Kinetic and thermodynamic study of the thorium phosphate–diphosphate dissolution

The dissolution of the thorium phosphate–diphosphate (TPD), which was proposed for the actinides immobilization, was systematically studied as a function of several parameters such as surface, leaching flow, temperature, acidity or basicity of the leachate and phosphate concentration. The dependence of the normalized leaching rate on the temperature leads to an activation energy equal to about 42±3 kJ mol−1. The normalized leaching rate is slightly increased when increasing the acidity or the basicity of the leachate. The partial orders related to proton and hydroxide ions are equal to 0.31–0.35 and 0.35, respectively. For the pH range studied, i.e., 1<pH<4 and 10<pH<13, the normalized dissolution rate is always lower than 10−5 g m−2 d−1 which confirms the good retention properties of the TPD even in a very corrosive medium. The presence of phosphate ions in the solution slightly increases the normalized leaching rate, but this increase is significant only for concentration higher than 0.1 M. Simultaneously to these experiments, the TEM study showed the formation of an amorphous external thin layer on the TPD surface and small corrosion pits during the first days of leaching in 5 M HNO3. Several crystallized phases coexist inside the gel finally obtained after the rather complete dissolution of TPD. When the saturation of the solution is obtained, the neoformed phase was identified as the thorium phosphate–hydrogen phosphate hydrate whose solubility product is very small. In these conditions, the thorium and phosphate concentrations measured in the leachate are controlled by the precipitation of such a compound and remain lower than 10−5 M when the initial TPD is partly dissolved.