Efficient Cr(VI) adsorption by a cryo-structured lignin biomonolith with tailored porosity and fast kinetics

The removal of hexavalent chromium from wastewater is a pressing environmental challenge due to its high toxicity and strong mobility in aquatic systems. To address this issue, we engineered a three-dimensional porous biomonolith via redox cryopolymerization, leveraging the abundant functional groups and structural advantages of covalently crosslinked lignin and chitosan for efficient chromium adsorption. Remarkably, the developed biomonolith exhibited a high chromium adsorption capacity of 232.2 mg/g at pH 2, reaching 63.6% of equilibrium uptake within the first 30 min and achieving full equilibrium in 90 min. This enhanced performance stems from the synergistic effects of the material’s macroporous structure, which facilitates rapid chromium diffusion, and its multifunctional surface chemistry, featuring amine, amide, sulfonic, and oxygen-containing groups. These functional groups promote chromium adsorption through electrostatic interactions and partial reduction of Cr(VI) to Cr(III), as confirmed by X-ray photoelectron spectroscopy (XPS). The positively charged Cr(III) further strengthens electrostatic interactions with lignin’s oxygen-containing groups, enhancing adsorption. The chromium adsorption process followed the pseudo-second-order kinetic model (R² = 0.972) and the Freundlich isotherm (R² = 0.93), indicating chemisorption and surface complexation as the dominant mechanisms. The developed biomonolith exhibits enhanced selectivity, improved shelf stability, and excellent reusability for chromium adsorption. This lignin-based biomonolith presents a promising solution for Cr(VI) removal, offering significant potential for environmental applications and advancing research in sustainable wastewater treatment.