Metaschoepite Dissolution in Sediment Column Systems-Implications for Uranium Speciation and Transport

Area of Science:

• Geochemistry
• Environmental Science
• Nuclear Waste Management

Background:

• Metaschoepite (UO3·nH2O) is a common uranium mineral in contaminated sites.
• Understanding uranium (U) mobility from metaschoepite is crucial for waste management strategies.
• Shallow or deep disposal of metaschoepite-bearing wastes necessitates knowledge of U dissolution and fate.

Purpose of the Study:

• To investigate the dissolution behavior and transformation of metaschoepite under varying redox conditions.
• To track the migration and speciation of uranium released from metaschoepite in simulated sediment/groundwater systems.
• To assess the impact of microbial sulfate reduction on uranium mobility and colloid formation.

Main Methods:

• Emplacement of metaschoepite particles in controlled sediment/groundwater columns (oxic and anoxic conditions).
• Monitoring of solution chemistry and uranium speciation using fluorescence spectroscopy and X-ray absorption spectroscopy (XAS).
• Analysis of solid-phase uranium using transmission electron microscopy (TEM) and XAS.

Main Results:

• Extensive uranium migration observed in oxic columns, with uranyl ions (UO2^2+) detected in effluents.
• Significant formation of uranium dioxide (UO2)-like colloids (>60% of added U) in anoxic, sulfate-reducing columns.
• Identification of U(VI), noncrystalline U(IV), and biogenic UO2 in reduced sediments, with UO2 increasing over time.

Conclusions:

• Metaschoepite dissolution under reducing conditions can lead to the formation of mobile U(IV) colloids.
• Microbially mediated sulfate reduction plays a key role in transforming U(VI) solids and influencing uranium mobility.
• The study highlights the complex biogeochemistry of uranium in dynamic subsurface environments and its implications for contaminant transport.