Leaching Effect of Organic Acids on Heavy Metal Contaminated Soil

Bing YU Mingxin MEN Peijia LIU Kening WU

AbstractIn order to improve electrokinetic remediation efficiency of cadmiumcontaminated soil, the effects of electric field intensity, remediation time and electrolyte on removal rate of total Cd in Cdcontaminated soil by electrokinetic remediation were studied through the preparation of Cdcontaminated soil and the construction of electrokinetic remediation equipment. The results showed that under the same condition, with the electric field intensity increasing from 2.5 to 3 V/cm, the total Cd removal rate increased by 10.62%, and with the increase of the electric field intensity from 3 to 3.5 V/cm, the removal rate increased by 1.87%; when the remediation time was prolonged from 72 to 96 h, the removal rate of total Cd increased by 6.68%, and with the remediation time prolonged from 96 to 120 h, the removal rate of total Cd increased by 8.75%; and with the remediation time prolonged from 120 to 144 h, the removal rate of total Cd only increased by 1.07%. Compared with citric acid as the electrolyte, the acetic acid group improved the remediation efficiency by 12.14% and the total energy consumption by 62.13%, while the hydrochloric acid group improved the remediation efficiency by 18.04% and the total energy consumption by 187.9%. Comprehensively from total Cd removal rate and energy consumption, the remediation effect was the best under the electric field intensity of 3 V/cm and the electrokinetic time of 120 h with acetic acid as the electrolyte, which achieved a total Cd removal rate of 41.95%.

Key wordsElectrokinetic remediation; Cadmium contamination; Electrolyte; Bentonite



Received: May 21, 2018Accepted: August 26, 2018

Supported by National Science Foundation of China (41641032).

Meng SHEN (1995-), female, P. R. China, master, devoted to research about soil remediation.

*Corresponding author. Email: wanyushan@126.com.

The area of cadmiumcontaminated soil has been increasing in recent years, and the contamination condition is getting severe［1］. Cd contamination not only causes environmental and economic losses, but also threatens human health. If Cd accumulates in human body, it would induced diseases including cancer［2］. The remediation of Cdcontaminated soil has attracted extensive attention from scholars at home and abroad.

Currently, the remediation methods of Cdcontaminated soil could be divided into three types: physical methods, chemical methods and biological methods［3］. Some of these methods might influence soil, while some might consume a lot of human and material resources［4-6］. Electrokinetic remediation as a kind of insitu soil remediation technique, has the advantages of simple operation and low environmental effect, with broad application prospect［7］. In the 1990s, the Netherlands conducted five largescale electrokinetic remediation projects of heavy metalcontaminated sites［8］. Peng［9］ studied the effect of voltage on remediation effect using distilled water as electrolyte, and concluded that conventional electrokinetic remediation had a poorer removal effect on Cd in soil. There are also many problems existing in electrokinetic remediation, such as poor uniformity in site remediation and the need to overcome strong corrosion to electrode and to enhance the migration capacity of heavy metals in soil. This study was conducted to optimize the working conditions for electrokinetic remediation of Cdcontamined soil.

Materials and Methods

Experimental equipment and soil

The experimental equipment［10-11］ is shown as Fig. 1. The cell body is made of acrylic material, and includes electrode chambers and a soil chamber. The equipment has a size of 276 mm ×110 mm×105 mm, the cathode and anode chambers are all of 80 mm×100 mm×100 mm. The soil chamber has an internal size of 100 mm×100 mm×100 mm. The equipment also includes a DC power supply, spacer plates, electrode bars, copper wires and a nonwoven filter screen.



Fig. 1Diagram of electrokinetic remediation equipment

The collected farmland soil was pulverized, airdried and grinded, and sieved with a 100mesh sieve. Cdcontaminated soil was prepared from the farmland soil, and determined to have a total Cd concentration of 4.99 mg/kg with a pH value of 7.61.

A certain amount of the Cdcontaminated soil (500 g) was weighed, placed into the soil chamber and compacted. Into each of the cathode and anode chamber, 600 ml of electrolyte was added, followed by powering on and applying a constant voltage. Current and pH values of cathode and anode electrolytes were measured every two hours. After turning off the power supply, the soil in the soil chamber was airdried and divided uniformly to 15 parts, which were determined for pH and concentrations of total Cd and various Cd forms.

Experimental scheme

Different electric field intensity gradients, different remediation time and different electrolytes were set, to discuss the optimal conditions for electrokinetic remediation. The specific experimental scheme is shown in Table 1.

Table 1Experimental scheme for optimization of electrokinetic remediation conditions

No.Electric fieldintensity∥V/cmElectrolyte0.1 mol/LRun timehNo.Electric fieldintensity∥V/cmElectrolyte0.1 mol/LRun timeh

T12Citric acid 120T63Citric acid96

T22.5T73144

T33T83Hydrochloric acid 120

T43.5T93Acetic acid

T5372

Analytical methods

Total Cd and various forms of Cd were determined by flame atomic absorption spectrometer. ① Plotting of standard curve: Because the flame atomic absorption spectrophotometer (240FSAA, Agilent) selected in this study has automatic dilution function, only 0.50 mg/L Cd standard solution was prepared. ② Total Cd was determined after nitric acidhydrofluoric acidperchloric acid. ③ The determination of various forms of Cd in soil adopted BCR continuous extraction［12］.

Calculation methods

Removal rate of total Cd=(Total Cd content before treatmentTotal Cd content after treatment)/Total Cd content before treatment.

Calculation of energy consumption: E=U∫Idt（1）

Wherein E is powder consumption required by electrokinetic remediation, kW?h; U is applied voltage, V; I is current intensity, A; and t is running time, h.

Optimization of Electrokinetic Remediation Technological Conditions

Effect of electric field intensity on the effect of electrokinetic remediation

It could be seen from the monitoring of current conditions in the four groups that a higher applied electric field led to a stronger current in the operational process. In the first 36 h, the currents in the four groups all showed an increasing trend; 36 h later, the currents in the four groups all increased slowly; and 96 later, groups T3 and T4 showed a current trend of decreasing at first and then becoming stable, while the currents in groups T1 and T2 continued to increase slowly and then tended to be stable, which was because H+ produced by the electrolysis on the anode changed heavy metal ions in soil into free state. Researches have shown that during electrokinetic process, the concentration of movable ions in soil directly influences the value of current［13］. The stronger the voltage was, the higher the concentration of free ions was, and the rapider the current changed. Meanwhile, the electrolysis on cathode produced OH-. Over time, positive ions produced hydroxide precipitate with OH-, a small part of H+ changed heavy metal ions in soil to free state, and therefore, the currents in the four groups showed a trend of increasing slowly. The reduction in currents of groups T3 and T4 in later period was because that under a stronger electric field intensity, positive ions in soil and H+ produced on the anode were neutralized by OH- produced on the cathode at first, which not only reduced movable freestate ions in soil solution, but also blocked soil voids［14］.

Average pH values and average total Cd contents in different parts of soils of various groups are shown in Fig. 2 and Fig. 3, respectively. It could be seen from Fig. 2 that pH values of soils in various groups all decreased compared with before treatment, and there was no big difference in soil pH between the nearanode and middle soils. However, the nearcathode soil showed a soil pH increasing with the increase of electric field.

It could be seen from Fig. 3 that after the completion of the running, except the group with the electric field intensity of 2 V/cm, total Cd contents in soils of other groups all showed a phenomenon that the content of the nearcathode part was higher than that of the nearanode part, suggesting that after powering on, Cd in soil migrated gradually from the anode to the cathode, and the closer the soil was to the anode, the stronger migration capacity Cd in the soil had. The total Cd contents in the nearanode and middle soils decreased with the electric field intensity increasing, indicating that the strong the electric field intensity was, the stronger migration capacity the Cd in the nearanode and middle soils had. However, the total Cd contents in nearcathode soils remarkably increased compared with the values in soils before remediation under the electric field intensities of 3 and 3.5 V/cm, suggesting that after the reaction, Cd accumulated in soil around the cathode, which might be because that H+ and OH- produced from hydrolysis met in the middle soil and neutralized each other when they migrated to cathode and anode through electromigration and diffusion, respectively, resulting in a sudden change of soil pH in the region, which promoted the precipitation reaction between free heavy metal ions in soil and OH-, producing heavy metal precipitate, which might block soil voids and result in a drop of the remediation effect.



Fig. 2Soil pH after electrokinetic remediation



Fig. 3Distribution of total Cd in soil after electrokinetic remediation

Meng SHEN et al. Study on Electrokinetic Remediation of Cadmiumcontaminated Soil

According to the calculation methods of the removal rate of total Cd and energy consumption, the removal rates of total Cd and energy consumption in the four groups were calculated, as shown in Table 2. The removal rate of total Cd increased with the electric field intensity increasing, which was because the intensity of electric field directly influenced the intensity of the hydrolysis reaction in the electrolyte. Generally, a higher current causes an intenser electrochemical reaction［15］. However, the higher the voltage, the higher the current, and the more the energy consumption［16］. When the electric field intensity increased from 2.5 to 3 V/cm, the removal rate of total Cd increased by 10.62%; and when the electric field intensity increased from 3 to 3.5 V/cm, the removal rate only increased by 1.87%. Comprehensively, the electric field intensity of 3 V/cm was more proper.

Table 2Removal rate of total Cd and energy consumption at the end of electrokinetic remediation

T1T2T3T4

Working solution∥V/cm2.002.53.003.50

Removal rate∥%18.4426.7937.4139.28

Energy consumption ∥kw?h51.5270.3198.07119.88

Effect of remediation time on the effect of electrokinetic remediation

It could be seen from the realtime current monitoring that in 72 and 96 h of electrokinetic remediation, the current increased gradually, and after 96 h of electrokinetic remediation, the current increased slowly and then decreased gradually.

Average pH values and average total Cd contents in different parts of soils of various groups are shown in Fig. 4 and Fig. 5, respectively. It could be seen from Fig. 4 that after the experiment, no big differences were observed between various soil regions, which might be because that the pH of the cathode and anode electrolytes basically remained unchanged 72 h later, and the current also increased slowly.

It could be seen from Fig. 5 that over time, total Cd content decreased gradually in the nearanode and middle soils, which was because the soil of the nearanode part was weakly acidic due to the influence from the anode electrolyte, and Cd in the nearanode soil thus had very good mobility. However, the total Cd content in nearcathode soil increased gradually, and 96 h later, the total Cd content in the nearcathode soil exceeded the total Cd content in the soil before remediation, which was because free Cd migrated toward the cathode, while the soil near cathode was finally become alkaline due to the influence from the cathode electrolyte, and OH- increased in the soil, and produced precipitate with positive ions in the soil, which hindered the migration of Cd ions which were then enriched in the soil around the cathode. With the progress of the remediation, the Cd content around the cathode increased higher and higher.



Fig. 4Soil pH after electrokinetic remediation



Fig. 5Distribution of total Cd in soil after electrokinetic remediation

The total Cd removal rates and energy consumption of the various groups are shown in Table 3. It could be seen that the energy consumption became larger over time. With the remediation time prolonged from 72 to 96 h, the removal rate of total Cd increased by 6.68%. With the remediation time prolonged from 96 to 120 h, the removal rate of total Cd increased by 8.75%, while when the remediation time was prolonged to 144 h, the removal rate of total Cd only increased by 1.07%. Therefore, comprehensively, the optimal remediation time was 120 h.

Table 3Removal rate of total Cd and energy consumption after electrokinetic remediation

T5T6T3T7

Time∥h7296120144

Removal rate∥%21.9828.6637.4138.48

Energy consumption∥kw?h54.8977.0798.07106.79

Effect of electrolyte on electrokinetic remediation

The realtime current monitoring showed that the current values ranked as hydrochloric acid>actuc acid group>citric acid group. The average pH values after the electrokinetic remediation using different electrolytes are shown in Fig. 6. The pH values of most soils were lower than those before remediation. The pH in the nearcathode soil of group T8 was slightly higher than that in soil before remediation, which might be because that within 12 h of the running, the pH in the cathode electrolyte of group T8 had reached and basically remained at a higher alkaline condition, and the cathode electrolyte influenced pH in the nearcathode soil.



Fig. 6Soil pH after electrokinetic remediation

In practice, Cd is converted to other chemical forms gradually in soil through dissolution, agglomeration, precipitation and adsorption. During electrokinetic remediation, different forms of Cd also differed in migration capacity in soil. Heavy metal could be divided into acetic acidextractable, reducible, oxidable and residual forms［16］. Three groups of soil samples were analyzed by BCR method, as shown in Fig. 7. Before and after the electrokinetic remediation, the residual form of Cd changed little in the nearanode and middle soils, which might be because the residual form hardly could be desorbed into the soil solution by the three kinds of acidic electrolytes, and thus hardly could be migrated by electric field force. However, the contents of residual form of Cd in the three nearcathode groups increased remarkably, by 115.5%, 71.83% and 63.38%, respectively, which might be because that Cd precipitated in the soil around the cathode during the process of migrating from the anode to the cathode, and was thus converted to residual form. When using hydrochloric acid as the electrolyte, the electrokinetic remediation was relatively intenser, the pH in the cathode electrolyte increased rapidly to 12, making the soil around the cathode alkaline, and more Cd migrating to the cathode was converted to precipitate, so there was the most residual form of Cd around the cathode. The weak acidextractable and reducible forms in the nearanode and middle soils both decreased remarkably compared with before remediation, suggesting that there was mainly a very good migration effect on weak acidextractable and reducible forms during the electrokinetic remediation. However, the content of the oxidable form of Cd was very little before the remediation, so the migration effect on the oxidable form of Cd could not be observed this time.



Fig. 7Distribution of total Cd in soil after electrokinetic remediation

As shown in Table 4, the remediation efficiency and total energy consumption ranked as hydrochloric acid＞acetic acid＞citric acid. Compared with citric acid as the electrolyte, acetic acid as the electrolyte improved the remediation efficiency by 12.14% and the total energy consumption by 62.13%, while hydrochloric acid as the electrolyte improved the remediation efficiency by 18.04% and the total energy consumption by 187.9%. Therefore, comprehensively from removal rate and energy consumption, acetic acid was more suitable as the electrolyte.

Table 4Removal rate of total Cd and energy consumption at the end of electrokinetic remediation

T3T8T9

Working solution∥0.1 mol/LCitric acidHydrochloric acidAcetic acid

Removal rate∥%37.4144.1541.95

Energy consumption∥kw?h98.07282.345159.97

Conclusions

Under the electrokinetic time of 120 h, with the electric field intensity increasing from 2.5 to 3 V/cm, the total Cd removal rate increased by 10.62%, and with the increase of the electric field intensity from 3 to 3.5 V/cm, the removal rate increased by 1.87%. The electric field intensity of 3 V/cm was more suitable.

When the remediation time was prolonged from 72 to 96 h, the removal rate of total Cd increased by 6.68%. With the remediation time prolonged from 96 to 120 h, the removal rate of total Cd increased by 8.75%. With the remediation time prolonged from 120 to 144 h, the removal rate of total Cd only increased by 1.07%. Therefore, comprehensively from total Cd removal rate and energy consumption, the optimal remediation time was selected to be 120 h.

Compared with citric acid as the electrolyte, the acetic acid group improved the remediation efficiency by 12.14% and the total energy consumption by 62.13%, while the hydrochloric acid group improved the remediation efficiency by 18.04% and the total energy consumption by 187.9%. Acetic acid was advantageous in total energy consumption.

Comprehensively from total Cd removal rate and energy consumption, the remediation effect was the best under the electric field intensity of 3 V/cm and the electrokinetic time of 120 h with acetic acid as the electrolyte, which achieved a total Cd removal rate of 41.95%.

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