Fe(II)-Fe(III) Electron Transfer at Fe-containing Clay Minerals and its Effect on Hg(II) and Cr(VI) Transformation

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The Fe(II)/Fe(III) redox couple plays an important role in biogeochemical cycles and for the dynamics of contaminant transformation in soils and aquifers. Despite the abundance of clay minerals in subsurface environments, redox processes of structural Fe in clay minerals as well as their impact on the mobility and toxicity of inorganic contaminants are not well understood. To address this knowledge gap, the proposed research aims to determine whether electron transfer between aqueous Fe(II), a reductant abundant in natural anaerobic environments, and structural Fe(III) in clay minerals occurs and how this reaction influences the transformation of the inorganic contaminants Hg(II) and Cr(VI). Isotope selective 57Fe-Mössbauer spectroscopy and infrared spectroscopy will be used to monitor the oxidation state and the structural environment of Fe in clay minerals and the Fe(III) product will be characterized with microscopic and spectroscopic methods. To gain a better understanding of inorganic contaminant reduction by clay mineral bound Fe(II), Hg(II) and Cr(VI) were chosen as exemplary contaminants, exhibiting complementary properties with regard to accessing the different surfaces of clay minerals. The application of stable isotope fractionation measurement to the non-traditional elements Hg and Cr will generate new insights into processes at the clay mineral-contaminant interface and will add to the further development of these novel isotope systems as source and process tracer. The findings will add to understanding the role of clay mineral bound Fe as a renewable source of redox equivalents in the environment and will improve the ability to assess the fate of contaminants, both inorganic and organic.