(179h) Effect of PFAS Molecular Structure on Interfacial Adsorption

Per- and poly- fluoroalkyl substances, PFAS, have gained significant attention in recent years. The adverse effects of PFAS on human health have motivated ongoing research to investigate and develop effective remediation strategies. Structurally, PFAS molecules vary in terms of headgroup and chain length. These structural variations impact their transport properties within their existing environment. Developing treatment technologies requires an understanding of the PFAS behavior in water systems. In this work, we investigate how PFAS molecular structure influences their adsorption at the air-water interface.

PFAS are surface-active agents, surfactants, due to their amphiphilic property where the hydrophobic fluorinated tails are oriented away from the water surface. This adsorption modifies the surface properties, particularly the surface tension. Capturing this change among various PFAS helps explain how the molecular structure influences surface tension values. We use a micro-Wilhelmy plate to measure the surface tension of several PFAS molecules. The obtained adsorption isotherms are fitted using a modified Langmuir model, to understand the kinetics and thermodynamics of the adsorption process for PFAS molecules of different molecular structures.

Our results show that PFAS with sulphonate head groups show a greater ability to reduce the surface tension value compared to PFAS that have the same chain length but with carboxylic head group. This suggests that sulphonate PFAS pack more tightly at the air- water interface. This behavior may be attributed to various factors such as electronegativity and geometry of the headgroup. Furthermore, the results reveal that longer chain length PFAS have greater surface activity than short chain ones. This is likely due to the greater hydrophobicity of these molecules.

These findings imply that there is no single remediation technique that is universally suited to all PFAS. The higher surface accumulation of long chain length PFAS molecules and sulphonate group PFAS molecules can make their remediation by traditional foam fractionation more viable. Other PFAS molecules need further investigation to obtain a sustainable method to increase their adsorption capacity onto different surfaces. This understanding helps the development of structure-specific treatment methods for different PFAS compounds.