Dual-site ionized covalent organic frameworks with enhanced charge density for highly efficient and selective...

Technetium-99 present in radioactive wastewater poses serious radiological and environmental threats, as it predominantly exists as highly mobile pertechnetate (⁹⁹TcO₄^-) with a long half-life and persistence in aquatic systems. Herein, we report a dual-site ionized covalent organic framework, Ion-N-COF, for rapid and selective adsorption of perrhenate (ReO₄^-), a non-radioactive surrogate for ⁹⁹TcO₄^-, from simulated radioactive wastewater. Ion-N-COF was constructed through the simultaneous quaternization of imine and pyridine nitrogen sites, generating dense cationic binding domains within the framework. Benefiting from this structural design, Ion-N-COF exhibited rapid ReO₄^- adsorption, reaching equilibrium within 3 min, and achieved a high maximum adsorption capacity of 1561 mg·g^-1. More importantly, it showed excellent selectivity in the presence of large excesses of NO₃^- and SO₄^2- and achieved 89% ReO₄^- removal with a distribution coefficient (K_d) of 3.24 × 10⁴ mL·g^-1 in simulated Hanford low-activity waste. Mechanistic investigations revealed that electrostatic attraction at dual cationic sites served as a major contributor to ReO₄^- capture, while partial ion exchange further facilitates the adsorption process. This work provides a molecular strategy for designing high-performance ionic porous adsorbents for hazardous oxyanion remediation in complex radioactive wastewater.