Cationic covalent organic frameworks electrode for 99TcO4- electro-adsorption with enhanced removal property

Abstract

Electro-adsorption approach has been exhibited enormous promise for radionuclides removal from radioactive wastewater, and the fabrication of functional electrodes is of great significance for achieving efficient and selective removal. Herein, a cationic covalent organic framework (COFs) was synthesized through aldimine condensation reaction between 3,8-diamino-5-methyl-6-phenylphenanthridinium bromide and 1,3,5-tris(p-formylphenyl)benzene and then modified onto carbon felt electrode for efficient separation of TcO4- by electro-adsorption. The cationic COFs functionalized electrode showed efficient separation properties for TcO4- with high selectivity, fast kinetic, and large uptake capacity. The adsorption equilibrium of the functional electrode for ReO4- (a surrogate for 99TcO4-) can be reached within only 60 s by electro-adsorption with 1.2 V applied voltage, much faster than physicochemical adsorption (10 min). The maximum electro-adsorption capacity was calculated to be 990.1 mg/g by Langmuir model, at least 2.1 times larger than the value of 473.9 mg/g acquired via physicochemical adsorption. In the meantime, the cationic COFs functionalized electrode exhibited high selectivity for ReO4-. The sorption mechanism was studied by XPS, EDX and DFT, the results revealed that the adsorption of ReO4- onto the cationic COFs mainly involve an ion exchange process between ReO4- and Br-. This work provides a new method for efficient and selective elimination of TcO4-, which can be extended to eliminate other contaminants through rational electrode design and manufacture.