The fission of uranium (U) for plutonium production was a major activity at the U.S. Department of Energy’s (DOE) Hanford Site in Washington State during World War II and Cold War. This endeavor resulted in the generation of over two million liters of high-level radioactive waste, most of which still remains in 177 underground storage tanks. Due to the improper storage and aging of these tanks in addition to other waste releases across the Site, approximately 200,000 kg of U have been released into the vadose zone. The objective of this study was to determine whether the application of the reactive gas, ammonia (NH3), could be effective for sequestration of U in vadose zone conditions such as those at the Hanford Site.
The goal of this novel technique is to elevate the pH and induce mineral dissolution. As the NH3 dissipates and the pH returns to neutral conditions, adsorption and co-precipitation processes are expected to immobilize U. The targeted mineral dissolution and secondary precipitate formation processes are not well understood at these conditions including their impact on U behavior.
The experimental results suggest that, as a result of pH manipulation with NH3, investigated minerals (illite, muscovite, and montmorillonite) undergo incongruent dissolution. In addition, several analytical techniques were applied to compare ammonia-treated and circumneutral pH-treated minerals. Characterization studies showed that physicochemical transformations occurred, such as recrystallization of mineral edges and particle size and surface area increase. These behaviors are indicative of secondary precipitate formation, which was confirmed by comparisons of Al:Si ratios in solution and the solid phase, suggesting U sequestration. Furthermore, U distribution calculations between the solid and liquid phases indicate a significant increase in solid phase U with treatment, while geochemical software modeling provided a way to predict U species and secondary mineral phases upon alkaline treatment.
These findings show the scientific community that NH3 gas injection is an effective technology to decrease the mobility of the uranyl ion. This technology may be particularly valuable to unsaturated areas where contamination remedies are needed in situ without the addition of liquid amendments
