Geosynthetic clay liners against inorganic chemical solutions

Transcription

1 Geosynthetic clay liners against inorganic chemical solutions Takeshi Katsumi 1, Atsushi Ogawa, Shugo Numata 3, Craig H. Benson 4, Dale C. Kolstad 5, Ho-Young Jo 6, Tuncer B. Edil 4, and Ryoichi Fukagawa 7 1 Associate Professor, Graduate School of Global Environmental Studies, Kyoto University, Sakyo, Kyoto, Japan; Formerly, Associate Professor, Department of Civil Engineering, Ritsumeikan University, Japan. Graduate Student, Department of Civil Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan. 3 Kuwaharagumi Co., Ltd., Shiga, Japan; Formerly, Undergraduate Student, Ritsumeikan University, Japan. 4 Professor, Dept. of Civil and Environ. Engrg., University of Wisconsin-Madison, Madison, Wisconsin, USA. 5 Barr Engineering, Minneapolis, Minnesota, USA; Formerly, Graduate Student, University of Wisconsin-Madison, Madison, Wisconsin, USA. 6 Graduate Student, University of Wisconsin-Madison, Madison, Wisconsin, USA. 7 Professor, Department of Civil Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan. ABSTRACT: Free swell and hydraulic conductivity tests were conducted on geotextile-sandwiched geosynthetic clay liners (GCLs) with and LiCl-CaCl solutions to evaluate their chemical compatibility and applicability to landfill liners. Flexible wall permeameters were used for the hydraulic conductivity tests. Main conclusions derived from the experimental results are: (1) the hydraulic conductivity of GCLs were correlated to the free swell power of ground bentonites, () powdered bentonites in GCL are more compatible than granular bentonites against chemicals, in particular aggressive (poly-valent and high concentration) solutions, and (3) divalent cations have more effects on the performance of GCLs than monovalent cations. 1 INTRODUCTION Geosynthetic clay liners (GCLs) provide a barrier layer in either cover or bottom liner systems of waste containment facilities, and can be a component of final covers applied for the remediation of contaminated sites. The performence of GCLs is attributed to the extremely low hydraulic conductivity provided by bentonite (<10-8 cm/s with water) and the high containment capacity due to the high swelling capacity. GCLs are made of factory-manufactured prefabricated barrier material, either bentonite granules sandwiched between two bentonite + adhesive Adhesive-bond clay to upper and lower Bentonite Stitch-Bonded clay between upper and lower Needle punched Bentonite fibers throughout Needle-punched clay through upper and lower bentonite + adhesive Adhesive-bond clay to a GM Stitch bonded in rows GM Figure 1. Cross sections of currently available geosynthetic clay liners (GCLs) geotextiles (woven or nonwoven) or bentonite glued to a geomembrane. The advantages of GCLs are that they are very thin (usually 5-10 mm thick) and less quality control is needed during their construction. Cross sections of typical GCLs are shown in Figure 1. Rowe (1998) and Bouazza (00) provided comprehensive state-of-the-art reviews on GCLs, and issues to be discussed include the following: Mechanical stability when they are used in side slopes Gas permeability when they are used in covers Durability against dry-wetting and freezing-thawing Punctures, bentonite thinning, and internal erosion Chemical compatibility Equivalency to other liner materials such as CCLs and geomembranes Chemical compatibility is presented in this paper among the listed issues. It is well known that bentonite does not swell against inorganic solutions or nonpolar liquids (Norrish and Quirk 1954). Therefore, when bentonite is used as a liner material, its chemical compatibility should be investigated according to the given conditions in the field. Recent studies have discussed the chemical compatibility and applicability of GCLs, but most of them are merely based on the experimental study on single chemical species (e.g., Daniel et al. 1993,

2 Petrov and Rowe 1997, Ruhl and Daniel 1997, Shackelford et al. 000, Jo et al. 001, Katsumi et al. 001). In this paper, free swell and hydraulic conductivity tests were conducted on geotextile-sandwiched geosynthetic clay liners (GCLs) with multi-species chemical solutions. GCLs having powdered bentonites and solutions were used for the experiments at Ritsumeikan University, and were compared with the results on GCLs having granular bentonites permeated with LiCl-CaCl solutions conducted at University of Wisconsin-Madison by Kolstad (000). BENTONITE-CHEMICAL INTERACTION Low hydraulic conductivity of GCLs is attributed to the high swelling of bentonites. Smectite group minerals, which are the main component in bentonite, exhibit a high swelling capacity and can provide an extremely low level of hydraulic conductivity against pure water. Many researchers have reported that, even though the hydraulic conductivity is low when the clays are permeated with pure water, permeation with chemical solutions will result in an increase (sometimes significant) in the hydraulic conductivity. These effects were summarized by Mitchell and Madsen (1987), Shackelford (1994), and Shackelford et al. (000), and categorized into (1) the dissolution of clay particles and chemical compounds, () the development of a diffuse double layer (DDL), and (3) the osmotic swelling of smectite clay. The dissolution of soil components results from strong acids and bases. Strong acids promote the dissolution of carbonates, iron oxides, and the alumina octahedral layers of clay minerals. Bases promote the dissolving of the silica tetrahedral layers. These effects can result in an increase in the hydraulic conductivity, although the reprecipitation of dissolved compounds might clog the pores and decrease the hydraulic conductivity (Mitchell and Madsen 1987). Since the water molecules in a diffuse double layer (DDL) are strongly attracted, they do not contribute to the water flow, and only molecules outside of the DDL can form the flow channel. Thus, the more DDLs develop, the lower the hydraulic conductivity becomes. If the permeant contains cations, the negative charge will attract these cations and fewer water molecules. Therefore, the existence of cations results in fewer DDLs being developed. Polyvalent cations promote the development of fewer DDLs rather than monovalent cations. Higher cation concentrations will also result in the development of fewer DDLs. The development of a DDLs is also affected by the polarity and the dielectric constant of pore fluids. Mesri and Olson (1971) conducted a series of consolidation tests on kaolinite, illite, and smectite, with several fluids, to investigate the effect of the volume and the shape of voids and the nature of fluids on the hydraulic conductivity. They showed that, when these clays are consolidated with nonpolar fluids (benzen and carbon tetrachloride) instead of water, the hydraulic conductivity is simply dependent on the void ratio regardless of the type of clay. Since these clays differ in particle size, namely, largest for kaolinite and smallest for smectite, the shapes of the flow channels should also be different from clay to clay. Nevertheless, all three types of clay yield the same void ratio versus hydraulic conductivity relationship as the nonpolar fluids. This is because nonpolar fluids do not form a DDL. Mesri and Olson (1971) also showed that the hydraulic conductivity is highest for nonpolar fluids, smaller for polar fluids of low dielectric constant (ethyl alcohol and methyl alcohol), and lowest for water, which is polar and has a high dielectric constant (~80). These results can be explained by the DDL theory. The permeation of pure organic liquids is not likely to occur in practice when clay liners are applied. Several researchers have reported that no significant increase in hydraulic conductivity occurred when the concentration of organic chemicals was lower than 50% (e.g., Bowders and Daniel 1987; Petrov et al. 1997). This is because dilution with water leads to enough of an increase in the dielectric constant to develop a DDL. Osmotic swelling is an important phenomenon for smectite clay. When dry smectite is hydrated, water molecules are strongly attracted to the internal and the external clay surfaces during the hydration phase. After this hydration process, if the exchange cations of the smectite clay are monovalent, the region of the interlayer may retain numerous layers of water molecules during the osmotic phase (Norrish and Quirk 1954). This condition can be observed as a significant amount of swelling; it is typically observed when Na-bentonites are hydrated with deionized water. When the smectite is monovalent at the exchange site and is permeated with the solution containing a low concentration of monovalent cations, a large fraction of water is attracted to the clays, less mobile water is available for the water flow, and the hydraulic conductivity is

3 low. When polyvalent cations exist at the exchange sites, the osmotic phase does not occur and less swelling is observed. 3 EXPERIMENTAL PROCEDURE Free swell and hydraulic conductivity tests were conducted on geotextile-sandwiched GCLs. Experimental procedures conducted at Ritsumeikan University are explained mainly herein, while procedures at University of Wisconsin-Madison are similar. The GCLs used at Ritsumeikan University and the University of Wisconsin in this study are from the same manufacturer but the different products. They contain natural sodium bentonite (Na-bentonite) encapsulated between a slit-film monofilament woven geotextile and a staple-fiber non-woven geotextile. Probably, the bentonites used at both laboratories have quite similar properties as discussed in the next chapter. However, the bentonites in GCL used at Ritsumeikan University are powered, while those at the University of Wisconsin are glanular. The geotextiles are bonded by needle-punched fibers that have been thermally fused to the woven geotextile. The initial thickness of the GCL ranged from 5.5 to 6.5 mm and the average initial gravimetric water content of the bentonite was 9-11%. The GCL has a mass of bentonite per area of approximately 5.0 kg/m. Chemical solutions used are solutions (Ritsumeikan University) and LiCl-CaCl solutions (University of Wisconsin). The solutions were set to have different values of ionic strength (I) and ratio of monovalent to divalent (RMD). Here, ionic strength and RMD are defined as follows: I =.5 c i z (1) 0 i c 1 RMD = () c where c i is the concentration of ith ion, z i is the valence of ith ion, c 1 is the concentration of monovalent cation, and c is the concentration of divalent cation. In this study, only the cations are considered to calculate the ionic strengths. Free swell tests were conducted according to ASTM D 5890 on -g ground dry bentonites either for powered and granular ones. Falling head hydraulic conductivity tests with constant tail water elevation were conducted on the GCL using flexible-wall permeameters. This test was performed in accordance to ASTM D An hydraulic gradient were and cell pressure was 30 kpa at Ritsumeikan University, instead of an average hydraulic gradient of 100 and effective stress of 0 kpa at University of Wisconsin. However, these differences are expected to derive the minor difference of the hydraulic conductivity compared to other factors such as types and concentration of chemical solutions. Aqueous solutions of the inorganic salts were used as the test permeants. Backpressure was not used to permit convenient collection of effluent samples for ph and electrical conductivity testing. A photograph of the test set-up in the laboratory is shown in Figure. 4 RESULTS AND DISCUSSIONS Free swell test results were presented in Figure 3. The general tendency exhibits that the increase in ionic strength results in the decrease in the swelling power, which is attributed to that the higher concentration of inorganic chemicals prevent the formation of DDL and osmotic swelling more. Ionic strength higher than 0.5 M does not cause the decrease in swelling power any more. The experimental results also indicate that the divalent Swell volume (ml/g-solid) LiCl-CaCl solution (Kolstad (000)) solution (RMD = 1.0) solution (RMD = 0.5) solution (RMD = 0.) CaCl solution (RMD = 0) Dl water Figure. Photograph of hydraulic conductivity test set-up Ionic strength for cations (M) Figure 3. Swell volume of ground bentonite from two different GCLs used at two different laboratories

4 Swell volume (ml/g-solid) Ca + Na + Hydraulic conductivity (cm/s) 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 LiCl-CaCl solution (Kolstad (000)) NaCl-CaCl solution Ionic strength for Na + or Ca + Figure 4. Effects of the concentration of Na + or Ca ++ in solutions on swell volume of bentonite 1E Swell volume (ml/g-solid) Figure 6. Effect of the chemical solutions on swell volume of bentonite in GCL and hydraulic conductivity of GCL cations have stronger effect on bentonites than monovalent cations, because, for the solutions having the ionic strength lower than 0.5 M, the higher RMD results in the larger swell volume. Another insight derived from these experimental results is that the bentonites used at two different laboratories have similar properties in terms of the swelling, because plots between ionic strength versus swell volume matches very well between Ritsumeikan University and University of Wisconsin. Note that these two laboratories used two different types of GCLs from the same manufacturer. From these results, the same aggressiveness of chemical solutions would result in the interaction on bentonites in terms of swelling. Figure 4 shows the effects of Na + or Ca ++ concentrations in the chemical solutions on the swell volume of bentonites. In the case that only Na + is present in the solutions, swell volume of bentonites will be governed by the concentration of Na + (e.g., Katsumi et al. 001). However, strong Hydraulic conductivity (cm/s) 1E-5 1E-6 1E-7 1E-8 LiCl-CaCl solution (Kolstad (000)) solution (RMD = 1.0) NaCl-CaCl solution (RMD = 0.5) 1E-9 solution (RMD = 0.) Dl water 1E Ionic sterngth for cations (M) Figure 5. Effects of the ionic strength of the permeants on the hydraulic conductivity values of GCLs correlation between Ca ++ concentration versus swell volume were observed while plots between Na + versus swell volume were scattered as shown in Figure 4. Therefore, divalent cations would have stronger effects than monovalent cations when multi-species cations are present in the solution. Effect of Ca ++ is significant such that Ca ++ higher than 0.1 M results in the complete prevention of swelling. This tendency is consistent with the results reported by Jo et al. (001) performing the swell and hydraulic conductivity tests on GCLs with a several types of chloride solutions having different levels of concentrations. Hydraulic conductivity values obtained for different chemical solutions are summarized in Figure 5. For the results obtained from both laboratories, hydraulic conductivity values are increase with the increase with ionic strength. If the ionic strengths for cations are lower than 0. M, the hydraulic conductivity values are similar to each other. However, for the chemical solutions having a higher concentration than 0.3 M, GCLs having granular bentonite permeated with LiCl-CaCl solutions exhibited greater hydraulic conductivity than powdered bentonite GCLs permeated with solutions. The experimental study presented by Jo et al. (001) showed that the difference of type of cations has no effect on GCL hydraulic conductivity if the valence of the cations is the same. Thus, it is considered that whether Li + or Na + will not cause a difference on GCL hydraulic conductivity. Figure 6 illustrates the swell volume of bentonites in the permeants versus the hydraulic conductivity values. According to the results of the swell volume shown in Figure 3, the two (powdered and ground) types of bentonite have almost the

5 same strength against chemicals (the swell volume is measured using ground bentonite). In contrast, Figure 6 indicates that both GCLs are affected by the chemical solutions, but that the powder bentonite may be more compatible than the granular bentonite, particularly with chemically strong solutions. This is probably because the pores between the granules are not blocked due to a lower level of swelling of the bentonite, especially for aggressive chemical solutions. This results in a significant increase in hydraulic conductivity. In contrast, the pores of the powdered bentonite are small even when the swelling is limited, and result in the lower hydraulic conductivity. 5 CONCLUSIONS The main results obtained from this study are as follows: 1. Hydraulic conductivity values of GCLs were correlated to the free swell power of ground bentonites. This tendency was observed for GCLs having both granular and powdered bentonites.. Powdered bentonites in GCL are more compatible than granular bentonites against chemicals, in particular aggressive (poly-valent and high concentration) solutions. This is probably because the pores between the granules are not blocked due to a lower level of swelling of the bentonite, especially for aggressive chemical solutions. 3. Divalent cations have more effects on the performance of GCLs than monovalent cations. Effect of Ca ++ is significant such that Ca ++ higher than 0.1 M results in the complete prevention of swelling. 4. Further research is needed to summarize the effect of multi-species of chemical solutions. Also, the effects of pre-hydration and modified chemical resistant bentonites will be under consideration. ACKNOLEDGEMENTS Support provided by Ritsumeikan University to the first author was greatly appreciated in preparing this paper. Support for the University of Wisconsin-Madison was provided in part by the US National Science Foundation under Grant No. CMS REFERENCES Bouazza, A. (00): Geosynthetic clay liners, Geotextiles and Geomembranes, Elsevier, Vol.0, No.1, pp Bowders, J.J. and Daniel, D.E. (1987): Hydraulic conductivity of compacted clay to dilute organic chemicals, Journal of Geotechnical Engineering, ASCE, Vol.113, No.1, pp Daniel, D.E., Shan, H.-Y., and Anderson, J.D. (1993): Effects of partial wetting on the performance of the bentonite component of a geosynthetic clay liner, Geosynthetics 93, IFAI, pp Jo, H.Y., Katsumi, T., Benson, C.H., and Edil, T.B. (001): Hydraulic conductivity and swelling of non-prehydrated GCLs permeated with single species salt solutions, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol.17, No.7, pp Katsumi, T., Onikata, M., Hasegawa, S., Lin, L., Kondo, M., and Kamon, M. (001): Chemical compatibility of modified bentonite permeated with inorganic solutions, Geoenvironmental Engineering Geoenvironmental Impact Management, R.N. Yond and H.R. Thomas (eds.), Thomas Telford, London, pp Kolstad, D.C. (000): Compatibility of Geosynthetic Clay Liners (GCLs) with Multi-Species Inorganic Solutions, MS Thesis, University of Wisconsin-Madison. Mesri, G. and Olson, R.E. (1971): Mechanisms controlling the permeability of clays, Clays and Clay Minerals, Vol.19, pp Mitchell, J.K. and Madsen, F.T. (1987): Chemical effects on clay hydraulic conductivity, Geotechnical Practice for Waste Disposal 87, R.D. Woods (ed.), ASCE, pp Norrish, K. and Quirk, J. (1954): Cristalline swelling of montmorillonite, Use of electrolytes to control swelling, Nature, Vol.173, pp Petrov, R.J. and Rowe, R.K. (1997): Geosynthetic clay liner (GCL) chemical compatibility by hydraulic conductivity testing and factors impacting its performance, Canadian Geotechnical Journal, Vol.34, pp Rowe, R.K. (1998): Geosynthetics and the minimization of contaminant migration through barrier systems beneath solid waste, Proceedings of the Sixth International Conference on Geosynthetics, IFAI, pp Ruhl, J.L. and Daniel, D.E. (1997): Geosynthetic clay liners permeated with chemical solutions and leachates, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol.13, No.4, pp Shackelford, C.D. (1994): Waste-soil interactions that alter hydraulic conductivity, Hydraulic Conductivity and Waste Contaminant Transport in Soils, ASTM

6 STP 114, D.E. Daniel and S.J. Trautwein (eds.), ASTM, pp Shackelford, C.D., Benson, C.H., Katsumi, T., Edil, T.B., and Lin, L. (000): Evaluating the hydraulic conductivity of GCLs permeated with non-standard liquids, Geotextiles and Geomembranes, Elsevier, Vol.18, Nos.-3, pp