Sorption, Desorption, and Isotope Exchange of Cesium (10-9-10 -3 M) on Chlorite

-Batch cation-exchange experiments were performed on chlorite, using synthetic ground water containing cesium in the concentration range 10-9-10 -3 M. The sorption behavior was complex; with increasing Cs concentration, the distribution ratio of Cs decreased at first, but passed through a pronounced maximum at high Cs loadings. The desorption data differed from the sorption data only by a shift towards larger distribution ratios in the region of the maximum. The distribution ratios for isotope exchange were even larger, but varied less with Cs loadings. Similar results were obtained for KCl-treated chlorite; however, grinding or acid-treatment of the chlorite considerably altered its behavior, the absence of the maximum being the most striking change. Most of the effects at higher Cs loadings can be explained by postulating structural changes induced by the sorpfion of Cs. Key Words--Acid treatment, Cation exchange, Cesium, Chlorite, Isotope exchange, Sorption. I N T R O D U C T I O N The plans of many countries to store nuclear wastes in geologic formations have led to a recent revival of sorption studies for radionuclides on a variety of rocks and minerals. Radionuclide sorption in geologic media may retard migration to the biosphere and the food chain. The understanding of sorption and desorption of long-lived fission products and actinide elements is, therefore, essential. Due to long half-lives (~ 106 yr for ~35Cs, 30 yr for ~37Cs) and high solubilities, isotopes of Cs are among the most hazardous nuclides. The chemistry of Cs is simple and suitable for an investigation of complex sorption processes in nature. Thus, the environmental behavior of Cs has been studied extensively. Glacio-fluvial sediments that cover large areas of the alpine countries contain shallow ground-water resources that are extensively used. These formations represent the final barrier to radionuclide migration from potential disposal sites before they reach the biosphere. The content of illite and chlorite in these formations governs the sorption of Cs. Whereas the sorption of Cs by illite has been studied by various authors (e.g., Jacobs and Tamura, 1961; Gaudette et al., 1966; Brouwer et al., 1983), a thorough investigation of Cs sorption on chlorite is, to our knowledge, presented here for the first time. Chlorite is also common in cracks and fractures of granite formations, which in many areas may be selected for the final storage of