Experimental redox transformations of uranium phosphate minerals and mononuclear species in a contaminated wetland

Lucie Stetten (1, 2) , Pierre Lefebvre (1) , Pierre Le Pape (1) , Arnaud Mangeret (2) , Pascale Blanchart (3) , Pauline Merrot (1) , Jessica Brest (1) , Anthony Julien (2) , John R Bargar (4) , Charlotte Cazala (2) , Guillaume Morin (1)

1 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie
2 IRSN/PSE-ENV/SEDRE/LELI - Laboratoire de recherche sur le devenir des pollutions de sites radioactifs
3 IRSN/PSE-ENV/SEDRE/USDR - Unité d'expertise des sites et des déchets radioactifs
4 SSRL SLAC - Stanford Synchrotron Radiation Lightsource

Reducing conditions and high organic carbon content make wetlands favorable to uranium (U) sequestration. However, such environments are subjected to water-table fluctuations that could impact the redox behavior of U and its mobility. Our previous study on U speciation in a contaminated wetland has suggested a major role of water-table redox fluctuations in the redistribution of U from U(IV)-phosphate minerals to organic U(VI) and U(IV) mononuclear species. Here, we investigate the mechanisms of these putative processes by mimicking drying or flooding periods via laboratory incubations of wetland samples. LCF-XANES and EXAFS analyses show the total oxidation/reduction of U(IV)/U(VI)-mononuclear species after 20 days of oxic/anoxic incubation, whereas U-phosphate minerals are partly oxidized/reduced. SEM-EDXS combined with μ-XRF and μ-XANES analyses suggest that autunite Ca(UO2)2(PO4)2⋅11H2O is reduced into lermontovite U(PO4)(OH)⋅H2O, whereas oxidized ningyoite CaU(PO4)2⋅2H2O is locally dissolved. The release of U from this latter process is observed to be limited by U(VI) adsorption to the soil matrix and further re-reduction into mononuclear U(IV) upon anoxic cycling. Analysis of incubation waters show, however, that dissolved organic carbon enhances U solubilization even under anoxic conditions. This study brings important information that help to assess the long-term stability of U in seasonally saturated organic-rich contaminated environments.

Domaines: Sciences de l'environnement; Planète et Univers [physics]

Lucie Stetten, Pierre Lefebvre, Pierre Le Pape, Arnaud Mangeret, Pascale Blanchart, et al.. Experimental redox transformations of uranium phosphate minerals and mononuclear species in a contaminated wetland. Journal of Hazardous Materials, 2020, 384, pp.121362. ⟨10.1016/j.jhazmat.2019.121362⟩. ⟨hal-02343440⟩