinetics and equilibrium studies,” Water SA 29, 147-156.

Aoki, N., Fukushima, K., Kurakata, H., Sakamoto, M., and Furuhata, K. (1999a). “6-deoxy-6-mercaptocellulose and its S-substituted derivatives as sorbents for metal ions,” Reactive and Functional Polymers 42(3), 223-233.

Aoki, Y., Tanaka, K., Sakamoto, M., and Furuhata, K. (1999b). “Sorption of metal ions by bead cellulose grafted with amidoximated polyacrylonitrile,” Sen’i Gakkaishi 55(12), 569-575.

Aoyama, M. (2003). “Removal of Cr(VI) from aqueous solution by London plane leaves,” J. Chem. Technol. Biotechnol. 78, 601-604.

Aoyama, M., Honma, S., Kasai, A., Iseda, Y, Nakajima, A., and Sakaguchi, T. (1991). “Uranium uptake by conifer leaves,” Holzforschung 45(1), 75-77.

Aoyama, M., Kishino, M., and Jo, T.-S. (2004). “Biosorption of Cr(VI) on Japanese cedar bark,” Sep. Sci. Technol. 35(5), 1149-1162.

Aoyama, M., Sugiyama, T.., Doi, S., Cho, N.-S., and Kim, H.-E. (1999). “Removal of hexavalent chromium from dilute aqueous solution by coniferous leaves,” Holzforschung 53, 365-368.

Aoyama, M., and Tsuda, M. (2001). “Removal of Cr(VI) from aqueous solutions by larch bark,” Wood Sci. Technol. 35(5), 425-432.

Apiratikul, R., and Pavasant, P. (2006). “Sorption isotherm model for binary component sorption of copper, cadmium, and lead ions using dried green macroalga, Caulerpa lentillifera,” Chem. Eng. J. 119(2-3), 135-145.

Araujo, G. C. L., Lemos, S. G. Ferreira, A. G., Freitas, H., and Nogueira, A. R. A. (2007). “Effect of pre-treatment and supporting media on Ni(II), Cu(II), Al(III) and Fe(III) sorption by plant root material,” Chemosphere 68(3), 537-545.

Aravindhan, R., Madhan, B., Rao J. R., and Nair, B. U. (2004a). “Recovery and reuse of chromium from tannery wastewaters using Turbinaria ornate seaweed,” J. Chem. Technol. Biotechnol. 79(11), 1251-1258.

Aravindhan, R., Madhan, B., Rao J. R., Nair, B. U., and Ramasami, T. (2004b). “Bioaccumulation of chromium from tannery wastewater: An approach for chrome recovery and reuse,” Environ. Sci. Technol. 38(1), 300-306.

Argun, M. E., and Dursun, Ş. (2008a). “A new approach to modification of natural adsorbent for heavy metal adsorption,” Bioresour. Technol. 99, 2516-2527.

Argun, M. E., and Dursun, Ş. (2008b). “Cadmium removal using activated pine bark,” J. Int. Environ. Appl. Sci. 3, 37-42.

Argun, M. E., Dursun, S., Gur, K., Ozdemir, C., Karatas, M., and Dogan, S. (2005a). “Adsorption of copper on modified wood (pine) materials,” Cellulose Chem. Technol. 39(5-6), 581-591.

Argun, M. E., Dursun, S., Gur, K., Ozdemir, C., Karatas, M., and Dogan, S. (2005b). “Nickel adsorption on the modified pine tree materials,” Environ. Technol. 26(5), 479-487.

Argun, M. E., Dursun, S., and Karatas, M. (2009). “Removal of Cd(II), Pb(II), Cu(II) and Ni(II) from water using modified pine bark,” Desalination 249(2), 519-527.

Argun, M. E., Dursun, S., Karatas, M., and Guru, M. (2008). “Activation of pine cone using Fenton oxidation for Cd(II) and Pb(II) removal,” Bioresour. Technol. 99(18), 8691-8698.

Argun, M. E., Dursun, Ş., Özdemir, C., and Karatas, M. (2007). “Heavy metal adsorption by oak sawdust: Thermodynamics and kinetics,” J. Hazard. Mater. 141(1), 77-85.

Arief, V. O., Trilestari, K., Sunarso, J., Indraswati, N., and Ismadji, S. (2008). “Recent progress on biosorption of heavy metals from liquids using low cost biosorbents: Characterization, biosorption parameters and mechanism studies,” Clean – Soil Air Water 36(12), 937-962.

Arica, M. Y., and Bayramoğlu, G. (2005). “Cr(VI) biosorption from aqueous solutions using free and immobilized biomass of Lentinus sajor-caju: Preparation and kinetic considerations,” Colloids Surf. A 253(1-3), 203-221.

Arica, M. Y., Tuzun, I., Yalcin, E., Ince, O., and Bayramoglu, G. (2005). “Utilization of native, heat and acid-treated microalgae Chlamydomonas reinhardtii preparations for biosorption of Cr(VI) ions,” Process Biochem. 40(7), 2351-2358.

Ariff, A. B., Mel, M., Hasan, M. A., and Karim, M. I. A. (1999). “The kinetics and mechanisms of lead(II) biosorption by powderized Rhizopus olgisporus,” World Journal of Microbiology Biotechnology 15, 291-298.

Arshad, M., Zafar, M. N., Younis, S, and Nadeem, R. (2008). “The use of neem biomass for the biosorption of zinc from aqueous solutions,” J. Hazard. Mater. 157, 534-540.

Arslan, G., and Pehlivan, E. (2008). “Uptake of Cr³⁺ from aqueous solution by lignite-based humic acids,” Bioresour. Technol. 99(16), 7597-7605.

Arulanantham, A., Balasubramanian, N., and Ramakrishna, T. V. (1989). “Coconut shell carbon for treatment of cadmium and lead containing wastewater,” Metal Finishing 87, 51-55.

Ash, B., Satapathy, D., Mukherjee, P. S., Nanda, B., Gumaste, J. L., and Mishra, B. K. (2006). “Characterization and application of activated carbon prepared from waste coir pith,” J. Sci. Indus. Res. 65(12), 1008-1012.

Ashkenazy, R., Gottlieb, L, and Yannai, S. (1997). Characterization of acetone-washed yeast biomass functional groups involved in lead biosorption, Biotechnol. Bioeng. 55, 1-10.

Atkinson, B. W., Bux, F., and Kasan, H. C. (1998). “Considerations for application of biosorption technology to remediate metal-contaminated industrial effluents,” Water SA 24, 129-135.

Avery, S. V., and Tobin, J. M. (1993). “Mechanism of adsorption of hard and soft metal ions to Saccharomyces cerevisiae and influence of hard and soft anions,” Applied and Environmental Microbiology 59, 2851-2856.

Azab, M. S., and Peterson, P. J. (1989). “The removal of Cd from wastewater by the use of biological sorbents,” Water Sci. Technol. 21(12), 1705-1706.

Aziz, A., Ouali, M. S., Elandaloussi, E. H., De Ménorval, L. C., and Lindheimer, M. (2009). “Chemically modified olive stone: A low-cost sorbent for heavy metals and basic dyes removal from aqueous solutions,” J. Hazard. Mater. 163, 441-447.

Babel, S., and Kurniawan, T. A. (2003). “Low-cost adsorbents for heavy metals uptake from contaminated water: A review,” J. Hazardous Mater. 97(1-3), 219-243.

Babel, S, and Kurniawan, T. A. (2004). “Cr(VI) removal from synthetic wastewater using coconut shell charcoal and commercial activated carbon modified with oxidizing agent and/or chitosan,” Chemosphere 54(7), 951-967.

Babula, P., Adam, V., Opatrilova, R., Zehnalek, J., Havel, L., and Kizek, R. (2008). “Uncommon heavy metals, metalloids and their plant toxicity: A review,” Environ. Chem. Lett. 6(4), 189-213.

Baccar, R., Bouzid, J., Feki, M., and Montiel, A. (2009). “Preparation of activated carbon from Tunisian olive-waste cakes and its application for adsorption of heavy metal ions,” J. Hazard. Mater. 162(2-3), 1522-1529.

Bai, R. S., and Abraham, T. E. (1998). “Studies on biosorption of chromium(VI) by dead fungal biomass,” J. Sci. Ind. Res. 57(10-11), 821-824.

Bai, R. S., and Abraham, T. E. (2001). “Biosorption of Cr(VI) from aqueous solution by Rhizopus nigricans,” Bioresour. Technol. 79, 73-81.

Bai, R. S., and Abraham, T. E. (2002). “Studies on enhancement of Cr(VI) biosorption by chemically modified biomass of Rhizopus nigricans,” Water Res. 36(5), 1224-1236.

Bai, R. S., and Abraham, T. E. (2003). “Studies on chromium(VI) adsorption-desorption using immobilized fungal biomass,” Bioresour. Technol. 87(1) 17-26.

Baillet, F., Magnin, J.-P., Cheruy, A., and Ozil, P. (1998). “Chromium precipitation by the acidophilic bacterium Thiobacillus ferrooxidans,” Biotechnology Letters 20(1), 95-99.

Bailey, S. E., Olin, T. J., Bricka, R. M., and Adrian, D. D. (1999). “A review of potentially low-cost sorbents for heavy metals,” Wat. Res. 33(11), 2469-2479.

Bakhti, M. Z., Selatnia, A., and Junter, G. A. (2008). “Biosorption of Ag⁺ from aqueous solution by Streptomyces rimosus biomass,” Intl. J. Environ. Pollution 34(1-4), 297-307.

Bakir, A., McLoughlin, P., Tofail, S. A. M., and Fitzgerald, E. (2009). “Competitive sorption of antimony with zinc, nickel, and aluminum in a seaweed based fixed-bed sorption column,” Clean – Soil Air Water 37(9), 712-719.

Bakkaloglu, I., Butter, T. J., Evison, L. M., Holland, F. S., and Hancock, I. C. (1998). “Screening of various types biomass for removal and recovery of heavy metals (Zn, Cu, Ni) by biosorption, sedimentation and desorption,” Water Sci. Technol. 38(6), 269-277.

Balan, C., Bilba, D., and Macoveanu, M. (2009). “Studies on chromium(III) removal from aqueous solutions by sorption on Sphagnum moss peat,” J. Serbian Chem. Soc. 74(8-9), 953-964.

Balasubramanian, R., Perumal, S. V., and Vijayaraghavan, K. (2009). “Equilibrium isotherm studies for the multicomponent adsorption of lead, zinc, and cadmium onto Indonesian peat,” Ind. Eng. Chem. Res. 48(4), 2093-2099.

Banarjee, S. S., Joshi, M. V., and Jayaram, R. V. (2004). “Removal of Cr(VI) and Hg(II) from aqueous solutions using fly ash and impregnated fly ash,” Sep. Sci. Technol. 39(7), 1611-1629.

Bansal, M., Garg, U., Singh, D., and Garg, V. K. (2009). “Removal of Cr(VI) from aqueous solutions using pre-consumer processing agricultural waste: A case study of rice husk,” J. Hazardous Mater. 162(1), 312-320.

Bansode, R. R., Losso, J. N., Marshall, W. E., Rao, R. M., and Portier, R. J. (2003). “Adsorption of metal ions by pecan shell-based granular activated carbons,” Bioresour. Technol. 89(2), 115-119.

Baral, S. S., Das, S. N., and Rath, P. (2006). “Hexavalent chromium removal from aqueous solution by adsorption on treated sawdust,” Biochem. Eng. J. 31(3), 216-222.

Baral, S. S., Das, S. N., Rath, P., Chaudhury, G. R., and Swamy, Y. V. (2007). “Removal of Cr(VI) from aqueous solution using waste weed, Salvinia cucullata,” Chem. Ecol. 23(2), 105-117.

Barkat, M., Nibou, D., Chearouche, S., and Mellah, A. (2009). “Kinetics and thermodynamics studies of chromium(VI) ions adsorption onto activated carbon from aqueous solutions,” Chem. Eng. Proc. 48(1), 38-47.

Basci, N., Kocadagistan, E., and Kocadagistan, B. (2003). “Biosorption of Cu II from aqueous solutions by wheat shells,” Desalination 164, 135-140.

Basha, S., Murthy, Z. V. P., and Jha, B. (2008). “Biosorption of hexavalent chromium by chemically modified seaweed, Cystoseira indica,” Chem. Eng. J. 137(3), 480-488.

Basha, S., Murthy, Z. V. P., and Jha, B. (2009). “Removal of Cu(II) and Ni(II) from industrial effluents by brown seaweed, Cystoseira indica,” Ind. Eng. Chem. Res. 48(2), 961-975.

Basso, M. C., Cerrella, E. G., and Cukierman, A. L. (2002a). “Lignocellulosic materials as potential biosorbents of trace toxic metals from wastewater,” Indus. Eng. Chem. Res. 41(15), 3580-3585.

Basso, M. C., Cerrella, E. G., and Cukeirman, A. L. (2002b). “Activated carbons developed from a rapidly renewable biosources for removal of cadmium (II) and nickel(II) ions from dilute aqueous solutions,” Ind. Eng. Chem. Res. 41(2), 180-189.

Basso, M. C., Cerrella, E. G., and Cukierman, A. L. (2004). “Cadmium uptake by lignocellulosic materials: Effect of lignin content,” Sep. Sci. Technol. 39(5), 1163-1175.

Basumajumdar, A., Pal, T. K., and Bhattacharyya, S. (2003). “Biosorption of cadmium by Aspergillus niger from multimetal solution,” J. Indian Chem. Soc. 80, 899-902.

Bayramoglu, G., Bektas, S., and Arica, M. Y. (2003). “Biosorption of heavy metal ions on immobilized white-rot fungus Trametes versicolor,” Journal of Hazardous Materials 101(3), 285-300.

Bayramoglu, G., Celik, G., Yalcin, E., Yilmaz, M., and Arica, M. Y. (2005). “Modification of surface properties of Lentinus sajor-caju mycelia by physical and chemical methods: Evaluation of their Cr⁺⁶ removal efficiencies from aqueous medium,” J. Hazard. Mater. 119, 219-229.

Belhalfaoui, B., Aziz, A., Elandaloussi, E., Ouali, M. S., and De Menorval, L. C. (2009). “Succinate-bonded cellulose: A regenerable and powerful sorbent for cadmium-removal from spiked high-hardness groundwater,” J. Hazardous Mater. 169(1-3), 831-837.

Bello, G., Cid, R., Garcia, R., and Arriagada, R. (1999). “Retention of Cr(VI) and Hg(II) in Eucalyptus globulus– and peach stone-activated carbons,” J. Chem. Technol. Biotech. 74(9), 904-910.

Benaissa, H. (2006). “Screening of new sorbent materials for cadmium removal from aqueous solutions,” J. Hazard. Mater. 132(2-3), 189-195.

Bendezu, S., Oyague, J., Romero, A., Garcia, R., Munoz, Y., and Escalona, N. (2005). “Chromium adsorption from tannery effluents by activated carbons prepared from coconut shells by chemical activation with KOH and ZnCl₂,” J. Chilean Chem. Soc. 50(4), 677-684.

Berenquer, R., Marco-Lozar, J. P., Quijada, C., Cazorla-Amoros, D., and Morallon, E. (2009). “Effect of electrochemical treatments on the surface chemistry of activated carbon.” Carbon47(4), 1018-1027.

Beveridge, T. J., and Murray, R. G. E. (1980). “Sites of metal deposition in the cell wall of Bacillus subtilis,” J. Biotechnol. 141, 876-887.

Bhainsa, K. C., and D’Souza, S. F. (1999). “Biosorption of uranium(VI) by Aspergillus fumigatus,” Biotechnol. Techniques 13(10), 695-699.

Bicak, N., Sherrington, D. C., and Senkal, B. F. (1999). “Graft copolymer of acrylamide onto cellulose as mercury selective sorbent,” Reactive and Functional Polymers 41(1-3), 69-76.

Bingol, A., Ucun, H., Bayhan, Y. K., Karagunduz, A., Cakici, A., and Keskinler, B. (2004). “Removal of chromate anions from aqueous stream by a cationic surfactant-modified yeast,” Bioresour. Technol. 94, 245-249.

Bischoff, R., and Schwedt, G. (1982). “Differentiation of the chemical forms of trace amounts of copper in peas and soy meal by means of ion exchange on cellulose materials and molecular exclusion chromatography,” Anal. Chem. Acta 142, 109-118.

Bishnoi, N. R., Bajaj, M., Sharma, N., and Gupta, A. (2004). “Adsorption of chromium (VI) on activated rice husk carbon and activated alumina,” Bioresour. Technol. 91(3), 305-307.

Bishnoi, N. R., and Garima. (2005). “Fungus – An alternative for bioremediation of heavy metal containing wastewater: A review,” J. Sci. Indus. Res. 64(2), 93-100.

Bishnoi, N. R., Kumar, R., Kumar, S., and Rani, S. (2007). “Biosorption of Cr(III) from aqueous solution using algal biomass Spirogyra spp.,” J. Hazardous Mater. 145(1-2), 142-147.

Blanco, A., Sanz, B., Llama, M. J., and Serra, J. L. (1999). “Biosorption of heavy metals to immobilized Phormidium laminosum biomass,” J. Biotechnol. 69, 227-240.

Blazquez, G., Hernainz, F., Calero, M., Martin-Lara, M. A., and Tenorio, G. (2009). “The effect of pH on the biosorption of Cr(III) and Cr(VI) with olive stone,” Chem. Eng. J. 148, 473-478.

Blazquez, G., Hernainz, F., Calero, M., and Ruiz-Nunez, L. F. (2005). “Removal of cadmium ions with olive stones: The effect of some parameters,” Proc. Biochem. 40(8), 2649-2654.

Bozic, D., Stankovic, V., Gorgievski, M., Bogdanovic, G., and Kovacevic, R. (2009). “Adsorption of heavy metal ions by sawdust of deciduous trees,” J. Hazardous Mater. 171(1-3), 684-692.

Brady, D., Stoll, A., and Duncan, J. R. (1994). “Biosorption of heavy metal cations by non-viable yeast biomass,” Environ. Technol. 15, 429-438.

Brady, J. M., and Tobin, J. M. (1995). “Binding of hard and soft metal ions to Rhizopus arrhizus biomass,” Enzyme Microb. Technol. 17(9), 791-796.

Brown, P., Jefcoat, I. A., Parrish, D., Gill, S., and Graham, E. (2000). “Evaluation of the adsorption capacity of peanut hull pellets for heavy metals in solution,” Adv. Environ. Res. 4, 19-29.

Budinova, T., Ekinci, E., Yardim, F., Grimm, A., Bjornbom, E., Minkova, V., and Goranova, M. (2006). “Characterization and application of activated carbon produced by H₃PO₄ and water vapor activation,” Fuel Proc. Technol. 87(10), 899-905.

Bueno, B. M., Molina, F. F., Torem, M. L., de Mesquita, L. M. S., and Pino, G. A. H. (2009). “Removal of Pb(II) from aqueous solutions by biosorption with R.opacus,” REM – Revista Esoca de Minas 62(4), 487-494.

Bueno, C. I. D., and Carvalho, W. A. (2007). “Lead(II) removal in discontinous systems by carbon activated by phosphoric acid and vapor,” Quimica Nova 30(8), 1911-1918.

Bulut, Y., and Tez, Z. (2007a). “Adsorption studies on ground shells of hazelnut and almond,” J. Hazardous Mater. 149(1), 35-41.

Bulut, Y., and Tez, Z. (2007b). “Removal of heavy metals from aqueous solution by sawdust adsorption,” J. Environ. Sci. 19(2), 160-166.

Bustard, M., and McHale, A. P. (1998). “Biosorption of heavy metals by distillery-derived biomass,” Bioprocess Engineering 19, 351-353.

Cabtingan, L. K., Agapay, R. C., Rakels, J. L. L., Ottens, M., and Van Der Wielen, L. A. M. (2001). “Potential of biosorption for the recovery of chromate in industrial wastewaters,” Ind. Eng. Chem. Res. 40(10), 2302-2309.

Cabuk, A., Akar, T., Tunali, S., and Gedikli, S. (2007). “Biosorption of Pb(II) by industrial strain of Saccharomyces cerevisiae immobilized on the biomatrix of cone biomass of Pinus nigra: Equilibrium and mechanism analysis,” Chem. Eng. J. 131(1-3), 293-300.

Calero, M., Hernainz, F., Blazquez, G., Martin-Lara, M. A., and Tenorio, G. (2009). “Biosorption kinetics of Cd(II), Cr(III) and Pb(II) in aqueous solutions by olive stone,” Bras. J. Chem. Eng. 26(2), 265-273.

Calero de Hoces, M., Bermudez de Castro, F. H., Blazquez Garcia, G., and Tenorio Rivas, G. (2006). “Equilibrium modeling of removal of cadmium ions by olive stones,” Environ. Prog. 25(3), 261-266.

Candela, M. P., Martinez, J. M., and Macia, R. T. (1995). “Chromium(VI) removal with activated carbons,” Water Res. 29(9), 2174-2180.

Capasso, R., Pigna, M., de Martino, A., Pucci, M., Sannino, F., and Violante, A. (2004). “Potential remediation of waters contaminated with Cr(III), Cu, and Zn by sorption on the organic polymeric fraction of olive mill wastewater (polymerin) and its derivatives,” Environ. Sci. Technol. 38(19), 5170-5176.

de Castro, Dantas, T. N., Dantas Neto, A. A., de A. Moura, M. C. P., Barrow Neto, E. L., and de Paiva Telemaco, E. (2010). “Chromium adsorption by chitosan impregnated with microemulsion,” Langmuir 17(14), 4256-4260.

Cay, S., Uyanık, A., and Zasık, A. O. (2004). “Single and binary component adsorption of copper(II) and cadmium(II) from aqueous solutions using tea-industry waste,” Sep. Purif. Technol. 38, 273-280.

Celik, A, and Demirbas, A. (2005). “Removal of heavy metal ions from aqueous solutions via adsorption onto modified lignin from pulping wastes,” Energy Sources 27(12), 1167-1177.

Chai, X.-S., Hou, Q. X., Zhu, J. Y., Chen, S.-L., Wang, S. F., and Lucia, L. (2003). “Carboxyl groups in wood fibers. 1. Determination of carboxyl groups by headspace gas chromatography,” Ind. Eng. Chem. Res. 42, 5440-5444.

Chakravarty, S., Pimple, S., Hema, S., Chaturvedi, T., Singh, S., and Gupta, K. K. (2008). “Removal of copper from aqueous solution using newspaper pulp as an adsorbent,” Journal of Hazardous Materials 159, 396-403.

Chamarthy, S., Seo, C. W., and Marshall, W. E. (2001). “Adsorption of selected toxic metals by modified peanut shells,” J. Chem. Technol. Biotechnol. 76(6), 593-597.

Chand, R., Narimura, K., Kawakita, H., and Ohto, K. (2009). “Grape waste as a biosorbent for removing Cr(VI) from aqueous solution,” J. Hazard. Mater. 163(1), 245-250.

Chadlia, A., Mohamed, K., Najah, L., and Farouk, M. M. (2009). “Preparation and characterization of new succinic anhydride grafted Posidonia for the removal of organic and inorganic pollutants,” J. Hazardous Mater. 172(2-3), 1579-1590.

Chandran, C. B., Subramanian, T. V., and Felse, P. A. (2002). “Parametric optimization of biotransportation of Cd by mixed function oxidase produced under controlled conditions in Saccharomyces cerevisiae,” Indian Chem. Engr. 44(1), 223-229.

Chang, L. W. (1996). Toxicology of Metals, Lewis Publ., Boca Raton, 1198 pp.

Chang, J. S., and Hong, J. (1994). “Biosorption of mercury by the inactivated cells of Pseudomonas aeruginosa PU21 (Rip 64),” Biotechnol. Bioengng. 44(8), 999-1006.

Chang, J. S., Law, R., and Chang, C. C. (1997). “Biosorption of lead, copper and cadmium by biomass of Pseudomonas aeruginosa PU21,” Water Research 31(7), 1651-1658.

Chauhan, G. S., Guleria, L., and Sharma R., (2005a). “Synthesis, characterization and metal ion sorption studies of graft copolymers of cellulose with glycidyl methacrylate and some comonomers,” Cellulose 12(1), 97-110.

Chauhan, G. S., Singh, B., Chauhan, S., Verma, M., and Mahajan, S. (2005b). “Sorption of some metal ions on cellulosic-based hydrogels,” Desalination 181(1-3), 217-224.

Chauhan, G. S., Singh, B., Sharma, R. K., Verma, M., Jaswal, S. C., and Sharma, R. (2006). “Use of biopolymers and acrylamide-based hydrogels for sorption of Cu²⁺, Fe²⁺ and Cr⁶⁺ ions from their aqueous solutions,” Desalination 197(1-3), 75-81.

Chaves, T. F., de Queiroz, Z. F., de Sousa, D. N. R., Girao, J. H. S., and Rodrigues, E. D. (2009). “Use of rice husk ash (RHA) obtained from thermal energy generation as Zn(II) adsorbent in aqueous solutions,” Quimica Nova 32(6), 1378-1383.

Chavez-Guerrero, L., Rangel-Mendez, R., Munoz-Sandoval, E., Cullen, D. A., Smith, D. J., Terrones, H., and Terrones, M. (2008). “Production and detailed characterization of bean husk-based carbon: Efficient cadmium (II) removal from aqueous solutions,” Water Res. 42(13), 3473-3479.

Chen, C., and Wang, J. L. (2007a). “Characteristics of Zn²⁺ biosorption by Saccharomyces cerevisiae,” Biomed. Environ. Sci. 20(6), 478-482.

Chen, C., and Wang, J. L. (2007b). “Correlating metal ionic characteristics with biosorption capacity using QSAR model,” Chemosphere 69(10), 1610-1616.

Chen, C., and Wang, J. L. (2007c). “Influence of metal ionic characteristics on their biosorption capacity by Saccharomyces cerevisiae,” Appl. Microbiol. Biotechnol. 74(4), 911-917.

Chen, C., and Wang, J. L. (2008a). “Investigating the interaction mechanism between zinc and Saccharomyces cerevisiae using combined SEM-EDX and XAFS,” Appl. Microbiol. Biotechnol. 79(2), 293-299.

Chen, C., and Wang, J. L. (2008b). “Removal of Pb²⁺, Ag⁺, Cs⁺, and Sr²⁺ from aqueous solution by brewery’s waste biomass,” J. Hazard. Mater. 151(1), 65-70.

Chen, C., and Wang, J. L. (2010). “Removal of heavy metal ions by waste biomass of Saccharomyces cerevisiae,” J. Environ. Eng. – ASCE 136(1), 95-102.

Chen, J. P., and Wu, S. (2004a). “Acid/base-treated activated carbons: Characterization of functional groups and metal adsorptive properties,” Langmuir 20(6), 2233-2242.

Chen, J. P., and Wu, S. (2004b). “Simultaneous adsorption of copper ions and humic acid onto an activated carbon,” J. Colloid Interface Sci. 280(2), 334-342.

Chen, J. P., Wu, S. N., and Chong, K. H. (2003). “Surface modification of a granular activated carbon by citric acid for enhancement of copper adsorption,” Carbon 41(10), 1979-1986.

Chen, J. P., and Yang, L. (2005). “Chemical modification of Sargassum sp. for prevention of organic leaching and enhancement of uptake during metal biosorption,” Ind. Eng. Chem. Res. 44(26), 9931-9942.

Chen, J. P., and Yang, L. (2006). “Study of a heavy metal biosorption onto raw and chemically modified Sargassum sp. via spectroscopic and modeling analysis,” Langmuir 22(21), 8906-8914.

Chen, S., Zou, Y., Yan, Z., Shen, W., Shi, S., Zhang, X., and Wang, H. (2009). “Carboxymethylated-bacterial cellulose for copper and lead ion removal,” Journal of Hazardous Materials161(2-3), 1355-1359.

Chen, S. X., Huang, Z. Z., Liang, J., Jin, Y., and Zeng, H. M. (2002). “Reduction adsorption of silver ions and their distribution on activated carbon fibers,” New Carbon Mater. 17(3), 6-10.

Chen, S. X., Xu, R. M., Huang, H. X., Yi, F. Y., Zhou, X., and Zeng, H. M. (2007). “Reduction-adsorption behavior of platinum ions on activated carbon fibers,” J. Mater. Sci. 42(23), 9572-9581.

Chen, S. X., and Zeng, H. M. (2003). “Improvement of the reduction capacity of activated carbon fiber,” Carbon 41(6), 1265-1271.

Chen, X. C.,Wang, Y. P., Lin, Q., Shi, J. Y., Wu, W. X., and Chen, Y. X. (2005). “Biosorption of copper(II) and zinc(II) from aqueous solution by Pseudomonas putida CZ1,” Colloids Surf. B-Biointerfaces 46(2), 101-107.

Chen, Y. H., Wu, Q. L., Pan, N., Gong, J. H., and Pan, D. (2008). “Rayon-based activated carbon fibers treated with both alkali metal salt and Lewis acid,” Micropor. Mesopor. Mater.109(1-3), 138-146.

Chergui, A., Bakhti, M. Z., Chahboub, A., Haddoum, S., Selatnia, A., and Junter, G. A. (2007). “Simultaneous biosorption of Cu²⁺, Zn ²⁺ and Cr⁶⁺ from aqueous solution by Streptomyces rimosus biomass,” Desalination 206(1-3), 179-184.

Cho, D. H., and Kim, E.Y. (2003). “Characterization of Pb²⁺ biosorption from aqueous solution by Rhodotorula glutinis,” Bioprocess Biosystem Engineering 25, 271-277.

Choi, S. B., and Yun, Y. S. (2004). “Lead biosorption by waste biomass of Corynebacterium glutamicum generated from lysine fermentation process,” Biotechnol. Lett. 26(4), 331-336.

Choi, Y. J., Ahn, Y. H., Kang, M. S., Jun, H. K., Kim, I. S., and Moon, S. H. (2004). “Preparation and characterization of acrylic acid-treated bacterial cellulose cation-exchange membrane,” J. Chem. Technol. Biotech. 79(1), 79-84.

Chojnacka, K. (2006). “Biosorption of Cr(III) ions by wheat straw and grass: A systematic characterization of new biosorbents,” Pol. J. Environ. Stud. 15(6), 845-852.

Chojnacka, K. (2009). Biosorption and Bioaccumulation in Practice, Nova Sci. Publ., New York.

Chojnacka, K. (2010). “Biosorption and bioaccumulation – The prospects for practical applications,” Environ. Intl. 36(3), 299-307.

Chojnacka, K., and Michalak, I. (2009). “Using wood and bone ash to remove metal ions from solutions,” Global Next J. 11(2), 205-217.

Choksi, P. M., and Joshi, V. Y. (2007). “Adsorption kinetic study for the removal of nickel (II) and aluminum (III) from an aqueous solution by natural adsorbents,” Desalination 208(1-3), 216-231.

Chu, K. H., and Hashim, M. A. (2002). “Adsorption and desorption characteristics of zinc on ash particles derived from oil palm waste,” J. Chem. Technol. Biotech. 77(6), 685-693.

Chuah, T. G., Jumasiah, A., Azni, I., Katayon, S., and Choong, S. Y. T. (2005). “Rice husk as a potentially low-cost biosorbent for heavy metal and dye removal: An overview,” Desalination 175(3), 305-316.

Chubar, N., Carvalho, J. R., and Correia, M. J. N. (2003). “Cork biomass as biosorbent for Cu(II), Zn(II) and Ni(II),” Colloids Surf. A – Physicochem. Eng. Aspects 230(1-3), 57-65.

Cimino, G., Passerini, A., and Toscano, G. (2000). “Removal of toxic cations and Cr(VI) from aqueous solution by hazelnut shell,” Water Res. 34(11), 2955-2962.

Cochrane, E. L., Lu, S., Gibb, S. W., and Villaescusa, I. (2006). “A comparison of low-cost biosorbents and commercial sorbents for the removal of copper from aqueous media,” J. Hazard. Mater. B 137(1), 198-206.

Cooney, D. O. (1998). Adsorption Design for Wastewater Treatment, Lewis Publ., New York, 190 pp.

Corapcioglu, M. O., and Huang, C. P. (1987). “The adsorption of heavy metals onto hydrous activated carbon,” Wat. Res. 21(9), 1031-1044.

Cordero, T., Rodriguez-Mirasol, J., Tancredi, N., Piriz, J., Vivo, G., and Rodriguez, J. J. (2002). “Influence of surface composition and pore structure on Cr(III) adsorption onto activated carbons,” Ind. Eng. Chem. Res. 41(24), 6042-6048.

Cossich, E. S., Da Silva, E. A., Tavares, C. R. G., Cardozo, L., and Ravagnani, T. M. K. (2004). “Biosorption of chromium(III) by biomass of seaweed Sargassum sp. in a fixed-bed column,” Adsorption – J. Int. Adsorp. Soc. 10(2), 129-138.

Cox, M., Pichugin, A. A., El-Shafey, E. I., and Appleton, Q. (2005). “Sorption of precious metals onto chemically prepared carbon from flax shive,” Hydrometallurgy 78(1-2), 137-144.

Crini, G. (2005). “Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment,” Progress in Polymer Science 30(1), 38-70.

Crist, D. R., Crist, R. H., and Martin, J. R. (2003). “A new process for toxic metal uptake by a kraft lignin,” J. Chem. Technol. Biotech. 78(2-3), 199-202.

Crist, R. H., Martin, J. R., and Crist, D R. (2002). “Heavy metal uptake by lignin: Comparison of biotic ligand models with an ion-exchange process,” Environ. Sci. Technol. 36(7), 1485-1490.

Crist, R. H., Martin, J. R., and Crist, D. R. (2004). “Use of a novel formulation of kraft lignin for toxic metal removal from process waters,” Separation Sci. Technol. 39(7), 1535-1545.

Cui, J. R., and Zhang, L. F. (2008). “Metallurgical recovery of metals from electronic waste: A review,” J. Hazardous Mater. 158(2-3), 228-256.

Dakiky, M., Khamis, M., Manassra, A., and Mereb, M. (2002). “Selective adsorption of chromium(VI) in industrial wastewater using low-cost abundantly available adsorbents,” Adv. Environ. Res. 6(4), 533-540.

Daneshvar, N., Salari, D., and Aber, S. (2002). “Chromium adsorption and Cr(VI) reduction to trivalent chromium in aqueous solutions by soya cake,” J. Hazard. Mater. 94(1), 49-61.

Dang, V. B. H., Doan, H. D., Dang-Vu, T., and Lohi, A. (2009). “Equilibrium and kinetics of biosorption of cadmium (II) and copper (II) ions by wheat straw,” Bioresour. Tech. 100(1), 211-219.

Das, D. D., Mahapatra, R., Pradhan, J., Das, S. N., and Thakur, R. S. (2004). “Removal of Cr(VI) from aqueous solution using activated cow dung carbon,” J. Colloid Interface Sci. 232(2), 235-240.

Das, N., Vimala, R., and Karthika, P. (2008). “Biosorption of heavy metals – An overview,” Indian J. Biotechnol. 7(2), 159-169.

Dastgheib, S. A., and Rockstraw, D. A. (2001). “Pecan shell activated carbon: Synthesis, characterization and application for the removal of copper from aqueous solution,” Carbon39(12), 1849-1855.

Dastgheib, S. A., and Rockstraw, D. A. (2002a). “A model for the adsorption of single metal ion solutes in aqueous solution onto activated carbon produced from pecan shells,” Carbon40(11), 1843-1851.

Dastgheib, S. A., and Rockstraw, D. A. (2002b). “A systematic study and proposed model of the adsorption of binary metal ion solutes in aqueous solution onto activated carbon produced from pecan shells,” Carbon 40(11), 1853-1861.

Davila-Jimenez, M. M., Elizalde-Gonzalez, M. P, Geyer, W., Mattusch, J., and Wennrich, R. (2003). “Adsorption of metal cationic from aqueous solution onto a natural and a model biocomposite,” Colloids Surf A 219(1-3), 243-252.

Davis, T. A., Volesky, B., and Mucci, A. (2003). “A review of the biochemistry of heavy metal biosorption by brown algae,” Water Res. 37(18), 4311-430.

Dean, S. A., and Tobin, J. M. (1999). “Uptake of chromium cations and anions by milled peat,” Resour. Conserv. Recycl. 27(1-2), 151-156.

de Britto, J. S., da Costa, A. C. A., Luna, A. S., and Henriques, C. A. (2007). “Comparative study of ion-exchange and biosorption processes for the removal of Cd²⁺ and Zn²⁺ ions from aqueous effluents,” Adsorp. Sci. Technnol. 25(9), 661-671.

Deepa, K. K., Sathishkumar, M., Binupriya, A. R., Murugesan, G. S., Swaminathan, K., and Yun, S. E. (2006). “Sorption of Cr(VI) from dilute solutions and wastewater by live and pretreated biomass of Aspergillus flavus,” Chemosphere 62(5), 833-840.

De la Rosa, G., Gardea-Torresdey, J. L., Peralta-Videa, J. R., Herrera, I., and Contreras, C. (2003). “Use of silica-immobilized humin for heavy metal removal from aqueous solution under flow conditions,” Bioresour. Technol. 90(1), 11-17.

de Mesquita, J. P., Martelli, P. B., and Gorgulho, H. D. F. (2006). “Characterization of copper adsorption on oxidized activated carbon,” J. Braz. Chem. Soc. 17(6), 1133-1143.

Demiral, H., Demiral, I., Tumsek, F., and Karabacakoglu, B. (2008). “Adsorption of chromium(VI) from aqueous solution by activated carbon derived from olive bagasse and applicability of different adsorption models,” Chem. Eng. J. 144(2), 188-196.

Demirbas, A. (2004). “Adsorption of lead and cadmium ions in aqueous solutions onto modified lignin from alkali glycerol delignication,” J. Hazard. Mater. 109(1-3), 221-226.

Demirbas, A. (2005). “Adsorption of Cr(III) and Cr(VI) ions from aqueous solutions on to modified lignin,” Energy Sources 27(15), 1449-1455.

Demirbas, A. (2007). “Adsorption of Co(II) and Hg(II) from water and wastewater onto modified lignin,” Energy Sources Pt. A – Recov. Util. Environ. Effects 29(2), 117-123.

Demirbas, A. (2008). “Heavy metal adsorption onto agro-based waste materials: A review,” J. Hazardous Mater. 157(2-3), 220-229.

Demirbas, E., Dizge, N., Sulak, M. T., and Kobya, M. (2009). “Adsorption kinetics and equilibrium of copper from aqueous solutions using hazelnut shell activated carbon,” Chem. Eng. J.148(2-3), 480-487.

Demirbas, E., Kobya, M., and Konukman, A. E. S. (2008). “Error analysis of equilibrium studies for the almond shell activated carbon adsorption of Cr(VI) from aqueous solutions,” J. Hazardous Mater. 154(1-3), 787-794.

Demirbas, E., Kobya, M., Oncel, S., and Sencan, S. (2002). “Removal of Ni(II) from aqueous solution by adsorption onto hazelnut shell activated carbon: Equilibrium studies,” Bioresour. Technol. 84(3), 291-293.

Demirbas, E., Kobya, M., Senturk, E., and Ozkan, T. (2004). “Adsorption kinetics for the removal of chromium (VI) from aqueous solutions on the activated carbons prepared from agricultural wastes,” Water SA. 30(4), 533-539.

Deng, S. B., and Ting, Y. P. (2005a). “Fungal biomass with grafted poly(acrylic acid) for enhancement of Cu(II) and Cd(II) biosorption,” Langmuir 21(13), 5940-5948.

Deng, S., and Ting, Y.-P. (2005b). “Characterization of PEI-modified biomass and biosorption of Cu(II), Pb(II) and Ni(II),” Water Res. 39(10), 2167-2177.

Deng, S. B., and Ting, Y. P. (2005c). “Polyethylenimine-modified fungal biomass as a high-capacity biosorbent for Cr(VI) anions: Sorption capacity and uptake mechanisms,” Environ. Sci., Technol. 39(21), 8490-8496.

Deng, S., Ting, Y. P., and Yu, G. (2006). “Chromate sorption and reduction kinetics onto an aminated biosorbent,” Water Sci. Technol. 54(10), 1-8.

Dias, J. M., Alvim-Ferraz, M. C. M., Almeida, M. F., Rivera-Utrilla, J., and Sanchez-Polo, M. (2007). “Waste materials for activated carbon preparation and its use in aqueous-phase treatment: A review,” J. Environ. Manag. 85(4), 833-846.

Diniz, V., and Volesky, B. (2005). “Biosorption of La, Eu and Yb using Sargassum biomass,” Water Res. 39(1), 239-247.

Dizhbite, T., Zakis, G., Kizima A., Lazareva, E., Rossinskaya, G., Jurkjane, V., Telysheva, G., and Viesturs, U. (1999). “Lignin – A useful bioresource for the production of sorption-active materials,” Bioresour. Technol. 67, 221-228.

Doan, H. D., Lohi, A., Dang, V. B. H, and Dang-Vu, T. (2008). “Removal of Zn⁺² and Ni⁺² by adsorption in a fixed bed of wheat straw,” Proc. Safety Environ. Prot. 86(B4), 259-267.

Dönmez, G., and Aksu, Z. (2002). “Removal of Cr(VI) from saline wastewaters by Dunaliella species,” Process Biochem. 38, 751-762.

Dönmez, G., and Aksu, Z., Öztürk, A., and Kutsal, T. (1999). “A comparative study on heavy meatal biosorption characteristics of some algae,” Process Biochem. 34(9), 885-892.

Doyurum, S., and Celik, A. (2006). “Pb(II) and Cd(II) removal from aqueous solutions by olive cake,” J. Hazard. Mater. 138(1), 22-28.

Dronnet, V. M., Renard, C. M. G. C., Axelos, M. A. V., and Thibault, J. F. (1997). “Binding of divalent metal cations by sugar-beet pulp,” Carbohyd. Poly. 34(1-2), 73-82.

Dubey, S. P., Gopal, K., and Bersillon, J. L. (2009). “Utility of adsorbents in the purification of drinking water: A review of characterization, efficiency and safety evaluation of various adsorbents,” J. Environ. Biol. 30(3), 327-332.

Dubey, S. P., and Krishna, G. (2007). “Adsorption of chromium (VI) on low cost adsorb-ents derived from agricultural waste material,” J. Hazard. Mater. 145(3), 465-470.

Dubinin, M. M., and Radushkevich, L. V. (1947). “Equation of the characteristic curve of activated charcoal,” Chemisches Zentralblatt 1, 875-889.

Duman, G., Onalt, Y., Okutucu, C., Onenc, S., and Yanik, J. (2009). “Production of activated carbon from pine cone and evaluation of its physical, chemical, and adsorption properties,” Energy Fuels 23(Special Issue), 2197-2204.

Duong, T. D., Hoang, M., and Nguyen, K. L. (2004). “Extension of Donnan theory to predict calcium ion exchange on phenolic hydroxyl sites of unbleached kraft fibers,” J. Colloid Interface Sci. 276(1), 6-12.

Dupont, L., and Guillon, E. (2003). “Removal of hexavalent chromium with a lignocellulosic substrate extracted from wheat bran,” Environmental Science and Technology 37(18), 4235-4241.

Dursun, A. Y. (2006). “A comparative study on determination of the equilibrium, kinetic and thermodynamic parameters of biosorption of copper(II) and lead(II) ions onto pretreated Aspergillus niger,” Biochem. Eng. J. 28(2), 187-195.

Edgar, T. F., and Himmelblau, D. M. (1989). Optimization of Chemical Processes, McGraw-Hill, N.Y., 208-241.

Edyvean, R. G. J., Williams, C. J., Wilson, M. W., and Aderhold, D. (1997). “Biosorption using unusual biomasses,” in: Biosorbents for Metal Ions, Wase, J., and Forster, C. (eds.), CRC Press, London, UK, 165-182.

Elangovan, R., Philip, L., and Chandraraj, K. (2008a). “Biosorption of chromium species by aquatic weeds: Kinetics and mechanism studies,” J. Hazard. Mater. 152(1), 100-112.

Elangovan, R., Philip, L, and Chandraraj, K. (2008b). “Biosorption of hexavalent and trivalent chromium by palm flower (Borassus aethiopum),” Chem. Eng. J. 141(1-3), 99-111.

El-Ashtoukhy, E.-S. Z., Amin, N. K., and Abdelwahab, O. (2008). “Removal of lead(II) and copper(II) from aqueous solution using pomegranate peel as new adsorbent,” Desalination223(1-3), 162-173.

El-Hendawy, A. N. A. (2003). “Influence of HNO₃ oxidation on the structure and adsorptive properties of corncob-based activated carbon,” Carbon 41(4), 713-722.

El-Hendawy, A. N. A. (2009). “The role of surface chemistry and solution pH on the removal of Pb²⁺ and Cd²⁺ ions via effective adsorbents from low-cost biomass,” J. Hazard. Mater.167(1-3), 260-267.

El-Hendawy, A. N. A., Samra, S. E., and Girgis, B. S. (2001). “Adsorption characteristics of activated carbons obtained from corncobs,” Colloids Surf. A – Physicochem. Eng. Aspects180(3), 209-221.

El Nemr, A. (2007). “Pomegranate husk as an adsorbent in the removal of toxic chromium from wastewater,” Chem. Ecol. 23(5), 409-425.

El Nemr, A., Khaled, A., Abdelwahab, O., and El-Sikaily, A. (2008). “Treatment of wastewater containing toxic chromium using new activated carbon developed from date palm seed,” J. Hazard. Mater. 152(1), 263-275.

El-Shafey, E. I. (2005). “Behaviour of reduction-sorption of chromium(VI) from an aqueous solution on a modified sorbent from rice husk,” Water Air Soil Pollution 163(1-4), 81-102.

El-Shafey, E. I. (2007a). “Removal of Se(IV) from aqueous solution using sulphuric acid-treated peanut shell,” J. Environ. Manag. 84(4), 620-627.

El-Shafey, E. I. (2007b). “Sorption of Cd(II) and Se(IV) from aqueous solution using modified rice husk,” J. Hazardous Mater. 147(1-2), 546-555.

El-Shafey, E. I. (2010). “Removal of Zn(II) and Hg(II) from aqueous solution on a carbonaceous sorbent chemically prepared from rice husk,” J. Hazardous Mater. 175(1-3), 319-327.

El-Shafey, E. I., and Canepa, P. (2003). “Remediation of a Cr (VI) contaminated soil: Soil washing followed by Cr (VI) reduction using a sorbent prepared from rice husk,” J. Physique IV107, 415-418.

El-Sikaily, A, El Nemr, A., Khaled, A., and Abdelwehab, O. (2007). “Removal of toxic chromium from waste water using green alga Ulva lactuca and its activated carbon,” J. Hazard. Mater. 148(1-2), 216-228.

Erdogan, S., Onal, Y., Akmil-Basar, C., Bilmez-Erdemoglu, S., Sarici-Ozdemir, C., Koseoglu, E., and Icduygu, G. (2005). “Optimization of nickel adsorption from aqueous solution by using activated carbon prepared from waste apricot by chemical activation,” Appl. Surf. Sci. 252(5), 1324-1331.

Escudero, C., Gabaldón, C., Marzal, P., and Villaescusa, I. (2008). “Effect of EDTA on divalent metal adsorption onto grape stalk and exhausted coffee wastes,” J. Hazard. Mater. 152(2), 476-485.

Esposito, A., Pagnanelli, F., and Vegliò, F. (2002). “pH-related equilibria models for biosorption in single metal systems,” Chem. Eng. Sci. 57, 307-313.

Fahim, N. F., Barsoum, B. N., Eid, A. E., and Khalil, M. S. (2006). “Removal of Cr (III) from tannery wastewater using activated carbon from sugar industrial waste,” J. Hazard. Mater.136(2), 303-309.

Fang, J., Gu, Z. M., Gang, D. C., Liu, C. X., Ilton, E. S., and Deng, B. L. (2007). “Cr(VI) removal from aqueous solution by activated carbon coated with quaternized poly(4-vinylpyridine),” Environ. Sci. Technol. 41(13), 4748-4753.

Farajzadeh, M. A., and Monji, A. B. (2004a). “Adsorption characteristics of wheat bran towards heavy metal cations,” Sep. Purif. Technol. 38(3), 197-207.

Farajzadeh, M. A., and Monji, A. B. (2004b). “Treated rice bran for scavenging Cr(III) and Hg(II) from acidic solution,” J. Chinese Chem. Soc. 51(4), 751-759.

Farinella, N. V., Matos, G. D., and Arruda, M. A. Z. (2007). “Grape bagasse as a potential biosorbent of metals in effluent treatments,” Bioresour. Technol. 98(10), 1940-1946.

Farinella, N. V., Matos, G. D., Lehmann, E. L., and Arruda, M. A. Z. (2008). “Grape bagasse as an alternative natural adsorbent of cadmium and lead for effluent treatment,” J. Hazardous Mater. 154(1-3), 1007-1012.

Faur-Brasquet, C., Reddad, Z., Kadirvelu, K., and Le Cloirec, P. (2002). “Modeling the adsorption of metal ions (Cu²⁺, Ni²⁺, Pb²⁺) onto ACCs using surface complexation models,” Appl. Surf. Sci. 196(1-4), 356-365.

Febrianto, J., Kosasih, A. N., Sunarso, J., Jua, Yi-H., Indraswati, N., and Ismadji, S. (2009). “Equilibrium and kinetic studies in adsorption of heavy metals using biosorbent:A summary of recent studies,” J. Hazard. Mater. 162(2-3), 616-645.

Filipovic-Kovacevic, Z., Sipos, L., and Briski, F. (2000). “Biosorption of chromium, copper, nickel and zinc ions onto fungal pellets of Aspergillus niger 405 from aqueous solutions,” Food Technol. Biotechnol. 38(3), 211-216.

Fiol, N., Poch, J., and Villaescusa, I. (2004). “Chromium(VI) uptake by grape stalks wastes encapsulated in calcium alginate beads: Equilibrium and kinetics studies,” Chem. Speciation Bioavail. 16(1-2), 25-33.

Fiol, N., Villaescusa, I., Martinez, M., Miralles, N., Poch, J., and Serarols, J. (2003). “Biosorption of Cr(VI) using low cost sorbents,” Environ. Chem. Lett. 1(2), 135-139.

Fiol, N., Villaescusa, I., Martinez, M., Miralles, N., Poch, J., and Serarols, J. (2006). “Sorption of Pb(II), Ni(II), Cu(II) and Cd(II) from aqueous solution by olive stone waste,” Separation Purification Technol. 50(1), 132-140.

Foglarova, M., Prokop, J., and Milichovsky, M. (2009). “Oxidized cellulose: An application in the form of sorption filter materials,” J. Appl. Polymer Sci. 112(2), 669-678.

Fourest, E., Canal, C., and Roux, J. C. (1994). “Improvement of heavy metal biosorption by mycelial dead biomasses (Rhizopus arrhizus, Mucor miehei and Penicillium chrysogenum) – pH control and cationic activation,” FEMS Microbiol. Rev. 14(4), 325-332.

Fourest, E., and Roux, J. (1992). “Heavy metal biosorption by fungal mycelial by-product mechanisms and influence of pH,” Appl. Microbiol. Biotechnol. 37(3), 399-403.

Fourest, E., and Volesky, B. (1996). “Contribution of sulfonate groups and alginate to heavy metal biosorption by the dry biomass of Sargassum fluitans,” Environ. Sci. Technol. 30(1), 277-282.

Fourest, E., and Volesky, B. (1997). “Alginate properties and heavy metal biosorption by marine alga,” Appl. Biochem. Biotechnol. 67, 33-34.

Fraga, C. G., and Oteiza, P. I. (2002). “Iron toxicity and antioxidant nutrients,” Toxicology 180(1), 23-32.

Freer, J., Baeza, J., Maturana, H., and Palma, G. (1989). “Removal and recovery of uranium by modified Pinus radiata D. don bark,” J. Chem. Technol. Biotechnol. 46(1), 41-48.

Freundlich, H. M. F. (1907). “Uber die adsorption in losungen,” Zeitschrift fur Physikalische Chemie (Leipzig). A 57, 385-470.

Friedman, M., and Waiss, A. C. (1972). “Mercury uptake by selected agricultural products and by-products,” Environ. Sci. Technol. 6(5), 457-458.

Fritz, W., and Schlünder, E. U. (1974). “Simultaneous adsorption equilibria of organics solutes in dilute aqueous solutions on activated carbon,” Chem. Eng. Sci. 29, 1279-1282.

Fritz, W., and Schlünder, E. U. (1981). “Competitive adsorption of two dissolved organics onto activated carbon – I,” Chem Eng Sci. 36, 721-730.

Fytianos, K. (2001). “Speciation analysis of heavy metals in natural waters: A review,” J. AOAC Intl. 84(6), 1763-1769.

Gabaldón, C., Izquierdo, M., Marzal, P., and Sempere, F. (2007). “Evaluation of biosorbents for Cu removal from wastewater in the presence of EDTA,” J. Chem. Technol. Biotechnol.82(10), 888-897.

Gabaldón, C., Marzal, P., Seco, A., and González, J. A. (2000). “Cadmium and copper removal by a granular activated carbon in laboratory column systems,” Sep. Sci. Technol. 35(7), 1039-1053.

Gaballah, I., and Kilbertus, G. (1998). “Recovery of heavy metal ions through decontamination of synthetic solutions and industrial effluents using modified barks,” Journal of Geochemical Exploration 62(1-3), 241-286.

Gabriel, J., Vosahlo, J., and Baldrian, P. (1996). “Biosorption of cadmium to mycelial pellets of wood rotting fungi,” Biotechnology Techniques 10, 345-348.

Gadd, G. M. (1993). “Interactions of fungi with toxic metals by biosorption: A review,” Hydrometallurgy 44, 301-316.

Gadd, G. M. (2009). “Biosorption: Critical review of scientific rationale, environmental importance and significance for pollution treatment,” J. Chem. Technol. Biotech. 84(1), 13-28.

Gaikwad, R. W. (2004). “Removal of Cd (II) from aqueous solution by activated charcoal derived from coconut shell,” Electron. J. Environ. Agric. Food Chem. 3(4), 702-709.

Gajghate, D. G., Saxena, E. R., and Aggarwal, A. L. (1992). “Removal of chromium (VI) as chromium diphenyl carbazide (CDC) complex from aqueous solution by activated carbon,” Water Air and Soil Pollution 65(3-4), 329-337.

Gajghate, D. G., Saxena, E. R., and Vittal, M. (1991). “Removal of lead from aqueous solution by activated carbon,” Ind. J. Environ. Health 33, 374-379.

Galiatsatou, P., Metaxas, M., and Kasselouri-Rigopoulou, V. (2002). “Adsorption of zinc by activated carbons prepared from solvent extracted olive pulp,” J. Hazard. Mater. 91(1-3), 187-203.

Gao, H., Liu, Y., Zeng, G., Xu, W., Li, T., and Xia, W. (2008). “Characterization of Cr(VI) removal from aqueous solutions by a surplus agricultural waste, rice straw,” J. Hazardous Mater.150(2), 446-452.

Garcia-Reyes, R. B., Rangel-Mendez, J. R., and Alfaro-De la Torre, M. C. (2009). “Chromium (III) uptake by agro-waste biosorbents: Chemical characterization, sorption-desorption studies, and mechanism,” J. Hazardous Mater. 170(2-3), 845-854.

Gardea-Torresdey, J. L., De La Rosa, G., and Peralta-Videa, J. R. (2004a). “Use of phytofilteration technologies to remove the heavy metals: A review,” Pure and Applied Chemistry 76(4), 801-803.

Gardea-Torresdey, J. L., Dokken, K., Tiemann, K. J., Parsons, J. G., Ramos, J., Pingitore, N. E., and Gamez, G. (2002). “Infrared and X-ray absorption spectroscopic studies on the mechanism of chromium(III) binding to alfalfa biomass,” Microchem. J. 71(2-3), 157-166.

Gardea-Torresdey, J. L., Gonzalez, J. H., Tiemann, K .J., Rodriguez, O., and Gamez, G. (1998). “Phytofilteration of hazardous cadmium, chromium, lead, and zinc ions by biomass of Medicago sativa (alfalfa). J. Hazard. Mater. 57(1-3), 29-39.

Gardea-Torresdey, J. L., Hejazi, M., Tiemann, K. J., Parsons, J. G., Duarte-Gardea, M., and Henning, J. (2002). “Use of hop (Humulus lupulus) agricultural by-products for the reduction of aqueous lead (II) environmental health hazards,” J. Hazard. Mater. 91(1-3), 95-112.

Gardea-Torresdey, J. L., Peralta-Videa, J. R., Montes, M., LaRosa, G., and Corral-Diaz, B. (2004b). “Bioaccumulation of cadmium, chromium and copper by Convolvulus arvensis L.: Impact on plant growth and uptake of nutritional elements,” Bioresour. Technol. 92, 229-235.

Gardea-Torresdey, J. L., Tang, L., and Salvador, J. M. (1996a). “Copper adsorption by esterified and unesterified fractions of sphagnum peat moss and its different humic substances,” J. Hazard. Mater. 48(1-3), 191-206.

Gardea-Torresdey, J. L., Tiemann, K. J., Armendariz, V., Bess-Oberto, L., Chianelli, R. R., Rios, J., Parsons, J. G., and Gamez, G. (2000a). “Characterization of chromium (VI) binding and reduction to chromium (III) by the agricultural byproduct of Avena monida (oat) biomass,” J. Hazard. Mater. 80(1-3), 175-188.

Gardea-Torresdey, J. L., Tiemann, K. J., Gamez, G., Dokken, K., Cano-Aguilera, I., Furenlid, L. R., and Renner, M. W. (2000b). “Reduction and accumulation of gold(III) by Medicago sativa alfalfa biomass: X-ray absorption spectroscopy, pH, and temperature dependence,” Environ. Sci. Technol. 34(20), 4392-4396.

Gardea-Torresdey, J. L., Tiemman, K. J., Gonzalez, J. H., Cano-Aguilera, I., Henning, J. A., and Towsend, M. S. (1996b). “Removal of nickel ions from aqueous solution by biomass and silica-immobilized biomass of Medicago sativa (alfalfa),” J. Hazard. Mater. 49(2-3), 205-216.

Garg, U. K., Kaur, M. P., Garg, V. K., and Sud, D. (2007). “Removal of hexavalent Cr from aqueous solutions by agricultural waste biomass,” J. Hazard. Mater. 140(1-2), 60-68.

Garg, U. K., Kaur, M. P., Garg, V. K., and Sud, D. (2008a). “Removal of Ni (II) from aqueous solution by adsorption on agricultural waste biomass using a response surface methodological approach,” Bioresour. Technol. 99(5), 1325-1331.

Garg, U., Kaur, M. P., Jawa, G. K., Sud, D., and Garg, V. K. (2008b). “Removal of cadmium(II) from aqueous solution by adsorption on agriculturalwaste biomass,” J. Hazard. Mater. 154(1-3), 1149-1157.

Garg, U. K., Kaur, M. P., Sud, D., and Garg, V. K. (2009). “Removal of hexavalent chromium from aqueous solution by adsorption on treated sugarcane bagasse using response surface methodological approach,” Desalination 249(2), 475-479.

Garg, V. K., Gupta, R., Kumar, R., and Gupta, R. K. (2004). “Adsorption of chromium from aqueous solution on treated sawdust,” Bioresour. Technol. 92(1), 79-81.

Geay, M., Marchetti, V., Clement, A., Loubinoux, B., and Gerardin, P. (2000). “Decon-tamination of synthetic solutions containing heavy metals using chemically modified sawdusts bearing polyacrylic acid chains,” Journal of Wood Science 46(4), 331-333.

Gérente, C., du Mensil, P. C., Andrès, Y., Thibault, J., and Le Cloirec, P. (2000). “Removal of metal ions from aqueous solution on low cost natural polysaccharides. Sorption mechanism approach,” React. Funct. Polym. 46(2), 135-144.

Gérente, C., Lee, V. K. C., Le Cloirec, P., and McKay, G. (2007). “Application of chitosan for the removal of metals from wastewaters by adsorption – Mechanisms and models review,” Crit. Rev. Env. Sci. Technol. 37(1), 41-127.

Ghimire, K. N., Inoue, J. I., Inoue, K., Kawakita, H., and Ohto, K. (2008). “Adsorptive separation of metal ions onto phosphorylated orange waste,” Separation Sci. Technol. 43(2), 362-375.

Ghimire, K. N., Inoue, K., Ohto, K., and Hayashida, T. (2007). “Adsorptive separation of metallic pollutants onto waste seaweeds, Porphyra yezoensis and Ulva japonica,” Separation Sci. Technol. 42(9), 2003-2018.

Ghimire, K. N., Inoue, K., Ohto, K., and Hayashida, T. (2008). “Adsorption study of metal ions onto crosslinked seaweed Laminaria japonica,” Bioresour. Technol. 99(1), 32-37.

Ghodbane, I., and Hamdaoui, O. (2008). “Removal of mercury(II) from aqueous media using eucalyptus bark: Kinetic and equilibrium studies,” J. Hazard. Mater. 160(2-3), 301-309.

Ghodbane, I. Nouri, L. Hamdaoui, O., and Chiha, M. (2007). “Kinetic and equilibrium study for the sorption of cadmium(II) ions from aqueous phase by eucalyptus bark,” J. Hazard. Mater. 152(1), 148-158.

Gill, R. I. S. (1989). “The use of potentiometric titration and polyelectrolyte titration to measure the surface charge of cellulose fibre,” In: Fundamentals of Papermaking: Transactions of the 9^th Fundamental Research Symposium Held at Cambridge: Sept. 1989, Vol. 1, Baker, C. F., and Punton, V. W. (eds.), Mechanical Engineering Publ. Ltd., London, 437-452.

Girgis, B. S., Attia, A. A., and Fathy, N. A. (2007). “Modification in adsorption characteristics of activated carbon produced by H₃PO₄ under flowing gases,” Colloids Surf. A – Physicochem. Eng. Aspects 299(1-3), 79-87.

Girgis, B. S., Smith, E., Louis, M. M., and El-Hendawy, A. N. A. (2009). “Pilot production of activated carbon from cotton stalks using H₃PO₄,” J. Anal. Appl. Pyrol. 86(1), 180-184.

Gloaguen, V., and Morvan, H. (1997). “Removal of heavy metal ions from aqueous solution by modified barks,” J. Environ. Sci. Heal A 32(4), 901-912.

Gokhale, S. V., Jyoti, K. K., and Lele, S. S. (2008). “Kinetic and equilibrium modeling of chromium (VI) biosorption on fresh and spent Spirulina platensis/Chlorella vulgaris biomass,” Bioresour. Technol. 99(9), 3600-3608.

Gomes, N. C. M., and Linardi, V. R. (1996). “Removal of gold, silver and copper by living and nonliving fungi from leach liquor obtained from the gold mining industry,” Rev. Microbiol. 27(4), 218-222.

Gomez-Serrano, V., Cuerda-Correa, E. M., Fernandez-Gonzalez, M. C., Alexandre-Franco, M. F., and Macias-Garcia, A. (2005). “Preparation of activated carbons from walnut wood: A study of microporosity and fractal dimension,” Smart Mater. Struc. 14(2), 363-368.

Gomez-Serrano, V., Macias-Garcia, A., Espinosa-Mansilla, A., and Valenzuela-Calahorro, C. (1998). “Adsorption of mercury, cadmium and lead from aqueous solution on heat-treated and sulphurized activated carbon,” Water Res. 32(1), 1-4.

Gomez-Tamayo, M. D., Macias-Garcia, A., Diez, M. A. D., and Cuerda-Correa, E. M. (2008). “Adsorption of Zn(II) in aqueous solution by activated carbons prepared from evergreen oak (Quercus rotundifolia L.),” J. Hazard. Mater. 153(1-2), 28-36.

Gonzalez, M. H., Araujo, G. C. L., Pelizaro, C. B., Menezes, E. A., Lemos, S. G., de Sousa, G. B., and Nogueira, A R. A. (2008). “Coconut coir as biosorbent for Cr(VI) removal from laboratory wastewater,” J. Hazardous Mater. 159(2-3), 252-256.

Gonzalez-Serrano, E., Cordero, T., Rodriguez-Mirasol, J., Cotoruelo, L., and Rodriguez, J. J. (2004). “Removal of water pollutants with activated carbons prepared from H₃PO₄ activation of lignin from kraft black liquors,” Water Res. 38(13), 3043-3050.

Goyal, N., Jain, S. C., and Banerjee, U. C. (2003). “Comparative studies on the microbial adsorption of heavy metals,” Adv. Environ. Res. 7(2), 311-319.

Grimm, A., Zanzi, R., Björnbom, E., and Cukierman, A. L. (2008). “Comparison of different types of biomasses for copper biosorption,” Bioresour. Technol. 99(7), 2559-2565.

Guclu, G., Gurdag, G., and Ozgumus, S. (2003). “Competitive removal of heavy metal ions by cellulose graft copolymers,” Journal of Applied Polymer Science 90(8), 2034-2039.

Guibaud, G., Tixier, N., Bouju, A., and Baudu, M. (2003). “Relation between extracellular polymers composition and its ability to complex Cd, Cu and Pb,” Chemosphere 52(10), 1701-1710.

Guillon, E., Merdy, P., Aplincourt, M., Dumonceau, J., and Vezin, H. (2001). “Structural characterization and iron(III) binding ability of dimeric and polymeric lignin models,” J. Colloid Interface Sci. 239(1), 39-48.

Guo, S. H., Peng, J. H., Li, W., Yang, K. B., Zhang, L. B., Zhang, S. M., and Xia, H. Y. (2009). “Effects of CO₂ activation on porous structures of coconut shell-based activated carbons,” Appl. Surf. Sci. 255(20), 8443-8449.

Guo, X. Y., Zhang, S. Z., and Shan, X. Q. (2008). “Adsorption of metal ions on lignin,” Journal of Hazardous Materials 151(1), 134-142.

Guo, Y. P., Qi, J. R., Yang, S. F., Yu, K. F., Wang, Z. C., and Xu, H. D. (2003). “Adsorption of Cr(VI) on micro- and mesoporous rice husk-based active carbon,” Mater. Chem. Phys. 78(1), 132-137.

Gupta, R., Ahuja, P., Khan, S., Saxena, R. K., and Mohapatra, H. (2000). “Microbial biosorbents: Meeting challenges of heavy metal pollution in aqueous solutions,” Current Sci. 78(8), 967-973.

Gupta, S., and Babu, B. V. (2009). “Utilization of waste product (tamarind seeds) for the removal of Cr(VI) from aqueous solutions: Equilibrium, kinetics, and regeneration studies,” J. Environ. Manag. 90(10), 3013-3022.

Gupta, V. K., and Ali, I. (2000). “Utilization of bagasse fly ash (a sugar industry waste) for the removal of copper and zinc from wastewater,” Separation and Purification Technol. 18(2), 131-140.

Gupta, V. K., and Ali, I. (2004). “Removal of lead and chromium from wastewater using bagasse fly ash – A sugar industry waste,” J. Colloid Interface Sci. 271(2), 321-328.

Gupta, V. K., Jain, C. K., Ali, I., Sharma, M., and Saini, V. K. (2003). “Removal of cadmium and nickel from wastewater using bagasse fly ash – A sugar industry waste,” Water Res.37(16), 4038-4044.

Gupta, V. K., Mohan, D., and Sharma, S. (1998). “Removal of lead from wastewater using bagasse fly ash – A sugar industry waste material,” Separation Sci. Technol. 33(9), 1331-1343.

Gupta, V. K., Mohan, D., Sharma, S., and Park, K. T. (1999). “Removal of chromium (VI) from electroplating industry wastewater using bagasse fly ash – A sugar industry waste material,” The Environmentalist 19, 129-136.

Gupta, V. K., and Rastogi, A. (2008a). “Biosorption of lead(II) from aqueous solutions by nonliving algal biomass Oedogonium sp. and Nostoc sp.—A comparative study,” Colloids Surf. B: Biointerfaces 64(2), 170-178.

Gupta, V. K., and Rastogi, A. (2008b). “Equilibrium and kinetic modeling of cadmium(II) biosorption by nonliving algal biomass Oedogonium sp. from aqueous phase,” J. Hazard. Mater. 153(1-2), 759-766.

Gupta, V. K., and Rastogi, A. (2009). “Biosorption of hexavalent chromium by raw and acid-treated green alga Oedogonium hatei from aqueous solutions,” J. Hazardous Mater. 163(1), 396-402.

Gupta, V. K., Shrivastava, A. K., and Jain, N. (2001). “Biosorption of chromium(VI) from aqueous solutions by green algae Spirogryra species,” Water Res. 35(17), 4079-4085.

Gurgel, L. V. A., Karnitz, Jr., O., Gil, R. P. F., and Gil, L. F. (2008). “Adsorption of Cu(II), Cd(II), and Pb(II) from aqueous single metal solutions by cellulose and mercerized cellulose chemically modified with succinic anhydride,” Bioresour. Technol. 99(8), 3077-3083.

Gurisik, E., Arica, M. Y., Bektas, S., and Genc, O. (2004). “Comparison of the heavy metal biosorption capacity of active, heat-inactivated and NaOH-treated Phanerochaete chrysosporium biosorbents,” Eng. Life Sci. 4(1), 86-89.

Guzel, F., Yakut, H., and Topal, G. (2008). “Determination of kinetic and equilibrium parameters of the batch adsorption of Mn(II), Co(II), Ni(II) and Cu(II) from aqueous solution by black carrot (Daucus carota L.) residues,” J. Hazard. Mater. 153(3), 1275-1287.

Hamadi, N. K., Chen, X.-D., Farid, M. M., and Lu, M. G. Q. (2001). “Adsorption kinetics for the removal of chromium (VI) from aqueous solution by adsorbents derived from used tyres and sawdust,” Chem. Eng. J. 84(2), 95-105.

Han, D., Halada, G. P., Spalding, B., and Brooks, S. C. (2009). “Electrospun and oxidized cellulosic materials for environmental remediation of heavy metals in groundwater,” In: Model Cellulosic Surfaces, Roman, M. (ed.), ACS Symp. Ser. 1019, Oxford Univ. Press, USA, Ch. 11, 243-257.

Han, I., Schlautman, M. A., and Batchelor, B. (2000). “Removal of hexavalent chromium from groundwater by granular activated carbon,” Water Environ. Res. 72(1), 29-39.

Hanafiah, M. A. K. M., and Ngah, W. S. W. (2009). “Preparation, characterization and adsorption mechanism of Cu(II) onto protonated rubber leaf powder,” Clean Solid Air Water 37(9), 696-703.

Hanif, M. A., Bhatti, H. N., Ali, M. A., Asgher, M., and Bhatti, I. A. (2010). “Heavy metals tolerance and biosorption potential of white rot fungi,” Asian J. Chem. 22(1), 335-345.

Hanif, M. A., Nadeem, R., Zafar, M. N., Akhtar, K., and Bhatti, H. N. (2007). “Nickel (II) biosorption by Casia fistula biomass,” J. Hazard. Mater. 139(2), 345-355.

Hanna, O. T., and Sandall, O. C. (1995), Computational Methods in Chemical Engineering, Prentice-Hall International, N.J., 127-130.

Harman, G., Patrick, R., and Spittler, T. (2007). “Removal of heavy metals from polluted waters using lignocellulosic agricultural waste products,” Indust. Biotechnol. (Winter), 366-374.

Harmita, H., Karthikeyan, K. G., and Pan, X. J. (2009). “Copper and cadmium sorption onto kraft and organosolv lignins,” Bioresour. Technol. 100(24), 6183-6191.

Harry, I. D., Saha, B., and Cumming, I. W. (2008). “Effect of electrochemical reduction and oxidation of a viscose rayon based activated carbon cloth for Cr(VI) sorption from aqueous solution,” Ind. Eng. Chem. Res. 47(17), 6734-6741.

Hasan, S. H., Ranjan, D., and Talat, M. (2009a). “’Rice polish’ for the removal of arsenic from aqueous solution: Optimization of process variables,” Ind. Eng. Chem. Res. 48(9), 4194-4201.

Hasan, S. H., Ranjan, D., and Talat, M. (2010). “Water hyacinth biomass (WHB) for the biosorption of hexavalent chromium: Optimization of process parameters,” BioRes. 5(2), 563-575.

Hasan, S. H., Srivastava P., Ranjan, D., and Talat, M. (2009b). “Biosorption of Cr(VI) from aqueous solution using A. hydrophila in up-flow column: Optimization of process variables,” Appl. Microbiol. Biotechnol. 83(3), 567-577.

Hasan, S. H., Talat, M., and Rai, S. (2007). “Sorption of cadmium and zinc from aqueous solutions by water hyacinth (Eichhornia crassipes),” Bioresour. Technol. 98, 918-928.

Hasar, H., Cuci, Y., Obek, E., and Dilekoglu, M. F. (2003). “Removal of zinc(II) by activated carbon prepared from almond husks under different conditions,” Adsorp. Sci. Technol. 21(9), 799-808.

Hashem, A. (2006). “Amidoximated sunflower stalks (ASFS) as a new adsorbent for removal of Cu(II) from aqueous solution,” Polymer–Plastics Technology and Engineering 45(1), 35-42.

Hashem, A., Abdel-Lateff, A., Farag, S., and Hussein, D. M. (2008). “Treatment of alhagi residues with tartaric acid for the removal of Zn(II) ions from aqueous solution,” Adsorp. Sci. Technol. 26(9), 661-678.

Hashem, A., Abdel-Halim, E. S., El-Tahlawy, K. F., and Hebeish, A. (2005a). “Enhancement of adsorption of Co (II) and Ni (II) ions onto peanut hulls through esterification using citric acid,” Adsorp. Sci. Technol. 23(5), 367-380.

Hashem, A., Akasha, R. A., Ghith, A., and Hussein, D. A. (2005b). “Adsorbent based on agricultural wastes for heavy metal and dye removal: A review,” Energy Edu. Sci. Technol. 19, 69-86.

Hashem, A., Abou-Okeil, A., El-Shafie, A., and El-Sakhawy, M. (2006a). “Grafting of high-cellulose pulp extracted from sunflower stalks for removal of Hg (II) from aqueous solution,” Polym.-Plast. Technol. Eng. 45(1), 135-141.

Hashem, A., Elhmmali, M. M., Hussein, H. A., and Senousi, M. A. (2006b). “Utilization of sawdust-based materials as adsorbent for wastewater treatment,” Polymer-Pastics Technol. Eng.45(7), 821-827.

Hawari, A. H., and Mulligan, C. N. (2006). “Biosorption of lead(II), cadmium(II), copper(II) and nickel(II) by anaerobic granular biomass,” Bioresour. Technol. 97(4), 692-700.

Hawari, A. H., and Mulligan, C. N. (2006). “Heavy metals uptake mechanisms in a fixed-bed column by calcium-treated anaerobic biomass,” Process Biochem. 41(1), 187-198.

Heinze, T., Helbig, K., and Klemm, D. (1993). “Investigation of metal-ion adsorption of carboyxmethyl cellulose gel beads,” Acta Polymerica 44(2), 108-109.

Hernainz, F., Calero, M., Blazquez, G., Martin-Lara, M. A., and Tenorio, G. (2008). “Comparative study of the biosorption of cadmium(II), chromium(III), and lead(II) by olive stone,” Environ. Prog. 27(4), 469-478.

Hernainz, F., Calero, M., Blazquez, G., Tenorio, G., and Martin-Lara, M. A. (2009). “Effect of the presence of chromium (III) on the removal of lead (II) from aqueous solutions by agricultural wastes,” J. Environ. Eng. ASCE 135(12), 1348-1356.

Herrero, R., Cordero, B., Lodeiro, P., Rey-Castro, C., and Sastre de Vicente, M. E. (2006). “Interactions of cadmium(II) and protons with dead biomass of marine algae Fucus sp.,” Mar. Chem. 99(1-4), 106-116.

Herrington, T. M., and Petzold, J. C. (1992). “An investigation into the nature of charge on the surface of papermaking woodpulps. 2. Analysis of potentiometric titration data,” Colloids Surf. 64, 109-118.

Hiemenz, P. C., and Rajagopalan, R. (1997). Principles of Colloid and Surface Chemistry, Marcel Dekker, New York.

Ho, Y. S. (2003). “Removal of copper ions from aqueous solution by tree fern,” Water Res. 37(10), 2323-2330.

Ho, Y. S. (2004). “Second order kinetic model for sorption of cadmium onto tree fern: A comparison of linear and non-linear methods,” Wat. Res. 40(1), 119-125.

Ho, Y. S. (2005). “Effect of pH on lead removal from water using tree fern as the sorbent,” Bioresour. Technol. 96(11), 1292-1296.

Ho, Y., Huang, C. T., and Haung, H. W. (2002). “Equilibrium sorption isotherm for metal ions on tree fern,” Process Biochem. 37(12), 1421-1430.

Ho, Y. S., and McKay, G. (1998b). “A comparison of chemisorption kinetic models applied to pollutant removal on various sorbents,” Process Safety and Environmental Protection 76(4), 332-340.

Ho, Y. S., and McKay, G. (1999a). “Pseudo-second order model for sorption processes,” Process Biochem. 34(5), 451-465.

Ho, Y. S., and McKay, G. (1999b). “Competitive sorption of copper and nickel ions from aqueous solution using peat,” Adsorption 5(4), 409-417.

Ho, Y. S., and McKay, E., (2000). “The kinetics of sorption of divalent metal ions onto Sphagnum moss peat,” Water Res. 34(3), 735-742.

Ho, Y. S., and McKay, G. (2003). “Sorption of dyes and copper ions onto biosorbents,” Process Biochem. 38(7), 1047-1061.

Ho, Y. S., McKay, G., Wase, D. A. J., and Foster, C. F. (2000a). “Study of the sorption of divalent metal ions on to peat,” Adsorp. Sci. Technol. 18(7), 639-650.

Ho, Y. S., Ng, J. C. Y., and McKay, G. (2000b). “Kinetics of pollutant sorption by biosorbents: Review,” Separ. Purif. Methods 29(2), 189-232.

Ho, Y. S., and Ofomaja, A. E. (2005). “Kinetics and thermodynamics of lead ion sorption on palm kernel fibre from aqueous solution,” Proc. Biochem. 40(11), 3455-3461.

Ho, Y. S., and Ofomaja, A. E. (2006a). “Biosorption thermodynamics of cadmium on coconut copra meal as biosorbent,” Biochem. Eng. J. 30(2), 117-123.

Ho, Y. S., and Ofomaja, A. E. (2006b). “Pseudo-second-order model for lead ion sorption from aqueous solutions onto palm kernel fiber,” J. Hazardous Mater. 129(1-3), 137-142.

Ho, Y. S., and Ofomaja, A. E. (2006). “Kinetic studies of copper ion adsorption on palm kernel fibre,” J. Hazardous Mater. 137(3), 1796-1802.

Ho, Y. S., Porter, J. F., and McKay, G. (2002). “Equilibrium isotherm studies for the sorption of divalent metal ions onto peat: Copper, nickel and lead single component systems,” Water, Air, and Soil Pollution 141, 1-33.

Ho, Y. S., Wase, D. A. J., and Forster, C. F. (1995). “Batch nickel removal from aqueous solution by sphagnum moss peat,” Water Res. 29(5), 1327-1332.

Ho, Y. S., Wase, D. A. J., and Forster, C. F. (1996). “Kinetic studies of competitive heavy metal adsorption by sphagnum moss peat,” Environ. Technol. 17(1), 71-77.

Holan, Z. R., and Volesky, B. (1995). “Accumulation of cadmium, lead and nickel by fungal and wood biosorbents,” Appl. Environ. Microbiol. 53(2), 133-146.

Holan, Z. R., Volesky, B., and Prasetyo, I. (1993). “Biosorption of cadmium by biomass of marine-algae,” Biotechnology and Bioengineering 41(8), 819-825.

Holan, Z. R., and Volesky, B. (1995). “Accumulation of cadmium, lead and nickel by fungal and wood biosorbents,” Applied Biochemistry Biotechnology 53(2), 133-146.

Horikoshi, T., Nakajima, A., and Sakaguchi, T. (1981). “Studies on the accumulation of heavy metal elements in biological systems, XIX: Accumulation of uranium by microorganisms,” Eur. J. Appl. Microbiol. Biotechnol. 12(2), 90-96.

Horsfall, M., and Abia, A. A. (2003). “Sorption of cadmium(II) and zinc(II) ions from aqueous solutions by cassava waste biomass (Manihot sculenta Cranz),” Water Res. 37(20), 4913-4923.

Horsfall Jr., M., Abia, A. A., and Spiff, A. I. (2006). “Kinetic studies on the adsorption of Cd²⁺, Cu²⁺ and Zn²⁺ ions from aqueous solutions by cassava (Manihot sculenta Cranz) tuber bark waste,” Bioresour. Technol. 97(2), 283-291.

Horsfall, M., Ogban, F., and Akporhonor, E. E. (2005). “Biosorption of Pb²⁺ from aqueous solution by waste biomass of aerial roots of Rhizophora mangle (red mangrove),” Chem. Biodivers. 2(9), 1246-1255.

Horsfall, M., and Spiff, A. I. (2004). “Studies on the effect of pH on the sorption of Pb²⁺ and Cd²⁺ ions from aqueous solutions by Caladium bicolor (Wild Cocoyam) biomass,” Electron. J. Biotechnol. 7(3), 313-323.

Horsfall, M., and Spiff, A. I. (2005a). “Effects of temperature on the sorption of Pb²⁺ and Cd²⁺ from aqueous solution by Caladium bicolor (Wild Cocoyam) biomass,” Electron J. Biotechnol. 8(2), 162-169.

Horsfall, M., and Spiff, A. I. (2005b). “Effect of 2-mercaptoethanoic acid treatment of fluted pumpkin waste (Telfairia occidentalis Hook. f.) on the sorption of Ni²⁺ ions from aqueous solution,” J. Sci. Indus. Res. 64(8), 613-620.

Horsfall, M., and Spiff, A. I. (2005c). “Sorption of lead, cadmium, and zinc on sulfur-containing chemically modified wastes of fluted pumpkin (Telfairia occidentalis Hook f.),” Chem. Biodivers. 2(3), 373-385.

Hosea, M., Greene, B., McPherson, R., Henzl, M., Alexander, M. D., and Darnall, D. W., (1986). “Accumulation of elemental gold on alga Chlorella vulgaris,” Inorg. Chim. Acta 123(3), 161-165.

Hu, Z., Lei, L., Li, Y., and Hi, Y. (2003). “Chromium adsorption on high-performance activated carbons from aqueous solutions,” Sep. Purif. Technol. 31(1), 13-18.

Huang, L. Y., Ou, Z. Y., Boving, T. B., Tyson, J., and Xing, B. S., (2009). “Sorption of copper by chemically modified aspen wood fibers,” Chemosphere 76(8), 1056-1061.

Huang, C. P., and Wu, M. H. (1977). “The removal of chromium (VI) from dilute solution by activated carbon,” Water Res. 11(8), 673-679.

Igwe, J. C., Abia, A. A., and Ibeh, C. A. (2008). “Adsorption kinetics and intra-particulate diffusivities of Hg, As and Pb ions on unmodified and thiolated coconut fiber,” Int. J. Environ. Sci. Technol. 5(1), 83-92.

Imamoglu, M., and Tekir, O. (2008). “Removal of copper (II) and lead (II) ions from aqueous solutions by adsorption on activated carbon from a new precursor hazelnut husks,” Desalination 228(1-3), 108-113.

Inbaraj, B. S., Sridharan, R., Nandhakumar, V., and Sulochana, N. (2006). “Sorption of mercury(II) by an adsorbent derived from fruit shell of Terminalia catappa,” J. Indian Chem. Soc.83(5), 446-452.

Inbaraj, B. S., and Sulochana, N. (2004). “Carbonised jackfruit peel as and adsorbent for the removal of Cd (II) from aqueous solution,” Bioresour. Technol. 94(1), 49-52.

Inbaraj, B. S., and Sulochana, N. (2006). “Mercury adsorption on a carbon sorbent derived from fruit shell of Terminalia catappa,” J. Hazardous Mater. 133(1-3), 283-290.

Iqbal, M., Saeed, A., and Akhtar, N. (2002). “Petiolar felt-seath of palm: A new biosorbent for the removal of heavy metals from contaminated water,” Bioresour. Technol. 81(2), 151-153.

Iqbal, M., and Saeed, A. (2002). “Removal of heavy metals from contaminated water by petiolar felt-sheath of palm,” Environ. Technol. 23(10), 1091-1098.

Iqbal, M., Saeed, A., Edyvean, R. G. J., O’Sullivan, B., and Styring, P. (2005). “Production of fungal biomass immobilized loofa sponge (FBILS)-discs for the removal of heavy metal ions and chlorinated compounds from aqueous solution,” Biotechnol. Lett. 27(17), 1319-1323.

Iqbal, M., Saeed, A., and Kalim, I. (2009a). “Characterization of adsorptive capacity and investigation of mechanism of Cu²⁺, Ni²⁺ and Zn²⁺ adsorption on mango peel waste from constituted metal solution and genuine electroplating effluent,” Separation Sci. Technol. 44(15), 3770-3791.

Iqbal, M., Saeed, A., and Zafar, S. I. (2007). “Hybrid biosorbent: An innovative matrix to enhance the biosorption of Cd(II) from aqueous solution,” J. Hazardous Mater. 148(1-2), 47-55.

Iqbal, M., Saeed, A., and Zafar, S. I. (2009b). “FTIR spectrophotometry, kinetics and adsorption isotherms modeling, ion exchange, and EDX analysis for understanding the mechanism of Cd²⁺ and Pb²⁺ removal by mango peel waste,” J. Hazardous Mater. 164(1), 161-171.

Izquierdo, M., Gabaldon, C., Marzal, P., and Alvarez-Hornos, F. J. (2010). “Modeling of copper fixed-bed biosorption from wastewater by Posidonia oceanica,” Bioresour. Technol. 101(2), 510-517.

Jain, J. S., and Snoeyink, V. L. (1973). “Adsorption from bisolute systems on active carbon,” J. WPCF 45(12), 2463-2479.

Jalali-Rad, R., Ghafourian, H., Asef, Y., Dalir, S. T., Sahafipour, M. H., and Gharanjik, B. M. (2004). “Biosorption of cesium by native and chemically modified biomass of marine algae: Introduce the new biosorbents for biotechnology applications,” J. Hazard. Mater. B 116(1-2), 125-134.

Jalali, R., Ghafourian, H., Asef, Y., Davarpanah, S. J., and Sepehr, S. (2002). “Removal and recovery of lead using nonliving biomass of marine algae,” J. Hazard. Mater. 92(3), 253-262.

Jaramillo, J., Gomez-Serrano, V., and Alvarez, P. M. (2009). “Enhanced adsorption of metal ions onto functionalized granular activated carbons prepared from cherry stones,” J. Hazard. Mater. 161(2-3), 670-676.

Jia, Y. F., and Thomas, K. M. (2000). “Adsorption of cadmium ions on oxygen surface sites in activated carbon,” Langmuir 16(3), 1114-1122.

Jia, Y. F., Steele, C. J., Hayward, I. P., and Thomas, K. M. (1998). “Mechanism of adsorption of gold and silver species on activated carbons,” Carbon 36(9), 1299-1308.

Jia, Y. F., Xiao, B., and Thomas, K. M. (2002). “Adsorption of metal ions on nitrogen surface functional groups in activated carbons,” Langmuir 18(2), 470-478.

Johns, M. M., Marshall, W. E., and Toles, C. A. (1998). “Agricultural byproducts as granular activated carbons for adsorbing dissolved metals and organics,” J. Chem. Technol. Biotechnol. 71(2), 131-140.

Joseph, O., Rouez, M., Metivier-Pignon, H., Bayard, R., Emmanuel, E., and Gourdon, R. (2009). “Adsorption of heavy metals on to sugar cane bagasse: Improvement of adsorption capacities due to anaerobic degradation of the biosorbent.” Environ. Technol. 30(13), 1371-1379.

Jusoh, A., Shiung, L. S., Ali, N., and Noor, M. J. M. M. (2007). “A simulation study of the removal efficiency of granular activated carbon on cadmium and lead,” Desalination 206(1-3), 9-16.

Kacar, Y., Arpa, C., Tan, S., Denizli, A., Genc, O., and Arica, M. Y. (2002). “Biosorption of Hg(II) and Cd(II) from aqueous solutions: Comparison of biosorptive capacity of alginate and immobilized live and heat inactivated Phanerochaete chrysosporium,” Process Biochem. 37(6), 601-610.

Kadirvelu, K., Faur-Brasquet, C., and Le Cloirec, P. (2000). “Removal of Cu(II), and Ni(II) by adsorption on activated carbon cloths,” Langmuir 16, 8404-8409.

Kadirvelu, K., Senthilkumar, P., Thamaraiselvi, K., and Subburam, V. (2002). “Activated carbon prepared from biomass as adsorbent: Elimination of Ni(II) from aqueous solution,” Bioresour. Technol. 81, 87-90.

Kadirvelu, K., Thamaraiselve, K., and Namasivayam, C. (2001). “Removal of heavy metal from industrial wastewaters by adsorption on to activated carbon prepared from an agricultural solid waste,” Bioresour. Technol. 76(1), 63-65.

Kaikake, K., Hoaki, K, Sunanda, H., Dhakal, R. P., and Baba, Y. (2007). “Removal characteristics of metal ions using degreased coffee beans: Adsorption equilibrium of cadmium(II),” Bioresour. Technol. 98(15), 2787-2791.

Kalavathy, M. H., Karthikeyan, T., Rajgopal, S., and Miranda, L. R. (2005). “Kinetic and isotherm studies of Cu(II) adsorption onto H₃PO₄-activated rubber wood sawdust,” J. Colloid Interf. Sci. 292(2), 354-362.

Kamel, S., Hassan, E. M., and El-Sakhawy, M. (2006). ”Preparation and application of acrylonitrile-grafted cyanoethyl cellulose for the removal of copper(II) ions,” Journal of Applied Polymer Science 100(1), 329-334.

Kang, H. Y., Park, S. S., and Rim, Y. S. (2006). “Preparation of activated carbon from paper mill sludge by KOH-activation,” Kor. J. Chem. Eng. 23(6), 948-953.

Kannan, N., and Rengasamy, G. (2005a). “Comparison of Cd adsorption on various activated carbon,” Water Air Soil Pollut. 163(1-4), 185-201.

Kannan, N., and Rengasamy, G. (2005b). “Studies on the removal of nickel(II) ions by adsorption using various carbons – A comparative study,” Fresenius Environ. Bull. 14(6), 435-443.

Kapoor, A., and Viraraghavan, T. (1995). “Fungal biosorption – An alternative treatment option for heavy metal bearing wastewaters: A review,” Biores. Technol. 53(3), 195-206.

Kapoor, A., and Viraraghavan, T. (1998a). “Biosorption of heavy metals on Aspergillus niger: Effect of pretreatment,” Bioresour. Technol. 63(2), 109-113.

Kapoor, A., and Viraraghavan, T . (1997). “Heavy metal biosorption sites in Aspergillus niger,” Biores. Technol. 61(3), 221-227.

Kapoor, A., and Viraraghavan, T. (1998b). “Removal of heavy metals from aqueous solutions using immobilized fungal biomass in continuous mode,” Water Res. 32(6), 1968-1977.

Kapoor, A., Viraraghavan, T., and Cullimore, D. R. (1999). “Removal of heavy metals using the fungus Aspergillus niger,” Bioresour. Technol. 70(1), 95-104.

Kadirvelu, K., Faur-Brasquet, C., and Le Cloirec, P. (2000). “Removal of Cu(II), and Ni(II) by adsorption activated carbon cloths,” Langmuir 16(22), 8404-8409.

Kalavathy, M. H., Karthikeyan, T., Rajgopal, S., and Miranda, L. R. (2005). “Kinetic and isotherm studies of Cu(II) adsorption onto H₃PO₄-activated rubber wood sawdust,” J. Colloid Interface Sci. 292(2), 354-362.

Karnitz, O., Gurgel, L. V. A., de Freitas, R. P., and Gil, L. F. (2009). “Adsorption of Cu(II), Cd(II), and Pb(II) from aqueous single metal solutions by mercerized cellulose and mercerized sugarcane bagasse chemically modified with EDTA dianhydride (EDTAD),” Carbohyd. Polymers 77(3), 643-650.

Karnitz, O., Gurgel, L. V. A., and Gil, L. F. (2010). “Removal of Ca(II) and Mg(II) from aqueous single metal solutions by mercerized cellulose and mercerized sugarcane bagasse grafted with EDTA dianhydride (EDTAD),” Carbohydrate Polymers 79(1), 184-191.

Karnitz, Jr., O., Gurgel, L. V. A., Melo, J. C. P., Botaro, V. R., Melo, T. M. S., Gil, R. P. F., and Gil, L. F. (2007). “Adsorption of heavy metal ion from aqueous single metal solution by chemically modified sugarcane bagasse,” Biores. Technol. 98(6), 1291-1297.

Karthikeyan, T., Rajgopal, S., and Miranda, L. R. (2005). “Chromium (VI) adsorption from aqueous solution by Hevea brasilinesis saw dust activated carbon,” J. Hazard. Mater. 124(1-3), 192-199.

Karthikeyan, K. G., Tshabalala, M. A., Wang, D., and Kalbasi, M. (2004). “Solution chemistry effects on orthophosphate adsorption by cationized solid wood residues,” Environ. Sci. Technol. 38, 904-911.

Karunasagar, D., Arunachalam, J., Rashmi, K., Latha, J. N. L., and Mohan, P. M. (2003). “Biosorption of inorganic and methyl mercury by a biosorbent from Aspergillus niger,” World J. Microbiol. Biotechnol. 9(3), 291-295.

Karunasagar, D., Krishna, M. V. B., Rao, S. V., and Arunachalam, J. (2005). “Removal and preconcentration of inorganic and methyl mercury from aqueous media using a sorbent prepared from the plant Coriandrum sativum,” J. Hazardous Mater. 118(1-3), 133-139.

Kelley, C., Mielke, R. E., Dimaquibo, D., Curtis, A. J., and Dewitt, J. G. (1999). “Adsorption of Eu(III) onto roots of water hyacinth,” Environ. Sci. Technol. 33(9), 1439-1443.

Keskinkan, O., Goksu, M. Z. L., Basibuyuk, M., and Forster, C. F. (2004). “Heavy metal adsorption properties of a submerged aquatic plant (Ceratophollum demersum),” Bioresour. Technol. 92, 197-200.

Khalid, N., Ahmad, S., Kiani, S. N., and Ahmed, J. (1998a). “Removal of lead from aqueous solutions using rice husk,” Sep. Sci. Technol. 33(15), 2349-2362.

Khalid, N., Ahmad, S., Kiani, S. N., and Ahmed, J. (1999). “Removal of mercury from aqueous solutions by adsorption to rice husks,” Separ. Sci. Technol. 34(16), 3139-3153.

Khalid, N., Rahman, A., Ahmad, S., Kiani, S. N., and Ahmed, J. (1998b). “Adsorption of cadmium from aqueous solutions on rice husk,” Radiochimica Acta 83(3), 157-162.

Khan, A. R., Al-Wahebam, I. R., and Al-Haddad, A. (1996). “A generalised equation for adsorption isotherms for multicomponent organic pollutants in dilute aqueous solution,” Envir. Technol. 17, 13-23.

Khan, M. N., and Wahab, M. F. (2007). “Characterization of chemically modified corncobs and its application in the removal of metal ions from aqueous solution,” J. Hazard. Mater. 141(1), 237-244.

Khezami, L., and Capart, R. (2005). “Removal of chromium (VI) from aqueous solution by activated carbons: Kinetic and equilibrium studies,” J. Hazard. Mater. 123(1-3), 223-231.

Kikuchi, Y., Qian, Q. R., Machida, M., and Tatsumoto, H. (2006). “Effect of ZnO loading to activated carbon on Pb(II) adsorption from aqueous solution,” Carbon 44(2), 195-202.

Kim, D. S. (2004). “Activated carbon from peach stones using phosphoric acid activation at medium temperatures,” J. Environ. Sci. Health Pt. A – Toxic/Haz. Subst. Environ. Eng. 39(5), 1301-1318.

Kim, Y. H., Park, J. Y., and Yoo, Y. J. (1998). “Modeling of biosorption by marine brown Undaria pinnatifida based on surface complexation mechanism,” Korean J. Chem. Eng. 15(2), 157-163.

Kim, Y. H., Park, J. Y., Yoo, Y. J., and Kwak, J. W. (1999). “Removal of lead using xanthated marine brown alga, Undaria pinnatifida,” Process Biochem. 34(6-7), 647-652.

Kim, Y. H., Park, J. Y., Yoo, Y. J., Ryoo, B. I., and Lee, W. W. (1996). “Removal of copper using marine brown alga, Undaria pinnatifida modified with oxime,” Biothecnol. Techniques10(12), 923-928.

Kim, Y. H., Yoo, Y. J., and Lee, H. Y. (1995). “Characteristic of lead adsorption by Undaria pinnatfida,” Biotechnology Letters 17(3), 345-350.

King, P., Anuradha, K., Lahari, S. B., Kumar, Y. P., and Prasad, V. S. R. K. (2008a). “Biosorption of zinc from aqueous solution using Azadirachta indica bark: Equilibrium and kinetic studies,” J. Hazard. Mater. 152(1), 324-329.

King, P., Rakesh, N., Beenalahari, S., Kumar, Y. P., and Prasad, V. S. R. K. (2007). “Removal of lead from aqueous solution using Syzygium cumini L.: Equilibrium and kinetic studies,” J. Hazardous Mater. 142(1-2), 340-347.

King, P., Rakesh, N., Lahari, S. B., Kumar, Y. P., and Prasad, V. S. R. K. (2008b). “Biosorption of zinc onto Syzygium cumini L.: Equilibrium and kinetic studies,” Chem. Eng. J. 144(2), 181-187.

King, P., Srinivasa, P., Kumar, Y. P., and Prasad, V. S. R. K. (2006). “Sorption of copper(II) ion from aqueous solution by Techtona grandis l.f. (teak leaves powder),” J. Hazard. Mater. B136(3), 560-566.

Kiran, B., and Kaushik, A. (2008). “Cyanobacterial biosorption of Cr(VI): Application of two parameter and Bohart Adams models for batch and column studies,” Chem. Eng. J. 144(3), 391-399.

Kiran, B., Kaushik, A., and Kaushik, C. P. (2007). “Biosorption of Cr(VI) by native isolate of Lyngbya putealis (HH-15) in the presence of salts,” J. Hazard. Mater. 141(3), 662-667.

Klimmek, S., Stan, H. J.,Wilke, A., Bunke, G., and Buchholz, R. (2001). “Comparative analysis of the biosorption of cadmium, lead, nickel, and zinc by algae,” Environ. Sci. Technol. 35(21), 4283-4288.

Klasson, K. T., Wartelle, L. H., Rodgers, J. E., and Lima, I. M. (2009). “Copper(II) adsorption by activated carbons from pecan shells: Effect of oxygen level during activation,” Indus. Crops Prod. 30(1), 72-77.

Kobya, M. (2004a). “Removal of Cr(VI) from aqueous solutions by adsorption onto hazelnut shell activated carbon: Kinetic and equilibrium studies,” Bioresour. Technol. 91(3), 317-321.

Kobya, M. (2004b). “Adsorption kinetics and equilibrium studies of Cr(VI) by hazelnut shell activated carbon,” Adsorp. Sci. Technol. 22(1), 51-64.

Kobya, M., Demirbas, E., and Bayramoglu, M. (2004). “Modelling the effects of adsorbent dose and particle size on the adsorption of Cr(VI) ions from aqueous solutions,” Adsorp. Sci. Technol. 22(7), 583-594.

Kobya, M., Demirbas, E., Senturk, E., and Ince, M. (2005). “Adsorption of heavy metal ions from aqueous solutions by activated carbon prepared from apricot stone,” Bioresour. Technol.96(13), 1518-1521.

Kobya, M., Demirbas, E., Oncel, M. S., and Sencan, S. (2002). “Adsorption kinetic models applied to nickel ions on hazelnut shell activated carbons,” Adsorp. Sci. Technol. 20(2), 179-188.

Kogej, A., and Pavko, A. (2001). “Comparison of Rhizopus nigricans in a pelleted growth form with some other types of waste microbial biomass as biosorbents for metal ions,” World J. Microbiol. Biotechnol. 17(7), 677-685.

Kongsuwan, A., Patnukao, P., and Pavasant, P. (2009). “Binary component sorption of Cu(II) and Pb(II) with activated carbon from Eucalyptus camaldulensis Dehn bark,” J. Indus. Eng. Chem. 15(4), 465-470.

Kostal, J., Prabhukumar, G., Lao, U. L., Chen, A., Matsumoto, M., Mulchandani, A., and Chen, W. (2005). “Customizable biopolymers for heavy metal remediation,” J. Nanopart. Res. 7(4-5), 517-523.

Kratochvil, D., Fourest, E., and Volesky, B. (1995). “Biosorption of copper by Sargassum fluitans biomass in fixed-bed column,” Biotechnol. Lett. 17(7), 777-782.

Kratochvil, D., Pimentel, P., and Volesky, B. (1998). “Removal of trivalent and hexavalent chromium by seaweed biosorbent,” Environ. Sci. Technol. 32(18), 2693-2698.

Kratochvil, D., and Volesky, B. (1998). “Advances in biosorption of heavy metals,” Trends. Biotechnol, 16: 291-300.

Kratochvil, D., Volesky, B., and Demopoulos, G. (1997). “Optimizing Cu removal/recovery in a biosorption column,” Water Res. 31(9), 2327-2339.

Kriaa, A., Hamdi, N., and Srasra, E. (2010). “Removal of Cu (II) from water pollutant with Tunisian activated lignin prepared by phosphoric acid activation,” Desalination 250(1), 179-187.

Krishnan, K. A., and Anirudhan, T. S. (2002a). “Removal of mercury(II) from aqueous solutions and chlor-alkali industry effluent by steam activated and sulphurised activated carbons prepared from bagasse pith: Kinetics and equilibrium studies,” J. Hazardous Mater. 92(2), 161-183.

Krishnan, K. A., and Anirudhan, T. S. (2002b). “Uptake of heavy metals in batch systems by sulfurized steam activated carbon prepared from sugarcane bagasse pith,” Indus. Eng. Chem. Res. 41(20), 5085-5093.

Krishnan, K. A., and Anirudhan, T. S. (2003). “Removal of cadmium(II) from aqueous solutions by steam-activated sulphurised carbon prepared from sugar-cane bagasse pith: Kinetics and equilibrium studies,” Water SA 29(2), 147-156.

Krishnan, K. A., and Anirudhan, T. S. (2008a). “Kinetic and equilibrium modelling of cobalt(II) adsorption onto bagasse pith based sulphurised activated carbon,” Chem. Eng. J. 137(2), 257-264.

Krishnani, K. K., and Ayyappan, S. (2006). “Heavy metals remediation of water using plants and lignocellulosic agrowastes,” Rev. Environ. Contam. Toxicol. 188, 59-84.

Krishnani, K. K., Meng, X. G., Christodoulatos, C., and Boddu, V. M. (2008b). “Biosorption mechanism of nine different heavy metals onto biomatrix from rice husk,” J. Hazardous Mater. 153(3), 1222-1234.

Krishnani, K. K., Parimala, V., and Meng, X. G. (2004). “Detoxification of chromium(VI) in coastal water using lignocellulosic agricultural waste,” Water SA 30(4), 541-545.

Kubota, H., and Shigehisa, Y. (1995). “Introduction of amidoxime groups into cellulose and its ability to adsorb metal ions,” Journal of Applied Polymer Science 56, 147-151.

Kula, I., Ugurlu, M., Karaoglu, H., and Celik, A. (2008). “Adsorption of Cd(II) ions from aqueous solutions using activated carbon prepared from olive stone by ZnCl₂ activation,” Bioresour. Technol. 99(3), 492-501.

Kumar, A.,. and Rao, N. N., and Kaul, S. N. (2000). “Alkali-treated straw and insoluble straw xanthate as low cost adsorbents for heavy meatl removal – Preparation, characterization, and application,” Bioresour. Technol. 92(2), 151-155.

Kumar, U. (2006). “Agricultural products and by-products as a low cost adsorbent for heavy metal removal from water and wastewater: A review,” Sci. Res. Essay 1(2), 33-37.

Kumar, R., Bishnoi, N. R, Garima, and Bishnoi, K. (2008). “Biosorption of chromium(VI) from aqueous solution and electroplating wastewater using fungal biomass,” Chem. Eng. J.135(3), 202-208.

Kumar, U., and Bandyopadhyay, M. (2006a). “Sorption of cadmium from aqueous solution using pretreated rice husk,” Bioresour. Technol. 97(1), 104-109.

Kumar, U., and Bandyopadhyay, M. (2006b). “Fixed bed column study for Cd(II) removal from wastewater using treated rice husk,” J. Hazardous Mater. 129(1-3), 253-259.

Kumar, Y. P., King, P., and Prasad, V. S. R. K. (2006a). “Equilibrium and kinetic studies for the biosorption system of copper(II) ion from aqueous solution using Tectona grandis L.f.leaves powder,” J. Hazard. Mater. B137, 1211-1217.

Kumar, Y. P., King, P., and Prasad, V. S. R. K. (2006b). “Zinc biosorption on Tectona grandis L.f. leaves biomass: Equilibrium and kinetic studies,” Chem. Eng. J. 124(1-3), 63-70.

Kumar, Y. P., King, P., and Prasad, V. S R. K. (2006c). “Comparison for adsorption modelling of copper and zinc from aqueous solution by Ulva fasciata sp.,” J. Hazardous Mater. 137(2), 1246-1251.

Kurek, E., and Majewska, M. (2004). “In vitro remobilization of Cd immobilized by fungal biomass,” Geoderma 122(2-4), 235-246.

Kurniawan, T. A., Chan, G. Y. S., Lo, W.-H., and Babel, S. (2006a). “Physicochemical treatment techniques for wastewater laden with heavy metals,” Chemical Engineering Journal118(1-2), 83-98.

Kurniawan, T. A., Chan, G. Y. S., Lo, W. H., and Babel, S. (2006b). “Comparison of low-cost adsorbents for treating wastewater laden with heavy metals,” Sci. Total Environ. 366(2-3), 409-426.

Kutahyali, C., and Eral, M. (2010). “Sorption studies of uranium and thorium on activated carbon prepared from olive stones: Kinetic and thermodynamic aspects,” J. Nuclear Mater.396(2-3), 251-256.

Kuyucak, N., and Volesky, B. (1989). “Accumulation of cobalt by marine alga,” Biotechnology & Bioengineering 33(7), 809-814.

Lacour, S., Bollinger, J. C., Serpaud, B., Chantron, P., and Arcos, R. (2001). “Removal of heavy metals in industrial wastewaters by ion-exchanger grafted textiles,” Anal. Chim. Acta428(1), 121-132.

Lalvani, S. B., Hubner, A., and Wiltowski, T. S. (2000). “Chromium adsorption by lignin,” Energy Sources. 22(1), 45-56.

Lalvani, S. B., Wiltowski, T., Hubner, A., Wetson, A., and Mandich, N. (1998). “Removal of hexavalent chromium and metal cations by selective and novel carbon adsorbent,” Carbon36(7-8), 1219-1226.

Lalvani, S. B., Wiltowski, T. S., Murphy, D., and Lalvani, L. S. (1997). “Metal removal from process water by lignin,” Environ. Technol. 18(11), 1163-1168.

Langmuir, I. (1916). “The constitution and fundamental properties of solids and liquids,” J. Am. Chem. Soc. 38, 2221-2295.

Langmuir, I. (1918). “The adsorption of gases on plane surface of glass, mica and platinum,” J. Am. Chem. Soc. 40, 1361-1403.

Larous, S., Meniai, A. H., and Lehocine, M. B. (2005). “Experimental study of the removal of copper from aqueous solutions by adsorption using sawdust,” Desalination 185(1-3), 483-490.

Laszlo, J. A., and Dintzis, F. R. (1994). “Crop residues as ion exchange materials. Treatment of soy bean hull and sugar beet fiber (pulp.) with epichlohydrin to improve cation exchange capacity and physical stability,” J. Appl. Polym. Sci. 52(4), 531-538.

Lawrance, G. A. (2010). Introduction to Coordination Chemistry, Wiley, Chichester, UK.

Lee, D. C., Park, C. J., Yung, J. E., and Jeong, Y. H. (2000). “Screening of hexavalent chromium biosorbent from marine algae,” Applied Microbiology and Biotechnology 54, 445-448.

Lee, H. S., and Suh, J. H. (2000). “Continuous biosorption of heavy metal ions by Ca-loaded Laminaria japonica in fixed bed column,” Kor. J. Chem. Eng. 17(4), 477-479.

Lee, I. H., Kuan, Y. C., and Chern, J. M. (2007). “Equilibrium and kinetics of heavy metal ion exchange,” J. Chinese Inst. Chem. Eng. 38(1), 71-84.

Lee, M. G., Lim, J. H., and Kam, S. K. (2002). “Biosorption characteristics in the mixed heavy metal solution by biosorbents of marine brown algae,” Korean J. Chem. Eng. 19(2), 277-284.

Lee, S. H., Jung, C. H., Chung, H., Lee, M. Y., and Yang, J. W. (1998). “Removal of heavy metals from aqueous solution by apple residues,” Process Biochem. 33(2), 205-211.

Lee, S. H., Shon, J. S., Chung, H. S., Lee, M. Y., and Yang, J. W. (1999). “Effect of chemical modification of carboxyl groups in apple residues on metal ion binding,” Kor. J. Chem. Eng. 16(5), 576-580.

Lee, S.-H., and Yang, J.-W. (1997). “Removal of copper in aqueous solution by apple wastes,” Sep. Sci. Technol. 32(8), 1371-1387.

Lehrfeld, J. (1996). “Conversion of agricultural residues into cation exchange materials,” J. Appl. Polym. Sci. 61(12), 2099-2105.

Leusch, A., Holan, Z. R., and Volesky, B. (1995). “Biosorption of heavy metals (Cd, Cu, Ni, Pb, Zn) by chemically reinforced biomass of marine algae,” J. Chem. Tech. Biot. 62(3), 279-288.

Leyva-Ramos R., Bemal-Jacome, L. A., and Acosta-Rodriguez, I. (2005). “Adsorption of cadmium(II) from aqueous solution on natural and oxidized corncob,” Sep. Puri. Technol. 45, 41-49.

Leyva-Ramos, R., Bernal-Jacome, L. A., Guerrero-Coronado, R. M., and Fuentes-Rubio, L. (2001). “Competitive adsorption of Cd(II) and Zn(II) from aqueous solution onto activated carbon,” Separ. Sci. Technol. 36(16), 3673-3687.

Leyva-Ramos, R., Diaz-Flores, P. E., Aragon-Pina, A., Mendoza-Barron, J., and Guerrero-Coronado, R. M. (2005). “Adsorption of cadmium(II) from an aqueous solution onto activated carbon cloth,” Separ. Sci. Technol. 40(10), 2079-2094.

Leyva-Ramos, R., Fuentes-Rubio, L., Guerrero-Coronado, R. M., and Mendoza-Barron, J. (1995). “Adsorption of Cr(III) from aqueous solutions onto activated carbon,” J. Chem. Technol. Biotechnol. 62(1), 64-67.

Leyva-Ramos, R., Martinez, J., and Guerrero-Coronado, R. (1994). “Adsorption of Cr(VI) from aqueous solutions onto activated carbon,” Water Sci. Technol. 30(9), 191-197.

Leyva-Ramos, R., Rangel-Mendez, J. R., Mendoza-Barron, J., Fuentes-Rubio, L., and Coronado, R. M. (1997). “Adsorption of cadmium (II) from aqueous solution onto activated carbon,” Wat. Sci. Tech. 35(7), 205-211.

Li, C., Chen, H., and Li, Z. (2004a). “Adsorptive removal of Cr(VI) by Fe-modified steam exploded wheat straw,” Process Biochem. 39(5), 541-545.

Li, K., and Wang, X. (2009). “Adsorptive removal of Pb(II) by activated carbon prepared from Spartina alterniflora: Equilibrium, kinetics and thermodynamics,” Bioresour. Technol. 100(11), 2810-2815.

Li, Q., Wu, S., Liu, G., Liao, X., Deng, X., Sun, D., Hu, Y., and Huang, Y. (2004b). “Simultaneous biosorption of cadmium(II) and lead(II) ions by pretreated biomass of Phanerochaete chrysosporium,” Sep. Purif. Technol. 34(1-3), 135-142.

Li, Q., Zhai, J., Zhang, W., Wan, M., and Zhou, J. (2007). “Kinetic studies of adsorption of Pb(II), Cr(III) and Cu(II) from aqueous solution by sawdust and modified peanut husk,” J. Hazard. Mater. 141(1), 163-167.

Li, X., Tang, Y., Cao, X., Lu, D., Luo, F., and Shao, W. (2008). “Preparation and evaluation of orange peel cellulose adsorbents for effective removal of cadmium, zinc, cobalt and nickel,” Colloids Surf. A. 317(1-3), 512-521.

Li, X. M., Tang, Y. R., Xuan, Z. X., Liu, Y. H., and Luo, F. (2007). ”Study on the preparation of orange peel cellulose adsorbents and biosorption of Cd(II) from aqueous solution,” Separation and Purification Technology 55(1), 69-75.

Li, X.-M., Liao, D.-X., Xu, X.-Q., Yang, Q., Zeng, G.-M., Zheng, W., and Guo, L., (2008). “Kinetic studies for the biosorption of lead and copper ions by Penicillium simplicissimumimmobilized within loofa sponga,” J. Hazard. Mater. 159(2-3), 610-615.

Lillo-Rodenas, M. A., Cazorla-Amoros, D., and Linares-Solano, A. (2003). “Understanding chemical reactions between carbons and NaOH and KOH – An insight into the chemical activation mechanism,” Carbon 41(2), 267-275.

Lin, C. C., and Lai, Y. T. (2006). “Adsorption and recovery of lead(II) from aqueous solutions by immobilized Pseudomonas aeruginosa PU21 beads,” J. Hazard. Mater. 137(1), 99-105.

Lindgren, J., Öhman, L.-O., Gunnars, S., and Wågberg, L. (2002). “Charge deter-minations of cellulosic fibers of different origin – Comparison between different methods,” Nordic Pulp Paper Res. J. 17(1), 89-96.

Liou, T. H., and Wu, S. J. (2009). “Characteristics of microporous/mesoporous carbons prepared from rice husk under base- and acid-treated conditions,” J. Harard Mater. 171(1-3), 693-703.

Lister, S. K., and Line, M. A. (2001). “Potential utilization of sewage sludge and paper mill waste for biosorption of metals from polluted waterways,” Bioresour. Technol. 79(1), 35-39.

Liu, M., Deng, Y., Zhan, H., and Zhang, X. (2002). “Adsorption and desorption of copper(II) from solutions on new spherical cellulose adsorbent,” Journal of Applied Polymer Science84(3), 478-485.

Liu, M., Deng, Y., Zhan, H., Zhang, X., Liu, W., and Zhan, H. (2001). “Removal and recovery of chromium(III) from aqueous solutions by a spheroidal cellulose adsorbent,” Water Environmental Research 73(3), 322-328.

Liu, S. X., Chen, X., Chen, X. Y., Liu, Z. F., and Wang, H. L. (2007). “Activated carbon with excellent chromium(VI) adsorption performance prepared by acid-base surface modification,” J. Hazard. Mater. 141(1), 315-319.

Liu, X. M., Dai, X. D., Zhang, J., Song, H., and Yan, Z. F. (2008). “Preparation of high surface area activated carbons by KOH activation promoted by alkali metal halides,” New Carbon Mater. 23(2), 133-138.

Lloyd-Jones, P. J., Rangel-Mendez, J. R., and Streat, M. (2004). “Mercury sorption from aqueous solution by chelating ion exchange resins, activated carbon and a biosorbent,” Process Safety Environ. Protec. 82(B4), 301-311.

Lo, W., Chua, H., Lam, K. H., and Bi, S. P. (1999). “A comparative investigation on the biosorption of lead by filamentous fungal biomass,” Chemosphere 39, 2723-2736.

Lodeiro, P., Barriada, J. L., Herrero, R., and Sastre de Vicente, M. E. (2006). “The marine microalga Cystoseira baccata as biosorbent for cadmium(II) and lead(II) removal: Kinetic and equilibrium studies,” Environ. Pollut. 142(2), 264-273.

Lodeiro, P., Cordero, B., Barriada, J. L., Herrero, R., and de Vicente, M. E. S. (2005). “Biosorption of cadmium by biomass of brown marine macroalgae,” Bioresour. Technol. 96(16), 1796-1803.

Lodeiro, P., Fuentes, A., Herrero, R., and de Vicente, M. E. S. (2008). “Cr-III binding by surface polymers in natural biomass: The role of carboxylic groups,” Environ. Chem. 5(5), 355-365.

Lodeiro, P., Herrero, R., and de Vicente, M. E. S. (2006). “Thermodynamic and kinetic aspects on the biosorption of cadmium by low cost materials: A review,” Environ. Chem. 3(6), 400-418.

Lohani, M. B., Singh, A., Rupainwar, D. C., and Dhar, D. N. (2008). “Studies on efficiency of guava (Psidium guajava) bark as bioadsorbent for removal of Hg (II) from aqueous solutions,” J. Hazard. Mater. 159(2-3), 626-629.

Lopez-Ramon, M. V., Stoeckli, F., Moreno-Castilla, C., and Carrasco-Marin, F. (1999). “On the characterization of acidic and basic surface sites on carbons by various techniques,” Carbon 37(8), 1215-1221.

Loukidou, M. X., Karapantsios, T. D., Zouboulis, A. I., and Matis, K. A. (2004a). “Diffusion kinetic study of cadmium (II) biosorption by Aeromonas caviae,” J. Chem. Technol. Biotechnol. 79(7), 711-719.

Loukidou, M. X., Zouboulis, A. I., Karapantsios, T. D., and Matis, K. A. (2004b). “Equilibrium and kinetic modeling of chromium(VI) biosorption by Aeromonas caviae,” Colloids Surf. A: Physicochem. Eng. Aspects 242(1-3), 93-104.

Low, K. S., Lee, C. K., and Lee, C. Y. (2001). “Quaternized wood as sorbent for hexavalent chromium,” Appl. Biochem. Biotech. 90(1), 75-87.

Low, K. S., Lee, C. K., and Lee, P. K. (1993). “Sorption of copper by dye-treated oil palm fibers,” Bioresour. Technol. 44(2), 109-112.

Low, K. S., Lee, C. K., and Lee, P. L. (1997). “Chromium(III) sorption enhancement through NTA – Modification of biological materials,” Bull Environ. Contam. Toxic 58(3), 380-386.

Low, K. S., Lee, C. K., and Leo, A. C. (1995a). “Removal of metals from electroplating wastes using banana pith,” Bioresour. Technol. 51(2-3), 227-231.

Low, K. S., Lee, C. K., and Liew, S. C. (2000). “Sorption of cadmium and lead from aqueous solutions by spent grain,” Process Biochem. 36(1-2), 59-64.

Low, K. S., Lee, C. K., and Mak, S.M. (2004). “Sorption of copper and lead by citric acid modified wood,” Wood Science and Technology 38, 629-640.

Low, K. S., Lee, C. K., and Tai, C. H. (1994). “Biosorption by water hyacinth roots,” J. Environ. Sci. Heal. A29(1), 171-188.

Low, K. S., Lee, C. K., Wong, S. Y., and Tang, P. L. (2000). “Metal sorption enhance-ment of rice hull through chemical modification,” Environ. Technol. 21(11), 1239-1244.

Low, K. S., Lee, C. K., and Yong, S. L. (1995b). “Effect of dye modification on the sorption of copper by coconut husk,” Environ. Technol. 16(9), 877-883.

Lu, G. Q., Low, J. C. F., Lin, C. Y., and Lua, A. C. (1995). “Surface area development of sewage sludge during pyrolysis,” Fuel 74(3), 344-348.

Lu, S., and Gibb, S. W. (2008). “Copper removal from wastewater using spent-grain as biosorbent,” Bioresour. Technol. 99(6), 1509-1517.

Luef, E., Prey, T., and Kubicek, C. P. (1991). “Biosorption of zinc by fungal mycelial wastes,” Appl. Microbiol. Biotechnol. 34(5), 688-692.

Luo, F., Liu, Y. H., Li, X. M., Xuan, Z. X., and Ma, J. T. (2006). “Biosorption of lead ion by chemically-modified biomass of marine brown algae Laminaria japonica,” Chemosphere64(7), 1122-1127.

Luoma, S. N., and Rainbow, P. S. (2008). Metal Contamination in Aquatic Environments: Science and Lateral Managment, Cambridge Univ. Press, New York.

Lyubchik, S. I., Lyubchik, A. I., Galushko, O. L., Tikhonova, L. P., Vital, J., Fonseca, I. M., and Lyubchik, S. B. (2004). “Kinetics and thermodynamics of the Cr(III) adsorption on the activated carbon from co-mingled wastes,” Colloids Surf. A – Physicochem. Eng. Aspects 242(1-3), 151-158.

Ma, W., and Tobin, J. M. (2004). “Determination and modeling of effects of pH on peat biosorption on chromium, copper and cadmium,” Biochem. Eng. J. 18(1), 33-40.

Macchi, G., Marani, D., and Tirivanti, G. (1986). “Uptake of mercury by exhausted coffee grounds,” Environ. Technol. Lett. 7(8), 431-444.

Machado, R., Carvalho, J. R. M., and Correia, J. N. (2002). “Removal of trivalent chromium(III) from solution by biosorption in cork powder,” J. Chem. Technol. Biotechnol. 77(12), 1340-1348.

Machida, M., Kikuchi, Y., Aikawa, M., and Tatsumoto, H. (2004). “Kinetics of adsorption and desorption of Pb(II) in aqueous solution on activated carbon by two-site adsorption model,” Colloids Surf. A 240(1-3), 179-186.

Macias-Garcia, A., Gomez-Serrano, V., Alexandre-Franco, M. F., and Valenzuela-Calahorro, C. (2003). “Adsorption of cadmium by sulphur dioxide treated activated carbon,” J. Hazard. Mater. 103(1-2), 141-152.

Macias-Garcia, A., Valenzuela-Calahorro, C., Espinosa-Mansilla, A., Bernalte-Garcia, A., and Gomez-Serrano, V. (2004). “Adsorption of Pb²⁺ in aqueous solution by SO₂-treated activated carbon,” Carbon 42(8-9), 1755-1764.

Madrid, Y., and Camara, C. (1997). “Biological substrates for metal preconcentration and speciation,” Trac-Trends Anal Chem 16(1), 36-44.

Maekawa, E., and Koshijima, T. (1984). “Properties of 2,3-dicarboxy cellulose combined with various metallic ions,” Journal of Applied Polymer Science 29(7), 2289-2297.

Maekawa, E., and Koshijima, T. (1990). “Preparation and characterisation of hydroxamic acid derivatives and its metal complexes derived from cellulose,” Journal of Applied Polymer Science 40(9-10), 1601-1613.

Malik, D. J., Strelko, V., Streat, M., and Puziy, A. M. (2002). “Characterisation of novel modified active carbons and marine algal biomass for the selective adsorption of lead,” Water Res.36(6), 1527-1538.

Malik, U. R., Hasany, S. M., and Subhani, M. S. (2005). “Sorptive potential of sunflower stem for Cr(III) ions from aqueous solutions and its kinetic and thermodynamic profile,” Talanta66(1), 166-173.

Malik, U. R., Hasany, S. M., and Subhani, M. S. (2006). “Sunflower stem: A novel and economical scavenger for Hg(II) ions from aqueous solutions and its kinetic and thermodynamic investigation,” Radiochimica Acta 94(3), 153-159; “Sunflower stem: a novel and economical scavenger for Hg(II) ions from aqueous solutions and its kinetic and thermodynamic investigation (vol 94, pg 153, 2006),” Radiochimica Acta 94(5), 300.

Malkoc, E. (2006). “Ni(II) removal from aqueous solutions using cone biomass of Thuja orientalis,” J. Hazard. Mater. 137(2), 899-908.

Malkoc, E., and Nuhoglu, Y. (2003). “The removal of chromium(VI) from synthetic wastewater by Ulothrix zonata,” Fresenius Environ. Bull. 12(4), 376-381.

Malkoc, E., and Nuhoglu, Y. (2005). “Investigation of Ni II removal from aqueous solutions using tea factory waste,” J. Hazard. Mater. B127(1-3), 120-128.

Malkoc, E., and Nuhoglu, Y. (2006a). “Fixed bed studies for the sorption of chromium(VI) onto tea factory waste,” Chem. Eng. Sci. 61(13), 4363-4372.

Malkoc, E., and Nuhoglu, Y. (2006b). “Removal of Ni(II) ions from aqueous solutions using waste of tea factory: Adsorption on a fixed-bed column,” J. Hazardous Mater. 135(1-3), 328-336.

Malkoc, E., and Nuhoglu, Y. (2007a). “Determination of kinetic and equilibrium parameters of the batch adsorption of Cr(VI) onto waste acorn of Quercus ithaburensis,” Chem. Eng. Processing 46(10), 1020-1029.

Malkoc, E., and Nuhoglu, Y. (2007b). “Potential of tea factory waste for chromium(VI) removal from aqueous solutions: Thermodynamic and kinetic studies,” Separ. Purif. Technol. 54(3), 291-298.

Malkoc, E., Nuhoglu, Y., and Abali, Y. (2006a). “Cr(VI) adsorption by waste acorn of Quercus ithaburensis in fixed beds: Prediction of breakthrough curves,” Chem. Eng. J. 119(1), 61-68.

Malkoc, E., Nuhoglu, Y., and Dundar, M. (2006b). “Adsorption of chromium(VI) on pomace – An olive oil industry waste: Batch and column studies,” J. Hazard. Mater. 138(1), 142-151.

Mameri, N., Boudries, N., Addour, L., Belhocine, D., Lounici, H., Grib, H., and Pauss, A. (1999). “Batch zinc biosorption by a bacterial nonliving Streptomyces rimosus biomass,” Water Res. 33(6), 1347-1354.

Manju, G. N., Raji, C., and Anirudhan, T. S. (1998). “Evaluation of coconut husk carbon for the removal of arsenic from water,” Water Res. 32(10), 3062-3070.

Maranon, E., and Sastre, H. (1991a). “Heavy metal removal in packed beds using apple wastes,” Bioresour. Technol. 38(1), 39-43.

Maranon, E., and Sastre, H. (1991b). “Ion-exchange equilibria of heavy metals onto chemically modified apple residues,” Solvent Extraction Ion Exchange 9(3), 515-531.

Maranon, E., and Sastre, H. (1991c). “Preparation of cation exchangers from apple residues. 2. Exchange properties,” Afinidad 48(434), 255-258.

Maranon, E., and Sastre, H. (1992a). “Behavior of lignocellulosic apple residues in the sorption of trace metals in packed beds,” Reactive Polymers 18(2), 173-176.

Maranon, E., and Sastre, H. (1992b). “Preconcentration and removal of trace metals from water by apple waste,” Bioresour. Technol. 40(1), 73-76.

Maranon, E., Contreras, A., and Sastre, H. (1991). “Preparation of cation exchangers from applie residues. 1. Treatments and characterization,” Afinidad 48(433), 180-182.

Marchetti, V., Clement, A., Gerardin, P., and Loubinoux, B. (2000a). “Synthesis and use of esterified sawdusts bearing carboxyl group for removal of cadmium(II) from water,” Wood Sci. Technol. 34(2), 167-173.

Marchetti, M., Clement, A., Loubinoux, B., and Gerardin, P. (2000b). “Decontamination of synthetic solutions containing heavy metals using chemically modified sawdusts bearing polyacrylic acid chains,” Journal of Wood Science 46(4), 331-333.

Marin, J., and Ayele, J. (2002). “Removal of some heavy metal cations from aqueous solutions by spruce sawdust. I. Study of the binding mechanism through batch experiments,” Environ. Technol. 23(10), 1157-1171.

Marin, J., and Ayele, J. (2003). “Removal of some heavy metal cations from aqueous solutions by spruce sawdust. II. Adsorption-desorption through column experiments,” Environ. Technol. 24(4), 491-502.

Marshall, W. E., and Johns, M. M. (1996). “Agricultural byproducts as metal adsorbents: Sorption properties and resistance to mechanical abrasion,” J. Chem. Tech. Biotechnol. 66(2), 192-198.

Marshall, W. E., and Wartelle, L. H. (2006). “Chromate (CrO₄^2-) and copper (Cu²⁺) adsorption by dual-functional ion exchange resins made from agricultural by-products,” Water Research 40 (13), 2541-2548.

Marshall, W. E., Wartelle, L. H., and Akin, D. E. (2007). “Flax shive as a source of activatived carbon for metals remediation,” BioResources 2(1), 82-90.

Marshall, W. E., Wartelle, L. H., Boler, D. E., John, M. M., and Toles, C. A. (1999). “Enhanced metal adsorption by soybean hulls modified with citric acid,” Bioresour. Technol. 69(3), 263-268.

Martínez, M., Miralles, N., Hidalgo, S., Fiol, N., Villaescusa, I., and Poch, J. (2006a). “Removal of lead(II) and cadmium(II) from aqueous solutions using grape stalk waste,” J. Hazard. Mater. B 133, 203-211.

Martinez-Garcia, G., Bachmann, R. T., Williams, C. J., Burgoyne, A., and Edyvean, R. G J. (2006b). “Olive oil waste as a biosorbent for heavy metals,” Internat. Biodeter. Biodegrad.58(3-4), 231-238.

Martin-Lara, M. A., Hernainz, F., Calero, M., Blazquez, G., and Tenorio, G. (2009). “Surface chemistry evaluation of some solid wastes from olive-oil industry used for lead removal from aqueous solutions,” Biochem. Eng. J. 44(2-3), 151-159.

Martin-Lara, M. A., Pagnanelli, F., Mainelli, S., Calero, M., and Toro, L. (2008). “Chemical treatment of olive pomace: Effect on acid-basic properties and metal biosorption capacity,” J. Hazardous Mater. 156(1-3), 448-457.

Mata, Y. N., Blazquez, M. L., Ballester, A., Gonzalez, F., and Munoz, J. A. (2008). “Characterization of the biosorption of cadmium, lead and copper with the brown alga Fucus vesiculosus,” J. Hazardous Mater. 158(2-3), 316-323.

Matheickal, J. T., and Yu, Q. M. (1999). “Biosorption of lead (II) and copper (II) from aqueous solutions by pre-treated biomass of Australian marine algae,” Bioresour. Technol. 69(3), 223-229.

Matheickal, J. T., Yu, Q., and Woodburn, G. M. (1999). “Biosorption of cadmium from aqueous solutions by pre-treated biomass of marine alga Duruvillaea potatorum,” Wat. Res. 33(2), 335-342.

McKay, G., Allen, S. J., and McConvey, I. F. (1984). “The adsorption of dyes from solution – Equilibrium and column studies,” Water, Air, and Soil Pollut. 21, 127-129.

Mehta, S. K., and Gaur, J. P. (2005). “Use of algae for removing heavy metal ions from wastewater,” Crit. Rev. Biotechnol. 25(3), 113-152.

Melo, J. S., and D’Souza, S. F. (2004). “Removal of chromium by mucilaginous seeds of Ocimum basilicum,” Bioresour. Technol. 92(2), 151-155.

Mengistie, A. A., Rao, T. S., Rao, A. V. P., and Singanan, M. (2008). “Removal of lead(II) ions from aqueous solutions using activated carbon from Militia ferruginea plan leaves,” Bull. Chem. Soc. Ethiopia 22(3), 349-360.

Merdy, P., Guillon, E., Aplincourt, M., and Duomonceau, J. (2000). “Interaction of metallic cations with lignins, Part 1: Stability of iron (III), manganese (II) and copper (II) complexes with phenolic lignin model compounds: coumaric, ferulic and sinapic acids and coniferyl alcohol,” J. Chem. Res. S (2), 76-77.

Merdy, P., Guillon, E., Aplincourt, M., Dumonceau, J., and Vezin, H. (2002). “Copper sorption on a straw lignin: Experiments and EPR characterization,” J. Colloid Interf. Sci. 245(1), 24-31.

Merdy, P., Guillon, E., Frapart, Y. M., and Aplincourt, M. (2003). “Iron and manganese surface complex formation with extracted lignin. Part 2: Characterisation of magnetic interaction between transition metal and quinonic radical by EPR microwave power saturation experiments,” New J. Chem. 27(3), 577-582.

Meunier, N., Blais, J. F., and Tyagi, R. D. (2002). “Selection of a natural sorbent to remove toxic metals from acidic leachate produced during soil decontamination,” Hydrometallurgy67(1-3), 19-30.

Meunier, N., Blais, J. F., and Tyagi, R. D. (2003a). “Selectivity of cococa shells for removal of metals from soil decontamination effluents,” J. Environ. Eng. Sci. 2(1), 1-8.

Meunier, N., Laroulandie, J., Blais, J. F., and Tyagi, R. D. (2003b). “Lead removal from acidic solutions by sorption on cocoa shells: Effect of some parameters,” J. Environ. Eng. ASCE129(8), 693-698.

Michie, R. I. (1961). “Sorption of copper by cellulose,” Nature 190(477), 803-805.

Milenkovic, D. D., Dasic, P. V., and Veljkovic, V. B. (2009). “Ultrasound-assisted adsorption of copper(II) ions on hazelnut shell activated carbon,” Ultrason. Sonochem. 16(4), 557-563.

Min, S. H., Han, J. S., Shin, E. W., and Park, J. K. (2004). “Improvement of cadmium ion removal by base treated juniper fiber,” Water Res. 38(5), 1289-1295.

Mise, S. R., and Shantha, G. M. (1993). “Adsorption studies of chromium(VI) from synthetic aqueous solution by activated carbon derived from bagasse,” J. Environ. Sci. Heath. Pt. A – Environ. Sci. Eng. Toxic Hazard. Subst. Control 28(10), 2263-2280.

Mishra, S. P., Prasad, S. K., Dubey, R. S., Mishra, M., Tiwari, D., and Lee, S. M. (2007). “Biosorptive behaviour of rice hulls for Cs-134 from aqueous solutions: A radiotracer study,” Appl. Radiation Isotopes 65(3), 280-286.

Mishra, S. P., Tiwari, D., Dubey, R. S., and Mishra, M. (1998). “Removal behaviour of rice (Oryza sativa) hulls for submicro concentrations of Hg²⁺ and Cr³⁺ from aqueous solutions: A radiotracer study,” Radiochem. Acta 80(1), 47-52.

Mishra, S. P., Tiwari, D., Prasad, S. K., Dubey, R. S., and Mishra, M. (2007). “Biosorptive behavior of mango (Mangifera indica) and neem (Azadirachta indica) barks for Cs-134 from aqueous solutions: A radiotracer study,” J. Radioanal. Nucl. Chem. 272(2), 371-379.

Mistova, E., Parschova, H., and Matejka, Z. (2007). “Selective sorption of metal oxoanions from dilute solution by bead cellulose sorbent,” Separation Sci. Technol. 42(6), 1231-1243.

Mistova, E., Parschova, H., Jelinek, L., Matejka, Z., Plichta, Z., and Benes, M. (2008). “Selective sorption of metal oxoanions from dilute solution by chemicaly modified brown seaweed Ascophyllum nodosum,” Separation Sci. Technol. 43(11-12), 3168-3182.

Miyauchi, T., Kuroki, K., Ishikawa, N., Takahashi, M, and Mori, H. (2007). “Preparation and characterization of cation exchanger based on woody biomass modified by synthetic polymer,” Bunseki Kagaku 56(1), 9-15.

Modak, J. M., and Natarajan, K A. (1995). “Biosorption of metals using nonliving biomass – A review,” Miner. Metall. Proc. 12(4), 189-196.

Modak, J. M., Natarajan, K. A., and Saha, B. (1996). “Biosorption of copper and zinc using waste Aspergillus niger biomass,” Miner. Metall. Process 13(2), 52-57.

Mohammad, A., Ajmal, M., Yousuf, R., and Ahmed, A. (1997). “Adsorption of Cu(II) from water on the seed and seed shell of Mangifera indica (Mango),” Ind. J. Chem. Technol. 4(5), 223-227.

Mohan, D., and Pittman, C. U. (2006). “Activated carbons and low cost adsorbents for remediation of tri- and hexavalent chromium from water,” J. Hazardous Mater. 137(2), 762-811.

Mohan, D., and Pittman, C. U. (2007). “Arsenic removal from water/wastewater using adsorbents – A critical review,” J. Hazard. Mater. 142(1-2), 1-53.

Mohan, D., Pittman, C. U., Bricka, M., Smith, F., Yancey, B., Mohammad, J., Steele, P. H., Alexandre-Franco, M. F., Gomez-Serrano, V., and Gong, H. (2007). “Sorption of arsenic, cadmium, and lead by chars produced from fast pyrolysis of wood and bark during bio-oil production,” J. Colloid Interface Sci. 310(1), 57-73.

Mohan, D., Pittman Jr., C. U., and Steele, P. H. (2006). “Single, binary and multi-component adsorption of copper and cadmium from aqueous solutions on kraft lignin – A biosorbent,” J. Colloid Interface Sci. 297(2), 489-504.

Mohan, D., Pittman, C. U., and Steele, P. H. (2006). “Pyrolysis of wood/biomass for bio-oil: A critical review,” Energy & Fuels 20(3), 848-889.

Mohan, D., and Singh, K. P. (2002). “Single- and multi-component adsorption of cadmium and zinc using activated carbon derived from bagasse-an agricultural waste,” Wat. Res. 36(9), 2304-2318.

Mohan, D., Singh, K. P., and Singh, V. K. (2005). “Removal of hexavalent chromium from aqueous solution using low-cost activated carbons derived from agricultural waste materials and activated carbon fabric cloth,” Ind. Eng. Chem. Res. 44(4), 1027-1042.

Mohan, D., Singh, K. P., and Singh, V. K. (2006). “Trivalent chromium removal from wastewater using low cost activated carbon derived from agricultural waste material and activated carbon fabric cloth,” J. Hazardous Mater. 135(1-3), 280-295.

Mohan, S., and Sreelakshmi, G. (2008). “Fixed bed column study for heavy metal removal using phosphate treated rice husk,” J. Hazard. Mater. 153(1-2), 75-82.

Mohanty, K., Jha, M., Biswas, M. N., and Meikap, B. C. (2005). “Removal of chromium (VI) from dilute aqueous solutions by activated carbon developed from Terminalia arjuna nuts activated with zinc chloride,” Chem. Eng. Sci. 60(11), 3049-3059.

Mohanty, K., Jha, M., Meikap, B. C., and Biswas, M. N. (2006). “Biosorption of Cr(VI) from aqueous solutions by Eichhornia crassipes,” Chem. Eng. J. 117(1), 71-77.

Montanher, S. F., Oliveira, E. A., and Rollemberg, M. C. (2005). “Removal of metal ions from aqueous solutions by sorption onto rice bran,” J. Hazard. Mater. B 117(2-3), 207-211.

Mor, S., Ravindra, K., and Bishnoi, N. R. (2007). “Adsorption of chromium from aqueous solution by activated alumina and activated charcoal,” Bioresour. Technol. 98, 954-957.

Mouni, L., Merabet, D., Bouzaza, A., and Belkhiri, L. (2010). “Removal of Pb²⁺ and Zn²⁺ from the aqueous solutions by activated carbon prepared from dates stone,” Desal. Water Treat.16(1-3), 66-73.

Mugisidi, D., Ranaldo, A., Soedarsono, J. W., and Hikam, M. (2007). “Modification of activated carbon using sodium acetate and its regeneration using sodium hydroxide for the adsorption of copper from aqueous solution,” Carbon 45(5), 1081-1084.

Mungasavalli, D. P., Viraraghavan, T., and Jin, Y. C., (2007). “Biosorption of chromium from aqueous solutions by pretreated Aspergillus niger: Batch and column studies,” Colloids Surf. A: Physicochem. Eng. Aspects 301(1-3), 214-223.

Muraleedharan, T. R., and Venkobachar, C. (1990). “Mechanism of biosorption of copper (II) by Ganoderma lucidium,” Biotechnology & Bioengineering 35(3), 320-325.

Murphy, V., Hughes, H., and McLoughlin, P. (2007). “Cu(II) binding by dried biomass of red, green and brown macroalgae,” Water Res. 41(4), 731-740.

Murphy, V., Hughes, H., and McLoughlin, P. (2008). “Comparative study of chromium biosorption by red, green and brown seaweed biomass,” Chemosphere 70(6), 1128-1134.

Murphy, V., Hughes, H., and McLoughlin, P. (2009a). “Enhancement strategies for Cu(II), Cr(III) and Cr(VI) remediation by a variety of seaweed species,” J. Hazardous Mater. 166(1), 318-326.

Murphy, V., Tofail, S. A. M., and Hughes, H., and McLoughlin, P. (2009b). “A novel study of hexavalent chromium detoxification by selected seaweed species using SEM-EDX and XPS analysis,” Chem. Eng. J. 148(2-3), 425-433.

Murugan, M., and Subramanian, E. (2003). “Biosorbent Cupressus female cone in the efficient treatment of effluent containing Cr(VI),” Journal of Scientific and Industrial Research 62, 1071-1078.

Mussatto, S. I., Fernandes, M., Rocha, G. J. M., Orfao, J. J. M., Teixeira, J. A., and Roberto, I. C. (2010). “Production, characterization and application of activated carbon from brewer’s spent grain lignin,” Bioresour. Technol. 101(7), 2450-2457.

Muthukumaran, K., and Balasubramanian, N., and Ramakrishna, T. V. (1995). “Removal and recovery of chromium from plating waste using chemically activated carbon,” Metal Finishing 93(11), 46-53.

Nada, A. A. M. A., Alkady, M. Y., and Fekry, H. M. (2007a). “Synthesis and characterization of grafted cellulose used in water and metal ions sorption,” BioResourses 3(1), 46-59.

Nada, A. M. A., Eid, M. A., El Bahnasawy, R. M., and Khalifa, M. N. (2002a). “Preparation and characterization of cation exchangers from agricultural residues,” J. Appl. Polymer Sci.85(4), 792-800.

Nada, A. M. A, Eid, M. A, Sabry, A., and Khalifa, M. N. (2003). “Preparation and some applications of phosphsulfonated bagasse and wood cation exchangers,” J. Appl. Polym. Sci. 90(1), 1250-1258.

Nada, A. M. A., Eid, M. A., El-Bahnsawy, R. M., and Khalifa, M. N. (2002b). “Preparation and characterization of cation exchangers from agricultural residue,” J. Appl. Poly, Sci. 85(4), 792-800.

Nada, A. M. A., El-Mongy, S. A., and El-Sayed, E. S. A. (2009a). “Effect of different treatments on cellulose toward carboxylation and its application for metal ion absorption,” BioResources 4(1), 80-93.

Nada, A. M. A., El-Sherif, S., Nasr, A., and Kamel, M. (2007b). “Infrared spectroscopic and thermal analysis of prepared cation exchangers from cellulose materials,” J. Mol. Structure841(1-3), 1-6.

Nada, A. M. A., and El-Wakil, N. A. (2006). “Molecular structure and ion exchange of amidoximate cellulosic materials,” J. Appl. Polym. Sci. 102(1), 303-311.

Nada, A. M. A., El-Wakil, N., Hassan, M. L., and Adel, A. (2006). “Differential adsorption of heavy metal ions by cotton stalks cation-exchangers containing multiple functional groups,” J. Appl. Polym. Sci. 101(6), 4124-4132.

Nada, A. M. A., Hamed, S. S., Soliman, S. I., and El-Mongy, S. (2005). “Spectroscopic and ion exchange studies on modified cotton linters,” J. Sci. & Ind. Res. 64, 1003.

Nada, A. M. A., and Hassan, M. L. (2003). “Phosphorylated cation exchangers from cotton stalks and its constituents,” J. Appl. Polym. Sci. 89(11) 2950-2956.

Nada, A. M A., and Hassan, M. (2006). “Ion exchange properties of carboxylated bagasse,” J. Appl. Polym. Sci. 102(2), 1399-1404.

Nada, A. M. A., Hassan, M. L., and Adel, A. (2008). “Application of phosphorylated chitin and wood pulp for removal of heavy metal ions,” O Papel 69(8), 84-97.

Nada, A. M. A., Kassem, N. F., and Mohamed, S. H. (2008). “Characterization and properties of ion exchangers produced from lignin precipitated after peroxyacid pulping,” BioResources3(2), 538-548.

Nada, A. A. M. A., Mahdy, A. A., and El-Gendy, A. A. (2009b). “Spectroscopy and chemical studies of cationic exchangers prepared from corn cobs,” BioResources 4(3), 1017-1031.

Naddafi, K., Nabizadeh, R., Saeedi, R., Mahvi, A. H., Vaezi, F., Yaghmaeian, K., Ghasri, A., and Nazmara, S. (2007). “Biosorption of lead(II) and cadmium(II) by protonated Sargassum glaucescens biomass in a continuous packed bed column,” J. Hazardous Mater. 147(3), 785-791.

Nadeem, M., Mahmood, A., Shahid, S. A., Shah, S. S., Khalid, A. M., and McKay, G. (2006). “Sorption of lead from aqueous solution by chemically modified carbon adsorbents,” J. Hazard. Mater. 138(3), 604-613.

Nakajima, A., and Baba, Y. (2004). “Mechanism of hexavalent chromium adsorption by persimmon tannin gel,” Water Res. 38(12), 2859-2864.

Nakajima, A., and Tsuruta, T. (2004). “Competitive biosorption of thorium and uranium by Micrococcus luteus,” J. Radioanal. Nucl. Chem. 260(1), 13-18.

Nakajima, A., Yasuda, M., Yokoyama, H., Ohya-Nishiguchi, H., and Kamada, H. (2001). “Copper biosorption by chemically treated Micrococcus luteus cells,” World J. Microbiol. Biotechnol. 17(4), 343-347.

Namasivayam, C., and Kadirvelu, K. (1997). “Agricultural solid wastes for the removal of heavy metals: Adsorption of Cu(II) by coirpith carbon,” Chemosphere 34(2), 377-399.

Namasivayam, C., and Kadirvelu, K. (1999). “Uptake of mercury (II) from wastewater by activated carbon from an unwanted agricultural solid by-product: coirpith,” Carbon 37(1), 79-84.

Namasivayam, C., and Periasamy, K. (1993). “Bicarbonate-treated peanut hull carbon for mercury(II) removal from aqueous solution,” Water Res. 27(11), 1663-1668.

Namasivayam, C., and Sureshkumar, M. V., (2008). “Removal of chromium(VI) from water and wastewater using surfactant modified coconut coir pith as a biosorbent,” Bioresour. Technol. 99(7), 2218-2225.

Namasivayam, C., and Sureshkumar, M. V. (2009). “Removal and recovery of molybdenum from aqueous solutions by adsorption onto surfactant-modified coir pith, a lignocellulosic polymer,” Clean – Soil Air Water 37(1), 60-66.

Namasivayam, C., and Yamuna, R. T. (1995a). “Adsorption of chromium(VI) by low-cost adsorbent: Biogas residual slurry,” Chemosphere 30(3), 561-578.

Namasivayam, C., and Yamuna, R. T. (1995b). “Waste biogas residual slurry as an adsorbent for the removal of Pb(II) from aqueous solution and radiator manufacturaing industry waste water,” Bioresour. Technol. 52(2), 125-131.

Namasivayam, C., and Yamuna, R. T. (1999). “Studies on chromium (III) removal from aqueous solution by adsorption onto biogas residual slurry and its application to tannery wastewater treatment,” Water Air Soil Pollution 133(1-4), 371-384.

Nasir, M. H., Nadeem, R., Akhtar, K., Hanif, M. A., and Khalid, A. M. (2007). “Efficacy of modified distillation sludge of rose (Rosa centifolia) petals for lead and zinc removal from aqueous solutions,” J. Hazard. Mater. 147(3), 1006-1014.

Navarro, R. R., Sumi, K., and Matsumura, M. (1999). “Improved metal affinity of chelating adsorbents through graft polymerization,” Water Research 33(9), 2037-2044.

Navarro, R. R., Sumi, K., Fujii, N., and Matsumura, M. (1996). “Mercury removal from wastewater using porous cellulose carrier modified with polyethyleneimine,” Water Research30(10), 2488-2494.

Navarro, R., Tatsumi, K., Sumi, K., and Matsumura, M. (2001). “Role of anions on heavy metal sorption of a cellulose modified with poly(glycidyl methacrylate) and polyethyleneimine,” Water Res. 35(11), 2724-2730.

Netzer, A., and Hughes, D. E. (1984). “Adsorpton of copper, lead and cobalt by activated carbon,” Water Res. 18(8), 927-933.

Ngah, W. S. W., and Hanafiah, M. A. K. M. (2008a). “Adsorption of copper on rubber (Hevea brasiliensis) leaf powder: Kinetic, equilibrium and thermodynamic studies,” Biochem. Eng. J. 39(3), 521-530.

Ngah, W. S. W., and Hanafiah, M. A. K. M. (2008b). “Biosorption of copper ions from dilute aqueous solutions on base treated rubber (Hevea brasiliensis) leaves powder: kinetics, isotherm, and biosorption mechanisms,” J. Environ. Sci. – China 20(10), 1168-1176.

Ngah, W. S. W., and Hanafiah, M. A. K. M. (2009). “Surface modification of rubber (Hevea brasiliensis) leaves for the adsorption of copper ions: Kinetic, thermodynamic and binding mechanisms,” J. Chem. Technol. Biotech. 84(2), 192-201.

Ninham, B. W., and Lo Nostro, P. (2010). Molecular Forces and Self Assembly: In Colloid, Nano Sciences and Biology, Cambridge Univ. Press, Cambridge, UK.

Niu, H., Xu, X. S., Wang, J. H., and Volesky, B. (1993). “Removal of lead from aqueous solutions by Penicillium biomass,” Biotechnol. Bioeng. 42(6), 785-787.

Nomanbhay, S. M., and Palanisamy, K. (2005). “Removal of heavy metal from industrial wastewater using chitosan coated oil palm shell charcoal,” Electron. J. Biotechnol. 8(1), 43-53.

Nourbakhsh, M., Sag, Y., Özer, D., Aksu, Z., Kutsal, T., and Caglar, A. (1994). “A comparative study of various biosorbents for removal of chromium(VI) ions from industrial wastewaters,” Process Biochem. 29(1), 1-5.

Nouri, L., Ghodbane, I., Hamdaoui, O., and Chiha, M. (2007). “Batch sorption dynamics and equilibrium for the removal of cadmium ions from aqueous phase using wheat bran,” J. Hazard. Mater. 149(1), 115-125.

Nuhoglu, Y., Malkoc, E., Gurses, A., and Canpolat, N. (2002). “The removal of Cu(II) from aqueous solutions by Ulothrix zonata,” Bioresour. Technol. 85(3), 331-333.

Nuhoglu, Y., and Oguz, E. (2003). “Removal of copper(II) from aqueous solutions by biosorptioni on the cone biomass of Thuja orientalis,” Process Biochem. 38, 1627-1631.

O’Connell, D. W., Aszalos, B., and Birkinshaw, C., and O’Dwyer, T. F. (2010). “A study of the mechanisms of divalent copper binding to a modified cellulose adsorbent,” J. Appl. Polymer Sci. 116(5), 2496-2503.

O’Connell, D. W., Birkinshaw, C., and O’Dwyer, T. F. (2006a). “A chelating cellulose adsorbent for the removal of Cu(II) from aqueous solutions,” Journal of Applied Polymer Science99(6), 2888-2897.

O’Connell, D. W., Birkinshaw, C., and O’Dwyer, T. F. (2006b). “A modified cellulose adsorbent for the removal of Ni(II) from aqueous solutions,” Journal of Chemical Technology and Biotechnology 81(11), 1820-1828.

O’Connell, D. W., Birkinshaw, C., and O’Dwyer, T. F. (2006c). “Removal of lead(II) from aqueous solutions using a modified cellulose adsorbent,”Adsorption Science and Technology24(4), 337-347.

O’Connell, D. W., Birkinshaw, C., and O’Dwyer, T. F. (2008). “Heavy metal adsorbents prepared from the modification of cellulose: A review,” Bioresour. Technol. 99(15), 6709-6724.

O’Cooney, D. (1998). Adsorption Design for Wastewater Treatment, CRC Press, Boca Raton, Florida, USA, pp. 9-25.

Ofomaja, A. E., and Ho, Y. S. (2007). “Effect of pH on cadmium biosorption by coconut copra meal,” J. Hazardous Mater. 139(2), 356-362.

Ofomaja, A. E., and Ho, Y. S. (2008). “Kinetic biosorption study of cadmium onto coconut copra meal as biosorbent,” Intl. J. Environ. Pollution 34(1-4), 466-476.

Ogden, K. L, Ogden, G. E., Hanners, J. L., and Unkefer, P. J. (1996). “Remediation of low-level mixed waste: Cellulose-based materials and plutonium,” J. Hazardous Mater. 51(1-3), 115-130.

Oguz, E. (2005). “Adsorption characteristics and the kinetics of Cr(VI) on Thuja orientalis,” Colloids Surf. A 252(2-3), 121-128.

Oh, M., and Tshabalala, M. A. (2007). “Pelletized ponderosa pine bark for adsorption of toxic heavy metals from water,” BioResources 2(1), 66-81.

Oh, W. C. (2005). “Effect of acid pretreatment on the metal loading of activated carbon,” J. Indus. Eng. Chem. 11(1), 137-144.

Okieimen, F. E., Okundia, E. U., and Ogbeifun, D. E. (1991). “Sorption of cadmium and lead ions on modified groundnut (Arachis hypogea) husks,” J. Chem. Tech. Biotechnol. 51(1), 97-103.

Oliveira, E. A., Montanher, S. F., Andnade, A. D., Nobrega, J. A., and Rollemberg, M. C. (2005). “Equilibrium studies for the sorption of chromium and nickel from aqueous solutions using raw rice bran,” Process Biochem. 40(11), 3485-3490.

Oliveira, W. E., Franca, A. S., Oliveira, L S., and Rocha, S. D. (2008). “Untreated coffee husks as biosorbents for the removal of heavy metals from aqueous solutions,” J. Hazard. Mater. 152(3), 1073-1081.

Omar, N. B., Merroun, M. L., Gonzalez-Munoz, M. T., and Arias, J. M. (1996). “Brewery yeast as a biosorbent for uranium,” Journal of Applied Bacteriology 81, 283-287.

Onyancha, D., Mavura, W., Ngila, J. C., Ongoma, P., and Chacha, J. (2008). “Studies of chromium removal from tannery wastewaters by algae biosorbents, Spirogyra con-densata and Rhizoclonium hieroglyphicum,” J. Hazardous Mater. 158(2-3), 605-614.

Oo, C. W., Kassim, M. J., and Pizzi, A. (2009). “Characterization and performance of Rhizophora apiculata mangrove polyflavonoid tannins in the adsorption of copper (II) and lead (II),” Industr. Crops Prods. 30(1), 152-161.

Orhan, Y., and Buyukgungor, H. (1993). “The removal of heavy metals by using agricultural wastes,” Wat. Sci.Technol. 28(2), 247-255.

Orhan, Y., Hrenovic, J., and Buyukgungor, H. (2006). “Biosorption of heavy metals from wastewater by biosolids,” Eng. Life Sci. 6(4), 399-402.

Orlando, U. S., Baes, A., Nishijima, W., and Okada, M. (2002). “Preparation of chelating agents from sugarcane bagasse by microwave radiation as an alternative ecologically benign procedure,” Green Chemistry 4(6), 555-557.

Owlad, M., Aroua, M. K., Daud, W. A. W., and Baroutian, S. (2009). “Removal of hexavalent chromium-contaminated water and wastewater: A review,” Water Air Soil Pollut. 200(1-4), 59-77.

Ozcan, A. S., Tunali, S., Akar, T., and Ozcan, A. (2009). “Biosorption of lead(II) ions onto waste biomass of Phaseolus vulgaris L.: Estimation of the equilibrium, kinetic and thermodynamic parameters,” Desalination 244(1-3), 188-198.

Ozdemir, G., Ceyhan, N., Ozturk, T., Akirmak, F., and Cosar, T. (2004). “Biosorption of chromium(VI), cadmium(II) and copper(II) by Pantoea sp. TEM 18,” Chem. Eng. J. 102(3), 249-253.

Ozer, A. (2007). “Removal of Pb(II) ions from aqueous solutions by sulphuric acid-treated wheat bran,” J. Hazard. Mater. 141(3), 753-761.

Ozer, A., Ekiz, H. I., Ozer, D., Kutsal, T., and Caglar, A. (1997). “A staged purification process to remove heavy metal ions from wastewater using Rhizopus arrhizus,” Process Biochem.32(4), 319-326.

Ozer, A., and Ozer, D. (2003). “Comparative study of the biosorption of Pb(II), Ni(II) and Cr(VI) ions onto S. cerevisiae: Determination of biosorption heats,” J. Hazard. Mater. 100(1-3), 219-229.

Ozer, A., and Ozer, D. (2004). “The adsorption of Cr(VI) on sulphuric acid-treated wheat bran,” Environ. Technol. 25(6), 689-697.

Ozer, A., Ozer, D., and Ekiz, H. I. (2004a). “The equilibrium and kinetic modeling of the biosorption of copper(II) ions on Cladophora crispate,” Adsorption 10(4), 317-326.

Ozer, A., Ozer, D., and Ozer, A. (2004b). “The adsorption of copper(II) ions on to dehydrated wheat bran (DWB): Determination of the equilibrium and thermodynamic parameters,” Process Biochem. 39(12), 2183-2191.

Ozer, A., and Pirincci, H. B. (2006). “The adsorption of Cd(II) ions on sulphuric acid treated wheat bran,” J. Hazard. Mater. B 137(2), 849-855.

Ozer, A., and Tumen, F. (2003). “Cd(II) adsorption from aqueous solution by activated carbon from sugar beet pulp impregnated with phosphoric acid,” Fresenius Environ. Bull. 12(9), 1050-1058.

Padilha, P. M., Campos, J. T. S., Moreira, J. C., and Federici, C. C. (1995). “Study on ion-exchange and adsorptive properties of cellulose and modified cellulose,” Quimica Nova 18(6), 529-533.

Padmesha, T. V. N., Vijayaraghavana, K., Sekaranb, G., and Velan, M. (2006). “Application of two-and three-parameter isotherm models: Biosorption of Acid Red 88 onto Azolla microphylla,” Bioremediation Journal 10(1-2), 37-44.

Pagnanelli, F., Mainelli, S., De Angelis, S., and Toro, L. (2005a). “Biosorption of protons and heavy metals onto olive pomace: Modelling of competition effects,” Water Res. 39(8), 1639-1651.

Pagnanelli, F., Mainelli, S., and Toro, L. (2005b). “Optimisation and validation of mechanistic models for heavy metal bio-sorption onto a natural biomass,” Hydrometallurgy 80(1-2), 107-125.

Pagnanelli, F., Mainelli, S., Vegliò, F., and Toro, L. (2003). “Heavy metal removal by olive pomace: Biosorbent characterisation and equilibrium modeling,” Chem. Eng. Sci. 58(20), 4709-4717.

Pagnanelli, F., Papini, M. P., Toro, L., Trifoni, M., and Veglio, F. (2000). “Biosorption of metal ions on Arthrobacter sp.: Biomass characterization and biosorption modeling,” Environ. Sci. Technol. 34(13), 2773-2778.

Palma, G., Freer, G. J., and Beeza, J. (2003). “Removal of metal ions by modified Pinus radiata bark and tannins from water solutions,” Water Res. 37(20), 4974-4980.

Panda, G. C., Das, S. K., Chatterjee, S., Maity, P. B., Bandopadhyay, T. S., and Guha, A. K. (2006). “Adsorption of cadmium on husk of Lathyrus sativus: Physico-chemical study,” Colloids Surf. B – Interfaces 50(1), 49-54.

Parab, H., Joshi, S., Shenoy, N., Lali, A., Sarma, U. S., and Sudersanan, M. (2006b). “Determination of kinetic and equilibrium parameters of the batch adsorption of Co(II), Cr(III) and Ni(II) onto coir pith,” Process Biochem. 41(3), 609-615.

Parab, H., Joshi, S., Shenoy, N., Verma, R., Lali, A., and Sudersanan, M. (2005). “Uranium removal from aqueous solution by coir pith: Equilibrium and kinetic studies,” Bioresour. Technol. 96(11), 1241-1248.

Parab, H., Joshi, S., Shenoy, N., Lali, A., Sarma, U. S., and Sudersanan, M. (2006a). “Determination of kinetic and equilibrium parameters of the batch adsorption of Co(II), Cr(III) and Ni(II) onto coir pith,” Process Biochem. 41(3), 609-615.

Parab, H., Joshi, S., Shenoy, N., Lali, A., Sarma, U. S., and Sudersanan, M. (2008). “Esterified coir pith as an adsorbent for the removal of Co(II) from aqueous solution,” Bioresour. Technol. 99(6), 2083-2086.

Parab, H., Joshi, S., Sudersanan, M., Shenoy, N., Lali, A., and Sarma, U. (2010). “Removal and recovery of cobalt from aqueous solutions by adsorption using low cost lignocellulosic biomass – coir pith,” J. Environ. Sci. Health Pt. A Toxic/Haz. Subst. Environ. Eng. 45(5), 603-611.

Parajuli, D., Adhikari, C. R., Kawakita, H., Kajiyama, K., Ohto, K., and Inoue, K. (2008). “Reduction and accumulation of Au(III) by grape waste: A kinetic approach,” Reactive Functional Poly. 68(8), 1194-1199.

Parajuli, D., Kawakita, H., Inoue, K., and Funaoka, M. (2006). “Recovery of gold(III), palladium(II), and platinum(IV) by aminated lignin derivatives,” Ind. Eng. Chem. Res. 45(19), 6405-6412.

Pardo, R., Herguedas, M., Barrado, E., and Vega, M. (2003). “Biosorption of cadmium, copper, lead and zinc by inactive biomass of Pseudomonas putida, Anal. Bioanal. Chem. 376(1), 26-32.

Park, D., Lim, S.-R., Yun, Y.-S., and Park, J. M. (2007a). “Reliable evidences that the removal mechanism of hexavalent chromium by natural biomaterials is adsorption-coupled reduction,” Chemosphere 70(2), 298-305.

Park, D., Lim, S.-R., Yun, Y.-S., and Park, J. M. (2008a). “Development of new Cr(VI)-biosorbent from agricultural biowaste,” Bioresour. Technol. 99(18), 8810-8818.

Park, D., Park, J. M., and Yun, Y. S. (2006a). “Mechanisms of the removal of hexavalent chromium by biomaterials or biomaterial-based activated carbons,” J. Hazardous Mater. 137(2), 1254-1257.

Park, D., Park, J. M., and Yun, Y. S. (2006). “Comment on “Sorption of Cr(VI) from dilute solutions and wastewater by live and pretreated biomass of Aspergillus flavus” by Deepa et al.,” Chemosphere 63(6), 1060-1062.

Park, D., Yun, Y. S., Ahn, C. K., and Park, J. M. (2007b). “Kinetics of the reduction of hexavalent chromium with the brown seaweed Ecklonia biomass,” Chemosphere 66(5), 939-946.

Park, D., Yun, Y. S., Cho, H. Y., and Park, J. M. (2004a). “Chromium biosorption by thermally treated biomass of the brown seaweed, Ecklonia sp.,” Indus. Eng. Chem. Res. 43(26), 8226-8232.

Park, D., Yun, Y. S., Jo, J. H., and Park, J. M. (2005a). “Effects of ionic strength, background electrolytes, heavy metals, and redox-active species on the reduction of hexavalent chromium by Ecklonia biomass,” J. Microbiol. Biotechnol. 15(4), 780-786.

Park, D., Yun, Y. S., Jo, J. H., and Park, J. M. (2005b). “Mechanism of hexavalent chromium removal by dead fungal biomass of Aspergillus niger,” Water Res. 39(4), 533-540.

Park, D., Yun, Y. S., Jo, J. H., and Park, J. M. (2006b). “Biosorption process for treatment of electroplating wastewater containing Cr(VI): Laboratory-scale feasibility test,” Ind. Eng. Chem. Res. 45(14), 5059-5065.

Park, D., Yun, Y. S., Kim, J. Y., and Park, J. M. (2008b). “How to study Cr(VI) biosorption: Use of fermentation waste for detoxifying Cr(VI) in aqueous solution,” Chem. Eng. J. 136(2-3), 173-179.

Park, D., Yun, Y. S., Lee, D. S., Lim, S. R., and Park, J. M. (2006c). “Column study on Cr(VI)-reduction using the brown seaweed Ecklonia biomass,” J. Hazardous Mater. 137(3), 1377-1384.

Park, D., Yun, Y. S., Lee, H. W., and Park, J. M. (2008c). “Advanced kinetic model of the Cr(VI) removal by biomaterials at various pHs and temperatures,” Bioresour. Technol. 99(5), 1141-1147.

Park, D., Yun, Y. S., and Park, J. M. (2004b). “Reduction of hexavalent chromium with the brown seaweed Ecklonia biomass,” Environ. Sci. Technol. 38(18), 4860-4864.

Park, D., Yun, Y. S., and Park, J. M. (2005c). “Use of dead fungal biomass for the detoxification of hexavalent chromium: Screening and kinetics,” Process Biochem. 40(7), 2559-2565.

Park, D., Yun, Y. S., and Park, J. M. (2005d). Studies on hexavalent chromium biosorption by chemically-treated biomass of Ecklonia sp,” Chemosphere 60(10), 1356-1364.

Park, D., Yun, Y. S., and Park, J. M. (2006). “Comment on “Chromate ion adsorption by agricultural byproducts modified with dimethyloldihydroxylethylene urea and choline chloride” by Wartelle and Marshall,” Water Res. 40(7), 1501-1504.

Park, D., Yun, Y. S., and Park, J. M. (2006). “Comment on the removal mechanism of hexavalent chromium by biomaterials or biomaterial-based activated carbons,” Indus. Eng. Chem. Res. 45(7), 2405-2407.

Park, D., Yun, Y. S., and Park, J. M. (2008d). “XAS and XPS studies on chromium-binding groups of biomaterial during Cr(VI) biosorption,” J. Colloid Interface Sci. 317(1), 54-61.

Park, S. J., Jang, Y. S., Shim, J. W., and Ryu, S. K. (2003). “Studies on pore structures and surface functional groups of pitch-based activated carbon fibers,” J. Colloid Interface Sci.260(2), 259-264.

Park, S. J., and Kim, Y. M. (2004). “Influence of anodic treatment on heavy metal ion removal by activated carbon fibers,” J. Colloid Interface Sci. 278(2), 276-281.

Parsons, J. G., Hejazi, M., Tiemann, K. J., Henning, J., and Gardea-Torresdey, J. L. (2002). “An XAS study of the binding of copper(II), zinc(II), chromium(III) and chromium(VI) to hops biomass,” Microchem. J. 71(2-3), 211-219.

Parsons, J. G., Tiemann, K. J., Peralta-Videa, J. R., and Gardea-Torresdey, J. L. (2006). “Sorption of uranyl cations onto inactivated cells of alfalfa biomass investigated using chemical modification, ICP-OES, and XAS,” Environ. Sci. Technol. 40(13), 4181-4188.

Pearson, R. G. (1963). “Hard and soft acids and bases,” J. Am. Chem. Soc. 85, 3533-3539.

Pehlivan, E., Altuna, T., Cetin, S., and Bhanger, M. I. (2009a). “Lead sorption by waste biomass of hazelnut and almond shell,” J. Hazardous Mater. 167(1-3), 1203-1208.

Pehlivan, E., Altun, T., and Parlayici, S. (2009b). “Utilization of barley straws as biosorbents for Cu²⁺ and Pb²⁺ ions,” J. Hazard. Mater. 164, 982-986.

Pehlivan, E., Cetin, S., and Yanik, B. H. (2006). “Equilibrium studies for the sorption of zinc and copper from aqueous solutions using sugar beet pulp and fly ash,” J. Hazardous Mater.135(1-3), 193-199.

Pehlivan, E., Yanık, B. H., Ahmetli, G., and Pehlivan, M. (2008). “Equilibrium isotherm studies for the uptake of cadmium and lead ions onto sugar beet pulp,” Bioresour. Technol. 99(9), 3520-3527.

Periasamy, K., and Namasivayamm, C. (1994). “Process development for removal and recovery of cadmium from waste water by a low-cost adsorbent – Adsorption rates and equilibrium studies,” Indus. Eng. Chem. Res. 33(2), 317-320.

Periasamy, K., and Namasivayam, C. (1995a). “Removal of nickel(II) from aqueous solution and nickel plating industry waste water using an agricultural waste – peanut hulls,” Waste Manag. 15(1), 63-68.

Periasamy, K., and Namasivayam, C. (1995b). “Adsorption of Pb(II) by peanut hull carbon from aqueous solution,” Separation Sci. Technol. 30(10), 2223-2237.

Periasamy, K., and Namasivayam, C. (1996). “Removal of copper(II) by adsorption onto peanut hull carbon from water and copper plating industry wastewater,” Chemosphere 32(4), 769-789.

Peternele, W. S., Winkler-Hechenleitner, A. A., and Gomez Pineda, E. A. (1999). “Adsorption of Cd(II) and Pb(II) onto functionalized formic lignin from sugar cane bagasse,” Bioresource Technology, Bioprocessing and Characterization of Lignocellulosics 68(1), 95-100.

Phan, N. H., Rio, S., Faur, C., Le Coq, L., Le Cloirec, P., and Nguyen, T. H. (2006). “Production of fibrous activated carbons from natural cellulose (jute, coconut) fibers for water treatment applications,” Carbon 44(12), 2567-2577.

Pino, G. H., de Mesquita, L. M. S., Torem, M. L., and Pinto, G. A. S. (2006). “Biosorption of heavy metals by powder of green coconut shell,” Sep. Sci. Technol. 41(14), 3141-3153.

Pokhrel, D., and Viraraghavan, T. (2006). “Arsenic removal from aqueous solution by iron oxide-coated fungal biomass: A factorial design analysis,” Water, Air, Soil Pollut. 173(1-4), 195-208.

Pokhrel, D., and Viraraghavan, T. (2008). “Arsenic removal from an aqueous solution by modified A-niger biomass: Batch kinetic and isotherm studies,” J. Hazardous Mater. 150(3), 818-825.

Poleo, N., Oliveros, S., Colina, M., Rincon, N., Mesa, J., and Colina, G. (2010). “Study of different chemical activatings on obtaining activated carbon from Hymenaea courbaril L shell for cadmium (II) removal,” Revista Tecnica Fac. Ingen. Univ. del Zulia 33(1), 29-38.

Pollard, S. J. T., Fowler, G. D., Sollars, C. J., and Perry, R. (1992). “Low-cost adsorbents for waste and waste-water treatment – A review,” Sci. Total Environ. 116(1-2), 31-52.

Popa, K., Cecal, A., Drochioiu, G., and Humelnicu, D. (2003). “Saccharomyces cerevisiae as uranium bioaccumulating material: The influence of contact time, pH and anion nature,” Nukleonika 48, 121-125.

Prabhakaran, S. K., Vijayaraghavan, K., and Balasubramanian, R. (2009). “Removal of Cr(VI) ions by spent tea and coffee dusts: Reduction to Cr(III) and biosorption,” Ind. Eng. Chem. Res. 48(4), 2113-2117.

Pradhan, S., Singh, S., and Rai, L. C. (2007). “Characterization of various functional groups present in the capsule of Microcystis and study of their role in biosorption of Fe, Ni, and Cr,” Bioresour. Technol. 98(3), 595-601.

Prakasham, R. S., Merrie, J. S., Sheela, R., Saswathi, N., and Ramkrishna, S. V. (1999). “Biosorption of chromium(VI) by free and immobilized Rhizopus arrhizus,” Environ. Pullut.104(3), 421-427.

Prasad, A. G. D., and Abdullah, M. A. (2010). “Biosorption of Cr(VI) from synthetic wasatewater using the fruit shell of gulmohar (Delonix regia): Application to electroplating wastewater,” BioResources 5(2), 838-853.

Prasanjit, B., and Sumathi, S. (2005). “Uptake of chromium by Aspergillus foetidus,” Journal of Material Cycles and Waste Management 7(2), 88-92.

Preetha, B., and Viruthagiri, T. (2005). “Biosorption of zinc (II) by Rhizopus arrhizus; equilibrium and kinetic modeling,” African J. Biotechnol. 4(6), 506-508.

Preetha, B., and Viruthagiri, T. (2007a). “Application of response surface methodology for the biosorption of copper using Rhizopus arrhizus,” J. Hazardous Mater. 143(1-2), 506-510.

Preetha, B., and Viruthagiri, T. (2007b). “Batch and continuous biosorption of chromium(VI) by Rhizopus arrhizus,” Separ. Purif. Technol. 57(1), 126-133.

Price, M. S., Classen, J. J., and Payne, G. A. (2001). “Aspergillus niger absorbs copper and zinc from swine wastewater,” Bioresour. Technol. 77(1), 41-49.

Puranik, P. R., and Paknikar, K. M. (1997). “Biosorption of lead and zinc from solutions using Streptoverticillium cinnamoneum waste biomass,” J. Biotechnol. 55(2), 113-124.

Puranik, P. R., and Paknikar, K. M. (1999). “Influence of co-cations on biosorption of lead and zinc – A comparative evaluation in binary and multimetal systems,” Bioresour. Technol. 70(3), 269-276.

Puranik, P. R., Chabukswar, N. S., and Paknikar, K. M. (1995). “Cadmium biosorption by Streptomyce pimprina waste biomass,” Appl. Microbiol. Biotechnol. 43(6), 1118-1121.

Puziy, A. M., Poddubnaya, O. I., Martinez-Alonso, A., Suarez-Garcia, F., Tascon, J. M. D. (2002). “Synthetic carbons activated with phosphoric acid – I. Surface chemistry and ion binding properties,” Carbon 40(9), 1493-1505.

Py, X., Guillot, A., and Cagnon, B. (2004). “Nanomorphology of activated carbon porosity: Geometrical models confronted to experimental facts,” Carbon 42(8-9), 1743-1754.

Pyrzynska, K. (2010). “Sorption of Cd(II) onto carbon-based materials-a comparative study,” Microchimica Acta 169(1-2), 7-13.

Pyrzynska, K., and Trojanowicz, M. (1999). “Functionalized cellulose sorbents for preconcentration of trace metals in environmental analysis,” Crit. Rev. Anal. Chem. 29(4), 313-321.

Qadeer, R., Hanif, I., and Hanif, J. (1996). “Effect of different cations on the adsorption of Dy³⁺, Gd³⁺, Eu³⁺ and Sm³⁺ ions on activated charcoal from aqueous solutions,” Adsorption Sci. Technol. 13(1), 27-29.

Qadeer, R., Hanif, J., and Hanif, I. (1995a). “Kinetics of strontium ions adsorption on activated charcoal from aqueous solutions,” J. Chem. Sco. Pakistan 17(2), 74-77.

Qadeer, R., Hanif, J., Saleem, M., and Afjal, M. (1992). “Selective adsorption of thorium on activated charcoal from electrolytic aqueous solution,” J. Radioanal. Nucl. Chem. 157(2), 321-334.

Qadeer, R., Hanif, J., Saleem, M., and Afzal, M. (1995b). “Kinetics and thermodynamics of some metal ions adsorption on activated charcoal from aqueous solutions,” J. Chem. Soc. Pakistan 17(2), 82-86.

Qaiser, S., Saleemi, A. R., and Ahmad, M. M. (2007). “Heavy metal uptake by agro based waste materials,” Elec. J. Biotechnol. 10(3), 409-416.

Qaiser, S., Saleemi, A. R., and Umar, M. (2009). “Biosorption of lead from aqueous solution by Ficus religiosa leaves: Batch and column study,” J. Hazardous Mater. 166(2-3), 998-1005.

Qi, B. C., and Aldrich, C. (2008). “Biosorption of heavy metals from aqueous solutions with tobacco dust,” Bioresour. Technol. 99(13), 5595-5601.

Quek, S. Y., Wase, D. A. J., and Forster, C. F. (1998). “The use of sago waste for the sorption of lead and copper,” Water SA 24, 251-256.

Quintana, G. C., Rocha, G. J. M., Gonçalves, A. R., and Velásquez, J. A. (2008). “Evaluation of heavy metal removal by oxidised lignins in acid media from various sources,” BioResources 3(4), 1092-1102.

Rai, P. K. (2009). “Heavy metal phytoremediation from aquatic ecosystems with special reference to macrophytes,” Crit. Rev. Environ. Sci. Technol. 39(9), 697-753.

Rajeshwarisivaraj and Subburam, V. (2002). “Activated parthenium carbon as an adsorbent for the removal of dyes and heavy metal ions from aqueous solution,” Biores. Technol. 85, 205-206.

Raji, C., and Anirudhan, T. S. (1997). “Chromium(VI) adsorption by sawdust carbon: Kinetics and equilibrium,” Ind. J. Chem. Technol. 4(5), 228-236.

Raji, C., and Anirudhan, T. S. (1998). “Batch Cr(IV) removal by polyacrylamide grafted sawdust: Kinetics and thermodynamics,” Water Research 32(12), 3772-3780.

Raji, C., Manju, G. N., and Anirudhan, T. S. (1997). “Removal of heavy metal ions from water using sawdust-based activated carbon,” Ind. J. Eng. Mater. Sci. 4(6), 254-260.

Randall, J. M., Bermann, R. L., Garret, V., and Waiss, A. C. (1974). “Use of bark to remove heavy metal ions,” Forest Products. J. 24(9), 80-84.

Rangel-Mendez, J. R., and Streat, M. (2002). “Adsorption of cadmium by activated carbon cloth: Influence of surface oxidation and solution pH,” Water Res. 36(5), 1244-1252.

Ranjan, D., Talat, M., and Hasan, S. H. (2009a). “Biosorption of arsenic from aqueous solution using agricultural residue ‘rice polish’,” J. Hazardous Mater. 166(2-3), 1050-1059.

Ranjan, D., Talat, M., and Hasan, S. H. (2009b). “Rice polish: An alternative to conventional adsorbents for treating arsenic bearing water by up-flow column method,” Indus. Eng. Chem. Res. 48(23), 10180-10185.

Rao, K. S., Anand, S., and Venkateswarlu, P. (2010). “Adsorption of cadmium(II) ions from aqueous solution by Tectona grandis L. F. (teak leaves powder),” BioResources 5(1), 438-454.

Rao, M., and Parwate, A. V. (2002). “Utilization of low-cost adsorbents for the removal of heavy metals from wastewater – A review,” J. Environ. Pollut. Control 5, 12-23.

Rao, M., Parwate, A. V., and Bhole, A. G. (2002). “Removal of Cr and Ni from aqueous solution using bagasse and fly ash,” Waste Manage. 22(7), 821-830.

Reddad, Z., Gerente, C., Andres, Y., Ralet, M.-C., Thibault, J.-F., and Cloirec, P. L. (2002d). “Ni (II) and Cu (II) binding properties of native and modified sugar beet pulp,” Carbohydr. Polym. 49(1), 23-31.

Reddad, Z., Gerente, C., Andres, Y., and Le Cloirec, P. (2002a). “Modeling of single and competitive metal adsorption onto a natural polysaccharide,” Environ. Sci. Technol. 36(10), 2242-2248.

Reddad, Z., Gerente, C., Andres, Y., and Le Cloirec, P. (2002b). “Adsorption of several metal ions onto a low cost biosorbent: Kinetic and equilibrium studies,” Environ. Sci. Technol. 36(9), 2067-2073.

Reddad, Z., Gerente, C., Andres, Y., and Le Cloirec, P. (2003). “Mechanisms of Cr(III) and Cr(VI) removal from aqueous solutions by sugar beet pulp,” Environ. Technol. 24(2), 257-264.

Reddad, Z., Gérente, C., Andrés, Y., Ralet, M. C., Thibault, J. F., and Le Cloirec, P. (2002e). “Ni(II) and Cu(II) binding properties of native and modified sugar beet pulp,” Carbohyd. Polymers 49(1), 23-31.

Rincon, C. D. G., Garcia, V., Quintero-Guzman, M. W., Moreno-Pirajan, J. C., and Giraldo-Gutierrez, L. (2007). “Evaluation of the properties of activated carbon fibers designed for Zn²⁺and Cd²⁺ adsorption,” AFINIDAD 64(531), 623-631.

Rincón, J., González, F., Ballester, A., Blázquez, M. L., and Muñoz, J. A. (2005). “Biosorption of heavy metals by chemically-activated alga Fucus vesiculosus,” J. Chem. Technol. Biotechnol. 80, 1403-1407.

Riversa-Utrilla, J., and Sanchez-Polo, M. (2003a). “Adsorption of Cr(III) on ozonised activated carbon. Importance of C-cation interactions,” Water Res. 37, 3335-3340.

Rivera-Utrilla, J., Toledo, I. B.., Ferro-Garcia, M. A., and Moreno-Satilla, C. (2003b). “Biosorption of Pb(II), Cd(II), and Cr(VI) on activated carbon from aqueous solutions,” Carbon41(2), 323-330.

Roberts, E. J., and Rowland, S. P. (1973). “Removal of mercury from aqueous solutions by nitrogen-containing chemically modified cotton,” Environ. Sci. Technol. 7(6), 552-555.

Rocha, S. C. S., Cavalcante, J. D., da Silva, M. G. C., and Pinho, C. G. (2006). “Influence of the drying conditions of Sargassum sp. alga on the bioadsorption of hexavalent chromium,” Environ. Technol. 27(9), 979-990.

Rocha, J. C., Toscano, I. A. S., and Burba, P. (1997). “Lability of heavy metal species in aquatic humic substances characterized by ion exchange with cellulose phosphate,” Talanta 44(1), 69-74.

Romera, E., Gonzalez, F., Ballester, A., Blazquez, M. L., and Munoz, J. A. (2006). “Biosorption with algae: A statistical review,” Crit. Rev. Biotechnol. 26(4), 223-235.

Romero-Gonzalez, J., Peralta-Videa, J. R., Rodriguez, E., Delgado, M., and Gardea-Torresdey, J. L. (2006). “Potential of Agave lechuguilla biomass for Cr(III) removal from aqueous solutions: Thermodynamic studies,” Bioresour. Technol. 97(1), 178-182.

Romero-Gonzalez, J., Peralta-Videa, J. R., Rodríguez, E., Ramirez, S. L., and Gardea-Torresdey, J. L. (2005). “Determination of thermodynamic parameters of Cr(VI) adsorption from aqueous solution onto Agave lechuguilla biomass,” J. Chem. Thermodyn. 37(4), 343-347.

Romero-Gonzalez, M. E., Williams, C. J., and Gardiner, P. H. E. (2001). “Study of the mechanisms of cadmium biosorption by dealginated seaweed waste,” Environ. Sci. Technol. 35, 3025-3030.

Rudie, A. W., Ball, A., and Patel, N. (2006). “Ion exchange of H⁺, Na⁺, Mg²⁺, Ca²⁺, Mn²⁺, and Ba²⁺ on wood pulp,” J. Wood Chem. Technol. 26(3), 259-272.

Saeed, A., Akhter, M. W., and Iqbal, M. (2005a). “Removal and recovery of heavy metals from aqueous solution using papaya wood as a new biosorbents,” Sep. Purif. Technol. 45(1), 25-31.

Saeed, A., and Iqbal, M. (2003). “Bioremoval of Cd from aqueous solution by black gram husk (Cicer arientinum),” Water Res. 37(14), 3472-3480.

Saeed, A., Iqbal, M., and Akhtar, M. W. (2005b). “Removal and recovery of lead (II) from single and multiple, (Cd, Ni, Cu, Zn) solutions by crop milling waste (black gram husk),” J. Hazard. Mater. 117(1), 65-73.

Saeed, A., Iqbal, M., and Höll, W. H. (2009). “Kinetics, equilibrium and mechanism of Cd²⁺ removal from aqueous solution by mungbean husk,” J. Hazardous Mater. 168(2-3), 1467-1475.

Sag, Y. (2001). “Biosorption of heavy metals by fungal biomass and modeling of fungal biosorption: A review,” Separation Purification Methods 30(1), 1-48.

Sag, Y., and Kutsal, T., (1996). “Fully competitive biosorption of Cr(VI) and Fe(III) ions from binary metal mixtures by R. arrhizus: Use of the competitive Langmuir model,” Process Biochem. 31(6), 573-585.

Saha, B., Tai, M.H., and Streat, M. (2003). “Adsorption of transition metals from aque-ous solutions by modified activated carbons,” Chem. Eng. Res. Des. 81, 1343-1353.

Şahin, Y., and Öztürk, A. (2005). “Biosorption of chromium(VI) ions from aqueous solution by the bacterium Bacillus thuringiensis,” Process Biochem. 40(5), 1895-1901.

Saito, T., and Isogai, A. (2005). “Ion-exchange behavior of carboxylate groups in fibrous cellulose oxidized by the TEMPO-mediated system,” Carbohydrate Polymers 61, 183-190.

Salam, A., Pawlak, J. J., Venditti, R. A., and El-Tahlawy, K. (2010). “Synthesis and characterization of starch citrate-chitosan foam with superior water and saline absorbance properties,” Biomacromol. 11, 1453-1459.

Salamatinia, B., Kamaruddin, A. H., and Abdullah, A. Z. ^. (2007). “Removal of Zn and Cu from wastewater by sorption on oil palm tree-derived biomasses,” Journal of Applied Sciences7(15), 2020-2027.

Salamatinia, B., Kamaruddin, A. H., and Abdullah, A. Z. (2010). “Regeneration and reuse of spent NaOH-treated oil palm frond for copper and zinc removal from wastewater,” Chem. Eng. J. 156(1), 141-145.

Saleem, M., Qadeer, R., Hanif, J., and Afzal, M. (1994). “Effects of pH on the adsorption of Ce³⁺, Sm³⁺, Eu³⁺ and Gd³⁺ ions on activated charcoal,” J. Radioanal. Nuclear Chem. 182(2), 415-419.

Salehizadeh, H., and Shojaosadati, S. A. (2003). “Removal of metal ions from aqueous solution by polysaccharide produced from Bacillus firmus,” Water Research 17, 4231-4235.

Saliba, R., Gauthier, H., and Gauthier, R. (2005). “Adsorption of heavy metal ions on virgin and chemically-modified lignocellulosic materials,” Adsorption Science and Technology 23(4), 313-322.

Saliba, R., Gauthier, H., Gauthier, R., and Petit-Ramel, M. (2000). “Adsorption of copper(II) and chromium(III) ions onto amidoximated cellulose,” J. Appl. Polymer Sci. 75(13), 1624-1631.

Saliba, R., Gauthier, H., Gauthier, R., and Petit-Ramel, M. (2001). “Amidoximated cellulose as scavenger for cadmium and nickel cations,” Cellulose Chem. Technol. 35(5-6), 435-449.

Saliba, R., Gauthier, H., Gauthier, R., and Petit-Ramel, M. (2002a). “The use of amidoximated cellulose for the removal of metal ions and dyes from waste waters,” Cellulose 9(2), 183-191.

Saliba, R., Gauthier, H., Gauthier, R., and Petit-Ramel, M. (2002b). “The use of eucalyptus barks for the adsorption of heavy metal ions and dyes,” Adsorp. Sci. Technol. 20(2), 119-129.

Salim, R., Al-Subu, M. M., and Sahrhage, E. (1992). “Uptake of cadmium from water by beech leaves,” J. Environ. Sci. Health. A 27(3), 603-627.

Salinas, E., Orellano, M. E. D., Rezza, I., Martinez, L., Marchesvky, E., and Tosetti, M. S. D. (2000). “Removal of cadmium and lead from dilute aqueous solutions by Rhodotorula rubra,” Bioresour. Technol. 72(2), 107-112.

Samantanoi, S., Mohanty, A. K., and Misra, M. (1997). “Removal of hexavalent chromium by Kendu fruit gum dust,” J. Appl. Polymer Sci. 66(8), 1485-1452.

Santhy, K., and Selvapathy, P. (2004). “Removal of heavy metals from wastewater by adsorption on coir pith activated carbon,” Separ. Sci. Technol. 39(14), 3331-3351.

Sanyahumbi, D., Duncan, J. R., Zhao, M., and VanHile, R. (1998). “Removal of lead from solution by the non-viable biomass of the water fern Azolla filiculoides,” Biotechnology Letters20(8), 745-747.

Sari, A., Mendil, D., Tuzen, M., and Soylak, M. (2008). “Biosorption of Cd(II) and Cr(III) from aqueous solution by moss (Hylocomium splendens) biomass: Equilibrium, kinetic and thermodynamic studies,” Chem. Eng. J. 144(1), 1-9.

Sari, A., Mendil, D., Tuzen, M., Soylak, M. (2009). “Biosorption of palladium(II) from aqueous solution by moss (Racomitrium lanuginosum) biomass: Equilibrium, kinetic and thermodynamic studies.” J. Hazardous Mater. 162(2-3), 874-879.

Sarı, A., and Tuzen, M., (2008). “Biosorption of total chromium from aqueous solution by red algae (Ceramium virgatum): Equilibrium, kinetic and thermodynamic studies,” J. Hazard. Mater. 160(2-3), 349-355.

Sari, A., and Tuzen, M. (2009a). “Biosorption of As(III) and As(V) from aqueous solution by macrofungus (Inonotus hispidus) biomass: Equilibrium and kinetic studies,” J. Hazardous Mater. 164(2-3), 1372-1378.

Sari, A., and Tuzen, M. (2009b). “Equilibrium, thermodynamic and kinetic studies on aluminum biosorption from aqueous solution by brown algae (Padina pavonica) biomass,” J. Hazardous Mater. 171(1-3), 973-979.

Sari, A., and Tuzen, M. (2009c). “Removal of mercury(II) from aqueous solution using moss (Drepanocladus revolvens) biomass: Equilibrium, thermodynamic and kinetic studies,” J. Hazardous Mater. 171(1-3), 500-507.

Sari, A., and Tuzen, M. (2009d). “Kinetic and equilibrium studies of biosorption of Pb(II) and Cd(II) from aqueous solution by macrofungus (Amanita rubescens) biomass,” J. Hazardous Mater. 164(2-3), 1004-1011.

Sari, A., Tuzen, M., Uluozlu, O. D., and Soylak, M. (2007). “Biosorption of Pb(II) and Ni(II) from aqueous solution by lichen (Cladonia furcata) biomass,” Biochem. Eng. J. 37(2), 151-158.

Sarin, V., and Pant, K. K. (2006). “Removal of chromium from industrial waste by using eucalyptus bark,” Bioresour. Technol. 97(1), 15-20.

Sarkar, B. (ed.) (2002). Heavy Metals in the Environment, Marcel Dekker, New York, 725 pp.

Savova, D., Petrov, N., Yardim, M. F., Ekinci, E., Budinova, T., Razvigorova, M., and Minkova, V. (2003). “The influence of the texture and surface properties of carbon adsorbents obtained from biomass products on the adsorption of manganese ions from aqueous solution,” Carbon 41(10), 1897-1903.

Sawalha, M. F., Peralta-Videa, J. R., Romero-Gonzalez, J., Duarte-Gardea, M., and Gardea-Torresdey, J. L. (2007). “Thermodynamic and isotherm studies of the biosorption of Cu(II), Pb(II), and Zn(II) by leaves of saltbush (Atriplex canescens),” J. Chem. Thermodyn. 39(3), 488-492.

Sawalha, M. F., Peralta-Videa, J. R., Romero-Gonzalez, J., and Gardea-Torresdey, J. L. (2006). “Biosorption of Cd(II), Cr(III), and Cr(VI) by saltbush (Atriplex canescens) biomass: Thermodynamic and isotherm studies,” J. Colloid Interface Sci. 300(1), 100-104.

Say, R., Denizli, A., and Arica, M. Y. (2001). “Biosorption of cadmium(II), lead(II) and copper(II) with the filamentous fungus Phanerochaete chrysosporium,” Bioresour. Technology76(1), 67-70.

Say, R., Yilmaz, N., and Denizli, A. (2003a). “Biosorption of cadmium, lead, mercury, and arsenic ions by the fungus Penicillium purpurogenum,” Sep. Sci. Technol. 38(9), 2039-2053.

Say, R., Yimaz, N., and Denizli, A. (2003b). “Removal of heavy metal ions using the fungus Penicillium canescens,” Adsorpt. Sci. Technol. 21(7), 643-650.

Say, R., Yilmaz, N., and Denizli, A. (2003c). “Removal of chromium(VI) ions from synthetic solutions by the fungus Penicillium purpurogenum,” Eng. Life Sci. 4(3), 276-280.

Sayan, E. (2006). “Ultrasound-assisted preparation of activated carbon from alkaline impregnated hazelnut shell: An optimization study on removal of Cu²⁺ from aqueous solution,” Chem. Eng. J. 115(3), 213-218.

Schneider, I. A. H., Rubio, J., Misra, M., and Smith, R.W. (1995). “Eichhornia crassipes as biosorbent for heavy metal ions,” Miner. Eng. 8(9), 979-988.

Schneider, I. A. H., Rubio, J., and Smith, R. W. (2001). “Biosorption of metals onto plant biomass: Exchange adsorption or surface precipitation?,” Int. J. Mineral Processing 62(1-4), 111-120.

Schiewer, S. (1999). “Modelling complexation and electrostatic attraction in heavy metal biosorption by Sargassum biomass,” J. Applied Phycology 11(1), 79-87.

Schiewer, S., and Volesky, B. (1995). “Modeling of the proton–metal ion-exchange in biosorption,” Environ. Sci. Technol. 29(12), 3049-3058.

Schiewer, S., and Volesky, B. (1996). “Modeling multi-metal ion exchange in biosorption,” Environ. Sci. Technol. 30(10), 2921-2927.

Schiewer, S., and Volesky, B. (1997a). “Ionic strength and electrostatic effects in biosorption of divalent metal ions and protons,” Environ. Sci. Technol. 31(9), 2478-2485.

Schiewer, S., and Wong, M. H. (1999). “Metal binding stoichiometry and isotherm choice in biosorption,” Environ. Sci. Technol. 33(21), 3821-3828.

Schiewer, S., and Wong, M. H. (2000). “Ionic strength effects in biosorption of metals by marine algae,” Chemosphere 41(1-2), 271-282.

Sciban, M., and Klasnja, M. (2004a). “Study of the adsorption of copper(II) ions from water onto wood sawdust, pulp and lignin,” Adsorp. Sci. Technol. 22(3), 195-206.

Sciban, M., and Klasnja, M. (2004b). “Wood saw dust and wood originate materials as adsorbents for heavy metal ions,” Holz Roh Werkst. 62(1), 69-73.

Sciban, M., Klasnja, M., and Skrbic, B. (2006a). ”Modified hardwood sawdust as adsorbent of heavy metal ions from water,” Wood Sci. Technol. 40(3), 217-227.

Sciban, M., Klasnja, M., and Skrbic, B. (2006b). “Modified softwood sawdust as adsorbent of heavy metal ions from water,” J. Hazardous Mater. 136(2), 226-271.

Sciban, M., Klasnja, M., and Skrbic, B. (2008). “Adsorption of copper ions from water by modified agricultural by-products,” Desalination 229(1-3), 170-180.

Sciban, M., Radetic, B., Kevresan, D., and Klasnja, M. (2007). “Adsorption of heavy metals from electroplating wastewater by wood sawdust,” Bioresour. Technol. 98(2), 402-409.

Scott, C. D. (1992). “Removal of dissolved metals by plant tissue,” Biotechnol. Bioengng. 39(10), 1064-1068.

Seidel, A., and Gelbin, D. (1988). “On applying the ideal adsorbed solution theory to multicomponent adsorption equilibria of dissolved organic components on activated carbon,” Chem. Eng. Sci. 43, 79-89.

Seidel-Morgenstern, A., and Guichon, G. (1993). “Modelling of the competitive isotherms and the chromatographic separation of two enantiomers,” Chem. Eng. Sci. 48, 2787-2797.

Sekar, M., Sakthi, V., and Rengaraj, S. (2004). “Kinetics and equilibrium adsorption study of lead(II) onto activated carbon prepared from coconut shell,” J. Colloid Interface Sci. 279(2), 307-313.

Sekhar, K. C., Subramanian, S., Modak, J. M., and Natarajan, K. A. (1998). “Removal of metal ions using an industrial biomass with reference to environmental control,” Intl. J. Mineral Proc. 53(1-2), 107-120.

Seki, K., Saito, N., and Aoyama, M. (1997). “Removal of heavy metal ions from solutions by coniferous barks,” Wood Sci. Technol. 31(6), 441-447.

Selatnia, A., Bakhti, M. Z., Madani, A., Kertous, L., and Mansouri, Y. (2004a). “Biosorption of Cd²⁺ from aqueous solution by a NaOH-treated bacterial dead Streptomyces rimosusbiomass,” Hydrometallurgy 75(1-4), 11-24.

Selatnia, A., Boukazoula, A., Kechid, N., Bakhti, M. Z., and Chergui, A. (2004b). “Biosorption of Fe³⁺ from aqueous solution by a bacterial dead Streptomyces rimosus biomass,” Process Biochem. 39(11), 1643-1651.

Selatnia, A., Boukazoula, A., Kechid, N., Bakhti, M. Z., Chergui, A., and Kerchich, Y. (2004c). “Biosorption of lead (II) from aqueous solution by a bacterial dead Streptomyces rimosusbiomass,” Biochem. Eng. J. 19(2), 127-135.

Selatnia, A., Madani, A., Bakhti, M. Z., Kertous, L., Mansouri, Y., and Yous, R. (2004d). “Biosorption of Ni²⁺ from aqueous solution by a NaOH-treated bacterial dead Streptomyces rimosus biomass,” Miner. Eng. 17(7-8), 903-911.

Selvaraj, K., Manomani, S., and Pattabhi, S. (2003). “Removal of hexavalent chromium using distillery sludge,” Bioresour. Technol. 89(2), 207-211.

Selvi, K., Pattabhi, S., and Kadirvelu, K. (2001). “Removal of Cr(VI) from aqueous solution by adsorption onto activated carbon,” Bioresour. Technol. 80(1), 87-89.

Selomulya, C., Meeyoo, V., and Amal, R. (1999). “Mechanisms of Cr(VI) removal from water by various types of activated carbons,” J. Chem. Technol. Biotechnol. 74(2), 111-122.

Sharma, D. C., and Forster, C. F. (1993). “Removal of hexavalent chromium using sphagnum moss peat,” Water Res. 27(7), 1201-1208.

Sharma, D. C., and Forster, C. F. (1994a). “A preliminary examination into the adsorption of hexavalent chromium using low-cost adsorbents,” Bioresour. Technol. 47(3), 257-264.

Sharma, D. C., and Forster, C. F. (1994b). “The treatment of chromium wastewaters using the sorptive potential of leaf mould,” Bioresour. Technol. 49(1), 31-40.

Sharma, D. C., and Forster, C. F. (1995a). “Column studies into the adsorption of chromium(VI) using sphagnum moss peat,” Bioresour. Technol. 52(3), 261-267.

Sharma D. C., and Forester, C. F. (1995b). “Continuous adsorption and desorption of chromium ions by sphagnum moss peat,” Process Biochem. 30(4), 293-298.

Sharma, D. C., and Forster, C. F. (1996a). “A comparison of the sorbtive characteristics of leaf mould and activated carbon columns for the removal of hexavalent chromium,” Proc. Biochem. 31(3), 213-218.

Sharma, D. C., and Forster, C. F. (1996b). “Removal of hexavalent chromium from aqueous solutions by granular activated carbon,” Water SA 22(2), 153-160.

Sharma, R. K., and Chauhan, G. S. (2009). “Synthesis and characterization of graft copolymers of 2-hydroxyethyl methacrylate and some comonomers onto extracted cellulose for use in separation technologies,” BioResources 4(3), 986-1005.

Sheindorf, C., Rebhum, M., and Sheintuch, M. (1981). “A Freundlich-type multicomponent isotherm,” J. Colloid Interface Sci. 79, 136-142.

Shen, J. C., and Duvnjak, Z. (2004). “Effects of temperature and pH on adsorption isotherms for cupric and cadmium ions in their single and binary solutions using corncob particles as adsorbent,” Separ. Sci. Technol. 39(13), 3023-3041.

Shen, J. C., and Duvnjak, Z. (2005a). “Adsorption isotherms for cupric and cadmium ions on corncob particles,” Separ. Sci. Technol. 40(7), 1461-1481.

Shen, J. C., and Duvnjak, Z. (2005b). “A reversible surface reaction model with an effectiveness factor and its application to sorption kinetics of cupric ions on corncob particles,” Separ. Purification Technol. 44(1), 69-77.

Shen, J. C., and Duvnjak, Z. (2005c). “Adsorption kinetics of cupric and cadmium ions on corncob particles,” Process Biochem. 40(11), 3446-3454.

Sheng, P. X., Tan, L. H., Chen, J. P., and Ting, Y. P. (2004a). “Biosorption performance of two brown marine algae for removal of chromium and cadmium,” J. Dispersion Sci. Technol.25(5), 679-686.

Sheng, P. X., Ting, Y. P., and Chen, J. P. (2007). “Biosorption of heavy metal ions (Pb, Cu, and Cd) from aqueous solutions by the marine alga Sargassum sp. in single- and multiple-metal systems,” Indus. Eng. Chem. Res. 46(8), 2438-2444.

Sheng, P. X., Ting, Y. P., Chen, J. P., and Hong, L. (2004b). “Sorption of lead, copper, cadmium, zinc and nickel by marine algal biomass: Characterization of biosorptive capacity and investigation of mechanisms,” J. Colloid Interface Sci. 275(1), 131-141.

Sheng, P. X., Wee, K. H., Ting, Y. P., and Chen, J. P. (2008). “Biosorption of copper by immobilized marine algal biomass,” Chem. Eng. J. 136(2-3), 156-163.

Shibi, I. G., and Anirudhan, T. S. (2002). “Synthesis, characterisation, and application as a mercury(II) sorbent of banana stalk-polyacrylamide grafted copolymer bearing carboxyl groups,” Industrial Engineering and Chemical Research 41(22), 5341-5352.

Shibi, I. G., and Anirudhan, T. S. (2005). “Adsorption of Co(II) by a carboxylate-functionalized polyacrylamide grafted lignocellulosics,” Chemosphere 58(8), 1117-1126.

Shibi, I. G., and Anirudhan, T. S. (2006). “Polymer-grafted banana (Musa paradisiaca) stalk as an adsorbent for the removal of lead(II) and cadmium(II) ions from aqueous solutions: Kinetic and equilibrium studies,” Journal of Chemical Technology and Biotechnology 81(3), 433-444.

Shim, J.-W., Park, S.-J., and Ryu, S.-K. (2001). “Effect of modification with HNO₃ and NaOH on metal adsorption by pitch-based activated carbon fibers,” Carbon 39(11), 1635-1642.

Shin, E. J., Lauve, A., Carey, M., Bukovsky, E., Ranville, J. F, Euans, R. J., and Herring, A. M. (2008). “The development of bio-carbon adsorbents from lodgepole pine to remediate acid mine drainage in the Rocky Mountains,” Biomass Bioenergy 32(3), 267-276.

Shin, E. W., Karthikeyan, K. G., and Tshabalala, M. A. (2007). “Adsorption mechanism of cadmium on juniper bark and wood,” Bioresour. Technol. 98(3), 588-594.

Shukla, A., Zhang, Y. H., Dubey, P., Margrave, J. L., and Shukla, S. S. (2002). “The role of sawdust in the removal of unwanted materials from water,” Journal of Hazardous Materials95(1-2), 137-152.

Shukla, S. R., Gaikar, V. G., Pai, R. S., and Suryavanshi, U. S. (2009). “Batch and column adsorption of Cu(II) on unmodified and oxidized coir,” Separation Sci. Technol. 44(1), 40-62.

Shukla, S. R., and Pai, R. S. (2005a). “Adsorption of Cu (II), Ni (II) and Zn (II) on modified jute fibres,” Bioresour. Technol. 96(13), 1430-1438.

Shukla, S., and Pai, R. S. (2005b). “Removal of Pb(II) from solution using cellulose-containing materials,” J. Chem. Technol. Biotech. 80(2), 176-183.

Shukla, S. R., and Pai, R. S. (2005c). “Adsorption of Cu(II), Ni(II) and Zn(II) on dye loaded groundnut shells and sawdust,” Separ. Purif. Technol. 43(1), 1-8.

Shukla, S. R., Pai, R. S., and Shendarkar, A. D. (2006). “Adsorption of Ni(II), Zn(II) and Fe(II) on modified coir fibres,” Separation Purification Technol. 47(3), 141-147.

Shukla, S. S., Yu, L. J., Dorris, K., and Shukla, A. (2005d). “Removal of nickel from aqueous solutions by saw dust,” J. Hazard. Mater. 121(1-3), 243-246.

Silva, J. I. R., Ferreira, A. C. D., and da Costa, A. C. A. (2009). “Uranium biosorption under dynamic conditions: Preliminary tests with Sargassum filipendula in real radioactive wastewater containing Ba, Cr, Fe, Mn, Pb, Ca and Mg,” J. Radioanal. Nuclear Chem. 279(3), 909-914.

Simkovic, I. (1999). “Preparation of anion exchangers from beach sawdust and wheat straw,” Industrial Crops and Products 10(3), 167-173.

Singh, C. K., Sahu, J. N., Mahalik, K. K., Mohanty, C. R., Mohan, B. R., and Meikap, B. C. (2007). “Studies on the removal of Pb(II) from wastewater by activated carbon developed from Tamarind wood activated with sulphuric acid,” J. Hazard. Mater. 153(1-2), 221-228.

Singh, K. K., Rastogi, R., and Hasan, S. H. (2005a). “Removal of Cr(VI) from wastewater using rice bran,” J. Colloid Interface Sci. 290(1), 61-68.

Singh, K. K., Rastogi, R., and Hasan, S. H. (2005b). “Removal of cadmium from waste water using agricultural waste ‘rice polish’,” J. Hazard. Mater. 121(1-3), 51-58.

Singh, K. K., Singh, A. K., and Hasan, S. H. (2006). “Low cost bio-sorbent ‘wheat bran’ for the removal of cadmium form wastewater: Kinetic and equilibrium studies,” Bioresour. Technol. 97(8), 994-1001.

Singh, V. K., and Tiwari, P. N. (1997). “Removal and recovery of chromium(VI) from industrial wastewater,” J. Chem. Technol. Biotechnol. 69, 376-382.

Sjöström, E. (1993). Wood Chemistry: Fundamentals and Applications, Academic Press, San Diego, CA, USA.

Skowronski, T., Pirszel, J., and Pawlik-Skowronska, B. (2001). “Heavy metal removal by the waste biomass of Penicillium chrysogenum,” Water Qual. Res. J. Can. 36(4), 793-803.

Sokker, H. H., Badawy, S. M., Zayed, E. M., Eldien, F. A. N., and Farag, A. M. (2009). “Radiation-induced grafting of glycidyl methacrylate onto cotton fabric waste and its modification for anchoring hazardous wastes from their solutions,” J. Hazardous Mater. 168(1), 137-144.

Soleimani, M., and Kaghazchi, T. (2007). “Agricultural waste conversion to activated carbon by chemical activation with phosphoric acid,” Chem. Eng. Technol. 30(5), 649-654.

Soleimani, M., and Kaghazchi, T. (2008). “The investigation of the potential of activated hard shell of apricot stones as gold adsorbents,” J. Indus. Eng. Chem. 14(1), 28-37.

Southichak, B., Nakano, K., Nomura, M., Chiba, N., and Nishimura, O. (2006a). “Phragmites australis: A novel biosorbent for the removal of heavy metals from aqueous solution,” Water Res. 40(12), 2295-2302.

Southichak, B., Nakano, K., Nomura, M., Chiba, N., and Nishimura, O. (2006b). “Pb(II) biosorption on reed biosorbent derived from wetland: Effect of pretreatment on functional groups,” Water Sci. Technol. 54(10), 133-141.

Southichak, B., Nakano, K., Nomura, M., Chiba, N., and Nishimura, O. (2009). “Differences in adsorption mechanisms of heavy metal by two different plant biomasses: Reed and brown seaweed,” Water Sci. Technol. 59(2), 339-346.

Spanelova, M., Machovic, V., and Brezina, M. (2003). “Characterization and sorption properties of Aspergillus niger waste biomass,” Cent. Eur. J. Chem. 1(3), 192-200.

Sreejalekshmi, K. G., Krishnan, K. A., and Anirudhan, T. S. (2009). “Adsorption of Pb(II) and Pb(II)-citric acid on sawdust activated carbon: Kinetic and equilibrium isotherm studies,” J. Hazardous Mater. 161(2-3), 1506-1523.

Sricharoenchaikul, V., Pechyen, C., Aht-ong, D., and Atong, D. (2008). “Preparation and characterization of activated carbon from the pyrolysis of physic nut (Jatropha curcas L.) waste,” Energy Fuels 22(1), 31-37.

Srinath, T., Verma, T., Ramteke, P. W., and Garg, S. K. (2002). “Chromium(VI) biosorption and bioaccumulation by chromate resistant bacteria,” Chemosphere 48, 427-435.

Srinivasan, K., and Hema, M. (2009). “Nickel removal from wastewater by using activated carbon prepared from agro industrial wastes,” Res. J. Chem. Env. 13(3), 54-65.

Srivastava, S. K., Singh, A. K., and Sharma, A. (1994). “Studies on the uptake of lead and zinc by lignin obtained from black liquor a paper industry waste material,” Environ. Technol. 15(4), 353-361.

Srivastava, V. C., Mall, I. D., and Mishra, I. M. (2006a). “Equilibrium modelling of single and binary adsorption of cadmium and nickel onto bagasse fly ash,” Chem. Eng. J. 117(1), 79-91.

Srivastava, V. C., Mall, I. D., and Mishra, I. M. (2006b). “Modelling individual and competitive adsorption of cadmium(II) and zinc(II) metal ions from aqueous solution onto bagasse fly ash,” Separ. Sci. Technol. 41(12), 2685-2710.

Srivastava, V. C., Mall, I. D., and Mishra, I. M. (2006c). “Characterization of mesoporous rice husk ash (RHA) and adsorption kinetics of metal ions from aqueous solution onto RHA,” J. Hazard. Mater. B 134(1-3), 257-267.

Srivastava, V. C., Mall, I. D., and Mishra, I. M. (2007). “Adsorption thermodynamics and isosteric heat of adsorption of toxic metal ions onto bagasse fly ash (BFA) and rice husk ash (RHA),” Chem. Eng. J. 132(1-3), 267-278.

Srivastava, V. C., Mall, I. D., and Mishra, I. M. (2008a). “Removal of cadmium(II) and zinc(II) metal ions from binary aqueous solution by rice husk ash,” Colloids Surf. A – Physicochem. Eng. Aspects 312(2-3), 172-184.

Srivastava, V. C., Mall, I. D., and Mishra, I. M. (2008b). “Adsorption of toxic metal ions onto activated carbon – Study of sorption behaviour through characterization and kinetics,” Chem. Eng. Proc. 47(8), 1275-1286.

Srivastava, V. C., Mall, I. D., and Mishra, I. M. (2009a). “Equilibrium modeling of ternary adsorption of metal ions onto rice husk ash,” J. Chem. Eng. Data 54(3), 705-711.

Srivastava, V. C., Mall, I. D., and Mishra, I. M. (2009b). “Competitive adsorption of cadmium(II) and nickel(II) metal ions from aqueous solution onto rice husk ash,” Chem. Eng. Proc.48(1), 370-379.

Srivastava, S., and Thakur, I. S. (2006). “Biosorption potency of Aspergillus niger for removal of chromium (VI),” Current Microbiol. 53(3), 232-237.

Stefan, D. S., Belcu, M., Stefan, M., and Marinescu, I. A. (2010). “Sorption of Hg²⁺ and Pb²⁺ ions from aqueous solutions on corn cobs biomaterials,” Revista de Chimie 61(1), 31-35.

Stone, J. E., and Scallan, A. M. (1966). “Influence of drying on the pore structures of the cell wall,” in Consolidation of the Paper Web, Trans. Symp. Cambridge, Sept. 1965, F. Bolam (ed.), Tech. Sec. British Paper and Board Makers’ Assoc. Inc, London, Vol. 1, 145-174.

Sud, D., Mahajan, G., and Kaur, M. P. (2008). “Agricultural waste material as potential adsorbent for sequestering heavy metal ions from aqueous solutions – A review,” Bioresour. Technol. 99, 6017-6027.

Sudha, B. R., and Abraham, E. (2003). “Studies on chromium (VI) adsorption using immobilized fungal biomass,” Bioresour. Technol. 87(1), 17-26.

Suemitsu, R., Uenishi, R., Akashi, I., and Nakan, M. (1986). “The use of dyestuff-treated rice hulls for the removal of heavy metals from waste water,” J. Appl. Polym. Sci. 31(1), 75-83.

Suh, J. H., Yun, J. W., and Kim, D. S. (1998). “Comparison of Pb²⁺ accumulation characteristics between live and dead cells of Saccharomyces cerevisiae and Aureobasidium pullulans,” Biotechnology Letters 20(3), 247-251.

Suhas, Carrott, P. J. M., and Carrott, M. M. L. R. (2007). “Lignin-from natural adsorbent to activated carbon: A review,” Bioresour. Technol. 98(12), 2301-2312.

Suksabye, P., Nakajima, A., Thiravetyan, P., Baba, Y, and Nakbanpote, W. (2009). “Mechanism of Cr(VI) adsorption by coir pith studied by ESR and adsorption kinetic,” J. Hazardous Mater. 161(2-3), 1103-1108.

Suksabye, P., Thiravetyan, P., Nakbanpote, W., and Chayabutra, S. (2007). “Chromium removal from electroplating wastewater by coir pith,” J. Hazard. Mater. 141(3), 637-644.

Sumathi, K. M. S., Mahimairaja, S., and Naidu, R. (2005). “Use of low-cost biological wastes and vermiculite for removal of chromium from tannery effluent,” Bioresour. Technol. 96(3), 309-316.

Sun, G., and Shi, W. X. (1998). “Sunflower stalks as adsorbents for the removal of metal ions from wastewater,” Ind. Eng. Chem. Res. 37(4), 1324-1328.

Sundman, O., and Öhman, L. O. (2006). “Acid/base and metal adsorption properties of CMC-type softwood Kraft pulps of different charge,” Nordic Pulp Paper Res. J. 21(3), 372-381.

Sundman, O., Persson, P., and Ohman, L. O. (2008). “A multitechnique study of the interactions between H⁺, Na⁺, Ca²⁺ and Cu²⁺, and two types of softwood Kraft fibre materials,” J. Colloid Interface Sci. 328(2), 248-256.

Sungur, S., and Babaoglu, S. (2005). “Synthesis of a new cellulose ion exchanger and use for the separation of heavy metals in aqueous solutions,” Separation Sci. Technol. 40(10), 2067-2078.

Suryavanshi, U. S., and Shukla, S. R. (2009). “Adsorption of Ga(III) on oxidized coir,” Ind. Eng. Chem. Res. 48(2), 870-876.

Svecova, L., Spanelova, M., Kubal, M., and Guibal, E. (2006). “Cadmium, lead and mercury biosorption on waste fungal biomass issued from fermentation industry. 1. Equilibrium studies,” Sep. Purif. Technol. 52(1), 142-153.

Tabakci, M., Erdemir, S., and Yilmaz, M. (2007). “Preparation, characterization of cellulose grafted with calix[4]arene polymers for the adsorption of heavy metals and dichromate anions,” J. Hazard. Mater. 148(1-2), 428-435.

Tandel, M. A., and Oza, B. N. (2005). “Adsorption and regeneration studies for the removal of chromium(VI) from the wastewater of electroplating industry using granular activated carbon,” Asian J. Chem. 17(4), 2524-2530.

Tangsathitkulchai, C., Ngernyen, Y., and Tangsathitkulchai, M. (2009). “Surface modification and adsorption of eucalyptus wood-based activated carbons: Effects of oxidation treatment, carbon porous structure and activation method,” Kor. J. Chem. Eng. 26(5), 1341-1352.

Tan, L., Zhu, D., Zhou, W., Mi, W., Ma, L., and He, W. (2008). “Preferring cellulose of Eichhornia crassipes to prepare xanthogenate to other plant materials and its adsorption properties on copper,” Bioresour. Technol. 99(10), 4460-4466.

Tan, W. T., Ooi, S. T., and Lee, C. K. (1993). “Removal of chromium (VI) from solution by coconut husk and palm pressed fibers,” Environ. Technol. 14(3), 277-282.

Tangaromsuk, J., Pokethitiyook, P., Kruatrachue, M., and Upatham, E. S. (2002). “Cadmium biosorption by Sphingomonas paucimobilis biomass,” Bioresour. Technol. 85(1), 103-105.

Tangjuank, S., Insuk, N., Udeye, V., and Tontrakoon, J. (2009). “Chromium (III) sorption from aqueous solutions using activated carbon prepared from cashew nut shells,” Intl. J. Phys. Sci. 4(8), 412-417.

TAPPI (1993). “Carboxyl content of pulp,” In: TAPPI Test Methods, Technical Association of the Pulp and Paper Industry, Atlanta, GA, USA, method T237 om-93.

Tarley, C. R .T., and Arruda, M. A. Z. (2004). “Biosorption of heavy metals using rice milling byproducts. Characterization and application for removal of metals from aqueous effluents,” Chemosphere 54(7), 987-995.

Tashiro, T., and Shimura, Y. (1982). “Removal of mercuric ions by systems based on cellulose derivatives,” Journal of Applied Polymer Science 27(2), 747-756.

Taty-Costodes, C. V., Fauduet, H., Porte, C., and Delacroix, A. (2003). “Removal of Cd(II) and Pb(II) ions, from aqueous solutions, by adsorption onto sawdust of Pinus sylvestris,” J. Hazard. Mater. B105, 121-142.

Tazrouti, N., and Amrani, M. (2009). “Chromium (VI) adsorption onto activated kraft lignin produced from alfa grass (Stipa tenacissima),” BioResources 4(2), 740-755.

Temkin, M. J., and Pyzhev, V. (1940). “Kinetics of ammonia synthesis on promoted iron catalysts,” Acta Physiochim. URSS 12, 217-222.

Tewari, N., Vasudevan, P., and Guha, B. K. (2005). “Study on biosorption of Cr(VI) by Mucor hiemalis,” Biochem. Eng. J. 23(2), 185-192.

Texier, A.C., Andrès, Y., and Le Cloirec, P. (1999). “Selective biosorption of lanthanide (La, Eu, Yb) ions by Pseudomonas aeruginosa,” Environ. Sci. Technol. 33, 489-495.

Texier, A. C., Andres, Y., and Le Cloirec, P. (2000). “Selective biosorption of lanthanide (La, Eu, Yb) ions by an immobilized bacterial biomass,” Water Sci. Technol. 42(5-6), 91-94.

Tiemann, K. J., Gardea-Torresdey, J. L., Gamez, G., Dokken, K., Cano-Aguilera, I., Renner, J. W., and Furenlid, L. R. (2000). “Effects of oxidation state on metal ion binding by Medico sativa (alfalfa): Atomic and X-ray absorption spectroscopic studies with Fe(II) and Fe(III),” Environ. Sci. Technol. 34(4), 693-698.

Tiemann, K. J., Gardea-Torresdey, J. L., Gamez, G., Dokken, K., and Sias, S. (1999). “Use of X-ray absorption spectroscopy and esterification to investigate chromium(III) and nickel(II) ligand in alfalfa biomass,” Environmental Science & Technology 33, 150-154.

Tiemann, K. J., Rascon, A. E., Gamez, G., Parsons, J. G., Baig, T., Cano-Aguilera, E., and Gardea-Torresdey, J. L. (2002). “Heavy metal binding by inactivated tissues of Solanum elaeagnifolium: Chemical and subsequent XAS studies,” Microchem. J. 71(2-3), 133-141.

Tiwari, D., Mishra, S. P., Mishra, M., and Dubey, R. S. (1999). “Biosorptive behaviour of mango (Mangifera indica) and neem (Azadirachta indica) bark for Hg²⁺, Cr³⁺, and Cd²⁺ toxic ions from aqueous solutions: A radiotracer study,”  Appl. Radiat. Isot. 50(4), 631-642.

Tobin, J. M., Cooper, D.G., and Neufeld, R. J. (1984). “Uptake of metal ions by Rhizopus arrhizus biomass.”Appl. Environ. Microbiol. 47(4), 821-824.

Tobin, J. M., and Roux, J. C. (1998). “Mucor biosorbent for chromium removal from tanning effluent,” Water Res. 32(5), 1407-1416.

Toles, C. A., Marshall, W. E., and Johns, M. M. (1997). “Granular activated carbons from nutshells for the uptake of metals and organic compounds,” Carbon 35(9), 1407-1414.

Toles, C. A., Marshall, W. E., and Johns, M. M. (1998). “Phosphoric acid activation of nutshells for metals and organic remediation. Process optimization,” J. Chem. Technol. Biotechnol.72(3), 255-263.

Toles, C. A., Marshall, W. E., and Johns, M. M. (1999). “Surface functional groups on acid-activated nutshell carbons,” Carbon 37(8), 1207-1214.

Townsley, C. C., and Ross, I. S. (1986). “Copper uptake in Aspergillus niger during batch growth and in non-growing mycelial suspensions,” Experimental Mycology 10(4), 281-288.

Tsezos, M., and Volesky, B. (1981). “Biosorption of uranium and thorium,” Biotechnology and Bioengineering 23(3), 583-604.

Tsezos, M., and Volesky, B. (1982). “The mechanism of uranium biosorption by Rhizopus arrhizus,” Biotechnology and Bioengineering 24(2), 385-401.

Tshabalala, M. A., Karthikeyan, K. G., and Wang, D. (2004). “Cationized milled pine bark as an adsorbent for orthophosphate anions,” J. Appl. Polymer Sci. 93(4), 1577-1583.

Tsui, M. T. K., Cheung, K. C., Tam, N. F. Y., and Wong, M. H. (2006). “A comparative study on metal sorption by brown seaweed,” Chemosphere 65(1), 51-57.

Tunali, S., Kiran, I., and Akar, T. (2005). “Chromium(VI) biosorption characteristics of Neurospora crassa fungal biomass,” Miner. Eng. 18, 681-689.

Tupkanjana, P., and Phalakornkule, C. (2007). “Development of activated carbons from sunflower seed husk for metal adsorption,” J. Chem. Eng. Japan 40(3), 222-227.

Tuzen, M., Sari, A., Mendil, D., and Soylak, M. (2009). “Biosorptive removal of mercury(II) from aqueous solution using lichen (Xanthoparmelia conspersa) biomass: Kinetic and equilibrium studies,” J. Hazardous Mater. 169(1-3), 263-270.

Ucer, A., Uyanik, A., and Aygün, Ş. F. (2006). “Adsorption of Cu(II), Cd(II), Zn(II), Mn(II), and Fe(III) ions by tannic acid immobilised activated carbon,” Sep. Purif. Technol. 47, 113-118.

Ucer, A., Uyanik, A., Cay, S., and Ozkan, Y. (2005). “Immobilisation of tannic acid onto activated carbon to improve Fe(III) adsorption,” Separ. Purif. Technol. 44(1), 11-17.

Ucun, H., Aksakal, O. and Yildiz, E. (2009). “Copper(II) and zinc(II) biosorption on Pinus sylvestris L,” Journal of Hazardous Materials 161(2-3), 1040-1045.

Ugurlu, M., Kula, I., Karaoglu, M. H., and Arslan, Y. (2009). “Removal of Ni(II) ions from aqueous solutions using activated-carbon prepared from olive stone by ZnCl₂ activation,” Environ. Prog. Sustain. Energy 28(4), 547-557.

Ulmanu, M., Maranón, E., Fernández, Y., Castrillón, L., Anger, I., and Dumitriu, D. (2003). “Removal of copper and cadmium ions from diluted aqueous solutions by low cost and waste material adsorbents,” Water, Air, Soil Pollut. 142(1-4), 357-373.

Uluozlu, O. D., Sari, A., Tuzen, M., and Soylak, M. (2008). “Biosorption of Pb(II) and Cr(III) from aqueous solution by lichen (Parmelina tiliaceae) biomass,” Bioresour. Technol. 99(8), 2972-2980.

Unnithan, M. R., and Anirudhan, T. S. (2001). “The kinetics and thermodynamics of sorption of chromium(VI) onto the iron(III) complex of a carboxylated polyacrylamide-grafted sawdust,” Ind. Eng. Chem. Res. 40(12), 2693-2701.

Unnithan, M. R., Vinod, V. P., Anirudhan, T. S. (2004). “Synthesis, characterization, and application as a chromium(VI) adsorbent of amine-modified polyacrylamide-grafted coconut coir pith,” Indus. Eng. Chem. Res. 43(9), 2247-2255.

Upendra, K., and Manas, B. (2006). “Sorption of cadmium from aqueous solution using pretreated rice husk,” Bioresour. Technol. 97(1), 104-109.

Valdman, E., Leite, S. G. F. (2000). “Biosorption of Cd, Zn and Cu by Sargassum sp. waste biomass,” Bioprocess Engineering 22, 171-173.

Valix, M., Cheung, W. H., and Zhang, K. (2006). “Role of heteroatom in activated carbon for the removal of hexavalent Cr from wastewater,” J. Hazard. Mater. 135(1-3), 395-405.

Valix, M., Cheung, W. H., and Zhang, K. (2008). “Textural and surface chemistry of activated bagasse and its role in the removal of chromium ions from solution,” Adsorp. J. Intl. Adsorp. Soc. 14(4-5), 711-718.

Vaughan, T., Seo, C. W., and Marshall, W. E. (2001). “Removal of selected metal ions from aqueous solution using modified corncobs,” Bioresour. Technol. 78(2), 133-139.

Veglio, F., and Beolcini, F. (1997). “Removal of metals by biosorption: A review,” Hydrometallurgy 44, 301-316.

Verma, V. K., Tewari, S., and Rai, J. P. N. (2008). “Ion exchange during heavy metal bio-sorption from aqueous solution by dried biomass of macrophytes,” Bioresour. Technol. 99(6), 1932-1938.

Vianna, L. N. L., Andrade, M. C., and Nicoli, J. R. (2000). “Screening of waste biomass from Saccharomyces cerevisiae, Aspergillus oryzae and Bacillus lentus fermentations for removal of Cu, Zn and Cd by biosorption,” World J. Microbiol. Biotechnol. 16(5), 437-440.

Vieira, M. G. A., Oisiovici, R. M., Gimenes, M. L., and Silva, M. G. C. (2008). “Biosorption of chromium(VI) using a Sargassum sp. packed-bed column,” Bioresour. Technol. 99(8), 3094-3099.

Viera, R. H. S. F., and Volesky, B. (2000). “Biosorption: A solution to pollution?” Internatl. Microbiol. 3(1), 17-24.

Vijayaraghavan, K., Lee, M. W., and Yun, Y. S. (2008). “Evaluation of fermentation waste (Corynebacterium glutamicum) as a biosorbent for the treatment of nickel(II)-bearing solutions,” Biochem. Eng. J. 41(3), 228-233.

Vijayaraghavan, K., Padmesh, T. V. N., Palanivelu, K., and Velan, M. (2006a). “Biosorption of nickel (II) ions onto Sargassum wightii: Application of two-parameter and three parameter isotherm models,” J. Hazard. Mater. 133(1-3), 304-308.

Vijayaraghavan, K., and Yun, Y. S. (2008). “Bacterial biosorbents and biosorption,” Biotechnol. Adv. 26(3), 266-291.

Villaescusa, I., Fiol, N., Martinez, M., Miralles, N., Poch, J., and Serarols, J. (2004). “Removal of copper and nickel ions from aqueous solutions by grape stalk wastes,” Water Research38(4), 992-1002.

Villaescusa, I., Martinez, M., and Miralles, N. (2000). “Heavy metal uptake from aqueous solutions by cork and Yohimber bark wastes,” J. Chem. Tech. Biotechnol. 75(9), 812-816.

Vinopal, S., Ruml, T., and Kotrba, P. (2007). “Biosorption of Cd²⁺ and Zn²⁺ by cell surface-engineered Saccharomyces cerevisiae,” Int. Biodeter. Biodegrad. 60(2), 96-102.

Volesky, B. (1994). “Advances in biosorption of metals: Selection of biomass types,” FEMS Microbiol. Rev. 14(4), 291-302.

Volesky, B. (2001). “Detoxification of metal bearing effluents: Biosorption for next century,” Hydrometallurgy 59(2-3), 203-216.

Volesky, B., and Holan, Z. R. (1995). “Biosorption of heavy metals,” Biotechnol. Prog. 11(3), 235-250.

Volesky, B., May, H., and Holan, Z. R. (1993). “Cadmium biosorption by Saccharomyces cerevisiae,” Biotechnol. Bioeng. 41(8), 826-829.

Volesky, B., and Prasetyo, I. (1994). “Cadmium removal in the biosorption column,” Biotechnology & Bioengineering 43, 1010-1015.

Volesky, B., Weber, J., and Park, J. M. (2003). “Continuous-flow metal biosorption in a regenerable Sargassum column,” Water Res. 37(2), 297-306.

Volpe, A., Lopez, A., and Pagano, M. (2003). “Olive husk – An alternative sorbent for removing heavy metals from aqueous streams,” Appl. Biochem. Biotechnol. 110(3), 137-149.

Wafwoyo, W., Chung, W. S., and Marshall, W. E. (1999). “Utilization of peanut shells as adsorbents for selected metals,” J. Chem. Tech. Biot. 74(11), 1117-1121

Wan Ngah, W. S., and Hanafiah, M. A. K. M. (2008b). “Removal of heavy metal ions from wastewater by chemically modified plant wastes as adsorbents: A review,” Bioresour. Tech. 99(10), 3935-3948.

Wang, J. L., and Chen, C. (2006). “Biosorption of heavy metals by Saccharomyces cerevisiae: A review,” Biotechnol. Adv. 24(5), 427-451.

Wang, J. L., and Chen, C. (2009). “Biosorbents for heavy metals removal and their future,” Biotechnol. Advances 27(2), 195-226.

Wang, J. L., Zhan, X. M., Ding, D. C., and Zhou, D. (2001). “Bioadsorption of lead(II) from aqueous solution by fungal biomass of Aspergillus niger,” J. Biotechnol. 87(3), 273-277.

Wang, X. S., Li, Z. Z., and Sun, C. (2009). “A comparative study of removal of Cu(II) from aqueous solutions by locally low-cost materials: Marine macroalgae and agricultural by-products,” Desalination 235(1-3), 146-159.

Wang, X. S., Tang, Y. P., and Tao, S. R. (2008). “Removal of Cr (VI) from aqueous solutions by the nonliving biomass of alligator weed: Kinetics and equilibrium,” Adsorption – J. Intl. Adsorp. Soc. 14(6), 823-830.

Wang, X. S., Tang, Y. P., and Tao, S. R. (2009). “Kinetics, equilibrium and thermodynamic study on removal of Cr(VI) from aqueous solutions using low-cost adsorbent alligator weed,” Chem. Eng. J. 148(2-3), 217-225.

Wang, Y. H., Lin, S. H., and Juang, R. S. (2003). “Removal of heavy metal ions from aqueous solutions using various low-cost adsorbents,” Journal of Hazardous Materials 102(2-3), 291-302.

Wartelle, L. H., and Marshall, W. E. (2000). “Citric acid modified agricultural by-products as copper ion adsorbents,” Adv. Environ. Res. 4(1), 1-7.

Wartelle, L. H., and Marshall, W. E. (2005). “Chromate ion adsorption by agricultural by-products modified with dimethyloldihydroxyethylene urea and choline chloride,” Water Res.39(13), 2869-2876.

Wase, J., and Forster, C. (1997). Biosorbents for Metal Ions, Taylor and Francis, 238 pp.; Chapters of particular note: Forster, C. F., and Wase, D. A J., “Biosorption of heavy metals: An introduction,” Ch. 1, pp. 1-10; Garnham, G. W., “The use of algae as metal biosorbents,” Ch. 2, pp. 11-37; Urrutia, M. M., “General bacterial sorption processes,” Ch. 3, pp. 39-66; Kapoor, A., and Viraraghavan, T., “Fungi as biosorbents,” Ch. 4, pp. 67-85.

Williams, C. J., Aderhold, D., and Edyvean, G. J. (1998). “Comparison between biosorbents for the removal of metal ions from aqueous solution,” Water Res. 32(1), 216-224.

Wing, R. E. (1996). “Corn fibre citrate: Preparation and ion exchange properties,” Ind. Crops Prod. 5(4), 301-305.

Wing, R. E. (1997). “Cellulosic citrates: Preparation and ion exchange properties,” J. Polymer Mater. 14(4), 303-309.

Wong, K. K., Lee, C. K., Low, K. S., and Haron, M. J. (2003). “Removal of Cu and Pb from electroplating wastewater using tartaric acid modified rice husk,” Proc. Biochem. 39(4), 437-445.

Wu, Y., Zhang, S. Z., Guo, X. Y., and Huang, H. L. (2008). “Adsorption of chromium(III) on lignin,” Bioresour. Technol. 99(16), 7709-7715.

Xie, J. Z., Chang, H. L., and Kilbane, J. J. (1996). “Removal and recovery of metal ions from wastewater using biosorbents and chemically modified biosorbents,” Bioresour. Technol.57(2), 127-136.

Xiu, G. H., and Li, P. (2000). “Prediction of breakthrough curves for adsorption of lead(II) on activated carbon fibers in a fixed bed,” Carbon 38(7), 975-981.

Xu, T., and Liu, X. Q. (2008). “Peanut shell activated carbon: Characterization, surface modification and adsorption of Pb²⁺ from aqueous solution,” Chinese J. Chem. Eng. 16(3), 401-406.

Xuan, Z. X., Tang, Y. R., Li, X. M., Liu, Y. H., and Luo, F. (2006). “Study on the equilibrium, kinetics and isotherm of biosorption of lead ions onto pretreated chemically modified orange peel,” Biochem. Eng. J. 31(2), 160-164.

Yan, G. Y., and Viraraghavan, T. (2001). “Heavy metal removal in a biosorption column by immobilized M. rouxii biomass,” Bioresour. Technol. 78(3), 243-249.

Yan, G., and Viraraghavan, T. (2003). “Heavy-metal removal from aqueous solution by fungus Mucor rouxii,” Water Research 37, 4486-4496.

Yang, J., and Volesky, B. (1999a). “Biosorption of uranium on Sargassum biomass,” Water Res. 33 (15), 3357-3363.

Yang, J., and Volesky, B. (1999b). “Cadmium biosorption rate in protonated Sargassum biomass,” Environ. Sci. Technol. 33(5), 751-757.

Yang, J., and Volesky, B. (1999c). “Modeling uranium—proton ion exchange in biosorption,” Environ. Sci. Technol. 33(22), 4079-4085.

Yang, L., and Chen, J. P. (2008). “Biosorption of hexavalent chromium onto raw and chemically modified Sargassum sp.,” Bioresour. Technol. 99(2), 297-307.

Yang, T., and Lua, A. C. (2006). “Textural and chemical properties of zinc chloride activated carbons prepared from pistachio-nut shells,” Mater. Chem. Phys. 100(2-3), 438-444.

Yi, W., Sakamoto, Y., and Kamiya, Y. (2009). “Remediation of actual groundwater polluted with nitrate by the catalytic reduction over copper-palladium supported on active carbon,” Appl. Catal. A Gen. 361(1-2), 123-129.

Yin, H., He, B., Peng, H., Ye, J., Yang, F., and Zhang, N. (2008). “Removal of Cr(VI) and Ni(II) from aqueous solution by fused yeast: Study of cations release and biosorption mechanism,” J. Hazard. Mater. 158(2-3), 568-576.

Yin, P., Yu, Q., Jin, B., and Ling, Z. (1999). “Biosorption removal of cadmium from aqueous solution by using pretreated fungal biomass cultured from starch wastewater,” Water Res. 33(8), 1960-1963.

Yoon, S., Han, S. S., and Rana, S. V. S. (2008). “Molecular markers of heavy metal toxicity – A new paradigm for health risk assessment,” J. Environ. Biol. 29(1), 1-14.

Youssef, A. M., El-Nabarawi, T., and Samra, S. E. (2004). “Sorption properties of chemically activated carbons. 1. Sorption of cadmium (II) ions,” Colloid Surf. A 235(1-3), 153-163.

Yu, B., Zhang, Y., Shukla, S. S., and Dorris, K. L. (2000). “The removal of heavy metal from aqueous solutions by sawdust adsorption – Removal of copper,” J. Hazard. Mater. B 80(1-3), 33-42.

Yu, B., Zhang, Y., Shukla, A., Shukla, S., and Dorris, K. L. (2001). “The removal of heavy metals from aqueous solutions by sawdust adsorption – Removal of lead and comparison of its adsorption with copper,” J. Hazard. Mater. 84(1), 83-94.

Yu, H., Covey, G. H., and O’Connor, A. J. (2008). “Innovative use of silvichemical biomass and its derivatives for heavy metal sorption from wastewater,” Int. J. Environ. Pollut. 34(1-4), 427-450.

Yu, J. X., Tong, M., Sun, X. M., and Li, B. H. (2007). “A simple method to prepare poly(amic acid)-modified biomass for enhancement of lead and cadmium adsorption,” Biochem. Eng. J.33(2), 126-133.

Yu, L. J., Shukla, S. S., Dorris, K. L., Shukla, A., and Margrave, J. L. (2003). “Adsorption of chromium from aqueous solutions by maple sawdust,” J. Hazard. Mater. 100(1-3), 53-63.

Yu, Q. M., Matheickal, J. T., Yin, P. H., and Kaewsarn, P. (1999). “Heavy metal uptake capacities of common marine macro algal biomass,” Water Res. 33(6), 1534-1537.

Yue, Z. R., Bender, S. E., Wang, J. W., and Economy, J. (2009). “Removal of chromium Cr(VI) by low-cost chemically activated carbon materials from water,” J. Hazard. Mater. 166(1), 74-78.

Yun, Y. S., Park, D., Park, J. M., and Volesky, B. (2001). “Biosorption of trivalent chromium on the brown seaweed biomass,” Environ. Sci. Technol. 35(21), 4353-4358.

Yun, Y. S., and Volesky, B. (2003). “Modeling of lithium interference in cadmium biosorption,” Environ. Sci. Technol. 37(16), 3601-3608.

Zafar, M. N., Nadeem, R., and Hanif, M. A. (2007). “Biosorption of nickel from protonated rice bran,” J. Hazardous Mater. 143(1-2), 478-485.

Zagorodni, A. A. (2007). Ion Exchange Materials: Properties and Applications, Elsevier, Amsterdam.

Zahra, N. (2010). “Arsenic contamination in water and its removal by using different low cost adsorbents,” J. Chem. Soc. Pakistan 32(2), 259-264.

Zatta, P. (ed.) (2003). Metal Ions and Neurodegenerative Disorders, World Scientific, River Edge, NJ, USA.

Zhang, K., Cheung, W. H., and Valix, M. (2005). “Roles of physical and chemical properties of activated carbon in the adsorption of lead ions,” Chemosphere 60(8), 1129-1140.

Zhang, L. N., Cai, J., Zhou, J. P., and Tang, Y. R. (2004). “Adsorption Cd²⁺ and Cu²⁺ on ion-exchange beads from cellulose/alginic acid blend,” Separation Sci. Technol. 39(5), 1203-1219.

Zhang, L., Zhao, L., Yu, Y., and Chen, C. (1998). “Removal of lead from aqueous solution by non-living Rhizopus nigricans,” Water Res. 32(5), 1437-1444.

Zhang, Y., and Banks, C. (2006). “A comparison of the properties of polyurethane immobilised Sphagnum moss, seaweed, sunflower waste and maize for the biosorption of Cu, Pb, Zn and Ni in continuous flow packed columns,” Water Res. 40(4), 788-798.

Zhang, Z. H., Wu, Q. L., Cai, Z. T., Yan, C., Song, H. Y., and Pan, D. (2009). “Effect of alkali metal salt on the macropore structure of rayon-based activated carbon fibers,” New Carbon Mater. 24(3), 265-269.

Zhao, B-X., Wang, P., Zheng, T., Chen, C.-Y., and Shu, J. (2006). “Preparation and adsorption performance of a cellulosic-adsorbent resin for copper(II),” J. Applied Poly. Sci. 99(6), 2951-2959.

Zhao, M., and Duncan, J. R. (1997a). “Batch removal of sexivalent chromium by Azolla filiculoides,” Biotechnol. Appl. Biochem. 26, 179-182.

Zhao, M., and Duncan, J. R. (1997b). “Use of formaldehyde cross-linked Saccharomyces cerevisiae in column bioreactors for removal of metals from aqueous solutions,” Biotechnol. Lett. 19(10), 953-955.

Zhao, M., and Duncan, J. R. (1998a). “Removal and recovery of nickel from aqueous solution and electroplating rinse effluent using Azolla filiculoides,” Proc. Biochem. 33(3), 249-255.

Zhao, M., and Duncan, J. R. (1998b). “Column sorption of Cr(VI) from electroplating effluent using formaldehyde cross-linked Saccharomyces cerevisiae,” Biotechnol. Lett. 20(6), 603-606.

Zhao, M., Duncan, J. R., and Van Hille, R. P. (1999). “Removal and recovery of zinc from solution and electroplating effluent using Azolla filiculoides,” Water Res. 33(6), 1516-1522.

Zheng, J. C., Feng, H. M., Lam, M. H. W., Lam, P. K. S., Ding, Y. W., and Yu, H. Q. (2009). “Removal of Cu(II) in aqueous media by biosorption using water hyacinth roots as a biosorbent material,” J. Hazard. Mater. 171, 780-785.

Zheng, L. C., Dang, Z., Yi, X. Y., and Zhang, H. (2010). “Equilibrium and kinetic studies of adsorption of Cd(II) from aqueous solution using modified corn stalk,” J. Harard. Mater.176(1-3), 650-656.

Zheng, W., Li, X. M., Wang, F., Yang, Q., Deng, P., and Zeng, G. M. (2008). “Adsorption removal of cadmium and copper from aqueous solution by areca – A food waste,” J. Hazardous Mater. 157(2-3), 490-495.

Zhou, D., Zhang, L., Zhou, J., and Guo, S. (2004). “Cellulose/chitin beads for adsorption of heavy metals in aqueous solution,” Water Research 38(11), 2643-2650.

Zhou, J. L. (1999). “Zn biosorption by Rhizopus arrhizus and other fungi,” Appl. Microbiol. Biotechnol. 51(5), 686-693.

Zhou, J. L., Huang, P. L., and Lin, R. G. (1998). “Sorption and desorption of Cu and Cd by macroalgae and microalgae,” Environ. Pollution 101(1), 67-75.

Zhou, J. L., and Kiff, R. J. (1991). “The uptake of copper from aqueous solution by immobilized fungal biomass,” J. Chem. Technol. 52(3), 317-330.

Zhou, M., Liu, Y. G., Zeng, G. M., Li, X., Xu, W. H., and Fan, T. (2007). “Kinetic and equilibrium studies of Cr(VI) biosorption by dead Bacillus licheniformis biomass,” World J. Microbiol. Biotechnol. 23(1), 43-48.

Zhu, C. S., Wang, L. P., and Chen, W. B. (2009a). “Removal of Cu(II) from aqueous solution by agricultural by-product: Peanut hull,” J. Hazardous Mater. 168(2-3), 739-746.

Zhu, H. J., Jia, Y. F., Wu, X., and Wang, H. (2009b). “Removal of arsenic from water by supported nano zero-valent iron on activated carbon,” J. Hazard. Mater. 172(2-3), 1591-1596.

Ziagova, M., Dimitriadis, G., Aslanidou, D., Papaioannou, X., Tzannetaki, E. L., and Kyriakides, M. L. (2007). “Comparative study of Cd(II) and Cr(VI) biosorption on Staphylococcus xylosus and Pseudomonas sp. in single and binary mixtures,” Bioresour. Technol. 98(15), 2859-2865.

Zouboulis, A. I., Matis, K. A., and Hancock, I. C. (1997). “Biosorption of metals from dilute aqueous solutions,” Separation Purification Methods 26(2), 255-295.

Zoumpoulakis, L., and Simitzis, J. (2001). “Ion exchange resins from phenol/formaldehyde resin-modified lignin,” Polymer Internat. 50(3), 277-283.

Zulkali, M. M. D., Ahmad, A. L., and Norulakmal, N. H. (2006). “Oriza sativa husk as heavy metal adsorbent: Optimization with lead as model solution,” Bioresour. Technol. 97(1), 21-25.

Zvinowanda, C. M., Okonkwo, J. O., Agyei, N. M., Forbes, P., Mpangela, V., Phaleng, J., Shabalala, P. N., Dennis, T., and Ozoemena, K. I. (2008a). “Biosorption of toxic metals: The potential of maize tassel for the removal of Pb (II) from aqueous solutions,” Fresen. Environ. Bull. 17(7), 814-818.

Zvinowanda, C. M., Okonkwo, J. O., Agyei, N. M., Sekhula, M. M., and Sadiku, R. (2009a). “Application of maize tassel for the removal of Pb, Se, Sr, U and V from borehole water contaminated with mine wastewater in the presence of alkaline metals,” J. Hazard. Mater. 164(2-3), 884-891.

Zvinowanda, C. M., Okonkwo, J. O., Shabalala, P. N., and Agyei, N. M. (2009b). “A novel absorbent for heavy metal remediation in aqueous environments,” Int. J. Environ. Sci. Tech.6(3), 425-434.