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Experimental study of the electrokinetic behaviour of kaolinite-smectite mixtures.

Introduction

It is very difficult to investigate the evolution of the fabric of clays subjected to mechanical, chemical or biological treatment due to the variety of clays found in nature and the variety of stresses applied to soils. In recent years, studies have been undertaken to investigate how the structure of clays changes under mechanical and hydric stresses (e.g. Al-Mukhtar et al. 1996; Guillot et al. 2001; Nowamooz and Masrouri 2010; Souli et al. 2010). Other studies have investigated fabric changes under chemo-hydro-mechanical stress (Jullien et al. 2002; Jozja et al. 2003; Souli et al. 2008, 2009). Hammad (2010) and Hattab et al. (2015) studied the evolution of the mechanical properties of kaolinite-smectite mixtures and showed that the compression index, the swelling index and the friction angle were highly dependent on the smectite amount; Hattab et al. (2015) showed that the compression and swelling indices increased with the amount of smectite, whereas the friction angle decreased. In addition, Hammad (2010) and Hattab et al. (2015) highlighted the fact that when the percentage of smectite was >35%, the compression curves presented two slopes in the (log [[sigma].sub.v] (where [[sigma].sub.v] is the vertical mechanical stress and e is the void rati) coordinate system and that the friction angle became nearly constant, close to that of the pure smectite. They related this behaviour to scanning electron microscopy (SEM) observations showing that when the percentage of smectite exceeded 35%, smectite was present at the contact points between the groups of kaolinite particles and dominated the behaviour of the mixtures due to the higher flexibility and deformability of its particles.

The application of an electrical field also affects the behaviour of clays. Indeed, electrokinetic treatment results in a change in the pH of the medium and, as a consequence, in the electrical charge of the clay particles. Two types of applications are often considered in the case of electrical treatment: depollution and consolidation of soil. In the case of depollution, significant effects of pH, ionic strength, pollutant type and metal concentration on the electrokinetic behaviour of some soils containing kaolinite have been reported (Saichek and Reddy 2003; Altin and Degirmenci 2005; Al-Shahrani and Roberts 2009; Ouhadi et al. 2010). More recently Ben Hassine et al. (2016) studied the behaviour of mixtures of kaolinite and carbonate in the presence of water and lead and showed that the electrokinetic behaviour of the sample is highly related to the concentration of lead, as well as the percentage of carbonate. Indeed, the presence of carbonate prevents the diffusion of [H.sup.+] cations in the soil, consequently preventing the evolution of electrical parameters during electrokinetic tests. Other studies have investigated the relationship between hydraulic permeability and electro-osmotic permeability. For example, Casagrande (1953) showed that the electro-osmotic permeability of soil samples with various compositions remained approximately equal to [10.sup.-9] [m.sup.2] [V.sup.-1] [s.sup.-1]. Jayasekera (2004) presented results of studies performed on a smectite ([k.sub.hydraulic] = 2.4 x [10.sup.-9] [ms.sup.-1]) and a kaolinite ([k.sub.hydraulic] = 1 x 1 x [10.sup.-7] m [s.sup.-1]) at different pH, confirming that the electro-osmotic permeability of both clays was around [10.sup.-9] [m.sup.2] [V.sup.-1] [s.sup.-1]. Asadi et al. (2013) reported that the electro-osmotic permeability changed slightly from one clay to another, with values of 2 x [10.sup.-9] [m.sup.2] [V.sup.-1] [s.sup.-1] for a Na-smectite and 5.7 x [10.sup.-9] [m.sup.2] [V.sup.-1] [s.sup.-1] for a kaolinite. Conversely, Mosavat et al. (2013) found that the electro-osmotic permeability of a kaolinite was higher than that of a smectite. Therefore, additional research is needed to clarify this important point.

The aim of the present study was to investigate the behaviour of kaolinite-smectite mixtures subjected to mechanical and electrokinetic tests over a large range of smectite content. Measurements of the Atterberg limits and compression indices were performed. With regard to the electrokinetic tests, particular interest was given to the evolution of electrical conductivity, pH and electro-osmotic permeability during and after the tests. One of the objectives of the study was to determine the minimum percentage of smectite at which this mineral plays a dominant part in the electrical properties of the mixtures, as well as to determine whether this percentage corresponds to a significant change in mechanical behaviour.

Materials and methods

For characterisation of samples, the liquid limit values were determined using the NF P 94-051 standard (Association Franjaise de Normalisation (AFNOR) 1993). Grain size distribution was determined using the NF P 94-057 standard (AFNOR 1992) and total specific surface area was measured using the P94-068 standard (AFNOR 1998). X-Ray diffraction measurements were performed on powdered samples, with scans recorded at diffraction angles in the range of 2[theta] 2.8[degrees]-70[degrees] on a Siemens D5000 diffractometer (Ni-filtered Cu K[alpha], [lambda] = 0.15418nm, 40 kV, 20 mA).

Oedometric tests

Mechanical tests were performed on mixtures containing different percentages of smectite and kaolin. Slurries of the mixtures were prepared at a moisture content equal to 1.5-fold their liquid limit. Then, samples were subjected to a step-by-step increase in vertical stress, each load being double that of the previous load. Deformation of samples was measured as a function of time under each mechanical stress until stabilisation.

Electrokinetic tests

Electrokinetic tests were performed on mixtures of kaolin and smectite. The samples were first prepared at water contents equal to 1.5-fold their liquid limit and then consolidated under 25 kPa vertical stress until stabilisation of the vertical strain. The electrokinetic device is shown in Fig. 1. The anode and cathode are placed in constant level reservoirs, with flat graphite electrodes connected to a direct current (DC) power supply (U= 20 V). Two steel electrodes (E3 and E4) are introduced in the sample to measure the potential difference in the soil during the tests. Electro-osmotic flow was measured by the cathodic reservoir outflow. The pH in the anodic and cathodic reservoirs was maintained acidic (pH = 3) by circulating a citric acid solution using peristaltic pumps. The electrokinetic tests were performed until stabilisation of the parameters. The duration of the tests depended on the composition of the mixtures. A Mettler Toledo pH meter equipped with a KC1 glass electrode was used for pH measurements. During the tests, the potential difference, the current intensity, the pH and the electro-osmotic flow were recorded as a function of time. The evolution of electrical conductivity during the tests was calculated using the current intensity and the measured potential difference values.

At the end of the tests, samples were removed from the electrokinetic cell and cut into five slices. The NF ISO 10390 procedure (AFNOR 2002) was used to measure soil pH, whereas electrical conductivity was measured using the NF P 94-050 standard (AFNOR 1995). These parameters are presented as a function of the normalised distance from the anode, which was obtained by dividing the distance by the length of the specimen.

Results

Characterisation of samples

The study was performed on two basic clays and on various mixtures made by mixing different percentages of these clays. The X-ray patterns of the two basic clays are shown in Fig. 2a, b. The first material, called 'kaolin', contains mainly kaolinite ([d.sub.001] = 0.7 nm, d is the interlayer spacing) and, at a smaller percentage, illite ([d.sub.001] = 1 nm) and quartz ([d.sub.001] =0.34nm). The X-ray pattern of the second clay sample shows a (001) reflection located at 1.5 nm, corresponding to calcic smectite. The specific surface area (SSa) of the kaolin is 15 [m.sup.2] [g.sup.-1], whereas the SSa of the smectite is 750 [m.sup.2] [g.sup.-1]. In the kaolin, 100% of grains have a diameter <80 [micro]m and 60% have a diameter <2 [micro]m. In the smectite, 100% of grains are <80 [micro]m in diameter and 40% are <2 [micro]m in diameter. The liquid limits of the two base kaolin and smectite materials are 40% and 170% respectively.

In addition to the two original materials, five mixtures were prepared with 5%, 10%, 25%, 50% and 95% (w/w) smectite. These mixtures are designated according to the percentage of smectite (e.g. 'S25' for the mixture with 25% smectite). The evolution of the liquid and plastic limits, as well as the water content, before the electrokinetic tests is presented as a function of smectite content in Fig. 3a. The tests show a non-linear increase in the liquid and plastic limits and in the initial water content (after consolidation under 25 kPa) with the amount of smectite.

Mechanical and hydraulic tests

The evolution of the void ratio of the slurries, consolidated under a vertical stress of 25 kPa, during the oedometric tests is shown in Fig. 3b as a function of the logarithm of the vertical effective stress, log [[sigma].sub.v]. The slopes of the curves called compression indices ([C.sub.c]s) are compared. The results show that, when the smectite percentage is <25%, the curves are the usual straight lines with a single slope [C.sub.c], characteristic of the 'Normally Consolidated' behaviour. In contrast, samples with a high amount of smectite exhibit bilinear curves with two slopes, [C.sub.c1] and [C.sub.c2]. The compression indices [C.sub.c1] and [C.sub.c2] correspond to the small stresses and high stresses loading domains respectively. The values of the compression indices as a function of smectite content are given in Table 1. These results arc consistent with those of Hammad (2010) and Hattab et al. (2015), with a transition between the two types of behaviour located between 25% and 50%, compared with the value of 35% indicated by Hattab et al. (2015).

The hydraulic permeability of the kaolin (SO) deduced from the oedometric tests is equal to 8 x [10.sup.-9] [ms.sup.-1], whereas that of the smectite (S100) is 5 x [10.sup.-12] [ms.sup.-1] . The hydraulic permeability values for the different mixtures are shown in Fig. 4. As expected, the results show a large decrease in hydraulic permeability with increases in smectite percentage.

Electrokinetic studies

Electrical conductivity

The evolution of the normalised electrical conductivity of the mixtures as a function of time is shown in Fig. 5. The normalised electrical conductivity is defined as the ratio of the electrical conductivity to its initial value before the application of the electric field. The results show that, for the sample of kaolin (SO), normalised electrical conductivity increased as a function of time. This increase is explained by the diffusion into the clay structure of [H.sup.+] cations resulting from the oxydoreduction reactions near the anode. It is also known that the oxydo-reduction reactions produce O[H.sup.-] anions at the cathode. Because the mobility of [H.sup.+] cations is higher than that of O[H.sup.-] anions because of their small size, the electrokinetic reactions are dominated by the effects of [H.sup.+]. Samples S5 and S10 also showed an increase in normalised electrical conductivity, with final values lower than that of the kaolin. For these samples (SO, S5 and S10), the slope of the curves decreases as the percentage of smectite increases. However, in the case of samples with [greater than or equal to] 50% smectite, the behaviour is different. These samples feature a decrease in normalised electrical conductivity followed by a slight increase, with a final value lower than the initial value. In the case of these samples (S50-S100), this decrease seems to be independent of the amount of smectite because the curves are nearly superimposed. The sample with 25% smectite (S25) exhibits intermediate behaviour, with a decrease in normalised electrical conductivity at the beginning of the test but a final value that is slightly higher than the initial value. In the case of samples containing <25% smectite, the increase in normalised electrical conductivity is due to the diffusion of [H.sup.+] cations and their fixation on the edge of the kaolinite particles. When the amount of smectite exceeds 25%, the changes observed in the normalised electrical conductivity are due to the slow diffusion of the [H.sup.+] into the structure with the increase in the amount of smectite. Conversely, the decrease in electrical conductivity for the samples S50, S95 and SI00 is due to the replacement of the interlayer cations present in the smectite, which are usually divalent cations, by [H.sup.+] protons, which are monovalent cations.

Electro-osmotic flow

The evolution of the volume of solution getting out of the specimen under the effect of the electric field is shown in Fig. 6 as a function of time for all the materials. In these tests, the solution flows from the anode to the cathode, suggesting that the pH of the samples is higher than the pH of the zero point charge (ZPC; Reddy et al. 2002). The results show that the electro-osmotic volume decreases when the amount of smectite increases from 0% (kaolin) to 100% (smectite). For the kaolin (SO) and the S5 and S10 mixtures, the slopes of the curves (i.e. the electro-osmotic flows) become the same after ~100 h. In contrast, for samples containing [greater than or equal to] 25% smectite (i.e. Samples S25, S50, S95 and SI00), a longer time is needed to reach a constant flow, with similar final values that are significantly lower than those of samples with <25% smectite.

The results show that the electro-osmotic permeability (i.e. the electro-osmotic flow divided by the electric field) of the kaolin is higher than that of the smectite (1.04 x [10.sup.-9] vs 3.64 x [10.sup.-10] [m.sup.2] [s.sup.-1] [V.sup.-1], respectively; Fig. 4). In the case of the mixtures, the evolution of the electro-osmotic permeability follows the same trend as that seen for hydraulic permeability. For smectite contents up to 25%, the electro-osmotic permeability decreases when the smectite percentage increases, to reach a quasi-constant value close to that of the smectite when the amount of smectite is [greater than or equal to] 50%. These results suggest that the electro-osmotic permeability depends on the hydraulic permeability and on the composition of the samples, but with a much smaller variation range: when the hydraulic permeability varies from 8000 to 5 x [10.sup.-12] m [s.sup.-1], the change in electro-osmotic permeability is only from 10 to 3.5 x [10.sup.-10] [m.sup.2] [s.sup.-1] [V.sup.-1].

Values of pH, electrical conductivity and water content before electrokinetic tests

The pH values and electrical conductivities of the different materials before the electrokinetic treatment are shown in Fig. 7. The results show that Samples SO and S5 have low electrical conductivity values. Conductivity increases with the smectite percentage up to 25%. For smectite percentages [greater than or equal to] 50%, electrical conductivity decreases to a value slightly higher than the initial value. The evolution of pH is nearly symmetrical to that of the electrical conductivity, with a minimum for approximately 25% smectite. The nonmonotonic change in the behaviour of the materials is the result of two opposite phenomena: (1) the increase in electrical conductivity induced by an increase in the smectite content and the related increase in cation exchange capacity; and (2) the decrease in electrical conductivity as a result of the increase in water content and porosity observed when the smectite percentage increases (Fig. 3a), as shown, for example, by Kaufhold et al. (2015). In the case of Samples SO, S5 and S10, and up to 25% smectite, the first phenomenon is dominant and the electrical conductivity increases due to the increase in smectite content and the corresponding increase in exchange capacity. The increase in the quantity of cation is related to the decrease in pH values. In contrast, for smectite contents strictly higher than 25%, the second phenomenon becomes dominant and the electrical conductivity decreases with the increase in water content. It has been suggested that at low water contents, corresponding in the present study to Samples SO-S25, the cations and water from the diffuse double layer are adsorbed on the clay particles (Mojid and Cho 2006; Tabbagh and Cosenza 2007; Kaufhold et al. 2015). In this case, the cations cover the particles, consequently increasing their electrical conductivity. The maximum electrical conductivity corresponds to the full development of the diffuse double layer. The particles come into contact through their diffuse double layer, which results in an increase in electrical conductivity (Fukue et al. 1999; Mojid et al. 2003). The increase in water content, corresponding in the present study to Samples S50, S95 and SI00, results in a progressive dissociation of the diffuse double layers and a decrease in electrical conductivity, associated with an increase in pH.

Values of pH and electrical conductivity after electrokinetic tests

The evolution of the pH and electrical conductivity of the samples after the electrokinetic tests are shown in Fig. 8 as a function of the normalised distance to the anode. The results show that, after the electrokinetic tests, the electrical conductivity is constant for samples containing a small amount of smectite (SO, S5 and S10), and the pH is low. These results suggest that the sites are totally occupied by [H.sup.+] cations and that the acidification of the sample is total. Sample S25 features higher pH values. These values are not constant along the samples. As the smectite content increases, in Samples S50, S95 and S100, the pH increases. The evolution of pH is not homogeneous along the sample, and the sample is more acidic near the anode. For these smectite contents, the electrical conductivity is higher near the anode than near the cathode. These results indicate that acidification of the samples is not complete in the case of samples containing a high amount of smectite. Indeed, because the cation exchange capacity (CEC) and the SSa of the smectite are important, all the cations are not totally exchanged with [H.sup.+], and therefore a longer time is needed to fill all the sites with [H.sup.+] cations. This behaviour also explains the lower values of pH near the anode compared with those near the cathode.

Conclusion

The findings of the present study show that:

* The Atterberg limits and the water content continuously increase with the amount of smectite. For samples containing a small percentage of smectite, the curves are straight lines in the [log [[sigma].sub.v],e] plane, with a slope [C.sub.c1]. When the amount of smectite is high, the curves becomes bilinear, with two slopes: [C.sub.c1] in the small stresses range, and [C.sub.c2] in the high stresses range. The second behaviour is characteristic of the domain in which the role of smectite becomes predominant, as shown by Hattab et al. (2015). The smectite content for which this change of behaviour is observed is situated between 25% and 50%.

* The electrokinetic results highlight an increase in normalised electrical conductivity as a function of time when the amount of smectite is lower than 25%. This increase is due to the diffusion of [H.sup.+] cations into the structure. The tendency is progressively reversed as the amount of smectite increases. When the amount of smectite is [greater than or equal to] 50%, the decrease in normalised electrical conductivity is due to the replacement of the divalent cations by [H.sup.+] cations. Indeed, the electrical conductivity is related to both the concentration of cations and their valence.

* The evolution of the electro-osmotic flow as a function of time is also affected by the amount of smectite. The increase in the amount of smectite results in a decrease in the electro-osmotic flow, compared with the electro-osmotic flow corresponding to the kaolin (SO). The electro-osmotic flow values remain close to that of the kaolin for samples containing <25% smectite. The hydraulic and electro-osmotic permeabilities decrease when the amount of smectite increases. This result highlights the dependence between both permeabilities; however, the variation range of the electro-osmotic permeability is much smaller than that of the hydraulic permeability: when the hydraulic permeability varies from 8000 to 5 x [10.sup.-12] m [s.sup.-1], the change in electro-osmotic permeability is only in the range from 10 to 3.5 x [10.sup.-10] [m.sup.2] [V.sup.-1] [s.sup.-1]. The decrease in electroosmotic flow in the presence of smectite is related to the low permeability and diffusivity of the smectite, which slow the displacement of ions.

* The initial bulk electrical conductivity of the mixtures is low when the amount of smectite is low. This is due to the strong presence of kaolinite, characterised by its low CEC. The increase in the electrical conductivity for higher amounts of smectite is explained by the fact that the particles come in contact with each other through their double layers, as explained by Kaufhold et al. (2015). When the amount of smectite is very high, electrical conductivity decreases. This is due to the dissociation of the diffuse double layer with the increase in water content. The evolution of pH is in agreement with the evolution of electrical conductivity.

The measurements performed after the electrokinetic tests show that when the amount of smectite is <25%, the pH values decrease after the test compared with initial values, whereas electrical conductivity values increase. The results show that the pH and electrical conductivity arc homogeneous in the totality of samples; this is due to the low SSa of the samples and to the small thickness of the diffuse double layers. The decrease in pH is due to the diffusion of the [H.sup.+] cations into the structure under the effect of the electrical field. The homogeneity of the pH and of the conductivity along the samples suggests that the [H.sup.+] cations are well diffused into the structure and occupy the accessible sites. When the amount of smectite becomes higher than 25%, the pH of the samples is more acidic near the anode; the pH is no longer homogeneous along the sample. Electrical conductivity is high near the anode and low near the cathode. The homogeneity of pH for smectite amounts <25% is at the origin of the fast stabilisation of the electro-osmotic flow compared with samples with higher amounts of smectite. The homogeneity of pH is due the high diffusivity of the samples when the amount of smectite is low. For higher amounts of smectite, the low diffusivity of the sample results in the accumulation of [H.sup.+] cations near the anode, leading to a decrease in pH in this part of the sample.

The results of the mechanical tests, as well as those of the electrokinetic tests, show that the behaviour of the kaolinite-smectite mixtures is highly affected by the amount of smectite. All aspects of the behaviour of the mixtures are dominated by the smectite when the smectite content is higher than a critical percentage, which, in our tests, is between 25% and 50%, whereas Hattab et al. (2015) suggest that this percentage could be close to 35%. Below the critical percentage, the behaviour of mixtures follows a typical mixture law. As the amount of smectite becomes higher than this critical percentage, the electrical conductivity and electro-osmotic permeability values become close to those of the smectite. The behaviour of the materials was studied in the presence of water; it would be interesting in a future study to determine what happens in the presence of solutions of pollutants, like metallic cations.

http://dx.doi.org/10.1071/SR16267

Received 6 October 2016, accepted 4 April 2017, published online 4 May 2017

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M. Ben Salah (A,B), H. Souli (A), P. Dubujet (A), M. Hattab (C), and M. Trabelsi Ayadi (B)

(A) Universite de Lyon, Ecole Nationale d'Ingenieurs de Saint Etienne (ENISE), Laboratoire de Tribologie et Dynamique des Systemes (LTDS), UM5513, 58 rue Jean Parot, 42023 Saint-Etienne, France.

(B) Laboratoire d'Application de la Chimie aux Ressources et Substances Naturelles et a I'Environnement (LACReSNE), Faculte des Sciences de Bizerte, 7021 Zarzouna, Bizerte, Tunisia.

(C) Universite de Lorraine, Laboratoire d'Etude des Microstructures et de Mecanique des Materiaux (LEM3), UMR CNRS 7239, He du Saulcy, Metz Cedex 1, France.

(D) Corresponding author. Email: hanene.souli@enise.fr

Caption: Fig. 1. Electrokinetic experimental device.

Caption: Fig. 2. X-Ray diffraction patterns of (a) kaolin and (h) smectite.

Caption: Fig. 3. Evolution of the (a) liquid limit ([w.sub.L]), plastic limit ([w.sub.P]) and initial water content as a function of the smectite percentage and (h) the void ratio as a function of the vertical stress in oedometer tests. SO, S10, S25, S50, SI00, mixtures with smectite content of 0%, 10%, 25%, 50% and 100% respectively; [C.sub.c1], [C.sub.c2], slopes of the curves.

Caption: Fig. 4. Evolution of the hydraulic and electro-osmotic permeability of the mixtures as a function of the smectite percentage.

Caption: Fig. 5. Evolution of normalised electrical conductivity as a function of time. S0, S5, S10, S25, S50, S95, SI00, mixtures with smectite content of 0%, 5%, 10%, 25%, 50%, 95% and 100% respectively.

Caption: Fig. 6. Evolution of the electro-osmotic volume getting out of the specimen as a function of time. S0, S5, S10, S25, S50, S90, SI00, mixtures with smectite content of 0%, 5%, 10%, 25%, 50%, 90% and 100% respectively.

Caption: Fig. 7. Evolution of electrical conductivity and pH as a function of the percentage of smectite.

Caption: Fig. 8. Evolution of (a) pH and (b) electrical conductivity as a function of the normalised distance from the anode, obtained by dividing the distance by the length of the specimen, after the electrokinetic tests. SO, S5, S10, S25, S50, S95, S100, mixtures with smectite content of 0%, 5%, 10%, 25%, 50%, 95% and 100% respectively.
Table 1. Evolution of the compression
indices of the mixtures as a function of
smectite content

              Compression index

% Smectite    [C.sub.c1]   [C.sub.c2]

0                0.24
5                0.3           -
10               0.37          -
25               0.65         0.64
50               1.3          1.21
95               1.78         1.27
100              1.81         1.32
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Author:Salah, M. Ben; Souli, H.; Dubujet, P.; Hattab, M.; Ayadi, M. Trabelsi
Publication:Soil Research
Article Type:Report
Date:Nov 1, 2017
Words:5278
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