A new framework for modeling the effect of salt on interfacial adsorption of PFAS in environmental systems

Abstract

Per- and polyfluoroalkyl substances (PFAS) are surface active contaminants of great environmental concern, due to their widespread historical use and their environmental persistence. Salts are known to have a profound influence on the interfacial behaviors of all ionic surfactants, including some of the most commonly detected PFAS. This work describes a new mass-action model for predicting the interfacial behavior of surfactants as a function of salt concentration. The three-parameter model is fit to interfacial tension data over a range of salt concentrations, and is then able to predict interfacial adsorption isotherms for the entire range from no added salt, up to 0.5 M added salt. The phenomenological nature of the model means that it is likely to provide more robust predictions for new systems and conditions than some of the existing empirical approaches, and the minimal number of adjustable parameters ensures that unique calibrations are possible with limited data. The model is found to be consistent with experimental data, and is bracketed by experimental values at low PFAS concentrations. Of particular interest, the model predicts the existence of sigmoidal adsorption isotherms at low salt concentrations, a deviation from isotherms calculated the commonly-used Szyszkowski equation; the observation is supported by a maximum in measured interfacial adsorption coefficient calculated from low-concentration surface tension measurements. Because adsorption affinities can vary by orders of magnitude with changing salt concentration, the ability to predict the effects of salt on adsorption is of critical importance for quantitative prediction of PFAS behavior in the environment.

Keywords: Air-water interface; Interfacial partition coefficient; Ion pairing; PFAS; Salt effects; Surface tension; Surfactants.