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

Abstract

Abstract 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 δ34S and δ18O 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 δ18O values to determine the relative contributions of H2O-derived and O2-derived oxygen in the newly formed sulfate. . Experiments were conducted under aerobic and anaerobic conditions using O2 and Fe(III)aq as the oxidants, respectively. Aerobic incubations with A. ferrooxidans, and So as the sole energy source were also conducted. The δ 34 S SO 4 values from both the biological and abiotic oxidation of ZnS and ZnSFe by Fe(III)aq produced sulfur isotope fractionations ( e 34 S SO 4 - ZnS ) of up to −2.6‰, 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 ( e 34 S SO 4 - 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 e 18 O SO 4 - H 2 O values of 8.2 ± 0.2‰ and 7.5 ± 0.1‰, respectively. Also, during the aerobic oxidation of ZnSFe and So by A. ferrooxidans, all of the sulfate oxygen was derived from water with similar measured e 18 O SO 4 - H 2 O values of 8.1 ± 0.1‰ and 8.3 ± 0.3‰, respectively. During biological oxidation of ZnS by O2, an estimated 8% of sulfate–oxygen was derived from O2, which is enriched in 18O relative to water, thus resulting in a larger apparent e 18 O SO 4 - H 2 O value of 9.5‰. Based on the data presented we hypothesize that the similar e 18 O SO 4 - H 2 O values of ∼8‰ from all of the aerobic and anaerobic experiments result from a common rate-limiting step that involves oxygen isotopic exchange between a sulfite ( SO 3 - ) intermediate and H2O. Our results indicate that the δ 18 O SO 4 values cannot be used to distinguish biological and abiotic, nor aerobic versus anaerobic, pathways of sphalerite oxidation. However, the e 18 O SO 4 - H 2 O values of ∼8‰ measured here are distinctly higher than e 18 O SO 4 - H 2 O values of ∼4‰ previously reported for pyrite oxidation indicating the influence of sulfide mineralogy on measured δ 18 O SO 4 values.