No abstract

The remarkable discovery of pyrite-induced hydrogen peroxide (H2O2) provides a key step in the evolution of oxygenic photosynthesis. Here we show that H2O2 can be generated rapidly via a reaction between pyrite and H2O in the absence of dissolved oxygen. The reaction proceeds in the dark, and H2O2 levels increase upon illumination with visible light. Since pyrite was stable in most photic environments prior to the rise of O2 levels, this finding represents an important mechanism for the formation of H2O2 on early Earth.

The oxidation-induced degradation of pyrite in mine tailing piles is of significant interest, since the resulting production of sulfuric acid has a severe detrimental impact on the surrounding environment. Hence, there has been much effort to understand the mechanism of pyrite oxidation. Much of the information concerned with the surface reactivity of pyrite, however, has been inferred from macroscopic observations during aqueous studies. Here, we directly investigated model FeS2(100) surfaces after exposure to well-defined oxidizing environments, with X-ray photoelectron spectroscopy, to evaluate the mechanism of pyrite oxidation at a microscopic level. Our studies showed that, in the pure H2O vapor environment, oxidation of FeS2(100) was spatially limited to nonstoichiometric or sulfur-deficient surface sites. Results further suggested that thiosulfate was a long-lived intermediate that ultimately converted to sulfate on the pyrite surface in this environment. Significant oxidation of the disulfide group of FeS2(100) only occurred if O2 was present along with H2O vapor (O2 alone resulted in only minor reaction). It is proposed that O2 adsorption on the stoichiometric region of FeS2(100) resulted in the formation of Fe3+ sites that facilitated the dissociation of H2O and the oxidation of the disulfide group.