Preparation and characterization of a novel xanthated chitosan
Introduction
Chitosan is a natural, cationic aminopolysaccharide copolymer of glucosamine and N-acetylglucosamine, obtained by the alkaline, partial deacetylation of chitin, which originates from shells of crustaceans such as crabs and prawns (Muzzarelli, 1977). Chitosan is a non-toxic, hydrophilic, biodegradable, biocompatible, mucoadhesive, and anti-bacterial, biopolymer which has led to a very diverse range of its applications (Lehr, Bouwstra, Schacht, & Junginger, 1992). It has attracted tremendous attention as novel functional materials and potentially important renewable agricultural resource, and has been widely applied in the fields of agriculture, medicine, pharmaceuticals, cosmetic, and food industries, environmental protection, and biotechnology in the last 20 years (Kurita, 1998). The presence of a large number of amine groups on the chitosan chain increases the adsorption capacity of chitosan (Evans et al., 2002, Lu et al., 2001, Wu et al., 2000).
Several methods have been used to modify raw chitosan flake either physical or chemical modifications (Guibal et al., 1999, Yang et al., 2002). Physical modifications (Onsoyen & Skaugrud, 1990) may increase the sorption properties: gel formation decreases the crystallinity of the sorbent and involves an expansion of the porous network. Chemical modifications also offer a wide spectrum of tools to enhance the sorption properties of chitosan for metals. Both hydroxyl and amine groups of chitosan can be chemically modified. They may increase the chemical stability of the sorbent in acid media and, especially, decrease the solubility in most mineral and organic acids. They also increase its resistance to biochemical and microbiological degradation (Guibal et al., 1999, Yang and Yuan, 2001). Cross-linking on chitosan is a convenient and effective way on improving its physical and mechanical properties for practical usages. Various reagents, including glutaraldehyde (Goissis et al., 1999, Guibal et al., 1998, Hsien and Rorrer, 1997), sulfuric acid (Huang, Pal, & Moon, 1999), epoxy compound (Wei, Hudson, Mayer, & Kaplan, 1992), and dialdehyde starch (Schmidt & Baier, 2000) etc. have been used as cross-linking agents for chitosan. Glutaraldehyde has been frequently used to cross-link chitosan and to stabilize it in acidic solutions whereby the reaction occurs through a Schiff’s base reaction between aldehyde groups of glutaraldehyde and some amine groups of chitosan. Although cross-linking may reduce the adsorption capacity, it enhances the resistance of chitosan against acid, alkali and chemicals. The chemically modified chitosan has a bright future and their development is practically boundless. For a breakthrough in utilization, chemical modification to introduce a variety of functional groups is a key point. For this purpose, more fundamental studies on chemical modification will be required. Although the chemical modification of cellulose is well studied and is still an active field, not much work has been reported on the chemical modification of chitin and chitosan.
Carbon dioxide and carbon disulfide have been utilized for polymer synthesis, both by their direct (co)polymerizations and polymerizations of monomers synthesized from them. Carbon disulfide, which also exists in the troposphere, has long been used as a sulfur source in organic chemistry and a solvent for various polymers, such as cellulose in industrial processes. Carbon disulfide finds use as a starting material for a variety of useful chemicals for agricultural, medicinal, and pharmaceutical applications. Sulfur compounds are very efficient at chelating metals and especially precious metals, according to Hard and Soft acid theory used by Pearson for the prediction of complexation reactions (Chanda & Rempel, 1990). Introducing sulfur chelating moieties gives a dual structure to the polymer: chelating and ion exchange capability. Since sulphur has a very strong affinity for most heavy metals, the metal–sulphur complex is very stable in basic conditions. Sorbents with donor N and more especially S atoms in their functional groups are thus performing resins. Muzzarelli and Tanfani produced dithiocarbamate chitosan by reaction of chitosan with carbon disulfide (Muzzarelli & Tanfani, 1982). Xanthation of carbohydrate materials, particularly starch, has been reported to assist removal of metals from aqueous solutions (Lokesh and Tare, 1989, Tare and Chaudhari, 1987). The xanthated rice straw was first used for removal of iron, copper, and chromium from synthetic and electroplating industry waste (Changgeng, 1991). The characterization of xanthated straw and the mechanism(s) of heavy metal removal (Kumar, Rao, & Kaul, 2000) have been studied. In this paper, sulphur groups were introduced onto glutaraldehyde cross-linked chitosan through a chemical reaction, and its characteristics of chromium adsorption were compared to that of control chitosan.
The chemical structure and physical properties of chitosan derivatives were characterized by FTIR, 13C NMR, TGA, and XRD. SEM-EDAX techniques were also used to characterize chemical modifications of the sorbent.
Section snippets
Materials
The sample of pure chitosan (Sigma) from crab shells was used; the degree of deacetylation was 85%. Chitosan was ground to fine powders (>140 mesh) and dried under vacuum at room temperature. Glutaraldehyde and carbondisulfide were purchased from Sigma–Aldrich and used without further purification. Potassium chromate (BDH chemicals), sodium hydroxide (BDH chemicals) and hydrochloric acid (merck) were used. All the inorganic chemicals used were of analytical grade. All the reagents were prepared
IR
Fig. 1a shows the basic characteristics of chitosan at: 3429 cm−1 (O–H stretch), 2828 cm−1 (C–H stretch), 1596 cm−1 (N–H bend), 1154 cm−1 (bridge-O-stretch), and 1081 cm−1 (C–O stretch) (Brugnerotto et al., 2001, Shigemasa et al., 1996). The IR spectrum of the chitosan showed strong peaks at 1030, 1076, and 1261 cm−1, characteristic peaks of a saccharide structure (due to O–H bending, C–O stretching, and C–N stretching). The strong peak around 3429 cm−1 could be assigned to the axial stretching
Conclusions
In this work, a novel chitosan derivative was prepared by chemical reaction with CS2 under alkaline conditions. It was characterized and evaluated for its adsorption capacity for Cr(VI). The introduction of sulphur group onto chitosan is of importance in enhancing the adsorption properties of plain chitosan. Such chemically modified Xanthated chitosan might find potential use as adsorbent in tannery wastewater treatment.
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