Geosynthetic clay liners (GCLs) are multi-component materials commonly used in hydraulic barrier systems in applications ranging from different types of landfills to waste ponds to additional liquid containment applications. Conventional GCLs consist of a thin layer of montmorillonite clay (i.e., bentonite) typically encased between two geotextile layers and in some cases an additional geofilm layer attached to one of the geotextile layers. Sodium (Na) montmorillonite (i.e., Na-bentonite) is the main clay mineral in GCLs manufactured and used in North America. Figure 1 presents a GCL installed along the slope of a bottom liner component in a municipal solid waste landfill.
GCLs provide high resistance to the transport of water. However, these materials are strongly affected by the chemistry of other liquids, such as liquids containing cations with higher charges than that of sodium due to the highly active montmorillonite mineral (Mitchell and Soga 2005). The barrier function of the GCLs may be significantly affected when in contact with highly concentrated solutions (NRC 2007, Benson et al. 2011), as presented in Figure 2.
Replacement of monovalent sodium (Na+) with polyvalent cations (e.g., Ca2+, Mg2+, Fe3+) through a cation exchange process significantly affects the material properties of the bentonite component and the overall engineering properties of GCLs. Adverse impacts of cation exchange on both fully saturated and variably saturated GCLs have been well demonstrated in the literature (e.g., Kolstad et al. 2004, Jo et al. 2005, Scalia and Benson 2011, Yesiller et al. 2019). While various studies have investigated effects of cation exchange on GCL behavior, studies on timing and rate of exchange processes are limited. Data and analysis have been reported for GCLs hydrated overlying/between different soils (type, water content, density/compaction conditions) including determination of cation exchange properties (e.g., Rowe and Abdelatty 2012, Bradshaw et al. 2013, Bradshaw and Benson 2014). However, temporal development of the cation exchange processes has not been widely investigated. Therefore, a comprehensive investigation was conducted to systematically analyze the cation exchange characteristics of a GCL as a function of time, concentration of hydration liquid, temperature and applied stress. Results of the time-dependent analysis conducted at 20°C (68°F) with hydration liquids of variable concentrations under no stress conditions is presented herein.
Experimental Test Program
Experimental analysis was conducted on a conventional medium-weight needlepunched nonwoven-nonwoven GCL with granular bentonite that is commonly used in the U.S. (CETCO Bentomat DN). Figure 3 presents the granular sodium bentonite that was removed from the sample roll used for testing in the study. The minimum bentonite dry mass per unit area (ASTM D5993), index flux (ASTM D5887), and maximum hydraulic conductivity (ASTM D5887 ) based on manufacturer data for the GCL are 3600 g/m2, 1×10-8 (m3/m2)/s, and 5×10-11 m/s, respectively. The hydration solutions used in the experiments were deionized (DI) water, 2 mM CaCl2, 50 mM CaCl2 and 200 mM CaCl2. The calcium chloride solutions were made by mixing appropriate masses of American Chemical Society reagent-grade CaCl2crystals with deionized water. The 2 mM, 50 mM and 200 mM CaCl2 solutions represent typical soil pore liquid, mild landfill leachate and harsh landfill leachate, respectively (Bradshaw and Benson 2014, Tian et al. 2019).
Tests were conducted on 150 x 150 mm (5.9 x 5.9 inch) square specimens of the GCL, which were cut from the sample roll following the process presented in Figure 4. Measurements made on the GCL specimens included determination of physical characteristics such as length, width, thickness and mass before each hydration period. The edges of the specimens were wrapped in duct tape to prevent bentonite loss (Figure 5). The specimens also were taped across the mid-lengths along both sides to aid in maintaining relatively uniform hydration conditions (i.e., prevent high swelling in the middle and low swelling near the edges). Each specimen was fully submerged in a plastic tub with 2 L of hydration solution (Figure 6). The individual tubs were placed in a temperature-controlled bath (Figure 7) for the testing program. A total of 12 tubs were placed in the temperature bath (maintained at 20°C or 68°F) at a given time. The GCLs were hydrated over eight distinct periods: 4 hours, 8 hours, 1 day, 2 days, 4 days, 8 days, 16 days and 32 days.
Water content of the GCL was determined in the immediate vicinity of the locations where the GCL specimens were removed from the GCL roll for approximately every 12 specimens prior to testing. After hydration testing, water content for each specimen was determined by peeling off the cover geotextile while cutting needlepunched fibers with a razor knife and carefully removing the bentonite component of the GCLs (Figures 8 and 9). Also, swell index of the bentonite component of the GCLs was determined using DI water (ASTM D5890) for all specimens after completion of each hydration period (Figure 10). Swell index (SI) data also was obtained on the virgin GCL that was not subjected to any of the hydration fluids. Daily measurements were made on the hydration fluid and included determination of electrical conductivity (EC), total dissolved solids (TDS) and temperature. All testing and post-conditioning sampling were conducted in regions associated with fully exposed conditions (i.e., not covered by duct tape).
After each hydration period, the bound cations (BC) and cation exchange capacity (CEC) were determined for the bentonite component of the GCL specimens. BC and CEC also were determined on the virgin GCL that was not subjected to any of the hydration fluids. Soluble cations (SC) and bound cations were determined using the procedure outlined in ASTM D7503. For sampling of bentonite from the GCL specimens for these tests, the bentonite was first dried in a microwave oven to arrest any further cation exchange in a timely manner to maintain representative conditions for the given hydration period. Bentonite specimens were filtered through a 0.45-µm filter and then analyzed for concentrations of major cations (Na, Ca, Mg and K) using inductively coupled plasma/optical emission spectrophotometry (ICP-OES). Tests were conducted using SPECTROBLUE FMT36 (Mahwah, N.J.) following USEPA SW-846 Test Method 6010B (USEPA 2018). CEC was determined using the procedure outlined in ASTM D7503 (Figure 10).
The cation exchange capacity of the GCL specimens varied from 70.4 cmol+/kg (average for 2 mM CaCl2 solution) to 70.5 cmol+/kg (average for DI water). The CEC for the virgin GCL was 72 cmol+/kg and the average CEC for the 32 specimens hydrated in the variable solutions was 70.5 cmol+/kg. These values are in line with CEC reported for similar conventional GCLs in previous studies (e.g., Bradshaw and Benson 2014). Variations of bound cations with time and concentration of hydration solution are presented in Figures 11 and 12 with focus on relative fractions and time variability, respectively. The main variations in bound cations were for Na+ and Ca2+ with low variations in the relative bound fractions of K+ and Mg2+. The sodium in the bentonite is replaced increasingly with the calcium in the hydration solution with increasing CaCl2 concentration. The relative fraction of Na+ in the virgin GCL was 66%, which decreased to 9% for the GCL hydrated with the 200 mM CaCl2 solution for 32 days. The relative fraction of Ca2+ in the virgin GCL was 23%, which increased to 87% for the GCL hydrated with the 200 mM CaCl2 solution for 32 days. The amount of bound Ca2+ in the bentonite increased rapidly for the 200 mM CaCl2solution followed by the 50 mM CaCl2 solution with negligible variations over time for the DI water and 2 mM CaCl2solutions. Similarly, the amount of bound Na+ in the bentonite decreased rapidly for the 200 mM CaCl2solution followed by the 50 mM CaCl2solution with negligible variations over time for the DI water and 2 mM CaCl2solutions. For the 200 mM CaCl2 solution, the maximum decrease in the relative fraction of Na+ was 14 percentage points and occurred for 1- and 2-day tests and the maximum increase in the relative fraction of Ca2+ was 10 percentage points and occurred for 8-hour and 1-day tests. For the 50 mM CaCl2solution, the maximum decrease in the relative fraction of Na+ was also 14 percentage points and occurred for 4- and 8-hour tests, and the maximum increase in the relative fraction of Ca2+ was 12 percentage points and occurred for 2- and 4-day tests. The variation in BC reduced significantly after 4 to 8 days for the two relatively high CaCl2 solutions used in the test program.
Correlations between swell index and amount of bound Na+ and Ca2+ in the conditioned bentonite are presented in Figures 13 and 14. Swell index increased with increasing Na+ and decreasing Ca2+ concentrations (Figure 13). Swell index has been demonstrated to be inversely proportional to CaCl2solution concentration used for testing in previous studies (Scalia et al. 2014). Results of this investigation demonstrate the permanence of the modified GCL structure upon cation exchange by providing similar trends for conditioned specimens when DI water is used for the swell index tests. To attain swell index higher than 19 mL/2 g, more than 35 cmol+/kg Na+ and less than 35 cmol+/kg of Ca2+ were required in the bound cations in this test program. The variation of swell index with time was in line and character with the time-dependent variation of the BCs as presented in a double y-axis plot in Figure 14. For the two solutions with high calcium concentration (50 and 200 mM CaCl2 solutions), the timings for the significant changes in swell index and bound Na+ and Ca2+ concentrations were generally consistent, yet the timings for maximum Ca2+ and minimum Na+ fractions were not entirely consistent as individual sacrificial specimens were used in the test program.
Variation of electrical conductivity with time is presented in Figure 15. The EC was determined to be relatively unchanged during the hydration tests irrespective of the concentration of the hydration solution. Electrical conductivity as an indication of chemical equilibrium between influent and effluent is used in experimental determination of hydraulic properties of GCLs with aqueous solutions (ASTM D6766). A low threshold variability between influent and effluent EC serves as a termination criterion for GCL flux/hydraulic conductivity tests. In this test program, EC did not provide an indication of the variation of cation exchange processes and bound cation fractions in the GCL specimens, and thus the use of EC as a termination criterion may not be a proper indication of attainment of chemical equilibrium.
This investigation was conducted to systematically analyze the cation exchange characteristics of a conventional GCL as a function of time and concentration of hydration fluids at 20°C (68°F). Tests were conducted on 150 x 150 mm (5.9 x 5.9 inch) square specimens of the GCL, which was a medium-weight needlepunched nonwoven-nonwoven GCL product with granular bentonite. The GCL specimens were hydrated over eight conditioning periods: 4 hours, 8 hours, 1 day, 2 days, 4 days, 8 days, 16 days and 32 days in four hydration solutions: DI water and 2 mM, 50 mM and 200 mM CaCl2solutions.
The average cation exchange capacity of the GCL specimens was determined to be 70.5 cmol+/kg. The concentration of hydration fluid and conditioning time affected cation exchange behavior of the GCL. For the 50 mM and 200 mM CaCl2 solutions, variations were significant, where the amount of bound Ca2+ in the bentonite increased rapidly and the amount of bound Na+ in the bentonite decreased rapidly, with negligible variations over time for the DI water and 2 mM CaCl2 solutions. The majority of the exchange processes was completed within days (approximatelyless than 8 days) at these relatively high calcium chloride solution hydration/exposure conditions. Relatively low changes in bound K+ and Mg2+ concentrations were observed. Electrical conductivity of the hydration solutions was not indicative of the physicochemical processes occurring in the GCLs. The timing of cation exchange processes is critical for various applications ranging from laboratory and field testing for fluid transport characteristics of GCLs to maintaining integrity of GCL characteristics in field applications with timely cover of constructed liners, potential prehydration and/or first/repeated exposure to contained liquids. This test program provides worst-case scenario conditions for timing of exchange processes. The timing of exchange processes may be different under different exposure conditions such as different hydration solutions and hydration over/under/between different variably saturated soils. The findings from this investigation improve understanding of the fundamental behavior of GCLs in the presence of chemical solutions.
This investigation was supported by a grant from the National Science Foundation (ROA Supplement to CMMI-1812550), CETCO/Mineral Technologies Inc. and the Global Waste Research Institute.
ASTM D5887. (2022). “Standard Test Method for Measurement of Index Flux Through Saturated Geosynthetic Clay Liner Specimens Using a Flexible Wall Permeameter.” ASTM, West Conshohocken, Pa.
ASTM D5890. (2022). “Standard Test Method for Swell Index of Clay Mineral Component of Geosynthetic Clay Liners.” ASTM, West Conshohocken, Pa.
ASTM D5993. (2022). “Standard Test Method for Measuring Mass per Unit Area of Geosynthetic Clay Liners.” ASTM, West Conshohocken, Pa.
ASTM D6766. (2022). “Standard Test Method for Evaluation of Hydraulic Properties of Geosynthetic Clay Liners Permeated with Potentially Incompatible Aqueous Solutions.” ASTM, West Conshohocken, Pa.
ASTM D7503. (2022). “Standard Test Method for Measuring the Exchange Complex and Cation Exchange Capacity of Inorganic Fine-Grained Soils.” ASTM, West Conshohocken, Pa.
Benson, C. H., Albright, W., Fratta, D., Tinjum, J., Kucukkirca, I. E., Lee, S., Scalia, J., Schlicht, P., and Wang, X. (2011). “Engineered covers for waste containment: Changes in engineering properties and complications for long-term performance assessment.” NUREG/CR-7028. Washington, D.C.: Office of Research, US Nuclear Regulatory Commission.
Bradshaw, S. L., and Benson, C. H. (2014). “Effect of municipal solid waste leachate on hydraulic conductivity and exchange complex of geosynthetic clay liners,” J. Geotech. Geoenviron. Eng., 140(4): 04013038.
Bradshaw, S. L., Benson, C. H., and Scalia, J. (2013). “Hydration and cation exchange during subgrade hydration and effect on hydraulic conductivity of geosynthetic clay liners.” J. Geotech. Geoenviron. Eng. 139(4): 526–538. 10.1061/(ASCE)GT.1943-5606.0000793.
Jo, H., Benson, C. H., Shackelford, C. D., Lee, J., and Edil., T. B. (2005). “Long-term hydraulic conductivity of a non-prehydrated geosynthetic clay liner permeated with inorganic salt solutions.” J. Geotech. Geoenviron. Eng., 131(4): 405–417. 10.1061/(ASCE)1090 -0241(2005)131:4(405).
Kolstad, D. C., Benson, C. H., and Edil, T. B. (2004). “Hydraulic conductivity and swell of non-prehydrated GCLs permeated with multispecies inorganic solutions.” J. Geotech. Geoenviron. Eng., 130(12): 1236–1249. 10.1061/(ASCE)1090-0241(2004)130:12(1236).
Mitchell, J. K., and Soga, K. (2005). Fundamentals of soil behavior, 3rd ed., John Wiley & Sons, Hoboken, N.J.
NRC (National Research Council). (2007). “Assessment of the performance of engineered waste containment barriers.” NRC, Washington, D.C.
Rowe, R. K., and Abdelatty, K. (2012). “Effect of a calcium-rich soil on the performance of an overlying GCL.” J. Geotech. Geoenviron. Eng., 138(4): 423-431. 10.1061/(ASCE)GT.1943-5606.0000614.
Scalia, J., and Benson, C. H. (2011). “Hydraulic conductivity of geosynthetic clay liners exhumed from landfill final covers with composite barriers.” J. Geotech. Geoenviron. Eng., 137(1): 1–13. 10.1061/(ASCE)GT.1943-5606.0000407.
Scalia, J., Benson, C. H., Bohnhoff, G. L., Edil, T. B., and Shackelford, C. D. (2014). “Long-term hydraulic conductivity of a bentonite-polymer composite permeated with aggressive inorganic solutions.” J. Geotech. Geoenviron. Eng., 140(3): 04013025. 10.1061/(ASCE) GT.1943-5606.0001040.
Tian, K., Likos, W. J., and Benson, C. H. (2019). “Polymer elution and hydraulic conductivity of bentonite–polymer composite geosynthetic clay liners.” J. Geotech. Geoenviron. Eng., 145(10): 04019071. 10.1061/(ASCE)GT.1943-5606.0002097.
USEPA. (2018). “Test Methods for Evaluating Solid Waste: Physical/Chemical Methods, Method 6010B.” EPASW-846, USEPA, Washington, D.C.
Yesiller, N., Hanson, J. L., Risken, J. L., Benson, C. H., Abichou, T., and Darius, J. B. (2019). “Hydration fluid and field exposure effects on moisture-suction response of geosynthetic clay liners.” J. Geotech. Geoenviron. Eng., 145(4): 04019010. 10.1061/(ASCE)GT.1943-5606.0002011.
Kurt Katzenberger is a staff geotechnical engineer at ENGEO Incorporated in Oakland, Calif. He received both his bachelor’s and master’s degrees in civil engineering from California Polytechnic State University, San Luis Obispo.
All figures courtesy of the authors unless otherwise noted.