THE SOLUBILITY OF (UO2)3(PO4)2.4H2O(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)-H3PO4 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: (UO2)3(PO4)2.4H2O(s) Analysis of these data indicates the formation of the predominant species UO2HPO4(aq) and UO2PO4- in the pH range 4-9. Their formation constants as well as the solubility constant Of (UO2)3(PO4)2.4H2O(s) have been determined. From the same experiments we have established the equilibrium constant of the hydroxide complex UO2(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 [PO43-]T/[CO32-]T is greater than 10(-1).