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Technical Papers
Jul 26, 2019

Polymer Elution and Hydraulic Conductivity of Bentonite–Polymer Composite Geosynthetic Clay Liners

Authors: Kuo Tian, M.ASCE [email protected], William J. Likos, M.ASCE [email protected], and Craig H. Benson, F.ASCE https://orcid.org/0000-0001-8871-382X [email protected]Author Affiliations
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 145, Issue 10
https://doi.org/10.1061/(ASCE)GT.1943-5606.0002097
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Abstract

Hydraulic conductivity tests were conducted on two commercially available bentonite–polymer composite (BPC) geosynthetic clay liners (GCLs) containing the same dry blend of sodium bentonite (NaB) and polymer sandwiched between two nonwoven geotextiles (GTs) bound by needlepunching: a bentonite–polymer GCL (BP GCL) and bentonite–polymer GCL with silt film (BPS GCL). The GCLs were essentially identical except that the BPS GCL contained a woven slit-film GT to constrain polymer elution. The slit-film GT was located inside the GCL adjacent to the downstream nonwoven GT. The test liquids were deionized (DI) water and 20–500 mM CaCl2 representing dilute to high-concentration permeant solutions. Effluent from the hydraulic conductivity tests was analyzed for total organic carbon (TOC) to quantify polymer elution. Scanning electron microscopy (SEM) was used to capture how the solution chemistry affected the fabric of the BPC and how hydraulic conductivity was related to pore-scale features in the fabric. Both BPC GCLs were approximately one to four orders of magnitude less permeable than a conventional NaB GCL permeated with the same solutions. Polymer elution data and SEM images of polymer structure suggest that polymer retention in the pore space and pore clogging by polymer hydrogel are responsible for the lower hydraulic conductivity of BPC GCLs. The BPS GCL had less polymer elution and hydraulic conductivity three orders of magnitude lower than the BP GCLs when permeated with 50 and 100 mM CaCl2 solutions because the slit-film GT reduced polymer elution from the bentonite. A conceptual framework is proposed relating solution chemistry, pore-scale fabric, and hydraulic conductivity of dry-blended BPC GCLs.

Introduction

Geosynthetic clay liners (GCLs) are widely used as hydraulic barriers in waste containment systems because they can have low hydraulic conductivity (<10−10  m/s) for a range of physical and chemical environments, are readily deployed, and have minimal impact on airspace (e.g., Ruhl and Daniel 1997; Petrov and Rowe 1997; Shackelford et al. 2000; Jo et al. 2001, 2005; Kolstad et al. 2004a; Scalia and Benson 2010; Tian et al. 2016). Conventional GCLs consist of a thin (approximately 7–10 mm) layer of powdered or granular sodium bentonite (NaB) sandwiched between two geotextiles (GTs) joined by needlepunching or stitching. The low hydraulic conductivity of NaB GCLs is attributed to osmotic swelling of montmorillonite, the primary mineral in bentonite (Norrish and Quirk 1954; Kolstad et al. 2004a; Scalia et al. 2014; Tian et al. 2016; Setz et al. 2017). Water molecules associated with osmotic swelling are strongly bound to the mineral surface and are effectively immobile, reducing the size, number, and connectivity of hydraulically active pores (Kolstad et al. 2004a; Scalia et al. 2014).
Osmotic swelling of bentonite is affected by the chemical composition of the hydrating and permeant solutions. Maintaining osmotic swell and low hydraulic conductivity requires that (1) the predominant cation in the exchange complex be monovalent (e.g., Na+) and (2) the ionic strength (I) of the pore fluid be relatively low (I<50  mM, Jo et al. 2001). Exposure of GCLs containing NaB to leachates having high ionic strength or a preponderance of divalent cations can reduce swelling and increase hydraulic conductivity (Shan and Daniel 1991; Petrov and Rowe 1997; Ruhl and Daniel 1997; Shackelford et al. 2000; Vasko et al. 2001; Jo et al. 2001, 2005, Kolstad et al. 2004a; Meer and Benson 2007; Guyonnet et al. 2009; Tian et al. 2016). Higher hydraulic conductivity is attributed to larger pores in the NaB that would be hydraulically inactive when osmotic swell is appreciable (Kolstad et al. 2004a; Bradshaw and Benson. 2014; Scalia et al. 2014; Tian et al. 2016; Chen et al. 2018).
Bentonites amended with polymers and other organic molecules have been introduced in recent years to improve the chemical compatibility and hydraulic conductivity of GCLs permeated with aggressive leachates (e.g., Trauger and Darlington 2000; Ashmawy et al. 2002; Kolstad et al. 2004b; Katsumi et al. 2008; Shackelford et al. 2010; Scalia et al. 2014; Bohnhoff and Shackelford 2014; Fehervari et al. 2016; Gates et al. 2016; Shaheen et al. 2016; Tian et al. 2016; Chen et al. 2019). Bentonite–polymer composites (BPCs) composed of polymer intercalated in the montmorillonite interlayer or dry blended with bentonite have maintained low hydraulic conductivity (<10−10  m/s) in solutions having high ionic strength, a preponderance of polyvalent cations, and extreme pH (Onikata et al. 1996, 1999; Kolstad et al. 2004b; Katsumi et al. 2008; Scalia et al. 2014; Tian and Benson 2014; Tian et al. 2016). Mechanisms controlling the hydraulic conductivity of BPCs have been hypothesized, including increased swelling capacity (Onikata et al. 1996, 1999; Katsumi et al. 2008), prevention of cation exchange (Trauger and Darlington 2000; Ashmawy et al. 2002; Deng et al. 2006), and pore clogging (Scalia et al. 2014; Tian et al. 2016). However, the underlying physical and chemical mechanisms responsible for the lower hydraulic conductivity of BPC GCLs to more concentrated permeant solutions are not completely understood.
The objective of this study was to evaluate how polymer elution affects the hydraulic conductivity of BPC GCLs comprising a dry mixture of bentonite and polymer and to propose mechanisms that control the hydraulic conductivity of BPC GCLs. Two commercially available GCLs containing the same dry mixture of bentonite and polymer were exposed to deionized (DI) water and CaCl2 solutions ranging in concentration from 20 to 500 mM. One GCL included a slit-film geotextile (woven-GT) on the interior as a filtration layer to control polymer elution. Total organic carbon (TOC) analysis of effluent from hydraulic conductivity tests was used to quantify polymer elution. Pore-scale images of specimens after permeation were obtained using scanning electron microscopy (SEM) to observe how the fabric of the BPC was altered by the permeant solution. The SEM images and TOC data are used to support a proposed conceptual framework to relate pore fluid chemistry, polymer elution, fabric, swelling, and hydraulic conductivity.

Background

Polymers and Polymer Conformation

Polymers are long-chain molecules composed of unit cells (monomers) linked in either straight or branched chains to form macromolecules (Painter and Coleman 1998). A single macromolecule may contain thousands of monomers. When dissolved in a solution, some polymers form a hydrogel—a web of polymer strands and associated water molecules with a gelatinous structure. The polymers used in BPCs generally form hydrogels when hydrated (Tian et al. 2016; Chen et al. 2019).
The conformation (structure) of polymers in solution is sensitive to environmental conditions such as pH, ionic strength, temperature, electrical potential, and photo-irradiation (Park and Hoffman 1992; Gudeman and Peppas 1995; Swann et al. 2010; Kim and Palomino 2011). Low pH or high ionic strength generally results in a coiled polymer conformation, whereas high pH or low ionic strength generally results in an extended polymer conformation (Fig. 1). Chemical environments leading to a coiled conformation have a destabilizing effect on polymers in solution, resulting in so-called salting out (precipitation), whereas conditions leading to extended polymer conformation have a so-called salting-in (solubilization) effect (Swann et al. 2010). Differences in polymer conformation affect the macroscopic properties of a polymer hydrogel (Kim and Palomino 2011).
Fig. 1. Conceptual illustration of effects of solution chemistry on polymer conformation.

Polymer–Clay Interactions

Polymers can associate with clay minerals through ion-dipole interaction, electrostatic forces, and hydrogen (H) bonding (Deng et al. 2006; Theng 2012). Cationic polymers bind to montmorillonite via electrostatic interaction, whereas anionic polymers bind to exchangeable cations of montmorillonite through cation bridging (Deng et al. 2006). Polymer can bind on clay mineral surfaces through H-bonding between the functional group of polymer (e.g., amide groups) and water molecules in the hydration shells of exchangeable cations (Theng 2012). Anionic polymers also bind to positively charged edge surfaces of montmorillonite and other clay minerals when the pH is less than 4 (Black et al. 1965; Heller and Keren 2003).
Interactions between montmorillonite and polymers can have three forms (Kim and Palomino 2011): (1) phase-separated interaction [Fig. 2(a)] where intact clay particles or aggregates of particles are dispersed within a polymer; (2) intercalated interaction [Fig. 2(b)] where polymer molecules are inserted into the interlayer space of clay minerals, but individual unit layers of the mineral maintain a well-defined and coherent layered structure; and (3) exfoliated interaction [Fig. 2(c)] where clay mineral interlayers are disassociated and dispersed in the polymer (Giannelis et al. 1999; Alexandre and Dubois 2000; Ray and Okamoto 2003; Ruiz-Hitzky and van Meerbeek 2006). The BPC synthesized by dry blending used in this study has phase-separated interaction, whereas BPCs prepared by polymerization or replacing exchangeable cations with polymer often are in intercalated forms (e.g., Onikata et al. 1996; Trauger and Darlington 2000; Kim and Palomino 2011; Bohnhoff and Shackelford 2014; Scalia et al. 2014; Tian et al. 2016; De Camillis et al. 2016; Chen et al. 2018, 2019). Waste containment facilities have made use of BPC GCLs with phase-separated and intercalated forms to contain more concentrated leachates (Gates et al. 2009; Gebka et al. 2018).
Fig. 2. Conceptual illustration of polymer–clay interactions in composite forms: (a) phase-separated; (b) intercalated; and (c) exfoliated. (Adapted Kim and Palomino 2011.)

Materials and Methods

Bentonite–Polymer GCLs

Tests were conducted on two commercially available GCLs containing the same dry-blended bentonite–polymer mixture (BP and BPS GCLs) sandwiched between two GTs bound by needlepunching. A woven slit-film GT intended to control polymer elution by filtering was included in the interior of the BPS GCL (Fig. 3). The slit-film GT was directly adjacent to the GT on the effluent side of the GCL.
Fig. 3. Cross section of GCL containing BP mixture and a woven slit-film geotextile (BPS).
Index properties, cation exchange capacity, and major bound cations of the bentonite–polymer mixture are summarized in Table 1. Standards followed to obtain these properties are summarized in the table [ASTM D5993 (ASTM 2018b); ASTM D5890 (ASTM 2019); ASTM D7348 (ASTM 2013); ASTM D7503 (ASTM 2018c); ASTM D422 (ASTM 2007)]. Montmorillonite is the major mineral component (79%), as determined by X-ray diffraction (XRD) using the methods in Scalia et al. (2014). The swell index [ASTM D5890 (ASTM 2019)] in DI water is 28  mL/2  g (Table 2). Polymer loading was defined as the dry mass of polymer per dry mass of bentonite in the BP and was quantified by loss on ignition (LOI) measurements under oxygenated conditions in accordance with ASTM D7348 (ASTM 2013). In addition to enabling quantification of polymer, the procedure also accounts for loss of strongly bound water, calcite, and organic matter associated within the bentonite fraction (Scalia et al. 2014; Tian et al. 2016). The polymer loading of the GCL was determined to be 5.1%. The LOI of conventional bentonite was 1.6%, which is attributed to a loss of strongly bound water molecules (Grim 1968), decomposition of calcite, and combustion of any organic matter in the bentonite (Scalia et al. 2014; Tian et al. 2016). Complete combustion of the polymer at 550°C was assumed.
Table 1. Index properties, cation exchange capacity, and bound cations of bentonite–polymer mixtures
PropertyASTM standardBPBPS
Dry mass per area (kg/m2)D59933.63.6
Swell index (mL/2  g)D58902828
Loss on ignition (LOI) (%)D73486.66.6
Polymer loading (%)5.15.1
Cation exchange capacity (CEC) (cmol+/kg)D750376.375.2
Bound Na+ (mole fraction)0.450.44
Bound K+ (mole fraction)0.030.04
Bound Ca2+ (mole fraction)0.420.44
Bound Mg2+ (mole fraction)0.080.06
D10 (mm)D4220.40.4
D30 (mm)0.60.6
D60 (mm)1.01.0

Note: Polymer loading calculated based on loss on ignition per ASTM D7348 after accounting for bentonite fraction. Bound cations and CEC measured on base bentonite in BP prepared using LOI to remove all polymer prior to test.

Table 2. Swell index of BP, BPS, and NaB GCLs in CaCl2 solutions
Permeant liquidSolution propertiesSwell index (mL/2  g)
EC (S/m)pHBP GCLBPS GCLNaB GCLa
DI water——282831.4
20 mM CaCl20.496.51515.314.2
50 mM CaCl20.986.41212.510.8
100 mM CaCl21.786.7109.7—
200 mM CaCl23.096.8999
500 mM CaCl26.816.987.87.6
a
Scalia et al. (2014).
Mole fractions of bound cations (BCs) and cation exchange capacity (CEC) of the base bentonite in the bentonite–polymer mixture were determined according to ASTM D7503 (ASTM 2018c). The BPC was subjected to the LOI procedure to remove the polymer prior to BC and CEC analysis.

Polymer Identification

Fourier transform infrared (FTIR) spectroscopy was used to identify the type and characteristics of the polymer blended with the bentonite. FTIR analysis of the dry bentonite–polymer mixture sampled directly from the GCL was unsuccessful owing to the predominance of the bentonite fraction in the mixture (>90% by mass). During subsequent hydraulic conductivity testing, however, the effluent often had viscosity visibly higher than the influent (Tian et al. 2016), suggesting that polymer was being eluted from the GCL (see subsequent discussion). Isopropanol (purity=99.9%, Sigma-Aldrich, St. Louis) was added to the effluent to precipitate the polymer, which was assumed to be the same polymer within the GCL (Tian et al. 2016). FTIR spectra of the white precipitate were obtained using an attenuated total reflection (ATR) spectrometer (Thermo Scientific iD5 Diamond ATR with Nicolet iS5 FTIR spectrometer, Thermo Scientific, Madison, Wisconsin). The polymer type was determined by comparing the measured FTIR spectra with spectra in the Thermo Scientific OMNIC Spectra Software library. The analysis indicated that 80% of the measured spectrum of the white precipitate was consistent with anionic polyacrylamide (PAM) in the library. FTIR spectra obtained for a commercially available anionic PAM (Polysciences, Warrington, Pennsylvania, Catalog No. 04652) were also comparable to the spectra for the precipitate (Fig. 4). Though the polymer in the GCL cannot be defined precisely, PAM is believed to be a major fraction.
Fig. 4. FTIR spectra for commercial PAM and polymer precipitate in effluent from BP GCL.
PAM consists of polymer chains having hydrophilic functional groups (e.g., carboxylic acid groups) that can bind water molecules through hydrogen bonding, forming a hydrogel (Soppirnath and Aminabhavi 2002; Ahmed 2015). PAM chains can connect with other chains to form a three-dimensional network structure. Interactions between ions (e.g., Ca2+, Cl−) in solution, and charged carbonyl and amide functional groups along the polymer chain (Besra et al. 2002) influence the conformation of PAM in the hydrogel. In low-ionic-strength solutions, the charged functional groups repel, extending the polymer chain and forming a three-dimensional gel network. In high-ionic-strength solutions, cations in solution neutralize the negatively charged carbonyl groups, causing the polymer chain to contract or coil (Klenina and Lebedeva 1983; Kurenkov 1997; Schweins et al. 2003).

Test Liquids

Swell index and hydraulic conductivity tests were conducted with DI water and calcium chloride (CaCl2) solutions (20, 50, 100, 200, and 500 mM), the latter prepared by dissolving reagent-grade dihydrate-calcium chloride (CaCl2•2xH2O) in Type II DI water per ASTM D1193 (ASTM 2018a). Concentrations of Ca2+ in solution were verified by inductively coupled plasma-optical emission spectroscopy (ICP-OES) (MPX ICP-OEX) (Varian, Palo Alto, California) following USEPA Method 6010 B. Electrical conductivity (EC) and pH of the test solutions (Table 2) were measured using Orion Star A215 Meters (Thermo Fisher Scientific, Waltham, Massachusetts).
CaCl2 solutions with concentrations ranging from 20 to 500 mM were used to represent conditions ranging from dilute leachate to high-concentration leachate. The 20 mM CaCl2 solution was considered a dilute permeant solution, the 50 and 100 mM CaCl2 solutions were considered modest-concentration solutions, and the 200 and 500 mM CaCl2 solutions were considered high-concentration solutions based on municipal solid waste leachate data reported in Bradshaw and Benson (2014), low-level radioactive waste leachate data in Tian et al. (2017), and coal combustion product leachate data in Chen et al. (2019). The BPC GCLs were anticipated to have lower hydraulic conductivity than NaB GCLs to some, if not all, of these solutions.
This range of CaCl2 concentrations was selected to investigate how the concentration of divalent cations affects the swelling and hydraulic conductivity of BPC GCLs and how BPC GCLs behave differently from NaB GCLs. The swelling and hydraulic conductivity of NaBs are strongly affected by Ca2+ concentration (Jo et al. 2001; Kolstad et al. 2004a; Lee and Shackelford 2005; Scalia et al. 2014). NaB GCLs permeated with relatively dilute CaCl2 solutions (5, 10, and 20 mM) have a hydraulic conductivity near 2×10−10  m/s after complete exchange of Ca2+ for native Na+, or approximately 10 times the hydraulic conductivity to DI water (Jo et al. 2001; Kolstad et al. 2004a; Lee and Shackelford 2005). As the CaCl2 concentration exceeds 50 mM, the hydraulic conductivity of NaB GCLs can increase by orders of magnitude (10−8 to 10−7  m/s).

Swell Index and Hydraulic Conductivity Tests

Swell index tests were conducted in general accordance with ASTM D5890 (ASTM 2019) on bentonite–polymer mixtures sampled from the GCLs. Prior to testing, the granular bentonite–polymer mixture was ground with a mortar and pestle to 100% passing the No. 100 sieve with a minimum of 65% passing the No. 200 sieve, oven-dried to constant mass at 105±5°C, and cooled to room temperature. The swell index was measured in 100-mL graduated cylinders using 2.0±0.001  g of processed bentonite–polymer mixture. DI water and each CaCl2 solution were used as hydrating liquids.
Hydraulic conductivity tests were conducted according to ASTM D6766 (ASTM 2012) in flexible-wall permeameters using the falling headwater-constant tailwater method. Influent permeant solution was contained in 50-mL graduated glass pipettes. Effluent was collected in 60-mL polyethylene bottles. EC, pH, cation concentration, and TOC were measured periodically on the effluent. The burettes and bottles were covered with parafilm to minimize evaporation and atmospheric interaction.
GCL specimens for hydraulic conductivity testing were prepared by cutting a circular sample directly from a roll provided by the GCL manufacturer using a razor knife and steel ring following the procedure in Jo et al. (2001). Paste prepared with bentonite–polymer mixture and permeant solution was placed around the perimeter of the specimen to prevent loss of the mixture during handling and preferential flow along the edge during permeation. Initial thickness of each specimen was measured with calipers at six equidistant points.
The BPS GCL specimen was installed in the permeameter with the slit-film GT at the bottom of the GCL, which is the same orientation used in the field. Specimens were hydrated with permeant solution for 48 h in the permeameters prior to permeation at an isotropic effective stress of 10 kPa by opening the inflow valve while keeping the effluent valve closed to prevent flow. Flow was oriented vertically downward (i.e., top to bottom flow). BP GCLs were permeated with 20, 50, 100, 200, and 500 mM CaCl2, and BPS GCLs were permeated with 50, 100, and 200 mM CaCl2. The effective confining stress was increased to 20 kPa prior to permeation in each case.
Permeation was conducted using an average hydraulic gradient of 130. Tests were continued until the hydraulic and chemical equilibrium in ASTM D6766 were satisfied. Hydraulic equilibrium was defined as steady hydraulic conductivity for three consecutive measurements with ratios of incremental volumetric outflow to inflow (flow ratio) within 1.00±0.25. Chemical equilibrium was assumed when the ratios of effluent to influent electrical conductivity (ECout/ECin) and effluent to influent pH (pHout/pHin) were steady and within 1.00±0.1. In addition, concentrations of major cations in the influent and effluent were required to be within 10% of those recommended in Jo et al. (2005). All tests met the termination criteria by the end of the study except those conducted with DI water, 20 mM and 50 mM CaCl2, which had very low hydraulic conductivity and transmitted insufficient volume of permeant solution to meet the chemical equilibrium criteria.
The permeameters used in this study employed oversize (approximately 7 mm) tubing and fittings to ensure that polymer eluted from the GCL would not clog the influent and effluent lines. The influent and effluent lines were checked and flushed periodically to ensure that clogging did not occur. The nonwoven GT installed on the upper and lower faces of the GCL specimen to distribute permeant liquid is the same nonwoven GT used in BP GCLs and is known not to constrain elution.

Total Organic Carbon Analysis

Effluent from the BP and BPS GCLs was analyzed periodically for TOC to quantify the mass of polymer eluted. Organic carbon concentrations were analyzed following ASTM D4839 (ASTM 2017) using a Shimadzu TOC analyzer (TOC-VCSH, Shimadzu, Kyoto, Japan) for nonpurgable organic carbon. Calibration solutions were prepared by diluting 1,000  mg/L anhydrous potassium hydrogen phthalate (carbon content=47.0%) to various concentrations with high-purity water. Samples were diluted 25 times to maintain the carbon content within the range of the standard solutions. TOC samples were acidified to pH 2 with 2 M HCl prior to injection into analyzer. Inorganic carbon was removed by sparging with CO2-free gas. The remaining carbon was converted to CO2 by combusting the sample, which was then dehydrated, scrubbed to remove chlorine and other halogens, and detected for CO2 content in a nondispersive infrared gas analyzer. Polymer concentration (mg/L) in the effluent was computed from the TOC using the carbon fraction of PAM repeat unit (−CH2 CHCONH2−).

Scanning Electron Microscopy

Following permeation with each test solution, SEM was conducted to observe the pore-scale clay fabric, polymer structure, and clay–polymer interactions in the BPCs. SEM specimens were prepared by immersing the fully hydrated postpermeation BPC in liquid nitrogen followed by freeze drying in a bench-top freeze-drying system (Labconco Model 7740020, Kansas City, Missouri). Fast freezing in liquid nitrogen prevents crystallization of water molecules and inhibits breakdown of the polymer structure due to the liquid-to-solid transition (Annabi et al. 2010; Nireesha et al. 2013). Drying under vacuum minimizes drying-induced disturbance by allowing direct sublimation of water from the solid to vapor phase and has been used to observe the microstructure of polymer hydrogels (Annabi et al. 2010; Tian et al. 2016).
Freeze-dried samples were cut into thin specimens (5×5  mm) using a razor knife, mounted on sample holders using carbon tape, and coated with gold using a sputtering system (Denton Vacuum Desk II, Denton Vacuum, Moorestown, New Jersey). SEM images of the cut surfaces were obtained using a LEO 1550 Gemini SEM (ZEISS, Jena, Germany) using a 3-keV electron beam and conventional secondary electron detector. Elemental analyses were conducted on select BPC specimens coated with carbon using energy dispersive spectroscopy (EDS) (Model 2227A-ASPS-SN, Thermo Scientific NORAN, Waltham, Massachusetts). EDS was used to differentiate between polymer and bentonite in the SEM images.
SEM images were also obtained for the commercially available PAM to assess how conformation of the anionic polymer may vary in dilute (20 mM) to concentrated (500 mM) CaCl2 solutions without the presence of bentonite. Specimens were prepared by hydrating 0.05 g PAM for 24 h in 10 mL of solution contained in a glass vial. Then the glass vial was submerged in liquid nitrogen followed by freeze drying in a bench-top freeze-drying system.

Results and Discussion

Results of the hydraulic conductivity tests are summarized in Table 3. The GCLs were permeated for up to 1.8 years when the tests were terminated. GCLs permeated with 50, 100, 200, and 500 mM CaCl2 solutions met all of the termination criteria in ASTM D6766 and the supplemental criteria regarding pH and the concentrations of major cations in the influent and effluent. The BP GCLs permeated with 20 mM CaCl2 and BPS GCL permeated with 20 and 50 mM CaCl2 solutions did not meet all of the termination criteria because of their very low hydraulic conductivity and low pore volumes of flow (PVF).
Table 3. Hydraulic conductivity of BP, BPS, and NaB GCLs to CaCl2 solutions
PermeantBP GCLBPS GCLNaB GCL
Test duration (Days)PVFHydraulic conductivity (m/s)Ave. hydraulic conductivity (m/s)Test duration (days)PVFHydraulic conductivity (m/s)Ave. hydraulic conductivity (m/s)Hydraulic conductivity (m/s)
DI water65214.83.7×10−123.7×10−123154.82.2×10−122.2×10−122.5×10−11
20 mM CaCl233110.51.2×10−111.2×10−111261.72.3×10−122.3×10−122.8×10−11
50 mM CaCl227228.93.1×10−103.1×10−101523.42.5×10−122.5×10−121.7×10−7
100 mM CaCl210234.11.1×10−88.9×10−913316.23.1×10−113.2×10−11—
12529.66.5×10−916822.33.4×10−11
200 mM CaCl21719.12.7×10−82.9×10−82819.22.3×10−82.6×10−84.3×10−7
3411.53.2×10−81914.13.8×10−8
500 mM CaCl2638.86.1×10−87.0×10−8————4.5×10−7
1030.28.0×10−8————

Swell Index

Swell indices of the bentonite–polymer mixtures from the BP and BPS GCLs are shown in Fig. 5 as a function of CaCl2 concentration, along with swell indices for conventional NaB GCLs reported by Scalia et al. (2014). A summary is in Table 2. The conventional NaB GCL used in Scalia et al. (2014) was provided by the same manufacturer as the BP and BPS GCLs and had the same base bentonite. The swell index for the BPCs and the conventional NaB decreases with increasing CaCl2 concentration (Fig. 5) from approximately 30  mL/2  g in DI water to 8  mL/2  g in 500 mM CaCl2, with most of the reduction between 5 and 50 mM CaCl2 (from around 28  mL/2  g to around 12  mL/2  g). Jo et al. (2001), Kolstad et al. (2004a), Lee and Shackelford (2005), Katsumi et al. (2008), Scalia et al. (2014), and Chen et al. (2019) report similar trends for NaB. Swell indices for the bentonite–polymer mixtures from the BP and BPS GCLs are nearly identical, which is expected as the same mixture is used to manufacture both GCLs. At CaCl2 concentrations ≥50  mM, swell indices for the BPC and the NaB are almost identical and are comparable to the swell index for Ca-bentonite (8–10  mL/2  g) (Jo et al. 2001). These findings suggest that the swelling behavior of the bentonite–polymer mixture obtained from the swell index test is controlled by the bentonite fraction and not the polymer. Another possibility is that the polymer became transparent after swelling in a saline solution and separated from the bentonite during sedimentation in the swell index test. Thus, the swell index test may not be able to capture the bulk swelling of BPCs. A study is ongoing to investigate this issue.
Fig. 5. Swell index for BP and NaB in DI water and CaCl2 solutions with concentrations ranging from 5 to 500 mM. DI water shown at 0.001 mM. (NaB data from Scalia et al. 2014.)

Hydraulic Conductivity

Hydraulic conductivity is shown in Fig. 6 versus PVF for the BP and BPS GCLs permeated with 20, 100, and 200 mM CaCl2 solutions. The PVF was calculated using the pore volume of the GCL specimens determined at the end of testing. The hydraulic conductivity of the BP GCL to 20 mM CaCl2 decreased for the first 3 PVF to 2.0×10−12  m/s and then increased gradually until reaching hydraulic equilibrium around 11 PVF (1.2×10−11  m/s). The hydraulic conductivity of the BPS GCL to 20 mM CaCl2 gradually decreased to approximately 2.3×10−12  m/s during the testing period. For BP GCLs permeated with modest-concentration solutions (100 and 200 mM CaCl2), the hydraulic conductivity increased rapidly (within around 10 PVF) from about 10−11−10−10  m/s to a steady state on the order of 10−8  m/s, with higher hydraulic conductivity obtained with higher concentrations. For the BPS GCL, the hydraulic conductivity was 3.2×10−11  m/s when permeated with 100 mM CaCl2 but increased to approximately 10−8  m/s with 200 mM CaCl2. The initial decrease in the hydraulic conductivity of the BP GCLs is attributed to transient conditions during the hydration process, during which bentonite swells and the polymer hydrogel forms, resulting in narrower and more torturous flow paths and reduction of hydraulic conductivity. Similar initial decreases in hydraulic conductivity are observed when NaB GCLs are permeated with DI water or dilute solutions (Jo et al. 2001, 2005; Kolstad et al. 2004a).
Fig. 6. Hydraulic conductivity versus PVF for BP and BPS GCLs permeated with 20, 100, and 200 mM CaCl2 solutions.
The hydraulic conductivities of the BP and BPS GCLs permeated with DI water and 20, 50, 100, 200, and 500 mM CaCl2 are compared in Fig. 7 along with the hydraulic conductivity of the NaB GCL reported by Scalia et al. (2014). The BP and BPS GCLs are approximately one to four orders of magnitude less permeable than the NaB GCL over the entire range of CaCl2 solutions. The greatest differences in hydraulic conductivity exist between 50 and 100 mM CaCl2. The hydraulic conductivity of the NaB and BP GCLs increases dramatically in this concentration range, whereas the hydraulic conductivity of the BPS GCL increases only modestly. When permeated with the most concentrated saline solutions (200 and 500 mM CaCl2), all of the GCLs were highly permeable (around 10−7  m/s or higher). The BP and BPS GCLs had similar hydraulic conductivity at 200 mM (2.9×10−8 and 2.6×10−8  m/s) and are only 15 times less permeable than the NaB (4.3×10−7  m/s).
Fig. 7. Hydraulic conductivity of BP and BPS GCLs to DI water and CaCl2 solutions. (NaB data from Scalia et al. 2014.)

Polymer Elution

Effluent from the hydraulic conductivity tests on the BP and BPS GCLs was visibly more viscous than the influent, suggesting that polymer was eluted during permeation. Scalia et al. (2014) and Tian et al. (2016) report similar observations. The surface of the BP GCL after permeation with the 100 mM CaCl2 solution is shown in Fig. 8(a). Strands of polymer hydrogel span between the surface of the GCL (gray) and the adjacent nonwoven GT (black) used for flow distribution in the permeameter.
Fig. 8. (a) Polymer eluted during permeation with 100 mM CaCl2 solution; and (b) concentration of eluted polymer in effluent of BP and BPS GCL permeated with 100 or 200 mM CaCl2 solutions as a function of PVF.
Polymer concentration in the effluent is shown in Fig. 8(b) as a function of PVF for the BP and BPS GCLs permeated with 100 and 200 mM CaCl2. The eluted polymer concentration was highest within a few PVF after permeation began and gradually decreased as permeation continued. For the 100 mM CaCl2 solution, polymer concentrations were higher for the BP than the BPS GCL, indicating greater elution for the BP GCL. For the 200 mM CaCl2 solution, the polymer concentration was initially higher for the BP GCL, but polymer was eluted later from the BPS GCL at higher concentration.
Hydraulic conductivity versus cumulative mass of polymer eluted is shown in Fig. 9(a) for the BP and BPS GCLs permeated with 100 and 200 mM CaCl2 solutions. Cumulative mass of eluted polymer was calculated by integrating the eluted polymer concentration with the outflow volume. The hydraulic conductivity of the GCL increased sharply with polymer elution [Fig. 9(a)], suggesting that pore spaces between bentonite granules that were initially blocked by hydrogel open up as polymer is eluted from the GCL. The hydraulic conductivities of the BP and BPS GCLs at equilibrium are shown in Fig. 9(b) as a function of percentage of total polymer eluted, calculated as the quotient of polymer mass eluted and the initial polymer mass. The percentage of polymer eluted increases as the CaCl2 concentration increases for a given GCL, and the hydraulic conductivity of both GCLs increases as the polymer eluted increases [Fig. 9(b)].
Fig. 9. Hydraulic conductivity of BP and BPS GCLs as a function of (a) cumulative polymer eluted during permeation with 100 and 200 mM CaCl2 solution; and (b) percentage polymer eluted with 50, 100, 200, and 500 mM CaCl2 solutions.
These findings suggest that the mobility of the polymer is affected by the solution concentration and that the release of polymer from the GCL opens pores and causes the hydraulic conductivity to increase. Neutralization of charged functional groups (carbonyl and amide) on the polymer chains by the ions in solution (e.g., Ca2+ and Cl−) reduces the electrostatic binding of polymers to mineral surfaces (Klenina and Lebedeva 1983; Kurenkov 1997; Schweins et al. 2003), making the polymer more mobile under a hydraulic gradient. Collapse and coiling of the polymer hydrogel in the more concentrated solutions also reduces the size and hydraulic diameter of the polymer (Ahmed 2015), which may further increase polymer mobility.
Less elution occurs for the BPS GCL with the 100 mM CaCl2 solution because the slit-film GT constrains movement of the polymer, increasing the retention of the polymer within the pores in the bentonite (Fig. 3). Consequently, the BPS GCL has lower hydraulic conductivity than the BP GCL to the 100 mM CaCl2 solution. The lower hydraulic conductivity of the BPS GCL relative to the BP GCL with the 50 mM CaCl2 solution is also probably due to polymer retention by the slit-film GT. However, polymer concentrations in effluent from the BPS GCL permeated with 50 mM CaCl2 solution were not measured, and retention for the 50 mM solution cannot be inferred. With the 200 mM CaCl2 solution, alterations to the charge and conformation of the polymer apparently were substantial enough that the slit-film GT was no longer effective in retaining the polymer hydrogel, permitting greater elution and much higher hydraulic conductivity of the BPS GCL. These findings are consistent with the inference that retaining polymer hydrogel in the intergranular pore space of bentonite is necessary to maintain a low hydraulic conductivity when the permeant solution inhibits osmotic swell of the bentonite fraction.

Bentonite–Polymer Microstructure

A colorized SEM image of BPC permeated with DI water and then freeze-dried for imaging is shown in Fig. 10(a). The netlike structure binding to the bentonite clusters in the image is the structure of the polymer hydrogel, as confirmed by EDS analysis. Silicon, aluminum, and oxygen, key elements in aluminosilicate but not in polymer, are higher at Point 6 than Point 1, suggesting that 1 is polymer and 6 is mineral solid [Fig. 10(b)].
Fig. 10. (a) SEM image of BP GCL after permeation with DI water; and (b) bentonite and polymer identification using EDS.
SEM images for BP GCL specimens after permeation with 20 and 200 mM CaCl2 solutions are shown in Fig. 11. The polymer hydrogel for the specimen permeated with 20 mM CaCl2 solution [circled in Fig. 11(a)] has thicker strands and is less extensive than the hydrogel present with DI water [Fig. 10(a)]. However, the hydrogel is sufficient to fill the intergranular pores and maintain a low hydraulic conductivity. With the 200 mM CaCl2 solution, polymer strands are nearly nonexistent in the image [Fig. 11(b)], and no evidence of a polymer web exists that would support a hydrogel. The absence of a hydrogel is consistent with the high hydraulic conductivity and substantial polymer elution for these specimens (Fig. 7). The CaCl2 solutions used for these tests had circumneutral pH (6.1 ≤PH ≤6.8) (Table 2), which suggests that differences in polymer conformation evident in the SEM images are likely due to differences in ionic strength (e.g., Besra et al. 2002).
Fig. 11. SEM images of BP GCLs after exposure to (a) 20 mM (BP to 20 mM, k=1.2×10−11  m/s); and (b) 200 mM (BP to 200 mM, k=2.9×10−8  m/s) CaCl2 solutions. Circles: polymer hydrogel.
SEM images of commercial anionic PAM polymer (no bentonite) hydrated in 20 mM CaCl2 and 500 mM CaCl2 are shown in Fig. 12 (SEM images were not collected for 200 mM CaCl2 due to experimental difficulties). PAM hydrated in 20 mM CaCl2 exhibits a coherent three-dimensional meshlike network of thin polymer strands [Fig. 12(a)]. For the 500 mM CaCl2 solution [Fig. 12(b)], the conformation is coiled or globular (“pearl necklace” conformation per Schweins and Huber 2004) and forms a sparsely distributed network. These changes in conformation of PAM with the chemistry of the hydrating solution are similar to the changes in the BPC polymer structure in the GCLs for the same solutions (Figs. 10 and 11) and suggest that an extensive and coordinated hydrogel forms at 20 mM, whereas limited and localized gels form at 200 mM.
Fig. 12. SEM images of freeze-dried anionic PAM hydrated with (a) 20 mM CaCl2; and (b) 500 mM CaCl2.

Pore-Scale Mechanisms and Hydraulic Conductivity

A conceptual model of the mechanisms controlling the hydraulic conductivity of BPC GCLs was created based on the data and observations and past conceptual models for mechanisms controlling the hydraulic conductivity of conventional NaB GCLs (Kolstad et al. 2004a; Scalia and Benson 2010). Under dry conditions, the interior of a conventional NaB GCL consists of granular NaB separated by relatively large intergranular pores (Fig. 13). When permeated with DI water or dilute solution, the granules swell in response to osmotic swelling in the montmorillonite, closing intergranular pores and forcing water to flow in small and tortuous pores within the granules, resulting in low hydraulic conductivity (approximately 10−11  m/s). Permeation with a low-concentration solution (e.g., ≤20  mM CaCl2) constrains osmotic swelling modestly, reducing the swelling of bentonite granules and the efficiency of closing intergranular pores [Fig. 13(c)], resulting in a modest increase in hydraulic conductivity (approximately 10−10−10−9  m/s). Permeation with more concentrated solutions (e.g., ≥50  m MCaCl2) precludes osmotic swelling of the montmorillonite interlayer, swelling of bentonite granules, and closure of intergranular pores [Fig. 13(d)]. Consequently, the GCL has a high hydraulic conductivity (approximately 10−8 to 10−7  m/s).
Fig. 13. Conceptual models for mechanisms controlling hydraulic conductivity of NaB GCL in dilute to aggressive permeant solutions: (a) dry bentonite granules; (b) osmotic swelling of NaB in DI water or dilute solution results in narrow and tortuous flow paths and low hydraulic conductivity; (c) permeation with low-concentration solution inhibits osmotic swelling modestly, resulting in intergranular flow channels and moderate hydraulic conductivity; and (d) NaB permeated with moderate- or high-concentration solution prevents osmotic swelling, resulting in increased pore sizes and higher hydraulic conductivity.
Prior to hydration, GCLs containing bentonite–polymer mixtures have granules of polymer interspersed in the intergranular pores between bentonite granules [Fig. 14(a)]. Permeation with DI water [Fig. 14(b)] induces osmotic swelling within the bentonite fraction, swelling of bentonite granules, and the formation of a polymer hydrogel. The bentonite and polymer function together as a composite that fills the pores [Fig. 14(b)]. Because the pore space is highly constrained, the hydraulic conductivity of BPC GCLs to DI water is very low—approximately one order of magnitude lower than the hydraulic conductivity of a NaB GCL (10−12 versus 10−11  m/s). Polymer elution is low owing to strong binding between the bentonite and polymer in the constrained pore space.
Fig. 14. Conceptual models for mechanisms controlling hydraulic conductivity of BP GCL in dilute and aggressive permeant solutions: (a) dry bentonite and polymer granules; (b) osmotic swelling of NaB and polymer hydrogel formation in DI water results in narrow and tortuous flow paths and low hydraulic conductivity; (c) permeation with dilute solution inhibits osmotic swelling of NaB, but the polymer hydrogel clogs the intergranular flow channels, resulting in low hydraulic conductivity; (d) permeation with moderate-concentration solution leads to polymer hydrogel collapse and polymer elution, but the polymer hydrogel clogs the intergranular pores, resulting in a moderate increase in hydraulic conductivity; and (e) permeation with high-concentration solution leads to further collapse of polymer hydrogel and higher polymer elution, resulting in high hydraulic conductivity.
Permeation with a dilute or low-concentration solution inhibits osmotic swelling of NaB modestly and may partially compromise the hydrogel [Figs. 14(c and d)]. The polymer becomes less extensive and the size of the pore space increases, allowing some polymer elution during permeation. However, together the bentonite and polymer function as a composite to maintain a closed intergranular pore network and low hydraulic conductivity, provided elution is not substantial.
Permeation with a moderate-concentration solution [Fig. 14(d)] inhibits osmotic swelling of the bentonite, constrains the formation of polymer hydrogel, and reduces binding of the hydrogel with the clay mineral surfaces, which promotes polymer elution. Larger pore spaces between the granules exist, resulting in a moderately higher hydraulic conductivity.
Permeation with a concentrated solution [Fig. 14(e)] prevents swelling of bentonite granules, prevents the formation of a hydrogel, and precludes binding between the polymer and mineral surfaces. Substantial polymer is eluted from the large intergranular pores and the hydraulic conductivity is high (10−8 to 10−7  m/s).
This conceptual model suggests that retaining polymer in the pore space is critical for a BPC GCL to maintain low hydraulic conductivity in applications where osmotic swelling of the bentonite would be constrained and a conventional NaB GCL would be too permeable. The service life of BPC GCL would be controlled by the rate of polymer elution, which is influenced by the chemistry of the permeant solution. Research on the service life of BPC GCLs is necessary to determine the period of effectiveness of BPC GCLs, especially in applications with more concentrated leachates.

Summary and Conclusions

Hydraulic conductivity and swell index tests were conducted on two commercially available bentonite–polymer composite (BPC) GCLs containing the same dry-blended mixture of bentonite and polymer. One of the GCLs contained bentonite–polymer mixture between two nonwoven geotextiles (BP GCL); the other GCL (BPS GCL) contained an additional woven slit-film geotextile to retain polymer within the GCL. The GCLs were permeated with DI water or CaCl2 solutions ranging in concentration (20–500 mM CaCl2) to represent dilute to high-concentration permeant solutions encountered in waste containment systems.
The following conclusions may be drawn based on the findings of this study:
•
BPC GCLs are approximately one to four orders of magnitude less permeable than conventional NaB GCLs over a wide range of permeant solution concentrations. When permeated with very dilute (DI water) or very concentrated solutions (500 mM CaCl2), BPC GCLs are 5–20 times less permeable than NaB GCLs. The largest differences in hydraulic conductivity between BPC and NaB GCLs occur for intermediate CaCl2 concentrations (50–200 mM), with the hydraulic conductivity of the BP and BPS GCLs as much as four orders of magnitude lower than that of NaB GCLs.
•
The lower hydraulic conductivity of BPC GCLs is attributed to polymer hydrogel filling and clogging pores between bentonite granules, as identified in this study using SEM images. The bentonite and polymer hydrogel function together as a composite material, with the bentonite having a much larger role in filling the pore space in dilute solutions and clogging by polymer hydrogel being more predominant at higher concentrations.
•
Elution of polymer hydrogel from the GCL opens pores, providing additional flow paths and resulting in higher hydraulic conductivity. For the same GCL, cumulative polymer elution is sensitive to the chemistry of the permeant solution, with greater elution at higher ionic strengths. The presence and extensiveness of the polymer hydrogel within the bentonite matrix diminishes as the CaCl2 concentration increases, most likely owing to changes in conformation of the polymer as well as binding between the polymer and aluminosilicate surfaces. At higher CaCl2 concentrations, the intergranular pores in the bentonite are larger, reducing the potential for contact and binding between the polymer and mineral surfaces and permitting polymer to elute more readily.
•
A woven slit-film geotextile incorporated in a BPC GCL reduced polymer elution, resulting in a BPC GCL with very lower hydraulic conductivity over a broader range of concentrations. The greater polymer retention and lower hydraulic conductivity of the GCL incorporating a slit-film geotextile is consistent with the conceptual model proposed for BPC GCLs.

Acknowledgments

Financial support for this study was provided by Minerals Technology Inc. and the US Department of Energy’s (DOE) Department of Environmental Management through the Consortium for Risk Evaluation with Stakeholder Participation (CRESP). The findings and conclusions presented herein are solely those of the authors and do not necessarily represent the policies or perspectives of Minerals Technology Inc. or DOE.

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Go to Journal of Geotechnical and Geoenvironmental Engineering
Journal of Geotechnical and Geoenvironmental Engineering
Volume 145 • Issue 10 • October 2019

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This work is made available under the terms of the Creative Commons Attribution 4.0 International license, http://creativecommons.org/licenses/by/4.0/.

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Received: Mar 14, 2018
Accepted: Mar 6, 2019
Published online: Jul 26, 2019
Published in print: Oct 1, 2019
Discussion open until: Dec 26, 2019

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Kuo Tian, M.ASCE [email protected]
Assistant Professor, Sid and Reva Dewberry Dept. of Civil, Environmental and Infrastructural Engineering, George Mason Univ., Fairfax, VA 22030. Email: [email protected]
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William J. Likos, M.ASCE [email protected]
Gary Wendt Professor, Geological Engineering, Civil and Environmental Engineering, Univ. of Wisconsin–Madison, Madison, WI 53706 (corresponding author). Email: [email protected]
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Craig H. Benson, F.ASCE https://orcid.org/0000-0001-8871-382X [email protected]
Dean and Hamilton Endowed Chair, School of Engineering, Univ. of Virginia, Charlottesville, VA 22904-4246. ORCID: https://orcid.org/0000-0001-8871-382X. Email: [email protected]
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