Uranium storage mechanisms in wet-dry redox cycled sediments

Area of Science:

• Environmental Science
• Geochemistry
• Microbial Ecology

Background:

• Biogeochemical redox processes in sediments are sensitive to water cycle variations.
• Naturally Reduced Zones (NRZs) accumulate uranium (U) under reducing conditions.
• Hydrological fluctuations can alter sediment redox states and contaminant mobility.

Purpose of the Study:

• To investigate the impact of redox cycling on uranium speciation and solubility in shallow NRZ-like sediments.
• To characterize uranium behavior in sediments experiencing seasonal moisture fluctuations.

Main Methods:

• Tracking of uranium, iron, and sulfur oxidation states and speciation.
• Analysis of shallow fine-grained NRZ-like sediments at a legacy uranium site.
• Investigation of sediments with repeated redox cycling due to seasonal moisture changes.

Main Results:

• Reducing conditions are necessary for uranium accumulation in sediments.
• Uranium accumulates as both U(IV) and U(VI) in roughly equal proportions, contrary to expectations of predominantly U(IV).
• High abundances of U(VI) in transiently saturated sediments suggest redox cycling promotes its accumulation.

Conclusions:

• Redox oscillations driven by water table fluctuations and evapotranspiration promote the conversion of U(IV) to immobile U(VI).
• Uranium accumulates in a redox-resistant form (U(VI)) in these dynamic environments.
• This challenges the notion that reduced uranium is always re-oxidized and mobilized.