Covalent organic frameworks for enhanced photocatalytic extraction of uranium via the modulation of charge transfer pathways

Abstract

Crystalline covalent organic frameworks (COFs) are emerging as promising candidates for uranium extraction from seawater and treatment of uranium-containing wastewater. Nevertheless, conventional donor–acceptor (D-A) COFs severely limit the catalytic efficiency due to stochastic charge transfer and scarcity of sites. Here, we tuned the charge transfer channel to achieve efficient electron transfer via a facile functionalization strategy. In contrast to the typical situation where the donor and acceptor are distributed alternately in two building blocks, the carboxyl-modified COF has an electron-withdrawing module that allows for the directional transfer of electrons to designated acceptor sites, which have adsorptive and catalytic capabilities, thereby enhancing uranium extraction. The carboxyl-modified COF (TFA-TAT-COF-Q) can efficiently extract over 97 % of uranyl without a sacrificial agent. Notably, TFA-TAT-COF-Q maintains a high removal efficiency of 74.3 % for uranyl in natural seawater at pH = 8.1, highlighting its potential for practical seawater uranium extraction applications. A complex correlation between uranyl extraction and hydrogen peroxide (H₂O₂) consumption was subsequently revealed, with uranyl eventually being enriched in the form of pure metastudite [(UO₂)O₂·2H₂O] rather than the typical UO₂. Ultimately, we demonstrate that optimized electron transfer mode and enhanced exciton dissociation are pivotal factors driving the performance enhancement of COFs. This study brings a novel strategy for the preparation of D-A COFs.