Beyond Ion Exchange: Unraveling the Complexity of Lithium-Ion Adsorption on H₂TiO₃

Layered titanium acid (H₂TiO₃ or HTO) has been extensively utilized as a lithium-ion sieve in studies on lithium extraction from salt lake brines. However, the actual lithium uptake by layered HTO, commonly known through an ion exchange mechanism, is much lower than the theoretical value. The hypothesis is that there is a clear need to further elucidate the mechanism of lithium-ion adsorption for the purpose of guiding the design of a lithium ion sieve (LIS). In this study, lithium adsorption on layered HTO as a function of pH was investigated to gain deeper insight into its underlying adsorption mechanism. Lithium adsorption was found to increase significantly with pH, in particular, at pH 14. To comprehend this strong pH dependency, thermogravimetric analysis-differential scanning calorimetry (TG-DSC) techniques were used to analyze the state of HTO layers before and after lithium adsorption. Surprisingly, HTO surfaces at pH 14 exhibited fewer hydration layers and a higher abundance of hydroxyl groups compared with values at lower pH values. Characterization of HTO before and after lithium adsorption using X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed an anatase type of TiO₂ in HTO at pH 14. Fourier transform infrared spectroscopy (FTIR) and solid-state nuclear magnetic resonance (SSNMR) measurements showed a significant alteration in the configuration of hydroxyl groups within HTO layers after Li⁺ adsorption at pH 14, distinguishing it from the behavior observed at other pH values. These findings demonstrate that lithium adsorption occurs predominantly through LiOH molecules, which displace interlayer water molecules. The results from density functional theory (DFT) calculations align well with our experimental findings. This research provides a more profound understanding of lithium adsorption in multilayer LIS materials and a scientific basis for the design of innovative multilayer structures for the extraction of lithium from salt lake brines.