Air-water interfacial adsorption coefficients for PFAS when present as a multi-component mixture.
Jeff A K Silva1, William A Martin2, John E McCray3
1Arclight Research & Consulting, LLC, Golden, CO, USA; GSI North America, Inc., Flemington, NJ 08822, USA.
Journal of Contaminant Hydrology
|November 13, 2020
Summary
Competitive adsorption of per- and polyfluoroalkyl substances (PFAS) in water was studied. Models accurately predicted adsorption for similar PFAS but struggled with mixtures of different PFAS types, highlighting the need for new models.
Area of Science:
- Environmental Chemistry
- Surface Chemistry
- Chemical Engineering
Background:
- Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants.
- Understanding PFAS behavior in water is crucial for remediation and risk assessment.
- Competitive adsorption at air-water interfaces influences PFAS fate and transport.
Purpose of the Study:
- To investigate the competitive adsorption of various per- and polyfluoroalkyl substances (PFAS) mixtures at the air-water interface.
- To evaluate the predictive accuracy of the multi-component extended Langmuir (EL) isotherm model for different PFAS mixture compositions.
- To identify limitations of current models and suggest improvements for predicting PFAS adsorption.
Main Methods:
- Preparation of binary, ternary, and eight-component mixtures of perfluorocarboxylic acids (PFCAs) and perfluoroalkane sulfonates (PFSAs).
- Measurement of surface tension isotherms and calculation of air-water interfacial (AWI) adsorption data.
- Application and evaluation of the multi-component extended Langmuir (EL) isotherm model.
Main Results:
- Observed non-ideal competitive adsorption, favoring the most surface-active PFAS components.
- The EL model accurately predicted adsorption for binary and ternary PFCA mixtures.
- EL model predictions diminished for mixtures containing both PFCAs and PFSAs due to differing hydrophile structures.
- Model over- and under-predictions occurred for mixed PFCA/PFSA solutions.
- Individual PFAS adsorptive affinities can change with concentration due to competitive adsorption, a factor not captured by the EL model.
Conclusions:
- The extended Langmuir model is insufficient for predicting adsorption in complex PFAS mixtures containing dissimilar compounds.
- Alternative mathematical models are needed to account for concentration-dependent affinity coefficients.
- Accurate prediction of individual PFAS transport in mixtures requires models that capture complex competitive adsorption dynamics.
