Influence of Molecular Structure on the Adsorption of Per - and Polyfluoroalkyl Substances (PFASs) to Activated Carbon in the Aqueous Phase

Per- and polyfluoroalkyl substances (PFASs) exhibit extreme biological, chemical, and physical stability due to the strong carbon–fluorine bond. Because of their unique chemical and physical properties, PFASs are not efficiently removed in wastewater treatment plants, leading these facilities to serve as point sources for PFASs entering the environment. However, the mechanisms of PFAS removal in treatment systems remain poorly understood, largely due to their unique chemical characteristics and the diverse structures of compounds with varying chain lengths and functional groups. This study examined how the structural characteristics of individual PFAS congeners, such as carbon-fluorine chain length and functional groups, affect their adsorption onto activated carbon during the removal process. Specifically, six PFCAs (F(CF<sub>2</sub>)<sub>n</sub>COO<sup>-</sup>, where 3≤n≤8) and six PFSAs (F(CF<sub>2</sub>)<sub>n</sub>SO<sub>3</sub><sup>-</sup>, where 4≤n≤8) were investigated. The interactions governing the mechanisms of PFAS removal by activated carbon were analyzed using thermodynamic and kinetic models. The adsorption of PFASs onto activated carbon was found to increase with the number of fluorinated carbons in the PFAS molecules. Furthermore, the adsorption of PFSAs was greater than that of PFCAs with the same number of fluorinated carbons. These findings suggest that long-chain PFASs exhibit increased hydrophobicity due to their extended hydrophobic tails, which enhance their adsorption capacity on the hydrophobic surface of activated carbon. Additionally, PFSAs, which possess larger steric hindrance and greater molecular size due to the sulfonate group, demonstrated higher hydrophobicity and, consequently, greater adsorption compared to PFCAs with an equivalent number of fluorinated carbons. Therefore, long-chain PFASs are more likely to compete aggressively for adsorption sites on activated carbon.