Oxygen and sulfur isotope systematics of sulfate produced during abiotic and bacterial oxidation of sphalerite and elemental sulfur

Studies of metal sulfide oxidation in acid mine drainage (AMD) systems have primarily focused on pyrite oxidation, although acid soluble sulfides (e.g., ZnS) are predominantly responsible for the release of toxic metals. We conducted a series of biological and abiotic laboratory oxidation experiments with pure and Fe-bearing sphalerite (ZnS & Zn0.88Fe0.12S), respectively, in order to better understand the effects of sulfide mineralogy and associated biogeochemical controls of oxidation on the resultant delta S-34 and delta O-18 values of the sulfate produced. The minerals were incubated in the presence and absence of Acidithiobacillus ferrooxidans at an initial solution pH of 3 and with water of varying delta O-18 values to determine the relative contributions of H2O-derived and O-2-derived oxygen in the newly formed sulfate.. Experiments were conducted under aerobic and anaerobic conditions using O-2 and Fe(III)(aq) as the oxidants, respectively. Aerobic incubations with A. ferrooxidans, and S-o as the sole energy source were also conducted. The delta S-34(SO4) values from both the biological and abiotic oxidation of ZnS and ZnSFe by Fe(III)(aq) produced sulfur isotope fractionations (epsilon S-34(SO4-ZnS)) of up to -2.6 parts per thousand, suggesting the accumulation of sulfur intermediates during incomplete oxidation of the sulfide. No significant sulfur isotope fractionation was observed from any of the aerobic experiments. Negative sulfur isotope enrichment factors (epsilon S-34(SO4-ZnS)) in AMD systems could reflect anaerobic, rather than aerobic pathways of oxidation. During the biological and abiotic oxidation of ZnS and ZnSFe by Fe(III)(aq) all of the sulfate oxygen was derived from water, with measured epsilon O-18(SO4-H2O) values of 8.2 +/- 0.2 parts per thousand and 7.5 +/- 0.1 parts per thousand, respectively. Also, during the aerobic oxidation of ZnSFe and S-o by A. ferrooxidans, all of the sulfate oxygen was derived from water with similar measured epsilon O-18(SO4-H2O) values of 8.1 +/- 0.1 parts per thousand and 8.3 +/- 0.3 parts per thousand, respectively. During biological oxidation of ZnS by O-2, an estimated 8% of sulfate-oxygen was derived from O-2, which is enriched in O-18 relative to water, thus resulting in a larger apparent epsilon O-18(SO4-H2O) value of 9.5 parts per thousand. Based on the data presented we hypothesize that the similar epsilon O-18(SO4-H2O) values of similar to 8 parts per thousand from all of the aerobic and anaerobic experiments result from a common rate-limiting step that involves oxygen isotopic exchange between a sulfite (SO3-) intermediate and H2O. Our results indicate that the delta O-18(SO4) values cannot be used to distinguish biological and abiotic, nor aerobic versus anaerobic, pathways of sphalerite oxidation. However, the epsilon O-18(SO4-H2O) values of similar to 8 parts per thousand measured here are distinctly higher than epsilon O-18(SO4-H2O) values of similar to 4 parts per thousand previously reported for pyrite oxidation indicating the influence of sulfide mineralogy on measured delta O-18(SO4) values. (C) 2011 Elsevier Ltd. All rights reserved.