Iron sulfides and sulfur species produced at hematite surfaces in the presence of sulfate-reducing bacteria

Abstract

In the presence of sulfate-reducing bacteria (desulfovibrio desulfuricans) hematite (a-Fe2O3) dissolution is affected potentially by a combination of enzymatic (hydrogenase) reduction and hydrogen sulfide oxidation. As a consequence, ferrous ions are free to react with excess H2S to form insoluble ferrous sulfides. X-ray photoelectron spectra indicate binding energies similar to ferrous sulfides having pyrrhotite-like structures (Fe2p3/2 708.4 eV; S2p3/2 161.5 eV). Other sulfur species identified at the surface include sulfate, sulfite and polysulfides. Thin film X-ray diffraction identifies a limited number of peaks, the principal one of which may be assigned to the hexagonal pyrrhotite (102) peak (d = 2.09 Å; 22 = 43.22°), at the hematite surface within 3 months exposure to sulfate-reducing bacteria (SRB). High-resolution transmission electron microscopy identifies the presence of a hexagonal structure associated with observed crystallites. Although none of the analytical techniques employed provide unequivocal evidence as to the nature of the ferrous sulfide formed in the presence of SRB at hematite surfaces, we conclude from the available evidence that a pyrrhotite stoichiometry and structure is the best description of the sulfides we observe. Such ferrous sulfide production is inconsistent with previous reports in which mackinawite and greigite were products of biological sulfate reduction (Rickard 1969a; Herbert et al., 1998: Benning et al., 1999). The apparent differences in stoichiometry may be related to sulfide activity as the mineral surface, controlled in part by H2S autooxidation in the presence of iron oxides. Due to the relative stability of pyrrhotite at low temperatures, ferrous sulfide dissolution is likely to be reduced compared to the more commonly observed products of SRB activity. Additionally, biogenic pyrrhotite formation will also have implications for geomagnetic field behavior of sediments.