Rational design of cooperative chelating sites on covalent organic frameworks for highly selective uranium extraction from seawater

Amidoxime-functionalized adsorbents have been pursued for uranium extraction from seawater, though competitive adsorption of vanadium and copper ions remains an issue. Herein, we report the successful design of a family of covalent organic framework (COF) adsorbents adopting an alternative chelating strategy for uranium binding comprising hydrazine-carbonyl sites with assistant groups for enhanced uranium uptake. A COF possessing hydroxypropoxy assistant groups (denoted as COF-R5) showed exceptional selectivity for uranium over vanadium and other metal ions, resulting in a fast uranium adsorption kinetics and high uptake capacity of 11.3 mg/g with a distribution coefficient over 9.2 × 105 mL/g in seawater, which exceeded most COFs previously reported in the literature. Mechanism studies revealed that hydroxypropoxy groups enhanced the binding of UO22+ at the COF’s hydrazine-carbonyl sites through hydrogen bonding interactions and charge stabilization effects, thus boosting the overall uranium affinity and adsorption capacity. Results validate the developed “uranium nanotraps” strategy for potential large-scale uranium extraction.