The long-term fate of uranium-contaminated sediments, especially downstream former mining areas, is a widespread environmental challenge. Essential for their management is the proper understanding of uranium (U) immobilization mechanisms in reducing environments. In particular, the long-term behavior of noncrystalline U(IV) species and their possible evolution to more stable phases in subsurface conditions is poorly documented, which limits our ability to predict U long-term geochemical reactivity. Here, we report direct evidence for the evolution of U speciation over 3,300 y in naturally highly U-enriched sediments (350–760 µg ⋅ g−1 U) from Lake Nègre (Mercantour Massif, Mediterranean Alps, France) by combining U isotopic data (δ238U and (234U/238U)) with U L3-edge X-ray absorption fine structure spectroscopy. Constant isotopic ratios over the entire sediment core indicate stable U sources and accumulation modes, allowing for determination of the impact of aging on U speciation. We demonstrate that, after sediment deposition, mononuclear U(IV) species associated with organic matter transformed into authigenic polymeric U(IV)–silica species that might have partially converted to a nanocrystalline coffinite (UIVSiO4·nH2O)-like phase. This diagenetic transformation occurred in less than 700 y and is consistent with the high silica availability of sediments in which diatoms are abundant. It also yields consistency with laboratory studies that proposed the formation of colloidal polynuclear U(IV)–silica species, as precursors for coffinite formation. However, the incomplete transformation observed here only slightly reduces the potential lability of U, which could have important implications to evaluate the long-term management of U-contaminated sediments and, by extension, of U-bearing wastes in silica-rich subsurface environments.

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Determination of uranium isotope ratios is of great expedience for assessing its origin in environmental samples. In particular, the 236 U/ 238 U isotope ratio provides a powerful tool to discriminate between the different sources of uranium (uranium ore, geochemical background and uranium from anthropogenic activities). However, in the environment, this ratio is typically below 10 -8 . This low abundance of 236 U and the presence in large excess of major isotopes (mainly 238 U and 235 U) complicates the accurate detection of 236 U signal by mass spectrometry and thus highly sensitive analytical instruments providing high abundance sensitivity are required. This work pushes the limits of triple quadrupole-based ICP-MS technology for accurate detection of 236 U/ 238 U isotope ratios down to 10 -10 , which is so far mainly achievable by AMS. Coupled with an efficient desolvating module, N 2 O was used as the reaction gas in the collision reaction cell of the ICP-MS/MS. This configuration allows a significant decrease of the uranium polyatomic interferences ( 235 UH + ions) and an accurate determination of low 236 U/ 238 U isotope ratios. This new methodology was successfully validated through measurements of certified reference material from 10 -7 to 10 -9 and then through comparisons with AMS measurement results for ratios down to 10 -10 . This is the first time that 236 U/ 238 U isotope ratios as low as 10 -10 were determined by ICP-MS/MS. The possibility of measuring low 236 U/ 238 U isotope ratios can offer a large variety of geochemical applications in particular for the determination of uranium sources in the environment. ASSOCIATED CONTENT Supporting InformationDemonstration of Eq. 1 and Eq. 2. and a figure on the influence of the 235 UH + interference on the measurement of 236 U/ 238 U isotope ratio for a given hydride formation rate.

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Sorbed U(IV) species can be major products of U(VI) reduction in natural reducing environments as sediments and waterlogged soils. These species are considered more labile than crystalline U(IV) minerals, which could potentially influence uranium migration in natural systems subjected to redox oscillations. In this study, we examined the role of oxygen and carbonate on the remobilization of uranium from lake sediments, in which ∼70% of the 150-300 ppm U is under the form of mononuclear U(IV) sorbed species. Our results show that both drying and oxic incubation only slightly increase the amount of remobilized U after 8 days, compared to anoxic drying and anoxic incubation. In contrast, the amount of remobilized U increases with the quantity of added bicarbonate even under anoxic conditions. Moreover, U L-edge XANES data show that a significant amount of the solid U(IV) is mobilized in such conditions. Thermodynamic speciation calculations based on the supernatant composition indicates the predominance of aqueous UO(CO) and, to a lesser extent, CaUO(CO) complexes. These results suggest that monomeric U(IV) species could be oxidized into aqueous U(VI) carbonate complexes even under anoxic conditions via carbonate promoted oxidative dissolution, which emphasizes the need for considering such a process when modeling U dynamics in reducing environments.

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