Simultaneous removal of U(VI) and humic acid on defective TiO_2−x investigated by batch and spectroscopy techniques

In this paper, the defective TiO_2−x with abundant oxygen vacancies were fabricated through a simple modified solvothermal strategy and characterized by SEM, TEM, XRD, FT-IR, EDX, Zeta-potential, and XPS techniques in detail. The prepared TiO_2−x was applied to remove U(VI) and humic acid (HA) from aqueous solutions to evaluate its sorption performance. The sorption of U(VI) and HA on TiO_2−x was examined under various environmental conditions (e.g., contact time, pH, ionic strength, initial HA/U(VI) concentrations, the addition sequences of HA/U(VI) and temperature). According to the Langmuir model simulation, the maximum sorption capacities of U(VI) and HA on TiO_2−x at pH = 5.0 were calculated to be 65 and 142 mg g⁻¹, respectively, which were higher than most current reported materials. The kinetic results indicated that the sorption of U(VI) and HA onto TiO_2−x was better described by the pseudo-second-order kinetic model. The presence of HA enhanced U(VI) sorption on TiO_2−x at pH = 5.0, meanwhile, the surface adsorbed U(VI) on TiO_2−x also enhanced the sorption capacity of HA, which was mainly attributed to the formation of U(VI)-HA-TiO_2−x ternary complexes. Combining FT-IR and XPS analysis, the oxygen vacancies could provide high chemical activity and trap U(VI) in the defective sites of TiO_2−x, while the sorption of HA was dominated by surface complexation. The findings might provide an opportunity to estimate and optimize the efficient simultaneous elimination of radionuclides and natural organic substances by using defective TiO_2−x.