Site-Enriched Quinoline Covalent Organic Frameworks for Selective Palladium Recovery under Acidic Radioactive Media

Palladium, as a member of the precious platinum group metals and a strategic resource, exhibits uneven global distribution and price volatility. This has sparked significant interest in its efficient recovery technologies. To tackle the challenge of highly selective Pd²⁺ recovery and Pt/Pd separation under extreme conditions, we developed a covalent organic framework (COF) adsorbent enriched with pyridinic nitrogen and carboxyl groups, synthesized via one-pot Povarov and Doebner multicomponent reactions. Intriguingly, the incorporation of multiple binding sites within the COF framework significantly enhances Pd²⁺ interactions, enabling rapid recognition and efficient Pd²⁺ extraction under highly acidic and simulated high-level radioactive conditions. The as-synthesized site-enriched COF (PQCOF-2) exhibits the highest Pd²⁺ adsorption capacity of 376 mg g^-1 in 3 M HNO₃ and achieves a Pt/Pd separation factor of 4.10. Further, PQCOF-2 showed an exceptional Pd²⁺ distribution coefficient (K_d = 6.79 × 10⁵ mL g^-1) and removal efficiency exceeding 99% in simulated high-level radioactive waste solutions, together with notable recovery ability. In brief, this work provides a versatile example for developing advanced adsorbents aimed at capturing radioactive metal ions.