Redox Cycling of Uranium Phosphate Minerals in a Mining-Contaminated Wetland
Résumé
Reduction of uranium (VI) to low soluble uranium (IV) species in wetlands is expected to limit the transfers of this toxic element to downstream waterways. However, in such environments, long-term uranium (U) scavenging may beperturbed by hydrological and redox fluctuations. Here, based on field [1] and laboratory investigations, we detailthe mechanisms of uranium redistribution from U-phosphate minerals in a heavily contaminated wetland from Brittany,France.Using U LIII-edge (micro-) X-ray absorption spectroscopy, electron microscopy and geochemical analyses, we show that uranium released by the oxidative dissolution of U(IV)-phosphate minerals, especially ningyoite CaU(PO4)2·2H2O, is rapidly converted to organicbound mononuclear U(VI) species. These latter can be then reduced to organic-bound U(IV) species under watersaturated conditions [1]. Moreover, oxic and anoxic incubations of soil samples reveal that specific U-phosphate minerals, autunite Ca(UO2)2(PO4)2·11H2O and lermontovite U(PO4)(OH)·H2O, are the most resistant uranium species to redox cycling occurring in the studied soils.Altogether, the results of this study bring important informations to assess the long-term stability of uranium inseasonally saturated organic-rich mining-impacted environments.[1] Stetten et al. (2018) Environ. Sci. Technol. 52 (22), 13099–13109