Prediction of the air-water interfacial adsorption of per- and polyfluoroalkyl substances (PFAS)

Abstract

Per- and polyfluoroalkyl substances (PFAS), also known as fluorinated surfactants, are a class of emerging contaminants. Historical use of PFAS for commercial applications has caused widespread contamination in surface water, groundwater and soil/sediments worldwide, with broad environmental and human health implications. As such, understanding the mechanisms that control the fate and transport of PFAS compounds in a range of environmental conditions is of particular interest.

Air-water interfacial adsorption is an important environmental process that contributes to PFAS fate and transport. It is well known that air-water interfacial behaviour of PFAS, and generally of surfactants, is strongly impacted by the molecular chemical structures (i.e., hydrophobicity of the carbon chains and hydrophilicity of functional groups) and environmental conditions (e.g., salinity). Two significant challenges related to the air-water interfacial adsorption of PFAS are (1) the large number of single PFAS compounds with diverse molecular structures in the environment, and (2) the influence of dynamic environmental conditions on interfacial behaviour. Limited research is currently available to adequately predict the air-water interfacial activity of PFAS compounds in natural conditions with varied salinity. Therefore, the aim of this thesis is to develop quantitatively predictive models to predict the interfacial behaviour for a wide range of environmentally relevant PFAS with differing composition and concentration of inorganic salts.

To achieve this aim, the first part of the thesis presents a group contribution model to quantitatively predict the interfacial affinity for PFAS based on PFAS chemical structure. Literature values for air-water surface tensions were collected for a range of PFAS and conventional hydrocarbon surfactants, and then fitted to the Langmuir-Szyszkowski equation to quantify the interfacial affinity for single surfactants. These data were subsequently used as input to the group contribution model in order to determine the specific molecular component parameters. Using these parameters, the interfacial affinity is then calculated for any PFAS with known molecular components. In the next part of this thesis, a new model (named UNSW-OU) was developed based on the mass action law. This model predicts the air-water interfacial affinity for different salt concentrations from 0 to 0.5 M. This model was then expanded to predict the impact of salt composition, with measured surface tension data for PFAS solutions containing diverse salt compositions and concentrations collected via laboratory experiments. These data were then used as model inputs to calculate parameters that are specific to surfactants and different salt types. With these parameters, interfacial affinity is calculated for different anionic PFAS solutions containing monovalent and divalent salt components with an ionic strength up to 0.5 M.

This thesis provides a quantitative approach to predict interfacial behaviour for a wide range of environmentally relevant PFAS under different inorganic salt concentrations. As salts are ubiquitous, and vary from site to site, a small change in salt concentration or composition is shown to have a substantial impact on PFAS interfacial behaviour. Therefore, the ability to calculate interfacial affinity in different salt conditions is important to achieve accurate predictions for PFAS transport in the vadose zone. Further, the knowledge obtained from this thesis is beneficial where the air-water interfacial area is significant, including in the long-range transport of PFAS due to interfacial adsorption via the sea-spray surface, and in PFAS treatment using gas bubbling and foam forming techniques.