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Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates
CLAY-03977; No of Pages 11 Applied Clay Science xxx (2016) xxx–xxx
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Research paper
Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates A. Fehervari a, W.P. Gates b,⁎, T.W. Turney c, A.F. Patti d, A. Bouazza a a
Department of Civil Engineering, Monash University, Melbourne, Australia Australian Centre for Infrastructure Durability, Institute for Frontier Materials, Deakin University, Melbourne, Australia Department of Materials Science and Engineering, Monash University, Melbourne, Australia d School of Chemistry, Monash University, Melbourne, Australia b c
a r t i c l e
i n f o
Article history: Received 23 May 2016 Received in revised form 6 September 2016 Accepted 7 September 2016 Available online xxxx Keywords: Bentonite Bentonite modification Osmotic desiccation Glycerol carbonate Propylene carbonate
a b s t r a c t Two cyclic organic carbonates (COC), propylene carbonate (PC) and glycerol carbonate (GC), were investigated as saline-resistant modifying agents of Na+-montmorillonite using X-ray diffraction (XRD) and Fourier transform infrared (FTIR). PC has been studied previously and has been used as an effective amendment material of Na+bentonite for saline applications. In this research GC is proposed as a more effective modifying agent for containing hyper saline leachates. Na+-montmorillonite was reacted with up to 1 N sodium chloride (NaCl) and calcium chloride (CaCl2) salt solutions to assess changes in the interlayer spacing (i.e., d-value of the 001 reflection in XRD traces) due to osmotic desiccation, as well as to investigate the mechanism and strength of bonding between GC/ PC and Na+-montmorillonite by FTIR. GC/Na+-montmorillonite was strongly resistant against strongly saline sodic salt solution compared to PC/Na+-montmorillonite. CaCl2 solution had a more detrimental effect on COC modified Na+-montmorillonite, however, GC/Na+-montmorillonite appeared to retain more intercalated COC than PC/Na+-montmorillonite when leached by strong calcic salt solutions. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Various mining and mineral processing industries produce large quantities of hypersaline liquid waste (Thiel and Smith, 2004; Smith, 2008). More recently, the advent of reverse osmosis plants in remote locations (e.g., from coal seam gas industry) results in large amounts of highly saline liquid waste stored in lined evaporation ponds (Fan et al., 2009; Pérez-González et al., 2016). Often, processing liquids from solid ore wastes, waste rock dumps or heap leach pads or brine ponds are stored in disposal impoundments lined with geotextiles, including geosynthetic clay liners (Breitenbach and Smith, 2006; Hornsey et al., 2010). Geosynthetic clay liners (GCL), have been widely used as components of hydraulic barriers in many waste containment applications, usually in combination with a polymeric liner referred to as geomembrane (Bouazza, 2002; Hornsey et al., 2010; Rowe, 2014) and within the past decade, have become an important component in engineered barriers in mining applications. A GCL product contains a thin layer of sodium (Na+)-bentonite contained within two layers of geotextile. The favourable geotechnical characteristics (e.g. high swelling and low hydraulic conductivity) of these liner materials originate
⁎ Corresponding author. E-mail address: [email protected] (W.P. Gates).
from the properties of their bentonite component (Egloffstein, 2001; Gates et al., 2009). As such, GCL-based materials have become one of the leading lining technologies in waste management and disposal facilities (Rowe, 2014). Environmental or hydraulic barrier lining systems that incorporate GCL are not generally limited to cases where they are expected to be in contact only with clean water during their service lives. While a given GCL may have a very low hydraulic conductivity to water and low salinity waters (ionic strength: I b 0.1 M), due to resulting chemical incompatibility, they often inadequately attenuate the transport of leachates having extremes of pH or ionic strength (Petrov et al., 1997, Petrov and Rowe, 1997, Shackelford et al., 2000; Jo et al., 2001; Kashir and Yanful, 2001; Kolstad et al., 2004; Jo et al., 2005; Benson et al., 2010; Gates and Bouazza, 2010; Bouazza and Gates, 2014; Liu et al., 2013, 2015). The chemical incompatibility associated with the GCL products, in part, stems from reactions that take place with Na+-montmorillonite (Mt), which is the active mineral component of Na+-bentonite. These reactions include a loss of crystalline swelling of Mt which results from the high osmotic strengths of hypersaline leachates (Norrish, 1954; Slade et al., 1991; Slade et al., 1991) leading to degradation in GCL hydraulic performance (Quirk and Schofield, 1955; Guyonnet et al., 2005). GCL performance is thus often limited by the ability of the Na+-bentonite component to swell and form and maintain strong gels (Bouazza and Gates, 2014).
http://dx.doi.org/10.1016/j.clay.2016.09.007 0169-1317/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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A. Fehervari et al. / Applied Clay Science xxx (2016) xxx–xxx
Osmotically induced desiccation and cation exchange reactions, which may occur in bentonites when exposed to saline and hypersaline leachates, can change the microstructure of the bentonite by removing the hydration water from the smectite interlayer space (Aylmore and Quirk, 1960; Suquet et al., 1975; Tessier, 1990; Guyonnet et al., 2005; Laird, 2006) and ultimately may result in increased porosity because of the collapse of the Mt gel structure (Norrish and Quirk, 1954; Quirk and Schofield, 1955; Slade and Quick, 1991; Slade et al., 1991). Furthermore, cations with higher charge and/or smaller ionic radii may cause ion exchange within Mt interlayers due to their greater affinity to the exchange sites (Guyonnet et al., 2005; Meer and Benson, 2007). Such effects have been identified as important in diminishing the tight seal that swollen bentonite normally forms when wetted by aqueous liquids (Shackelford et al., 2000; Chertkov and Ravina, 2001; Rowe, 2005; Gates and Bouazza, 2010; Bouazza and Gates, 2014). In consequence, questions on the long term compatibility of GCL due to exposure of saline and hypersaline leachates remain. There therefore exists a need to (i) optimise GCL hydraulic behaviour when in contact with nonstandard liquids (Malusis and Shackelford, 2002; Shackelford and Lee, 2003), and (ii) develop possible solutions for applications where calcium-enriched leachates may have significant detrimental effects on the hydraulic performance of GCL (Guyonnet et al., 2005; Shackelford and Lee, 2003; Benson et al., 2010). The objective of this study was to examine whether glycerol carbonate (GC) modified Na+-bentonite can overcome or minimise the adverse effects associated with osmotic desiccation as well as Ca2 + for Na+ exchange in hypersaline leachates. Previous studies have shown that PC-modified Na+-bentonite retains good swelling in up to 0.3 M CaCl2 (Onikata et al., 1999) and has been proposed as an alternative material for liner systems (Katsumi et al., 2008). GC is a non-toxic cyclic organic carbonate solvent having good solubility in water and high dielectric permittivity (Chernyak, 2006). Moreover, GC can be relatively inexpensively synthesized by green chemical methods (Turney et al., 2013), and GC and its derivatives form stable intercalates with Na+-Mt (Gates et al., 2016). The hypothesis tested here is that GC offers a stronger interaction with interlayer Na+ than does PC, partly due to its higher permittivity (Chernyak, 2006), but also because it has an additional pendant OH functional group, which allows it to H-bond with both interlayer water and with the interlayer surfaces of Na+-Mt (Gates et al., 2016), thereby promoting resistance to the osmotically induced desiccation of saline leachates (Fig. 1). Incorporation of GC modified Na+-bentonite would thus be expected to improve the hydraulic performance of GCL materials to saline leachates. Stability of glycerol carbonate is considered to be crucial when used as a modifying agent in an environmental liner application, especially where the liner may be exposed to variable conditions for considerable lengths of time. Any degradation due to heat, sunlight, oxygen or microbiological activity can reduce its efficiency in improving barrier performance of Na+-bentonites against saline leachates. While investigation of long-term chemical stability of the cyclic organic carbonates was out of the scope of this project, some preliminary research has been carried out in order to obtain basic information on the stability and possible degradation of glycerol carbonate and propylene carbonate.
Table 1 Mineralogical properties of the natural sodium bentonite. Property Bulk mineralogy (% of total)a Montmorillonite (Mt) Quartz Opal/cristobalite-tridymite Feldspar b0.2 μmb (% of fraction) Montmorillonite Opal/cristobalite-tridymite Particle size (% of bulk) b0.2 μm N0.2 μm CEC (meq/100 g) Bulkc b0.2 μmd
Value 73 ± 1 18 ± 1 6±1 3±1 98 ± 1 2±1 53 ± 3 44 ± 3 85 104
a X-ray diffraction and Reitfeld analysis (performed at CSIRO Land and Water, Adelaide, Australia). b The b0.2 μm fraction was separated by centrifugation (e.g., Gates et al., 2002). c The methylene blue CEC tests were on bulk materials by CSIRO Land and Water (Adelaide, Australia) following the tetra-sodium pyrophosphate (TSPP) pre-treatment method of Wang et al. (1996). d The Ba CEC tests were conducted using X-ray fluorescence on oriented films of the b0.2 μm fractions by CSIRO Land and Water following in-house methods.
2. Materials and methods A natural Na+-bentonite (Miles, Queensland, Australia) marketed by Sibelco (Melbourne, Australia), commonly used in GCL applications in Australia, was selected in this study. Mineralogical and chemical properties of the bentonite are summarized in Table 1 (see also Gates et al. (2016)). Propylene carbonate (PC) (anhydrous, 99.7%) was purchased from Sigma-Aldrich and glycerol carbonate (GC) was synthesized according to the method developed by Turney et al. (2013). Basic chemical properties of GC and PC are summarized in Table 2. To determine the specific mechanism of interaction between the cyclic organic carbonates (COC) and the bentonites, the effects of COC addition and exposure to salt solutions on COC-bentonite complexes were investigated by Fourier transform infrared (FTIR) and X-ray powder diffraction (XRD) techniques. To reduce the amount of non-swelling components of the samples the bentonite was purified by sedimentation: 5 g bentonite was dispersed in 1 L distilled water by using an ultrasonic bath and the suspension was left undisturbed for two days to allow sedimentation of the non-swelling components. The upper half of the suspension was then decanted into a ceramic vessel and placed in an oven at 105 °C until an oven-dried material was obtained. The dry materials were hand ground with an agate mortar and pestle. This procedure effectively removed quartz and feldspar components, and significantly reduced the fraction of opal CT (Table 1), and was intended to improve investigations of the reactions of COC with the Na+-Mt component of Na+-bentonite.
Fig. 1. Proposed interactions of GC/Na+-montmorillonite with liquids having low and high salinity.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
A. Fehervari et al. / Applied Clay Science xxx (2016) xxx–xxx
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Table 2 Chemical properties of glycerol carbonate and propylene carbonate.
Formula Molar mass (g/mol) Form Boiling point at atmospheric press (°C) Density at 25 °C (g/mL) Solubility in water Purity (%) a b
GC
PC
C4H6O4 118.09 Liquid 137a 1.400a Yesa ≥99b
C4H6O3 102.09 Liquid 242a 1.205a Yesa ≥99a
http://www.sigmaaldrich.com/australia.html. Shaheen (2014).
Preparation (and step-wise analysis) of COC/Na+-bentonite complexes consisted of the following: 1. Orientated films of Na+-bentonite were deposited under suction onto ceramic tiles from suspensions in water (~100 mg clay/ceramic tile) with a diaphragm vacuum pump. 2. FTIR and XRD measurements were collected on the air dried samples (see specific methodology below). All measurements were referenced to this air-dry state. 3. The samples were then saturated with COC by vacuum filtering ~ 0.6 mL of a 20% COC in water mixture through the pre-deposited bentonite film. 4. FTIR and XRD measurements were re-collected on the air-dried COC/ Na+-bentonite complexes. 5. The films were then washed by vacuum-filtering 6.4 mL of NaCl or 4.3 mL of CaCl2 salt solution of an appropriate concentration. 6. FTIR and XRD measurements were re-collected on the air-dried saline-treated COC/Na+-bentonite complexes. An (XRD) technique was used to measure the d-value of the 001 reflection of various bentonites and modified bentonites. All the experiments were carried out on a Philips 1140 Diffractometer (copper Kα radiation) operated at 40 kV and 25 mA between 2 and 65° two theta (°2θ) with 0.02° step size and 0.5°/min scanning speed. For calculating the d(001) values from the XRD traces, the centre of the 001 reflection, as determined at mid-intensity, followed the widely known Bragg's (1913) law. The location and intensity of fundamental vibrations and associated rotational-vibrational features of functional groups in the COC molecules and Mt structure provide clues regarding specific interactions. In order to study the interactions of PC and GC with the Mt, infrared spectra from the mid-IR region were collected with a Thermo Scientific Nicolet 6700 FTIR spectrometer accessorised with an Attenuated Total Reflectance (ATR) diamond crystal window. The parameters of the FTIR measurements were as follows: Min. range limit: 650 cm−1 Max. range limit: 4000 cm−1 Number of scans: 512 Resolution: 4 cm−1 Spectral conversion: Kubelka-Munk (Yang and Miklavicic, 2005) A reference background was collected from atmosphere before each run using the same experimental setup as above. To investigate the strength of the GC-Na+-bentonite reaction and the stability of the GC/Na+-bentonite complex in salt solutions, a reverse-phase high-performance liquid chromatography (RP-HPLC) technique was selected. An Agilent 1200 series chromatograph with a quaternary pump, degasser, photo diode array ultra-violet (UV) detector (220 nm) and auto-sampler was used with a 5 μm, 3.9 × 150 mm Waters Symmetry C18 column to separate 10 mL of sample at a flow rate of 1 mL/min in a gradient mixture of acetonitrile (ACN) and water (5% ACN for 2 min, then increased to 50% ACN by 12 min).
Fig. 2. XRD traces of GC/ (a) and PC/ (b) Na+-bentonite complexes washed with NaCl or CaCl2 solutions.
3. Results and discussion 3.1. XRD of PC/ and GC/Na+-bentonite The XRD traces (Fig. 2) of GC/ and PC/Na+-bentonite indicated that both COC molecules effectively penetrated the interlayer spaces of Na+-Mt. Addition of GC and PC increased the size of the interlayer space of Na+-Mt to a similar extent. The d-values (Table 3) of the Na+-Mt and COC/Na+-Mt complexes were determined from the centre of the reflection at 1/2 full intensity. An application of the COC at 1:1 (wt/wt) to the bentonite increased the Na+-Mt d-value from 1.29 nm to 1.89 nm (GC) and 1.83 nm (PC). The measured 1.29 nm d(001) value of the untreated Na+-Mt represented an intermediate state between the 1-layer and 2-layer hydrates (Laird, 2006), whereas the 1.89/1.83 nm d(001) values of the COC/Na+-Mt component of the modified bentonites corresponded to the interlayer spaces of bi-layers of COC molecules around the interlayer cations (Onikata et al., 1999; Gates et al., 2016). Both GC and PC modification of bentonite at rates of Table 3 d-Values of Na+-Mt, Ca2+-Mt, GC/ and PC/Mt washed with NaCl or CaCl2.
Na+-Mt Ca2+-Mt PC/Na+-Mt Washed with CaCl2 Washed with NaCl GC/Na+-Mt Washed with CaCl2 Washed with NaCl
d-Value (nm)
FWHM (nm)
1.29 ± 0.03 1.58 ± 0.03 1.83 ± 0.03 1.55 ± 0.04 1.61 ± 0.04 1.89 ± 0.03 1.61 ± 0.04 1.86 ± 0.04
2.2 ± 0.04 2.4 ± 0.04 3.5 ± 0.04 2.4 ± 0.06 3.0 ± 0.06 3.3 ± 0.04 3.6 ± 0.06 2.7 ± 0.06
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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Fig. 3. IR spectra and characteristic bands of Na+-Mt within (a) 1250–4000 cm−1 range and (b) 650–1250 cm−1 range.
1:1 COC:Na+-bentonite resulted in both sharp and symmetric Na+-Mt 001 reflections with full width at half maximum (FWHM) narrower than those of the Na+-bentonite indicating no discernible interstratifications of COC/Na+-Mt intercalates (Fig. 2). Thus the COC effectively and orderly intercalated Na+-Mt creating a uniform COC/Na+-bentonite complex. 3.2. FTIR spectroscopy of PC/ and GC/Na+-bentonite complexes Intercalation of the COC into the interlayer space of Na+-Mt allows for potential interaction of COC with (i) the interlayer cation, (ii)
hydration water surrounding the cation and (iii) the interlayer surfaces of the Mt. The FTIR spectra of the PC/ and GC/Na+-bentonite complexes were examined to further probe which of these scenarios best described the intercalation of GC and PC into Na+-Mt. For the untreated Mt, absorbance bands characteristic of Mt (Madejova et al., 1997; Bishop et al., 2002; Gates, 2005) were identified (Fig. 3), including δ(OH) vibrations at 910 cm−1 (Al3+ \OH), 881 cm−1 2 \ (Fe3+ Al3+\\OH), 843 cm−1 (Al3+ Mg2+\\OH) as well as δ(AlO) deformation at 696 cm−1 (out of plane) and ν(SiO) vibrations at 1115 cm−1 (in plane) and 982 cm− 1 (out of plane). The octahedral cation–OH stretching complex band (ʋ(O\\H)str, Madejova et al., 1997) was
Fig. 4. Functional groups of GC.
Fig. 5. C_O stretching region of COC when intercalated in Na+-bentonite (a) GC and (b) PC.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
A. Fehervari et al. / Applied Clay Science xxx (2016) xxx–xxx Table 4 Positions of bands for COC, Na+-Mt and PC/ GC/Na+-Mt under study. Band
Anhydrous PCa
Wavenumbers (cm−1) ʋ(OH) − ʋ(C_O) 1790 ʋ(C\ \C) 1180 ʋ(C\ \O) 1079
PC/Na+-Mt
Anhydrous GCb
GC/Na+-Mt
− 1782 1190 Disappeared
3413 1783 1177 1085
3408 1774 1184 Disappeared
Band
Na+-Mt
PC/Na+-Mt
Na+-Mt
GC/Na+-Mt
Wavenumbers (cm−1) ʋ(OH)str ʋ(Si\ \O)in plane ʋ(Si\ \O)out of plane ʋ(Al\ \OH) ʋ(Fe\ \Al\ \OH) ʋ(Al\ \Mg\ \OH)
3618 1115 982 910 881 843
3622 1115 989 910 883 845
3618 1115 982 910 881 843
3622 1108 989 910 885 841
a b
NIST (2016). National Institute of Standards and Technology. Shaheen (2014).
observed centred at 3622 cm−1. The broad bands near 3400 and 3250 are ʋ(H\\O\\H) of adsorbed water and the water δ(H\\O\\H) associated with the interlayer cation was observed at 1632 cm−1. The position of the latter band is consistent with interlayer water associated with Na+ (Johnston et al., 1992). These bands are expected to be significantly affected by the saline solutions. A decrease in the intensity of both bands is expected due to interlayer water loss in hypersaline leachates. PC and GC have four functional groups in common (Fig. 4): a carbonate ester group, the carbonyl part of the carbonate group and two nonequivalent ether groups. GC differs from PC in having a hydroxymethyl group instead of a methyl group connected to the cyclic carbonate.
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The effect of inorganic salts on the COC/Na+-bentonites were generally observed by intensity changes of the characteristic bands of cyclic carbonyl (C_O) stretches at 1783 cm−1 (GC) and 1790 cm−1 (PC) (Onikata et al., 1999; Indran et al., 2014) as well as the C\\C stretching bands at 1177 cm−1 (GC) and 1180 cm−1 (PC) (Indran et al., 2014; NIST, 2016). Interactions between carbonyl groups and interlayer cations are expected to result in a red shift (Joseph and Jemmis, 2007) of the C_O bond in the IR spectrum of GC because, as the partial negative charge of the highly electronegative oxygen atom is decreased through electrostatic interaction with interlayer cations, the bond energy of the carbonyl group is decreased (it becomes more stable) (Parker and Frost, 1996; Onikata et al., 1999). Direct interaction between the interlayer cation and the carbonyl group of PC was shown by Onikata et al. (1999) to result in a shift greater than the shift due to an interaction through hydration water. A similar effect was observed for GC/Na+-Mt (Gates et al., 2016). The Mt interlayer surface oxygens potentially act as adsorption sites via hydrogen bonding (Sandi, 2005) which could affect the OH group of GC, because the hydroxyl may increase the electron density on the Si\\O bond. Onikata et al. (1999) and Gates et al. (2016) showed that the positions of the IR absorption bands of COC functional groups, which may interfere with water, are highly dependent on the water content of the carbonate. The presence of μM water results in shifts to the characteristic IR frequencies (Shaheen, 2014; Gates et al., 2016). Due to the high affinities of COC to water, drying at 105 °C was insufficient to eliminate all water. In order to gain a valid assessment of the changes/shifts to the IR spectra of GC and PC when intercalated into bentonites, the IR spectrum on anhydrous GC provided by Shaheen (2014) and anhydrous PC (National Institute of Standards and Technology, webbook.nist.gov) were used for comparison. All other samples (e.g., untreated and
Fig. 6. OH stretching region of COC when intercalated in (a) GC/ and (b) PC/Na+-bentonite complexes.
Fig. 7. Blue shifts of ʋ(Si\ \O)str for Na+-Mt modified with (a) GC (b) PC.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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Fig. 8. ʋ(C\ \C) stretching region of COC when intercalated in Na+-bentonite (a) GC and (b) PC.
Fig. 9. (a) ʋ(C = O)str and (b) ʋ(C\ \C)str of GC−/Na+-bentonite washed with I = 2 M of NaCl solution.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
A. Fehervari et al. / Applied Clay Science xxx (2016) xxx–xxx
modified bentonites, samples washed with salt solutions, etc.) were collected as part of this study for comparison. Formation of the GC/Na+-bentonite and subsequent intercalation of GC into the interlayer space of Na+-Mt resulted in a small to medium red shift of the characteristic carbonyl stretch (ʋ(C_O)) (Fig. 5; Table 4). Onikata et al. (1999) and Gates et al. (2016) observed similar red shifts for, respectively, PC/ and GC/ intercalated complexes of Na+-Mt. In GC/Na+ -Mt a small (5 cm− 1 ) red shift (3413 cm − 1 → 3408 cm− 1) of ʋ(O\\H)str for glycerol carbonate intercalation was observed (Fig. 6). The red shift of ʋ(O\\H)str may be attributed to either a H-bonding interaction between the hydroxyl group of GC and the surface oxygen of Mt or H-bonding of GC-OH with interlayer water (Gates et al., 2016). The red shift of ʋ(Si\\O)in plane indicates that the interlayer urface oxygen probably forms some H-bonding interaction with the hydroxyl of GC. This additional H-bond, which is precluded in PC/Na +-bentonite, possibly further stabilises GC molecules within the interlayer. The absence of the red shift of ʋ(Si\\O)in plane in PC/Na+-bentonite supports this theory. In Na+-Mt a small (4 cm−1) blue shift (3618 cm−1 → 3622 cm−1) of ʋ(O\\H)str (Fig. 6) and a small (7 cm−1) blue shift (982 cm−1 → 989 cm− 1) of ʋ(Si\\O)out of plane (Fig. 7) were observed when intercalated with either GC or PC. The blue shift of the ʋ(O\\H)str band is related to hydrogen bonding between the structural O\\H groups of Mt and COC. All C\\C
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stretches (ʋ(C\\C)) of both GC and PC blue shifted upon intercalation into Na+-Mt and varied from 7 cm−1 to 10 cm−1 (Fig. 8). 3.3. Effect of salt washing on intercalate stability To address some basic aspects of stability of the complexes when exposed to saline leachates, the COC/Na+-Mt complexes were washed with various salt solutions. Two general observations were made: (i) washing the samples with 4.3 mL of 1 M (I = 3 M) CaCl2 resulted in a larger shift of the 001 reflection (Fig. 2) compared to the effect of 6.4 mL of 2 M (I = 2 M) NaCl; and (ii) the GC/Na+-bentonite complex had a greater resistance against these high concentrations of inorganic salts compared to PC/Na+-bentonite. After washing the samples with ~ 140 × the amount of NaCl solution required to replace all interlayer cations (~ 100 mg clay/ceramic tile with ~ 80 meq/100 g), the d(001) value for the Na+-Mt component of the PC/Na+-bentonite decreased from 1.83 nm to 1.61 nm (Table 3). The same treatment only decreased the d(001) value of the Na+-Mt component of the GC/Na+-bentonite complex from 1.89 nm to 1.86 nm. The relatively large difference in the decrease of interlayer space indicates a stronger interaction between GC and Na+-Mt compared to the strength of interaction between PC and Na+-Mt. Samples exposed to 1 M CaCl2 solution, at a rate of ~95× the amount of Ca2+ needed to fully exchange all Na+ from the interlayer spaces,
Fig. 10. (a) ʋ(C = O)str and (b) ʋ(C\ \C)str of PC/Na+-bentonite washed with I = 2 M of NaCl solution.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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resulted in a similar magnitude of decrease in the interlayer space for both GC/ and PC/Na+-bentonite complexes. The d(001) values of the Na+-Mt components for both GC/ and PC/Na+-bentonite complexes decreased by 0.28 nm. However, the original interlayer thickness of Na+-Mt in the GC/Na+-bentonite complex (1.89 nm) was greater than the basal spacing of that in the PC/Na+-bentonite complex (1.83 nm) and hence a greater proportion of GC/Na+-Mt remained in a non-collapsed state (1.61 nm compared to 1.55 nm). XRD results presented here agree with geotechnical aspects of COC modified bentonites (Fehervari et al., 2016) where it was shown that at these high ionic strengths (2 M of NaCl and 3 M of CaCl2) GC/Na+-bentonite had a better barrier performance than PC/Na+-bentonite. Degradation of GC or PC in the COC modified bentonites due to exposure to either strongly saline NaCl or CaCl2 solutions was followed by FTIR as well. A decrease in intensity, or outright disappearance of the ʋ(C_O)str at 1774 (GC/Na+-bentonite), 1782 cm−1 (PC/Na+-bentonite) as well as the intensity decrease of ʋ(C\\C)str at 1184 cm−1 (GC/ Na+-bentonite), 1190 cm−1 (PC/Na+-bentonite) (Indran et al., 2014) was observed (Figs. 9-12). In general, these shifts or changes agree
well with the XRD analysis presented above. NaCl (I = 2 M) solution had less of an effect on the ʋ(C_O)str and ʋ(C\\C)str bands of the GC/ Na+-bentonite: only moderate decreases in intensity were observed (Fig. 9). The comparable characteristic bands for PC in PC/Na+-bentonite underwent larger reduction in intensity when washed with NaCl (Fig. 10). Exposure of both GC and PC modified samples to strongly saline CaCl2 solution (I = 3 M) resulted in relatively little increased resistance against the salt solution, as was indicated by the significant intensity losses of the ʋ(C_O)str (1700–1850 cm−1) and ʋ(C\\C)str (1100– 1250 cm−1) bands for both GC/ and PC/Na+-bentonite complexes (Figs. 11 and 12). These bands were nearly completely lost in the PC/ Na+-bentonite when washed with either NaCl or CaCl2, but the losses amounted to ~20% loss in the ʋ(C_O)str and ~10% loss in ʋ(C\\C)str intensities for GC/Na+-bentonite in NaCl. When washed with CaCl2 however, these bands were largely lost from GC/Na+-bentonite. These results raise some concern regarding the long-term stability of GC/ and PC/-Na+-Mt complexes to strongly saline leachates such as tested here.
Fig. 11. (a) ʋ(C = O)str and (b) ʋ(C\ \C)str of GC/Na+-bentonite washed with I = 3 M of CaCl2 solution.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
A. Fehervari et al. / Applied Clay Science xxx (2016) xxx–xxx
Fig. 12. (a) ʋ(C = O)str and (b) ʋ(C\ \C)str when PC/Na+-bentonite washed with I = 3 M of CaCl2 solution.
3.4. Stability of GC-bentonite complexes in salt solution To further investigate possible leaching of GC from the GC/Na+-bentonite complex to hypersaline leachate an RP-HPLC technique was used. Mixtures (~ 6% solid concentration) of GC/Na+-bentonite complex in 4.5 M CaCl2 solutions were allowed to react for up to 30 days after which time effluent was filtered and examined by RP-HPLC (Fig. 13). The main outcome of the RP-HPLC test was that the prepared GCbentonite complex was stable in I = 4.5 M CaCl2 solution for at least 30 days. The loss of the GC was negligible; there was an average of ~1.5% (absolute), which is within the error of the method. In companion studies (Fehervari, 2015) the GC/Na+ bentonite complex performance when reacted within I = 4.5 M CaCl2 resulted in essentially the same fluid loss values (67.5 ± 2 mL) independent of sample aging. The stability of GC in the Mt interlayer at low hydration water content or in the absence of interlayer water probably originated from the ion-dipole interaction between the interlayer cation and the C_O group of the carbonate (Onikata et al., 1999; Shaheen, 2014; Gates et al., 2016), which was further strengthened by the co-ordination of the GC-OH group with the surface oxygen of Mt (Gates et al., 2016).
Fig. 13. Results of RP-HPLC measurements on 1:1 GC/Na+-bentonite.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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3.5. Stability of the COC/Na+-bentonite complexes
4. Summary and conclusions
Previous work in our laboratory has indicated that PC/Na+-bentonite complexes were highly stable. Some basic tests to determine the stability of the GC/Na+ bentonite complexes were undertaken to evaluate complex stability. These included apparent evaporation at room temperature (RT) and at elevated temperatures. GC/Na+-bentonite complexes subjected to 105 °C for 24 h had no more evaporative loss than untreated Na+ bentonite. As such, the mass losses were considered to be solely due to the evaporation of water and not GC. The aleatory degradation of COC/Na+-bentonite complexes, primarily due to contact with air, was also investigated over the long-term (Fig. 14). 22.5 g of either oven dried or as received Na+-bentonite was mixed with 22.5 g of either GC or PC to obtain a 1:1 ratio between the Na+-bentonite and the COC. Then the mixtures were placed in open containers and the masses of the composites with time were followed for approximately a 20 month-period under laboratory conditions. Oven dried and as received (with ~9% gravimetric water content) materials were used to check whether the initial water content had any effect on the interaction of components in the COC/Na+-bentonite complex and their possible degradation. When analysing the mass changes of the prepared composites it was important to consider laboratory conditions (especially temperature and humidity). These factors could have impacted the rate of mass loss, or even have affected mass gain of the samples. In the current study the air temperature showed moderate fluctuation: during the period of investigation the ambient temperature was 25 °C ± 5 °C. Regarding humidity it was observed that there was a small increase in the mass of a sample on rainy or humid days, however, relative humidity had no effect on the overall long-term trends. In general ~2 g differences were measured between the mass of the samples prepared by mixing oven dried bentonite with COC compared with mixing as received bentonite with COC because the water content of the air dried bentonite was ~9%. Hence, it is likely that the oven dried samples contained ~2 g more clay in the COC/Na+-bentonite complex. Any other effect of drying the samples was not observed. Over the period of investigation, ~2.7% and ~6.3% mass losses were observed for the oven-dried and air-dried GC/Na+-bentonite complex. Respective specimens with propylene carbonate showed ~ 20.2% and 24.6% weight losses over the same time period. While the results would represent that when COC/Na+-bentonite complexes are fully exposed to the surrounding atmospheric conditions, PC/Na+-bentonites resulted in losses of 41% (9.17 g) and 50% (11.25 g) of initial PC added, which is an undesirable and potentially worrisome result since PC-bentonite is currently marketed as multiswellable bentonite for geotechnical applications in saline environments (Scalia et al., 2014; Bohnhoff et al., 2013).
The main goal of this paper was to investigate COC-modified bentonite complexes for enhanced and extended containment of hypersaline (sodic and calcic) leachates. For this purpose attributes of GC-modified Na+-bentonite was investigated with NaCl and CaCl2 solutions and compared to unmodified Na+-bentonite and PC-modified Na+-bentonite. Both XRD and FTIR results indicated that the GC/Na+-bentonite complex was strongly resistant against sodic salt solution compared to the PC/Na+-bentonite complex. CaCl2 solution had a more detrimental effect on both modified bentonites, however, the GC/Na+-bentonite appeared to retain more COC than the PC/Na+-bentonite when leached by strong calcic salt solutions. It is proposed that in both GC and PC the carbonyl groups of the carbonates interact with the interlayer cations through hydration water molecules at low ionic strengths (Onikata et al., 1999). However, at higher ionic strengths, the carbonyl interacts directly with the cation. In this study GC was found to be more effective in linking directly to interlayer Na+ compared to PC at high concentrations of NaCl and CaCl2. Consequently, at very high salt concentrations the GC/Na+-bentonite complex maintained a more swollen state and better hydraulic barrier performance because of this stronger interaction. In addition, GC was able to interact with the smectite interlayer surfaces through its hydroxyl groups. While these interactions slightly limited the crystalline swelling of GC/Na+-Mt in low salinity liquids (e.g., in water) (Fehervari et al., 2016), the stronger interlayer Na+–GC–Mt surface interaction was beneficial in hypersaline solutions because GC stuck in the smectite interlayer and could not be removed easily. The long-term laboratory and field testings of COC/Na+-bentonites have yet to be conducted including the stability of COC when exposed to microbial activity. Such studies are required before the long-term use of these modifying agents and their complexes with bentonite can be fully promoted. Research carried out so far showed that depending on the specific engineering application, the use of PC/Na+-bentonite complexes could be an appropriate option for saline conditions where I b 0.3 M (Katsumi et al., 2008). However, the current study clearly indicated that the GC/Na+-bentonite complex is more stable and therefore may be a better option in applications where contact with hypersaline leachates with I b 1 M is anticipated. In addition, GC can be advantageously produced using a cost effective green chemical method (Turney et al., 2013). GC thus has strong potential for its use as an alternative additive to bentonite under hypersaline and calcium-enriched applications.
Acknowledgements This study was supported under the Australian Research Council's Discovery Projects funding scheme (project number ARC DP1095129). The authors' sincere appreciation is extended to the council. The first author is grateful for the funding provided by the Faculty of Engineering and the Department of Civil Engineering at Monash University to support his PhD studies. The authors would like to express their gratitude to Ms. Gyögyvér Engloner for her assistance with creating figures and the Graphical Abstract.
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Fig. 14. Mass changes of bentonite-COC complexes with time.
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Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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Research paper
Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates A. Fehervari a, W.P. Gates b,⁎, T.W. Turney c, A.F. Patti d, A. Bouazza a a
Department of Civil Engineering, Monash University, Melbourne, Australia Australian Centre for Infrastructure Durability, Institute for Frontier Materials, Deakin University, Melbourne, Australia Department of Materials Science and Engineering, Monash University, Melbourne, Australia d School of Chemistry, Monash University, Melbourne, Australia b c
a r t i c l e
i n f o
Article history: Received 23 May 2016 Received in revised form 6 September 2016 Accepted 7 September 2016 Available online xxxx Keywords: Bentonite Bentonite modification Osmotic desiccation Glycerol carbonate Propylene carbonate
a b s t r a c t Two cyclic organic carbonates (COC), propylene carbonate (PC) and glycerol carbonate (GC), were investigated as saline-resistant modifying agents of Na+-montmorillonite using X-ray diffraction (XRD) and Fourier transform infrared (FTIR). PC has been studied previously and has been used as an effective amendment material of Na+bentonite for saline applications. In this research GC is proposed as a more effective modifying agent for containing hyper saline leachates. Na+-montmorillonite was reacted with up to 1 N sodium chloride (NaCl) and calcium chloride (CaCl2) salt solutions to assess changes in the interlayer spacing (i.e., d-value of the 001 reflection in XRD traces) due to osmotic desiccation, as well as to investigate the mechanism and strength of bonding between GC/ PC and Na+-montmorillonite by FTIR. GC/Na+-montmorillonite was strongly resistant against strongly saline sodic salt solution compared to PC/Na+-montmorillonite. CaCl2 solution had a more detrimental effect on COC modified Na+-montmorillonite, however, GC/Na+-montmorillonite appeared to retain more intercalated COC than PC/Na+-montmorillonite when leached by strong calcic salt solutions. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Various mining and mineral processing industries produce large quantities of hypersaline liquid waste (Thiel and Smith, 2004; Smith, 2008). More recently, the advent of reverse osmosis plants in remote locations (e.g., from coal seam gas industry) results in large amounts of highly saline liquid waste stored in lined evaporation ponds (Fan et al., 2009; Pérez-González et al., 2016). Often, processing liquids from solid ore wastes, waste rock dumps or heap leach pads or brine ponds are stored in disposal impoundments lined with geotextiles, including geosynthetic clay liners (Breitenbach and Smith, 2006; Hornsey et al., 2010). Geosynthetic clay liners (GCL), have been widely used as components of hydraulic barriers in many waste containment applications, usually in combination with a polymeric liner referred to as geomembrane (Bouazza, 2002; Hornsey et al., 2010; Rowe, 2014) and within the past decade, have become an important component in engineered barriers in mining applications. A GCL product contains a thin layer of sodium (Na+)-bentonite contained within two layers of geotextile. The favourable geotechnical characteristics (e.g. high swelling and low hydraulic conductivity) of these liner materials originate
⁎ Corresponding author. E-mail address: [email protected] (W.P. Gates).
from the properties of their bentonite component (Egloffstein, 2001; Gates et al., 2009). As such, GCL-based materials have become one of the leading lining technologies in waste management and disposal facilities (Rowe, 2014). Environmental or hydraulic barrier lining systems that incorporate GCL are not generally limited to cases where they are expected to be in contact only with clean water during their service lives. While a given GCL may have a very low hydraulic conductivity to water and low salinity waters (ionic strength: I b 0.1 M), due to resulting chemical incompatibility, they often inadequately attenuate the transport of leachates having extremes of pH or ionic strength (Petrov et al., 1997, Petrov and Rowe, 1997, Shackelford et al., 2000; Jo et al., 2001; Kashir and Yanful, 2001; Kolstad et al., 2004; Jo et al., 2005; Benson et al., 2010; Gates and Bouazza, 2010; Bouazza and Gates, 2014; Liu et al., 2013, 2015). The chemical incompatibility associated with the GCL products, in part, stems from reactions that take place with Na+-montmorillonite (Mt), which is the active mineral component of Na+-bentonite. These reactions include a loss of crystalline swelling of Mt which results from the high osmotic strengths of hypersaline leachates (Norrish, 1954; Slade et al., 1991; Slade et al., 1991) leading to degradation in GCL hydraulic performance (Quirk and Schofield, 1955; Guyonnet et al., 2005). GCL performance is thus often limited by the ability of the Na+-bentonite component to swell and form and maintain strong gels (Bouazza and Gates, 2014).
http://dx.doi.org/10.1016/j.clay.2016.09.007 0169-1317/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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Osmotically induced desiccation and cation exchange reactions, which may occur in bentonites when exposed to saline and hypersaline leachates, can change the microstructure of the bentonite by removing the hydration water from the smectite interlayer space (Aylmore and Quirk, 1960; Suquet et al., 1975; Tessier, 1990; Guyonnet et al., 2005; Laird, 2006) and ultimately may result in increased porosity because of the collapse of the Mt gel structure (Norrish and Quirk, 1954; Quirk and Schofield, 1955; Slade and Quick, 1991; Slade et al., 1991). Furthermore, cations with higher charge and/or smaller ionic radii may cause ion exchange within Mt interlayers due to their greater affinity to the exchange sites (Guyonnet et al., 2005; Meer and Benson, 2007). Such effects have been identified as important in diminishing the tight seal that swollen bentonite normally forms when wetted by aqueous liquids (Shackelford et al., 2000; Chertkov and Ravina, 2001; Rowe, 2005; Gates and Bouazza, 2010; Bouazza and Gates, 2014). In consequence, questions on the long term compatibility of GCL due to exposure of saline and hypersaline leachates remain. There therefore exists a need to (i) optimise GCL hydraulic behaviour when in contact with nonstandard liquids (Malusis and Shackelford, 2002; Shackelford and Lee, 2003), and (ii) develop possible solutions for applications where calcium-enriched leachates may have significant detrimental effects on the hydraulic performance of GCL (Guyonnet et al., 2005; Shackelford and Lee, 2003; Benson et al., 2010). The objective of this study was to examine whether glycerol carbonate (GC) modified Na+-bentonite can overcome or minimise the adverse effects associated with osmotic desiccation as well as Ca2 + for Na+ exchange in hypersaline leachates. Previous studies have shown that PC-modified Na+-bentonite retains good swelling in up to 0.3 M CaCl2 (Onikata et al., 1999) and has been proposed as an alternative material for liner systems (Katsumi et al., 2008). GC is a non-toxic cyclic organic carbonate solvent having good solubility in water and high dielectric permittivity (Chernyak, 2006). Moreover, GC can be relatively inexpensively synthesized by green chemical methods (Turney et al., 2013), and GC and its derivatives form stable intercalates with Na+-Mt (Gates et al., 2016). The hypothesis tested here is that GC offers a stronger interaction with interlayer Na+ than does PC, partly due to its higher permittivity (Chernyak, 2006), but also because it has an additional pendant OH functional group, which allows it to H-bond with both interlayer water and with the interlayer surfaces of Na+-Mt (Gates et al., 2016), thereby promoting resistance to the osmotically induced desiccation of saline leachates (Fig. 1). Incorporation of GC modified Na+-bentonite would thus be expected to improve the hydraulic performance of GCL materials to saline leachates. Stability of glycerol carbonate is considered to be crucial when used as a modifying agent in an environmental liner application, especially where the liner may be exposed to variable conditions for considerable lengths of time. Any degradation due to heat, sunlight, oxygen or microbiological activity can reduce its efficiency in improving barrier performance of Na+-bentonites against saline leachates. While investigation of long-term chemical stability of the cyclic organic carbonates was out of the scope of this project, some preliminary research has been carried out in order to obtain basic information on the stability and possible degradation of glycerol carbonate and propylene carbonate.
Table 1 Mineralogical properties of the natural sodium bentonite. Property Bulk mineralogy (% of total)a Montmorillonite (Mt) Quartz Opal/cristobalite-tridymite Feldspar b0.2 μmb (% of fraction) Montmorillonite Opal/cristobalite-tridymite Particle size (% of bulk) b0.2 μm N0.2 μm CEC (meq/100 g) Bulkc b0.2 μmd
Value 73 ± 1 18 ± 1 6±1 3±1 98 ± 1 2±1 53 ± 3 44 ± 3 85 104
a X-ray diffraction and Reitfeld analysis (performed at CSIRO Land and Water, Adelaide, Australia). b The b0.2 μm fraction was separated by centrifugation (e.g., Gates et al., 2002). c The methylene blue CEC tests were on bulk materials by CSIRO Land and Water (Adelaide, Australia) following the tetra-sodium pyrophosphate (TSPP) pre-treatment method of Wang et al. (1996). d The Ba CEC tests were conducted using X-ray fluorescence on oriented films of the b0.2 μm fractions by CSIRO Land and Water following in-house methods.
2. Materials and methods A natural Na+-bentonite (Miles, Queensland, Australia) marketed by Sibelco (Melbourne, Australia), commonly used in GCL applications in Australia, was selected in this study. Mineralogical and chemical properties of the bentonite are summarized in Table 1 (see also Gates et al. (2016)). Propylene carbonate (PC) (anhydrous, 99.7%) was purchased from Sigma-Aldrich and glycerol carbonate (GC) was synthesized according to the method developed by Turney et al. (2013). Basic chemical properties of GC and PC are summarized in Table 2. To determine the specific mechanism of interaction between the cyclic organic carbonates (COC) and the bentonites, the effects of COC addition and exposure to salt solutions on COC-bentonite complexes were investigated by Fourier transform infrared (FTIR) and X-ray powder diffraction (XRD) techniques. To reduce the amount of non-swelling components of the samples the bentonite was purified by sedimentation: 5 g bentonite was dispersed in 1 L distilled water by using an ultrasonic bath and the suspension was left undisturbed for two days to allow sedimentation of the non-swelling components. The upper half of the suspension was then decanted into a ceramic vessel and placed in an oven at 105 °C until an oven-dried material was obtained. The dry materials were hand ground with an agate mortar and pestle. This procedure effectively removed quartz and feldspar components, and significantly reduced the fraction of opal CT (Table 1), and was intended to improve investigations of the reactions of COC with the Na+-Mt component of Na+-bentonite.
Fig. 1. Proposed interactions of GC/Na+-montmorillonite with liquids having low and high salinity.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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Table 2 Chemical properties of glycerol carbonate and propylene carbonate.
Formula Molar mass (g/mol) Form Boiling point at atmospheric press (°C) Density at 25 °C (g/mL) Solubility in water Purity (%) a b
GC
PC
C4H6O4 118.09 Liquid 137a 1.400a Yesa ≥99b
C4H6O3 102.09 Liquid 242a 1.205a Yesa ≥99a
http://www.sigmaaldrich.com/australia.html. Shaheen (2014).
Preparation (and step-wise analysis) of COC/Na+-bentonite complexes consisted of the following: 1. Orientated films of Na+-bentonite were deposited under suction onto ceramic tiles from suspensions in water (~100 mg clay/ceramic tile) with a diaphragm vacuum pump. 2. FTIR and XRD measurements were collected on the air dried samples (see specific methodology below). All measurements were referenced to this air-dry state. 3. The samples were then saturated with COC by vacuum filtering ~ 0.6 mL of a 20% COC in water mixture through the pre-deposited bentonite film. 4. FTIR and XRD measurements were re-collected on the air-dried COC/ Na+-bentonite complexes. 5. The films were then washed by vacuum-filtering 6.4 mL of NaCl or 4.3 mL of CaCl2 salt solution of an appropriate concentration. 6. FTIR and XRD measurements were re-collected on the air-dried saline-treated COC/Na+-bentonite complexes. An (XRD) technique was used to measure the d-value of the 001 reflection of various bentonites and modified bentonites. All the experiments were carried out on a Philips 1140 Diffractometer (copper Kα radiation) operated at 40 kV and 25 mA between 2 and 65° two theta (°2θ) with 0.02° step size and 0.5°/min scanning speed. For calculating the d(001) values from the XRD traces, the centre of the 001 reflection, as determined at mid-intensity, followed the widely known Bragg's (1913) law. The location and intensity of fundamental vibrations and associated rotational-vibrational features of functional groups in the COC molecules and Mt structure provide clues regarding specific interactions. In order to study the interactions of PC and GC with the Mt, infrared spectra from the mid-IR region were collected with a Thermo Scientific Nicolet 6700 FTIR spectrometer accessorised with an Attenuated Total Reflectance (ATR) diamond crystal window. The parameters of the FTIR measurements were as follows: Min. range limit: 650 cm−1 Max. range limit: 4000 cm−1 Number of scans: 512 Resolution: 4 cm−1 Spectral conversion: Kubelka-Munk (Yang and Miklavicic, 2005) A reference background was collected from atmosphere before each run using the same experimental setup as above. To investigate the strength of the GC-Na+-bentonite reaction and the stability of the GC/Na+-bentonite complex in salt solutions, a reverse-phase high-performance liquid chromatography (RP-HPLC) technique was selected. An Agilent 1200 series chromatograph with a quaternary pump, degasser, photo diode array ultra-violet (UV) detector (220 nm) and auto-sampler was used with a 5 μm, 3.9 × 150 mm Waters Symmetry C18 column to separate 10 mL of sample at a flow rate of 1 mL/min in a gradient mixture of acetonitrile (ACN) and water (5% ACN for 2 min, then increased to 50% ACN by 12 min).
Fig. 2. XRD traces of GC/ (a) and PC/ (b) Na+-bentonite complexes washed with NaCl or CaCl2 solutions.
3. Results and discussion 3.1. XRD of PC/ and GC/Na+-bentonite The XRD traces (Fig. 2) of GC/ and PC/Na+-bentonite indicated that both COC molecules effectively penetrated the interlayer spaces of Na+-Mt. Addition of GC and PC increased the size of the interlayer space of Na+-Mt to a similar extent. The d-values (Table 3) of the Na+-Mt and COC/Na+-Mt complexes were determined from the centre of the reflection at 1/2 full intensity. An application of the COC at 1:1 (wt/wt) to the bentonite increased the Na+-Mt d-value from 1.29 nm to 1.89 nm (GC) and 1.83 nm (PC). The measured 1.29 nm d(001) value of the untreated Na+-Mt represented an intermediate state between the 1-layer and 2-layer hydrates (Laird, 2006), whereas the 1.89/1.83 nm d(001) values of the COC/Na+-Mt component of the modified bentonites corresponded to the interlayer spaces of bi-layers of COC molecules around the interlayer cations (Onikata et al., 1999; Gates et al., 2016). Both GC and PC modification of bentonite at rates of Table 3 d-Values of Na+-Mt, Ca2+-Mt, GC/ and PC/Mt washed with NaCl or CaCl2.
Na+-Mt Ca2+-Mt PC/Na+-Mt Washed with CaCl2 Washed with NaCl GC/Na+-Mt Washed with CaCl2 Washed with NaCl
d-Value (nm)
FWHM (nm)
1.29 ± 0.03 1.58 ± 0.03 1.83 ± 0.03 1.55 ± 0.04 1.61 ± 0.04 1.89 ± 0.03 1.61 ± 0.04 1.86 ± 0.04
2.2 ± 0.04 2.4 ± 0.04 3.5 ± 0.04 2.4 ± 0.06 3.0 ± 0.06 3.3 ± 0.04 3.6 ± 0.06 2.7 ± 0.06
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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Fig. 3. IR spectra and characteristic bands of Na+-Mt within (a) 1250–4000 cm−1 range and (b) 650–1250 cm−1 range.
1:1 COC:Na+-bentonite resulted in both sharp and symmetric Na+-Mt 001 reflections with full width at half maximum (FWHM) narrower than those of the Na+-bentonite indicating no discernible interstratifications of COC/Na+-Mt intercalates (Fig. 2). Thus the COC effectively and orderly intercalated Na+-Mt creating a uniform COC/Na+-bentonite complex. 3.2. FTIR spectroscopy of PC/ and GC/Na+-bentonite complexes Intercalation of the COC into the interlayer space of Na+-Mt allows for potential interaction of COC with (i) the interlayer cation, (ii)
hydration water surrounding the cation and (iii) the interlayer surfaces of the Mt. The FTIR spectra of the PC/ and GC/Na+-bentonite complexes were examined to further probe which of these scenarios best described the intercalation of GC and PC into Na+-Mt. For the untreated Mt, absorbance bands characteristic of Mt (Madejova et al., 1997; Bishop et al., 2002; Gates, 2005) were identified (Fig. 3), including δ(OH) vibrations at 910 cm−1 (Al3+ \OH), 881 cm−1 2 \ (Fe3+ Al3+\\OH), 843 cm−1 (Al3+ Mg2+\\OH) as well as δ(AlO) deformation at 696 cm−1 (out of plane) and ν(SiO) vibrations at 1115 cm−1 (in plane) and 982 cm− 1 (out of plane). The octahedral cation–OH stretching complex band (ʋ(O\\H)str, Madejova et al., 1997) was
Fig. 4. Functional groups of GC.
Fig. 5. C_O stretching region of COC when intercalated in Na+-bentonite (a) GC and (b) PC.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
A. Fehervari et al. / Applied Clay Science xxx (2016) xxx–xxx Table 4 Positions of bands for COC, Na+-Mt and PC/ GC/Na+-Mt under study. Band
Anhydrous PCa
Wavenumbers (cm−1) ʋ(OH) − ʋ(C_O) 1790 ʋ(C\ \C) 1180 ʋ(C\ \O) 1079
PC/Na+-Mt
Anhydrous GCb
GC/Na+-Mt
− 1782 1190 Disappeared
3413 1783 1177 1085
3408 1774 1184 Disappeared
Band
Na+-Mt
PC/Na+-Mt
Na+-Mt
GC/Na+-Mt
Wavenumbers (cm−1) ʋ(OH)str ʋ(Si\ \O)in plane ʋ(Si\ \O)out of plane ʋ(Al\ \OH) ʋ(Fe\ \Al\ \OH) ʋ(Al\ \Mg\ \OH)
3618 1115 982 910 881 843
3622 1115 989 910 883 845
3618 1115 982 910 881 843
3622 1108 989 910 885 841
a b
NIST (2016). National Institute of Standards and Technology. Shaheen (2014).
observed centred at 3622 cm−1. The broad bands near 3400 and 3250 are ʋ(H\\O\\H) of adsorbed water and the water δ(H\\O\\H) associated with the interlayer cation was observed at 1632 cm−1. The position of the latter band is consistent with interlayer water associated with Na+ (Johnston et al., 1992). These bands are expected to be significantly affected by the saline solutions. A decrease in the intensity of both bands is expected due to interlayer water loss in hypersaline leachates. PC and GC have four functional groups in common (Fig. 4): a carbonate ester group, the carbonyl part of the carbonate group and two nonequivalent ether groups. GC differs from PC in having a hydroxymethyl group instead of a methyl group connected to the cyclic carbonate.
5
The effect of inorganic salts on the COC/Na+-bentonites were generally observed by intensity changes of the characteristic bands of cyclic carbonyl (C_O) stretches at 1783 cm−1 (GC) and 1790 cm−1 (PC) (Onikata et al., 1999; Indran et al., 2014) as well as the C\\C stretching bands at 1177 cm−1 (GC) and 1180 cm−1 (PC) (Indran et al., 2014; NIST, 2016). Interactions between carbonyl groups and interlayer cations are expected to result in a red shift (Joseph and Jemmis, 2007) of the C_O bond in the IR spectrum of GC because, as the partial negative charge of the highly electronegative oxygen atom is decreased through electrostatic interaction with interlayer cations, the bond energy of the carbonyl group is decreased (it becomes more stable) (Parker and Frost, 1996; Onikata et al., 1999). Direct interaction between the interlayer cation and the carbonyl group of PC was shown by Onikata et al. (1999) to result in a shift greater than the shift due to an interaction through hydration water. A similar effect was observed for GC/Na+-Mt (Gates et al., 2016). The Mt interlayer surface oxygens potentially act as adsorption sites via hydrogen bonding (Sandi, 2005) which could affect the OH group of GC, because the hydroxyl may increase the electron density on the Si\\O bond. Onikata et al. (1999) and Gates et al. (2016) showed that the positions of the IR absorption bands of COC functional groups, which may interfere with water, are highly dependent on the water content of the carbonate. The presence of μM water results in shifts to the characteristic IR frequencies (Shaheen, 2014; Gates et al., 2016). Due to the high affinities of COC to water, drying at 105 °C was insufficient to eliminate all water. In order to gain a valid assessment of the changes/shifts to the IR spectra of GC and PC when intercalated into bentonites, the IR spectrum on anhydrous GC provided by Shaheen (2014) and anhydrous PC (National Institute of Standards and Technology, webbook.nist.gov) were used for comparison. All other samples (e.g., untreated and
Fig. 6. OH stretching region of COC when intercalated in (a) GC/ and (b) PC/Na+-bentonite complexes.
Fig. 7. Blue shifts of ʋ(Si\ \O)str for Na+-Mt modified with (a) GC (b) PC.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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Fig. 8. ʋ(C\ \C) stretching region of COC when intercalated in Na+-bentonite (a) GC and (b) PC.
Fig. 9. (a) ʋ(C = O)str and (b) ʋ(C\ \C)str of GC−/Na+-bentonite washed with I = 2 M of NaCl solution.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
A. Fehervari et al. / Applied Clay Science xxx (2016) xxx–xxx
modified bentonites, samples washed with salt solutions, etc.) were collected as part of this study for comparison. Formation of the GC/Na+-bentonite and subsequent intercalation of GC into the interlayer space of Na+-Mt resulted in a small to medium red shift of the characteristic carbonyl stretch (ʋ(C_O)) (Fig. 5; Table 4). Onikata et al. (1999) and Gates et al. (2016) observed similar red shifts for, respectively, PC/ and GC/ intercalated complexes of Na+-Mt. In GC/Na+ -Mt a small (5 cm− 1 ) red shift (3413 cm − 1 → 3408 cm− 1) of ʋ(O\\H)str for glycerol carbonate intercalation was observed (Fig. 6). The red shift of ʋ(O\\H)str may be attributed to either a H-bonding interaction between the hydroxyl group of GC and the surface oxygen of Mt or H-bonding of GC-OH with interlayer water (Gates et al., 2016). The red shift of ʋ(Si\\O)in plane indicates that the interlayer urface oxygen probably forms some H-bonding interaction with the hydroxyl of GC. This additional H-bond, which is precluded in PC/Na +-bentonite, possibly further stabilises GC molecules within the interlayer. The absence of the red shift of ʋ(Si\\O)in plane in PC/Na+-bentonite supports this theory. In Na+-Mt a small (4 cm−1) blue shift (3618 cm−1 → 3622 cm−1) of ʋ(O\\H)str (Fig. 6) and a small (7 cm−1) blue shift (982 cm−1 → 989 cm− 1) of ʋ(Si\\O)out of plane (Fig. 7) were observed when intercalated with either GC or PC. The blue shift of the ʋ(O\\H)str band is related to hydrogen bonding between the structural O\\H groups of Mt and COC. All C\\C
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stretches (ʋ(C\\C)) of both GC and PC blue shifted upon intercalation into Na+-Mt and varied from 7 cm−1 to 10 cm−1 (Fig. 8). 3.3. Effect of salt washing on intercalate stability To address some basic aspects of stability of the complexes when exposed to saline leachates, the COC/Na+-Mt complexes were washed with various salt solutions. Two general observations were made: (i) washing the samples with 4.3 mL of 1 M (I = 3 M) CaCl2 resulted in a larger shift of the 001 reflection (Fig. 2) compared to the effect of 6.4 mL of 2 M (I = 2 M) NaCl; and (ii) the GC/Na+-bentonite complex had a greater resistance against these high concentrations of inorganic salts compared to PC/Na+-bentonite. After washing the samples with ~ 140 × the amount of NaCl solution required to replace all interlayer cations (~ 100 mg clay/ceramic tile with ~ 80 meq/100 g), the d(001) value for the Na+-Mt component of the PC/Na+-bentonite decreased from 1.83 nm to 1.61 nm (Table 3). The same treatment only decreased the d(001) value of the Na+-Mt component of the GC/Na+-bentonite complex from 1.89 nm to 1.86 nm. The relatively large difference in the decrease of interlayer space indicates a stronger interaction between GC and Na+-Mt compared to the strength of interaction between PC and Na+-Mt. Samples exposed to 1 M CaCl2 solution, at a rate of ~95× the amount of Ca2+ needed to fully exchange all Na+ from the interlayer spaces,
Fig. 10. (a) ʋ(C = O)str and (b) ʋ(C\ \C)str of PC/Na+-bentonite washed with I = 2 M of NaCl solution.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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resulted in a similar magnitude of decrease in the interlayer space for both GC/ and PC/Na+-bentonite complexes. The d(001) values of the Na+-Mt components for both GC/ and PC/Na+-bentonite complexes decreased by 0.28 nm. However, the original interlayer thickness of Na+-Mt in the GC/Na+-bentonite complex (1.89 nm) was greater than the basal spacing of that in the PC/Na+-bentonite complex (1.83 nm) and hence a greater proportion of GC/Na+-Mt remained in a non-collapsed state (1.61 nm compared to 1.55 nm). XRD results presented here agree with geotechnical aspects of COC modified bentonites (Fehervari et al., 2016) where it was shown that at these high ionic strengths (2 M of NaCl and 3 M of CaCl2) GC/Na+-bentonite had a better barrier performance than PC/Na+-bentonite. Degradation of GC or PC in the COC modified bentonites due to exposure to either strongly saline NaCl or CaCl2 solutions was followed by FTIR as well. A decrease in intensity, or outright disappearance of the ʋ(C_O)str at 1774 (GC/Na+-bentonite), 1782 cm−1 (PC/Na+-bentonite) as well as the intensity decrease of ʋ(C\\C)str at 1184 cm−1 (GC/ Na+-bentonite), 1190 cm−1 (PC/Na+-bentonite) (Indran et al., 2014) was observed (Figs. 9-12). In general, these shifts or changes agree
well with the XRD analysis presented above. NaCl (I = 2 M) solution had less of an effect on the ʋ(C_O)str and ʋ(C\\C)str bands of the GC/ Na+-bentonite: only moderate decreases in intensity were observed (Fig. 9). The comparable characteristic bands for PC in PC/Na+-bentonite underwent larger reduction in intensity when washed with NaCl (Fig. 10). Exposure of both GC and PC modified samples to strongly saline CaCl2 solution (I = 3 M) resulted in relatively little increased resistance against the salt solution, as was indicated by the significant intensity losses of the ʋ(C_O)str (1700–1850 cm−1) and ʋ(C\\C)str (1100– 1250 cm−1) bands for both GC/ and PC/Na+-bentonite complexes (Figs. 11 and 12). These bands were nearly completely lost in the PC/ Na+-bentonite when washed with either NaCl or CaCl2, but the losses amounted to ~20% loss in the ʋ(C_O)str and ~10% loss in ʋ(C\\C)str intensities for GC/Na+-bentonite in NaCl. When washed with CaCl2 however, these bands were largely lost from GC/Na+-bentonite. These results raise some concern regarding the long-term stability of GC/ and PC/-Na+-Mt complexes to strongly saline leachates such as tested here.
Fig. 11. (a) ʋ(C = O)str and (b) ʋ(C\ \C)str of GC/Na+-bentonite washed with I = 3 M of CaCl2 solution.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
A. Fehervari et al. / Applied Clay Science xxx (2016) xxx–xxx
Fig. 12. (a) ʋ(C = O)str and (b) ʋ(C\ \C)str when PC/Na+-bentonite washed with I = 3 M of CaCl2 solution.
3.4. Stability of GC-bentonite complexes in salt solution To further investigate possible leaching of GC from the GC/Na+-bentonite complex to hypersaline leachate an RP-HPLC technique was used. Mixtures (~ 6% solid concentration) of GC/Na+-bentonite complex in 4.5 M CaCl2 solutions were allowed to react for up to 30 days after which time effluent was filtered and examined by RP-HPLC (Fig. 13). The main outcome of the RP-HPLC test was that the prepared GCbentonite complex was stable in I = 4.5 M CaCl2 solution for at least 30 days. The loss of the GC was negligible; there was an average of ~1.5% (absolute), which is within the error of the method. In companion studies (Fehervari, 2015) the GC/Na+ bentonite complex performance when reacted within I = 4.5 M CaCl2 resulted in essentially the same fluid loss values (67.5 ± 2 mL) independent of sample aging. The stability of GC in the Mt interlayer at low hydration water content or in the absence of interlayer water probably originated from the ion-dipole interaction between the interlayer cation and the C_O group of the carbonate (Onikata et al., 1999; Shaheen, 2014; Gates et al., 2016), which was further strengthened by the co-ordination of the GC-OH group with the surface oxygen of Mt (Gates et al., 2016).
Fig. 13. Results of RP-HPLC measurements on 1:1 GC/Na+-bentonite.
Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007
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3.5. Stability of the COC/Na+-bentonite complexes
4. Summary and conclusions
Previous work in our laboratory has indicated that PC/Na+-bentonite complexes were highly stable. Some basic tests to determine the stability of the GC/Na+ bentonite complexes were undertaken to evaluate complex stability. These included apparent evaporation at room temperature (RT) and at elevated temperatures. GC/Na+-bentonite complexes subjected to 105 °C for 24 h had no more evaporative loss than untreated Na+ bentonite. As such, the mass losses were considered to be solely due to the evaporation of water and not GC. The aleatory degradation of COC/Na+-bentonite complexes, primarily due to contact with air, was also investigated over the long-term (Fig. 14). 22.5 g of either oven dried or as received Na+-bentonite was mixed with 22.5 g of either GC or PC to obtain a 1:1 ratio between the Na+-bentonite and the COC. Then the mixtures were placed in open containers and the masses of the composites with time were followed for approximately a 20 month-period under laboratory conditions. Oven dried and as received (with ~9% gravimetric water content) materials were used to check whether the initial water content had any effect on the interaction of components in the COC/Na+-bentonite complex and their possible degradation. When analysing the mass changes of the prepared composites it was important to consider laboratory conditions (especially temperature and humidity). These factors could have impacted the rate of mass loss, or even have affected mass gain of the samples. In the current study the air temperature showed moderate fluctuation: during the period of investigation the ambient temperature was 25 °C ± 5 °C. Regarding humidity it was observed that there was a small increase in the mass of a sample on rainy or humid days, however, relative humidity had no effect on the overall long-term trends. In general ~2 g differences were measured between the mass of the samples prepared by mixing oven dried bentonite with COC compared with mixing as received bentonite with COC because the water content of the air dried bentonite was ~9%. Hence, it is likely that the oven dried samples contained ~2 g more clay in the COC/Na+-bentonite complex. Any other effect of drying the samples was not observed. Over the period of investigation, ~2.7% and ~6.3% mass losses were observed for the oven-dried and air-dried GC/Na+-bentonite complex. Respective specimens with propylene carbonate showed ~ 20.2% and 24.6% weight losses over the same time period. While the results would represent that when COC/Na+-bentonite complexes are fully exposed to the surrounding atmospheric conditions, PC/Na+-bentonites resulted in losses of 41% (9.17 g) and 50% (11.25 g) of initial PC added, which is an undesirable and potentially worrisome result since PC-bentonite is currently marketed as multiswellable bentonite for geotechnical applications in saline environments (Scalia et al., 2014; Bohnhoff et al., 2013).
The main goal of this paper was to investigate COC-modified bentonite complexes for enhanced and extended containment of hypersaline (sodic and calcic) leachates. For this purpose attributes of GC-modified Na+-bentonite was investigated with NaCl and CaCl2 solutions and compared to unmodified Na+-bentonite and PC-modified Na+-bentonite. Both XRD and FTIR results indicated that the GC/Na+-bentonite complex was strongly resistant against sodic salt solution compared to the PC/Na+-bentonite complex. CaCl2 solution had a more detrimental effect on both modified bentonites, however, the GC/Na+-bentonite appeared to retain more COC than the PC/Na+-bentonite when leached by strong calcic salt solutions. It is proposed that in both GC and PC the carbonyl groups of the carbonates interact with the interlayer cations through hydration water molecules at low ionic strengths (Onikata et al., 1999). However, at higher ionic strengths, the carbonyl interacts directly with the cation. In this study GC was found to be more effective in linking directly to interlayer Na+ compared to PC at high concentrations of NaCl and CaCl2. Consequently, at very high salt concentrations the GC/Na+-bentonite complex maintained a more swollen state and better hydraulic barrier performance because of this stronger interaction. In addition, GC was able to interact with the smectite interlayer surfaces through its hydroxyl groups. While these interactions slightly limited the crystalline swelling of GC/Na+-Mt in low salinity liquids (e.g., in water) (Fehervari et al., 2016), the stronger interlayer Na+–GC–Mt surface interaction was beneficial in hypersaline solutions because GC stuck in the smectite interlayer and could not be removed easily. The long-term laboratory and field testings of COC/Na+-bentonites have yet to be conducted including the stability of COC when exposed to microbial activity. Such studies are required before the long-term use of these modifying agents and their complexes with bentonite can be fully promoted. Research carried out so far showed that depending on the specific engineering application, the use of PC/Na+-bentonite complexes could be an appropriate option for saline conditions where I b 0.3 M (Katsumi et al., 2008). However, the current study clearly indicated that the GC/Na+-bentonite complex is more stable and therefore may be a better option in applications where contact with hypersaline leachates with I b 1 M is anticipated. In addition, GC can be advantageously produced using a cost effective green chemical method (Turney et al., 2013). GC thus has strong potential for its use as an alternative additive to bentonite under hypersaline and calcium-enriched applications.
Acknowledgements This study was supported under the Australian Research Council's Discovery Projects funding scheme (project number ARC DP1095129). The authors' sincere appreciation is extended to the council. The first author is grateful for the funding provided by the Faculty of Engineering and the Department of Civil Engineering at Monash University to support his PhD studies. The authors would like to express their gratitude to Ms. Gyögyvér Engloner for her assistance with creating figures and the Graphical Abstract.
References
Fig. 14. Mass changes of bentonite-COC complexes with time.
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Please cite this article as: Fehervari, A., et al., Cyclic organic carbonate modification of sodium bentonite for enhanced containment of hyper saline leachates, Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.09.007