Alkali and alkaline earth ion exchange affinity in ETS-10 toward aqueous lithium separation

Continuous ion exchange is a more sustainable alternative to current methods for removing common impurities from lithium sources. In this work, we examine ion-adsorbent interactions for Mg²⁺ and Ca²⁺ with microporous titanosilicate ETS-10, an ion exchange solid with promising performance, using experimental and computational (density functional theory, DFT) methods. Ion exchange affinity for Mg²⁺ and Ca²⁺ using the Na⁺-form of ETS-10 are quantified from measured equilibrium isotherms, analyzed using a modified Langmuir isotherm accounting for overall stoichiometric desorption/adsorption in the cation exchange process. The equilibrium constant for ion exchange using Na-ETS-10 is greatest for Ca and decreases in order of Mg, K, and Li, respectively. These differences in ion exchange affinity are consistent with trends in DFT-derived ion exchange energies, which account for hydration and solvation of cations using a thermochemical cycle. These equilibrium constant values and ion exchange energies suggest that exchange of Ca²⁺, Mg²⁺, and K⁺ using Na-ETS-10 is more favorable than that of Li⁺. Indeed, competitive ion exchange of equimolar aqueous mixtures of Li⁺ and each of K⁺, Mg²⁺, or Ca²⁺ demonstrate selective uptake of the non-lithium cation into the solid, thereby concentrating Li⁺ in the aqueous solution while removing impurity cations.