Geosynthetics October/November 2021 - 13

Using bentonite-polymer
composite GCLs to contain
CCR leachates
By Jiannan Chen, Sarah A. Gustitus, Yu Tan and Craig H. Benson
C
oal combustion residuals (CCRs) are the byproducts from combustion in coalfired
boilers that are disposed in lined landfills when they cannot be used beneficially
in other applications (EPRI 2009) (Figure 1). Land disposal of CCRs is regulated
under the " coal ash rule " incorporated into Subtitle D of the Resource Conservation
and Recovery Act (RCRA) (Federal Register 2015). The coal ash rule requires that coal
ash disposal facilities be lined with a composite liner consisting of a geomembrane
underlain by a compacted clay liner at least 2 feet (0.6 m) thick having a hydraulic
conductivity no greater than 1 × 10-9 m/s (Bittner et al. 2019). Geosynthetic clay liners
(GCLs) can be used in lieu of compacted clay liners provided that the GCL meets
the equivalency criteria in the coal ash rule. This generally requires that the hydraulic
conductivity of the GCL be less than 3 × 10-11 m/s when tested with the CCR leachate
to be contained (Bittner et al. 2019).
CCR leachates contain a variety of cations and anions that can affect the hydraulic
conductivity of the GCL (Chen et al. 2018). Jo et al. (2001), Kolstad et al. (2004) and Xu
et al. (2009) evaluated how ionic strength, cation valence and pH of permeant solutions
affect the swelling and hydraulic conductivity of sodium bentonite (NaB) GCLs. They
show that the hydraulic conductivity of NaB GCLs increase as the ionic strength of
the leachate increases, and that GCLs are more permeable to solutions with polyvalent
cations (e.g., calcium, magnesium, aluminum) than those with monovalent cations
(e.g., sodium, potassium, lithium), all other factors being equal. Extreme pH (pH>13
or pH<2) also alters the hydraulic conductivity of NaB GCLs. Kolstad et al. (2004)
evaluated how multispecies solutions affect swelling and hydraulic conductivity of NaB
GCLs. They report that the ionic strength and the relative abundance of monovalent
and polyvalent cations of a solution are master variables affecting swelling and hydraulic
conductivity of NaB GCLs. Chen et al. (2018) evaluated the hydraulic conductivity
of NaB GCLs permeated with CCR leachates representing a broad range of conditions
in the U.S. They found that the hydraulic conductivity of NaB GCLs increased from
10-10 to 10-6 m/s as the ionic strength of the CCR leachate increased from 40 to 755
mM. Thus, for many CCRs, a conventional NaB GCL may not satisfy the criteria in the
coal ash rule as an alternative to a compacted clay liner. In contrast to an NaB GCL, the
Jiannan (Nick) Chen is an
Assistant Professor at the
University of Central Florida in
Orlando, Fla.
Sarah A. Gustitus, Ph.D., EIT, is
Senior Staff Professional for
Geosyntec Consultants Inc. in
Tampa, Fla.
Yu Tan is a visiting scholar in
the School of Engineering at
the University of Virginia in
Charlottesville, Va.
Craig H. Benson, Ph.D.,
P.E., N.A.E., is Wisconsin
Distinguished Professor Emeritus
at the University of WisconsinMadison
in Madison, Wisc.
All figures courtesy of the authors except as noted.
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Geosynthetics October/November 2021

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