Optimization of electroosmotic flow to enhance the removal of contaminants from low‑permeable soils

Electrokinetic is an effective method for the extraction of contaminants from low-permeable soils. This work examines the influence of Ca+2 ions on the electroosmotic flow and the effect of electric potential, current, and pH variations on the removal of Pb2+, Na+, and Clˉ ions from artificially contaminated soil during vertical electrokinetic experiments. The DC electric field of 1 Vcm−1 was applied across the soil specimen via steel mesh electrodes for 24, 48, and 72 h of the experiment. In this work, the vertical electrokinetic cell was used to avoid the deposition of Na+ and Pb +2 ions near the soil surface after the treatment. making it even less permeable. The results show that the formation of acidic and alkaline environments in soil specimens affects the transport of ionic species by reducing the effect of electromigration and electroosmotic water flow. The enhancement of electroosmosis using Ca+2 ions as an electrolyte increased the extraction efficiency of Pb2+ (i.e., 41%) and Na+ (i.e., 82%) ions instead of the Clˉ (i.e., 69%) ions due to the high electroosmotic flow (i.e., 81 mL) from anode to the cathode. For relatively low electroosmotic flow (i.e., 19 mL), the extraction efficiency of Clˉ ions (i.e., 76%) was higher than that of Pb2+ (i.e., 27%) and Na+ (i.e., 44%) ions. The results demonstrated that the extraction efficiency of ions and energy consumption increased with treatment time and were higher during the first 24 h of the experiments.

possible solutions. The common practice for the reclamation of contaminated soils is to apply excess water to leach salts out of the surface soil. However, salt-affected soils are usually low-permeable [4], whereby leaching water penetration is slow and inefficient. As a result, harmful salts, especially sodium salts, deposit near the soil surface, making it even less permeable [5].
In addition, the other conventional soil-reclamation methods, e.g., chemical amendment, soil washing, and bioremediation have other limitations, such as environmental concerns, costly input, and time consuming [6]. In order to overcome the limitations posed by the conventional methods, electrokinetic method was proposed for the reclamation of low hydraulic conductivity areas, particularly with high clay content [7]. In the EK method, DC electric potential is applied across the contaminated soil through electrodes, ions migrate toward the corresponding electrodes (i.e., anode and cathode) due to the electroosmosis (i.e., net flux of water toward one of the electrodes), electromigration (i.e., migration of charged ions), and electrophoresis (i.e., migration of charged particles) [6]. Electroosmosis is a key factor to improve the mechanical strength of soil. The direction of electroosmotic flow (EOF) depends on the net charge on soil surface, usually, the net charge on soil surface is negative, so EOF is directed from positive to negative electrode [8], [9]. The EOF in soils depends on zeta potential, applied electric field intensity, and pH of the soil and fluid [10].
After applying the electric field, the H + are formed in the fluid at the anode side and tightly bound with the soil particles to maintain the electric-neutrality region. This region is called the electrical double layer (EDL) [11], while the potential between these two regions is called zeta potential. The zeta potential depends on the thickness of EDL, the net surface charges of the particles, and pH of pore fluid [12]. Yeung and Gu found that generally, the pH of soils varies from 2 to 11, and the zeta potential of clayey soils varies from − 50 to 50 mV [13]. The EDL is divided into three layers: the inner fixed layer, the outer fixed layer, and a diffuse layer or moveable layer. The diffuse layer is responsible for the flow of water from the negative to the positive electrode by viscous force in the soil. Meanwhile, the adsorption of H + ions and other cations in negatively charged particles of soil causes a reduction in the thickness of the diffuse layer near the anode side [14]. The thin diffuse layer can affect the repulsive forces and the van der Waals attractive forces among the soil particles, which can cause the precipitation of soil particles and restrict the EOF [15]. Nevertheless, many scientists have used chemical solutions, such as CaSO 4 , Ca(OH) 2 , CaCl 2 , Al 2 (SO 4 ) 3 , and ZnSO 4 to improve the hydraulic permeability and EOF in the low-permeable soil over the last few decades. Under the DC electric field strength, these chemicals cause cation-exchange reactions between chemical solutions and soil-charged particles [16].
However, the significant drawbacks of EK are the electrode corrosion, dehydration, thermal effects, and focusing effects (which occur in soil, where acidic (H + ions) and alkaline (OHˉ ions) fronts collide with each other and cause the accumulation of metal ions in the soil and restrict their emissions) [17]. In particular, the focusing effects are the main thing that makes it hard to remove heavy metals over a long period of time [18]. In the last few years, many different buffer solutions have been tried to overcome the acidic and alkaline front collision (focusing effects) by many researchers during the EK process. To control the alkaline front on the cathode side, the acidic materials such as citric, acetic, and nitric acid can be used, while alkaline materials such as calcium hydroxide and calcium carbonate can be used to control the acidic front on the anode side [19]. In practical conditions, calcium ions can be added to the irrigation system to remove sodium and lead ions from the root zone and improve the permeability of low-permeable soils as well as reduce the salinity level on the soil surface.
The aim of this manuscript is to examine the effect of CaCl 2 on enhancing the EOF in low-permeable soil for the removal of Na + , Clˉ, Ca 2+ , and Pb 2+ ions from artificially contaminated soil. In this work, to prevent the dryness of the soil specimen, a wet sponge was placed on the soil surface, which was connected to the electrolyte reservoir (i.e., filled with the solution of 1 L distilled water and 5.5 g CaCl 2 ) as shown in Fig. 1. In this study, the vertical electrokinetic cell was used for to remove the Na + ions that deposit near the soil surface, making it even less permeable, and to reduce the electrical conductivity of the soil to less than 2 dS m −1 (which is a non-toxic and safe limit for the plant's development). Also, the effect of Ca 2+ ions and treatment time (i.e., 24, 48, and 72 h) on pH change of soil specimens (measured by pH strips after electrokinetic experiments), electric current (measured by digital multimeter every 15 min during experiments), and voltage distribution (measured by data logger (i.e., Labjack U12) every hour during experiments) were measured.

Experimental soil
The soil specimens used in the experiments was collected from the research field of the University of Agriculture Faisalabad (31° 26′ 2.18″ N, 73° 3′ 53.6″ E). The soil was air dried at room temperature (298 K) and then mechanically fine powdered with pestle and mortar and mixed thoroughly after being sieved through a mesh (2 mm). Some physicochemical properties of soil specimens are given in Table 1. The initial extractable cations (i.e., Na + and Ca +2 ) concentration was measured using the 10 g soil with 50 ml of 1 N NH 4 OAc solution and stirred for 30 min at 300 rpm, and then the colloidal solution was filtered using a Whatman No.1 filter paper [20]. The concentration of extractable Na + ions was determined by a flame photometer (Sherwood-360), while Ca +2 ions were determined by ethylenediaminetetraacetic acid disodium (EDTA) titration [21]. Therefore, to determine the initial concentration of Clˉ ions and electrical conductivity (EC) of soil, we used the [1:2] soil and distilled water ratio and filtered the colloidal solution with Whatman No.42 filter paper after 2 h of shaking with digital mechanical shaker (RJH-5005). The Clˉ concentration was determined by silver nitrate titration [22] and EC was determined by conductivity meter (HANNA, HI99300), whereas we used the [1:1] ratio of soil and distilled water to determine the pH of soil by pH digital meter (HANNA, HI98128). To extract the concentration of Pb 2+ and Fe 2+ ions from soil, block-digestion method was performed using one gram soil specimen and concentrated HClO 4 -HF-HCl with the ratio of 4-20-8 mL, respectively, and the concentration of lead and iron in soil specimen was measured by atomic absorption spectroscopy [20]. The initial ionic concentration, pH, and EC of soil are given in Table 1.
The grain size distribution was determined by hydrometer method [23] while the saturated hydraulic conductivity or permeability coefficient of soil was determined by the Darcy's law (i.e., constant head method) by Eq. (1) [24] [25].
where "K" is the hydraulic conductivity of the soil [cm s −1 ], "Q" is the total discharge volume of water through the soil per unit time [cm 3 A" is the area of the soil column [cm 2 ], "∆H" is the difference in height between the hydraulic head and the soil column [cm], "H" is the height of the hydraulic head [cm], and "L" is the length of the soil column [cm]. The hydraulic flux (Ѵ) was determined according to the following equation [V = Ki] , where "K" is the hydraulic conductivity of the soil [cm s −1 ], and "i" is the hydraulic gradient (-).
However, we used the 10 g of dry soil in a laboratory furnace (F30420C) at 823 K temperature for 3 h to determine the organic matter of the soil specimen by the loss-onignition method using Eq. (2) [26].
where "W i " is the initial dry weight of soil specimen at room temperature [g] and "W f " is the final dry weight of soil specimen at 823 K [g]. The mean value of bulk density (g cm −3 ) and particle density (g cm −3 ) of soil was determined by core method and graduated cylinder method, respectively, whereas the soil porosity (%) was determined using Eq. (3) [20].
The mean value of soil porosity and standard deviation of pore size are given in Table 1.

Electrokinetic cell and specimen preparation
The schematic vertical diagram of an electrokinetic (EK) cell for the reclamation of contaminated soil is shown in Fig. 1. The EK setup consisted of the soil column being made of PVC (polyvinyl chloride) pipe, and the length was 9 cm while the diameter was 5.8 cm. The stainless steel mesh electrodes (i.e., anode and cathode) were connected Soil porosity (%) = Particle density − Bulk density Particle density × 100 on the top and bottom of the soil specimen to provide an electric field of 1 Vcm −1 across the artificially contaminated soil specimen. However, the anode side was connected to the electrolyte by a wet sponge to keep the soil wet, and filter paper was inserted between the two ends of the soil column to prevent the loss of wet soil particles. To achieve the goal of this study, two vertical electrokinetic (VEK) experiments (i.e., VEK-1 and VEK-2) were performed with three different treatment times, i.e., 24, 48, and 72 h. In each vertical soil column, 370 g of artificially contaminated soil with 34% moisture was used to remove the contamination. To prepare the contaminated soil, 7.6 g kg −1 of NaCl and 1.342 g kg −1 of lead chloride were mixed with the soil for each of the VEK experiments. In VEK-1, one liter of distilled water was added as an electrolyte in an anode-side beaker that was connected to the soil surface via a wet sponge. While in VEK-2, 5.5 g of CaCl 2 in one liter of distilled water was added as an electrolyte in the anodeside beaker. After applying the electric field of 1 Vcm −1 across each soil column, the variation in electric current was measured by the multimeter (UT61A UNI-T) every 15 min, periodically, up to the completion of the experiments. To measure the distribution of electric potential in a soil specimen, a data logger device (Lab Jack U12) was connected to the platinum wires in a parallel configuration. The platinum wires were inserted into the specimen every 1 cm to measure the electric potential variation. During both VEK experiments, the electroosmotic flow of water moved from the top to the bottom of the vertical soil column due to the effects of gravity and the applied electric potential. This water was collected in a beaker placed below the cathodw side of the vertical electrokinetic cell.

Analytical methods
After completion of the VEK experiments, the soil specimens were equally divided into nine segments, and then the segments were kept in a laboratory dry oven (DHG 9030A) at 378 ± 5 K for 5 h. To determine the Na + and Ca +2 concentrations, 10 g of soil was mixed with 50 ml of 1 N NH 4 OAc solution and stirred with a magnetic stirrer for 30 min at 300 rpm, and the colloidal solution was filtered using a Whatman No.1 filter paper. The concentration of extractable Na + ions was determined by a flame photometer (Sherwood-360), whereas the concentration of Ca +2 ions was determined by EDTA titration. Therefore, to determine the concentration of soluble Clˉ ions and electrical conductivity (EC) of soil, the ratio of soil to distilled water was kept [1:2]. Thereafter, the colloidal solution was filtered with Whatman No. 42 filter paper after shaking with a digital mechanical shaker (RJH-5005) for 2 h. The Clˉ ions concentration was determined by the silver nitrate titration method, and EC was determined by a conductivity meter (HANNA, HI99300). The pH of the soil specimen was determined by pH strips after VEK experiments. To extract Pb 2+ and Fe 2+ ions from the soil specimens, a block-digestion method was used where one gram of soil specimen was mixed with the concentrated HClO 4 -HF-HCl at a ratio of 4-20-8 mL, and the concentration of lead and iron in the soil specimen was measured using atomic absorption spectroscopy (AAS). All the measurements were repeated three times, and only the average values are presented over here.

Results and discussion
In this part of the manuscript, the effect of calcium ions on the distribution of electric potential, current, and pH variation during vertical electrokinetic (VEK) experiments was studied. To determine the effect of treatment time on the removal of ionic species from artificially contaminated soil specimens, two VEK experiments (i.e., VEK-1 and VEK-2) were performed for three different treatment times (i.e., 24, 48, and 72 h). In each of the VEK experiments, a DC electric field of 1Vcm −1 was applied across the soil specimens.

Electric potential, current, and pH of soil
The variation in electric potential profiles VEK-1 under an applied electric potential of 9 V across the soil specimen is shown in Fig. 2a. In the case of the VEK-1, distilled water was used as electrolyte at anode-side reservoir. At the beginning of the experiment, a linear distribution of the electrical potential profile across the soil specimen is observed, and this linearity is sustained for up to 10 h of the experiment. This linear electric potential profile shows that the moisture and ionic species in the soil sample were at first spread out evenly [27]. During VEK-1, the initial value of electric current was 131.3 mA, as shown in Fig. 2c, a comparatively low current intensity flowing through the soil specimen. It represents the low ionic concentration between the soil and electrode interfaces, which causes a potential drop at the anode and cathode electrode-soil interfaces as shown in the zero-hour profile of Fig. 2a. Almost 2 h after the start of the experiment, the potential drop at the electrode-soil interface decreases due to the migration of ionic species, and the maximum current passes through the soil. However, after 10 h of the experiment, the electric potential variation initiates to deviate from its linearity. While the current also decreases and continuously decreases up to the completion of the experiment (i.e., 72 h), this is attributed to the development of a transition region (where acidic (i.e., H + ions) and alkaline (part from the anode-side is observed as shi.e., OHˉ ions) fronts meet with each other in the soil specimen is called the focusing effect). At 14 h, a sharp drop of electric potential in the soil specimen at almost 1/3 own in Fig. 2c, whereas maximum current flows through the soil specimen during the 14 h of the experiment (Fig. 2c), indicating maximum ionic species transport via electromigration and electroosmosis. The development of a sharp electric potential gradient in the soil specimens due to collision of H + and OHˉ fronts causes the accumulation of metal ions in soil and restricts their transport. The development of H + and OHˉ front positions was tracked by pH strips and is shown in Fig. 2e. A similar trend in electric potential was observed by [28][22] [29].
In the case of VEK-2, CaCl 2 (i.e., 5.5 g L −1 ) was used at an anode-side electrolyte reservoir to enhance the process of electromigration and electroosmosis. Calcium ions are commonly used in low-permeable soils to improve hydraulic permeability and cation-exchange reactions [21]. However, the deviation in electric potential profile from its linearity and the variation in the profile of electric current with respect to time is shown in Fig. 2b, d. It can be seen that the potential variation in soil specimens in Fig. 2b shows a different trend than that was observed for the VEK-1 (i.e., Fig. 2a) when only distilled water was used in the anode-side reservoir. The linearity of electric potential is sustained only up to 6 h, and it deviates 4 h early as compared to the VEK-1 profile (Fig. 2a). The development of a very small transition zoon (where acidic and alkaline fronts meet with each other) occurred almost within 10 h into the soil specimen in VEK-2. It is attributed to the high production rate of Ca(OH) 2 and high buffering capacity of soil against acidic environment. During the first 6 h of the experiment, maximum current flows through the soil specimen (Fig. 2d), which shows the maximum ionic species removed from the soil specimen by electromigration and an initially high electroosmotic flow rate as shown in Fig. 3a. The initial high value of electric current intensity (i.e., 174.1 mA) shows the presence of the calcium and chloride ions, which are already present in the anode-side reservoir. For this reason, the same applied electric field of 1 Vcm −1 across the soil specimens resulted in different current intensities for VEK-1 and VEK-2.
During VEK-1 and VEK-2, the acidic and alkaline front positions in the soil specimen during different treatment times are shown in Fig. 2e, f. It can be seen clearly in Fig. 2a, e that the position of sharp electric potential drop and the position of acidic and alkaline fronts were sustained at the 1/3 part of the soil specimen from the anode side for up to 14 to 72 h. But in the case of VEK-2 (i.e., Fig. 2b, f), the position of the sharp transition zone of electric potential and the position of acidic and alkaline environments are sustained at 1/3 part of the soil specimen from the anode side for only 10 to 16 h. Thereafter, the alkaline front moves toward the anode-side, i.e., against the direction of motion of H + ions and the gravity. The alkaline front meets the anode-side electrode completely after 54 h, which is attributed to the soil specimen's high buffering capacity against the acidic environment, the soil specimen's initial high pH value (i.e., pH = 8.3), and the formation of Ca(OH) 2 and NaOH compounds at the cathode side of the soil specimen. Meanwhile, when the alkaline front meets the anode-side electrode, the potential profile was linear for 54 to 72 h, as shown in Fig. 2b. Since the pH changes in soil specimens are caused by the formation of H + and OHˉ ions in the vicinity of electrodes and can be tracked by the pH strips. In Fig. 2g, h, the red color indicates the acidic environment and the blue color shows the alkaline environment, while, during the experiment, both acidic and alkaline fronts move toward the opposite polarity electrodes due to the electromigration, electroosmosis, diffusion process, as well as gravity effect. The pH of the soil specimen was measured by the pH strips at 72 h after the completion of experiments, as shown in Fig. 2g, h. The collision of H + and OHˉ ions produces pure water. Hence, a sharp concentration gradient of hydrogen Fig. 3 a Electroosmotic flow rate b Cumulative flow of water for VEK-1 and VEK-2 after applying a 1 Vcm −1 electric field across the soil specimen (Note: the variation in electroosmotic permeability with respect to treatment time is shown in the inset of Fig. 3a) and hydroxide ions is observed in this part of the soil specimen. Moreover, at this stage, the diffusion process is predominant as compared to electromigration and electroosmosis. We concluded that wet sponges are unable to suppress the pH changes in soil specimens and that calcium chloride can be used in a water reservoir on the anode side during electrokinetic soil remediation to reduce the focusing effect.

Electroosmosis
In vertical EK experiments, the electroosmotic flow (EOF) was observed from anode to cathode side due to negative zeta potential, which exists between the soil pores and the pores solution. After applying the electric field, the pH changes, and the adsorption of hydrogen ions and other cations in negatively charged particles of soil causes a reduction in the zeta potential and thus in the thickness of the diffuse layer near the anode side [14]. The thin diffuse layer can affect the repulsive forces and the van der Waals attractive forces among the porous materials [30], which can cause the precipitation of soil particles and restrict the EOF [14]. The EOF depends on the zeta potential, salt concentration, applied electric field intensity, and pH of soil and fluid [10]. The EOF rate can be expressed according to the Helmholtz-Smoluchowski theory as presented in Eq. (4).
where "Q e " is the electroosmotic flow rate [m 3 s −1 ], "k e " is the coefficient of electroosmotic permeability [m 2 V −1 s −1 ], "I e " is the electric field [Vm −1 ], "A" is the area of soil, which is perpendicular to the flow of fluid [m 2 ], "ε 0 " is the permittivity of fluid [C 2 N −1 m −2 ], "η" is the viscosity of fluid [Nsm −2 ], "n" is the porosity of soil (-), and "ζ" is the zeta potential [V]. The Eq. (4) is in accordance with Darcy's law, which describes the hydraulic flow due to a hydraulic gradient.
However, for both VEK experiments (i.e., VEK-1 and VEK-2), the EOF was measured during different time intervals up to 72 h. The variation in electroosmotic flow rate and cumulative flow of water is shown in Fig. 3a, b. In this study, the EOF rate (Q e ) was determined according to the following equation: 3 ] is the total cumulative volume of water, and t [sec] is time. It can be seen in Fig. 3a that initially the EOF rate of water increased due to the presence of high moisture content and the absence of acidic and alkaline environments in the soil specimen. After the formation of acidic and alkaline fronts during VEK experiments, the EOF rate gradually decreased due to increased corrosion of the steel mesh electrode at the vicinity of the anode and electrostatic interaction between the hydrated water molecules and soil particles [31]. Due to water electrolysis, (4) Q e = k e I e A = 0 n I e A an acidic environment developed on the anode side, which caused its corrosion. Hence, Fe 2+ ions produced by corrosion of anode entered into the soil specimens, and these cations enter into the soil specimen due to electromigration and electroosmosis. Meanwhile, it can reduce current density and prevent the EOF of water through soil specimens in the form of Fe(OH) 2 [32].
In VEK-1, after 10 h of the experiment, the EOF became negligibly small due to several factors, including the electrical resistance of soil due to the electric potential drop within the soil [33] and the formation of acidic and alkaline environments on the electrode side [34], [35] and their propagation into the soil specimen. The EOF stoped after 10 h and it remained ceased up to the completion of experiment (i.e., 72 h). Since the pH values can change the magnitude and the direction of EOF by influencing the zeta potential at the soil water interface and a decrease in the EOF rate with the decrease in pH values (pH <7) at the anode side and the reversal in the direction of EOF (cathode side to anode side) at lower pH values (pH<3) was observed by Zhang et al., [35]. However, in our case, the initial pH value of anode electrolyte was ~ 7 and after the start of all VEK experiments, the pH value of soil at the anode side started to decrease and become low (~ 3) due to the production of H + ions as shown in (Fig. 4d).
However, in VEK-2, the EOF of water is higher than in VEK-1 due to the presence of calcium ions in the soil specimen. After injecting CaCl 2 into the water reservoir on the anode side, the Ca 2+ ions moved toward the negative electrode, which increased the current density of the soil specimen and enhanced the EOF rate due to the flocculation [36] (where Ca 2+ ions adsorb onto the surface of the soil particles and displace the hydrated ions of water and other ions such as Na + ions, which allows the EOF toward the cathode side, which is called flocculation). In our case by using calcium ions, the pH of soil increased (~ 13) as shown in (Fig. 2h) due to the alkaline nature of soil, higher buffering capacity of soil against the acid environment, and the formation of Ca(OH) 2 and Pb(OH) 2 compounds. During the experiment, the adsorption rate of H + ions onto the soil surface decreased due to the exchangeable reaction of cations with Ca 2+ ions, which allowed the enhancement of EOF from soil. Zhang et al. also observed an increase in the EOF rate from anode to cathode side with for high pH values (~ 13) at the anode side [35].
After 72 h, the total cumulative flow of water for VEK-1 and VEK-2 is 19 ± 1 mL and 81 ± 1 mL, respectively. The EOF rate is proportional to the electroosmotic permeability and is shown in the inset of Fig. 3a. The electroosmotic permeability (K e ) was determined according to the following equation [K e = Q e I e A ] where Q e (cm 3 s −1 ) is the total discharge volume of water through the soil per unit time, I e (Vcm −1 ) is the applied electric field across the soil specimen, and A (cm 2 ) is the cross-sectional area of the soil column (πR 2 ). The electroosmotic permeability varies during the EK experiments due to the adsorption of cationic charges on soil surfaces and decreases the zeta potential [37]. The total electroosmotic permeability of soil up to 72 h during VEK-1 and VEK-2 was 2.59 × 10 -6 cm 2 V −1 s −1 and 1.17 × 10 -5 cm 2 V −1 s −1 , respectively. We concluded that calcium ions have a significant impact on the enhancement of electroosmotic permeability of low-permeable soil. The Pb 2+ profile shows that ions are moved toward the cathode side due to the electromigration and electroosmosis processes. In both VEK-1 and VEK-2, the removal of Pb 2+ ions was high near the anode side due to the acidic fronts (H + ions). Because acidic front causes the desorption of Pb 2+ ions from the soil surface, therefore the lead ions become more mobile and it becomes relatively easy to remove them from the contaminated soil. However, the stagnation of Pb 2+ ions was observed at the transition zone of electric potential during VEK-1 (i.e., Fig. 4a) due to the absence of electroosmosis and electromigration. Moreover, the electromigration of alkaline environment from the cathode side into the soil sample reduces the transport of lead ions due to the their precipitation in the form of Pb(OH) 2 ; hence, the concentration of lead ions was high in alkaline environment. In this study, steel mesh electrodes were used to apply an electric field of 1 Vcm −1 across the soil specimens. As a result, the Fe 2+ ions are produced by the corrosion of electrodes at the anode side, so the concentration of  (a, b), Clˉ ions (c, d), Ca +2 ions (e, f) and the EC is shown in (g, h) in the soil segments for 24, 48, and 72 h after VEK-1 and VEK-2 experiments Fe 2+ ions is high at the anode side from the reference line throughout all the soil segments. As shown in Eq. (5) and Eq. (6), the Pb 2+ and Fe 2+ ions precipitate in the alkaline region of the soil specimen in the form of compounds such as Pb(OH) 2 and Fe(OH) 2 [32]. These compounds reduce the soil pore space and mobility of Pb 2+ ions during VEK experiments.

Distribution of ions and electrical conductivity in soil segments
However, when the calcium chloride solution was used at the anode-side reservoir during VEK-2, the production of Fe 2+ ions was high due to the higher rate of corrosion at the positive electrode. Hence, the concentration of Fe 2+ ions after the treatments was higher in VEK-2 than in VEK-1 as shown in Fig. 4c, d. The removal efficiency of Pb 2+ ions for VEK-1 and VEK-2 after 24, 48, and 72 h experiment is given in Table 2 and was calculated by Eq. (8). We concluded that in VEK-1, more treatment time is less effective for the removal of Pb 2+ ions because electromigration and electroosmosis phenomena are stopped due to the focusing effect, so the removal of Pb 2+ ions was very slow after the first 24 of the experiment during VEK-1; while in VEK-2 (where calcium chloride solution was used as an electrolyte in the anode reservoir), though the removal of Pb 2+ ions was higher than VEK-1 but the alkaline environment limited its transport. By adding calcium chloride, the alkalinity in soil increased and the alkaline fronts continued progressing toward the anode side until they reached at the soi surface in contact with at the anode after 54 h and due to the chemical nature of soil and formation of compounds such as Ca(OH) 2 , Pb(OH) 2 , and Fe(OH) 2 in the soil specimen. It was observed that the introduction of calcium ions enhances the removal of cations ions due to an increase in EOF rate. Moreover, the removal of salt ions can be increased by increasing the treatment time. for the removal of ions from soil.
Under the effect of an electric field, the Na + ions were moved from the anode to the cathode side due to the influence of electroosmosis and electromigration processes. However, due to electromigration, the movement of Clˉ ions occurred from the cathode side to the anode side of the specimen. due to the electromigration. During VEK-1, the profile of Na + ions indicates (Fig. 5a, b) that the extraction efficiency was low relative to that of VEK-2 due to the absence of electroosmosis and electromigration by the formation of transition zone the by sharp electric potential gradient within the soil. Ishaq et al. observed the stagnation of sodium ions at the transition zone of electric potential during the electrokinetic desalination of fired-clay bricks [22]. However, during VEK-2, it was found that the Na + ions were removed efficiently from the soil segments when calcium chloride were used as an electrolyte at the anode-side reservoir as shown in Fig. 5b. The leachability of sodium ions was good from soil particles due to the calcium and sodium exchangeable reactions. Moreover, the sustainability of the EOF and electromigration during the experiment allows better transportation of Na + and Ca +2 ions from anode to cathode. Therefore, during the oxidation reaction at the anode side, some chloride ions are converted into chlorine gas as shown in Eq. (7) [38].
The removal efficiency of Pb 2+ , Na + , and Clˉ ions for both VEK experiments with different treatment times of 24, 48, and 72 h was calculated by Eq. (8) [39].
where "C o " is the initial concentration of ions in soil samples before electrokinetic experiments [g kg −1 ], and "C" is the final concentration of ions in soil samples after electrokinetic experiments [g kg −1 ]. The removal efficiency of Clˉ ions (i.e., 76%) is higher than Na + ions (i.e., 44%) in VEK-1 after 72 h of the experiment due to the absence of EOF and the higher mobility of Clˉ ions than Na + ions (the effective-ionic mobility of Clˉ ions (i.e., 166.3 × 10 -6 cm 2 V −1 s −1 ) is 1.5 times higher than Na + ions (i.e., 108.9 × 10 -6 cm 2 V −1 s −1 )) [40]; while in VEK-2 (where calcium ions were applied), the removal efficiency of Na + ions (i.e., 82%) was higher than that of Clˉ ions (i.e., 69%) due to the dominance of the electroosmosis phenomenon, because the EOF and electromigration of Na + ions have the same direction (i.e., anode to cathode). During both VEK experiments, the electromigration phenomena were dominant when electroosmosis was absent. Therefore, with an increase in treatment time, the removal of all ionic species is increased. The removal efficiencies of Pb 2+ , Na + , and Clˉ ions for VEK-1 and VEK-2 after 24, 48, and 72 h of the experiment are given in Table 2.
The Ca +2 ions profiles during VEK-1 (i.e., Fig. 5e) indicate an increase in Ca +2 concentration on the anode sides of the specimens with an increase in treatment time. This increase in Ca +2 can be attributed to an increase in its solubility in the acidic environment.
While in VEK-2, the maximum concentration of Ca +2 ions at the anode side of the specimen was observed after the completion of 24 h experiment. However, after further increasing the treatment time the Ca +2 concentration dropped. The initial increase in Ca +2 at the anode side of the specimen is due to their introduction from the electrolyte reservoir containing calcium chloride. However, at later stages of experiments the Ca +2 ions precipitated due to the spread of alkaline environment in the entire specimen. In addition, we observed that the calcium ions are inversely proportional to the pH value of soil. Hence, it is supposed that some Ca +2 ions are utilized in the formation of calcium hydroxide, which explains the lower number of exchangeable calcium ions.
The EC variation in soil segments after 24, 48, and 72 h is shown in Fig. 5 g, h. The profiles of EC show the same trend as the distribution of ionic species in soil specimens after the two VEK experiments. Under an applied electric potential, the Pb 2+ , Fe 2+ , Na + , Clˉ, and Ca 2+ ions move toward their the respective electrodes due to electroosmosis and electromigration phenomena. Hence, the EC value of soil segments is high at both ends of the soil segments and lower at the center of the soil specimens after both VEK-1 and VEK-2.
The reduction in the EC of soil with respect to treatment time is given in Table 2. In our case, after the completion of VEK-1 and VEK-2 experiments, the average EC of soil was 2.18 dS m −1 and 0.99 dS m −1 , respectively. For practical purposes these values of soil EC are non-toxic and safe limit for the plant's development.

Concentration of ions in cumulative water
After applying the electric field of 1 Vcm −1 across the soil specimen, the flow of water in both VEK experiments occurred from anode to cathode due to electroosmosis (due to the net negative charge on soil particles). Hence, cations are transported from the anode to the cathode side under the combined effect of electroosmosis and electromigration. In VEK-1, the flow of water due to electroosmosis decreased after approximately 10 h of the experiment due to the formation of a transition zone of electric potential and low pH of the soil at the anode side (i.e., 3), so the removal rate of ions decreased after the first 24 h of the experiment. Therefore, in VEK-2, the electroosmotic flow of water was high due to the exchangeable calcium ion, so the removal of ionic species was high with increase in treatment time. However, the pH of the cumulative flow of water was almost equal to 13 throughout both VEK experiments due to the formation of an alkaline environment in the soil at the cathode side. The value of the cumulative flow of water and the concentration of ionic species in collected water at cathode side are given in Table 3 with respect to treatment time.

Economical evaluation
Energy consumption is a significant parameter to estimate the economic viability of EK experiments in large-scale agriculture fields. The energy consumption per unit volume and the removal efficiency of ionic charge species depend on the applied electric field intensities. At the end of the experiment, the energy consumption per unit volume was calculated by Eq. (9) [41]. For each VEK experiment, "E v " is the energy density [kWhr m −3 ], "I" is the electric current [A], "t" is the treatment time [hour], "V" is the applied voltage [V], and "V s " is the volume of used soil samples [m 3 ]. The volume of the soil sample was ~ 2.38 × 10 -4 m 3 , and electric current was measured every 15 min during all the VEK experiments after applying an electric field of 1 Vcm −1 across the soil specimens. The value of energy consumption per unit volume is given in Table 2 during different treatment times: 24, 48, and 72 h for VEK-1 and VEK-2. When the treatment time goes up, more ionic-charged species are taken out of the soil and more energy is consumed. Low-permeable soils containing sodium and chloride require less time for removal from the soil and therefore less cost and energy consumption. While the lead ions are difficult to remove from the alkaline soil due to their precipitation with soil particles, therefore, more time is required for the removal of lead ions from alkaline soil. Hence, before starting the EK experiments, the type of targeted ions is important for the choice of optimal treatment time in order to save energy. In some EK applications, using fuel to supply the electrical generator can lead to excessive emissions of carbon dioxide (CO 2 ). So, assessing the nature and initial concentration of contaminants is needed first before starting EK experiments for the optimal approach to ecological and economic needs [6].

Conclusion
The objective of this research was to determine the effect of Ca +2 ions on electroosmosis and the removal of Pb 2+ , Na + , and Clˉ ions from artificially contaminated soil during vertical electrokinetic experiments. During experiments, an electric field of 1 Vcm −1 was applied across the soil specimen via steel mesh electrodes for 24, 48, and 72 h. of the experiment.
The outcomes depict that the Ca +2 ions effectively enhanced the electroosmosis flow of water, i.e., 81 mL from anode to cathode side after 72 h during VEK-2. Hence, in the presence of Ca +2 ions, the removal efficiency of Pb 2+ and Na + ions, i.e., 41% and 82%, respectively, increased instead of the Clˉ ions, i.e., 69% due to the same direction of electroosmotic flow and electromigration. For relatively low electroosmotic flow, i.e., 19 mL, the removal efficiency of Clˉ ions, i.e., 76%, was higher than the Pb 2+ and Na + ions , i.e., 27% and 44%, respectively, attributed to the higher mobility of Clˉ ions. However, we observed that the Ca +2 ions are not beneficial for the removal of heavy metals (i.e., Pb 2+ ) from alkaline soils due to the precipitation of Pb 2+ ions with the advancement of an alkaline environment. In Idt addition, the maximum ionic removal rate was achieved within the first 24 h due to the high initial electroosmotic flow. This study offered valuable evidence on the impact of calcium ions and treatment time on the removal of sodium ions from the soil during the vertical electrokinetic experiments. The optimum treatment time for the field-scale experiments can be determined by considering the soil type, concentration, and the contaminants' nature.