Modulating phenolic–lignocellulosic synergy in rapeseed hull food waste for enhanced and sustainable Cr(VI) remediation

Rapeseed hulls are an abundant food-processing by-product whose improper disposal contributes to biomass waste and the loss of potentially valuable functional materials. This study investigated whether modulating endogenous phenolic–lignocellulosic interactions in rapeseed hull food waste could enhance chromium(VI) adsorption and reduction while supporting sustainable biomass valorization. Cr(VI) is a highly toxic, mobile, and persistent contaminant that poses serious environmental and public health risks. Defatting and phenolic extraction were applied to improve surface accessibility, expose reactive functional groups, and strengthen the adsorption–reduction synergy of rapeseed hull-derived biosorbents. Structural characterization using Fourier-transform infrared spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy confirmed substantial changes in surface morphology, elemental composition, and functional-group distribution following treatment. Among the prepared materials, the defatted and phenolic-extracted rapeseed hull residue, designated dRSH-R, exhibited the highest Cr(VI) adsorption capacity of 65.11 mg g⁻¹ in multi-metal systems, together with strong Cr(VI) selectivity and excellent regeneration performance. The pseudo-second-order kinetic model provided the best fit to the experimental data, indicating that adsorption was primarily governed by surface-controlled interactions. The Freundlich isotherm described the equilibrium behavior, suggesting heterogeneous and multilayer adsorption. Spectroscopic analyses showed that oxygen- and nitrogen-containing functional groups played important roles in Cr(VI) binding and transformation. Phenolic extraction converted conjugated phenolic compounds into more accessible free forms with enhanced redox activity, thereby promoting the reduction of highly toxic Cr(VI) to the less harmful Cr(III) species. Recombination experiments further confirmed the synergistic contribution of phenolic compounds and lignocellulosic components to Cr(VI) removal. By regulating the natural coexistence and accessibility of biomass constituents without introducing external functionalizing agents, the proposed strategy offers a comparatively simple, sustainable, and industrially applicable modification route. Overall, the findings demonstrate that rapeseed hull food waste can be converted into an efficient and regenerable biosorbent for chromium-contaminated wastewater. This approach simultaneously advances food-waste valorization, wastewater remediation, resource recovery, and circular bioeconomy development.