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

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 Ucontaining 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 2 ) 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.

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

“…Uranium mining operations and legacies have been shown to be responsible for local increases of U contaminations in surrounding surface environments. For instance, U concentrations exceeding the geochemical background were found in aquifer sediments [4], lacustrine sediments [5], soils [6] and wetlands [7][8][9][10] in the vicinity of former U mining sites.…”

Influence of Surface Compositions on the Reactivity of Pyrite toward Aqueous U(VI)

“…This study shows that surface S 2- 66,67 In comparison, the pyrite precipitating at ambient temperature in flooded soils and sediments, and at former mining and processing sites is usually characterized by poorer crystallinity, smaller size, more lattice defects, more impurity doping, and higher solubility, and thus is expected to be more reactive towards U(VI). 13 This kind of pyrite is able to reduce U(VI) and accumulate U effectively in the surroundings if the soil/sediment can stay reducing (i.e. depending on the persistence of redox oscillations 68 ), which is evidenced by the widespread co-existence of pyrites and U precipitates in sediments.…”

Assessment of the Mode of Occurrence and Radiological Impact of Radionuclides in Nigerian Coal and Resultant Post-Combustion Coal Ash Using Scanning Electron Microscopy and Gamma-Ray Spectroscopy

“…Such an apparent observation could be the result of inherent sampling bias; however, this may conversely arise from their non-existence within the coal, rather occurring encapsulated within the bulk carbonaceous material. This encapsulation would serve to partially protect the U in the coal from the prevalent oxidizing environmental conditions, which would significantly reduce its environmental mobility over oxidized (+6) species of U [33]. It is following combustion of the coal materials that such particle fragments are liberated into the coal ash material.…”