Flexible Units in Covalent Organic Frameworks Promote Photocatalytic Uranium Extraction from Wastewater

Covalent organic frameworks (COFs) are increasingly being utilized in photocatalysis due to their programmable band gaps and pore geometries. Flexible COFs are currently of interest for selective sorption, though their use in photocatalysis has received little attention to date. Herein, we propose a strategy for modulating the flexible units of COFs to facilitate efficient charge carrier separation and transfer during photocatalysis. Rotational freedom of flexible linkers in our COFs led to controllable local pore curvature, thereby optimizing optoelectronic properties and boosting photocatalytic activity. One of our developed COFs (COF-3) showed excellent photocatalytic performance for uranium removal from contaminated groundwater and contaminated tap water. Mechanistic studies revealed that COF-3 photocatalytically generated H ₂ O ₂ under visible light irradiation in water, which transformed the UO ₂ ²⁺ to a solid (UO ₂ )O ₂ ·2H ₂ O through a redox pathway. The results demonstrate an effective design strategy towards flexible COF photocatalysts for uranium removal from water and potentially many other applications.