Search for: All records

Abstract Per‐ and poly‐fluoroalkyl substances (PFAS) are interfacially‐active contaminants that adsorb at air‐water interfaces (AWIs). Water‐unsaturated soils have abundant 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 of soil grain surfaces and the subsequently modified chemical potential of PFAS at the AWI may significantly change the PFAS adsorption at the thin‐water‐film AWI 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 AWI. We develop a thermodynamic‐based mathematical model to quantify this anomalous adsorption. The model couples the chemical equilibrium of PFAS with the Poisson‐Boltzmann equation that governs the distribution of electrical potential in a thin water film. Our model analyses suggest that PFAS adsorption at thin‐water‐film AWI can deviate significantly (up to 82%) from that at bulk AWIs. The deviation increases for lower porewater ionic strength, thinner water film, and higher soil grain surface charge. These results highlight the importance of accounting for the anomalous adsorption of PFAS at the thin‐water‐film AWI when modeling PFAS fate and transport in the vadose zone. 

Abstract Air–water interfacial adsorption complicates per‐ and polyfluoroalkyl substance (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: (a) The mass‐transfer limitations between bulk capillary water and thin water films inside a pore are negligible due to surface diffusion. (b) 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. (c) 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. 

Per- and polyfluoroalkyl substances (PFAS) have become one of the most important contaminants due to their ubiquitous presence in the environment and potentially profound impacts on human health and the environment even at parts per trillion (ppt) concentration levels.  A growing number of field investigations have revealed that soils act as PFAS reservoirs at many contaminated sites, with significant amounts of PFAS accumulating over several decades. Because PFAS accumulated in soils may migrate downward to contaminate groundwater resources, understanding the fate and transport of PFAS in soils is of paramount importance for characterizing, managing, and mitigating long-term groundwater contamination risks.Many PFAS are surfactants that adsorb at air–water and solid–water interfaces, which leads to complex transport behaviors of PFAS in soils. Concomitantly, PFAS present in porewater can modify surface tension and other interfacial properties, which in turn may impact variably saturated flow and PFAS transport. Furthermore, some PFAS are volatile (i.e., can migrate in the gas phase) and/or can transform under environmental conditions into persistent PFAS. These nonlinear and coupled processes are further complicated by complexities of the soil environment such as thin water films, spatial heterogeneity, and complex geochemical conditions.In this commentary, we present an overview of the current challenges in understanding the fate and transport of PFAS in the environment. Building upon that, we identify a few potential areas where porous media research may play an important role in addressing the problem of PFAS contamination in groundwater.