Covalent organic framework photocatalysts: Design and photocatalytic U(VI) separation

The highly selective and effective separation of radioactive uranium from wastewater containing uranium and seawater through photocatalytic technique holds great significance for alleviating the energy crisis and protecting the environment, yet it also presents great challenges. Covalent organic frameworks (COFs) are considered desirable photocatalytic candidates for separating hexavalent uranium (U(VI)) on account of their tunable and programmable structure. Herein, the crucial advances in COF-based photocatalysts for U(VI) photocatalytic reduction from radioactive wastewater and seawater are presented, emphasizing particularly the relationship between structure-property-activity. A detailed discussion is conducted on the design of linkage of COF for important factors affecting photocatalysis. Subsequently, the performance enhancement strategies (such as morphology control, skeleton functionalization, hybrid materials, metalized, elemental doping and piezo-photocatalysis) are systematically highlighted. The experimental parameters of COF photocatalysts for practical application in radioactive wastewater or seawater are discussed, and the underlying mechanisms of U(VI) photocatalytic reduction by COF materials carefully analyzed. Finally, the challenges and prospects of COFs-based photocatalysts on separation U(VI) are envisaged, and a variety of strategies for improving the performance are elaborated. This review establishes the structure-activity relationship between COF linkages, band structure, and uranium reduction efficiency, which aims to and affords guidance for the design and synthesis of diverse COF-based photocatalyst with excellent photocatalytic performance to achieve the high-efficiency photoreduction U(VI) from radioactive wastewater and seawater.