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, stability, and ion exchange thermodynamics of microporous titanosilicate, ETS-10. Samples are prepared by exchanging with concentrated Li⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺ for multiple times. The exchanged samples were analyzed by solid-state NMR, elemental analysis, p-XRD, and UV-vis techniques, with support from density functional theory (DFT) calculations. ETS-10 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 solutions using the sodium form of 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. Equilibrium exchange constants indicate a trend in exchange affinity: K_Ca2+:K_Mg2+:K_K+:K_Li+= 77:16:6:~1, suggesting Li⁺ is the least favorable ion to be exchanged. This affinity difference is corroborated by DFT calculations. Energy differences from DFT were interpreted using a thermochemical cycle to account for the hydration and solvation of cations. DFT-derived energy differences revealed the same ion affinity trend observed experimentally. These studies provide a foundation to tune the composition of the porous material and measure the cation exchange selectivity using exchange isotherms. Selectivity studies confirm the selective adsorption of other alkali and alkali-earth ions from multi-cation aqueous streams using ETS-10.