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

Adsorption-based separation of cationic pollutants, typically ammonia nitrogen (NH₄⁺−N), from water holds great potential for environmental decontamination and resource recycling. However, NH₄⁺ 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 NH₄⁺ 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 NH₄⁺ and Na⁺ at the -SO₃Na site and the formation of hydrogen bonds (N─H─N and N─H─O) between H of NH₄⁺ 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.