Magnetite (Fe3O4) is a common biomineralization product of microbial iron respiration and is often found in subsurface anoxic environments, such as groundwater aquifers where aqueous FeII is present. We investigated the reaction between aqueous FeII and magnetite using the isotopic selectivity of 57Fe Mössbauer spectroscopy and revisited the reduction of nitrobenzene by magnetite. Similar to our previous findings with Fe3+ oxides, we did not observe the formation of a stable sorbed FeII species; instead, we observed oxidation of the FeII to a partially oxidized magnetite phase. Oxidation of FeII was accompanied by reduction of the octahedral Fe3+ atoms in the underlying magnetite to octahedral Fe2+ atoms. The lack of a stable, sorbed FeII species on magnetite prompted us to reevaluate what is controlling the extent of FeII uptake on magnetite, as well as contaminant reduction in the presence of magnetite and FeII. Uptake of FeII by magnetite appears to be limited by the stoichiometry of the magnetite particles, rather than the surface area of the particles. More oxidized (or less stoichiometric) magnetite particles take up more FeII, with the formation of stoichiometric magnetite (Fe2+/ Fe3+ = 0.5) limiting the extent of FeII uptake. We also show that stoichiometric magnetite, in the absence of aqueous FeII, can rapidly reduce nitrobenzene. Based on these results, we speculate that contaminant reduction that was previously attributed to FeII sorbed on magnetite is due to a process similar to negative (n) doping of a solid, which increases the stoichiometry of the magnetite and alters the bulk redox properties of the particle to make reduction more favorable.
All Science Journal Classification (ASJC) codes
- Environmental Chemistry