The results of our studies on the properties of hydrous titanium (IV) -silicon oxides (TS) and hydrous titanium (IV) -zirconium oxides (TZ) as solidification media of liquid radioactive wastes have been reviewed. TS exchangers with 37 to 67 mol% Ti have much larger cation-exchange capacity than the component simple hydrous metal oxides, while the cation-exchange capacity of TZ increases linearly with titanium content in the exchangers. Though the affinity of these exchangers for representative metal ions increases with the acidity of the exchange sites, no such simple relation holds for transition metal ions; in the sorption of transition metal ions, the formation of a covalent bond is a key factor in determining the selectivity of the exchangers, along with the remote coulombic interaction.These exchangers do not suffer from any drastic change in the crystallinity upon heating up to 600 °C, and are much more heat-resistant than simple hydrous metal oxides.The dissolutions of Sr²⁺, Co²⁺, Eu³⁺and Th⁴⁺ from the exchangers calcined at ≥ 900°C are very slow due to the crystallization of strontium titanium silicate or titanates of Co²⁺, Eu³⁺ and Th⁴⁺ ions. The leaching rates of Cs⁺ and UO₂²⁺ from TS significantly decrease with increasing temperature for thermal treatment, while such effect is not observed in TZ due to the formation of soluble Cs₂O and α -U₃O₈. In calcined TScontaining Cs⁺ and UO₂²⁺, most of the intense X-ray diffraction peaks cannot be assigned to known compounds, indicating the formation of crystallographically unknown hardly soluble caesium titanium silicate and uranium titanium silicate. Based on these results, we can conclude that TS is a promising material for the solidification of radwaste.

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Catalytic activities for dehydration of alcohols over lithium taeniolites (LTN) exchanged with some cations (M=Fe³⁺, Al³⁺, Zr⁴⁺) were studied.The catalytic activity of the taeniolite exchanged with Fe³⁺ (Fe/LTN) is the highest among various M/LTN, depending on both the reaction temperature and the pretreatment temperature.The catalytic behavior can be explained in terms of the temperature dependence of the layer structure of Fe/LTN, suggesting the active sites of dehydration reaction are located in the interlayer of Fe/LTN.The unique shape--selectivity such as molecular size and shape is not recognized.The activities of Fe-exchanged montmorillonite (Fe/Mont) and Fe-exchanged lithium hectrorite (Fe/LHT) are much lower than that of Fe/LTN, indicating Fe species in those catalysts are different from that of Fe/LTN, in which the active species for dehydration of alcohols is α -FeOOH located in the interlayer.The good correlation between the catalytic activity and the amount of desorbed pyridine is obtained.The amount of the acid sites in M/LTN could be estimated by pyridine-TPD measurements, which is found to be a good technique for the characterization of the acid sites of hydrated M/LTN.

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