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

Abstract

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 SO₄²⁻ 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.