Solubility of U(VI) in chloride solutions. II. The stable oxides/hydroxides in alkaline KCl solutions: Thermodynamic description and relevance in cementitious systems

Highlights: • Systematic solubility study with U(VI) in dilute to concentrated KCl systems. • K₂U₂O₇·1.5H₂O(cr) controls the solubility of U(VI) above pH_m ≈ 9.5. • Comprehensive thermodynamic, chemical and SIT activity models derived. • Accurate description of the UO₂²⁺–H⁺–K⁺–Na⁺–Cl^––OH^––H₂O(l) system provided. • K₂U₂O₇·1.5H₂O(cr) postulated as relevant U(VI) phase in cement degradation stage I. The solubility of U(VI) is investigated from undersaturation conditions in 0.1, 0.51, 1.03, 3.31 and 4.58 m KCl–KOH solutions at pH_m = 7.5–14.6 (with pH_m = –log [H⁺] in molal units). All experiments were performed under Ar atmosphere at T = (22 ± 2)°C. XRD, quantitative chemical analysis, SEM–EDS and TG–DTA confirm that K₂U₂O₇·1.5H₂O(cr) is the solid phase controlling the solubility in all evaluated systems at pH_m ≥ 9.5. Below this pH_m and with decreasing KCl concentration, the formation of sub-stoichiometric phases with K:U < 1 is indicated by XRD and solubility data. The concentration of uranium in equilibrium with K₂U₂O₇·1.5H₂O(cr) shows a pH-independent behaviour up to pH_m ≈ 11 regardless of ionic strength, whereas an increase of the solubility with a well–defined slope of +1 (log [U] vs. pH_m) is observed at pH_m ≥ 11. These results are consistent with the predominance of UO₂(OH)₃^– and UO₂(OH)₄²⁻ species as previously reported in the literature. The combination of solubility data obtained in the present study with K₂U₂O₇·1.5H₂O(cr) and the U(VI) hydrolysis scheme reported in Altmaier et al. (2017) yields a solubility product of log ^∗K°_s,0{0.5K₂U₂O₇·1.5H₂O(cr)} = (12.0 ± 0.2). SIT ion interaction coefficients for UO₂(OH)₃^– and UO₂(OH)₄²⁻ with K⁺ are derived based on the newly generated experimental data in dilute to concentrated KCl systems and analogy with NaCl systems. This work extends the thermodynamic database available for U(VI) and allows more accurate source term calculations in the context of nuclear waste disposal under boundary conditions where significant K concentrations may be present and redox conditions lie in the stability field of U(VI). The K₂U₂O₇·1.5H₂O(cr) solid phase can be considered to control the solubility of U(VI) in the degradation phase I of cement and cementitious materials.