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Abstract [Objectives] This study was conducted to investigate in-situ leaching and electrokinetic combined remediation of composite heavy metal-contaminated soil.
[Methods] Leaching and electrokientic combined remediation was applied to remediate artificially-simulated composite Cd， Cu， Pb， and Zn-contaminated soil. The electrokinentic remediation of the soil was carried out using EDTA and acetic acid as the eluents with electrodes placed on the top and bottom of the soil， to investigate the effects of different types and concentrations of eluents， reaction time and energization methods on the remediation.
[Results] Applying electrodynamic force to the leaching remediation process achieved good removal effects of Cd， Cu， Pb， and Zn in the soil. Compared with pure leaching， the leaching and electrokinetic combined remediation saved 23.8% of EDTA consumption. EDTA showed the ability to complex heavy metals better than acetic acid， and after 10 d of reaction， the 0.1 mol/L EDTA achieved very good removal effects on Cd， Cu， Pb， and Zn from the upper and middle layers of the soil column， exhibiting removal rates of 94.4%， 93.3%， 91.4% and 92.8% in the upper layer and 87.2%， 88.2%， 83.3% and 84.6%， respectively. Compared with the conventional experiments， the intermittent energization method and the voltage-increasing method improved the removal rates of Cd， Cu， Pb and Zn by 6.3%， 6.1%， 5.9% and 6%， and 0.6%， 0.9%， 0.5% and 0.4%， respectively， and saved 46.8% and 10.3% of energy consumption， respectively.
[Conclusions] The study provides reference for the remediation of composite heavy metal-contaminated soil.
Key words EDTA; Acetic acid; Heavy metals; Removal rate; Energy consumption
In recent years， the area of heavy metal-contaminated land has been increasing， and the degree of contamination also has been increasing[1]. Heavy metal pollution not only causes damage to the environment and economy， but also threatens human health[2]. The problem of remediation of heavy metal-contaminated soil has caused widespread concern from scholars at home and abroad. At present， the remediation techniques of heavy metal-contaminated soil can be divided into three categories： physical methods， chemical methods and biological methods[3]. Some of these methods may affect the soil itself， and some consume a lot of manpower and material resources[4-6]. Electrokinetic remediation is an in-situ soil remediation technology， which has the advantages of simple operation and low environmental impact， and has a wide application prospect[7]. In the 1990s， the Netherlands launched five large-scale electrokinetic remediationn projects for heavy metal-contaminated sites， involving removal of copper， lead， cadmium and nickel in the soil[8]. Peng[9] used distilled water as an electrolyte in the experimental study to investigate the effects of different voltages on the remediation effect and concluded that the traditional electrokinetic remediation had a poor removal effect of heavy metals in the soil and a low remediation efficiency. Researchers at home and abroad have begun to try to combine electrokinetic remediation technique with other methods to improve the remediation efficiency， with an attempt to improve the treatment efficiency by degrading， destroying or even changing the original state of pollutants. In order to further improve the efficiency of electrokinetic remediation， Tan et al.[10] increased a leaching process to avoid heavy metal ions forming a concentrated zone in the soil. When combining the electrokinetic remediation technique with organic acid leaching， a large amount of heavy metal ions are released from the soil by organic acids， overcoming the inhibition of pH on electrokinetic remediation， and the leaching effect can be promoted by electric field. Materials and Methods
Experimental apparatuses and equipment
MS-605D DC power supply （Maisheng）; PHS-3C pH meter （AIPLI Instrument Co.， Ltd.）; flame atomic absorption spectrophotometer （Agilent 240FSAA）; YH-A2003 balance （Yingheng）; H3018DR centrifuge （Shanghai Zhixin Experimental Instrument Technology Co.， Ltd.）.
Soil samples
The test soil was taken from the vegetable field near Changzhou Science and Education City， Jiangsu Province， with a sampling depth of 0-20 cm. After air drying and removal of stones and weeds， the soil samples were sieved through a 5 mm nylon sieve and placed in a cool place for later use. Heavy metal solutions were prepared to artificially contaminate soil in accordance with "Risk Screening Guideline Value for Soil Contamination of Developed Land （Third Draft for Seeking Opinions）". The reagents used in the contamination of soil were CdCl2·5H2O， Pb （NO3）2， and CuSO4·5H2O. The soil was added into a rotary mixer， and sprayed with stirring. The soil， which was contaminated evenly， was then stood for one year. The basic properties of the soil after the simulation are shown in Table 1.
Experimental device
The reaction device is composed of plexiglass （175 mm×100 mm×100 mm）， inside which are disposed two division plates of 3 mm thick. The size of the intermediate soil reaction chamber is 100 mm×100 mm×100 mm. Several small holes are distributed on the division plates. Two layers of non-woven fabric are placed on each division plate， with a layer of filter paper sandwiched therebetween， to separate the soil from the electrolytic cell and prevent soil particles from entering the electrolytic cell. A shower nozzle is arranged on the top of the reaction device， and a peristaltic pump is connected outside to introduce an eluent into the shower nozzle. Meanwhile， a layer of 3 mm thick quartz sand is laid on the non-woven fabric of the upper division plate to make the leaching solution pass through the quartz sand uniformly. A leachate collection bottle is placed at the bottom of the reaction device to collect the solution flowing out of the reaction device. The top electrode of the soil is set as the anode， and the bottom electrode is set as the cathode. The electrolytic cell is collected to an external ammeter and a DC power supply. The schematic diagram of the device is shown in Fig.1[11-12].
Experimental program
Group experiment
The prepared composite contaminated soil was weighed and filled into the soil reaction chamber. At the same time， a certain amount of deionized water was added， and the soil was continuously stirred to make the soil moisture content at about 33%. After aging for 24 h， the electrokinetic leaching was performed. Before the experiments， EDTA and acetic acid at concentrations of 0.05 and 0.1 mol/L were prepared， and the pH was adjusted to 4.5 with HNO3 and NaOH. Operation of electrokinetic leaching experiments： Corresponding eluent was added into the soil column， and the power was switched on when the first drop of leachate flew out of the small holes of the lower layer; the voltage was constant at 20 V， and the voltage gradient was maintained at 2 V/cm; and the flow was controlled with the peristaltic pump， to keep the upper liquid level at about 3 mm. The design of the group experiments is shown in Table 2. Enhanced experiments
Intermittent power on experiment （K1）： The system was energized for 120 h and then powered off for 12 h， forming a cycle， followed by another 120 h of energization， and the leaching remediation was kept on when the system was powered off.
Voltage-increasing method （K2）： The voltage was constant at 20 V for 120 h and then increased to 30 V for 12 h， which was a cycle， and the leaching remediation was kept on when power off.
Testing method
During the experiments， the current value was recorded every 12 h. After the reaction， soil samples were taken for analysis at different distances （1， 3， 5， 7 and 9 cm） from the anode， and numbered 1#， 2#， 3#， 4# and 5#. The pH， moisture content and contents of various metals were measured. Each experiment was performed in parallel.
Results and Discussion
Effect of electrokinetic force on the effect of leaching remediation
After treating the soil for 10 d with EDTA at a concentration of 0.1 mol/L as the eluent， the comparison of heavy metal residues between the combined electrokinetic leaching test （M7） and the pure leaching test （M8） for which no power was applied is shown in Fig. 2. It can be seen that the residual amounts of Cd， Cu， Pb， and Zn of M8 were 4.84， 193.06， 278.43 and 398.46 mg/kg in the upper layer and 8.09， 279.28， 440.26 and 669.62 mg/kg in the middle layer， respectively. M7 reduced the residual amounts of Cd， Cu， Pb and Zn by 203.5%， 145.2%， 136.7% and 141.8% in the upper layer and 48.9%， 40.3%， 19.6% and 31.2% in the middle layer， respectively. The removal rates of Cd， Cu， Pb and Zn were 57.9%， 59.6%， 42.3% and 43.7% in experiment M7， and 53.4%， 55.2%， 46.7% and 49.2% in experiment M8， respectively， so both the two experiments had certain removal effects on Cd， Cu， Pb and Zn. However， compared with M8， the introduction of the voltage in M7 could promote the heavy metals to move toward the cathode at a faster speed， which helped the heavy metals to be concentrated in the lower layer of the soil column[13]. Moreover， it was found that the total removal amounts of heavy metals by M8 and M7 were not much different， but the consumption amount of EDTA in M7 was 232 g/kg， which was 23.8% less than the consumption amount of EDTA in M8. Applying voltage can save the consumption of EDTA， and strengthen the leaching and desorption of heavy metals.
Hui HAN et al. Experimental Study on In-situ Leaching and Electrokinetic Remediation of Composite Heavy Metal-contaminated Soil Change of current
As shown in Fig. 3， the experimental currents of the M1 to M7 groups had approximately the same change trend. In the initial reaction stage， the current value of the system was low， then gradually increased， and gradually decreased after reaching an extreme value. The change in current reflects the amount of charged ions transferred. After the system was powered on， electrode reactions occurred at the two electrodes[14]. Under the action of the electric field， H+， OH- and acid group anions migrated to the soil， and heavy metals in the soil changed from the adsorbed phase to the dissolved phase， and then undergone complexation reaction with acid group ions. These processes took some time， so the current was initially weak. As the reaction progressed， the ions migrated in the system gradually increased， and the current value started to rise， reflecting that the heavy metal ions were continuously flowing to the electrode liquid. After a period of reaction， the current began to slowly decrease and approached equilibrium， indicating that the content of dissolved heavy metals in the system decreased， and the number of ions that can be migrated gradually decreased. The migration of other ions in the soil maintained the systems equilibrium current. It could also be seen from the figure that the currents in the experiments （M5， M6 and M7） using EDTA disodium salt as the eluent were higher than those in the experiments （M2， M3 and M4） using acetic acid as the eluent， which reflected that Na+ played a strong role in promoting the improvement of current and conductivity[15]. On the other hand， acetic acid is a weak acid， so the concentration of the acid group ions forming metal complex in the liquid was small， which made the increase of the current in the system not easy. Furthermore， it could also be seen from the figure that the magnitude of the current value was directly proportional to the concentration of the electrolyte， indicating that the current of an electrokinetic remediation system is closely related to the concentration of migratable ions in the system. A larger concentration is more conductive to the improvement of current.
Change of pH value
As shown in Fig. 4， the pH values of the soil in various test groups had the same trend， which gradually increased from the anode to the cathode， but could eventually remain stable. It was mainly due to the consumption of electrolyte and electrode polarization， which reduced the current and weakened the electrolysis， resulting in the reduction of OH- production[16]. Meanwhile， the OH- ions generated would be migrated， part of which would be consumed by reaction with soil material and part of which will be consumed by H+. Finally， the pH was relatively stable. From the effects of different eluents on soil pH， in the test （M1） using deionized water as the eluent， the soil near the cathode was under the action of a large amount of OH-， and the pH could reach 9.42. Compared with the experiments （M2， M3， M4） with acetic acid as the eluent， the experiments （M5， M6， M7） with EDTA as the eluent showed the pH of the soil near the cathode changing much more， and their pH reached 9.6 at the highest. From the effect of eluent concentration on soil pH， the pH values of the soil near the anode in the experiments （M4， M7） with a concentration of 0.1 mol/L were lower than those in the experiments （M3， M6） with a concentration of 0.05 mol/L， while the pH values of the soil near the cathode were higher than those in the experiments （M3， M6） with a concentration of 0.05 mol/L. From the perspective of the reaction time， the longer the reaction time was， the greater effect the remediation had on the soil pH. The experiments for 10 d （M3， M4， M6， M7） produced higher pH changes in the soil close to the cathode than the experiments for 5 d （M2， M5）， and the maximum difference was 1.34. Distribution of heavy metals in soil
Leaching and electrokinetic combined remediation gradually enriched Cd， Cu， Pb， and Zn in the soil to the bottom of the soil column. The soil reaction chamber was divided into upper， middle and lower layers and the leachate. The percentage contents of heavy metals after the reaction in each part is shown in Fig. 5. The eluent of the M1 group was deionized water， which had a weak ability to promote the desorption of heavy metals in the soil， and could not react with heavy metals， resulting in a poor remediation effect. Using EDTA or acetic acid as the eluent could effectively promote the downward migration of heavy metals. It was mainly due to the acidic effect and complexation of the eluent， which released the heavy metals that are difficult to escape from the soil during electrokinetic remediation. Meanwhile， the participation of the electric field promoted metal dissolution and migration through electrophoresis and electrodialysis. The combination of the two had achieved significant remediation results. The M2-M7 groups showed that the enrichment effect of heavy metals to the bottom of the soil column was directly proportional to the concentration of the eluent and the reaction time. From the perspective of the removal effect of heavy metals， the remediation ability of EDTA was stronger than acetic acid. Among them， M7 treated the soil for 10 d with 0.1mol/L EDTA as the eluent， and achieved good removal effects on Cd， Cu， Pb， and Zn， the percentage contents of which in the leachate were 57.9%， 59.6%， 42.3% and 43.7%， respectively. After treating the soil with this device， M7 made heavy metals move to the bottom of the soil column， and the removal rates of Cd， Cu， Pb， and Zn were as high as 94.4%， 93.3%， 91.4% and 92.8% in the upper layer， and 87.2%， 88.2%， 83.3% and 84.6% in the middle layer， respectively.
Analysis of experimental results of intermittent energization method and voltage-increasing method
Fig. 6 shows the change of heavy metal content in each part of soil after enhanced experiments （K1， K2）. It can be seen from the figure that after the remediation of K1 and K2， the percentage contents of heavy metals in the upper and middle soil samples near the anode was low， close to 0， exhibiting a very good remediation effect. K1 showed removal rates of 99.7%， 99.2%， 97.3%， and 98.4% for Cd， Cu， Pb and Zn in the upper layer， and removal rates of 90.6%， 92.3%， 87.9% and 90.2% in the middle layer， respectively. As to K2， the removal rates for Cd， Cu， Pb and Zn were as high as 96.3%， 95.1%， 94.3% and 94.6% in the upper layer， respectively， and those for Cd， Cu， Pb and Zn in the middle layer were 89.5%， 90.4%， 85.4% and 87.6%， respectively. However， the percentage contents of heavy metals in the lower soil sample of K2 near the cathode were very high， which meant that a large amount of heavy metal ions were concentrated in this part of soil sample， so the overall removal effect was not improved. Compared with M7 under normal energization， the overall removal rates of K1 and K2 for Cd， Cu， Pb and Zn increased by 6.3%， 6.1%， 5.9% and 6%， and 0.6%， 0.9%， 0.5%， and 0.4%， respectively， and the electricity consumption was saved by 46.8% and 10.3%， respectively. It showed that the intermittent energization method was an effective method to improve electrokinetic and leaching remediation of heavy metal-contaminated soil[17]. Comparing the current changes of continuous power supply and intermittent power off， Fig. 7 shows that the current value suddenly increased after powering off and re-energization， which was nearly twice the value before powering off， but quickly decreased to the current value before powering off. After the second time of powering off and powering on， the current in the soil sample increased briefly and continued to maintain at a higher level than before powering off， that is， from 0.05-0.06 A to 0.1-0.12 A. It might be because that the transient powering off caused the precipitated metal to melt.
Conclusions
The upper and lower electrodes could combine the leaching technique and the electrokinetic technique well， so that heavy metals were enriched to the bottom of the soil column， and the removal of Cd， Cu， Pb and Zn in the soil achieved good results. The combined remediation technique was 23.8% less in the average consumption of EDTA than pure leaching remediation without power supply.
EDTA had a stronger ability to complex heavy metals than acetic acid. When using 0.1 mol/L EDTA as the eluent to treat the soil for 10 d， the removal rates of Cd， Cu， Pb and Zn reached 94.4%， 93.3%， 91.4% and 92.8% in the upper layer （1#） and 87.2%， 88.2%， 83.3% and 84.6% in the middle layer （3#） of the soil column， respectively.
The intermittent power-off method can increase the intensity of the current in the soil during a period of time following powering off and powering on， so that the decline period of current becomes longer， thereby improving the remediation effect of the electrokinetic method. The voltage-increasing method can increase the current during the period with increased voltage. Although it can also improve the removal rate， the power consumption is higher than the intermittent power off method.
References
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[2] XU Y， XU X， HOU H， et al. Moisture content-affected electrokinetic remediation of Cr （VI）-contaminated clay by a hydrocalumite barrier[J]. Environmental Science and Pollution Research， 2016， 23（7）： 6517-6523.
[3] MA CY， CAI DJ， YAN H. Research progress of soil cadmium pollution and its treatment technology[J]. Henan Chemical Industry， 2013， 30（16）： 17-22. （in Chinese）
[4] GUO XF， WEI ZB， WU QT. Degradation ad residue of EDTA used for soil repair in heavy metal-contaminated soil[J]. Transactions of the Chinese Society of Agricultural Engineering， 2015， 31（7）： 272-278. （in Chinese） [5] LI YS， FENG CL， WU XF， et al. Research progress on microbial function in phytoremediation of heavy metal-contaminated soil[J]. Acta Ecologica Sinica， 2015， 35（20）： 6881-6890. （in Chinese）
[6] XU YZ， FANG ZQ. Research progress on biochar remediation of heavy metals in soil[J]. Environmental Engineering， 2015， 33（2）： 156-159， 172. （in Chinese）
[7] FAN YL， WANG ZZ. Research progress of in-situ soil remediation technology[J]. Agriculture and Technology， 2015， 35（18）： 29-30. （in Chinese）
[8] LAGEMAN R. Electroreclamation applications in the Netherlands[J]. Environmental Science &Technology， 1993， 27（13）： 2648-2650.
[9] PENG LM. Experimental study on electrokinetic method and its enhancement technology to remediate cadmium-contaminated soil[D]. Chengdu： Chengdu University of Technology， 2013. （in Chinese）
[10] TAN XY， LI D， LI Y， et al. Experiment on electrokinetic and flushing jointed ex-situ remediation of Pb-contaminated soil[J]. Journal of Chongqing Technology and Business University： Natural Science Edition， 2014， 31（11）： 89-92. （in Chinese）
[11] LI YL. Study on removal of cadmium from soil by electrokinetic remediation[J]. Journal of Henan University of Engineering： Natural Science Edition， 2016， 28（1）： 42-46. （in Chinese）
[12] FU RB， LIU F， MA J， et al. Electrokinetic remediation of chromium （VI）-contaminated soil by permeable reactive composite electrode method[J]. Environmental Science， 2012， 33（1）： 280-285. （in Chinese）
[13] ZHOU M， TANG HY， ZHU SF， et al. EDTA-enhanced electrokinetic remediation of heavy metals co-contaminated soil[J]. Chinese Journal of Environmental Engineering， 2014， 8（3）： 1197-1202. （in Chinese）
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[Methods] Leaching and electrokientic combined remediation was applied to remediate artificially-simulated composite Cd， Cu， Pb， and Zn-contaminated soil. The electrokinentic remediation of the soil was carried out using EDTA and acetic acid as the eluents with electrodes placed on the top and bottom of the soil， to investigate the effects of different types and concentrations of eluents， reaction time and energization methods on the remediation.
[Results] Applying electrodynamic force to the leaching remediation process achieved good removal effects of Cd， Cu， Pb， and Zn in the soil. Compared with pure leaching， the leaching and electrokinetic combined remediation saved 23.8% of EDTA consumption. EDTA showed the ability to complex heavy metals better than acetic acid， and after 10 d of reaction， the 0.1 mol/L EDTA achieved very good removal effects on Cd， Cu， Pb， and Zn from the upper and middle layers of the soil column， exhibiting removal rates of 94.4%， 93.3%， 91.4% and 92.8% in the upper layer and 87.2%， 88.2%， 83.3% and 84.6%， respectively. Compared with the conventional experiments， the intermittent energization method and the voltage-increasing method improved the removal rates of Cd， Cu， Pb and Zn by 6.3%， 6.1%， 5.9% and 6%， and 0.6%， 0.9%， 0.5% and 0.4%， respectively， and saved 46.8% and 10.3% of energy consumption， respectively.
[Conclusions] The study provides reference for the remediation of composite heavy metal-contaminated soil.
Key words EDTA; Acetic acid; Heavy metals; Removal rate; Energy consumption
In recent years， the area of heavy metal-contaminated land has been increasing， and the degree of contamination also has been increasing[1]. Heavy metal pollution not only causes damage to the environment and economy， but also threatens human health[2]. The problem of remediation of heavy metal-contaminated soil has caused widespread concern from scholars at home and abroad. At present， the remediation techniques of heavy metal-contaminated soil can be divided into three categories： physical methods， chemical methods and biological methods[3]. Some of these methods may affect the soil itself， and some consume a lot of manpower and material resources[4-6]. Electrokinetic remediation is an in-situ soil remediation technology， which has the advantages of simple operation and low environmental impact， and has a wide application prospect[7]. In the 1990s， the Netherlands launched five large-scale electrokinetic remediationn projects for heavy metal-contaminated sites， involving removal of copper， lead， cadmium and nickel in the soil[8]. Peng[9] used distilled water as an electrolyte in the experimental study to investigate the effects of different voltages on the remediation effect and concluded that the traditional electrokinetic remediation had a poor removal effect of heavy metals in the soil and a low remediation efficiency. Researchers at home and abroad have begun to try to combine electrokinetic remediation technique with other methods to improve the remediation efficiency， with an attempt to improve the treatment efficiency by degrading， destroying or even changing the original state of pollutants. In order to further improve the efficiency of electrokinetic remediation， Tan et al.[10] increased a leaching process to avoid heavy metal ions forming a concentrated zone in the soil. When combining the electrokinetic remediation technique with organic acid leaching， a large amount of heavy metal ions are released from the soil by organic acids， overcoming the inhibition of pH on electrokinetic remediation， and the leaching effect can be promoted by electric field. Materials and Methods
Experimental apparatuses and equipment
MS-605D DC power supply （Maisheng）; PHS-3C pH meter （AIPLI Instrument Co.， Ltd.）; flame atomic absorption spectrophotometer （Agilent 240FSAA）; YH-A2003 balance （Yingheng）; H3018DR centrifuge （Shanghai Zhixin Experimental Instrument Technology Co.， Ltd.）.
Soil samples
The test soil was taken from the vegetable field near Changzhou Science and Education City， Jiangsu Province， with a sampling depth of 0-20 cm. After air drying and removal of stones and weeds， the soil samples were sieved through a 5 mm nylon sieve and placed in a cool place for later use. Heavy metal solutions were prepared to artificially contaminate soil in accordance with "Risk Screening Guideline Value for Soil Contamination of Developed Land （Third Draft for Seeking Opinions）". The reagents used in the contamination of soil were CdCl2·5H2O， Pb （NO3）2， and CuSO4·5H2O. The soil was added into a rotary mixer， and sprayed with stirring. The soil， which was contaminated evenly， was then stood for one year. The basic properties of the soil after the simulation are shown in Table 1.
Experimental device
The reaction device is composed of plexiglass （175 mm×100 mm×100 mm）， inside which are disposed two division plates of 3 mm thick. The size of the intermediate soil reaction chamber is 100 mm×100 mm×100 mm. Several small holes are distributed on the division plates. Two layers of non-woven fabric are placed on each division plate， with a layer of filter paper sandwiched therebetween， to separate the soil from the electrolytic cell and prevent soil particles from entering the electrolytic cell. A shower nozzle is arranged on the top of the reaction device， and a peristaltic pump is connected outside to introduce an eluent into the shower nozzle. Meanwhile， a layer of 3 mm thick quartz sand is laid on the non-woven fabric of the upper division plate to make the leaching solution pass through the quartz sand uniformly. A leachate collection bottle is placed at the bottom of the reaction device to collect the solution flowing out of the reaction device. The top electrode of the soil is set as the anode， and the bottom electrode is set as the cathode. The electrolytic cell is collected to an external ammeter and a DC power supply. The schematic diagram of the device is shown in Fig.1[11-12].
Experimental program
Group experiment
The prepared composite contaminated soil was weighed and filled into the soil reaction chamber. At the same time， a certain amount of deionized water was added， and the soil was continuously stirred to make the soil moisture content at about 33%. After aging for 24 h， the electrokinetic leaching was performed. Before the experiments， EDTA and acetic acid at concentrations of 0.05 and 0.1 mol/L were prepared， and the pH was adjusted to 4.5 with HNO3 and NaOH. Operation of electrokinetic leaching experiments： Corresponding eluent was added into the soil column， and the power was switched on when the first drop of leachate flew out of the small holes of the lower layer; the voltage was constant at 20 V， and the voltage gradient was maintained at 2 V/cm; and the flow was controlled with the peristaltic pump， to keep the upper liquid level at about 3 mm. The design of the group experiments is shown in Table 2. Enhanced experiments
Intermittent power on experiment （K1）： The system was energized for 120 h and then powered off for 12 h， forming a cycle， followed by another 120 h of energization， and the leaching remediation was kept on when the system was powered off.
Voltage-increasing method （K2）： The voltage was constant at 20 V for 120 h and then increased to 30 V for 12 h， which was a cycle， and the leaching remediation was kept on when power off.
Testing method
During the experiments， the current value was recorded every 12 h. After the reaction， soil samples were taken for analysis at different distances （1， 3， 5， 7 and 9 cm） from the anode， and numbered 1#， 2#， 3#， 4# and 5#. The pH， moisture content and contents of various metals were measured. Each experiment was performed in parallel.
Results and Discussion
Effect of electrokinetic force on the effect of leaching remediation
After treating the soil for 10 d with EDTA at a concentration of 0.1 mol/L as the eluent， the comparison of heavy metal residues between the combined electrokinetic leaching test （M7） and the pure leaching test （M8） for which no power was applied is shown in Fig. 2. It can be seen that the residual amounts of Cd， Cu， Pb， and Zn of M8 were 4.84， 193.06， 278.43 and 398.46 mg/kg in the upper layer and 8.09， 279.28， 440.26 and 669.62 mg/kg in the middle layer， respectively. M7 reduced the residual amounts of Cd， Cu， Pb and Zn by 203.5%， 145.2%， 136.7% and 141.8% in the upper layer and 48.9%， 40.3%， 19.6% and 31.2% in the middle layer， respectively. The removal rates of Cd， Cu， Pb and Zn were 57.9%， 59.6%， 42.3% and 43.7% in experiment M7， and 53.4%， 55.2%， 46.7% and 49.2% in experiment M8， respectively， so both the two experiments had certain removal effects on Cd， Cu， Pb and Zn. However， compared with M8， the introduction of the voltage in M7 could promote the heavy metals to move toward the cathode at a faster speed， which helped the heavy metals to be concentrated in the lower layer of the soil column[13]. Moreover， it was found that the total removal amounts of heavy metals by M8 and M7 were not much different， but the consumption amount of EDTA in M7 was 232 g/kg， which was 23.8% less than the consumption amount of EDTA in M8. Applying voltage can save the consumption of EDTA， and strengthen the leaching and desorption of heavy metals.
Hui HAN et al. Experimental Study on In-situ Leaching and Electrokinetic Remediation of Composite Heavy Metal-contaminated Soil Change of current
As shown in Fig. 3， the experimental currents of the M1 to M7 groups had approximately the same change trend. In the initial reaction stage， the current value of the system was low， then gradually increased， and gradually decreased after reaching an extreme value. The change in current reflects the amount of charged ions transferred. After the system was powered on， electrode reactions occurred at the two electrodes[14]. Under the action of the electric field， H+， OH- and acid group anions migrated to the soil， and heavy metals in the soil changed from the adsorbed phase to the dissolved phase， and then undergone complexation reaction with acid group ions. These processes took some time， so the current was initially weak. As the reaction progressed， the ions migrated in the system gradually increased， and the current value started to rise， reflecting that the heavy metal ions were continuously flowing to the electrode liquid. After a period of reaction， the current began to slowly decrease and approached equilibrium， indicating that the content of dissolved heavy metals in the system decreased， and the number of ions that can be migrated gradually decreased. The migration of other ions in the soil maintained the systems equilibrium current. It could also be seen from the figure that the currents in the experiments （M5， M6 and M7） using EDTA disodium salt as the eluent were higher than those in the experiments （M2， M3 and M4） using acetic acid as the eluent， which reflected that Na+ played a strong role in promoting the improvement of current and conductivity[15]. On the other hand， acetic acid is a weak acid， so the concentration of the acid group ions forming metal complex in the liquid was small， which made the increase of the current in the system not easy. Furthermore， it could also be seen from the figure that the magnitude of the current value was directly proportional to the concentration of the electrolyte， indicating that the current of an electrokinetic remediation system is closely related to the concentration of migratable ions in the system. A larger concentration is more conductive to the improvement of current.
Change of pH value
As shown in Fig. 4， the pH values of the soil in various test groups had the same trend， which gradually increased from the anode to the cathode， but could eventually remain stable. It was mainly due to the consumption of electrolyte and electrode polarization， which reduced the current and weakened the electrolysis， resulting in the reduction of OH- production[16]. Meanwhile， the OH- ions generated would be migrated， part of which would be consumed by reaction with soil material and part of which will be consumed by H+. Finally， the pH was relatively stable. From the effects of different eluents on soil pH， in the test （M1） using deionized water as the eluent， the soil near the cathode was under the action of a large amount of OH-， and the pH could reach 9.42. Compared with the experiments （M2， M3， M4） with acetic acid as the eluent， the experiments （M5， M6， M7） with EDTA as the eluent showed the pH of the soil near the cathode changing much more， and their pH reached 9.6 at the highest. From the effect of eluent concentration on soil pH， the pH values of the soil near the anode in the experiments （M4， M7） with a concentration of 0.1 mol/L were lower than those in the experiments （M3， M6） with a concentration of 0.05 mol/L， while the pH values of the soil near the cathode were higher than those in the experiments （M3， M6） with a concentration of 0.05 mol/L. From the perspective of the reaction time， the longer the reaction time was， the greater effect the remediation had on the soil pH. The experiments for 10 d （M3， M4， M6， M7） produced higher pH changes in the soil close to the cathode than the experiments for 5 d （M2， M5）， and the maximum difference was 1.34. Distribution of heavy metals in soil
Leaching and electrokinetic combined remediation gradually enriched Cd， Cu， Pb， and Zn in the soil to the bottom of the soil column. The soil reaction chamber was divided into upper， middle and lower layers and the leachate. The percentage contents of heavy metals after the reaction in each part is shown in Fig. 5. The eluent of the M1 group was deionized water， which had a weak ability to promote the desorption of heavy metals in the soil， and could not react with heavy metals， resulting in a poor remediation effect. Using EDTA or acetic acid as the eluent could effectively promote the downward migration of heavy metals. It was mainly due to the acidic effect and complexation of the eluent， which released the heavy metals that are difficult to escape from the soil during electrokinetic remediation. Meanwhile， the participation of the electric field promoted metal dissolution and migration through electrophoresis and electrodialysis. The combination of the two had achieved significant remediation results. The M2-M7 groups showed that the enrichment effect of heavy metals to the bottom of the soil column was directly proportional to the concentration of the eluent and the reaction time. From the perspective of the removal effect of heavy metals， the remediation ability of EDTA was stronger than acetic acid. Among them， M7 treated the soil for 10 d with 0.1mol/L EDTA as the eluent， and achieved good removal effects on Cd， Cu， Pb， and Zn， the percentage contents of which in the leachate were 57.9%， 59.6%， 42.3% and 43.7%， respectively. After treating the soil with this device， M7 made heavy metals move to the bottom of the soil column， and the removal rates of Cd， Cu， Pb， and Zn were as high as 94.4%， 93.3%， 91.4% and 92.8% in the upper layer， and 87.2%， 88.2%， 83.3% and 84.6% in the middle layer， respectively.
Analysis of experimental results of intermittent energization method and voltage-increasing method
Fig. 6 shows the change of heavy metal content in each part of soil after enhanced experiments （K1， K2）. It can be seen from the figure that after the remediation of K1 and K2， the percentage contents of heavy metals in the upper and middle soil samples near the anode was low， close to 0， exhibiting a very good remediation effect. K1 showed removal rates of 99.7%， 99.2%， 97.3%， and 98.4% for Cd， Cu， Pb and Zn in the upper layer， and removal rates of 90.6%， 92.3%， 87.9% and 90.2% in the middle layer， respectively. As to K2， the removal rates for Cd， Cu， Pb and Zn were as high as 96.3%， 95.1%， 94.3% and 94.6% in the upper layer， respectively， and those for Cd， Cu， Pb and Zn in the middle layer were 89.5%， 90.4%， 85.4% and 87.6%， respectively. However， the percentage contents of heavy metals in the lower soil sample of K2 near the cathode were very high， which meant that a large amount of heavy metal ions were concentrated in this part of soil sample， so the overall removal effect was not improved. Compared with M7 under normal energization， the overall removal rates of K1 and K2 for Cd， Cu， Pb and Zn increased by 6.3%， 6.1%， 5.9% and 6%， and 0.6%， 0.9%， 0.5%， and 0.4%， respectively， and the electricity consumption was saved by 46.8% and 10.3%， respectively. It showed that the intermittent energization method was an effective method to improve electrokinetic and leaching remediation of heavy metal-contaminated soil[17]. Comparing the current changes of continuous power supply and intermittent power off， Fig. 7 shows that the current value suddenly increased after powering off and re-energization， which was nearly twice the value before powering off， but quickly decreased to the current value before powering off. After the second time of powering off and powering on， the current in the soil sample increased briefly and continued to maintain at a higher level than before powering off， that is， from 0.05-0.06 A to 0.1-0.12 A. It might be because that the transient powering off caused the precipitated metal to melt.
Conclusions
The upper and lower electrodes could combine the leaching technique and the electrokinetic technique well， so that heavy metals were enriched to the bottom of the soil column， and the removal of Cd， Cu， Pb and Zn in the soil achieved good results. The combined remediation technique was 23.8% less in the average consumption of EDTA than pure leaching remediation without power supply.
EDTA had a stronger ability to complex heavy metals than acetic acid. When using 0.1 mol/L EDTA as the eluent to treat the soil for 10 d， the removal rates of Cd， Cu， Pb and Zn reached 94.4%， 93.3%， 91.4% and 92.8% in the upper layer （1#） and 87.2%， 88.2%， 83.3% and 84.6% in the middle layer （3#） of the soil column， respectively.
The intermittent power-off method can increase the intensity of the current in the soil during a period of time following powering off and powering on， so that the decline period of current becomes longer， thereby improving the remediation effect of the electrokinetic method. The voltage-increasing method can increase the current during the period with increased voltage. Although it can also improve the removal rate， the power consumption is higher than the intermittent power off method.
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