Adsorption of antimony(III) and antimony(V) on bentonite: Kinetics, thermodynamics and anion competition

Abstract

This research attempted to study the adsorption of Sb(III) and Sb(V) on bentonite using batch experiments. The effects of reaction time, temperature, initial Sb concentration, and competitive anions at different concentrations on the adsorption of Sb(III) and Sb(V) were investigated. Kinetic studies suggested that the adsorption equilibriums for both Sb(III) and Sb(V) were reached within 24 h. The desorption of Sb adsorbate on the bentonite was observed following Sb(III) adsorption, probably due to the oxidation of Sb(III) on the bentonite surface and subsequent desorption of Sb(V). In addition, oxidation of Sb(III) can occur in the solution medium also, which decreases the concentration of Sb(III) in the solution thereby driving the equilibrium in the direction of desorption from the surface. The adsorption data at three temperatures were successfully modeled using Langmuir (r 2 > 0.82) and Freundlich (r 2 > 0.99) isotherms. The thermodynamic parameters (ΔG 0, ΔH 0, and ΔS 0) were calculated from the temperature dependence, suggesting that the adsorption process of Sb(III) is spontaneously exothermic, while the adsorption process of Sb(V) is spontaneously endothermic. Competitive anions such as NO 3 − , SO 4 2 − , and PO 4 3 − hardly affected the Sb(III) adsorption on bentonite, while SO 4 2 − and PO 4 3 − could compete with Sb(OH) 6 − for adsorption sites. The competition between PO 4 3 − and Sb(OH) 6 − on adsorption sites was presumably due to the formation of surface complexes and the surface accumulation or precipitation of PO 4 3 − on bentonite surface.