Redox chemistry of uranium in reducing, dilute to concentrated NaCl solutions

The redox behaviour of uranium was investigated in 0.1 and 5.0 M NaCl solutions at 2 ≤ pHm ≤ 14.5 (pHm = -log [H+]) in the presence of different reducing chemical systems (Sn(II), Na2S2O4, Sn(II) + TiO2, Sn(II) + Fe(0), Sn(II) + Fe3O4). All experiments were performed under Ar atmosphere at T = (22 ± 2) °C. Uranium was added to independent batch samples as U(VI) (with [U]0 = 3.0⋅10−5 or 4.2⋅10−4 M), and the evolution of uranium concentration monitored for t ≤ 635 days. After attaining equilibrium conditions, [U] was found in all cases clearly below the solubility of U(VI) solid phases (UO3⋅2H2O(cr) or Na2U2O7⋅H2O(cr)) and in good agreement with the solubility of tetravalent UO2(am, hyd) as calculated with available thermodynamic data. This observation is in line with (pe + pHm) measurements, which in all cases fell in the stability field of U(IV). Solvent extraction and XANES confirmed also that uranium is predominantly found as U(IV) in the aqueous and solid phases investigated. No evidence on the formation of anionic hydrolysis species of U(IV) was obtained up to pHm = 14.5. Based on our long-term redox study, we conclude that previous investigations reporting the formation of U(OH)5- and U(OH)62− are possibly flawed by insufficient equilibration time, which prevented the complete reduction of U(VI) to U(IV). Our results further confirm that experimental pHm and Eh values measured in buffered systems can be considered as reliable parameters to predict the redox behaviour of U in dilute to concentrated NaCl systems.