Past nuclear weapon production activities have left a significant legacy of uranium (U) contamination in the vadose zone (VZ) of the Department of Energy (DOE) Hanford Site. This U is a source of groundwater (GW) contamination. There is a concern that elevated U concentration would slowly infiltrate through the VZ, reach the GW water table, and then end up in nearby rivers and lakes. Remediation of U-contaminated low moisture content soil is a challenging task considering the VZ depth, where contamination is found between 70 and 100 m below the ground surface, and the formation of highly soluble and stable CaUO2CO3 complexes is influenced by Hanford’s soil rich in carbonate.
Injection of reactive gasses (e.g., NH3) is a promising technology to decrease U migration in through the VZ. The NH3 injection creates alkaline conditions that would alter the pore water chemistry (e.g., dissolving some aluminosilicates). Over time as the pH neutralizes, U(VI) could precipitate as uranyl mineral (e.g., Na-boltwoodite). Also, the dissolved U(VI) could be incorporated into the structure of some mineral phases or be coated by non-U minerals. These chemical reactions could control the U(VI) mobility to the GW. However, there is a lack of knowledge on how the VZ pore water constituents (e.g., Si, Al3+, HCO3-, and Ca2+) would affect U(VI) removal/precipitation in alkaline conditions.
This study quantified the role of the major pore water constituents on the U(VI) removal and evaluated the uranyl minerals that could precipitate from a variety of SPW solutions. Results showed that the percentage of U(VI) removal was controlled by Si/Al ratios and Ca2+ concentration regardless of HCO3- concentration tested. XRD revealed the presence of uranyl minerals by analyzing precipitates formed from SPW solutions, but none of them were identified as uranyl silicates as expected from speciation modeling. The SEM images displayed dense amorphous regions high in silica content, where EDS elemental analysis unveiled higher U atomic percentage in some samples. U(VI) silicate and carbonate minerals were predicted by the speciation modeling.