Due to their amphiphilic properties, per and polyfluoralkyl substances (PFAS) are known to accumulate at the interfaces between air/organics and water phases. Therefore, the presence of nonaqueous phase liquid (NAPL) at sites where aqueous film forming foam (AFFF) was used for fire training and suppression activities can potentially impact the transport and distribution of PFAS in the subsurface. In this work, batch and column experiments were coupled with mathematical modeling to investigate the influence of a representative residual NAPL, tetrachloroethylene (PCE), on the transport and retention of PFAS in 60-80 mesh Ottawa sand. Perfluorooctanesulfonic acid (PFOS), Perfluorononanoic acid (PFNA), a 1:1 mixture of PFOS and PFNA, and a mixture of 6 PFAS compounds including PFOS, PFNA, Perfluorooctanoic acid (PFOA), Perfluoroheptanoic acid (PFHpA), Perfluorohexane sulfonate (PFHxS), and Perfluorobutane sulfonate (PFBS) were used to assess PFAS interactions with PCE-NAPL. Batch studies showed that PFOS partitioning into PCE-NAPL and adsorption on the solid phase were minimal at pH = 7.5±0.3, as indicated by NAPL-water partition coefficients (Knw) of less than 0.13 and adsorption coefficients (Kd) of less than 0.0003 (L/g). Interfacial tension (IFT) measurements revealed that PFOS exhibited greatest interfacial adsorption activity when present either as a single solute or in the mixture, followed by PFNA and PFOA. Column results showed that the presence of residual PCE-NAPL (~16% saturation) increased 100 µg/L PFOS and PFNA retardation factors (R) from 1.1 and 1.2 to 1.7 and 3.0, respectively, which was attributed to adsorption of PFAS at the NAPL-water interface. The breakthrough curves obtained for PFNA and PFOS when introduced alone or in a 1:1 mixture were nearly identical, indicating competitive PFAS adsorption at the NAPL-water interface was minimal at the lower concentration (100 µg /L). In addition, breakthrough curve obtained for PFOS remained nearly unchanged when the pore-water velocity was reduced from 1 m/day to 0.28 m/day, suggesting that PFAS adsorption at the NAPL-water interfacial was not rate-limited under these experimental conditions. A mathematical model that accounts for NAPL-water interfacial sorption accurately reproduced PFAS transport and retention behavior under all investigated conditions, providing a tool to predict the influence of residual NAPL on PFAS fate and transport in co-contaminated subsurface environments.

Influence of residual nonaqueous-phase liquid (NAPL) on the transport and retention of perfluoroalkyl substances (PFAS) in quartz sand

ACS Fall 2021

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