Phosphonate-Decorated Covalent Organic Frameworks for Actinide Extraction: A Breakthrough Under Highly Acidic Conditions

Although solid-phase extraction is a useful approach for metal ion separation from aqueous solutions, existing sorbents suffer from low extraction efficiencies and/or instability when in contact with strong acidic media. We report here the first study on rational design and fabrication of phosphonatedecorated covalent organic frameworks, COF-IHEP1 and COF-IHEP2, for efficient and selective extraction of of uranium (VI) [U(VI)] and plutonium(IV) [Pu(IV)] from highly acidic solutions. We found that the negatively charged frameworks with excellent stability under harsh conditions and strong chelating ability from incorporated phosphonate moieties make the COFs superb U(VI) and Pu(IV) sorbents. In 1 M HNO 3 solution, COF-IHEP1 achieved recorded U(VI) uptake of 112 mgg -1 , with extremely high selectivity toward Pu(IV) even in the presence of large excess of competing metal cations. Our mechanistic study confirms a sandwich-type microstructure of hydrated U(VI) and Pu(IV) cations bound by the oxygen sites of phosphonate groups. These newly fabricated tailored hybrid porous materials could afford new opportunities to achieve highly efficient actinide extraction from acidic nuclear fuel effluents.