Aqueous Oxidation of Iron Monosulfide (FeS) by Molecular Oxygen

Aqueous oxidation of iron monosulfide (FeS) by oxygen at initial pH between 2.5 and 5 was investigated in a closed system at different temperatures (25, 35, and 40°C). It was found that the rate of aqueous oxidation of FeS increases when initial H⁺] and temperature increase. The reaction order with respect to H⁺] was 0.16 ± 0.02 at 25°C. The activation energy was found to be 23 ± 5 kJ mol^–1 at initial pH 2.5. This value suggests that aqueous oxidation of FeS by oxygen is controlled by a mixed regime of diffusion and surface reaction control. FTIR analysis of the initial and reacted FeS samples has shown that during the aqueous oxidation of FeS by O₂ a sulfur-rich layer is formed on the mineral surface. The experimental results indicate that the protons adsorb on the mineral surface and catalyze Fe²⁺ release into solution (by Couloumbic repulsion) and S(-II) oxidation to higher oxidation states.