Pyrite Oxidation under initially neutral pH conditions and in the presence of Acidithiobacillus ferrooxidans and micromolar hydrogen peroxide

Abstract. Hydrogen peroxide (H₂O₂) at a micromolar level played a role in the microbial surface oxidation of pyrite crystals under initially neutral pH. When the mineral-bacteria system was cyclically exposed to 50 μM H₂O₂, the colonization of \textit{Acidithiobacillus ferrooxidans} onto the mineral surface was markedly enhanced, as compared to the control (no added H₂O₂). This can be attributed to the effects of H₂O₂ on increasing the roughness of the mineral surfaces, as well as the acidity and Fe²⁺ concentration at the mineral-solution interfaces. All of these effects tended to create more favourable nano- to micro-scale environments in the mineral surfaces for the cell adsorption. However, higher H₂O₂ levels inhibited the attachment of cells onto the mineral surfaces, possibly due to the oxidative stress in the bacteria when they approached the mineral surfaces where high levels of free radicals are present as a result of Fenton-like reactions. The more aggressive nature of H₂O₂ as an oxidant caused marked surface flaking of the mineral surface. The XPS results suggest that H₂O₂ accelerated the oxidation of pyrite-S and consequently facilitated the overall corrosion cycle of pyrite surfaces. This was accompanied by pH drop in the solution in contact with the pyrite cubes.