Gram-scale green synthesis of a highly stable cationic covalent organic framework for efficient and selective removal of ReO4-/99TcO4-

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 99ReO4- as a surrogate for 99TcO4- show rapid adsorption kinetics, reaching near 100% removal efficiency within 1 min, 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 SO42- and NO3-. These findings highlight that the hydrothermal synthesis is an effective method to synthesize COF adsorbents for effective removal of 99TcO4- and offers a sustainable approach for practical applications.