The role of functional-group-tuning in adsorption-photoreduction of U(VI) onto β-ketoenamine covalent organic frameworks photosystem

The visible-light-induced conversion of soluble U(VI) into insoluble U(IV) has been considered as an effective method for uranium recovery. However, covalent organic frameworks (COFs) have rarely been applied for uranium extraction by photocatalytic reduction, which may be mainly due to the low charge transfer efficiency between the monomers, resulting in weak photocatalytic activity. Structural monomer functionalization as an effective strategy to modulate the physical and chemical properties of COFs. Herein, we synthesized a series of isostructural β-ketoenamine COFs with different functional groups (denoted as COF-TpPa-X, where X was –(OH)2, –Cl2, –SO3H, –COOH and –NO2) for photocatalytic U(VI) reduction under visible light. Research results revealed that the microenvironment of pore surface and the photoelectric properties of functional groups played pivotal roles in determining their energy band structures, photoelectric characteristics and hydrophilicity. These TpPa-X derivates were shown as n-type semiconductors, and the introduction of functional groups amplified the built-in electric field strength between the donor (Tp) and acceptor (Pa-X) moieties. As a result, compared with TpPa-1(X = H2), the U(VI) uptake capacity of TpPa-X was significantly improved by combining advantages of chemical coordination adsorption and photocatalytic reduction, and the order of photocatalytic removal rate was TpPa-COOH (0.00569 min−1) > TpPa-SO3H (0.00368 min−1) > TpPa-Cl2 (0.00274 min−1) > TpPa-NO2 (0.00255 min−1) > TpPa-(OH)2 (0.00211 min−1) > TpPa-1 (0.00122 min−1). By virtue of theoretical calculations, and coupled with spectral analysis, we demonstrated that the photocatalytic generation of electrons and •O2– radical lead to the reduction of adsorbed U(VI) to U(IV) in the presence of (UO2)O2·2H2O and UO2. Therefore, it was helpful to understand the structure-performance dependencies of the multivariate COFs adsorption-photocatalysts at the molecular level for achieving beneficial photoactivity.