Heterogeneous redox chemistry on iron-rich clay minerals can control the mobility and bioavailability of nutrients and contaminants in soils. Structural iron Fe(II)/Fe(III) couple in the octahedral sheet of clay minerals can directly participate in electron transfer reactions. To study the controls on the reactivity of clay structural Fe(II)/Fe(III) couple, we use natural Fe-rich nontronite NAu-1 (Clay Minerals Society) and synthetic Fe-phyllosilicate produced in our laboratory. We characterized the synthetic Fe-phyllosilicate using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, Small Angle X-ray Scattering (SAXS), and Mössbauer spectroscopy. The NAu-1 and synthetic Fe-phyllosilicate have similar crystallographic structure and particle geometry, and the synthetic substrate has a lower degree of crystallinity and smaller average particle size as compared to NAu-1. Based on the analysis of its Mössbauer spectra, the Fe(II) content in the synthetic Fe-phyllosilicate is on the order of 1-3% of total Fe, and this material is free of Fe-oxide impurities. We performed preliminary batch experiments to screen the redox transformations in the presence of NAu-1 clay and synthetic Fe-phyllosilicate. These substrates were reacted with redox active elements - arsenic (As), chromium (Cr), and selenium (Se) in separate batches. The results indicate that the reactivity depends on the Fe(II)/Fe(III) ratio in clay structure and the identity and speciation of the “probe” element (e.g. cation CrOH2+ vs. neutral H3AsO30 vs. anion SeO32-). Fully oxidized NAu-1 readily oxidizes Cr(III); however, it does not oxidize As(III). Partially reduced (dithionite-treated) NAu-1 and synthetic Fe-phyllosilicate promote the oxidation of As(III), but not Se(IV), under both oxic and anoxic conditions. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Mössbauer measurements and data analysis were performed using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Controls on the reactivity of clay structural Fe(II)/Fe(III) redox couple

249th National Meeting (2015)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.