Influence of magnetite stoichiometry on Fe^II uptake and nitrobenzene reduction

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 Fe^II is present. We investigated the reaction between aqueous Fe^II 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 Fe^II species; instead, we observed oxidation of the Fe^II to a partially oxidized magnetite phase. Oxidation of Fe^II was accompanied by reduction of the octahedral Fe³⁺ atoms in the underlying magnetite to octahedral Fe²⁺ atoms. The lack of a stable, sorbed Fe^II species on magnetite prompted us to reevaluate what is controlling the extent of Fe^II uptake on magnetite, as well as contaminant reduction in the presence of magnetite and Fe^II. Uptake of Fe^II 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 Fe^II, with the formation of stoichiometric magnetite (Fe²⁺/ Fe³⁺ = 0.5) limiting the extent of Fe^II uptake. We also show that stoichiometric magnetite, in the absence of aqueous Fe^II, can rapidly reduce nitrobenzene. Based on these results, we speculate that contaminant reduction that was previously attributed to Fe^II 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.