Sorption of perfluoroalkyl ether carboxylic acids (PFECAs) at the air-water interface in porous media: Modeling perspectives

Abstract

Perfluoroalkyl ether carboxylic acids (PFECAs) are a growing environmental concern, and thus their migration in unsaturated soils is receiving increased interest. In this study, sorption of four PFECAs at the air-water interface was measured in unsaturated sand columns under static (no flow) conditions following gravity drainage to assess the appropriateness of two commonly applied sorption models (Langmuir and Freundlich) in predicting interfacial uptake. This static method was employed to eliminate any potential artifacts due to flow bypassing and/or interfacial sorption kinetics that might occur in dynamic column experiments. The largest interfacial adsorption coefficient measured was 2.8 ± 1.6×10−5 m for ammonium perfluoro(2-methyl-3-oxahexanoate) (GenX). A commonly employed Langmuir-based model based on interfacial tension data substantially underpredicted GenX and nonafluoro-3,6-dioxaheptanoic acid (PFO2HpA) interfacial sorption measured at low (∼1,000 ng/L) aqueous concentrations in unsaturated sand columns, while a Freundlich-based model substantially overpredicted GenX and PFO2HpA interfacial sorption. These results highlight the need for improved interfacial modeling approaches to predict PFECA fate and transport at relatively low concentrations in unsaturated media.