Anomalous Adsorption Of Pfas At The Thin-Water-Film Air–Water Interface And The Impact On Pfas Leaching In The Vadose Zone
Presenter: Wenqian Zhang1
Co-Author(s): Baike Xi
Advisor(s): Bo Guo
1Department of Hydrology and Atmospheric Sciences, University of Arizona
Per- and poly-fluoroalkyl substances (PFAS) are interfacially-active contaminants that adsorb at air–water interfaces (AWIs). The vadose zone has abundant with AWIs, which generally consist of two types: one is associated with the pendular rings of water between soil grains (i.e., bulk AWI) and the other arises from the thin water films covering the soil grains. To date, the two types of AWIs have been treated the same when modeling PFAS retention in vadose zones. However, the presence of electrical double layers at electronically charged soil grain surfaces and the subsequently modified electrical potential at the AWI may significantly change the adsorption of electronically charged PFAS at the thin-water-film AWIs relative to that at the bulk AWI. Given that thin water films contribute to over 90% of AWIs in the vadose zone under many field-relevant wetting conditions, it is critical to quantify the potential anomalous adsorption of PFAS at the thin-water-film AWIs. We present a mathematical model to quantify this anomalous adsorption. The model couples the chemical equilibrium of PFAS with the Poisson-Boltzmann equation that governs the electrical potential distribution in a thin water film. A up-scaling framework is derived to implement the anomalous adsorption model into a model for PFAS transport in the vadose zone. Our model analyses suggest that PFAS adsorption at thin-water-film AWIs can deviate significantly from that at bulk AWIs. The deviation increases for lower porewater ionic strength, thinner water film, and higher soil grain surface charge. The PFAS retention in the vadose zone can either be enhanced or reduce depending on the charge sign of PFAS. These results highlight the importance of accounting for the anomalous adsorption of PFAS at the thin-water-film AWIs when modeling PFAS fate and transport in the vadose zone.