Coal combustion residuals (CCRs) are the byproducts from combustion in coal-fired 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 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 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.
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–/SO42-), 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):
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).
The relationship between RMD and ionic strength for the CCR leachates is shown in Figure 2 along with leachates in the Electric Power Research Institute (EPRI) leachate database reported by Chen et al. (2018, 2019). The CCR leachates in this study (closed symbols in the figure) predominantly have ionic strengths in the upper portion of the ionic strengths in the EPRI leachate database and tend to have lower RMD (more polyvalent).
Geosynthetic clay liners
Seven commercially available BPC GCLs were evaluated in this study. All of the BPC GCLs are manufactured by dry mixing one or more granular proprietary polymers (cross-linked or linear polymers) with granular sodium bentonite. The BPC GCLs are labeled as BP4.0, BP5.8, BP6.3, BP8.6, BP8.9, BP9.0 and BP9.7, with the numerical suffix representing the polymer loading in percent by dry mass as measured based on loss of ignition (LOI) of the BPC and NaB using the procedure in Scalia et al. (2014).
Hydraulic conductivity of GCL
Hydraulic conductivity tests were conducted on 6-inch (150-mm) diameter GCL specimens using flexible-wall permeameters in accordance with ASTM D6766. The falling headwater-constant tailwater method was used (Figure 3). GCL specimens were trimmed from the rolls following the method described in Jo et al. (2001). Excess geotextile fibers were carefully removed from the edge of GCL specimens, and bentonite paste prepared with the permeant liquid was applied to the edge of specimens to prevent bentonite loss that could potentially induce sidewall leakage. The GCL specimens were placed between two nonwoven geotextiles (mass per area = 7.1 ounces per square yard [240 g/m2]) to evenly distribute flow and to prevent fouling with polymer eluted from the GCL. The effective stress was set at 2.9 psi (20 kPa) and the average hydraulic gradient was 190.
GCL specimens were hydrated with CCR leachate for 48 hours prior to permeation. Hydration was conducted in the permeameters with the cell pressure applied, the headwater applied and the effluent valve closed. Permeation followed immediately afterward and was conducted until the hydraulic conductivity was steady, the ratio of incremental outflow to inflow (Qout/Qin) was within 1.00 ± 0.25, and the pH and EC of the effluent were within 10% of those of the influent. Duplicate tests were conducted to ensure the results were reproducible. When relatively high hydraulic conductivities (10-8 m/s) were obtained, the influent was spiked with Rhodamine WT dye to stain flow paths through the GCL.
Results Hydraulic conductivity
Typical graphs of hydraulic conductivity, Qin/Qout, pHout/pHin and ECout/ECin versus cumulative inflow are shown in Figures 4a and 4b, which show data from the test on GCL BP6.3 permeated with the CCR-VA2 leachate. The final hydraulic conductivity was 4.2 × 10-12 m/s. Hydraulic conductivity of the GCL gradually decreased and then leveled out after 3.4 ounces (100 mL) of inflow. The ratio Qin/Qout was near 1.0 after 1.7 ounces (50 mL) cumulative inflow, indicating the GCL was hydrated. The EC ratio diminished as salts were flushed from the pore space, with ECout/ECin within 1 ± 0.1 after 8.1 ounces (240 mL) of inflow. The ratio pHout/pHin was always within 1± 0.1, indicating pH equilibrium.
Effect of ionic strength
Hydraulic conductivity of the BPC GCLs as a function of ionic strength of the CCR leachates is shown in Figure 5. Hydraulic conductivities of the NaB GCLs to CCR leachates from Chen et al. (2019) are also shown in the figure. Hydraulic conductivities of the BPC GCLs are one to four orders of magnitude lower than hydraulic conductivities of NaB GCLs for similar ionic strength. Hydraulic conductivity of the NaB GCL is strongly affected by ionic strength, whereas hydraulic conductivity of the BPC GCL is nearly independent of ionic strength. All of the BPC GCLs had hydraulic conductivity less than 10-10 m/s, even when permeated with leachate of high ionic strength (i.e., CCR-WY with I = 681 mM). Most were less than 1 × 10-11 m/s.
Effect of RMD and anion ratio
Hydraulic conductivity of the BPC GCL is shown as a function of RMD and anion ratio of the CCR leachates in Figures 6a and 6b on page 18. No trends are evident with RMD or anion ratio, except the highest hydraulic conductivities were obtained for leachates with lower RMD and lower anion ratio, and the lowest polymer loading. The low RMD may have suppressed swelling of the bentonite, rendering the polymer loading insufficient to achieve low hydraulic conductivity for these more permeable BPC GCLs.
Mechanisms controlling hydraulic conductivity of BPC GCLs to CCR leachate
Tian et al. (2019) and Chen et al. (2019) indicate that the hydraulic conductivity of BPC GCLs is controlled by polymer hydrogel clogging intergranular pores. Higher polymer loading in the BPC GCL provides greater likelihood that flow paths are clogged and the hydraulic conductivity is low. Hydraulic conductivity to the CCR leachates is shown in Figure 7 as a function of polymer loading of the BPC GCL (open symbols). Hydraulic conductivities of NaB GCLs in the figure are reported as zero polymer loading (closed symbols). As the polymer loading increases, the maximum hydraulic conductivity of the BPC GCL decreases, and the hydraulic conductivities become increasingly consistent. The hydraulic conductivity is consistently less than 10-10 m/s when the polymer loading exceeds 4%, and less than 10-11 m/s when the polymer loading exceeds 6%. The type of polymer used in the BPC probably influences the threshold as well, but was not provided by the GCL manufacturers. These hydraulic conductivities represent the equilibrium condition obtained with the aforementioned methods. In the long-term, polymer elution may alter the hydraulic conductivity of the BPC GCL (Tian et al. 2019).
Hydraulic conductivity tests were conducted on seven BPC GCLs using seven CCR leachates. The GCLs were permeated directly with leachate (no prehydration) at an effective stress of 2.9 psi (20 kPa). The following conclusions are drawn:
BPC GCLs can have appreciably lower hydraulic conductivity than NaB GCLs when permeated with CCR leachates. The BPC GCLs evaluated in this study have hydraulic conductivities to CCR leachates ranging from 10-12 to 10-10 m/s, whereas NaB GCLs had hydraulic conductivities to the CCR leachates ranging from 10-10 to 10-6 m/s.
BPC GCLs with higher polymer loading tend to have lower hydraulic conductivity to CCR leachates. BPC GCLs consistently had hydraulic conductivity less than 10-10 m/s when the polymer loading was greater than 4%, and less than 10-11 m/s when the polymer loading was greater than 6%. Polymer type likely affects these thresholds, but was not explored in this study.
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 Wisconsin-Madison in Madison, Wisc.
All figures courtesy of the authors except as noted.
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