Modulating Anion Nanotraps via Halogenation for High-Efficiency ⁹⁹TcO₄^–/ReO₄^– Removal under Wide-Ranging pH Conditions

Abstract

Efficient and sustainable methods for ⁹⁹TcO₄^- removal from acidic nuclear waste streams, contaminated water, and highly alkaline tank wastes are highly sought after. Herein, we demonstrate that ionic covalent organic polymers (iCOPs) possessing imidazolium-N⁺ nanotraps allow the selective adsorption of ⁹⁹TcO₄^- under wide-ranging pH conditions. In particular, we show that the binding affinity of the cationic nanotraps toward ⁹⁹TcO₄^- can be modulated by tuning the local environment around the nanotraps through a halogenation strategy, thereby enabling universal pH ⁹⁹TcO₄^- removal. A parent iCOP-1 possessing imidazolium-N⁺ nanotraps showed fast kinetics (reaching adsorption equilibrium in 1 min), a high adsorption capacity (up to 1434.1 ± 24.6 mg/g), and exceptional selectivity for ⁹⁹TcO₄^- and ReO₄^- (nonradioactive analogue of ⁹⁹TcO₄^-) removal in contaminated water. By introducing F groups near the imidazolium-N⁺ nanotrap sites (iCOP-2), a ReO₄^- removal efficiency over 58% was achieved in 60 min in 3 M HNO₃ solution. Further, introduction of larger Br groups near the imidazolium-N⁺ binding sites (iCOP-3) imparted a pronounced steric effect, resulting in exceptional adsorption performance for ⁹⁹TcO₄^- under super alkaline conditions and from low-activity waste streams at US legacy Hanford nuclear sites. The halogenation strategy reported herein guides the task-specific design of functional adsorbents for ⁹⁹TcO₄^- removal and other applications.