Geochemical
Control on Uranium(IV) Mobility in a Mining-Impacted
Wetland
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Abstract
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 (FeS<sub>2</sub>) 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