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Abstract [Objectives] This study was conducted to explore the effect of electrokinetic remediation of Cd-contaminated soil.
[Methods] By preparing Cd-contaminated soil and constructing an electrokinetic remediation experimental device， the effects of changing remediation time， voltage gradient， electrolyte type and power-on method on the removal rate of Cd were studied.
[Results] When the voltage gradient was increased from 0.5 to 1.0 V/cm， the total Cd removal rate increased by 13%; and when the voltage gradient was increased from 1.0 to 1.5 V/cm， the total Cd removal rate only increased by 3%. Compared with 72 h of electrokinetic remediation， 96 h of remediation increased the remediation rate by 4%， and the total energy consumption increased by 13%， while 120 h of remediation increased the remediation rate by 6.5%， and the total energy consumption increased by 67%. Compared with citric acid as electrolyte， the remediation rate of the acetic acid group increased by 12%， and the total energy consumption increased by 42%， while the remediation rate of the hydrochloric acid group increased by 18%， and the total energy consumption increased by 70.5%. Compared with the conventional remediation method， the total Cd removal rates achieved by the different power-on methods increased by 9% and 31%， respectively， and the total energy consumption decreased by 58% and 45%， respectively.
[Conclusions] The electrokinetic remediation technology has certain advantage in remediating Cd-contaminated soil.
Key words Voltage gradient; Electrolyte type; Intermittent energization; Removal rate; Enerey consumption
Soil， as a main natural resource that people depend on， provides necessary material and energy for human survival and development. However， with the development of industry and urbanization and the increase in the use of chemicals， the level of soil cadmium pollution has continued to increase[1-3]. Cadmium pollution not only causes damage to the environment and economy， but also poses a threat to human health. Cd is concentrated in the human body and is likely to cause cancer and other diseases[4]. At present， the remediation techniques of soil cadmium contamination are mainly divided into three kinds of methods： chemical methods， physical methods and biological methods[5]， but these methods have certain limitations.
Electrokinetic remediation has become a research hotspot because it has no secondary pollution， less disturbance to the soil， and is suitable for sticky soils[6]. It is also listed as a new type of "green soil remediation technique". The principle of electrokinetic remediation technique is to apply an electric field at both ends of the contaminated soil to make the heavy metals in the soil move relative to each other under the action of electroosmosis， electromigration and electrophoresis， migrate to both ends of the electrode and enrich in the electrode areas， thereby achieving heavy metal removal. Peng[7] used distilled water as the electrolyte in the experimental research to study the effects of different voltages on the remediation effect and concluded that the traditional electrokinetic remediation technique had a poor removal effect on Cd in the soil and a low remediation efficiency. Li et al.[8] discussed the remediation effects of electrokinetic techniques based on different enhancers on heavy metal-contaminated soil. In this study， Cd-contaminated soil was prepared by artificial simulation and an electrokinetic remediation test device was set up to explore the optimization of working conditions for electrokinetic remediation of Cd-contaminated soil， aiming to provide a basis for effective treatment of cadmium-contaminated soil.
Materials and Methods
Experimental device
The experimental device shown in Fig. 1 includes an electrode chamber， a soil chamber， a DC power supply， partition plates， electrode rods， a peristaltic pump， copper wires， and a non-woven filter. The sizes of the cathode and anode electrode chambers are 100 mm×150 mm×100 mm， and the size of the soil chamber is 200 mm×150 mm×100 mm.
Instruments and materials
Experimental instruments： MS605D DC power supply （Maisheng）; PHS-3C pH meter （Aipu Metrical Instrument Co.， Ltd.）; flame atomic absorption spectrophotometer （Agilent 240FSAA）.
Experimental soil samples： The soil sample was taken from the vacant land of Changzhou Science and Education City， and air-dried and ground through a 100-mesh sieve. The Cd-contaminated soil was prepared and determined to have a pH of 7.50 and a Cd concentration of 1 600 mg/kg.
Before the experiment， the prepared Cd-contaminated soil was weighed， input to the soil chamber layer by layer， and compacted. An electrolyte was added to the positive and negative electrode chambers， and the electric reaction unit was connected to the power supply with wires and applied with a constant voltage. According to the different distances from the anode （2， 6， 10， 14， 18 cm）， sampling holes were set up and numbered 1， 2， 3， 4 and 5， respectively. The current was recorded every 6 h， and the pH was measured every 12 h. After the power was turned off， the soil in the soil chamber was air-dried， mixed， and divided into 16 parts to determine the total soil Cd concentration.
Calculation of removal rate and energy consumption
Total Cd removal rate of=（Total Cd concentration before treatment-Total Cd concentration after treatment）/Total Cd concentration before treatment
Electric energy consumption： E=U∫Idt（1）
Wherein E is the power consumption required by electrokinetic remediation （kw·h）; U is the applied voltage （V）; I is the current intensity （A）; and t is the operation time （h）.
Experimental scheme
The effects of changing the remediation time， voltage gradient， electrolyte and power-on method on the remediation effect were discussed. The specific experimental schemes are shown in Table 1 and Table 2. Results and Discussion
Effect of changing voltage on the remediation effect
It can be seen from Fig. 2 that the current change trends of groups A1 to A4 were basically the same. The current first increased rapidly， then reached a maximum value， and gradually decreased and tended to be stable. As the voltage increased， the current increased， which was due to the replacement of heavy metal ions in the soil to free state by the H+ produced through anodic electrolysis. Under a higher voltage， the free ion concentration was higher， and the current changed faster. The reason for the decrease of the current in the A1-A4 groups in the later stage was that the positive ions in the soil and the H+ generated by the anode were neutralized with the OH- generated by the cathode first， which not only reduced the mobile free ions in the soil solution， but also might block the soil voids[9].
The total Cd removal rate and energy consumption of groups A1 to A4 are shown in Fig. 3. The Cd removal rate increased with the increase of the applied voltage gradient， because the strength of the voltage gradient directly affected the strength of the electrolyte hydrolysis reaction. In general， the greater the current， the stronger the electrochemical reaction[10]. However， the higher the voltage， the greater the current and the more the energy consumption[11]. When the voltage gradient was increased from 0.5 to 1.0 V/cm， the total Cd removal rate increased by 13%; and when the voltage gradient was increased from 1.0 to 1.5 V/cm， the total removal rate only increased by 3%. Comprehensively， the voltage gradient of 1.0 V/cm was more suitable.
Effect of Changing remediation time on the remediation effect
As can be seen from Table 3， the total Cd removal rate of the soil gradually increased with the progress of the reaction; and until 96 h after the reaction， the total Cd removal rate decreased and tended to be stable. Compared with B2， the remediation rate of B3 increased by 4%， and the total energy consumption increased by 13%， while the remediation rate of B4 increased by 6.5%， and the total energy consumption increased by 67%. Taken together， the optimal response time was 96 h.
Effect of changing electrolyte on the remediation effect
Fig. 4 and Fig. 5 show that the current values during operation ranked as hydrochloric acid>acetic acid>citric acid. Compared with citric acid， the acetic acid group increased the remediation rate by 12% and the total energy consumption by 42%， while the hydrochloric acid group increased the remediation rate by 18% and the total energy consumption by 70.5%. The stronger the acidity of the electrolyte， the higher the removal rate. When the entire soil reaction chamber is kept in an acidic environment， it is beneficial to release the heavy metals on the surface of the soil particles， and can promote the migration of heavy metals to the cathode， thereby ensuring the direction of electroosmosis from the anode to the cathode and improving the heavy metal removal rate[12]. The effect of changing power-on method on the remediation effect
It can be seen from Fig. 6a that the current value suddenly increased when the power was turned on again at several points after power off， and the current in the soil sample had a short peak， but quickly dropped back to the current value before power off. It might be because after power off， the heavy metals in precipitated state dissolved again， so free ions increased and the current increased， after the current was turned on. Then， the dissolution was saturated， the precipitation was generated， and the current was reduced. It can be seen from Fig. 6b that under the T2 power-on mode， the pH value changed significantly. It was mainly due to that under the action of electrodynamic force， electrolysis occurred near the two electrodes and H+ and OH- in the aqueous solution moved to the two electrodes at the same time， causing the pH of the negative and positive electrodes to rise and fall， respectively; and after the power was turned off， the electrolytic reaction stopped， the alkaline soil neutralized the acid soil， and the pH of the negative and positive electrodes changed reversely.
As shown in Fig. 7a， the electrode directions were exchanged at 48 h. As a result， the current value increased rapidly within 6 h， and then gradually decreased to stable. It could be seen that after the exchange of the electric field， a large amount of heavy metal ions that were precipitated or left in the excited state migrated， so the current value increased and the heavy metal removal rate increased， which can effectively improve the effect of electrokinetic remediation. It can be seen from Fig. 7b that after the termination of the remediation activity， compared with the T1 group， the pH of the soil in the T3 power-on mode changed more gently， and basically remained within the neutral range. After exchanging the directions of the electrodes， the OH- around the original anode was neutralized by the H+ produced by the later anode， and the original cathode producing OH- at first became the anode producing H+， thereby preventing a large change in pH and avoiding focusing. Moreover， placing the contaminated soil under neutral conditions could help reduce the amount of Cd ions precipitate， and the Cd concentration was thus reduced， ultimately achieving a higher remediation effect， which was more prominent than the conventional T1 remediation group.
It can be seen from Table 4 that compared with T1， T2 improved the removal rate of heavy metals from 36% to 45%， and reduced the power consumption from 82 to 34.5 kW·h. Under the premise of the same other conditions， the remediation effect of the T2 group was more significant than that of the T1 group. It can also be seen from Table 4 that compared with T1， the removal rate of heavy metals in T3 increased from 36% to 67%， and the power consumption decreased from 82 to 45 kW·h.
Comparing T2 and T3 with T1， the total Cd removal rates increased by 9% and 31%， respectively， and the energy consumption decreased by 58% and 45%， respectively. Compared with T2， T3 showed slightly higher energy consumption， but the remediation effect was significant.
Conclusions
When the voltage gradient was increased from 0.5 to 1.0 V/cm， the total Cd removal rate increased by 13%; and when the voltage gradient was increased from 1.0 to 1.5 V/cm， the total Cd removal rate only increased by 3%. Comprehensively， the voltage gradient of 1.0 V/cm was more suitable.
Compared with 72 h of electrokinetic remediation， 96 h of remediation increased the remediation rate by 4%， and the total energy consumption increased by 13%， while 120 h of remediation increased the remediation rate by 6.5%， and the total energy consumption increased by 67%. Comprehensively， 96 h of remediation was more suitable.
Compared with citric acid as electrolyte， the remediation rate of the acetic acid group increased by 12%， and the total energy consumption increased by 42%， while the remediation rate of the hydrochloric acid group increased by 18%， and the total energy consumption increased by 70.5%. The acetic acid group had an advantage in total energy consumption.
Compared with the conventional remediation method， the removal rate of Cd by the intermittent power-on method increased by 9%， and the energy consumption decreased by 58%. The removal rate of Cd by the method of exchanging electrode direction increased by 31%， and the energy consumption decreased by 45%.
The electrokinetic remediation technology has certain advantage in remediating Cd-contaminated soil.
References
[1] WANG N， WEI Y. Investigation on sources of soil heavy metal cadmium pollution and its remediation techniques[J]. Environment and Developmen， 2019（8）： 55-56， 58. （in Chinese）
[2] VIRKUTYT J， SILLANP M， LATOSTENMAA P. Electrokinetic soil remediation-critical overview[J]. Science of the Total Environment， 2002， 289（1-3）： 97.
[3] WANG JY， ZHANG DS， STABNIKOVA O， et al. Evaluation of electrokinetic removal of heavy metal from sewage sludge[J]. Journal of Hazardous Materials， 2005， 124（1-3）： 139-146.
[4] 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. [5] XU F. Characteristics and prospect analysis of soil heavy metal pollution remediation techniques[J]. Xiandai Nongcun Keji， 2019， （3）： 85. （in Chinese）
[6] LUO QS， WANG H， ZHANG XH， et al. Enhancement of in situ bioremediation by electrokinetic technology[J]. Environmental Pollution & Control， 2014， 26（4）： 268-271. （in Chinese）
[7] PENG LM. Experimental study on electrokinetic method and its enhancement technique to remediate cadmium-contaminated soil[D]. Chengdu： Chengdu University of Technology， 2013. （in Chinese）
[8] LI WJ， WANG P， XU HY， et al. Effects of electrokinetic technology with different enhancing agents on cadmium removal and enzyme activity in soil[J]. Chinese Journal of Environmental Engineering， 2018（8）： 2320-2327. （in Chinese）
[9] WANG QY， ZHOU DM， CANG L， et al. Application of bioassays to evaluate a copper contaminated soil before and after a pilot-scale electrokinetic remediation[J]. Environmental Pollution， 2009， 157（2）： 410.
[10] LIU H. Field-scale electrokientic remediation of heavy metal contaminated sites[J]. Chinese Journal of Environmental Engineering， 2016， 10（7）： 3877-3883. （in Chinese）
[11] LI YL， LIU L， DUAN WC， et al. Effect of electrokinetic remediation technology on cadmium migration in soil[J]. Chinese Journal of Environmental Engineering，2016（10）： 6021-6027. （in Chinese）
[12] ZHOU DM， DENG CF， ALSHAWABKEH AN， et al. Effects of catholyte conditioning on electrokinetic extraction of copper from mine tailings[J].Environment International，2005，31（6）， 885-890.
[Methods] By preparing Cd-contaminated soil and constructing an electrokinetic remediation experimental device， the effects of changing remediation time， voltage gradient， electrolyte type and power-on method on the removal rate of Cd were studied.
[Results] When the voltage gradient was increased from 0.5 to 1.0 V/cm， the total Cd removal rate increased by 13%; and when the voltage gradient was increased from 1.0 to 1.5 V/cm， the total Cd removal rate only increased by 3%. Compared with 72 h of electrokinetic remediation， 96 h of remediation increased the remediation rate by 4%， and the total energy consumption increased by 13%， while 120 h of remediation increased the remediation rate by 6.5%， and the total energy consumption increased by 67%. Compared with citric acid as electrolyte， the remediation rate of the acetic acid group increased by 12%， and the total energy consumption increased by 42%， while the remediation rate of the hydrochloric acid group increased by 18%， and the total energy consumption increased by 70.5%. Compared with the conventional remediation method， the total Cd removal rates achieved by the different power-on methods increased by 9% and 31%， respectively， and the total energy consumption decreased by 58% and 45%， respectively.
[Conclusions] The electrokinetic remediation technology has certain advantage in remediating Cd-contaminated soil.
Key words Voltage gradient; Electrolyte type; Intermittent energization; Removal rate; Enerey consumption
Soil， as a main natural resource that people depend on， provides necessary material and energy for human survival and development. However， with the development of industry and urbanization and the increase in the use of chemicals， the level of soil cadmium pollution has continued to increase[1-3]. Cadmium pollution not only causes damage to the environment and economy， but also poses a threat to human health. Cd is concentrated in the human body and is likely to cause cancer and other diseases[4]. At present， the remediation techniques of soil cadmium contamination are mainly divided into three kinds of methods： chemical methods， physical methods and biological methods[5]， but these methods have certain limitations.
Electrokinetic remediation has become a research hotspot because it has no secondary pollution， less disturbance to the soil， and is suitable for sticky soils[6]. It is also listed as a new type of "green soil remediation technique". The principle of electrokinetic remediation technique is to apply an electric field at both ends of the contaminated soil to make the heavy metals in the soil move relative to each other under the action of electroosmosis， electromigration and electrophoresis， migrate to both ends of the electrode and enrich in the electrode areas， thereby achieving heavy metal removal. Peng[7] used distilled water as the electrolyte in the experimental research to study the effects of different voltages on the remediation effect and concluded that the traditional electrokinetic remediation technique had a poor removal effect on Cd in the soil and a low remediation efficiency. Li et al.[8] discussed the remediation effects of electrokinetic techniques based on different enhancers on heavy metal-contaminated soil. In this study， Cd-contaminated soil was prepared by artificial simulation and an electrokinetic remediation test device was set up to explore the optimization of working conditions for electrokinetic remediation of Cd-contaminated soil， aiming to provide a basis for effective treatment of cadmium-contaminated soil.
Materials and Methods
Experimental device
The experimental device shown in Fig. 1 includes an electrode chamber， a soil chamber， a DC power supply， partition plates， electrode rods， a peristaltic pump， copper wires， and a non-woven filter. The sizes of the cathode and anode electrode chambers are 100 mm×150 mm×100 mm， and the size of the soil chamber is 200 mm×150 mm×100 mm.
Instruments and materials
Experimental instruments： MS605D DC power supply （Maisheng）; PHS-3C pH meter （Aipu Metrical Instrument Co.， Ltd.）; flame atomic absorption spectrophotometer （Agilent 240FSAA）.
Experimental soil samples： The soil sample was taken from the vacant land of Changzhou Science and Education City， and air-dried and ground through a 100-mesh sieve. The Cd-contaminated soil was prepared and determined to have a pH of 7.50 and a Cd concentration of 1 600 mg/kg.
Before the experiment， the prepared Cd-contaminated soil was weighed， input to the soil chamber layer by layer， and compacted. An electrolyte was added to the positive and negative electrode chambers， and the electric reaction unit was connected to the power supply with wires and applied with a constant voltage. According to the different distances from the anode （2， 6， 10， 14， 18 cm）， sampling holes were set up and numbered 1， 2， 3， 4 and 5， respectively. The current was recorded every 6 h， and the pH was measured every 12 h. After the power was turned off， the soil in the soil chamber was air-dried， mixed， and divided into 16 parts to determine the total soil Cd concentration.
Calculation of removal rate and energy consumption
Total Cd removal rate of=（Total Cd concentration before treatment-Total Cd concentration after treatment）/Total Cd concentration before treatment
Electric energy consumption： E=U∫Idt（1）
Wherein E is the power consumption required by electrokinetic remediation （kw·h）; U is the applied voltage （V）; I is the current intensity （A）; and t is the operation time （h）.
Experimental scheme
The effects of changing the remediation time， voltage gradient， electrolyte and power-on method on the remediation effect were discussed. The specific experimental schemes are shown in Table 1 and Table 2. Results and Discussion
Effect of changing voltage on the remediation effect
It can be seen from Fig. 2 that the current change trends of groups A1 to A4 were basically the same. The current first increased rapidly， then reached a maximum value， and gradually decreased and tended to be stable. As the voltage increased， the current increased， which was due to the replacement of heavy metal ions in the soil to free state by the H+ produced through anodic electrolysis. Under a higher voltage， the free ion concentration was higher， and the current changed faster. The reason for the decrease of the current in the A1-A4 groups in the later stage was that the positive ions in the soil and the H+ generated by the anode were neutralized with the OH- generated by the cathode first， which not only reduced the mobile free ions in the soil solution， but also might block the soil voids[9].
The total Cd removal rate and energy consumption of groups A1 to A4 are shown in Fig. 3. The Cd removal rate increased with the increase of the applied voltage gradient， because the strength of the voltage gradient directly affected the strength of the electrolyte hydrolysis reaction. In general， the greater the current， the stronger the electrochemical reaction[10]. However， the higher the voltage， the greater the current and the more the energy consumption[11]. When the voltage gradient was increased from 0.5 to 1.0 V/cm， the total Cd removal rate increased by 13%; and when the voltage gradient was increased from 1.0 to 1.5 V/cm， the total removal rate only increased by 3%. Comprehensively， the voltage gradient of 1.0 V/cm was more suitable.
Effect of Changing remediation time on the remediation effect
As can be seen from Table 3， the total Cd removal rate of the soil gradually increased with the progress of the reaction; and until 96 h after the reaction， the total Cd removal rate decreased and tended to be stable. Compared with B2， the remediation rate of B3 increased by 4%， and the total energy consumption increased by 13%， while the remediation rate of B4 increased by 6.5%， and the total energy consumption increased by 67%. Taken together， the optimal response time was 96 h.
Effect of changing electrolyte on the remediation effect
Fig. 4 and Fig. 5 show that the current values during operation ranked as hydrochloric acid>acetic acid>citric acid. Compared with citric acid， the acetic acid group increased the remediation rate by 12% and the total energy consumption by 42%， while the hydrochloric acid group increased the remediation rate by 18% and the total energy consumption by 70.5%. The stronger the acidity of the electrolyte， the higher the removal rate. When the entire soil reaction chamber is kept in an acidic environment， it is beneficial to release the heavy metals on the surface of the soil particles， and can promote the migration of heavy metals to the cathode， thereby ensuring the direction of electroosmosis from the anode to the cathode and improving the heavy metal removal rate[12]. The effect of changing power-on method on the remediation effect
It can be seen from Fig. 6a that the current value suddenly increased when the power was turned on again at several points after power off， and the current in the soil sample had a short peak， but quickly dropped back to the current value before power off. It might be because after power off， the heavy metals in precipitated state dissolved again， so free ions increased and the current increased， after the current was turned on. Then， the dissolution was saturated， the precipitation was generated， and the current was reduced. It can be seen from Fig. 6b that under the T2 power-on mode， the pH value changed significantly. It was mainly due to that under the action of electrodynamic force， electrolysis occurred near the two electrodes and H+ and OH- in the aqueous solution moved to the two electrodes at the same time， causing the pH of the negative and positive electrodes to rise and fall， respectively; and after the power was turned off， the electrolytic reaction stopped， the alkaline soil neutralized the acid soil， and the pH of the negative and positive electrodes changed reversely.
As shown in Fig. 7a， the electrode directions were exchanged at 48 h. As a result， the current value increased rapidly within 6 h， and then gradually decreased to stable. It could be seen that after the exchange of the electric field， a large amount of heavy metal ions that were precipitated or left in the excited state migrated， so the current value increased and the heavy metal removal rate increased， which can effectively improve the effect of electrokinetic remediation. It can be seen from Fig. 7b that after the termination of the remediation activity， compared with the T1 group， the pH of the soil in the T3 power-on mode changed more gently， and basically remained within the neutral range. After exchanging the directions of the electrodes， the OH- around the original anode was neutralized by the H+ produced by the later anode， and the original cathode producing OH- at first became the anode producing H+， thereby preventing a large change in pH and avoiding focusing. Moreover， placing the contaminated soil under neutral conditions could help reduce the amount of Cd ions precipitate， and the Cd concentration was thus reduced， ultimately achieving a higher remediation effect， which was more prominent than the conventional T1 remediation group.
It can be seen from Table 4 that compared with T1， T2 improved the removal rate of heavy metals from 36% to 45%， and reduced the power consumption from 82 to 34.5 kW·h. Under the premise of the same other conditions， the remediation effect of the T2 group was more significant than that of the T1 group. It can also be seen from Table 4 that compared with T1， the removal rate of heavy metals in T3 increased from 36% to 67%， and the power consumption decreased from 82 to 45 kW·h.
Comparing T2 and T3 with T1， the total Cd removal rates increased by 9% and 31%， respectively， and the energy consumption decreased by 58% and 45%， respectively. Compared with T2， T3 showed slightly higher energy consumption， but the remediation effect was significant.
Conclusions
When the voltage gradient was increased from 0.5 to 1.0 V/cm， the total Cd removal rate increased by 13%; and when the voltage gradient was increased from 1.0 to 1.5 V/cm， the total Cd removal rate only increased by 3%. Comprehensively， the voltage gradient of 1.0 V/cm was more suitable.
Compared with 72 h of electrokinetic remediation， 96 h of remediation increased the remediation rate by 4%， and the total energy consumption increased by 13%， while 120 h of remediation increased the remediation rate by 6.5%， and the total energy consumption increased by 67%. Comprehensively， 96 h of remediation was more suitable.
Compared with citric acid as electrolyte， the remediation rate of the acetic acid group increased by 12%， and the total energy consumption increased by 42%， while the remediation rate of the hydrochloric acid group increased by 18%， and the total energy consumption increased by 70.5%. The acetic acid group had an advantage in total energy consumption.
Compared with the conventional remediation method， the removal rate of Cd by the intermittent power-on method increased by 9%， and the energy consumption decreased by 58%. The removal rate of Cd by the method of exchanging electrode direction increased by 31%， and the energy consumption decreased by 45%.
The electrokinetic remediation technology has certain advantage in remediating Cd-contaminated soil.
References
[1] WANG N， WEI Y. Investigation on sources of soil heavy metal cadmium pollution and its remediation techniques[J]. Environment and Developmen， 2019（8）： 55-56， 58. （in Chinese）
[2] VIRKUTYT J， SILLANP M， LATOSTENMAA P. Electrokinetic soil remediation-critical overview[J]. Science of the Total Environment， 2002， 289（1-3）： 97.
[3] WANG JY， ZHANG DS， STABNIKOVA O， et al. Evaluation of electrokinetic removal of heavy metal from sewage sludge[J]. Journal of Hazardous Materials， 2005， 124（1-3）： 139-146.
[4] 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. [5] XU F. Characteristics and prospect analysis of soil heavy metal pollution remediation techniques[J]. Xiandai Nongcun Keji， 2019， （3）： 85. （in Chinese）
[6] LUO QS， WANG H， ZHANG XH， et al. Enhancement of in situ bioremediation by electrokinetic technology[J]. Environmental Pollution & Control， 2014， 26（4）： 268-271. （in Chinese）
[7] PENG LM. Experimental study on electrokinetic method and its enhancement technique to remediate cadmium-contaminated soil[D]. Chengdu： Chengdu University of Technology， 2013. （in Chinese）
[8] LI WJ， WANG P， XU HY， et al. Effects of electrokinetic technology with different enhancing agents on cadmium removal and enzyme activity in soil[J]. Chinese Journal of Environmental Engineering， 2018（8）： 2320-2327. （in Chinese）
[9] WANG QY， ZHOU DM， CANG L， et al. Application of bioassays to evaluate a copper contaminated soil before and after a pilot-scale electrokinetic remediation[J]. Environmental Pollution， 2009， 157（2）： 410.
[10] LIU H. Field-scale electrokientic remediation of heavy metal contaminated sites[J]. Chinese Journal of Environmental Engineering， 2016， 10（7）： 3877-3883. （in Chinese）
[11] LI YL， LIU L， DUAN WC， et al. Effect of electrokinetic remediation technology on cadmium migration in soil[J]. Chinese Journal of Environmental Engineering，2016（10）： 6021-6027. （in Chinese）
[12] ZHOU DM， DENG CF， ALSHAWABKEH AN， et al. Effects of catholyte conditioning on electrokinetic extraction of copper from mine tailings[J].Environment International，2005，31（6）， 885-890.