Fe Atom Exchange between Aqueous Fe2+ and Magnetite

The reaction between magnetite and aqueous Fe2+ has been extensively studied due to its role in contaminant reduction, trace-metal sequestration, and microbial respiration. Previous work has demonstrated that the reaction of Fe2+ with magnetite (Fe3O4) results in the structural incorporation of Fe2+ and an increase in the bulk Fe2+ content of magnetite. It is unclear, however, whether significant Fe atom exchange occurs between magnetite and aqueous Fe2+, as has been observed for other Fe oxides. Here, we measured the extent of Fe atom exchange between aqueous Fe2+ and magnetite by reacting isotopically “normal” magnetite with 57Fe-enriched aqueous Fe2+. The extent of Fe atom exchange between magnetite and aqueous Fe2+ was significant (54–71%), and went well beyond the amount of Fe atoms found at the near surface. Mössbauer spectroscopy of magnetite reacted with 56Fe2+ indicate that no preferential exchange of octahedral or tetrahedral sites occurred. Exchange experiments conducted with Co-ferrite (Co2+Fe2 3+O4) showed little impact of Co substitution on the rate or extent of atom exchange. Bulk electron conduction, as previously invoked to explain Fe atom exchange in goethite, is a possible mechanism, but if it is occurring, conduction does not appear to be the rate-limiting step. The lack of significant impact of Co substitution on the kinetics of Fe atom exchange, and the relatively high diffusion coefficients reported for magnetite suggest that for magnetite, unlike goethite, Fe atom diffusion is a plausible mechanism to explain the rapid rates of Fe atom exchange in magnetite.