Mobile uranium(IV)-bearing colloids in a mining-impacted wetland

Uranium is known to accumulate in wetland soils, where the precipitation of the sparingly soluble U(IV) mineral uraninite (UO2) under reducing conditions is considered a promising strategy for U immobilization. Here, we investigate the mobility of U in a mining impacted wetland in France that exhibited locations harboring U concentrations of up to 4,000 ppm. A distinct release of U into the stream passing through the wetland is observable. We examine soil and porewater composition as a function of depth at two U hotspots to assess the geochemical conditions leading to this release. The analyses show that U is present in soil as a non-crystalline U(IV) species sorbed onto amorphous Fe-Al-P-Si aggregates through phosphate groups, and that high U(IV) concentrations in porewater are due to the association of U with Fe- and organic matter-containing colloids. These results show that tetravalent U in soil is labile and releases U(IV) to form mobile colloids that ultimately result release of U into the stream. This is the first report of mobile U(IV) colloids in the environment and strongly brings into question the common assumption that U is immobile when present in a tetravalent oxidation state.