pH as an Overlooked Variable in GAC PFAS Treatment
Source water pH is a parameter that water treatment operators monitor routinely, yet its effect on PFAS adsorption by GAC is frequently underappreciated. Most GAC performance models and pilot studies are conducted at or near neutral pH (6.5–7.5), which represents typical finished drinking water conditions. However, groundwater sources treated for PFAS contamination can range from pH 5 to pH 9, and this variability has measurable consequences for treatment system design and performance predictions.
The pH effect on PFAS adsorption is not a single mechanism — it operates through at least three distinct pathways: changes in PFAS speciation, changes in GAC surface charge, and indirect effects on competing natural organic matter. Understanding how these pathways interact is necessary for accurately projecting system performance across real-world source water conditions.
PFAS Speciation and the pKa Question
Perfluoroalkyl sulfonates (PFSA) such as PFOS are strong acids with pKa values well below 1 — they exist in their anionic (deprotonated) form across essentially the entire practical pH range for water treatment. Perfluoroalkyl carboxylates (PFCA) such as PFOA have pKa values in the range of 0–3, also ensuring near-complete ionization at drinking water pH conditions.
The ionization state matters because GAC surface charge and electrostatic interaction with PFAS change with pH. While the hydrophobic tail of PFAS is pH-independent, the charged head group interacts differently with the GAC surface depending on whether that surface is net positive or net negative. At low pH (< 5), some carbon surface functional groups become protonated and positively charged, which can enhance electrostatic attraction to anionic PFAS head groups. At high pH (> 8), the carbon surface becomes increasingly negative, introducing electrostatic repulsion that partially counteracts hydrophobic adsorption.
Observed pH Effects in Adsorption Studies
Laboratory isotherm studies with GAC under controlled pH conditions consistently show higher PFAS adsorption capacity at lower pH. Studies with PFOA and PFOS on bituminous and coal-based GAC typically show 15–40% higher adsorption at pH 5 compared to pH 8, depending on the carbon type and PFAS chain length. The effect is most pronounced for shorter-chain PFCAs where the hydrophobic driving force is weaker and the electrostatic contribution represents a larger fraction of total adsorption energy.
For short-chain PFAS (C4–C6), the pH effect can be substantial. At pH 8, the already-marginal hydrophobic interaction between the short fluoroalkyl tail and the GAC surface may be further undermined by electrostatic repulsion, leading to adsorption capacities that are insufficient for practical treatment goals. This pH sensitivity creates a design vulnerability for systems treating alkaline source waters.
Natural Organic Matter and pH Coupling
pH also affects the character of NOM in source water, introducing an indirect effect on PFAS removal. At higher pH, NOM humic fractions carry greater negative charge, making them more mobile in the water column and increasing their competition for positively-charged sites on the GAC surface. This indirect competition effect adds to the direct electrostatic disadvantage of alkaline conditions for PFAS adsorption.
Conversely, lower pH can reduce NOM–GAC competition, which is one reason that some pre-acidification strategies have been investigated as a means of improving GAC PFAS performance. However, pH adjustment as a treatment preprocess introduces chemical handling costs, corrosion risks, and pH re-stabilization requirements that limit its practical applicability in full-scale systems.
Implications for System Design
GAC system designers should incorporate pH as an explicit variable in design calculations, not an assumed constant. Pilot studies should be conducted at the actual pH range expected in source water, not normalized to a standard pH. For source waters with pH above 7.5 — particularly alkaline groundwaters or treated surface waters with elevated carbonate alkalinity — performance projections based on neutral-pH literature data will overestimate actual removal capacity.
For alkaline source waters where PFAS MCLs must be met reliably, the pH sensitivity of GAC is one additional factor supporting the evaluation of pH-independent removal technologies, including adsorption media that do not rely on the pH-sensitive hydrophobic mechanism for PFAS capture.