Redox Fluctuations and Organic Complexation Govern Uranium Redistribution from U(IV)-Phosphate Minerals in a Mining-Polluted Wetland Soil, Brittany, France

Lucie Stetten (1) , Pascale Blanchart (2) , Arnaud Mangeret (3) , Pierre Lefebvre (4) , Pierre Le Pape (4) , Jessica Brest (1) , Pauline Merrot (4) , Anthony Julien (3) , Olivier Proux (5) , Samuel Webb (6) , John R Bargar (6) , Charlotte Cazala (3) , Guillaume Morin (7)

1 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie
2 IRSN/PSE-ENV/SEDRE/USDR - Unité d'expertise des sites et des déchets radioactifs
3 IRSN/PSE-ENV/SEDRE/LELI - Laboratoire de recherche sur le devenir des pollutions de sites radioactifs
4 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie
5 OSUG - Observatoire des Sciences de l'Univers de Grenoble
6 SLAC - SLAC National Accelerator Laboratory
7 IMPMC - Institut de minéralogie et de physique des milieux condensés

Wetlands have been proposed to naturally attenuate U transfers in the environment via U complexation by organic matter and potential U reduction. However, U mobility may depend on the identity of particulate/dissolved uranium source materials and their redox sensitivity. Here, we examined the fate of uranium in a highly contaminated wetland (up to 4500 mg·kg–1 U) impacted by former mine water discharges. Bulk U LIII-EXAFS and (micro-)XANES combined with SEM-EDXS analyses of undisturbed soil cores show a sharp U redox boundary at the water table, together with a major U redistribution from U(IV)-minerals to U(VI)-organic matter complexes. Above the water table, U is fully oxidized into mono- and bidentate U(VI)-carboxyl and monodentate U(VI)-phosphoryl complexes. Minute amounts of U(VI)-phosphate minerals are also observed. Below the water table, U is fully reduced and is partitioned between U(IV)-phosphate minerals (i.e., ningyoite and a lermontovite-like phase), and bidentate U(IV)-phosphoryl and monodentate U(IV)-carboxyl complexes. Such a U redistribution from U-minerals inherited from mine water discharge deposits could result from redox cycling nearby the water table fluctuation zone. Oxidative dissolution of U(IV)-phosphate minerals could have led to U(VI)-organic matter complexation, followed by subsequent reduction into U(IV)-organic complexes. However, uranium(IV) minerals could have been preserved in permanently waterlogged soil.

Domaines: Autre; Chimie de coordination; Radiochimie

Lucie Stetten, Pascale Blanchart, Arnaud Mangeret, Pierre Lefebvre, Pierre Le Pape, et al.. Redox Fluctuations and Organic Complexation Govern Uranium Redistribution from U(IV)-Phosphate Minerals in a Mining-Polluted Wetland Soil, Brittany, France. Environmental Science and Technology, 2018, 52 (22), pp.13099-13109. ⟨10.1021/acs.est.8b03031⟩. ⟨hal-02367827⟩