EVANGELOU, V. P. and SETA, A. K., Soils Division, Univ. of Kentucky, Lexington, KY 40546-0091

A mechanism explaining enhancement of abiotic pyrite (FeS₂) oxidation in circumneutral pH environments is OH^- involvement in an inner-sphere electron transfer process. In this process an OH^- and an electron are exchanged simultaneously between pyrite surface-Fe(III)(OH)_n3-n and pyrite surface disulfide. Data in the literature suggest that bicarbonate (HCO₃^-) could promote abiotically pyrite oxidation but this mechanism is not understood and evidence is lacking. This study was carried out to elucidate the potential role of HCO₃^- on pyrite oxidation. Evidence obtained from our previous studies, using FT-IR spectroscopy, demonstrated that pyrite exposed to humidified CO₂ plus O₂ formed pyrite surface-Fe(II)-HCO₃ complexes. Based on the above results, it was postulated that HCO3-, having the potential to form pyrite surface Fe(II)-HCO₃ complexes, would promote abiotic pyrite oxidation by accelerating oxidation of Fe(II). The latter (Fe(II)) would oxidize the disulfide (-S₂) 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 mmol L^-1 NaHCO₃. The data showed that 100 mmol L^-1 NaHCO₃ increased significantly the oxidation rate of FeS2. Furthermore, the data showed that FeS₂ oxidation kinetics were dependent on H₂O₂ concentration at the 10 mmol L^-1 HCO₃^- but were less dependent on H₂O₂ concentration at 100 mmol L^-1 HCO₃-. These results are consistent with the findings of our FT-IR spectroscopic studies and published data which showed that Fe(II) oxidation in solution was enhanced in the presence of solution HCO₃^-.
 

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