Influence of cationic and nonionic hydrocarbon surfactants on the retention and transport of PFOA under saturated and unsaturated conditions.

Abstract

Per- and polyfluoroalkyl substances (PFAS) often occur with hydrocarbon surfactants at aqueous film-forming foam (AFFF) contaminated sites, yet the impact of surfactant activity on PFAS transport remains poorly understood. In this study, the effects of cationic and nonionic surfactants on the transport of perfluorooctanoic acid (PFOA) under saturated and unsaturated conditions were investigated. The impact of different concentrations of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the nonionic surfactant Triton X-100 (TX-100) on PFOA transport and interfacial adsorption was studied via miscible-displacement experiments and surface tension measurements. Under saturated conditions, CTAB minimally affects PFOA adsorption at the water-solid interface, while the K_d value increases from 0.052 to 0.106 cm³ g^-1 with the addition of TX-100. Retardation of PFOA becomes greater when CTAB and TX-100 are present under unsaturated conditions. The retardation factor (R) for PFOA, due to adsorption at the air-water interface, increases with CTAB concentration. Conversely, when the ratio of PFOA to TX-100 is 1 : 1, the R value of PFOA is significantly enhanced relative to conditions devoid of TX-100, but further addition of TX-100 has only a limited effect. The air-water interface accounts for 81% and more than 90% of the total retardation without and with the addition of hydrocarbon surfactants, respectively. CTAB has a greater effect on PFOA retention and transport than the nonionic TX-100 because of its stronger electrostatic interaction with PFOA, which is confirmed by our density-functional-theory (DFT) computational modeling. The results of this study enhance the understanding of PFAS transport at AFFF-contaminated sites where PFAS co-exists with hydrocarbon surfactants.