Immobilization of U(VI) onto covalent organic frameworks with the different periodic structure by photocatalytic reduction

Abstract

In recent years, the donor (D)-acceptor (A) covalent organic frameworks (COFs) have attracted more and more attention in photocatalysis, but the photoinduced electron-transfer in the COFs have not been revealed. Herein, three 2D COFs materials (Tp-Tapb, Tp-Taz and TpTt, among them, Tp= 1,3,5-Triformylphloroglucinol, Tapb=1,3,5-tris(4-aminophenyl)-benzene, Tt=melamine, Taz= 4,4′,4′’-(1,3,5-triazine-2,4,6-triyl)-trianiline) with similar structure, different building monomers with number acceptors and the distance between the two acceptors were synthesized. COFs were considered as potential adsorbents to capture uranium (U(VI)) from wastewater due to high specific surface area, stability and heteroatom content. Compared with Tp-Tapb, Tp-Taz and TpTt shown the better photocatalytic performance and the higher separation efficiency of e^-- h⁺, due to the coexistence of triazine and ketone groups. Furthermore, TpTt possessed two short-range acceptor units (triazine and ketone) with the high content of heteroatoms, which exhibited the highest adsorption (505 mg/g) and photoreduction (0.22 h⁻¹) performances. The photocatalytic reduction of U(VI) by these three COFs materials, U(VI) was photo-reduced by e^- and •O₂^- in the frameworks of photoactive COFs under visible-light irradiation, so then the insoluble U(IV) species (UO₂, UO_2.9, UO_2.87, UO_2.82) were deposited in the open channels. COFs served as the composite platform with high catalytic activity and chelating coordination ability. The mutual enhancement of adsorption and photogenerated electron-transfer can realize the efficient removal of U(VI).