Adsorption isotherms are the fundamental scientific tool for characterizing how much PFAS a given filter material can capture under specific conditions. Understanding this concept illuminates why filter performance ratings are conditions-dependent and how researchers evaluate new filtration materials.
What an Adsorption Isotherm Measures
An adsorption isotherm describes the relationship between the concentration of a contaminant in solution (the liquid phase) and the amount of that contaminant adsorbed onto the filter material (the solid phase) at equilibrium. By measuring how much PFAS adsorbs to a carbon sample across a range of PFAS concentrations, researchers generate a curve — the isotherm — that characterizes the material’s capacity and affinity for that specific PFAS compound.
Freundlich and Langmuir Models
PFAS adsorption onto activated carbon is commonly described by either the Freundlich or Langmuir isotherm equations. The Freundlich model captures the empirical finding that adsorption capacity increases with solution concentration in a nonlinear fashion and has no theoretical maximum capacity. The Langmuir model assumes monolayer adsorption with a finite number of sites and a defined maximum capacity. Both models are used in engineering calculations to predict how long a carbon system will perform before exhaustion.
Practical Implications
High adsorption capacity (favorable isotherm position) means a carbon material can treat more water per unit mass before reaching saturation. Higher affinity (steeper isotherm slope) means the material adsorbs PFAS effectively even at very low concentrations, which is critical for achieving regulatory targets at the parts-per-trillion level. These isotherm parameters, determined in laboratory studies, directly inform how much filter media must be installed and how frequently it must be replaced in real treatment systems.