Method for removing cadmium from soil by microbial combined electrokinetic remediation

Abstract

The present application relates to a kind of microbial combined electric repair method for removing soil Cd, using electric repair contaminated soil device, the device includes soil chamber, anode chamber and cathode chamber at both ends of soil chamber, anode chamber and cathode chamber are equipped with anode plate, cathode plate respectively, direct current stabilized power supply is equipped between anode plate and cathode plate, soil chamber is equipped with solid-liquid separation baffle between anode chamber and cathode chamber, including the following steps: S1, fill Cd contaminated soil in soil chamber, add electrolyte solution to Cd contaminated soil, cover soil upper surface, soak 20-30h;S2, add organic acid buffer solution to cathode chamber;S3, input direct current;S4, filamentous fungi are inoculated in soil close to cathode region, repair 7-9d;S5, remove filamentous fungi from soil, complete the repair of contaminated soil.The present application makes Cd in soil migrate to soil close to cathode region, filamentous fungi enrich heavy metal Cd, after removing filamentous fungi, make soil Cd concentration reach farmland soil standard.

Description

Technical Field

[0001] This invention relates to a method for removing Cd from soil using a combination of microbial and electrodynamic remediation, belonging to the field of soil remediation technology. Background Technology

[0002] With industrial development, substandard discharge of wastewater from the chemical industry has led to severe Cd pollution in soil. This soil Cd pollution directly results in excessive Cd levels in crops such as rice, seriously threatening human health. Current Cd remediation technologies for farmland mainly include flooding, chemical passivation, and hyperaccumulating plants. Flooding remediation can easily reduce crop yields and is only suitable for farmland with mild Cd pollution. Chemical passivation can reduce the bioavailability of Cd and decrease its absorption by rice, but when the environment changes, the less reactive Cd can easily transform into a free state and be reabsorbed by crops. Hyperaccumulating plants have a long remediation cycle and a low plant survival rate.

[0003] Chinese patent application CN105855290A discloses a fungal remediation method for heavily chromium-contaminated soil. This method uses spores of *Penicillium oxalicum* SL2 as the inoculum for remediation. The heavily chromium-contaminated soil is mixed with potato culture medium in a specific ratio to create a slurry suitable for spore germination and growth. A certain amount of oxalic acid is added to accelerate the remediation efficiency of chromium contamination. Chinese patent application CN106391690A discloses a method to improve the remediation effect of cadmium-contaminated soil, employing a DC electric field-plant combined remediation method. This method effectively improves the remediation effect of cadmium-contaminated soil, increasing the remediation efficiency by 20%-38%, but the remediation cycle is too long. Chinese patent application CN104801537A relates to an electro-microbial combined remediation method and apparatus for heavy metal-contaminated soil. The apparatus includes a pretreatment system, a DC regulated power supply, electric electrodes, and a post-treatment system. The apparatus features an anode and cathode electrolytic cell, with electrodes directly inserted into the soil. The regulated DC power supply is directly connected to a graphite plate in the electrodes. The electric pretreatment system includes the cultivation and acclimatization of indigenous microorganisms. This method primarily involves adding *Thiobacillus acidophilus* to the soil to activate the heavy metal Cd, thereby enhancing Cd's migration ability. The Cd removal rate in the soil is 37.5%. However, after electrokinetic remediation, Cd still accumulates in the cathode area and is not removed, posing a risk of Cd re-release. Furthermore, this patented method requires the bacterial solution to be cultured in the soil for 15 days before electrokinetic treatment, resulting in a long remediation cycle.

[0004] Therefore, there is an urgent need for a device and method that has a short adsorption cycle for heavy metal Cd and high efficiency in remediating soil cadmium pollution, so as to reduce the Cd content in the soil, effectively prevent the risk of Cd pollution re-release, and achieve safe production of crops. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a method for removing Cd from soil through microbial combined electrokinetic remediation. This method involves migrating Cd in the soil to the area near the cathode, using microorganisms to remediate cadmium-contaminated soil near the cathode, and effectively reducing the Cd content in the soil by removing microorganisms that accumulate heavy metal Cd.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0007] A method for removing Cd from soil using microbial combined electrodynamic remediation employs an electrodynamic remediation device. The device includes a soil chamber, with an anode chamber and a cathode chamber located at opposite ends. An anode plate and a cathode plate are respectively installed in the anode and cathode chambers. A DC regulated power supply is provided between the anode and cathode plates. Solid-liquid separation partitions are installed between the soil chamber and both the anode and cathode chambers. The method includes the following steps:

[0008] S1. Fill the soil chamber with Cd-contaminated soil, add sodium nitrate solution or potassium nitrate solution to the Cd-contaminated soil, and soak for 20-30 hours.

[0009] S2. Add organic acid buffer solution to the cathode chamber;

[0010] S3, Apply DC power;

[0011] S4. Inoculate the soil near the cathode area with filamentous fungi and remediate for 7-9 days. The filamentous fungi are Aspergillus fumigatus.

[0012] S5. Remove filamentous fungi from the soil to complete the remediation of contaminated soil.

[0013] In this process, Cd-contaminated soil is filled into a soil chamber, and sodium nitrate or potassium nitrate solution is added as an electrolyte solution to replenish the soil's sodium, potassium, and nitrogen sources. This process is harmless to the soil. Soaking for 20-30 hours ensures full contact and penetration of the electrolyte solution with the contaminated soil. Direct current is then applied for electrokinetic remediation, driving Cd to move directionally within a short time, causing Cd to accumulate in the soil near the cathode area. Simultaneously, in the initial stage of electrokinetic remediation, filamentous fungi are inoculated into the soil near the cathode area. These fungi grow and accumulate Cd in this area, remediating the Cd-contaminated soil. After remediation, the Cd-accumulated filamentous fungi are removed, reducing the Cd content in the soil and effectively preventing the re-release of Cd contamination, thus ensuring safe crop production. Adding an organic acid buffer solution to the cathode chamber not only regulates the pH balance of the system and promotes electrokinetic remediation of Cd-contaminated soil but also provides the carbon source needed by the filamentous fungi, enhancing the electrolysis effect and facilitating the remediation of Cd-contaminated soil by the filamentous fungi.

[0014] Electrostatic remediation requires the application of a constant voltage and external organic acids. Appropriate voltage and organic acids can effectively promote the growth of microorganisms. Microbial growth can enrich the Cd element in the soil, and they interact and synergistically promote each other.

[0015] Organic acid buffer not only enhances the electrolysis effect, but also provides a carbon source for Aspergillus fumigatus, promoting the enrichment of Cd in the soil by Aspergillus fumigatus.

[0016] Furthermore, the electro-hydraulic soil remediation device is a known device for electro-hydraulic soil remediation in the prior art.

[0017] Furthermore, in step S4, the filamentous fungi are purchased directly from the Wuhan Microbial Culture Collection Center. The filamentous fungi can complex heavy metals through functional groups on the surface of the cell or extracellular polymers, and can also transport heavy metals into the cell for enrichment.

[0018] Further, in step S1, sodium nitrate solution or potassium nitrate solution is added as an electrolyte solution, and the concentration of the electrolyte solution is 0.01 mol / L-0.02 mol / L.

[0019] Further, the flow rate of the organic acid is 1.0 ml / min-2.0 ml / min; the concentration of the organic acid is 0.1 mol / L-0.2 mol / L. In some preferred embodiments of the present invention, the organic acid is acetic acid, which is used to regulate the pH balance of the system and provide a carbon source required by microorganisms.

[0020] Furthermore, in step S2, the pH range within the device is 4-9. Acidic conditions promote the growth of filamentous fungi and prevent heavy metal precipitation, thus enhancing the electrokinetic remediation effect. However, if the pH of the system is too low, it is detrimental to plant growth in the remediated soil.

[0021] Furthermore, in step S3, the voltage of the direct current is 1V / cm-3V / cm. Excessive voltage is detrimental to microbial growth, while insufficient voltage results in ineffective electroremediation and prevents sufficient accumulation of Cd in the soil near the cathode area.

[0022] Furthermore, the device is equipped with a buffer solution tank, and the cathode chamber is connected to the buffer solution tank via a conduit equipped with a peristaltic pump. This facilitates control over the amount of buffer solution added.

[0023] In some embodiments of the present invention, a microbial chamber is disposed at the end of the soil chamber near the cathode chamber, and a partition is disposed between the microbial chamber and the soil chamber. Thus, the microbial chamber is used to cultivate filamentous fungi. Under the influence of an electric current, Cd in the soil migrates towards the soil near the cathode area and accumulates in the soil within the microbial chamber. The filamentous fungi in the microbial chamber float on the liquid surface, and the hyphae enter the soil and accumulate the heavy metal Cd accumulated at the cathode during electrokinetic remediation through their root systems. The filamentous fungi enriched with heavy metal Cd are located within the microbial chamber, avoiding the risk of Cd re-release. The partition is used for solid-liquid separation and prevents the filamentous fungi from overflowing from the microbial chamber into other areas of the soil chamber, facilitating their removal by physical removal or retrieval.

[0024] In some embodiments of the present invention, a control valve is provided between the anode chamber and the anode collection tank, and a control valve is provided between the cathode chamber and the cathode collection tank. This facilitates control of the electrolyte solution level in the soil chamber, allowing excess electrolyte solution to flow into the anode collection tank and / or the cathode collection tank.

[0025] Furthermore, in step S1, the electrolyte solution level is kept above the soil surface. This ensures full contact between the electrolyte solution and the contaminated soil, allowing for complete soil saturation. The soaking time is determined by the soil's permeability; soils with high permeability require shorter soaking times, while those with low permeability require longer soaking times.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] (1) This invention enables Cd in the soil to migrate rapidly to the soil near the cathode area through electro-remediation, so that the Cd concentration in the soil in other areas reaches the farmland soil standard; it utilizes filamentous fungi to enrich the heavy metal Cd accumulated at the cathode during the electro-remediation process through the root system, and removes the microorganisms that enrich the heavy metal Cd to achieve the purpose of soil Cd removal, so that the Cd concentration in the soil near the cathode area reaches the farmland soil standard.

[0028] (2) The method of the present invention can effectively reduce the Cd content in farmland, avoid the risk of Cd being released again, and is environmentally friendly with no secondary pollution; the treatment cycle is short and the removal rate of Cd in the soil is high. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the structure of the electric soil remediation device of the present invention.

[0030] Explanation of reference numerals in the attached diagram: 1-Anode collection tank, 2-Cathode collection tank, 3-Anode chamber, 4-Cathode chamber, 5-Separator, 6-Soil chamber, 61-Microbial chamber, 7-Buffer solution tank, 8-Peristaltic pump, 9-DC regulated power supply, 10-Cd contaminated soil, 11-Filamentous fungi, 12-Cathode plate, 13-Anode plate. Detailed Implementation

[0031] The present invention will be described in detail below with reference to embodiments. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0032] Example 1

[0033] Aspergillus fumigatus was purchased from the China Microbial Culture Collection Center, accession number ATCC96918.

[0034] A method for removing Cd from soil using a microbial-electro-mechanical remediation device is described, see reference. Figure 1 The device includes a soil chamber 6 with dimensions of 28×15×14cm (length×width×height). An anode chamber 3 with dimensions of 6×15×14cm is located on the left side of the soil chamber 6, and a cathode chamber 4 with dimensions of 6×15×14cm is located on the right side of the soil chamber 6. A partition 5 is provided between the soil chamber 6 and the anode chamber 3 and the cathode chamber 4, respectively. The soil chamber 6 is filled with 3.3kg of contaminated soil (the soil has a Cd content of 1.2mg / kg, that is, the Cd content in 1kg of soil is 1.2mg, and the pH is 6.0). The partition 5 is placed vertically in the soil. DSA electrodes are installed in anode chamber 3 and cathode chamber 4 respectively. A DC regulated power supply 9 is connected to the DSA electrodes. The upper part of anode chamber 3 is connected to anode collection tank 1, and the upper part of cathode chamber 4 is connected to cathode collection tank 2. The top of cathode chamber 4 is connected to buffer tank 7 through a conduit, on which a peristaltic pump 8 is installed. The soil chamber 6 is divided into two parts by a partition 5 at the end near cathode chamber 4. The part near cathode chamber 4 is the microbial chamber 61 (7×15×14cm). The partition 5 has several holes with a diameter of 0.5-2mm to achieve solid-liquid separation and ensure liquid flow in the device. Anode collection tank 1 and cathode collection tank 2 are used to collect the electrolyte solution in the device. The position where anode collection tank 1 is connected to anode chamber 3 is higher than the soil surface, and the position where cathode collection tank 2 is connected to cathode chamber 4 is higher than the soil surface, so that the electrolyte solution can cover the soil and maintain a certain liquid level. Buffer tank 7 contains buffer solution, which is added to cathode chamber 4 through peristaltic pump 8.

[0035] The method for removing Cd from soil using microbial combined electroremediation includes the following steps:

[0036] S1. Fill the soil chamber with Cd-contaminated soil, add sodium nitrate electrolyte solution to the soil chamber to cover the soil surface, and soak for 24 hours. The concentration of sodium nitrate solution is 0.01 mol / L.

[0037] S2. Add acetic acid to the cathode chamber at a flow rate of 1.5 ml / min and a concentration of 0.1 mol / L, and adjust the pH of the system to 5.

[0038] S3. Apply DC current, voltage 2V / cm;

[0039] S4. Add 1 ml of Aspergillus fumigatus suspension (purchased directly from Wuhan Microbial Culture Collection Center) to the microbiology room, and start the electric repair process;

[0040] S5. After 7 days of remediation, remove Aspergillus fumigatus from the soil to complete the remediation of the contaminated soil.

[0041] The soil chamber was divided into four equal areas, designated as Zone 1, Zone 2, Zone 3, and Zone 4 along the direction from the anode chamber to the cathode chamber. The microbial chamber was located in Zone 4. The Cd content in the soil of Zones 1-4 was measured, and the Cd content in Aspergillus fumigatus removed from the soil was also measured. The results are shown in Table 1.

[0042] Table 1. Cd content in soil samples and Aspergillus fumigatus from different regions

[0043]

[0044] After treating Cd-contaminated soil using the method of this invention, the Cd content in soils of zones 1-4 was significantly reduced compared to before treatment, with zone 1 showing the lowest Cd content, followed by zone 2, and zone 3 showing the highest. This indicates that Cd levels in the soil vary along the distribution of Cd. Figure 1 Moving in the direction indicated by the middle arrow, from the anode to the cathode, the removal rate of heavy metal Cd in Cd-contaminated soil can reach over 75.0%.

[0045] Comparative Example 1

[0046] This comparative example used the same electric remediation device as Example 1 to remove Cd from the soil. The difference from Example 1 is that Aspergillus fumigatus suspension was not added in step S4. The Cd content in the soil of each area was measured, and the results are shown in Table 2.

[0047] Table 2. Cd content in soil samples from different regions after treatment with no addition of Aspergillus fumigatus suspension.

[0048] sample Zone 1 soil Soil in Zone 2 Soil in Zone 3 Soil in Zone 4 Cd concentration (mg / kg) 0.16 0.25 0.28 3.8

[0049] Table 2 shows that the Cd content in the soil of zones 1-4 increases sequentially, with zone 4 having the highest Cd content, significantly higher than before treatment. Cd in the soil migrates from the anode to the cathode and accumulates in the region near the cathode. Comparative Example 1, using only electrokinetic remediation, cannot effectively remove Cd from Cd-contaminated soil; it only causes Cd to accumulate in the region near the cathode. The high Cd content in zone 4 indicates that the contaminated soil cannot be completely remediated.

[0050] Comparative Example 2

[0051] The difference between this comparative example and Example 1 is that step S2 was omitted, acetic acid was not added to the cathode chamber, and the Cd content in the soil of zones 1-4 was measured after repair. The results are shown in Table 3.

[0052] Table 3. Cd content in soil samples from different regions after treatment with acetic acid.

[0053]

[0054] Table 3 shows that without the addition of acetic acid, the Cd content in the soil of zones 1-3 was relatively high, with the highest Cd content in zone 4. The Cd content in the Aspergillus fumigatus strain removed from the soil was 0, indicating that Aspergillus fumigatus did not enrich Cd in the soil. This suggests that acetic acid not only enhances the electrolysis effect but also provides a carbon source for Aspergillus fumigatus, promoting its enrichment of Cd in the soil.

[0055] Comparative Example 3

[0056] Using the apparatus and method described in Example 1, the effect of different pH values ​​of the system in step S2 on the Cd content in the soil was investigated. The experimental results are shown in Table 4.

[0057] Table 4 shows the Cd content in soil samples from different regions after treatment with different pH values.

[0058]

[0059] The acid-base balance in the system is controlled by adjusting the concentration and flow rate of acetic acid. Acidic conditions promote the growth of filamentous fungi and prevent heavy metal precipitation, thus enhancing the electroremediation effect. However, if the pH of the system is too low, it is detrimental to plant growth in the remediated soil.

[0060] Comparative Example 4

[0061] The difference between this embodiment and embodiment 1 is that the remediation time in step S4 is different. The effect of different remediation times on the Cd content in the soil was investigated, and the results are shown in Table 5.

[0062] Table 5 shows the Cd content in soil samples from different regions after different remediation days.

[0063]

[0064] As shown in Table 5, the Cd content in the soil of zones 1-4 was still relatively high after 5 days of remediation, indicating that the short remediation time meant that the Cd in the soil had not fully migrated to the area near the cathode. When the remediation time was 11 days, the Cd content in the soil of zones 1-4 did not change significantly compared with the remediation times of 7 and 9 days. The excessively long remediation time not only resulted in a waste of electrical energy but also reduced work efficiency.

[0065] Comparative Example 5

[0066] The difference between this embodiment and embodiment 1 is that no direct current is applied in step S2. The effect of adding Aspergillus fumigatus alone on the Cd content in the soil is investigated, and the Cd content in the soil of each region is measured. The results are shown in Table 6.

[0067] Table 6 shows the effect of adding Aspergillus fumigatus alone on the Cd content in soil of different areas after soil remediation.

[0068]

[0069] As shown in Table 6, when Aspergillus fumigatus was added to the soil in Zone 4, the Aspergillus fumigatus grew and accumulated heavy metal Cd in the soil. After a certain period of remediation, the Aspergillus fumigatus was removed, and the Cd content in the soil in Zone 4 was greatly reduced. However, the Cd content in the soil in Zones 1-3 did not change, and it could not effectively remediate Cd-contaminated soil.

[0070] This indicates that the combined microbial and electrokinetic remediation method for removing Cd from soil, with the synergistic effect of current and microbial remediation, improves the remediation effect of Cd pollution in soil, effectively prevents the risk of Cd pollution re-release, and ensures safe crop production.

[0071] As can be seen from Example 1, Comparative Example 1 and Comparative Example 5, the treatment effect of Example 1 is significantly better than the combined effect of Comparative Example 1 and Comparative Example 5, indicating that current and microbial remediation have a synergistic promoting effect.

[0072] The above embodiments should be understood as being used only to illustrate the present invention more clearly, and not to limit the scope of the present invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art fall within the scope defined by the appended claims.

Claims

1. A method for removing Cd from soil by microbial combined electrokinetic remediation, characterized in that, The device is used for remediation of contaminated soil, and comprises a soil chamber (6), an anode chamber (3) and a cathode chamber (4) arranged at two ends of the soil chamber (6) respectively, an anode plate (13) and a cathode plate (12) arranged in the anode chamber (3) and the cathode chamber (4) respectively, a direct current stabilized power supply (9) arranged between the anode plate (13) and the cathode plate (12), a solid-liquid separation partition plate (5) arranged between the soil chamber (6) and the anode chamber (3) and the cathode chamber (4), and a microorganism chamber (61) arranged at one end of the soil chamber (6) close to the cathode chamber (4), wherein a partition plate (5) is arranged between the microorganism chamber (61) and the soil chamber (6); The method comprises the following steps: S1, filling Cd contaminated soil into the soil chamber, adding sodium nitrate solution or potassium nitrate solution to the Cd contaminated soil, and soaking for 20-30 h; S2, adding an organic acid buffer solution to the cathode chamber; S3, passing direct current; S4, culturing filamentous fungi in the microorganism chamber, inoculating the filamentous fungi in the soil close to the cathode zone, and remediation for 7-9 d, wherein the filamentous fungi are at least one of Aspergillus fumigatus, brown rot fungi, wood-rotting fungi and Aspergillus niger; S5, in order to avoid the filamentous fungi overflowing from the microorganism chamber to other areas of the soil chamber, the filamentous fungi are removed by physical removal or fishing, and the remediation of the contaminated soil is completed; The organic acid is acetic acid. In step S2, the pH value in the device ranges from 4 to 9.

2. The method of claim 1, wherein, In step S1, the concentration of the sodium nitrate solution or the potassium nitrate solution ranges from 0.01 mol / L to 0.02 mol / L.

3. The method of claim 1, wherein, The flow rate of the organic acid ranges from 1.0 ml / min to 2.0 ml / min, and the concentration of the organic acid ranges from 0.1 mol / L to 0.2 mol / L.

4. The method of claim 1, wherein, In step S3, the voltage of the direct current ranges from 1 V / cm to 3 V / cm.

5. The method according to any one of claims 1 to 4, characterized in that, A buffer solution pool (7) is arranged on the device, the cathode chamber (4) and the buffer solution pool (7) are communicated through a conduit, and a peristaltic pump (8) is arranged on the conduit.

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