Organic complexation of U(VI) in reducing soils at a natural analogue site : implications for uranium transport

Fuller, Adam J. and Leary, Peter and Gray, Neil D. and Davies, Helena S. and Mosselmans, J.Frederick W. and Cox, Filipa and Robinson, Clare H. and Pittman, Jon K. and McCann, Clare M. and Muir, Michael and Graham, Margaret C. and Utsunomiya, Satoshi and Bower, William R. and Morris, Katherine and Shaw, Samuel and Bots, Pieter and Livens, Francis R. and Law, Gareth T. W. (2020) Organic complexation of U(VI) in reducing soils at a natural analogue site : implications for uranium transport. Chemosphere, 254. 126859. ISSN 0045-6535

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    Abstract

    Understanding the long-term fate, stability, and bioavailability of uranium (U) in the environment is important for the management of nuclear legacy sites and radioactive wastes. Analysis of U behavior at natural analogue sites permits evaluation of U biogeochemistry under conditions more representative of long-term equilibrium. Here, we have used bulk geochemical and microbial community analysis of soils, coupled with X-ray absorption spectroscopy and µ-focus X-ray fluorescence mapping, to gain a mechanistic understanding of the fate of U transported into an organic-rich soil from a pitchblende vein at the UK Needle’s Eye Natural Analogue site. U is highly enriched in the Needle’s Eye soils (~1600 mg kg-1). We show that this enrichment is largely controlled by U(VI) complexation with soil organic matter and not U(VI) bioreduction. Instead, organic-associated U(VI) seems to remain stable under microbially-mediated Fe(III)-reducing conditions. U(IV) (as non-crystalline U(IV)) was only observed at greater depths at the site (>25 cm); the soil here was comparatively mineral-rich, organic-poor, and sulfate-reducing / methanogenic. Furthermore, nanocrystalline UO2, an alternative product of U(VI) reduction in soils, was not observed at the site, and U did not appear to be associated with Fe-bearing minerals. Organic-rich soils appear to have the potential to impede U groundwater transport, irrespective of ambient redox conditions.