Speaker
Description
The alpha-radioactive alkaline effluent generated from the alkali washing course in the Purex Process represents a major challenge in downstream waste management. This necessitates the design of metal-organic frameworks (MOFs) or covalent organic frameworks (COFs) with tailored pore structures and functionalities for the highly selective capture of actinide ions under alkaline conditions. Given that radionuclides in such waste primarily exist as anionic complexes, this study employs the Williams reaction to post-synthesize COFs functionalized with quaternary ammonium (QA) groups of varying alkyl chain lengths. By controlling the grafting sites and density of these cationic groups, we effectively regulate the pore size distribution within the COFs. Twelve hierarchically porous QA-functionalized COFs were synthesized. We investigated the effects of QA group concentration and interlayer packing density on adsorption performance, revealing how pore volume effects and functional group content jointly influence the adsorption capacities for uranium and thorium (as a non-radioactive surrogate for plutonium) under alkaline conditions. The optimized material, COF-2QA-50, exhibits a high specific surface area, large pore volume, and outstanding adsorption performance, with static adsorption capacities of 568.7 mg/g for U(VI) and 508.8 mg/g for Th(IV). These highly crystalline COFs also show fast adsorption kinetics, reaching equilibrium within 30 minutes, and excellent reusability, retaining their original adsorption capacity over five consecutive adsorption-desorption cycles. Combined XPS and DFT analyses elucidated the adsorption mechanism, confirming the critical role of the balanced cavity structure and strategic positioning of QA groups in the effective capture of actinide radionuclides.