Diisocyanate-mediated covalent grafting of lignin onto chitosan enables hyperconjugation-driven dramatic enhancement in methylene blue adsorption.

Lignin (LG) is widely incorporated into chitosan (CS) via physical blending to increase adsorption sites and thereby enhance dye removal. However, chemical grafting of LG onto CS has received limited attention due to persistent doubts about its superiority over physical blending and unclear the synergistic mechanism. Herein, we demonstrate a diisocyanate-mediated strategy to graft LG onto CS for enhancing the adsorption capacity of methylene blue (MB) by hyperconjugation-driven: the synergistic effects of expanded adsorption sites, strengthened binding affinity, and establishment of a benzene-dominated σ → π⁎ hyperconjugation system. Specifically, LG-diisocyanate-CS (LG-D-CS) composites were successfully formed via covalent bonding with three different diisocyanates, which exhibited improved dispersion and hydrophobicity owing to the introduction of lipophilic groups compared to CS. Subsequently, LG-diphenylmethane diisocyanate-CS (LG-M-CS) was then evaluated for MB adsorption. Comprehensive isotherm, kinetic, and thermodynamic analyses confirmed that LG-M-CS (132.279 mg/g) showed four-fold boost in adsorption capacity than CS (32.514 mg/g). Furthermore, molecular simulations (MS) were performed to elucidate the adsorption mechanism, encompassing HOMO-LUMO energy gaps, molecular electrostatic potentials (MEP), Bond dissociation energy (BDE), binding energies, mean square displacements (MSD) and radial distribution functions (RDF). The results revealed benzene-rich LG and MDI segments dramatically proposed more adsorption sites, stronger intrinsic affinity, enhanced electron-donating capability and intensified π-cation interactions, providing the microscopic evidence of this electronic effect in bio-based adsorbents. This finding establishes hyperconjugation as a powerful new design principle for dramatically boosting the dye removal performance of renewable polysaccharide adsorbents.