Fundamental study on the separation and solidification of radwaste with hydrous titanium(IV) oxides modified with either silicon or zirconium

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Inorganic ion-exchangers are a group of promising materials for the solidification of radioactive liquid waste, since they have high sorption capacity for radioactive species, and can be converted into stable waste forms suitable for disposal by simple thermal treatment. The potentiality of the selective removal and immobilizatin of radioactive Cs^+, Sr^<2+>, Co^<2+>, Eu^<3+>, Th^<4+>, and UO_2^<2+> ions in hydrous Ti^<IV>-Si oxide (TS) and hydrous Ti^<IV>-Zr oxide (TZ) has been studied experimentally : The distribution coefficient and sorption capacity for these ions at various conditions in solutions, the structure and the composition of the exchangers in relevant ionic forms at high temperatures, and the leaching rate from calcined exchangers into water and a 0,01M HNO_3 solution at room temperature.TS uptakes these cations much more than TZ does, but the separation of Th^<4+> and UO_2^<2+> from Cs^+ and Sr^<2+> is performed more completely on TZ than TS.The dissolutions of Sr^<2+>, Co^<2+>, Eu^<3+> and Th^<4+> from calcined TS and TZ exchangers at (〕SY.gtoreq.〔)900ﾟC are very slow due to crystallization of strontium titanium silicate or titanates of Co^<2+>, Eu^<3+> and Th^<4+>. The leaching rate of Cs^+ and UO_2^<2+> from TS significantly decreases with increasing temperature for thermal treatment, while such effect is not observed in TZ due to the formation of soluble Cs_2O and alpha-U_3O_8. In calcined TS exchangers in the Cs^+ and UO_2^<2+> forms, most of the intense X-ray diffraction peaks cannot be assigned to known compounds, indicating the formation of crystallographycally unknown caesium titanium silicate and uranium titanium silicate.Based on these results, TS is superior to TZ in the sorption capacity and fixation capability of various kinds of radionuclides and considered as a promising material for the solidification of radwaste.