Enrichment of poly- and perfluoroalkyl substances (PFAS) in the surface microlayer and foam in synthetic and natural waters

Per- and Polyfluoroalkyl Substances Enrichment in the Surface Microlayer of a Freshwater System Impacted by Aqueous Film-Forming Foams

“…A Freundlich isotherm cannot be regressed for the CaCl 2 dataset due to the lack of sufficient data points from other laboratory studies, but there appears to be an increase in K aw with decreasing concentration (consistent with a Freundlich isotherm), even if it is assumed the surface water electrolyte composition is entirely CaCl 2 . The field-estimated K aw for both PFOS and PFOA were also within the same order of magnitude of laboratory experiments conducted with natural and synthetic surface waters . Because the field-estimated air–water adsorption coefficients were within the same order of predicted equilibrium air–water adsorption coefficients, air–water interfacial adsorption is the only mechanism needed to explain PFAS enrichment in the SML.…”

“…Literature K aw values for PFOS under low ionic strength (<0.001 M NaCl left) and higher ionic strength (>0.001 M NaCl) obtained by residual air column transport, pan film, Garrett screen, drainage-based column transport, and field-measured values (this study). Langmuir and Freundlich models , are calculated from interfacial tension data for 0.01 M NaCl and the Freundlich regression is fit to residual air column data for low-ionic-strength PFOS.…”

“…The average field-measured EFs for PFOA (1.5 ± 0.3) and PFOS (7.9 ± 3.5) (Figure 3 and Table S4) are in the range as those measured by Schaefer et al who brought natural water in the laboratory and measured the SML. 41 They are also in the same range as field-measured EF for PFOA (1.2−4.7) and PFOS (2−109) for marine waters at higher salinities. 13,15 Casas et al 14 reported average EFs < 3 for seven PFAS, but the EFs did not vary by fluorinated chain length.…”

“…FHxSA, PFOS, and FOSA) (Figure 3 and Table 4). Data from the microscope slides were used to calculate K aw values because slides gave a fixed SML volume with one dip (e.g., a known surface area) that was used to compute the operationally defined, average Literature K aw values for PFOS under low ionic strength (<0.001 M NaCl left) and higher ionic strength (>0.001 M NaCl) obtained by residual air column transport, 28 pan film, 10 Garrett screen, 41 drainage-based column transport, 27 and field-measured values (this study). Langmuir 12 and Freundlich models 10,28 are calculated from interfacial tension data for 0.01 M NaCl and the Freundlich regression is fit to residual air column data for low-ionic-strength PFOS.…”

“…Data from the microscope slides were used to calculate K aw values because slides gave a fixed SML volume with one dip (e.g., a known surface area) that was used to compute the operationally defined, average field-measured SML thicknesses (σ) 71 μm. Values for K aw and EF are related using eq K aw = ( EF − 1 ) 0.25em * σ where K aw is the air–water adsorption coefficient, EF is the enrichment factor, and σ is the SML thickness.…”