- Email: [email protected]
Development of a novel base liner material for offshore final disposal sites and the assessment of its hydraulic conductivity
Waste Management 102 (2020) 190–197
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Development of a novel base liner material for offshore final disposal sites and the assessment of its hydraulic conductivity Xin Xu a,b,⇑, Xiaofeng Liu c, Myounghak Oh d, Junboum Park a,* a
Department of Civil and Environmental Engineering, Seoul National University, Seoul 08826, South Korea Institute of Engineering Research, Seoul National University, Seoul 08826, South Korea c College of Civil Engineering, Taiyuan University of Technology, Taiyuan 030024, China d Maritime ICT R&D Center, Korea Institute of Ocean Science & Technology, Busan 49111, South Korea b
a r t i c l e
i n f o
Article history: Received 24 November 2018 Revised 22 October 2019 Accepted 24 October 2019
Keywords: Offshore waste disposal Bentonite Saline water Novel liner material Hydraulic conductivity Compressive strength
a b s t r a c t In the process of landfilling, leachates resulting from the waste landfill are likely to cause secondary environmental pollution, and installation of a basal liner is essential under a landfill site to block and reduce permeation of leachate flowing into the subsurface environment. The research aims to develop a saltresistant bentonite and a novel base liner material for offshore waste disposal. The liner materials consist of core materials and coating materials in which mixtures of bentonite, sepiolite, and guar gum were used to overcome the shortcomings in bentonite to realise a high water-resistance and a permeability coefficient of below 1.0 10 7 cm/s under saline water conditions. The optimal mixing ratio of bentonite, sepiolite, and guar gum was confirmed as 76:19:5 by conducting drying shrinkage cracking tests, free swelling tests, and hydraulic conductivity tests. The hydraulic conductivities of spherical particles, as measured in a rigid-wall permeameter and a flexible-wall permeameter, were less than 1.0 10 7 cm/ s under saline water conditions. The compressive properties of spherical particles were evaluated through triaxial compression testing. The engineering characteristics of the liner material were studied in the present research, but the long-term biodegradation characteristics of polymer additives were also important, yet remained unclear. The long-term biostability of the additives, and its effect on basal liner performance should be evaluated in future research. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction With growing economic development, increasing urbanization, and the constant expansion of the world’s population, all types of municipal solid waste are also increasing both by mass and volume (Hoornweg and Bhada-Tata, 2012). South Korea is a nation with small land area, a high population density, and many mountainous regions, therefore on-land waste disposal space is scarce. Meanwhile, capacity in existing on-land waste disposal facilities is reaching saturation and it is difficult to build new facilities: because South Korea is surrounded by sea on three sides, it has an abundance of sea space and offshore waste disposal will become a possible effective waste disposal method in the future (Renou et al., 2008). Some countries have already built offshore waste disposal facilities, but not yet South Korea. In recent years, the government has input more time and funding into offshore waste
⇑ Corresponding authors at: Department of Civil and Environmental Engineering, Seoul National University, Seoul 08826, South Korea (X. Xu). E-mail addresses: [email protected] (X. Xu), [email protected] (J. Park). https://doi.org/10.1016/j.wasman.2019.10.047 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
disposal research, and the planned offshore waste disposal facility will be used to dispose of the municipal solid waste incineration ash generated from the capital city area in South Korea. This research into a base liner material for an offshore waste disposal site was a part of a project entitled ‘‘Development of Technology for Offshore Waste Final Disposal”. For offshore waste disposal, the effects on oceanic environments and leachate leakage remain a high priority. In modern landfills, solid waste disposal facilities must contain a liner system which controls intrusion of groundwater into landfill facilities and protects the soil and ground water from pollution originating in the landfill. The hydraulic conductivity of a liner material used in landfill should be less than 1.0 10 7 cm/s (Mor et al., 2006; Alsabahi et al., 2009), but in offshore waste disposal facilities, it is difficult to lay a complete liner system underwater. The construction standard in major offshore waste disposal countries (Japan and Singapore) is as follows: if there is a vertical impermeable layer under the seafloor, and the thickness of layer is greater than 5 m and its hydraulic conductivity is less than 1.0 10 5 cm/s, a liner system is not required during offshore
191
X. Xu et al. / Waste Management 102 (2020) 190–197
waste disposal facility construction (MEJ, 2007; Chai and Miura, 2002), however, the seafloor is not smooth everywhere, it often exhibits non-uniform thickness and soft soil areas in impermeable layers, that can affect construction operations. To tackle this problem, a novel bottom-liner system construction method (a spherical particle liner system) for offshore waste disposal was proposed. The novel bottom liner materials consist of core and coating materials. When the particles make contact with saline water (or fresh water), coating materials swell to fill the voids to constitute a liner system with low permeability (Fig. S2). The high-strength core materials can provide enough bracing force for offshore waste final disposal and cement beads were used as core materials; because cement mortars exhibit good mechanical properties and great durability, they can be hydrated in aqueous solution, and have excellent strength and high stability. The spherical particles will be deposited into the expected bottom layer where engineers need to enhance the impermeability and strength of the impermeable layer during waste disposal facility construction. Bentonite, as a waterproof material, has been widely used in waste disposal works and it belongs to the smectite group of clay minerals and is an absorbent aluminium phyllosilicate, generally found as impure clay, consisting mostly of montmorillonite (Kong, 2010; Millard et al., 2016; Liu and Zhou, 2010; Khan, 2017). Bentonite can be used as a liner material in waste disposal sites owing to its good characteristics, such as low hydraulic conductivity, high swelling capacity, and adsorption properties (Arabzai and Honma, 2014; Panjaitan and Andi, 2017). Some researchers have undertaken in-depth research into the properties of bentonite as a liner material in on-land waste disposal facilities (Tay et al., 2001; Tang et al., 2011). There remain several problems when using bentonite as a liner system material in coastal areas: low swelling capacity, high permeability under saline conditions, and, because of bentonite’s powder state and high liquidity, it is difficult to use bentonite to construct bottom liners in coastal areas and construction quantities are usually large (Komine et al., 2007). In addition, the spherical particles were prepared by using bentonite slurry as a coating material to cover around the cement beads, and dried in air to obtain the base liner materials. To avoid coating material separation from the core material, and for convenience of storage and transportation, drying shrinkage cracking of the coating material must be prevented. To deal with these problems, bentonite was used as the main material and mixed with some additives to enhance workability under saline conditions. Sepiolite is shot with randomly distributed fibres that can be used in adhesives to fill joints and cracks in engineering works. Its structure is stable even in saline environments with high salt concentrations, thus can be added to bentonite to prevent drying cracks (Alvarez, 1984.). Many researchers have found that using a polymer, to modify the bentonite can increase its swelling capacity and impermeability under saline conditions (Camillis et al., 2016; Wang and Wang, 2009). Modified coating materials can reduce drying shrinkage cracking and have low hydraulic conductivity under saline conditions. The objective of this research is to develop a novel bottom liner system and investigate the engineering properties of spherical particles. Sepiolite was added to bentonite to prevent drying shrinkage cracks and guar gum was added to bentonite to maintain the swelling capacity of bentonite under saline conditions. The spherical particles can enhance the workability of bentonite as a bottom liner material. The optimum mixing ratio of the coating materials was determined through drying shrinkage crack testing, free swelling testing, and hydraulic conductivity testing. The hydraulic conductivity was determined by use of the falling head method and compressive properties were evaluated through triaxial compression testing.
2. Materials 2.1. Bentonite and sepiolite Natural sodium bentonite (Na-bentonite) mined in India, and sepiolite produced in South Korea, were used in the current research. Sepiolite was added to the bentonite to prevent drying shrinkage cracking. Both bentonite and sepiolite were powders and were sieved through a No. 200 sieve. The physical and chemical properties of bentonite and sepiolite are summarized in Tables 1 and 2, Fig. S3 shows the microstructures of bentonite and sepiolite as captured by scanning electron microscope (SEM). As the bentonite has a plate structure, a large amount of water can penetrate the void spaces therein, which cause a change from a platelike structure to a house-of-cards structure. This exacerbates the volumetric expansion of the bentonite. As may be seen from the microstructure of the sepiolite, multiple fibres are present, which results in the interconnection between it and the bentonite after being mixed with sepiolite. Therefore, fewer fissures were generated in the bentonite during drying shrinkage (Ekici et al., 2006; Guney et al., 2014).
2.2. Salt-resistant materials Four types of salt-resistant materials were selected for this research: diatomite, guar gum, sodium pyrophosphate, and xanthan gum. Due to the high specific surface and multi-pore structure, diatomite can adsorb cations in saline solution and therefore prevent ion exchange between the bentonite and a saline solution and reduce the salt ionic effect on bentonite swelling capacity (Nenadovic´ et al., 2009; Tian et al., 2016). The other three materials are high-molecular weight polymers, which could surround, and combine with the bentonite, then reduce the salt ionic effect on the swelling capacity in saline solution. Polymers have high cohesion and are often used as additives to enhance the interaction between themselves and other materials (Sandolo et al., 2007). Polymers can be mixed with bentonite to enhance some properties of the mixed material such as decreasing the number of cracks and their hygroscopic nature (Levy et al., 1995).
2.3. Core materials The core materials were prepared by mixing cement and sand in a mass ratio of 3:2 to form spherical particles of different diameters. Salt-resistant cement with strength grade of 42.5 (produced in South Korea) was used, and the sand was a Jumunjin standard sand (mined in South Korea). There are two sizes of core materials used in this research: 10 mm in diameter and 20 mm in diameter. The amount of leaching of hazardous material in core materials was below corresponding standard limits set by the Ministry of Environment of Korea (MEK, 2016).
Table 1 Physical properties of bentonite and sepiolite as used in this study.
Specific gravity Liquid limit (%) Plastic index (%) Unified Soil Classification System Percentage passing sieve No. 200 Specific surface area (m2/g) Cation exchange capacity (meq/100 g)
Bentonite
Sepiolite
2.7 397 360 CH 100% 40–50 100–130
2.5 133 47 MH 100% 80–100 25–50
192
X. Xu et al. / Waste Management 102 (2020) 190–197
Table 2 Chemical composition of bentonite and sepiolite as determined by X-ray fluorescence spectrometer (XRF).
Bentonite Sepiolite
SiO2
MgO
Al2O3
Na2O
K2O
Fe2O3
TiO2
MnO
CaO
P2O5
L.O.I
Total
61.30 28.60
1.64 17.71
14.62 1.41
2.54 0.11
0.28 0.25
3.80 0.47
0.52 0.04
0.07 0.02
4.13 23.98
0.01 0.03
11.09 27.92
100 100
2.4. Permeant solutions To observe the properties of bentonite under fresh and saline conditions, distilled water and artificial sea water were used in this research. The saline water was prepared according to ASTM D 1141-98 (2008) which represents standard practice for the preparation of substitute ocean water. Saline water was prepared through dissolving NaCl (2.45%), MgCl2 (0.52%), Na2SO4 (0.41%), and CaCl2 (0.12%) in distilled water to simulate sea water. The properties of the artificial sea water used in this study were as follows: a pH of 8.2, a salinity of 3.5%, an electrical conductivity of 4.3 Xm and an ionic strength of 0.66 mol/L. 3. Methods Bentonite, sepiolite, and salt-resistant materials were mixed to develop a coating material which has high swelling capacity and low permeability under saline water. Drying shrinkage crack tests and free swell index tests were carried out to determine the optimum mixing ratio of sepiolite in mixed clay and confirm most effective salt-resistant materials for modified bentonite by hydraulic conductivity testing. Hydraulic conductivity test and triaxial compression test were used to evaluate the properties of base liner materials under saline conditions. 3.1. Drying shrinkage crack test Adding some fibrous structural materials to bentonite can reduce desiccation cracking. Sepiolite is often used to this end. To determine the optimum mixing ratio of bentonite and sepiolite in mixed clay (bentonite-sepiolite mixture), drying shrinkage crack tests were conducted (Table S3). The mixed clay with sepiolite contents of 0%, 5%, 10%, 15%, 20%, 25%, 30%, 40%, 50%, and 60% was prepared, and then mixed with 1.5 times the mass of water, afterwards, the mixed clay slurry was shaped in a mould measuring 30 mm in diameter and 3 mm in height. Test specimens were dried at room temperature for 48 h and drying shrinkage cracking was then assessed. 3.2. Free swell index test The free swell index test was conducted to determine the swelling capacity of a mixed clay and to observe the effects of saltresistant materials on the chemical resistance of bentonite against saline conditions. The optimum salt-resistant materials could be selected for bentonite modification using free swell index tests. Guar gum, diatomite, sodium pyrophosphate (TSPP), and xanthan gum were selected as alternative materials for bentonite modification. The amounts of salt-resistant materials in the modified bentonite were 0%, 1%, 1.5%, 2.5%, 5%, and 7.5% (Table S4). According to the aforementioned mix proportions, the diatomite powder was directly added to the mixed clay and mixed thoroughly to obtain a modified bentonite. The three polymers were separately dissolved in distilled water to prepare aqueous solutions thereof, then mixed with the bentonite base in the aforementioned proportions and uniformly stirred at 60 °C for 2 h using an electric blender. The mixtures were kept at room temperature until completion of cross-linking polymerization wherein, the bentonite
and polymers can polymerize together through chemical bounding and chain-formation to form a stable polymeric substance. They were then oven-dried at 80 °C, and the dried mixtures were ground and sieved to obtain the modified bentonite. Based on ASTM D 5890 (2006), gradated cylinders were prepared with 90 mL fresh water or saline water, 2 g of each specimen was poured into separate cylinders in 0.1 g increments. After 2 g of specimen had been dispensed, the cylinders were filled to a total volume of 100 mL with fresh, or saline water. After 16 to 24 h, the equilibrium volumes of the swelling specimens were recorded. 3.3. Spherical particle preparation The workability of bentonite was improved using modified bentonite (i.e., a bentonite-sepiolite-guar gum mixture) as a slurry cover around cement beads (the core material). The optimum mixing ratio of modified bentonite has been determined by drying shrinkage crack testing, free swelling index testing, and hydraulic conductivity testing. The coating material was prepared by mixing modified bentonite with water in a solid-liquid mixing ratio of 2:3. Then the coating material was used to cover cement beads to make spherical particles and the spherical particles were dried in air for 48 h. To explore the relationship between the hydraulic conductivity and coating material thickness, hydraulic conductivity and particle size, six different sizes of spherical particles were prepared. The spherical particles measured 10 mm in diameter with coating material thicknesses making up 1 mm, 2 mm, 3 mm, and 20 mm diameters (i.e., coating material thicknesses of 2 mm, 3 mm, and 4 mm, respectively). 3.4. Hydraulic conductivity test Hydraulic conductivity tests were conducted on coating materials and spherical particles. The hydraulic conductivity test was carried out using both a rigid-wall permeameter and a flexible-wall permeameter under saline water or fresh water conditions, and the falling head method was used in all cases. For the rigid-wall permeameter, the hydraulic conductivity was measured according to ASTM D 5856 (2007) and an influent pressure was applied using nitrogen gas at 34.47 kPa and the hydraulic conductivity of coating materials and spherical particles could be determined. For the flexible-wall permeameter, the hydraulic conductivity was measured according to ASTM D 5084 (2004), and an influent pressure of 34.47 kPa and confining pressure of 55.15 kPa were applied by way of compressed nitrogen gas. The void fraction of specimens was determined before and after testing. Both short-term and long-term tests were performed at a constant temperature of 24 °C. 3.5. Triaxial compression test (CU) Consolidated-undrained triaxial compression testing was undertaken to assess the compressive strength of the proposed base liner materials. Tests were conducted in accordance with ASTM D4767 (2011), and three confining pressures (100, 300, and 500 kPa) were used. Test specimens were 100 mm in diameter and 210 mm in height, and specimens were saturated by using a vacuum pumping system. The tests were conducted at a strain rate of 0.5% per minute until the axial strain reached 20%. Two sizes of
193
X. Xu et al. / Waste Management 102 (2020) 190–197
spherical particles were used in the test: 10 and 20 mm in diameter (core size). To simulate offshore conditions, artificial seawater was used as the permeant solution in CU tests. 4. Results and discussion 4.1. Mixing ratio of coating material (modified bentonite) 4.1.1. Drying shrinkage crack testing Through drying shrinkage crack tests, the optimum ratio of bentonite to sepiolite in a mixed clay was determined. After the water had evaporated, the diameter of test specimens decreased from 30 mm to 20 mm after air-drying (Table 3). The amount of shrinkage cracks decreased with increasing sepiolite content and cracks were not found when the sepiolite content exceeded 15% in the mixed clay; however, upon increasing the sepiolite content, the plasticity and shrinkage capacity of the mixed clay decreased and at a sepiolite content of more than 30%, the shape of the samples was not preserved after drying. The best ratio of bentonite and sepiolite was found to be 4:1 with regard to it safely preventing shrinkage cracks after drying. 4.1.2. Free swell index testing Table 4 shows the free swell index of bentonite, sepiolite and mixed clay under fresh and saline conditions. In saline conditions, the free swell index of bentonite decreased to about one third of that in fresh water conditions. The free swell index of mixed clay (at a bentonite: sepiolite ratio of 4:1) was 18.5 mL per 2 g and 6 mL per 2 g under fresh, and saline, conditions, respectively. To enhance the swelling capacity of bentonite under saline condition, some salt-resistant materials were added to the mixed clay to impart a saline effect on the bentonite. Four types of alternative salt-resistant materials were added into the mixed clay respectively before measuring the free swell index, and the alternative salt-resistant materials were selected on the basis of published evidence of previous use. Fig. 1 illustrates the free swell index test results for mixed clays with added salt-resistant materials under saline conditions: the swell index increased with increasing added doses of salt-resistant materials. All salt-resistant additives (except diatomite) have high salinity-tolerances. Although the swelling index does not have a proportional correlation to hydraulic conductivity, a high swelling capacity was considered indicative of the quality of a mixed clay. Owing to its salinity tolerance, viscosity, and low cost, guar gum was confirmed as optimal salt-resistant material for bentonite modification. When the mixing ratio of mixed clay and guar gum was 37:3, the free swell index under saline conditions was close to that in fresh water. 4.1.3. Hydraulic conductivity testing of coating materials A hydraulic conductivity test was performed to study the permeability of the coating material and validate the optimum mixing Table 3 Crack parameters for the dried mixed clay. Sepiolite content (wt.%)
0, 5, 10
15
20, 25, 30, 40, 50, 60
Crack length
1 cm
1 cm
No cracking
ratio thereof. The guar gum contents in the modified bentonite were 0%, 1%, 1.5%, 2.5%, 5%, and 7.5% (Table S5). For test specimens, a coating material slurry was formed into small globules. The specimens were dried at room temperature for 48 h and the diameter of the specimens decreased to 7–9 mm after drying. Then the specimens were placed into a rigid-wall permeameter without compaction. Fig. 2 shows hydraulic conductivity of mixed clay (bentonite-sepiolite mixture) under fresh, and saline, conditions. The hydraulic conductivity of the mixed clay was below 1.0 10 9 cm/s under fresh water conditions, but under saline conditions, it exceeded 1.75 10 5 cm/s (a 20,000-fold increase) under fresh water condition. Similar results were obtained in other research (Anandganarayanan and Murugaiyan, 2014; Shariatmadari et al., 2011). The hydraulic conductivity exceeded 1.0 10 7 cm/s, thus did not satisfy EPA standard (2001) requirements for use as a bottom liner material for waste disposal facilities. Fig. 3 displays hydraulic conductivity test results of coating materials (bentonite-sepiolite mixtures with various added guar gum contents) under saline conditions. The hydraulic conductivity of the coating materials decreased with increasing guar gum content. When the guar gum content was greater than 2.5%, the hydraulic conductivity was below 1.0 10 7 cm/s under saline conditions. To ensure that the hydraulic conductivity of coating materials remained below 1.0 10 7 cm/s under saline conditions, the guar gum content must be above 2.5%. These results confirmed 5% as the guar gum content when added to this modified bentonite. To overcome the weaknesses of bentonite under saline conditions, the weight ratio of bentonite, sepiolite and guar gum in modified bentonite was 76:19:5. The solid–liquid ratio in the coating material slurry was 2:3 (weight ratio of modified bentonite to water). 4.2. Hydraulic conductivity of spherical particles 4.2.1. Hydraulic conductivity measured in a rigid-wall permeameter The coating material was prepared by mixing modified bentonite with 1.5 times the mass of distilled water, thus forming a coating material slurry which was used to cover the cement beads to form spherical particles. The spherical particles were air-dried for 48 h. The spherical particles measured 10 mm in diameter (core size) with coating material thicknesses of 1 mm, 2 mm, and 3 mm, and 20 mm in diameter (core size) with coating material thicknesses of 2 mm, 3 mm, and 4 mm. Fig. 4 demonstrates hydraulic conductivity test results of spherical particles of 10 mm (or 20 mm) diameter core size with various coating thicknesses under saline conditions. The hydraulic conductivity of spherical particles decreased with increasing coating thickness. When the coating thickness exceeded 2 mm (or 3 mm), the hydraulic conductivity was less than 1.0 10 7 cm/s under saline conditions. To ensure that the hydraulic conductivity was below 1.0 10 7 cm/s under saline conditions, the coating thickness must be greater than 2 mm (or 3 mm). The range of initial void fractions was 0.34– 0.37, and the final void fractions of specimens decreased to 0.0031–0.044 after rigid-wall hydraulic conductivity testing. Based on the short-term hydraulic conductivity test results, spherical particles which have a hydraulic conductivity of less than 1.0 10 7 cm/s were selected as specimens for long-term testing
Table 4 Free swell index of bentonite, sepiolite and mixed clay under fresh and saline water conditions. Free swell index of bentonite (mL/2 g)
Free swell index of sepiolite (mL/2 g)
Free swell index of mixed clay (mL/2 g)
Fresh
Saline
Fresh
Saline
Fresh
Saline
22
7
5.5
5.5
18.5
6
194
X. Xu et al. / Waste Management 102 (2020) 190–197
Fig. 1. Free swell index test results of mixed clay modified by salt-resistant additive (a: Diatomite, b: Xanthan gum, c: Guar gum, d: TSPP).
Fig. 2. Hydraulic conductivity of mixed clay under fresh and saline water conditions (without guar gum).
Fig. 3. Hydraulic conductivity of coating materials modified by guar gum under saline water conditions.
(6 months). Test specimens included 10 mm diameter cores with 2 mm and 3 mm coating thicknesses, and 20 mm diameter cores with 3 mm and 4 mm coating thicknesses. The long-term hydraulic conductivity tests were continued for 6 months and were described by using pore volumes of flow (PVF). Fig. 5 illustrates the hydraulic conductivities of such spherical particles under saline conditions. All test results (each with four replicates) show the relationship between hydraulic conductivity and PVF. The physical equilibrium was evaluated by the ratio of volume of outflow to volume of inflow. The hydraulic conductivities of each sample decreased to equilibrium after about 0.8 to 1 PVF under saline
conditions. In Fig. 5(a), the hydraulic conductivities of two types of spherical particles were between 2.1 10 8 cm/s and 8 9 2.5 10 cm/s and from 6.4 10 cm/s to 7.2 10 9 cm/s. In Fig. 5(b), the hydraulic conductivities of two types of spherical particles were between 3.0 10 8 cm/s and 3.4 10 8 cm/s and 7.7 10 9 cm/s to 8.4 10 9 cm/s. The hydraulic conductivities underwent minor fluctuations as the PVF increased and the hydraulic conductivities were similar under short-term and longterm test conditions. The test results indicate that spherical particles were stable and had a low permeability under saline conditions, however, the tests were not designed to investigate the
X. Xu et al. / Waste Management 102 (2020) 190–197
195
Fig. 4. Hydraulic conductivity of spherical particles of various coating thicknesses under saline water conditions.
Fig. 5. Hydraulic conductivity of spherical particles under saline conditions.
potential effects of biodegradation of the polymers over a much longer time-frame and the related biodegradation characteristics of polymers warrant investigation in future research. The test results show that the hydraulic conductivities of these spherical particles decreased with increasing coating thickness for both core sizes (10 mm and 20 mm). At a given coating thickness, spherical particles with smaller cores had a lower hydraulic conductivity than particles with a larger core. Fig. 6 shows the relationship between hydraulic conductivity and coating thickness ratio for spherical particles under saline conditions. The hydraulic conductivity decreased with increasing ratio of coating thickness to core diameter. To acquire a hydraulic conductivity of less than 1.0 10 7 cm/s under saline conditions, the ratio of coating thickness to core diameter must be greater than 0.15. To obtain more accurate experimental data, small spherical particles were used in these laboratory hydraulic conductivity tests: when using spherical particles as a bottom liner system in an offshore waste disposal facility, the appropriate size of spherical particle remains unknown. Based on the ratio of coating thickness to core diameter, spherical particles can be produced in various sizes for on-site application.
4.2.2. Hydraulic conductivity measured in a flexible-wall permeameter To determine the hydraulic conductivity of spherical particles under field seawater pressure, a flexible-wall permeameter was used to simulate field conditions. Spherical particles with a
Fig. 6. Relationship between coating thickness ratio and hydraulic conductivity under saline water conditions (spherical particles).
hydraulic conductivity (2.1–9.3 10 8 cm/s) of less than 1.0 10 7 cm/s were selected as test specimens for flexible-wall permeameter testing, we compared the hydraulic conductivity of spherical particles as measured with a rigid-wall permeameter in Fig. 7. The hydraulic conductivity was three to four times higher when measured in the flexible-wall permeameter at a given salinity, this finding is in agreement with other research results (Ahn
196
X. Xu et al. / Waste Management 102 (2020) 190–197
Fig. 7. Hydraulic conductivity of spherical particles under saline water conditions: rigid, and flexible-wall permeameters.
and Jo, 2009). The range of specimen initial void fractions was 0.34–0.37, and the final void fractions decreased to 0.0047–0.052 after flexible-wall hydraulic conductivity testing; however, the hydraulic conductivity also satisfied the EPA standard requirements in that it was less than 1.0 10 7 cm/s under saline conditions. 4.3. Triaxial compression test (CU) Fig. 8 shows compressive stress-strain curves for the proposed base liner material under saline water conditions. As shown in the figures, the deviator stress increased with the confining pressure at the same axial strain. At the same axial strain and confining pressure, the specimen containing the bigger spherical particles had a higher deviator stress at failure. This is owing to the test specimens with spherical particles of 20 mm diameter (core size) containing more core material. This trend is similar to that reported elsewhere (Xia, et al., 2017; Wang, et al., 2016; Xie, et al., 2005). From the Mohr-Coulomb envelopes, the cohesion (c) and friction angle (u) were estimated. The cohesion and friction angle of specimens were as follows: spherical particles of 10 mm diameter (core size): 131.6 kPa and 19.5°; spherical particles of 20 mm diameter (core size): 98.7 kPa and 18.9°. The effect of spherical particle size on friction angle was negligible yet that on
cohesion was significant. These data can be used as a basis for practical construction work using such materials. 5. Conclusion A laboratory study was conducted to develop a novel base liner material for offshore waste disposal facilities. The hydraulic conductivity and compressive strength of engineered spherical particles were evaluated. A salt-resistant bentonite was developed as a coating material for the spherical particles by adding sepiolite and guar gum to a bentonite base, and the optimum mixing ratio of bentonite, sepiolite, and guar gum was confirmed as 76:19:5. The salt-resistant bentonite exhibits a high swelling capacity, low hydraulic conductivity, and a low incidence of drying shrinkage cracking. At a given salinity, the hydraulic conductivity of spherical particles as measured in a rigid-wall permeameter was lower than that measured in a flexible-wall permeameter. The hydraulic conductivity decreased as the ratio of coating thickness to core diameter increased. To obtain a hydraulic conductivity of less than 1.0 10 7 cm/s under saline conditions, the ratio of coating thickness to core diameter was greater than 0.15. The triaxial compression test results show that the effect of spherical particle size on friction angle was negligible yet that on the apparent cohesion was significant.
Fig. 8. Variation of deviator stress with axial strain under different confining pressures.
X. Xu et al. / Waste Management 102 (2020) 190–197
In further research, three-dimensional water tank testing, and large column testing, will be used to investigate the in situ performance of spherical particles. Both the permeability and compressive strength of novel bottom liner systems will be evaluated under simulated field conditions. In addition, the long-term biodegradation characteristics of polymer additives and the resultant effect on basal liner performance will also be investigated, and this basal liner material should not be used in field until these issues have been addressed. Funding This research was supported by a part of the project titled ‘‘Development on Technology for Offshore Waste Final Disposal”, funded by the Ministry of Oceans and Fisheries, South Korea, and the Korea Ministry of the Environment as ‘‘The SEM (Subsurface Environment Management) projects; 2019”. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.10.047. References Anandganarayanan, G., Murugaiyan, V., 2014. Effects of salt solution and sea water on the geotechnical properties of soil-a review. Int. J. Eng. Res. Technol. 3 (3), 1819–1824. Ahn, H.S., Jo, H.Y., 2009. Influence of exchangeable cations on hydraulic conductivity of compacted bentonite. Appl. Clay Sci. 44 (1), 144–150. Alsabahi, E., Rahim, S.A., Zuhairi, W.Y.W., Alnozaily, F., Alshaebi, F., 2009. The characteristics of leachate and groundwater pollution at municipal solid waste landfill of Ibb city, Yemen. Am. J. Environ. Sci. 5 (3), 256–266. Arabzai, A., Honma, S., 2014. Adsorption and transport of heavy metal ions in saturated soils. Proc. School Eng. Tokai Univ. 39, 27–32. ASTM D1141-98, 2008. Standard Practice for the Preparation of Substitute Ocean Water. American Society for Testing and Materials International. ASTM D5890-11, 2006. Standard Test Method for Swell Index of Clay Mineral Component of Geosynthetic Clay Liners. American Society for Testing and Materials International. ASTM D5856-15, 2007. Standard Test Method for Measurement of Hydraulic Conductivity of Porous Material using a Rigid-wall, Compaction-mold Permeameter. American Society for Testing and Materials International. ASTM D5084-03, 2004. Standard Test Method for Measurement of Hydraulic Conductivity of Saturated Porous Materials using a Flexible Wall Permeameter. American Society for Testing and Materials International. ASTM D4767, 2011. Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils. American Society for Testing and Materials International. Alvarez, A., 1984. Sepiolite: properties and uses. Dev. Sedimentol. 37, 253–287. Camillis, M.D., Emidio, G.D., Bezuijen, A., Verástegui-Flores, R.D., 2016. Hydraulic conductivity and swelling ability of a polymer modified bentonite subjected to wet-dry cycles in seawater. Geotext. Geomembr. 44 (5), 739–747. Chai, J.C., Miura, N., 2002. Comparing the performance of landfill liner systems. J. Mater. Cycl. Waste Manage. 4 (2), 135–142.
197
Ekici, S., Isßıkver, Y., Saraydın, D., 2006. Poly(acrylamide-sepiolite) composite hydrogels: preparation, swelling and dye adsorption properties. Polym. Bull. 57 (2), 231–241. EPA530-F-97-002, 2001. Geosynthetic Clay Liners Used in Municipal Solid Waste Landfills, Solid Waste and Emergency Response. United States Environmental Protection Agency. Guney, Y., Cetin, B., Aydilek, A.H., Tanyu, B.F., Koparal, S., 2014. Utilization of sepiolite materials as a bottom liner material in solid waste landfills. Waste Manage. 34 (1), 112–124. Hoornweg, D., Bhada-Tata, P., 2012. What a Waste: A Global Review of Solid Waste Management. World Bank, Washington DC. Komine, H.K., Yasuhara, K.Y., Murakami, S.M., 2007. Swelling characteristics of bentonites in artificial seawater. Can. Geotech. J. 46 (46), 177–189. Kong, W., 2010. Study on swelling performance of bentonite in aqueous solution. Non-Metall. Mines 33 (1), 42–44. Khan, M.R., 2017. Adsorption of lead by bentonite clay. Int. J. Sci. Res. Manage. Stud. 5 (7), 5800–5804. Liu, Z.R., Zhou, S.Q., 2010. Adsorption of copper and nickel on Na-bentonite. Process Saf. Environ. Prot. 88 (1), 62–66. Levy, N., Garti, N., Magdassi, S., 1995. Flocculation of bentonite suspensions with cationic guar. Colloids Surf., A 97 (2), 91–99. MEJ, 2007. The Specification for Design and Construction Offshore Waste Disposal Facilities. Ministry of Environment, Japan. MEK 2016-4, 2016.. The Standard of Water Quality of Groundwater and River. Ministry of Environment, Seoul, S. Korea. Millard, A., Mokni, N., Barnichon, J.D., Thatcher, K.E., Bond, A.E., Fraser-Harris, A., 2016. Comparative modelling of laboratory experiments for the hydromechanical behaviour of a compacted bentonite-sand mixture. Environ. Earth Sci. 75 (19), 1311. Mor, S., Ravindra, K., Dahiya, R.P., Chandra, A., 2006. Leachate characterization and assessment of groundwater pollution near municipal solid waste landfill site. Environ. Monit. Assess. 118 (1–3), 435–456. Nenadovic´, S., Nenadovic´, M., Kovacˇevic´, R., Matovic´, Lj, 2009. Influence of diatomite microstructure on its adsorption capacity for Pb(Ⅱ). Sci. Sinter. 41 (3), 309–317. Panjaitan, N., Andi, A., 2017. Electrokinetic phenomena of cation exchange and its effect on the behaviour of expansive clays. Int. J. Geomate. 13 (38), 173–177. Renou, S., Givaudan, J.G., Poulain, S., Dirassouyan, F., Moulin, P., 2008. Landfill leachate treatment: review and opportunity. J. Hazard. Mater. 150 (3), 468–493. Shariatmadari, N., Salami, M., Karimpour, F.M., 2011. Effect of inorganic salt solutions on some geotechnical properties of soil-bentonite mixtures as barriers. 9(2), 103–110. Sandolo, C., Matricardi, P., Alhaique, F., Coviello, T., 2007. Dynamo-mechanical and rheological characterization of guar gum hydrogels. Eur. Polym. J. 43 (8), 3355– 3367. Tay, Y.Y., Stewart, D.I., Cousens, T.W., 2001. Shrinkage and desiccation cracking in bentonite-sand landfill liners. Eng. Geol. 60 (1), 263–274. Tian, L., Zhang, J., Shi, H., Li, N., Ping, Q., 2016. Adsorption of malachite green by diatomite: equilibrium isotherms and kinetic studies. J. Dispers. Sci. Technol. 37 (7), 1059–1066. Tang, C.S., Shi, B., Liu, C., Suo, W.B., Gao, L., 2011. Experimental characterization of shrinkage and desiccation cracking in thin clay layer. Appl. Clay Sci. 52 (1–2), 69–77. Wang, J.Y., Cao, W.G., Jiang, Z.M., Zhao, Z.P., 2016. Large-scale triaxial tests on deformation and mechanical behavior of soil-rock aggregate mixture under different stress paths. Rock Soil Mech. 37 (2), 424–431. Wang, W., Wang, A., 2009. Synthesis, swelling behaviors, and slow-release characteristics of a guar gum-g-poly (sodium acrylate)/sodium humate superabsorbent. J. Appl. Polym. Sci. 112 (4), 2102–2111. Xia, J., Hu, R., Qi, S., Gao, W., Sui, H., 2017. Large-scale triaxial shear testing of soil rock mixtures containing oversized particles. Chin. J. Rock Mech. Eng. 36 (8), 2031–2039. Xie, W.L., Wang, J.D., Zhang, L.H., 2005. Testing study on characteristics of strength and deformation for coarse materials. Chin. J. Rock Mech. Eng. 24 (3), 430–438.
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Development of a novel base liner material for offshore final disposal sites and the assessment of its hydraulic conductivity Xin Xu a,b,⇑, Xiaofeng Liu c, Myounghak Oh d, Junboum Park a,* a
Department of Civil and Environmental Engineering, Seoul National University, Seoul 08826, South Korea Institute of Engineering Research, Seoul National University, Seoul 08826, South Korea c College of Civil Engineering, Taiyuan University of Technology, Taiyuan 030024, China d Maritime ICT R&D Center, Korea Institute of Ocean Science & Technology, Busan 49111, South Korea b
a r t i c l e
i n f o
Article history: Received 24 November 2018 Revised 22 October 2019 Accepted 24 October 2019
Keywords: Offshore waste disposal Bentonite Saline water Novel liner material Hydraulic conductivity Compressive strength
a b s t r a c t In the process of landfilling, leachates resulting from the waste landfill are likely to cause secondary environmental pollution, and installation of a basal liner is essential under a landfill site to block and reduce permeation of leachate flowing into the subsurface environment. The research aims to develop a saltresistant bentonite and a novel base liner material for offshore waste disposal. The liner materials consist of core materials and coating materials in which mixtures of bentonite, sepiolite, and guar gum were used to overcome the shortcomings in bentonite to realise a high water-resistance and a permeability coefficient of below 1.0 10 7 cm/s under saline water conditions. The optimal mixing ratio of bentonite, sepiolite, and guar gum was confirmed as 76:19:5 by conducting drying shrinkage cracking tests, free swelling tests, and hydraulic conductivity tests. The hydraulic conductivities of spherical particles, as measured in a rigid-wall permeameter and a flexible-wall permeameter, were less than 1.0 10 7 cm/ s under saline water conditions. The compressive properties of spherical particles were evaluated through triaxial compression testing. The engineering characteristics of the liner material were studied in the present research, but the long-term biodegradation characteristics of polymer additives were also important, yet remained unclear. The long-term biostability of the additives, and its effect on basal liner performance should be evaluated in future research. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction With growing economic development, increasing urbanization, and the constant expansion of the world’s population, all types of municipal solid waste are also increasing both by mass and volume (Hoornweg and Bhada-Tata, 2012). South Korea is a nation with small land area, a high population density, and many mountainous regions, therefore on-land waste disposal space is scarce. Meanwhile, capacity in existing on-land waste disposal facilities is reaching saturation and it is difficult to build new facilities: because South Korea is surrounded by sea on three sides, it has an abundance of sea space and offshore waste disposal will become a possible effective waste disposal method in the future (Renou et al., 2008). Some countries have already built offshore waste disposal facilities, but not yet South Korea. In recent years, the government has input more time and funding into offshore waste
⇑ Corresponding authors at: Department of Civil and Environmental Engineering, Seoul National University, Seoul 08826, South Korea (X. Xu). E-mail addresses: [email protected] (X. Xu), [email protected] (J. Park). https://doi.org/10.1016/j.wasman.2019.10.047 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
disposal research, and the planned offshore waste disposal facility will be used to dispose of the municipal solid waste incineration ash generated from the capital city area in South Korea. This research into a base liner material for an offshore waste disposal site was a part of a project entitled ‘‘Development of Technology for Offshore Waste Final Disposal”. For offshore waste disposal, the effects on oceanic environments and leachate leakage remain a high priority. In modern landfills, solid waste disposal facilities must contain a liner system which controls intrusion of groundwater into landfill facilities and protects the soil and ground water from pollution originating in the landfill. The hydraulic conductivity of a liner material used in landfill should be less than 1.0 10 7 cm/s (Mor et al., 2006; Alsabahi et al., 2009), but in offshore waste disposal facilities, it is difficult to lay a complete liner system underwater. The construction standard in major offshore waste disposal countries (Japan and Singapore) is as follows: if there is a vertical impermeable layer under the seafloor, and the thickness of layer is greater than 5 m and its hydraulic conductivity is less than 1.0 10 5 cm/s, a liner system is not required during offshore
191
X. Xu et al. / Waste Management 102 (2020) 190–197
waste disposal facility construction (MEJ, 2007; Chai and Miura, 2002), however, the seafloor is not smooth everywhere, it often exhibits non-uniform thickness and soft soil areas in impermeable layers, that can affect construction operations. To tackle this problem, a novel bottom-liner system construction method (a spherical particle liner system) for offshore waste disposal was proposed. The novel bottom liner materials consist of core and coating materials. When the particles make contact with saline water (or fresh water), coating materials swell to fill the voids to constitute a liner system with low permeability (Fig. S2). The high-strength core materials can provide enough bracing force for offshore waste final disposal and cement beads were used as core materials; because cement mortars exhibit good mechanical properties and great durability, they can be hydrated in aqueous solution, and have excellent strength and high stability. The spherical particles will be deposited into the expected bottom layer where engineers need to enhance the impermeability and strength of the impermeable layer during waste disposal facility construction. Bentonite, as a waterproof material, has been widely used in waste disposal works and it belongs to the smectite group of clay minerals and is an absorbent aluminium phyllosilicate, generally found as impure clay, consisting mostly of montmorillonite (Kong, 2010; Millard et al., 2016; Liu and Zhou, 2010; Khan, 2017). Bentonite can be used as a liner material in waste disposal sites owing to its good characteristics, such as low hydraulic conductivity, high swelling capacity, and adsorption properties (Arabzai and Honma, 2014; Panjaitan and Andi, 2017). Some researchers have undertaken in-depth research into the properties of bentonite as a liner material in on-land waste disposal facilities (Tay et al., 2001; Tang et al., 2011). There remain several problems when using bentonite as a liner system material in coastal areas: low swelling capacity, high permeability under saline conditions, and, because of bentonite’s powder state and high liquidity, it is difficult to use bentonite to construct bottom liners in coastal areas and construction quantities are usually large (Komine et al., 2007). In addition, the spherical particles were prepared by using bentonite slurry as a coating material to cover around the cement beads, and dried in air to obtain the base liner materials. To avoid coating material separation from the core material, and for convenience of storage and transportation, drying shrinkage cracking of the coating material must be prevented. To deal with these problems, bentonite was used as the main material and mixed with some additives to enhance workability under saline conditions. Sepiolite is shot with randomly distributed fibres that can be used in adhesives to fill joints and cracks in engineering works. Its structure is stable even in saline environments with high salt concentrations, thus can be added to bentonite to prevent drying cracks (Alvarez, 1984.). Many researchers have found that using a polymer, to modify the bentonite can increase its swelling capacity and impermeability under saline conditions (Camillis et al., 2016; Wang and Wang, 2009). Modified coating materials can reduce drying shrinkage cracking and have low hydraulic conductivity under saline conditions. The objective of this research is to develop a novel bottom liner system and investigate the engineering properties of spherical particles. Sepiolite was added to bentonite to prevent drying shrinkage cracks and guar gum was added to bentonite to maintain the swelling capacity of bentonite under saline conditions. The spherical particles can enhance the workability of bentonite as a bottom liner material. The optimum mixing ratio of the coating materials was determined through drying shrinkage crack testing, free swelling testing, and hydraulic conductivity testing. The hydraulic conductivity was determined by use of the falling head method and compressive properties were evaluated through triaxial compression testing.
2. Materials 2.1. Bentonite and sepiolite Natural sodium bentonite (Na-bentonite) mined in India, and sepiolite produced in South Korea, were used in the current research. Sepiolite was added to the bentonite to prevent drying shrinkage cracking. Both bentonite and sepiolite were powders and were sieved through a No. 200 sieve. The physical and chemical properties of bentonite and sepiolite are summarized in Tables 1 and 2, Fig. S3 shows the microstructures of bentonite and sepiolite as captured by scanning electron microscope (SEM). As the bentonite has a plate structure, a large amount of water can penetrate the void spaces therein, which cause a change from a platelike structure to a house-of-cards structure. This exacerbates the volumetric expansion of the bentonite. As may be seen from the microstructure of the sepiolite, multiple fibres are present, which results in the interconnection between it and the bentonite after being mixed with sepiolite. Therefore, fewer fissures were generated in the bentonite during drying shrinkage (Ekici et al., 2006; Guney et al., 2014).
2.2. Salt-resistant materials Four types of salt-resistant materials were selected for this research: diatomite, guar gum, sodium pyrophosphate, and xanthan gum. Due to the high specific surface and multi-pore structure, diatomite can adsorb cations in saline solution and therefore prevent ion exchange between the bentonite and a saline solution and reduce the salt ionic effect on bentonite swelling capacity (Nenadovic´ et al., 2009; Tian et al., 2016). The other three materials are high-molecular weight polymers, which could surround, and combine with the bentonite, then reduce the salt ionic effect on the swelling capacity in saline solution. Polymers have high cohesion and are often used as additives to enhance the interaction between themselves and other materials (Sandolo et al., 2007). Polymers can be mixed with bentonite to enhance some properties of the mixed material such as decreasing the number of cracks and their hygroscopic nature (Levy et al., 1995).
2.3. Core materials The core materials were prepared by mixing cement and sand in a mass ratio of 3:2 to form spherical particles of different diameters. Salt-resistant cement with strength grade of 42.5 (produced in South Korea) was used, and the sand was a Jumunjin standard sand (mined in South Korea). There are two sizes of core materials used in this research: 10 mm in diameter and 20 mm in diameter. The amount of leaching of hazardous material in core materials was below corresponding standard limits set by the Ministry of Environment of Korea (MEK, 2016).
Table 1 Physical properties of bentonite and sepiolite as used in this study.
Specific gravity Liquid limit (%) Plastic index (%) Unified Soil Classification System Percentage passing sieve No. 200 Specific surface area (m2/g) Cation exchange capacity (meq/100 g)
Bentonite
Sepiolite
2.7 397 360 CH 100% 40–50 100–130
2.5 133 47 MH 100% 80–100 25–50
192
X. Xu et al. / Waste Management 102 (2020) 190–197
Table 2 Chemical composition of bentonite and sepiolite as determined by X-ray fluorescence spectrometer (XRF).
Bentonite Sepiolite
SiO2
MgO
Al2O3
Na2O
K2O
Fe2O3
TiO2
MnO
CaO
P2O5
L.O.I
Total
61.30 28.60
1.64 17.71
14.62 1.41
2.54 0.11
0.28 0.25
3.80 0.47
0.52 0.04
0.07 0.02
4.13 23.98
0.01 0.03
11.09 27.92
100 100
2.4. Permeant solutions To observe the properties of bentonite under fresh and saline conditions, distilled water and artificial sea water were used in this research. The saline water was prepared according to ASTM D 1141-98 (2008) which represents standard practice for the preparation of substitute ocean water. Saline water was prepared through dissolving NaCl (2.45%), MgCl2 (0.52%), Na2SO4 (0.41%), and CaCl2 (0.12%) in distilled water to simulate sea water. The properties of the artificial sea water used in this study were as follows: a pH of 8.2, a salinity of 3.5%, an electrical conductivity of 4.3 Xm and an ionic strength of 0.66 mol/L. 3. Methods Bentonite, sepiolite, and salt-resistant materials were mixed to develop a coating material which has high swelling capacity and low permeability under saline water. Drying shrinkage crack tests and free swell index tests were carried out to determine the optimum mixing ratio of sepiolite in mixed clay and confirm most effective salt-resistant materials for modified bentonite by hydraulic conductivity testing. Hydraulic conductivity test and triaxial compression test were used to evaluate the properties of base liner materials under saline conditions. 3.1. Drying shrinkage crack test Adding some fibrous structural materials to bentonite can reduce desiccation cracking. Sepiolite is often used to this end. To determine the optimum mixing ratio of bentonite and sepiolite in mixed clay (bentonite-sepiolite mixture), drying shrinkage crack tests were conducted (Table S3). The mixed clay with sepiolite contents of 0%, 5%, 10%, 15%, 20%, 25%, 30%, 40%, 50%, and 60% was prepared, and then mixed with 1.5 times the mass of water, afterwards, the mixed clay slurry was shaped in a mould measuring 30 mm in diameter and 3 mm in height. Test specimens were dried at room temperature for 48 h and drying shrinkage cracking was then assessed. 3.2. Free swell index test The free swell index test was conducted to determine the swelling capacity of a mixed clay and to observe the effects of saltresistant materials on the chemical resistance of bentonite against saline conditions. The optimum salt-resistant materials could be selected for bentonite modification using free swell index tests. Guar gum, diatomite, sodium pyrophosphate (TSPP), and xanthan gum were selected as alternative materials for bentonite modification. The amounts of salt-resistant materials in the modified bentonite were 0%, 1%, 1.5%, 2.5%, 5%, and 7.5% (Table S4). According to the aforementioned mix proportions, the diatomite powder was directly added to the mixed clay and mixed thoroughly to obtain a modified bentonite. The three polymers were separately dissolved in distilled water to prepare aqueous solutions thereof, then mixed with the bentonite base in the aforementioned proportions and uniformly stirred at 60 °C for 2 h using an electric blender. The mixtures were kept at room temperature until completion of cross-linking polymerization wherein, the bentonite
and polymers can polymerize together through chemical bounding and chain-formation to form a stable polymeric substance. They were then oven-dried at 80 °C, and the dried mixtures were ground and sieved to obtain the modified bentonite. Based on ASTM D 5890 (2006), gradated cylinders were prepared with 90 mL fresh water or saline water, 2 g of each specimen was poured into separate cylinders in 0.1 g increments. After 2 g of specimen had been dispensed, the cylinders were filled to a total volume of 100 mL with fresh, or saline water. After 16 to 24 h, the equilibrium volumes of the swelling specimens were recorded. 3.3. Spherical particle preparation The workability of bentonite was improved using modified bentonite (i.e., a bentonite-sepiolite-guar gum mixture) as a slurry cover around cement beads (the core material). The optimum mixing ratio of modified bentonite has been determined by drying shrinkage crack testing, free swelling index testing, and hydraulic conductivity testing. The coating material was prepared by mixing modified bentonite with water in a solid-liquid mixing ratio of 2:3. Then the coating material was used to cover cement beads to make spherical particles and the spherical particles were dried in air for 48 h. To explore the relationship between the hydraulic conductivity and coating material thickness, hydraulic conductivity and particle size, six different sizes of spherical particles were prepared. The spherical particles measured 10 mm in diameter with coating material thicknesses making up 1 mm, 2 mm, 3 mm, and 20 mm diameters (i.e., coating material thicknesses of 2 mm, 3 mm, and 4 mm, respectively). 3.4. Hydraulic conductivity test Hydraulic conductivity tests were conducted on coating materials and spherical particles. The hydraulic conductivity test was carried out using both a rigid-wall permeameter and a flexible-wall permeameter under saline water or fresh water conditions, and the falling head method was used in all cases. For the rigid-wall permeameter, the hydraulic conductivity was measured according to ASTM D 5856 (2007) and an influent pressure was applied using nitrogen gas at 34.47 kPa and the hydraulic conductivity of coating materials and spherical particles could be determined. For the flexible-wall permeameter, the hydraulic conductivity was measured according to ASTM D 5084 (2004), and an influent pressure of 34.47 kPa and confining pressure of 55.15 kPa were applied by way of compressed nitrogen gas. The void fraction of specimens was determined before and after testing. Both short-term and long-term tests were performed at a constant temperature of 24 °C. 3.5. Triaxial compression test (CU) Consolidated-undrained triaxial compression testing was undertaken to assess the compressive strength of the proposed base liner materials. Tests were conducted in accordance with ASTM D4767 (2011), and three confining pressures (100, 300, and 500 kPa) were used. Test specimens were 100 mm in diameter and 210 mm in height, and specimens were saturated by using a vacuum pumping system. The tests were conducted at a strain rate of 0.5% per minute until the axial strain reached 20%. Two sizes of
193
X. Xu et al. / Waste Management 102 (2020) 190–197
spherical particles were used in the test: 10 and 20 mm in diameter (core size). To simulate offshore conditions, artificial seawater was used as the permeant solution in CU tests. 4. Results and discussion 4.1. Mixing ratio of coating material (modified bentonite) 4.1.1. Drying shrinkage crack testing Through drying shrinkage crack tests, the optimum ratio of bentonite to sepiolite in a mixed clay was determined. After the water had evaporated, the diameter of test specimens decreased from 30 mm to 20 mm after air-drying (Table 3). The amount of shrinkage cracks decreased with increasing sepiolite content and cracks were not found when the sepiolite content exceeded 15% in the mixed clay; however, upon increasing the sepiolite content, the plasticity and shrinkage capacity of the mixed clay decreased and at a sepiolite content of more than 30%, the shape of the samples was not preserved after drying. The best ratio of bentonite and sepiolite was found to be 4:1 with regard to it safely preventing shrinkage cracks after drying. 4.1.2. Free swell index testing Table 4 shows the free swell index of bentonite, sepiolite and mixed clay under fresh and saline conditions. In saline conditions, the free swell index of bentonite decreased to about one third of that in fresh water conditions. The free swell index of mixed clay (at a bentonite: sepiolite ratio of 4:1) was 18.5 mL per 2 g and 6 mL per 2 g under fresh, and saline, conditions, respectively. To enhance the swelling capacity of bentonite under saline condition, some salt-resistant materials were added to the mixed clay to impart a saline effect on the bentonite. Four types of alternative salt-resistant materials were added into the mixed clay respectively before measuring the free swell index, and the alternative salt-resistant materials were selected on the basis of published evidence of previous use. Fig. 1 illustrates the free swell index test results for mixed clays with added salt-resistant materials under saline conditions: the swell index increased with increasing added doses of salt-resistant materials. All salt-resistant additives (except diatomite) have high salinity-tolerances. Although the swelling index does not have a proportional correlation to hydraulic conductivity, a high swelling capacity was considered indicative of the quality of a mixed clay. Owing to its salinity tolerance, viscosity, and low cost, guar gum was confirmed as optimal salt-resistant material for bentonite modification. When the mixing ratio of mixed clay and guar gum was 37:3, the free swell index under saline conditions was close to that in fresh water. 4.1.3. Hydraulic conductivity testing of coating materials A hydraulic conductivity test was performed to study the permeability of the coating material and validate the optimum mixing Table 3 Crack parameters for the dried mixed clay. Sepiolite content (wt.%)
0, 5, 10
15
20, 25, 30, 40, 50, 60
Crack length
1 cm
1 cm
No cracking
ratio thereof. The guar gum contents in the modified bentonite were 0%, 1%, 1.5%, 2.5%, 5%, and 7.5% (Table S5). For test specimens, a coating material slurry was formed into small globules. The specimens were dried at room temperature for 48 h and the diameter of the specimens decreased to 7–9 mm after drying. Then the specimens were placed into a rigid-wall permeameter without compaction. Fig. 2 shows hydraulic conductivity of mixed clay (bentonite-sepiolite mixture) under fresh, and saline, conditions. The hydraulic conductivity of the mixed clay was below 1.0 10 9 cm/s under fresh water conditions, but under saline conditions, it exceeded 1.75 10 5 cm/s (a 20,000-fold increase) under fresh water condition. Similar results were obtained in other research (Anandganarayanan and Murugaiyan, 2014; Shariatmadari et al., 2011). The hydraulic conductivity exceeded 1.0 10 7 cm/s, thus did not satisfy EPA standard (2001) requirements for use as a bottom liner material for waste disposal facilities. Fig. 3 displays hydraulic conductivity test results of coating materials (bentonite-sepiolite mixtures with various added guar gum contents) under saline conditions. The hydraulic conductivity of the coating materials decreased with increasing guar gum content. When the guar gum content was greater than 2.5%, the hydraulic conductivity was below 1.0 10 7 cm/s under saline conditions. To ensure that the hydraulic conductivity of coating materials remained below 1.0 10 7 cm/s under saline conditions, the guar gum content must be above 2.5%. These results confirmed 5% as the guar gum content when added to this modified bentonite. To overcome the weaknesses of bentonite under saline conditions, the weight ratio of bentonite, sepiolite and guar gum in modified bentonite was 76:19:5. The solid–liquid ratio in the coating material slurry was 2:3 (weight ratio of modified bentonite to water). 4.2. Hydraulic conductivity of spherical particles 4.2.1. Hydraulic conductivity measured in a rigid-wall permeameter The coating material was prepared by mixing modified bentonite with 1.5 times the mass of distilled water, thus forming a coating material slurry which was used to cover the cement beads to form spherical particles. The spherical particles were air-dried for 48 h. The spherical particles measured 10 mm in diameter (core size) with coating material thicknesses of 1 mm, 2 mm, and 3 mm, and 20 mm in diameter (core size) with coating material thicknesses of 2 mm, 3 mm, and 4 mm. Fig. 4 demonstrates hydraulic conductivity test results of spherical particles of 10 mm (or 20 mm) diameter core size with various coating thicknesses under saline conditions. The hydraulic conductivity of spherical particles decreased with increasing coating thickness. When the coating thickness exceeded 2 mm (or 3 mm), the hydraulic conductivity was less than 1.0 10 7 cm/s under saline conditions. To ensure that the hydraulic conductivity was below 1.0 10 7 cm/s under saline conditions, the coating thickness must be greater than 2 mm (or 3 mm). The range of initial void fractions was 0.34– 0.37, and the final void fractions of specimens decreased to 0.0031–0.044 after rigid-wall hydraulic conductivity testing. Based on the short-term hydraulic conductivity test results, spherical particles which have a hydraulic conductivity of less than 1.0 10 7 cm/s were selected as specimens for long-term testing
Table 4 Free swell index of bentonite, sepiolite and mixed clay under fresh and saline water conditions. Free swell index of bentonite (mL/2 g)
Free swell index of sepiolite (mL/2 g)
Free swell index of mixed clay (mL/2 g)
Fresh
Saline
Fresh
Saline
Fresh
Saline
22
7
5.5
5.5
18.5
6
194
X. Xu et al. / Waste Management 102 (2020) 190–197
Fig. 1. Free swell index test results of mixed clay modified by salt-resistant additive (a: Diatomite, b: Xanthan gum, c: Guar gum, d: TSPP).
Fig. 2. Hydraulic conductivity of mixed clay under fresh and saline water conditions (without guar gum).
Fig. 3. Hydraulic conductivity of coating materials modified by guar gum under saline water conditions.
(6 months). Test specimens included 10 mm diameter cores with 2 mm and 3 mm coating thicknesses, and 20 mm diameter cores with 3 mm and 4 mm coating thicknesses. The long-term hydraulic conductivity tests were continued for 6 months and were described by using pore volumes of flow (PVF). Fig. 5 illustrates the hydraulic conductivities of such spherical particles under saline conditions. All test results (each with four replicates) show the relationship between hydraulic conductivity and PVF. The physical equilibrium was evaluated by the ratio of volume of outflow to volume of inflow. The hydraulic conductivities of each sample decreased to equilibrium after about 0.8 to 1 PVF under saline
conditions. In Fig. 5(a), the hydraulic conductivities of two types of spherical particles were between 2.1 10 8 cm/s and 8 9 2.5 10 cm/s and from 6.4 10 cm/s to 7.2 10 9 cm/s. In Fig. 5(b), the hydraulic conductivities of two types of spherical particles were between 3.0 10 8 cm/s and 3.4 10 8 cm/s and 7.7 10 9 cm/s to 8.4 10 9 cm/s. The hydraulic conductivities underwent minor fluctuations as the PVF increased and the hydraulic conductivities were similar under short-term and longterm test conditions. The test results indicate that spherical particles were stable and had a low permeability under saline conditions, however, the tests were not designed to investigate the
X. Xu et al. / Waste Management 102 (2020) 190–197
195
Fig. 4. Hydraulic conductivity of spherical particles of various coating thicknesses under saline water conditions.
Fig. 5. Hydraulic conductivity of spherical particles under saline conditions.
potential effects of biodegradation of the polymers over a much longer time-frame and the related biodegradation characteristics of polymers warrant investigation in future research. The test results show that the hydraulic conductivities of these spherical particles decreased with increasing coating thickness for both core sizes (10 mm and 20 mm). At a given coating thickness, spherical particles with smaller cores had a lower hydraulic conductivity than particles with a larger core. Fig. 6 shows the relationship between hydraulic conductivity and coating thickness ratio for spherical particles under saline conditions. The hydraulic conductivity decreased with increasing ratio of coating thickness to core diameter. To acquire a hydraulic conductivity of less than 1.0 10 7 cm/s under saline conditions, the ratio of coating thickness to core diameter must be greater than 0.15. To obtain more accurate experimental data, small spherical particles were used in these laboratory hydraulic conductivity tests: when using spherical particles as a bottom liner system in an offshore waste disposal facility, the appropriate size of spherical particle remains unknown. Based on the ratio of coating thickness to core diameter, spherical particles can be produced in various sizes for on-site application.
4.2.2. Hydraulic conductivity measured in a flexible-wall permeameter To determine the hydraulic conductivity of spherical particles under field seawater pressure, a flexible-wall permeameter was used to simulate field conditions. Spherical particles with a
Fig. 6. Relationship between coating thickness ratio and hydraulic conductivity under saline water conditions (spherical particles).
hydraulic conductivity (2.1–9.3 10 8 cm/s) of less than 1.0 10 7 cm/s were selected as test specimens for flexible-wall permeameter testing, we compared the hydraulic conductivity of spherical particles as measured with a rigid-wall permeameter in Fig. 7. The hydraulic conductivity was three to four times higher when measured in the flexible-wall permeameter at a given salinity, this finding is in agreement with other research results (Ahn
196
X. Xu et al. / Waste Management 102 (2020) 190–197
Fig. 7. Hydraulic conductivity of spherical particles under saline water conditions: rigid, and flexible-wall permeameters.
and Jo, 2009). The range of specimen initial void fractions was 0.34–0.37, and the final void fractions decreased to 0.0047–0.052 after flexible-wall hydraulic conductivity testing; however, the hydraulic conductivity also satisfied the EPA standard requirements in that it was less than 1.0 10 7 cm/s under saline conditions. 4.3. Triaxial compression test (CU) Fig. 8 shows compressive stress-strain curves for the proposed base liner material under saline water conditions. As shown in the figures, the deviator stress increased with the confining pressure at the same axial strain. At the same axial strain and confining pressure, the specimen containing the bigger spherical particles had a higher deviator stress at failure. This is owing to the test specimens with spherical particles of 20 mm diameter (core size) containing more core material. This trend is similar to that reported elsewhere (Xia, et al., 2017; Wang, et al., 2016; Xie, et al., 2005). From the Mohr-Coulomb envelopes, the cohesion (c) and friction angle (u) were estimated. The cohesion and friction angle of specimens were as follows: spherical particles of 10 mm diameter (core size): 131.6 kPa and 19.5°; spherical particles of 20 mm diameter (core size): 98.7 kPa and 18.9°. The effect of spherical particle size on friction angle was negligible yet that on
cohesion was significant. These data can be used as a basis for practical construction work using such materials. 5. Conclusion A laboratory study was conducted to develop a novel base liner material for offshore waste disposal facilities. The hydraulic conductivity and compressive strength of engineered spherical particles were evaluated. A salt-resistant bentonite was developed as a coating material for the spherical particles by adding sepiolite and guar gum to a bentonite base, and the optimum mixing ratio of bentonite, sepiolite, and guar gum was confirmed as 76:19:5. The salt-resistant bentonite exhibits a high swelling capacity, low hydraulic conductivity, and a low incidence of drying shrinkage cracking. At a given salinity, the hydraulic conductivity of spherical particles as measured in a rigid-wall permeameter was lower than that measured in a flexible-wall permeameter. The hydraulic conductivity decreased as the ratio of coating thickness to core diameter increased. To obtain a hydraulic conductivity of less than 1.0 10 7 cm/s under saline conditions, the ratio of coating thickness to core diameter was greater than 0.15. The triaxial compression test results show that the effect of spherical particle size on friction angle was negligible yet that on the apparent cohesion was significant.
Fig. 8. Variation of deviator stress with axial strain under different confining pressures.
X. Xu et al. / Waste Management 102 (2020) 190–197
In further research, three-dimensional water tank testing, and large column testing, will be used to investigate the in situ performance of spherical particles. Both the permeability and compressive strength of novel bottom liner systems will be evaluated under simulated field conditions. In addition, the long-term biodegradation characteristics of polymer additives and the resultant effect on basal liner performance will also be investigated, and this basal liner material should not be used in field until these issues have been addressed. Funding This research was supported by a part of the project titled ‘‘Development on Technology for Offshore Waste Final Disposal”, funded by the Ministry of Oceans and Fisheries, South Korea, and the Korea Ministry of the Environment as ‘‘The SEM (Subsurface Environment Management) projects; 2019”. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.10.047. References Anandganarayanan, G., Murugaiyan, V., 2014. Effects of salt solution and sea water on the geotechnical properties of soil-a review. Int. J. Eng. Res. Technol. 3 (3), 1819–1824. Ahn, H.S., Jo, H.Y., 2009. Influence of exchangeable cations on hydraulic conductivity of compacted bentonite. Appl. Clay Sci. 44 (1), 144–150. Alsabahi, E., Rahim, S.A., Zuhairi, W.Y.W., Alnozaily, F., Alshaebi, F., 2009. The characteristics of leachate and groundwater pollution at municipal solid waste landfill of Ibb city, Yemen. Am. J. Environ. Sci. 5 (3), 256–266. Arabzai, A., Honma, S., 2014. Adsorption and transport of heavy metal ions in saturated soils. Proc. School Eng. Tokai Univ. 39, 27–32. ASTM D1141-98, 2008. Standard Practice for the Preparation of Substitute Ocean Water. American Society for Testing and Materials International. ASTM D5890-11, 2006. Standard Test Method for Swell Index of Clay Mineral Component of Geosynthetic Clay Liners. American Society for Testing and Materials International. ASTM D5856-15, 2007. Standard Test Method for Measurement of Hydraulic Conductivity of Porous Material using a Rigid-wall, Compaction-mold Permeameter. American Society for Testing and Materials International. ASTM D5084-03, 2004. Standard Test Method for Measurement of Hydraulic Conductivity of Saturated Porous Materials using a Flexible Wall Permeameter. American Society for Testing and Materials International. ASTM D4767, 2011. Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils. American Society for Testing and Materials International. Alvarez, A., 1984. Sepiolite: properties and uses. Dev. Sedimentol. 37, 253–287. Camillis, M.D., Emidio, G.D., Bezuijen, A., Verástegui-Flores, R.D., 2016. Hydraulic conductivity and swelling ability of a polymer modified bentonite subjected to wet-dry cycles in seawater. Geotext. Geomembr. 44 (5), 739–747. Chai, J.C., Miura, N., 2002. Comparing the performance of landfill liner systems. J. Mater. Cycl. Waste Manage. 4 (2), 135–142.
197
Ekici, S., Isßıkver, Y., Saraydın, D., 2006. Poly(acrylamide-sepiolite) composite hydrogels: preparation, swelling and dye adsorption properties. Polym. Bull. 57 (2), 231–241. EPA530-F-97-002, 2001. Geosynthetic Clay Liners Used in Municipal Solid Waste Landfills, Solid Waste and Emergency Response. United States Environmental Protection Agency. Guney, Y., Cetin, B., Aydilek, A.H., Tanyu, B.F., Koparal, S., 2014. Utilization of sepiolite materials as a bottom liner material in solid waste landfills. Waste Manage. 34 (1), 112–124. Hoornweg, D., Bhada-Tata, P., 2012. What a Waste: A Global Review of Solid Waste Management. World Bank, Washington DC. Komine, H.K., Yasuhara, K.Y., Murakami, S.M., 2007. Swelling characteristics of bentonites in artificial seawater. Can. Geotech. J. 46 (46), 177–189. Kong, W., 2010. Study on swelling performance of bentonite in aqueous solution. Non-Metall. Mines 33 (1), 42–44. Khan, M.R., 2017. Adsorption of lead by bentonite clay. Int. J. Sci. Res. Manage. Stud. 5 (7), 5800–5804. Liu, Z.R., Zhou, S.Q., 2010. Adsorption of copper and nickel on Na-bentonite. Process Saf. Environ. Prot. 88 (1), 62–66. Levy, N., Garti, N., Magdassi, S., 1995. Flocculation of bentonite suspensions with cationic guar. Colloids Surf., A 97 (2), 91–99. MEJ, 2007. The Specification for Design and Construction Offshore Waste Disposal Facilities. Ministry of Environment, Japan. MEK 2016-4, 2016.. The Standard of Water Quality of Groundwater and River. Ministry of Environment, Seoul, S. Korea. Millard, A., Mokni, N., Barnichon, J.D., Thatcher, K.E., Bond, A.E., Fraser-Harris, A., 2016. Comparative modelling of laboratory experiments for the hydromechanical behaviour of a compacted bentonite-sand mixture. Environ. Earth Sci. 75 (19), 1311. Mor, S., Ravindra, K., Dahiya, R.P., Chandra, A., 2006. Leachate characterization and assessment of groundwater pollution near municipal solid waste landfill site. Environ. Monit. Assess. 118 (1–3), 435–456. Nenadovic´, S., Nenadovic´, M., Kovacˇevic´, R., Matovic´, Lj, 2009. Influence of diatomite microstructure on its adsorption capacity for Pb(Ⅱ). Sci. Sinter. 41 (3), 309–317. Panjaitan, N., Andi, A., 2017. Electrokinetic phenomena of cation exchange and its effect on the behaviour of expansive clays. Int. J. Geomate. 13 (38), 173–177. Renou, S., Givaudan, J.G., Poulain, S., Dirassouyan, F., Moulin, P., 2008. Landfill leachate treatment: review and opportunity. J. Hazard. Mater. 150 (3), 468–493. Shariatmadari, N., Salami, M., Karimpour, F.M., 2011. Effect of inorganic salt solutions on some geotechnical properties of soil-bentonite mixtures as barriers. 9(2), 103–110. Sandolo, C., Matricardi, P., Alhaique, F., Coviello, T., 2007. Dynamo-mechanical and rheological characterization of guar gum hydrogels. Eur. Polym. J. 43 (8), 3355– 3367. Tay, Y.Y., Stewart, D.I., Cousens, T.W., 2001. Shrinkage and desiccation cracking in bentonite-sand landfill liners. Eng. Geol. 60 (1), 263–274. Tian, L., Zhang, J., Shi, H., Li, N., Ping, Q., 2016. Adsorption of malachite green by diatomite: equilibrium isotherms and kinetic studies. J. Dispers. Sci. Technol. 37 (7), 1059–1066. Tang, C.S., Shi, B., Liu, C., Suo, W.B., Gao, L., 2011. Experimental characterization of shrinkage and desiccation cracking in thin clay layer. Appl. Clay Sci. 52 (1–2), 69–77. Wang, J.Y., Cao, W.G., Jiang, Z.M., Zhao, Z.P., 2016. Large-scale triaxial tests on deformation and mechanical behavior of soil-rock aggregate mixture under different stress paths. Rock Soil Mech. 37 (2), 424–431. Wang, W., Wang, A., 2009. Synthesis, swelling behaviors, and slow-release characteristics of a guar gum-g-poly (sodium acrylate)/sodium humate superabsorbent. J. Appl. Polym. Sci. 112 (4), 2102–2111. Xia, J., Hu, R., Qi, S., Gao, W., Sui, H., 2017. Large-scale triaxial shear testing of soil rock mixtures containing oversized particles. Chin. J. Rock Mech. Eng. 36 (8), 2031–2039. Xie, W.L., Wang, J.D., Zhang, L.H., 2005. Testing study on characteristics of strength and deformation for coarse materials. Chin. J. Rock Mech. Eng. 24 (3), 430–438.