Hossein Beyrami

National Salinity Research Center,Agricultural Research,Education and Extension Organization,Yazd,Iran

Keywords:Decontamination Soil moisture Voltage gradient Cadmium Lead Zinc

ABSTRACT Electrokinetic remediation is a promising method to decontamination of the heavy metals from soils.In this paper,the remediation of a contaminated calcareous soil with Zn,Cd and Pb sampled from around Zanjan province of Iran,was investigated using electrokinetic method.In this paper,the soil contain a high concentration of Zn (1400 mg.kg-1),Cd (15 mg.kg-1) and Pb (250 mg.kg-1).Electrokinetic decontamination consists of two series of experiments as follows:(1) the effect of five treatments including the use of distilled water,acetic acid and EDTA electrolyte solutions,and approaching anodes systems,and the circulation flow of electrolyte at two different voltage gradient (i.e.1.33 and 2.66 V.cm-1),and(2) the effect of moisture content (saturated,FC and 0.7FC,FC indicated soil moisture at “Field Capacity”) with a voltage gradient of 1.33 V.cm-1.After applying electric current for 5 days,the results of experiments indicated that the removal efficiency of heavy metals can be increased by raising the voltage gradient.In this matter,the highest remediation can be observed among different treatments in EDTA(Ethylene diamine tetra acetic acid)treatment(40.11%,43.10%and 24.7%for Zn,Cd and Pb,respectively).Moreover,the heavy metals removal at the saturated moisture was at the highest level so that 32.62%cadmium,31.33% zinc and 18.82% lead being removed after 120 h of electric current application.By decreasing moisture to 0.7FC,the removal percentage for the three heavy metals obtained 20.97%,18.44% and 12.25%,respectively.Furthermore,Cd had the highest removal,and Zn and Pb were next among the three heavy metals in question.

1.Introduction

Among the popular technologies developed to reclamation heavy metals contaminated soils,the electrokinetic remediation has been well known as an appropriate method,especially in soils with low hydraulic conductivity [1–3].Here,it is worthwhile to mention that the electrokinetics is an advanced technology that its aim is to separate and extract heavy metals,radiolucides,and organic contaminants from soils,sludge,sediments,and groundwater [4–6].The basis of Electrokinetic remediation is formed based on the migration of contaminating ions to the electrode with the opposite charge in the contaminated environment under the influence of an electric field applied through the anode electrodes(positive pole) and the cathode (i.e.negative pole) located in the soil [7–9].This phenomenon occurs when the soil is charged with a low-voltage electric current [10–12].

Overall,the electrokinetic method involves the three processes of ion electromigration,electroosmosis,and electrophoresis.This method can be employed to remediation one or more contaminants simultaneously in both saturated and unsaturated soils[8,13,14].Meanwhile,the soil electrokinetic remediation is a very promising method of soil decontamination that is highly effective to removal of heavy metals from low-permeability soils such as clay soils [12,15].Another dominant and important phenomenon of soil improvement is water electrolysis in the vicinity of electrodes.In the vicinity of the anode,water is oxidized to generate an acidic front by producing oxygen gas and hydrogen ions.At the same time,in the vicinity of the cathode,water reduction may lead to the production of hydrogen gas and hydroxyl ions,forming a basic front [9,16,17].The reaction of two processes is as follows:

Other reactions may take place under these conditions,which mainly depend on the concentration of the existing species as well as the electrochemical potential of the reactions.It should be noted that although secondary reactions may be preferable to cathodes due to their low electrochemical potential,the half-water reduction reaction (H2O/H2) is predominant during the Electrokinetic process [9,18,19].

More recently,many researchers have utilized this method to remove the heavy metal contamination [20].Reddy et al.[21]employed this method to remove 62%–100% of cadmium,chromium and nickel from the soil.After six days,Wang et al.[22]indicated that the application of electric current in the soil by Electrokinetic method 62% and 35% Cu and Pb can be removed from soil.Other researchers,such as Yang et al.[5],Kim et al.[23] and Giannis et al.[24] reported successful results in the removal of heavy metals.The efficiency of pollutant removal using the electrokinetic remediation depends on many factors.The effects of electrode material on the heavy metal removal efficiency were investigated in various studies.However,it is important to select a good electrode material,especially when electrochemical technologies are used.Ideally,graphite electrodes are supposed to be inert and permit electron transfer without entering into the reaction,but the electrode material commonly forms bonds with the species in solution,and oxide formation as well as species adsorption onto the electrodes occurs [25].For research purposes,the most suitable electrode materials include graphite,platinum,gold,silver,and so on.However,for practical applications and lower cost,it is more appropriate to use much cheaper electrode materials,such as titanium,stainless steel and even plastic[26,27].Numerous laboratory and site studies have shown that the other variables affecting the efficiency of removing contaminants from the soil include as follows:the chemical processes in electrodes,soil moisture content,soil structure,pH and pH slope,type and concentration of chemicals in the soil,the applied voltage and current intensity,the type of electrolyte solution used and other sample conditions [28].Therefore,this research aims to explore the effect of increasing the gradient of voltage in different treatments on the efficiency of Electrokinetic correction as well as the effect of soil initial moisture content under three levels of saturation,FC (Field Capacity) and 0.7 FC on the removal of heavy metals in the soil.

2.Materials and Methods

The soil sample was prepared from zero to 15 cm depth of a contaminated clay loam soil via heavy metals Zn,Cd and Pb along with the features mentioned in Table 1 of the field near the lead and zinc factory in Zanjan,Iran.After drying and passing through a 2 mm sieve,the desired soil was utilized to prepare soil columns with an apparent density of 1.3 g.cm-3.The soil columns were made of PVC,in which the electrodes used at the two ends of the column were made of graphite with a diameter of 62 mm and a thickness of 5 mm.In this way,to prevent soil outflow from the columns,two fabric filters and filter paper were considered at the end of the columns.Besides,the back surface of the electrodes was insulated using a compressed plastic material.In this paper,two series of experiments were carried out as follows.

The soil properties are given in Table 1.The soil texture was clay loam.Due to the concentration of Cd,Zn and Pb,the soil had a concentration higher than the critical range [29] and was highlycontaminated.

Table 1The soil properties and heavy metal concentrations

In the first series of experiments,the effect of two voltage gradient (1.33 and 2.66 V.cm-1) on electrokinetic remediation in soil columns with an internal diameter of 6.2 cm and a length of 22 cm (Fig.1),and using three electrolyte solutions of distilled water (T1),acetic acid 0.01 mol.L-1(T2) and EDTA (Ethylene diamine tetra acetic acid) 0.01 mol.L-1(T3) and two anode displacement systems (approaching anodes) (T4) and circulation of the electrolyte (T5) were investigated over a five days (120 h).Before applying the electric current,the soil columns were first saturated with each of the desired solutions and then kept saturated during the experiment.In these treatments,to achieve this aim,there was a gap of 1 cm between the soil column and the electrodes,which did not cause any disturbance in the electrical current during the experiment due to the existence of complete saturation (as illustrated in Fig.1).

In the anode displacement system (T4),the electrolyte solution was distilled water.In this system,in addition to the two end electrodes,two cylindrical electrodes with a diameter of 20 mm and an effective length of 48 mm with a distance of 8 cm from each other were placed in the middle of the columns(as depicted in Fig.2).In this system,the cathode electrode was fixed and the end anode electrode,or one of the two middle electrodes,was employed as an anode for 24-h periods,and then the other two anode electrodes were out of the electric current circuit.During each five-day period,on the first,third,and fifth days,the end anode electrode was placed in circuit as an anode,and on the second day,the first intermediate electrode,and on the fourth day,the second intermediate electrode was placed in circuit.The purpose was to transfer the acid production zone to the middle parts of the soil column to explore its effect on increasing the efficiency of the heavy metals.

In this paper,the circulation flow system(T5)was implemented to adjust the pH of the electrolyte and increase the efficiency of heavy metal removal.In this system,distilled water solution was employed as the electrolyte.The system involved the use of a circulation pump to generate flow from the catholyte to anolyte reservoir (Fig.3).This pump prepared a flow of 0.3 ml of distilled water electrolyte solution (0.3 ml.min-1).Before evaluating in the same columns,this flow was specified by the average soil hydraulic conductivity.Now,to prevent the transfer of heavy metals by the electrolyte flow from the catholyte reservoir to anolyte reservoir and the entry of these metals into the soil,a filter containing zeolite and activated carbon was utilized as the adsorbent at the end of the soil column on the cathode side.Two direct current power supplies with a potential difference of 32 and 64 V were utilized to establish the electric current.

The second series of experiments was carried out to investigate the effect of soil moisture content on the efficiency of heavy metal removal.After the soil was treated with distilled water to a certain moisture content (saturation,FC,and 0.7 FC),the soil columns were prepared with a length of 24 cm and an inner diameter of 6.2 cm.In these columns,the graphite electrode was in direct contact with the soil and the electric current with a potential difference of 1.33 V.cm-1,was established in time periods of 5 days(120 h)in the columns.In both series of experiments,some narrowtubes were placed above the electrode placement area to remove the gases from the electrolysis to prevent the accumulation of air bubbles,which were a strong insulator for electric current.The voltage and the electric current for both series of experiments were measured during the 24-h test,through a multimeter and the pH of the catholyte and anolyte reservoir solutions of the first series of experiments by the pH meter.

Fig.1.Schematic of electrokinetic set-up.

Fig.2.The location of electrodes in the anode displacement system.

Fig.3.A schematic design of circulation flow system.

Furthermore,the soil columns were separated from the system and cut into two-centimeter pieces at the end of each period.After drying,the total amount of Zn,Cd and Pb of soil was extracted by nitric acid(4 mol.L-1)[30]and then measured with atomic absorption spectrophotometer.Besides,the pH was measured in a ratio of 2.5:1 (water:soil) in the cut pieces to evaluate the pH changes along the soil column.

3.Results and Discussion

3.1.The changes in current intensity

In both voltage gradient in all treatments,except for the treatment of approaching anodes,the intensity of the electric current first increased slightly and decreased over time.This decrease in voltage gradient 2.66 V.cm-1is significant in the first 24 h.This is because the ion transfer intensity was higher at this voltage gradient.The voltage gradient,especially in the first 24 h,the electric current intensity is always higher than the voltage gradient 1.33 V.cm-1.Cameselle et al.[6] and Al-Hamdan and Reddy [11]described the increase in electrical current intensity in the early hours of the experiment to the production of H+ions by electrolysis and the transferring of these ions with electromigration to the cathode,which caused most contaminants by creating low pH in ion forms.Among all the treatments,the treatment of approaching anodes has many fluctuations,which can be interpreted due to the approach of the anode electrode at the end of the second and fourth days with the orbit of the second and third middle electrodes.

Regarding the voltage is constant,the intensity of the electric current increases rapidly as the anode approaches the cathode and shortens the path.At the end of the first,third,and fifth days,as the end electrode acts as an anode,the current intensity decreases.With the exception of the treatment of approaching anodes on both voltage gradient among the other four treatments,the EDTA treatment contains a higher electrical current intensity than the other treatments.Note that Giannis et al.[24] have reported the high current intensity in the EDTA treatment and the oscillations of the electric current intensity by other researchers [5,31].Yuan and Chiang [32] described that the intensity of electric current in a soil column is proportional to the concentration of free or moving ions.These free ions include two types:(1) H+and OH-ions,which are generated by electrolysis;and (2)species that exist in the soil itself and are released into the soil because of mechanisms such as ion exchange,desorption,dissolution,and so on.It should be mentioned that when there are no free ion species in the soil,the intensity of the electric current would decrease.The intensity of the electric current passing through the soil depends on several variables such as the amount of water in the soil,the properties of the electrode,the electrical conductivity of the electrolyte solution,or the concentration of ion species in the pore solution [11,33].

Fig.4.Changes of anolyte and catholyte pH in different treatments during the experiment in 5 days with a voltage gradient of 1.33 and 2.66 V.cm-1(the experiment of the first series).

By decreasing the moisture from saturation to 0.7 FC (FC indicated soil moisture at “Field Capacity”),as well as increasing the path resistance (reducing the cross-sectional area of the electric current transmission),the intensity of the electric current passing through the soil column can be reduced.On the first day,due to the dissolution of ions due to the production of H+(acid)on the anode side by electrolysis of water and increase of soluble ions,the electrical conductivity of the path increased slightly and the intensity of the passing current increased slightly;nevertheless,the ions moved to the electrode over time.With the reverse charge due to the ion electromigration phenomenon that results in the depletion of ions and the reduction of the electrical conductivity of the path,and also with the transfer of moisture due to the electrosmosis phenomenon from the anode to the cathode,which increases the resistance of the path on the anode,the moisture level in all three levels will gradually be decreased.Zhu et al.[14],Altaee et al.[34]and Cameselle and Pena [35] have also confirmed such reasons.

3.2.The changes in anolyte and catholyte pH

Fig.4(a–j)shows the pH changes of anolyte and catholyte under different treatments at voltage gradient of 1.33 and 2.66 V.cm-1.In all treatments,the anolyte pH has an almost constant trend after a rapid decrease on the first day onwards,in which in most treat-ments,the pH is between 2 and 4.Unlike anolyte in various treatments,catholyte pH fluctuates in the range of 12–13 in the following days after a rapid increase on the first day.The decrease in pH of the anolyte and the increase in the pH of the catholyte are due to the production of H+and OH-due to water electrolysis [6,36,37].Here,anolyte pH has intermittent fluctuations in the treatment of approaching anodes among the treatments,which is caused by transferring the anode to the central electrodes at the end of the second and fourth day.In this treatment,at a voltage gradient 2.66 V.cm-1,can be decreased in pH by raising the electric current intensity and further,despite moving away from the proton production center.Nevertheless,its effect on the pH of the anolyte is slightly less,whereas the increase in pH at the end of the second and fourth days is less.The voltage gradient was equal to 1.33 V.cm-1(see Fig.4g).

Fig.5.Changes in pH along the soil column at different treatment(in first series of experiments)on two gradient of voltage 1.33 and 2.66 V.cm-1 at the end of each period.

In the circulation flow treatment,due to the flow from the catholyte reservoir to the anolyte reservoir,the pH of the electrolyte was nearly controlled and the decrease in pH of the anolyte and the increase of the pH of the catholyte were less than the other treatments(Fig.4i and j).Due to the high intensity of electric current at the voltage gradient 2.66 V.cm-1,particulary in the early days,the ability of the circulation of the electrolyte system to control the pH of the anolyte and catholyte has noticeably been less.In EDTA treatment,by increasing the voltage gradient and increasing the current intensity,we have witnessed a decrease in the pH of the anolyte and an increase in the pH of the catholyte with a higher intensity in this treatment (Fig.4e and f).Similar changes have been reported by other researchers,such as Cameselle et al.[6],Giannis et al.[24] and Yuan et al.[33] about the changes in pH of anolyte and catholyte,and their reasons for modulating pH changes of anolyte and catholyte have been suggested by them.It should be considered that such changes in the pH of the catholyte and anolyte reservoirs might affect the pH of the soil,which will finally affect the metal removal efficiency.

3.3.The changes in soil pH

In this matter,Fig.5(a–e) reveals the pH changes in the soil of the experimental columns at relative distances from the anode in the different treatments of the first series of experiments (in the presence of different electrolyte solutions).In some treatments such as EDTA and the approaching anode system,where the electric current intensity was high,the pH changes in the soil as the electrolysis process intensified and the migration of proton and hydroxyl ions increased.Note that both distilled water and acetic acid had almost the same results,and with little change except for areas close to the anode and cathode that saw a slight decrease and increase in soil pH(Fig.5a and b).As EDTA treatment contains higher current intensity than other treatments,changes in soil pH have been more due to increased electrolysis intensity and production of H+and OH-ions and their faster transfer.However,the decrease in pH on the anode side at the voltage gradient 1.33 V.cm-1is not significant due to the buffering nature of EDTA(Fig.5c).Nevertheless,the soil pH reduction on the anode side can be reduced with the increase of the voltage gradient to 2.66 V.cm-1.

Regarding the transfer of the anode to the central electrodes on the second and fourth days,In the treatment of approaching anodes,or in other words,the transfer of the proton production center,we see two pH zones at least in the middle of the column(Fig.5d).The high electrical current intensity in this treatment has expanded the basic front in the soil column.However,as can be observed in the case of the pH of the anolyte (Fig.4),the pH increases on the anode side on days So that the second and fourth indicate a strong decrease in pH and the development of acidic fronts in the soil column.In circulation flow treatment,pH adjustment by electrolyte circulation prevented significant changes in soil pH,and only at a voltage gradient 2.66 V.cm-1we saw a decrease in pH on the anode side,which was not significant.These increase and decrease have also been reported by Cameselle et al.[6],Li et al.[38],Ryu et al.[39],Kim et al.[40],and Yuan et al.[41].

Fig.6(a–c) indicates the soil pH changes of the experimental columns at relative distances from the anode at three moisture levels (second series of experiment).As can be observed,the pH of the soil on the anode side has the lowest value and then increases rapidly and reaches the initial soil value,and there is a steady trend until the soil pH increases rapidly again in the vicinity of the cathode.Yuan and Chiang[32]mentioned the changes in pHto water electrolysis.The migration of H+and OH-ions to opposite electrodes creates an acidic and basic front,and to some extent affects the soil pH during the electrokinetic process.This trend of increasing and decreasing pH has also been reported by other researchers such as Altaee et al.[34],Al-Hamdan and Reddy [11]and Yuan et al [33].

Fig.6.Changes in pH along the soil column at different moisture levels at the end of each period.

Fig.7.Residual Zn concentration in soil in different treatments after 5 days of applying electric current with two voltage gradients of 1.33 and 2.66 V.cm-1.

3.4.Heavy metal removal efficiency

Figs.7–9 show some changes in the residual concentrations of Zn,Cd,and Pb in different treatments under two voltages of 1.33 and 2.66 V.cm-1.As the voltage gradient increases in all treatments,the intensity of heavy metal removal increases,but the amount of increase in different treatments is not the same.The electrokinetic treatment requires shorter time at high voltage,but the energy cost is much higher [6].Distilled water treatment(T1) and acetic acid treatment (T2) increased the average percentage of removal of Zn,Cd and Pb by increasing the voltage gradient,as compared to other treatments.EDTA treatment (T3) had a high efficiency to remove heavy metals,based on which by increasing voltage gradient,the average percentage of Zn,Cd and Pb metals was removed from 33.17%,33.31% and 18.55% in voltage gradient 1.33 V.cm-1were reached 40.11%,43.10% and 24.07% at a voltage gradient 2.66 V.cm-1(Table 2).

At the meantime,the approaching anode treatment(T4)had the highest removal efficiency after EDTA treatment on both voltage gradients.In this treatment,by increasing gradient of voltage,the average removal percentage of three metals increased from 22.53%,22.37% and 12.12% to 37.95%,37.74% and 23.60%,respectively(Table 2).In this treatment,the increase in removal percentage was higher with increasing voltage gradient than all treatments (Figs.7d–9d).On the other hand,in circulation flow treatment (T5),increasing the voltage gradient caused a slight increase in the average amount of Zn,Cd and Pb removal of this treatment by increasing the voltage gradient (Figs.7e–9e).It is worthwhile to mention that the use of acetic acid as an acidic substance to increase solubility and desorption of heavy metals relative to distilled water treatment in increasing the efficiency of removal was not very successful.The reason for this challenge can be interpreted due to the high amount of equivalent calcium carbonate in the soil(23%).Other researchers,such as Altin and Degirmenci[42],investigated the effects of substances such as lime and gypsum on reducing the effectiveness of acidic treatments.

EDTA treatment had the highest removal efficiency on both voltage gradient among the several treatments.This increase in removal efficiency can be related to the high complexity of EDTA.Gu et al.[31] and Yuan and Chiang [32] also reported thatEDTA,due to its high complexing ability to extract heavy metals without severe acidification of the environment,is a suitable enhancement in Electrokinetic remediation of heavy metals.In EDTA treatment,the highest amount of removal can be observed on the anode side.In this area,both the dissolution and desorption of heavy metals contain the highest value due to low pH.By increasing pH,heavy metal precipitation also occurred on the cathode side.

Fig.8.Residual Cd concentration in soil in different treatments after 5 days of applying electric current with two voltage gradients of 1.33 and 2.66 V.cm-1.

In the treatment of approaching anodes(Figs.7d–9d),two areas with the lowest percentage of residual concentration can be seen along the soil column.These two areas are the location of the central electrodes.Their use as an anode in these two areas has led to the production of H+and the expansion of the acidic front in these areas and the further removal of heavy metals [36].In these two areas,the expansion of the acidic front is also distinguishable by both the pH changes curves and the soil column (Figs.7d–9d).

The circulation flow treatment with partial pH adjustment of electrolyte,caused that in the voltage gradient 1.33 V.cm-1we have an increase percentage in the removal of all three metals,as compared to the distilled water treatment (Figs.7e-9e).Nonetheless,by increasing voltage gradient to 2.66 V.cm-1due to the flow of electrolyte solution from the catholyte chamber to the anolyte chamber has a low flow rate,it is not able to control the pH and as a result,a significant increase in the average removal percentage has not been achieved.In this regard,the circulation flow increases the efficiency of heavy metal removal by preventing the reduction of electrochemical flow by the presence of high H+concentration on the anode side and preventing the precipitation of heavy metals on the cathode side [18,43,44].The effect of electrolyte pH adjustment has also been reported by various methods on increasing the efficiency of the electrokinetic heavy metal removal by Lee and Yang[18],Saichek and Reddy[25],Zhou et al.[45]and Cherifi et al.[36].

Figs.10–12 represent the residual concentrations of Zn,Cd,and Pb,respectively,in the 0.7FC,FC,and saturation moisture treatments.Reducing the residual concentration of each metal denotes an increase in its removal rate.By increasing soil moisture over a period of 5 days,the amount of removal of all three metals from the soil has increased.In this matter,increasing soil moisture has increased the intensity of the electric current passing through the soil column as well as increasing the amount of heavy metal removal.That is,the saturation treatment had the highest average removal percentage.At the end of 5 days,the average percentage of Zn,Cd and Pb removal in the mentioned treatment reached 27.23%,28.09%and 15.77%,respectively.Removal percentage is the ratio of the removed metal to the initial amount of metal in the soil,which is expressed as a percentage.After averaging the percentage of removal of heavy metals cut into two-centimeter pieces,the average total percentage of metal removal from each column of soil).As such,with a decrease in moisture to 0.7 FC,the percentage ofremoval of the three metals reached its lowest value,i.e.13.74%,15.71% and 8.70% in the 5-day period (Table 3).The saturated treatment contains more fluctuations in the residual concentration curve of heavy metals,which is a reason for the high intensity of electric current,as well as the greater intensity of electromigration of heavy metal ions.Here,Cd contains the highest average percentage of removal (28.9%) and Zn (27.23%) and then Pb (15.77%)among the heavy metals in question (Table 3).For the heavy metals,their adsorption capacities on the soil surface and mobilities in soil have significant effects on the removal efficiency.In the tests,the lower amount of removed Pb and Zn was caused by the greater adsorption capacity and the immobility of the Lead and Zinc com-pared to The Cadmium[29].Decreasing the moisture in the vicinity of the anode can reduce the cross-sectional area of the ion path,as well as can increase the tortuosity of the path;so,increasing the path resistance.The effect of soil moisture has also been reported on the efficiency of the electrokinetic heavy metal removal by Reddy et al.[46] and Shin et al [47].

Table 2Mean removal efficiency for Zn,Cd and Pb in different voltage gradient

Table 3Mean removal efficiency for Zn,Cd and Pb in different moisture level

Fig.9.Residual Pb concentration in soil in different treatments after 5 days of applying electric current with two voltage gradients of 1.33 and 2.66 V.cm-1.

Fig.10.The residual concentration of soil Zn at different moisture levels after applying electric current with voltage gradient 1.33 V.cm-1.

Fig.11.The residual concentration of soil Cd at different moisture levels after applying electric current with voltage gradient 1.33 V.cm-1.

Fig.12.The residual concentration of soil Pb at different moisture levels after applying electric current with voltage gradient 1.33 V.cm-1.

4.Conclusions

Overall,the effect of increasing the voltage gradient in different treatments can lead to obtaining some different results on the heavy metal removal efficiency,whereas in the treatment of approaching anodes and EDTA treatment,increasing the voltage gradient(from 1.33 V.cm-1to 2.66 V.cm-1)is the highest increase in removal efficiency of Zinc,cadmium and lead.Some chemicals such as EDTA have significantly increased the electrokinetic remediation of heavy metal efficiency due to their high complexing ability.It should be noted that the use of acidic material in the experimented soil was not very effective to enhance the heavy metal removal efficiency due to the existence of high amounts of equivalent calcium carbonate in the soil (23% lime).The use of a circulation flow system as a pH adjustment in low-permeability soils at high voltage gradients has not been very successful to enhance the heavy metal removal efficiency.As a result,it was higher in the saturated moisture.On the other hand,the percentage of removal of heavy metals could be decreased due to the increasing in path resistance and reducing the moisture.Finally,among the three heavy metals,in the electrokinetic method,Cd had the highest amount of removal from the soil in terms of average removal percentage,while Zn and Pb were next.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.