Review Articles

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.

Lignin is readily available as a by-product from the pulp and paper industry. It is considered to be a promising substitute for phenol in phenol-formaldehyde (PF) resin synthesis, given the increasing concerns of the shortage of fossil resources and the environmental impact from petroleum-based products. One hurdle that prevents the commercial utilization of lignin is its low reactivity due to its chemical structure. Many efforts have been made to improve its reactivity by modification and/or depolymerization of lignin molecules. Methylolation and phenolation are the two most studied modification approaches aimed at introducing reactive functional groups to lignin molecules. Modified lignin from these two methods could partially replace phenol in PF resin synthesis. Demethylation of lignin could effectively increase the reactivity of lignin by forming catechol moieties in the lignin macromolecule. Other methods, including reduction, oxidation, and hydrolysis, have also been studied to improve the reactivity of lignin as well as to produce phenolic compounds from lignin. Most current methods of lignin modification are not economically attractive. One can expect that efforts will be continued, aimed at improving the utilization of lignin for value-added products.

A review of the literature reveals potential advantages that papermakers can achieve by placing minerals in the lumens or cell walls of fibers before the pulp is formed into paper. Loading of filler into the fiber lumen by mechanical deposition or within the cell wall by in-situ precipitation has been reported to generally result in a moderate reduction in light scattering coefficient and increased strength properties of laboratory handsheets, as well as in paper manufactured with pilot plant equipment, when compared to conventional addition of filler. However, there are some exceptions to this general observation, where the fiber loading is reported to decrease the tensile strength of paper. Some related effects can be achieved by either precipitating mineral onto fiber surfaces or co-flocculating mineral particles with cellulosic fines. Challenges remain with respect to the implementation of fiber-loading concepts at a commercial scale. Also, there is a need for further research aimed at establishing high-end applications in which it may be an advantage to load cellulosic fiber cell walls or lumens with minerals or other substances.

The primary aim of modern biorefineries is the efficient conversion of lignocellulosic materials into valuable products. Sugars and oils can be converted into valuable chemicals, but processing of lignin is still a challenge. A vast amount of lignin is incinerated to produce process steam and energy, and only a very small part is used for the production of value-added products. Technical lignins are isolated as by-streams in lignocellulosic refineries, e.g., as kraft, soda, organosolv, and hydrolysis lignins, as well as lignosulphonates. They have a modified structure and contain impurities that are dependent on the processing method. The structure and the composition of technical lignins restrict their subsequent applications. This paper reviews limiting factors in utilization of technical lignins. Four major classes of problems are identified, and approaches to overcoming these problems are suggested.

For the thermomechanical pulping (TMP) process both wood chip quality and the refining process have important effects on the resulting pulp and paper quality. Properties of wood raw material give a framework for final pulp properties. During TMP refining the specific energy consumption and refining intensity strongly impact fibre and pulp qualities. Increasing specific energy consumption benefits the development of fibres and improves their properties. However, high intensity refining tends to shorten the fibres and produces more fines content when compared with low intensity refining. This review focuses on the influence of key variables of chip qualities and the refining process on TMP pulp and paper qualities.

Wood and cellulose derivatives, in both fibrous and water-soluble macromolecular form, are emerging as outstanding candidates for organic electronics applications due to their large-scale availability, low cost, and easy processability. Paper and wood fibre-based derivatives are considered to be materials of choice as supports for communication world-wide. The interest in producing inexpensive and universally available conducting polymer/cellulose fibres substrates resides in the possibility of creating new materials that can be used for a broad range of advanced applications. For instance, PPy/cellulose fibres composites can be used for the preparation of energy storage devices thanks to the conjugation of the high specific area of cellulose fibres and the electrochemical properties of PPy. Other possible applications of such composites are in the area of the antistatic materials, sensors, electromagnetic interference shielding materials, smart packaging, and tissues. Concerning the woody polymers, some of them (i.e. cellulose derivatives) also exhibit biocompatibility, as well as film-forming properties and transparency. In combination with the electrical properties of PPy, these features make PPy/macromolecular cellulose composites suitable for applications as displays, lighting, and photovoltaics. Due to their chemical structure, macromolecular wood derivatives have been proposed with success as enhancing conductivity additives in Py polymerisation. The aim of the present review is to provide an overview of PPy chemistry and of the most relevant advances attained in the production of PPy/wood derived materials conducting composites.

Lignocellulosic carbohydrates, i.e. cellulose and hemicellulose, have abundant potential as feedstock for production of biofuels and chemicals. However, these carbohydrates are generally infiltrated by lignin. Breakdown of the lignin barrier will alter lignocelluloses structures and make the carbohydrates accessible for more efficient bioconversion. White-rot fungi produce ligninolytic enzymes (lignin peroxidase, manganese peroxidase, and laccase) and efficiently mineralise lignin into CO2 and H2O. Biological pretreatment of lignocelluloses using white-rot fungi has been used for decades for ruminant feed, enzymatic hydrolysis, and biopulping. Application of white-rot fungi capabilities can offer environmentally friendly processes for utilising lignocelluloses over physical or chemical pretreatment. This paper reviews white-rot fungi, ligninolytic enzymes, the effect of biological pretreatment on biomass characteristics, and factors affecting biological pretreatment. Application of biological pretreatment for enzymatic hydrolysis, biofuels (bioethanol, biogas and pyrolysis), biopulping, biobleaching, animal feed, and enzymes production are also discussed.

Retting is the main challenge faced during the processing of bast plants for the production of long fibre. The traditional methods for separating the long bast fibres are by dew and water retting. Both methods require 14 to 28 days to degrade the pectic materials, hemicellulose, and lignin. Even though the fibres produced from water retting can be of high quality, the long duration and polluted water have made this method less attractive. A number of other alternative methods such as mechanical decortication, chemical, heat, and enzymatic treatments have been reported for this purpose with mixed findings. This paper reviews different types of retting processes used for bast plants such as hemp, jute, flax, and kenaf, with an emphasis on kenaf. Amongst the bast fibre crops, kenaf apparently has some advantages such as lower cost of production, higher fibre yields, and greater flexibility as an agricultural resource, over the other bast fibres. The fibres produced from kenaf using chemical retting processes are much cleaner but low in tensile strength. Enzymatic retting has apparent advantages over other retting processes by having significantly shorter retting time and acceptable quality fibres, but it is quite expensive.

To deeply understand the factors that affect the conversion of lignocellulosic biomass to fermentable sugars, experimental results should be bridged with process simulations. The objective of this paper is to review published research on modeling of the pretreatment process using leading technologies such as dilute acid, alkaline, and steam explosion pretreatment, as well as the enzymatic hydrolysis process for converting lignocellulose to sugars. The most commonly developed models for the pretreatment are kinetic models with assumptions of a first-order dependence of reaction rate on biomass components and an Arrhenius-type correlation between rate constant and temperature. In view of the heterogeneous nature of the reactions involved in the pretreatment, the uses of severity factor, artificial neural network, and fuzzy inference systems present alternative approaches for predicting the behavior of the systems. Kinetics of the enzymatic hydrolysis of cellulosic biomass has been simulated using various modeling approaches, among which the models developed based on Langmuir-type adsorption mechanism and the modified Michaelis-Menten models that incorporate appropriate rate-limiting factors have the most potential. Factors including substrate reactivity, enzyme activity and accessibility, irreversible binding of enzymes to lignin, and enzyme deactivation at high conversion levels, need to be considered in modeling the hydrolysis process. Future prospects for research should focus on thorough understanding of the interactions between biomass reactants and chemicals/enzymes — the key to developing sophisticated models for the entire conversion process.

Paper aging and conservation are matters of concern to those responsible for archives and library collections. Wood-derived fibers are mainly composed of cellulose, hemicelluloses, and lignin, but paper composition can also include additives, such as starch, minerals, and synthetic polymers. Therefore, paper is a multi-component material, and because of its complex and varied nature, research findings in paper chemistry can be difficult to interpret. Deterioration of paper is caused by many factors such as acid hydrolysis, oxidative agents, light, air pollution, or the presence of microorganisms. The origin of the cellulosic material, as well as pulping and papermaking procedures, additives, and storage conditions play a crucial role. The chemical changes occurring within paper thus involve multi-parameter processes. The purpose of this review, which mainly focuses on the most recent decade, is to provide a description of the more important changes produced by aging and an update of the new tools available for the study of paper deterioration and its conservation.