- Email: [email protected]
Swelling and hydraulic conductivity of bentonites permeated with landfill leachates
CLAY-03999; No of Pages 9 Applied Clay Science xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Swelling and hydraulic conductivity of bentonites permeated with landfill leachates Ali Hakan Ören ⁎, R. Çağrı Akar a b
Dokuz Eylül University, Dept. of Civil Engineering, Tınaztepe Yerleşkesi, 35160 Buca-İzmir, Turkey Dokuz Eylül University, Graduate School of Natural and Applied Sciences, Tınaztepe Yerleşkesi, 35160 Buca-İzmir, Turkey
a r t i c l e
i n f o
Article history: Received 17 March 2016 Received in revised form 4 September 2016 Accepted 25 September 2016 Available online xxxx Keywords: Bentonite Free swell GCLs Hydraulic conductivity Landfill leachate
a b s t r a c t This study investigates and discusses the free swell and hydraulic conductivities of bentonites which were gathered from the local companies in Turkey. Total of 26 hydraulic conductivity tests were carried out along the study 12 of which were permeated with deionized (DIW) and tap water (TW) and the rest of them were permeated with landfill leachates (LLs). The free swell volumes of bentonites decreased when the pore fluid type was changed from water to LL. The recorded final swell volumes were within the range of 14.5–27.0 mL/2g in water, whereas 5.0–19.5 mL/2g in LLs. The hydraulic conductivity tests were performed on artificially prepared geosynthetic clay liners (AP-GCLs) which were prepared in the laboratory. The hydraulic conductivities of APGCLs were within the range of 5.2×10−12 to 3.0×10−11 m/s when TW and DIW were used as the permeant. The results also showed that the hydraulic conductivities permeated with LLs were almost the same as with those permeated with DIW or TW. The hydraulic conductivities of AP-GCLs to LLs were within the range of 2.3×10−12 to 2.0×10−11 m/s. Electrical conductivity and pH measurements were also conducted on influents and effluents along the test duration. The effluent to influent ratio of electrical conductivity was generally less than 1.0 which indicates the continuation of cation exchange process between bentonites and LLs. This conclusion was further approved with the ICP analysis conducted on influent and effluent samples. The exchangeable Ca2+ concentrations in the effluents were still less than those in the influents when the tests were terminated. In contrast, Na+ concentrations in effluent were greater than those of influent, suggesting the fully replacement of cations were not completed at the time of termination. The uncompleted cation exchange process was physically observed by performing free swell tests on post-test samples. The free swell values of bentonites were in between characteristics of the values obtained in water and LLs. It was expected that the hydraulic conductivity of AP-GCLs would increase when LLs are used as the permeant. However, comparable results were measured as with those of water. This is possibly due to the greater effective stress applied during the study which masked the negative influence of cation exchange on the hydraulic conductivity of the AP-GCLs. © 2016 Elsevier B.V. All rights reserved.
1. Introductıon Geosynthetic clay liners (GCLs) are carpet-like thin materials which are composed of bentonites and geotextiles. In most geoenvironmental applications, GCLs are used for having successful barrier performance. Bentonites are natural materials that satisfy low hydraulic conductivity to water. The hydraulic conductivity behaviors of bentonites are governed by montmorillonite which are major mineral component of bentonites. Bentonite can swell around ten times of their dry volumes when they are faced with water (Jo et al., 2001; Kolstad et al., 2004; Komine, 2004). Hence, there are little pore spaces available between clay particles for mobile water, resulting low hydraulic conductivity for bentonite (Mitchell and Soga, 2005). However, swelling of bentonite is affected from the pore fluid chemistry. In this case, the thickness of ⁎ Corresponding author. E-mail address: [email protected] (A.H. Ören).
diffuse double layer surrounding the particles decreases and the volume of pore spaces between particles increase, resulting increase in the hydraulic conductivity (Sridharan, 1991; Sivapullaiah et al., 2000). Researchers put effort into understanding how pore fluid chemistry change the hydraulic conductivity of bentonites, hence GCLs. The studies reported in the literature are categorized into two sections in terms of permeants used during the hydraulic conductivity tests: i) inorganic salt solutions and ii) real waste leachates. In the first category studies, GCLs were permeated with salt solutions that had monovalent cations, divalent cations or both. The influences of solution concentration, cation valence and pH on the hydraulic conductivity of GCLs were investigated. It was found that increase in the cation valence and solution concentration or decrease in the pH resulted in an increase in the hydraulic conductivity of GCLs. In these studies, researchers addressed the relationship between the free swell of bentonites and hydraulic conductivity of GCL as well. That is, the greater the free swell, the lower is the hydraulic conductivity (Petrov et al., 1997; Shackelford et al., 2000; Jo
http://dx.doi.org/10.1016/j.clay.2016.09.029 0169-1317/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
2
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
et al., 2001; Kolstad et al., 2004; Lee and Shackelford, 2005; Katsumi et al., 2007, 2008; Benson et al., 2010; Di Emidio et al., 2015). In contrast, complex environment of waste leachates (or landfill leachates) made it difficult to express the hydraulic conductivity results. That is why the second category studies are scarce in the literature when compared to those of the studies conducted on GCLs with inorganic salt solutions (Ruhl and Daniel, 1997; Ashmawy et al., 2002; Shan and Lai, 2002). In these studies, it was concluded that leachate compositions had great influences on the hydraulic conductivities. Thus, it is hard to say a unique tendency for hydraulic conductivity of GCLs when landfill leachates are used as the permeants. On account of technological development and extended knowledge from the previous studies conducted with ionorganic salt solutions, recent studies have reconsidered the hydraulic conductivities of GCLs with real landfill leachates (Katsumi et al., 2007; Guyonnet et al., 2009; Bradshaw and Benson, 2013). However, these studies are still limited and deserved to pay more attention. The aim of this study is to investigate and discuss the free swell and hydraulic conductivities of six bentonites which are potential candidate for GCL manufacturing. For this purpose, artificial GCLs were prepared in the laboratory by placing the bentonites between geotextiles without needle punching (i.e. fiber free). Then, GCLs were directly permeated with three landfill leachates using flexible-wall permeameters. The hydraulic conductivity results of GCLs are interpreted with the free swell of bentonites as well.
2.2. Methods 2.2.1. Index properties of bentonites The natural water contents and specific gravities of bentonites were determined in accordance with ASTM:D2216-10, 2010 and ASTM:D854-14, 2014, respectively. To determine the particle size distribution curves of bentonites, wet-sieving method was carried out as specified in ASTM:D422-63, 2007. Consistency limits of bentonites were determined according to ASTM:D4318-05, 2005. The soil index properties of bentonites are summarized in Table 3. The natural water contents (i.e. air-dried) were within the range of 9– 12%, whereas specific gravities were within the range of 2.67–2.76. Based on the particle size distribution curves shown in Fig. 1, bentonites contain negligible amount of sand grains. The fines content of the bentonites were greater than 96%, whereas the clay contents were as low as 31%. The liquid limits of bentonites were in broad range (149– 552%). In contrast, plastic limits were between 34% and 48% (Table 3). 2.2.2. Mineralogical analysis of bentonites The mineralogical compositions of bentonites were determined using an X-ray diffractometer (XRD). The samples were sieved from No. 200 (75 μm) and then oven dried at 60 °C. The XRD patterns were recorded with a GE Seifert 3003-PTS diffractometer using Cu-Kα radiation. The Rietveld method was used to estimate the quantitative amounts of mineral phases in the bentonites. Based on the XRD patterns, the montmorillonite contents of the bentonites were between 61% and 77% (Table 3).
2. Materials and methods 2.1. Materials 2.1.1. Bentonites This study was conducted with six bentonites. Two bentonites were already available in the laboratory at the time of study. The other four bentonites were supplied from local companies in Turkey (Table 1). Bentonites are commercial products which were either natural or treated. In addition, Bentonite-1 was in clod sized form, whereas others were in the powdered form. To get small particles for Bentonite-1, clods were crushed and sieved from No. 40 sieve (0.425 mm). The particles passing through No. 40 sieve were used for Bentonite-1 in this study. The other bentonites (Bentonite 2–6) were used as obtained. The names of manufacturers and brief information about the bentonites are summarized in Table 1.
2.1.2. Permeants Free swell and hydraulic conductivity tests were conducted using deionized water (DIW), tap water (TW) and landfill leachates (LLs). Deionized water was collected from Milli-Q Gradient water purification system. Tap water was the natural drinking water of İzmir. LLs were taken from the landfills which are located at the west part of Turkey. All leachates were stored into plastic bags and were kept in refrigerator along the test duration. The cation concentrations (i.e. Na+, K+, Mg2+, Ca2+), pH and electrical conductivities of LLs are presented in Table 2.
Table 1 Manufacturers and brief information about bentonites supplied from companies. Bentonite 1 2 3 4 5 6
Manufacturer
Location
Brief information
Particle form
Süd-Chemie Karakaya Eczacıbaşı Eczacıbaşı Çanbensan Çanbensan
Balıkesir Ankara İstanbul İstanbul Çankırı Çankırı
Na-treated Na-treated Activated Unactivated Na-Ca treated Polymer treated
Granular Powdered Powdered Powdered Powdered Powdered
2.2.3. Free swell test Free swell tests were conducted in DIW and LLs (ASTM:D5890-11, 2011). For this purpose, graduated cylinders were filled with test liquids to 90 mL levels. Then, 2 g bentonites were poured with 0.1 g increments into the graduated cylinders. About 10 min were allowed between the increments for hydrating and swelling the bentonite particles. After all particles were poured, graduated cylinders were filled to the 100 mL level with the same liquids and open ends of all cylinders were covered with parafilm to obstruct evaporation. Then, they were left for swelling. After 24 h of hydration, the final volumes of swollen bentonites were recorded. 2.2.4. Sample preparation for hydraulic conductivity tests Hydraulic conductivity tests were conducted on artificially prepared GCLs (AP-GCLs). For this purpose, sample diameters were kept rather high (i.e. 15 cm) to accomplish laying homogeneous bentonite layers between geotextiles. The target initial heights and mass per unit areas for the AP-GCLs were 0.6 cm and 0.5 kg/m2, respectively. To prepare the sample precisely, the plexiglass base pedestal was dismantled from the permeameter and placed over the balance. Instead of a porous stone, a heavy non-woven geotextile (Drefon S-1000) was placed on the pedestal and a woven carrier geotextile was laid on it. Adequate amount of air-dried bentonite was weighed and poured on the woven geotextile. Then, a non-woven geotextile was placed over the bentonite. After sitting the upper heavy non-woven geotextile, the base pedestal with AP-GCL was slightly removed from the balance. The circumference of the sample was moistened using a squirt bottle. Hence, AP-GCL retained itself without bentonite loss until attaching the base pedestal to the permeameter. Then, the top pedestal was placed over the heavy geotextile. Finally, latex membrane was placed on the AP-GCL and three O-rings were mounted on each pedestals. 2.2.5. Hydraulic conductivity tests The average effective stress applied during the hydraulic conductivity tests was 90 kPa and the hydraulic gradient was around 200. DIW, TW and three LLs were used as the permeant in the hydraulic conductivity tests. Before commencing the permeation, GCLs were soaked in the permeameter for 48 h with the test liquids (i.e. non-prehydrated).
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
3
Table 2 Cation concentrations, pH and electrical conductivity of landfill leachates (LLs). Landfill leachate
Na+ (mg/L)
K+ (mg/L)
Mg2+ (mg/L)
Ca2+ (mg/L)
pH
Electrical conductivity (mS/cm)
LL-1 LL-2 LL-3
1011 2903 1507
892 1135 1643
92 265 173
82 101 409
8.0 7.6 8.3
13.3 20.8 21.1
No backpressure was applied during the permeation and the outflow end of permeameter was open to the atmosphere. The direction of flow was from top to bottom in all tests. The hydraulic conductivity tests were lasted at least 6 months. Total of 26 hydraulic conductivity tests were carried out along the study.
2.2.6. Chemical analyses Chemical analyses were conducted on landfill leachates, influents and effluents along the test duration. The analyses were expressed in terms of cation concentrations, pH and electrical conductivity. The cation concentrations of landfill leachates (i.e. Na+, K+, Mg2+, Ca2+) as well as influent and effluents were determined with ICP-OES. Accumet XL500 pH meter and corresponding probes were used for the pH and electrical conductivity measurements. Prior to the measurement, the probes were calibrated with the relevant standard liquids.
3. Results and discussions 3.1. Free swell The free swells of bentonites in different pore fluids are summarized in Table 4. When this test was conducted in DIW, the highest free swell volumes were obtained for Bentonite-3 (25 mL/2g). In contrast, Bentonite-4 had the lowest swell volume (i.e. 8.0 mL/2g). The free swell value for Bentonite-4 is typical of Ca-Bentonite (Jo et al., 2001). The free swells of the other bentonites in DIW were within the range of 14.5 to 21.5 mL/2g. On the other hand, the free swell of bentonites in three LLs decreased significantly with respect to those in DIW. However, Bentonite-2 had rather greater free swell volumes in LLs when compared to the other bentonites. That is, the free swells of Bentonite-2 were 19.5, 12.0 and 18.0 mL/2g in LL-1, LL-2, and LL-3, respectively. The greater free swell for Bentonite-2 may be attributed to the montmorillonite content (Table 3) which was the highest among all bentonites (i.e. 77%). The free swells obtained in LLs are consistent with the literature and are typical for the free swells of bentonites in salt solutions or waste leachates (Jo et al., 2001; Lee et al., 2005; Katsumi et al., 2007). The reductions in the free swells of bentonites in LLs are due to the chemical composition of landfill leachates. Since chemical compositions were determined in terms of exchangeable cation concentrations, it can be seen from Table 2 that landfill leachates contain considerable amount of Na+, K+, Mg2+, Ca2+. During the free swell tests, bentonite particles are bombarded with the cations when faced with LLs. Thus, the thicknesses of diffuse double layers surrounding the particles are suppressed, resulting lower free swell volumes for bentonites in LLs.
3.2. Hydraulic conductivity Some post-test properties (GCL height and bentonite water content) as well as the final hydraulic conductivities of AP-GCLs are summarized in Table 5. Note that the final hydraulic conductivities were the average of last four readings. During the test program, 12 tests were conducted with DIW and TW. The final hydraulic conductivities of AP-GCLs were within the range of 5.2×10−12 to 3.0×10−11 m/s when TW and DIW were used as the permeant (Table 5). Among others, only one bentonite had been treated with polymer (Bentonite-6). This modified bentonite had comparable hydraulic conductivities to tap water and DIW (Table 5). These values are slightly less than the hydraulic conductivities of the GCLs reported in the literature (Petrov et al., 1997; Jo et al., 2001; Katsumi et al., 2007). This can be attributed to the greater effective stress applied during this study (i.e. 90 kPa). The hydraulic conductivity tests with LLs were conducted on four AP-GCLs which were selected based on the hydraulic conductivity values of bentonites to DIW. Thus, Bentonite-4 (AP-GCL-4) and Bentonite-6 (AP-GCL-6) were not permeated with LLs. Since Bentonite-6 was polymer treated, it was intended to conduct hydraulic conductivity tests with LLs on this bentonite. However, the free swell values in LLs were low (7.5–11.0 mL/2g) and very close to free swell of Bentonite-5. Thus, Bentonite-5 was selected to perform the hydraulic conductivity tests. Moreover, although Bentonite-1 (AP-GCL-1) had one of the greatest hydraulic conductivity to water with respect to other bentonites (i.e. 1.3×10−9 cm/s), it was kept under the test program to see whether LL-2 and LL-3 would have any influence on the hydraulic conductivity of this bentonite. But it did not seem to be necessary to conduct the hydraulic conductivity test on Bentonit-1 (AP-GCL-1) with LL-1. Moreover, Bentonite-3 (AP-GCL-3) had the lowest hydraulic conductivity to DIW (5.2×10−10 cm/s). Because of this property, Bentonite-3 was tested two times with each LL one of which was permeated for the long term. Long term hydraulic conductivity tests were lasted about one year. The total of hydraulic conductivity tests permeated with LLs was 14. The hydraulic conductivity behaviors of AP-GCLs permeated with LLs were similar within each other. Thus, only the results with LL-1 are given herein as a function of pore volumes of flow (PVF) (Fig. 2). The details about the other tests conducted with LL-2 and LL-3 can be found elsewhere (Ören et al., 2015). Table 5 also presents the final hydraulic conductivities of AP-GCLs to LLs. The physical equilibrium (i.e. Qout/Qin) was also checked by dividing the outflow volumes to inflow volumes (Fig. 2a–c). The results showed that there was almost twofold decrease in the hydraulic conductivity of AP-GCL-2 along the test duration (Fig. 2a). Similar reductions in the hydraulic conductivities were also measured for AP-GCL-3 and AP-GCL-5 as the PVFs increased
Table 3 Basic soil index properties of bentonites. Bentonite
Specific gravity
Natural water content (%)
Clay content (%)
Liquid limit (%)
Plastic limit (%)
Montmorillonite content (%)
1 2 3 4 5 6
2.76 2.69 2.67 2.72 2.76 2.71
12 12 9 10 12 12
67 73 78 31 78 78
283 529 552 149 397 417
48 38 41 42 34 44
61 77 72 67 63 68
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
4
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
Fig. 1. Particle size distribution of bentonites that were used in AP-GCLs.
(Fig. 2b–c). The short and long term hydraulic conductivity behaviors of AP-GCL-3 were almost the same. Although long term test was lasted twice of the short term test, the PVFs at the termination were the same for both tests. This is due to the fact that short term hydraulic conductivity was about two-fold greater than the long term hydraulic conductivity (8.2×10−10 cm/s versus 4.5×10−10 cm/s). The hydraulic conductivities of AP-GCLs with LLs were within the range of 2.3×10−12 to 2.0×10−11 m/s (Table 5). However, the hydraulic conductivities of AP-GCL-1 to LLs were about one order of magnitude greater than those of other AP-GCLs. There may be two possible explanations for the greater hydraulic conductivity for AP-GCL-1: i) larger air-dried particle size and ii) poor physical and mineralogical properties of Bentonite-1. As mentioned before, Bentonite 2–6 were in the powdered form while preparing AP-GCLs. However, Bentonite-1 had sand sized particles for the reason that the particles passing through No. 40 sieve were used in AP-GCL-1. Thus, inter-particle pore spaces are expected to be greater in AP-GCL-1 than in other AP-GCLs. The other reason may be due to having lower montmorillonite content for Bentonite1 (i.e. 61%). This may also lead to have lower clay content (67%) and liquid limit value (283%) for the bentonite. It is interesting to note that, however, the hydraulic conductivities of AP-GCL-2 to LLs were up to 2.7 times less than the hydraulic conductivity to DIW (Fig. 3). It is possibly due to the minor differences between samples that may have been generated while preparing the AP-GCLs. In contrast, the hydraulic conductivities of AP-GCL-3 increased with LL permeation. Fig. 3 shows that the short term hydraulic conductivities AP-GCL-3 to LL-2 and LL-3 were up to two-fold, whereas the long term hydraulic conductivities to LL-1 and LL-3 were 1.6 times and 3.8 times, respectively, greater than the hydraulic conductivities to DIW. Unlike AP-GCL-2 and AP-GCL-3, the influence of LLs had no effect on the hydraulic conductivity of AP-GCL-1 and AP-GCL-5 when compared to the hydraulic conductivity of the same GCLs to DIW. Regarding to the obtained results, it can be stated that there was no practical difference between the hydraulic conductivities of AP-GCLs when permeated
Table 4 Free swell of bentonites in different pore fluids. Bentonite
1 2 3 4 5 6
Free swell (mL/2g) DI water
LL-1
LL-2
LL-3
14.5 21.5 25.0 8.0 19.5 21.0
5.0 19.5 8.0 7.0 10.0 10.0
5.0 12.0 7.0 7.0 8.0 7.5
5.0 18.0 8.0 9.0 10.0 11.0
with LLs and DIW. In other words, hydraulic conductivities of AP-GCLs to LLs were comparable to the hydraulic conductivities to DIW or TW (Fig. 3). Physical factors such as using powdered bentonite in AP-GCLs and applying higher effective stress (i.e. 90 kPa) during the hydraulic conductivity tests are responsible for the low hydraulic conductivities to LLs. Due to low flow rate, the cation exchange process that took place between bentonites and LLs may not have been completed when hydraulic conductivity tests were ceased. The chemical equilibrium was checked along the test duration by measuring pH and electrical conductivity (EC) of influents and effluents. Then, these values were normalized with the corresponding influent values. The normalized pH and EC values are given just in case of permeation with LL-1 and shown in Fig. 4a–c as a function of PVF. The final normalized pH and EC values for the other tests conducted with LL-2 and LL-3 are summarized in Table 6 as well. At the time of termination, it was seen that the most pHout/pHin values were around 1.2 which is Table 5 The final heights, water contents and hydraulic conductivities of GCLs permeated with DIW, TW and landfill leachates. Test GCL type number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-4 AP-GCL-5 AP-GCL-6 AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-4 AP-GCL-5 AP-GCL-6 AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5 AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5 AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5
Permeant Final type height (cm)
Final water content (%)
Final hydraulic conductivity (cm/s)
DIW DIW DIW DIW DIW DIW TW TW TW TW TW TW LL-1 LL-1 LL-1 LL-1 LL-2 LL-2 LL-2 LL-2 LL-2 LL-3 LL-3 LL-3 LL-3 LL-3
81 145 225 101 93 87 163 139 203 108 93 87 101 107 100 85 113 106 112 104 77 83 151 120 105 91
1.3×10−9 6.1×10−10 5.2×10−10 3.0×10−9 5.9×10−10 8.1×10−10 1.5×10−9 6.3×10−10 8.5×10−10 3.0×10−9 6.2×10−10 6.9×10−10 2.8×10−10 4.5×10−10 8.2×10−10 5.9×10−10 1.2×10−9 2.3×10−10 9.9×10−10 3.5×10−10 6.1×10−10 1.94×10−9 5.0×10−10 7.8×10−10 2.0×10−9 4.6×10−10
0.48 1.10 1.38 0.79 0.84 0.79 0.99 1.10 1.41 0.79 0.84 0.79 0.85 0.89 0.88 0.74 0.72 0.73 0.92 0.91 0.73 0.76 – 0.97 – 0.67
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
5
Fig. 2. Hydraulic conductivity behaviors of AP-GCLs when permeated with Landfill Leachate-1 (LL-1): a) AP-GCL-2, b) AP-GCL-3, c) AP-GCL-5.
about 10% above the upper limit suggested in ASTM D6766 (1.0 ± 0.1). It is due to the reduction in the influent pH values along the test duration that result in an increase in the pHout/pHin values. These values are in agreement with the findings of Jo et al. (2005) who drawn similar conclusion with the salt solutions. However, although normalized EC values were within the range of 1.0 ± 0.1 during permeation with LL-1 and LL-3, they were as low as ~ 0.5 in the case of permeation
with LL-2. Based on these findings, one can argue that pH and EC measurements give little information about the completion of cation exchange process that took place between bentonites and LLs. In order to figure out the cation replacement between bentonite and LL, the effluent concentrations as well as the ratio of effluent to influent concentrations of AP-GCL-3 when permeated with LL-1 are given in Fig. 5 as a function of PVF. The circles denote the long term, whereas
Fig. 3. Final hydraulic conductivities of AP-GCLs.
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
6
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
Fig. 4. Normalized values of pH and electrical conductivity (EC) as a function of pore volumes of flow (PVF): a) AP-GCL-2, b) AP-GCL-3, c) AP-GCL-5.
the squares denote the short term test results. Moreover, close and open symbols show the effluent concentrations and the ratio of effluent to influent concentrations, respectively. It is indicated in Fig. 5a that the sodium concentration in the effluent was greater than that of in the influent when the tests were terminated, resulting greater normalized sodium values with respect to magnesium and potassium. The effluent
Table 6 pH and electrical conductivities normalized by dividing the effluent values to influent values. Test number
GCL type
Permeant type
pHout/pHin
ECout/ECin
13 14 15 16 17 18 19 20 21 22 23 24 25 26
AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5 AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5 AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5
LL-1 LL-1 LL-1 LL-1 LL-2 LL-2 LL-2 LL-2 LL-2 LL-3 LL-3 LL-3 LL-3 LL-3
1.24 1.20 1.18 1.24 1.01 1.20 1.18 1.15 1.19 1.29 1.15 1.17 1.25 1.32
0.94 1.05 1.03 0.87 0.86 0.54 0.58 0.88 0.55 1.20 1.09 1.08 0.74 1.00
to influent ratios for potassium and magnesium were close to 1.0 which shows the concentration equilibrium was almost reached at the end of hydraulic conductivity tests (Fig. 5b–c). In contrast, the effluent concentration of calcium was lower than that of influent, resulting lower effluent to influent ratio Ca2+ (i.e. 0.3) (Fig. 5d). Based on greater sodium and lower calcium concentrations in the effluents, it can be concluded that the cation replacement between AP-GCL-3 and LL-1 was not completed at the end of the tests. The similar conclusions were also drawn between other AP-GCLs which were permeated with LL-2 and LL-3. The details about the effluent concentrations of these tests can be found in Ören et al. (2015) as well. The solid/liquid ratio in free swell test is much lower than that in hydraulic conductivity test and therefore, the values obtained from these tests may not be related within each other (Bouazza et al., 2007). The lower solid/liquid ratio may result in release of some bound cations that were already adsorbed onto the bentonite particles into the free swell test solution (i.e. DIW) even cation exchange process complete. Hence, the free swell volume may be greater than its real volume. However, the free swell tests conducted on post test bentonites gave valuable information about the level of the cation exchange process at the end of hydraulic conductivity tests. Fig. 6 shows the free swell volumes of post test bentonites in DIW in comparison to the free swell volumes of new (or untreated) bentonites in DIW and LLs (Table 4). The only exception is for LL-3. Since the degree of cation exchange process was presented well with free swells of post test bentonites permeated with LL-1
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
7
Fig. 5. Effluent concentrations and the ratio of effluent to influent concentration for AP-GCL-3 when permeated with LL-1: a) sodium, b) potassium, c) magnesium, d) calcium.
and LL-2, this test was not performed on the bentonites permeated with LL-3. As a result, it is found that the free swell volumes of post test bentonites were between those of new samples in DIW and LLs, suggesting the cation exchange processes were not completed (Fig. 6a–b). The final AP-GCL heights and the final water contents of bentonite are also give some information about the swell of bentonites during hydraulic conductivity tests. Except AP-GCL-1 permeated with DIW, the final heights of all AP-GCLs were greater than the initial heights of APGCLs (i.e. 0.6 cm) even they were permeated with LLs. The final height for AP-GCL-1 was 0.48 cm which shows a reduction in the height (Table 5). This sample was the first sample prepared and tested in the permeameter during the testing program. Thus, this reduction was possibly generated from the operator who may not have achieved the target thickness at the first time of sample preparation. The heights of other AP-GCLs were within the ranges of 0.79–1.41 cm and
0.72–0.97 cm when permeated with DIW and LLs, respectively (Table 5). Since AP-GCLs were not needle-punched during sample preparation, the swell was not restricted. However, although the final heights for AP-GCLs permeated with LLs were less than those of the samples permeated with DIW, they were still greater than the initial heights, indicating swell behavior of bentonites during permeation. This also suggests osmotic swelling of bentonites. It is expected that the concentration of pore water decreases across the height of AP-GCLs. It is high where LL is in contact with GCLs. In contrast, the concentration of pore water decreases downwards because some of positively charged cations in LLs will be adsorbed on the bentonite. Since flow was from top to bottom, the swelling behavior of bentonites may be crystalline at the upper sections, whereas it may be crystalline and osmotic at the lower sections of the AP-GCLs. The final water contents of AP-GCLs also confirm this
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
8
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
hydraulic conductivity test results of some studies reported in the literature that were conducted under effective stresses between 20 and 34.5 kPa. Fig. 7 shows that the hydraulic conductivities of this study are less than the hydraulic conductivities given with the curve. The results can be separated into two groups based on free swell value of 15 mL/2g. The greater effective stress (90 kPa) applied in this study is responsible alone for the lower hydraulic conductivity with respect to literature when the free swell of bentonites is greater than 15 mL/2g. The values above this level generally obtained by the tests performed with DIW in this study and thus, there would be no cation exchange expected between bentonites and DIW. It is already known that increase in the effective stress result in a reduction in the pore spaces between bentonite particles (Estornell and Daniel, 1992; Petrov et al., 1997). However, the findings of this study are significantly below the proposed curve when the free swell value is between 5 mL/2g and 15 mL/2g. This can be attributed to the combined effects of greater effective stress applied during the study and the uncompleted cation exchange process between bentonite and LLs. 4. Conclusions
Fig. 6. Comparison of the free swell of post test bentonites in DIW with those of new samples in DIW and in: a) LL-1 and b) LL-2.
statement because the water contents were within the range of 77% and 151% (Table 5). The greater the final water content the lower is the hydraulic conductivity of AP-GCL (Petrov et al., 1997; Meer and Benson, 2007). Finally, the influence of effective stress on the hydraulic conductivity of AP-GCLs is shown in Fig. 7 as a function of free swell. A curve is also added on the graph which was drawn based on an equation proposed by Katsumi et al. (2008). Note that the proposed equation covers the
This study discusses the free swell and hydraulic conductivity behavior of six bentonites with water and landfill leachates (LLs). The free swell test results showed that the swell volumes of bentonites in DIW were greater than those in LLs. This was an expected behavior because the thickness of diffuse double layer surrounding the bentonite particles compressed on account of cations in LLs. Subsequently, hydraulic conductivity tests were conducted on bentonites. For this purpose, artificial geosynthetic clay liners (GCLs) were prepared in the laboratory by placing bentonites between non-woven and woven geotextiles without needle punching. This kind of GCL was named as “artificially prepared GCLs” and denoted by AP-GCLs. The hydraulic conductivities of AP-GCLs to DIW and TW were within the range of 5.2×10−12 to 3.0×10−11 m/s. The lower hydraulic conductivities of AP-GCLs to DIW with respect to those of GCLs reported in the literature can be attributed to the greater effective stress applied during the study (90 kPa). The greater the effective stress, the lower is the hydraulic conductivity for AP-GCLs. The hydraulic conductivities with LLs were between 2.3×10−12 m/s and 2.0×10−11 m/s which are comparable to the hydraulic conductivities to DIW and TW. However, the hydraulic conductivity of AP-GCL-1 had about an order of magnitude greater hydraulic conductivity to LLs when compared with those of other AP-GCLs. This is attributed to the larger particle size and poor physical and mineralogical characteristics of bentonite used in AP-GCL-1. On the other hand, AP-GCL-3 was
Fig. 7. The comparison of the findings of this study to those of the study reported in the literature.
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
permeated two times one of which was tested to determine long term (~one year) hydraulic conductivity with LLs. It is concluded that there was no practical difference between the short and long term hydraulic conductivities of AP-GCL-3. Hydraulic conductivity results with LLs were supported with pH and electrical conductivity measurements conducted on the influent and effluent samples. Effluent to influent ratio for electrical conductivity was generally less than one, indicating cation exchange was not completed when hydraulic conductivity tests were terminated. This was further confirmed by the cation analysis performed on the effluents and influents of LLs. Based on the cation analysis, it is found that effluent to influent of potassium and magnesium concentrations were close to one. However, calcium concentrations were lower, whereas sodium concentration were greater than one for all samples, suggesting cation exchange was still under progress at the end of the tests. This is further approved with the free swell tests conducted on post test bentonites which were permeated with LLs. The free swell volumes then were compared with those of new (or untreated) bentonites in DIW and LLs. It is found that the swell volumes of post test bentonites were between those of new bentonites in DIW and LLs. Thus, the lower hydraulic conductivity to LLs can be attributed not only to the greater effective stress applied during the study but also to the uncompleted cation exchange process at the end of tests. Acknowledgement This study was funded by the Scientific and Technical Research Council of Turkey, TUBITAK (Grant No: 111M718). The authors are grateful for this funding. Reference Ashmawy, A.K., El-Hajji, D., Sotelo, N., Muhammad, N., 2002. Hydraulic performance of untreated and polymer-treated bentonite in inorganic landfill leachates. Clay Clay Miner. 50, 546–552. http://dx.doi.org/10.1346/000986002320679288. ASTM:D2216-10, 2010. Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass. ASTM International, West Conshohocken, PA, USA, pp. 1–7 http://dx.doi.org/10.1520/D2216-10.2. ASTM:D422-63, 2007. Standard Test Method for Particle-Size Analysis of Soils. ASTM International, West Conshohocken, PA, USA, pp. 1–8 http://dx.doi.org/10.1520/ D0422-63R07E01.2. ASTM:D4318-05, 2005. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. ASTM International, West Conshohocken, PA, USA, pp. 1–14 http:// dx.doi.org/10.1520/D4318. ASTM:D5890-11, 2011. Standard Test Method for Fluid Loss of Clay Component of Geosynthetic Clay Liners. ASTM International, West Conshohocken, PA, USA, pp. 7–9 http://dx.doi.org/10.1520/D5890-11.2. ASTM:D854-14, 2014. Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer. ASTM International, West Conshohocken, PA, USA, pp. 1–8 http://dx. doi.org/10.1520/D0854-10. Benson, C.H., Ören, A.H., Gates, W.P., 2010. Hydraulic conductivity of two geosynthetic clay liners permeated with a hyperalkaline solution. Geotext. Geomembr. 28, 206–218. http://dx.doi.org/10.1016/j.geotexmem.2009.10.002. Bouazza, A., Jefferis, S., Vangpaisal, T., 2007. Investigation of the effects and degree of calcium exchange on the Atterberg limits and swelling of geosynthetic clay liners when
9
subjected to wet-dry cycles. Geotext. Geomembr. 25, 170–185. http://dx.doi.org/10. 1016/j.geotexmem.2006.11.001. Bradshaw, S.L., Benson, C.H., 2013. Effect of municipal solid waste leachate on hydraulic conductivity and exchange complex of geosynthetic clay liners. J. Geotech. Geoenviron. Eng. 140, 131003220201000. http://dx.doi.org/10.1061/(ASCE)GT. 1943-5606.0001050. Di Emidio, G., Mazzieri, E., Verastegui-Flores, R.-D., Van Impe, W., Bezuijen, A., 2015. Polymer-treated bentonite clay for chemical-resistant geosynthetic clay liners. Geosynth. Int. 22, 125–137. Estornell, P., Daniel, D.E., 1992. Hydraulic conductivity of three geosynthetic clay liners. J. Geotech. Eng. 118, 1592–1606. http://dx.doi.org/10.1061/(ASCE)07339410(1992)118:10(1592). Guyonnet, D., Touze-Foltz, N., Norotte, V., Pothier, C., Didier, G., Gailhanou, H., Blanc, P., Warmont, F., 2009. Performance-based indicators for controlling geosynthetic clay liners in landfill applications. Geotext. Geomembr. 27, 321–331. http://dx.doi.org/ 10.1016/j.geotexmem.2009.02.002. 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. J. Geotech. Geoenviron. Eng. 127 (7), 557–567. Jo, H.Y., Benson, C.H., Shackelford, C.D., Lee, J.-M., Edil, T.B., 2005. Long-term hydraulic conductivity of a geosynthetic clay liner permeated with inorganic salt solutions. J. Geotech. Geoenviron. Eng. 131, 405–417. http://dx.doi.org/10.1061/(ASCE)10900241(2005)131:4(405). Katsumi, T., Ishimori, H., Ogawa, A., Yoshikawa, K., Hanamoto, K., Fukagawa, R., 2007. Hydraulic conductivity of nonprehydrated geosynthetic clay liners permeated with inorganic solutions and waste leachates. Soils Found. 47, 79–96. http://dx.doi.org/10. 3208/sandf.47.79. Katsumi, T., Ishimori, H., Onikata, M., Fukagawa, R., 2008. Long-term barrier performance of modified bentonite materials against sodium and calcium permeant solutions. Geotext. Geomembr. 26, 14–30. http://dx.doi.org/10.1016/j.geotexmem.2007.04.003. Kolstad, D., Benson, C., Edil, T., 2004. Hydraulic conductivity and swell of nonprehydrated geosynthetic clay liners permeated with multispecies inorganic solutions. J. Geotech. 130, 1236–1249. http://dx.doi.org/10.1061/(ASCE)1090-0241(2004)130. Komine, H., 2004. Simplified evaluation for swelling characteristics of bentonites. Eng. Geol. 71, 265–279. http://dx.doi.org/10.1016/S0013-7952(03)00140-6. Lee, J.-M., Shackelford, C.D., 2005. Impact of bentonite quality on hydraulic conductivity of geosynthetic clay liners. J. Geotech. Geoenviron. Eng. 131, 64–77. http://dx.doi.org/ 10.1061/(ASCE)1090-0241(2005)131:1(64). Lee, J.-M., Shackelford, C.D., Benson, C.H., Jo, H.-Y., Edil, T.B., 2005. Correlating index properties and hydraulic conductivity of geosynthetic clay liners. J. Geotech. Geoenviron. Eng. 131, 1319–1329. http://dx.doi.org/10.1061/(ASCE)1090-0241(2005)131: 11(1319). Meer, S.R., Benson, C.H., 2007. Hydraulic conductivity of geosynthetic clay liners exhumed from landfill final covers. J. Geotech. Geoenviron. Eng. 133, 550–563. Mitchell, J.K., Soga, K., 2005. Fundamentals of Soil Behavior. 3rd Edition. John Wiley & Sons. Ören, A.H., Yukselen Aksoy, Y., Önal, O., 2015. Investigating the hydraulic conductivity of geosynthetic clay liners and detecting the suitable bentonites for manufacturing, Tubitak Report 111M718 (In Turkish with an English abstract). Petrov, R., Rowe, R., Quigley, R., 1997. Selected factors influencing GCL hydraulic conductivity. J. Geotech. Geoenviron. Eng. 123, 683–695. Ruhl, J.L., Daniel, D.E., 1997. Geosynthetic clay liners permeated with chemical solutions and leachates. J. Geotech. Geoenviron. Eng. 123, 369–381. http://dx.doi.org/10. 1061/(ASCE)1090-0241(1997)123:4(369). Shackelford, C.D., Benson, C.H., Katsumi, T., Edil, T.B., Lin, L., 2000. Evaluating the hydraulic conductivity of GCLs permeated with non-standard liquids. Geotext. Geomembr. 18, 133–161. http://dx.doi.org/10.1016/S0266-1144(99)00024-2. Shan, H.Y., Lai, Y.J., 2002. Effect of hydrating liquid on the hydraulic properties of geosynthetic clay liners. Geotext. Geomembr. 20, 19–38. http://dx.doi.org/10.1016/ S0266-1144(01)00023-1. Sivapullaiah, P.V., Sridharan, A., Stalin, V.K., 2000. Hydraulic conductivity of bentonitesand mixtures. Can. Geotech. J. 37, 406–413. Sridharan, A., 1991. Annual Lecture: Engineering behaviour of fine grained soils—a fundamental approach. Indian Geotech. J. 1–94.
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Swelling and hydraulic conductivity of bentonites permeated with landfill leachates Ali Hakan Ören ⁎, R. Çağrı Akar a b
Dokuz Eylül University, Dept. of Civil Engineering, Tınaztepe Yerleşkesi, 35160 Buca-İzmir, Turkey Dokuz Eylül University, Graduate School of Natural and Applied Sciences, Tınaztepe Yerleşkesi, 35160 Buca-İzmir, Turkey
a r t i c l e
i n f o
Article history: Received 17 March 2016 Received in revised form 4 September 2016 Accepted 25 September 2016 Available online xxxx Keywords: Bentonite Free swell GCLs Hydraulic conductivity Landfill leachate
a b s t r a c t This study investigates and discusses the free swell and hydraulic conductivities of bentonites which were gathered from the local companies in Turkey. Total of 26 hydraulic conductivity tests were carried out along the study 12 of which were permeated with deionized (DIW) and tap water (TW) and the rest of them were permeated with landfill leachates (LLs). The free swell volumes of bentonites decreased when the pore fluid type was changed from water to LL. The recorded final swell volumes were within the range of 14.5–27.0 mL/2g in water, whereas 5.0–19.5 mL/2g in LLs. The hydraulic conductivity tests were performed on artificially prepared geosynthetic clay liners (AP-GCLs) which were prepared in the laboratory. The hydraulic conductivities of APGCLs were within the range of 5.2×10−12 to 3.0×10−11 m/s when TW and DIW were used as the permeant. The results also showed that the hydraulic conductivities permeated with LLs were almost the same as with those permeated with DIW or TW. The hydraulic conductivities of AP-GCLs to LLs were within the range of 2.3×10−12 to 2.0×10−11 m/s. Electrical conductivity and pH measurements were also conducted on influents and effluents along the test duration. The effluent to influent ratio of electrical conductivity was generally less than 1.0 which indicates the continuation of cation exchange process between bentonites and LLs. This conclusion was further approved with the ICP analysis conducted on influent and effluent samples. The exchangeable Ca2+ concentrations in the effluents were still less than those in the influents when the tests were terminated. In contrast, Na+ concentrations in effluent were greater than those of influent, suggesting the fully replacement of cations were not completed at the time of termination. The uncompleted cation exchange process was physically observed by performing free swell tests on post-test samples. The free swell values of bentonites were in between characteristics of the values obtained in water and LLs. It was expected that the hydraulic conductivity of AP-GCLs would increase when LLs are used as the permeant. However, comparable results were measured as with those of water. This is possibly due to the greater effective stress applied during the study which masked the negative influence of cation exchange on the hydraulic conductivity of the AP-GCLs. © 2016 Elsevier B.V. All rights reserved.
1. Introductıon Geosynthetic clay liners (GCLs) are carpet-like thin materials which are composed of bentonites and geotextiles. In most geoenvironmental applications, GCLs are used for having successful barrier performance. Bentonites are natural materials that satisfy low hydraulic conductivity to water. The hydraulic conductivity behaviors of bentonites are governed by montmorillonite which are major mineral component of bentonites. Bentonite can swell around ten times of their dry volumes when they are faced with water (Jo et al., 2001; Kolstad et al., 2004; Komine, 2004). Hence, there are little pore spaces available between clay particles for mobile water, resulting low hydraulic conductivity for bentonite (Mitchell and Soga, 2005). However, swelling of bentonite is affected from the pore fluid chemistry. In this case, the thickness of ⁎ Corresponding author. E-mail address: [email protected] (A.H. Ören).
diffuse double layer surrounding the particles decreases and the volume of pore spaces between particles increase, resulting increase in the hydraulic conductivity (Sridharan, 1991; Sivapullaiah et al., 2000). Researchers put effort into understanding how pore fluid chemistry change the hydraulic conductivity of bentonites, hence GCLs. The studies reported in the literature are categorized into two sections in terms of permeants used during the hydraulic conductivity tests: i) inorganic salt solutions and ii) real waste leachates. In the first category studies, GCLs were permeated with salt solutions that had monovalent cations, divalent cations or both. The influences of solution concentration, cation valence and pH on the hydraulic conductivity of GCLs were investigated. It was found that increase in the cation valence and solution concentration or decrease in the pH resulted in an increase in the hydraulic conductivity of GCLs. In these studies, researchers addressed the relationship between the free swell of bentonites and hydraulic conductivity of GCL as well. That is, the greater the free swell, the lower is the hydraulic conductivity (Petrov et al., 1997; Shackelford et al., 2000; Jo
http://dx.doi.org/10.1016/j.clay.2016.09.029 0169-1317/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
2
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
et al., 2001; Kolstad et al., 2004; Lee and Shackelford, 2005; Katsumi et al., 2007, 2008; Benson et al., 2010; Di Emidio et al., 2015). In contrast, complex environment of waste leachates (or landfill leachates) made it difficult to express the hydraulic conductivity results. That is why the second category studies are scarce in the literature when compared to those of the studies conducted on GCLs with inorganic salt solutions (Ruhl and Daniel, 1997; Ashmawy et al., 2002; Shan and Lai, 2002). In these studies, it was concluded that leachate compositions had great influences on the hydraulic conductivities. Thus, it is hard to say a unique tendency for hydraulic conductivity of GCLs when landfill leachates are used as the permeants. On account of technological development and extended knowledge from the previous studies conducted with ionorganic salt solutions, recent studies have reconsidered the hydraulic conductivities of GCLs with real landfill leachates (Katsumi et al., 2007; Guyonnet et al., 2009; Bradshaw and Benson, 2013). However, these studies are still limited and deserved to pay more attention. The aim of this study is to investigate and discuss the free swell and hydraulic conductivities of six bentonites which are potential candidate for GCL manufacturing. For this purpose, artificial GCLs were prepared in the laboratory by placing the bentonites between geotextiles without needle punching (i.e. fiber free). Then, GCLs were directly permeated with three landfill leachates using flexible-wall permeameters. The hydraulic conductivity results of GCLs are interpreted with the free swell of bentonites as well.
2.2. Methods 2.2.1. Index properties of bentonites The natural water contents and specific gravities of bentonites were determined in accordance with ASTM:D2216-10, 2010 and ASTM:D854-14, 2014, respectively. To determine the particle size distribution curves of bentonites, wet-sieving method was carried out as specified in ASTM:D422-63, 2007. Consistency limits of bentonites were determined according to ASTM:D4318-05, 2005. The soil index properties of bentonites are summarized in Table 3. The natural water contents (i.e. air-dried) were within the range of 9– 12%, whereas specific gravities were within the range of 2.67–2.76. Based on the particle size distribution curves shown in Fig. 1, bentonites contain negligible amount of sand grains. The fines content of the bentonites were greater than 96%, whereas the clay contents were as low as 31%. The liquid limits of bentonites were in broad range (149– 552%). In contrast, plastic limits were between 34% and 48% (Table 3). 2.2.2. Mineralogical analysis of bentonites The mineralogical compositions of bentonites were determined using an X-ray diffractometer (XRD). The samples were sieved from No. 200 (75 μm) and then oven dried at 60 °C. The XRD patterns were recorded with a GE Seifert 3003-PTS diffractometer using Cu-Kα radiation. The Rietveld method was used to estimate the quantitative amounts of mineral phases in the bentonites. Based on the XRD patterns, the montmorillonite contents of the bentonites were between 61% and 77% (Table 3).
2. Materials and methods 2.1. Materials 2.1.1. Bentonites This study was conducted with six bentonites. Two bentonites were already available in the laboratory at the time of study. The other four bentonites were supplied from local companies in Turkey (Table 1). Bentonites are commercial products which were either natural or treated. In addition, Bentonite-1 was in clod sized form, whereas others were in the powdered form. To get small particles for Bentonite-1, clods were crushed and sieved from No. 40 sieve (0.425 mm). The particles passing through No. 40 sieve were used for Bentonite-1 in this study. The other bentonites (Bentonite 2–6) were used as obtained. The names of manufacturers and brief information about the bentonites are summarized in Table 1.
2.1.2. Permeants Free swell and hydraulic conductivity tests were conducted using deionized water (DIW), tap water (TW) and landfill leachates (LLs). Deionized water was collected from Milli-Q Gradient water purification system. Tap water was the natural drinking water of İzmir. LLs were taken from the landfills which are located at the west part of Turkey. All leachates were stored into plastic bags and were kept in refrigerator along the test duration. The cation concentrations (i.e. Na+, K+, Mg2+, Ca2+), pH and electrical conductivities of LLs are presented in Table 2.
Table 1 Manufacturers and brief information about bentonites supplied from companies. Bentonite 1 2 3 4 5 6
Manufacturer
Location
Brief information
Particle form
Süd-Chemie Karakaya Eczacıbaşı Eczacıbaşı Çanbensan Çanbensan
Balıkesir Ankara İstanbul İstanbul Çankırı Çankırı
Na-treated Na-treated Activated Unactivated Na-Ca treated Polymer treated
Granular Powdered Powdered Powdered Powdered Powdered
2.2.3. Free swell test Free swell tests were conducted in DIW and LLs (ASTM:D5890-11, 2011). For this purpose, graduated cylinders were filled with test liquids to 90 mL levels. Then, 2 g bentonites were poured with 0.1 g increments into the graduated cylinders. About 10 min were allowed between the increments for hydrating and swelling the bentonite particles. After all particles were poured, graduated cylinders were filled to the 100 mL level with the same liquids and open ends of all cylinders were covered with parafilm to obstruct evaporation. Then, they were left for swelling. After 24 h of hydration, the final volumes of swollen bentonites were recorded. 2.2.4. Sample preparation for hydraulic conductivity tests Hydraulic conductivity tests were conducted on artificially prepared GCLs (AP-GCLs). For this purpose, sample diameters were kept rather high (i.e. 15 cm) to accomplish laying homogeneous bentonite layers between geotextiles. The target initial heights and mass per unit areas for the AP-GCLs were 0.6 cm and 0.5 kg/m2, respectively. To prepare the sample precisely, the plexiglass base pedestal was dismantled from the permeameter and placed over the balance. Instead of a porous stone, a heavy non-woven geotextile (Drefon S-1000) was placed on the pedestal and a woven carrier geotextile was laid on it. Adequate amount of air-dried bentonite was weighed and poured on the woven geotextile. Then, a non-woven geotextile was placed over the bentonite. After sitting the upper heavy non-woven geotextile, the base pedestal with AP-GCL was slightly removed from the balance. The circumference of the sample was moistened using a squirt bottle. Hence, AP-GCL retained itself without bentonite loss until attaching the base pedestal to the permeameter. Then, the top pedestal was placed over the heavy geotextile. Finally, latex membrane was placed on the AP-GCL and three O-rings were mounted on each pedestals. 2.2.5. Hydraulic conductivity tests The average effective stress applied during the hydraulic conductivity tests was 90 kPa and the hydraulic gradient was around 200. DIW, TW and three LLs were used as the permeant in the hydraulic conductivity tests. Before commencing the permeation, GCLs were soaked in the permeameter for 48 h with the test liquids (i.e. non-prehydrated).
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
3
Table 2 Cation concentrations, pH and electrical conductivity of landfill leachates (LLs). Landfill leachate
Na+ (mg/L)
K+ (mg/L)
Mg2+ (mg/L)
Ca2+ (mg/L)
pH
Electrical conductivity (mS/cm)
LL-1 LL-2 LL-3
1011 2903 1507
892 1135 1643
92 265 173
82 101 409
8.0 7.6 8.3
13.3 20.8 21.1
No backpressure was applied during the permeation and the outflow end of permeameter was open to the atmosphere. The direction of flow was from top to bottom in all tests. The hydraulic conductivity tests were lasted at least 6 months. Total of 26 hydraulic conductivity tests were carried out along the study.
2.2.6. Chemical analyses Chemical analyses were conducted on landfill leachates, influents and effluents along the test duration. The analyses were expressed in terms of cation concentrations, pH and electrical conductivity. The cation concentrations of landfill leachates (i.e. Na+, K+, Mg2+, Ca2+) as well as influent and effluents were determined with ICP-OES. Accumet XL500 pH meter and corresponding probes were used for the pH and electrical conductivity measurements. Prior to the measurement, the probes were calibrated with the relevant standard liquids.
3. Results and discussions 3.1. Free swell The free swells of bentonites in different pore fluids are summarized in Table 4. When this test was conducted in DIW, the highest free swell volumes were obtained for Bentonite-3 (25 mL/2g). In contrast, Bentonite-4 had the lowest swell volume (i.e. 8.0 mL/2g). The free swell value for Bentonite-4 is typical of Ca-Bentonite (Jo et al., 2001). The free swells of the other bentonites in DIW were within the range of 14.5 to 21.5 mL/2g. On the other hand, the free swell of bentonites in three LLs decreased significantly with respect to those in DIW. However, Bentonite-2 had rather greater free swell volumes in LLs when compared to the other bentonites. That is, the free swells of Bentonite-2 were 19.5, 12.0 and 18.0 mL/2g in LL-1, LL-2, and LL-3, respectively. The greater free swell for Bentonite-2 may be attributed to the montmorillonite content (Table 3) which was the highest among all bentonites (i.e. 77%). The free swells obtained in LLs are consistent with the literature and are typical for the free swells of bentonites in salt solutions or waste leachates (Jo et al., 2001; Lee et al., 2005; Katsumi et al., 2007). The reductions in the free swells of bentonites in LLs are due to the chemical composition of landfill leachates. Since chemical compositions were determined in terms of exchangeable cation concentrations, it can be seen from Table 2 that landfill leachates contain considerable amount of Na+, K+, Mg2+, Ca2+. During the free swell tests, bentonite particles are bombarded with the cations when faced with LLs. Thus, the thicknesses of diffuse double layers surrounding the particles are suppressed, resulting lower free swell volumes for bentonites in LLs.
3.2. Hydraulic conductivity Some post-test properties (GCL height and bentonite water content) as well as the final hydraulic conductivities of AP-GCLs are summarized in Table 5. Note that the final hydraulic conductivities were the average of last four readings. During the test program, 12 tests were conducted with DIW and TW. The final hydraulic conductivities of AP-GCLs were within the range of 5.2×10−12 to 3.0×10−11 m/s when TW and DIW were used as the permeant (Table 5). Among others, only one bentonite had been treated with polymer (Bentonite-6). This modified bentonite had comparable hydraulic conductivities to tap water and DIW (Table 5). These values are slightly less than the hydraulic conductivities of the GCLs reported in the literature (Petrov et al., 1997; Jo et al., 2001; Katsumi et al., 2007). This can be attributed to the greater effective stress applied during this study (i.e. 90 kPa). The hydraulic conductivity tests with LLs were conducted on four AP-GCLs which were selected based on the hydraulic conductivity values of bentonites to DIW. Thus, Bentonite-4 (AP-GCL-4) and Bentonite-6 (AP-GCL-6) were not permeated with LLs. Since Bentonite-6 was polymer treated, it was intended to conduct hydraulic conductivity tests with LLs on this bentonite. However, the free swell values in LLs were low (7.5–11.0 mL/2g) and very close to free swell of Bentonite-5. Thus, Bentonite-5 was selected to perform the hydraulic conductivity tests. Moreover, although Bentonite-1 (AP-GCL-1) had one of the greatest hydraulic conductivity to water with respect to other bentonites (i.e. 1.3×10−9 cm/s), it was kept under the test program to see whether LL-2 and LL-3 would have any influence on the hydraulic conductivity of this bentonite. But it did not seem to be necessary to conduct the hydraulic conductivity test on Bentonit-1 (AP-GCL-1) with LL-1. Moreover, Bentonite-3 (AP-GCL-3) had the lowest hydraulic conductivity to DIW (5.2×10−10 cm/s). Because of this property, Bentonite-3 was tested two times with each LL one of which was permeated for the long term. Long term hydraulic conductivity tests were lasted about one year. The total of hydraulic conductivity tests permeated with LLs was 14. The hydraulic conductivity behaviors of AP-GCLs permeated with LLs were similar within each other. Thus, only the results with LL-1 are given herein as a function of pore volumes of flow (PVF) (Fig. 2). The details about the other tests conducted with LL-2 and LL-3 can be found elsewhere (Ören et al., 2015). Table 5 also presents the final hydraulic conductivities of AP-GCLs to LLs. The physical equilibrium (i.e. Qout/Qin) was also checked by dividing the outflow volumes to inflow volumes (Fig. 2a–c). The results showed that there was almost twofold decrease in the hydraulic conductivity of AP-GCL-2 along the test duration (Fig. 2a). Similar reductions in the hydraulic conductivities were also measured for AP-GCL-3 and AP-GCL-5 as the PVFs increased
Table 3 Basic soil index properties of bentonites. Bentonite
Specific gravity
Natural water content (%)
Clay content (%)
Liquid limit (%)
Plastic limit (%)
Montmorillonite content (%)
1 2 3 4 5 6
2.76 2.69 2.67 2.72 2.76 2.71
12 12 9 10 12 12
67 73 78 31 78 78
283 529 552 149 397 417
48 38 41 42 34 44
61 77 72 67 63 68
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
4
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
Fig. 1. Particle size distribution of bentonites that were used in AP-GCLs.
(Fig. 2b–c). The short and long term hydraulic conductivity behaviors of AP-GCL-3 were almost the same. Although long term test was lasted twice of the short term test, the PVFs at the termination were the same for both tests. This is due to the fact that short term hydraulic conductivity was about two-fold greater than the long term hydraulic conductivity (8.2×10−10 cm/s versus 4.5×10−10 cm/s). The hydraulic conductivities of AP-GCLs with LLs were within the range of 2.3×10−12 to 2.0×10−11 m/s (Table 5). However, the hydraulic conductivities of AP-GCL-1 to LLs were about one order of magnitude greater than those of other AP-GCLs. There may be two possible explanations for the greater hydraulic conductivity for AP-GCL-1: i) larger air-dried particle size and ii) poor physical and mineralogical properties of Bentonite-1. As mentioned before, Bentonite 2–6 were in the powdered form while preparing AP-GCLs. However, Bentonite-1 had sand sized particles for the reason that the particles passing through No. 40 sieve were used in AP-GCL-1. Thus, inter-particle pore spaces are expected to be greater in AP-GCL-1 than in other AP-GCLs. The other reason may be due to having lower montmorillonite content for Bentonite1 (i.e. 61%). This may also lead to have lower clay content (67%) and liquid limit value (283%) for the bentonite. It is interesting to note that, however, the hydraulic conductivities of AP-GCL-2 to LLs were up to 2.7 times less than the hydraulic conductivity to DIW (Fig. 3). It is possibly due to the minor differences between samples that may have been generated while preparing the AP-GCLs. In contrast, the hydraulic conductivities of AP-GCL-3 increased with LL permeation. Fig. 3 shows that the short term hydraulic conductivities AP-GCL-3 to LL-2 and LL-3 were up to two-fold, whereas the long term hydraulic conductivities to LL-1 and LL-3 were 1.6 times and 3.8 times, respectively, greater than the hydraulic conductivities to DIW. Unlike AP-GCL-2 and AP-GCL-3, the influence of LLs had no effect on the hydraulic conductivity of AP-GCL-1 and AP-GCL-5 when compared to the hydraulic conductivity of the same GCLs to DIW. Regarding to the obtained results, it can be stated that there was no practical difference between the hydraulic conductivities of AP-GCLs when permeated
Table 4 Free swell of bentonites in different pore fluids. Bentonite
1 2 3 4 5 6
Free swell (mL/2g) DI water
LL-1
LL-2
LL-3
14.5 21.5 25.0 8.0 19.5 21.0
5.0 19.5 8.0 7.0 10.0 10.0
5.0 12.0 7.0 7.0 8.0 7.5
5.0 18.0 8.0 9.0 10.0 11.0
with LLs and DIW. In other words, hydraulic conductivities of AP-GCLs to LLs were comparable to the hydraulic conductivities to DIW or TW (Fig. 3). Physical factors such as using powdered bentonite in AP-GCLs and applying higher effective stress (i.e. 90 kPa) during the hydraulic conductivity tests are responsible for the low hydraulic conductivities to LLs. Due to low flow rate, the cation exchange process that took place between bentonites and LLs may not have been completed when hydraulic conductivity tests were ceased. The chemical equilibrium was checked along the test duration by measuring pH and electrical conductivity (EC) of influents and effluents. Then, these values were normalized with the corresponding influent values. The normalized pH and EC values are given just in case of permeation with LL-1 and shown in Fig. 4a–c as a function of PVF. The final normalized pH and EC values for the other tests conducted with LL-2 and LL-3 are summarized in Table 6 as well. At the time of termination, it was seen that the most pHout/pHin values were around 1.2 which is Table 5 The final heights, water contents and hydraulic conductivities of GCLs permeated with DIW, TW and landfill leachates. Test GCL type number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-4 AP-GCL-5 AP-GCL-6 AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-4 AP-GCL-5 AP-GCL-6 AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5 AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5 AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5
Permeant Final type height (cm)
Final water content (%)
Final hydraulic conductivity (cm/s)
DIW DIW DIW DIW DIW DIW TW TW TW TW TW TW LL-1 LL-1 LL-1 LL-1 LL-2 LL-2 LL-2 LL-2 LL-2 LL-3 LL-3 LL-3 LL-3 LL-3
81 145 225 101 93 87 163 139 203 108 93 87 101 107 100 85 113 106 112 104 77 83 151 120 105 91
1.3×10−9 6.1×10−10 5.2×10−10 3.0×10−9 5.9×10−10 8.1×10−10 1.5×10−9 6.3×10−10 8.5×10−10 3.0×10−9 6.2×10−10 6.9×10−10 2.8×10−10 4.5×10−10 8.2×10−10 5.9×10−10 1.2×10−9 2.3×10−10 9.9×10−10 3.5×10−10 6.1×10−10 1.94×10−9 5.0×10−10 7.8×10−10 2.0×10−9 4.6×10−10
0.48 1.10 1.38 0.79 0.84 0.79 0.99 1.10 1.41 0.79 0.84 0.79 0.85 0.89 0.88 0.74 0.72 0.73 0.92 0.91 0.73 0.76 – 0.97 – 0.67
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
5
Fig. 2. Hydraulic conductivity behaviors of AP-GCLs when permeated with Landfill Leachate-1 (LL-1): a) AP-GCL-2, b) AP-GCL-3, c) AP-GCL-5.
about 10% above the upper limit suggested in ASTM D6766 (1.0 ± 0.1). It is due to the reduction in the influent pH values along the test duration that result in an increase in the pHout/pHin values. These values are in agreement with the findings of Jo et al. (2005) who drawn similar conclusion with the salt solutions. However, although normalized EC values were within the range of 1.0 ± 0.1 during permeation with LL-1 and LL-3, they were as low as ~ 0.5 in the case of permeation
with LL-2. Based on these findings, one can argue that pH and EC measurements give little information about the completion of cation exchange process that took place between bentonites and LLs. In order to figure out the cation replacement between bentonite and LL, the effluent concentrations as well as the ratio of effluent to influent concentrations of AP-GCL-3 when permeated with LL-1 are given in Fig. 5 as a function of PVF. The circles denote the long term, whereas
Fig. 3. Final hydraulic conductivities of AP-GCLs.
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
6
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
Fig. 4. Normalized values of pH and electrical conductivity (EC) as a function of pore volumes of flow (PVF): a) AP-GCL-2, b) AP-GCL-3, c) AP-GCL-5.
the squares denote the short term test results. Moreover, close and open symbols show the effluent concentrations and the ratio of effluent to influent concentrations, respectively. It is indicated in Fig. 5a that the sodium concentration in the effluent was greater than that of in the influent when the tests were terminated, resulting greater normalized sodium values with respect to magnesium and potassium. The effluent
Table 6 pH and electrical conductivities normalized by dividing the effluent values to influent values. Test number
GCL type
Permeant type
pHout/pHin
ECout/ECin
13 14 15 16 17 18 19 20 21 22 23 24 25 26
AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5 AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5 AP-GCL-1 AP-GCL-2 AP-GCL-3 AP-GCL-3 AP-GCL-5
LL-1 LL-1 LL-1 LL-1 LL-2 LL-2 LL-2 LL-2 LL-2 LL-3 LL-3 LL-3 LL-3 LL-3
1.24 1.20 1.18 1.24 1.01 1.20 1.18 1.15 1.19 1.29 1.15 1.17 1.25 1.32
0.94 1.05 1.03 0.87 0.86 0.54 0.58 0.88 0.55 1.20 1.09 1.08 0.74 1.00
to influent ratios for potassium and magnesium were close to 1.0 which shows the concentration equilibrium was almost reached at the end of hydraulic conductivity tests (Fig. 5b–c). In contrast, the effluent concentration of calcium was lower than that of influent, resulting lower effluent to influent ratio Ca2+ (i.e. 0.3) (Fig. 5d). Based on greater sodium and lower calcium concentrations in the effluents, it can be concluded that the cation replacement between AP-GCL-3 and LL-1 was not completed at the end of the tests. The similar conclusions were also drawn between other AP-GCLs which were permeated with LL-2 and LL-3. The details about the effluent concentrations of these tests can be found in Ören et al. (2015) as well. The solid/liquid ratio in free swell test is much lower than that in hydraulic conductivity test and therefore, the values obtained from these tests may not be related within each other (Bouazza et al., 2007). The lower solid/liquid ratio may result in release of some bound cations that were already adsorbed onto the bentonite particles into the free swell test solution (i.e. DIW) even cation exchange process complete. Hence, the free swell volume may be greater than its real volume. However, the free swell tests conducted on post test bentonites gave valuable information about the level of the cation exchange process at the end of hydraulic conductivity tests. Fig. 6 shows the free swell volumes of post test bentonites in DIW in comparison to the free swell volumes of new (or untreated) bentonites in DIW and LLs (Table 4). The only exception is for LL-3. Since the degree of cation exchange process was presented well with free swells of post test bentonites permeated with LL-1
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
7
Fig. 5. Effluent concentrations and the ratio of effluent to influent concentration for AP-GCL-3 when permeated with LL-1: a) sodium, b) potassium, c) magnesium, d) calcium.
and LL-2, this test was not performed on the bentonites permeated with LL-3. As a result, it is found that the free swell volumes of post test bentonites were between those of new samples in DIW and LLs, suggesting the cation exchange processes were not completed (Fig. 6a–b). The final AP-GCL heights and the final water contents of bentonite are also give some information about the swell of bentonites during hydraulic conductivity tests. Except AP-GCL-1 permeated with DIW, the final heights of all AP-GCLs were greater than the initial heights of APGCLs (i.e. 0.6 cm) even they were permeated with LLs. The final height for AP-GCL-1 was 0.48 cm which shows a reduction in the height (Table 5). This sample was the first sample prepared and tested in the permeameter during the testing program. Thus, this reduction was possibly generated from the operator who may not have achieved the target thickness at the first time of sample preparation. The heights of other AP-GCLs were within the ranges of 0.79–1.41 cm and
0.72–0.97 cm when permeated with DIW and LLs, respectively (Table 5). Since AP-GCLs were not needle-punched during sample preparation, the swell was not restricted. However, although the final heights for AP-GCLs permeated with LLs were less than those of the samples permeated with DIW, they were still greater than the initial heights, indicating swell behavior of bentonites during permeation. This also suggests osmotic swelling of bentonites. It is expected that the concentration of pore water decreases across the height of AP-GCLs. It is high where LL is in contact with GCLs. In contrast, the concentration of pore water decreases downwards because some of positively charged cations in LLs will be adsorbed on the bentonite. Since flow was from top to bottom, the swelling behavior of bentonites may be crystalline at the upper sections, whereas it may be crystalline and osmotic at the lower sections of the AP-GCLs. The final water contents of AP-GCLs also confirm this
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
8
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
hydraulic conductivity test results of some studies reported in the literature that were conducted under effective stresses between 20 and 34.5 kPa. Fig. 7 shows that the hydraulic conductivities of this study are less than the hydraulic conductivities given with the curve. The results can be separated into two groups based on free swell value of 15 mL/2g. The greater effective stress (90 kPa) applied in this study is responsible alone for the lower hydraulic conductivity with respect to literature when the free swell of bentonites is greater than 15 mL/2g. The values above this level generally obtained by the tests performed with DIW in this study and thus, there would be no cation exchange expected between bentonites and DIW. It is already known that increase in the effective stress result in a reduction in the pore spaces between bentonite particles (Estornell and Daniel, 1992; Petrov et al., 1997). However, the findings of this study are significantly below the proposed curve when the free swell value is between 5 mL/2g and 15 mL/2g. This can be attributed to the combined effects of greater effective stress applied during the study and the uncompleted cation exchange process between bentonite and LLs. 4. Conclusions
Fig. 6. Comparison of the free swell of post test bentonites in DIW with those of new samples in DIW and in: a) LL-1 and b) LL-2.
statement because the water contents were within the range of 77% and 151% (Table 5). The greater the final water content the lower is the hydraulic conductivity of AP-GCL (Petrov et al., 1997; Meer and Benson, 2007). Finally, the influence of effective stress on the hydraulic conductivity of AP-GCLs is shown in Fig. 7 as a function of free swell. A curve is also added on the graph which was drawn based on an equation proposed by Katsumi et al. (2008). Note that the proposed equation covers the
This study discusses the free swell and hydraulic conductivity behavior of six bentonites with water and landfill leachates (LLs). The free swell test results showed that the swell volumes of bentonites in DIW were greater than those in LLs. This was an expected behavior because the thickness of diffuse double layer surrounding the bentonite particles compressed on account of cations in LLs. Subsequently, hydraulic conductivity tests were conducted on bentonites. For this purpose, artificial geosynthetic clay liners (GCLs) were prepared in the laboratory by placing bentonites between non-woven and woven geotextiles without needle punching. This kind of GCL was named as “artificially prepared GCLs” and denoted by AP-GCLs. The hydraulic conductivities of AP-GCLs to DIW and TW were within the range of 5.2×10−12 to 3.0×10−11 m/s. The lower hydraulic conductivities of AP-GCLs to DIW with respect to those of GCLs reported in the literature can be attributed to the greater effective stress applied during the study (90 kPa). The greater the effective stress, the lower is the hydraulic conductivity for AP-GCLs. The hydraulic conductivities with LLs were between 2.3×10−12 m/s and 2.0×10−11 m/s which are comparable to the hydraulic conductivities to DIW and TW. However, the hydraulic conductivity of AP-GCL-1 had about an order of magnitude greater hydraulic conductivity to LLs when compared with those of other AP-GCLs. This is attributed to the larger particle size and poor physical and mineralogical characteristics of bentonite used in AP-GCL-1. On the other hand, AP-GCL-3 was
Fig. 7. The comparison of the findings of this study to those of the study reported in the literature.
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029
A.H. Ören, R.Ç. Akar / Applied Clay Science xxx (2016) xxx–xxx
permeated two times one of which was tested to determine long term (~one year) hydraulic conductivity with LLs. It is concluded that there was no practical difference between the short and long term hydraulic conductivities of AP-GCL-3. Hydraulic conductivity results with LLs were supported with pH and electrical conductivity measurements conducted on the influent and effluent samples. Effluent to influent ratio for electrical conductivity was generally less than one, indicating cation exchange was not completed when hydraulic conductivity tests were terminated. This was further confirmed by the cation analysis performed on the effluents and influents of LLs. Based on the cation analysis, it is found that effluent to influent of potassium and magnesium concentrations were close to one. However, calcium concentrations were lower, whereas sodium concentration were greater than one for all samples, suggesting cation exchange was still under progress at the end of the tests. This is further approved with the free swell tests conducted on post test bentonites which were permeated with LLs. The free swell volumes then were compared with those of new (or untreated) bentonites in DIW and LLs. It is found that the swell volumes of post test bentonites were between those of new bentonites in DIW and LLs. Thus, the lower hydraulic conductivity to LLs can be attributed not only to the greater effective stress applied during the study but also to the uncompleted cation exchange process at the end of tests. Acknowledgement This study was funded by the Scientific and Technical Research Council of Turkey, TUBITAK (Grant No: 111M718). The authors are grateful for this funding. Reference Ashmawy, A.K., El-Hajji, D., Sotelo, N., Muhammad, N., 2002. Hydraulic performance of untreated and polymer-treated bentonite in inorganic landfill leachates. Clay Clay Miner. 50, 546–552. http://dx.doi.org/10.1346/000986002320679288. ASTM:D2216-10, 2010. Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass. ASTM International, West Conshohocken, PA, USA, pp. 1–7 http://dx.doi.org/10.1520/D2216-10.2. ASTM:D422-63, 2007. Standard Test Method for Particle-Size Analysis of Soils. ASTM International, West Conshohocken, PA, USA, pp. 1–8 http://dx.doi.org/10.1520/ D0422-63R07E01.2. ASTM:D4318-05, 2005. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. ASTM International, West Conshohocken, PA, USA, pp. 1–14 http:// dx.doi.org/10.1520/D4318. ASTM:D5890-11, 2011. Standard Test Method for Fluid Loss of Clay Component of Geosynthetic Clay Liners. ASTM International, West Conshohocken, PA, USA, pp. 7–9 http://dx.doi.org/10.1520/D5890-11.2. ASTM:D854-14, 2014. Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer. ASTM International, West Conshohocken, PA, USA, pp. 1–8 http://dx. doi.org/10.1520/D0854-10. Benson, C.H., Ören, A.H., Gates, W.P., 2010. Hydraulic conductivity of two geosynthetic clay liners permeated with a hyperalkaline solution. Geotext. Geomembr. 28, 206–218. http://dx.doi.org/10.1016/j.geotexmem.2009.10.002. Bouazza, A., Jefferis, S., Vangpaisal, T., 2007. Investigation of the effects and degree of calcium exchange on the Atterberg limits and swelling of geosynthetic clay liners when
9
subjected to wet-dry cycles. Geotext. Geomembr. 25, 170–185. http://dx.doi.org/10. 1016/j.geotexmem.2006.11.001. Bradshaw, S.L., Benson, C.H., 2013. Effect of municipal solid waste leachate on hydraulic conductivity and exchange complex of geosynthetic clay liners. J. Geotech. Geoenviron. Eng. 140, 131003220201000. http://dx.doi.org/10.1061/(ASCE)GT. 1943-5606.0001050. Di Emidio, G., Mazzieri, E., Verastegui-Flores, R.-D., Van Impe, W., Bezuijen, A., 2015. Polymer-treated bentonite clay for chemical-resistant geosynthetic clay liners. Geosynth. Int. 22, 125–137. Estornell, P., Daniel, D.E., 1992. Hydraulic conductivity of three geosynthetic clay liners. J. Geotech. Eng. 118, 1592–1606. http://dx.doi.org/10.1061/(ASCE)07339410(1992)118:10(1592). Guyonnet, D., Touze-Foltz, N., Norotte, V., Pothier, C., Didier, G., Gailhanou, H., Blanc, P., Warmont, F., 2009. Performance-based indicators for controlling geosynthetic clay liners in landfill applications. Geotext. Geomembr. 27, 321–331. http://dx.doi.org/ 10.1016/j.geotexmem.2009.02.002. 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. J. Geotech. Geoenviron. Eng. 127 (7), 557–567. Jo, H.Y., Benson, C.H., Shackelford, C.D., Lee, J.-M., Edil, T.B., 2005. Long-term hydraulic conductivity of a geosynthetic clay liner permeated with inorganic salt solutions. J. Geotech. Geoenviron. Eng. 131, 405–417. http://dx.doi.org/10.1061/(ASCE)10900241(2005)131:4(405). Katsumi, T., Ishimori, H., Ogawa, A., Yoshikawa, K., Hanamoto, K., Fukagawa, R., 2007. Hydraulic conductivity of nonprehydrated geosynthetic clay liners permeated with inorganic solutions and waste leachates. Soils Found. 47, 79–96. http://dx.doi.org/10. 3208/sandf.47.79. Katsumi, T., Ishimori, H., Onikata, M., Fukagawa, R., 2008. Long-term barrier performance of modified bentonite materials against sodium and calcium permeant solutions. Geotext. Geomembr. 26, 14–30. http://dx.doi.org/10.1016/j.geotexmem.2007.04.003. Kolstad, D., Benson, C., Edil, T., 2004. Hydraulic conductivity and swell of nonprehydrated geosynthetic clay liners permeated with multispecies inorganic solutions. J. Geotech. 130, 1236–1249. http://dx.doi.org/10.1061/(ASCE)1090-0241(2004)130. Komine, H., 2004. Simplified evaluation for swelling characteristics of bentonites. Eng. Geol. 71, 265–279. http://dx.doi.org/10.1016/S0013-7952(03)00140-6. Lee, J.-M., Shackelford, C.D., 2005. Impact of bentonite quality on hydraulic conductivity of geosynthetic clay liners. J. Geotech. Geoenviron. Eng. 131, 64–77. http://dx.doi.org/ 10.1061/(ASCE)1090-0241(2005)131:1(64). Lee, J.-M., Shackelford, C.D., Benson, C.H., Jo, H.-Y., Edil, T.B., 2005. Correlating index properties and hydraulic conductivity of geosynthetic clay liners. J. Geotech. Geoenviron. Eng. 131, 1319–1329. http://dx.doi.org/10.1061/(ASCE)1090-0241(2005)131: 11(1319). Meer, S.R., Benson, C.H., 2007. Hydraulic conductivity of geosynthetic clay liners exhumed from landfill final covers. J. Geotech. Geoenviron. Eng. 133, 550–563. Mitchell, J.K., Soga, K., 2005. Fundamentals of Soil Behavior. 3rd Edition. John Wiley & Sons. Ören, A.H., Yukselen Aksoy, Y., Önal, O., 2015. Investigating the hydraulic conductivity of geosynthetic clay liners and detecting the suitable bentonites for manufacturing, Tubitak Report 111M718 (In Turkish with an English abstract). Petrov, R., Rowe, R., Quigley, R., 1997. Selected factors influencing GCL hydraulic conductivity. J. Geotech. Geoenviron. Eng. 123, 683–695. Ruhl, J.L., Daniel, D.E., 1997. Geosynthetic clay liners permeated with chemical solutions and leachates. J. Geotech. Geoenviron. Eng. 123, 369–381. http://dx.doi.org/10. 1061/(ASCE)1090-0241(1997)123:4(369). Shackelford, C.D., Benson, C.H., Katsumi, T., Edil, T.B., Lin, L., 2000. Evaluating the hydraulic conductivity of GCLs permeated with non-standard liquids. Geotext. Geomembr. 18, 133–161. http://dx.doi.org/10.1016/S0266-1144(99)00024-2. Shan, H.Y., Lai, Y.J., 2002. Effect of hydrating liquid on the hydraulic properties of geosynthetic clay liners. Geotext. Geomembr. 20, 19–38. http://dx.doi.org/10.1016/ S0266-1144(01)00023-1. Sivapullaiah, P.V., Sridharan, A., Stalin, V.K., 2000. Hydraulic conductivity of bentonitesand mixtures. Can. Geotech. J. 37, 406–413. Sridharan, A., 1991. Annual Lecture: Engineering behaviour of fine grained soils—a fundamental approach. Indian Geotech. J. 1–94.
Please cite this article as: Ören, A.H., Akar, R.Ç., Swelling and hydraulic conductivity of bentonites permeated with landfill leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.029