A simplified equilibrium model for sorption of heavy metal ions from aqueous solutions on chitosan

The amounts of sorption of Cu²⁺, Ni²⁺, and Zn²⁺ from water on cross-linked chitosan were measured. Experiments were performed as a function of initial pH (2-5), total metal concentration (0.77-17mol/m³), and metal concentration ratio (0.25-4) at 25°C in single- and binary-metal systems. The sorption was so highly pH dependent that the isotherm could not be described by one specific equation. A simplified equilibrium model was thus proposed considering competitive sorption of proton and metal ions. The number of active sites on chitosan bound with one metal ion was adjustable and model parameters could be graphically determined. Given initial metal concentrations and solution pH, the proposed model could predict the amounts of sorption of proton and metals as well as the equilibrium pH. In general, application of the model parameters calculated in single-metal systems to the prediction of sorption in binary-metal systems was not satisfactory due to a remarkable effect of competitive sorption.