Efficient capture of ⁹⁹TcO₄^-/ReO₄^- via node and linker bifunctional anion exchange covalent organic frameworks

In nuclear wastewater treatment, ion-scavenging materials designed to trap ⁹⁹TcO₄^- is urgently needed. However, strong acid/base, high radiation and high salt concentration of nuclear wastewater usually result in inadequate stability and adsorption capacity of the adsorbent. Herein, we report a new class of bifunctional anion-exchange olefin-linked COF (BPDC-MTMP) prepared via Knoevenagel condensation reactions, the first example exploring the synergistic integration of positively charged fragments at both nodes and linkers. Surprisingly, BPDC-MTMP exhibits a record ReO₄^- (a non-radioactive surrogate of ⁹⁹TcO₄^-) adsorption capacity up to 1593.21 mg g^-1, its outstanding adsorption capacity can be attributed to the synergistic enhancement of the positively charged fragments of the nodes and linkers leading to a significant increase in the positive charge density and the number of anion exchange sites. BPDC-MTMP's hydrophobicity is enhanced by the highly conjugated bulky alkyl skeleton, the affinity toward ReO₄^- and chemical stability are therefore significantly improved, ReO₄^- can be selectively and reversibly extracted even under strong acid/base and high salt concentration solutions. This study illustrates that the node and linker bifunctional anion exchange COF is of great potential for ReO₄^-/⁹⁹TcO₄^- trapping, which provides a new method to design high-performance adsorbents for the treatment of nuclear wastewater.