Cation exchange processes in GCLs

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

Materials

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). 

Methods

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).

Results

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.

Conclusions

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.

Acknowledgments

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.  

References

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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.