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Sodium polyacrylate modification method to improve the permeant performance of bentonite in chemical resistance
Journal of Cleaner Production 213 (2019) 242e250
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Sodium polyacrylate modification method to improve the permeant performance of bentonite in chemical resistance Chuang Yu a, Raoping Liao a, Xiaoqing Cai b, Xiaoniu Yu a, c, * a
College of Civil Engineering and Architecture, Wenzhou University, Wenzhou, 325035, China College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325035, China c School of Environment, Tsinghua University, Beijing, 100084, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 November 2017 Received in revised form 13 December 2018 Accepted 18 December 2018 Available online 18 December 2018
Bentonite, an important geotechnical material in the fields of environmental geotechnical engineering, can effectively adsorb harmful substances from the environment and fuel oil. This study investigated the formulation of sodium polyacrylate-modified bentonite that improves the permeant performance in the context of chemical resistance. The optimum material mix ratio and synthesis process were determined through multiple experiments. To better understand the sodium polyacrylate/bentonite interaction, the swelling index, filtrate loss, cation exchange capacity (CEC) and hydraulic conductivity performance in solutions of Pb(NO3)2 and acid were compared. The microstructures of raw bentonite and sodium polyacrylate bentonite were analyzed using scanning electron microscopy and infrared spectroscopy. The results showed that the microstructure of modified bentonite was characterized by isolated cavities, whereas the original configuration comprised a stacked layer, and the free swell index (FSI) increased from 25 to 50 mL/2 g. The modified bentonite showed a significant improvement in terms of chemical resistance through FSI testing and filtrate loss experiments, particularly in hydraulic conductivity tested in acidic and saline solutions. The permeability decreased by an order of magnitude through the modification process under the deionized water condition, and the chemical resistance increased to more than 40 mM in the Pb(NO3)2 solution. In addition, the CEC increased from 62.06 to 96.1 meq./100 g as a result of sodium polyacrylate modification. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Bentonite Polymer modified Permeability Swelling index Microstructure
1. Introduction Bentonite is considered one of the most favorable materials for hydraulic barriers in terms of impermeability and cost and is widely used within environmental- and energy-related materials (ElKorashy et al., 2016; Choi et al., 2017; Freitas et al., 2018). Geosynthetic clay liners (GCLs) made from bentonite are widely used in bottom liners and cover systems at refuse landfills and mining facilities worldwide. However, due to aggressive leachates of acids and cations the permeability of sodium bentonite based hydraulic barrier increases in magnitude as a result of cation exchange (Liu et al., 2015; Ahn et al., 2009). In this reaction, cations in ionexchange functional groups are reversibly exchanged with isotropic ions in bulk solution. The permeability and swelling
* Corresponding author. College of Civil Engineering and Architecture, Wenzhou University, Wenzhou, 325035, China. E-mail address: [email protected] (X. Yu). https://doi.org/10.1016/j.jclepro.2018.12.179 0959-6526/© 2018 Elsevier Ltd. All rights reserved.
behavior of bentonite are easily affected by salt solutions and acidic environment. Due to the reaction of cation exchange the permeability coefficient of bentonite greatly increases, and the antiseepage ability reduces accordingly. According to the literature reports, an increase in concentration of the electrolytic solution would result in an increase in hydraulic conductivity (Jo et al., 2001; Katsumi et al., 2007; Kolstad et al., 2004a; Petrov and Rowe, 1997; Ruhl and Daniel, 1997; Shackelford et al., 2000; Shan and Lai, 2002; Yang et al., 2014). As a result, there is a high probability of failure of bentonite in harsh environments. Unfortunately, the leachate formed by landfill, industrial wastewater, and tailings, for example, is often acidic and contains a high concentration of heavy metals. In addition, a long-term effectiveness of bentonite has been called into question for hydraulic containment applications (Malusis and Shackelford, 2002; Shackelford and Lee, 2003). Therefore, the chemical resistance of bentonite is an active research area focusing on making bentonite properties more compatible with the surrounding environment, often through chemical modifications. For example, calcium bentonite activated by sodium carbonate has
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favorable swelling characteristics and low hydraulic conductivity, but its long-term presence can affect the electronic equilibrium and hydraulic performance. Polymer-modified bentonite is attracting increasing attention. According to Theng (1979), a clay-polymer mixture can be created by decomposing the surface of the polymer into uncharged polymers, anions and cations. The mechanism of cation, uncharged polymer and polyanion interaction is electrostatic and associated with the negatively charged crystal interlayers of montmorillonite, the edge of the crystal and the formation of peripheral complexes, respectively. A significant amount of polymers with a very low molecular weight can adsorb to the interlayers of montmorillonite at pH ¼ 2.5 (Razakamanantsoa et al., 2012). Both intralamellar and extralamellar interactions can occur in polyionic polymers. To reduce voids, extralamellar interactions are employed to block the flow path in landfill applications. The polymers enhance the hydraulic performance of GCLs. Previous studies show that the influence of ash fill leachates on the polymer treated bentonite is less severe than the untreated sample (not sure if the correction still deliver the same information as what you originally did) and that high-swell bentonite is more resistant to multispecies solutions (Elhajji et al., 2001; Ashmawy et al., 2002; Didier and Comeaga, 1997). Contradictory results have emerged because the formulations of commercialized modified bentonite are often confidential. In addition, the information of manufactured polymers is incomplete, making it challenging to understand the interaction mechanism of bentonite-polymer and to find a suitable polymer additive. The behavior of clay-polymer has been widely researched in terms of chemical properties and microscopic approaches. However, there is a lack of information concerning the influence of raw material on polymer mixtures in terms of the hydraulic parameters of the liner geo-material, namely, the hydraulic conductivity, swelling, and microstructure. The object of the present study was to develop a formulation of modified bentonite with sodium polyacrylate, referred to as a sodium polyacrylate bentonite (SPB), to improve the hydraulic performance in the presence of saline solution and to compare it with that of the raw bentonite (RB) used in GCLs. Fig. 1 shows a schematic diagram of the synthetic reaction. Using calcium bentonite as the raw material, SPB was synthesized using acrylic acid after the sodium transition of sodium carbonate. The formulation of SPB was investigated sufficiently despite numerous influencing factors. Comparison between SPB and RB in terms of the swelling, filtrate loss, cation exchange capacity (CEC), hydraulic performance and microstructure is also included in the current study. 2. Material and methods 2.1. Materials 2.1.1. Development of modified materials The CEC of calcium bentonite (as determined by ammonium cation exchange) was 62.06 meq./100 g, and its swell index was 25 mL/2 g. The other materials used were acrylic acid, N-
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isopropylacrylamide (NIPA), potassium persulfate (K2S2O8), and sodium carbonate (Na2CO3). 2.1.2. Testing solutions Testing solutions were classified into four types: (1) deionized water (DIW), (2) 0.1% CaCl2 solution, (3) Pb(NO3)2 solution, and (4) HCl solution with various pH values. The free swell index (FSI) of bentonite in the 0.1% CaCl2 solution for 168 h was used to characterize the durability, which was an important parameter used to reflect the long-term swelling character. Pb(NO3)2 and HCl solutions with different pH values were also used to measure the FSI of bentonite. 2.2. Methods 2.2.1. Synthesis of SPB The flow chat of SPB synthesis was shown in was produced with acrylic acid and a conventional calcium bentonite and synthesized using a solution polymerization reaction, as shown in Fig. 2 (Scalia et al., 2012). First, sodium carbonate solution was prepared in a flask, followed by adding RB and NIPA to form a slurry; Second, the slurry was then neutralized by a certain volume of acrylic acid under continuous stirring condition to remove all bubbles; Third, the temperature of this bentonite-monomer slurry was raised, and polymerization was initiated after adding potassium persulfate. The molecules of initiator (potassium persulfate) decomposed into free radicals which reacted with the acrylic acid monomer to form more free radicals, which then in turn reacted with additional monomer to proliferate the polymer chain. Finally, the SPB was dried at 105 C and characterized by Colloid Environmental Technologies (CETCO). 2.2.2. Free swelling and CEC test Both RB and SPB samples were subjected to the free swelling test using the volume change according to ASTM D 5890. First, 90 mL of permeant solution and 2 g of sample were added to a 100 mL of graduated cylinder. The sample was allowed to stand for 24 h, and the first reading was recorded at room temperature. The durability of the samples was tested when the standing time was 168 h. The CEC values could be calculated and measured according to the literature (Borden and Giese, 2001) and following formula (Busenberg and Clemency, 1973):
CEC ðmeq=100 g Þ ¼
50 c w 105
where, c is the measured concentration of ammonia (mol/L), 50 mL is the volume of water, w is the mass of the air-dried clay sample (mg), and 105 is a conversion factor. 2.2.3. Structural analysis X-ray diffraction (XRD, Bruker Company, Germany) was carried out on samples at room temperature using a D8-Advance X
Sodium carbonate
Mix Acrylic acid Mix
Polymerization
Potassium persulfate Fig. 1. Schematic diagram of the synthetic reaction.
Slurry
N-isopropylacrylamide Bentonite
Modified bentonite
Fig. 2. Flow chart of synthetic bentonite.
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diffraction meter (40 kV, 40 mA) with Cu (l ¼ 1.5406 Å) irradiation at the rate of 0.15 s/step rate in the range of 10e80 . Fourier transform infrared spectroscopy (FTIR) of the bentonite samples was performed in transmission mode at room temperature on a 1725X Perkin Elmer instrument using the KBr pellet technique (1:20) at a resolution of 2 cm1. KBr was dried at 200 C for 24 h and homogenized with the bentonite sample in a ball grinder. The specimens (radius 1 cm, thickness 0.1 cm) were prepared using a hydraulic press. Scanning electron microscopy (SEM) was carried out in Japan by using a TENSOR27 Fourier transform infrared spectrometer produced by Zhengzhou Great Wall Branch Industry and Trade Co., electronic production JSM-6700F-type scanning electron microscope, magnification: 5000e30,000 times. Transmission electron microscope (TEM) images were obtained on a JEM-2100F tool. TEM grids were prepared using a few drops of nanoparticles followed by drying.
Fig. 4. Schematic diagram of the hydraulic control system.
3. Results and analysis 2.2.4. Filtrate loss test Filtrate loss tests were performed according to ASTM D5891; this was a standard test originally developed to qualify slurry for drilling applications. In this study, the filtrate loss test was measured to evaluate the short-term behavior of the bentonite permeability in Pb(NO3)2 solutions.
2.2.5. Hydraulic conductivity test To prepare the specimen, granular RB and granular SPB were loosely packed in molds with diameter and height of 70 and 10 mm, respectively. A schematic diagram of the preparation was illustrated in Fig. 3. The dry density of the molded specimen was approximately 15.4 g, or 4 kg/m2. These conditions were designed to simulate the application of bentonite to GCLs. Next, the prepared specimen was sandwiched between two filter papers attached with a porous stone larger than the mound and placed in the apparatus. The specimen was permeated with chemical solutions for 48 h to swell and saturate the specimen. Specimen prehydration shortened the duration of permeate testing substantially. After the specimen was prepared, it was transferred to the membrane testing apparatus. The hydraulic conductivity test was performed using flexible-wall permeameters according to ASTM D 5084, “Standard test method for measurement of hydraulic conductivity of saturated porous materials using a flexible wall permeameter”. A flexible-wall membrane cell and associate hydraulic control system similar to that of Kang and Shackelford used in their study (Kang and Shackelford, 2009). A schematic diagram of the hydraulic control system was illustrated in Fig. 4. The test consisted of two stages: a consolidation stage and a chemic-osmotic testing stage. During the consolidation stage, specimens were permeated at a cell pressure of 35 kPa and a base and back pressure of 15 kPa at a constant room temperature of 20 C for 48 h. During this stage, the swelling press equaled to the effective stress of 20 kPa as it ultimately adapted to the surrounding pressure. Following consolidation, the specimens were permeated from the bottom to the top by increasing the headwater pressure by 30 kPa.
Fig. 3. Schematic diagram of specimen preparation.
3.1. Influence of the amount of reactants on the swelling properties of SPB Based on the amount of acrylic acid used, the optimum mixing proportions of SPB with bentonite, sodium carbonate, NIPA, and potassium persulfate was determined. Furthermore, the parameters of the synthesis process such as temperature and the ratio of solid to liquid were also investigated. The amount of reactants was converted to the mass percentage of acrylic acid (%wt). Several studies have investigated the correlation between the FSI and the hydraulic conductivity (k) of GCLs (Lee et al., 2005; Schackelford et al., 2000), showing that the free swelling index decreased with an increase in permeability. To simplify the detection index of SPB, the mixture ratio of each component was characterized using the FSI. 3.1.1. Influence of sodium carbonate The measured FSI value with the addition of sodium carbonate in the polymerization was shown in Fig. 5(a). The FSI initially increased with an increase in sodium carbonate content and stayed relatively constant when high sodium carbonate content had reached. In the increasing stage, the excess Hþ restrained the swelling of SPB. Thus, for greater swelling of SPB, the acrylic acid and sodium carbonate must react fully. In this case, the relative amount of sodium carbonate was 64.6%wt (weight percentage wt %¼(mass of B/mass of A þ mass of B) 100%). 3.1.2. Influence of NIPA The FSI with NIPA addition was shown in Fig. 5(b), which indicates that the FSI does not change considerably with increasing NIPA. This was because NIPA was a crosslinking agent in the reaction, which changed the linear polymerization of sodium acrylic into a three-dimensional network structure; the fixed bentonite particles improve the degree of recombination. This condition was also reflected in the microstructure. The optimum content of NIPA was 2.5%wt. 3.1.3. Influence of potassium persulfate Fig. 5(c) showed the effect of addition of potassium persulfate in the polymerization on the FSI. The result showed that the optimal dosage of potassium sulphate was 1.25%wt. The polymerization reaction was exothermic and proceeds spontaneously after being triggered by an appropriate concentration of potassium persulfate. However, high concentrations of potassium persulfate resulted in an excessively fast and uneven reaction.
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D
E
F
G
H
I
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Fig. 5. Influence of the amount of reactants on the swelling properties of SPB.
3.1.4. Influence of bentonite Fig. 5(d) showed the results of the addition of bentonite in the polymerization. The recombination rate of SPB decreased with an increase in bentonite content. The swelling performance declined because the swelling was equivalent to the dilution of sodium polyacrylate. For increased performance, the amount of bentonite can be reduced appropriately, albeit with increasing SPB cost. Considering the factors of cost and performance, a bentonite dosage of 625%wt was recommended. 3.1.5. Influence of temperature Fig. 5(e) showed the temperature during polymerization. The organic polymerization was particularly sensitive to temperature as the chemical reactions usually associate with entropy reduction and heat release. The FSI of modified bentonite in DIW was not
synchronized with the mass fraction of the 0.1% CaCl2 solution. Considering the chemical resistance and the energy consumption of the process, it is recommended that the polymerization temperature be controlled at 75 C.
3.1.6. Influence of the solid-to-liquid ratio Fig. 5(f) showed the effect of solid-liquid ratio during polymerization on the FSI. It is necessary to control this ratio to determine the remaining reactant concentrations. In general a greater solidto-liquid ratio results in a larger FSI. This was because high concentration of sodium polyacrylate and bentonite was conducive to improve the utilization of the elements while increasing the ratio of. However, it is also necessary to ensure that the slurry can be stirred and dispersed evenly in solution.
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Fig. 6. Relationship between CEC and addition of materials.
Fig. 7. Comparison of the infrared absorption spectra of RB and SPB.
3.2. CEC analysis
hybrid chain favorably enhances the water absorption and swelling properties of modified bentonite, enabling the modified bentonite to exhibit a decreased solution permeability. FTIR spectroscopy was used to characterize the interfacial interaction between RB and SPB, as shown in Fig. 7. The origins of the peaks were shown in Table 1. Relative to RB, SPB featured additional characteristic bands at 2950 cm1 (eCH2 asymmetric stretching), 1575 cm1, and 1414 cm1 (COOe and C]O symmetric stretching, respectively) in its FTIR spectrum. Sodium polyacrylate modification is responsible for the new peaks observed in the FTIR spectrum of SPB. In addition, some peaks appeared to strengthen in the FTIR spectroscopy results. The interlayer water molecule OeH stretching superposition of the organic carbon chain of the OeH vibration was 3438 cm1. The peak at 1636 cm1 arise from HeOeH bending in water molecules and the vibration of amide and C]C. The peak at 2950 cm1 arise from eCH3 asymmetric vibration. The peak at 1575 cm1 arise from carboxylate radical (COOe) vibration and CeH asymmetric stretching vibration. Peak strengthening produced a peak at 1445 cm1 owing to carbonate and the introduction of the -(CH2)n-structure. The absorption peaks at 975 and 1036 cm1 arise from SieOeSi antisymmetric stretching vibration in bentonite. The enhancement at 621 cm1 was attributed to the induced reaction with potassium persulfate. The absorption peaks at 523 and 470 cm1 were caused by the coupling of montmorillonite SieO-M (where M was a metal cation) and M-O. The composition of RB and SPB was analyzed by XRD instrument, as shown in Fig. 8. Diffraction peaks of the specimens can be
Fig. 6 (a, b) showed the results obtained from the CEC test of SPB synthesized. The CEC value of RB was 62.06 meq./100 g, and the SPB performed best at 96.1 meq./100 g. These values were notably higher than those typically reported for conventional sodium bentonite (Lee and Shackelford, 2005). Fig. 6(a) showed the CEC and FSI changed in SPB with the addition of potassium persulfate. The CEC increased with increasing modification and decreased with increasing potassium persulfate and RB. Moreover, a relationship between the CEC and FSI could be derived from the figure that the FSI increased with increasing CEC. A greater CEC always meant a higher adsorption capacity. 3.3. FTIR and XRD analysis The sodium polyacrylate with the hydrophilic group has the characteristics of water absorption and swelling, and the modification via introduction of a hydrophilic group of COOe leads to less crystallinity of bentonite. Therefore, the polymerization of the
Fig. 8. XRD patterns of RB and SPB.
Table 1 Origins of peak formation. Peaks (cm1)
Reason for peak formation
3627 3438 2950 1636 1575 1445 1414 1320 1036 918 795 621 523 470
AleOeH stretching vibration HeOeH stretching vibration of interlayer water molecules eCH3 asymmetric vibration HeOeH bending vibration of water molecules, amide radical, C]C bond Carboxylate radical (COOe) vibration and CeH asymmetric stretching vibration - (CH2) n- plane rocking vibrations and carbonates Carboxylate radical (COOe) vibration and C]O symmetric stretching vibration Vibration of CeC bond in polymer SieOeSi antisymmetric stretching vibration eOH vibration of Montmorillonite SieOeSi A covalent organometallic compound, M-C and potassium sulphate Coupling vibration of montmorillonite SieO-M (M is metal cation) and M-O Coupling vibration of montmorillonite SieO-M (M is metal cation) and M-O
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Table 2 Experimental results of RB and SPB in FSI and FLV testing in Pb (NO3)2 solutions. C(Pb2þ)
FSI test (mL/2 g)
mM
RB
SPB
RB
SPB
0 10 20 30 40 50
25 16.5 13 12 11.5 10
50 28 25 21 19 17
24 52.6 102.8 130.2 157.2 177
4 4.8 6 15 64 126
FLV test (mL)
Fig. 11. Hydraulic conductivity of SPB and RB in Pb(NO3)2 solutions.
3.4. Filtrate loss analysis
Fig. 9. RB and SPB performance in FSI and FLV in Pb (NO3)2 solutions.
indexed by the MDI Jade 5.0 program. The bentonite could be readily indexed to the reported structures of SiO2 (JCPDS No. 14e0260). The sodium polyacrylate bentonite could be identified as the mixture of C34H32O6 (JCPDS No. 25e1749) and SiO2 (JCPDS No. 05e6466). The sodium polyacrylate modified bentonite was less crystalline than the raw bentonite, as indicated by the intensity of the XRD peaks. Therefore, the main components of bentonite and sodium polyacrylate modified bentonite are SiO2, and their structures exhibit different cell parameters.
Fig. 10. Hydraulic conductivity of SPB and RB under acidic conditions.
Table 2 and Fig. 9 summarized the FSI and FLV results of RB and SPB in Pb(NO3)2 solution. SPB performs better, as shown in Fig. 8. The FSI of SPB in DIW is 50 mL/2 g, which was about twice greater than that of RB in DIW (25 mL/2 g). Even in solutions with high concentration SPB showed better swelling performance than RB. Although both values increased rapidly with a continuous increase in concentration, the FLV of SPB at a concentration of 30 mM of Pb (NO3)2 was still lower than that of RB in DIW. The results clearly showed that the chemical resistance of SPB was significantly improved. 3.5. Permeability 3.5.1. Comparison of the hydraulic conductivity to that of acidic solution A summary of the hydraulic conductivity (k) test results with acidic solutions was shown in Fig. 10. An increasing concentration
Fig. 12. Relationship between the permeability (k) and FSI.
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Fig. 13. SEM (a) and TEM (b, c) images of RB, SEM (d) and TEM (e, f) images of SRB.
of acidic solutions was found to have a positive effect (resulting in greater k values) on the hydraulic performance of the two types of specimen tested in the present study. The cake prepared using SPB was more stable against acid attack at high acid concentration, where interaction with acidic solution resulted in only a negligible increase in k at pH ¼ 2. In contrast, there was a rapid increase in k at pH ¼ 2 for RB. It can be concluded that SPB is substantially better than RB at resisting acid erosion.
3.6. Comparison of the permeability behavior with that of Pb(NO3)2 solution Fig. 11 showed the hydraulic conductivity of RB and SPB in the
presence of Pb(NO3)2 solutions. Lead as a heavy metal is widely used in industry and commonly found in leachates. The hydraulic 10 conductivity (k) of RB with DIW (C2þ m/s. Pb ¼ 0 mM) was 1.61 10 However, k of RB increased rapidly by an order of magnitude. For example, at a Pb2þ concentration of 40 mM, k increased to 2.42 108 m/s, which showed the damaging influences of cation exchange reactions, particularly multivalent heavy metal solutions, in high-salinity environments (without knowing what you want to say, it is hard to correct this sentence). However, the modified bentonite, SPB, showed a relatively stable low permeability (k ¼ 1.2 1011 m/s, C2þ k ¼ 2.68 1011 m/s, Pb ¼ 0 mM; 2þ C2þ concentrations below 40 mM. In addition, k Pb ¼ 40 mM) for Pb of SPB showed a similar tendency to FLV when the Pb2þ
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concentration continued to increase. One possible explanation for the observed behavior of SPB was that the bentonite modified with sodium polyacrylate encapsulated the negatively charged crystal interlayers of montmorillonite when activated with sodium. This process made it challenging to neutralize crystal interlayers with cations owing to the negatively charged polymer network in the periphery. The mechanism of anionic polymer modified bentonite could alleviate only damage to the structure of the bentonite (negatively charged crystal interlayers of montmorillonite) caused by the electrolyte. Therefore, as the ionic strength continued to increase, SPB exhibited a rapidly increasing trend in permeability, similar to RB.
owing to the modification, which indicated a significant improvement in the adsorption capacity that merit further study. These observations were confirmed by FSI and permeant tests, indicating that a strong correlation existed between the bentonite FSI and permeability (k). In field, the damage to the structure of the bentonite caused by the electrolyte could not be eliminated. However, polymer modification was one of the methods that could enhance the resistance of bentonite to the permeant solution. The benefits of the modified bentonite in real application can be further evaluated through long-term field trail.
3.7. Relationship between the hydraulic conductivity and FSI
This work was supported by the National Nature Science Foundation of China (Grant No. 51578427, 51572197, 41372264, and 51702238), the Plan Project of Science and Technology of Zhejiang Province (No. 2014C33015 and 2015C33220), the Opening Funds of Jiangsu Key Laboratory of Construction Materials (CM2018-02), and the Plan Project of Science and Technology of Wenzhou (No. ZS2017002).
The relationship between k and the FSI has been investigated by several researchers (Katsumi et al., 2008; Andry et al., 2015). Liu et al. suggested that a strong correlation existed between the types of permeant liquids and bentonite FSI and k for different k proposed the relationship lg 1:1310 ¼ e0:14ðx14:95Þ , where 11 k ¼ hydraulic conductivity (m/s) and x ¼ FSI (mL/2 g) [1]. The correlation between the FSI and k was plotted in Fig. 12. The FSI and k values of the clays were measured in corresponding acid or saline solution. For each specimen, a higher FSI resulted in a lower k. This tendency was in accordance with Liu's observations that k increases exponentially for FSI values below 15 mL/2 g. 3.8. Microstructures of RB and SPB SEM and TEM were used to observe the morphological changes in RB and SPB for the modified polymer. Fig. 13 showed the rather different microstructures between RB and SPB. SPB has isolated cavities and empty spaces, whereas RB exhibited patchy aggregation and close stacking, as shown in Fig. 13a and d. TEM further confirmed that the RB and SPB structures exhibited irregular flakiness and flocculent particles, respectively (Fig. 13b and c, Fig. 13e and f). One possible explanation of the observed difference in microstructure was that the bentonite layers were rearranged when the negatively charged acrylic monomers were polymerized. In addition, the linear chain of polyacrylic was polymerized to form a three-dimensional network structure with the addition of the crosslinking agent NIPA. Because this cavity structure caused numerous barriers when liquid flows through the matrix, the permeability of the material decreased. 4. Conclusions This work focused on the formulation of bentonite modified with sodium polyacrylate and determination of the optimum material mix ratio and synthesis process. The proposed percentages of reactants relative to the acrylic acid were as follows: 625 wt% of bentonite, 64.6 wt% of sodium carbonate (complete reaction), 2.5 wt% of NIPA, 1.25 wt% of potassium persulfate, 1:2 solid-toliquid ratio, and 75 C reaction temperature. The SPB that was modified according to this formulation showed a significant improvement in chemical resistance. The permeability was reduced by an order of magnitude through the modification process under the DIW condition, and the chemical resistance increased to more than 40 mM in the Pb(NO3)2 solution. The microstructure analyzed by SEM clarified the mechanism of anionic polymer modified bentonite, which explained the improvement in hydraulic conductivity performance. The microstructure of modified bentonite changed from lamellar to cavity, which increased the specific surface area and permeant barriers. In addition, the CEC value increased from 62.06 to 96.1 meq./100 g
Acknowledgements
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Cr(VI) sorption: a combined basic study and industrial application. J. Clean. Prod. 137, 40e50. Scalia, J., 2012. Bentoniteepolymer Composites for Containment Applications (PhD Dissertation). University of Wisconsin, Madison, Wisconsin, USA. Shackelford, C.D., Benson, C.H., Katsumi, T., Edil, T.B., Lin, L., 2000. Evaluating the hydraulic conductivity of GCLs permeated with nonstandard liquids. Geotext. Geomembranes 18, 133e161. Shackelford, C.D., Lee, J.M., 2003. The destructive role of diffusion on clay
membrane behavior. Clay Clay Miner. 51 (2), 186e196. Shan, H.Y., Lai, Y.J., 2002. Effect of hydrating liquid on the hydraulic properties of geosynthetic clay liners. Geotext. Geomembranes 20, 19e38. Theng, B.K.G., 1979. Formation and Clay Properties of Clay Polymer Complexes. Elsevier scientific Publishing Company, Netherland. Yang, Z.X., Jardine, R.J., Zhu, B.T., Rimoy, S., 2014. Stresses developed round displacement piles penetrating in sand. J. Geotech. Geoenviron. 140, 04013027.
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Sodium polyacrylate modification method to improve the permeant performance of bentonite in chemical resistance Chuang Yu a, Raoping Liao a, Xiaoqing Cai b, Xiaoniu Yu a, c, * a
College of Civil Engineering and Architecture, Wenzhou University, Wenzhou, 325035, China College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325035, China c School of Environment, Tsinghua University, Beijing, 100084, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 November 2017 Received in revised form 13 December 2018 Accepted 18 December 2018 Available online 18 December 2018
Bentonite, an important geotechnical material in the fields of environmental geotechnical engineering, can effectively adsorb harmful substances from the environment and fuel oil. This study investigated the formulation of sodium polyacrylate-modified bentonite that improves the permeant performance in the context of chemical resistance. The optimum material mix ratio and synthesis process were determined through multiple experiments. To better understand the sodium polyacrylate/bentonite interaction, the swelling index, filtrate loss, cation exchange capacity (CEC) and hydraulic conductivity performance in solutions of Pb(NO3)2 and acid were compared. The microstructures of raw bentonite and sodium polyacrylate bentonite were analyzed using scanning electron microscopy and infrared spectroscopy. The results showed that the microstructure of modified bentonite was characterized by isolated cavities, whereas the original configuration comprised a stacked layer, and the free swell index (FSI) increased from 25 to 50 mL/2 g. The modified bentonite showed a significant improvement in terms of chemical resistance through FSI testing and filtrate loss experiments, particularly in hydraulic conductivity tested in acidic and saline solutions. The permeability decreased by an order of magnitude through the modification process under the deionized water condition, and the chemical resistance increased to more than 40 mM in the Pb(NO3)2 solution. In addition, the CEC increased from 62.06 to 96.1 meq./100 g as a result of sodium polyacrylate modification. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Bentonite Polymer modified Permeability Swelling index Microstructure
1. Introduction Bentonite is considered one of the most favorable materials for hydraulic barriers in terms of impermeability and cost and is widely used within environmental- and energy-related materials (ElKorashy et al., 2016; Choi et al., 2017; Freitas et al., 2018). Geosynthetic clay liners (GCLs) made from bentonite are widely used in bottom liners and cover systems at refuse landfills and mining facilities worldwide. However, due to aggressive leachates of acids and cations the permeability of sodium bentonite based hydraulic barrier increases in magnitude as a result of cation exchange (Liu et al., 2015; Ahn et al., 2009). In this reaction, cations in ionexchange functional groups are reversibly exchanged with isotropic ions in bulk solution. The permeability and swelling
* Corresponding author. College of Civil Engineering and Architecture, Wenzhou University, Wenzhou, 325035, China. E-mail address: [email protected] (X. Yu). https://doi.org/10.1016/j.jclepro.2018.12.179 0959-6526/© 2018 Elsevier Ltd. All rights reserved.
behavior of bentonite are easily affected by salt solutions and acidic environment. Due to the reaction of cation exchange the permeability coefficient of bentonite greatly increases, and the antiseepage ability reduces accordingly. According to the literature reports, an increase in concentration of the electrolytic solution would result in an increase in hydraulic conductivity (Jo et al., 2001; Katsumi et al., 2007; Kolstad et al., 2004a; Petrov and Rowe, 1997; Ruhl and Daniel, 1997; Shackelford et al., 2000; Shan and Lai, 2002; Yang et al., 2014). As a result, there is a high probability of failure of bentonite in harsh environments. Unfortunately, the leachate formed by landfill, industrial wastewater, and tailings, for example, is often acidic and contains a high concentration of heavy metals. In addition, a long-term effectiveness of bentonite has been called into question for hydraulic containment applications (Malusis and Shackelford, 2002; Shackelford and Lee, 2003). Therefore, the chemical resistance of bentonite is an active research area focusing on making bentonite properties more compatible with the surrounding environment, often through chemical modifications. For example, calcium bentonite activated by sodium carbonate has
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favorable swelling characteristics and low hydraulic conductivity, but its long-term presence can affect the electronic equilibrium and hydraulic performance. Polymer-modified bentonite is attracting increasing attention. According to Theng (1979), a clay-polymer mixture can be created by decomposing the surface of the polymer into uncharged polymers, anions and cations. The mechanism of cation, uncharged polymer and polyanion interaction is electrostatic and associated with the negatively charged crystal interlayers of montmorillonite, the edge of the crystal and the formation of peripheral complexes, respectively. A significant amount of polymers with a very low molecular weight can adsorb to the interlayers of montmorillonite at pH ¼ 2.5 (Razakamanantsoa et al., 2012). Both intralamellar and extralamellar interactions can occur in polyionic polymers. To reduce voids, extralamellar interactions are employed to block the flow path in landfill applications. The polymers enhance the hydraulic performance of GCLs. Previous studies show that the influence of ash fill leachates on the polymer treated bentonite is less severe than the untreated sample (not sure if the correction still deliver the same information as what you originally did) and that high-swell bentonite is more resistant to multispecies solutions (Elhajji et al., 2001; Ashmawy et al., 2002; Didier and Comeaga, 1997). Contradictory results have emerged because the formulations of commercialized modified bentonite are often confidential. In addition, the information of manufactured polymers is incomplete, making it challenging to understand the interaction mechanism of bentonite-polymer and to find a suitable polymer additive. The behavior of clay-polymer has been widely researched in terms of chemical properties and microscopic approaches. However, there is a lack of information concerning the influence of raw material on polymer mixtures in terms of the hydraulic parameters of the liner geo-material, namely, the hydraulic conductivity, swelling, and microstructure. The object of the present study was to develop a formulation of modified bentonite with sodium polyacrylate, referred to as a sodium polyacrylate bentonite (SPB), to improve the hydraulic performance in the presence of saline solution and to compare it with that of the raw bentonite (RB) used in GCLs. Fig. 1 shows a schematic diagram of the synthetic reaction. Using calcium bentonite as the raw material, SPB was synthesized using acrylic acid after the sodium transition of sodium carbonate. The formulation of SPB was investigated sufficiently despite numerous influencing factors. Comparison between SPB and RB in terms of the swelling, filtrate loss, cation exchange capacity (CEC), hydraulic performance and microstructure is also included in the current study. 2. Material and methods 2.1. Materials 2.1.1. Development of modified materials The CEC of calcium bentonite (as determined by ammonium cation exchange) was 62.06 meq./100 g, and its swell index was 25 mL/2 g. The other materials used were acrylic acid, N-
243
isopropylacrylamide (NIPA), potassium persulfate (K2S2O8), and sodium carbonate (Na2CO3). 2.1.2. Testing solutions Testing solutions were classified into four types: (1) deionized water (DIW), (2) 0.1% CaCl2 solution, (3) Pb(NO3)2 solution, and (4) HCl solution with various pH values. The free swell index (FSI) of bentonite in the 0.1% CaCl2 solution for 168 h was used to characterize the durability, which was an important parameter used to reflect the long-term swelling character. Pb(NO3)2 and HCl solutions with different pH values were also used to measure the FSI of bentonite. 2.2. Methods 2.2.1. Synthesis of SPB The flow chat of SPB synthesis was shown in was produced with acrylic acid and a conventional calcium bentonite and synthesized using a solution polymerization reaction, as shown in Fig. 2 (Scalia et al., 2012). First, sodium carbonate solution was prepared in a flask, followed by adding RB and NIPA to form a slurry; Second, the slurry was then neutralized by a certain volume of acrylic acid under continuous stirring condition to remove all bubbles; Third, the temperature of this bentonite-monomer slurry was raised, and polymerization was initiated after adding potassium persulfate. The molecules of initiator (potassium persulfate) decomposed into free radicals which reacted with the acrylic acid monomer to form more free radicals, which then in turn reacted with additional monomer to proliferate the polymer chain. Finally, the SPB was dried at 105 C and characterized by Colloid Environmental Technologies (CETCO). 2.2.2. Free swelling and CEC test Both RB and SPB samples were subjected to the free swelling test using the volume change according to ASTM D 5890. First, 90 mL of permeant solution and 2 g of sample were added to a 100 mL of graduated cylinder. The sample was allowed to stand for 24 h, and the first reading was recorded at room temperature. The durability of the samples was tested when the standing time was 168 h. The CEC values could be calculated and measured according to the literature (Borden and Giese, 2001) and following formula (Busenberg and Clemency, 1973):
CEC ðmeq=100 g Þ ¼
50 c w 105
where, c is the measured concentration of ammonia (mol/L), 50 mL is the volume of water, w is the mass of the air-dried clay sample (mg), and 105 is a conversion factor. 2.2.3. Structural analysis X-ray diffraction (XRD, Bruker Company, Germany) was carried out on samples at room temperature using a D8-Advance X
Sodium carbonate
Mix Acrylic acid Mix
Polymerization
Potassium persulfate Fig. 1. Schematic diagram of the synthetic reaction.
Slurry
N-isopropylacrylamide Bentonite
Modified bentonite
Fig. 2. Flow chart of synthetic bentonite.
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diffraction meter (40 kV, 40 mA) with Cu (l ¼ 1.5406 Å) irradiation at the rate of 0.15 s/step rate in the range of 10e80 . Fourier transform infrared spectroscopy (FTIR) of the bentonite samples was performed in transmission mode at room temperature on a 1725X Perkin Elmer instrument using the KBr pellet technique (1:20) at a resolution of 2 cm1. KBr was dried at 200 C for 24 h and homogenized with the bentonite sample in a ball grinder. The specimens (radius 1 cm, thickness 0.1 cm) were prepared using a hydraulic press. Scanning electron microscopy (SEM) was carried out in Japan by using a TENSOR27 Fourier transform infrared spectrometer produced by Zhengzhou Great Wall Branch Industry and Trade Co., electronic production JSM-6700F-type scanning electron microscope, magnification: 5000e30,000 times. Transmission electron microscope (TEM) images were obtained on a JEM-2100F tool. TEM grids were prepared using a few drops of nanoparticles followed by drying.
Fig. 4. Schematic diagram of the hydraulic control system.
3. Results and analysis 2.2.4. Filtrate loss test Filtrate loss tests were performed according to ASTM D5891; this was a standard test originally developed to qualify slurry for drilling applications. In this study, the filtrate loss test was measured to evaluate the short-term behavior of the bentonite permeability in Pb(NO3)2 solutions.
2.2.5. Hydraulic conductivity test To prepare the specimen, granular RB and granular SPB were loosely packed in molds with diameter and height of 70 and 10 mm, respectively. A schematic diagram of the preparation was illustrated in Fig. 3. The dry density of the molded specimen was approximately 15.4 g, or 4 kg/m2. These conditions were designed to simulate the application of bentonite to GCLs. Next, the prepared specimen was sandwiched between two filter papers attached with a porous stone larger than the mound and placed in the apparatus. The specimen was permeated with chemical solutions for 48 h to swell and saturate the specimen. Specimen prehydration shortened the duration of permeate testing substantially. After the specimen was prepared, it was transferred to the membrane testing apparatus. The hydraulic conductivity test was performed using flexible-wall permeameters according to ASTM D 5084, “Standard test method for measurement of hydraulic conductivity of saturated porous materials using a flexible wall permeameter”. A flexible-wall membrane cell and associate hydraulic control system similar to that of Kang and Shackelford used in their study (Kang and Shackelford, 2009). A schematic diagram of the hydraulic control system was illustrated in Fig. 4. The test consisted of two stages: a consolidation stage and a chemic-osmotic testing stage. During the consolidation stage, specimens were permeated at a cell pressure of 35 kPa and a base and back pressure of 15 kPa at a constant room temperature of 20 C for 48 h. During this stage, the swelling press equaled to the effective stress of 20 kPa as it ultimately adapted to the surrounding pressure. Following consolidation, the specimens were permeated from the bottom to the top by increasing the headwater pressure by 30 kPa.
Fig. 3. Schematic diagram of specimen preparation.
3.1. Influence of the amount of reactants on the swelling properties of SPB Based on the amount of acrylic acid used, the optimum mixing proportions of SPB with bentonite, sodium carbonate, NIPA, and potassium persulfate was determined. Furthermore, the parameters of the synthesis process such as temperature and the ratio of solid to liquid were also investigated. The amount of reactants was converted to the mass percentage of acrylic acid (%wt). Several studies have investigated the correlation between the FSI and the hydraulic conductivity (k) of GCLs (Lee et al., 2005; Schackelford et al., 2000), showing that the free swelling index decreased with an increase in permeability. To simplify the detection index of SPB, the mixture ratio of each component was characterized using the FSI. 3.1.1. Influence of sodium carbonate The measured FSI value with the addition of sodium carbonate in the polymerization was shown in Fig. 5(a). The FSI initially increased with an increase in sodium carbonate content and stayed relatively constant when high sodium carbonate content had reached. In the increasing stage, the excess Hþ restrained the swelling of SPB. Thus, for greater swelling of SPB, the acrylic acid and sodium carbonate must react fully. In this case, the relative amount of sodium carbonate was 64.6%wt (weight percentage wt %¼(mass of B/mass of A þ mass of B) 100%). 3.1.2. Influence of NIPA The FSI with NIPA addition was shown in Fig. 5(b), which indicates that the FSI does not change considerably with increasing NIPA. This was because NIPA was a crosslinking agent in the reaction, which changed the linear polymerization of sodium acrylic into a three-dimensional network structure; the fixed bentonite particles improve the degree of recombination. This condition was also reflected in the microstructure. The optimum content of NIPA was 2.5%wt. 3.1.3. Influence of potassium persulfate Fig. 5(c) showed the effect of addition of potassium persulfate in the polymerization on the FSI. The result showed that the optimal dosage of potassium sulphate was 1.25%wt. The polymerization reaction was exothermic and proceeds spontaneously after being triggered by an appropriate concentration of potassium persulfate. However, high concentrations of potassium persulfate resulted in an excessively fast and uneven reaction.
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D
E
F
G
H
I
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Fig. 5. Influence of the amount of reactants on the swelling properties of SPB.
3.1.4. Influence of bentonite Fig. 5(d) showed the results of the addition of bentonite in the polymerization. The recombination rate of SPB decreased with an increase in bentonite content. The swelling performance declined because the swelling was equivalent to the dilution of sodium polyacrylate. For increased performance, the amount of bentonite can be reduced appropriately, albeit with increasing SPB cost. Considering the factors of cost and performance, a bentonite dosage of 625%wt was recommended. 3.1.5. Influence of temperature Fig. 5(e) showed the temperature during polymerization. The organic polymerization was particularly sensitive to temperature as the chemical reactions usually associate with entropy reduction and heat release. The FSI of modified bentonite in DIW was not
synchronized with the mass fraction of the 0.1% CaCl2 solution. Considering the chemical resistance and the energy consumption of the process, it is recommended that the polymerization temperature be controlled at 75 C.
3.1.6. Influence of the solid-to-liquid ratio Fig. 5(f) showed the effect of solid-liquid ratio during polymerization on the FSI. It is necessary to control this ratio to determine the remaining reactant concentrations. In general a greater solidto-liquid ratio results in a larger FSI. This was because high concentration of sodium polyacrylate and bentonite was conducive to improve the utilization of the elements while increasing the ratio of. However, it is also necessary to ensure that the slurry can be stirred and dispersed evenly in solution.
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Fig. 6. Relationship between CEC and addition of materials.
Fig. 7. Comparison of the infrared absorption spectra of RB and SPB.
3.2. CEC analysis
hybrid chain favorably enhances the water absorption and swelling properties of modified bentonite, enabling the modified bentonite to exhibit a decreased solution permeability. FTIR spectroscopy was used to characterize the interfacial interaction between RB and SPB, as shown in Fig. 7. The origins of the peaks were shown in Table 1. Relative to RB, SPB featured additional characteristic bands at 2950 cm1 (eCH2 asymmetric stretching), 1575 cm1, and 1414 cm1 (COOe and C]O symmetric stretching, respectively) in its FTIR spectrum. Sodium polyacrylate modification is responsible for the new peaks observed in the FTIR spectrum of SPB. In addition, some peaks appeared to strengthen in the FTIR spectroscopy results. The interlayer water molecule OeH stretching superposition of the organic carbon chain of the OeH vibration was 3438 cm1. The peak at 1636 cm1 arise from HeOeH bending in water molecules and the vibration of amide and C]C. The peak at 2950 cm1 arise from eCH3 asymmetric vibration. The peak at 1575 cm1 arise from carboxylate radical (COOe) vibration and CeH asymmetric stretching vibration. Peak strengthening produced a peak at 1445 cm1 owing to carbonate and the introduction of the -(CH2)n-structure. The absorption peaks at 975 and 1036 cm1 arise from SieOeSi antisymmetric stretching vibration in bentonite. The enhancement at 621 cm1 was attributed to the induced reaction with potassium persulfate. The absorption peaks at 523 and 470 cm1 were caused by the coupling of montmorillonite SieO-M (where M was a metal cation) and M-O. The composition of RB and SPB was analyzed by XRD instrument, as shown in Fig. 8. Diffraction peaks of the specimens can be
Fig. 6 (a, b) showed the results obtained from the CEC test of SPB synthesized. The CEC value of RB was 62.06 meq./100 g, and the SPB performed best at 96.1 meq./100 g. These values were notably higher than those typically reported for conventional sodium bentonite (Lee and Shackelford, 2005). Fig. 6(a) showed the CEC and FSI changed in SPB with the addition of potassium persulfate. The CEC increased with increasing modification and decreased with increasing potassium persulfate and RB. Moreover, a relationship between the CEC and FSI could be derived from the figure that the FSI increased with increasing CEC. A greater CEC always meant a higher adsorption capacity. 3.3. FTIR and XRD analysis The sodium polyacrylate with the hydrophilic group has the characteristics of water absorption and swelling, and the modification via introduction of a hydrophilic group of COOe leads to less crystallinity of bentonite. Therefore, the polymerization of the
Fig. 8. XRD patterns of RB and SPB.
Table 1 Origins of peak formation. Peaks (cm1)
Reason for peak formation
3627 3438 2950 1636 1575 1445 1414 1320 1036 918 795 621 523 470
AleOeH stretching vibration HeOeH stretching vibration of interlayer water molecules eCH3 asymmetric vibration HeOeH bending vibration of water molecules, amide radical, C]C bond Carboxylate radical (COOe) vibration and CeH asymmetric stretching vibration - (CH2) n- plane rocking vibrations and carbonates Carboxylate radical (COOe) vibration and C]O symmetric stretching vibration Vibration of CeC bond in polymer SieOeSi antisymmetric stretching vibration eOH vibration of Montmorillonite SieOeSi A covalent organometallic compound, M-C and potassium sulphate Coupling vibration of montmorillonite SieO-M (M is metal cation) and M-O Coupling vibration of montmorillonite SieO-M (M is metal cation) and M-O
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Table 2 Experimental results of RB and SPB in FSI and FLV testing in Pb (NO3)2 solutions. C(Pb2þ)
FSI test (mL/2 g)
mM
RB
SPB
RB
SPB
0 10 20 30 40 50
25 16.5 13 12 11.5 10
50 28 25 21 19 17
24 52.6 102.8 130.2 157.2 177
4 4.8 6 15 64 126
FLV test (mL)
Fig. 11. Hydraulic conductivity of SPB and RB in Pb(NO3)2 solutions.
3.4. Filtrate loss analysis
Fig. 9. RB and SPB performance in FSI and FLV in Pb (NO3)2 solutions.
indexed by the MDI Jade 5.0 program. The bentonite could be readily indexed to the reported structures of SiO2 (JCPDS No. 14e0260). The sodium polyacrylate bentonite could be identified as the mixture of C34H32O6 (JCPDS No. 25e1749) and SiO2 (JCPDS No. 05e6466). The sodium polyacrylate modified bentonite was less crystalline than the raw bentonite, as indicated by the intensity of the XRD peaks. Therefore, the main components of bentonite and sodium polyacrylate modified bentonite are SiO2, and their structures exhibit different cell parameters.
Fig. 10. Hydraulic conductivity of SPB and RB under acidic conditions.
Table 2 and Fig. 9 summarized the FSI and FLV results of RB and SPB in Pb(NO3)2 solution. SPB performs better, as shown in Fig. 8. The FSI of SPB in DIW is 50 mL/2 g, which was about twice greater than that of RB in DIW (25 mL/2 g). Even in solutions with high concentration SPB showed better swelling performance than RB. Although both values increased rapidly with a continuous increase in concentration, the FLV of SPB at a concentration of 30 mM of Pb (NO3)2 was still lower than that of RB in DIW. The results clearly showed that the chemical resistance of SPB was significantly improved. 3.5. Permeability 3.5.1. Comparison of the hydraulic conductivity to that of acidic solution A summary of the hydraulic conductivity (k) test results with acidic solutions was shown in Fig. 10. An increasing concentration
Fig. 12. Relationship between the permeability (k) and FSI.
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Fig. 13. SEM (a) and TEM (b, c) images of RB, SEM (d) and TEM (e, f) images of SRB.
of acidic solutions was found to have a positive effect (resulting in greater k values) on the hydraulic performance of the two types of specimen tested in the present study. The cake prepared using SPB was more stable against acid attack at high acid concentration, where interaction with acidic solution resulted in only a negligible increase in k at pH ¼ 2. In contrast, there was a rapid increase in k at pH ¼ 2 for RB. It can be concluded that SPB is substantially better than RB at resisting acid erosion.
3.6. Comparison of the permeability behavior with that of Pb(NO3)2 solution Fig. 11 showed the hydraulic conductivity of RB and SPB in the
presence of Pb(NO3)2 solutions. Lead as a heavy metal is widely used in industry and commonly found in leachates. The hydraulic 10 conductivity (k) of RB with DIW (C2þ m/s. Pb ¼ 0 mM) was 1.61 10 However, k of RB increased rapidly by an order of magnitude. For example, at a Pb2þ concentration of 40 mM, k increased to 2.42 108 m/s, which showed the damaging influences of cation exchange reactions, particularly multivalent heavy metal solutions, in high-salinity environments (without knowing what you want to say, it is hard to correct this sentence). However, the modified bentonite, SPB, showed a relatively stable low permeability (k ¼ 1.2 1011 m/s, C2þ k ¼ 2.68 1011 m/s, Pb ¼ 0 mM; 2þ C2þ concentrations below 40 mM. In addition, k Pb ¼ 40 mM) for Pb of SPB showed a similar tendency to FLV when the Pb2þ
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concentration continued to increase. One possible explanation for the observed behavior of SPB was that the bentonite modified with sodium polyacrylate encapsulated the negatively charged crystal interlayers of montmorillonite when activated with sodium. This process made it challenging to neutralize crystal interlayers with cations owing to the negatively charged polymer network in the periphery. The mechanism of anionic polymer modified bentonite could alleviate only damage to the structure of the bentonite (negatively charged crystal interlayers of montmorillonite) caused by the electrolyte. Therefore, as the ionic strength continued to increase, SPB exhibited a rapidly increasing trend in permeability, similar to RB.
owing to the modification, which indicated a significant improvement in the adsorption capacity that merit further study. These observations were confirmed by FSI and permeant tests, indicating that a strong correlation existed between the bentonite FSI and permeability (k). In field, the damage to the structure of the bentonite caused by the electrolyte could not be eliminated. However, polymer modification was one of the methods that could enhance the resistance of bentonite to the permeant solution. The benefits of the modified bentonite in real application can be further evaluated through long-term field trail.
3.7. Relationship between the hydraulic conductivity and FSI
This work was supported by the National Nature Science Foundation of China (Grant No. 51578427, 51572197, 41372264, and 51702238), the Plan Project of Science and Technology of Zhejiang Province (No. 2014C33015 and 2015C33220), the Opening Funds of Jiangsu Key Laboratory of Construction Materials (CM2018-02), and the Plan Project of Science and Technology of Wenzhou (No. ZS2017002).
The relationship between k and the FSI has been investigated by several researchers (Katsumi et al., 2008; Andry et al., 2015). Liu et al. suggested that a strong correlation existed between the types of permeant liquids and bentonite FSI and k for different k proposed the relationship lg 1:1310 ¼ e0:14ðx14:95Þ , where 11 k ¼ hydraulic conductivity (m/s) and x ¼ FSI (mL/2 g) [1]. The correlation between the FSI and k was plotted in Fig. 12. The FSI and k values of the clays were measured in corresponding acid or saline solution. For each specimen, a higher FSI resulted in a lower k. This tendency was in accordance with Liu's observations that k increases exponentially for FSI values below 15 mL/2 g. 3.8. Microstructures of RB and SPB SEM and TEM were used to observe the morphological changes in RB and SPB for the modified polymer. Fig. 13 showed the rather different microstructures between RB and SPB. SPB has isolated cavities and empty spaces, whereas RB exhibited patchy aggregation and close stacking, as shown in Fig. 13a and d. TEM further confirmed that the RB and SPB structures exhibited irregular flakiness and flocculent particles, respectively (Fig. 13b and c, Fig. 13e and f). One possible explanation of the observed difference in microstructure was that the bentonite layers were rearranged when the negatively charged acrylic monomers were polymerized. In addition, the linear chain of polyacrylic was polymerized to form a three-dimensional network structure with the addition of the crosslinking agent NIPA. Because this cavity structure caused numerous barriers when liquid flows through the matrix, the permeability of the material decreased. 4. Conclusions This work focused on the formulation of bentonite modified with sodium polyacrylate and determination of the optimum material mix ratio and synthesis process. The proposed percentages of reactants relative to the acrylic acid were as follows: 625 wt% of bentonite, 64.6 wt% of sodium carbonate (complete reaction), 2.5 wt% of NIPA, 1.25 wt% of potassium persulfate, 1:2 solid-toliquid ratio, and 75 C reaction temperature. The SPB that was modified according to this formulation showed a significant improvement in chemical resistance. The permeability was reduced by an order of magnitude through the modification process under the DIW condition, and the chemical resistance increased to more than 40 mM in the Pb(NO3)2 solution. The microstructure analyzed by SEM clarified the mechanism of anionic polymer modified bentonite, which explained the improvement in hydraulic conductivity performance. The microstructure of modified bentonite changed from lamellar to cavity, which increased the specific surface area and permeant barriers. In addition, the CEC value increased from 62.06 to 96.1 meq./100 g
Acknowledgements
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