Heterogeneous oxidation of Fe(II) on iron oxides in aqueous systems: Identification and controls of Fe(III) product formation

The aqueous Fe(II)-oxide Fe(III) system is a reactant for many classes of redox sensitive compounds via an interfacial Fe(II) sorption and electron transfer process. The poorly soluble Fe(III) products formed as a result of contaminant reduction and Fe(II) oxidation on iron oxides may be capable of modifying iron oxide surfaces and affecting subsequent reduction rates of contaminants such as halogenated ethenes or nitroaromatic compounds. The scope of this study was to identify the secondary Fe(III) mineral phases formed after Fe(II) oxidation on common iron oxides during heterogeneous contaminant reduction by directly targeting the secondary minerals using Mossbauer-active isotopes. Fe(III) mineral characterization was performed using Fe-57-Mossbauer spectroscopy, mu-X-ray diffraction, and electron microscopy after oxidation of dissolved Fe-57(II) using nitrobenzenes as a model oxidant in pH-buffered suspensions of (56)hematite, (56)goethite, (56)magnetite, and (56)maghemite. Mossbauer spectra confirmed sorbed Fe-57(II) becomes oxidized by the parent Fe-56(III)-oxide sorbent and assimilated as the sorbent oxide prior to any nitrobenzene reduction, consistent with several reports in the literature. In addition to oxide sorbent growth, Fe(II) sorption and oxidation by nitrobenzene result also in the formation of secondary Fe(III) minerals. Goethite formed on three hematite morphologies (rhombohedra, needles, and hexagonal platelets), and acicular needle shapes typical of goethite appeared on the micron-sized hexagonal platelets, at times aligned in 60 degrees orientations on (001) faces. The proportion of goethite formation on the three hematites was linked to number of surface sites. Only goethite was observed to form on a goethite sorbent. In contrast, lepidocrocite was observed to form on magnetite and maghemite sorbents (consistent with homogeneous Fe(II) oxidation by O-2) and assumed spherulite morphologies. All secondary Fe(III) phases were confirmed within mu-X-ray diffraction patterns. On hematite, the directed formation of goethite as opposed to lepidocrocite suggests hematite may possess a templating ability for the alpha-FeOOH atom arrangement as opposed to gamma-FeOOH. The initiation of all secondary Fe-oxide formations occurred after four to six equivalents of monolayer coverage on the supporting mineral sorbent. Overall, Fe(III) product identity formed during heterogeneous Fe(II) oxidation appears to be governed mainly by the identity of the underlying sorbent and partly by the amount of available surface sites. (C) 2012 Elsevier Ltd. All rights reserved.