PFAS compounds exist in drinking water sources predominantly as anionic species—their carboxylate and sulfonate functional groups are fully deprotonated at the pH values typical of natural waters (6.5–8.5). However, pH still exerts meaningful influence on adsorption efficiency through its effects on adsorbent surface charge, the degree of ionization of competing species, and the electrostatic environment at the adsorbent–water interface. Understanding these pH effects is important for predicting treatment performance across different source water chemistries.
pKa Values and PFAS Speciation
Long-chain perfluoroalkyl carboxylic acids (PFCAs) have pKa values generally below 3.5, meaning they are virtually completely ionized as carboxylate anions at any realistic drinking water pH. Perfluoroalkyl sulfonic acids (PFSAs) like PFOS have even lower pKa values (near 0) and are always anionic in environmental water. Short-chain PFCAs show slightly elevated pKa values—PFBA (C4) has a pKa around 0.4—but still remain fully anionic at water treatment pH ranges.
PFAS precursors such as fluorotelomer alcohols (FTOHs) are neutral species with no ionizable group. These compounds behave differently from ionic PFAS at adsorbent surfaces, generally showing higher affinity for activated carbon due to the absence of electrostatic repulsion. However, FTOHs are not directly regulated under the current MCL framework and are less commonly measured in routine monitoring.
pH Effects on GAC and Selective Adsorbent Surfaces
Activated carbon surfaces carry a net charge that depends on pH. At pH values below the carbon’s point of zero charge (PZC), typically around pH 6–8 depending on carbon type, the surface is positively charged and can attract anionic PFAS. Above the PZC, the surface becomes negatively charged, and electrostatic repulsion with anionic PFAS increases. This explains the commonly observed trend of slightly higher PFAS adsorption on GAC at lower pH values.
Ion-exchange type PFAS-selective adsorbents exhibit stronger pH dependence. Anion-exchange resins carry fixed positive charges that attract PFAS anions through Coulombic interactions, and their performance is relatively insensitive to pH within the drinking water range because PFAS remain fully anionic throughout. However, competing anions—particularly sulfate, nitrate, and bicarbonate—increase in importance at higher ionic strength, potentially reducing effective capacity.
For practical treatment system design, pH adjustment solely to optimize PFAS adsorption is rarely warranted, as the improvement in adsorption efficiency from pH reduction is modest and must be weighed against corrosion risk, chemical costs, and impacts on other treatment steps. More consequential is ensuring that pH swings during source water events (spring snowmelt, storm runoff) are considered in performance models and that pilot testing encompasses the full seasonal pH range of the source water.