Diffusion Rate of Na + and Cs + in Hydrous Titanium(iv) Oxide
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DIFFUSION RATE OF Na+ AND Cs' IN HYDROUS TITANIUM(IV) OXIDE
Fuminori Kasuga, Hiromichi Yamazaki, and Yasushi Inoue Department of Nuclear Engineering, Tohoku University, Sendai 980-77, Japan
ABSTRACT The diffusion rates of Na4 and Cs+ in two types of hydrous titanium(IV) oxide, precipitated at pH 6 (exchanger A) and pH 13 (exchanger B), were determined radiochemically. The rate of Cs' in exchanger A is faster than that of Na+; diffusion coefficients are 4.9x10-11 and 1.9x10-11 m2 s-1 for Cs÷ and Na4 , respectively (pH 12, 5.0 °C). The activation energy for Cs+ diffusion is 21 kJ mol-1 and slightly smaller than that for Na+ diffusion, 29 UJ mol"1. The rates of these ions in exchanger B are much slower than those in exchanger A, and can be explained by assuming the existence of two kinds of independently diffusing ions (fast and slow species) in the exchanger. The diffusion coefficients are of the order of 10-12 and 10-13 m2 s"l for the fast and the slow species, respectively. The activation energies are 48-60 kU mol"1 for Na4 diffusion and 32-33 kJ mol"1 for Cs4 diffusion (pH 12). The marked difference in kinetics between two exchangers was interpreted in terms of the difference in the acid-base property and in the microstructure of the matrix. INTRODUCTION Hydrous titanium(IV) oxide is one of the promising materials for use in the processing of radioactive liquid waste.
This material, however, has a problem of having different
equilibrium properties depending on the synthetic conditions.1 For use of this material, it is important to know whether kinetic behavior is also affected by the conditions for preparation. In order to provide data useful for elucidating ion-exchange kinetics, radiochemical experiments were carried out to measure the isotopic exchange rates of Na+ and Cs4 between hydrous titanium(IV) oxide and aqueous solutions. We used two types of hydrous titanium(IV) oxide precipitated at pH 6 and pH 13, compared the results with each other, and discussed them in terms of the differences in their ion-exchange properties. EXPERIMENTAL Preparation of the exchangers in salt form spiked with radioisotope 3 Hydrous titanium(IV) oxide was precipitated by two ways: (1) By adding 1 mol din 3 NaOH solution into 1 mol dm" TiC14 solution to make pH 6 (exchanger A) and (2) by mixing these solutions in the reverse order to make pH 13 (exchanger B). After being washed and dried at 70 °C, the precipitate was ground and classified into several particle sizes by using Japan Industrial Standard sieve; it was then converted to H+ form by treating it with a 0.05 mol din 3 HC1 solution. After being washed with water, the exchanger was air-dried and stored in a desiccator containing a saturated NH 4 CI solution (relative humidity, 79 % at 25 °C). The exchanger in the H+ form was then converted to salt (Na+ or Cs+) form by treating it with solutions having the same chemical composition as those used for the rate measurement. The exchanger was then labeled with radioisotope by equilibrating it
Mat. Res. Soc. Symp. Proc. Vol.
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