Adsorption of N-heterocyclic compounds from aqueous solutions by sulfonic acid-functionalized hypercrosslinked resins in batch experiments

N-heterocyclic compounds (NHCs) in industrial wastewater, such as pyridine, have potential toxicity to organisms and are difficult to biodegrade, causing great harm to the environment and human health. In this work, four sulfonic acid-functionalized resins (30-R, 40-R, 60-R and 70-R) with different sulfonation degree were prepared on the basis of the conventional hypercrosslinked resin by controlling the reaction temperature with concentrated sulfuric acid. The batch experiments showed that the adsorption capacities of pyridine on the sulfonic resins are pH-dependent, and the optimum pH increases with the increase of sulfonation degree. The adsorption of pyridine on the resin was monolayer, and the maximum adsorption capacity fitted by Langmuir model was positively correlated with sulfonation degree. The fitting results of Weber-Morris model showed that high sulfonation degree meant high intraparticle diffusion rate. The driving forces of adsorption included electrostatic attraction, pi-pi conjugation and hydrophobic interaction. Although the electrostatic attraction, which played the most important role, significantly improved the adsorption capacity of the sulfonic resin, it weakened the effect of the other two. This phenomenon was caused by the introduction of sulfonic groups significantly promoted the adsorption performance of the resin for single-ring NHCs, but that for fused-ring NHCs with pi-electron-rich regions and strong hydrophobicity was unremarkable, or even decreased. Under the synergism of the three driving forces, sulfonic acid-functionalized hypercrosslinked resin exhibited superior adsorption performance for NHCs with easy protonation, high pi-electron cloud density and strong hydrophobicity.

Automated summary

In this study, sulfonic acid-functionalized resins with different sulfonation degrees were prepared and found to exhibit superior adsorption performance for N-heterocyclic compounds with characteristics such as easy protonation, high π-electron cloud density and strong hydrophobicity. The adsorption capacities of pyridine on the resins were pH-dependent, with the optimum pH increasing with the sulfonation degree. Additionally, high sulfonation degree was related to high intraparticle diffusion rate and different driving forces of adsorption, including electrostatic attraction, pi-pi conjugation, and hydrophobic interaction, influenced the adsorption capacity of the resins.