Characterization and solubility measurement of synthetic uranophane and sklodowskite under oxic groundwater conditions

Naturally occurring uranyl silicates can serve as solubility-limiting solid phases (SLSPs) of U(VI) in the oxic environment of granitic groundwater, which is likely to occur in crystalline host rock media in Korea. In this study, two synthetic uranyl silicates, uranophane (Ca[UO₂SiO₃OH]₂·5H₂O) and sklodowskite (Mg[UO₂SiO₃OH]₂·6H₂O) were prepared and used to measure the U(VI) solubility in simulated groundwater (sGW), mimicking the composition of samples collected from the underground research tunnel at the Korea Atomic Energy Research Institute (KAERI). The solid- and solution-phase species involved were examined in detail using various characterization methods. According to the powder X-ray diffraction pattern and elemental analysis results, the synthetic mineral phases, which were identified as uranophane-α and sklodowskite having layered crystal structures, were retained intact in sGW; however, in 0.1 M NaClO₄, emergence of other solid phases was observed. Enhanced FTIR and Raman spectroscopic data, particularly in the regions of 790–860 and 940–1020 cm⁻¹ for the UO₂²⁺ and SiO₄⁴⁻ ions, respectively, enable monitoring the changes in the solid phases. Ternary Ca–U(VI)-tricarbonato complexes were identified as the dominant dissolved U(VI) species in sGW using time-resolved laser-induced luminescence spectroscopy. Furthermore, the solubility constant of uranophane was calculated (log K_s,0° = 10.3 ± 0.4) and compared with the predicted values based on our geochemical modeling analysis and previously reported ones.