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Hydraulic capability of polymer-treated GCLs in saline solutions at elevated temperatures
Applied Clay Science 161 (2018) 364–373
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Research paper
Hydraulic capability of polymer-treated GCLs in saline solutions at elevated temperatures Hakki O. Ozhan
T
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Altinbas University, Department of Civil Engineering, Mahmutbey Dilmenler Cad., No. 26, Bagcilar, Istanbul, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Anionic polymer Cationic polymer Geosynthetic clay liner Permeability Triaxial permeability test Viscosity
Geosynthetic clay liners (GCLs) can be used effectively as barriers in waste containment facilities. However, interaction of the GCL with a saline solution might cause a decrease in the hydraulic performance of the GCL. In order to simulate the field conditions, triaxial permeability tests were performed on a GCL permeated with 0.5 M and 0.1 M MgCl2 solutions at 20 °C, 40 °C and 60 °C. The temperature increase resulted in an increase in the permeability of the GCL and this increase is attributed to a decrease in the viscosity of the permeant fluid. In order to improve the hydraulic capability of the GCL, an anionic and a cationic polymer with a polymer content of 1% and 2% by mass in the bentonite-polymer mixture was added to the GCL. According to the results, polymer treatment caused up to two orders of magnitude decrease in permeability and improved the hydraulic capability of the GCL. Generally, anionic polymer treatment resulted in slightly lower permeabilities than cationic polymer treatment. Furthermore, 2% anionic polymer treatment caused almost no decrease in permittivity when compared with 1% polymer treatment. However, the permeability of 1% cationic polymer-treated GCL was the lowest among all the other anionic and cationic polymer-treated GCLs that were permeated with 0.1 MgCl2 solution at 60 °C.
1. Introduction Geosynthetic clay liner (GCL) is a barrier material that is composed of a bentonite layer sandwiched between two geotextile layers. GCL is preferred as a lining over a soil deposit with its very low permeability to water, high self-healing capacity and low thickness and cost (Bouazza, 2002). Generally, GCLs are used as hydraulic barriers in various waste containment facilities such as mine tailings dams, hazardous waste landfills and heap leach pads (Rowe, 2014; Touze-Foltz et al., 2016). In these geoenvironmental applications, GCLs are usually in contact with either acidic or saline solutions. These liquids can be considered as leachates that might deteriorate the hydraulic capability of the GCLs by infiltrating through the bentonite component of the GCLs (Rowe, 2005; Abdelaal et al., 2014; Tian et al., 2016). Due to the cation exchange between the bentonite in the GCL and the leachate that the GCL was permeated with, permeability of the GCL might increase drastically. In order to prevent the interaction between the GCL and the leachate, a geomembrane layer is routinely laminated to the GCL. However, the geomembrane might be damaged when angular particles of a soil deposit are in contact with it or a high leachate level applies extra hydraulic load on it (Ozhan and Guler, 2013). After a possible crack or wrinkle is formed on the surface of the geomembrane, the leachate
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Corresponding author. E-mail address: [email protected].
https://doi.org/10.1016/j.clay.2018.05.007 Received 9 February 2018; Received in revised form 7 May 2018; Accepted 9 May 2018 0169-1317/ © 2018 Elsevier B.V. All rights reserved.
might infiltrate through the damaged geomembrane into the GCL and the hydraulic performance of the GCL might decrease again (Azad et al., 2012). Furthermore, organic materials that are collected in a waste containment facility decompose biologically which results in an increase in the temperature of the wastes (Rowe, 2005). Barrier materials used in mining and industrial applications might remain exposed to high temperatures ranging from 60 °C to 80 °C when the GCL has been in contact with the wastes for more than five years (Thiel and Smith, 2004; Bouazza et al., 2014). As a result, an increase in the temperature of the leachate probably deteriorates the hydraulic capability and the durability of the lining material (Rowe, 2005; Bouazza et al., 2014). Ishimori and Katsumi (2012) performed hydraulic conductivity tests on a GCL that was composed of powdered sodium bentonite sandwiched between a woven and a nonwoven geotextile. The GCL was permeated with 0.4 M NaCl solution at 20 °C and 60 °C. Permeation continued 98 days and hydraulic conductivity was measured as 5.8 × 10−11 m/s at 20 °C whereas 5.1 × 10−10 m/s at 60 °C. Test results indicated that temperature increase resulted in an increase of almost one order of magnitude in permeability. Saline solutions having divalent cations caused a drastical increase in the permeability of GCLs. Jo et al. (2001) performed hydraulic
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prevented the formation of effective bridging between bentonite particles and polymer chains (Lage et al., 2009). The main objective of this study was to investigate the effects of adding an anionic and a cationic polymer to a GCL in order to evaluate the hydraulic performance of the GCL usage in waste containment facilities. To simulate an aqueous solution with a high salinity content that might be collected in a waste containment facility, 0.5 M and 0.1 M MgCl2 solutions were used as the permeant fluids. The hydraulic performance of bentonite permeated with CaCl2 solutions has been investigated many times before (Gleason et al., 1997; Shan and Lai, 2002; Lee and Shackelford, 2005; Scalia and Benson, 2011; Scalia et al., 2014; Geng et al., 2016; Shen et al., 2016). MgCl2 solution is another saline solution with divalent cations but has not been tested with polymertreated GCLs before. For this reason, the permeant fluid was chosen as MgCl2 solution. Triaxial permeability tests were performed on the polymer-treated GCLs and the GCL without polymer treatment. The concentration of the polymer added to the GCLs was chosen as 1% and 2% by mass. Due to the biodegradation of organic substances in a waste containment facility, the temperature of the leachates might increase up to 60 °C (Rowe and Islam, 2009). For this reason, the effect of the solution temperature on the hydraulic capability of the GCLs was investigated by setting the temperature of the permeant fluid to 20 °C, 40 °C and 60 °C.
conductivity tests on GCLs permeated with 0.1 M CaCl2 solution. Permeability increased three orders of magnitude when compared with that of the same GCL permeated with deionized water. Chun-Ming et al. (2013) indicated that an increase in the molarity of an aqueous solution caused both an increase in the permeability and a decrease in the swelling capability of the bentonite. Hydraulic performance of the bentonite also decreased when the cations of the chemical solution that the bentonite was in contact with were divalent or trivalent instead of monovalent (McBride, 1994; Jo et al., 2001). In order to improve the hydraulic capability of the bentonite that was in contact with acidic or saline solutions, polymers can be added to the bentonite. Test results indicated that polymer treatment generally lowered the permeability of the bentonites permeated with either saline solutions or deionized water even for long periods of time (Razakamanantsoa et al., 2012; Scalia et al., 2014; Shen et al., 2016; Tian et al., 2016). However, Ashmawy et al. (2002) experienced GCL failure and no improvement in permeability as a consequence of polymer addition. Although cationic polymer treatment lowered the permeability (Razakamanantsoa et al., 2012; Tian et al., 2016), anionic polymer-treated bentonite had usually lower permeability than cationic polymer-treated bentonite (Liu et al., 2012a; Razakamanantsoa et al., 2012; Theng, 2012; Haase and Schanz, 2016). However, swelling capacity of cationic polymer-treated bentonite was higher than that of anionic polymer-treated bentonite in some studies (Liu et al., 2012b; Razakamanantsoa et al., 2012). This could be considered as an advantage of cationic polymer over anionic polymer for the applications where swelling is crucial. Theng (2012) stated that the interaction between bentonite particles and polymers is governed by electrostatic forces. Negatively charged (anionic) polymers are repelled from the net negatively charged bentonite but are attracted to the edge particles of the bentonite that are positively charged (Theng, 1982; Lu et al., 2002). Hydrogen bonding which is a weak attachment governs the bridges between bentonite particles and anionic polymers (Lu et al., 2002). Aggregated fabric is the structure formed by the cohesion of colloidal particles whereas flocculated fabric is the structure formed by colloidally unstable aggregates. (Haase and Schanz, 2015). When the interlayer cations are multivalent, flocculated bentonite fabrics are formed due to anionic polymer treatment. Flocculated fabrics lead to an increase in the homogeneity of the pore size distribution by decreasing the size of the maximum pore radii. As a result, a net-like fabric with small pores is formed due to a homogeneous pore size distribution (Mesri and Olson, 1971; Haase and Schanz, 2016). On the other hand, Coulomb bonding which is a much stronger attachment than hydrogen bonding governs the bridges between bentonite particles and cationic polymers. By the attraction between positively charged (cationic) polymers and the net negatively charged bentonite particles, the bentonite particles are stacked to form aggregates. Due to the fact that the pore size distribution is not homogeneous in aggregated fabrics and due to the existence of flow paths, this nonhomogeneous pore size distribution causes an increase in permeability at the same global void ratio (Haase and Schanz, 2016). Vryzas et al. (2017) performed viscosity tests on 7% sodium bentonite by mass that was mixed with deionized water. They indicated that temperature increase from 25 °C to 80 °C caused a decrease in the viscosity of the fluid. Ratkievicius et al. (2016) performed viscosity tests on bentonite-cationic polymer mixtures permeated with an aqueous solution. Cationic polymer addition resulted in an increase in the viscosity of the permeant fluid. Geng et al. (2016) conducted viscosity tests on sodium bentonite-anionic polymer mixtures permeated with deionized water, CaCl2 and NaCl solutions. Test results indicated that viscosity increased with an increase in the anionic polymer content. Moreover, viscosity was lower in CaCl2 (divalent) solutions than in NaCl (monovalent) solutions. Low viscosity resulted in a contracted polymer conformation whereas high viscosity caused an extended polymer conformation to be formed. As a result, contracted polymers
2. Materials The GCL tested consisted of a granular sodium bentonite placed between a woven and a nonwoven geotextile without reinforcement. The difference in permittivity of a needle-punched and an unreinforced GCL was found to be only of approximately 0.04 order of magnitude (Ozhan and Guler, 2013). By considering that internal confinement due to needle-punching did not cause a decrease in permittivity and it was not easy to add polymer to the bentonite component of a reinforced GCL due to the fibers that held the carrier and the cover geotextiles together, an unreinforced GCL was used in the triaxial permeability tests. The nonwoven geotextile of the GCL was a polypropylene, needlepunched geotextile with a mass/unit area of 250 g/m2 (ASTM D5261, 2010) and an apparent opening size of 0.2 mm (ASTM D4751, 2016). The woven geotextile of the GCL was a polypropylene, slit-film geotextile that had a mass/unit area and an apparent opening size of 100 g/ m2 (ASTM D5261, 2010) and 0.4 mm (ASTM D4751, 2016) respectively. The nonwoven geotextile was the carrier and the woven geotextile was the cover of the GCL. Specific gravity (ASTM D854, 2014), liquid and plastic limits (ASTM D4318, 2010), mass/unit area (ASTM D5993, 2014), cation exchange capacity (CEC), mineralogical and chemical compositions (Karakaya Bentonit, 2014) of the bentonite component of the GCL are listed in Table 1. The polymers that were mixed with the bentonite component of the GCL were air-dry, powdered anionic and cationic polymers. 2% of the polymer particles had a grain size of greater than 2 mm and 6% of the particles had a grain size of smaller than 0.15 mm (SNF, 2016). The anionic polymer was composed of negatively charged copolymers of acrylamide (C3H5NO) and contained almost nine times more soluble anions than cations in terms of mass. In contrast, the cationic polymer consisted of positively charged copolymers of acrylamide (C3H5NO) and was composed of approximately 11 times more soluble cations than anions in terms of mass. To specify the main soluble anions and cations in the polymers, a chemical analysis by Inductively Coupled Plasma–Atomic Emission Spectrometry (Van de Wiel, 2003) was performed. 1 g of the polymer was dissolved into 1 L of deionized water and the concentrations of the main ions on the polymer were identified. According to the results, the anionic polymer was composed of 0.026 mg/ L Cl−, 0.0083 mg/L NO3−, 0.0058 mg/L SO42-, 0.0032 Na+ and 0.0014 mg/L K+ whereas the main ions of the cationic polymer were 365
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Table 1 Material characterization of the bentonite. Engineering properties
Value
Specific gravity (−) Liquid limit (%) Plastic limit (%) Mass/unit area (g/m2) CEC (meq/100 g) Mineralogical composition Smectite (%) Quartz (%) Feldspar (%) Calcite (%) Biotite (%) Chemical composition SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) Na2O (%) MgO (%) K2O (%)
2.69 640 28 4800 77 Value 91 4 2 2 1 Value 61.28 17.79 3.01 4.54 2.70 2.10 1.24
0.032 mg/L Na+, 0.0095 mg/L Ca2+, 0.0062 mg/L K+ and 0.0044 SO42−. Triaxial permeability tests were conducted on the GCLs that were permeated with 0.5 M and 0.1 M MgCl2 solutions. These chemical solutions with divalent cations and high concentrations simulated saline solutions that could be collected in waste containment facilities.
Fig. 2. Triaxial permeability test set-up.
flexible-wall triaxial permeability cells. From top to bottom, triaxial permeability test set-up consisted of stainless steel top cap, porous stone, filter paper, GCL, filter paper, porous stone and stainless steel base cap as shown in Fig. 2. Then, the top and bottom caps were lubricated with high-vacuum silicone grease in order to attach the latex membrane to the GCL and prevent side leakage. Afterwards, latex membrane was wrapped around the test set-up and o-rings were attached to the membrane (ASTM D6766, 2012). The triaxial permeability cell was placed in a stainless steel tank with heater. The tank was filled with tap water and both the triaxial permeability cell and the plastic tubes that provided the fluid permeation through the GCL were sunk into the water as shown in Fig. 3 in order to set the temperature of the permeant fluid to the desired values. The temperature of the permeant fluid was measured from the outlet of the plastic tube by using a digital termometer. For setting the temperature of the permeant fluid to 20 °C, 40 °C and 60 °C, the heater of the tank was programmed to 21.1 °C, 41.3 °C ve 61.4 °C respectively. These slight differences between the programmed and the measured temperatures were due to the interaction between the fluid in the tank and the fluid in the plastic tubes. In conclusion, the heat transfer between the water in the tank and the fluid in the the plastic tubes was completed and the temperature of the permeant fluid was set to 20 °C, 40 °C and 60 °C. First, tap water was transferred from the water supply tank to the pressure supply unit and then, from the pressure supply unit to the triaxial permeability cell through a plastic tube as shown in Fig. 3. In order to saturate the GCL with water and then, consolidate it, cell pressure and backpressure were set to 550 and 515 kPa respectively.
3. Methodology and testing program First, the geotextile components of the GCL with a diameter of 100 mm were cut from geotextile rolls (ASTM D6766, 2012). Then, the granular bentonite was dried in an oven for 24 h at 105 °C and placed on the nonwoven carrier geotextile as shown in Fig. 1a. Afterwards, the bentonite was wetted with the permeant fluid homogenously in order to provide bonding to the geotextiles. Finally, the woven cover geotextile was attached to the wetted bentonite without any reinforcement effort such as needle-punching or stich-bonding as shown in Fig. 1b (Ozhan and Guler, 2013). For the sample preparation of the polymer-treated GCLs, dry polymers with a content of 1% and 2% by mass were added to the bentonite. The bentonite-polymer mixtures were mixed homogenously by using a spatula and then, the mixtures were poured into polyethylene bottles to avoid hydration. The bottles were closed and shaken by hand for five minutes in order to obtain homogeneous mixtures. Finally, the mixtures remained in the bottles for 24 h (Razakamanantsoa et al., 2012). Then, the polymer-treated GCLs were assembled by following the same procedure that was used for assembling the GCL without polymer treatment. After the sample preparation, constant-head triaxial permeability tests were performed on the GCLs. First, the GCLs were placed in
Fig. 1. (a): Sample preparation of GCL without reinforcement; (b): GCL specimen with the woven cover geotextile. 366
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Fig. 3. Diagram of the triaxial permeability test configuration.
MgCl2 solution whereas this period was 32–34 days for the GCLs permeated with 0.1 M MgCl2 solution. After measuring almost constant permittivity values, the triaxial permeability tests continued for a while and then, the tests were terminated. The GCL specimen in each test was designated by considering the polymer content and type (2AP for 2% anionic polymer, 0.5CP for 0.5% cationic polymer, etc.) and the permeant fluid concentration (0.5MC for 0.5 M MgCl2 solution and 0.1MC for 0.1 M MgCl2 solution). For example, 1% anionic polymer-treated GCL that was permeated with 0.5 MgCl2 solution and 2% cationic polymer-treated GCL that was permeated with 0.1 MgCl2 solution were designated as GCL-1AP-0.5MC and GCL-2CP-0.1MC respectively. GCL without polymer treatment that was permeated with 0.5 M MgCl2 solution was designated as GCL0.5MC. The repeatibility of the flow rates passing through the GCL was checked by testing duplicate GCL specimens for GCL-0.5MC and GCL0.1MC. According to the results, only a difference of up to 0.02 order of magnitude was obtained between the permittivities of the duplicate GCL specimens. For this reason, the rest of the triaxial permeability tests were performed on one GCL specimen for each condition. Furthermore, the tests under different temperatures were not performed consecutively on the same GCL specimen. After the termination of each test at the selected temperature, the permeability coefficient of the GCL specimen was measured and the GCL specimen was not used again in the following tests. In triaxial permeability tests, permittivity was measured instead of permeability because it was not possible to measure the thickness of the GCL in the triaxial permeability cell and the GCL could have various thickness values at the same time due to the fact that the GCL was a very thin and compressible material (Koerner, 1998). However, permeability of the GCLs could be measured at the end of each test. After the termination of the tests, the GCLs were taken from the triaxial permeability cell and then, the thickness of the GCLs was measured
After the termination of saturation and consolidation, the water supply tank was filled with the permeant fluid, then, the permeant fluid was transferred to the influent pressure reservoir and then, to the GCL. Afterwards, the GCL was permeated from bottom to top by increasing the influent pressure at the bottom of the GCL to 518 kPa and keeping the effluent pressure at the top of the GCL at 515 kPa (ASTM D6766, 2012). As a result, 3 kPa difference between the influent and effluent pressure resulted in an upward flow through the GCL. A hydraulic pressure of 3 kPa represented a hydraulic head of almost 30 cm which could be considered as the maximum leachate level collected in a waste containment facility (Weber and Zornberg, 2005). The permeant fluid was collected in the effluent pressure reservoir. The plastic tubes that provided permeant fluid transfer from the influent pressure reservoir to the triaxial permeability cell and from the triaxial permeability cell to the effluent pressure reservoir are shown in Fig. 3. During the triaxial permeability tests, permittivity (Ψ) of the GCL was measured and a correlation between permittivity and elapsed time was established. Permittivity is expressed in Eq. (1) as follows (Shan and Chen, 2003):
Ψ=
ΔQ A⋅Δh⋅Δt
(1)
Where Ψ (1/s) is the permittivity of the GCL, ΔQ (cm ) is the average of inflow and outflow of the permeant fluid through the GCL for a specific time interval, A (cm2) is the cross-sectional area of the GCL, Δh (cm) is the hydraulic head difference acting on the GCL and Δt (s) is the time interval where ΔQ is measured. Several permittivity values were measured until the termination of each triaxial permeability test. These tests were completed when the permeation became steady. However, it took a long time for the permittivity values to be stable. Almost constant permittivity values were measured 13–15 days after the beginning of the permeation with 0.5 M 3
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1.E-04
Permittivity (1/s)
1.E-05
1.E-06
1.E-07
GCL-0.5MC GCL-1AP-0.5MC GCL-2AP-0.5MC GCL-1CP-0.5MC GCL-2CP-0.5MC
1.E-08
1.E-09 0
50
100
150
200
250
300
350
400
450
Elapsed Time (hour) Fig. 4. Permittivity change of the GCLs permeated with 0.5 M MgCl2 solution at 20 °C with respect to time.
treated with 1% cationic polymer was approximately 0.6 order of magnitude lower than that of the GCLs treated with 2% cationic polymer. By causing a decrease of almost two orders of magnitude in permittivity, 1% cationic polymer-treated GCL had even lower permittivity than that of the anionic polymer-treated GCLs at 60 °C. For all the tested GCLs, permittivity increased similarly with an increase in temperature regardless of polymer treatment. Temperature increase from 20 °C to 60 °C caused an increase of 1–1.5 orders of magnitude in permittivity for all the GCLs permeated with either 0.5 M or 0.1 M MgCl2 solution. Changes in final equilibrium permeability coefficient with respect to salinity of MgCl2 solution for different polymer additions are shown in Figs. 10, 11 and 12 at 20 °C, 40 °C and 60 °C respectively. As the salinity was increased from 0.1 M to 0.5 M, the permeability coefficient also increased. The permeability coefficient of all the polymer-treated GCLs except 2% cationic polymer-treated GCL was almost 1.5 orders of magnitude lower than that of the GCL without polymer treatment when the GCLs were permeated with 0.1 M MgCl2 solution at 20 °C and 40 °C. The permeability coefficient of 2% cationic polymer-treated GCL was less than one order of magnitude lower than that of the GCL without polymer treatment. However, 1% cationic polymer-treated GCL had the lowest permeability at 60 °C. 1% cationic polymer treatment caused a decrease of two orders of magnitude in permeability, from 2.7 × 10−8 to 2.6 × 10−10 m/s. Permeability coefficients of the anionic polymer-
(ASTM D5199, 2012). Permeability coefficient of the GCLs is expressed in Eq. 2 as follows (Koerner, 1998): (2)
k = Ψ. L
Where k (m/s) is the final equilibrium permeability coefficient and L is the final thickness (m) of the GCL. 4. Results In order to evaluate the effects of polymer addition to the GCL, changes in permittivity with respect to elapsed time are shown in Figs. 4, 5 and 6 for the GCLs permeated with 0.5 M MgCl2 solution at 20 °C, 40 °C and 60 °C respectively. As shown in Figs. 4, 5 and 6, the permittivity of the GCLs treated with 1% anionic polymer was almost two orders of magnitude lower than that of the GCL without polymer treatment whereas the cationic polymer-treated GCLs had permittivity values 0.6–0.8 order of magnitude higher than those of the anionic polymer-treated GCLs. However, 2% polymer treatment resulted in almost no decrease in permittivity. Changes in permittivity with respect to elapsed time are shown in Figs. 7, 8 and 9 for the GCLs permeated with 0.1 M MgCl2 solution at 20 °C, 40 °C and 60 °C respectively. Both anionic and cationic polymer treatment resulted in a decrease in permittivity. However, the effect of polymer content on permittivity was not the same as that of the GCLs permeated with 0.5 M MgCl2 solution. The permittivity of the GCLs 1.E-04
Permittivity (1/s)
1.E-05
1.E-06
1.E-07
GCL-0.5MC GCL-1AP-0.5MC GCL-2AP-0.5MC GCL-1CP-0.5MC GCL-2CP-0.5MC
1.E-08
1.E-09 0
50
100
150
200
250
300
350
400
450
Elapsed Time (hour) Fig. 5. Permittivity change of the GCLs permeated with 0.5 M MgCl2 solution at 40 °C with respect to time. 368
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1.E-03
Permittivity (1/s)
1.E-04
1.E-05
1.E-06
1.E-07 GCL-0.5MC GCL-1AP-0.5MC GCL-2AP-0.5MC GCL-1CP-0.5MC GCL-2CP-0.5MC
1.E-08
1.E-09 0
50
100
150
200
250
300
350
400
450
Elapsed Time (hour) Fig. 6. Permittivity change of the GCLs permeated with 0.5 M MgCl2 solution at 60 °C with respect to time.
Schanz, 2016). In this study, either anionic or cationic polymer addition improved the hydraulic capability of the GCL by decreasing permeability. This decrease might be attributed to the polymer treatment that caused the pores to become partially blocked and the pore sizes to be reduced. As a result, pore size distribution governed the decrease in permeability due to polymer addition to the bentonite component of the GCL (Li and Zhang, 2009). 1% anionic polymer addition was probably enough to reduce the pore sizes to a minimum level regardless of the salinity of the permeant fluid. 2% anionic polymer treatment resulted in only a very small amount of decrease in permeability; in some cases, no decrease at all particularly for 0.1 M MgCl2 solution. On the other hand, 1% cationic polymer addition probably blocked most of the pores in the bentonitepolymer mixture for the GCL permeated with 0.1 M MgCl2 solution. The increase in the permeability of 2% cationic polymer-treated GCL when compared with that of 1% cationic polymer-treated GCL might be attributed to the aggregation of more bentonite particles that resulted in a nonhomogeneous pore size distribution (Haase and Schanz, 2015). During the triaxial permeability tests, 2–2.5 and 1–1.5 orders of magnitude increases in permittivity were measured for the GCLs permeated with 0.5 M and 0.1 M MgCl2 solution respectively. Then, the permittivity values became constant. The increase in permittivity might be attributed to the interaction of divalent Mg2+ cations with monovalent Na+ cations in the bentonite that caused a decrease in the
treated GCLs were measured lower than those of the cationic polymertreated GCLs when the GCLs were permeated with 0.5 M MgCl2 solution. 2% polymer treatment did not cause a significant decrease in permeability when compared with 1% polymer treatment. At 60 °C, 1% anionic polymer treatment resulted in a decrease of more than two orders of magnitude in permeability. Because of the addition of 1% anionic polymer, the permeability coefficient decreased from 7.2 × 10−7 to 6.8 × 10−9 m/s. However, the permeability coefficient of 1% cationic polymer-treated GCL was measured as 5.9 × 10−8 m/s, almost one order of magnitude higher than that of 1% anionic polymertreated GCL. Moreover, temperature increase from 20 °C to 60 °C resulted in an increase of up 1.5 orders of magnitude in permeability.
5. Discussion Both flocculation of the bentonite fabrics due to anionic polymer treatment and aggregation of the bentonite fabrics due to cationic polymer treatment result in differentiated pore size distribution that might change the permeability of the bentonite-polymer mixture. (Haase and Schanz, 2016). Flow of the permeant fluid through the GCL is controlled by the flow path with the maximum pore size. By posessing the maximum pore radii, this flow path has the lowest resistance against flow. Due to polymer addition, the size of the maximum pore radii decreases which results in a decrease in permeability (Haase and 1.E-06
Permittivity (1/s)
1.E-07
1.E-08
GCL-0.1MC GCL-1AP-0.1MC GCL-2AP-0.1MC GCL-1CP-0.1MC GCL-2CP-0.1MC
1.E-09
1.E-10
Elapsed Time (hour) Fig. 7. Permittivity change of the GCLs permeated with 0.1 M MgCl2 solution at 20 °C with respect to time. 369
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1.E-06
Permittivity (1/s)
1.E-07
1.E-08
GCL-0.1MC GCL-1AP-0.1MC GCL-2AP-0.1MC GCL-1CP-0.1MC GCL-2CP-0.1MC
1.E-09
1.E-10
0
Elapsed Time (hour) Fig. 8. Permittivity change of the GCLs permeated with 0.1 M MgCl2 solution at 40 °C with respect to time.
counter ion. Debye length is defined as the thickness of the diffuse double layer (Mitchell, 1993), κ is taken as 80.20, 73.17 and 66.73 for 20 °C, 40 °C and 60 °C respectively (Archer and Wang, 1990). By using Eq. (3), the increase in the salinity of MgCl2 solution caused a decrease in the thickness of the diffuse double layer. This could be a likely reason for the higher permeability of the GCLs permeated with MgCl2 solution that had a higher concentration. Moreover, increase in the temperature of the MgCl2 solution also resulted in a decrease in the thickness of the diffuse double layer. This could be a probable effect of temperature on the interlayer swelling of the bentonite contributing to the loss of hydraulic capability with temperature. In order to investigate the effects of viscosity and temperature on the permeability of the GCLs, Eq. (4) was derived by Rowe (1998) as follows:
thickness of the diffuse double layer. The valence of the counterions and the salinity of the permeant fluid caused a change in permeability. When a GCL is permeated with a saline solution, multivalent cations in the solution can easily replace Na+ ions in the bentonite component of the GCL. (McBride, 1994; McRory and Ashmawy, 2005; Haase and Schanz, 2016). Due to this ion exchange, the thickness of the diffuse double layer decreases and flocculation of the bentonite particles occurs. As a result, large pore channels that enables the flow of the permeant fluid are created among the flocculated bentonite particles (Mitchell, 1993; Gleason et al., 1997). The thickness of the diffuse double layer decreases with the salinity of the solution. As the salinity increases, the openings among the bentonite particles can be easily enlarged (Liu et al., 2014). Consequently, the swelling capacity of the bentonite decreases and the permeability of the GCL increases due to a decrease in the thickness of the diffuse double layer (McBride, 1994; Jo et al., 2001). The thickness of the diffuse double layer is expressed in Eq. (3) as follows (Mitchell, 1993; Gleason et al., 1997):
θ=
ρ1 μ 2 kT 1 = ⋅ kT 2 ρ2 μ1
(4)
kT 1 kT 2
ε 0⋅κ⋅R⋅T 2F 2⋅c⋅υ2
Where is the ratio of the permeability coefficient (kt1) of the GCL permeated with 0.5 M or 0.1 M MgCl2 solution at temperature t1 to the permeability coefficient (kt2) of the same GCL permeated with the same fluid at temperature t2, ρ1 and ρ2 (g/cm3) are the densities and μ1 and μ2 (mPa.s) are the dynamic viscosities of the permeant fluid at temperatures t1 and t2 respectively. Density and dynamic viscosity of 0.5 M and 0.1 M MgCl2 solutions at 20 °C, 40 °C and 60 °C are listed in Table 2. As can be seen in Table 2, the increase in the temperature of the
(3)
Where θ is the Debye length (m), ε0 is the vacuum permittivity (8.86 × 10−12 F/m), κ is the dielectric constant of deionized water, R is the universal gas constant (8.134 J/mol/K), T is the absolute temperature (K), F is Faraday's constant (9.65 × 104 C/mol), c is the concentration of the permeant fluid (mol/m3), and ν is the valence of the 1.E-05
Permittivity (1/s)
1.E-06
1.E-07
1.E-08 GCL-0.1MC GCL-1AP-0.1MC GCL-2AP-0.1MC GCL-1CP-0.1MC GCL-2CP-0.1MC
1.E-09
1.E-10
Elapsed Time (hour) Fig. 9. Permittivity change of the GCLs permeated with 0.1 M MgCl2 solution at 60 °C with respect to time. 370
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Final Equilibrium Permeability Coefficient (m/s)
1.E-07
1.E-08 GCL GCL-1AP GCL-2AP GCL-1CP GCL-2CP
1.E-09
1.E-10
1.E-11 0
0.1
0.2
0.3
0.4
0.5
0.6
Salinity of MgCl2 Solution (mol/L) Fig. 10. Final equilibrium permeability coefficient change with respect to salinity of MgCl2 solution at 20 °C.
permeant fluid resulted in a decrease in viscosity. As the density and dynamic viscosity values that are listed in Table 2 are used in Eq. (4), the permeability coefficient of the GCL permeated with either 0.5 M or 0.1 M MgCl2 solution at a higher temperature (kt2) is found to be higher than that of the same GCL permeated with the same fluid at a lower temperature (kt1). Hence, the increase in the permeability of the GCLs due to an increase from 20 °C to 60 °C in the temperature of either 0.5 M or 0.1 M MgCl2 solution is attributed to the decrease in the viscosity of the permeant fluid. Viscosity is defined as the shear resistance of the fluid against flow (Andrade, 1930). When the temperature of either 0.5 M or 0.1 M MgCl2 solution increased from 20 °C to 40 °C or 60 °C, the interaction between the bentonite particles and water became weaker which resulted in a decrease in the shear resistance of the fluid and easier flow through the bentonite particles (Wang et al., 2016). On the other hand, the viscosity increases due to an increase in polymer concentration. This can be attributed to the formation of more hydrogel structures as the polymer chains in a solution are increased. Due to the addition of more polymer to the bentonite component of the GCL that was in contact with a permeant fluid, absorption of water molecules onto the polymer chains increases. As a result, this strong interaction between the polymer and water caused an increase in the shear resistance of the permeant fluid and the increase in shear resistance could be expressed as an increase in the viscosity of the permeant fluid due to polymer treatment (Geng et al., 2016). Interaction between a cationic polymer with Cl− ions and bentonite causes an increase in the repulsive forces between the polymer chains and the
bentonite particles. This interaction might prevent effective hydrogel formation and results in lower viscosity. However, cationic polymers that consist of SO42− ions generally result in higher viscosity than cationic polymers having Cl− ions when mixed with aqueous solutions. (Geng et al., 2016). In this study, the cationic polymer contained SO42− ions but not Cl− ions. For this reason, the decrease in the permeability of the GCLs that were treated with the cationic polymer can be attributed to the presence of SO42− ions in the cationic polymer. 6. Conclusions The effects of both an anionic and a cationic polymer addition on the hydraulic capability of a GCL were investigated by performing triaxial permeability tests on the GCL permeated with 0.5 M and 0.1 M MgCl2 solutions at 20 °C, 40 °C and 60 °C respectively. These solutions represented aqueous solutions with high salinity contents that could be collected in a waste containment facility. The following conclusions are reached as a result of this study: From the beginning till the end of the tests, permittivity increased 2–2.5 orders of magnitude for the GCLs permeated with 0.5 M MgCl2 solution and 1–1.5 orders of magnitude for the GCLs permeated with 0.1 M MgCl2 solution. As the temperature of the permeant fluid increased from 20 °C to 60 °C, final equilibrium permeability coefficient of the GCLs increased 1–1.5 orders of magnitude. Both an increase in the temperature and the salinity of MgCl2 solution resulted in a decrease in the thickness of the diffuse double layer
Final Equilibrium Permeability Coefficient (m/s)
1.E-06 1.E-07 GCL GCL-1AP GCL-2AP GCL-1CP GCL-2CP
1.E-08 1.E-09 1.E-10 1.E-11 0
0.1
0.2
0.3
0.4
0.5
0.6
Salinity of MgCl2 Solution (mol/L) Fig. 11. Final equilibrium permeability coefficient change with respect to salinity of MgCl2 solution at 40 °C. 371
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Final Equilibrium Permeability Coefficient (m/s)
1.E-05 1.E-06
1.E-07
GCL GCL-1AP
1.E-08
GCL-2AP GCL-1CP
1.E-09
GCL-2CP
1.E-10 1.E-11 0
0.1
0.2
0.3
0.4
0.5
0.6
Salinity of MgCl2 Solution (mol/L) Fig. 12. Final equilibrium permeability coefficient change with respect to salinity of MgCl2 solution at 60 °C.
The decrease in the permeability of the GCLs treated with the polymers can be attributed to the hydrogel formation due to the absorption of water molecules onto the polymer chains. This strong interaction resulted in an increase in the shear strength of the permeant fluid. Polymer treatment caused an increase in the viscosity of MgCl2 solutions (Geng et al., 2016). In conclusion, either the anionic or the cationic polymer-treated GCL can effectively be used as a barrier material in waste containment facilities. According to the results, 1% anionic and 1% cationic polymer-treated GCLs can be considered to have the highest hydraulic performance in 0.5 M and 0.1 M MgCl2 solutions at 60 °C respectively. Although the triaxial permeability tests were performed in a long period of time in which the flow became stable, the permeability of the GCLs might be measured in much longer permeation periods in order to investigate the long-term effects of polymers on the hydraulic capability of the GCLs as a further study. Moreover, additional triaxial permeability tests might be performed by using acidic solutions to simulate leachates.
Table 2 Density (Connaughton et al., 1986; Kabiraz et al., 2011) and dynamic viscosity (Phang and Stokes, 1980; Afsal et al., 1989; Kabiraz et al., 2011) of the permeant fluids at 20 °C, 40 °C and 60 °C. Permeant fluid
0.5 M MgCl2 Solution 0.1 M MgCl2 Solution
20 °C
40 °C
60 °C
ρ (g/ cm3)
μ (mPa.s)
ρ (g/ cm3)
μ (mPa.s)
ρ (g/ cm3)
μ (mPa.s)
1.04
1.21
1.03
0.79
1.02
0.67
1.00
1.04
0.99
0.68
0.98
0.52
of the bentonite. The deterioration of the hydraulic capability of the GCL due to an increase in either temperature or salinity can be attributed to the decrease in the thickness of the diffuse double layer. In order to prevent this deterioration, an anionic and a cationic polymer were added to the GCL respectively. Both the anionic and the cationic polymer addition resulted in a decrease in permeability. The hydraulic performance of the anionic polymer was better than that of the cationic polymer when the GCLs were permeated with 0.5 M MgCl2 solution. 1% anionic polymer addition caused a decrease of approximately two orders of magnitude in permeability regardless of the temperature of the permeant fluid. Final equilibrium permeability coefficient decreased from 7.2 × 10−7 to 6.8 × 10−9 m/s at 60 °C. However, 2% anionic polymer treatment resulted in an additional decrease of only 0.15–0.25 order of magnitude in permeability. The hydraulic performance of 1% cationic polymer-treated GCL was better than that of the anionic polymer-treated GCLs in 0.1 M MgCl2 solution at 60 °C. 1% cationic polymer treatment caused a decrease of two orders of magnitude in permeability at 60 °C. Final equilibrium permeability coefficient decreased from 2.7 × 10−8 m/s to 2.6 × 10−10 m/s. However, 2% cationic polymer treatment resulted in an increase of approximately 0.5 order of magnitude in permeability when compared with that of 1% cationic polymer-treated GCL in 0.1 M MgCl2 solution. The change in permeability due to polymer treatment can be attributed to the pore size distribution of the bentonite-polymer mixtures. Flow of 0.5 M MgCl2 solution through the GCL was controlled by the flow path with the maximum pore size. Polymer addition caused the pores to be partially clogged and the size of the pores to be reduced (Li and Zhang, 2009; Haase and Schanz, 2016). The temperature increase resulted in a decrease in the viscosity of the permeant fluid and this decrease caused an increase in the permeability of the GCL.
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Research paper
Hydraulic capability of polymer-treated GCLs in saline solutions at elevated temperatures Hakki O. Ozhan
T
⁎
Altinbas University, Department of Civil Engineering, Mahmutbey Dilmenler Cad., No. 26, Bagcilar, Istanbul, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Anionic polymer Cationic polymer Geosynthetic clay liner Permeability Triaxial permeability test Viscosity
Geosynthetic clay liners (GCLs) can be used effectively as barriers in waste containment facilities. However, interaction of the GCL with a saline solution might cause a decrease in the hydraulic performance of the GCL. In order to simulate the field conditions, triaxial permeability tests were performed on a GCL permeated with 0.5 M and 0.1 M MgCl2 solutions at 20 °C, 40 °C and 60 °C. The temperature increase resulted in an increase in the permeability of the GCL and this increase is attributed to a decrease in the viscosity of the permeant fluid. In order to improve the hydraulic capability of the GCL, an anionic and a cationic polymer with a polymer content of 1% and 2% by mass in the bentonite-polymer mixture was added to the GCL. According to the results, polymer treatment caused up to two orders of magnitude decrease in permeability and improved the hydraulic capability of the GCL. Generally, anionic polymer treatment resulted in slightly lower permeabilities than cationic polymer treatment. Furthermore, 2% anionic polymer treatment caused almost no decrease in permittivity when compared with 1% polymer treatment. However, the permeability of 1% cationic polymer-treated GCL was the lowest among all the other anionic and cationic polymer-treated GCLs that were permeated with 0.1 MgCl2 solution at 60 °C.
1. Introduction Geosynthetic clay liner (GCL) is a barrier material that is composed of a bentonite layer sandwiched between two geotextile layers. GCL is preferred as a lining over a soil deposit with its very low permeability to water, high self-healing capacity and low thickness and cost (Bouazza, 2002). Generally, GCLs are used as hydraulic barriers in various waste containment facilities such as mine tailings dams, hazardous waste landfills and heap leach pads (Rowe, 2014; Touze-Foltz et al., 2016). In these geoenvironmental applications, GCLs are usually in contact with either acidic or saline solutions. These liquids can be considered as leachates that might deteriorate the hydraulic capability of the GCLs by infiltrating through the bentonite component of the GCLs (Rowe, 2005; Abdelaal et al., 2014; Tian et al., 2016). Due to the cation exchange between the bentonite in the GCL and the leachate that the GCL was permeated with, permeability of the GCL might increase drastically. In order to prevent the interaction between the GCL and the leachate, a geomembrane layer is routinely laminated to the GCL. However, the geomembrane might be damaged when angular particles of a soil deposit are in contact with it or a high leachate level applies extra hydraulic load on it (Ozhan and Guler, 2013). After a possible crack or wrinkle is formed on the surface of the geomembrane, the leachate
⁎
Corresponding author. E-mail address: [email protected].
https://doi.org/10.1016/j.clay.2018.05.007 Received 9 February 2018; Received in revised form 7 May 2018; Accepted 9 May 2018 0169-1317/ © 2018 Elsevier B.V. All rights reserved.
might infiltrate through the damaged geomembrane into the GCL and the hydraulic performance of the GCL might decrease again (Azad et al., 2012). Furthermore, organic materials that are collected in a waste containment facility decompose biologically which results in an increase in the temperature of the wastes (Rowe, 2005). Barrier materials used in mining and industrial applications might remain exposed to high temperatures ranging from 60 °C to 80 °C when the GCL has been in contact with the wastes for more than five years (Thiel and Smith, 2004; Bouazza et al., 2014). As a result, an increase in the temperature of the leachate probably deteriorates the hydraulic capability and the durability of the lining material (Rowe, 2005; Bouazza et al., 2014). Ishimori and Katsumi (2012) performed hydraulic conductivity tests on a GCL that was composed of powdered sodium bentonite sandwiched between a woven and a nonwoven geotextile. The GCL was permeated with 0.4 M NaCl solution at 20 °C and 60 °C. Permeation continued 98 days and hydraulic conductivity was measured as 5.8 × 10−11 m/s at 20 °C whereas 5.1 × 10−10 m/s at 60 °C. Test results indicated that temperature increase resulted in an increase of almost one order of magnitude in permeability. Saline solutions having divalent cations caused a drastical increase in the permeability of GCLs. Jo et al. (2001) performed hydraulic
Applied Clay Science 161 (2018) 364–373
H.O. Ozhan
prevented the formation of effective bridging between bentonite particles and polymer chains (Lage et al., 2009). The main objective of this study was to investigate the effects of adding an anionic and a cationic polymer to a GCL in order to evaluate the hydraulic performance of the GCL usage in waste containment facilities. To simulate an aqueous solution with a high salinity content that might be collected in a waste containment facility, 0.5 M and 0.1 M MgCl2 solutions were used as the permeant fluids. The hydraulic performance of bentonite permeated with CaCl2 solutions has been investigated many times before (Gleason et al., 1997; Shan and Lai, 2002; Lee and Shackelford, 2005; Scalia and Benson, 2011; Scalia et al., 2014; Geng et al., 2016; Shen et al., 2016). MgCl2 solution is another saline solution with divalent cations but has not been tested with polymertreated GCLs before. For this reason, the permeant fluid was chosen as MgCl2 solution. Triaxial permeability tests were performed on the polymer-treated GCLs and the GCL without polymer treatment. The concentration of the polymer added to the GCLs was chosen as 1% and 2% by mass. Due to the biodegradation of organic substances in a waste containment facility, the temperature of the leachates might increase up to 60 °C (Rowe and Islam, 2009). For this reason, the effect of the solution temperature on the hydraulic capability of the GCLs was investigated by setting the temperature of the permeant fluid to 20 °C, 40 °C and 60 °C.
conductivity tests on GCLs permeated with 0.1 M CaCl2 solution. Permeability increased three orders of magnitude when compared with that of the same GCL permeated with deionized water. Chun-Ming et al. (2013) indicated that an increase in the molarity of an aqueous solution caused both an increase in the permeability and a decrease in the swelling capability of the bentonite. Hydraulic performance of the bentonite also decreased when the cations of the chemical solution that the bentonite was in contact with were divalent or trivalent instead of monovalent (McBride, 1994; Jo et al., 2001). In order to improve the hydraulic capability of the bentonite that was in contact with acidic or saline solutions, polymers can be added to the bentonite. Test results indicated that polymer treatment generally lowered the permeability of the bentonites permeated with either saline solutions or deionized water even for long periods of time (Razakamanantsoa et al., 2012; Scalia et al., 2014; Shen et al., 2016; Tian et al., 2016). However, Ashmawy et al. (2002) experienced GCL failure and no improvement in permeability as a consequence of polymer addition. Although cationic polymer treatment lowered the permeability (Razakamanantsoa et al., 2012; Tian et al., 2016), anionic polymer-treated bentonite had usually lower permeability than cationic polymer-treated bentonite (Liu et al., 2012a; Razakamanantsoa et al., 2012; Theng, 2012; Haase and Schanz, 2016). However, swelling capacity of cationic polymer-treated bentonite was higher than that of anionic polymer-treated bentonite in some studies (Liu et al., 2012b; Razakamanantsoa et al., 2012). This could be considered as an advantage of cationic polymer over anionic polymer for the applications where swelling is crucial. Theng (2012) stated that the interaction between bentonite particles and polymers is governed by electrostatic forces. Negatively charged (anionic) polymers are repelled from the net negatively charged bentonite but are attracted to the edge particles of the bentonite that are positively charged (Theng, 1982; Lu et al., 2002). Hydrogen bonding which is a weak attachment governs the bridges between bentonite particles and anionic polymers (Lu et al., 2002). Aggregated fabric is the structure formed by the cohesion of colloidal particles whereas flocculated fabric is the structure formed by colloidally unstable aggregates. (Haase and Schanz, 2015). When the interlayer cations are multivalent, flocculated bentonite fabrics are formed due to anionic polymer treatment. Flocculated fabrics lead to an increase in the homogeneity of the pore size distribution by decreasing the size of the maximum pore radii. As a result, a net-like fabric with small pores is formed due to a homogeneous pore size distribution (Mesri and Olson, 1971; Haase and Schanz, 2016). On the other hand, Coulomb bonding which is a much stronger attachment than hydrogen bonding governs the bridges between bentonite particles and cationic polymers. By the attraction between positively charged (cationic) polymers and the net negatively charged bentonite particles, the bentonite particles are stacked to form aggregates. Due to the fact that the pore size distribution is not homogeneous in aggregated fabrics and due to the existence of flow paths, this nonhomogeneous pore size distribution causes an increase in permeability at the same global void ratio (Haase and Schanz, 2016). Vryzas et al. (2017) performed viscosity tests on 7% sodium bentonite by mass that was mixed with deionized water. They indicated that temperature increase from 25 °C to 80 °C caused a decrease in the viscosity of the fluid. Ratkievicius et al. (2016) performed viscosity tests on bentonite-cationic polymer mixtures permeated with an aqueous solution. Cationic polymer addition resulted in an increase in the viscosity of the permeant fluid. Geng et al. (2016) conducted viscosity tests on sodium bentonite-anionic polymer mixtures permeated with deionized water, CaCl2 and NaCl solutions. Test results indicated that viscosity increased with an increase in the anionic polymer content. Moreover, viscosity was lower in CaCl2 (divalent) solutions than in NaCl (monovalent) solutions. Low viscosity resulted in a contracted polymer conformation whereas high viscosity caused an extended polymer conformation to be formed. As a result, contracted polymers
2. Materials The GCL tested consisted of a granular sodium bentonite placed between a woven and a nonwoven geotextile without reinforcement. The difference in permittivity of a needle-punched and an unreinforced GCL was found to be only of approximately 0.04 order of magnitude (Ozhan and Guler, 2013). By considering that internal confinement due to needle-punching did not cause a decrease in permittivity and it was not easy to add polymer to the bentonite component of a reinforced GCL due to the fibers that held the carrier and the cover geotextiles together, an unreinforced GCL was used in the triaxial permeability tests. The nonwoven geotextile of the GCL was a polypropylene, needlepunched geotextile with a mass/unit area of 250 g/m2 (ASTM D5261, 2010) and an apparent opening size of 0.2 mm (ASTM D4751, 2016). The woven geotextile of the GCL was a polypropylene, slit-film geotextile that had a mass/unit area and an apparent opening size of 100 g/ m2 (ASTM D5261, 2010) and 0.4 mm (ASTM D4751, 2016) respectively. The nonwoven geotextile was the carrier and the woven geotextile was the cover of the GCL. Specific gravity (ASTM D854, 2014), liquid and plastic limits (ASTM D4318, 2010), mass/unit area (ASTM D5993, 2014), cation exchange capacity (CEC), mineralogical and chemical compositions (Karakaya Bentonit, 2014) of the bentonite component of the GCL are listed in Table 1. The polymers that were mixed with the bentonite component of the GCL were air-dry, powdered anionic and cationic polymers. 2% of the polymer particles had a grain size of greater than 2 mm and 6% of the particles had a grain size of smaller than 0.15 mm (SNF, 2016). The anionic polymer was composed of negatively charged copolymers of acrylamide (C3H5NO) and contained almost nine times more soluble anions than cations in terms of mass. In contrast, the cationic polymer consisted of positively charged copolymers of acrylamide (C3H5NO) and was composed of approximately 11 times more soluble cations than anions in terms of mass. To specify the main soluble anions and cations in the polymers, a chemical analysis by Inductively Coupled Plasma–Atomic Emission Spectrometry (Van de Wiel, 2003) was performed. 1 g of the polymer was dissolved into 1 L of deionized water and the concentrations of the main ions on the polymer were identified. According to the results, the anionic polymer was composed of 0.026 mg/ L Cl−, 0.0083 mg/L NO3−, 0.0058 mg/L SO42-, 0.0032 Na+ and 0.0014 mg/L K+ whereas the main ions of the cationic polymer were 365
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Table 1 Material characterization of the bentonite. Engineering properties
Value
Specific gravity (−) Liquid limit (%) Plastic limit (%) Mass/unit area (g/m2) CEC (meq/100 g) Mineralogical composition Smectite (%) Quartz (%) Feldspar (%) Calcite (%) Biotite (%) Chemical composition SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) Na2O (%) MgO (%) K2O (%)
2.69 640 28 4800 77 Value 91 4 2 2 1 Value 61.28 17.79 3.01 4.54 2.70 2.10 1.24
0.032 mg/L Na+, 0.0095 mg/L Ca2+, 0.0062 mg/L K+ and 0.0044 SO42−. Triaxial permeability tests were conducted on the GCLs that were permeated with 0.5 M and 0.1 M MgCl2 solutions. These chemical solutions with divalent cations and high concentrations simulated saline solutions that could be collected in waste containment facilities.
Fig. 2. Triaxial permeability test set-up.
flexible-wall triaxial permeability cells. From top to bottom, triaxial permeability test set-up consisted of stainless steel top cap, porous stone, filter paper, GCL, filter paper, porous stone and stainless steel base cap as shown in Fig. 2. Then, the top and bottom caps were lubricated with high-vacuum silicone grease in order to attach the latex membrane to the GCL and prevent side leakage. Afterwards, latex membrane was wrapped around the test set-up and o-rings were attached to the membrane (ASTM D6766, 2012). The triaxial permeability cell was placed in a stainless steel tank with heater. The tank was filled with tap water and both the triaxial permeability cell and the plastic tubes that provided the fluid permeation through the GCL were sunk into the water as shown in Fig. 3 in order to set the temperature of the permeant fluid to the desired values. The temperature of the permeant fluid was measured from the outlet of the plastic tube by using a digital termometer. For setting the temperature of the permeant fluid to 20 °C, 40 °C and 60 °C, the heater of the tank was programmed to 21.1 °C, 41.3 °C ve 61.4 °C respectively. These slight differences between the programmed and the measured temperatures were due to the interaction between the fluid in the tank and the fluid in the plastic tubes. In conclusion, the heat transfer between the water in the tank and the fluid in the the plastic tubes was completed and the temperature of the permeant fluid was set to 20 °C, 40 °C and 60 °C. First, tap water was transferred from the water supply tank to the pressure supply unit and then, from the pressure supply unit to the triaxial permeability cell through a plastic tube as shown in Fig. 3. In order to saturate the GCL with water and then, consolidate it, cell pressure and backpressure were set to 550 and 515 kPa respectively.
3. Methodology and testing program First, the geotextile components of the GCL with a diameter of 100 mm were cut from geotextile rolls (ASTM D6766, 2012). Then, the granular bentonite was dried in an oven for 24 h at 105 °C and placed on the nonwoven carrier geotextile as shown in Fig. 1a. Afterwards, the bentonite was wetted with the permeant fluid homogenously in order to provide bonding to the geotextiles. Finally, the woven cover geotextile was attached to the wetted bentonite without any reinforcement effort such as needle-punching or stich-bonding as shown in Fig. 1b (Ozhan and Guler, 2013). For the sample preparation of the polymer-treated GCLs, dry polymers with a content of 1% and 2% by mass were added to the bentonite. The bentonite-polymer mixtures were mixed homogenously by using a spatula and then, the mixtures were poured into polyethylene bottles to avoid hydration. The bottles were closed and shaken by hand for five minutes in order to obtain homogeneous mixtures. Finally, the mixtures remained in the bottles for 24 h (Razakamanantsoa et al., 2012). Then, the polymer-treated GCLs were assembled by following the same procedure that was used for assembling the GCL without polymer treatment. After the sample preparation, constant-head triaxial permeability tests were performed on the GCLs. First, the GCLs were placed in
Fig. 1. (a): Sample preparation of GCL without reinforcement; (b): GCL specimen with the woven cover geotextile. 366
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Fig. 3. Diagram of the triaxial permeability test configuration.
MgCl2 solution whereas this period was 32–34 days for the GCLs permeated with 0.1 M MgCl2 solution. After measuring almost constant permittivity values, the triaxial permeability tests continued for a while and then, the tests were terminated. The GCL specimen in each test was designated by considering the polymer content and type (2AP for 2% anionic polymer, 0.5CP for 0.5% cationic polymer, etc.) and the permeant fluid concentration (0.5MC for 0.5 M MgCl2 solution and 0.1MC for 0.1 M MgCl2 solution). For example, 1% anionic polymer-treated GCL that was permeated with 0.5 MgCl2 solution and 2% cationic polymer-treated GCL that was permeated with 0.1 MgCl2 solution were designated as GCL-1AP-0.5MC and GCL-2CP-0.1MC respectively. GCL without polymer treatment that was permeated with 0.5 M MgCl2 solution was designated as GCL0.5MC. The repeatibility of the flow rates passing through the GCL was checked by testing duplicate GCL specimens for GCL-0.5MC and GCL0.1MC. According to the results, only a difference of up to 0.02 order of magnitude was obtained between the permittivities of the duplicate GCL specimens. For this reason, the rest of the triaxial permeability tests were performed on one GCL specimen for each condition. Furthermore, the tests under different temperatures were not performed consecutively on the same GCL specimen. After the termination of each test at the selected temperature, the permeability coefficient of the GCL specimen was measured and the GCL specimen was not used again in the following tests. In triaxial permeability tests, permittivity was measured instead of permeability because it was not possible to measure the thickness of the GCL in the triaxial permeability cell and the GCL could have various thickness values at the same time due to the fact that the GCL was a very thin and compressible material (Koerner, 1998). However, permeability of the GCLs could be measured at the end of each test. After the termination of the tests, the GCLs were taken from the triaxial permeability cell and then, the thickness of the GCLs was measured
After the termination of saturation and consolidation, the water supply tank was filled with the permeant fluid, then, the permeant fluid was transferred to the influent pressure reservoir and then, to the GCL. Afterwards, the GCL was permeated from bottom to top by increasing the influent pressure at the bottom of the GCL to 518 kPa and keeping the effluent pressure at the top of the GCL at 515 kPa (ASTM D6766, 2012). As a result, 3 kPa difference between the influent and effluent pressure resulted in an upward flow through the GCL. A hydraulic pressure of 3 kPa represented a hydraulic head of almost 30 cm which could be considered as the maximum leachate level collected in a waste containment facility (Weber and Zornberg, 2005). The permeant fluid was collected in the effluent pressure reservoir. The plastic tubes that provided permeant fluid transfer from the influent pressure reservoir to the triaxial permeability cell and from the triaxial permeability cell to the effluent pressure reservoir are shown in Fig. 3. During the triaxial permeability tests, permittivity (Ψ) of the GCL was measured and a correlation between permittivity and elapsed time was established. Permittivity is expressed in Eq. (1) as follows (Shan and Chen, 2003):
Ψ=
ΔQ A⋅Δh⋅Δt
(1)
Where Ψ (1/s) is the permittivity of the GCL, ΔQ (cm ) is the average of inflow and outflow of the permeant fluid through the GCL for a specific time interval, A (cm2) is the cross-sectional area of the GCL, Δh (cm) is the hydraulic head difference acting on the GCL and Δt (s) is the time interval where ΔQ is measured. Several permittivity values were measured until the termination of each triaxial permeability test. These tests were completed when the permeation became steady. However, it took a long time for the permittivity values to be stable. Almost constant permittivity values were measured 13–15 days after the beginning of the permeation with 0.5 M 3
367
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1.E-04
Permittivity (1/s)
1.E-05
1.E-06
1.E-07
GCL-0.5MC GCL-1AP-0.5MC GCL-2AP-0.5MC GCL-1CP-0.5MC GCL-2CP-0.5MC
1.E-08
1.E-09 0
50
100
150
200
250
300
350
400
450
Elapsed Time (hour) Fig. 4. Permittivity change of the GCLs permeated with 0.5 M MgCl2 solution at 20 °C with respect to time.
treated with 1% cationic polymer was approximately 0.6 order of magnitude lower than that of the GCLs treated with 2% cationic polymer. By causing a decrease of almost two orders of magnitude in permittivity, 1% cationic polymer-treated GCL had even lower permittivity than that of the anionic polymer-treated GCLs at 60 °C. For all the tested GCLs, permittivity increased similarly with an increase in temperature regardless of polymer treatment. Temperature increase from 20 °C to 60 °C caused an increase of 1–1.5 orders of magnitude in permittivity for all the GCLs permeated with either 0.5 M or 0.1 M MgCl2 solution. Changes in final equilibrium permeability coefficient with respect to salinity of MgCl2 solution for different polymer additions are shown in Figs. 10, 11 and 12 at 20 °C, 40 °C and 60 °C respectively. As the salinity was increased from 0.1 M to 0.5 M, the permeability coefficient also increased. The permeability coefficient of all the polymer-treated GCLs except 2% cationic polymer-treated GCL was almost 1.5 orders of magnitude lower than that of the GCL without polymer treatment when the GCLs were permeated with 0.1 M MgCl2 solution at 20 °C and 40 °C. The permeability coefficient of 2% cationic polymer-treated GCL was less than one order of magnitude lower than that of the GCL without polymer treatment. However, 1% cationic polymer-treated GCL had the lowest permeability at 60 °C. 1% cationic polymer treatment caused a decrease of two orders of magnitude in permeability, from 2.7 × 10−8 to 2.6 × 10−10 m/s. Permeability coefficients of the anionic polymer-
(ASTM D5199, 2012). Permeability coefficient of the GCLs is expressed in Eq. 2 as follows (Koerner, 1998): (2)
k = Ψ. L
Where k (m/s) is the final equilibrium permeability coefficient and L is the final thickness (m) of the GCL. 4. Results In order to evaluate the effects of polymer addition to the GCL, changes in permittivity with respect to elapsed time are shown in Figs. 4, 5 and 6 for the GCLs permeated with 0.5 M MgCl2 solution at 20 °C, 40 °C and 60 °C respectively. As shown in Figs. 4, 5 and 6, the permittivity of the GCLs treated with 1% anionic polymer was almost two orders of magnitude lower than that of the GCL without polymer treatment whereas the cationic polymer-treated GCLs had permittivity values 0.6–0.8 order of magnitude higher than those of the anionic polymer-treated GCLs. However, 2% polymer treatment resulted in almost no decrease in permittivity. Changes in permittivity with respect to elapsed time are shown in Figs. 7, 8 and 9 for the GCLs permeated with 0.1 M MgCl2 solution at 20 °C, 40 °C and 60 °C respectively. Both anionic and cationic polymer treatment resulted in a decrease in permittivity. However, the effect of polymer content on permittivity was not the same as that of the GCLs permeated with 0.5 M MgCl2 solution. The permittivity of the GCLs 1.E-04
Permittivity (1/s)
1.E-05
1.E-06
1.E-07
GCL-0.5MC GCL-1AP-0.5MC GCL-2AP-0.5MC GCL-1CP-0.5MC GCL-2CP-0.5MC
1.E-08
1.E-09 0
50
100
150
200
250
300
350
400
450
Elapsed Time (hour) Fig. 5. Permittivity change of the GCLs permeated with 0.5 M MgCl2 solution at 40 °C with respect to time. 368
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1.E-03
Permittivity (1/s)
1.E-04
1.E-05
1.E-06
1.E-07 GCL-0.5MC GCL-1AP-0.5MC GCL-2AP-0.5MC GCL-1CP-0.5MC GCL-2CP-0.5MC
1.E-08
1.E-09 0
50
100
150
200
250
300
350
400
450
Elapsed Time (hour) Fig. 6. Permittivity change of the GCLs permeated with 0.5 M MgCl2 solution at 60 °C with respect to time.
Schanz, 2016). In this study, either anionic or cationic polymer addition improved the hydraulic capability of the GCL by decreasing permeability. This decrease might be attributed to the polymer treatment that caused the pores to become partially blocked and the pore sizes to be reduced. As a result, pore size distribution governed the decrease in permeability due to polymer addition to the bentonite component of the GCL (Li and Zhang, 2009). 1% anionic polymer addition was probably enough to reduce the pore sizes to a minimum level regardless of the salinity of the permeant fluid. 2% anionic polymer treatment resulted in only a very small amount of decrease in permeability; in some cases, no decrease at all particularly for 0.1 M MgCl2 solution. On the other hand, 1% cationic polymer addition probably blocked most of the pores in the bentonitepolymer mixture for the GCL permeated with 0.1 M MgCl2 solution. The increase in the permeability of 2% cationic polymer-treated GCL when compared with that of 1% cationic polymer-treated GCL might be attributed to the aggregation of more bentonite particles that resulted in a nonhomogeneous pore size distribution (Haase and Schanz, 2015). During the triaxial permeability tests, 2–2.5 and 1–1.5 orders of magnitude increases in permittivity were measured for the GCLs permeated with 0.5 M and 0.1 M MgCl2 solution respectively. Then, the permittivity values became constant. The increase in permittivity might be attributed to the interaction of divalent Mg2+ cations with monovalent Na+ cations in the bentonite that caused a decrease in the
treated GCLs were measured lower than those of the cationic polymertreated GCLs when the GCLs were permeated with 0.5 M MgCl2 solution. 2% polymer treatment did not cause a significant decrease in permeability when compared with 1% polymer treatment. At 60 °C, 1% anionic polymer treatment resulted in a decrease of more than two orders of magnitude in permeability. Because of the addition of 1% anionic polymer, the permeability coefficient decreased from 7.2 × 10−7 to 6.8 × 10−9 m/s. However, the permeability coefficient of 1% cationic polymer-treated GCL was measured as 5.9 × 10−8 m/s, almost one order of magnitude higher than that of 1% anionic polymertreated GCL. Moreover, temperature increase from 20 °C to 60 °C resulted in an increase of up 1.5 orders of magnitude in permeability.
5. Discussion Both flocculation of the bentonite fabrics due to anionic polymer treatment and aggregation of the bentonite fabrics due to cationic polymer treatment result in differentiated pore size distribution that might change the permeability of the bentonite-polymer mixture. (Haase and Schanz, 2016). Flow of the permeant fluid through the GCL is controlled by the flow path with the maximum pore size. By posessing the maximum pore radii, this flow path has the lowest resistance against flow. Due to polymer addition, the size of the maximum pore radii decreases which results in a decrease in permeability (Haase and 1.E-06
Permittivity (1/s)
1.E-07
1.E-08
GCL-0.1MC GCL-1AP-0.1MC GCL-2AP-0.1MC GCL-1CP-0.1MC GCL-2CP-0.1MC
1.E-09
1.E-10
Elapsed Time (hour) Fig. 7. Permittivity change of the GCLs permeated with 0.1 M MgCl2 solution at 20 °C with respect to time. 369
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1.E-06
Permittivity (1/s)
1.E-07
1.E-08
GCL-0.1MC GCL-1AP-0.1MC GCL-2AP-0.1MC GCL-1CP-0.1MC GCL-2CP-0.1MC
1.E-09
1.E-10
0
Elapsed Time (hour) Fig. 8. Permittivity change of the GCLs permeated with 0.1 M MgCl2 solution at 40 °C with respect to time.
counter ion. Debye length is defined as the thickness of the diffuse double layer (Mitchell, 1993), κ is taken as 80.20, 73.17 and 66.73 for 20 °C, 40 °C and 60 °C respectively (Archer and Wang, 1990). By using Eq. (3), the increase in the salinity of MgCl2 solution caused a decrease in the thickness of the diffuse double layer. This could be a likely reason for the higher permeability of the GCLs permeated with MgCl2 solution that had a higher concentration. Moreover, increase in the temperature of the MgCl2 solution also resulted in a decrease in the thickness of the diffuse double layer. This could be a probable effect of temperature on the interlayer swelling of the bentonite contributing to the loss of hydraulic capability with temperature. In order to investigate the effects of viscosity and temperature on the permeability of the GCLs, Eq. (4) was derived by Rowe (1998) as follows:
thickness of the diffuse double layer. The valence of the counterions and the salinity of the permeant fluid caused a change in permeability. When a GCL is permeated with a saline solution, multivalent cations in the solution can easily replace Na+ ions in the bentonite component of the GCL. (McBride, 1994; McRory and Ashmawy, 2005; Haase and Schanz, 2016). Due to this ion exchange, the thickness of the diffuse double layer decreases and flocculation of the bentonite particles occurs. As a result, large pore channels that enables the flow of the permeant fluid are created among the flocculated bentonite particles (Mitchell, 1993; Gleason et al., 1997). The thickness of the diffuse double layer decreases with the salinity of the solution. As the salinity increases, the openings among the bentonite particles can be easily enlarged (Liu et al., 2014). Consequently, the swelling capacity of the bentonite decreases and the permeability of the GCL increases due to a decrease in the thickness of the diffuse double layer (McBride, 1994; Jo et al., 2001). The thickness of the diffuse double layer is expressed in Eq. (3) as follows (Mitchell, 1993; Gleason et al., 1997):
θ=
ρ1 μ 2 kT 1 = ⋅ kT 2 ρ2 μ1
(4)
kT 1 kT 2
ε 0⋅κ⋅R⋅T 2F 2⋅c⋅υ2
Where is the ratio of the permeability coefficient (kt1) of the GCL permeated with 0.5 M or 0.1 M MgCl2 solution at temperature t1 to the permeability coefficient (kt2) of the same GCL permeated with the same fluid at temperature t2, ρ1 and ρ2 (g/cm3) are the densities and μ1 and μ2 (mPa.s) are the dynamic viscosities of the permeant fluid at temperatures t1 and t2 respectively. Density and dynamic viscosity of 0.5 M and 0.1 M MgCl2 solutions at 20 °C, 40 °C and 60 °C are listed in Table 2. As can be seen in Table 2, the increase in the temperature of the
(3)
Where θ is the Debye length (m), ε0 is the vacuum permittivity (8.86 × 10−12 F/m), κ is the dielectric constant of deionized water, R is the universal gas constant (8.134 J/mol/K), T is the absolute temperature (K), F is Faraday's constant (9.65 × 104 C/mol), c is the concentration of the permeant fluid (mol/m3), and ν is the valence of the 1.E-05
Permittivity (1/s)
1.E-06
1.E-07
1.E-08 GCL-0.1MC GCL-1AP-0.1MC GCL-2AP-0.1MC GCL-1CP-0.1MC GCL-2CP-0.1MC
1.E-09
1.E-10
Elapsed Time (hour) Fig. 9. Permittivity change of the GCLs permeated with 0.1 M MgCl2 solution at 60 °C with respect to time. 370
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Final Equilibrium Permeability Coefficient (m/s)
1.E-07
1.E-08 GCL GCL-1AP GCL-2AP GCL-1CP GCL-2CP
1.E-09
1.E-10
1.E-11 0
0.1
0.2
0.3
0.4
0.5
0.6
Salinity of MgCl2 Solution (mol/L) Fig. 10. Final equilibrium permeability coefficient change with respect to salinity of MgCl2 solution at 20 °C.
permeant fluid resulted in a decrease in viscosity. As the density and dynamic viscosity values that are listed in Table 2 are used in Eq. (4), the permeability coefficient of the GCL permeated with either 0.5 M or 0.1 M MgCl2 solution at a higher temperature (kt2) is found to be higher than that of the same GCL permeated with the same fluid at a lower temperature (kt1). Hence, the increase in the permeability of the GCLs due to an increase from 20 °C to 60 °C in the temperature of either 0.5 M or 0.1 M MgCl2 solution is attributed to the decrease in the viscosity of the permeant fluid. Viscosity is defined as the shear resistance of the fluid against flow (Andrade, 1930). When the temperature of either 0.5 M or 0.1 M MgCl2 solution increased from 20 °C to 40 °C or 60 °C, the interaction between the bentonite particles and water became weaker which resulted in a decrease in the shear resistance of the fluid and easier flow through the bentonite particles (Wang et al., 2016). On the other hand, the viscosity increases due to an increase in polymer concentration. This can be attributed to the formation of more hydrogel structures as the polymer chains in a solution are increased. Due to the addition of more polymer to the bentonite component of the GCL that was in contact with a permeant fluid, absorption of water molecules onto the polymer chains increases. As a result, this strong interaction between the polymer and water caused an increase in the shear resistance of the permeant fluid and the increase in shear resistance could be expressed as an increase in the viscosity of the permeant fluid due to polymer treatment (Geng et al., 2016). Interaction between a cationic polymer with Cl− ions and bentonite causes an increase in the repulsive forces between the polymer chains and the
bentonite particles. This interaction might prevent effective hydrogel formation and results in lower viscosity. However, cationic polymers that consist of SO42− ions generally result in higher viscosity than cationic polymers having Cl− ions when mixed with aqueous solutions. (Geng et al., 2016). In this study, the cationic polymer contained SO42− ions but not Cl− ions. For this reason, the decrease in the permeability of the GCLs that were treated with the cationic polymer can be attributed to the presence of SO42− ions in the cationic polymer. 6. Conclusions The effects of both an anionic and a cationic polymer addition on the hydraulic capability of a GCL were investigated by performing triaxial permeability tests on the GCL permeated with 0.5 M and 0.1 M MgCl2 solutions at 20 °C, 40 °C and 60 °C respectively. These solutions represented aqueous solutions with high salinity contents that could be collected in a waste containment facility. The following conclusions are reached as a result of this study: From the beginning till the end of the tests, permittivity increased 2–2.5 orders of magnitude for the GCLs permeated with 0.5 M MgCl2 solution and 1–1.5 orders of magnitude for the GCLs permeated with 0.1 M MgCl2 solution. As the temperature of the permeant fluid increased from 20 °C to 60 °C, final equilibrium permeability coefficient of the GCLs increased 1–1.5 orders of magnitude. Both an increase in the temperature and the salinity of MgCl2 solution resulted in a decrease in the thickness of the diffuse double layer
Final Equilibrium Permeability Coefficient (m/s)
1.E-06 1.E-07 GCL GCL-1AP GCL-2AP GCL-1CP GCL-2CP
1.E-08 1.E-09 1.E-10 1.E-11 0
0.1
0.2
0.3
0.4
0.5
0.6
Salinity of MgCl2 Solution (mol/L) Fig. 11. Final equilibrium permeability coefficient change with respect to salinity of MgCl2 solution at 40 °C. 371
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Final Equilibrium Permeability Coefficient (m/s)
1.E-05 1.E-06
1.E-07
GCL GCL-1AP
1.E-08
GCL-2AP GCL-1CP
1.E-09
GCL-2CP
1.E-10 1.E-11 0
0.1
0.2
0.3
0.4
0.5
0.6
Salinity of MgCl2 Solution (mol/L) Fig. 12. Final equilibrium permeability coefficient change with respect to salinity of MgCl2 solution at 60 °C.
The decrease in the permeability of the GCLs treated with the polymers can be attributed to the hydrogel formation due to the absorption of water molecules onto the polymer chains. This strong interaction resulted in an increase in the shear strength of the permeant fluid. Polymer treatment caused an increase in the viscosity of MgCl2 solutions (Geng et al., 2016). In conclusion, either the anionic or the cationic polymer-treated GCL can effectively be used as a barrier material in waste containment facilities. According to the results, 1% anionic and 1% cationic polymer-treated GCLs can be considered to have the highest hydraulic performance in 0.5 M and 0.1 M MgCl2 solutions at 60 °C respectively. Although the triaxial permeability tests were performed in a long period of time in which the flow became stable, the permeability of the GCLs might be measured in much longer permeation periods in order to investigate the long-term effects of polymers on the hydraulic capability of the GCLs as a further study. Moreover, additional triaxial permeability tests might be performed by using acidic solutions to simulate leachates.
Table 2 Density (Connaughton et al., 1986; Kabiraz et al., 2011) and dynamic viscosity (Phang and Stokes, 1980; Afsal et al., 1989; Kabiraz et al., 2011) of the permeant fluids at 20 °C, 40 °C and 60 °C. Permeant fluid
0.5 M MgCl2 Solution 0.1 M MgCl2 Solution
20 °C
40 °C
60 °C
ρ (g/ cm3)
μ (mPa.s)
ρ (g/ cm3)
μ (mPa.s)
ρ (g/ cm3)
μ (mPa.s)
1.04
1.21
1.03
0.79
1.02
0.67
1.00
1.04
0.99
0.68
0.98
0.52
of the bentonite. The deterioration of the hydraulic capability of the GCL due to an increase in either temperature or salinity can be attributed to the decrease in the thickness of the diffuse double layer. In order to prevent this deterioration, an anionic and a cationic polymer were added to the GCL respectively. Both the anionic and the cationic polymer addition resulted in a decrease in permeability. The hydraulic performance of the anionic polymer was better than that of the cationic polymer when the GCLs were permeated with 0.5 M MgCl2 solution. 1% anionic polymer addition caused a decrease of approximately two orders of magnitude in permeability regardless of the temperature of the permeant fluid. Final equilibrium permeability coefficient decreased from 7.2 × 10−7 to 6.8 × 10−9 m/s at 60 °C. However, 2% anionic polymer treatment resulted in an additional decrease of only 0.15–0.25 order of magnitude in permeability. The hydraulic performance of 1% cationic polymer-treated GCL was better than that of the anionic polymer-treated GCLs in 0.1 M MgCl2 solution at 60 °C. 1% cationic polymer treatment caused a decrease of two orders of magnitude in permeability at 60 °C. Final equilibrium permeability coefficient decreased from 2.7 × 10−8 m/s to 2.6 × 10−10 m/s. However, 2% cationic polymer treatment resulted in an increase of approximately 0.5 order of magnitude in permeability when compared with that of 1% cationic polymer-treated GCL in 0.1 M MgCl2 solution. The change in permeability due to polymer treatment can be attributed to the pore size distribution of the bentonite-polymer mixtures. Flow of 0.5 M MgCl2 solution through the GCL was controlled by the flow path with the maximum pore size. Polymer addition caused the pores to be partially clogged and the size of the pores to be reduced (Li and Zhang, 2009; Haase and Schanz, 2016). The temperature increase resulted in a decrease in the viscosity of the permeant fluid and this decrease caused an increase in the permeability of the GCL.
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