(222c) Aqueous Separation of Lithium from Alkali and Alkaline Earth Ion Mixtures Via Ion Exchange Using ETS-10 Titanosilicate

Complex lithium streams are potential alternative sources to meet the growing lithium demand for battery applications. These streams contain various cations with similar properties to lithium, making the separation process challenging. Current separation methods require large quantities of hazardous organic solvents and high energy input. Consequently, severe environmental impacts, such as water level depletion at the Great Salt Lake, highlight the need for sustainable and environmentally friendly separation methods, particularly a separation method that can be done in the aqueous phase. Continuous ion exchange and ion chromatography are promising alternatives to reduce environmental impact. Their adoption requires a fundamental understanding of ion-adsorbent interactions to develop effective adsorbent materials.

We examine the structural properties, compositional and structural stability, and ion exchange thermodynamics of microporous titanosilicate, ETS-10. Samples are prepared through exchanging with concentrated Li⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺ for multiple times. The exchanged samples were collected and analyzed by solid-state NMR, elemental analysis, p-XRD, and UV-vis techniques, with support from density functional theory (DFT) calculations. ETS-10 demonstrated great exchange capacity towards tested ions and retains its structural integrity after concentrated ion exchange. Ion-adsorbent interactions are quantified by measuring ion exchange isotherms for Li⁺, K⁺, Mg²⁺, and Ca²⁺ in dilute aqueous solution using the sodium form of ETS-10, Na-ETS-10. A modified Langmuir isotherm equation, accounting for the collective adsorption/desorption in the cation exchange process, was used to analyze exchange equilibrium and capacity. We determined the equilibrium constants for the exchange of Na⁺ with cation i, K_i,Na (where i = Li⁺, K⁺, Mg²⁺, or Ca²⁺), and the exchange capacity of Na⁺ at equilibrium with cation i, Q_i,e. The results indicate the following trend: K_Ca2+:K_Mg2+:K_K+:K_Li+ = 77:16:6:~1, suggesting Li⁺ is the least favorable ion to be exchanged into Na-ETS-10. This affinity difference is corroborated by DFT calculations of ion affinity to the Na-ETS-10 framework. Energy differences from DFT were interpreted using a thermochemical cycle to account for hydration and solvation of cations, thus enabling comparison of experiment and theory. DFT-derived energy differences also revealed the same trend that we observed experimentally and showed the binding affinity difference of the various cation sites of Na-ETS-10 among Li⁺, K⁺, Mg²⁺, and Ca²⁺. These studies provide a foundation to tune the composition of the porous material and measure the selectivity based on exchange isotherms. Ongoing selectivity studies aim to examine the separation of lithium from complex aqueous lithium sources and explore the role of framework substitution in ion affinity.