Thorium and uranium phosphates were synthetized via soft chemistry and then calcined at high temperature, respectively up to 850 and 1170°C. Th3(PO4)4 was produced in agreement with published results.while the analog uranium phosphate : U3(PO4)4 was not found. Instead, we found we had made a new mixed valence compound : U(UO2)(PO4)2, for which crystal structure, absorption spectrum in ultraviolet and visible, and X-ray photoelectron spectroscopy were done. All of these experiments confirmed the presence of uranium in both tetravalent and hexavalent states.
Lixiviation of thorium phosphate labelled with radioactive isotopes : 227Th and 223Ra allowed us to determine the upper limit of the solubility for this compound = 7.6xl0–11 M and, the percentage of 223Ra released in deionized water (about 5xl0–3%).
Similar lixiviation tests on U(UO2)(PO4)2 labelled with 230U (radiochemically produced) gave an uranium solubility value 10 times greater than uranium measured by laser spectrofluorimetry. This discrepancy is attributed to the radiation chemistry inherent in the use of radioactive isotope at high specific activity. The lixiviated uranium fraction from U(UO2)(PO4)2 is seven to eight orders of magnitude higher than for thorium. Unexpected behaviour was observed for uranium solubility which increases by two orders of magnitude after 2000 hours of lixiviation. This phenomenon is not yet understood.