Biogenic Synthesis and Reduction of Fe(III)-hydroxysulfates

The purpose of the study was to produce schwertmannite (ideally Fe8O8(OH)6(SO4)) and jarosite ((K, NH4, H3O)Fe3(SO4)2 (OH)6) through bacterial iron oxidation and use these Fe(III)-hydroxysulfates as electron acceptors for sulfate-reducing microorganisms. Iron-oxidizing microorganisms were used to oxidize soluble Fe(II) to Fe(III) at pH 2, resulting in precipitation of schwertmannite and jarosite. The precipitates were used as solid-phase electron acceptors for sulfate-reducing bacteria with lactate as the electron donor under mesophilic (22C) and thermophilic (45 and 60C) anaerobic conditions. Gypsum (CaSO42H2O) was also used as a reference electron acceptor because it is moderately soluble and previous studies had shown that the sulfate entity was reduced to sulfide by sulfate-reducing bacteria. The formation of greigite (Fe3S4) from schwertmannite was verified with X-ray diffraction after 2 weeks of incubation in the 60C sulfate-reducing culture. Greigite was also identified in solids from media that had gypsum and jarosite as the sole sulfate source but was much less abundant. In all cases, the relative amount and crystallinity of greigite increased with the incubation temperature. The results showed that Fe(III)-hydroxysulfates are not biologically stable and can be reduced to Fe-sulfides under anoxic conditions. The cycling of Fe between oxidized and reduced solid phases may also involve solubilization or immobilization of potentially toxic metals or other ions if they are associated with precipitates through sorption or solid solution.