Pore-Scale Modeling Of Pfas Transport In Water-Unsaturated Soils: Air--Water Interfacial Adsorption And Mass-Transfer Processes In Thin Water Films On Soil Grain Surfaces
Presenter: Siden Chen1
Co-Author(s): -
Advisor(s): Dr. Bo Guo
1Department of Hydrology and Atmospheric Sciences, University of Arizona
Per-and polyfluoroalkyl substances (PFAS) are a group of interfacially-active contaminants that adsorb at air–water interfaces in water-unsaturated soils. Because of this unique property, understanding and quantifying PFAS adsorption at air–water interfaces is critical for characterizing and remediating PFAS-contaminated soils. In the water-unsaturated soils, air–water interfaces arise from the pendular rings between soil grains (bulk capillary air–water interfaces) and the thin water films on grain surfaces (thin-water-film air–water interfaces). The latter accounts for over 90% of air–water interfaces under most field-relevant conditions. However, it remains unknown whether all thin-water-film air–water interfaces are accessible by PFAS. Potential mass-transfer limitations in thin water films may prevent PFAS from accessing the thin-water-film air–water interfaces, which can significantly reduce the retention of PFAS in water-unsaturated soils. We develop the first pore-scale modeling framework to tackle this critical knowledge gap. The model accounts for a set of PFAS-unique processes, including adsorption at bulk capillary and thin-water-film air–water interfaces as well as mass-transfer processes in thin water films. Our results and analyses suggest: (1) Mass-transfer limitations in the thin water films leads to strong nonequilibrium behaviors for the transport of PFAS through water-unsaturated soils, that is, PFAS exit the soil earlier than expected during contamination and continue to leach from the soil after the contamination process; (2) Despite the mass-transfer limitations, all of the air–water interfaces in the thin water films are accessible by PFAS due to greatly enhanced mass-transfer rates by surface diffusion of adsorbed PFAS at the air–water interfaces of thin water films. These findings highlight the importance of accounting for mass-transfer limitations in thin water films and surface diffusion for modeling PFAS transport in the variably water-unsaturated zone between land surface and groundwater table.