Kinetic and thermodynamic studies of the dissolution of thorium–uranium (IV) phosphate–diphosphate solid solutions

The dissolution of thorium–uranium (IV) phosphate–diphosphate solid solutions (TUPD) was studied as a function of the temperature and leachate acidity. The dependence of the normalized dissolution rate on the temperature leads to an activation energy equal to about 40 kJ mol−1, close to that obtained for the pure thorium phosphate–diphosphate (42±3 kJ mol−1) and for thorium–plutonium (IV) phosphate–diphosphate solid solutions (41±1 kJ mol−1). The normalized dissolution rate of TUPD slightly increases with the leachate acidity. The partial order related to the proton concentration, n, is equal to 0.40±0.02 while the apparent normalized dissolution rate constant, k′T,I, reaches (2.8±0.7)×10−4 g m−2 d−1 at 90°C and for [H3O+]=1 M. When the saturation of the leachate is reached, the concentration of thorium, uranium and phosphate ions measured in the solution are controlled by the precipitation of the uranyl phosphate pentahydrate (UO2)3(PO4)2·5H2O and the thorium phosphate–hydrogenphosphate Th2(PO4)2(HPO4)·H2O. Both solids were extensively characterized using XRD, infrared and UV–visible spectroscopies or electron probe microanalysis (EPMA). Their solubility products, K°S,0, were determined and extrapolated to I=0. They are equal to 10−55.2±0.5 and 10−66.6±1.2, respectively. All the samples leached were characterized using EPMA, SEM and TEM. These techniques showed that during the dissolution process, thorium and uranium are completely separated as (UO2)3(PO4)2·5H2O, on one hand, and Th2(PO4)2(HPO4)·H2O, on the other hand. In the first days of leaching tests, an amorphous additional phase, identified as Th2(PO4)2(HPO4)·nH2O was also observed. Several leaching tests performed on sintered TUPD samples revealed that the dissolution rates measured in 10−1 M HNO3 is very low (6.5×10−5 g d−1) by comparison to other ceramics studied in the same objective. In these conditions, the thorium phosphate–diphosphate (TPD) appears as a promising ceramic for the immobilization of tetravalent actinides like uranium, neptunium or plutonium.