PFAS Are Not All Negatively Charged
The conventional understanding of PFAS as negatively charged anionic compounds (due to their carboxylate or sulfonate functional groups) has shaped both ion exchange treatment design and regulatory thinking. But a significant fraction of PFAS compounds—including zwitterionic, cationic, and neutral variants—do not carry net negative charge and thus behave fundamentally differently in treatment systems designed for anionic PFAS.
Zwitterionic PFAS: Positive and Negative Charges Together
Zwitterionic PFAS include fluorinated betaines and amino acids present in aqueous film-forming foam formulations. These compounds carry both positive and negative charges simultaneously, producing complex adsorption behavior. In anion exchange systems designed to capture negatively charged PFAS through electrostatic attraction, zwitterionic PFAS may show dramatically reduced capture efficiency due to competing positive charge effects.
Cationic PFAS in AFFF
Some AFFF formulations contain cationic PFAS—fluorinated quaternary ammonium compounds—that carry net positive charge. Anion exchange resins, which capture PFAS through attraction between positive resin sites and negative PFAS charges, have essentially no affinity for cationic PFAS; they may actually repel these compounds. GAC provides some adsorption for cationic PFAS through hydrophobic mechanisms, but performance is poorly characterized.
Monitoring and Treatment Design Implications
Water utilities treating AFFF-contaminated water should recognize that PFAS contamination includes structurally diverse compounds not captured by treatment systems designed around anionic PFAS chemistry. Comprehensive contamination characterization using non-targeted analysis can identify zwitterionic and cationic PFAS requiring targeted treatment approaches beyond standard GAC or ion exchange systems.