Improving the Hydrophobicity of Powder Activated Carbon to Enhance the Adsorption Kinetics of Per- and Polyfluoroalkyl Substances.

Per- and polyfluoroalkyl substances (PFAS) are difficult to treat by using conventional drinking water treatment technologies. Herein, we upgrade a commercially available powder activated carbon (PAC) via an acid wash and pyrolysis to amplify hydrophobicity and enhance PFAS adsorption. Minimal differences in overall surface area, micropore volume and area, and external surface area were observed between acid-washed and pyrolyzed PACs. X-ray photoelectron spectroscopy, contact angle measurements, and scanning electron microscopy evidenced ∼5% reduced oxygen content and noticeable hydrophobicity increases for the pyrolyzed PAC, without altering morphology. Adsorption isotherms of perfluorooctanoic acid (PFOA) showed no major increases to adsorption capacity, but more rapid adsorption kinetics of PFOA and perfluorobutanesulfonic acid (PFBS) to the pyrolyzed PAC, in both low and high PFAS concentration tests, were revealed in both reagent water and synthetic natural organic matter, with overall greater removal values (e.g., ∼90% removal vs 60%, in water after 1 h at 2 mg/L PFOA). PFOA and PFBS adsorption behavior adhered to pseudo-second-order kinetics (R ² = 0.843-0.992). Density functional theory calculations quantitatively evaluated adsorption energies of PFOA and PFBS onto a graphene skeleton containing different organic functional groups, finding supportive outcomes. This work greater informs the importance of hydrophobicity for PFAS adsorption onto PAC.