Comparison of the cadmium and lead removal from three soils by electrokinetic remediation

doi:10.21203/rs.3.rs-3913546/v1

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Comparison of the cadmium and lead removal from three soils by electrokinetic remediation

https://doi.org/10.21203/rs.3.rs-3913546/v1

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The use of citric acid and ethylenediaminetetraacetic acid disodium as electrolytes on the removal of lead and cadmium from three soils were investigated for the first time by electrokinetic remediation. The experimental results showed that: under the voltage gradient of 2 V/cm and remediation time of 4 d, the best electrolyte for brown soil (ZR) was EDTA, and the removal rates of Cd and Pb were 17.8% and 13.2%, which were lower than those of red soil (HR)and black soil (BR). It seemed that electrokinetic remediation was less effective for ZR due to the high soil pH, and could combined with other remediation techniques to improve the efficiency. For HR, extending the remediation time to 10 d, the Cd removal rate reached 91.1% when the electrolyte was citric acid, and the Pb removal rate reached 63.2% when the electrolyte was EDTA. For BR, it was more sensitive to electrokinetic remediation technology, and the focusing phenomenon was serious. The removal rate of Cd in S1 of BR1 was 99%, and the focusing phenomenon occurs in S4 and S5 zones, resulting in an average removal rate of 22%, which could be improved by cathodic control; the removal rate of Pb in S5 of BR2 was 84.8%, and the focusing phenomenon occurred in S1, which could be improved by anodic control.

electrokinetic remediation

lead-cadmium

three soils

Soil pollution is extensive and serious in China, the total exceedance rate of soil was 16.1%, mainly inorganic pollution. The distribution is as follows: soil pollution in the south is heavier than in the north, the Yangtze River Delta, the Pearl River Delta, the old industrial bases in the northeast and other parts of the soil pollution problem was more prominent, the southwest central and southern areas of the soil heavy metals exceed the standard range, Cd, Hg, As, Pb content distribution from the northwest to the southeast, from the northeast to the southwest direction gradually rising trend[1]. Researchers had created a cornerstone map that provides precise geographic coordinates and heavy metal concentrations[2]。Heavy metal contaminated soil has posed a growing health risk to the local population and needs to be remediated urgently[3, 4]. For heavy metal contaminated soil, remediation treatment can be carried out by drenching, curing-stabilization, cement kiln co-disposal, and phytoremediation[5–7]. However, for dense soils, all of the above techniques have certain limitations. It had been proved that electrokinetic remediation is suitable for low permeability and non-homogeneous soils[6–8], and it can completely remove pollutants from the soil, which is an effective and promising remediation technology for heavy metal complex contaminated soils. This technology removes heavy metals by applying a voltage at both ends of the electrode under the action of electromigration, electrodialysis, electrophoresis, and diffusion[6, 9]. It has the advantages of low impact on soil structure, low secondary pollution, and flexibility to adapt in-situ or ex-situ remediation methods. The effect of electrokinetic remediation of heavy metal contaminated soil is influenced by many factors such as physical and chemical properties of soil, electrolyte, pH, electrode materials and electric field deployment, etc. Control of electrolyte addition or pH is still the main measure to enhance the electrokinetic remediation effect at the laboratory scale and field scale[10]. Ethylenediaminetetraacetic acid (EDTA) is the most commonly used chelating agent because of its strong chelating ability for heavy metals, low impact on soil properties, slow degradation by microorganisms, etc[11–13]. Citric acid is a small molecule organic acid that can increase the electro-osmotic flow during the electrokinetic remediation process[14, 15] and can form complexes with Cd and Pb, is relatively inexpensive, and easy to handle[16]. Thus, EDTA and citric acid are quality candidates for enhanced electrokinetic remediation。

Soil heterogeneity has a great influence on electrokinetic remediation of heavy metal contaminated soils[17–19]. Due to the variability of soil-forming factors (climate, parent material, biology, topography, time, and human activities), soil distribution has zonal regularity, resulting in great variability of soil physicochemical properties between southern and northern China. Researchers had studied electrokinetic remediation of heavy metal contaminated soil for a longer period. Yuan Lizhu et al [20] found that the removal of Pb and Cd in black soil was 95.85% and 98.44% under a steady potential gradient of 1V/cm and running time of 705h when the electrolyte was 0.2M citric acid mixed with 0.05M CaCl₂; Zhou, Dong Mei et al [21] showed that the removal rate of cd in red soil could reach 79.18% after 96h of electrokinetic remediation; The experimental results of Zhongyang Yang[22] showed that the removal rate of Pb in brown soil was 37.25% and the power consumption was 80.5 k W-h/g when the restoration time and voltage were 96 h and 1.0 V/cm. It was impossible to directly compare the removal of contaminants since the inconsistency of experimental conditions (like electrolyte type, running time, and soil type et al).

This paper explored the effect of citric acid and EDTA as electrolyte on the effectiveness of electrokinetic remediation for three soils(ZR、BR and HR) contaminated with lead and cadmium, analyzed the effects of soil physicochemical properties on the removal, and investigated the heavy metal migration patterns and removal mechanisms.

2.1 Soil

Soils were taken from unpolluted sites in different cities. Red soil (HR): Jiujiang City, Jiangxi Province, is located in the subtropical transition zone, with rich water and heat conditions and relatively strong chemical-physical weathering; Brown soil (ZR): Xuzhou City, Jiangsu Province, is located in the mid-latitude region and belongs to the warm-temperate monsoon climate zone; Black soil (BR): Harbin, Heilongjiang Province, is located in the north temperate zone and has a temperate monsoon climate. The soil samples were naturally air-dried, removed plant residues and stones, and passed through a 10-mesh sieve to determine the physicochemical properties of the soil, and the results are shown in Table 1. A certain amount of Pb(NO₃)₂ (analytically pure) and Cd(NO₃)₂ (analytically pure) were weighed separately and dissolved in a quantitative amount of deionized water, and the Pb(NO₃)₂ and Cd(NO₃)₂ solutions were mixed with the soil and equilibrated for one month to prepare a composite contaminated soil with Pb and Cd contents of 700 mg/kg and 90 mg/kg, respectively.

Table 1

physical and chemical characteristics of the tested sediment
Soils	ZR	HR	BR
pH	7.73	5.47	6.67
Cd(mg/kg)	0.46	0.082	0.023
Pb(mg/kg)	26.6	70.4	22.3
Organic matter	10.5	3.82	30.2
Fe₂O₃(%)	4.36	8.03	4.8
CaO(%)	0.88	0.35	1.11
CEC(cmol/Kg(+))	27.1	27.12	27.1

2.2 Experimental setup

The experimental setup is shown in Fig. 1. It is made of Plexiglas and mainly consists of a soil chamber (L × H × W = 25 cm × 10 cm × 10 cm) and a cathode chamber (L × H × W = 6 cm × 15 cm × 15 cm). The soil chamber and the electrode chamber are separated by a porous Plexiglas plate and filter paper, which is removable. The sampling ports are equally spaced above the soil chamber, and the gas generated during electrolysis is discharged from above the electrode chamber.

For the results of the indoor experiments to better guide the field experiments, the pre-treatment of soil samples was minimized. The prepared 4000g soil samples were placed in the soil chamber, pressed well, and the electrolyte was added to the anode and cathode chambers (the electrolyte was kept at the same height as the soil) with a voltage gradient of 2 V/cm. During the experiment, the current and the pH of the solution in the cathode and anode chambers were recorded daily. At the end of the experiment, the soil samples were divided into five equal parts from anode to cathode, numbered S1, S2, S3, S4, and S5, and the soil samples were removed from the partitions, air-dried, and their pH and Pb and Cd contents were measured. After the experiment, the cathode electrolyte was collected and the Ca²⁺, Mn^2+, and Fe³⁺ contents of the solution were determined. The experimental conditions are shown in Table 2.

Table 2

Operating parameters in tests
Test	Anolyte	Catholyte	Voltage gradient	Time/d
ZR0、HR0、BR0	deionized water	deionized water	2 V/cm	10
ZR1、HR1、BR1	0.3 mol/L CA	0.1 mol/L CA	2 V/cm	10
ZR2、HR2、BR2	0.05 mol/L EDTA	0.05 mol/L EDTA	2 V/cm	10

2.3 Analysis Method

The physical and chemical properties of soils are presented in Table 1; pH value and CEC content were measured according to the Chinese DZ/T 0279.34–2016 and LY/T 1243–1999 standards, respectively. Organic matter and Cd content were measured by the Chinese Rock Mineral Analysis (Fourth Edition) at 84.2.37 and 84.2.6, respectively. CaO, Fe₂O_3, and Pb content were measured according to the Chinese DZ/T 0279.2–2016.

3.1 soil pH

The H⁺ produced at the anode and the OH⁻ produced at the cathode causes the pH of the electrolyte to decrease and increase respectively. The pH change of catholyte and anolyte during electrokinetic remediation were shown in Fig. 2, and it could be seen that, when distilled water, citric acid, and EDTA were used as electrolyte, the pH of anolyte decreased to 2.23, 1.66, and 3.16, respectively, and the catholyte increased to 11.6, 3.35, and 9.05, respectively. Soil pH changed in the electromigration of H⁺ and diffusion of electrolyte, and the results were shown in Fig. 3. As shown in Fig. 4, when deionized water was used as the electrolyte, the pH of S1 zone of ZR0 (original soil pH 7.73) decreased to 4.86, while S5 increased to 8.71; the pH of S1 zone of HR0 (original soil pH 5.47) decreased to 3.1 after remediation, while the pH of S5 increased to 5.82; the pH of the soil in S1 and S5 zones of BR0 (original soil pH 6.67) after remediation was less altered when compared to ZR0 and HR0. When citric acid was used as electrolyte, the pH of the S1 decreased to 3.12 after ZR1 remediation, and the pH of the S5 decreased slightly to 7.05 under the effect of free diffusion of catholyte. BR1 and HR1 soil pH both changed dramatically after remediation. In test of BR1, the soil pH of S1 decreased to 2.52, meanwhile, the soil pH of S2 ~ S4 decreased dramatically to 4 ~ 4.6, and there was almost no change in the pH of S5. When EDTA was used as the electrolyte, the pH of soil samples in S1 ~ S5 showed a small increasing trend after remediation in ZR2, HR2 and BR2 tests, among which, the pH of the soil in S2 ~ S4 area increased more in BR2. In general, the pH of soil sections from S2 to S4 in ZR did not show any significant changes, indicating that ZR had a strong buffering capacity and a small effect on soil pH during the 4d operating time; while in the BR experiment, soil pH was greatly influenced by electrolyte, and S2 to S4 changed under different restoration conditions; For HR test, due to its strong acidic nature, the soil pH was greatly affected by citric acid as the electrolyte, while it was less affected by EDTA.

3.2 variation of current

The magnitude of current was positively correlated with the amount of mobile ions in the soil pore water[9, 23], the current changed during the repair process was shown in Fig. 4. As can be seen in Fig. 4, the current increased firstly and then decreased, and in the early stage of the experiment, water-soluble compounds in the soil were dissolved in the pore water, and some ions were displaced by H⁺, and the number of mobile ions in the pore water was gradually increased, which resulted in a gradual increase of the current. Subsequently, the charged ions migrate to the two poles, and the number of mobile ions in the pore water decreased. In addition, OH⁻ generated at the cathode encountered metal ions, such as Ca²⁺, Mg²⁺, and so on, and produced precipitation[24], which lead to an increase in the resistance of the soil, and a gradual decrease in the current. From Fig. 4(b)(c), it could be seen that BR1 and BR2 currents were higher than those of the same group, indicating that the number of movable ions in their soil aqueous solutions was higher, which was attributed to the large pH modification of BR soil and the obvious desorption of ions from soil components.

3.3 Pb, Cd removal

The Cd residual rates after electrokinetic remediation were shown in Fig. 5. As shown in Fig. 5(a), the average residual rates of Cd in ZR0, HR0 and BR0 were 97.3%, 81.6% and 94%, respectively; among them, the lowest Cd residual rate was 44.9% in the S1 zone in HR0; as shown in Fig. 5(b), the residual rates from S1 to S5 in the experiment of HR1 showed a gradual increase, with an average residual rate of 66.2%, and the Cd residual rate in the S1 zone (pH 2.8) only reached 17.9%; compared with the blank control group, the soil residue rate in BR1, ZR1 and HR1 decreased significantly, especially in the S1 zone. It could be seen that when deionized water and citric acid were used as electrolytes, soil pH was an important factor affecting the Cd removal rate in ZR, HR and BR, and the lower the soil pH was, the higher the Cd removal rate was. The average residual rate of Cd in ZR2, HR2, and BR2 was 82.%, 82.0%, and 78.8%, respectively, which indicated that the background value of soil pH had less influence on the Cd removal rate when EDTA was used as electrolyte. After electrokinetic remediation, the Cd residual rate in the soil showed a trend of "low at both ends and high in the middle", and there was a heavy metal aggregation phenomenon in the S3 section, especially, the Cd residual rate in the S5 and S4 sections in the BR2 was 2.4% and 16.8%, which was much lower than that in the ZR2 and HR2, which showed that when EDTA was used as the electrolyte, anionic complexes with Cd in BR more easily formed (e.g., [Cd-EDTA]²⁻) under the effect of electromigration to anode[12, 13].

The Pb removal rates after electrokinetic remediation were shown in Fig. 6. As seen in Fig. 6(a), the lowest value of Pb residual rate in HR0 occurred in the S1 zone (pH = 3.1) with 84.1%, while the average Pb residual rate in the S2 ~ S5 zone was as high as 99.5%. As shown in Fig. 6(b), comparing ZR1 and BR1, the average residual rate of Pb in HR1 was the lowest at 80.0%, and the removal of Pb in S1 (pH = 2.82), S2 (pH = 3.28), S4 (pH = 5.50), and S5 (pH = 5.80) zones were 60.9%, 82.1%, 84.3%, and 84.5%, respectively, with a focusing in S3 section phenomenon, indicating that citric acid and Pb form anionic complexes (e.g., Pb-(Citrate)₂⁴⁻) that migrated toward the anode under the action of electric field. Researches shown that Pb was desorbed from the soil particle surface only when the soil pH was < 4[25], which explains the low removal of Pb in ZR0, ZR1, BR0 and BR1 after 4 d of electrokinetic remediation. As could be seen in Fig. 6(c), when EDTA was used as the remediation solution, ZR2 and BR2 showed a focusing phenomenon in S3, and HR2 showed a focusing phenomenon at S1, and both had the lowest residual rate in S5, followed by the S4, and this phenomenon was especially prominent in BR2, indicating that when the soil pH was higher than 5.4, it was mainly through the formation of complexes with anionic with EDTA ( such as [Pb-EDTA]²⁻) to remove Pb[13].

In comparison, it was found that the removal of Pb and Cd in ZR were lower than those in HR and BR. There were several reasons to explain this phenomenon: firstly, the change of soil pH during electrokinetic remediation caused a series of physicochemical changes, such as precipitation/dissolution of minerals, adsorption/desorption of pollutants, and ion exchange between the solid and liquid phases of the soil[26]. Among the three soils, ZR has the largest pH background value of 7.73, which is alkaline (Table 1), with a strong soil buffering capacity and no significant change in soil pH after remediation (Fig. 4), and the high pH value made the desorption of heavy metals from soil particles longer; secondly, ZR had a high organic matter content, which was dominated metal binding and generated insoluble complexes with heavy metals through electrostatic adsorption or complexation (chelation)[17]; finally, the combined effect of soil pH and organic matter content resulted in the firm adsorption of heavy metals Pb-Cd in the soil compared to either single factor[27]. In addition, factors such as soil cation exchange capacity, mucilage content, and redox status also affect the heavy metals removal. In conclusion, the strong buffering capacity of ZR soil, the slow change of soil pH, and the strong binding ability of soil particles with heavy metals Pb-Cd made the electrokinetic remediation ineffective, and it was recommended to combine with other remediation techniques (e.g., leaching[28, 29], PRB[4, 30], chemical amendment[5, 31], etc.) to improve the remediation efficiency.

Time played a crucial role in the migration of substances in the soil and the desorption/dissolution of contaminants[32]. Extending the remediation time of HR and BR tests, the removal of Cd and Pb in the soil after 10 d of remediation was shown in Fig. 7 and Fig. 8. As seen in Fig. 7: extending the remediation time could significantly improve the removal of Cd; the average removal rate of Cd in HR1 was 91.1%, the highest removal rate was in the S1 area, 99.5% and the lowest removal rate was in S5 area, 71.2%,, and at same time, the pH values of S1 ~ S5 zones were lower than 3.5. The pH values of S1 ~ S5 zones in BR1 were 2.21, 2.81, 3.32, 5.54, and 7.98, and the removal of S1 ~ S3 zones were 99%, 90%, and 66%, respectively, whereas the focusing phenomenon appeared in the S4 and S5 zones, which resulted in an average Cd removal rate of only 22%. This was because with the prolongation of time, citrate ions in the cathodic liquid were neutralized by OH⁻ produced by electrolysis, and OH⁻ entered into the soil aqueous solution under the sway of electromigration, which made heavy metals and other metal ions meet OH⁻ to form a precipitation (the realization of which revealed that there was a slate phenomenon in the soil of S5 zone with white precipitates), which could be used to increase the removal rate of heavy metals through the cathodic control method. The average removal rate of Cd in both HR2 and BR2 was 44%, and the migration pattern of Cd in both soils was consistent, with the removal rates in the order of S5 > S4 > S3 > S1 > S2. Cd in the soil of S1 zone migrated to the cathode in the form of Cd²⁺ at low pH (< 4.5), while in S5, S4 and S3 zones migrated to the anode as a complex [Cd-EDTA]²⁻ at high pH, and focusing in the S2 region. Therefore, the method of controlling the anode pH could be used to prevent the focusing phenomenon and improve removal. In conclusion, low soil pH could significantly increase the removal of Cd, while EDTA could also promote the desorption of Cd in high pH soil, and the method of controlling the pH value of anolyte could be used to improve its removal rate.

The average removal rate of Pb in HR1 and BR1 were 40% and 20%, respectively, and the highest removal appeared in S1 section, which were 84.8% and 59.6%, respectively, while the lowest removal appeared in S5 (5%) and S3 section (7.6%), which were mainly affected by the pH value and organic matter content of the soil, and the existence of different morphologies of Pb in the soil and its complexation with electrolytic solution led to the differences in its migration pattern. The removal rate of Pb in HR1 was much lower than that of Cd (91.1%), which was due to the competitive adsorption capacity of Cd in soil was poor, and it was easy to be replaced and migrated into the soil aqueous solution and be removed; when EDTA was used as the electrolyte, the average value of the removal rate of Pb in HR2 was 63%, and the rate of the removal rate of the S5 to S1 section decreased in order, and the change of the pH value of soil was small compared with that of HR1 at this time. The average value of Pb removal rate in BR2 was 42%, and the aggregation effect appeared in S1 area, and the overall removal rate was lower than that of HR, which was because the pH value of BR soil was higher compared with that of HR, and its organic matter content was nearly three times of that of HR, and the heavy metal Pb formed insoluble chelates with the organic matter, which was more difficult to be desorbed from the surface of the soil particles.

In comparison, it was found that citric acid and EDTA as electrolytes have different mechanisms of action on heavy metals Pb and Cd. Citric acid accelerated soil acidification and desorbed heavy metals from the surface of soil particles, or formed negatively charged complexes with metal ions, which was more effective than Pb in the removal of Cd from the soil, while EDTA mainly formed negatively charged complexes by complexing with heavy metals, which was more effective than Cd in the removal of Pb from the soil. It was further demonstrated that EDTA formed anionic complexes (e.g. [Pb-EDTA]²⁻) to remove heavy metal Pb.

(1) Under the voltage gradient of 2 V/cm and remediation time of 4 d, the optimal electrolyte in ZR was EDTA, and the removal of Cd and Pb were 17.8% and 13.2%, respectively, which were lower than those of HR and BR due to the high soil pH. It could be tentatively inferred that electrokinetic remediation was less effective for ZR, with compound heavy metal pollution and can be combined with other remediation techniques.

(2) Electrokinetic remediation technology was better for HR because of the low soil pH and organic matter content. Extending the remediation time to 10 d, the Cd removal rate reached 91.1% when the electrolyte was citric acid, and the Pb removal rate reached 63.2% when the electrolyte was EDTA.

(3) Because of the high organic matter content of BR, it was more sensitive to electrokinetic remediation technology, and the focusing phenomenon was serious. The removal rate of Cd in S1 of BR1 was 99%, and the focusing phenomenon occurs in S4 and S5 zones, resulting in an average removal rate of 22%, which could be improved by cathodic control; the removal rate of Pb in S5 of BR2 was 84.8%, and the focusing phenomenon occurred in S1, which could be improved by anode control.

(4) The selective adsorption and specialized adsorption ability of Pb and Cd to soil components were different, before the start of electrokinetic remediation technology, with reference to the remediation target value, the degree of single-factor contamination of Pb and Cd in the soil could be assessed, if the degree of contamination of Cd was higher, citric acid was preferred to be used as electrolyte, and if the degree of contamination of Pb was higher, EDTA was preferred to be used as electrolyte

Funding

This study is supported by the Special fund for science and technology development of Xuzhou City Science and Technology Project (KC21334).

Author Contribution

May Zhang was responsible forwriting and revising of the manuscript.

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No competing interests reported.

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Comparison of the cadmium and lead removal from three soils by electrokinetic remediation

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Abstract

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1. Introduction

2 Materials and methods

2.1 Soil

2.2 Experimental setup

2.3 Analysis Method

3 Results and discussion

3.1 soil pH

3.2 variation of current

3.3 Pb, Cd removal

4 Results and Suggestion

Declarations

Funding

Author Contribution

References

Additional Declarations

Supplementary Files

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