Pyrite dissolution in acidic media

Oxidation of pyrite in aqueous solutions in contact with air (oxygen 20%) was studied at 25°C using short-term batch experiments. Fe²⁺ and SO₄²⁻ were the only dissolved Fe and S species detected in these solutions. After a short period, R = S]_tot/Fe]_tot stabilized from 1.25 at pH = 1.5 to 1.6 at pH = 3. These R values were found to be consistent with previously published measurements (as calculated from the raw published data). This corresponds to a nonstoichiometric dissolution (R < 2) resulting from a deficit in aqueous sulfur. Thermodynamics indicate that S(−I) oxidation can only produce S_(s)⁰ and SO₄²⁻ under these equilibrium conditions. However, Pourbaix diagrams assuming the absence of SO₄²⁻ indicate that S₂O₃²⁻ and S₄O₆²⁻ can appear in these conditions. Using these species the simplest expected oxidation mechanism is