Mechanism of pyrite catalysis of As(III) oxidation in bioleaching solutions at 30 °C and 70 °C

Arsenic, present as impurity in many sulfide ores, dissolves during the bioleaching process and the resulting high As(III) concentration can inhibit bacterial activity, compromising the whole process. Oxidation of As(III) to As(V) and its further precipitation as ferric arsenate is probably the most accurate way to face the problem. Thermodynamically, As(III) should be easily oxidized by Fe(III), but the reaction is very slow and only in the presence of pyrite, significant oxidation of As(III) to As(V) takes place. In the present study, the catalysis mechanism of arsenic oxidation on pyrite surface and the role of Acidithiobacillus ferrooxidans at 30 °C and Sulfolobus metallicus at 70 °C were analysed. Anaerobic experiments demonstrated that ferric ions participate as electron acceptors during the oxidation of As(III) to As(V) but that electron transfer only takes place at the pyrite surface. As(III) oxidation is only observed at high oxidoreduction potential (Eh > 450 mV vs. Ag/AgCl). At lower potential, As(V) is reduced to As(III), oxidising the pyrite. The role of microorganisms is mainly to maintain a high potential. Ferric ions produced by bacterial oxidation are reduced at the pyrite surface, oxidising both pyrite and As(III). Only a small fraction of electrons transferred comes from As(III).