Typical containment systems for landfills include the

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1 Organically Modified Bentonite as a Part of Geosynthetic Clay Liner System Puvvadi V. Sivapullaiah 1 and Vandana Sreedharan 2 1 Professor, Department of Civil Engineering, Indian Institute of Science, Bangalore 2 Research scholar, Department of Civil Engineering, Indian Institute of Science, Bangalore Geosynthetics are versatile materials for geotechnical and geoenvironmental applications. The art, science and engineering of their use for geotechnical applications is relatively well documented. Geo membranes and Geosynthetic clay liners are increasingly used to control the migration of leachates from waste disposal facilities. While Geo membranes require well constructed clay back up and possess many limitations, geosynthetics clay liners are relatively easy to construct and performance better and also reducing the volume of lining system for disposal facilities. Geosyntheic clay will have bentonite glued to geomembrane to improve the mechanical properties, reducing permeability and improve the retention capacity for various pollutants present in the leachates. However the retention capacity of bentonite is high only with respect to inorganic ions. To improve the retention capacity of bentonite for organic pollutants the clay has to be organically modified. Thus to improve the performance of geosyntheic clay for organic contaminants, organically modified clay needs to be incorporated. The contents of bentonite and organic modified bentonite needs to chosen from the consideration of their geotechnical and sorption capacities. Typical containment systems for landfills include the combined use of geosynthetics and earthen material components. A wide variety of geosynthetic products find application in environmental protection projects. They include geomembranes, geosynthetic clay liners (GCL), geonets, geocomposites and geopipes. Of the various types of geosynthetics used, GCL liners are one of the newest and their use is rapidly expanding. Geosynthetic clay liners (GCLs) were first developed in the 1980 s in USA, and since then their demand has increased many folds. Geosynthetic clay liners represent a composite material consisting of bentonite and geosynthetics. The geosynthetics used are either geotextiles or a geomembranes. Bentonite is contained by geotextiles on both sides and the geotextiles are bonded with an adhesive by needle-punching or by stitch-bonding. The GCLs have very low hydraulic conductivity to water of the order of m/s and are relatively low cost and are of limited thickness. However due to the limited thickness of the GCLs they are vulnerable to mechanical accidents. The sorption capacity of GCLs is limited when compared to conventional clay liners. Therefore significant increase of diffusive transport is likely in the absence of any underlying attenuation mineral layer. Moreover, when hydrated with leachates other than Advantages Rapid installation/less skilled labor /low cost Very low hydraulic conductivity to water Can withstand large differential settlement Excellent self healing characteristics Mot dependent on availability of local soils Easy to repair Resistant to freeze/thaw cycles More air space due to lower thickness No field hydraulic conductivity testing required Hydrated GCL is an effective gas barrier Reduces overburden pressure Table 1. Advantages and disadvantages of GCLs (modified from Bouzza,1997) Disadvantages Low shear strength of hydrated bentonite Can be punctured during and after puncture Possible loss of bentonite during installation Low moisture bentonite is permeable to gas Strength problem at interfaces Smaller leachate attenuation capacity Post-peak strength is small Higher long term flux due to lower thickness Compatibility problem with contaminants Higher diffusive flux of contaminants in comparison with compacted clay Prone to ion Exchange and desiccation 128 The Masterbuilder - October

2 water, bentonite will show a limited swelling which results in reduced efficiency of the hydraulic barrier. The Table 1 summarizes the major advantages and disadvantages of GCLs. Extensive investigations on the hydraulic and diffusion characteristics, chemical compatibility, mechanical behaviour, and durability and gas migration of GCL have been taken up by many researchers. (Bouazza et al. 1996; Petrov et al. 1997a, b; Lake & Rowe 2000; Shackelford et al. 2000) The hydraulic performance of GCLs is largely controlled by hydraulic conductivity of the bentonite. The hydraulic conductivities of GCLs with water vary between m/s and 10-10m/s, depending on applied confining stress. A reduction in GCL hydraulic conductivity observed with increasing confining stress is due to lower bulk void ratios resulting from higher confining stresses as in Figure 1. (Petrov et al a). Figure 1. Variation of hydraulic conductivity versus confining stress after (Bouazza 2002) As GCLs are often used to contain liquids other than water, the evaluation of hydraulic conductivity of GCLs when acted upon by chemical solutions is important. Hence compatibility tests are performed to assess the hydraulic conductivity to the actual permeant liquid. GCL compatibility with various permeants has been well researched by a number of researchers (Petrov et al. 1997a, b; Petrov & Rowe 1997; Rowe 1998). The main factors pertaining to the GCL that influence the hydraulic conductivity with liquids other than water; itemized as aggregate size, montmorillonite content, thickness of adsorbed layer, pre hydration and void ratio of the mineral component. The major factor related to the pernmeant which influence the hydraulic conductivity of GCL is the concentration of monovalent and divalent cations. The research carried out (Shackelford et al. 2000) has provided with an improved understanding on the importance of the clay component of the GCL. And this has lead to the improvement in the performance of GCLs. A significant effect on both the short- and long-term performance of the GCL in the liner system is predictable on small changes within bentonite mineralogy, clay chemistry and particle size. Bentonite in GCLS Bentonite is a rock type generally derived from the chemical alteration of volcanic ash previously deposited in shallow seas. They find a lot of industrial applications because of their fine particle size, surface area, layer charge and swelling capacity. Bentonites with high smectite content are widely used for lining/containment applications owing to their higher sorption capacity for inorganic contaminants along with better hydraulic performance. Montmorillonite is a swelling layered silicate composed of a sheet of octahedrally coordinated cations bound on two sides by sheets of tetrahedrally coordinated cations. Each combination of two tetrahedral sheets and one octahedral sheet makes a crystallite layer and two adjacent crystallites are separated by a largely water-filled space called the interlayer. Hydrated cations occupy the interlayer space of montmorillonite to neutralize the layer charge due to substitutions in the tetrahedral and octahedral sheets. Crystalline swelling in montmorillonites results due to gain and loss of water by these cations. The impurity content of commercial bentonite used in GCL manufacture is to be leimited to less than30% by mass. However research by Benson et al. (2010) and Gates and Bouazza (2009) has showed that the addition of certain non-swelling impurities can improve the performance of a GCL due to formation of secondary mineral phase owing to the reaction with high ph solution and thus can clog pores within the bentonite The other aspects of the bentonite which are important are fundamental particle size of the smectite and the relative differences in their sizes. Two types of bentonites are predominantly used in GCL manufacture viz., powdered and granular. In the case of GCLs with powdered bentonite water slowly wets the entire bentonite layer forming a thin uniform layer of hydrated bentonite particles and these results in an effective seal against advective water movement. While in GCLs with granular bentonite water penetrates the full thickness of the bentonite layer and wets the external surfaces of the granules. Particles within the granules are wet more slowly from the pores. Initial advective flow is higher in the case of GCLs with granular bentonite. Figure 2 Shows the Hydration Powdered Bentonite Granular Bentonite Figure 2. Hydration in GCLs with powdered and Granular bentonite (after Gates and Benson 2009) The Masterbuilder - October

3 in GCLs with powdered and Granular bentonite. The range of values for different properties of bentonite used in GCL is given in Table 2 Property Range or value Montmorillonite content >70% Octahedral Mg content atoms per unit cell(2-4% by eight) Octahedral Fe content atoms per unit cell(3-5 %by weight ) Layer charge <0.85 e- per cell Layer charge location Predominantly (>50%) Cation exchange capacity <110meq/100g Exchangeable cation Sodium Table 2. The range of values desirable for different properties for bentonite used in GCL (after Gates and Benson 2009) Parameters Controlling the Hydraulic Conductivity of Bentonite in GCL The hydraulic conductivity of bentonite and that of GCL are controlled by the following four major chemical-interaction parameters 1) Dielectric constant of permeating liquid, 2) Salt concentration of the permeating liquid, 3) Predominant cation of the bentonite vs. those in the permeating liquid, and 4) ph of the permeating liquid Dielectric constant of the permeating liquid. The lower the dielectric constant of the permeating liquid, lesser is the swelling of the bentonite and hence higher is its hydraulic conductivity. Therefore all organic liquids which have a much lower dielectric constant than water, therefore can cause potentially large increases in the hydraulic conductivity of bentonite. Dilute organics however, do not impede swelling in bentonite, and hence do not increase its hydraulic conductivity. Salt concentration of the permeating liquid. High concentrations of salts in the permeating liquid impart a negative effect on the hydraulic conductivity. When concentrations of salts are high at 1000ppm or more, they become large enough to cause concern. For concentrations less than about 500 ppm, it is the type of salt rather than the concentration that is critical. The cations The charge of the cation present in the bentonite relative to the cations in the permeating liquid plays a major role in deciding the hydraulic conductivity. The major cations found in GCLs are Na+, K+, Ca++, Mg++, and Al+++. The bentonite becomes more permeable with catons of higher positive charges. Thus, the bentonite with Na+ cations is the most beneficial one. The least favorable cations are the polyvalent cations, which have a charge of +2 or more. Calcium cations tend to produce the most significant adverse effects on bentonite swelling and its sealing capacity. ph of the permeating liquid The ph of the permeating liquid can also affect the hydraulic conductivity of bentonite. Extremely acidic or caustic liquids may be aggressive and may dissolve some of the bentonite clay and dramatically increase hydraulic conductivity. Tests for Bentonite in GCLs Earlier studies have shown the effect of pore fluid chemistry on the soil properties, such as permeability and shear strength. Dielectric constant and fluid viscosity have been found to have the major influence on clay behavior. The variation in the chemical composition of the montmorillonite together with crystalline size and shape are found to significantly influence the functional properties of the bentonite. Therefore a broad range of chemical, mineralogical and functional properties of bentonite used in GCL are to be evaluated to ensure their performance. The common tests recommended are: - Fluid loss ASTMD Swell index ASTMD Hydraulic conductivity ASTM D Plate water absorption ASTME Methylene blue CEC - Soluble Calcium - Soluble Magnesium - Leachable Ca++& CO3- - Leachable Mg++ - ph - Conductivity for TSD - Moisture - Loss on ignition mesh non dispersible minerals Use of Modified Bentonite in GCLs Numerous studies have examined the applicability and chemical compatibility of bentonites and GCLs, and have shown that the type and concentration of chemicals affect the hydraulic conductivity. It has been reported that the hydraulic conductivity value increases as the concentration of the electrolytic solution increases (Jo et al., 2001; Katsumi et al., 2007; Kolstad et al., 2004; Petrov and 130 The Masterbuilder - October

4 Rowe, 1997; Shackelford et al., 2000). The use of chemical resistant bentonite is reported as a considerable measure against the action of aggressive electrolytic solutions. Several types of modified bentonite materials have been developed to improve the chemical incompatibility of natural bentonite (Kolstad et al., 2004b; Lo et al., 1994, 1997; Onikata et al., 1996, 1999a, b; Gates et al., 2004;). The use of multi swellable bentonite and prehydrated GCLs and their effect when hydraulic conductivity values when -prehydrated geosynthetic clay liner (DPH-GCL) permeated with NaCl and/or CaCl2 permeant solutions were investigated by Katsumi et al.(2008). Onikata et al. (1996) discovered that propylene carbonate (PC) can be utilized as a swelling activation material which can exhibit sufficient swelling toward electrolytic chemical solutions and fresh water (Katsumi et al., 2004, Onikata et al., 1996, 1999a; Shackelford et al., 2000). Another effective method to enhance the chemical resistance of GCL is reported as to hydrate bentonite component before exposing to an electrolytic solution. A further method is to use consolidate bentonite as they exhibit lower hydraulic conductivity than unconsolidated bentonite (Katsumi and Fukagawa, 2005). Although GCLs are able to minimize the advective movement of contaminants the transport due to molecular diffusion can be a significant transport mechanism (Lake and Rowe, 2000, 2004). The mass flux of organic contaminants across composite of compacted clay and geomembrane liners by diffusion is reported to be significant. Since GCLs have an even shorter diffusive path than CCLs, the diffusion is a significant source of solute transport through GCLs as well. The inability of GCLs to impede movement of organic contaminants due to diffusive transport is of importance as GCLs are often used as barrier systems in applications where they may come into direct contact with organic solutes including landfills, barriers for highway construction and as temporary barriers for accidental spills. One method of minimizing the flux of organic pollutants through GCLs is by enhancing organic pollutant sorption by amending the bentonite layer with a material capable of strongly sorbing organic pollutants. cations can be incorporated in the GCLs and this will result in minimizing the flux of organic compounds by increasing sorption. However the huge cost of making OMB limits its large scale application. Laboratory diffusion and hydraulic conductivity testing were performed on GCLs amended with two different types of organo bentonites: and compared to the results for a GCL constructed with conventional sodium bentonite by Lorenzetti et al (2005). The results from the hydraulic conductivity testing have pointed out that the addition of organo bentonite result in an increase in hydraulic conductivity. Only a small increase in hydraulic conductivity was observed for both types of amended GCLs up to 20% organo bentonite amendment. At higher organo bentonite contents, measured hydraulic conductivity for both types of amended GCLs was higher by three orders of magnitude. The effective diffusion coefficient of [3H] water was measured for both the organo bentonite-amended GCLs and a laboratory-constructed sodium bentonite GCL were similar. One dimensional benzene solute transport is reported to significantly reduce with organo bentonite when compared to the conventional GCL. Geotechnical Evaluation of Bentonite and Modified Bentonite It is well established that the main requirements of clay liners are to ensure the minimization of pollutant migration over the long-term, low swelling and shrinkage, and resistance to shearing. (Brandl, 1992; Kayabal, 1997.). Sealing capacity of the clay depends on its swelling capacity. The swelling capacity of mixtures of bentonite and organic bentonite in different fluids, as assessed by free swell index, is presented in Figure 3. It is seen that the modified bentonite cannot have significant swelling with water or Organically Modified Bentonites (OMB) Long and short chain quaternary alkyl ammonium compounds are the generally adopted modifiers for the preparation of organo clays. The efficiency and mechanism of sorption of organo clay are largely dependent on the characteristics of the organically modified clay. Exhaustive studies have suggested that the characteristics of organo clays of different origin vary considerably and depend on bentonite and organic molecules used for modification. Organophilic bentonite produced by exchanging some of the naturally occurring sodium or calcium ions on the internal and external surfaces of the bentonite with organic Figure 3. Variation of swell index of bentonite-omb (Vandana and Sivapullaiah 2012) 132 The Masterbuilder - October

5 fluids of high dielectric constant as they are hydrophobic in nature. On the other hand modified clay has affinity for organic molecules. The adsorption of organic molecules and subsequent bonding by modified clay can alter their fabric and thus can very often exhibit swelling nature. The higher the ability of the organo clay to interact with any fluid the higher is the swelling with those organic fluids. The lower the dielectric constant of the fluid, the greater is the tendency of the organic molecules to be adsorbed by the organo clay. Thus the free swell of organo clay generally increases with decrease in dielectric constant, but there is no one to one correspondence because of differences in the type of bonding of the organic molecules with the clay. Sorption of Organic Contaminants by Bentonite and Organic Contaminants The influence of organic modification on the properties on the organic sorption of a bentonite modified with alkyl ammonium compound has been studied by Vandana and Sivapullaiah (2012). The potential of the organically modified clay for sorption of organic pollutant increased with increase in the concentration of organic pollutants in the leachate and can be as high as 0.9 mg/g of clay. Figure 4. Effect of organic modification on the adsorption capacity of bentonite The degree of adsorption of organic compound to the mineral surface can be estimated by the linear adsorption isotherms. When organic contaminants are sorbed on to the organo clay, it can partition into the organic phase attached to the clay. The sorption capacty is determined by the properties of the contaminant as well as by the properties of the organo clay it is introduced to. The adsorption of hydrophobic organic phase on clay is governed by the partitioning mechanism resulting in a higher adsorption for contaminants with higher octanol/ water partitioning coefficients (Kow). (Chang et al. 2009). Several studies have found that linear relations hold for adsorption of organic compounds to the soil matrix (Ma et al., 2007). Linear sorption isotherms have been derived experimentally and used to estimate the amount of adsorbed organic compounds. Conclusions - Mixture of bentonite and organically modified performs better than either of them as a component of Geosynthetic clay liner system when the pore fluid contains a pure organic phase. - The swell of mixture containing 50-70% of bentonite and OMB is maximum in organic fluids and hence can reduce the leachate permeability better. - Sorption of organic pollutants increases linearly with initial concentration of contaminant for any bentonite. - The adsorption of organic pollutants is far higher for OMB than bentonite. References - Benson, C.H., Oren A.H. and Gates W.P. (2010). Hydraulic conductivity of two geosynthetic clay liners permeated with a hyperalkalie solution. Geotextiles and Geomembranes, 28, Bouazza, A., and Bowders, J. (2009). GCLs in Waste Containment Applications. Taylor Francis, U.K. - Bouazza, A., Van Impe, W.F., Van Den Broeck, M. (1996). Hydraulic conductivity of a geosynthetic clay liner under various conditions. Proceedings of the Second International Congress on Environmental Geotechnics, Vol. 1, Osaka, Japan Brandl (1992). Mineral liners for hazardous waste containment, Geotechnique, 42, Chang, P. H., Li, Z., Jiang, W.-T., and Jean, J. S. (2009). Adsorption and intercalation of tetracycline by swelling clay minerals. Applied Clay Science, 46, Gates, W. P., Hornsey, W. P., and Buckely, J. L. (2009). Geosynthetic Clay Liners - Is the key component being overlooked GIGSA GeoAfrica 2009 Conference Cape Town, Gates, W.P., (2004). Crystalline swelling of organo-modi?ed clays in ethanol-water solutions. Applied Clay Science 27, Jo, H. Y., Katsumi, T., Benson, C.H. & Edil, T.B. (2001). Hydraulic conductivity and swelling of nonprehydrated GCLs permeated with single species salt solutions. Journal of Geotechnical and Geoenvironmental Engineering 127(7), Katsumi, T., Fukagawa, R., (2005). Factors affecting chemical compatibility and barrier performance of GCLs. In: Proceedings of the 16th International Conference on Soil Mechanics and Geotechnical Engineering. Millpress Science Publishers, Rotterdam, Netherlands, ; - Katsumi, T., Ishimori, H., Onikata, M., and Fukagawa, R. (2008). Longterm barrier performance of modified bentonite materials against sodium and calcium permeant solutions. Geotextiles and Geomembrnes, 26(1), The Masterbuilder - October

6 - Kayabali, K. (1997). Engineering aspects of a novel landfill liner material: bentonite amended natural zeolite. Engineering Geology, 46, Kolstad, D. C., Benson, C. H. and Edil, T. B. (2004). Hydraulic conductivity and swell of nonprehydrated geosynthetic clay liners permeated with multispecies inorganic solutions, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 130, Lake, C. B., and Rowe, R. K. (2004). Volatile organic compound diffusion and sorption coefficients for a needle punched GCL, Geosynthetics International (special issue on GCLs), 11, No. 4, Lake, C.B., Rowe, R.K., (2000). Diffusion of sodium and chloride through geosynthetic clay liners. Geotextiles and Geomembranes, 18, Lo, I. M. C., Liljestrand, H. M., and Daniel, E. D. (1994). Hydraulic conductivity and adsorption parameters for pollutant transport through montmorillonite and modi?ed montmorillonite clay liner materials. ASTM Special Technical Publication, Issue 1142, ASTM, West Conshohocken, Pa., Lo, I.M. C., Mak, R.K. M., Lee, S.C.-H., (1997). Modi?ed clays for waste containment and pollutant attenuation. Journal of Environmental Engineering 123 (1), L Ma, C., Wu, Y., Sun, C., and Lee, L. (2007). Adsorption characteristics of perchloroethylene in natural sandy materials with low organic carbon content. Environmental Geology, 52, Onikata, M., Kondo, M., Hayashi, N., Yamanaka, S., (1999). Complex formation of cation-exchanged montmorillonites with propylene carbonate: osmotic swelling in aqueous electrolyte solutions. Clays and Clay Minerals 47, Onikata, M., Kondo, M., Kamon, M., (1996). Development and characterization of multi swellable bentonite. In: Environmental Geotechnics, A.A. Balkema, Rotterdam, The Netherlands, Petrov, R.J., and Rowe, R.K. (1997a). GCL-chemical compatibility by hydraulic conductivity testing and factors impacting its performance. Canadian Geotechnical Journal, 34(6), Petrov, R.J., Rowe, R.K. and Quigley, R.M. (1997b). Selected factors influencing GCL hydraulic conductivity. Journal of Geotechnical and Geoenvironmental Engineering 123(8), Petrov, R.J., Rowe, R.K. and Quigley, R.M. (1997c). Comparison of laboratory measured GCL hydraulic conductivity based on three permeameter types. Geotechnical Testing Journal, 20 1, Rowe, R.K (1998). Geosynthetics and the minimization of contaminant migration through barrier systems beneath solid waste, Proceedings 6th International Conference on Geosynthetics, Atlanta 1, Schackelford, C.D., Benson, C.H., Katsumi, T., Edil, T.B. and Lin, L. (2000). Evaluating the hydraulic conductivity of GCLs permeated with nonstandard liquids. Geotextiles and Geomembranes 18, Vandana S. and Sivapullaiah P. V. (2012). Physico chemical and geotechnical evaluation of organically modified bentonite to contain organic contaminants, clay minerals (under review) Publisher s Note: This paper was presented during the One day conference on Geosynthetic Lining Solutions and Related Issues by ASCE IS SR in association with Department of Civil Engineering, Indian Institute of Science, Bengaluru, Indian Chapter of International Geosynthetic Society, New Delhi, Karnataka Geotechnical Center of Indian Geotechnical Cociety, Bengaluru, The Masterbuilder at IISc, Bengaluru, Karnataka on 25th February The Masterbuilder - October