Tailoring Alfalfa-Derived Biochar via Pyrolysis Temperature for Enhanced Adsorption of Diverse Per- and Polyfluoroalkyl Substances in Water

This study investigated the removal of long- and short-chain per- and polyfluoroalkyl substances (PFAS) such as perfluorooctanoic acid (PFOA) and perfluorobutanoic acid (PFBA) using alfalfa-derived biochar produced at different pyrolysis temperatures (300–900 °C). Increasing pyrolysis temperature substantially altered the physicochemical properties of the biochars, resulting in higher surface area, higher fixed carbon content, lower O/C and H/C ratios, and more condensed carbon structures. Among the tested materials, the biochar produced at 900 °C (AF900) showed the highest adsorption performance for both PFOA (0.39417 mg/g BC) and PFBA (0.0972 mg/g BC). Adsorption was strongly pH-dependent, with the highest removal observed at pH 3.0, and PFOA consistently showed much greater adsorption than PFBA. Possible mechanisms may include electrostatic, hydrophobic, and π–π interactions. The batch adsorption studies indicated that PFOA was better fitted to the pseudo-second-order and Freundlich models, and PFBA to the Elovich and Temkin models, while the adsorption of PFOA and PFBA followed endothermic reactions. The reusability experiment for AF900 showed a gradual decrease in regeneration efficiency over three cycles for PFOA and PFBA, with a relatively higher decrease for PFBA. Adsorption of PFOA and PFBA on AF900 in groundwater was lower than that in DI water, although increasing the loading rate of AF900 substantially improved PFAS removal in groundwater. Overall, AF900 was the best-performing material for adsorption of PFOA and PFBA among the alfalfa-derived biochars prepared at various temperatures in this study, whereas its removal performance in groundwater needs to be further studied.