Heat-induced changes in speciation and extraction of uranium associated with sheet silicate minerals |
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| Institution: | 1. Department of Civil and Environmental Engineering, Ruhr-Universität Bochum, Germany;2. Geoenvironmental Research Centre, School of Engineering, Cardiff University, UK;3. Indian National Science Academy, India;1. Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing 210098, China;2. Institute for Underground Space and Geoenvironment, School of Earth Sciences and Engineering Nanjing University, Nanjing 210046, China;3. Ecole des Ponts ParisTech, Laboratoire Navier/CERMES, 6 et 8 Av. B. Pascal, F-77455 Marne-la-Vallée, Cedex 2, France;4. Institute of Geotechnical Engineering, College of Civil Engineering, Fuzhou University, Fuzhou 350108, China;1. Ruhr-Universität Bochum, Germany;2. Geoenvironmental Research Centre, School of Engineering, Cardiff University, Queens Buildings, West Groove, Newport Road, Cardiff CF24 3AA, UK;3. Indian National Science Academy, New Delhi, India;4. Indian Institute of Science Bangalore, India |
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| Abstract: | Three samples of gouge from a U-mineralised fault, and two model samples, montmorillonite and muscovite, spiked with U, were heat-treated at a range of temperatures up to 1100 °C. Mineralogical changes were followed by thermal analysis, powder XRD and electron microscopy, and U extractability was measured by extraction with NH4+. Changes in U speciation in the montmorillonite sample were followed using EXAFS spectroscopy. On heating, the minerals progressively dehydrate, dehydroxylate and eventually decompose to form new phases in a glassy matrix. In the case of montmorillonite (90% of U extractable from unheated material), U extractability increased slightly on heating to temperatures around 400 °C. Almost 50% of U was extracted from unheated muscovite, and this increased slightly by 450 °C. Above 500–600 °C, U extractability from both montmorillonite and muscovite declined to very low levels, reflecting dehydration of the uranyl ion and trapping in the new phases and glassy matrix. Uranium extractability from the natural samples was much lower in all cases (0.25–5% of the total before heating). In 2 samples, a significant increase in U extraction was associated with dehydroxylation at around 600 °C, followed by a decrease to very low levels at higher temperatures. Uranium extraction from the third natural sample, which contained X-ray amorphous U minerals, decreased steadily on heating. The results show that changes in U extraction can be related to structural and morphological changes in sheet silicate minerals. Heat treatment has potential to fix U but only if temperatures above 800 °C are reached. If only lower temperatures, in the range 400–600 °C, are used, then U extraction may increase. |
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