posted on 2024-01-10, 22:10authored byThomas
A. Wanzek, Jennifer A. Field, Konstantinos Kostarelos
Historical
practices at firefighter-training areas involved
repeated
aqueous film-forming foams (AFFFs) applications, resulting in source
zones characterized by high concentrations of perfluoroalkyl and polyfluoroalkyl
substances (PFAS). Repeated applications of AFFF composed of 14 anionic
and 23 zwitterionic perfluoroalkyl substances (PFAS) were conducted
on a single one-dimensional saturated soil column to quantify PFAS
retention. An electrofluorination-based (3M) Milspec AFFF, which was
above the mixture’s critical micelle concentration (CMC), was
at application strength (3%, v/v). Retention and retardation of PFAS
mass increased with each successive AFFF addition, although the PFAS
concentration profiles for subsequent applications differed from the
initial. Greater degree of mass retention and retardation correlated
with longer PFAS carbon–fluorine chain length and charged-headgroup
type and as a function of AFFF application number.
Anionic PFAS were increasingly retained with each subsequent AFFF
application, while zwitterionic PFAS exhibited an alternating pattern
of sorption and desorption. Surfactant–surfactant adsorption
and competition during repeat AFFF applications that are at concentrations
above the CMC resulted in adsorbed PFAS from the first application,
changing the nature of the soil surface with preferential sorption
of anionic PFAS and release of zwitterionic PFAS due to competitive
elution. Applying a polyparameter quantitative structure–property
relationship developed to describe sorption of AFFF-derived PFAS to
uncontaminated, saturated soil was attempted for our experimental
conditions. The model had been derived for data where AFFF is below
the apparent CMC and our experimental conditions that included the
presence of mixed micelles (aggregates consisting of different kinds
of surfactants that exhibit characteristics properties different from
micelles composed of a single surfactant) resulted in overall PFAS
mass retained by an average of 27.3% ± 2.7% (standard error)
above the predicted values. The correlation was significantly improved
by adding a “micelle parameter” to account for cases
where the applied AFFF was above the apparent CMC. Our results highlight
the importance of interactions between the AFFF components that can
only be investigated by employing complex PFAS mixtures at concentrations
present in actual AFFF at application strength, which are above their
apparent CMC. In firefighter-training areas (AFFF source zones), competitive
desorption of PFAS may result in downgradient PFAS retention when
desorbed PFAS become resorbed to uncontaminated soil.