STUDIES ON THE ION-EXCHANGE BEHAVIOR OF THORIUM IONS WITH ZEOLITES

Commercial, synthetic zeolites, have been evaluated for the removal of thorium ions from aqueous solution by ion-exchange. Kinetics of uptake of thorium ions from solutions of various initial Th concentrations by four zeolites of different types viz., 13X, AR-1, ZSM-5 and 4A, was studied in batch experiments. The Th-Na exchange process is accompanied by side reactions such as H-Na exchange and surface precipitation of Th-hydroxide, making it difficult to derive any conclusive evidence about the nature of diffusion of thorium ions. Acidity of the solution, produced due to the hydrolysis of Th ions, has an adverse effect on the stability of the zeolites. X-ray diffraction and Infra red spectral studies were used for the characterization of the zeolites with respect to crystallinity, surface precipitation of Th-hydroxide etc., before and after exchange. Stirring was observed to dramatically affect the establishment of equilibrium requiring about 2 weeks for unstirred and just 4 hours for thoroughly stirred solutions. Adsorption isotherms obtained for Zeolite-13X, the one performing the best, with isonormal solutions of Th-nitrate at total ionic concentrations (Th + Na) of 0.024 N in one case and 0.1 N in the other, displayed unique shapes attributable to the accompanying side reactions. Distributions coefficient (K(d)) values varied from above 2000 ml/g at 0.1 mM Th to 15 ml/g at 10 mM Th concentration. In column experiments with 1 mM Th-nitrate solution and Zeolite-13X, percent breakthrough of 10, 50 and 96% were obtained on passing 125, 350 and 700 bed volumes respectively. Exchange capacity of 1.2 meq/g, corresponding to the passage of 300 bed volumes, was observed.