Triple-oxygen-isotope determination of molecular oxygen incorporation in sulfate produced during abiotic pyrite oxidation (pH = 2–11)

Abstract

Aqueous oxidation of sulfide minerals to sulfate is an integral part of the global sulfur and oxygen cycles. The current model for pyrite oxidation emphasizes the role of Fe 2+–Fe 3+ electron shuttling and repeated nucleophilic attack by water molecules on sulfur. Previous δ 18O-labeled experiments show that a variable fraction (0–60%) of the oxygen in product sulfate is derived from dissolved O 2, the other potential oxidant. This indicates that nucleophilic attack cannot continue all the way to sulfate and that a sulfoxyanion of intermediate oxidation state is released into solution. The observed variability in O 2% may be due to the presence of competing oxidation pathways, variable experimental conditions (e.g. abiotic, biotic, or changing pH value), or uncertainties related to the multiple experiments needed to effectively use the δ 18O label to differentiate sulfate–oxygen sources. To examine the role of O 2 and Fe 3+ in determining the final incorporation of O 2 oxygen in sulfate produced during pyrite oxidation, we designed a set of aerated, abiotic, pH-buffered (pH = 2, 7, 9, 10, and 11), and triple-oxygen-isotope labeled solutions with and without Fe 3+ addition. While abiotic and pH-buffered conditions help to eliminate variables, triple oxygen isotope labeling and Fe 3+ addition help to determine the oxygen sources in sulfate and examine the role of Fe 2+–Fe 3+ electron shuttling during sulfide oxidation, respectively. Our results show that sulfate concentration increased linearly with time and the maximum concentration was achieved at pH 11. At pH 2, 7, and 9, sulfate production was slow but increased by 4× with the addition of Fe 3+. Significant amounts of sulfite and thiosulfate were detected in pH ⩾ 9 reactors, while concentrations were low or undetectable at pH 2 and 7. The triple oxygen isotope data show that at pH ⩾ 9, product sulfate contained 21–24% air O 2 signal, similar to pH 2 with Fe 3+ addition. Sulfate from the pH 2 reactor without Fe 3+ addition and the pH 7 reactors all showed 28–29% O 2 signal. While the O 2% in final sulfate apparently clusters around 25%, the measurable deviations (>experimental error) from the 25% in many reaction conditions suggest that (1) O 2 does get incorporated into intermediate sulfoxyanions (thiosulfate and sulfite) and a fraction survives sulfite–water exchange (e.g. the pH 2 with no Fe 3+ addition and both pH 7 reactors); and (2) direct O 2 oxidation dominates while Fe 3+ shuttling is still competitive in the sulfite–sulfate step (e.g. the pH 9, 10, and 11 and the pH 2 reactor with Fe 3+ addition). Overall, the final sulfate–oxygen source ratio is determined by (1) rate competitions between direct O 2 incorporation and Fe 3+ shuttling during both the formation of sulfite from pyrite and from sulfite to final sulfate, and (2) rate competitions between sulfite and water oxygen exchange and the oxidation of sulfite to sulfate. Our results indicate that thiosulfate or sulfite is the intermediate species released into solution at all investigated pH and point to a set of dynamic and competing fractionation factors and rates, which control the oxygen isotope composition of sulfate derived from pyrite oxidation.