The Physical Chemistry of Successful PFAS Adsorption
Understanding why activated carbon works well for PFOA and PFOS—and why this success does not extend to other PFAS—requires examining the molecular properties that drive adsorption. The exceptional performance of GAC for these specific compounds reflects a fortunate alignment of molecular characteristics that doesn’t generalize to the broader PFAS family.
Hydrophobicity: The Primary Driver
PFOA and PFOS are highly hydrophobic despite containing oxygen and fluorine atoms, because the fluorocarbon chain repels water molecules strongly. Log Kow values for PFOA (~6) and PFOS (~6.3) indicate strong hydrophobic character that drives these compounds from the aqueous phase onto carbon surfaces. This hydrophobic interaction is the primary mechanism for their high GAC adsorption capacity and makes carbon highly selective for these specific compounds.
Molecular Size Contribution
PFOA and PFOS molecules are large enough to interact with multiple carbon surface sites simultaneously through van der Waals forces, increasing binding stability. The optimal size match between these specific compounds and the micropore structure of granular activated carbon contributes to their exceptional adsorption—a structural coincidence that does not hold for shorter or differently structured PFAS.
Electrostatic Effects at Typical pH
At typical drinking water pH, both PFOA (pKa ~0-3.5) and PFOS (pKa ~-3.3) are fully dissociated and carry negative charge. Activated carbon surfaces are typically also slightly negative at ambient pH, creating mild electrostatic repulsion. The hydrophobic interaction overwhelms this repulsion for PFOA/PFOS—but for short-chain PFAS with weaker hydrophobicity, the electrostatic repulsion becomes a proportionally larger barrier to adsorption, further explaining their poor GAC performance.