Solubility of U(VI) in chloride solutions. I. The stable oxides/hydroxides in NaCl systems, solubility products, hydrolysis constants and SIT coefficients

Abstract The solubility of uranium(VI) is studied at T  = (22 ± 2) °C in carbonate-free 0.03–5.61 mol·kg w −1 NaCl solutions at pH m  = 4–14.5 (pH m  = −log [H + ]). The solid phases metaschoepite UO 3 ·2H 2 O(cr) and sodium uranate Na 2 U 2 O 7 ·H 2 O(cr) (or NaUO 2 O(OH)(cr)) are characterized by XRD, SEM–EDS, thermogravimetry and quantitative chemical analysis. UO 3 ·2H 2 O(cr) controls the solubility of U(VI) in acidic to near neutral pH conditions regardless of NaCl concentration. Na 2 U 2 O 7 ·H 2 O(cr) forms in alkaline systems and is stable at pH m  ≥ 7 over the complete range of NaCl concentrations investigated. The solubility of UO 3 ·2H 2 O(cr) at pH m * K ° s,0 {UO 3 ·2H 2 O(cr)} = (5.35 ± 0.13). Titration experiments and UV–vis absorption spectra of saturated solutions at pH m  = 4–5 in dilute to concentrated NaCl solutions confirm the speciation calculated in the present study and show the predominance of the trinuclear hydroxide complex (UO 2 ) 3 (OH) 4 2+ at high chloride concentrations. Solubility data obtained at 7 ≤ pH m  ≤ 14.5 with Na 2 U 2 O 7 ·H 2 O(cr) and in solutions equilibrated simultaneously with Na 2 U 2 O 7 ·H 2 O(cr) and UO 3 ·2H 2 O(cr) yield a solubility product of log  * K ° s,0 {0.5 Na 2 U 2 O 7 ·H 2 O(cr)} = (12.2 ± 0.2) and the formation constants of the anionic hydrolysis species UO 2 (OH) 3 − (log  * β ° (1,3)  = −(20.7 ± 0.4)) and UO 2 (OH) 4 2− (log  * β ° (1,4)  = −(31.9 ± 0.2)). SIT coefficients for these species are derived based on the solubility data obtained in dilute to concentrated NaCl systems. The chemical, thermodynamic and SIT activity models provided in this work for the system U VI –Na + –H + –Cl − –OH − –H 2 O(l) represent an accurate and robust tool for the calculation of U(VI) solubility and aqueous speciation in a variety of geochemical conditions relevant in the context of nuclear waste disposal.