Foam fractionation of per- and polyfluoroalkyl substances (PFASs) in landfill leachate using different cosurfactants

Foam fractionation is one solution to recover per- and polyfluoroalkyl substances (PFASs) from aqueous sources. The separation process is based on the sorption of PFASs to the air-water interface of bubbles. In many practical cases, the PFAS concentration in the polluted liquid is too low to sustain foam formation and requires the support of a cosurfactant not only to act as a collector of PFAS but also to produce and sustain foam for effective separation. However, there is a lack of information regarding the appropriate choice of cosurfactant and its quantitative effect on the interfacial partitioning of PFASs on the air bubbles. This study is directed to (i) evaluate the effectiveness of four cosurfactants with different-charged headgroups (i.e., anionic, cationic, zwitterionic and nonionic) for foam fractionation of PFASs, and (ii) estimate the air-water interfacial partitioning (K_i) of PFASs in the presence of four different types of cosurfactants. The K_i values span over 4 orders of magnitude with good correlation with PFASs molar volume. All of the cosurfactants were effective for the removal of the long chain PFASs (1.2-4 logs). The cationic and zwitterionic surfactants have oppositely charged head groups with respect to the anionic PFASs and therefore facilitate increased separation due to charge interactions. Some short chain PFASs (e.g., Perfluorobutanesulfonic acid (PFBS), Perfluoropentanesulfonic acid (PFPeS)) can be effectively removed using cationic and zwitterionic cosurfactants.