Zwitterionic adsorbents derived from self-Exfoliated ionic covalent organic frameworks for efficient Co-Removal of anionic and cationic radionuclides

Exfoliating Covalent Organic Frameworks (COFs) into few-layer or even monolayer nanosheets not only improves the exposure of active sites and facilitates mass transfer efficiency, but also enhances their solubility or dispersion to achieve solution processability. As the Coulomb repulsion can weaken the stacking interactions, the construction of ionic COFs shows promising prospects for obtaining self-stripping nanosheets, but precisely controlling the self-exfoliated degree remains a challenge. Herein, we found for the first time that varying the number of hydroxyl groups in the building blocks could significantly affect the self-exfoliated degree for ionic COFs, and thus successfully produced a series of ionic COFs with stepwise solubility differences. Interestingly, the trend of self-exfoliated degree and solubility of cationic (EBCOFs) and anionic COFs (SDCOFs) were completely opposite to each other. Mechanistic studies indicated that more hydroxyl groups as electron donating groups increased the electron density of ionic COFs, resulting in a decrease in the Coulomb repulsion of cationic COFs (EBCOFs) but an increase in the Coulomb repulsion of anionic COFs (SDCOFs). Furthermore, in order to achieve the efficient co-removal of anionic and cationic radionuclides, we assembled cationic and anionic COF nanosheets through electrostatic attraction to obtain a novel zwitterionic COF (EB@SD COF) with both anion and cation adsorption sites. Impressively, EB@SD COF demonstrated rapid and efficient co-adsorption of ReO 4 - (a substitute for TcO 4 - ) and Sr 2+ with the maximum adsorption capacities of 117.6 and 122.4 mg/g, respectively. This work not only developed an effective zwitterionic COF adsorbent for the co-removal of radionuclides but also broadened the horizon of preparation methods for self-exfoliated ionic COFs.