Elemental sulfur on oxidized sulfide mineral surfaces

The effects of different microbial populations on the oxidative dissolution of pyrite, marcasite, and arsenopyrite were examined in order to better understand the mechanism and kinetics of dissolution under conditions that simulate acid mine drainage. An increase in dissolution rate was observed only in the presence of iron-oxidizing microorganisms. The chemical speciation at the mineral surface in the presence of these iron-oxidizing species is indistinguishable from that of abiotic control reactions; both are dominated by elemental sulfur (S₈). In contrast, experiments with a sulfur-oxidizing species indicate that the quantity of S₈ is less than 1% of the amount observed on samples exposed to an iron oxidizer. Surprisingly, removal of the S₈ from the mineral surface is not accompanied by an increase in the dissolution rate of the mineral.; This finding suggests that S₈ must be distributed very heterogeneously on the mineral surface. To test this hypothesis, spatially resolved spectroscopic studies of oxidized pyrite and arsenopyrite surfaces were conducted using a Raman imaging microscope. The resulting chemical maps of the mineral surface reveal that S₈ is present in randomly distributed, isolated patches on the order of tens of microns in size.; Although spectroscopic investigations of oxidized arsenopyrite surfaces revealed information about the speciation and spatial distribution of reaction products, further study of the mechanism of arsenopyrite oxidation required the development of a quantitative technique for the determination of S ₈. After laboratory oxidation of the mineral sample, S₈ is extracted from the surface in an organic solvent and quantitatively analyzed by high-performance liquid chromatography. Subsequent studies using this method revealed that S₈ accounts for more than half of the total reacted sulfur when arsenopyrite is oxidized by Fe3+. This finding has significant implications for the mechanism of arsenopyrite oxidation. If arsenopyrite is oxidized in a scheme whereby the sulfur is initially oxidized to thiosulfate, S₈ could be formed by the decomposition of thiosulfate in the acidic solution. This scheme, however, limits S₈ to no more than 50% of the total reacted sulfur. Instead, arsenopyrite must initially form lower oxidation state products such as polysulfides and S₈ rather than sulfoxy anions.