Kinetics and equilibrium modeling of Se(IV) removal from aqueous solutions using metal oxides

The efforts in this study were devoted to investigate the removal efficiency of selenium from waste solutions using iron and silicon oxides. Experiments were carried out as a function of pH, equilibrium time, initial concentration and temperature. The adsorption process was very fast initially and the removal efficiency reached more than 90% after 3 h of contact for initial selenium concentration of 20 mu g L-1. High removal efficiency of selenite occurred in a wide range of pH (i.e., 2-8), but the efficiency decreased at pH values higher than 8. The immobilization of Se(IV) anions onto the surface of oxides may proceed through ligand-exchange interactions and/or inner-sphere complexation. Selenite interaction with oxide particles followed second-order kinetics with a correlation coefficient extremely high and closer to unity and the rate constant (k(s)) had the values 7.69 x 10(-5) and 3.59 x 10(-5) g mu g(-1) min(-1) for adsorption onto Fe2O3 and SiO2, respectively. The values of equilibrium sorption capacity (q(e)) are consistent with the modeled data and attained the value 1.8 mg g(-1). Kinetically, both pore and film diffusions are participating in ruling the diffusion of Se(IV) anions. Equilibrium adsorption data were analyzed using Freundlich, Langmuir and Dubinin-Radushkevich (D-R) isotherm models. Of the models tested, D-R isotherm expressions were found to give better fit to the experimental data compared to the other models. The revealed evidences argue that metal oxides could be used as efficient adsorbents for removal of selenium from wastewaters. (C) 2010 Elsevier B.V. All rights reserved.