Adsorption behaviors and mechanisms of porous hypercrosslinked polymers with adjustable functional groups toward doxycycline hydrochloride from water

Imino hypercrosslinked polymers (NH-HCPs), amino hypercrosslinked polymers (NH2-HCPs), and carboxyl hypercrosslinked polymers (COOH-HCPs) were synthesized through cross-linking and Friedel-Crafts reactions to serve as highly efficient adsorbents for doxycycline hydrochloride (DOX) in water. These polymers, NH-HCPs, NH2-HCPs, and COOH-HCPs, exhibited specific surface areas measuring 450, 267.576, and 94.39 m(2)/g, respectively. The adsorption kinetics of DOX onto these polymers were consistent with the pseudo-second-order model, while the adsorption isotherms followed the Langmuir model (NH-HCPs) and Freundlich model (NH2-HCPs and COOH-HCPs), respectively. The maximum DOX adsorption capacities for NH-HCPs, NH2-HCPs, and COOH-HCPs were 166.82, 132.43, and 72.07 mg/g, respectively. Simulation results indicated that COOH-HCPs exhibited the strongest adsorption capability due to a substantial presence of oxygen and nitrogen groups on its surface, enabling the formation of hydrogen bonds with DOX. However, its actual adsorption capacity was the lowest among the polymers, indicating that structural adjustments played a more significant role in improving adsorption performance compared to functional adjustments. Adsorption experiments conducted with NH-HCPs and NH2-HCPs further supported this hypothesis. The primary DOX adsorption mechanism of NH-HCPs, NH2-HCPs, and COOH-HCPs involved the H-bonding of oxygen and nitrogen functional groups, along with other mechanisms such as pi-pi conjugated effects, pore-filling effects, electrostatic interactions, and acid-base interactions. Overall, this study demonstrates the effectiveness of NH-HCPs, NH2-HCPs, and COOH-HCPs in DOX removal from water, highlighting the significant influence of structural adjustments on adsorption performance.