Accumulation of PFOA and PFOS at the Air–Water Interface

Abstract

Knowledge of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) accumulation at the air−water interface is critical to understanding the fate and transport of these substances in subsurface environments. The surface tension of aqueous solutions containing PFOA and PFOS at concentrations ranging from 0.1 to >1000 mg/L and with dissolved solids (i.e., cations and anions) commonly found in groundwater was measured using the Wilhelmy plate method. The surface tensions of solutions containing dissolved solids were lower than those for ultrapure water, indicating an increase in the surface excess of PFOA and PFOS in the presence of dissolved solids. An equation for the surface excess of PFOA and PFOS with total dissolved solids was developed by fitting the measured surface tension values, which ranged from 72.0 to 16.7 mN/m, to the Szyszkowski equation. On the basis of mass distribution calculations for a representative unsaturated, fine-grained soil, up to 78% of the PFOA and PFOS mass will accumulate at the air−water interface, with the remaining mass dissolved in water and adsorbed on the solids.

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

“…The impact of fluid‐fluid interfacial adsorption on PFAS retention and transport in soil was examined initially by Brusseau (), who employed surface tension data for PFAS including two of primary concern—perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS)—and measured air‐water interfacial areas along with a comprehensive retention model to conduct a theoretical assessment. Additional surface‐tension‐based theoretical analyses of PFAS retention have since been reported (Brusseau, , ; Brusseau & Van Glubt, ; Costanza et al, ; Silva et al, ). Miscible‐displacement laboratory experiments demonstrating that adsorption of PFAS at air‐water and NAPL‐water interfaces can be an important retention process in soil and sand materials have also been reported (Brusseau et al, ; Lyu et al, ).…”

Comparing PFAS to other groundwater contaminants: Implications for remediation

“…Most sources resulted from releases of PFAS at the ground surface, and some of the PFAS has leached through the unsaturated zone to the capillary fringe, and then slowly partitioned to groundwater in the saturated zone. This is why, at some PFAS sites, the concentrations of PFAS in the capillary fringe are much higher than PFAS concentrations in deeper groundwater (Costanza, Arshadi, Abriola, & Pennell, 2019).…”

“…The implications of this surfactant behavior are seen in a compartment model developed by Brusseau et al (2019), where the retardation factor for PFOS through unsaturated sand was 7, compared to a retardation factor of 1.8 for completely saturated conditions. Other work has shown that a significant fraction of the PFAS (well over half the total mass in coarse soils) can be retained in the unsaturated zone due to this accumulation at air‐water interfaces (Costanza et al, 2019). Overall the presence of long‐term sustained PFAS source processes increases the need for some type of active remediation (e.g., removal or containment of source materials) but with the realization that not all the source mass can likely be removed at larger PFAS sites.…”

Infrared Hide-and-Seek: Vibrational Excitons Conceal Surfactants at the Air/Water Interface

“…Three main peaks are present in the C-F vibrational mode region of the IRRAS spectra of PFOA in In response to this apparent deviation from Hofmeister "salting-out" effects, 20,21 to the Szyszkowski equation, 23,24 130…”