Synthesis of phosphorylated covalent organic frameworks with different linkages–Critical factors controlling the extraction of uranium from highly acidic media

The ability to adsorb uranium from acidic solutions is generally inadequate due to the instability of adsorbents and the significant protonation of functional groups. In this study, we synthesized two phosphorylated covalent organic frameworks (COFs, P-St-TzDa-Phos and TzPa-Phos) in addition to the previously examined TzDa-Phos, that are similar in chemical composition and structure but differ in their linkage units, ranging from strong quinoline and −C(H 2 PO 3 )–NH- linkages to dynamic imine bonds. Adsorption tests indicated that the ability to adsorb uranium followed the order of P-St-TzDa-Phos > TzDa-Phos > TzPa-Phos at a pH of 3.0 and in more acidic conditions. This trend was attributed to the more negative charge of the superior framework or lower positive charge, which enhances the electrostatic attraction of cationic uranium species to the adsorbent, as well as the higher grafting ratios of phosphonic acid groups, which are the main sites for uranium binding. Notably, P-St-TzDa-Phos demonstrated uranium adsorption capacities of 106 mg/g at pH 1.0 and 45 mg/g under 3 M HNO 3 conditions, making it one of the most effective adsorbents in this area. This research could inform the design and creation of new solid-phase adsorbents for the efficient extraction of uranium from highly acidic solutions.