Site engineering of covalent organic frameworks to increase charge transfer channels and provide hydrogen bond toward enhanced photocatalytic reduction of U(VI)

Integrating multiple photogenerated charge transfer channels and rich [UO2(H2O)(2)](2+) confinement sites into one covalent organic frameworks (COFs) photocatalyst to enhance U(VI) removal from wastewater is highly desired but remains huge challenges. Herein, a proof-of-concept site engineering strategy was proposed to not only increase charge transfer channels in COFs but also provide hydrogen bond to strengthen [UO2(H2O)(2)](2+) adsorption. Specially, after replacing anthracene unit in COF-A by anthraquinone unit, the charge transfer channels from donor to acceptor increase from two to four in the resulting COF-AQ owing to the electron-withdrawing character of two oxygen atoms in anthraquinone unit. Additionally, as demonstrated by the theoretical calculations, the hydrogen bond is formed between [UO2(H2O)(2)](2+) and oxygen atom in anthraquinone unit of COF-AQ, thereby decreasing the energy barrier for [UO2(H2O)(2)](2+) adsorption and facilitating the transfer of hot electrons to U(VI) along the hydrogen bond bridge. As a result, without the usage of sacrificial agents, the U(VI) removal ratio in COF-AQ reaches as high as 99.2 % under visible-light irradiation, which is much higher than that in COF-A (81.6 %). This work for the first time demonstrates the huge potential of COFs site engineering strategy for addressing U(VI) contamination in wastewater, which may open a new avenue for rationally designing efficient photocatalysts to removal various radionuclides.