Mechanisms of Sb(III) Oxidation by Pyrite-Induced Hydroxyl Radicals and Hydrogen Peroxide

Antimony (Sb) is an element of growing interest, and its toxicity and mobility are strongly influenced by redox processes. Sb(III) oxidation mechanisms in pyrite suspensions were comprehensively investigated by kinetic measurements in oxic and anoxic conditions and simulated sunlight. Sb(III) was oxidized to Sb(V) in both solution and on pyrite surfaces in oxic conditions; the oxidation efficiency of Sb(III) was gradually enhanced with the increase of pH. The pyrite-induced hydroxyl radical (center dot OH) and hydrogen peroxide (H2O2) are the oxidants for Sb(III) oxidation. center dot OH is the oxidant for Sb(III) oxidation in acidic solutions, and H2O2 becomes the main oxidant in neutral and alkaline solutions. center dot OH and H2O2 can be generated by the reaction of previously existing Fe-(pyrite)(III) and H2O on pyrite in anoxic conditions. The oxygen molecule is the crucial factor in continuously producing center dot OH and H2O2 for Sb(III) oxidation. The efficiency of Sb(III) oxidation was enhanced in surface-oxidized pyrite (SOP) suspension, more center dot OH formed through Fenton reaction in acidic solutions, but Fe(IV) and H2O2 were formed in neutral and alkaline solutions. Under the illumination of simulated sunlight, more center dot OH and H2O2 were produced in the pyrite suspension, and the oxidation efficiency of Sb(III) was remarkably enhanced. In conclusion, Sb(III) can be oxidized to Sb(V) in the presence of pyrite, which will greatly influence the fate of Sb(III) in the environment.