The solubility of (UO 2) 3(PO 4) 2 · 4H 2O(s) and the formation of U(VI) phosphate complexes: Their influence in uranium speciation in natural waters
Uranium is commonly associated in natural waters with O-containing ligands both in the aqueous and the solid phases. Phosphate is present in most of these systems; however, the thermodynamics of the U(VI)-H 3PO 4 system are poorly known, particularly in the pH range of interest 6 to 9. Hence, the effect of phosphate on the migration of uranium in natural waters is not well understood. We have investigated the solubility of a well-characterized U (VI)-phosphate phase in the pH range 3 to 9: (UO 2) 3(PO 4) 2 · 4H 2O(s) Analysis of these data indicates the formation of the predominant species UO 2HPO 4(aq) and UO 2PO -4 in the pH range 4-9. Their formation constants as well as the solubility constant of (UO 2) 3(PO 4) 2 · 4H 2O(s) have been determined. From the same experiments we have established the equilibrium constant of the hydroxide complex UO 2(OH) 3-. A discussion of the hydrolysis of U (VI) is also reported. The effect of phosphate on the mobility of U in natural waters is assessed in the light of these new data. They indicate that in the pH range of most natural waters, 6 to 9, U (VI) will be associated to aqueous phosphate complexes when the total concentration ratio [PO 3-4] T/[CO 2-3] T is greater than 10 -1.