Pyrolysis-dependent performance of Miscanthus-derived biochar for simultaneous removal of short-and long-chain PFAS from water

초록

Porous carbon-based adsorbents, including activated carbon and biochar, are widely used for controlling per- and polyfluoroalkyl substances (PFAS) in water, though their effectiveness varies by congener. Long-chain PFAS are typically well removed, while short-chain PFAS exhibit lower adsorption affinity. This study evaluates biochars derived from Miscanthus by slow pyrolysis at 400 C-degrees (BC400) and 700 C-degrees (BC700) for the simultaneous removal of six perfluoroalkyl carboxylic acids and six perfluoroalkyl sulfonic acids. Adsorption isotherms, kinetics, and thermodynamics were integrated with surface and pore structure characterization to elucidate congener-specific mechanisms and interactions. BC700 consistently showed higher adsorption coefficients and faster adsorption across all congeners, attributable to its higher carbonization, lower surface polarity, and more accessible pore network. In contrast, BC400 exhibited greater surface polarity and higher concentrations of leachable divalent cations, supporting stronger headgroup-mediated interactions with short-chain PFAS. Kinetic data revealed preferential early-time site occupation by long-chain PFAS, which induced competitive exclusion effects and pore-blocking that suppressed short-chain adsorption. Thermodynamic signatures (Delta G < 0; Delta H < 0) distinguished enthalpy-driven polar interactions from entropy-favored desolvation-based partitioning. These results highlight the potential of pairing biochars with complementary properties for mixed-congener PFAS treatment. A sequential treatment strategy is proposed: BC700 is used as a front-end medium for preferential long-chain PFAS removal, followed by BC400 as a polishing stage to enhance short-chain PFAS adsorption once competitive effects are reduced.

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