Influence of magnetite stoichiometry on Fe(II) uptake and nitroaromatic reduction

Magnetite (Fe₃O₄) is an iron oxide that contains both Fe(II) and Fe(III). It is a common constituent of soils, an important industrial ore, and a product of iron-respiring bacteria. Magnetite has an inverse spinel structure, which provides unique electronic and magnetic properties. Maghemite (γ-Fe₂O₃) is an oxidation product of magnetite, and contains only Fe(III). Maghemite is isostructural with magnetite, which makes distinguishing between the two phases challenging. Magnetite has been shown to reduce several environmental contaminants, including some chlorinated solvents and heavy metals. Nitroaromatic compounds, such as nitrobenzene and RDX, however, appear to be stable in the presence of magnetite unless aqueous Fe(II) is added. Here we revisit the reduction of nitrobenzene by magnetite synthesized with varying Fe(II) to Fe(III) stoichiometries. We also use the isotopic specificity of ⁵⁷Fe Mössbauer spectroscopy to investigate the electron transfer reaction between aqueous Fe(II) and magnetite. We hypothesize that the redox behavior of magnetite with regards to Fe(II) sorption and contaminant reduction is strongly influenced by particle stoichiometry and diffusion through an oxidized coating.