Activated carbon prepared from hazelnut shell waste and magnetized by Fe₃O₄ nanoparticles for highly efficient adsorption of fluoride

This study involved the preparation of magnetized activated carbon (Fe₃O₄–HSAC) by first activating hazelnut shell waste, followed by coating it by Fe₃O₄ nanoparticles. The Fe₃O₄–HSAC thus prepared was evaluated as an adsorbent possessing the potential for fluoride to elimination, under a variety of conditions. From the findings, it is evident that by using the Fe₃O₄–HSAC as an adsorbent 100% fluoride removal could be accomplished under the optimum conditions cited (adsorbent dose = 0.75 g/L; pH = 3–5; and temperature = 323 K). The Halsey and Freundlich isotherm models both concurred strongly with the equilibrium adsorption data, and from the results of the Langmuir model, the maximum adsorption capacity was achieved at 146.2 mg/g when the temperature was 298 K and pH was 5. The pseudo-second-order kinetic model offered the best explanation for the adsorption process. Besides, both the intra-particle diffusion and liquid film diffusion models were found to control the kinetic mechanism of the fluoride adsorption onto the Fe₃O₄–HSAC. The quantity of adsorption energy provided using the Dubinin–Radushkevich model was 4.59 kJ/mol, indicating that the physical adsorption was predominant. Further, the negative values of Gibbs free energy change (ΔG° = -2.73 to -8.11 kJ/mol at temperature = 288 to 318 K, respectively) and the positive values of enthalpy change (ΔH° = 56.01 kJ/mol) and entropy change (ΔS° = 0.198 kJ/mol. K) suggest that the nature of the adsorption thermodynamics is endothermic, spontaneous, and physical. From this study, the observation of the outstanding performance of the Fe₃O₄–HSAC helped to conclude that this is a material of promise as a treatment agent in the fluoride elimination from contaminated water and wastewater.