Potential role of bicarbonate during pyrite 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. We 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) we 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 (1682 and 1653 cm(-1)) on the surface of pyrite upon exposure to CO2 which suggested that pyrite surface carbonate 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)]. We hypothesized that these pyrite surface-CO2 complexes could promote abiotic oxidation of pyrite by accelerating the abiotic oxidation of Fe2+. Iron (III) would oxidize the disulfide (-S-2) by accepting its electrons. Using a miscible displacement technique, oxidation of FeS2 with H2O2 was carried out in the absence or presence of 10 or 100 mM NaHCO3. The data show that 100 mM NaHCO3 significantly increased the oxidation rate of FeS2. Furthermore, the data show that FeS2 oxidation kinetics were more dependent on HCO3- but were less dependent on H2O2 for the range of HCO3- and H2O2 concentrations tested.