The effect of Si and Al concentrations on the removal of U(VI) in the alkaline conditions created by NH3 gas |
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| Institution: | 1. Applied Research Center, Florida International University, 10555 W. Flagler Street, Miami, FL 33174, USA;2. Pacific Northwest National Laboratory, PO Box 999, K3-62, Richland, WA 99352, USA;1. Montana State University, Department of Chemical and Biological Engineering, Bozeman, MT 59717, USA;2. Montana State University, Center for Biofilm Engineering, Bozeman, MT 59717, USA;1. Department of Chemistry, University of Helsinki, P.O. Box 55, 00014, Finland;2. AMPHOS 21 CONSULTING S.L, Passeig de Garcia i Fària, 49-51 08019 Barcelona, Spain;3. Posiva Oy, Finland;4. Nagra (National Cooperative for the Disposal of Radioactive Waste), Wettingen, Switzerland;1. Department of Geosciences and Geography, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland;2. Posiva Oy, Olkiluoto, 27160 Eurajoki, Finland;3. Department of Geosciences, Swedish Museum of Natural History, P.O. Box 50 007, SE-10405 Stockholm, Sweden;1. Department of Environmental Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, PR China;2. Department of Civil Engineering, Huali College Guangdong University of Technology, Guangdong 511325, PR China;1. Institute for Crop Science and Resource Conservation (INRES) – Soil Science and Soil Ecology, University of Bonn, Nussallee 13, 53115, Bonn, Germany;2. Institute for Bio- and Geosciences – IBG-3: Agrosphere, Forschungszentrum Jülich GmbH, 52425, Jülich, Germany;3. Soil Science and Soil Protection, Martin Luther University Halle-Wittenberg, von Seckendorff-Platz 3, 06120, Halle (Saale), Germany |
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| Abstract: | Remediation of uranium in the deep unsaturated zone is a challenging task, especially in the presence of oxygenated, high-carbonate alkalinity soil and pore water composition typical for arid and semi-arid environments of the western regions of the U.S. This study evaluates the effect of various pore water constituencies on changes of uranium concentrations in alkaline conditions, created in the presence of reactive gases such as NH3 to effectively mitigate uranium contamination in the vadose zone sediments. This contaminant is a potential source for groundwater pollution through slow infiltration of soluble and highly mobile uranium species towards the water table. The objective of this research was to evaluate uranium sequestration efficiencies in the alkaline synthetic pore water solutions prepared in a broad range of Si, Al, and bicarbonate concentrations typically present in field systems of the western U.S. regions and identify solid uranium-bearing phases that result from ammonia gas treatment. In previous studies (Szecsody et al. 2012; Zhong et al. 2015), although uranium mobility was greatly decreased, solid phases could not be identified at the low uranium concentrations in field-contaminated sediments. The chemical composition of the synthetic pore water used in the experiments varied for silica (5–250 mM), Al3+ (2.8 or 5 mM), HCO3? (0–100 mM) and U(VI) (0.0021–0.0084 mM) in the solution mixture. Experiment results suggested that solutions with Si concentrations higher than 50 mM exhibited greater removal efficiencies of U(VI). Solutions with higher concentrations of bicarbonate also exhibited greater removal efficiencies for Si, Al, and U(VI). Overall, the silica polymerization reaction leading to the formation of Si gel correlated with the removal of U(VI), Si, and Al from the solution. If no Si polymerization was observed, there was no U removal from the supernatant solution. Speciation modeling indicated that the dominant uranium species in the presence of bicarbonate were anionic uranyl carbonate complexes (UO2(CO3)2?2 and UO2(CO3)3?4) and in the absence of bicarbonate in the solution, U(VI) major species appeared as uranyl-hydroxide (UO2(OH)3? and UO2(OH)4?2) species. The model also predicted the formation of uranium solid phases. Uranyl carbonates as rutherfordine UO2CO3], cejkaite Na4(UO2)(CO3)3] and hydrated uranyl silicate phases as Na-boltwoodite Na(UO2)(SiO4)·1.5H2O] were anticipated for most of the synthetic pore water compositions amended from medium (2.9 mM) to high (100 mM) bicarbonate concentrations. |
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| Keywords: | Uranium Silica Ammonia gas Vadose zone Removal efficiency |
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