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Abstract In order to improve the efficiency of traditional electrokinetic remediation method of heavy metalcontaminated soil, cadmiumcontaminated soil was remediated by two enhanced electrokinetic techniques, namely, the approaching anode method and intermittent current method. The remediation effect was verified under the conditions that the electric field strength was 2 V/cm, 0.1 mol/L citric acid was used as the electrolyte, and the remediation reaction time was 96 h. The results showed that for the approaching anode method, when the average electromigration distance was 5.48, the removal rate was 88.12%, which was 10.47% higher than the unenhanced technique, and the energy consumption per unit volume was 7.36 kWh/m3; and as to the intermittent current method, the removal rate was 80.81%, and the energy consumption per unit volume was 8.12 kWh/m3, which was relatively reduced by 33.07%. After the remediation, the proportions of the weak acid extractive form and reducible form of calcium decreased, and the proportions of the oxidizable form and residual form of cadmium did not change significantly.
Key words Cadmium; Soil pollution; Electrodynamics; Approaching anode method; Intermittent current method
Received: June 21, 2019Accepted: September 29, 2019
Wanmeng WANG (1987-), male, P. R. China, intermediate engineer, master, devoted to research about water and soil pollution and its prevention.
*Corresponding author. Email: [email protected].
In recent years, with the rapid development of Chinas economy, industry and agriculture, unreasonable exploitation of resources and improper use of agrochemical products, the problem of heavy metal cadmium (Cd) pollution in Chinas soil has become increasingly serious［1］. Soil is the basis of life, and soil pollution can cause extremely serious potential harm to animals, plants and humans. The persistence and irreversibility of heavy metal pollution in soil has become a difficult point in the research field. Studies at home and abroad have shown that chemical, physical and bioremediation methods have been applied to the remediation of polluted soil. Among them, the electrokinetic method has the advantages of a wide range of application, less consumption of chemical reagents, no secondary pollution and little disturbance to the soil［2］, and has become a research hotspot of new insitu remediation technology. Meanwhile, there are some problems in this technology. Due to the polarization phenomenon in the electrokinetic remediation process, the remediation efficiency will be slowed and the energy consumption will increase［3-4］. Therefore, it is a research focus to find a way to increase soil remediation efficiency while reducing energy consumption. The two kinds of enhanced electrokinetic techniques in the study, namely, the approaching anode method and the intermittent current method were adopted. In the approaching anode method, by fixing the cathode position and changing the anode position during the remediation process to shorten the distance between the anode and the cathode, the purpose of improving the remediation effect was achieved. In the intermittent current method, the power was turned off after a period of remediation and then turned on again to increase the remediation current and increases the removal rate of cadmium, so as to achieve the purpose of further reducing energy consumption. This study provides relevant data basis for the deep treatment of heavy metal cadmium in soil.
Experimental Part
Experimental materials
The soil of this experiment was taken from the surrounding area of Science and Education City of Changzhou City, Jiangsu Province. The soil was sampled by the checkerboard sampling method. There were 10 sampling points, where the surface soil was removed, and the soil at a depth of 20 cm was taken. The sampling depth, width and size were unified at various sampling points. The sample was evenly mixed, then tiled and dried, ground through a 100 mesh sieve, and reserved. The physical and chemical properties of the soil were as follows: pH 6.78, conductivity 136 μs/cm, organic matter 7.63 g/kg, and moisture content 4.37%.
FE20K pH meter (Shanghai Kenli Instrument Co., Ltd.) was used to measure soil pH. Artificial cadmiumcontaminated soil was prepared with CdCl2·2.5H2O at a watersoil ratio of 1∶1. The conductivity meter used was DDS11A (Hangzhou Aolilong Instrument Co., Ltd.). Cadmium in soil was digested by triacid digestion method at a high temperature. The form of cadmium in the soil before and after remediation was analyzed by BCR method and determined by AAS (flame atomic absorption spectrophotometer).
Experimental device
In this experiment, a twodimensional orthohexagonal electrode configuration was adopted, and an electrokinetic remediation experiment device was designed and built in laboratory. The experiment device is shown in Fig. 1. A discshaped electric reaction chamber was made of organic glass, with a total radius of 16 cm and a depth of 14 cm, and the distance between two electrodes was 15 cm. The cathode of the device had a radius of 3 cm, and the anodes were arranged in a regular shape. The cathode material was a highpurity graphite rod (with a diameter of 1 cm), and the anode was a stainless steel electrode. Experimental design and process
Before the experiment, 4 kg of artificiallysimulated cadmiumcontaminated soil was weighed and compacted in a soil sample chamber, and a certain amount of soil saturation liquid was added to the soil chamber and the cathode chamber to saturate the soil under force. The experiment used a DC power supply, and the electric field strength was 2 V/cm. The cathode used 0.1 mol/L citric acid as the electrolyte which was kept at pH<4 (under acidic conditions). The current was measured once every 2 h. From the anodes to the cathode, five sampling points were set, each of which had a radius of 1 cm, and the distance between any two of the sampling points was 2.5 cm. The pH value and conductivity at each sampling point were determined, and the cadmium concentration and other properties were determined after the remediation.
The approaching anode method was run for 96 h in the same device. After starting the experiment, the anodes were moved 3 cm toward the cathode every 24 h of remediation, and the applied power supply voltage (30, 24, 18, 12 V) was changed while maintaining the electric field strength of 2 V/cm. After the remediation, sampling, analysis and determination were carried out. The intermittent current method was to repeat the cycle of turning off the powder for 6 h and then turning on the power for 28 h again, The experiment was done in three replicates, to improve accuracy.
Results and Discussion
Current change during the enhanced electrokinetic remediation
The value of current indicates the number of ions that undergo electromigration. Fig. 2 shows the change state of current with the remediation time in the two enhanced electrokinetic methods. In the approaching anode method, as the distance between the two electrodes was shortened, the voltage between the two electrodes was also continuously reduced. The current change was similar to that in the unenhanced remediation. In the initial six times of current measurement, it was obvious that the current grew at a high rate, because the amount of mobile ions used for the reaction in the soil at the beginning of the experiment was large, the H+ produced by the electrolysis at the anodes rapidly migrated in the soil to the cathode, and in the acidic environment, the soluble ions in the soil were resolved more on the surface, resulting in an increase of the current［5-6］. After adjusting the anode distance three times, the current tended to be stable, because the normal positive ions and H+ in the soil were neutralized by OH- in the soil, the amount of mobile ions in the soil was reduced, and the precipitation of heavy metal ions blocked the soil voids, resulting in a decrease in the current. At the beginning of the intermittent current experiment, the current value increased and then rapidly decreased after reaching a peak. The current drop rate slowed down after 12-24 h. At this time, the power was turned off at first and then turned on, the current value suddenly became twice as large as before the power failure, and then quickly dropped to the current value before the power was cut off. This procedure was repeated over and over again, and the entire remediation process was maintained at a current level higher than before the power failure. It can be seen from Fig. 2 that within 24 h of increasing the voltage, the current value was increased by one level, and after the voltage was restored, the current value was stabilized at about 50 mA.
Changes in soil pH and conductivity after the enhanced electrokinetic remediation
In the experiment, the pH value of each sampling point was analyzed by the approaching anode method. Fig. 3 shows that in the late stage of the experiment, in the same period of time, the pH values of the soil at different points decreased obviously. The soil pH of the A1 sampling point 2 cm from the anode dropped from 6.78 to less than 4.0, and the pH of the A2 sampling point also dropped to about 4. At 72 h, after the anodes were 6 cm from the cathode, the pH decreased significantly, and after 96 h, the pH of the A3 sampling point decreased to about 5. It indicated that the distance between the two electrodes in the approaching anode method was decreasing, and the H+ generated by the anodes flew to the cathode, and migrated at a high speed. The soil pH of the whole reaction tank was between 3.8 and 5.5, indicating that the soil in the whole reaction zone was in the acid migration zone which was more acidic than the above reaction, and the more acidic the soil environment was, the easier the heavy metal dissolves［7-8］.
In experiments, if the pH of the cathode is not controlled, since the migration speed of H+ to the cathode is twice the migration speed of OH- to the anode［9］, an acidbase transition zone would appear in the soil sample. In this experiment, the pH of the cathode was controlled, and citric acid was continuously added to the cathode tank, so that the pH of the whole soil chamber was acidic, but due to the continuous production of OH- in the cathode tank, the pH value increased from the anodes to the cathode.
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The curve of conductivity was distributed in a bowl shape. The soil samples near the cathode and anodes had higher conductivity, while the conductivity in the soil chamber was very low. Near the anodes, the pH was very low, and the soil contained a large amount of H+, so the conductivity was high. Near the cathode, there was not only a large amount of OH-, Cd2+ also diffused into the soil sample in the cathode tank, resulting in a very high conductivity. Migration and distribution of heavy metals in soil after the enhanced electrokinetic remediation
In the approaching anode experiment, sampling was carried out at 48, 72 and 96 h, respectively, and the concentration of Cd at 2 cm from the anodes reached the lowest value. After 72 h of remediation, the concentration of Cd in most areas of the soil decreased to 2 mg/kg. As can be seen in Fig. 4, the residual form near the cathode became more, because in the Cd migration process, Cd underwent precipitation reaction and was changed to the residual form due to the alkaline environment near the cathode［10-12］. Near the anodes, the weak acid extractive state and the reducible form decreased very significantly, indicating that under the soil property, it had a good migration effect on the weak acid extractive form and the reducible form. It can be seen from the figure that in the soil near the anodes in intermittent current method, the residual form changed little, while the reducible form and the weak acid extractive state were significantly decreased. The decline ratio of the weak acid extractive form at the sampling points near the cathode was 25%, and the decline ratio of the reducible form was about 8%.
As shown in Fig. 5, the removal rates of the approaching anode method and the intermittent current method were 88.12% and 80.81%, respectively. When the average electromigration distance was 6.3, the energy consumption per unit volume of the approaching anode method was 7.36 kWh/m3, and the remediation time was 36 h. As to the the intermittent current method, the energy consumption per unit volume was 8.12 kWh/m3 at the average electromigration distance of 5.48. It indicated that the two enhanced remediation methods further reduced the energy consumption per unit volume while increasing the removal rate.
Conclusions
Under the condition of electric field strength of 2 V/cm, with 0.1 mol/L citric acid as electrolyte and the remediation reaction time of 96 h, two kinds of enhanced electrokinetic techniques, namely, the approaching anode method and intermittent current method, were adopted to improve the traditional electric remediation of heavy metalcontaminated soil, and the remediation effect on cadmiumcontaminated soil was verified.
(1) Compared with the unenhanced remediation technique, the approaching anode method and the intermittent current method had relatively low pH value and high electrical conductivity. A lower pH and higher conductivity are beneficial to the migration and resolution of Cd. (2) As the distance between the cathode and anodes was gradually shortened, or the current decreased and increased intermittently, the soil that had been remediated did not need to consume electric energy. Therefore, under the condition of maintaining a high Cd removal rate, the two enhanced remediation methods had lower energy consumption per unit volume while maintaining a higher removal rate.
(3) The study shows that the approaching anode method and the intermittent current method are simple in operation, remarkable in remediation effect, extremely advantageous, and have a good application prospect.
References
［1］ KIM, WOOSEUNG, PARK, et al. In situ field scale electrokinetic remediation of multimetals contaminated paddy soil: Influence of electrode configuration［J］. Electrochimica Acta, 2012, 86(1): 89-95.
［2］ KIM DH, JO SU, CHOI JH, et al. Hexagonal two dimensional electrokinetic systems for restoration of saline agricultural lands: A pilot study［J］. Chemical Engineering Journal, 2012(8): 110-121.
［3］ CAMESELLE C. Enhancement of electro-osmotic flow during the electrokinetic treatment of a contaminated soil［J］. Electrochimica Acta, 2015, 181(3): 345-364.
［4］ BES C, MENCH M. Remediation of copper-contaminated topsoils from a wood treatment facility using in situ stabilisation［J］. Environmental Pollution, 2008, 156(3): 1128-1138.
［5］ REDDY KR, CAMESELLE C, REDDY KR, et al. Electrochemical remediation technologies for polluted soils, sediments and groundwater2009,97(4):6645-6657.
［6］ GANG LI, GUO S, LIA S, et al. Comparison of approaching and fixed anodes for avoiding the ‘focusing’ effect during electrokinetic remediation of chromium-contaminated soil［J］. Chemical Engineering Journal, 2012, 203(5):231-238.
［7］ HOEHUN H, OLSON JR, LING B, et al. Analysis of heavy metal sources in soil using kriging interpolation on principal components ［J］. Environmental Science & Technology, 2014, 48(9): 4999-5007.
［8］ DANMALIKI GI, SALEH TA, SHAMSUDDEEN AA. Response surface methodology optimization of adsorptive desulfurization on nickel/activated carbon ［J］. Chemical Engineering Journal, 2016, 313(8):7765-7789.
［9］ QUINTON JN, CATT JA. Enrichment of heavy metals in sediment resulting from soil erosion on agricultural fields［J］. Environmental Science & Technology, 2007, 41(10): 3495-3500.
［10］ CHENG X, DANEK T, DROZDOVA J, et al. Soil heavy metal pollution and risk assessment associated with the Zn-Pb mining region in Yunnan, Southwest China［J］. Environmental Monitoring & Assessment, 2018, 190(4): 1194-1203.
［11］ WEI Z, WANG D, ZHOU H, et al. Assessment of soil heavy metal pollution with principal component analysis and geoaccumulation index ［J］. Procedia Environmental Sciences, 2011, 10(1): 1946-1952.
［12］ YARGICOGLU EN, REDDY KR. Review of biological diagnostic tools and their applications in geoenvironmental engineering［J］. Reviews in Environmental Science & Bio/technology, 2015, 14(2): 161-194.
Key words Cadmium; Soil pollution; Electrodynamics; Approaching anode method; Intermittent current method
Received: June 21, 2019Accepted: September 29, 2019
Wanmeng WANG (1987-), male, P. R. China, intermediate engineer, master, devoted to research about water and soil pollution and its prevention.
*Corresponding author. Email: [email protected].
In recent years, with the rapid development of Chinas economy, industry and agriculture, unreasonable exploitation of resources and improper use of agrochemical products, the problem of heavy metal cadmium (Cd) pollution in Chinas soil has become increasingly serious［1］. Soil is the basis of life, and soil pollution can cause extremely serious potential harm to animals, plants and humans. The persistence and irreversibility of heavy metal pollution in soil has become a difficult point in the research field. Studies at home and abroad have shown that chemical, physical and bioremediation methods have been applied to the remediation of polluted soil. Among them, the electrokinetic method has the advantages of a wide range of application, less consumption of chemical reagents, no secondary pollution and little disturbance to the soil［2］, and has become a research hotspot of new insitu remediation technology. Meanwhile, there are some problems in this technology. Due to the polarization phenomenon in the electrokinetic remediation process, the remediation efficiency will be slowed and the energy consumption will increase［3-4］. Therefore, it is a research focus to find a way to increase soil remediation efficiency while reducing energy consumption. The two kinds of enhanced electrokinetic techniques in the study, namely, the approaching anode method and the intermittent current method were adopted. In the approaching anode method, by fixing the cathode position and changing the anode position during the remediation process to shorten the distance between the anode and the cathode, the purpose of improving the remediation effect was achieved. In the intermittent current method, the power was turned off after a period of remediation and then turned on again to increase the remediation current and increases the removal rate of cadmium, so as to achieve the purpose of further reducing energy consumption. This study provides relevant data basis for the deep treatment of heavy metal cadmium in soil.
Experimental Part
Experimental materials
The soil of this experiment was taken from the surrounding area of Science and Education City of Changzhou City, Jiangsu Province. The soil was sampled by the checkerboard sampling method. There were 10 sampling points, where the surface soil was removed, and the soil at a depth of 20 cm was taken. The sampling depth, width and size were unified at various sampling points. The sample was evenly mixed, then tiled and dried, ground through a 100 mesh sieve, and reserved. The physical and chemical properties of the soil were as follows: pH 6.78, conductivity 136 μs/cm, organic matter 7.63 g/kg, and moisture content 4.37%.
FE20K pH meter (Shanghai Kenli Instrument Co., Ltd.) was used to measure soil pH. Artificial cadmiumcontaminated soil was prepared with CdCl2·2.5H2O at a watersoil ratio of 1∶1. The conductivity meter used was DDS11A (Hangzhou Aolilong Instrument Co., Ltd.). Cadmium in soil was digested by triacid digestion method at a high temperature. The form of cadmium in the soil before and after remediation was analyzed by BCR method and determined by AAS (flame atomic absorption spectrophotometer).
Experimental device
In this experiment, a twodimensional orthohexagonal electrode configuration was adopted, and an electrokinetic remediation experiment device was designed and built in laboratory. The experiment device is shown in Fig. 1. A discshaped electric reaction chamber was made of organic glass, with a total radius of 16 cm and a depth of 14 cm, and the distance between two electrodes was 15 cm. The cathode of the device had a radius of 3 cm, and the anodes were arranged in a regular shape. The cathode material was a highpurity graphite rod (with a diameter of 1 cm), and the anode was a stainless steel electrode. Experimental design and process
Before the experiment, 4 kg of artificiallysimulated cadmiumcontaminated soil was weighed and compacted in a soil sample chamber, and a certain amount of soil saturation liquid was added to the soil chamber and the cathode chamber to saturate the soil under force. The experiment used a DC power supply, and the electric field strength was 2 V/cm. The cathode used 0.1 mol/L citric acid as the electrolyte which was kept at pH<4 (under acidic conditions). The current was measured once every 2 h. From the anodes to the cathode, five sampling points were set, each of which had a radius of 1 cm, and the distance between any two of the sampling points was 2.5 cm. The pH value and conductivity at each sampling point were determined, and the cadmium concentration and other properties were determined after the remediation.
The approaching anode method was run for 96 h in the same device. After starting the experiment, the anodes were moved 3 cm toward the cathode every 24 h of remediation, and the applied power supply voltage (30, 24, 18, 12 V) was changed while maintaining the electric field strength of 2 V/cm. After the remediation, sampling, analysis and determination were carried out. The intermittent current method was to repeat the cycle of turning off the powder for 6 h and then turning on the power for 28 h again, The experiment was done in three replicates, to improve accuracy.
Results and Discussion
Current change during the enhanced electrokinetic remediation
The value of current indicates the number of ions that undergo electromigration. Fig. 2 shows the change state of current with the remediation time in the two enhanced electrokinetic methods. In the approaching anode method, as the distance between the two electrodes was shortened, the voltage between the two electrodes was also continuously reduced. The current change was similar to that in the unenhanced remediation. In the initial six times of current measurement, it was obvious that the current grew at a high rate, because the amount of mobile ions used for the reaction in the soil at the beginning of the experiment was large, the H+ produced by the electrolysis at the anodes rapidly migrated in the soil to the cathode, and in the acidic environment, the soluble ions in the soil were resolved more on the surface, resulting in an increase of the current［5-6］. After adjusting the anode distance three times, the current tended to be stable, because the normal positive ions and H+ in the soil were neutralized by OH- in the soil, the amount of mobile ions in the soil was reduced, and the precipitation of heavy metal ions blocked the soil voids, resulting in a decrease in the current. At the beginning of the intermittent current experiment, the current value increased and then rapidly decreased after reaching a peak. The current drop rate slowed down after 12-24 h. At this time, the power was turned off at first and then turned on, the current value suddenly became twice as large as before the power failure, and then quickly dropped to the current value before the power was cut off. This procedure was repeated over and over again, and the entire remediation process was maintained at a current level higher than before the power failure. It can be seen from Fig. 2 that within 24 h of increasing the voltage, the current value was increased by one level, and after the voltage was restored, the current value was stabilized at about 50 mA.
Changes in soil pH and conductivity after the enhanced electrokinetic remediation
In the experiment, the pH value of each sampling point was analyzed by the approaching anode method. Fig. 3 shows that in the late stage of the experiment, in the same period of time, the pH values of the soil at different points decreased obviously. The soil pH of the A1 sampling point 2 cm from the anode dropped from 6.78 to less than 4.0, and the pH of the A2 sampling point also dropped to about 4. At 72 h, after the anodes were 6 cm from the cathode, the pH decreased significantly, and after 96 h, the pH of the A3 sampling point decreased to about 5. It indicated that the distance between the two electrodes in the approaching anode method was decreasing, and the H+ generated by the anodes flew to the cathode, and migrated at a high speed. The soil pH of the whole reaction tank was between 3.8 and 5.5, indicating that the soil in the whole reaction zone was in the acid migration zone which was more acidic than the above reaction, and the more acidic the soil environment was, the easier the heavy metal dissolves［7-8］.
In experiments, if the pH of the cathode is not controlled, since the migration speed of H+ to the cathode is twice the migration speed of OH- to the anode［9］, an acidbase transition zone would appear in the soil sample. In this experiment, the pH of the cathode was controlled, and citric acid was continuously added to the cathode tank, so that the pH of the whole soil chamber was acidic, but due to the continuous production of OH- in the cathode tank, the pH value increased from the anodes to the cathode.
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The curve of conductivity was distributed in a bowl shape. The soil samples near the cathode and anodes had higher conductivity, while the conductivity in the soil chamber was very low. Near the anodes, the pH was very low, and the soil contained a large amount of H+, so the conductivity was high. Near the cathode, there was not only a large amount of OH-, Cd2+ also diffused into the soil sample in the cathode tank, resulting in a very high conductivity. Migration and distribution of heavy metals in soil after the enhanced electrokinetic remediation
In the approaching anode experiment, sampling was carried out at 48, 72 and 96 h, respectively, and the concentration of Cd at 2 cm from the anodes reached the lowest value. After 72 h of remediation, the concentration of Cd in most areas of the soil decreased to 2 mg/kg. As can be seen in Fig. 4, the residual form near the cathode became more, because in the Cd migration process, Cd underwent precipitation reaction and was changed to the residual form due to the alkaline environment near the cathode［10-12］. Near the anodes, the weak acid extractive state and the reducible form decreased very significantly, indicating that under the soil property, it had a good migration effect on the weak acid extractive form and the reducible form. It can be seen from the figure that in the soil near the anodes in intermittent current method, the residual form changed little, while the reducible form and the weak acid extractive state were significantly decreased. The decline ratio of the weak acid extractive form at the sampling points near the cathode was 25%, and the decline ratio of the reducible form was about 8%.
As shown in Fig. 5, the removal rates of the approaching anode method and the intermittent current method were 88.12% and 80.81%, respectively. When the average electromigration distance was 6.3, the energy consumption per unit volume of the approaching anode method was 7.36 kWh/m3, and the remediation time was 36 h. As to the the intermittent current method, the energy consumption per unit volume was 8.12 kWh/m3 at the average electromigration distance of 5.48. It indicated that the two enhanced remediation methods further reduced the energy consumption per unit volume while increasing the removal rate.
Conclusions
Under the condition of electric field strength of 2 V/cm, with 0.1 mol/L citric acid as electrolyte and the remediation reaction time of 96 h, two kinds of enhanced electrokinetic techniques, namely, the approaching anode method and intermittent current method, were adopted to improve the traditional electric remediation of heavy metalcontaminated soil, and the remediation effect on cadmiumcontaminated soil was verified.
(1) Compared with the unenhanced remediation technique, the approaching anode method and the intermittent current method had relatively low pH value and high electrical conductivity. A lower pH and higher conductivity are beneficial to the migration and resolution of Cd. (2) As the distance between the cathode and anodes was gradually shortened, or the current decreased and increased intermittently, the soil that had been remediated did not need to consume electric energy. Therefore, under the condition of maintaining a high Cd removal rate, the two enhanced remediation methods had lower energy consumption per unit volume while maintaining a higher removal rate.
(3) The study shows that the approaching anode method and the intermittent current method are simple in operation, remarkable in remediation effect, extremely advantageous, and have a good application prospect.
References
［1］ KIM, WOOSEUNG, PARK, et al. In situ field scale electrokinetic remediation of multimetals contaminated paddy soil: Influence of electrode configuration［J］. Electrochimica Acta, 2012, 86(1): 89-95.
［2］ KIM DH, JO SU, CHOI JH, et al. Hexagonal two dimensional electrokinetic systems for restoration of saline agricultural lands: A pilot study［J］. Chemical Engineering Journal, 2012(8): 110-121.
［3］ CAMESELLE C. Enhancement of electro-osmotic flow during the electrokinetic treatment of a contaminated soil［J］. Electrochimica Acta, 2015, 181(3): 345-364.
［4］ BES C, MENCH M. Remediation of copper-contaminated topsoils from a wood treatment facility using in situ stabilisation［J］. Environmental Pollution, 2008, 156(3): 1128-1138.
［5］ REDDY KR, CAMESELLE C, REDDY KR, et al. Electrochemical remediation technologies for polluted soils, sediments and groundwater2009,97(4):6645-6657.
［6］ GANG LI, GUO S, LIA S, et al. Comparison of approaching and fixed anodes for avoiding the ‘focusing’ effect during electrokinetic remediation of chromium-contaminated soil［J］. Chemical Engineering Journal, 2012, 203(5):231-238.
［7］ HOEHUN H, OLSON JR, LING B, et al. Analysis of heavy metal sources in soil using kriging interpolation on principal components ［J］. Environmental Science & Technology, 2014, 48(9): 4999-5007.
［8］ DANMALIKI GI, SALEH TA, SHAMSUDDEEN AA. Response surface methodology optimization of adsorptive desulfurization on nickel/activated carbon ［J］. Chemical Engineering Journal, 2016, 313(8):7765-7789.
［9］ QUINTON JN, CATT JA. Enrichment of heavy metals in sediment resulting from soil erosion on agricultural fields［J］. Environmental Science & Technology, 2007, 41(10): 3495-3500.
［10］ CHENG X, DANEK T, DROZDOVA J, et al. Soil heavy metal pollution and risk assessment associated with the Zn-Pb mining region in Yunnan, Southwest China［J］. Environmental Monitoring & Assessment, 2018, 190(4): 1194-1203.
［11］ WEI Z, WANG D, ZHOU H, et al. Assessment of soil heavy metal pollution with principal component analysis and geoaccumulation index ［J］. Procedia Environmental Sciences, 2011, 10(1): 1946-1952.
［12］ YARGICOGLU EN, REDDY KR. Review of biological diagnostic tools and their applications in geoenvironmental engineering［J］. Reviews in Environmental Science & Bio/technology, 2015, 14(2): 161-194.