Investigation on Strontium Adsorption Selectivity of Hydrothermally Synthesized Layered Sodium Titanates

Two layered sodium titanate phases, sodium nonatitanate (Na 4 Ti 9 O 20 ) and sodium trititanate (Na 2 Ti 3 O 7 ), have been hydrothermally synthesized and their Sr 2+ adsorption selectivity was investigated in the coexistence of Cs + with ionic equivalent concentration. Although both phases exhibit Sr 2+ selective adsorption, Na 4 Ti 9 O 20 adsorbed both Sr 2+ and Cs + , while the adsorption of Cs + was not detected on Na 2 Ti 3 O 7 , despite its higher adsorption capacity. To investigate the causes for the high Sr 2+ selectivity of Na 2 Ti 3 O 7 , additional adsorption tests were carried out in different pH, which can be interpreted as the Sr 2+ –H + binary system, and in single and ternary systems of Al 3+ , Sr 2+ and K + with ionic equivalent concentrations. When changing the pH, the adsorption amount of Sr 2+ 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 Al 3+ –Sr 2+ –K + ternary system, the adsorption amount decreased in the order of Sr 2+ , Al 3+ and K + . The adsorption amount of K + was low compared to that of Sr 2+ and Al 3+ in both the single and ternary systems. Meanwhile, the adsorption amount of Sr 2+ significantly decreased compared to that in the single system, unlike in the Sr 2+ –Cs + binary system where the adsorption of Sr 2+ was almost the same. From these results, the high Sr 2+ selectivity of Na 2 Ti 3 O 7 in the Sr 2+ –Cs + binary system was anticipated to be due to the size effect. The smaller interlayer spacing of Na 2 Ti 3 O 7 compared to that of Na 4 Ti 9 O 20 appears to inhibit the intercalation of Cs + due to its large ionic radius.