Cationic surfactant-assisted foam fractionation for the removal of per- and polyfluoroalkyl substances (PFAS) from synthetic water

Abstract

The present work focused on the cationic surfactant-assisted foam fractionation as an efficient approach for the removal of per- and polyfluoroalkyl substances (PFAS) from synthetic water. The study included six PFAS, namely PFNA, PFOA, HFPO-DA (Gen X), PFOS, PFHxS and PFBS, for which the US National Primary Drinking Water Regulation (NPDWR) mandates enforceable maximum contaminant levels and calculated Hazard Index. The NPDWR limits, 4 ng/L for PFOA and PFOS, and 10 ng/L for PFNA, PFHxS and HFPO-DA, were used as thresholds for efficient PFAS removal in the study. PFBS was included in the Hazard Index calculation. The initial PFAS concentration in synthetic water was ca. 160 ng/L for each PFAS. The foam fractionation was done in a laboratory-scale single-column setup. Three commercial cationic trimethylammonium bromide surfactants with carbon chain lengths, C14, C16 and C18, were used as co-formulants at 25 mg/L. The PFAS concentration analyses were done with the EPA Method 1633 using HPLC-MS/MS. The PFAS removal efficacy between the three surfactants was found to be on average 94.7 – 100 % in the present study. The short-chain PFAS (HFPO-DA and PFBS) were removed on average with a higher efficacy than the long-chain ones (PFNA, PFOS, PFOA and PFHxS). This was unexpected as earlier research had reported challenges, especially in the removal of the short-chain PFAS. The removal efficacy of PFOS was the lowest one in the study (90.6 %) and the PFOS residuals exceeded the NPDWR limits. The residuals of other PFAS remained below their respective NPDWR thresholds. None of the samples exceeded the Hazard Index threshold. As the removal favored the short-chain PFAS and, especially, PFOS was removed with a lower efficacy, it was hypothesized that interference may have occurred in the solution by the surfactant and/or PFAS mixture. The highest average removal efficacies were reached with C14 and C16, which can be attributed to their higher foam stability compared to that of C18. These findings underscore the applicability of cationic surfactant-assisted foam fractionation in advanced water treatment systems, particularly for addressing PFAS contamination in engineered and natural water bodies.