| Due to the high reactivity and ubiquity of iron (hydr)oxides, biogeochemical cycling of many elements is controlled by the speciation and conversion of Fe. Accordingly, this research investigates geochemical controls on the extent, rate, and mechanisms of bacterial- and abiotic-induced reductive dissolution and conversion of Fe (hydr)oxides.{09}Bacterial reduction of ferrihydrite-coated sands under advective flow results in Fe(III) reduction and formation of goethite and magnetite. Equivalent mineral assemblages are obtained, however, following abiotic reaction of ferrous Fe with ferrihydrite indicating that bacteria serve solely as a source of Fe(II). Reaction of Fe(II) with the ferrihydrite surface induces ripening to goethite (via dissolution/reprecipitation) or solid-state conversion to magnetite. Bacterial- and flow-regulated ferrous Fe concentrations dictate the conversion of ferrihydrite to either goethite or magnetite (mutually competitive precipitation mechanisms). Lepidocrocite is consistently observed as a precursor for either goethite or magnetite, depending on Fe(II) concentration. The rate and products of ferrihydrte conversion are also impacted by pH, carbonate concentration, and stabilizing ligands. Thus, reaction of Fe(II) with ferrihydrite results in formation of more recalcitrant mineral phases, thus lowering the microbial reducing capacity. Introduction of chromate (CrVI), however, results in re-oxidation of Fe(II) and precipitation of Cr(III)-Fe(III) (hydr)oxides, thus regenerating microbially available Fe(III).; While ferrihydrite is expected to be the most available Fe (hydr)oxide for microbial reduction, the reactivity of ferrihydrite is transient and compromised by its incorporation of stabilizing ions. For pure Fe (hydr)oxides, the steady-state reactivity of ferrihydrite is equivalent to that of goethite and hematite due to rapid microbial reduction and passivation (via magnetite precipitation) of the ferrihydrite surface. Furthermore, ferrihydrite converts rapidly to goethite, lepidocrocite, and hematite upon reaction with minor amounts of Fe(II) (<40 muM). These more stable phases do not convert to magnetite upon either bioreduction or abiotic reaction with Fe(II). Moreover, the reactivity of natural, estuarine ferrihydrite is compromised due to preferential incorporation of stabilizing ions (e.g. Al3+). Thus, this research reveals that the long-term reduction and sorption capacity of sediments are controlled by more crystalline (e.g. goethite, hematite) and/or recalcitrant (e.g. metal-substituted ferrihydrite) phases that impose limitations on microbial reduction and secondary mineralization. |
