Kinetics, equilibria and thermodynamics of ion exchange in substituted tobermorites

The Al+Na and Al+K substituted tobermorites have been hydrothermally synthesized at 175°C under saturated steam pressure and their ion-exchange characteristics for K⁺ and Cs⁺ ions have been investigated from kinetics, equilibria and thermodynamics points of view. The initial exchange of Na⁺ in Al+Na substituted tobermorite with K⁺ and Cs⁺ ions was found to progress by diffusion of K⁺ and Cs⁺ ions. The activation energy for the Na → K exchange was evaluated to be smaller than that for Na → Cs exchange. The standard free energy changes for the cation exchange reactions were estimated at two total molarities of 1.0 × 10^-3 and 3.0 × 10^-3 mol/l which were found to be negative and the affinity of univalent cations for both Al+Na and Al+K substituted tobermorites at 25°C was found to follow the order: Na⁺ < K⁺ < Cs⁺. These basic studies suggest that the substituted tobermorites are selective for certain ions such as Cs⁺ and therefore they can be used in cation exchange separations.