Influence of Fe(II) on Sb(III) oxidation and adsorption by MnO2 under acidic conditions

The transformation and transport of Sb are significantly influenced by strong oxides (e.g. MnO2) in the natural environment. Furthermore, Fe(II) can coexist with Sb(III) and MnO2 in waters contaminated by acidic mine drainage. However, role of Fe(II) in Sb(III) oxidation and adsorption by MnO2 remains unclear. Therefore, in the present study, the effects of Fe(II) on the oxidation and adsorption of Sb(III) by MnO2 under acidic conditions (pH 3) and the mechanism thereof were comprehensively investigated. The results of kinetic experiments showed that, in the presence of soluble Fe(II), Sb(III) oxidation is inhibited, but adsorption is promoted. Further characterization confirmed that Fe(III) compounds are formed around MnO2 particles and that these inhibit Sb(III) oxidation. However, two different Fe(III) compounds are formed around MnO2 particles depending on how the Fe(II) is introduced into the experimental system. In the simultaneous oxidation system, poorly crystallized or amorphous FeSb precipitates are formed (probably FeSbO4) around MnO2 particles, while in the Fe(II) pretreated oxidation system, schwertmannite is formed. Thus, the present study revealed that Fe(II) is critical to Sb(III) oxidation and adsorption by MnO2 and that the mechanism of its action is depend upon how it is introduced into the reaction system. This information is of relevance to predicting the fate of Sb.