Thermodynamics of Adsorption and Temperature Dependence
Adsorption of organic compounds on activated carbon is generally an exothermic process — heat is released as molecules transfer from the liquid phase to the solid surface. A fundamental consequence of Le Chatelier’s principle applied to this exothermic equilibrium is that decreasing temperature favors the adsorption reaction, increasing equilibrium adsorbed-phase concentration, while increasing temperature shifts the equilibrium toward desorption. For PFAS adsorption on GAC, this temperature dependence is experimentally well-documented: lower water temperatures (typical of winter months) produce higher adsorption capacity (higher K_F) and more complete removal at a given liquid-phase PFAS concentration, while higher summer temperatures reduce effective capacity.
The magnitude of this temperature effect for PFAS is compound-dependent and typically modest relative to the chain-length effect, but operationally significant. Published studies report Freundlich K_F decreases of 15-30% for PFOA when temperature increases from 10°C to 25°C. For PFOS, the temperature sensitivity is similar in relative terms. Short-chain PFAS show comparable or slightly larger relative temperature effects, though the absolute change in K_F is smaller due to their lower baseline adsorption affinity. The practical consequence is that a GAC system designed to meet a performance target in winter may fall marginally short in summer — or a system operating with minimal safety margin in summer may have adequate capacity in winter.
Seasonal Performance Variation in Full-Scale GAC Systems
In full-scale municipal GAC systems treating PFAS-contaminated source water, seasonal performance variation is a documented operational reality. Effluent PFAS concentrations from fixed-bed GAC contactors typically show a seasonal pattern: lower concentrations (better removal) in winter months when source water temperature is 5-12°C, higher concentrations (reduced removal) in summer months when source water temperature may reach 18-25°C. This pattern is superimposed on the long-term trend of gradually increasing effluent concentration as the bed approaches exhaustion, making it important to separate temperature effects from exhaustion effects when interpreting monitoring data.
The seasonal temperature effect interacts with other seasonal variables that affect PFAS removal. Summer months also typically bring higher NOM loads from algal growth and runoff, increased biological activity in open-bed GAC systems, and in some regions higher source water PFAS concentrations from reduced dilution flows. The combination of reduced adsorption capacity (temperature effect), increased competition (NOM effect), and potentially higher PFAS loading creates a seasonal worst-case scenario for GAC performance that should inform operational planning and bed replacement scheduling.
Quantifying the Temperature Effect for Design Purposes
The van’t Hoff equation relates the temperature dependence of equilibrium constants to the enthalpy of adsorption: ln(K2/K1) = -(ΔH/R) × (1/T2 – 1/T1), where K represents the Freundlich K_F at temperatures T1 and T2 (in Kelvin), ΔH is the isosteric enthalpy of adsorption, and R is the gas constant. Published isosteric enthalpies of adsorption for PFAS on GAC range from approximately -10 to -40 kJ/mol, with higher-affinity long-chain compounds generally showing larger enthalpy magnitudes. Using these values, a 15°C temperature increase (10°C to 25°C) corresponds to a 10-25% reduction in the Freundlich K_F constant, depending on the specific PFAS compound and GAC type.
For engineering design, this means that column models and breakthrough predictions calibrated against bench-scale isotherm data generated at a single temperature (typically laboratory ambient, approximately 20-22°C) will underpredict summer performance degradation in cold-climate source water systems and overpredict summer performance in warm-climate source water systems. Temperature-adjusted isotherm parameters should be used in design calculations, with the worst-case summer temperature applied when sizing GAC beds for compliance with low MCLs such as the 4 ppt PFOA limit.
Operational Implications for Compliance Monitoring Programs
Water utilities operating GAC systems for PFAS compliance should design their effluent monitoring programs to account for seasonal temperature effects. Monitoring frequency should be higher during summer months when the combined effects of elevated temperature, increased NOM competition, and higher biological activity create peak stress on GAC performance. Operational safety factors in bed life calculations should be set using summer-condition performance data rather than annual averages or winter performance benchmarks.
For point-of-use filter applications in residential or commercial settings, temperature variation within buildings is smaller than in open source water systems, limiting the practical significance of the temperature effect for indoor filter cartridges. However, in outdoor or semi-outdoor applications — distribution point-of-entry systems, outdoor treatment stations — seasonal temperature variation can be substantial, and replacement intervals calibrated on laboratory data at 20°C may require adjustment for high-temperature deployment environments. Filter manufacturers specifying replacement intervals based on rated service volume at a fixed temperature should clearly state the test temperature and provide guidance for performance adjustment at higher operating temperatures.