Investigation on Strontium Adsorption Selectivity of Hydrothermally Synthesized Layered Sodium Titanates

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.