Geosynthetics October/November 2021 - 22

Hybrid GCLs for attenuation of PFAS in landfill leachates
GCL performance
GCLs are typically assessed for their
hydraulic performance in a triaxial permeability
testing apparatus. Standardized
test methodologies, such as American
Society for Testing and Materials
(ASTM) D6766, are used to hydrate and
permeate GCLs with potentially incompatible
aqueous solutions. Traditional
assessment using these methods results
in several pore volumes of permeating
liquid passing through the GCL until
hydraulic and chemical equilibrium is
reached. With precise volumetric measurement
changes of the influent and
effluent levels taken from the connected
pipettes over periods of documented
time, plus measurements including sample
area, hydraulic head and hydrated
clay thickness, index flux (m3/m2/s),
and hydraulic conductivity (m/s) of the
GCL in contact with the test liquid can
be calculated.
To assess PFAS concentrations in
the permeating liquid, the leachate is
sampled and analyzed for 27 PFAS analytes
before it is introduced to the GCL,
and then later sampled at repeated intervals
throughout the permeation period
by collecting the effluent liquid that has
passed through the GCL. From this data
the PFAS attenuation capacity of GCLs
can then be determined.
Initial studies on a landfill leachate
FIGURE 2 Hydraulic conductivity data for standard and polymer-modified bentonite (PMB) GCL
when hydrated and permeated with Leachate A
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Geosynthetics | October November 2021
(Leachate A) containing ~35 ppb (~35
µg/L) total PFAS were initiated with a
powdered natural sodium bentonite,
with and without polymer modification.
When the termination criteria
were reached, the hydraulic conductivity
was 5.74 × 10-11 m/s for the standard
GCL and 4.42 × 10-11 m/s for the polymer-modified
bentonite (PMB) GCL
(Figure 2). After two pore volumes
flow through the GCLs, concentrations
of the three nationally regulated
compounds (perfluorooctanoic acid
[PFOA], perfluorooctanesulfonic acid
[PFOS] and perfluorohexane sulfonate
[PFHxS]) show that PFOS reduced
from an initial concentration level of
0.24 ppb (0.24 µg/L) to below the level
of reporting (LOR) in both GCL types
(Figure 3). The PFOA concentration
reduced from an initial level of 0.69 to
0.50 ppb (0.69 to 0.50 µg/L) in the standard
GCL to 0.40 ppb (0.40 µg/L) in
the PMB GCL. PFHxS concentrations
reduced from an initial level of 0.94
to 0.71 ppb (0.94 to 0.71 µg/L) in the
standard GCL to 0.56 ppb (0.56 µg/L)
in the PMB GCL. This initial study
demonstrated that the hydraulic performance
does not seem to be negatively
affected by PFAS within landfill leachate
at these concentrations, and that
some attenuation is provided by the
standard and PMB GCLs. As only low
pore volumes were passed through the
GCLs in these preliminary investigations,
it would be unreasonable to draw
conclusions for long-term performance
from this dataset.
Geosynthetics October/November 2021

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