STUDIES OF HYDROUS SILICON TITANIUM(IV) OXIDE-ION EXCHANGER .3. THE RATE OF THE ISOTOPIC EXCHANGE OF NA+ AND CS+ IONS BETWEEN THE EXCHANGER IN THE RELEVANT IONIC FORM AND AQUEOUS-SOLUTIONS

The rate of the isotopic exchange of Na+ and Cs+ between hydrous silicon-titanium(IV) oxide in the relevant ionic form and aqueous solution was determined radiochemically. The rate was controlled by the diffusion of the ions in the exchanger particles. The diffusion coefficients at 5 degrees C are (3.9+/-0.1)x10(-11) and (2.4+/-0.1)x10(-11) m(2) s(-1), respectively, for Na+ and Cs+ in the exchanger equilibrated with solutions at pH 6. The activation energies are 31+/-5 kJ mol(-1) and 20+/-5 kJ mol(-1) for Na+ and Cs+ diffusion, respectively. The diffusion coefficients of the ions decreases with increasing pH of the solutions equilibrated with the exchanger, whereas their activation independent of pH. The results were interpreted in terms of the strength of the electrostatic interaction between the counter ions and the ion-exchange sites.