Facile synthesis of cationic polymer networks with ultrahigh charge density for effective capture of ReO4− (surrogate of radioactive 99TcO4−)

The proper management of radioactive nuclear waste, specifically the removal of ⁹⁹TcO₄⁻, is of strategic importance for environmental protection. Herein, we presented the novel cationic polymer networks (CPN-32, CPN-33, and CPN-34) with high-density positive charges from the straightforward quaternization of N,N,N′,N″,N″-pentamethyldiethylenetriamine. As a result, the as-prepared CPN-33 displayed a theoretical cationic content of 7.59 × 10⁻³ mol g⁻¹, which was higher than the reported cationic polymer networks. Batch adsorption experiments indicated that CPN-33 achieved the equilibrium adsorption within only 5 min and an exceptional adsorption capacity of 1658 mg g⁻¹ toward ReO₄⁻ (a safe surrogate for ⁹⁹TcO₄⁻), surpassing many reported adsorbents. Furthermore, CPN-33 maintained adsorption stability under strong acid/base, high ionic strength conditions, and radiation condition (200 kGy). They also showed effective removal for ReO₄⁻ even in the simulated Hanford low activity waste melter recycle stream and simulated Savannah River Site high-level radioactive waste stream. Moreover, the column adsorption indicated that CPN-33 could dynamically purify the ReO₄⁻ solution (5 mg L⁻¹) with a treatment capacity of 2200 column volume. The characterization analyses and Density Functional Theory calculation elucidated that the abundant quaternary ammonium sites in CPN-33 contributed to the effective ReO₄⁻ adsorption. Therefore, this study validates that the CPN-33 with high charge density holds potential values in radioactive ⁹⁹TcO₄⁻ wastewater remediation.