Influence of Magnetite Stoichiometry on Fe^II Uptake and Nitrobenzene Reduction

Abstract

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 showthat 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.

U(VI) Sorption and Reduction Kinetics on the Magnetite (111) Surface

“…Gorski and Scherer (2009) and Latta et al (2012) showed that the addition of aqueous Fe(II) to suspensions of oxidized magnetite resulted in reduction of U(VI) to UO 2 , consistent with Fe(II) taken up from solution and increasing the magnetite stoichiometry. 15,46 Thus as U(VI) reduction proceeds and the magnetite surface becomes oxidized, the released aqueous Fe(II) could locally recharge the oxidized surface creating new nucleation sites for further UO 2 growth. Because it appears probable that in our system Fe(II) is more soluble at the defect sites relative to the flat (111) surface, the release of Fe(II) could recharge oxidized Fe surface sites nearby allowing for additional U(VI) reduction, and restricting U(VI) reduction to the spatial domain around the defect sites.…”

Microbiological, Geochemical and Hydrologic Processes Controlling Uranium Mobility: An Integrated Field Scale Subsurface Research Challenge Site at Rifle, Colorado, February 2011 to January 2012

“…However, the oxidized product does not resemble the hematite, lepidocrocite, or large particle goethite and magnetite which are present in the background sediment sample. In many studies on the reaction of Fe(II) with pure Fe oxides (goethite, magnetite), the oxidized product resembles the structure of the sorbent mineral phase [Gorski and Scherer, 2009;Handler et al, 2009]. The fact that we did not observe this suggests that goethite and magnetite were probably not the predominant phases with which the 57 Fe(II) reacted.…”

“…This was followed by injection of a pulse of groundwater containing deuterium as a conservative tracer in 3 multi-level sampling wells upgradient of the Fe-reducing zone to allow us to track geochemical changes in groundwater, particularly dissolved U(VI), as it passed through the Fe-reducing zone. [Gorski and Scherer, 2009;Pedersen et al, 2005;Williams and Scherer, 2004]. Fe(II) oxidation on a Fe-free clay mineral (synthetic montmorillonite) has also been demonstrated -in this case the authors hypothesized that H + was the electron acceptor, producing H 2 that is closely associated with the mineral surface along with the oxidized Fe(III) [Gehin et al, 2007].…”

4th Annual DOE-ERSP PI Meeting: Abstracts

“…extracellularly during growth in the pH range 5-7. The amount of PO 4 3-liberated from an exogenous organophosphorus substrate, provided as the sole carbon and phosphorus (P) source, was sufficient to precipitate >95% of uranium [U(VI)] as low solubility uraniumphosphate minerals even at pH 5. Presently, the range of phosphatase activities in subsurface microbial communities exposed to long-term metal and radionuclide contamination is unknown, and their potential role in immobilizing metals and radionuclides remains poorly characterized.…”