Simulating PFAS transport influenced by rate-limited multi-process retention

Abstract

The transport of per-and poly-fluoroalkyl substances (PFAS) in the vadose zone is complicated by the fact that multiple mass-transfer processes can contribute to their retention and retardation. In addition, PFAS transport at some sites can be further complicated by the presence of organic immiscible liquids (OIL). Mass-transfer processes are inherently rate limited and, therefore, have the potential to cause nonideal transport of PFAS. The objectives of this research were to: (1) develop a solute-transport model that explicitly accounts for multiple retention processes, including adsorption at air-water and OIL-water interfaces, adsorption by the solid phase, and diffusive mass-transfer between advective and nonadvective domains, and (2) apply the model to measured transport data to delineate which processes are rate limited and contribute to observed nonideal transport. Breakthrough curves for transport of two PFAS and one hydrocarbon surfactant in sand obtained from prior miscible-displacement experiments exhibited nonideal transport. The multiprocess model effectively simulated the measured transport data. The results of the analyses indicate that adsorption at the air-water and OIL-water interface can generally be treated as effectively instantaneous for transport in porous media. The rate limitations associated with solidphase adsorption and diffusive mass transfer between advective and nonadvective domains were of greater significance.

A Mathematical Model for the Release, Transport, and Retention of Per‐ and Polyfluoroalkyl Substances (PFAS) in the Vadose Zone

“…The solid‐phase and air‐water interfacial adsorption formulations have been used to model PFAS transport in soil‐/sand‐packed columns under steady‐state unsaturated flow. Good agreement of breakthrough curves between simulation and experiment were reported (Brusseau et al, ; Brusseau, ).…”

“…We first present formulations of equilibrium isotherms for and . The solid‐phase adsorption can be modeled by the nonlinear Freundlich isotherm as suggested by prior experimental measurements (Brusseau et al, ; Brusseau, ; Higgins & Luthy, ; Wei et al, ) where and are fitting parameters to experimental data.…”

“… where is the fraction of sorbent for which sorption is instantaneous ( ); is the first‐order rate constant for kinetic solid‐phase adsorption ( ). For air‐water interfacial adsorption of PFAS, diffusion from the bulk to the air‐water interface has been considered as the primary controlling factor for kinetic adsorption (e.g., Brusseau, ; Day et al, ; Loppinet & Monteux, ; Miller et al, ; Sekine et al, ; Valkovska et al, ). In general, diffusive mass transfer needs to be included to model the air‐water interfacial kinetic adsorption.…”

“…The parameters for the Freundlich isotherm for the Accusand and Vinton soil were reported in Brusseau () and Brusseau et al (), respectively. Note that in Brusseau () and Brusseau et al () was for PFOS aqueous concentrations in mg/L and solid‐phase adsorption concentration in mg/kg. Here we have converted to the values for PFOS aqueous concentrations in mol/cm and solid‐phase adsorption in mol/g.…”

Combined leaching and plant uptake simulations of PFOA and PFOS under field conditions

“…The assessment of model fit was hampered by highly uncertain PFOA input values. In unsaturated laboratory soil columns, several reversible mass transfer processes such as adsorption at the air-water interface were used for a simulation of PFAAs leaching (Brusseau 2020 ). Thus, even though NER formation was repeatedly found in experimental studies, this process was not included in the simulation of PFAAs leaching up until now.…”