Removal of U(VI) in the Alkaline Conditions Created by NH₃ Gas - 17288

Injection of ammonia (NH₃) gas in the unsaturated zone is an innovative technology to mitigate uranium contamination in the presence of oxygenated, high-carbonate, alkaline soil, and pore water composition typical for the Hanford Site. A uranium-contaminated plume is a potential source for groundwater pollution due to migration of soluble uranium species towards the water table. This study evaluates the effect of various pore water constituents such as Ca, Al, Si and bicarbonate ions on changes in uranium concentrations in alkaline conditions, created by the injection of NH₃ gas to be used to effectively mitigate uranium contamination in the vadose zone sediments. The main target in this research was the speciation modeling calculations conducted via the Geochemist Workbench (GWB) software. For this purpose, the GWB's database was updated with the most recently published U thermodynamic data on dissolution reactions and revised stability and solubility product constants. The simulations were carried out for varied pore water compositions with 5% of NH₃(g) to increase the initial pore water pH from 8.0 to 11.87. The model assumed the system is closed to the atmosphere; so, no exchange of CO₂ with the atmosphere. The predicted U(VI) aqueous species and saturation indices [SI= log(IAP/Ksp)] were plotted as a function of pH values. The aqueous uranium carbonate species, UO₂(CO₃)₂^4- and UO₂(CO₃)₂^2-, were more abundant in the absence of Ca²⁺ and at a low bicarbonate concentration. In pore water compositions containing Ca²⁺ and HCO₃^-, the major predicted aqueous species were Ca₂UO₂(CO₃)₃⁰, CaUO₂(CO₃)₂, and UO₂(CO₃)₃^-4. Also, the speciation modeling predicted the formation of boltwoodite [Na,K(H₃O) UO₂(SiO₄)], uranophane [Ca(UO₂)₂(HSiO₄)₂ 5H₂O], and grimselite [NaK₃UO₂(CO₃)3.H₂O] minerals. The computed saturation indices (SI) indicated saturation of boltwoodite minerals in the simulated Ca-free compositions at zero and low (0 and 2.9 mM) HCO₃^- concentrations. In the presence of Ca ions, 5 mM and 10 mM, and similar HCO₃^- concentrations, speciation modeling predicted the formation of uranophane. However, grimselite phase was prevailing when simulated solutions contained Ca²⁺ ions and higher bicarbonate concentrations of 50 mM. (authors)