Soil contaminated with aqueous film-forming
foams (AFFFs) containing
per- and polyfluoroalkyl substances (PFASs) at firefighting training
sites has become a major concern worldwide. To date, most studies
have focused on assessing soil–water partitioning behavior
of PFASs and the key factors that can affect their sorption, whereas
PFASs leaching from contaminated soils have not yet been widely investigated.
This study evaluated the leaching and desorption of a wide range of
PFASs from twelve contaminated soils using the Australian Standard
Leaching Procedure (ASLP), the U.S. EPA Multiple Extraction Procedure
(MEP), and Leaching Environmental Assessment Framework (LEAF). All
three leaching tests provided a similar assessment of PFAS leaching
behavior. Leaching of PFASs from soils was related to C-chain lengths
and their functional head groups. While short-chain (CF2 ≤ 6) PFASs were easily desorbed and leached, long-chain PFASs
were more difficult to desorb. PFASs with a carboxylate head group
were leached more readily and to a greater extent than those with
a sulfonate or sulfonamide head group. Leaching of long-chain PFASs
was pH-dependent where leaching increased at high pH, while leaching
of short-chain PFASs was less sensitive to pH. Comparing different
leaching tests showed that the results using the alkaline ASLP were
similar to the cumulative MEP data and the former might be more practical
for routine use than the MEP. No single soil property was adequately
able to describe PFAS leaching from the soils. Overall, the PFAS chemical
structure appeared to have a greater effect on PFAS leaching from
soil than soil physicochemical properties.
This study investigated the mobilization
of a wide range of per- and polyfluoroalkyl substances (PFASs) present
in aqueous film-forming foams (AFFFs) in water-saturated soils through
one-dimensional (1-D) column experiments with a view to assessing
the feasibility of their remediation by soil desorption and washing.
Results indicated that sorption/desorption of most of the shorter-carbon-chain
PFASs (C ≤ 6) in soil reached greater than
99% rapidlyafter approximately two pore volumes (PVs) and
were well predicted by an equilibrium transport model, indicating
that they will be readily removed by soil washing technologies. In
contrast, the equilibrium model failed to predict the mobilization
of longer-chain PFASs (C ≥ 7), indicating
the presence of nonequilibrium sorption/desorption (confirmed by a
flow interruption experiment). The actual time taken to attain 99%
sorption/desorption was up to 5 times longer than predicted by the
equilibrium model (e.g., ∼62 PVs versus ∼12 PVs predicted
for perfluorooctane sulfonate (PFOS) in loamy sand). The increasing
contribution of hydrophobic interactions over the electrostatic interactions
is suggested as the main driving factor of the nonequilibrium processes.
The inverse linear relationship (R
2 =
0.6, p < 0.0001) between the nonequilibrium mass
transfer rate coefficient and the Freundlich sorption coefficient
could potentially be a useful means for preliminary evaluation of
potential nonequilibrium sorption/desorption of PFASs in soils.
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