Adsorption Behavior of Enhanced Sr²⁺ Removal Using Cake-like TiO₂/K₂Ti₄O₉ Nanocomposite: Experimental and DFT Studies

A novel Ti-based adsorbent (a derivative of Ti-MOF, denoted as NTCO) was prepared by using a solvothermal synthesis-solid-phase melting modification method, combining and melting. Microstructural characterization suggested that the as-synthesized NTCO possesses a regular cake-like morphology and was mainly composed of TiO₂ and K₂Ti₄O₉. Fourier-transform infrared (FT-IR) characterization showed that NTCO had abundant Ti-O, O-H and -C-N functional groups. That adsorbent displayed excellent adsorption performance for Sr²⁺ with an adsorption capacity of 81.7 mg/g. The adsorption of Sr²⁺ on NTCO was verified to be spontaneous and endothermic, and this adsorption conformed to the pseudo-second-order model (R² = 0.997) and the Freundlich model (R² = 0.997). NTCO could effectively adsorb Sr²⁺ across a wide pH range of 4 to 9, and the coexisting ions showed slight effects on Sr²⁺ adsorption at low concentrations. The spectral investigation and density functional theory analysis validated that the adsorption was mainly dominated by ion exchange and surface complexation, where the surface complexation of Ti-O coordination sites made the biggest contribution. The findings suggest that NTCO would be an efficient adsorbent for Sr²⁺ removal and may hold promise in water treatment.