Potential role of bicarbonate during pyrite oxidation

The need to prevent the development of acid mine drainage (AMD) by oxidation of pyrite has triggered numerous investigations into the mechanisms of its oxidation. According to Frontier molecular orbital (FMO) theory, the surface-exposed sulfur atom of pyrite possesses an unshared electron pair which produces a slightly negatively charged pyrite surface that can attract cations such as Fe2+. Because of surface electroneutrality and pH considerations, however, the pyrite surface Fe2+ coordinates OH. The authors proposed that this surface Fe2+ OH when in the presence of CO2 is converted to -FeCO3 or -FeHCO3, depending on pH. In this study, using Fourier transform infrared spectroscopy (FT-IR) they demonstrated that such complexes form on the surface of pyrite and continue to persist even after a significant fraction of the surface Fe2+ was oxidized to Fe3+. FT-IR spectra also showed the presence of two carbonyl absorption bands (1,682 and 1,653 cm-1) on the surface of pyrite upon exposure to CO2 which suggested that pyrite surface carbon complexes existed in two different surface chemical environments, pointing out two potential mechanisms of pyrite surface-CO2 interactions. One potential mechanism involved formation of a pyrite surface-Fe(II)HCO3 complex, whereas a second potential mechanism involved formation of a pyrite surface-carboxylic acid group complex [-Fe(II)SSCOOFe-(II)]