Valorization of agricultural hazelnut waste into high-surface-area activated carbon: sustainable methodology for accurate VOC capture and thermodynamic assessment

The conversion of agricultural residues into high-performance carbonaceous materials is a pivotal strategy for the circular economy and sustainable environmental remediation. In this study, we report the synthesis of high-surface-area activated carbon (AC) from hazelnut shells—a major agricultural byproduct in Turkey—via an eco-friendly steam activation protocol. The synthesized AC, characterized by a significant surface area of 820 m 2 /g and a well-developed microporous structure, was evaluated for the dynamic capture of a series of alkyl-substituted benzene volatile organic compounds (VOCs). A major methodological challenge in gas-phase adsorption—achieving true thermodynamic equilibrium—was addressed by implementing an innovative closed-loop reservoir (balloon) system. This novel setup provided sufficient residence time to overcome steric hindrance within the porous network, ensuring rigorous adsorption-desorption equilibrium. Comparative analysis revealed that traditional single-pass systems significantly underestimate VOC recovery by up to 22% and fail to provide reliable thermodynamic parameters. Based on high-precision equilibrium data, the adsorption process was found to be spontaneous (ΔG 0 < 0) and exothermic (ΔH 0 < 0), with a clear mechanistic correlation between the size of the alkyl substituent and adsorption efficiency due to enhanced Van der Waals interactions. These findings demonstrate that hazelnut shell-derived AC is a sustainable and highly efficient adsorbent for VOC abatement, provided that methodological accuracy is maintained through closed-loop equilibrium designs.