Adsorption-photocatalysis processes: The performance and mechanism of a bifunctional covalent organic framework for removing uranium ions from water

Photocatalytic reaction could become an effective way to solve the energy crisis and environmental restoration and governance. Herein, we reported a covalent organic framework (COF-TpBpy) was connected by polyhydroxy-containing unit (Tp) and bipyridine unit (Bpy), which was utilized as photocatalytic semiconductor for photocatalytic detoxification of U(VI). Firstly, TpBpy was comprehensively characterized by physical chemistry (PXRD, SEM, TEM, FT-IR, BET, EA) and photoelectrochemistry (Vis-DRS, M-S plots, photocurrenttime, EIS, PL). Hereafter, the adsorption capacity of TpBpy for U(VI) was investigated by batch adsorption experiment, reaching 455 mg/g within 60 min. The adsorption experimental data conformed to the second-order kinetics, suggesting the chemisorption or surface complexation mechanism. Following that, TpBpy exhibited the narrower optical band gap (E-g = 2.2 V, E-CB =-0.95 and E-VB = 1.25 V vs. NHE) due to its extended pi-conjugation system and electron-rich units of the skeleton (N-14.56%, O-27.73%). After visible light irradiation for 420 min, the photoreduction removal rate of U(VI) by TpBpy was about 55.4%. ESR spectra corroborated that O-center dot(2)- radicals and photoelectrons were the main active species involved in the photoreduction process. XPS analysis revealed the formation of N-U bond and U-O bond, whether in the process of adsorption or photoreduction. In short, the structure of TpBpy possessed strong coordination ability with U(VI) ions, and promoted the transfer of electrons from electron-rich groups to U(VI), thereby reducing to U(IV). As a multifunctional material, TpBpy possess the advantage in eliminating nuclear waste streams.