Rational design of cooperative chelating sites on covalent organic frameworks for highly selective uranium extraction from seawater
Yinghui Xie, You Wu, Xiaolu Liu, Mengjie Hao, Zhongshan Chen, Geoffrey I. N. Waterhouse, Xiangke Wang, Hui Yang, Shengqian Ma
Abstract
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.