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

Geochimica et Cosmochimica Acta
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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 δ34SSO4 values from both the biological and abiotic oxidation of ZnS and ZnSFe by Fe(III)aq produced sulfur isotope fractionations (ε34SSO4-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 (ε34SSO4-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 ε18OSO4-H2O values of 8.2 ± 0.2‰ and 7.5 ± 0.1‰, respectively. Also, during the aerobic oxidation of ZnSFe and So by Aferrooxidans, all of the sulfate oxygen was derived from water with similar measured ε18OSO4-H2O 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 ε18OSO4-H2O value of 9.5‰. Based on the data presented we hypothesize that the similar ε18OSO4-H2O 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 (SO3-) intermediate and H2O. Our results indicate that the δ18OSO4 values cannot be used to distinguish biological and abiotic, nor aerobic versus anaerobic, pathways of sphalerite oxidation. However, the ε18OSO4-H2O values of ∼8‰ measured here are distinctly higher than ε18OSO4-H2O values of ∼4‰ previously reported for pyrite oxidation indicating the influence of sulfide mineralogy on measured δ18OSO4 values.

Publication type Article
Publication Subtype Journal Article
Title Oxygen and sulfur isotope systematics of sulfate produced during abiotic and bacterial oxidation of sphalerite and elemental sulfur
Series title Geochimica et Cosmochimica Acta
DOI 10.1016/j.gca.2011.10.022
Volume 77
Year Published 2012
Language English
Publisher Elsevier
Contributing office(s) Geology, Geophysics, and Geochemistry Science Center
Description 17 p.
First page 335
Last page 351
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