Alkali metal ion exchange selectivity of Al-substituted tobermorite

Alkali metal ion exchange isotherms at a total ionic strength of 0.001 M were determined at 25 °C on a 1.13 nm anomalous [Al³⁺ + Na⁺]-substituted tobermorite with the formula, Ca₅Na_0.75Al_0.9Si_5.1O₁₆(OH)₂ • 6.03H₂O. The Kielland plots of Na⁺/K⁺, Na⁺/Rb⁺, and Na⁺/Cs⁺ exchange reactions showed straight lines. The slopes were steeper for Cs⁺ and Rb⁺ exchange reactions, as compared to K⁺ exchange. The cation-exchange selectivity for alkali metals was found to increase as follows: Cs⁺ > Rb⁺ > K⁺ > Na⁺. A new evaluation method of the separation factor (α^M _N≡K^M _d/K^N _d,K_d: Distribution coefficient) for a combination of two cations at infinite dilution was proposed in terms of the selectivity coefficient (K^M _Na) which can be easily determined from the Kielland plot. An extremely large separation factor for Cs (α^Cs _Na = 112) was found at infinite dilution. These basic studies of cation exchange selectivity are of relevance in cation separation and purification and nuclear waste disposal.