Adsorption of Forever Chemical Pollutants: The Physical Chemistry of PFAS near Surfaces

Owing to their great stability and rich physical chemistry, the impact and fate of per- and polyfluoroalkyl substances (PFAS) in natural media raise increasing concerns that challenge existing water remediation strategies. In this context, while adsorption-based solutions appear to be promising efficient remediation approaches, their development is hampered by important knowledge gaps in the fundamental mechanisms governing the behavior of PFAS near solid surfaces. This review provides a comprehensive state of the art on the theoretical and experimental aspects of PFAS adsorption. By adopting a fundamental physical chemistry standpoint, we report on recent advances in understanding the complex adsorption phenomena arising from their rich and intriguing behavior in combination with the various thermodynamic and chemical conditions encountered in practical situations. First, we define the main properties of PFAS by focusing on chemical aspects such as composition, molecular structure and dissociation. We also examine their physical properties -- including the phase diagram, solubility and aggregation of PFAS. Second, we introduce the fundamental interactions involved in the adsorption of individual molecules on solid surfaces before evaluating the collective behaviors associated with the self-aggregation of PFAS molecules, the formation of ionic bridges between PFAS molecules and the solid surface, the competition with organic matter (OM) and/or the adsorption of OM-PFAS complexes. We also present the fundamentals of the thermodynamics and kinetics of PFAS adsorption by introducing classical adsorption isotherm models and highlighting the theoretical principles needed to provide a robust description framework. In particular, an accurate definition of the adsorption and desorption rates for a physical modeling of PFAS kinetics is discussed along with the key factors determining the kinetic order (i.e. first, second or mixed order kinetics). More complex aspects are then presented by emphasizing the specific characteristics of PFAS adsorption. In particular, both batch and kinetic adsorption experiments are analyzed to identify the role of the solid surface geometry and chemistry, as well as the influence of PFAS structure and chemistry on the adsorption mechanisms. The role of the water matrix is also evaluated by examining how the surface capacity and affinity for PFAS molecules is modified by environmental parameters such as pH and salinity. The impact of copollutants and OM on the adsorption kinetics is also evaluated. We conclude this review by identifying the fundamental and practical challenges that remain to be addressed and provide perspectives for the characterization and modeling of PFAS adsorption on solid surfaces and in porous materials.