Gram-scale green synthesis of a highly stable cationic covalent organic framework for efficient and selective removal...

Covalent organic frameworks (COFs) have developed as efficient and selective adsorbents to mitigate ⁹⁹TcO₄ ^- contamination. However, the eco-friendly and scalable production of COF-based adsorbents for the removal of ⁹⁹TcO₄ ^- has not yet been reported. This study explores the potential of a cationic COF (TpDB-COF) synthesized via a green hydrothermal method, achieving gram-scale yields per batch, thereby addressing a significant limitation of existing COF production methods. The TpDB-COF demonstrates an exceptional stability in strongly acidic conditions (2 weeks in 3 M HNO₃), as well as in various organic solvents, making it suitable for harsh nuclear waste environments. Adsorption experiments using ReO₄ ^- as a surrogate for ⁹⁹TcO₄ ^- show rapid adsorption kinetics, reaching nearly 100% removal efficiency within 1 min (with initial concentration of 28 ppm at a solid-to-liquid ratio of 1 g L^-1), a maximum adsorption capacity of 570 mg g^-1 and excellent stability. Moreover, the COF maintains high selectivity for ReO₄ ^- even in the presence of competing anions such as SO₄ ^2- and NO₃ ^-. These findings highlight that the hydrothermal synthesis is an effective method to synthesize COF adsorbents for efficient removal of ⁹⁹TcO₄ ^- and offers a sustainable approach for practical applications.