The
objective of this research was to examine the influence of
nonideal sorption/desorption on the transport of polyfluorinated alkyl
substances (PFASs) in soil, with a specific focus on characterizing and quantifying
potential extended, mass-transfer-limited elution behavior. Perfluorooctane
sulfonic acid (PFOS) was used as a representative PFAS, and miscible-displacement
experiments were conducted with two soils comprising contrasting geochemical
properties. The influence of nonlinear, rate-limited, hysteretic,
and irreversible sorption/desorption on transport was investigated
through experiments and model simulations. The breakthrough curves
measured for PFOS transport in the two soils were asymmetrical and
exhibited extensive elution tailing, indicating that sorption/desorption
was significantly nonideal. The widely used two-domain sorption kinetics
model could not fully simulate the observed transport behavior, whereas
a multirate model employing a continuous distribution of sorption
domains was successful. The overall results indicated that sorption/desorption
was significantly rate-limited and that nonlinear, hysteretic, and
irreversible sorption/desorption had minimal impact on PFOS transport.
Comparison of PFOS transport data to data reported for two hydrophobic
organic contaminants (HOCs) showed that the HOCs exhibited much more
extensive elution tailing, likely reflecting differences in sorption/desorption
mechanisms. The projected influence of rate-limited sorption/desorption
on PFOS transport at the field scale was investigated through simulation.
The results of the study suggest that rate-limited sorption/desorption
may affect the field-scale transport of PFOS and other PFAS for systems
influenced by transient or short-residence-time conditions and in
some cases could possibly increase the amount of flushing required
to reduce PFOS concentrations to levels below those associated with
human-health concerns.
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