Iron (Fe) is a highly abundant element in the lithosphere, and its biogeochemistry in many aquatic and terrestrial environments is driven largely by microbial activity. Fe(II)-bearing minerals (<i>e.g</i>., green rust, magnetite, siderite, clays, etc.) provide a reservoir of reducing capacity in many subsurface environments that may contribute to the reduction of redox active contaminants such as uranium, which often exists as U(VI) and U(IV) and has, on occasion, been observed as U(V). We have examined the interactions between U(VI) and many different Fe(II)-containing minerals. Using synchrotron-based techniques including x-ray absorption spectroscopy and x-ray fluorescence imaging, coupled with wet chemical- and genomic-based approaches, we have sought to understand the interactions among biotic and abiotic constituents of soils and sediments and contaminants such as uranium. In recent work we have investigated the interaction of U(VI) with reduced Fe-containing clay minerals (SWy-2, NAu-1, NAu-2). Results indicate that these clay minerals are able to transfer electrons to aqueous U(VI). In instances where there is stoichiometric excess of Fe(II) in the system, U(VI) is reduced to U(IV) only at high solids loadings, whereas a mixed-valence U(V)-U(VI) oxide is stabilized at lower loadings. By varying the amounts of reductant in the system we find that the transformation of U(VI) to the U(V) or the U(IV) state is controlled by the reducing capacity. These results and results from previous examinations of uranium interactions with reduced iron oxides have enabled us to develop a model to explain the distribution of U(IV) between uraninite and adsorbed U(IV) species which is dependent upon mineral surface site ability. Past results, our most recent results examining uranium interactions with reduced clays, and our model will be presented.

Redox dynamic iron mineralogy and its effect on uranium transformations

255th National Meeting (2018)

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