DFT calculation and experiments for Li+/H+ ion-exchange on titanium-based lithium ion-sieves

Titanium-based lithium ion-sieves (H2TiO3) with the layered structure is an excellent adsorbent for lithium recovery from brines, since it has a high theoretical Li+ ions adsorption amount (∼142 mg/g) and a stable structure under acid regeneration. However, the formation of the strong H–O bonds in H2TiO3 leads to a greater energy barrier for re-adsorption of lithium ions, so it would be difficult to reach the theoretical lithium adsorption capacity in practical application. Moreover, the influence of H+ content in brine (pH value) on the Li+ ion adsorption amount is obvious, that limits the lithium recovery efficiency from brines. In this work, Li+/H+ ion-exchange mechanism of layered H2TiO3 (HTO) ion-sieve is investigated through DFT calculation, where the Li+ ion adsorption energies for three kinds of Li+/H+ ion-exchange pathways are calculated, and the order of priority for Li+/H+ ion-exchange is deduced and validated based on the experimental data of Li+ ions adsorption on the prepared nanometer HTO ion-sieve. Then, a quantitative relationship between Li+ ion adsorption amounts on HTO ion-sieve and pH values in Li+-containing solution is developed on the basis of the experimental data. When the nanometer HTO ion-sieve powders are formed into the millimeter PVB-HTO ion-sieve granules for industrial application, it is found that the formation of an acidic micro-environment inside of this PVB-HTO granule obviously reduces the Li+ ions adsorption rate, especially in the initial stage of Li+/H+ ion-exchange process. Finally, an improved strategy of Li+/H+ ion-exchange rate on PVB-HTO granules from a carbonate-type brine is experimentally demonstrated utilizing a batch and a fixed-bed adsorber, respectively.