Efficient capture of thorium ions by the hydroxyl-functionalized sp²c-COF through nitrogen-oxygen cooperative mechanism

Since the continuous development of nuclear energy, substantial amounts of radioactive thorium wastewater are inevitably produced. The discharge of radioactive thorium wastewater not only pollutes the natural environment but also endangers human health. Due to its affordability, simplicity, and high effectiveness, the adsorption method has emerged as the most often used method for treatment. Covalent organic framework (COF) materials are excellent adsorbents with various characteristics, including superior chemical stability, design flexibility, and various architectures, and thus are widely used in separating radioactive nuclides. Herein, we synthesized two structurally similar COFs that vary in their pore dimensions and the connectivity of their modules. After incorporating hydroxyl groups into the structure of Tb-TMT formed by benzene-1,3,5-tricarbaldehyde (Tb) and 2,4,6-trimethyl-1,3,5-triazine (TMT), the uptake capacity of thorium ions is significantly enhanced. The differences in solution pH, contact time, initial concentration, and competitive ion experiments between the materials before and after hydroxyl functionalization were studied. Additionally, the research assessed their reuse capabilities. In this research, the Hb-TMT exhibits an outstanding adsorption capacity for Th(Ⅳ) ions, with a remarkable adsorptive capacity reaching 543.5 mg g⁻¹, and it showes good uptake efficiency within 5 min with excellent selectivity (K_d = 1.2 × 10⁴). After three cycles of regeneration, Hb-TMT still maintains a high level of adsorption capacity for Th(Ⅳ) (> 80%) and has good reusability. Furthermore, the role of nitrogen-oxygen synergistic effect on hydroxyl-functionalized COF is highlighted by density functional theory (DFT) calculations. This study provides fresh insights for choosing functional groups in functionalized COFs, specifically for radionuclide adsorption.