Antimonite oxidation and adsorption onto two tunnel-structured manganese oxides: Implications for antimony mobility

Abstract

Manganese (Mn) oxides have high oxidation and sorption capacities, which enables them to play important roles in antimony (Sb) migration and transformation. In this study, the oxidation and adsorption behaviors of antimonite (Sb(III)) on two tunnel-structured Mn oxides, pyrolusite (β-MnO2) and manganite (γ-MnOOH), were investigated and compared using batch kinetic techniques; the adsorption isotherms, edges and spectroscopic characteristics of Sb(III) and Sb(V) on the two minerals were also compared and modeled. The adsorption isotherms and edges indicated that the affinity of Sb(III) and Sb(V) toward the two Mn oxides depended on the Sb species, the pH of the solution and the characteristics of the two minerals. Acidic pH favored the sorption of both Sb(III) and Sb(V) onto the two minerals, with the rate of sorption decreasing subsequently with the increase in pH. The sorption capacity of β-MnO2 for both Sb(III) and Sb(V) was much lower than that of the γ-MnOOH. Sb(V) was the main species after Sb(III) and Sb(V), which adsorbed onto the surface of both β-MnO2 and γ-MnOOH. Furthermore, Sb(V) combined with γ-MnOOH through inner-sphere complexation and with β-MnO2 through outer-sphere complexation. These results therefore provide new insights into the mechanism of Sb(III) oxidation and adsorption on the surfaces of two tunnel-structured Mn oxides and indicates that γ-MnOOH would exhibit a huge capacity in Sb oxidation and sorption.