EXPERIMENTAL STUDIES OF MICROBIAL MECHANISMS OF METAL SULFIDE OXIDATION: STABLE ISOTOPIC SYSTEMATICS OF SULFATE AND IRON OXIDE

The oxidation of pyrite and other metal sulfides in acid mine drainage (AMD) systems can proceed biotically and abiotically by various  mechanisms.  In order to better understand the mechanisms and relative influence of bacteria on metal sulfide oxidation, we have conducted biotic and abiotic metal sulfide oxidation experiments. A focus of this investigation is to determine whether the stable isotopic values of sulfate (d³⁴S_SO4 and d¹⁸O_SO4) and iron oxides (d⁵⁶Fe and d¹⁸O), the products of metal sulfide oxidation in AMD systems, are dependent on the parent metal sulfide, microbial reaction pathways , and sulfide oxidation reaction stoichiometry. The d¹⁸O_SO4 formed from metal sulfide oxidation can vary depending on the relative contributions from O₂ and H₂O, which varies depending on the relative influence of bacteria and whether oxidation proceeds aerobically or anaerobically.  We have conducted experiments with the bacterium, Acidithiobacillus ferrooxidans, a common resident bacterium in AMD systems. In experiments designed for d¹⁸O_SO4 and d³⁴S_SO4 measurements, we varied the type of metal sulfide (e.g., ZnS and FeS₂) and d¹⁸O_H2O value. For experiments that measured d⁵⁶Fe values of iron oxides formed from Fe(II) oxidation, we varied temperature or the type of Fe(II) salt provided in order to determine whether these variations affected iron isotopic fractionation. Initial results of metal sulfide oxidation experiments indicate  that sulfate formed from anaerobic oxidation of ZnS has a much higher d¹⁸O_SO4  value than sulfate formed from oxidation of pyrite under identical conditions. This difference probably reflects differences in reaction mechanisms or pathways. These results suggest that the parent metal sulfide oxidized must also be considered in order to make accurate (paleo)environmental assessment of metal sulfide oxidation pathways and mechanisms based on  measured d¹⁸O_SO4 values.  Fe(II) oxidation  experiments, consistently show a smaller iron isotope fractionation effect (~1.5-2.5 per mil) for biological oxidation than for abiotic oxidation experiments (~3-4 per mil). Several possible reasons  might explain  this difference  in fractionation  and these will be discussed.