Chemical conditions of synthesis of Th4(PO4)4P2O7. Preparation of thorium phosphate-hydrogenphosphate as precursor

Thorium phosphate−diphosphate, Th4(PO4)4P2O7, is obtained, at high temperature (800−1250 °C), for a given ratio r = PO4/Th equal to 3/2 from a thorium compound and a phosphating reactant whatever the chemical conditions. This compound, whose solubility is very low, can be loaded in situ and used as a host matrix for radwaste storage. One of the methods for its synthesis involves the precipitation of a precursor, thorium phosphate−hydrogenphosphate. This precursor is used as an adsorber for low concentrations of radionuclides and, after heating in air in the range 800−1200 °C, it is transformed into doped Th4(PO4)4P2O7. The optimum conditions for the precipitation process have been determined. Starting from 0.5−2 M solutions of thorium nitrate or chloride and (NH4)2HPO4 at pH = 9−9.5 at the ratio r = 3/2, a compound of general formula Th2(PO4)2HPO4·nH2O (n = 3−7) is synthesized. Its physicochemical properties have also been studied.