Hydraulic conductivity and swelling ability of a polymer modified bentonite subjected to wet–dry cycles in seawater
Introduction
Geosynthetic clay liners (GCLs) are increasingly used for waste disposal facilities and they have been subject of numerous investigations (Shackelford et al., 2000, Egloffstein, 2001, Bouazza, 2002, Lee and Shackelford, 2005, Jo et al., 2005, Chevrier et al., 2012, Ishimori and Katsumi, 2012). GCLs are employed to isolate waste disposals from the environment, to avoid infiltration of water to the waste and to prevent the release of leachate. GCLs are factory-manufactured hydraulic barriers containing a thin uniform layer of sodium bentonite sandwiched between two geotextiles or glued to a geomembrane. Bouazza (2002) described the advantages of GCLs with respect to compacted clay bottom and cover lining and explained why GCLs are nowadays considered an efficient alternative. The main advantages include the very low hydraulic conductivity to water, excellent self-healing capacity, capability of withstanding differential settlement, limited thickness and cost effectiveness. On the other hand, the bentonite is sensitive to chemical interactions in presence of liquids other than water. Sodium ions present between the clay platelets usually confer low hydraulic conductivity to the clay. However, exposure to leachate or to electrolyte solutions can cause a loss of efficiency of the clay as hydraulic barrier, which could increase the vulnerability of the environment.
Several laboratory studies focused on the hydraulic conductivity of bentonites in contact with various permeant liquids (Ruhl and Daniel, 1997, Shackelford et al., 2000, Bouazza, 2002, Jo et al., 2005, Katsumi et al., 2008, Rosin-Paumier et al., 2011, Barral et al., 2012, Chun-Ming et al., 2013, Hosney and Rowe, 2014, Bradshaw et al., 2016). For instance, Jo et al. (2005) conducted long-term hydraulic conductivity tests on GCLs permeated with inorganic single-species salt solutions with CaCl2 concentrations ranging from 5 mM to 500 mM and 100 mM for NaCl and KCl solutions. The hydraulic conductivity obtained with NaCl and KCl were comparable to those obtained with deionized water, even considering the different hydrated radii of Na+ and K+. However, the hydraulic conductivity increased of about 3 orders of magnitude with the 100 mM of the divalent solution. Strong Ca2+ concentrated solutions (≥50 mM) led to a rapid exchange for Na+, resulting in high hydraulic conductivity (≥10−8 cm/s). Katsumi et al. (2008) performed hydraulic conductivity tests on granular sodium bentonite with 1 M NaCl solution and the permeability increased two orders of magnitude (>10−9 m/s). The increase of salinity in the infiltrating solution considerably affects both swelling and permeability, as was also demonstrated by Chun-Ming et al. (2013).
The service life of a GCL cover can also be affected by other factors like environmental loading. Heat waves, seasonal rainfall and groundwater migration may negatively affect the hydraulic performance of the liners subjected to wet–dry aging. Bouazza et al. (2014) quantified heat and moisture migration in a geomembrane-GCL composite liner overlying a compacted subgrade subjected to high temperatures and low vertical stresses. Continuous elevated temperature on the liner, up to 70 °C, influenced the temperature profile for around 200 mm below the GCL cover liner. Although no cracks were detected, moisture migration from the GCL to the subgrade material was observed. In general, the consequences of wet–dry cycles are a dramatic increase of permeability and loss of self-healing capacity, due to the combination of ion exchange and simultaneous desiccation (Egloffstein, 2001). In particular, desiccation may damage the barrier layer forming crack networks in the bentonite and gaps between GCLs panels. In such a scenario, size, distribution and connectivity of cracks govern the flow of solutes in the soil (Tang et al., 2011).
Several investigations have been undertaken to evaluate the effect of wet and dry cycles combined with cation exchange on hydraulic conductivity and swelling ability of clay barriers (Hewitt and Philip, 1999; Lin and Benson, 2000, Bouazza et al., 2006, Bouazza et al., 2007, Thiel et al., 2006, Komine et al., 2009, Rowe et al., 2011, Tang et al., 2011, De Camillis et al., 2014, Mukunoki et al., 2014, He et al., 2015).
Amended clays have been developed to improve their resistance in aggressive environments. Several studies have proposed new alternatives (Kondo, 1996, Katsumi et al., 2007, Katsumi et al., 2008, Mazzieri et al., 2009, Di Emidio, 2010, Di Emidio et al., 2012, Malusis and McKeehan, 2013, Scalia et al., 2013, Malusis and Di Emidio, 2014, Di Emidio et al., 2015, Mazzieri and Di Emidio, 2015, Razakamanantsoa et al., 2016). Kondo (1996) developed Multiswellable Bentonite (MSB), which is a bentonite compounded with Propylene Carbonate (PC). The PC is able to activate the osmotic swelling capacity of the clay in both fresh water and electrolyte solutions. Another manufactured patented GCL is the Dense PreHydrated GCL (DPH GCL). This material is densified by vacuum extrusion after prehydration with a polymeric solution containing Na-CMC, sodium polyacrylate and methanol. Katsumi et al. (2008) reported long-term permeability tests on MSB and DPH GCL. The results have shown appreciable resistance to electrolyte solution of both GCLs. The main issue of these modified bentonites is that the polymer adsorption onto the clay might not be permanent (Mazzieri and Pasqualini, 2006, Di Emidio, 2010).
In this study the HYPER clay technology is investigated. HYPER clay is a polymer-treated bentonite created by combining natural Na-bentonite with carboxymethyl cellulose (CMC). Once the CMC intercalates the clay platelets, the diffuse double layer is maintained open even in presence of factors that generally produce the collapse of the interlayer (Di Emidio, 2010). A feature of this bentonite is the irreversible adsorption of the polymer into the clay, following the HYPER clay treatment, due to a dehydration step. Moreover, the intercalation of the polymer in the interlayer region of the clay was demonstrated.
The objective of this research is to evaluate the performance of the innovative polymer-amended HYPER clay subjected to wet–dry aging in contact with seawater. The results have implications on the barrier resistance of HYPER clay against wet and dry cycles combined with highly concentrated solution, e.g. seawater.
Section snippets
Properties of sodium bentonite
Bentonite is a naturally occurring clay widely used in GCLs as the low-permeability element. It can be mainly sodium or calcium bentonite, depending on the dominant exchangeable cation. The quality of bentonite is related to the montmorillonite content, the surface area, the surface charge deficiency, and the composition of the exchange complex (Shackelford et al., 2000). Sodium montmorillonite is part of the smectite family characterized by a high specific surface area, weak interlayer bonds
Bentonites
Basic and treated bentonites have been used in this research. The tested base bentonite used is sodium bentonite (NaB), composed of 91% smectite-mica, 4% quartz-opal and 2% feldspars. Table 1 illustrates an overview of the untreated sodium bentonite characterization. HYPER clay, polymer treated bentonite, was produced according to the procedure proposed by Di Emidio (2010). The principle of HYPER clay is to combine powdered Na-bentonite (NaB) with an anionic polymer (sodium carboxymethyl
Swell index tests
Swell index tests were performed for quality assessment, as mentioned in the Geosynthetic Research Institute (GRI) GCL-3 (Koerner and Koerner, 2005), which describes proper test methods and minimum required values. The followed testing method was in accordance with ASTM D5890 (2006). Swell index tests were carried out to study the effect of polymer treatment on the swelling capacity of the clays analyzed. The bentonite was prepared as mentioned in the section above. Specimens of 2 g of NaB,
Swell index
The swelling ability of treated and untreated clays was quantified by means of the standard swell index test. Swell index (SI) values of Na-bentonite, HYPER clay 2% and 8% in deionized water are given in Fig. 1.
As it can be seen in Fig. 1, the treatment with the anionic polymer improved the swelling ability of the untreated clay. Results also showed that the swell index increased with increasing polymer dosage. The SI of HC + 8% was more than two times the swell index of NaB (52 mL/2 g versus
Conclusions
The combination of cation exchange and wet and dry aging process was analyzed by means of swell index, free swell and hydraulic conductivity tests. The purpose was to evaluate the effects of wet–dry cycles on untreated sodium bentonite, HYPER clay 2% and HYPER clay 8% in contact with a strong electrolyte solution, such as seawater.
The swell index showed that the water adsorption and swelling ability increase as the polymer content increases.
Similar results were obtained from free swell tests.
Practical implication
Lin and Benson (2000) performed wet and dry cycles on a needled-punched GCL containing loose granular Na-bentonite. Three different permeant liquids, DI water, tap water, and 0.0125-M CaCl2 solution were used in their study.
The GCL specimen initially permeated with DI and then with CaCl2 maintained low hydraulic conductivity through 6 wet–dry cycles. In particular, the hydraulic conductivity was 1.0 × 10−11 m/s at the end of the fourth cycle. Conversely the hydraulic conductivity of untreated
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