Geosynthetics October/November 2021 - 14

Using bentonite-polymer composite GCLs to contain CCR leachates
hydraulic conductivity of most compacted
clay liners is relatively insensitive to CCR
leachates (Benson et al. 2018).
The chemical compatibility of GCLs
has been enhanced in some applications
by adding polymers to the bentonite (Di
Emedio et al. 2011; Mazzieri et al. 2010;
Scalia et al. 2014; Katsumi et al. 2008).
These polymeric additions may be interlayer
substitution or polymer surface
treatment of the montmorillonite fraction
(e.g., a polymer-modified bentonite
[PMB]) or consist of a dry mixture of
bentonite and polymer granules to form
a bentonite-polymer composite (BPC)
material. Some polymer additions are
effective, whereas others are not. For
example, Shackelford et al. (2010) report
that the hydraulic conductivity of a PMB
GCL was nine to 21 times higher than
that of an NaB GCL when permeated
with the same solution. In contrast, Scalia
et al. (2014) report that the hydraulic
conductivity of a BPC GCL permeated
to calcium chloride (CaCl2) solutions
was up to four orders of magnitude less
permeable than an NaB GCL prepared
with the same bentonite and permeated
with the same CaCl2 solutions.
The hydraulic conductivity of several
FIGURE 2 RMD versus ionic strength for CCR
leachates in this study. Leachates from EPRI
database shown with open symbols.
CCR Leachates
pH
EC@25oC (S/m)
Ionic Strength (mM)*
RMD (M1/2)
Cl-/SO4
2CCR-VA1
4.3
0.4
46
0.2
0.09
CCR-VA2
8.3
0.3
35
0.4
154.8
TABLE
1 Bulk chemical parameters of CCR leachates
14
Geosynthetics | October November 2021
BPC GCLs to CCR leachates was evaluated
in this study as part of chemical compatibility
testing conducted to identify
suitable GCL products for CCR disposal
facilities. Polymer loading was also measured
to understand mechanisms affecting
hydraulic conductivity of the BPC GCLs.
CCR-VA3
9.9
1.5
233
2.4
2.4
CCR-VA4
8.4
0.07
14
0.07
2
CCR-WY
8.5
4.4
681
1.2
1.1
*Calculated using Visual MINTEQ, charge differences <5%
Materials
CCR leachates
Seven CCR leachates were obtained from
coal ash disposal facilities in Virginia
(CCR-VA1, CCR-VA2, CCR-VA3 and
CCR-VA4); Wyoming (CCR-WY); and
Minnesota (CCR-MN1, CCR-MN2).
Two of the synthetic CCR leachates (i.e.,
flue gas desulfurization [FGD] and high
strength [HS]) from Chen et al. (2018)
were also used. Bulk chemical parameters,
including pH, electrical conductivity
(EC), ionic strength (I), relative
abundance of monovalent and polyvalent
cations (RMD) and anion ratio (molar
ratio of chloride to sulfate, Cl-/SO4
2-),
are summarized in Table 1. The pH of
the CCR leachates ranges from 4.3 to 9.9,
the ionic strength ranges from 33 to 681
mM and the EC ranges from 0.3 to 4.4
S/m at 77°F (25°C). The leachates range
from chloride rich (CCR-VA2) to sulfate
rich (CCR-VA1).
Kolstad et al. (2004) defined the
parameter RMD to quantify the relative
abundance of monovalent and polyvalent
cations in a permeant liquid (Equation 1):
RMD =
MM
(1)
where MM is the total molarity of the
monovalent cations and MD is the total
molarity of polyvalent cations in the
permeant solution. RMD of the CCR
leachates ranges from 0.07 to 2.4 M1/2, or
predominantly polyvalent (low RMD) to
predominantly monovalent (high RMD).
CCR-MN1
7.2
0.3
33
0.3
0.4
CCR-MN2
9
0.8
85
0.07
0.2
FGD
8
0.8
94
0.4
0.8
HS
8
1.4
174
1
1

Geosynthetics October/November 2021

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