Environ. Sci. Technol. 2004, 38, 4905-4909
Natural Polysaccharides and Their Interactions with Dye Molecules: Applications in Effluent Treatment† RICHARD S. BLACKBURN* Green Chemistry Group, Centre for Technical Textiles, University of Leeds, Leeds, U.K.
Dyeing effluent is one of the largest contributors to textile effluent and such colored wastewater has a seriously destructive impact on the environment. Adsorption can be a very effective treatment for decolorization of textile dyeing effluent, but current techniques employ adsorption chemistry that is not particularly environmentally friendly, such as the use of alum. In this study, natural polysaccharides were used as adsorbents for removal of dye molecules from effluent. The results showed that naturally cationic polysaccharides such as chitin and chitosan gave excellent levels of color removal, and this was attributed to a combination of electrostatic attraction, van der Waals forces, and hydrogen bonding. Nonionic galactomannans (locust bean gum, guar gum, cassia gum) were also highly effective in removing dye from effluent, whereas other nonionic polysaccharides, such as starch, were not effective. This was attributed to the structure of the polysaccharides and the relative degree of inter- and intramolecular interactions between separate polymer chains. The pendant galactose residues of galactomannans prevented strong interaction, allowing greater hydrogen bonding with dye; comparatively, starch has extensive chain interactions, and as such had limited potential for hydrogen bonding with the dye molecules at the temperature of application. In addition, hydrophobic interactions between the hydrophobic parts of the dye and the R-face of the pendant galactose residues may have contributed to the superior performance. Repulsion between anionic polysaccharides and the dye anions prevented any hydrogen bonding and as such pectin, carrageenans, and alginic acid were not effective in dye removal from effluent. The use of galactomannans derived from plants in this system presents a sustainable method of effluent treatment. The raw materials are derived from renewable plant sources and are available in tonnage quantities, the adsorption system itself is highly effective and does not involve any additional chemical input or treatment other than the use of the adsorbent, and the adsorption agents themselves are nontoxic and biodegradable.
Introduction Wastewater discharged from dyeing processes can be one of the biggest contributors to textile effluent; this comprises mainly residual dyes and auxiliary chemicals. Over 50 000 * E-mail:
[email protected]; tel: +44 (0)113 343 3757; fax: +44 (0)113 343 3704. † Some preliminary findings discussed in this paper were presented at the 225th American Chemical Society National Meeting, March 23-27, 2003, New Orleans, LA (1). 10.1021/es049972n CCC: $27.50 Published on Web 08/06/2004
2004 American Chemical Society
ton (2) of dye is discharged into effluent annually, with the proportion of this total varying according to dye type, fiber of application, and relative degree of dye fixation. The relative degree of dye loss to effluent for the main dye-fiber application systems is shown in Table 1 (3). The presence of dyeing effluent in a watercourse is aesthetically undesirable, but has a more serious environmental impact. Dyeing effluent has high biochemical oxygen demand (BOD; measure of organic matter in effluent and the oxygen required by bacteria in respiratory processes to break down the organic matter) and consequently algae, which are normally controlled by bacteria, overpopulate watercourses and block sunlight transmission into water. This, combined with the spectral absorption of the dye itself, can affect photosynthetic processes of flora in the immediate environment (4), hence a reduction in oxygen levels in the water is observed; in severe cases this can result in suffocation of aquatic flora and fauna. In addition, the dyestuffs may be toxic to aquatic organisms due to the presence of subsistent metals and chlorine (5). Several methods have been developed to remove color from dyehouse effluent, varying in effectiveness, economic cost, and environmental impact (of the treatment process itself). Traditionally, biological treatments using activated sludge (bacterial oxidation) have been used. They can reduce BOD, but do not effectively remove color, as the oxidation rate is very slow (6). Chemical oxidation can be employed, using treatments based on hydrogen peroxide with an activator (7). Fenton’s reagent (H2O2 + Fe(II) salts) decolorizes both soluble and insoluble dyes, but the process can generate sludge through flocculation (8). However, the latest developments (9) in lowpressure oxidation technology for treating toxic and nonbiodegradable wastewater components have led to an efficient low-cost system based on Fenton’s reagent, which requires relatively little H2O2 because it uses mainly air, and the Fenton’s catalyst can be recycled. Usually, the degradation products are just CO2 and water, but some biodegradable matter still remains. H2O2-activation by ultraviolet radiation is also effective, but can often generate hazardous byproducts (10). Another chemical treatment that has been developed in recent years is ozonation, which decolorizes dyehouse effluent, but at a high cost as a result of the constant requirement of ozone in the decolorization process due to its short half-life (10). Some electrochemical methods have been described that are highly effective with no chemical consumption, but the technology is relatively very expensive and hence not economically viable (11). An additional problem with techniques that partly break down the dye molecule is that, although the color may be removed, toxic derivatives, such as primary aromatic amines and heavy metals, may still exist in the treated liquor. Physical methods of effluent decolorization are available. Membrane filtration technology is capable of separation, which is not possible in classic wastewater treatment; it has the ability to clarify, concentrate, and separate dye continuously from effluent (12). However, the process is disadvantaged in that it has a high capital cost and the membranes are prone to pore clogging (13). Ion-exchange resins can be employed, a method in which both cationic and anionic dyes are removed. There is no loss of adsorbent on regeneration and the solvent is reclaimed after use (14). However, this method is not effective for all dyes, so it has not been widely used for the treatment of dye effluent (15). Sorption of dye molecules onto a substrate (adsorbent) can be a very effective, low-cost method of color removal. VOL. 38, NO. 18, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Dye Loss to Effluent for Different Dye-Fiber Systems dye class
fiber
loss to effluent (%)
acid basic direct disperse metal-complex reactive sulfur vat
polyamide acrylic cotton polyester wool cotton cotton cotton
5-20 0-5 5-30 0-10 2-10 10-50 10-40 5-20
Adsorption has particular advantages, in that, techniques that change or destroy the dye chromophore to remove color do not always remove the residual moieties from the effluent, and as such may still present environmental problems. Adsorption removes the complete molecule, leaving no fragments in the wastewater; this is particularly relevant for metal-containing dyes, where the coordination metal (e.g., Cr, Co, Cu) would remain in the effluent after nonadsorptive treatments, potentially in a more hazardous uncomplexed form. Activated carbon is the most commonly used method of dye removal by adsorption (16), although performance is dependent on the type of carbon used, the characteristics of the wastewater, and the type of dye. Activated carbon is expensive (4), so the carbon also has to be reactivated, otherwise disposal of the concentrates has to be considered; reactivation results in 10-15% loss of the sorbent. Low cost adsorbents have been used, such as peat, wood, fly ash, and coal, but their effectiveness is limited and certainly inferior to that of activated carbon. Other current techniques employ adsorbents that are not entirely eco-friendly themselves, such as alum, polyaluminum chloride, and silica gel. The industry requires highly effective, low-cost adsorbents, that are available in tonnage quantities, and that do not present an environmental problem themselves (17). Chitin, poly-β-(1f4)-N-acetyl-D-glucosamine, is widely distributed in nature, especially in marine invertebrates, insects, fungi, and yeasts (18). On an industrial scale, it is extracted from the shell waste of crabs, shrimps, and prawns by an acid/alkali process. Chitosan is found in nature, but is mainly produced by deacetylation of chitin and has been shown to be superior to chitin for adsorption of metals, due to the higher number of possible chelation sites as a result of the primary amino functionality (19). Chitin (4, 20) and chitosan (21) have been successfully employed to adsorb dye molecules from dyehouse effluent. However, the traditional method of extraction of chitin creates its own environmental problems as it generates large quantities of waste (22), and the production of chitosan involves a chemical deacetylation process. The purpose of this paper is to examine other natural polysaccharides derived from renewable plant sources, and to investigate their interactions with dye molecules, in an attempt to provide alternative, sustainable, low cost, biodegradable adsorbents for dyehouse effluent treatment.
Experimental Section Materials. The dyes used in this study (23) were C. I. reactive red 238 (1a), supplied by Ciba; C. I. direct black 22 (2), supplied by Ciba; and C. I. acid blue 193 (3), supplied by Crompton & Knowles (France). These are widely used dyes and the structures are typical of most dyes in their class. Although the exact structure of C. I. reactive red 238 is not disclosed by the manufacturer, it is believed that the chromophore is based on H-acid (1-amino-8-naphthol-3,6-disulfonic acid); again this is typical of most red reactive dyes. Adsorbents used were chitin, chitosan (low, medium, and high Mr), locust bean gum, guar gum, corn starch, wheat starch, pectin, 4906
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carrageenan (ι- and κ-forms), dextrin, and alginic acid, supplied by Aldrich, tamarind gum, supplied by Krystal Colloids (India), and cassia gum (Diagum CS), supplied by Noveon Pharma GmbH & Co KG (Germany). All other chemicals were of general laboratory grade, supplied by Aldrich.
Effluent Preparation. To mimic the effluent produced by a typical dyehouse, synthetic dyeing effluents were prepared. The dyes were dissolved separately in water at concentrations of 150 mg dm-3, which was determined to be a typical concentration (high end) of dye in effluent, and was in agreement with proposals from the literature (4, 20). To the C. I. reactive red 238 solution was added 50 g dm-3 sodium sulfate and the solution was heated to 60 °C, the pH was raised to 10.5 by addition of 0.1 M sodium hydroxide, and the solution was stirred at 60 °C for 60 min. This procedure effectively mimicked the dyeing process. The dye remaining at the end of the process was the completely hydrolyzed form of the dye (1b), with no potential for further nucleophilic reaction. To the C. I. direct black 22 solution was added 20 g dm-3 sodium sulfate and the solution was heated to 98 °C, the pH was maintained at pH 7.0, and the solution was stirred at 98 °C for 60 min. Again, this procedure effectively mimicked the dyeing process. The C. I. acid blue 193 solution was heated to 98 °C with no salt addition. The pH was reduced to pH 4.0 by addition of 0.1 M formic acid, and the solution was stirred at 98 °C for 60 min. Again, this procedure effectively mimicked the dyeing process. The prepared stock solutions were cooled to room temperature. Spectroscopic analysis of the prepared dyeing effluents was carried out in a 1-cm quartz cell using a Perkin-Elmer Lambda 9 UV/VIS/NIR spectrophotometer. The wavelength of maximum absorption (λmax) of each dyeing effluent was determined, and a calibration graph of absorbance versus dye concentration formulated at λmax. The equation of the line for each calibration plot, y ) mx + c (y ) absorbance; m ) gradient; x ) concentration; c ) 0), was used to calculate the concentration of residual solution from the absorbency tests. Dye Effluent Absorbance Procedure. Polysaccharide adsorbent (1 g) was added to 100 cm3 of prepared effluent solution and stirred at room temperature for 12 h in a sealed container, with no incident light. After the treated solution was stirred, it was allowed to settle over 1 h. An aliquot of the remaining top solution was taken and the absorbance was measured spectrophotometrically, and the residual dye concentration was calculated.
Results and Discussion Table 2 describes the % dye removal by the polysaccharide absorbents measured after 12 h of treatment; this is also
FIGURE 1. Degree of color removal by various polysaccharides.
TABLE 2. Dye Removal (%) by Various Polysaccharides polysaccharide
C. I. reactive red 238
C. I. direct black 22
C. I. acid blue 193
chitin chitosan (high Mr) chitosan (med. Mr) chitosan (low Mr) locust bean gum guar gum cassia gum tamarind gum corn starch wheat starch dextrin κ-carrageenan ι-carrageenan pectin alginic acid
86.5 86.6 94.2 86.6 85.0 78.6 80.5 23.4 1.6 26.5 1.1 4.1 3.2 5.5 1.7
86.5 73.5 92.2 41.6 87.9 70.3 78.5 19.3 14.8 23.0 1.0 13.3 11.3 14.4 15.4
93.1 99.4 99.0 74.3 92.2 60.7 75.3 44.8 36.7 38.7 2.6 20.2 4.2 2.5 12.9
represented graphically in Figure 1. It can be seen from the data that chitin and the different Mr chitosans were effective in removing dye from the effluent, in agreement with previous work (4, 20, 21). Many authors have proposed that the mechanism of adsorption of anionic dye molecules by chitin and chitosan is mainly based on electrostatic attraction to cationic centers in the polymer (-NH2+COCH3 and -NH3+, respectively) that exist in equilibrium with the respective unprotonated form. However, both chitin and chitosan proved to be effective at higher pH, when the equilibrium would favor the unprotonated form of the polysaccharide with almost complete exclusion of the protonated form, suggesting that additional mechanisms of interaction were in operation. Indeed, other workers (4) have observed that adsorption of anionic dyes onto chitin fitted both Langmuir (adsorption at specific sites) and Freundlich (no specific sites, possibility of formation of multi-molecular layer) isotherms; typical of adsorption of anionic dyes onto polyamide (electrostatic attraction) and cellulose (hydrogen bonding) fibers, respectively. Hence, it would suggest that adsorption of anionic dyes onto chitin and chitosan occurs through a combination of electrostatic forces, van der Waals interactions, and hydrogen bonding, with the electrostatic forces only having a significant effect at low pH. The observation of most interest within this work was the adsorption performance of the galactomannans: locust bean gum, guar gum, and cassia gum. Galactomannans occur in large amounts in the endosperm of the seeds of many
FIGURE 2. Galactomannan structure (locust bean gum shown).
TABLE 3. Mannose/Galactose Ratios of Natural Galactomannans
galactomannan
plant source
mannose/ galactose (ref 26)
guar gum tara gum locust bean gum cassia gum
Cyamopsis tetragonoloba Cesalpinia spinosa Ceratonia siliqua Cassia obtusifolia
1.5-2.0:1.0 2.5-3.0:1.0 3.0-4.0:1.0 5.0-7.0:1.0
Leguminoseae (24) and consist of a (1f4)-linked β-Dmannopyranose backbone with branch points from their O-6 positions linked to R-D-galactose (i.e., 1f6-linked R-Dgalactopyranose) (25) (Figure 2; locust bean gum). Table 3 shows the ratio of mannose/galactose units of naturally occurring galactomannans. Guar gum (guaran) is extracted from the seed of the leguminous shrub Cyamopsis tetragonoloba, having between 1.5 and 2.0 mannose residues for every galactose residue, it is an economical thickener and stabilizer used commercially in the printing and food industries. Locust bean gum (Carob bean gum) is extracted from the seed (kernels) of the carob tree (Ceratonia siliqua), it is polydisperse, consisting of nonionic molecules made up of about 2000 residues, it is less soluble and has lower viscosity than guar gum, and being nonionic, is not affected by ionic strength or pH. Other commercial galactomannans with different substitution include cassia gum, obtained from Cassia tora (also known as Cassia obtusifolia), and tara gum, obtained from Cesalpinia spinosa. Galactomannans may be engineered with lower substitution by the specific removal VOL. 38, NO. 18, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. K-carrageenan (R ) H) and ι-carrageenan (R ) SO3-).
FIGURE 3. Dye-polysaccharide interactions: (a) dipole-dipole hydrogen bonding interactions between polysaccharide hydroxyl groups and electronegative residues in the dye molecule; (b) Yoshida H-bonding between polysaccharide hydroxyl groups and aromatic residues in dye. of some of their pendant galactose groups using certain R-galactosidases. The adsorption performance of the galactomannans was comparable with that of chitin and chitosan, and superior to other polysaccharides, yet they are nonionic polymers with no potential for cationic interaction, hence it would be expected that interaction and adsorption would mainly be based on van der Waals forces and hydrogen bonding. However, other nonionic polysaccharides, such as corn starch, wheat starch, and dextrin (partially hydrolyzed starch), did not secure significant adsorption of dye. One reason for this observation lies in the structural hydrogen bonding relationships of the different nonionic polymers. Starch consists of two types of molecules, amylose (normally 20-30%) and amylopectin (normally 70-80%). Both consist of polymers of R-D-glucose units in the 4C1 conformation, and in this sense starch is similar in structure to cellulose. However, in amylose and cellulose these are linked -(1f4)- whereas in amylopectin about one residue in every twenty or so is also linked -(1f6)- forming branchpoints (27). Cellulose requires high temperatures to break down inter- and intramolecular hydrogen bonding between polymer chains in order and to allow hydrogen bonding with dye molecules (Figure 3), for affinity, adsorption, and diffusion (28). Such extensive inter- and intramolecular hydrogen bonding is also observed between the chains of main components of starch (amylose and amylopectin) (29), so at the temperature of effluent treatment (20 °C) there was limited potential for additional hydrogen bonding with effluent dye molecules to remove them from solution. There is also evidence (30) that tamarind gum, a xyloglucan, has extensive chain interactions that would lead to extensive intermolecular hydrogen bonding, explaining why the adsorption of dye onto this polysaccharide was low. There is evidence (31) that the branched galactose residues in galactomannans prevent strong chain interactions, and that this limits the extent of intra- and, particularly, intermolecular hydrogen bonding as the individual polymer chains are separated. Therefore, at the temperatures of effluent treatment there is greater potential for hydrogen bonding with dye molecules in solution, with respect to other polysaccharides that have extensive inter- and intramolecular interactions, hence securing superior performance of locust bean gum, guar gum, and cassia gum. An additional factor may be that, unlike glucose, galactose displays a relatively hydrophobic R-face that may develop van der Waals interactions with the hydrophobic parts of the dyes (e.g., the aromatic nuclei). This phenomenon is well-evidenced in the X-ray structures of some sugar-protein complexes with, for instance, a tryptophan residue stacked on the sugar hydrophobic face (32). It may also play a role in this case considering 4908
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the fact that the starches are based on glucose residues and the galactomannans have pendant galactose residues. Although all three galactomannan polysaccharides were effective in removing color, locust bean gum proved the most effective; this may be because locust bean gum has an optimum degree of galactose substitution above (guar gum) and below (cassia gum) which the ability to adsorb vagrant dye is reduced as the potential for hydrogen bonding is reduced, due to increased intermolecular hydrogen bonding between the separate galactomannan polymer chains. The anionic polysaccharides that were tested, carrageenans, pectin, and alginic acid, did not work effectively in dye adsorption. The main reason for this was the extensive electrostatic repulsion between the anionic polymer and the dye anions. Although electrostatic repulsion between anionic species in cellulose and dye anions can be overcome by electrolyte addition, this was not observed for these polysaccharides as the anionic groups in the polymers were strongly negatively charged groups, such as sulfonic acid. This was exemplified by slightly better performance of κ-carrageenan to ι-carrageenan, as the former has half the number of anionic centers with respect to the latter (Figure 4). The hydrogen-bonding between the dyes and the galactomannans, and hence their efficiency in effluent treatment, was not effected by variation in pH or by the presence/ absence of electrolyte. This observation reiterates the strong potential for hydrogen-bonding and van der Waals interactions, making the systems entirely practical for industrial application. It is recognized that some may regard “end-of-pipe” treatments as not true “green chemistry”; an ideal situation would be to not create the effluent in the first place. However, until research develops such technology, where 100% fixation of dye to fiber is achieved, the problem of such effluent needs to be addressed, and as such the green chemistry in this research is an advance in the method of effluent treatment to a greener system. The use of galactomannans derived from plants in this system presents a sustainable method of effluent treatment. The raw materials are derived from renewable plant sources and are available in tonnage quantities, the adsorption system itself is highly effective and does not involve any additional chemical input or treatment other than the use of the adsorbent, and the adsorption agents themselves are nontoxic and biodegradable. This system is also particularly applicable for the dyeing industry in the Indian subcontinent and Southeast Asia, which represents a high concentration of the global dye application industry, so green solutions are a necessity. These natural galactomannans could provide a sustainable method of effluent treatment, being low cost and readily available in these countries, already finding extensive industrial application in textile printing and food production. Water is at a premium in these parts of the world. Potentially, if color could be removed from the dyeing effluent through adsorption, the wastewater could be reused several times, as the remainder would contain only electrolyte as a main component. Additionally, this could enable a reduction in the annual level of sodium employed and emitted. The author has previously discussed the importance of such reductions (33, 34). The disposal of the recovered adsorption material and the bound dye needs to be addressed, however,
the presence of a natural food source with the dye molecules may aid the biodegradation of the dyes in subsequent processes, but this would require further investigation. In any case, the disposal of such adsorbent/dye complex would be preferable to the disposal of complexes with alum, silicates, or activated carbon. Research is ongoing to further analyze the interactions between dye molecules and polysaccharides, extending the work to other polysaccharides, especially other galactomannans, and extending the work to include other dye types, particularly nonionic disperse dyes involved in polyester coloration. It is noted that some textile wet processors concentrate textile waste liquors from different processes, which may lead to concentration much higher than those used in this study (150 g dm-3). As such there is a requirement for these proposed adsorption techniques to be able to perform at these higher concentrations. Additionally, such higher concentration studies related to adsorption isotherms would provide further insight into the mechanisms and capacity for adsorption of dyes on galactomannans and other polysaccharides.
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Received for review January 5, 2004. Revised manuscript received June 18, 2004. Accepted July 1, 2004. ES049972N
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