Lignin-functionalized hypercrosslinked polymers for phenol removal from aqueous solutions

This study reports the synthesis of lignin-functionalized hypercrosslinked polymers (HCPs-Fe and HCPs-Al) for efficient phenol removal from aqueous media. The materials were synthesized via a two-step strategy: lignin grafting onto chloromethylated polystyrene (CMPs), followed by FeCl₃- or AlCl₃-catalyzed Friedel-Crafts alkylation. Comprehensive characterization by FT-IR, XPS, XRD, TGA, SEM, TEM, and N₂ adsorption-desorption analysis confirmed lignin incorporation and demonstrated hierarchical porosity. HCPs-Fe exhibited a high specific surface area of 421 m²/g, with 69.4% micropores, 28.5% mesopores, and 2.1% macropores. In contrast, HCPs-Al showed a lower surface area of 215 m²/g, with 43.3% micropores, 53.6% mesopores, and 3.1% macropores. The adsorbents exhibited high phenol adsorption capacities of 172.8 mg/g and 135.3 mg/g at 293 K, respectively, attributed to synergistic effects of hydrogen bonding, π-π stacking interactions, and micropore filling. Kinetic studies indicated that the adsorption process followed pseudo-second-order kinetics (R² > 0.98), whereas the isotherm analysis showed good agreement with the Freundlich model (R² > 0.99). Thermodynamic investigations demonstrated that the adsorption process was exothermic. Optimal performance was observed at neutral pH, and an adsorbent dosage of 1.0 g per 50 mL achieved approximately 90.3% phenol removal. In addition, zeta potential analysis of HCPs-Fe revealed that its isoelectric point (IEP) was within the pH range of 5-7. At pH < IEP, the adsorbent surface was protonated, yielding a positive zeta potential; whereas at pH > IEP, the surface was deprotonated, yielding a negative zeta potential. Notably, HCPs-Fe maintained 90.8% of its initial capacity after five regeneration cycles, demonstrating good reusability. The combination of renewable lignin with the robust structure of HCPs provides an environmentally friendly and effective approach for phenol-contaminated wastewater treatment.