Transport of perfluorooctanoic acid in unsaturated porous media mediated by SDBS

Abstract

Per– and polyfluoroalkyl substances (PFAS) contamination is ubiquitous and likely co–exist with hydrocarbon surfactants in the subsurface environment. The transport of PFAS in unsaturated porous media (quartz sand–packed columns) with an anionic hydrocarbon surfactant was investigated using perfluorooctanoic acid (PFOA) and sodium dodecyl benzene sulfonate (SDBS) as representative PFAS and hydrocarbon surfactant at 1.5 and 30 mM NaCl, respectively. Our results show that SDBS inhibited the retardation of PFOA during transport. The degree of the inhibited retardation was more significant at higher concentration of SDBS (CSDBS) for a given concentration of PFOA (CPFOA) and higher ionic strength. In contrast, PFOA only marginally affected the transport of SDBS under all tested conditions. This is mainly due to the greater surface activity of SDBS with respect to PFOA, favoring the competitive preferential adsorption of SDBS at the air–water interface and then the transport of PFOA in the unsaturated sand. This assumption is further supported by the greater air–water adsorption coefficient (Kai) and surface excess (Γ) for SDBS than those for PFOA, as well as the surface tension isotherms that fairly weighted toward to SDBS in the binary PFOA–SDBS system. Our findings indicate the critical role of anionic hydrocarbon surfactant in retarding PFOA transport in unsaturated zone. This should be considered when assessing the transport potential and environmental risk, and developing remedial strategies of PFAS in the subsurface environment.