influence of the addition of bentonite in alternative
INFLUENCE OF THE ADDITION OF BENTONITE IN ALTERNATIVE MIXTURES FOR CCL PURPOSE OF LANDFILL L.A. MARTINS*, K.M.W REBELO**, Y.G. CUNHA° AND P.J.M. PIRES°° * Department of Civil Engineering, Federal University of Espírito Santo, Brazil ** Department of Civil Engineering, Federal University of Espírito Santo, Brazil ° Department of Civil Engineering, Federal University of Espírito Santo, Brazil °° Department of Civil Engineering, Federal University of Espírito Santo, Brazil
SUMMARY: Impermeable barriers are a need in several engineering works, especially in compacted clay liners (CCL) for waste containment systems. In that sense, the soil should have appropriate characteristics, such as low permeability (under 10-7 cm/s). This requirement can be restrictive considering the fact that very clayey soils are note available at all sites. For these cases, the bentonite addition is a potential alternative to improve the CCL performance. Soilbentonite used to backfill consisting of on-site soil and bentonite, are extensively used as engineering barriers for achieving low hydraulic conductivity. To avoid major environmental impacts and to minimize the risk of contamination for soil and groundwater under waste containment systems, zeolite, minerals as adsorptive material used in permeable reactive barriers for remediation of groundwater contaminated by heavy metals, can be used as alternative material in constructions of landfill bottom liners. In that sense, various ratios of bentonite and on-site soil (S/B) and bentonite and zeolite (Z/B) compacted at optimum water content (wopt) were test to determine the hydraulic conductivity (k) to found an ideal landfill liner material and study the behavior of the soils with the addition of bentonite. For the characterization of pure soil and mixtures grain size tests were performed (NBR 7181/16), density by pycnometer method (NBR 6458/16), liquid limit (NBR 6459/16) and plastic limit (NBR 7180/16). To determine the hydraulic the laboratory test involves flexible wall permeameter (ASTM D-5084-10). The results indicate that, for all mixtures soil-bentonite and for the mixture zeolite-bentonite with 7% of bentonite, the hydraulic conductivity showed adequate values for the construction of impermeable barriers.
1. INTRODUCTION The growing concern of society with the degradation of the environment and what it will entail for future generations has led to the use of management techniques that aim to reduce, recycle and reuse waste produced by different human activities, as well as technical solutions that make safe and adequate disposal of these wastes. In this context, the most widely adopted alternative for disposal and treatment of solid waste in Brazil and in most countries is the sanitary and industrial landfill, which still corresponds to the most viable technical and economic solution. One of the major concerns in the construction of a sanitary landfill is the contamination, by the leachate, of the soil and watercourses in the vicinity of the landfill. For that reason, impermeableing bottom barriers has been used in the Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
foundation of the landfill. The impermeableing layer, to maintain good performance during its useful life, must have a hydraulic conductivity lower than 10-7 cm / s, meeting the standards of several countries. In places where the natural soil is inadequate for the construction of impermeable barriers, as it happens in several regions of the earth globe, additives to the soil are used for improvements in the hydraulic and mechanical behavior. According to Das (2007) the soil can be mixed with external clay minerals, such as sodium bentonite, to establish the desired range of hydraulic conductivity. Some already published studies have analyzed the influence of additives in soils of several Brazilian regions, among them Lukiantchuki (2007), Camargo (2012) and Morandini & Leite (2015) and wordwide Villar and Rivas (1994), Sivapullaiah et al. (2000), Abichou et al. (2004) and Akgun (2010). Despite the impermeableing characteristics of the liner of sanitary landfills when they are made of a compacted layer of clay, cracks can appear causing leaks, making it easier for the leachate to reach and contaminate the surrounding soils and groundwater. In view of this situation, the use of zeolite, a mineral used in reactive permeable barriers, can be an alternative as a base material for impermeableing layer. The purpose of zeolite is to reduce the concentration and/or removal of pollutants to acceptable levels under current legislation. The high cation exchange capacity and the high adsorption capacity enable zeolites, among other uses, to recover areas affected by heavy metal contamination (MONTE & RESENDE, 2005). In this sense, it is necessary to carry out research in Laboratory scale in order to analyze the viability of using zeolitic concentrates in impermeableing layers, since the natural zeolites can be highly effective in the remediation of leaks coming from the base layer of the landfill. In this scenario, the hydraulic behavior of two soil-additive mixtures, among them the mixture local soil-bentonite and zeolite-bentonite, will be investigated by means of hydraulic conductivity tests for use as impermeableing barriers. The obtained results will contribute to the expansion of the database related to the study of materials that can be used as impermeableing barriers. It should be noted that this approach is of great importance within a landfill project since it comprises aspects that affect the efficiency and performance of the impermeableing system, compromising the integrity of the system as a whole. 2. MATERIALS 2.1 Tropical soil A tropical soil from the state of Espírito Santo, Brazil, was used to carry out this research. The choice of this soil was due to the great presence of tropical soils throughout the country and the terrestrial globe that, due to their sandy characteristics, are generally not suitable for the "in natura" use in base layer of landfill. The samples were collected, packed in bags and transported to the soil mechanics laboratory of the Federal University of Espírito Santo (UFES). The soil was previously separated by mechanical trap and characterized prior to addition of the percentages of additive. The physical and chemical characterization of the material is presented in item 4.1.1. The local soil was classified as silty sand (SM) according to the Unified Soil Classification System (USCS). 2.2 Zeolite The zeolite used in this study is of the natural type clinoptilolite and it was donated by the company "Celta Brasil". By analyzing previous work, it was requested 25 kg of material with a diameter of 0.0045 mm and 75 kg with a diameter of 0.4 to 1 mm. The samples were taken from
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
the state of São Paulo, Brazil, and they were collected by the company itself, packed in bags and transported to the soil mechanics laboratory of the Federal University of Espírito Santo (UFES). For the study, a previous mixture of all received material with the two granulometries was done. After the initial mixing, the material was separated by mechanical block in four fractions and characterized before the additions of the percentages. The physical and chemical characterization of the zeolite is presented in Item 4.2.1. The material was classified according to the USCS as silty sand. 2.3 Bentonite The bentonite here used is commercialized by the company "Bentonisa - Bentonita do Nordeste SA" and it was classified as sodium bentonite, thus presenting the sodium as interlamellar cation, and thus it presents, as characteristic, the swelling in water, beneficial to the structure to be analyzed. Therefore, when expanding the bentonite, it has self-healing power reducing the probability of the leachate to cross the liner by the possible cracks that can appear with the time of operation. Also due to their expansive characteristic under confined conditions, as in vertical barriers, the expanded bentonite particles are forced against each other, filling the voids between the soil particles, forming a barrier against the passage of the fluid (Heineck, 2002). The bentonite was classified, according to the USCS, as clay of high plasticity (CH) and the physical characterization of the material is presented in item 4.1.1. By the parameter of Liquidity Limit between 300% and 500%, adopted in HEINECK (2002), it can be classified as of medium quality. 3. METHODS 3.1 Sample characterization 3.1.1 Geotechnical analyses The geotechnical characterization of the pure soil and soil-bentonite mixtures, in the previously defined proportions, as well as pure zeolite and zeolite-bentonite mixtures, was performed according to the tests provided for in the Brazilian Association of Technical Standards (ABNT): grain size (NBR 7181/16) and density of grains by the pycnometer method (NBR 6458/16). The local soil was tested by means of thermal bath, while the zeolite was tested by the application of vacuum, Liquidity Limit (NBR 6459/16 ), Plasticity Limit (NBR 7180/16) and compaction test with Proctor Normal energy and with reuse (NBR 7182/16). For the characterization of the additive used in this study, due to its expansive characteristic and its adsorptive power in which it was verified the no complete hydration in a period of 24 hours, some modifications were made in the normative procedures for an accomplishment of the tests of granulometry and specific mass. For the granulometry test, a test sample was reduced from 70 g to 25 g and, for complete hydration, the material was left for 7 days in a solution of 125 ml of sodium hexametaphosphate and distilled water, and regarding the mass test, a tested sample was reduced from 50g to 15g and the material for its complete hydration was allowed to stand for 7 days in distilled water. The other tests performed, Liquidity Limit and Plasticity Limit, followed the guidelines of current standards NBR 6459/16 and NBR 7180/16, respectively.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3.1.2 Chemical analyses The chemical characterization was performed for pure local soil samples, pure zeolite and bentonite by the Brazilian Institute of Analyzes (IBRA), located in Jardim Nova Veneza Sumaré SP. For the analysis, 200g of each material were separated, packed in plastic bags and sent to the company's laboratory. The following tests were performed: pH in CaCl2 and SMP, CTC, saturation of H + Al bases, macronutrients and micronutrients by the Mehlich extractor. The results were received seven days after receiving the samples. 3.2 Hydraulic conductivity testing All hydraulic conductivity tests, for both the pure materials and their respective mixtures, were performed in flexible wall permeameter, and the test method was performed according to ASTM D5084-10 Method A - Constant loading. To perform the test, the samples were molded to the dimensions of approximately 10 cm in diameter and 5 cm in height, packed with Normal Proctor's energy, using the mechanical compactor, and humidity near the optimum moisture obtained by the compaction test. The assembly of the test was carried out in the following sequence: over the equipment pedestal, a previously saturated porous stone, a filter paper, the specimen, another filter paper and another porous stone were all set right in the center, along with the head of the equipment. Finally, to wrap the specimen and to avoid lateral flow, the coating of the assembly was carried out with a flexible latex membrane, fixed to the pedestal and the head by two o'rings. Figure 1 shows the assembly assay sequence in the flexible wall permeameter.
Figura 1. Assembly assay sequence in the flexible wall permeameter. Initially, the filling of the chamber of the equipment was performed. The saturation was performed by percolation of water and the test body was considered totally saturated after the passage of a water volume of three to five times the void volume of the specimen, following Guidelines. These values were adopted because it was not possible to verify the saturation by means of parameter B of Skempton by limiting the apparatus in the measurement of the variation of the neutral pressure. After the saturation process, the loads were kept constant and, thus, consecutive measurements of the volume of percolated water were made during the test. The hydraulic conductivity calculation was performed using Darcy's law, as explained in equation 1.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Where: k is the hydraulic conductivity (cm / s); ΔV is percolated water volume variation (cm³); L is the length of the specimen (cm); A is the cross-sectional area of the specimen (cm²); Δt is the time variation between the measurements (s) and Δh is the load variation applied during the test; After the test ended, the humidity of the specimen was measured and the saturation was calculated using correlations of physical indices. Due to the limitations of the test, saturation values above 95% were considered satisfactory.
4. RESULTS AND DISCUSSIONS For a better interpretation, it was first performed an analysis for each separate base material and, after that, a comparison between the alternative blends was then performed. 4.1 Soil – Bentonite mixtures (S/B) The following nomenclatures S00, S03, S05, S07 and S09 were adopted to designate, respectively, pure soil, Soil with addition of 3% bentonite, Soil with addition of 5% bentonite, Soil with addition of 7% of bentonite and Soil with addition of 9% of bentonite. 4.1.1 Characterization The purpose of this analysis is to predict the behavior of both the pure soil and its mixtures for use as impermeableing barrier. The physical properties of the materials are summarized in Table 1, and the granulometric curves of the materials are shown in Figure 2. Table 1. Physical properties of pure soil, bentonite and soil-bentonite mixtures used in the study. Materials S00 S03 S05 S07 S09 Bentonite ρd (g/cm³) 2.75 3.05 Clay (%) 6.45 13.02 14.24 14.58 16.92 77.98 Silte (%) 13.39 22.64 23.39 21.59 22.82 20.43 Sand (%) 55.26 43.11 43.36 47.18 43.22 1.59 LL (%) NL 41.11 57.19 70.24 81.65 378.12 Atterberg PL (%) NP 20.53 21.52 21.10 21.21 46.46 Limits IP(%) 20.57 35.67 49.14 60.44 331.66 USCS SM SC SC SC SC CH
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
(a) (b) Figure 2. Granulometric curves for the samples: (a) Pure soil and Bentonite (b) Pure soil and its mixtures. The pure soil, the bentonite and all mixtures was classified as silty sand (SM), high plasticity clay (CH) and clayey sand (SC), respectively, according to the Unified Soil Classification System (USCS). Considering the pure soil classification, this one presents properties that are not compatible with impermeableing barriers. With the addition of bentonite, the mixtures showed characteristics more pertinent to base materials of landfill. Besides this, the bentonite addition didn’t bring any substancial modification to the granulometric curves of the mixtures. On the other hand, the Plasticy Index (PI) shows linear correlations as the bentonite content increases. Figura 3 shows this effect. It can also be seen that PI mainly depend on Liquid Limits (LL), since Plastic Limits (PL) show small variation between the mixtures.
Figure 3. Plasticy Index as function of the bentonite content for the soil – bentonite mixtures. The Figure 4 indicate the consequences of the addition of bentonite on the compactation curves such as decrease in the maximum dry density and increase in optimum water content. The results are presented in Table 2. Table 2. Standard Proctor compactation parameters for soil - bentonite mixtures. Parameter S00 S03 S05 S07 S09 ρdmax (g/cm³) 1.99 1.94 1.92 1.90 1.89 wopt (%) 10.75 11.70 11.80 12.15 12.50
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 4. Standard Proctor compactation curves for soil – bentonite mixtures The optimum humidity and maximum dry density parameters will be adopted as reference for molding the specimen of the hydraulic conductivity test. Upon analyzing the chemical characterization, the pure soil presented a cation exchange capacity (CEC) of 1.86 meq / 100g, which means the material presents low content of exchangeable cations. The result of the pH test presented values within the usable sanitary landfills parameter of 7.21. 4.1.2 Hydraulic conductivity In this case, no tests were performed for S09 because the hydraulic conductivity and stability of the parameter in previous samples were already reached. For each percentage of additive, two specimens were examined. For the study of the influence of bentonite percentage in the hydraulic conductivity, the value used for the analysis was obtained by the average between the two tests results performed for each sample. Table 3 shows the molding conditions and results for the soil-bentonite mixtures. Table 3. Molding conditions and results for the soil-bentonite mixtures Sample wmold (%) ∆w* (%) ρd (g/cm³) DC (%) Saturation (%) S00(I) 10.56 -0.19 1.91 96 98.16 S00 S00(II) 10.86 -0.07 1.92 96 98.95 S03(I) 11.35 -0.35 1.85 95 99.02 S03 S03(II) 11.36 -0.34 1.89 97 97.76 S05(I) 12.01 0.21 1.87 97 98.64 S05 S05(II) 12.06 0.26 1.82 95 98.87 S07(I) 11.80 -0.35 1.88 99 97.80 S07 S07(II) 11.88 -0.27 1.86 98 98.34 *∆w = wopt - wmold kmed = k medium
k (cm/s) 2.66 E-06 2.29 E-06 6.54 E-08 4.63 E-08 1.02 E-08 1.81 E-08 8.47 E-09 7.91 E-09
kmed (cm/s) 2.64 E-06 5.58 E-08 1.61 E-08 7.88 E-09
It is clear from Table 3 that the increase in the bentonite content considerably reduces the hydraulic conductivity (k) of soil sample, as demonstrated by the plots of k value vesus bentonite content describle in Figure 5.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 5. Hydraulic conductivity versus bentonite content for soil-bentonite mixtures It worth noting that the value of k = 10-7 cm/s was not archieved for the compacted soil sample. For CCL this value is the maximum required at many regulations worldwide. Comparing the k-values of the soil – bentonite mixtures, from Table 4 and Figure 5, one may conclude that the addition of bentonite results in a considerable reduction of the hydraulic conductivity of the soil sample. It is also worth mentioning that the results indicate that, for all alternative mixtures, the hydraulic conductivity showed adequate values for the construction of impermeable barriers. Figure 5 shows that the graph generated by the hydraulic conductivity versus percentage of bentonite showed adherence to the predicted behavior in previous researches such as Luckiantchuki (2007), Camargo (2012) and Morandini & Leite (2015). The curve denoted the characteristic of a logarithmic exponential with R² = 0.93 with stability starting from the addition of 4% of bentonite. 4.2 Zeolite – Bentonite mixtures (Z/B) Even though in many studies the zeolitic material is used as an additive, in this research, the zeolite will act as the base soil. Like the mixture studed on topic 4.1 the nomenclatures Z00; Z03; Z05; e Z07 were adopted to designate the pure zeolite; mixture with 3% of bentonite; mixture with 5% addition of bentonite and mixture with 7% of addition of bentonite, respectively. 4.2.1 Characterization The results of the characterization of the additive will not be exposed in this topic, since it was the same used in item 4.1. This stage is to predict the behavior of the pure zeolite and its mixtures for use as impermeable barriers. The physical properties of the pure zeolite and their mixtures are summarized in Table 4, and the granulometric curves oh the materials are shown in Figure 6.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 4. Physical properties of pure zeolite and mixtures zeolite - bentonite used in the study. Materials Z00 Z03 Z05 Z07 ρd (g/cm³) 2.49 Clay (%) 4.27 4.58 6.86 8.53 Silte (%) 18.31 19.35 17.60 17.83 Sand (%) 77.42 76.07 75.54 73.64 LL (%) 50.65 68.19 81.76 94.12 Atterberg PL (%) NP 18.13 19.72 18.67 Limits IP(%) 50.60 62.04 75.45 USCS SM SC SC SC
Figure 6. Granulometric curves for the samples: (a) Pure zeolite and Bentonite (b) Pure zeolite and their mixtures. Similar the 4.1.1 item, Table 4 show the effect of the bentonite addition in most zeolite parameter. The pure zeolite was classified as silty sand (SM) and all mixtures was classified as clayey sand (SC), according to the Unified Soil Classification System (USCS). Analyzing the expected behavior of the pure zeolite as a silty sand (SM) with 77.42% of sand, this one presents properties that are not compatible with impermeable barries. The mixtures, with the increase of bentonite, showed characteristics more pertinets to base materials of landfill. The bentonite addition didn’t bring any substancial modification to the granulometric curves or the Plastic Limit (PL) of the mixtures. On the other hand the index properties shows that the Liquid Limits (LL) and Plasticity Index (PI) increases as the bentonite content increases. Figure 7 shows this effect. It is worth mentioning that the Plastic Limit increases between the pure zeolite and the mixtures.
Figure 7. Plasticy Index as function of the bentonite content for the zeolite – bentonite mixtures.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
With the increasing of bentonite content, optimum water also increases, while the dry density decreases. However, increases in bentonite content do not result in high variations, neither in dry density or optimum moisture content. The results are present on the Table 5, and the Figure 8. Table 5. Standard Proctor compactation parameters for soil - bentonite mixtures. Parameter Z00 Z03 Z05 Z07 ρdmax (g/cm³) 1.35 1.32 1.31 1.30 wopt (%) 29.6 30.7 31.7 31.9 Table 5 shows high optimum water and low dry density, it can be explain by the high adsorption capacity of the zeolite. This parameters will be adopted as reference for molding the specimen of the hydraulic conductivity test.
Figure 8. Standard Proctor compactation curves for soil – bentonite mixtures Analysing the parameter of the chemical analysis, the zeolite showed a CEC of 65.17 meq/100g. Comparing with the other studies of brazilian zeolite like Englert & Rubio (2005) and Oliveira (2011) the amount is in the expected margin. However this value can be inferior to the reality of the material seen, noticing that it wasn’t made the full saturation of the zeolite on CEC test, because according Kitsopoulos apud Oliveira (2011) the time to the effective saturation is 12 days, wich was not followed by IBRA. The zeolite shows a high CEC, what helps the reduction of environmental inpacts reduzing the contamination of the environment by heavy metals in case of a leakage of leached thru the cracks on the liner. About the pH, the material showed a result of 7.47 attending the brazilian criteria specefied to landfills. By mean of the mineralogical analysis by X-ray spectrometry made by the Celta Brasil Company is possible to afirm that the zeolite in study is composed in 71.3% of yours particles from SiO2 and 12.7% from AL2O3. The others oxides of your composition are presented in the Table 6. Table 6. Mineralogical composition of the zeolite sample according to the X-ray spectrometry results Oxides Results (%) Oxides Results (%) Oxides Results (%) SiO2 71.30 CaO 2.73 MnO
SUMMARY: Impermeable barriers are a need in several engineering works, especially in compacted clay liners (CCL) for waste containment systems. In that sense, the soil should have appropriate characteristics, such as low permeability (under 10-7 cm/s). This requirement can be restrictive considering the fact that very clayey soils are note available at all sites. For these cases, the bentonite addition is a potential alternative to improve the CCL performance. Soilbentonite used to backfill consisting of on-site soil and bentonite, are extensively used as engineering barriers for achieving low hydraulic conductivity. To avoid major environmental impacts and to minimize the risk of contamination for soil and groundwater under waste containment systems, zeolite, minerals as adsorptive material used in permeable reactive barriers for remediation of groundwater contaminated by heavy metals, can be used as alternative material in constructions of landfill bottom liners. In that sense, various ratios of bentonite and on-site soil (S/B) and bentonite and zeolite (Z/B) compacted at optimum water content (wopt) were test to determine the hydraulic conductivity (k) to found an ideal landfill liner material and study the behavior of the soils with the addition of bentonite. For the characterization of pure soil and mixtures grain size tests were performed (NBR 7181/16), density by pycnometer method (NBR 6458/16), liquid limit (NBR 6459/16) and plastic limit (NBR 7180/16). To determine the hydraulic the laboratory test involves flexible wall permeameter (ASTM D-5084-10). The results indicate that, for all mixtures soil-bentonite and for the mixture zeolite-bentonite with 7% of bentonite, the hydraulic conductivity showed adequate values for the construction of impermeable barriers.
1. INTRODUCTION The growing concern of society with the degradation of the environment and what it will entail for future generations has led to the use of management techniques that aim to reduce, recycle and reuse waste produced by different human activities, as well as technical solutions that make safe and adequate disposal of these wastes. In this context, the most widely adopted alternative for disposal and treatment of solid waste in Brazil and in most countries is the sanitary and industrial landfill, which still corresponds to the most viable technical and economic solution. One of the major concerns in the construction of a sanitary landfill is the contamination, by the leachate, of the soil and watercourses in the vicinity of the landfill. For that reason, impermeableing bottom barriers has been used in the Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
foundation of the landfill. The impermeableing layer, to maintain good performance during its useful life, must have a hydraulic conductivity lower than 10-7 cm / s, meeting the standards of several countries. In places where the natural soil is inadequate for the construction of impermeable barriers, as it happens in several regions of the earth globe, additives to the soil are used for improvements in the hydraulic and mechanical behavior. According to Das (2007) the soil can be mixed with external clay minerals, such as sodium bentonite, to establish the desired range of hydraulic conductivity. Some already published studies have analyzed the influence of additives in soils of several Brazilian regions, among them Lukiantchuki (2007), Camargo (2012) and Morandini & Leite (2015) and wordwide Villar and Rivas (1994), Sivapullaiah et al. (2000), Abichou et al. (2004) and Akgun (2010). Despite the impermeableing characteristics of the liner of sanitary landfills when they are made of a compacted layer of clay, cracks can appear causing leaks, making it easier for the leachate to reach and contaminate the surrounding soils and groundwater. In view of this situation, the use of zeolite, a mineral used in reactive permeable barriers, can be an alternative as a base material for impermeableing layer. The purpose of zeolite is to reduce the concentration and/or removal of pollutants to acceptable levels under current legislation. The high cation exchange capacity and the high adsorption capacity enable zeolites, among other uses, to recover areas affected by heavy metal contamination (MONTE & RESENDE, 2005). In this sense, it is necessary to carry out research in Laboratory scale in order to analyze the viability of using zeolitic concentrates in impermeableing layers, since the natural zeolites can be highly effective in the remediation of leaks coming from the base layer of the landfill. In this scenario, the hydraulic behavior of two soil-additive mixtures, among them the mixture local soil-bentonite and zeolite-bentonite, will be investigated by means of hydraulic conductivity tests for use as impermeableing barriers. The obtained results will contribute to the expansion of the database related to the study of materials that can be used as impermeableing barriers. It should be noted that this approach is of great importance within a landfill project since it comprises aspects that affect the efficiency and performance of the impermeableing system, compromising the integrity of the system as a whole. 2. MATERIALS 2.1 Tropical soil A tropical soil from the state of Espírito Santo, Brazil, was used to carry out this research. The choice of this soil was due to the great presence of tropical soils throughout the country and the terrestrial globe that, due to their sandy characteristics, are generally not suitable for the "in natura" use in base layer of landfill. The samples were collected, packed in bags and transported to the soil mechanics laboratory of the Federal University of Espírito Santo (UFES). The soil was previously separated by mechanical trap and characterized prior to addition of the percentages of additive. The physical and chemical characterization of the material is presented in item 4.1.1. The local soil was classified as silty sand (SM) according to the Unified Soil Classification System (USCS). 2.2 Zeolite The zeolite used in this study is of the natural type clinoptilolite and it was donated by the company "Celta Brasil". By analyzing previous work, it was requested 25 kg of material with a diameter of 0.0045 mm and 75 kg with a diameter of 0.4 to 1 mm. The samples were taken from
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
the state of São Paulo, Brazil, and they were collected by the company itself, packed in bags and transported to the soil mechanics laboratory of the Federal University of Espírito Santo (UFES). For the study, a previous mixture of all received material with the two granulometries was done. After the initial mixing, the material was separated by mechanical block in four fractions and characterized before the additions of the percentages. The physical and chemical characterization of the zeolite is presented in Item 4.2.1. The material was classified according to the USCS as silty sand. 2.3 Bentonite The bentonite here used is commercialized by the company "Bentonisa - Bentonita do Nordeste SA" and it was classified as sodium bentonite, thus presenting the sodium as interlamellar cation, and thus it presents, as characteristic, the swelling in water, beneficial to the structure to be analyzed. Therefore, when expanding the bentonite, it has self-healing power reducing the probability of the leachate to cross the liner by the possible cracks that can appear with the time of operation. Also due to their expansive characteristic under confined conditions, as in vertical barriers, the expanded bentonite particles are forced against each other, filling the voids between the soil particles, forming a barrier against the passage of the fluid (Heineck, 2002). The bentonite was classified, according to the USCS, as clay of high plasticity (CH) and the physical characterization of the material is presented in item 4.1.1. By the parameter of Liquidity Limit between 300% and 500%, adopted in HEINECK (2002), it can be classified as of medium quality. 3. METHODS 3.1 Sample characterization 3.1.1 Geotechnical analyses The geotechnical characterization of the pure soil and soil-bentonite mixtures, in the previously defined proportions, as well as pure zeolite and zeolite-bentonite mixtures, was performed according to the tests provided for in the Brazilian Association of Technical Standards (ABNT): grain size (NBR 7181/16) and density of grains by the pycnometer method (NBR 6458/16). The local soil was tested by means of thermal bath, while the zeolite was tested by the application of vacuum, Liquidity Limit (NBR 6459/16 ), Plasticity Limit (NBR 7180/16) and compaction test with Proctor Normal energy and with reuse (NBR 7182/16). For the characterization of the additive used in this study, due to its expansive characteristic and its adsorptive power in which it was verified the no complete hydration in a period of 24 hours, some modifications were made in the normative procedures for an accomplishment of the tests of granulometry and specific mass. For the granulometry test, a test sample was reduced from 70 g to 25 g and, for complete hydration, the material was left for 7 days in a solution of 125 ml of sodium hexametaphosphate and distilled water, and regarding the mass test, a tested sample was reduced from 50g to 15g and the material for its complete hydration was allowed to stand for 7 days in distilled water. The other tests performed, Liquidity Limit and Plasticity Limit, followed the guidelines of current standards NBR 6459/16 and NBR 7180/16, respectively.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3.1.2 Chemical analyses The chemical characterization was performed for pure local soil samples, pure zeolite and bentonite by the Brazilian Institute of Analyzes (IBRA), located in Jardim Nova Veneza Sumaré SP. For the analysis, 200g of each material were separated, packed in plastic bags and sent to the company's laboratory. The following tests were performed: pH in CaCl2 and SMP, CTC, saturation of H + Al bases, macronutrients and micronutrients by the Mehlich extractor. The results were received seven days after receiving the samples. 3.2 Hydraulic conductivity testing All hydraulic conductivity tests, for both the pure materials and their respective mixtures, were performed in flexible wall permeameter, and the test method was performed according to ASTM D5084-10 Method A - Constant loading. To perform the test, the samples were molded to the dimensions of approximately 10 cm in diameter and 5 cm in height, packed with Normal Proctor's energy, using the mechanical compactor, and humidity near the optimum moisture obtained by the compaction test. The assembly of the test was carried out in the following sequence: over the equipment pedestal, a previously saturated porous stone, a filter paper, the specimen, another filter paper and another porous stone were all set right in the center, along with the head of the equipment. Finally, to wrap the specimen and to avoid lateral flow, the coating of the assembly was carried out with a flexible latex membrane, fixed to the pedestal and the head by two o'rings. Figure 1 shows the assembly assay sequence in the flexible wall permeameter.
Figura 1. Assembly assay sequence in the flexible wall permeameter. Initially, the filling of the chamber of the equipment was performed. The saturation was performed by percolation of water and the test body was considered totally saturated after the passage of a water volume of three to five times the void volume of the specimen, following Guidelines. These values were adopted because it was not possible to verify the saturation by means of parameter B of Skempton by limiting the apparatus in the measurement of the variation of the neutral pressure. After the saturation process, the loads were kept constant and, thus, consecutive measurements of the volume of percolated water were made during the test. The hydraulic conductivity calculation was performed using Darcy's law, as explained in equation 1.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Where: k is the hydraulic conductivity (cm / s); ΔV is percolated water volume variation (cm³); L is the length of the specimen (cm); A is the cross-sectional area of the specimen (cm²); Δt is the time variation between the measurements (s) and Δh is the load variation applied during the test; After the test ended, the humidity of the specimen was measured and the saturation was calculated using correlations of physical indices. Due to the limitations of the test, saturation values above 95% were considered satisfactory.
4. RESULTS AND DISCUSSIONS For a better interpretation, it was first performed an analysis for each separate base material and, after that, a comparison between the alternative blends was then performed. 4.1 Soil – Bentonite mixtures (S/B) The following nomenclatures S00, S03, S05, S07 and S09 were adopted to designate, respectively, pure soil, Soil with addition of 3% bentonite, Soil with addition of 5% bentonite, Soil with addition of 7% of bentonite and Soil with addition of 9% of bentonite. 4.1.1 Characterization The purpose of this analysis is to predict the behavior of both the pure soil and its mixtures for use as impermeableing barrier. The physical properties of the materials are summarized in Table 1, and the granulometric curves of the materials are shown in Figure 2. Table 1. Physical properties of pure soil, bentonite and soil-bentonite mixtures used in the study. Materials S00 S03 S05 S07 S09 Bentonite ρd (g/cm³) 2.75 3.05 Clay (%) 6.45 13.02 14.24 14.58 16.92 77.98 Silte (%) 13.39 22.64 23.39 21.59 22.82 20.43 Sand (%) 55.26 43.11 43.36 47.18 43.22 1.59 LL (%) NL 41.11 57.19 70.24 81.65 378.12 Atterberg PL (%) NP 20.53 21.52 21.10 21.21 46.46 Limits IP(%) 20.57 35.67 49.14 60.44 331.66 USCS SM SC SC SC SC CH
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
(a) (b) Figure 2. Granulometric curves for the samples: (a) Pure soil and Bentonite (b) Pure soil and its mixtures. The pure soil, the bentonite and all mixtures was classified as silty sand (SM), high plasticity clay (CH) and clayey sand (SC), respectively, according to the Unified Soil Classification System (USCS). Considering the pure soil classification, this one presents properties that are not compatible with impermeableing barriers. With the addition of bentonite, the mixtures showed characteristics more pertinent to base materials of landfill. Besides this, the bentonite addition didn’t bring any substancial modification to the granulometric curves of the mixtures. On the other hand, the Plasticy Index (PI) shows linear correlations as the bentonite content increases. Figura 3 shows this effect. It can also be seen that PI mainly depend on Liquid Limits (LL), since Plastic Limits (PL) show small variation between the mixtures.
Figure 3. Plasticy Index as function of the bentonite content for the soil – bentonite mixtures. The Figure 4 indicate the consequences of the addition of bentonite on the compactation curves such as decrease in the maximum dry density and increase in optimum water content. The results are presented in Table 2. Table 2. Standard Proctor compactation parameters for soil - bentonite mixtures. Parameter S00 S03 S05 S07 S09 ρdmax (g/cm³) 1.99 1.94 1.92 1.90 1.89 wopt (%) 10.75 11.70 11.80 12.15 12.50
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 4. Standard Proctor compactation curves for soil – bentonite mixtures The optimum humidity and maximum dry density parameters will be adopted as reference for molding the specimen of the hydraulic conductivity test. Upon analyzing the chemical characterization, the pure soil presented a cation exchange capacity (CEC) of 1.86 meq / 100g, which means the material presents low content of exchangeable cations. The result of the pH test presented values within the usable sanitary landfills parameter of 7.21. 4.1.2 Hydraulic conductivity In this case, no tests were performed for S09 because the hydraulic conductivity and stability of the parameter in previous samples were already reached. For each percentage of additive, two specimens were examined. For the study of the influence of bentonite percentage in the hydraulic conductivity, the value used for the analysis was obtained by the average between the two tests results performed for each sample. Table 3 shows the molding conditions and results for the soil-bentonite mixtures. Table 3. Molding conditions and results for the soil-bentonite mixtures Sample wmold (%) ∆w* (%) ρd (g/cm³) DC (%) Saturation (%) S00(I) 10.56 -0.19 1.91 96 98.16 S00 S00(II) 10.86 -0.07 1.92 96 98.95 S03(I) 11.35 -0.35 1.85 95 99.02 S03 S03(II) 11.36 -0.34 1.89 97 97.76 S05(I) 12.01 0.21 1.87 97 98.64 S05 S05(II) 12.06 0.26 1.82 95 98.87 S07(I) 11.80 -0.35 1.88 99 97.80 S07 S07(II) 11.88 -0.27 1.86 98 98.34 *∆w = wopt - wmold kmed = k medium
k (cm/s) 2.66 E-06 2.29 E-06 6.54 E-08 4.63 E-08 1.02 E-08 1.81 E-08 8.47 E-09 7.91 E-09
kmed (cm/s) 2.64 E-06 5.58 E-08 1.61 E-08 7.88 E-09
It is clear from Table 3 that the increase in the bentonite content considerably reduces the hydraulic conductivity (k) of soil sample, as demonstrated by the plots of k value vesus bentonite content describle in Figure 5.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 5. Hydraulic conductivity versus bentonite content for soil-bentonite mixtures It worth noting that the value of k = 10-7 cm/s was not archieved for the compacted soil sample. For CCL this value is the maximum required at many regulations worldwide. Comparing the k-values of the soil – bentonite mixtures, from Table 4 and Figure 5, one may conclude that the addition of bentonite results in a considerable reduction of the hydraulic conductivity of the soil sample. It is also worth mentioning that the results indicate that, for all alternative mixtures, the hydraulic conductivity showed adequate values for the construction of impermeable barriers. Figure 5 shows that the graph generated by the hydraulic conductivity versus percentage of bentonite showed adherence to the predicted behavior in previous researches such as Luckiantchuki (2007), Camargo (2012) and Morandini & Leite (2015). The curve denoted the characteristic of a logarithmic exponential with R² = 0.93 with stability starting from the addition of 4% of bentonite. 4.2 Zeolite – Bentonite mixtures (Z/B) Even though in many studies the zeolitic material is used as an additive, in this research, the zeolite will act as the base soil. Like the mixture studed on topic 4.1 the nomenclatures Z00; Z03; Z05; e Z07 were adopted to designate the pure zeolite; mixture with 3% of bentonite; mixture with 5% addition of bentonite and mixture with 7% of addition of bentonite, respectively. 4.2.1 Characterization The results of the characterization of the additive will not be exposed in this topic, since it was the same used in item 4.1. This stage is to predict the behavior of the pure zeolite and its mixtures for use as impermeable barriers. The physical properties of the pure zeolite and their mixtures are summarized in Table 4, and the granulometric curves oh the materials are shown in Figure 6.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 4. Physical properties of pure zeolite and mixtures zeolite - bentonite used in the study. Materials Z00 Z03 Z05 Z07 ρd (g/cm³) 2.49 Clay (%) 4.27 4.58 6.86 8.53 Silte (%) 18.31 19.35 17.60 17.83 Sand (%) 77.42 76.07 75.54 73.64 LL (%) 50.65 68.19 81.76 94.12 Atterberg PL (%) NP 18.13 19.72 18.67 Limits IP(%) 50.60 62.04 75.45 USCS SM SC SC SC
Figure 6. Granulometric curves for the samples: (a) Pure zeolite and Bentonite (b) Pure zeolite and their mixtures. Similar the 4.1.1 item, Table 4 show the effect of the bentonite addition in most zeolite parameter. The pure zeolite was classified as silty sand (SM) and all mixtures was classified as clayey sand (SC), according to the Unified Soil Classification System (USCS). Analyzing the expected behavior of the pure zeolite as a silty sand (SM) with 77.42% of sand, this one presents properties that are not compatible with impermeable barries. The mixtures, with the increase of bentonite, showed characteristics more pertinets to base materials of landfill. The bentonite addition didn’t bring any substancial modification to the granulometric curves or the Plastic Limit (PL) of the mixtures. On the other hand the index properties shows that the Liquid Limits (LL) and Plasticity Index (PI) increases as the bentonite content increases. Figure 7 shows this effect. It is worth mentioning that the Plastic Limit increases between the pure zeolite and the mixtures.
Figure 7. Plasticy Index as function of the bentonite content for the zeolite – bentonite mixtures.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
With the increasing of bentonite content, optimum water also increases, while the dry density decreases. However, increases in bentonite content do not result in high variations, neither in dry density or optimum moisture content. The results are present on the Table 5, and the Figure 8. Table 5. Standard Proctor compactation parameters for soil - bentonite mixtures. Parameter Z00 Z03 Z05 Z07 ρdmax (g/cm³) 1.35 1.32 1.31 1.30 wopt (%) 29.6 30.7 31.7 31.9 Table 5 shows high optimum water and low dry density, it can be explain by the high adsorption capacity of the zeolite. This parameters will be adopted as reference for molding the specimen of the hydraulic conductivity test.
Figure 8. Standard Proctor compactation curves for soil – bentonite mixtures Analysing the parameter of the chemical analysis, the zeolite showed a CEC of 65.17 meq/100g. Comparing with the other studies of brazilian zeolite like Englert & Rubio (2005) and Oliveira (2011) the amount is in the expected margin. However this value can be inferior to the reality of the material seen, noticing that it wasn’t made the full saturation of the zeolite on CEC test, because according Kitsopoulos apud Oliveira (2011) the time to the effective saturation is 12 days, wich was not followed by IBRA. The zeolite shows a high CEC, what helps the reduction of environmental inpacts reduzing the contamination of the environment by heavy metals in case of a leakage of leached thru the cracks on the liner. About the pH, the material showed a result of 7.47 attending the brazilian criteria specefied to landfills. By mean of the mineralogical analysis by X-ray spectrometry made by the Celta Brasil Company is possible to afirm that the zeolite in study is composed in 71.3% of yours particles from SiO2 and 12.7% from AL2O3. The others oxides of your composition are presented in the Table 6. Table 6. Mineralogical composition of the zeolite sample according to the X-ray spectrometry results Oxides Results (%) Oxides Results (%) Oxides Results (%) SiO2 71.30 CaO 2.73 MnO
