A novel device and construction method for the combined zoned desorption and electrokinetic remediation of heavy metal contaminated soil.

Abstract

This invention discloses a device and construction method for the combined zoned desorption and electro-optic remediation of heavy metal contaminated soil, belonging to the field of soil remediation technology. The method divides the remediation area into an anode control zone, a pre-desorption enhancement zone, a main migration and transport zone, a terminal enrichment and stabilization zone, and a cathode water collection zone. Zoned desorption is performed according to a gradient: anode pH > enhancement zone pH ≥ main migration zone pH ≥ stabilization zone pH. This invention uses low-molecular-weight organic acids as desorbents, combined with zoned gradient desorption and ion-selective blocking structures, effectively mitigating electrode polarization, anode soil acidification, and cathode heavy metal precipitation problems, reducing the impact of reverse electroosmosis, decreasing reagent dosage, and lowering remediation costs. Verification shows that this technology can stably control the physicochemical parameters of the system, significantly improve heavy metal migration and removal efficiency, and has a remediation efficiency superior to traditional overall immersion desorption processes, making it suitable for large-scale remediation of heavy metal contaminated soil.

Description

Technical Field

[0001] The invention belongs to the field of heavy metal contaminated soil remediation technology, specifically a novel device and construction method for the combined zoned desorption and electrokinetic remediation treatment of heavy metal contaminated soil. Background Technology

[0002] Soil pollution is a global environmental problem, not only disrupting ecosystem balance and threatening the quality and safety of agricultural products, but also continuously harming human health through the food chain. With the accelerating pace of industrialization and urbanization, soil pollution is becoming increasingly severe, with inorganic pollution accounting for 82.8%, particularly heavy metal pollution. Heavy metals can migrate freely in soil and cannot be naturally degraded, making remediation extremely difficult. Therefore, the remediation of heavy metal-contaminated soil has become a key research direction in the environmental field.

[0003] Electrokinetic remediation is a rapidly developing new soil remediation technology in recent years. Its core principle involves deploying electrodes at the contaminated site to create an electric field. Through electromigration and electroosmosis, heavy metals in the soil are directionally migrated to the electrode area and removed. This technology boasts advantages such as safe and simple operation, scalability, wide applicability, and strong compatibility, and has promising prospects for engineering applications.

[0004] In electroremediation systems, heavy metal pollutants migrate primarily through two mechanisms: electromigration and electroosmosis. Electromigration refers to the movement of charged ions towards electrodes with opposite charges under the influence of a potential gradient; heavy metal cations gradually migrate towards the cathode, while anions migrate towards the anode. Electroosmosis manifests as the overall flow of soil pore water under an electric field: soil particles typically carry a negative charge and form a double electric layer structure. Under the influence of an electric field, cations within the double electric layer carry pore water and flow towards the cathode. To enhance the migratory capacity of heavy metals in soil, the industry commonly adds inorganic acids, organic acids, and complexing agents to contaminated soil to increase heavy metal solubility and enhance migration. However, the effects of different agents vary significantly: after adding complexing agents, heavy metals form negatively charged complexes and migrate towards the anode, while the direction of electroosmosis is from the anode to the cathode. These opposing directions directly hinder ion migration and reduce heavy metal removal efficiency.

[0005] Adding acid to the soil to desorb heavy metals before electrokinetic remediation is a relatively mature and optimized approach. This method aligns the electromigration direction of heavy metal ions with the electroosmotic flow direction (both from the anode to the cathode). However, three major problems remain in practical applications: First, excessive acid addition may lead to soil acidification, while insufficient acid addition cannot effectively increase the electromigration rate. Second, adding acid can lower the soil zeta potential, or even cause it to turn from negative to positive, thus altering the electroosmotic flow direction and hindering the electromigration of heavy metal ions. Third, there is the cost issue; electrokinetic remediation itself is expensive, and using excessive amounts of additional chemical reagents will further increase costs. Although it may increase removal efficiency to some extent, its cost-effectiveness is relatively low compared to other methods.

[0006] The current mainstream desorption method is immersion desorption, which involves mixing contaminated soil with a desorption reagent, allowing it to stand, and then applying an electroremediation agent. In this mode, the pH value of the entire soil system tends to be uniform. However, these methods do not consider the dynamic nonlinear changes in pH caused by electrode reactions and pollutant migration during electroremediation, nor can they achieve quantitative addition of the desorption reagent. This ultimately leads to uncontrolled remediation costs, poor heavy metal removal, and severe damage to the original physicochemical properties of the soil. For example, existing patents CN 115446103 A and CN 118341818 B both employ a pre-acidification combined with electroremediation process, optimizing the electrode structure and adding a cathode buffer chamber and a wastewater treatment unit. These technologies have achieved some improvements in increasing heavy metal removal rates and avoiding secondary pollution. However, both technologies use a single, integrated soil remediation chamber, which suffers from the aforementioned problems.

[0007] Furthermore, patent CN 113118201 A proposes an electrode-based zoned remediation technology using a compressed electric field. This technology achieves zoned control of the electric field through a multi-electrode layout, utilizing a non-isochronous compressed electric field to enhance the synergistic removal of heavy metals and organic pollutants. However, this process relies on multiple electrodes being energized at different times to achieve electric field compression, resulting in long remediation cycles, high equipment energy consumption, complex overall control procedures, stringent requirements for on-site operation and maintenance capabilities, and high construction and operating costs. Therefore, it is not suitable for large-scale, heavy metal-contaminated soil remediation projects, making its widespread adoption difficult.

[0008] In summary, current electrokinetic remediation technologies suffer from uneven remediation effects with integrated cavity structures, and the electrode zoning approach is time-consuming, energy-intensive, and impractical. Therefore, electrokinetic remediation technology for contaminated soil still requires improvement. Summary of the Invention

[0009] Technical issues Existing electrostatic remediation technologies suffer from several problems, including difficulty in quantitatively adding desorption agents, easy alteration of soil potential and physicochemical properties, high remediation costs, and low heavy metal removal rates.

[0010] Technical solution To address the aforementioned technical problems, this invention provides a novel device and construction method for the combined regional desorption and electrokinetic remediation of heavy metal contaminated soil. Compared to traditional immersion desorption and electrokinetic remediation of copper-contaminated soil, this invention utilizes a novel method to determine regional desorption based on the soil's isoelectric point. It considers the nonlinear changes in soil zeta potential during electroosmosis and the impact of soil zeta potential on the electroosmosis effect, proposing a more scientific and effective approach. This method involves regional desorption followed by electrokinetic remediation, scientifically quantifying the desorption reagents required in the soil, avoiding soil acidification, and reducing the cost of electrokinetic remediation. Furthermore, by using regional desorption followed by electrokinetic remediation, the impact of reverse electroosmotic flow on electrokinetic remediation is effectively reduced, the cathodic focusing effect is mitigated, and the efficiency of heavy metal removal from the soil is significantly improved.

[0011] To achieve the above objectives, the present invention provides a technical solution: a method for gradient-regional desorption-electrokinetic co-remediation of heavy metal contaminated soil, the method comprising the following steps: (1) Test the electrokinetic potential of the target soil at different pH values ​​to determine the desorption pH of the zone; (2) Mix the contaminated soil and desorbent, and fill the soil chamber. The pH in the soil chamber is controlled at pH 1. 阳极 >pH 强化 ≥pH 近迁移 >Isoelectric point ≥ pH 稳定 ; (3) Start the electric repair and complete the repair after a period of time.

[0012] In some of these examples, the electro-remediation device used in the above method includes a soil chamber, an anode plate, a cathode plate, a cathode chamber, a DC power supply, and wires. The anode plate and cathode plate are located at both ends of the soil chamber and are connected to a DC power supply via wires; the anode plate is a titanium-plated metal plate, and the cathode plate is a perforated titanium-plated metal plate, through which water is discharged into the cathode chamber; the material used for partitioning is acrylic sheet. The cathode chamber is located behind the cathode plate and has a drain pipe at the bottom; The soil chamber is vertically divided into four regions from the anode to the cathode: the anode control zone, the pre-desorption enhancement zone, the main migration and transport zone, and the terminal enrichment and stabilization zone. Each region accounts for 23-27% of the total volume of the soil chamber, and the total volume of the regions is 100%. A cation exchange membrane is set between the main migration and transport zone and the terminal enrichment and stabilization zone, while no barrier medium is set between the other regions.

[0013] In some of these instances, the electrokinetic potential can be measured using a Malvern zeta potential meter.

[0014] In some of these instances, the contaminated soil is soil contaminated with heavy metals, including copper (Cu), lead (Pb), cadmium (Cd), zinc (Zn), chromium (Cr), nickel (Ni), and arsenic (As).

[0015] In some of these examples, the desorbents include citric acid, oxalic acid, and tartaric acid.

[0016] In some examples, the pH of contaminated soil in the anodic control zone is 5-6, in the pre-desorption enhancement zone it is 4-5, in the main migration and transport zone it is 3-4, and in the terminal enrichment and stabilization zone it is 2-3. The pH of the anodic control zone is adjusted primarily based on the soil's electrochemical properties to prevent reverse electroosmotic flow during electrokinetic remediation; it is generally 5-6. The pH of the pre-desorption enhancement zone and the main migration zone is adjusted according to the optimal desorption pH for the corresponding ions. Considering that hydrogen ions migrate with the electric field, the pH of the pre-desorption enhancement zone is 4-5, and the pH of the main migration and transport zone is 3-4. The pH of the terminal enrichment and stabilization zone is adjusted based on the pH at which the corresponding ions begin to precipitate, aiming to mitigate the cathode "focusing effect." The easier an ion is to precipitate, the lower the initial pH in this zone, generally 2-3.

[0017] In some of these instances, the contaminated soil and desorbent are mixed and stirred, then left to stand for 10-20 minutes.

[0018] In some of these instances, the mixing of contaminated soil and desorbent can be carried out either before or after the contaminated soil is loaded into the soil chamber.

[0019] In some of these examples, the voltage for electric repair is 25-30V, the current is 0.1-1A, and the repair time is 20-48h.

[0020] Beneficial effects This invention employs a gradient-zone pH control mode, which can effectively solve problems such as insufficient heavy metal desorption, severe pH polarization at the anode and cathode, and excessive OH groups at the cathode. - Reverse migration leads to problems such as heavy metal redeposition and uneven remediation. At the same time, regional regulation can significantly alleviate electrode polarization, reduce the severe acid-base differentiation between the two poles, avoid excessive acidification of the anodic soil, and protect the original physicochemical properties of the soil.

[0021] This invention incorporates cation exchange membranes within functional zones, which can block the back diffusion of hydroxide ions from the cathode, effectively suppressing the cathode focusing effect caused by heavy metal precipitation, preventing soil pore blockage, and ensuring continuous unobstructed seepage channels. The device is divided into multi-level functional zones, forming a complete treatment chain of desorption, migration, and enrichment. The system current and electroosmotic flow operate stably, significantly improving the overall removal efficiency of heavy metals.

[0022] Compared to traditional overall soaking processes, this invention allows for precise dosing of desorbents according to the needs of different zones, reducing the amount of reagents used and thus lowering remediation costs while mitigating secondary pollution. The entire system is simple in structure and easy to operate and maintain, suitable for soils contaminated with various heavy metals such as copper, lead, and cadmium, and can meet the needs of large-scale remediation projects. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of an electric repair device, which includes soil chamber S1, soil chamber S2, soil chamber S3, soil chamber S4, anode plate 5, cathode plate 6, cathode chamber 7, DC power supply 8, drainage pipe 9, and wire 10. The DC power supply 8 is connected to the anode 5 and cathode 6 through the wire. Figure 2 A graph showing the variation of zeta potential in Taihu Lake sediment with environmental pH. Figure 3 A comparison chart of current changes between immersion desorption electroosmosis and zoned desorption electroosmosis. Figure 4 A comparison chart of the changes in drainage volume between immersion desorption electroosmosis and zoned desorption electroosmosis; Figure 5 A comparison chart of pH changes between immersion desorption electroosmosis and zoned desorption electroosmosis; Figure 6 A comparison chart of conductivity changes between immersion desorption electroosmosis and zoned desorption electroosmosis. Figure 7 A comparison chart showing the changes in copper ion removal rates between immersion desorption electroosmosis and zoned desorption electroosmosis. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. The described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] This invention provides a novel device for the combined regional desorption and electrokinetic remediation of heavy metal-containing soil. The structure of the novel device is as follows: Figure 1 As shown. The main structure of the device includes soil chamber S1, soil chamber S2, soil chamber S3, soil chamber S4, anode plate 5, cathode plate 6, cathode chamber 7, DC power supply 8, drainage pipe 9, and wire 10. The DC power supply 8 is connected to the anode 5 and cathode 6 through the wire. A current sensor is connected in series in the entire circuit to detect changes in current within the system.

[0026] Soil chambers S1 to S4 are respectively the anodic control zone, the pre-desorption enhancement zone, the main migration and transport zone, and the terminal enrichment and stabilization zone, with all four zones being of equal size. A cation exchange membrane, acting as a barrier, is placed between the terminal enrichment and stabilization zone and the main migration and transport zone, while no barrier medium is placed between the remaining anodic control zone, the pre-desorption enhancement zone, and the main migration and transport zone.

[0027] The desorbent used in the aforementioned novel device is citric acid, a low-molecular-weight organic acid that can improve the solubility and mobility of heavy metals in soil. However, as the concentration increases, it reduces the absolute value of the soil zeta potential, thereby decreasing the electroosmotic flow rate. In subsequent embodiments, after copper ions are desorbed from the soil, they move towards the cathode via electromigration and electroosmotic flow. As electroosmosis proceeds, electrode reactions occur at both ends, with H+ being generated at the anode. + OH is generated at the cathode - This leads to a rapid decrease in anode pH, resulting in reverse electroosmotic flow, and a rapid increase in cathode pH, causing precipitation reactions, clogging drainage channels in the soil, and reducing the removal rate of heavy metals. Therefore, according to this invention, by rationally adjusting the initial desorption pH of the soil, desorbing the soil to be remediated in zones, and simultaneously setting a cation exchange membrane between the terminal enrichment and stabilization zone and the main migration and transport zone, the magnitude and direction of electroosmotic flow can be controlled, the electromigration effect can be increased, and the time of cathode precipitation reaction can be delayed, thereby improving the removal rate of heavy metals.

[0028] Example 1 use Figure 1 The experiment was conducted using the electric remediation device shown. The experimental soil was collected from Taihu Lake silt in Wuxi, Jiangsu Province. After the soil was dried, it was ground through a 10-mesh sieve, and then copper salt solution was added to make the copper ion concentration in the contaminated soil 1000 mg / kg.

[0029] Before the experiment, the zeta potential of the experimental soil was determined as a function of pH. The specific method is as follows: a certain amount of soil samples with different pH values ​​were dispersed in deionized water (mass fraction of 0.1%), ultrasonically vibrated for 5 min, and the zeta potential was measured using a nanoparticle size analyzer (model ZS-90) from Malvern Instruments, UK. The average value was taken from three tests.

[0030] A control group was set up for Example 1. In both groups, 2000g of soil was placed in the soil chamber using two different methods. One method was to directly place 2000g of soil into the soil chamber without dividing it into sections (treatment 1); the other method was to divide 2000g of soil into 500g portions and place them into four sections (treatment 2).

[0031] Both treatments required desorption followed by electroosmosis experiments. In Treatment 1, the citric acid concentration was controlled at 20 g / kg, dissolved in water and thoroughly mixed with the soil samples for soaking, with a moisture content of 50% and a pH between 3 and 4. In Treatment 2, the citric acid concentration was controlled at 20 g / kg, dissolved in water and thoroughly mixed with soil samples from each zone for soaking, with pH values ​​of 5-6, 4-5, 3-4, and 2-3 for the four zones, respectively, and a moisture content of 50%. The electroosmosis experiment lasted 24 hours at a DC voltage of 30 V. During the experiment, the current and electroosmotic flow of the entire system were measured. After the experiment, soil samples were taken from each zone, dried, ground, and passed through a 100-mesh sieve. The soil pH, electrical conductivity, and copper content of each zone were then measured.

[0032] Example 2 use Figure 1 The electric remediation device shown was used in an experiment. The contaminated soil was collected from a contaminated site in Wuxi. The soil's physicochemical properties were: pH 7.29, and total copper content 1000 mg / kg.

[0033] Before the experiment, the zeta potential of the experimental soil was determined as a function of pH. The specific method is as follows: a certain amount of soil samples with different pH values ​​were dispersed in deionized water (mass fraction of 0.1%), ultrasonically vibrated for 5 min, and the zeta potential was measured using a nanoparticle size analyzer (model ZS-90) from Malvern Instruments, UK. The average value was taken from three tests.

[0034] A control group was set up for Example 2. In both groups, 2000g of soil was placed in the soil chamber in two different ways. One method was to directly place 2000g of soil into the soil chamber without dividing it into sections (treatment 3); the other method was to divide 2000g of soil into 500g portions and place them into four sections (treatment 4).

[0035] Both treatments required desorption followed by electroosmosis experiments. In treatment 3, the citric acid concentration was controlled at 20 g / kg, dissolved in water and thoroughly mixed with the soil samples for soaking, with a moisture content of 50% and a pH between 3 and 4. In treatment 4, the citric acid concentration was controlled at 20 g / kg, dissolved in water and thoroughly mixed with soil samples from each zone for soaking, with pH values ​​of 5-6, 4-5, 3-4, and 2-3 for the four zones, respectively, and a moisture content of 50%. The electroosmosis experiment lasted 24 hours at a DC voltage of 30 V. During the experiment, the current and electroosmotic flow of the entire system were measured. After the experiment, soil samples were taken from each zone, dried, ground, and passed through a 100-mesh sieve. The soil pH, electrical conductivity, and copper content of each zone were then measured.

[0036] Table 1

[0037] Figure 3 and Figure 4The figures show the changes in current and electroosmotic flow under different treatments during electroremediation. As can be seen from the figures, before 12 hours, the current during immersion desorption electroremediation was greater than that during zonal desorption; after 12 hours, the current during zonal desorption was greater than that during immersion desorption. Overall, the current change during zonal desorption was more gradual. The electroosmotic flow of both immersion and zonal desorption gradually increased with electroremediation time, eventually leveling off, with the final electroosmotic flow of immersion desorption being less than that of zonal desorption. These results indicate that electroosmosis following zonal desorption can slow the rate of current decrease during electroosmosis and increase the current in the later stages of the electroremediation system, thus improving the migration of heavy metals.

[0038] Figure 5 This figure shows the pH changes in different areas under different treatments after the electroremediation process. As can be seen from the figure, the pH changes drastically from anode to cathode under the immersion desorption treatment, with the anode pH as low as 2.4 and the cathode pH as high as 7.8. In contrast, the pH change from anode to cathode under the partitioned desorption treatment is gradual, slowly increasing from 3.0 to 5.1. This result indicates that under the immersion desorption treatment, the reactions at both electrodes are intense. An excessively low anode pH leads to a decrease in electroosmotic flow, which may even reverse. An excessively high cathode pH leads to a greater precipitation reaction in the soil, producing a "focusing effect." A large amount of precipitate accumulates near the cathode, gradually clogging the pores, resulting in reduced drainage efficiency and thus a lower heavy metal removal rate. The method provided by this invention ensures a slow change in soil pH, preventing the anode from becoming too low, the electroosmotic flow from decreasing too rapidly, and the cathode from becoming too high. This minimizes precipitation reactions within 24 hours, which is beneficial for the removal of heavy metals.

[0039] Figure 6 This figure shows the changes in conductivity in different regions under different treatments after electrokinetic remediation. As can be seen from the figure, after electrokinetic remediation under the immersion desorption treatment, the conductivity in S1 and S2 is greater than that under the partitioned desorption treatment. This is because the former has a lower pH and more free ions in the soil. The conductivity in S3 and S4 is less than that under the partitioned desorption treatment, because the former has a higher pH, leading to precipitation. This result further verifies that the present invention can significantly reduce cathodic precipitation reactions and avoid anodic soil acidification, effectively improving the removal rate of heavy metals in the soil.

[0040] Figure 7 This indicates the heavy metal removal rate in different areas under different treatments after electro-remediation. From anode to cathode, the heavy metal removal rate in electro-remediation via desorption was greater than that in traditional immersion desorption electro-remediation. The anode showed an increase of over 10%, and other areas also showed improvements. This result demonstrates that the application of this invention can significantly increase electroosmotic flow, inhibit cathodic precipitation reactions, and effectively improve the removal rate of heavy metals from soil.

[0041] The results show that this technical solution, by combining zoned desorption with electrokinetic remediation, regulates the overall pH and conductivity of the soil, enhances electromigration and electroosmotic flow in the soil, thereby strengthening the effect of the electrokinetic remediation experiment and improving the heavy metal removal rate.

[0042] The above embodiments merely illustrate several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of this invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A method for gradient-regional desorption-electrokinetic co-remediation of heavy metal contaminated soil, characterized in that, The method includes the following steps: The contaminated soil and desorbent are mixed and stirred, then loaded into the soil chamber. The electric remediation is started, and the remediation is completed after a period of time. The method is performed using an electric remediation device, which includes a soil chamber, an anode plate, a cathode plate, a cathode chamber, a DC power supply, and wires. The soil chamber is vertically divided into four regions from the anode to the cathode: the anode control region, the pre-desorption enhancement region, the main migration and transport region, and the terminal enrichment and stabilization region. Each region accounts for 23-27% of the total volume of the soil chamber, and the total volume of the regions is 100%. A cation exchange membrane is set between the main migration and transport region and the terminal enrichment and stabilization region, while no barrier medium is set between the other regions. The pH of the contaminated soil in the anode control zone is 5-6, the pH of the contaminated soil in the pre-desorption enhancement zone is 4-5, the pH of the contaminated soil in the main migration and transport zone is 3-4, and the pH of the contaminated soil in the terminal enrichment and stabilization zone is 2-3.

2. The method according to claim 1, characterized in that, In the electric remediation device, the anode plate and cathode plate are located at both ends of the soil chamber and are connected to a DC power supply via wires; the cathode chamber is located behind the cathode plate and is equipped with a drainage pipe at the bottom.

3. The method according to claim 1, characterized in that, The anode plate is a titanium-plated metal plate, and the cathode plate is a perforated titanium-plated metal plate through which water is discharged into the cathode chamber.

4. The method according to claim 1, characterized in that, Contaminated soil is soil contaminated with heavy metals, including copper, lead, cadmium, zinc, chromium, nickel, and arsenic.

5. The method according to claim 1, characterized in that, Desorbents include citric acid, oxalic acid, and tartaric acid.

6. The method according to claim 1, characterized in that, After mixing and stirring the contaminated soil and desorbent, let it stand for 10-20 minutes.

7. The method according to claim 1, characterized in that, The mixing and stirring of contaminated soil and desorbent can be carried out before the contaminated soil is loaded into the soil chamber, or after the contaminated soil is loaded into the soil chamber.

8. The method according to claim 1, characterized in that, The voltage for electric repair is 25-30V, the current is 0.1-1A, and the repair time is 20-48 hours.

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