Sonochemical precipitation of amorphous uranium phosphates from trialkyl phosphate solutions and their thermal conversion to UP2O7

• Amorphous uranyl phosphates were precipitated by sonication.
• Solutions contained uranyl precursors in triesters of phosphoric acid.
• Crystalline UP2O7 is obtained on heating the precipitates to 1000 °C.
• The efficiency of uranium removal was typically 30–35%.

Insoluble amorphous precipitates containing uranyl and phosphate ions are obtained by sonication of solutions of three uranyl precursors, UO2(X)2, X = NO3, CH3COO, CH3C(O)CHC(O)CH3 (acetylacetonate, acac), in triesters of phosphoric acid, OP(OR)3, R = Me (trimethyl phosphate, TMP), Et (triethyl phosphate, TEP). TMP and TEP are used as high-boiling solvents and they serve also as a source of phosphate anions. Sonolysis experiments were carried out under flow of Ar at 40 °C on a Sonics and Materials VXC 500 W system (f = 20 kHz, Pac = 0.49 W cm−3). Powder X-ray diffraction (PXRD) reveals amorphous character of all obtained precipitates. The presence of uranyl and phosphate is evidenced by IR spectroscopy and ICP-OES analysis reveals the content of both U (38.6–43.4 wt%) and P (11.0–13.6 wt%). The thermal behavior of the substances was studied by TG/DSC analysis, which shows weight losses in the range of 19.21–24.08%. On heating the amorphous precipitates to 1000 °C, crystalline uranium diphosphate UP2O7 is obtained in all cases as the only crystalline phase. Uranyl(VI) is reduced during thermolysis to U(IV) as there is no characteristic vibration of UO22+ in the IR spectra of solid UP2O7 products. The ICP-OES analysis of U and P content in precipitates allowed us to calculate the efficiency of precipitation of uranium from mother liquor and to compare it with the efficiency calculated from the data received by the PXRD and TG/DSC analyses. The efficiency of the uranium removal attained by our sonoprecipitation procedure was typically 30–35%. These sonochemical precipitation reactions providing insoluble uranium phosphates may be potentially interesting models for the description of behavior of uranium-containing waste or reprocessing streams.