Surfactant-induced flow compromises determination of air-water interfacial areas by surfactant miscible-displacement

Abstract

Surfactant miscible-displacement (SMD) column experiments are used to measure air-water interfacial area (A) in unsaturated porous media, a property that influences solute transport and phase-partitioning. The conventional SMD experiment results in surface tension gradients that can cause water redistribution and/or net drainage of water from the system ("surfactant-induced flow"), violating theoretical foundations of the method. Nevertheless, the SMD technique is still used, and some suggest that experimental observations of surfactant-induced flow represent an artifact of improper control of boundary conditions. In this work, we used numerical modeling, for which boundary conditions can be perfectly controlled, to evaluate this suggestion. We also examined the magnitude of surfactant-induced flow and its impact on A measurement during multiple SMD flow scenarios. Simulations of the conventional SMD experiment showed substantial surfactant-induced flow and consequent drainage of water from the column (e.g., from 75% to 55% S) and increases in actual A of up to 43%. Neither horizontal column orientation nor alternative boundary conditions resolved surfactant-induced flow issues. Even for simulated flow scenarios that avoided surfactant-induced drainage of the column, substantial surfactant-induced internal water redistribution occurred and was sufficient to alter surfactant transport, resulting in up to 23% overestimation of A. Depending on the specific simulated flow scenario and data analysis assumptions used, estimated A varied by nearly 40% and deviated up to 36% from the system's initial A. We recommend methods for A determination that avoid generation of surface-tension gradients and urge caution when relying on absolute A values measured via SMD.

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

“…The prior modeling studies incorporated solid-phase adsorption for the transport of surfactants, but not fluid-fluid interfacial adsorption. More recently, Costanza-Robinson and Henry (2017) modified HYDRUS-1D (Simunek et al, 2008) to simulate surfactant-induced flow to evaluate the aqueous air-water interfacial partitioning tracer test (IPTT) method. Both solid-phase and air-water interfacial adsorption were included.…”

Influence of Tension-Driven Flow on the Transport of AFFF in Unsaturated Media

“…The effects of TDF have been studied previously, ,− with a focus on the transport of single-component surface active agents as IFT sorbents, causing water movement and TDF in unsaturated porous media. These studies demonstrated that the presence of the surfactant in unsaturated media creates concentration-dependent surface tension gradients between surfactant-free and surfactant-impacted zones, which causes capillary pressure gradients and moisture transition across the media. ,− Costanza-Robinson and Henry modified the HYDRUS-1D model (publicly unavailable) to include the impact of surfactant-induced flow on IFT changes and subsequent impacts on water movement. They concluded that surfactant-induced flow significantly affects the air–water interfacial areas (increasing the magnitude up to 43%), with obvious implications for vadose-zone fluid physics.…”

Determining air-water interfacial areas for the retention and transport of PFAS and other interfacially active solutes in unsaturated porous media

“…These studies include laboratory miscible-displacement experiments (e.g., Lyu et al, 2018 , 2022 ; Lyu and Brusseau, 2020 ; Lyu et al, 2020 ; Brusseau et al, 2019 , 2021 ; Yan et al, 2020 ; Li et al, 2021 ), model-based analyses ( Brusseau, 2018 , 2020 ; Brusseau et al, 2019 ; Guo et al, 2020 , 2022 ; Silva et al, 2020 ; Newell et al, 2021 ; Zeng et al, 2021 ; Gnesda et al, 2022 ; Wallis et al, 2022 ), and field-scale investigations ( Brusseau and Guo, 2022 ; Schaefer et al, 2022 ). The results of prior studies have demonstrated that adsorption at the air-water interface can also influence the retention and transport of other solutes, including hydrocarbon surfactants (e.g., Kimetal.,1997 ; Allred and Brown, 2001 ; Brusseau et al, 2007 ; Costanza-Robinson and Henry, 2017 ), hydrophobic organic compounds (e.g., Brusseau et al, 1997 ; Kim et al, 1998 , 2001 , 2005 ; Popovicova and Brusseau, 1998 ), and pharmaceuticals (e.g., Dai et al, 2020 ; Hamdollahi et al, 2022 ). Characterizing and quantifying the impact of air-water interfacial adsorption on the retention of PFAS and other interfacially active solutes requires knowledge of the amount of air-water interfacial area present in the porous medium.…”