What Isotherms Measure
Adsorption isotherms characterize the equilibrium relationship between dissolved PFAS concentration and the amount of PFAS adsorbed onto carbon media at a constant temperature. Freundlich and Langmuir isotherm models are most commonly applied to PFAS-carbon systems. Isotherm data allows engineers to estimate the maximum adsorption capacity of a given carbon product for specific PFAS compounds and compare different carbon types under standardized conditions.
Compound-Specific Isotherm Variation
PFAS compounds vary enormously in their carbon adsorption affinity. Long-chain perfluorooctane sulfonates (PFOS) and perfluorooctanoic acid (PFOA) exhibit Freundlich adsorption capacity constants an order of magnitude or more higher than short-chain compounds like PFBS or PFBA. This difference explains why standard GAC systems that perform well for PFOA and PFOS may fail to adequately treat short-chain PFAS at the low part-per-trillion concentrations now regulated under EPA MCLs.
Natural Organic Matter Competition
Isotherm data generated in clean laboratory water systematically overestimates adsorption capacity compared to performance in real surface or groundwater containing natural organic matter (NOM). NOM molecules compete with PFAS for carbon adsorption sites, and some NOM fractions are preferentially adsorbed, reducing available capacity for PFAS. Site-specific isotherm testing in actual source water provides more accurate capacity estimates for design purposes than published literature values generated in reagent-grade water.
Temperature Effects on PFAS Adsorption
Adsorption capacity decreases at higher temperatures because the adsorption of most PFAS compounds onto carbon is an exothermic process — it releases heat. Treatment plants in warmer climates or treating warmer source water (such as groundwater in summer months) will observe reduced carbon adsorption capacity relative to colder conditions. This seasonal variation should be incorporated into media replacement scheduling, particularly for systems operating near breakthrough thresholds.