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

In the presence of sulfate-reducing bacteria (Desulfovibrio desulfuricans) hematite (alpha -Fe2O3) dissolution is affected potentially by a combinatio n of enzymatic (hydrogenase) reduction and hydrogen sulfide oxidation. As a consequence, ferrous ions are free to react with excess H2S to form insolu ble ferrous sulfides. X-ray photoelectron spectra indicate binding energies similar to ferrous sulfides having pyrrhotite-like structures (Fe2p(3/2) 7 08.4 eV; S2p(3/2) 161.5 eV). Other sulfur species identified at the surface include sulfate, sulfite and polysulfides. Thin film X-ray diffraction ide ntifies a limited number of peaks, the principal one of which may be assign ed to the hexagonal pyrrhotite (102) peak (d = 2.09 Angstrom; 2 theta = 43. 22 degrees), at the hematite surface within 3 months exposure to sulfate-re ducing bacteria (SRB), High-resolution transmission electron microscopy ide ntifies the presence of a hexagonal structure associated with observed crys tallites. Although none of the analytical techniques employed provide unequ ivocal evidence as to the nature of the ferrous sulfide formed in the prese nce of SRB at hematite surfaces, we conclude from the available evidence th at a pyrrhotite stiochiometry and structure is the best description of the sulfides we observe. Such ferrous sulfide production is inconsistent with p revious reports in which mackinawite and greigite were products of biologic al sulfate reduction (Rickard 1969a; Herbert et al., 1998; Benning et al., 1999). The apparent differences in stoichiometry may be related to sulfide activity at the mineral surface, controlled in part by H2S autooxidation in the presence of iron oxides. Due to the relative stability of pyrrhotite a t low temperatures, ferrous sulfide dissolution is lilly to be reduced comp ared to the more commonly observed products of SRB activity, Additionally, biogenic pyrrhotite formation will also have implications for geomagnetic f ield behavior of sediments. Copyright (C) 2001 Elsevier Science Ltd.