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.

Automated summary

Photocatalytic reaction is an effective way to solve the energy crisis and environmental restoration and governance. In this study, a covalent organic framework (COF-TpBpy) was developed as a photocatalytic semiconductor for the photocatalytic detoxification of U(VI). The results showed that COF-TpBpy had strong adsorption capacity for U(VI) and effectively removed U(VI) through a photoreduction mechanism.