High Li-Ion Selectivity of Five-Coordinate Layered Titanate K₂Ti₂O₅

Selectivity of ion exchangers is an important topic in adsorption science owing to its specific application in resource recovery and environmental remediation. In this study, the cation exchange property of the submillimeter-sized five-coordinate K₂Ti₂O₅ (KTO) crystals is demonstrated. Adsorption isotherm measurements were performed on KTO crystals ion-exchanged with alkali metal cations including Li⁺, Na⁺, Rb⁺, and Cs⁺. The maximum adsorption amounts of Li⁺, Na⁺, Rb⁺, and Cs⁺ on KTO were 2.70, 1.15, 0.59, and 0.42 mmol g^-1, respectively, which is contradictory to the "normal" selectivity sequence (Cs⁺ > Rb⁺ > K⁺ > Na⁺ > Li⁺) of conventional ion exchangers, including clays and organic resins. The Kielland plots for the Li⁺ and Cs⁺ exchange experiments showed preferential Li⁺ adsorption on KTO, which supports the high Li⁺ selectivity. The interlayer distance for M⁺-exchanged KTO (M = Li, Na, Rb, and Cs) was dependent on cation type. Raman and X-ray absorption near-edge structure spectroscopic analyses of the KTO samples indicated that certain Ti species in KTO underwent hydrolysis, and thereby formed hydroxyl groups on the KTO surface during ion exchange. The origin of the high Li⁺ selectivity of KTO is discussed herein based on experimental characterization results.