Abstract
Adsorption-based separation of cationic pollutants, typically ammonia nitrogen (NH(4) (+)-N), from water holds great potential for environmental decontamination and resource recycling. However, NH(4) (+) 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(4) (+) 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(-1) 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(4) (+) and Na(+) at the -SO(3)Na site and the formation of hydrogen bonds (N─H─N and N─H─O) between H of NH(4) (+) 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.