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Diagenetic formation of uranium-silica polymers in lake sediments over 3,300 years

Abstract : The storage of large volumes of uranium (U) mine waste and consequent contaminated sites is a widespread environmental challenge. Essential for their management is the proper understanding of U immobilization mechanisms in reducing environments. In particular, the long-term behavior of recently discovered non-crystalline U(IV) species in (sub-)surface 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 years in naturally highly U-enriched sediments from Lake Nègre (Mercantour Massif, Mediterranean Alps, France) by combining U isotopic data (δ238U and (234U/238U)) with U L3-edge X-Ray Absorption Spectroscopy. Constant isotopic ratios over the entire sediment core indicate stable U sources and accumulation modes, allowing for determination of the sole impact of ageing on U speciation. We demonstrate that non-crystalline U(IV) associated to organic matter polymerizes to a nano-crystalline coffinite (UIVSiO4·nH2O)-like phase in less than 700 years. This transformation does not significantly impact U remobilization with respect to its non-crystalline precursor. Sustainable reducing conditions are thus required to allow long-term U sequestration in widely encountered organic and silica-rich environments.
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https://hal.archives-ouvertes.fr/hal-03151596
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Submitted on : Wednesday, February 24, 2021 - 9:01:10 PM
Last modification on : Wednesday, March 31, 2021 - 3:39:06 AM

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Pierre Lefebvre, Alkiviadis Gourgiotis, Arnaud Mangeret, Pierre Sabatier, Pierre Le Pape, et al.. Diagenetic formation of uranium-silica polymers in lake sediments over 3,300 years. Proceedings of the National Academy of Sciences of the United States of America , National Academy of Sciences, 2021, 118 (4), pp.e2021844118. ⟨10.1073/pnas.2021844118⟩. ⟨hal-03151596⟩

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