Characterization and microbial reduction of bacteriogenic iron oxides

The main objectives of the present study were to first determine the chemical and mineralogical composition of bacteriogenic iron oxides (BIOS) collected from various locations (i.e., wetlands near Deep River, Ontario, hydrothermal sea vents along the Kermadec Arc between New-Zealand and Fiji, and from the Aspo tunnel in Sweden), to assess their sorptive capacity for Sr and to measure their rates of reduction in the presence of a well known iron-reducing bacterium. X-ray diffraction (XRD) and selected area electron diffraction indicated that both marine and freshwater BIOS were composed of the iron mineral, 2-line ferrihydrite, in combination with cellular structures of known iron-oxidizing microbes such as Gallionella spp., Leptothrix spp., and Mariprofundus spp. Mossbauer spectroscopy of marine BIOS confirmed 2-line ferrihydrite as the dominant mineral phase, but a small fraction of more crystalline oxides may also have been present (as detected by XRD). Using electron microscopy, the mineral phases were typically observed as 100-200 nm thick crusts of poorly structured material, attached to the surfaces of the associated microbial debris. Some well-defined crystal habits were observed in one marine sample. Sorption experiments showed that BIOS were effective adsorbents of dissolved strontium. Sorption isotherms indicated that Sr was bound to the BIOS via reversible outer-sphere complexation. Extended X-ray absorption fine structure (EXAFS) confirmed binding of hydrated Sr2+ but failed to elucidate whether sorption occurred to sites specific to the iron or organic component of the BIOS. All BIOS samples were more susceptible to microbial Fe(III) reduction than was synthetic 2-line ferrihydrite. This suggests that BIOS are not stabilized by precipitation along microbial surfaces, and that the use of synthetic analogs to model BIOS reactivity is insufficient. Reduction of BIOS resulted in the complete dissolution of the iron mineral phase, and a nearly complete solubilization of the sorbed Sr. This suggests that sorption occurs primarily to sites within the iron oxide component. In a natural BIOS deposit, levels of dissolved Sr in BIOS porewaters were elevated at depths where Fe(II) was also elevated, and levels of dissolved Sr were 2 to 3 times higher in aquifer waters that did not contain BIOS. These data suggest that BIOS may represent a natural means of Sr attenuation, provided oxidizing conditions are maintained