Preparation of hydrochar from hazelnut shell optimized by Box–Behnken design for efficient dye adsorption

Hydrochar was prepared from hazelnut shell waste, which is widely available, by hydrothermal treatment. Key process variables (initial solution pH, activation solution pH, and hydrothermal temperature) were optimized using a Box–Behnken design based on response surface methodology, with methylene blue (MB) adsorption percentage as the response variable. The resulting materials were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). Adsorption performance of the optimized hydrochar was evaluated for both MB and Congo red (CR). Maximum adsorption capacities of 167.71 mg g −1 for MB and 162 mg g −1 for CR were achieved at 150 ppm initial dye concentration and pH 7. The primary adsorption mechanisms were complex formation, π – π interaction, and hydrogen bonding, with electrostatic interactions contributing to a lesser extent. Beyond its dye adsorption efficiency, the hydrochar's carbon‐rich structure and moderate heating value indicate its potential for biomass valorization, low‐grade fuel use, and integration into sustainable bioeconomy frameworks.