Potential for U sequestration with select minerals and sediments via base treatment

Area of Science:

• Environmental Science
• Geochemistry
• Remediation Technologies

Background:

• Heavy metal contamination, particularly uranium, poses risks to groundwater and soil ecosystems.
• Temporary base treatment offers a potential in-situ remediation strategy for immobilizing contaminants.
• Ammonia gas injection is a novel approach for pH manipulation in vadose zones, avoiding liquid flux.

Purpose of the Study:

• To evaluate the effectiveness of temporary base treatments (NaOH, NH4OH, NH3 gas) on uranium mobility.
• To investigate the impact of these treatments on mineral dissolution and secondary phase precipitation.
• To assess the suitability of ammonia gas for vadose zone uranium remediation.

Main Methods:

• Laboratory batch experiments were conducted using Hanford site minerals and natural sediments.
• Uranium mobility was assessed following treatments with sodium hydroxide, ammonium hydroxide, and ammonia gas.
• Mineral dissolution and secondary phase formation were analyzed using geochemical techniques.

Main Results:

• Base treatments significantly increased uranium partitioning to the solid phase by several orders of magnitude.
• Elevated pH facilitated uranium precipitation and co-precipitation, influenced by dissolved calcium and carbonate.
• Incongruent mineral dissolution led to the formation of secondary mineral phases that sequestered uranium.

Conclusions:

• Temporary base treatments, especially ammonia gas, are effective in reducing uranium mobility in soils.
• The formation of secondary uranium-bearing minerals is a key mechanism for contaminant immobilization.
• Ammonia gas injection presents a viable, low-flux method for in-situ uranium remediation in vadose zones.