Layered H2TiO3 has been studied as an ionic sieve material for the selective concentration of lithium from solutions. The accepted mechanism of lithium adsorption on H2TiO3 ion sieves is that it occurs via Li+-H+ ion exchange with no chemical bond breakage. However, in this work, we demonstrate that lithium adsorption on H2TiO3 occurs via O-H bond breakage and the formation of O-Li bonds, contrary to previously proposed mechanisms. Thermogravimetric analysis results show that the weight loss due to dehydroxylation decreases from 2.96 wt % to 0.8 wt % after lithium adsorption, indicating that surface hydroxyl groups break during lithium adsorption. Raman and Fourier transform infrared spectroscopy studies indicate that H2TiO3 contains isolated OH groups and hydrogen-bonded OH groups. Among these two hydroxyl groups, isolated OH groups present in the HTi2 layers are more actively involved in lithium adsorption than hydrogen-bonded OH groups. As a result, the actual adsorption capacity is limited by the number of isolated OH groups, whereas hydrogen-bonded OH groups involved are for stabilizing the layered structure. We also show that H2TiO3 contains a high concentration of stacking faults and structural disorders which play a crucial role in controlling lithium adsorption properties.
Keywords: adsorption mechanism; ion exchange; lithium; lithium titanium oxide; surface hydroxyls.