Nitrogen Abundant Polyimide Covalent Organic Frameworks as Nanotraps toward Uranium Sequestration

The design and synthesis of efficient and highly selective UO22+ nanotraps still pose difficulties in the field of radionuclide removal. Herein, a stable polyimide covalent organic framework (PI-COF-6) was rationally synthesized for targeted uranium adsorption through the irreversible imidization reaction between nitrogen-rich monomer 5,5′,5″-(1,3,5-triazine-2,4,6-triyl)tris(pyridin-2-amine) (TTPA) and pyromellitic dianhydride (PMDA). The PI-COF-6 showed satisfactory uranium adsorption performance with high capacity (424.5 mg g–1), fast kinetics (98.0% removal rate in 50 min), ultrahigh selectivity among multiple metal ions, excellent stability, and outstanding recyclability. XPS characterization and DFT calculations identified that the uranium adsorption of PI-COF-6 mainly relied on the N–N–O nanotraps which were rationally constructed by pyridine nitrogen in TTPA coupling with the tertiary amine nitrogen and carbonyl oxygen in the imide ring. Moreover, the results of high-resolution U 4f XPS spectroscopy revealed that part of U(VI) was reduced into U(IV). Hence, according to the design of rational nanotraps for targeted radioactive nuclides, highly effective adsorbents for practical application (rare earth mining waste remediation) were successfully achieved.