The redox state and speciation of the metalloid arsenic (As) determine its environmental fate and toxicity. Knowledge about biogeochemical processes influencing arsenic redox state is therefore necessary to understand and predict its environmental behavior. Here we quantified arsenic redox changes by pH-neutral goethite [alpha-Fe(III)OOH] mineral suspensions amended with Fe(II) using wet-chemical and synchrotron X-ray absorption (XANES) analysis. Goethite itself did not oxidize As(III) and, in contrast to thermodynamic predictions, Fe(II)-goethite systems did not reduce As(V). However, we observed rapid oxidation of As(III) to As(V) in Fe(II)-goethite systems. Mössbauer spectroscopy showed initial formation of (57)Fe-goethite after (57)Fe(II) addition plus a so far unidentified additional Fe(II) phase. No other Fe(III) phase could be detected by Mössbauer, EXAFS, SEM, XRD, or HR-TEM. This suggests that reactive Fe(III) species form as an intermediate Fe(III) phase upon Fe(II) addition and electron transfer into bulk goethite but before crystallization of the newly formed Fe(III) as goethite. In summary this study indicates that in the simultaneous presence of Fe(III) oxyhydroxides and Fe(II), as commonly observed in environments inhabited by iron-reducing microorganisms, As(III) oxidation can occur. This potentially explains the presence of As(V) in reduced groundwater aquifers.

We collected Mössbauer spectra of 57Fe(II) interacting with 56hematite (alpha-Fe2O3) over a range of Fe(II) concentrations and pH values to explore whether a sorbed Fe(II) species would form. Several models of Fe(II) sorption (e.g., surface complexation models) assume that stable, sorbed Fe(II) species form on ligand binding sites of Fe(III) oxides and other minerals. Model predictions of changes in both speciation and concentration of sorbed Fe(II) species are often invoked to explain Fe(II) sorption patterns, as well as rates of contaminant reduction and microbial respiration of Fe(III) oxides. Here we demonstrate that, at low Fe(II) concentrations, sorbed Fe(II) species are transient and quickly undergo interfacial electron transfer with structural Fe(III) in hematite. At higher Fe(II) concentrations, however, we observe the formation of a stable, sorbed Fe(II) phase on hematite that we believe to be the first spectroscopic confirmation for a sorbed Fe(II) phase forming on an iron oxide. Low-temperature Mössbauer spectra suggest that the sorbed Fe(II) phase contains varying degrees of Fe(II)-Fe(II) interaction and likely contains a mixture of adsorbed Fe(II) species and surface precipitated Fe(OH)2(s). The transition from Fe(II)-Fe(III) interfacial electron transfer to formation of a stable, sorbed Fe(II) phase coincides with the macroscopically observed change in isotherm slope, as well as the estimated surface site saturation suggesting that the finite capacity for interfacial electron transfer is influenced by surface properties. The spectroscopic demonstration of two distinctly different sorption endpoints, that is an Fe(III) coating formed from electron transfer or a stable, sorbed Fe(II) phase, challenges us to reconsider our traditional interpretations and modeling of Fe(II) sorption behavior (as well as, we would argue, of any other redox active sorbate-sorbent couple).

RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), a nitramine explosive, is often found as a subsurface contaminant at military installations. Though biological transformations of RDX are often reported, abiotic studies in a defined medium are uncommon. The work reported here was initiated to investigate the transformation of RDX by ferrous iron (Fe(II)) associated with a mineral surface. RDX is transformed by Fe(II) in aqueous suspensions of magnetite (Fe3O4). Negligible transformation of RDX occurred when it was exposed to Fe(II) or magnetite alone. The sequential nitroso reduction products (MNX, DNX, and TNX) were observed as intermediates. NH4+, N2O, and HCHO were stable products of the transformation. Experiments with radiolabeled RDX indicate that 90% of the carbon end products remained in solution and that negligible mineralization occurred. Rates of RDX transformation measured for a range of initial Fe(II) concentrations and solution pH values indicate that greater amounts of adsorbed Fe(II) result in faster transformation rates. As pH increases, more Fe(II) adsorbs and k(obs) increases. The degradation of RDX by Fe(II)-magnetite suspensions indicates a possible remedial option that could be employed in natural and engineered environments where iron oxides are abundant and ferrous iron is present.

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Electron exchange between aqueous Fe(II) and structural Fe(III) in iron oxides and oxyhydroxides is important for understanding degradation of environmental pollutants through its apparent constitutive role underlying highly reactive "sorbed Fe(II)" and by catalyzing phase interconversion among these minerals. Although a mechanistic understanding of relationships between interfacial Fe(II)(ads)-Fe(III)(oxide) electron transfer, bulk electron conduction, Fe(II) release, and phase transformation behavior is emerging, much remains unclear, in part due to poorly interconnected investigations. The focus of this study is on reconciling two mutually similar observations of Fe(II)-catalyzed hematite growth documented spectroscopically and microscopically under substantially different chemical conditions. Here, we employ iron isotopic labeling to demonstrate that hematite grown on the (001) surface in Fe(II)-oxalate solution at pH 2.10 and 348 K has magnetic properties that closely correspond to those of hematite grown in Fe(II) solution at pH 7.4 and room temperature. The temperature evolution and extent of the Morin transition displayed in these two materials strongly suggest a mechanistic link involving trace structural Fe(II) incorporation into the growing hematite. Our findings indicate that Fe(II) catalyzed growth of hematite on hematite can occur under environmentally relevant conditions and may be due to bulk electron conduction previously demonstrated for hematite single crystals.