The pH of source water affects PFAS behavior in filtration systems in ways that matter for treatment performance, particularly for activated carbon and membrane-based systems. While most municipal water falls within pH ranges where these effects are modest, understanding pH interactions helps optimize treatment in atypical water conditions.
pH and PFAS Charge State
PFAS carboxylic acids (like PFOA) and sulfonic acids (like PFOS) are fully deprotonated (negatively charged) at the pH of typical drinking water (6.5 to 8.5), given their low pKa values. However, some PFAS precursor compounds and very short-chain acids may be partially protonated at lower pH values, affecting their interaction with filter surfaces and membranes. In very acidic conditions, partially protonated (neutral) PFAS may adsorb more strongly to carbon due to reduced electrostatic repulsion from carbon’s slightly negative surface.
pH and Activated Carbon Performance
Studies examining PFAS adsorption on activated carbon across pH ranges show generally decreasing adsorption as pH increases in the range of 5 to 9. At higher pH, both carbon surfaces and PFAS become more negatively charged, increasing electrostatic repulsion that partially counteracts hydrophobic adsorption. The practical magnitude of this effect in the pH range of drinking water is typically modest — a few percent difference in removal rate — but can be meaningful when optimizing treatment for high-removal-efficiency targets.
pH and Membrane Rejection
NF and RO membrane surface charge becomes more negative as pH increases, enhancing electrostatic repulsion of anionic PFAS at higher pH. Nanofiltration in particular shows improved PFAS rejection at higher pH due to this charge effect. For utilities or researchers optimizing NF system design for PFAS control, operating at or adjusting to higher pH within regulatory ranges can improve rejection efficiency with appropriate engineering consideration.