β-Ketoenamine and Benzoxazole-Linked Covalent Organic Frameworks Tailored with Phosphorylation for Efficient Uranium Removal from Contaminated Water Systems

The complexity of nuclear wastewater treatment motivates the design and synthesis of highly selective and stable uranium adsorbents to provide environmental remediation. Herein, a post-synthetic modification method is adopted to synthesize two beta-ketoenamine and benzoxazole-linked COFs decorated with abundant phosphonic groups for the uranium removal, which overcome the disadvantage of amidoxime groups with stronger affinity for vanadium than uranium. The U-uptake capacity of beta-ketoenamine-based TpPa-2OH-PHos reaches 410.9 mg g-1 (pH 5, C0 = 100 mg L-1 and m/V = 1/10 g L-1). Notably, U-uptake capacity of TpDBD-PHos reaches 633.3 mg g-1 at the same conditions. More abundant phosphonic groups and the synergistic effect between the benzoxazole rings and phosphonic groups may be responsible for the good uranium adsorption performance for TpDBD-PHos. Moreover, TpDBD-PHos exhibits a satisfactory uranium removal rate of 87.71% in actual rainwater samples. The X-ray pHotoelectron spectroscopy (XPS) analysis suggests that the strong interaction between U(VI) and oxygen/nitrogen atoms in the frameworks plays a vital role for the adsorption of TpPa-2OH-PHos and TpDBD-PHos, which agrees well with the results of density functional theory (DFT) calculations. The good chemostability and excellent selectivity demonstrate that TpDBD-PHos is an outstanding candidate for the removal of uranium from contaminated water systems. Two beta-ketoenamine and benzoxazole-linked COFs decorated with abundant phosphonic groups are successfully synthesized for the removal of U(VI). In comparison with TpPa-2OH-PHos, TpDBD-PHos shows better uranium adsorption performance due to the synergistic effect between the benzoxazole rings and phosphonic groups. Impressively, TpDBD-PHos still displays high removal rate (87.71%) for uranium in actual rainwater sample.image