Physical and Chemical Behavior of 'Permanent Chemical' PFOA and Its Substitute GenX at the Air-Water Interface: A Systematic Review

As one of the representative emerging contaminants (ECs), perfluorooctanoic acid (PFOA) is facing global prohibitions due to its environmental persistence and biological toxicity. Consequently, hexafluoropropylene oxide dimer acid, commonly known as GenX, has been widely used since its introduction as a major substitute for PFOA. However, its high potential of long-distance transport and low degradation rate result in a continuous accumulation, raising significant environmental and health concerns. During the transport, enrichment, and transformation processes of PFOA and GenX, air-water interface (AWI) serves as a critical medium with its contribution to their environmental fate remaining insufficiently characterized. This review systematically compares the adsorption and degradation behavior of PFOA and GenX at AWI. The AWI adsorption capacity of PFOA and GenX is closely related to molecular structure, especially chain length. Meanwhile, ionic strength and types in the solution are also important factors. Compared with the AWI of bulk water, the microdroplet AWI significantly accelerates PFOA degradation through interfacial reactive species, including hydroxyl radicals (•OH), hydrated electrons (e aq − ), and superoxide radicals (O 2 • − ), involving one- and two-carbon degradation mechanism, S N 2 mechanism and reductive defluorination mechanism (H/F exchange and DHEH pathways). Interfacial environments, including microdroplet size, inorganic ions, pH, and mineral particles, together with external conditions such as gas sparging, ultraviolet irradiation and plasma, regulate not only the degradation kinetics but also reactive-species chemistry, pathway selectivity, and the fate of degradation products. To date, studies on GenX degradation are still limited to aqueous phase, leaving a critical knowledge gap regarding its degradation at AWI. The present review establishes a theoretical comparison for PFOA and GenX interfacial behavior, highlights the importance of AWI in evaluating the environmental fate and biological risks of PFAS substitutes, offering support for their risk assessment and regulatory control.