Electrokinetic restoration of local saline soil

Materials Today: Proceedings xxx (xxxx) xxx Contents lists available at ScienceDirect Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr Electrokinetic restoration of local saline soil Faiza Klouche a,⇑, Karim Bendani a, Ahmed Benamar b, Hanifi Missoum a, Mustapha Maliki a, Nadia Laredj a a Civil Engineering and Architecture Department, Faculty of Sciences and Technology, Construction, Transport and Protection of Environment Laboratory (LCTPE), University Abdelhamid Ibn Badis of Mostaganem, 27000 Mostaganem, Algeria b LOMC Laboratory, UMR 6294, CNRS-University of Havre, France a r t i c l e i n f o Article history: Received 22 June 2019 Accepted 5 August 2019 Available online xxxx Keywords: Silty clay Electrokinetic Electromigration Remediation Salinity a b s t r a c t Electrokinetics is a development technology used for the removal and extraction of contaminants, such as: heavy metals, radionuclides and organic contaminants from fine-grained and low-permeability soils. The goal of electrokinetic remediation is to perform the migration of contaminants under the application of an electric field imposed on two electrodes inserted into a mass of soil. Three phenomena can occur when the soil is electrically charged, which are electro-osmosis, electromigration and electrophoresis. In this paper, an experimental investigation was conducted to study the effect of electrokinetic treatment on the mobility, extraction and transfer of ionic species in a saline soil collected from Ain Nouissy town near Mostaganem city in Algeria. The soil under consideration is a silty clay with a moderate plasticity. A series of electrokinetic tests was carried out on an experimental cell developed at the LCTPE laboratory (University of Mostaganem-Algeria), where electric current, effluent flow, pH and water content were measured along the sample at different applied voltages. The results obtained during the tests have shown that the electro-osmotic flow increases and the electric current decreases during the treatment, while a significant reduction in the water content is clearly noticed, i.e. dry zone at the anode and wet one at the cathode. With regard to the electromigration, removal of solutes from the soil, sodium was better recovered than calcium. The results have shown that the electrokinetic process is a quick and easy solution for treating saline soils. This technique is a cost-effective method for achieving sanitation objectives of sites contaminated by chemical species and protect agricultural infrastructure and soils from deleterious effects of salinity. Ó 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Congress: Applied Materials for the Environment CIMAE-2018. 1. Introduction In recent years, soil salinity has become a major soil degradation problem in the fields of Agriculture and Civil Engineering, especially in arid and semi-arid areas. For civil engineering structures, soil salinity causes adverse damage to infrastructure, including buildings, roads, bridges, steel structures and underground networks [1]. Salinity leads to an increase in osmotic pressure which makes water more difficult to mobilize, a toxicity for certain ions for plants (chloride, sodium, etc.) and a clear degradation of the soil. Among the main factors contributing to the increasing salinity are the rare rainfall, high evaporation, saline water irrigation and human intervention in cultivable practices in the near coastal, arid ⇑ Corresponding author. E-mail address: faiza.klouche@univ-mosta.dz (F. Klouche). zones and semi-arid. Saline water occupies 71% of the earth’s surface and about half of the uncultivated land in the world is under the influence of salinization. In agriculture, such degraded, lowfertility soils are generally unsuitable for agricultural production, leading to unacceptable reductions in yields [2,3]. Also, excessive accumulation of salts can cause adverse effects on human health, such as goiter, hypertension, cardiovascular diseases and various cancers [4,5]. Effective remediation of low permeability porous salt media (clays, silts) is a challenge not yet solved by practitioners. In this context, a multitude of remediation techniques, such as biological, chemical and physical treatments are often inadequate, expensive and inefficient, especially for fine-grained soils [5–8]. Electrokinetic (EK) decontamination has considerable potential for the remediation of saline soils with low permeability. This treatment technology is an innovative, sustainable and inexpensive method used for the stabilization and restoration of soils in general and fine sediments in particular [9–14]. https://doi.org/10.1016/j.matpr.2019.08.082 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Congress: Applied Materials for the Environment CIMAE-2018. Please cite this article as: F. Klouche, K. Bendani, A. Benamar et al., Electrokinetic restoration of local saline soil, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.082 2 F. Klouche et al. / Materials Today: Proceedings xxx (xxxx) xxx Thus, the electrokinetics is based on the placement of the electrodes in a mass of contaminated soil and in the application of a low electric potential, which induces several solute transport mechanisms, such as: the electro-osmosis, electrolysis and electro-migration, which allows the mobilization of salts and the facilitation of their transport and their elimination. However, electrolysis is defined as a conversion of electrical energy into chemical energy. As a result, oxidation occurs at the anode with generation of the oxygen (O2) gas and hydrogen ions (H). While at the cathode, there is a reduction, with generation of hydrogen gas (H2) and hydroxide ions (OH ) [15,16]. As for electro-osmosis, it is defined as a movement of water under the application of a potential difference. When a voltage is applied within the soil matrix, a clear electroosmotic flow occurs [15]. Moreover, electro-migration is known as the migration of ions present in the porous medium to the electrodes of opposite charge [17,18]. In this work, the effects of electrical potential and treatment time on transport and salt removal efficiency were examined. In addition, the intensity of the current, the rate of cation removal, as well as pH changes were determined. 2. Materials and experimental methods 2.1. The study area Fig. 1. Location of the Mostaganem region in Algeria. The study area is located in the north-west of Algeria, in the lower valleys of West Mostaganem, especially in the region of Ain Nouissy, whose area is 680 km2 and which hardly exceeds 105 m altitude. It is 17 km from Mostaganem, chief town of the wilaya (Figs. 1 and 2) and is characterized by a semi-arid climate, according to the classification of Koppen-Geiger (BSk-Csa) [19] with cold-dry winter and warm-dry summer. Soil salinity problems in this region have become more serious over the last twenty years; this is due to the low rate of precipitation, which causes salts to accumulate on the surface of the soil at an alarming rate. The study area has low vegetation on one hand and poor crop yields on the other, making agricultural and construction activities very difficult. 2.2. Soil properties Fig. 3 shows the local saline soil from the Ain Nouissy – Mostaganem area used in this study. The physical properties were obtained from tests carried out in accordance with the French standards. The chemical properties were obtained by X-ray fluorescence. All these properties are summarized in Tables 1 and 2. The soil under consideration is a silty clay with a moderate plasticity. 2.3. Experimental cell Fig. 2. Satellite photo of the Ain Nouissy study area (Mostaganem). The electrokinetic experiments were conducted in a parallelepiped shaped cell with 8 mm thick glass walls and whose Fig. 3. Saline soil of Ain Nouissy region (Mostaganem). Please cite this article as: F. Klouche, K. Bendani, A. Benamar et al., Electrokinetic restoration of local saline soil, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.082 F. Klouche et al. / Materials Today: Proceedings xxx (xxxx) xxx 3 2.4. Experimental procedure Table 1 Physico-chemical properties of the soil. Properties Values Physical properties pH Specific gravity Liquid limit Plastic limit 8.5 2.65% 42.8% 22.1% Chemical composition SiO2 Al2O3 Fe2O3 46.71% 11.09% 4.73% Table 2 Initial ions concentrations of soil specimen. Ions Initial concentration (mg/kg) Naþ Ca2þ Kþ Mg 2þ 1917.2 1165 136 94 dimensions are as follows: length 30 cm; width 10 cm and height 10 cm, comprising three compartments corresponding to: the anode, the soil sample and the cathode (Fig. 4). The tanks are made of glass because it is a chemically and electrically inert material to minimize soil reactions. A quantity of 2600 g of dry soil was used for each electrokinetic experiment. A homogeneous and smooth paste of our soil was poured into the central reservoir of the experimental cell in three successive layers where each layer was subjected to vibrations in order to eliminate the air voids formed during the placement of the soil. Paper filters [Whatman # 4] were placed on each side of the cell to prevent the movement of soil particles to the anode and cathode compartments. Two cylindrical copper tubes were introduced into the cell at a distance of 2 cm from the ends of the transverse walls, to serve as electrodes. The mass of the fine soil is subjected to a potential gradient of 1.5 V/cm for a duration of 5, 8, 10 and 15 days (Fig. 5). The variation of soil pH, the intensity of the electric current and the rate of extraction of the ions of salts was measured during and at the end of the EK test. The variation of electrical intensity was controlled using a data acquisition card. The cumulative electroosmotic flow was measured by means of a graduated column. And, the concentration of cations, such as sodium and calcium was evaluated using a flame spectrophotometer. At the end of test, the soil sample was subdivided into equal sections to measure the electrical conductivity and pH and this was done by using a multiple parameter Hach pH 156/conductivity/dissolved oxygen apparatus. 3. Results and discussion 3.1. Electrical current The intensity of the electric current is the main parameter studied, because it led to the electrokinetic process, then its efficiency. The variation of the intensity of the electric current as a function of the duration of the treatment is illustrated in Fig. 6. The electric current has shown the same trend. It increased to a maximum value, then declined sharply and finally remained at a constant value. Indeed, when the voltage gradient was established, the electric current in the soil cell was low because it took a while for the electrolyte to enter a contaminated soil and for the contaminants and minerals to dissolve and desorb from the soil surface. 3.2. pH variation Fig. 4. Diagram of the electrokinetic cell. The pH distribution of the soil sections studied at the end of the electrokinetic tests is illustrated in Fig. 7. Electrolysis produces hydrogen and hydroxide ions respectively at the anode and at the cathode. As a result, the pH increased around the cathode Fig. 5. Experimental device of the electrokinetic process. Please cite this article as: F. Klouche, K. Bendani, A. Benamar et al., Electrokinetic restoration of local saline soil, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.082 4 F. Klouche et al. / Materials Today: Proceedings xxx (xxxx) xxx and acidification occurred around the anodic region. As such, there was a large pH variation with the operating time, due to the different applied periods of the current electric. The latter, penetrating the soil, was strongly dependent on the electrical conductivity (EC), which changed with the ionic concentration in the interstitial water. For this, the current increased in proportion to the concentration of the existing ions. The strongly alkaline environment in the cathodic zones causes precipitation of ionic compounds in the soil, resulting in a decrease in the intensity of the electric current [5,9]. 3.3. Evolution of salt removal with treatment time During the electrokinetic treatment, the electric current favors the desorption and the mobilization of the salts, facilitating their elimination. This is ensured by two main mechanisms, such as electromigration and electro-osmosis. Electromigration results in the migration and movement of ionic compounds, such as cationic salts through the cathode, while the electroosmosis mechanism is associated with the flow of water, allowing for the drawing and transport of these solutes from anode to the cathode. From the results of Table 3, it can be seen that by increasing the electrokinetic treatment time in the range of 5–15 days, better ion extraction is obtained. Also, divalent ions need more energy to be removed compared to monovalent ions. Monovalent cations have a lower atomic size than divalent cations, which explains the high ionic mobility in aqueous solution and the strong extraction of sodium from calcium cations. In our tests, calcium decreased near the anode and accumulated near the cathode (Fig. 8). The acid zone was generated near the anode due to an electrolysis reaction and it is possible that the desorption of calcium at the surface of our soil was improved near the anode. The desorbed calcium in the interstitial water migrated and accumulated near the cathode. In the case of sodium (Fig. 8), it was desorbed from the soil surface by the ion exchange reaction between adsorbed sodium on the soil surface and hydrogen ions transported by electromigration. The elimination of these ions leads to a significant reduction of soil salinity, which has reduced the harmful risks of this phenomenon. Therefore, a positive influence on the management of the works, both in agriculture and in the public works and civil engineering sectors can be obtained. Fig. 6. Variation of the electric current with time at 30 V. Fig. 7. pH distribution of soil section after EK treatment. Table 3 Ion extraction efficiency rate during time. Extraction of ions (%) Applied voltage 30 V Duration of treatment (days) Naþ Ca2þ 5 49.12 28.75 8 43.12 34.65 10 75.14 46.32 15 82.75 57.54 4. Conclusion This study presents the experimental results realized in the LCTPE research laboratory on a local soil of Ain Nouissy – Mosta- Fig. 8. Distribution of ions a) calcium b) sodium after electrokinetic treatment. Please cite this article as: F. Klouche, K. Bendani, A. Benamar et al., Electrokinetic restoration of local saline soil, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.082 F. Klouche et al. / Materials Today: Proceedings xxx (xxxx) xxx ganem. In our research, it was particularly interested to study the performance of electrokinetic treatment for the improvement and remediation of fine soils. The results were obtained on the variation of the electric current, the pH as a function of the electrokinetic treatment time and the removal rate of calcium and sodium. The proposed technique achieved removal rates of 83% and 58% for sodium and calcium ions respectively after 15 days of EK treatment. The increase of the treatment time allows a better transport of the ionic species. It is observed that removal of Na+ is more effective than Ca2+, as it is noted that a majority of Na+ ions migrated to the cathode. These Na+ ions moved easily and rapidly and most of them that migrated from the anode to the cathode occurred within the first ten days. Divalent cations such as Ca2+ appear to be less mobile than monovalent cations like Na+. This, is related to the chemical form of the calcium ions, forming complexes with anions present in the pore fluid, reducing their mobility, their solubility and preventing their elimination. This electrokinetic experiment has shown that this technique is promising and inexpensive for the treatment and restoration of finegrained saline soils. 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Please cite this article as: F. Klouche, K. Bendani, A. Benamar et al., Electrokinetic restoration of local saline soil, Materials Today: Proceedings, https://doi. org/10.1016/j.matpr.2019.08.082