The molecular form of reduced uranium in the subsurface is affected by common sediment constituents.
Clay minerals are ubiquitous native components of sediments and soils, as well as a material used in the engineered barriers of spent nuclear fuel storage facilities. A recent study examined the molecular form of uranium (IV) in the presence of montmorillonite clays and found that they can inhibit the predicted precipitation of the mineral uraninite.
The effect of environmental surfaces on the form of reduced uranium is currently not accounted for in computational models. This study used state-of-the-art spectroscopy techniques to provide the molecular-level information needed for accurate prediction of uranium transport in subsurface environments.
The mobility of uranium in the subsurface depends strongly on its oxidation state, with U(IV) being significantly less soluble than U(VI). However, solubility also depends on the molecular form of the contaminant, which can be affected by adsorption to the surface of minerals, bacterial membranes and other constituents in the surrounding environment. A team of scientists led by Argonne National Laboratory examined the ability of montmorillonite clay minerals to adsorb U(IV) resulting from the reduction of U(VI), and compared it to that of iron and titanium oxide surfaces. The valence and molecular structure of uranium was tracked by synchrotron X-ray absorption spectroscopy. The team found that at low clay surface:U ratios the reduction of U(VI) in the presence of SYn-1 montmorillonite leads to the formation of the mineral uraninite (UO2). However, at high clay surface:U ratios (more typical of environmental conditions) a significant fraction of the resulting U(IV) is present as adsorbed U(IV) ions (up to 50% of total U). The threshold U(IV) surface coverage above which uraninite formation begins was determined to be significantly lower for montmorillonite than for iron or titanium oxides, suggesting that metal oxides may play a more important role than clay minerals in stabilizing the nonuraninite species observed in natural sediments.
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Program Officer, U.S. DOE Office of Science
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Kenneth M. Kemner
Argonne National Laboratory
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This research is part of the Subsurface Science Scientific Focus Area at Argonne National Laboratory (ANL), which is supported by the Subsurface Biogeochemical Research Program of the Office of Biological and Environmental Research, within the U.S. Department of Energy DOE) Office of Science. Use of the Electron Microscopy Center and the Advanced Photon Source at ANL is supported by the Office of Basic Energy Sciences within the DOE Office of Science. MRCAT/EnviroCAT operations are supported by DOE and the MRCAT/EnviroCAT member institutions. All work at ANL was under Contract DE-AC02-06CH11357.
Boyanov, M. I., D. E. Latta, M. M. Scherer, E. J. O’Loughlin, and K. M. Kemner. “Surface area effects on the reduction of UVI in the presence of synthetic montmorillonite.” Chemical Geology 464, 110–17 (2017). [DOI:10.1016/j.chemgeo.2016.12.016].
Subsurface Science Scientific Focus Area at Argonne National Laboratory
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