Solubility and solubility product at 22°C of UO2(c) precipitated from aqueous U(IV) solutions

Abstract

Solubility studies on UO2(c), precipitated at 90°C from low-pH U(IV) solutions, were conducted under rigidly controlled redox conditions maintained by EuCl2 as a function of pH and from the oversaturation direction. Samples were equilibrated for 24 days at 90°C and then for 1 day at 22°C. X-ray diffraction (XRD) analyses of the solid phases, along with the observed solubility behavior, identified UO2(c) as the dominant phase at pH≳1.2 and UO2(am) as the dominant phase at pH≳1.2. The UV-Vis-NIR spectra of the aqueous phases showed that aqueous uranium was present in the tetravalent state. Our ability to effectively maintain uranium in the tetravalent state during experiments and the recent availability of reliable values of Pitzer ion-interactionparameters for this system have helped to set reliable upper limits for the log Ko value of ≤ −60.2 + 0.24 for the UO2(c) solubility [UO2(c) + 2H2O ⇌ U4+ + 4OH−] and of >−11.6 for the formation of U(OH)4(aq) [U4++ 4H2O ⇌ U(OH)4(aq) + 4H+]