PORE FLUID CHARACTERISTICS EFFECT ON DISPERSIVITY BEHAVIOUR OF BENTONITE FROM MACRO AND MICRO STRUCTURE ASPECTS

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1 PORE FLUID CHARACTERISTICS EFFECT ON DISPERSIVITY BEHAVIOUR OF BENTONITE FROM MACRO AND MICRO STRUCTURE ASPECTS Vahid R. Ouhadi, Associate. Prof., Bu-Ali Sina University, Hamedan, Iran Amir R. Goodarzi, Graduate Student, Bu-Ali Sina University, Hamedan, Iran ABSTRACT The soil dispersivity-sedimentation behaviour is a physico-chemical phenomenon that usually happens at low sodium concentration. In all common factors for the monitoring of dispersivity behaviour, less attention is given to the source of sodium ion. However, in waste disposal projects or in the characteristics of pore fluid interacting with soil, it is very common to face sodium ions with combination of anions like sulfate or chloride. In this paper, an attention is given to the role of sodium ion with different anion type to the dispersivity behaviour of bentonite. This study performed from macro and microstructure point of view. The achieved results indicate that with a change on the chemical characteristics of pore fluid, and specifically with a change on the anion type having similar cation, the dispersivity characteristics of bentonite differ leading to a change on the soil erosion potential and availability of contaminant to the ground water surface. RÉSUMÉ Le comportement sédimentation - dispersé des sols est un phénomène physico-chimique qui se crée quand la concentration de sodium est assez peu. Entre tout les facteurs influençant le comportement dispersé des sols, l effet des ions est moins étudié. Cependant dans les caractèrictiques de l interaction de l eau interstitielle et sol, les ions sodium sont généralement regardés avec la combinaison d anions comme sulfate ou chloride. Dans cet article nous avons fait l attention sur le rôle des ions sodium avec différent type d anions pour vérifier le comportement dispersé de bentonite. Cet étude est fait au point vue macro et microstructure. les résultats obtenus montrent qu avec un changement sur les caractéristiques chimiques de fluide interstitielle, en particulier un changement sur le type d anion ayant les cations similaire, les caracteristiques dispersés de bentonite conduisent de façon différent au changement sur le potentiel d érosion de sol et la capacité de la contamination de l eau en surface. 1. INTRODUCTION The use of waste disposal facilities such as landfill is known as a major tool to prevent the contaminant migration to the groundwater. Clayey soils are very suitable to be used in these facilities due to the low permeability, high buffering capacity and high ion exchange capability. It is known that a mixture of sand and bentonite has the above-mentioned characteristics with a high workability (Alawaji 1999). Since montmorilonite mineral usually is used as a major mineral for clay liners, it is expected that the permeability, compressibility, and soil-contaminant interaction of these sites is governed by the pore fluid properties variation of soil (Yong et al. 1992, Ouhadi 1998, Ouhadi & Sedighi, 3). On the other hand, it is reported that dispersivity behaviour of clay barrier has been responsible for some leachates from waste disposal facilities to the ground water surface (Goodarzi 3). Therefore, for the evaluation of dispersivity behaviour of soils including the erosion process during the seepage of water in clay barrier, there is a need to take into consideration the soil mineral s type and chemical properties of pore fluid. In this regard it is known that dispersive behaviour is usually accompanied by the presence of montmorilonite rich in sodium ion as a major exchangeable cation (Sherard et al. 1976). The recent research also indicates that, the dispersivity behaviour is significantly affected by the anion type (Abend and Lagaly ). In fact, anion may accumulate around clay edges, causing neutrality on the positive charge of clay edges. This will eliminate the edge (+) to face (-) interaction providing a decrease in shear strength of slurry and an increase in dispersivity (Penner and Lagaly ). Based on the above points, it can be concluded that the mechanism of anion effect of contamination on the soil behaviour is a function of the mechanism of ion adsorption by soil. Usually, anion such as chloride and sulfate is considered in a similar category (Yong et al. 1992). The recent research confirms the effect of anion valence on the dispersivity behaviour of soils. (Abend & Lagaly ). Therefore with an attention to the relation of dispersivity mechanism with chemical properties of pore fluid, it is necessary to take attention on the difference between anion type effects on the dispersivity behaviour of clay liner of waste disposal facilities. This research is aimed to focus attention on the anion valence effect and the role of sodium cation with different anion fraction on the dispersivity of bentonite. These different processes may happen in any landfill disposal projects. A part of this study evaluates such a different behaviour from macroscopic and microscopic point of view using XRD and SEM experiments for microscopic behaviour evaluation. 2. MATERIALS AND METHODS This study was performed using a bentonite sample supplied by the Iran Barit Co. The chemical analysis performed according to the methods presented by Yong et al. (1992). Atomic Absorption Spectrophotometer (AAS- GBC 932, AB Plus) was used to evaluate the soil pore fluid

2 Table 1. Physical properties of bentonite. Characteristic Quantity measured Plasticity index Soil classification CH Gs Clay fraction% 76 Silt fraction% 23 Sand% 1 Table 2. Chemical properties of bentonite. Characteristic Quantity measured ph (1:5, soil-water) 1 EC(dS/m) (1:5,soil-water).64 Na + (cmol/kg) 48.5 Ca 2+ (cmol/kg) 14.2 Mg 2+ (cmol/kg) 3.4 K + (cmol/kg) 2.1 CEC, (cmol/kg) 68.2 SSA, (m 2 /g) 418 Carbonate, % 8 Organic, % <1% analysis. Physical and mechanical properties are measured based on ASTM (1992) Standard. Tables 1 and 2 present some of the physical and mechanical properties of bentonite sample. For dispersivity measurement, the double hydrometer test is used (ASTM 1992). In this experiment, a series of one-liter slurry of bentonite-electrolyte is prepared (Soil-electrolyte ratio of 1:5). These samples are kept for 96 hours for equilibrium. After equilibrium condition, each sample is shake for 15 minutes, to provide a homogeneous sample. Following this process, with application of hydrometer test, for different samples having different electrolyte, the variation of particle distribution is evaluated. The percent dispersion is defined as the ratio of clay size particles (.5 mm) while no additive is used as a dispersive agent, on the clay size particles with the same size while mechanical and chemical agitation is used. The electrolyte concentration varies within the range of.5 to 1 meq/l. This is based on the fact that dispersivity behaviour is usually happens in the low electrolyte concentration (Guler and Balci 1998, Chen et al. 199). In addition, an attention is taken to the concentration range between 2 to meq/l, since in this range the highest dispersivity happens (Abend and Lagaly ). For consideration of different anion effect with similar cation type, three types of salt including, Na 2SO 4, and K 2SO 4, is used. For microscopic study, XRD and SEM analysis performed. For these purposes, with making solution of 1:5 bentoniteelectrolyte concentration, on the specific salt concentration and after equilibrium condition, few drops of solution is poured on the glass slide. After air-drying process, with the use of Siemens-Diffraktometer-D5 XRD equipment, an analysis is provided. To prepare sample for SEM analysis, a slurry sample, as addressed before, is prepared. After equilibrium condition, samples are placed on the developed consolidating mold (Ouhadi & Sedighi, 3). With application of 25-kPa loading, slurry samples are consolidated. In this process, the excess pore fluid is drained from sample. Following that, samples are air-dried and kept in an oven for 48 hours under the temperature of 6 degrees. Samples are broken to the pieces of 1 cm 3 and for achieving a homogeneous broken surface; each broken surface is covered 1 times with a strip tape to provide a very well trimmed surface. In such a case, one can assume the appearance of real situation of soil particles at the broken surface. Following this sample preparation process, they analyzed by SEM Jeol-Jsm 84A equipment and SEM pictures are provided. 3. RESULTS AND DISCUSSION The experimental results can be studied from different aspects as are discussed in this section. 3.1 Effect of Electrolyte Concentration on the Grain Size Distribution of Soil Usually in sandy and silty soils, the soil performance is not a function of variation of pore water characteristics. However, in clay soils due to the presence of active soil surface, the grain size distribution of soil is a function of soil mineral and its interaction to the pore fluid characteristics. As an example, for a montmorilonite particle at the low electrolyte concentration, particles behave independently, however with an increase in electrolyte concentration we are facing with the formation of tactoids, clusters and peds, respectively (Yong 1999). In other words, with an increase in electrolyte concentration, due to the flocculated behaviour of particles, the weight of each floc increases as the electrolyte concentration increases. This will change the speed of particle settlement, which is directly in conjunction with grain size distribution of soil. Based on these points, to study the effect of salt concentration on the grain size distribution of soil, the results of hydrometer testing of soil samples having different salt concentration are monitored (Figure 1). Percent Finer Pore Fluid Characteristics Effect on Grain-Size Distribution Curve Particle Size (mm) As it is.5 meq/l 1 meq/l 5 meq/l 1 meq/l 25 meq/l 5 meq/l 1 meq/l Figure 1. The effect of salt concentration on the grain size distribution of contaminated bentonite 1

3 As can be seen in this Figure, with an increase in salt concentration, at the relatively low electrolyte concentration, due to the reduction of the repulsive forces, and due to an increase on the weight of flocculated particles, the settlement velocity increases. Following that, with an increase on the electrolyte concentration and a decrease on the repulsive forces, the aggregate structure takes place causing a further increase on the settlement velocity. In addition, at the similar concentration, these mainly physical graphs do not show any difference among several-different electrolytes used in this study. 3.2 Effect of Exchangeable Cation on the Dispersivity To evaluate the effect of cation type on the dispersivity behaviour of soil, two types of solution, including K 2SO 4 and Na 2SO 4 is used. Figure 2 shows the results of hydrometer testing of bentonite samples with these two solutions as electrolyte. As can be seen in this graph, potassium ion has relatively low dispersivity effect on the bentonite sample, in comparison with sodium ion. This performance can be attributed to the lower hydrated ion radius of potassium ion and consequently an increase in thickness of double layer of soil once potassium presents as a pore fluid. On the other hand, with an increase in sodium concentration, the thickness of double layer decreases. However, once soil gets in contact with fresh water, due to a decrease on the electrolyte concentration, and due to the osmotic behaviour of particles, the water adsorption characteristic of soil increases. This makes an increase on the volume change of soil and makes particles to get away from each other. This performance can be attributed to the reversible behaviour of sodium cation. However, this behaviour cannot be observed for other cations such as potassium and calcium (Di Miao 1996). This phenomenon of soil erosion reported at some waste disposal projects once they are experienced high quantity of rainfall (Tallin 1984). Percent Dispersion K2SO Salt Conc. (meq/l) Figure 2. Effect of Cation Type on the Dispersivity of Bentonite (Double Hydrometer Testing) 3.3 Effect of Anion on the Dispersivity Behaviour of Bentonite Even though it is reported that the anion type may have a significant effect on dispersivity behaviour of soils (Yong and Sethi 1977, Ouhadi and Yong 1, Yong 2), there is few researches in this phenomena. In this paper to take into consideration the effect of anion on the dispersivity behaviour of soils, sodium chloride and sodium sulfate are used as electrolyte. Figure 3 shows the effect of these two electrolytes on the dispersivity performance of bentonite, while the double hydrometer testing is used to measure and monitor the dispersivity rate of artificial samples. As can be seen in this Figure, with an increase in salt concentration the dispersivity percentage increases, in which at 5 meq/l of electrolyte we are facing with a highest percentage of dispersivity. However, the effect of anion type cannot be seen in this experiment, since one cannot observe a major change between the effects of these two solutions on the dispersivity behaviour of bentonite. Based on previous research (Penner & Lagaly 1) one may expect that with an increase on anion valence, and consequently an increase on negative charge of clay particles, the soil tendency for having a stable suspension increases. To investigate these observed conflicts, an attention is given to the adsorption characteristics of soil due to the presence of sodium ion with different source of anion. Percent Dispersion Salt Conc. (meq/l) Figure 3. Anion type effect on dispersivity of bentonite 3.4 Sodium Adsorption Effect on Dispersivity Potential It is shown that dispersivity behaviour happens in two different mechanisms (Yong & Sethi, 1977). I) With an increase in soil volume and sodium adsorption. II) By repulsing of exchangeable cation and an increase in clay particle distance. To investigate on the above-mentioned points, batch equilibrium testing performed with preparing samples of 1 g soil and 5 ml electrolyte with a specific salt concentration. After 24 hours of shaking and keeping samples for 96 hours to achieve equilibrium, the supernatant of sample is separated following 12 minutes of centrifuging with 5 rpm. With an attention to the initial soluble sodium of sample and the quantity of artificial sodium added to the sample, the adsorbed sodium is calculated with the use of atomic adsorption spectrophotometer. Figure 4 shows the effect of anion type on the adsorption characteristics of bentonite. As can be observed in this Figure, with an increase on anion valence, the ability of bentonite to adsorb the sodium ions increases. The quantity of adsorbed sodium with sulfate source is 2

4 almost 1% more than the quantity of adsorbed sodium with chloride source. This difference can be explained as follows: At the presence of sulfate ion, the thickness of diffuse double layer is relatively more than that of the presence of chloride ion. This conclusion is in agreement with the results presented by other authors (Abend & Lagaly ). In addition, it is already reported that the shear strength of mixtures of sodium sulfate and bentonite in 2% slurry is less than the shear strength of bentonite and sodium chloride in similar concentration (Guler & Balci, 1988). These results also confirms the conclusion already been addressed. Figure 4. The effect of anion type on the adsorption characteristic of bentonite Density (gr/cm 3 ) Adsorption of Na + (%) Salt Conc.(meq/L) Salt Conc.(meq/L) Based on the above results, one can conclude that a dispersivity phenomenon at swelling soils is governed with the adsorption of sodium ions. Therefore one may expect a change on the physico-chemical characteristics of soil at the peak of dispersivity graph. 3.5 Effect of Dispersivity on the Volume Change of Bentonite A set of experiment performed to investigate the effect of dispersivity on the mechanical-physical-macro-behaviour of bentonite. Figure 5 shows the effect of salt concentration on the variation of density and volume change characteristics of bentonite. The results of this Figure indicate that with an increase in salt concentration (in the range of highest dispersivity according to the double hydrometer testing), first the maximum dry density of soil decreases, and soil s void ratio increases. Then, with an increase of salt concentration, due to the formation of flocculated structure, and due to the increase of peds pieces of particles, soil s void ratio noticeably decreases. The phenomena of a decrease of density and an increase of void ratio at the highest percentage of dispersivity can be responsible for a reduction of shear strength of dispersive soils. This might me also responsible for failure mechanism happens during the rainfall in dispersive slopes. On the other hand, it is shown that the elastic settlement is a function of elasticity coefficient (E) and the Poisson ratio. In which, with decreasing of elasticity coefficient, and assuming constant Poisson ratio, the immediate settlement increases (Hicher 1996, Sivapullaiah ). Based on the above points, one may come to this conclusion that with an increase in void ratio at dispersivity point, and consequently a decrease on elasticity coefficient, the soil settlement at the dispersivity point will be more than the soil settlement of non dispersive soil. Since dispersivity starts at some specific point of liner, and develops through the liner, one may observe a differential settlement in the liner, happening at the same time that dispersivity develops. 3.6 Dispersivity Evaluation from Microscopic Point of View To investigate on the microscopic aspect of dispersivity, XRD analysis and SEM analysis performed based on the sample preparation addressed before. The results of these experiments are as follows: Void Ratio Salt Conc.(meq/L) Results of XRD Analysis Some results are reported on the effect of salt concentration on the variation of basal spacing and intensity of major peaks of montmorilonite. The recent results show that the basal spacing of montmorilonite decreases as the salt concentration increases (Herbert & Moog 1999). Figure 6 shows the results of XRD analysis of bentonite sample with different salt concentration. Figure 5. Pore fluid effect on mechanical properties of bentonite The XRD analyses are presented for bentonite sample without any salt as an additive, bentonite having 5 meq/l, bentonite having 5 meq/l Na 2SO 4, and bentonite 3

5 Int = Å 9.84 Å Air-dry Natural Bentonite Oven-dry (55ºc) 3 34 Å 3.2 Å having 1 meq/l of Na 2SO 4. The appeared basal spacing of angstrom is in accordance to the basal spacing of montmorilonite-illite (ICDD1988). This basal spacing is transferred to the basal spacing of 9.98 angstrom after heat treatment. This performance can be attributed to the removal of water content within the basic structure of montmorilonite. This reduction of basal spacing observed in all XRD analysis of samples after heat treatment at 55 oven dry. Furthermore, with addition of salt concentration, Natural Bentonite Int = Å Bentonite with 5 meq/l Air-dry Condition 3 2 Å 9.84 Å Oven-dry (55 ºc) 4.3 Å 3 33 Å 3.19 Å Bentonite with 5meq/L Int = Å Bentonite with 5 meq/l Na 2SO 4 Air-dry Condition 3 2 Å Bentonite with 5 meq/l Na 2SO Å 1 98 Å Oven-dry (55ºc) 3.33 Å 3 18 Å Å Int =22 Bentonite with 1 meq/l Na 2SO 4 Bentonite with 1 meq/l Na 2SO Å AIR-DRY CONDITION Oven-dry (55º c) 4.45 Å 4.1 Å 3.34 Å 3 2 Å Figure 6. Results of XRD analysis Figure 7. Results of SEM analysis 4

6 the required salt quantity for making soil as a dispersive sample, on the basal spacing of sample having sodium sulfate as an electrolyte increases in comparison with other samples. This increase of basal spacing from angstrom to angstrom can be attributed to the formation of dispersive structure having an increase on the thickness of double layer once sulfate is the anion fraction of salt. The higher intensity of this sample in comparison with that of sample having sodium chloride as an electrolyte confirms the higher oriented structure once sulfate anion presents in the soil solution. It is important to note that as the salt concentration increases from 5 meq/l of sodium sulfate to the 1 meq/l concentration of sodium sulfate, due to the formation of flocculated structure and decreasing the thickness of double layer, one can observes a decrease in the peak intensity and basal spacing as well Results of SEM Experiments It is shown that at the point with the highest dispersivity potential, the repulsive forces among clay particles are the highest. This repulsive force makes the clay particles to get away from each other (Mitchell 1993). To investigate on the effect of sodium ion with difference source of anion on the dispersivity behaviour of soil from microscopic point of view, some SEM pictures (Figure 7) are taken according to the sample preparation already addressed. As can be seen in Figure 7, with an increase in electrolyte concentration, first, due to the formation of repulsive forces, a dispersive structure forms. With an increase in salt concentration, the structure changes to the flocculated structure. In addition to the above, the scanning electro microscope pictures show that soil sample having sodium sulfate as an electrolyte is more dispersed than soil sample having sodium chloride as an electrolyte. This difference which can be observed from open structure of sample having sodium sulfate as pore fluid, is in a good agreement with the result of sodium adsorption of soil at different electrolyte type as addressed in this paper. 4. CONCUDING REMARKS The results of this research provide the following conclusions; 1- Dispersivity-sedimentation is a physico-chemical phenomena in which it is a function of chemical characteristic of pore fluid. The highest dispersivity can be observed at the low concentration of electrolyte. In addition anion type of pore fluid has a major effect on dispersivity behaviour even for a similar sodium ion concentration. 2- Adsorption measurement is much more effective tool for dispersivity mechanism evaluation than some common experiments such as double hydrometer testing. This experiment is capable of considering the effect of different anion on the dispersivity behaviour of soils having similar concentration of sodium ion. 3- Sine the physical performance of soils is affected by the chemical properties of pore fluid; the void ratio variation of soil due to the dispersivity-sedimentation phenomena may cause a differential settlement in the soil liner. 5. ACKNOWLEDGEMENTS The authors would like to express their acknowledgement to Prof. M. Maleki for preparing the French abstract. 6. REFERENCES American Society for Testing and Materials, ASTM, Annual Book of ASTM Standards, Philadelphia, V.4, 8. Abend, S., and Lagaly, G.. Sol-gel transitions of sodium montmorilonite dispersions. Applied clay science 16, pp Alawaji, A., H Swell and compressibility characteristics of sand-bentonite mixtures inundated with liquids. Applied clay science 15, pp Chen, J. S., Cushman, J. H., and Low, P. F Rheological behavior of Na-montmorilonite suspensions at low electrolyte concentration. J. Clay and clay minerals. Vol. 38, No. 1. pp DI Miao, C Exposure of bentonite to salt solution: osmotic and mechanical effects. Geotechnic 46, No.4, pp GÜler, C., and Balci, E Effect of some salts on the viscosity of slip casting. Applied clay science 13, pp Goodarzi, A.R., 3. Effect of pore fluid characteristics on the dispersivity behaviour of soils, specific attention to the microstructure interaction, Master thesis, Civil Eng. Dept., Bu-Ali Sina University, Iran. Herbert, H. J. and Moog, H. C Cation exchange, interlayer spacing, and water content of MX-8 bentonite in high molar saline solution. J. Eng. Geology 54. pp Hicher, P. Y Elastic properties of soils. J. Geotech. Eng., ASCE 122, No. 8, pp ICDD, International Centre for Diffraction Data, Mineral Powder Diffraction File Search Manual. Joint Committee on Powder Diffraction Standards. Swarthmore, Pennsylvania Mitchell, J. K Fundamentals of soil behaviour. John Wiley & Sons. Inc., pp

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