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Experimental data on the exchange between Rb and Na on kaolinite and bentonite clays were examined in terms of the exchange equation of Heald et al. The results could be interpreted in terms of a single selectivity factor, describing the relative tendencies of the two ions to form ion pairs with the charge on the clay surface when it was assumed that the amount of each ion present in the form of ion pairs was a constant fraction of the total amount of that ion present.

Two layered sodium titanate phases, sodium nonatitanate (Na4Ti9O20) and sodium trititanate (Na2Ti3O7), have been hydrothermally synthesized and their Sr2+ adsorption selectivity was investigated in the coexistence of Cs+ with ionic equivalent concentration. Although both phases exhibit Sr2+ selective adsorption, Na4Ti9O20 adsorbed both Sr2+ and Cs+, while the adsorption of Cs+ was not detected on Na2Ti3O7, despite its higher adsorption capacity. To investigate the causes for the high Sr2+ selectivity of Na2Ti3O7, additional adsorption tests were carried out in different pH, which can be interpreted as the Sr2+ –H+ binary system, and in single and ternary systems of Al3+, Sr2+ and K+ with ionic equivalent concentrations. When changing the pH, the adsorption amount of Sr2+ showed a high and nearly constant value at pH above 4 and drastically decreased at pH below 3, reaching nearly zero at pH 2. In the Al3+–Sr2+ –K+ ternary system, the adsorption amount decreased in the order of Sr2+, Al3+ and K+. The adsorption amount of K+ was low compared to that of Sr2+ and Al3+ in both the single and ternary systems. Meanwhile, the adsorption amount of Sr2+ significantly decreased compared to that in the single system, unlike in the Sr2+–Cs+ binary system where the adsorption of Sr2+ was almost the same. From these results, the high Sr2+ selectivity of Na2Ti3O7 in the Sr2+–Cs+ binary system was anticipated to be due to the size effect. The smaller interlayer spacing of Na2Ti3O7 compared to that of Na4Ti9O20 appears to inhibit the intercalation of Cs+ due to its large ionic radius.