Hazelnut Husk Waste-Derived Activated Carbon for Adsorption of a Remdesivir-Containing Commercial Formulation: Equilibrium, Kinetic, and Thermodynamic Studies

The occurrence of antiviral pharmaceutical residues in aquatic environments has raised increasing concern regarding their environmental persistence and removal by conventional wastewater treatment processes. In this study, activated carbon was prepared from hazelnut husk agro-waste using KOH-assisted chemical activation and evaluated for the adsorption of a remdesivir-containing commercial pharmaceutical formulation from aqueous media. Textural and morphological characterization by Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) indicated the development of a porous carbon structure with a BET surface area of 441.72 ± 7.72 m2 g−1. Batch adsorption experiments were performed to investigate the effects of adsorbent dose, solution pH, initial concentration, contact time, and temperature. The equilibrium data were described by Langmuir, Temkin, Freundlich, and Dubinin–Radushkevich models using nonlinear regression and multiple error functions. The Langmuir model provided the best statistical description among the tested models, with a calculated maximum capacity of 199.11 mg g−1 (R2 = 0.9759), whereas the highest experimentally determined uptake reported in the equilibrium experiments was 205.60 mg g−1. The adsorption kinetics were best described by the pseudo-second-order model (R2 = 0.999); however, this empirical kinetic fit was not interpreted as independent proof of possible chemical contribution. Thermodynamic analysis based on the temperature-dependent equilibrium constants yielded a negative enthalpy change of −49.29 kJ mol−1 and negative Gibbs free-energy values over the investigated temperature range, indicating a favorable and exothermic adsorption process under the experimental conditions. The results suggest that the adsorption behavior may involve a combination of pore accessibility and non-covalent adsorbent–adsorbate interactions. Because a commercial formulation containing sulfobutylether-β-cyclodextrin (SBECD) was used, possible contributions of the excipient to adsorption and chromatographic response cannot be completely excluded. Therefore, the present findings should be regarded as an initial assessment of the adsorption behavior of a remdesivir-containing pharmaceutical formulation rather than direct evidence of treatment performance in real wastewater.