Geochemical Control on Uranium(IV) Mobility in a Mining-Impacted Wetland

Wetlands often act as sinks for uranium and other trace elements. Our previous work at a mining-impacted wetland in France showed that a labile noncrystalline U­(IV) species consisting of U­(IV) bound to Al–P–Fe−Si aggregates was predominant in the soil at locations exhibiting a U-containing clay-rich layer within the top 30 cm. Additionally, in the porewater, the association of U­(IV) with Fe­(II) and organic matter colloids significantly increased U­(IV) mobility in the wetland. In the present study, within the same wetland, we further demonstrate that the speciation of U at a location not impacted by the clay-rich layer is a different noncrystalline U­(IV) species, consisting of U­(IV) bound to organic matter in soil. We also show that the clay-poor location includes an abundant sulfate supply and active microbial sulfate reduction that induce substantial pyrite (FeS₂) precipitation. As a result, Fe­(II) concentrations in the porewater are much lower than those at clay-impacted zones. U porewater concentrations (0.02–0.26 μM) are also considerably lower than those at the clay-impacted locations (0.21–3.4 μM) resulting in minimal U mobility. In both cases, soil-associated U represents more than 99% of U in the wetland. We conclude that the low U mobility reported at clay-poor locations is due to the limited association of Fe­(II) with organic matter colloids in porewater and/or higher stability of the noncrystalline U­(IV) species in soil at those locations