Conversion of waste from the vegetable oil industry into efficient adsorbents for dye removal: thermo-chemical modification of hydrochar with green modifiers, adsorption kinetics and mechanisms

Abstract The discharge of synthetic dyes into aquatic environments poses a significant environmental threat. Waste from the vegetable oil industry, such as hemp oil cake, offers a low-cost feedstock for the production of carbonaceous adsorbents for water purification due to its high carbon content and availability. In this study, hydrochars produced from hemp oil cake and cheese whey were thermo-chemically modified using natural vinegars and compared with conventional modifiers, then tested for adsorption of bromocresol green (BCG) and methylene blue (MB) dyes. According to FTIR, BET and SEM-EDS analyses, modifications with alcoholic and wine vinegar, acetic acid and KOH significantly altered the structural properties of the hydrochars, increasing the specific surface area from 1.60 to 234.35, 260.73, 71.32 and 582.84 m²/g, respectively. Among the acid-modified hydrochars, the hydrochar modified with alcoholic vinegar was the most effective, with an adsorption capacity of 15.45 mg/g for BCG and 62.16 mg/g for MB, as determined by Langmuir kinetic model, while the unmodified hydrochar had lower capacities. In comparison, the highest adsorption capacities for KOH-modified hydrochar were 640.15 mg/g for MB and 38.17 mg/g for BCG. Adsorption kinetics mostly followed a pseudo-second order model, indicating a combination of chemical and physical adsorption mechanisms driven byelectrostatic interactions, hydrogen bonding and π–π interactions. Isothermal kinetic modelling revealed heterogeneous adsorption behaviour, with the Redlich-Peterson and Freundlich models providing the best fit to the experimental data. The results emphasise the enhanced effectiveness of thermo-chemically modified hemp oil cake hydrochars for sustainable dye removal and water purification applications. Graphical Abstract