Sorption of cadmium on hydrous ferric oxide at high sorbate/sorbent ratios: Equilibrium, kinetics, and modeling

Abstract

Equilibrium and kinetics data for cadmium sorption on hydrous ferric oxide are presented for a wide range of sorbate/sorbent ratios. The equilibrium data follow a Langmuir isotherm at low Cd 2+ concentrations (<10 −6 M), a Freundlich isotherm at low to moderate concentrations (10 −6–10 −3 M), and exhibit a maximum sorption density at the highest concentrations. A two-site surface complexation model is needed to fit the equilibrium data. To describe the expected transition from adsorption to precipitations as Cd(OH) 2(s) saturates, the surface precipitation model (Farley, K. J., Dzombak, D. A., and Morel, F. M. M., J. Colloid Interface Sci. 106, 226, 1985) can be incorporated in the two-site complexation model without addition of fitting parameters. The kinetics of cadmium sorption slow considerably as the sorbate/sorbent ratio is increased, and these results are consistent with the equilibrium data in that rate expressions based on two site types are necessary to fit the kinetics data corresponding to high sorbate/sorbent ratios.