Synergistic reinforcement of lignin–chitosan hydrogels using vanillin as a sustainable green crosslinker

While bio-based hydrogels derived from lignin and chitosan are promising sustainable adsorbents, their practical application is hindered by poor mechanical stability. This study addresses this limitation by synthesizing lignin-chitosan hydrogels using vanillin as a green, bio-derived crosslinker via a facile solution-casting method. The formation of imine (Schiff base) linkages between the aldehyde groups of vanillin and the primary amines of chitosan was confirmed by FTIR spectroscopy (C = N stretching at ~ 1602 cm − 1 ). SEM and TGA results demonstrated a reduction in the pore size of the crosslinked hydrogel and an enhanced residual thermal mass. Rheological and tensile testing revealed robust network reinforcement, with a 32.3% increase in tensile strength compared to non-crosslinked analogs. Despite a slight decrease in swelling capacity due to a denser network, the crosslinked hydrogels exhibited versatile, dual-functional adsorption, effectively sequestering both cationic and anionic dyes. These achieved ~ 95% removal of methylene blue, ~ 94% removal of rhodamine 590 chloride, and ~ 92% removal of methyl orange. The adsorption adhered to the Langmuir isotherm and pseudo-first-order kinetics, suggesting a controlled, monolayer chemisorption process. These findings highlight the potential of vanillin-mediated crosslinking to produce high-performance, fully bio-based adsorbents for sustainable wastewater remediation.