Deciphering Physicochemical Principles Guiding PFAS Adsorption and Thermal Degradation on Regenerable Waste-Derived Biochar

Sustainable biochar materials are promising alternatives to activated carbon for PFAS removal, yet they are facing the same challenges of poor short-chain PFAS capture and low regeneration potential. The diverse biochar properties also complicated the interpretation of PFAS removal mechanisms. To this end, we comprehensively investigated 17 biochar of varying physicochemical characteristics for their adsorption of two legacy long-chain PFAS and two emerging short-chain PFAS. While batch adsorption tests confirmed the importance of hydrophobic/electrostatic interactions, divalent cation (e.g., Ca²⁺) bridging of PFAS anions and biochar surface groups is also found essential. Likewise, micropore filling played a larger role in the adsorption of short-chain PFAS, which were prone to displacement by long-chain PFAS. Finally, spent wheat straw biochar could be effectively reactivated through 900 °C N₂ thermolysis with complete PFAS degradation, which surprisingly enhanced the adsorption of short-chain PFAS in three cycles due to micropore formation. The findings from this study underscore the importance of pore structure and indigenous ionic species in guiding biochar selections for PFAS removal, informing new strategies in the future design and engineering of biochar adsorbents toward both short- and long-chain PFAS.