Pore‐Scale Modeling of PFAS Transport in Water‐Unsaturated Porous Media: Air–Water Interfacial Adsorption and Mass‐Transfer Processes in Thin Water Films

Abstract

Air–water interfacial adsorption complicates PFAS transport in vadose zones. Air–water interfaces can arise from pendular rings between soil grains and thin water films on grain surfaces, the latter of which account for over 90% of the total air–water interfaces for most field‐relevant conditions. However, whether all thin‐water‐film air–water interfaces are accessible by PFAS and how mass‐transfer limitations in thin water films control PFAS transport in soils remain unknown. We develop a pore‐scale model that represents both PFAS adsorption at bulk capillary and thin‐water‐film air–water interfaces and the mass‐transfer processes between bulk capillary water and thin water films (including advection, aqueous diffusion, and surface diffusion along air–water interfaces). We apply the pore‐scale model to a series of numerical experiments—constrained by experimentally‐determined hydraulic parameters and air–water interfacial area data sets—to examine the impact of thin‐water‐film mass‐transfer limitations in a sand medium. Our analyses suggest: (1) The mass‐transfer limitations between bulk capillary water and thin water films inside a pore are negligible due to surface diffusion. (2) However, strong mass‐transfer limitations arise in thin water films of pore clusters where pendular rings disconnect. The mass‐transfer limitations lead to early arrival and long tailing behaviors even if surface diffusion is present. (3) Despite the mass‐transfer limitations, all air–water interfaces in the thin water films were accessed by PFAS under the simulated conditions. These findings highlight the importance of incorporating the thin‐water‐film mass‐transfer limitations and surface diffusion for modeling PFAS transport in vadose zones.This article is protected by copyright. All rights reserved.

The Dynamics of Per- and Polyfluoroalkyl Substances (PFAS) at Interfaces in Porous Media: A Computational Roadmap from Nanoscale Molecular Dynamics Simulation to Macroscale Modeling

“…Therefore, surface diffusion along the air−water interfaces is likely to become the dominant mass-transfer process for PFAS in thin water films. 25,36 We note here that a change in the abundance of PFAS molecules at the interface also temporally and spatially changes the surface tension which results in changes to contact angles and again to the interfacial area (i.e., PFAS are not passive scalars 37 ). To the best of our knowledge, surface diffusion along the interfaces has been overlooked in the literature of PFAS transport in the vadose zone.…”

“…Diffusion of adsorbed hydrocarbon surfactants at air–water interfaces has been reported in the surface science literature. , The magnitude of the surface diffusivity of hydrocarbon surfactants at air–water interfaces is close to that of aqueous diffusivity. However, unlike the flux due to aqueous diffusion, surface diffusion does not scale with the film thickness which means the surface diffusion flux does not decrease in thinner water films . Furthermore, for PFAS molecules, especially those with longer carbon chains and greater interfacial activity, the adsorbed mass at the air–water interfaces may represent the majority of their mass in a thin water film.…”

“…This may be even more pronounced when the thickness of the water films is down to <100 nm. Therefore, surface diffusion along the air–water interfaces is likely to become the dominant mass-transfer process for PFAS in thin water films. , …”

“…Furthermore, independent microscale studies of PFAS resembling porous media analogues of soil–water–NAPL interfaces are not numerous. Excluding industrial studies on microfabricated structures, the only study identified looked at pore scale PFAS dynamics in porous media . The pore-scale model mimicked PFAS adsorption at bulk capillary and thin-water-film air–water interfaces.…”

“…To the best of our knowledge, surface diffusion along the interfaces has been overlooked in the literature of PFAS transport in the vadose zone. 36 We believe this phenomenon, which is called the solutocapillary Marangoni (solutal Marangoni) effect, 32,33 can be significant for PFAS dynamics. Given that the solutal Marangoni effect/flow occurs under interfacial tension gradients, such an effect is expected in the presence of PFAS, being highly surface active.…”