Hydraulic Conductivity of Geosynthetic Clay Liners to Coal Combustion Product Leachates

Transcription

1 Geosynthetics 2015 February 15-18, Portland, Oregon Hydraulic Conductivity of Geosynthetic Clay Liners to Coal Combustion Product Leachates Chen, J., University of Wisconsin-Madison, USA Benson, C. H., University of Wisconsin-Madison, USA, Edil, T. B., University of Wisconsin-Madison, USA, ABSTRACT Experiments were conducted to evaluate the compatibility of geosynthetic clay liners (GCLs) containing natural sodium bentonite when permeated with coal combustion product (CCP) leachates. Five synthetic leachates were selected from a database of CCP leachates to represent a range of CCP disposal facilities: typical CCP, strongly divalent cation fly ash, flue gas desulfurization () residual, high ionic strength ash, and trona ash leachate. Hydraulic conductivity tests were conducted on non-prehydrated and subgrade hydrated GCL specimens at effective stresses ranging from 20 to 450 kpa. At 20 kpa, the GCL had high hydraulic conductivity (>10-6 m/s) to trona leachate and moderate to high hydraulic conductivity (10-10 to 10-7 m/s) to the other CCP leachates. Hydraulic conductivity was strongly related to the ionic strength of the leachate and inversely related to the swell index of the bentonite when hydrated in leachate. Increasing the effective stress from 20 to 450 kpa caused the hydraulic conductivity to decrease up to three orders of magnitude. Hydration on a subgrade before permeation had no beneficial effect on hydraulic conductivity. 1. INTRODUCTION Coal combustion products (CCPs) are residuals from coal-fired boilers that are disposed in waste containment facilities when they cannot be used beneficially in other applications. When contacted with water, CCPs can release a variety of cations and anions into solution. Geosynthetic clay liners (GCLs) are used as hydraulic barriers in disposal facilities for CCPs. GCLs consist of a thin layer of bentonite clay (5-10 mm) sandwiched between two geotextiles bonded by stitching or needlepunching. In some cases, a geomembrane is laminated to the GCL. The effectiveness of a GCL as a barrier to fluid flow is controlled by the hydraulic conductivity of the bentonite, which is comprised primarily of the clay mineral montmorillonite (Mesri and Olson 1971, Shackelford et al. 2000). Species and valence of cations in the exchange complex associated with the montmorillonite surface strongly affect the hydraulic conductivity of GCLs. For most new GCLs, sodium (Na + ) is the primary cation bound to the surface of the bentonite, but other cations may be present and their relative abundance varies with the bentonite source (Shackelford et al. 2000, Jo et al. 2005). Because Na + generally is the primary cation in the exchange complex in a new GCL, the bentonite in GCLs is referred to as Na-bentonite or NaB (valence subscripts dropped henceforth for brevity). Na-bentonites have very low hydraulic conductivity to water (~10-11 m/s) because osmotic swelling occurs when monovalent cations (e.g., Na) bound to the montmorillonite surface hydrate (Norrish and Quirk 1954, van Olphen 1977, McBride 1994, Prost et al. 1998, Jo et al. 2001, Kolstad et al. 2004). Osmotic swelling diminishes the size and frequency of pores for water flow, and makes the flow pathways tortuous (Jo et al. 2001). However, replacement of monovalent cations on the surface by divalent or polyvalent cations, or increasing the ionic strength (I) of the surrounding pore water, can reduce or eliminate osmotic swelling, resulting in higher hydraulic conductivity (Egloffstein 1997, Shackelford et al. 2000, Jo et al. 2001, Kolstad et al. 2004). Replacement of Na in bentonite by cations with higher valence is thermodynamically favorable (Sposito 1981 and 1989, McBride 1994) and occurs when Na-bentonite is in environments where divalent or polyvalent cations are present. CCP leachates contain an abundance of Ca and Mg cations, which can replace Na, suppress osmotic swell, and potentially alter the hydraulic conductivity. This study was conducted to evaluate how CCP leachates affect the hydraulic conductivity of a GCL containing natural granular sodium bentonite. Hydraulic conductivity tests were conducted on GCL specimens with five different CCP leachates and deionized (DI) water. GCL specimens were permeated directly with leachate or hydrated on a subgrade prior to permeation to simulate field conditions more realistically. Tests were conducted at effective stresses ranging from 20 to 450 kpa to evaluate how the hydraulic conductivity may vary as a CCP disposal facility is filled. 173

2 2. MATERIALS 2.1 Geosynthetic Clay Liner The GCL used in this study contained granular Na-bentonite encapsulated between a woven and a nonwoven geotextile bound together by needle-punching. The dry bentonite mass per unit area was 3.66 kg/m 2 (per ASTM D5993), the average initial moisture content was 5.1%, and the initial thickness of GCL ranged from 7.9 to 10.4 mm (average = 9.1 mm). The granule size distribution (GSD) of the Na-bentonite is shown in Fig. 1 along with the GSD of the Na-bentonite used by Jo et al. (2001). The GSDs are similar in both studies, with more than 50% of the bentonite granules larger than 0.85 mm. Cation exchange capacity (CEC) and concentrations of initial bound cations (Ca 2+, Na +, Mg 2+ and K + ) were determined by ASTM D7503 (Table 1). The average CEC of the bentonite is 73.2 ± 4.9 cmol + /kg (5 tests) with Na the predominant bound cation (33.3 cmol + /kg). X-ray diffraction showed the mineralogy of the Na-bentonite consists of 84% montmorillonite, 9% quartz, 1% mica, 3% feldspar, and 1% calcite. 2.2 Synthetic CCP Leachate Kolstad et al. (2004) indicates that ionic strength and the relative abundance of monovalent and polyvalent cations in a permeant liquid are master chemical variables controlling the hydraulic conductivity of GCLs containing natural Nabentonite. The latter is quantified by the parameter RMD: RMD M m M d [1] where M M is the total molarity of monovalent cations and M D the total molarity of polyvalent cations in the solution. Leachates used in the testing program were selected after analyzing the ionic strength and RMD of leachates from CCP disposal facilities throughout the US. Concentrations in the leachates were verified by geochemical equilibrium computations made with Visual MINTEQ (US EPA) and ChemEQL (Swiss Federal Institute of Aquatic Science and Technology). Na, K, Ca, Mg, chloride, and sulfate were determined to be the major ions in the CCP leachates. The relationship between RMD and ionic strength for the leachates in the database is shown in Fig. 2. Five CCP leachates were selected for testing that represent different conditions encountered in CCP disposal facilities (solid triangles in Fig. 2): (1) typical CCP leachate (I=39.5 mm and RMD=0.16 M 1/2 ), (2) strongly divalent ( low RMD ) cation ash leachate (I=48.0 mm, RMD=0.014M 1/2 ), (3) flue gas desulfurization () residue leachate (I=96.8 mm, RMD=0.39M 1/2 ), (4) high ionic strength ash leachate (I=177 mm, RMD=1.0M 1/2 ), and (5) trona ash leachate (I=755 mm, RMD=4.5M 1/2 ). Synthetic leachates representing these conditions were created by dissolving reagent-grade CaSO 4, Na 2SO 4, MgSO 4, K 2SO 4, NaCl, and CaCl 2 in DI water (Benson et al. 2013). Table 1. Exchange complex and CEC of bentonite in GCL before and after subgrade hydration and hydraulic conductivity testing. Tests not conducted on GCLs permeated with high ionic strength leachate. GCL Original subgrade permeation permeation permeation permeation NaB hydration with Typ. CCP. with Low RMD with with Na < Mole K Fraction Ca Mg < CEC (cmol + /kg) Swell Index to DI (ml/2 g) Water Content (%) a a Measured after reaching hydraulic equilibrium at 450 kpa. 174

3 Percent FIner (%) This study Na-B (Jo et al. 2001) Fly Ash Mixed Coal Ash Synthetic Leachate (5) (3) (4) (1) Granule Size (mm) Figure 1. Granule size distribution of bentonite from GCLs in this study and in Jo et al. (2001) (2) Leachate Key: (1) Typ. CCP (2) Str. divalent (3) (4) High strength (5) Ionic Strength, I (mm) Figure 2. RMD vs. ionic strength of CCP leachates, including leachates used in study. 2.3 Subgrade Soil A silty clay subgrade soil was selected for subgrade hydration that had pore water with similar ionic strength but slightly lower RMD (I=3.8 mm and RMD=0.021 M 1/2 ) than the average subgrade pore water reported by Scalia and Benson (2010) (Fig. 3). The major cations in the pore water were determined by batch tests following the procedure described in Meer and Benson (2007) followed by chemical analysis (see subsequent discussion in METHODS). The major cation concentrations were: Na = 0.70 mm, Ca = 0.99 mm, K = 0.17 mm and Mg = 0.68 mm. The subgrade soil has a specific gravity of solids (G s) = 2.61, optimum moisture content (w opt) = 18.8% (standard Proctor), maximum dry unit weight (γ dmax) = 16.5 kn/m 3, and plasticity index (PI) = METHODS 3.1 Subgrade Hydration GCL specimens were hydrated on the subgrade soil following the methods described in Bradshaw et al. (2013) and Bradshaw and Benson (2013). The subgrade soil was compacted to the maximum dry unit weight (16.5 kn/m 3 ) at optimum water content (18.8%) per standard Proctor in a 152 mm (inside diameter) x 116 mm (height) compaction mold. compaction, the subgrade soil was extruded and placed on a 152-mm-diameter PVC bottom plate. A 152-mmdiameter GCL specimen was then placed on top of the subgrade (woven side of the GCL contacting the subgrade), which was overlain by a 1.5-mm geomembrane disk, a non-woven geotextile, and a PVC top plate. A latex membrane was placed around the entire setup and two O-rings were used on each end to seal the latex membrane to the PVC top and bottom plates. The entire assembly was then placed into a water-filled chamber with 10 kpa applied to simulate the stress applied by a leachate collection system. GCL specimens were hydrated on the subgrade for 60 ± 2 d to simulate the time period between completion of a liner installation and placement of CCPs. the hydration period, the 152-mm GCL specimen was trimmed to a diameter of 100 mm for hydraulic conductivity testing. The remainder of the specimen was used for water content, exchange complex, CEC, and swell index testing. 175

4 Scalia and Benson (2010) Meer and Benson (2007) Bradshaw (2008) Average Water (Scalia and Benson (2010)) Ionic Strength (M) This Study Benson and Meer (2009) bound for alteration in hydraulic conductivity due to wet-dry cycling RMD (M ) Figure 3. Ionic strength and RMD of subgrade pore water in this study (star) and from Scalia and Benson (2010). 3.2 Hydraulic Conductivity Hydraulic conductivity tests were conducted on the 100-mm-diameter GCL specimens using flexible-wall permeameters in accordance with ASTM D5084 and ASTM D6766. The falling headwater-constant tailwater method was used. A 20 kpa effective stress was applied initially, and the average hydraulic gradient was 190. GCL specimens were prepared in accordance with the procedure in Jo et al. (2001) and hydrated with leachate in the permeameter for 48 h with the effluent valve closed. Permeation followed immediately afterwards and was conducted until the hydraulic conductivity was steady, the incremental inflow and outflow were equal, and the ph, major cation concentrations (Na, Ca, Mg. K), and electrical conductivity of the effluent were within 10% of those of the influent. each test reached chemical equilibrium at 20 kpa, the effective stress was increased incrementally to 100, 250 and 450 kpa to evaluate how effective stress affects the hydraulic conductivity after chemical exchange processes have occurred. At the time this paper was submitted, all tests with direct permeation had been completed except for the test with high strength leachate (currently at 100 kpa effective stress). However, not all of the tests on GCLs hydrated on subgrade soil had reached chemical equilibrium at the time this paper was prepared. Thus, the hydraulic conductivities presented herein for those specimens represent a snapshot in time and may not represent final equilibrium conditions. 3.3 Swell Index Swell index (SI) tests were conducted on Na-bentonite with each leachate and DI water (control) in accordance with ASTM D5890. Bentonite from the GCL was ground to fine powder by a mortar and pestle until at least 65% passed the No. 200 US standard sieve. The powdered sample was oven-dried at 105 o C for 24 h prior to testing. A 100-mL graduated cylinder was filled to 90 ml with testing liquid, and 2 g of ground bentonite was added to the graduated cylinder in 0.1 g increments. The graduated cylinder was then filled to 100 ml with leachate. Bentonite volumes were recorded as the SI after 24 h of hydration. 4. RESULTS AND DISSCUSSIONS 4.1 Hydraulic Conductivity of GCLs with Direct Permeation Hydraulic conductivities of the Na-bentonite GCL to the CCP leachates at 20 kpa are shown in Fig.4 as a function of leachate ionic strength. The GCL had low hydraulic conductivity (2.6 x m/s) to DI water and higher hydraulic conductivity (>10-10 m/s) to all CCP leachates, with increasing hydraulic conductivity with higher ionic strength leachate (I typccp < I lowrmd < I < I highstr < I trona). The hydraulic conductivity increased up to four orders of magnitude (10-10 vs m/s) as the ionic strength of the leachate increased from 39.5 to 755 mm. Hydraulic conductivity of the GCL was also strongly related to swell index (Fig. 5), as previously reported by Jo et al. (2001) and Kolstad et al. (2004). Based on this study, when the swell index exceeds 24.0 ml/2g, the hydraulic 176

5 conductivity to CCP leachate is less than 10-9 m/s at 20 kpa. This good correspondence indicates that swell index can be used as an indicator of the hydraulic conductivity of the Na-B GCL used in this study Hydraulic Conductivity, K (m/s) DI High St. Low RMD Ty. CCP High St. Low RMD Ty. CCP DI Ionic Strength, I (mm) Figure 4. Hydraulic conductivity of GCLs vs ionic strength of leachate Swell Index (ml/2g) Figure 5. Hydraulic conductivity of GCLs vs. swell index. The exchange complex of the bentonites after permeation is summarized in Table 1. For GCLs permeated with Ca-rich leachate (i.e., Typ. CCP, Low RMD and leachate), nearly all of the Na naturally present in the exchange complex was replaced by Ca from the leachate (mole fraction of Na decreased from 0.44 to < 0.001, mole fraction of Ca increased from 0.28 to 0.90; Table 1). The swell index of the bentonite decreased to ml/2 g, which indicates that the Na-bentonite had been transformed to a Ca-bentonite and no longer undergoes osmotic swell when hydrated (Jo et al. 2001, Meer and Benson 2007). In contrast, the specimen permeated with the Na-rich leachate trona leachate had more Na and fewer divalent cations in the exchange complex than the original bentonite (Na mole fraction increased from 0.44 to 0.62; mole fraction of divalent cations decreased from 0.44 to 0.17). Nevertheless, the bentonite had low swell index in trona leachate (16.0 ml/2g) because the high ionic strength of the leachate suppressed osmotic swell. 4.2 Hydraulic Conductivity of GCLs with Subgrade Hydration Hydraulic conductivity of GCL specimens hydrated on a subgrade and then permeated with CCP leachates are shown in Fig. 6 relative to hydraulic conductivities to the same leachates with direct permeation. All of the data in Fig. 6 correspond to 20 kpa effective stress. The hydraulic conductivities obtained by both methods are similar and within a factor of 10, with GCLs hydrated on a subgrade having slightly higher hydraulic conductivity except for the test with Typ. CCP leachate. The higher hydraulic conductivities of GCLs hydrated on a subgrade is likely due to subtle cation exchange during hydration, despite the increase in average water content to 68.3% during the 60-d hydration period. Although major changes in the exchange complex did not occur during subgrade hydration (Na mole fraction decreased by 0.04, Ca increased by 0.10, Table 1), bentonite from GCLs hydrated on subgrades had an average post-hydration swell index in DI water of 22 ml/2 g, whereas bentonite from the new GCL had a swell index of 36 ml/2 g (Table 1). Thus, some of the osmotic swelling capacity of the bentonite was lost during subgrade hydration. 177

6 Hydraulic Conductivity, K (m/s) Hydraulic Conductivity (m/s) after Subgrade Hydration Typ CCP Low RMD High St : :1 1:2 1:1 1: Hydraulic Conductivity (m/s) after Direct Permeation Figure 6. Hydraulic conductivity of GCLs with subgrade hydration prior to permeation vs. GCLs with direct permeation. 4.3 Influence of Effective Stress on Hydraulic Conductivity Increasing the effective stress from 20 kpa to 450 kpa caused the hydraulic conductivity to decrease up to three orders of magnitude for the GCLs directly permeated with leachate (Fig. 7a). For Typ. CCP, low RMD, and leachate, the hydraulic conductivities decreased to no more than 3.3 x m/s when the stress was increased to 250 kpa, which is similar to the baseline hydraulic conductivity to DI water at 20 kpa (2.6 x m/s). Even for the tests with the high ionic strength trona leachate, the hydraulic conductivity decreased to approximately 1.0 x10-9 m/s when the effective stress was 450 kpa. Thus, once a disposal facility is filled, a GCL should have low hydraulic conductivity regardless of the CCP being contained. The relationship between normalized hydraulic conductivity (hydraulic conductivity at given stress hydraulic conductivity at 20 kpa, or K/K 20) and effective stress is shown in Fig. 7b. The decrease in hydraulic conductivity with increasing effective stress is similar regardless of the leachate being used for permeation, which suggests that the decrease in hydraulic conductivity is controlled by physical phenomena (i.e., decrease in void ratio) rather than chemical phenomena. The functional relationships in Fig. 7b can be used to estimate the expected and upper bound hydraulic conductivities at any stress between 20 and 450 kpa if the hydraulic conductivity at 20 kpa is known (a) Typ CCP Low RMD High St Upper Bound Line: log (K/K 20 ) = 38.4 log ' R 2 = Baseline with DI water at 20 kpa (b) Effective Stress, ' (kpa) Effective Stress, ' (kpa) Figure 7. Relationship between hydraulic conductivity (a) and normalized hydraulic conductivity (b) and effective stress. K/K Regression Line: log (K/K 20 )= * log ' R 2 =

7 5. CONCLUSIONS The hydraulic conductivity of a geosynthetic clay liner (GCL) containing natural sodium bentonite to coal combustion product (CCP) leachates was evaluated in this study. Hydraulic conductivity tests were conducted on GCLs directly permeated with leachate and GCLs hydrated on a subgrade prior to permeation at effective stresses ranging from 20 kpa to 450 kpa. The following conclusions are drawn: At low effective stress (20 kpa), the Na-bentonite GCL had high hydraulic conductivity (>10-6 m/s) to trona leachate; moderate or high hydraulic conductivity to the low RMD, high ionic strength, and leachates (10-10 to 10-7 m/s); and low hydraulic conductivity (10-11 m/s) to DI water. The hydraulic conductivity was strongly and directly related to the ionic strength of the leachate, and inversely related to the swell index of the bentonite when hydrated with leachate. Hydraulic conductivity less than 10-9 m/s is realized when the swell index in leachate exceeds 24 ml/2 g. Subgrade hydration had no beneficial effect on hydraulic conductivity, and in most cases GCLs hydrated on a subgrade were more permeable to leachate (albeit slightly) than GCLs directly permeated with leachate. Increasing the effective stress from 20 kpa to 450 kpa caused the hydraulic conductivity to decrease up to three orders of magnitude. Equations are provided to estimate the expected and lower bound reduction in hydraulic conductivity that occurs as the effective stress increases. ACKNOWLEDGEMENT The Electric Power Research Institute (EPRI) provided financial support for this study through a grant to the Office of Sustainability at the University of Wisconsin-Madison. Bruce Hensel of Natural Resource Technology Inc. provided input regarding the leachate database. REFERENCES Benson, C., Chen, J. and Edil, T. (2013). Hydraulic Conductivity of Geosynthetic Clay Liners to Coal Combustion Product Leachates Sustainability Report No. OS-13-07, Office of Sustainability, University of Wisconsin-Madison, Madison, WI. Benson, C., and Meer, S. (2009). Relative Abundance of Monovalent and Divalent Cations and the Impact of Desiccation on Geosynthetic Clay Liners, J. Geotech. Geoenviron. Eng., 135(3), Bradshaw, S., (2008). Effect of Cation Exchange During Subgrade Hydration and Leachate Permeation. MS Thesis, University of Wisconsin-Madison, WI, USA. Bradshaw, S., Benson, C., and Scalia, J. (2013). Hydration and Cation Exchange during Subgrade Hydration and Effect on Hydraulic Conductivity of Geosynthetic Clay Liners, J. Geotech. Geoenvironmental Eng., 139(4), Bradshaw, S. and Benson, C. (2013). Effect of Municipal Solid Waste Leachate on Hydraulic Conductivity and Exchange Complex of Geosynthetic Clay Liners, J. Geotech. Geoenvironmental Eng., to 17. Egloffstein, T. (1997). Geosynthetic Clay Liners, Part Six: Ion Exchange, Geotech. Fabrics Report, 15(5), Jo, H., Katsumi, T., Benson, C., and Edil, T. (2001). Hydraulic Conductivity and Swelling of Nonprehydrated GCLs Permeated with Single-Species Salt Solutions. J. Geotech. Geoenviron. Eng., 127(7), Jo, H., Benson, C., Shackelford, C., Lee, J., and Edil, T. (2005). Long-Term Hydraulic Conductivity of a Non-Prehydrated Geosynthetic Clay Liner Permeated with Inorganic Salt Solutions, J. of Geotech. Geoenviron. Eng., 131(4), Kolstad, D., Benson, C., and Edil, T. (2004). Hydraulic Conductivity and Swell of Nonprehydrated GCLs Permeated With Multi-Species Inorganic Solutions, J. Geotech. Geoenvironmental Eng., 130(12), McBride (1994). Environmental Chemistry of Soils, Oxford University Press, New York, 406 p. Mesri, G. and Olson, R. (1971). Mechanisms Controlling the Permeability of Clays. Clays and Clay Minerals, 19,

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