Novel model development for sorption of arsenate on chitosan

The sorption of arsenate onto chitosan flakes has been studied. Chitosan, a natural, non-toxic, biodegradable polysaccharide is derived by the deacetylation of chitin, a major component of crustacean shells of prawn, crab or shrimp. Its main attributes correspond to its polycationic nature and the abundance of amine functional groups. Chitosans have received increasing attention as renewable polymeric materials for the treatment of metal contaminated water and wastewater. The effect of initial pH on the sorption isotherm has been studied for two initial concentration ranges of arsenate (0-3000 μg L^-1 and 0-10,000 μg L^-1) on chitosan. The equilibrium data have been modelled using Langmuir and Freundlich type isotherms at three initial pH values. The maximum adsorption capacity occurs at an initial pH 3.5 and empirical correlations have been developed to model the effect of pH on the sorption isotherm parameters. Each initial pH_i value, namely, 3.5, 4.0 and 4.5, corresponded to a fixed final pH_e value, namely, 4.69, 6.40 and 6.73 respectively. A series of batch kinetic experiments has been carried out at different initial pH values. The arsenate sorption process appears to be completed after 30 min, however, a previously unreported phenomenon was observed, namely, a steady desorption of arsenate. There is a natural buffering effect from the chitosan. The batch kinetic data have been correlated using the pseudo-first order, pseudo-second order and pseudo-first order reversible models; this latter model was modified to incorporate the arsenate desorption step as a function of the changing system pH.