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 .