Method for repairing heavy metal contaminated soil by combining an iron-based nanocomposite modified electrode with a microbial electrochemical system

Abstract

This invention discloses a method for remediating heavy metal contaminated soil using an iron-based nanocomposite modified electrode combined with a microbial electrochemical system. The method involves first preparing a suspension of Fe₃O₄ and GO, then uniformly coating the surface of a graphite rod with Fe₃O₄ and GO using a soaking-drying method to obtain an iron-based nanocomposite modified electrode. The modified electrode is then used as the anode, and the unmodified graphite rod electrode as the cathode. The anode and cathode are connected by wires and placed in a galvanic cell containing a culture medium and heavy metal contaminated soil. A domesticated microbial strain is then introduced for leaching. The modified electrode prepared using iron-based nanocomposite materials exhibits excellent electrochemical performance, high compatibility with microorganisms, and demonstrates good remediation performance for heavy metal contaminated soil through a combination of electrochemical and biological methods.

Description

Technical Field

[0001] This invention belongs to the field of modified electrode technology and relates to a method for remediating heavy metal contaminated soil using an iron-based nanocomposite modified electrode combined with a microbial electrochemical system. Background Technology

[0002] With the rapid development of society, the types and pathways through which pollutants generated and emitted by industries and other sectors enter the soil environment have increased dramatically. Pollutants such as electronic waste from industrial development, as well as dust, particulate matter, wastewater, and waste residue generated from metal smelting and coal combustion, cause varying degrees of soil pollution after entering the soil. Once the heavy metal content in the soil exceeds the standard, microorganisms cannot fully degrade these heavy metal pollutants. These pollutants accumulate in the human body through the food chain, posing a worrying threat to agricultural productivity, food safety, and human health. Furthermore, because metals are non-biodegradable and highly persistent in soil, they accumulate over long periods, rendering the soil unusable and causing irreversible damage to the ecosystem. Researchers have made numerous attempts to achieve the harmless remediation of polluted soil.

[0003] Bioelectrochemical systems (BESs), as an emerging co-remediation technology, overcome the shortcomings of traditional single-remediation techniques and have attracted considerable attention due to their excellent performance in pollutant purification. They utilize electroactive bacteria to oxidize anodic pollutants and reduce cathodic pollutants through extracellular electron transfer (EET). Compared with other remediation technologies for contaminated soil, bioelectrochemical technology offers milder conditions, lower cost, and the electrodes can provide a large number of electron donors and acceptors, promoting the redox reaction of microorganisms and thus avoiding the use of oxidants and reductants, preventing secondary pollution of the surrounding soil. Current research has demonstrated the feasibility of BESs in degrading pollutants such as heavy metals in soil, and they have been successfully applied. However, due to the lack of suitable electron acceptors and slow mass transfer, their application remains limited, and further research is needed to achieve highly efficient application of bioelectrochemicals. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for improving the removal efficiency of heavy metals in contaminated soil by combining an iron-based nanocomposite modified electrode with a microbial electrochemical system. This invention improves the conductivity and specific surface area of ​​the electrode by modifying the surface structure of graphite rods, while simultaneously optimizing the biocompatibility of the electrode material. The modified electrode prepared with iron-based nanocomposite material exhibits excellent electrochemical performance, high compatibility with microorganisms, and demonstrates good remediation performance for heavy metal contaminated soil by combining electrochemical and biological methods.

[0005] The technical solution of the present invention is described in detail below.

[0006] This invention provides a method for remediating heavy metal contaminated soil using an iron-based nanocomposite material modified electrode combined with a microbial electrochemical system, the specific steps of which are as follows:

[0007] Step S1: Soil sample pretreatment

[0008] The naturally air-dried heavy metal contaminated soil was ground and sieved to obtain a soil sample to be treated.

[0009] Step S2: Strain domestication and culture

[0010] Select strains for domestication and cultivation to improve their tolerance to heavy metals;

[0011] Step S3, Pretreatment of graphite rod electrodes

[0012] The graphite rod electrode is cleaned 2-3 times in sequence with dilute alkali, dilute acid and deionized water, and then dried after cleaning to achieve pretreatment.

[0013] Step S4: Preparation of iron-based nanocomposite modified electrodes

[0014] Using Fe3O4 oxide and GO oxide as raw materials, a Fe3O4-GO suspension was prepared; the pretreated graphite rods were immersed in the Fe3O4-GO suspension, then removed and air-dried naturally. This process was repeated 2-4 times to obtain an iron-based nanocomposite modified electrode.

[0015] Step S5: Constructing a bioelectrochemical system to remediate heavy metal contaminated soil

[0016] A modified iron-based nanocomposite electrode was used as the anode, and an unmodified pretreated graphite rod electrode was used as the cathode. The anode and cathode were connected by wires and placed in a galvanic cell device. The reaction vessel contained the soil sample to be treated and a culture medium. An acclimatized bacterial strain was inoculated, and leaching was carried out at 25-30℃ and 120-150 r / min for 5-10 days. During the leaching process, the pH and Fe content of the solution were monitored. 2+ The concentration, ORP, and heavy metal leaching rate changed with leaching time, and leaching ended after 5 to 10 days.

[0017] In this invention, in step S1, the heavy metal contaminated soil is collected from contaminated soil in a mining area, and the heavy metals include Zn, Cu, Cd and Pb; each kilogram of contaminated soil contains 2000-4000 mg of Zn, 10000-30000 mg of Cu, 50-200 mg of Cd and 300-1000 mg of Pb; the sieve mesh size is 80 mesh.

[0018] In this invention, in step S2, the bacterial strain is *Thiobacillus acidophilus* (…). Acidithiobacillusferrooxidans hereinafter referred to as " A.f The steps for domesticating and cultivating bacteria are as follows: A.f The bacteria were inoculated into 9K culture medium solution containing soil contaminated with a quantitative amount of complex heavy metals, and subjected to continuous subculturing and acclimatization culture. At the end of each generation, the optimal strain was selected and transferred to the next generation to establish a screening system. A.f The bacteria's tolerance to heavy metal concentrations.

[0019] In this invention, in step S3, based on the porous nature of graphite, the graphite rod surface exposed to air will adsorb dust and other impurities, affecting subsequent experiments. Therefore, the graphite rod is pre-treated by cleaning. The dilute alkali is a NaOH solution with a concentration of 0.5-2 mol / L, and the dilute acid is a HCl solution with a concentration of 0.5-2 mol / L. The cleaning is done by ultrasonic cleaning, with each cleaning time being 15-40 min. The rod is then dried in an oven at a temperature of 60-85℃.

[0020] In this invention, the preparation method of the Fe3O4-GO suspension in step S4 is as follows:

[0021] 50 mg of graphene oxide nanosheets were dispersed in 20-60 ml of deionized water and stirred for 4-6 h to form a uniform dispersion. Then, 0.8-1.2 g of iron oxide nanoparticles were added and ultrasonically stirred for 1-3 h. The electrostatic repulsion between the nanoparticles was used to fully mix the iron oxide nanoparticles and graphene oxide nanosheets to obtain a Fe3O4-GO suspension.

[0022] In this invention, in step S5, the culture medium is a 9K culture medium solution, the inoculum size is 8-15 vol%, and the ratio of the soil sample to be treated to the culture medium is 1:50~1:20 g / mL. Preferably, the pH of the culture medium is between 2 and 3; the inoculum size is 8-12 vol%; and the ratio of the soil sample to be treated to the culture medium is 1:50~1:40 g / mL.

[0023] In this invention, in step S5, the wire used to connect the graphite rod cathode and the graphite rod anode is a copper wire, and the part in contact with the graphite rod is a stainless steel alligator clip.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] (1) Graphene oxide possesses conductive properties and a porous structure. Studies have shown that atomic doping can effectively create suitable wettability and positively charged microregions on carbon sheets to facilitate cell adhesion; iron is a key component of cytochrome c and iron-sulfur protein, both of which are essential for most electrogenic microorganisms. A.f In solutions where bacteria leach heavy metal-contaminated soil, the presence of this electrode system can accelerate redox reactions and electron transfer rates in the reaction solution, thereby increasing... A.f This method enhances the oxidizing ability of bacteria against heavy metal ions, thereby improving leaching efficiency. The Fe content in the leachate is determined using a UV spectrophotometer. 2+ The concentration changed over time, and the ORP electrode monitored the change in redox potential in the solution over time. ICP-AES was used to analyze the concentration of metal ions in the leachate. Based on the changes in parameters over time, it can be concluded that the iron-based nanocomposite modified graphite rod electrode system can improve... A.f The leaching efficiency of bacteria.

[0026] (2) Compared with the blank control group, the graphene oxide-modified graphite rod electrode system of this invention improves the remediation efficiency of *Thiobacillus ferrooxidans* for heavy metal contaminated soil. This invention has a simple operation method, low raw material cost, and is environmentally friendly, and has a practical basis for industrial application.

[0027] (3) The present invention utilizes iron-based nanocomposite materials to modify the anode, which not only gives it good electrical conductivity, but also allows it to carry highly electroactive microorganisms. This effectively solves the problem of low electron transfer efficiency in soil microbial electrochemical remediation. At the same time, it solves the common key problem of soil mass transfer by using a conductive network to mediate long-distance electron transfer instead of pollutant migration. This invention achieves a breakthrough technology in soil microbial electrochemistry for remediating soils with high concentrations of heavy metal pollution. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art without innovative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0029] In this example, the heavy metal contaminated soil was collected from a mining area. The total metal content of the soil sample was analyzed using an HNO3-HF-HCl digestion system. Inductively coupled plasma optical emission spectrometry (ICP-OES) revealed that the main heavy metals in the contaminated soil were Zn, Cu, Pb, and Cd, with a content of 3785 mg·kg⁻¹. -1 26176mg·kg -1 75.9 mg·kg -1 703 mg·kg -1It far exceeds the farmland risk screening value of the "Soil Environmental Quality Agricultural Land Soil Risk Control Standard (Trial)" (GB15618-2018). Example 1

[0030] Pretreatment of graphite rods: The graphite rods were all commercially available untreated conductive graphite rods (6×60mm). The graphite rod electrode surfaces were cleaned by immersing them in 1mol / L NaOH solution and 1mol / L HCl solution in sequence. Then, the graphite rod electrodes were cleaned with deionized water 2-3 times. The graphite rods were cleaned by ultrasonic cleaning for 30 minutes each time. The cleaned graphite rods were then dried in an oven at 80℃. The pretreatment was completed after drying.

[0031] Using unmodified graphite rods as cathodes and graphite rods modified with iron-based nanocomposite materials as anodes, the specific fabrication process is as follows: 50 mg of graphene oxide nanosheets are dispersed in 50 ml of deionized water and stirred for 5 hours to form a uniform suspension. Then, 1 g of iron(III) oxide nanoparticles are added, and the mixture is sonicated for 1 hour and stirred for 1 hour. Utilizing the electrostatic repulsion between the nanoparticles, the iron(III) oxide nanoparticles and graphene oxide nanosheets are thoroughly mixed to obtain a Fe3O4-GO suspension. The pretreated graphite rods are immersed in the Fe3O4-GO suspension for 30 minutes and then air-dried. This process is repeated three times until the Fe3O4-GO suspension is stably and uniformly fixed on the surface of the graphite rods.

[0032] According to the electron microscope of the obtained modified electrode material, it can be seen that the Fe spherical nanoparticles have no obvious agglomeration and good dispersion; the Fe spherical nanoparticles are attached to the GO nanosheets. According to the preliminary XRD analysis, the composition of the obtained modified electrode material remains unchanged during the modification preparation process.

[0033] The electrochemical performance of the modified electrode was tested using a three-electrode system. A Pt wire electrode was used as the auxiliary electrode, a graphite electrode (modified / unmodified) as the working electrode, and an Ag / AgCl electrode as the reference electrode. The electrochemical test results showed that the modified electrode exhibited a larger electrochemical active area, and its electrochemical performance was significantly improved under the same scan range and scan rate conditions. Example 2

[0034] In the microbial electrochemical system, the selected bacterial strain is A.f The bacteria are first subjected to domestication and cultivation; the steps of domestication and cultivation are as follows:

[0035] Will A.f The bacteria were inoculated into 9K culture medium solution containing soil contaminated with certain heavy metals for acclimatization and culture. Continuous subculturing was performed to gradually screen out bacteria with high tolerance to heavy metals. A.f bacteria.

[0036] The iron-based nanocomposite modified electrode prepared in Example 1 was used as the anode, and the unmodified pretreated graphite rod electrode was used as the cathode. The anode and cathode were connected by wires and placed in a galvanic cell device containing 90 ml of 9K culture medium. The culture medium composition was: 44.2 g / L FeSO4•7H2O, 3.5 g / L (NH4)2SO4, 0.5 g / L K2HPO4, 0.1 g / L KCl, 0.5 g / L MgSO4•7H2O, and 0.01 g / L Ca(NO3)2, with a pH between 2 and 3. 4 g of heavy metal-contaminated soil was added to the culture medium, and acclimatized bacterial strains were inoculated. A.f The inoculum was 10% (v / v) and acclimatized in an incubator at 30°C and 130 r / min. The pH and ORP values ​​of the solution were measured periodically, as was the ICP.

[0037] Comparative Example 1

[0038] The unmodified electrode from Example 1, after pretreatment, was used as the anode, and the unmodified pretreated graphite rod electrode was used as the cathode. The anode and cathode were connected by a wire and placed in a galvanic cell device containing 90 ml of 9K culture medium. The culture medium composition was: 44.2 g / L FeSO4•7H2O, 3.5 g / L (NH4)2SO4, 0.5 g / L K2HPO4, 0.1 g / L KCl, 0.5 g / L MgSO4•7H2O, and 0.01 g / L Ca(NO3)2, with a pH between 2 and 3. 4 g of heavy metal-contaminated soil was added to the culture medium, and acclimatized bacterial strains were inoculated. A.f The inoculum was 10% (v / v) and acclimatized in an incubator at 30°C and 130 r / min. The pH and ORP values ​​of the solution were measured periodically, as was the ICP.

[0039] Comparative Example 2

[0040] Add 90 ml of 9K culture medium to a reaction vessel. The culture medium composition is: 44.2 g / L FeSO4•7H2O, 3.5 g / L (NH4)2SO4, 0.5 g / L K2HPO4, 0.1 g / L KCl, 0.5 g / L MgSO4•7H2O, and 0.01 g / L Ca(NO3)2, with a pH between 2 and 3. Add 4 g of heavy metal contaminated soil to the culture medium and inoculate with acclimatized bacterial strains. A.f The inoculum was 10% (v / v) and acclimatized in an incubator at 30°C and 130 r / min. The pH and ORP values ​​of the solution were measured periodically, as was the ICP.

[0041] Based on the removal rates of different heavy metals after different treatments (leaching time was 7 days for all), it can be seen that the removal rates of different heavy metals obtained by the modified electrode system in Example 2 are higher than those of the other two treatments (Comparative Example 1 and Comparative Example 2), namely 78% (Cu), 91% (Zn), 86% (Cd), and 89% (Pb).

Claims

1. A method for remediating heavy metal contaminated soil using an iron-based nanocomposite material modified electrode combined with a microbial electrochemical system, characterized in that, The specific steps are as follows: Step S1: Soil sample pretreatment The naturally air-dried heavy metal contaminated soil was ground and sieved to obtain a soil sample to be treated. Step S2: Strain domestication and culture Select strains for domestication and cultivation to improve their tolerance to heavy metals; Step S3, Pretreatment of graphite rod electrodes The graphite rod electrode is cleaned 2-3 times in sequence with dilute alkali, dilute acid and deionized water, and then dried after cleaning to achieve pretreatment. Step S4: Preparation of iron-based nanocomposite modified electrodes Fe3O4 and graphene oxide (GO) were used as raw materials to prepare Fe3O4-GO suspension. Pretreated graphite rods were immersed in Fe3O4-GO suspension for 20-40 minutes, then removed and air-dried. This process was repeated 2-4 times to obtain iron-based nanocomposite modified electrodes. Step S5: Constructing a bioelectrochemical system to remediate heavy metal contaminated soil Using an iron-based nanocomposite modified electrode as the anode and an unmodified pretreated graphite rod electrode as the cathode, the anode and cathode were connected by wires and placed in a galvanic cell device. The reaction vessel contained the soil sample to be treated and a culture medium, and acclimatized bacterial strains were inoculated. Leaching was carried out at 25–30°C and 120–150 r / min for 5–10 days. During the leaching process, the pH and Fe content of the solution were monitored. 2+ Changes in concentration, ORP, and heavy metal leaching rate with leaching time; leaching ends after 5-10 days; among which: In step S2, the bacterial strain is *Thiobacillus acidophilus* (…). Acidithiobacillus ferrooxidans , Af The steps for domesticating and cultivating it are as follows: Af The bacteria were inoculated into 9K culture medium solution containing soil contaminated with a quantitative amount of complex heavy metals, and subjected to continuous subculturing and acclimatization culture. At the end of each generation, the optimal strain was selected and transferred to the next generation to establish a screening system. Af Bacterial tolerance to heavy metal concentrations; In step S3, the dilute alkali is a NaOH solution with a concentration of 0.5-2 mol / L, and the dilute acid is a HCl solution with a concentration of 0.5-2 mol / L. The cleaning is performed by ultrasonic cleaning, with each cleaning session lasting 15-40 minutes. The product is then dried in an oven at a temperature of 60-85℃.

2. The method according to claim 1, characterized in that, In step S1, the heavy metal contaminated soil was collected from contaminated soil in a mining area, and the heavy metals included Zn, Cu, Cd and Pb.

3. The method according to claim 1, characterized in that, In step S4, the preparation method of the Fe3O4-GO suspension is as follows: 50 mg of graphene oxide nanosheets were dispersed in 20-60 mL of deionized water and stirred for 4-6 h to form a uniform dispersion. Then, 0.8-1.2 g of iron oxide nanoparticles were added and ultrasonically stirred for 1-3 h. The electrostatic repulsion between the nanoparticles was used to fully mix the iron oxide nanoparticles and graphene oxide nanosheets to obtain a Fe3O4-GO suspension.

4. The method according to claim 1, characterized in that, In step S5, the culture medium is 9K medium solution with a pH value between 2 and 4; the inoculum amount is 8-15 vol%; and the ratio of the soil sample to be treated to the culture medium is 1:50~1:20 g / mL.

5. The method according to claim 1, characterized in that, In step S5, the pH of the culture medium is between 2 and 3; the inoculum size is 8-12 vol%; and the ratio of the soil sample to the culture medium is 1:50 to 1:40 g / mL.