Multivariate Covalent Organic Frameworks for High-Performance Ammonia Nitrogen Separation: Structure-Property-Function Relationships.

Adsorption-based separation of cationic pollutants, typically ammonia nitrogen (NH -N), from water holds great potential for environmental decontamination and resource recycling. However, NH is more challenging to adsorb than other cations due to its stable structure and relatively large ionic radius. In this study, a "multivariate" synthetic strategy is applied to construct covalent channels through rational encoding sulfonic acid groups to enhance NH adsorption and to investigate the structure-property-function relationships of sulfonated covalent organic frameworks (COFs). The optimal sulfonic acid group density is 50%, with an adsorption capacity of 17.09 mg g and an equilibrium time of 5 min, far surpassing most adsorbents. The crystallinity of COFs significantly enhances both adsorption capacity and kinetics. Surface area and hydrophilicity primarily increaseadsorption capacity, with minimal influence on kinetics. In contrast, a large pore size correlates negatively with adsorption capacity but facilitates kinetics. N K-edge near-edge X-ray absorption fine structure spectroscopy validates atomic-level adsorption mechanisms of ion exchange between NH and Na at the -SONa site and the formation of hydrogen bonds (N─H─N and N─H─O) between H of NH and pyrrolic N as well as O of carbonyl on COFs. This study provides directions for designing ultrafast and high-capacity adsorbents for cation capture.