Mechanisms of Long-Term U Transport under Oxidizing Conditions
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Mechanisms of Long-Term U Transport under Oxidizing Conditions Takashi Murakami Department of Earth and Planetary Science, Univ. of Tokyo, Tokyo 113-0033, Japan ABSTRACT Understandings of U transport at the Earth’s surface are quite important especially for the disposal of radioactive waste and the remediation of contaminated ground water. We examined long-term U transport in and around the Koongarra U deposit, Australia, which has been subjected to weathering for the past 2 million years. The formation of saléeite [Mg(UO2)2(PO4)2•10H2O] occurred at the rim of apatite [Ca5(PO4)3(F,OH)], where the ground waters had about 200 - 400 ppb U (~10-7 mol•l-1) and were undersaturated with respect to saléeite. The laboratory experiments using apatite and U-containing solutions and Rutherford backscattering spectrometry of the apatite surface have suggested that a leached layer of about 100 nm thick is formed on dissolving apatite, saléeite is precipitated only in the leached layer by local saturation, and saléeite precipitated is added to the interface between the leached layer and solution. At lower U concentrations (~10-8 mol•l-1), U occurred as nanocrystals (20-100 nm in size) of uranyl phosphates such as metatorbernite [Cu(UO2)2(PO4)2•8H2O], scattered between and attached firmly to nanocrystals (2-50 nm in size) of goethite and hematite. A possible mechanism is that U, P, and Mg or Cu adsorbed onto ferrihydrite form nanocrystals of uranyl phosphates during crystallization of goethite and hematite from ferrihydrite. The above two mechanisms significantly lower the U concentration in the ground water that flows into the Koongarra creek, 200 m downstream from the ore deposit, where the U concentration is ~10-10 mol•l-1. We conclude that the two mechanisms of uranyl-phosphate mineralization control the long-term U transport at Koongarra. INTRODUCTION Uranium(IV) is essentially insoluble and thus immobile, but becomes unstable and oxidized to U(VI) under oxidizing conditions. Uranium(VI) is much more mobile, and dissolved U(VI) in ground water forms complex ions, depending on geochemical conditions such as pH, Eh and the presence of other dissolved ions [1]. Duff et al. [2] have summarized that the U(VI) mobility is controlled by sorption, occlusion by clay and oxide coatings, microbial uptake, precipitation, and coprecipitation. Because sorption can be the most probable mechanism to control U transport, quite a few experiments of U(VI) sorption have been carried out (e.g., [3]). On the other hand, Fe-(hydr)oxides such as goethite [FeOOH] and hematite [Fe2O3] are widely used as substrates for U(VI) sorption experiments because they are widespread at the Earth’s surface and ubiquitous in weathered rocks, soils and sediments [4]. A dominant chemical form of sorbed U(VI) is an inner-sphere, bidentate uranyl complex [3,5,6] or an inner-sphere U(VI)-
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carbonato ternary complexes in a solution equilibrium with atmospheric CO2 [7]. At higher U concentrations (more than 4x10-5 mol•l-1 of U), U mineral
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