Quantifying the Affinity Difference between Lithium and Close Identity Ions Using ETS-10 Titanosilicate

The affinity difference between lithium and potassium ions for the adsorption to a titanosilicate adsorbent is quantified. Experimental and computational methods examine the thermodynamics of Na+–K+ and Na+–Li+ ion exchange from dilute aqueous solution using modified ETS-10 titanosilicate. Equilibrium uptake data were analyzed with a modified Langmuir isotherm that accounts for ion exchange involving the desorption of Na+ from the modified ETS-10 framework together with the adsorption of Li+ or K+ to the framework. The equilibrium constant for ion exchange of Na+ from the modified ETS-10 framework with K+ from aqueous solution is 6-fold greater than the equilibrium constant for Li+ exchange. This affinity difference is supported by density functional theory (DFT) calculations of ion affinity to the modified ETS-10 framework. Energy differences from DFT were interpreted using a thermochemical cycle to account for hydration and solvation of cations, thus enabling comparison between the experiment and theory. DFT-derived energy differences also revealed the binding affinity differences of the various sites of ETS-10 among Li+, Na+, and K+.