Subsurface Transport Potential of Perfluoroalkyl Acids
at Aqueous Film-Forming Foam (AFFF)-Impacted Sites
- Publication date
- Publisher
Abstract
Subsurface
transport potential of a suite of perfluoroalkyl acids
(PFAAs) was studied in batch sorption experiments with various soils
and in the presence of co-contaminants relevant to aqueous film-forming
foam (AFFF)-impacted sites. Specifically, PFAA sorption to multiple
soils in the presence of nonaqueous phase liquid (NAPL) and nonfluorinated
AFFF surfactants was examined. This study is the first to report on
sorption of perfluorobutanoate (PFBA) and perfluoropentanoate (PFPeA)
(log <i>K</i><sub>oc</sub> = 1.88 and 1.37, respectively)
and found that sorption of these compounds does not follow the chain-length
dependent trend observed for longer chain-length PFAAs. Sorption of
PFBA was similar to that of perfluorooctanoate (PFOA, log <i>K</i><sub>oc</sub> = 1.89). NAPL and nonfluorinated AFFF surfactants
all had varying impacts on sorption of longer chain (>6 CF<sub>2</sub> groups) PFAAs. The primary impact of NAPL was observed in
low <i>f</i><sub>oc</sub> soil (soil A) where Freundlich <i>n</i>-values increased when NAPL was present. Impacts of nonfluorinated
AFFF surfactants varied with surfactant and soil. The anionic surfactant
sodium decyl sulfate (SDS) illicited PFAA chain-length dependent impacts
in two negatively charged soils with varying <i>f</i><sub>oc</sub>. In soil A, <i>K</i><sub>d</sub> values for perfluoroheptanoate
(PFHpA) increased 91% with SDS, whereas values for perfluorodecanoate
(PFDA) increased only 28%. An amphoteric surfactant, <i>n</i>,<i>n</i>-dimethyldodecylamine <i>n</i>-oxide
(AO), had the most notable impact on PFAA sorption to a positively
charged soil (soil C). In this soil, AO oxide significantly increased
sorption for the longer chain PFAAs (i.e., 528% increase in <i>K</i><sub>d</sub> for PFDA). Changes in sorption caused by SDS
and AO may be due to mixed hemimicelle formation, competitive sorption,
or changes to PFAA solubility. Short-chain PFAA behavior in the presence
of NAPL, SDS, and AO was again notable. Co-contaminants generally
increased the sorption of these compounds to all soils. Log <i>K</i><sub>d</sub> values of PFBA in soil A increased 85%, 372%,
and 32% in the presence of NAPL, SDS, and AO, respectively. Use of <i>K</i><sub>d</sub> values to calculate retardation factors (<i>R</i><sub>f</sub>) of PFAAs demonstrates the variability of
co-contaminant impacts on PFAA transport. Whereas NAPL and nonfluorinated
surfactants decreased the sorption of perfluorooctanesulfonate (PFOS)
at lower PFOS concentrations (1 μg/L), they led to increases
in sorption at higher PFOS concentrations (500 μg/L). These
results demonstrate that PFAA groundwater transport will depend on
the solid phase characteristics as well as PFAA concentration and
chain length. Detailed site-specific information will likely be needed
to accurately predict PFAA transport at AFFF-impacted sites