A simplified equilibrium model for sorption of heavy metal ions from aqueous solutions on chitosan
Introduction
Waste streams containing low-to-medium level of heavy metals are often encountered in metal plating facilities, mining operations, fertilizer and electronic device manufactures. Most of heavy metals are highly toxic and are not biodegradable, therefore, they must be removed from the polluted streams in order to meet increasingly stringent environmental quality standards. Many methods including chemical precipitation, electrodeposition, ion exchange, membrane separation, and adsorption have been used to treat such streams. Of these methods, traditional chemical precipitation (OH−, S2−, etc.) is the most economic but is inefficient for dilute solutions. Ion exchange and reverse osmosis are generally effective, but have rather high maintenance and operation costs and subject to fouling. Adsorption is one of the few promising alternatives for this purpose, especially using low-cost natural sorbents such as agricultural wastes, clay materials, biomass, and seafood processing wastes [1], [2], [3].
Chitosan is a partially acetylated glucosamine biopolymer found in the cell wall of some fungi. It is produced cheaply because it mainly results from deaceylation of chitin, the second-most abundant biopolymer in nature next to cellulose [4], [5]. Chitosan has many useful characteristics such as hydrophilicity, biocompatibility, biodegradability, and anti-bacterial properties. It is also a known sorbent for heavy metal ions because the amino (NH2) and/or hydroxy (OH) groups on chitosan chains serve as coordination sites [3], [6], [7], [8].
The sorption of heavy metals including Cu2+, Ni2+, Zn2+, Cd2+, Hg2+, Pb2+, Cr3+, VO2+, and UO22+ by raw and chemically modified chitosans has been extensively studied [3], [6], [9], [10], [11], [12], [13], [14], [15], [16]. Most of these studies either obtained the sorption isotherms or purely compared the selectivity series based on the results of single-metal systems. The isotherms were usually force-fitted to the common two-parameter Langmuir or Freundlich equation [3], [9], [10], [11], [15], [16] although such equation was generally valid only over a very restricted concentration range. Because, the present process is strongly pH-dependent due to the competition of proton and metal ions, a more cautious manner for data presentation and a generalized model are required to describe the sorption equilibria of metal ions. This is especially the case in binary metal systems. For example, Sag et al. [17] recently used an extension of the Freunlich equation to describe the sorption isotherms of binary Pb2+, Cu2+, or Zn2+ on Rhizopus arrhizus. The Freundlich parameters are varied with solution pH. In fact, Jang et al. [18] have applied the above competition concept to model the isotherms of binary Cu2+ and Zn2+ sorption using alginate gel under acidic conditions; however, the binding is assumed to be bidentate.
In this work, a simplified equilibrium model was proposed to investigate competitive sorption behavior of proton and metal ions on glutaraldehyde cross-linked chitosan beads in single-metal systems. The solution pH was incorporated into model equations as an independent variable. The amounts of metal sorption in binary-metal systems were predicted purely based on the model parameters determined in single-metal systems. The sorption of Cu2+, Ni2+, and Zn2+ in binary-metal systems was finally examined, and was compared with that in single-metal systems.
Section snippets
Materials
Chitosan produced from lobster shell wastes was offered as flakes from Ying-Huah Co., Kaohsiung, Taiwan, without further purification. The degree of deacetylation of the flakes was obtained to be 87 mol% following the method of Tan et al. [19]. The MW of chitosan was found to be 4.1×105 by the Mark–Houwink equation from viscosity data of solutions containing chitosan in 3.5 vol% acetic acid.
Nitrate salts of heavy metals and inorganic chemicals were supplied by Merck Co. as analytical-reagent
Model assumptions
In order to avoid the possible complexation of sulfate and chloride with metals in the aqueous phase, nitrate salts are used [20]. The sorption of heavy metal ions can be attributed to several mechanisms including ion exchange, coordination (complexation), electrostatic attraction, and micro-precipitation [18], [21]. For chitosan, coordination was shown to play an important role in metal binding with the NH2 groups [7], [8], [22], [23]. On the other hand, competitive sorption of proton and
Amount of sorption in single-metal systems
The effects of equilibrium pH (pHeq) on the amounts of sorption of Cu2+, Ni2+, and Zn2+ on chitosan beads are shown in Fig. 1, Fig. 2, Fig. 3. Under comparable conditions (e.g., pHeq), chitosan exhibits the highest sorption ability for Cu2+, which is consistent with the earlier results [3], [11], [15]. It is found that qM normally increases with increasing pH. This is due to competitive sorption of proton and metal ions, which was supported by experiments that the solution pH becomes higher
Conclusions
Sorption equilibria of Cu2+, Ni2+, and Zn2+ from single- and binary-metal solutions on glutaraldehyde cross-linked chitosan beads were studied at 25°C. The amount of metal sorption (qM) increased with increasing pH, confirming the occurrence of competitive sorption of proton and metal ions. A simplified model was proposed considering possible competitive reactions and the model parameters were graphically determined. The validity of the model was justified from the consistent values of
Acknowledgements
Financial support for this work by the ROC National Science Council under Grant No. NSC89-2214-E-155-002 is gratefully appreciated.
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