Abstract: Influence Of Bicarbonate On Pyrite Oxidation

EVANGELOU, V. P., and A. K. SETA

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-.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky