Rational construction of covalent organic frameworks with multi-site functional groups for highly efficient removal of low-concentration U(vi) from water

Abstract

Uranium, a key nuclide in the nuclear industry, has substantially damaged the ecological environment and human health due to its high toxicity and radioactivity. Therefore, its efficient separation and extraction from water are of great significance for environmental safety. However, the low concentration of uranium in water requires capture materials with high adsorption affinity, which poses a severe challenge to conventional materials. Herein, we designed a novel covalent organic framework (COF-TpDd-AO₂) with two amidoxime groups in each structure unit, to provide multiple capture sites towards uranium, as the trap material. This COF exhibited different adsorption behaviors towards uranium with different concentrations, with a maximum affinity of 8.27 × 10⁵ mL g⁻¹, which was 6.82 and 9.81 times higher than those of the control sample COF-TpDb-AO with a single amidoxime group and the pristine COF-TpDd. The removal efficiency exceeded 99.2% with the initial uranium concentration of 0.5 mg L⁻¹. Mechanism studies showed that the chelation interaction between multi-site amidoxime groups and uranium acted as the dominant adsorption force. Moreover, with the increasing density of the amidoxime group in COF-TpDd-AO₂, the chelation effect changed from one single site to multiple sites, thus further enhancing the uranium affinity. This work suggested that the purposeful design of COFs with multi-site amidoxime groups could be an effective strategy to improve the adsorption affinity of COFs towards low-concentration uranium.