A method for improving the bioleaching efficiency of heavy metal contaminated soil by using graphene oxide modified carbon rod electrode system

Abstract

This invention discloses a method for improving the bioleaching efficiency of heavy metal contaminated soil using a carbon rod electrode system modified with graphene oxide (GO). The invention involves preparing a carbon nanomaterial, GN-GO, from traditional graphene rod electrodes using a GO suspension through a multiple soaking and drying process. The GN-GO material is then used as the anode, and the unmodified graphene rod electrode as the cathode. The anode and cathode, connected to an external power source via wires, are placed in a container containing a culture medium and a heavy metal contaminated soil sample. An acclimatized bacterial strain is then introduced, and leaching is performed at 25–30°C and 120–150 r / min. This invention improves the conductivity and specific surface area of ​​the electrode by altering the electrode surface structure of the bioelectrochemical system, while simultaneously optimizing the biocompatibility of the electrode material. The method combines electrochemical and biological approaches, resulting in high remediation efficiency and good remediation effects for heavy metal contaminated soil.

Description

A method for improving the bioleaching efficiency of heavy metal contaminated soil using a carbon rod electrode system modified with graphene oxide. Technical Field

[0001] This invention belongs to the field of bioremediation technology, and specifically relates to a method for improving the bioleaching efficiency of heavy metal contaminated soil using a carbon rod electrode system modified with graphene oxide. Background Technology

[0002] Heavy metals in soil exist primarily in water-soluble, carbonate-bound, iron-manganese oxide-bound, organically bound, and residual forms, exhibiting environmental persistence. Their mobility, activity, toxicity, and availability decrease within the soil. In-situ remediation technology is widely used due to its low cost, minimal disturbance to the ecosystem, and ease of operation. Based on principles, in-situ remediation technology can be further divided into physical, chemical, and bioremediation techniques. Bioremediation is considered an environmentally beneficial approach, but its long remediation cycle is a drawback due to insufficient acceptors. Bioremediation technologies, including phytoremediation, animal remediation, and microbial remediation, are considered greener and more economical compared to physical and chemical remediation techniques. However, these bioremediation strategies are limited by long remediation cycles and slow microbial community activity. Furthermore, the toxicity mechanisms of heavy metals to microorganisms and the lack of electron acceptors in the soil significantly impact remediation effectiveness. Bioelectrochemical systems (BES) technology has recently been used for soil remediation because it provides an adaptive platform for removing organic pollutants and heavy metals from contaminated soils. Electroactive microorganisms and electrode materials are important determinants of remediation systems. Therefore, it is urgent to focus on developing and improving economically feasible new electrode materials with high electrochemical activity and biocompatibility to promote the formation of electroactive biofilms on new electrodes with large surface areas.

[0003] Chinese patent application CN201610222686.8 discloses a method for improving the microbial leaching of metallic copper from waste circuit boards using a graphene-modified carbon rod electrode system. This method modifies the electrode with graphene to leach elemental copper. Chinese patent application CN201510193868.2 discloses a carbon nanosphere-modified electrode, its preparation method, and its application in microbial fuel cells. The prepared carbon nanosphere-modified electrode has a large specific surface area, good redox properties, and electrochemical performance. Applying it to microbial dye batteries can increase the amount of microorganisms attached, improving the battery's power generation performance and wastewater treatment efficiency. However, the modification process generates a large amount of waste gas and consumes a significant amount of heat, making it less environmentally friendly. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a method for improving the bioleaching efficiency of heavy metal contaminated soil using a carbon rod electrode system modified with graphene oxide. The leachate used in this method is environmentally friendly and harmless. This invention utilizes graphene oxide-modified electrode materials that not only possess excellent electrical conductivity but also can be highly loaded with electroactive microorganisms, solving the problem of low electron transfer efficiency in soil microbial electrochemical remediation. Simultaneously, by using a conductive network to mediate long-distance electron transport instead of pollutant migration, it addresses the common key challenge of difficult soil mass transfer, achieving a breakthrough technology in soil microbial electrochemical remediation of soils with high concentrations of heavy metal contaminated soil.

[0005] This invention prepares carbon nanomaterials modified from traditional graphite rod electrodes using a GO suspension through a multiple soaking and drying process. These modified materials are then adapted to electroactive microorganisms through culture, forming a 3D large-pore conductive network structure. The modified electrode surface exhibits excellent biocompatibility and electrochemical properties, and the large pore size allows for extracellular electron transfer by microorganisms to the centimeter level. This invention improves the conductivity and specific surface area of ​​the electrode by altering its surface structure in a bioelectrochemical system, while simultaneously optimizing the biocompatibility of the electrode material. The 3D carbon nanomaterial modified electrode prepared with GO suspension exhibits excellent electrochemical performance and high compatibility with microorganisms, demonstrating superior and efficient remediation performance for heavy metal contaminated soils through a combination of electrochemical and biological methods. The specific technical solution of this invention is as follows.

[0006] This invention provides a method for improving the bioleaching efficiency of heavy metal contaminated soil using a carbon rod electrode system modified with graphene oxide. The specific steps are as follows:

[0007] Step 1: The heavy metal contaminated soil is dried, ground and sieved in sequence to obtain the soil sample to be treated;

[0008] Step 2: Select bacterial strains for acclimatization and cultivation in heavy metal contaminated soil and DC regulated power supply to improve the strains' tolerance to heavy metals and system current; wherein: the bacterial strain is Acidithiobacillus ferrooxidans (Af).

[0009] Step 3: Under ultrasonic conditions, the graphite rod is immersed in a 5-10 mg / mL graphene oxide (GO) suspension to ensure that the graphene oxide is fully dispersed and in full contact with the graphite rod. Then, the graphite rod is removed and placed in an oven for drying to obtain a carbon nanotube modified electrode material for one cycle. The immersion and drying are repeated 2-4 times until GO is stably and uniformly fixed on the graphite rod to obtain the graphene oxide modified graphite rod GN-GO electrode.

[0010] Step 4: Using a GN-GO electrode as the anode and an unmodified graphite rod electrode as the cathode, the anode and cathode are inserted into an electrolytic cell containing the culture medium and the soil sample to be treated. The anode and cathode are connected to the positive and negative terminals of an external power supply via wires, respectively. An acclimatized bacterial strain is introduced, and leaching is performed at a temperature of 25–30°C and a rotation speed of 120–150 r / min for 3–7 days. During the leaching process, the pH and Fe content of the solution are monitored. 2+ Variation of concentration, ORP, and heavy metal leaching rate with leaching time.

[0011] In this invention, in step 1, the heavy metals in the heavy metal contaminated soil include Cu, Zn, Cd, and Pb; the heavy metal contaminated soil is ground and then passed through a 100-mesh sieve.

[0012] In this invention, step 2, the process of acclimatizing and culturing the bacterial strain is as follows: First, Af bacteria are inoculated into 9K culture medium solution, and heavy metal contaminated soil of different concentration gradients is added for acclimatization and culturing, and Af bacterial strains with high tolerance to heavy metals are gradually screened out; then, a regulated current is applied to the culture medium system through a DC regulated power supply to acclimatize the screened Af bacteria with high tolerance to heavy metals, so that they become tolerant to the micro electric field.

[0013] In this invention, in step 3, when the GO suspension concentration is 5-10 mg / mL, the GO suspension more easily forms a stable three-dimensional network structure on the electrode surface, overcoming the technical difficulties of low adhesion rate and uneven adhesion between the suspension and the graphite rod electrode. The ultrasonic frequency is 100 Hz, and the ultrasonic treatment time is 30-60 min.

[0014] In this invention, in step 3, the oven temperature is set to 100-110℃ and the drying time is 80-150min. By precisely controlling the drying temperature and time, the drying speed is faster than natural drying and other methods, ensuring that GO can be fully diffused and fixed on the surface of the graphite rod electrode.

[0015] In this invention, step 4 involves leaching under a constant current of 15-25 mA, using 9K medium as the culture medium, with an inoculum size of 8-15 vol%, and a feed ratio of 1:80-1:30 g / mL between the soil sample and the culture medium. The initial pH of the system is between 1.5 and 2.5.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] (1) The present invention combines carbon nanomaterials with electrochemically active microorganisms to form a conductive network that mediates long-distance electron transfer in the electrode system, thereby solving the problem of low electron transfer efficiency in soil remediation.

[0018] (2) By optimizing the concentration of GO suspension, precise soaking and drying conditions and its adaptive culture with Af bacteria, this invention achieves efficient modification of commercially available traditional conductive graphite rods, significantly improves electrochemical performance and biocompatibility, and improves the problems of electrode corrosion and low repair efficiency in bioelectrochemical systems.

[0019] (3) Graphene oxide has conductive properties and a porous structure. It is modified on the surface of graphite rods and together with the graphite rod anode and physical wires, it constitutes a graphene oxide-modified carbon rod electrode system. In the solution of A. f. acidophilus leaching heavy metal contaminated soil, the presence of this electrode system can accelerate the redox reaction and electron transfer rate in the reaction solution, thereby improving the oxidation ability of A. f. acidophilus to heavy metal ions, converting heavy metal ions from the adsorbed state to the free state, and improving the leaching efficiency.

[0020] (4) Compared with the blank control group, the carbon rod electrode system modified with graphene oxide in this invention improves the remediation efficiency of *Thiobacillus ferrooxidans* for heavy metal contaminated soil. This method has advantages such as simple process, low investment, and environmental friendliness, and has a practical basis for industrial application. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 is a schematic diagram of the experimental procedure of the present invention.

[0023] Figure 2 is a schematic diagram of the process route of the present invention.

[0024] Figure 3 is a photograph of the graphene oxide modified electrode prepared in Example 1.

[0025] Figure 4 is a scanning electron microscope image of the graphene oxide modified electrode with large pores prepared in Example 1.

[0026] Figure 5 is a scanning electron microscope image of the embedded bacterial graphene oxide modified electrode prepared in Example 2. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0028] This invention utilizes an electrode system modified with graphene oxide (GO) combined with microorganisms to achieve the leaching of various metal ions from soil. While the structure of GO is similar to graphene, it contains numerous carbonyl (C=O), carboxyl (COOH), and hydroxyl (OH) functional groups on its basal surface and edges. These oxygen-containing functional groups increase the water solubility of GO, resulting in excellent dispersibility and reactivity. Simultaneously, GO, as a thin and uniform graphene derivative, exhibits excellent conductivity and chemical stability. After forming a stable suspension in water, it can form a uniform and dense coating on the electrode surface, effectively increasing the electrode surface area and thus improving electrode reactivity and electrochemical performance, such as electron transfer and ion diffusion rates. Furthermore, GO can act as a growth promoter for microorganisms, increasing cell attachment and proliferation through non-specific (π-π stacking). Therefore, this invention significantly improves the efficiency of leaching different metal ions from soil.

[0029] Figure 1 is a schematic diagram of the experimental process of the present invention. Figure 2 is a schematic diagram of the process route of the present invention.

[0030] In this embodiment, soil contaminated with heavy metals near the electronic waste dismantling site was selected as the acclimatization soil sample. The Af strain was obtained from soil samples from the Dexing Copper Mine in Jiangxi Province through strict screening and multiple acclimatization and enrichment in the laboratory to improve its tolerance and degradation to the contaminated soil in the copper mine. A 20mA regulated current was applied to the culture medium system through a DC regulated power supply to acclimatize the Af bacteria and make it have a certain tolerance to the micro electric field.

[0031] 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 -1 .

[0032] In the examples, the domestication method of *Acidithiobacillus acidophilus* is as follows:

[0033] The inoculum of *Acidithiobacillus ferrooxidans* was 10% (v / v) and inoculated into a reaction vessel containing 9K medium. The 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. 1 g / L of heavy metal contaminated soil was added to the medium, and the mixture was acclimatized and cultured in an incubator at 30°C and 130 r / min. The pH value of the solution was measured. When the pH began to decrease and tended to stabilize, the strain was in the logarithmic growth phase. The above acclimatization process was repeated again with an inoculum of 10% (volume ratio). The amount of heavy metal contaminated soil added was 2.5 g / L. The above steps were repeated, and the amount of heavy metal contaminated soil added was increased in a gradient, namely 1, 2.5, 5, 7.5, 10, 15, and 20 g / L. Acidophilus ferrooxidans with high tolerance to heavy metals was screened out.

[0034] Example 1

[0035] The preparation method of graphene oxide modified graphite rod electrode is as follows:

[0036] Graphite rods, all 4cm in length and 7mm in diameter, were purchased from the market. Unmodified graphite rods served as cathodes, while graphene oxide-modified graphite rods served as anodes. The specific preparation process was as follows: 5mg / L of GO was placed in a small beaker, and the graphite rods were added. The beaker was then placed in an ultrasonic instrument and sonicated for 1 hour to ensure the graphene oxide was fully dispersed and in full contact with the graphite rods, allowing the carbon nanomaterials to uniformly cover the anode. The graphite rods were then removed and dried in an oven at 105°C for 2 hours. This soaking and drying process was repeated three times to load the carbon nanomaterials onto the graphite rods, resulting in graphene oxide-modified graphite rod electrodes for later use. Figure 3 shows a photograph of the graphene oxide-modified electrode prepared in Example 1. Figure 4 is a scanning electron microscope image of the large-pore graphene oxide-modified electrode prepared in Example 1.

[0037] Example 2

[0038] Modified electrodes were used as anodes for treating heavy metal contaminated soil: A 9K liquid culture medium was prepared, comprising: 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. H2SO4 (concentrated sulfuric acid and water, volume ratio 1:1) was added to adjust the pH to approximately 2.00. Acclimated *Thiobacillus acidophilus* was inoculated into the culture medium in the electrolytic cell at an inoculum size of 10% (v / v) to form a leachate. The ratio of heavy metal contaminated soil to leachate was 1:50 (g / cm³). 3 A graphene oxide-modified graphite rod electrode system was added to the leaching solution. The graphite rods of both the anode and cathode were 4 cm long and φ7 mm in diameter. A silver / silver chloride saturated electrode was used as the reference electrode. An external power supply was connected to construct a three-electrode system. Leaching was carried out under a constant current of 20 mA. During the leaching process, the concentration of heavy metal ions in the filtrate was determined by ICP. The results showed that after 3 days of treatment, the leaching rates of Cu, Zn, Cd and Pb in the soil were 93.9%, 89.9%, 85.9% and 83.4%, respectively, by using the GO-modified bioelectrochemical system as the anode to remediate heavy metal soil.

[0039] Figure 5 is a scanning electron microscope image of the bacterium-embedded graphene oxide modified electrode obtained after leaching for 3 days in Example 2. A comparison with Figure 4 shows obvious microbial growth on the electrode surface.

[0040] Comparative Example 1

[0041] Unmodified electrodes were used as anodes to treat heavy metal contaminated soil: A 9K liquid culture medium was prepared, comprising: 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. H2SO4 (concentrated sulfuric acid and water, volume ratio 1:1) was added to adjust the pH to approximately 2.00. Acclimated *Thiobacillus acidophilus* was inoculated into the culture medium in the leaching tank at an inoculum size of 10% (v / v) to form a leachate. The ratio of heavy metal contaminated soil to leachate was 1:50 (g / cm³). 3An unmodified graphite rod electrode system was added to the leaching solution. The graphite rods of both the anode and cathode were 4 cm long and φ7 mm in diameter. A silver / silver chloride saturated electrode was used as the reference electrode. An external power supply was connected to construct a three-electrode system. Leaching was carried out under a constant current of 20 mA. During the leaching process, the concentration of heavy metal ions in the filtrate was determined by ICP. The results showed that after 3 days of treatment, the leaching rates of Cu, Zn, Cd and Pb in the soil were 81.9%, 68.9%, 70.9% and 72.4%, respectively, when using the unmodified bioelectrochemical system anode to remediate heavy metal soil.

[0042] Comparative Example 2

[0043] A modified electrode was used as the anode to treat heavy metal contaminated soil without an external power source. A 9K liquid culture medium was prepared, comprising: 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. H2SO4 solution (concentrated sulfuric acid and water, volume ratio 1:1) was added to adjust the pH to approximately 2.00. Acclimated *Thiobacillus acidophilus* was inoculated into the culture medium of the galvanic cell device at a 10% (v / v) inoculation rate to form an extract. The ratio of heavy metal contaminated soil to extract was 1:50 (g / cm³). 3 A graphene oxide-modified graphite rod electrode system was added to the leachate. Both the anode and cathode graphite rods were 4 cm long and φ7 mm in diameter. During the leaching process, the concentration of heavy metal ions in the filtrate was measured by ICP. The results showed that after 3 days of treatment, the leaching rates of Cu, Zn, Cd, and Pb in the soil were 82.1%, 77.4%, 75.2%, and 76.3%, respectively, achieved by using the unmodified bioelectrochemical system as the anode to remediate heavy metal-containing soil.

Claims

1. A method for improving the bioleaching efficiency of heavy metal contaminated soil using a graphene oxide-modified graphite rod electrode system, characterized in that, The specific steps are as follows: Step 1, the heavy metal contaminated soil is dried, ground, and sieved sequentially to obtain the soil sample to be treated; Step 2, a bacterial strain is selected for acclimatization culture of the heavy metal contaminated soil and DC regulated power supply to improve the strain's tolerance to heavy metals and system current; the bacterial strain is *Acidithiobacillus ferrooxidans* (Af); Step 3, under ultrasonic conditions, the graphite rod is immersed in a 5-10 mg / mL graphene oxide (GO) suspension to ensure that the graphene oxide is fully dispersed and hydrated with the graphite rod. The graphite rods were then removed and placed in an oven for drying to obtain a single-cycle carbon nanotube modified electrode material. This process of soaking and drying was repeated 2-4 times until GO was stably and uniformly fixed on the graphite rods, resulting in a graphene oxide-modified graphite rod GN-GO electrode. In step 4, using the GN-GO electrode as the anode and the unmodified graphite rod electrode as the cathode, the anode and cathode were inserted into an electrolytic cell containing culture medium and the soil sample to be treated. The anode and cathode were connected to the positive and negative terminals of an external power supply via wires, respectively. An acclimatized bacterial strain was introduced, and leaching was performed at 25-30℃ and 120-150 r / min for 3-7 days. During leaching, the pH and Fe content of the solution were monitored. 2+ The changes in concentration, ORP, and heavy metal leaching rate with leaching time; wherein: in step 1, the heavy metals in the heavy metal contaminated soil include Cu, Zn, Cd, and Pb; the heavy metal contaminated soil is ground and passed through a 100-mesh sieve; in step 2, the steps for acclimatizing and culturing the bacterial strain are as follows: first, Af bacteria are inoculated into 9K culture medium solution, and heavy metal contaminated soil of different concentration gradients is added for acclimatization and culturing, and Af bacteria with high tolerance to heavy metals are gradually screened out; then, a regulated current is applied to the culture medium system through a DC regulated power supply to acclimatize the screened Af bacteria with high tolerance to heavy metals, so that they become tolerant to the micro electric field.

2. The method according to claim 1, characterized in that, In step 3, the ultrasonic frequency is 100 Hz and the ultrasonic treatment time is 30-60 min.

3. The method according to claim 1, characterized in that, In step 3, the oven temperature is set to 100-110℃ and the drying time is 80-150 minutes.

4. The method according to claim 1, characterized in that, In step 4, leaching is carried out under a constant current of 15-25mA. The culture medium is 9K medium, the inoculum is 8-15 vol%, the ratio of the soil sample to be treated to the culture medium is 1:80-1:30 g / mL, and the initial pH of the system is between 1.5 and 2.5.

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