Antimony(III) oxidation and antimony(V) adsorption reactions on synthetic manganite

Oxidation and adsorption processes critically affect the mobility of antimony (Sb) in the environment. Minerals such as manganese oxides appear to be important adsorbents or oxidants for Sb in soils and sediments. In this study, Sb oxidation and adsorption onto synthetic manganite (γ-MnOOH) in aqueous suspensions were comprehensively investigated. The oxidation of Sb(III) by manganite occurred on a time scale of minutes and Sb(V) was released into the suspension as soon as the reaction began. X-ray absorption near edge structure (XANES) analyses showed that Sb(V) was the dominant species adsorbed on manganite, suggesting that manganite acts as a strong oxidant of Sb(III). The effects of temperature on the adsorption rates of Sb(V) were investigated. Thermodynamic parameters indicated that adsorption of Sb(V) by manganite is an exothermic process and is spontaneous at the specific temperatures investigated. A change in ionic strength from 0.001 to 0.1 M NaNO3 had little effect on the adsorption of Sb(V) onto manganite, indicating that Sb(V) forms inner-sphere complexes at the mineral surface. Anions such as phosphate, sulfate, silicate and carbonate, which may be present in natural waters, were ineffective competitors with Sb(V) for sorption sites.