A MnOx biochar cathode biocathodic electrochemical system for remediating lead and cadmium contaminated soil and applications

Abstract

The application discloses a MnOx biochar cathode bioelectrochemical system for repairing lead and cadmium contaminated soil and application, which comprises an anode and a MnOx modified biochar cathode. 2+ , Cd 2+ electrons are used as direct electron acceptors instead of oxygen, the cathode area is expanded by using biochar, and the internal electric field strength of the bioelectrochemical system is increased, so that the effective repair of heavy metals in soil is promoted. The electron flow and electric field generated by the system itself are source power, which drives the migration, transformation and concentrated removal of heavy metals, realizes that the power generation directly contributes to the removal of heavy metals, and does not need external energy input. Meanwhile, the system has good repair effect on lead contaminated soil, cadmium contaminated soil and lead and cadmium combined contaminated soil.

Description

Technical Field

[0001] This invention relates to the field of heavy metal soil pollution remediation technology, and in particular to a MnOx biochar cathode bioelectrochemical system and its application for remediating lead and cadmium contaminated soil. Background Technology

[0002] Heavy metal pollution in soil is a global problem, with over 10 million major soil contaminated sites worldwide. More than 50% of these sites are contaminated with heavy metals or metalloids, covering 20 million hectares of land. Heavy metals are inorganic pollutants; because they are non-biodegradable, highly toxic, and bioaccumulative, they persist in soil for extended periods, exerting a continuous impact on soil ecosystems and accumulating in organisms through the food chain, posing a serious threat to human health. Due to the expansion of modern industrial and agricultural production and the widespread use of metallic materials, lead (Pb) and cadmium (Cd) have become severely polluting soil pollutants in my country. Cd can accumulate in the edible parts of plants, reaching levels harmful to humans. Long-term intake of high concentrations of Cd through diet can lead to serious health problems such as kidney disease, bone damage, and even cancer. Soil Pb pollution reduces crop productivity, and the persistence, bioaccumulation, and non-degradability of Pb have raised significant concerns about food security and human health. Therefore, there is an urgent need to develop economical and efficient treatment methods to effectively remove heavy metals from soil.

[0003] Currently, bioelectrochemical methods (bioelectrochemical systems) for remediating soil heavy metal pollution have shown broad development prospects due to their advantages such as mild conditions, no need for external power sources, and strong adaptability, making them a hot topic in the field of soil remediation. In soil remediation, bioelectrochemical remediation provides an adaptive platform for removing organic pollutants and heavy metals from contaminated soil. Bioelectrochemical systems combine electrochemical and biological methods and have been widely applied to the remediation of environmental pollution such as wastewater, sludge, sediments, and soil. Soil bioelectrochemical systems utilize the dual aerobic and anaerobic characteristics of the soil surface and interior, and their structure naturally facilitates the formation of microbial fuel cells (MFCs). Using the electrical energy (electron flow and electric field) generated by the soil MFC as the driving force, they promote the migration, transformation, and removal of heavy metals in the soil, achieving the goal of directly contributing to the removal of heavy metals from the soil through the electricity generated by the soil MFC. Although soil bioelectrochemical systems can be used for the remediation of soil heavy metal pollution, the electric field generated by the soil bioelectrochemical system itself is relatively weak, resulting in slow efficiency in driving the electromigration and cathodic reduction of heavy metals in the soil, leading to low remediation efficiency for heavy metals Pb and Cd in soil. Summary of the Invention

[0004] To address the technical problem that existing soil bioelectrochemical systems generate weak electric fields, resulting in slow efficiency in driving the electromigration and cathodic reduction of heavy metals in the soil and thus low remediation efficiency for heavy metals Pb and Cd in soil, this invention provides a MnOx biochar cathodic bioelectrochemical system and its application for remediating lead-cadmium contaminated soil.

[0005] A bioelectrochemical system for remediating lead-cadmium contaminated soil using MnOx biochar cathode is characterized in that the bioelectrochemical system includes an anode and an MnOx modified biochar cathode, the MnOx modified biochar cathode and the anode being disposed opposite each other within the heavy metal contaminated soil, the MnOx modified biochar cathode and the anode forming a closed current path outside the heavy metal contaminated soil, and an external resistor being provided on the current path.

[0006] Furthermore, the anode is loaded with electrochemically active microorganisms.

[0007] Furthermore, the external resistor is 500~1500 Ω.

[0008] Furthermore, the MnOx-modified biochar cathode is composed of an MnOx-modified biochar catalyst layer, a current collector, and a diffusion layer, wherein the catalyst layer is loaded on the current collector to form a catalytically active surface.

[0009] Furthermore, the preparation method of the MnOx modified biochar cathode includes the following steps:

[0010] S1: Preparation of the current collector: pretreatment of carbon felt, dilution of 50-70% polytetrafluoroethylene emulsion, complete soaking of graphite felt with 4-6% polytetrafluoroethylene emulsion, heating at 350-400℃ for 30-45 min to obtain the current collector.

[0011] S2: Preparation of the catalytic mixture: MnOx modified biochar and F900 carbon black are weighed and mixed evenly at a mass ratio of 9~11:1. Isopropanol and anhydrous ethanol are then added, with a volume ratio of 4~6:1. The mixture is ultrasonically mixed in a beaker to obtain a catalytic mixture with a content of 36~84 mg / mL of MnOx modified biochar.

[0012] S3: Catalytic layer loading: 5-15% polytetrafluoroethylene emulsion is added to the catalytic mixture. The mass ratio of the polytetrafluoroethylene emulsion to the MnOx modified biochar is 2-4:1. The mixture is heated and stirred at 75-80°C until the mixture separates into layers with isopropanol and anhydrous ethanol and forms micelles. The micelles are uniformly coated and lightly pressed onto one side of the current collector. The mixture is heated in a muffle furnace at 350-400°C for 30-45 min to form a catalytic layer with micropores on the surface.

[0013] S4: Diffusion layer loading: A 50-70% polytetrafluoroethylene (PTFE) emulsion is uniformly coated on the other side of the current collector, and heated at 350-400℃ for 30-45 min to form a diffusion layer. The PTFE loading in the diffusion layer reaches 80-140 mg / cm³. 2 .

[0014] Further, the preparation method of the MnOx modified biochar is as follows: potassium permanganate and biochar are dissolved in water at a mass ratio of 1:1~4, and the amount of water is 5~35 times the total mass of potassium permanganate and biochar. The mixture is stirred at 75~85℃ for 1~1.5 h, and after the solution is allowed to stand for 2~2.5 h, the solid is collected after solid-liquid separation, washed with deionized water, and dried at 75~80℃ for 2~3 h. The dried biochar is then activated at 450~500℃ for 1.5~2 h, and after cooling, MnOx modified biochar is obtained.

[0015] Furthermore, the bioelectrochemical system is also equipped with a reactor for loading heavy metal contaminated soil, the moisture content of which is maintained at 15-25%, and artificial nutrient solution is added daily to the soil around the anode during the reaction process.

[0016] Furthermore, the bioelectrochemical system has a soil remediation cycle of 35-45 days.

[0017] Furthermore, the artificial nutrient solution is a mixed solution prepared by adding 2 g CH3COONa, 0.31 g NH4Cl, 0.2 g MgSO4, 0.13 g KCl, and 0.008 g CaCl2 to 1 L of water.

[0018] In summary, the present invention has the following beneficial effects:

[0019] First, the cathode of the bioelectrochemical system of this invention uses Pb with a high reaction kinetic constant. 2+ Cd 2+ Electron acceptors replace oxygen as direct electron acceptors, and the large specific surface area of ​​biochar expands the cathode area, increasing the electric field strength within the bioelectrochemical system, thereby promoting the effective remediation of heavy metals in soil. Compared to the experimental group without MnOx biochar modification, the experimental group using MnOx biochar as the cathode catalyst showed improved power generation performance, such as an increase in maximum power density (from 21.534 to 39.774 mW / m³). 2Significant improvements were observed in the biochar cathode modification, including maximum voltage (increased from 336 mV to 466 mV) and total power generation (increased from 76.712 mW·h), indicating that the bioelectrochemical system modified with biochar cathode exhibited improved electrochemical performance and cathode catalytic activity.

[0020] Secondly, the bioelectrochemical system of this invention uses its own generated electron flow and electric field as the driving force to drive the migration, transformation, and concentrated removal of heavy metals in the soil. This allows the generated electricity to directly contribute to the removal of heavy metals in the soil, without requiring external energy input. It also exhibits good remediation effects on lead-contaminated, cadmium-contaminated, and lead-cadmium combined heavy metal contaminated soils. One mechanism of the MnOx biochar cathode bioelectrochemical system for remediating heavy metal cations is the migration of charged heavy metal ions from the anode to the cathode. After the bioelectrochemical system is running, the heavy metal cations in the anode chamber are concentrated and removed in the cathode chamber. The main principle of the soil bioelectrochemical system for remediating heavy metal pollution is to enrich metal cations to the cathode through electromigration and electroosmosis in a bioelectric field environment. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 The images show SEM images of the unmodified biochar (a) and MnOx-modified biochar (b) of this invention.

[0023] Figure 2 The XRD patterns are those of the unmodified biochar and MnOx-modified biochar of this invention.

[0024] Figure 3 Power density curves for the remediation of lead-cadmium pollution (a) and single lead or cadmium pollution (b) by the bioelectrochemical system of the present invention.

[0025] Figure 4 The polarization curves are for the remediation of lead-cadmium pollution (a) and single lead or cadmium pollution (b) by the bioelectrochemical system of the present invention.

[0026] Figure 5 This invention presents a statistical chart showing the changes in cadmium (a) and lead (b) content in soil contaminated with lead and cadmium using a bioelectrochemical system for remediation, as well as the removal rates of cadmium (c) and lead (d) in the anode region.

[0027] Figure 6The graph shows the changes in cadmium (a) and lead (b) content in soil contaminated with lead or cadmium alone, as well as the statistical graphs of cadmium (c) and lead (d) removal rates in the anode region, for the remediation of soil contaminated with lead or cadmium by the bioelectrochemical system of this invention.

[0028] Figure 7 This is a distribution diagram of cadmium (a) and lead (b) after the remediation of soil with lead-cadmium combined pollution by the bioelectrochemical system of this invention.

[0029] Figure 8 This is a distribution diagram of cadmium (a) and lead (b) after soil remediation by the bioelectrochemical system of this invention for single lead or cadmium pollution.

[0030] Figure 9 This is a redundancy analysis diagram of lead-cadmium combined pollution (a), single cadmium pollution (b), and single lead pollution (c) environments in the bioelectrochemical system remediation of this invention.

[0031] Figure 10 This is a schematic diagram of the bioelectrochemical system of the present invention.

[0032] In the diagram: 1. External resistor; 2. Heavy metal contaminated soil; 3. MnOx modified biochar cathode; 4. Anode; 5. Reactor. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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 some embodiments of the present invention, 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.

[0034] Please see Figure 10 This invention provides a bioelectrochemical system for remediating lead-cadmium contaminated soil using MnOx biochar cathode. The bioelectrochemical system includes an anode 5 and an MnOx modified biochar cathode 3. The MnOx modified biochar cathode 3 and the anode 5 are disposed opposite each other in the heavy metal contaminated soil 2. The MnOx modified biochar cathode 3 and the anode 5 form a closed current path outside the heavy metal contaminated soil 2. An external resistor 1 is provided on the current path.

[0035] The cathode of the bioelectrochemical system in this scheme uses Pb with a high reaction kinetic constant. 2+ Cd 2+ Electron acceptors replace oxygen as direct electron acceptors, and the large specific surface area of ​​biochar expands the cathode area, increasing the electric field strength inside the bioelectrochemical system, thereby promoting the effective remediation of heavy metals in soil.

[0036] Furthermore, electrochemically active microorganisms are loaded onto anode 5.

[0037] This scheme loads electrochemically active microorganisms on the anode, which transfer electrons to the cathode through catalytic oxidation reactions, thereby improving the system's energy conversion efficiency and power generation capacity. These microorganisms form a biofilm on the anode surface, utilize substrates for metabolism and generate current, further enhancing the overall efficiency of the bioelectrochemical system.

[0038] Furthermore, the external resistor 1 is 500~1500 Ω.

[0039] This solution optimizes the constant resistance of the external load, regulates and controls the electron flow in the system, thereby affecting the activity of anode microorganisms, the electron-accepting effect of the cathode, and the overall performance and output power of the system. This improves energy conversion efficiency while ensuring system stability.

[0040] Furthermore, the MnOx modified biochar cathode 3 is composed of an MnOx modified biochar catalyst layer, a current collector, and a diffusion layer, wherein the catalyst layer is loaded on the current collector to form a catalytically active surface.

[0041] This scheme constructs a system with a current collector as a carrier and an integrated high-efficiency supported catalytic layer and diffusion layer structure on both sides, which significantly improves the MnOx catalytic efficiency and thus enhances the electrocatalytic performance and behavior of the bioelectrochemical system.

[0042] Furthermore, the preparation method of the MnOx modified biochar cathode 3 includes the following steps:

[0043] S1: Preparation of the current collector: pretreatment of carbon felt, dilution of 50-70% polytetrafluoroethylene emulsion, complete soaking of graphite felt with 4-6% polytetrafluoroethylene emulsion, heating at 350-400℃ for 30-45 min to obtain the current collector.

[0044] S2: Preparation of the catalytic mixture: MnOx modified biochar and F900 carbon black are weighed and mixed evenly at a mass ratio of 9~11:1. Isopropanol and anhydrous ethanol are then added, with a volume ratio of 4~6:1. The mixture is ultrasonically mixed in a beaker to obtain a catalytic mixture with a content of 36~84 mg / mL of MnOx modified biochar.

[0045] S3: Catalytic layer loading: 5-15% polytetrafluoroethylene emulsion is added to the catalytic mixture. The mass ratio of the polytetrafluoroethylene emulsion to the MnOx modified biochar is 2-4:1. The mixture is heated and stirred at 75-80°C until the mixture separates into layers with isopropanol and anhydrous ethanol and forms micelles. The micelles are uniformly coated and lightly pressed onto one side of the current collector. The mixture is heated in a muffle furnace at 350-400°C for 30-45 min to form a catalytic layer with micropores on the surface.

[0046] S4: Diffusion layer loading: A 50-70% polytetrafluoroethylene (PTFE) emulsion is uniformly coated on the other side of the current collector, and heated at 350-400℃ for 30-45 min to form a diffusion layer. The PTFE loading in the diffusion layer reaches 80-140 mg / cm³. 2 .

[0047] Further, the preparation method of the MnOx modified biochar is as follows: potassium permanganate and biochar are dissolved in water at a mass ratio of 1:1~4, and the amount of water is 5~35 times the total mass of potassium permanganate and biochar. The mixture is stirred at 75~85℃ for 1~1.5 h, and after the solution is allowed to stand for 2~2.5 h, the solid is collected after solid-liquid separation, washed with deionized water, and dried at 75~80℃ for 2~3 h. The dried biochar is then activated at 450~500℃ for 1.5~2 h, and after cooling, MnOx modified biochar is obtained.

[0048] This scheme provides a method for realizing MnOx-modified biochar cathodes. The MnOx-modified biochar cathodes prepared by this scheme significantly improve the electrocatalytic performance of bioelectrochemical systems and promote electron transfer efficiency and stability due to their unique structural characteristics and enhanced MnOx catalytic efficiency.

[0049] The present invention also provides an application of a MnOx biochar cathode bioelectrochemical system for remediating lead-cadmium contaminated soil. The bioelectrochemical system is further provided with a reactor 4 for loading heavy metal contaminated soil 2. The moisture content of the heavy metal contaminated soil 2 is maintained at 15-25%. During the reaction process, artificial nutrient solution is added daily to the soil around the anode 5.

[0050] Furthermore, the bioelectrochemical system has a soil remediation cycle of 35-45 days.

[0051] This scheme discloses the practical application of a bioelectrochemical system in lead-cadmium contaminated soil. Under these conditions, the electron flow and electric field generated by the bioelectrochemical system itself serve as the driving force to drive the migration, transformation, and concentrated removal of heavy metals in the soil. The generated electricity directly contributes to the removal of heavy metals in the soil without the need for external energy input. At the same time, it achieves good remediation effects on lead-contaminated, cadmium-contaminated, and lead-cadmium combined heavy metal contaminated soils within a remediation period of 35-45 days.

[0052] Furthermore, the artificial nutrient solution is a mixed solution prepared by adding 2 g CH3COONa, 0.31 g NH4Cl, 0.2 g MgSO4, 0.13 g KCl, and 0.008 g CaCl2 to 1 L of water.

[0053] This solution provides a formulation of necessary ions and trace elements for electrochemical systems, supporting the growth, metabolic activities, and electrocatalytic processes of electrochemically active microorganisms, thereby improving the electrochemical performance and energy conversion efficiency of the system.

[0054] Example 1

[0055] Preparation of the cathode and anode of the MnOx biochar cathode bioelectrochemical system

[0056] (1) Preparation of biochar by slow pyrolysis: The leaf stems of fallen leaves of the Chinese parasol tree were collected, and the surface dust was washed with tap water and distilled water. The leaves were then dried at 60°C, chopped, ground, and passed through a 40-mesh sieve. The biomass material was placed in a crucible and pyrolyzed in a muffle furnace at 500°C (heating rate 10°C / min) under oxygen-limited conditions for 2 h. The pyrolysis temperature of 500°C allowed the biochar to retain abundant functional groups such as -OH, -COOH, and -C=O. After cooling, the biochar after preliminary pyrolysis was filtered and washed repeatedly with deionized water. After drying, the obtained sample was ground, and the biochar material that passed through a 100-mesh sieve was stored for further use.

[0057] (2) Preparation of modified biochar by hydrothermal impregnation: 3.328 g of potassium permanganate crystals and 6.0 g of biochar were dissolved together in 100 mL of deionized water. The mixed solution was initially purplish-black. The mixed solution was heated and stirred at 80℃ for 1 h. During this period, the solution changed from purplish-black to brownish-yellow. After the solution was allowed to stand for 2 h, it was washed repeatedly by vacuum filtration with deionized water. After washing, the modified biochar was placed in an oven and dried at 80℃ for 2 h. Then, the hydrothermally impregnated biochar was placed in a muffle furnace and subjected to secondary pyrolysis at 450℃ for 2 h. After cooling, the MnOx modified biochar was sieved, packaged, and stored.

[0058] (3) Preparation of MnOx-modified biochar cathode: First, the carbon felt was pretreated. Polytetrafluoroethylene (PTFE) was used to enhance the structural strength of the carbon felt current collector. After diluting 60% PTFE emulsion, the graphite felt was completely soaked in 5% PTFE emulsion and then heated in a muffle furnace at 370℃ for 30 min to stabilize the structural strength of the electrode current collector. Then, MnOx-modified biochar and F900 carbon black were weighed and mixed evenly. Isopropanol and an appropriate amount of anhydrous ethanol were added to a beaker and ultrasonically mixed to prepare the biochar catalyst layer. The contents of each substance in the catalyst layer preparation are as follows: the mass of MnOx-modified biochar is 240 mg, the mass of conductive carbon black is 24 mg, and the volume of isopropanol is 5 mL. The loading of MnOx-modified biochar in the catalyst layer is 12~28 mg / cm³. 2 After adding 600 μL of 10% polytetrafluoroethylene (PTFE) emulsion to a uniformly dispersed mixture, the mixture was heated and stirred at 80°C. When the mixture separated into layers with isopropanol and anhydrous ethanol and presented a dough-like consistency, the adhesive and extensible dough was coated and gently pressed to adhere to one side of the graphite felt. Then, the electrode with the catalyst layer loaded was heated at 370°C for 30 min in a muffle furnace to form micropores on the catalyst layer surface. After loading the catalyst layer, 60% PTFE emulsion was coated onto the other side of the current collector. This was then heated at 370°C for 30 min in a muffle furnace. Through multiple coating and sintering processes, the PTFE loading of the diffusion layer was adjusted to 80–140 mg / cm². 2 .

[0059] (4) Anode biofilm acclimatization: First, the anode carbon felt is pretreated. Pretreatment removes the wax on the surface and inside of the carbon felt during production, allowing microorganisms to attach effectively. The cut carbon felt is soaked in acetone solution for 12 h, rinsed repeatedly, and then soaked in 0.1 mol / L sulfuric acid solution and 0.1 mol / L sodium hydroxide solution for 2 h. After rinsing with deionized water, the pretreatment is completed. A dedicated reactor for anode biofilm cultivation is constructed using a dual-chamber reactor configuration. The pretreated carbon felt is used as the anode electrode for loading electrogenic bacteria, and a carbon brush is used as the redox cathode. An appropriate concentration of activated sludge is filled into the anode chamber of the bioelectrochemical system. A mixed solution of 0.1 mol / L potassium dihydrogen phosphate and 0.1 mol / L potassium ferricyanide is used as the electron acceptor in the cathode chamber. The anode and cathode chambers are separated by a proton exchange membrane to ensure proton exchange between the electrodes. The electrodes are connected by titanium wires, with an external resistor of 1000 Ω added in the middle to consume excess electrons. The catholyte in the acclimatization reactor is replaced every three days, and the activated sludge on the anode is replaced every two days. Once the reactor voltage stabilizes at around 500 mV, the biofilm on the anode can be considered successfully acclimatized. The acclimatization period is approximately 15 days.

[0060] Example 2

[0061] The bioelectrochemical system was used to remediate Cd and Pb contamination in the soil. This embodiment was also named Group Cs1. The lead content in the heavy metal contaminated soil was 1000 mg / kg and the cadmium content was 100 mg / kg.

[0062] A single-chamber soil bioelectrochemical system was used. Anode 5 was a carbon felt loaded with electrochemically active microorganisms prepared in Example 1, and cathode 3 was a MnOx-modified biochar cathode prepared in Example 1. Reactor 4 of the bioelectrochemical system consisted of a cylindrical plexiglass container with an inner diameter of 3.8 cm and a length of 12 cm. 120 g of heavy metal-contaminated soil 2 was placed inside reactor 4, and 30 mL of deionized water was added. The soil was moistened daily to maintain a moisture content of approximately 20%. During the reaction, 2 mL of artificial nutrient solution was added daily to the soil around anode 5. The solution consisted of 2 g / L CH3COONa, 0.31 g / L NH4Cl, 0.2 g / L MgSO4, 0.13 g / L KCl, and 0.008 g / L CaCl2. Anode 5 and MnOx-modified biochar cathode 3 were connected by alligator clips to form a closed circuit. The external resistor 1 was 1000 Ω, and the remediation period was 40 days.

[0063] Example 3

[0064] The bioelectrochemical system was used to remediate Cd and Pb contamination in the soil. This embodiment was also named the Cs2 group. The lead content in the heavy metal contaminated soil was 1500 mg / kg and the cadmium content was 200 mg / kg.

[0065] A single-chamber soil bioelectrochemical system was used. Anode 5 was a carbon felt loaded with electrochemically active microorganisms prepared in Example 1, and cathode 3 was a MnOx-modified biochar cathode prepared in Example 1. Reactor 4 of the bioelectrochemical system consisted of a cylindrical plexiglass container with an inner diameter of 3.8 cm and a length of 12 cm. 120 g of heavy metal-contaminated soil 2 was placed inside reactor 4, and 30 mL of deionized water was added. The soil was moistened daily to maintain a moisture content of approximately 20%. During the reaction, 2 mL of artificial nutrient solution was added daily to the soil around anode 5. The solution consisted of 2 g / L CH3COONa, 0.31 g / L NH4Cl, 0.2 g / L MgSO4, 0.13 g / L KCl, and 0.008 g / L CaCl2. Anode 5 and MnOx-modified biochar cathode 3 were connected by alligator clips to form a closed circuit. The external resistor 1 was 1000 Ω, and the remediation period was 40 days.

[0066] Example 4

[0067] The bioelectrochemical system was used to remediate Cd and Pb contamination in the soil. This embodiment was also named the Cs3 group. The lead content in the heavy metal contaminated soil was 2000 mg / kg and the cadmium content was 250 mg / kg.

[0068] A single-chamber soil bioelectrochemical system was used. Anode 5 was a carbon felt loaded with electrochemically active microorganisms prepared in Example 1, and cathode 3 was a MnOx-modified biochar cathode prepared in Example 1. Reactor 4 of the bioelectrochemical system consisted of a cylindrical plexiglass container with an inner diameter of 3.8 cm and a length of 12 cm. 120 g of heavy metal-contaminated soil 2 was placed inside reactor 4, and 30 mL of deionized water was added. The soil was moistened daily to maintain a moisture content of approximately 20%. During the reaction, 2 mL of artificial nutrient solution was added daily to the soil around anode 5. The solution consisted of 2 g / L CH3COONa, 0.31 g / L NH4Cl, 0.2 g / L MgSO4, 0.13 g / L KCl, and 0.008 g / L CaCl2. Anode 5 and MnOx-modified biochar cathode 3 were connected by alligator clips to form a closed circuit. The external resistor 1 was 1000 Ω, and the remediation period was 40 days.

[0069] Example 5

[0070] The bioelectrochemical system was used to remediate Cd contamination in the soil. This embodiment was also named Group Ls1, and the cadmium content in the heavy metal contaminated soil was 100 mg / kg.

[0071] A single-chamber soil bioelectrochemical system was used. Anode 5 was a carbon felt loaded with electrochemically active microorganisms prepared in Example 1, and cathode 3 was a MnOx-modified biochar cathode prepared in Example 1. Reactor 4 of the bioelectrochemical system consisted of a cylindrical plexiglass container with an inner diameter of 3.8 cm and a length of 12 cm. 120 g of heavy metal-contaminated soil 2 was placed inside reactor 4, and 30 mL of deionized water was added. The soil was moistened daily to maintain a moisture content of approximately 20%. During the reaction, 2 mL of artificial nutrient solution was added daily to the soil around anode 5. The solution consisted of 2 g / L CH3COONa, 0.31 g / L NH4Cl, 0.2 g / L MgSO4, 0.13 g / L KCl, and 0.008 g / L CaCl2. Anode 5 and MnOx-modified biochar cathode 3 were connected by alligator clips to form a closed circuit. The external resistor 1 was 1000 Ω, and the remediation period was 40 days.

[0072] Example 6

[0073] The bioelectrochemical system was used to remediate Cd contamination in the soil. This embodiment was also named Ls2 group, and the cadmium content in the heavy metal contaminated soil was 200 mg / kg.

[0074] A single-chamber soil bioelectrochemical system was used. Anode 5 was a carbon felt loaded with electrochemically active microorganisms prepared in Example 1, and cathode 3 was a MnOx-modified biochar cathode prepared in Example 1. Reactor 4 of the bioelectrochemical system consisted of a cylindrical plexiglass container with an inner diameter of 3.8 cm and a length of 12 cm. 120 g of heavy metal-contaminated soil 2 was placed inside reactor 4, and 30 mL of deionized water was added. The soil was moistened daily to maintain a moisture content of approximately 20%. During the reaction, 2 mL of artificial nutrient solution was added daily to the soil around anode 5. The solution consisted of 2 g / L CH3COONa, 0.31 g / L NH4Cl, 0.2 g / L MgSO4, 0.13 g / L KCl, and 0.008 g / L CaCl2. Anode 5 and MnOx-modified biochar cathode 3 were connected by alligator clips to form a closed circuit. The external resistor 1 was 1000 Ω, and the remediation period was 40 days.

[0075] Example 7

[0076] The bioelectrochemical system was used to remediate Cd contamination in the soil. This embodiment was also named Ls3 group, and the cadmium content in the heavy metal contaminated soil was 250 mg / kg.

[0077] A single-chamber soil bioelectrochemical system was used. Anode 5 was a carbon felt loaded with electrochemically active microorganisms prepared in Example 1, and cathode 3 was a MnOx-modified biochar cathode prepared in Example 1. Reactor 4 of the bioelectrochemical system consisted of a cylindrical plexiglass container with an inner diameter of 3.8 cm and a length of 12 cm. 120 g of heavy metal-contaminated soil 2 was placed inside reactor 4, and 30 mL of deionized water was added. The soil was moistened daily to maintain a moisture content of approximately 20%. During the reaction, 2 mL of artificial nutrient solution was added daily to the soil around anode 5. The solution consisted of 2 g / L CH3COONa, 0.31 g / L NH4Cl, 0.2 g / L MgSO4, 0.13 g / L KCl, and 0.008 g / L CaCl2. Anode 5 and MnOx-modified biochar cathode 3 were connected by alligator clips to form a closed circuit. The external resistor 1 was 1000 Ω, and the remediation period was 40 days.

[0078] Example 8

[0079] The bioelectrochemical system was used to remediate Pb contamination in the soil. This embodiment was also named Ls4 group, and the lead content in the heavy metal contaminated soil was 1000 mg / kg.

[0080] A single-chamber soil bioelectrochemical system was used. Anode 5 was a carbon felt loaded with electrochemically active microorganisms prepared in Example 1, and cathode 3 was a MnOx-modified biochar cathode prepared in Example 1. Reactor 4 of the bioelectrochemical system consisted of a cylindrical plexiglass container with an inner diameter of 3.8 cm and a length of 12 cm. 120 g of heavy metal-contaminated soil 2 was placed inside reactor 4, and 30 mL of deionized water was added. The soil was moistened daily to maintain a moisture content of approximately 20%. During the reaction, 2 mL of artificial nutrient solution was added daily to the soil around anode 5. The solution consisted of 2 g / L CH3COONa, 0.31 g / L NH4Cl, 0.2 g / L MgSO4, 0.13 g / L KCl, and 0.008 g / L CaCl2. Anode 5 and MnOx-modified biochar cathode 3 were connected by alligator clips to form a closed circuit. The external resistor 1 was 1000 Ω, and the remediation period was 40 days.

[0081] Example 9

[0082] The bioelectrochemical system was used to remediate Pb contamination in the soil. This embodiment was also named Ls5 group, and the lead content in the heavy metal contaminated soil was 1500 mg / kg.

[0083] A single-chamber soil bioelectrochemical system was used. Anode 5 was a carbon felt loaded with electrochemically active microorganisms prepared in Example 1, and cathode 3 was a MnOx-modified biochar cathode prepared in Example 1. Reactor 4 of the bioelectrochemical system consisted of a cylindrical plexiglass container with an inner diameter of 3.8 cm and a length of 12 cm. 120 g of heavy metal-contaminated soil 2 was placed inside reactor 4, and 30 mL of deionized water was added. The soil was moistened daily to maintain a moisture content of approximately 20%. During the reaction, 2 mL of artificial nutrient solution was added daily to the soil around anode 5. The solution consisted of 2 g / L CH3COONa, 0.31 g / L NH4Cl, 0.2 g / L MgSO4, 0.13 g / L KCl, and 0.008 g / L CaCl2. Anode 5 and MnOx-modified biochar cathode 3 were connected by alligator clips to form a closed circuit. The external resistor 1 was 1000 Ω, and the remediation period was 40 days.

[0084] Example 10

[0085] The bioelectrochemical system was used to remediate Pb contamination in the soil. This embodiment was also named Ls6 group, and the lead content in the heavy metal contaminated soil was 2000 mg / kg.

[0086] A single-chamber soil bioelectrochemical system was used. Anode 5 was a carbon felt loaded with electrochemically active microorganisms prepared in Example 1, and cathode 3 was a MnOx-modified biochar cathode prepared in Example 1. Reactor 4 of the bioelectrochemical system consisted of a cylindrical plexiglass container with an inner diameter of 3.8 cm and a length of 12 cm. 120 g of heavy metal-contaminated soil 2 was placed inside reactor 4, and 30 mL of deionized water was added. The soil was moistened daily to maintain a moisture content of approximately 20%. During the reaction, 2 mL of artificial nutrient solution was added daily to the soil around anode 5. The solution consisted of 2 g / L CH3COONa, 0.31 g / L NH4Cl, 0.2 g / L MgSO4, 0.13 g / L KCl, and 0.008 g / L CaCl2. Anode 5 and MnOx-modified biochar cathode 3 were connected by alligator clips to form a closed circuit. The external resistor 1 was 1000 Ω, and the remediation period was 40 days.

[0087] Comparative Example 1

[0088] The bioelectrochemical system was used to remediate Cd and Pb contamination in the soil. This comparative example was also named the Cg group. The lead content in the heavy metal contaminated soil was 1000 mg / kg and the cadmium content was 100 mg / kg.

[0089] A single-chamber soil bioelectrochemical system was used. Anode 5 was a carbon felt loaded with electrochemically active microorganisms prepared in Example 1, and cathode was a carbon felt without MnOx-modified biochar catalyst. Reactor 4 of the bioelectrochemical system consisted of a cylindrical plexiglass container with an inner diameter of 3.8 cm and a length of 12 cm. 120 g of heavy metal-contaminated soil 2 was placed in reactor 4, along with 30 mL of deionized water. The soil was moistened daily to maintain a moisture content of approximately 20%. During the reaction, 2 mL of artificial nutrient solution was added daily to the anode region 5. The solution consisted of 2 g / L CH3COONa, 0.31 g / L NH4Cl, 0.2 g / L MgSO4, 0.13 g / L KCl, and 0.008 g / L CaCl2. The anode 5 and cathode of the bioelectrochemical system were connected by alligator clips to form a closed circuit. The external resistance was 1000 Ω, and the remediation period was 40 days.

[0090] Comparative Example 2

[0091] The bioelectrochemical system was used to remediate Cd and Pb contamination in soil. This comparative example was also named the RS group. The lead content in the heavy metal contaminated soil was 1000 mg / kg and the cadmium content was 100 mg / kg.

[0092] A single-chamber soil bioelectrochemical system was used. Anode 5 was a carbon felt loaded with electrochemically active microorganisms prepared in Example 1, and cathode 3 was a MnOx-modified biochar cathode prepared in Example 1. Reactor 4 of the bioelectrochemical system consisted of a cylindrical plexiglass container with an inner diameter of 3.8 cm and a length of 12 cm. 120 g of metal-contaminated soil was placed in reactor 4, and 30 mL of deionized water was added. The soil moisture content was maintained at approximately 20% daily. During the reaction, 2 mL of artificial nutrient solution was added daily to the anode area. The solution consisted of 2 g / L CH3COONa, 0.31 g / L NH4Cl, 0.2 g / L MgSO4, 0.13 g / L KCl, and 0.008 g / L CaCl2. No treatment was performed between anode 5 and MnOx-modified biochar cathode 3 to prevent the formation of a closed circuit. The remediation period was 40 days.

[0093] The examples and comparative examples, along with their corresponding test groups and parameters, are shown in Table 1.

[0094] Table 1. Summary of Examples, Comparative Examples, Corresponding Experimental Groups, and Bioelectrochemical System Parameters

[0095]

[0096] Experimental Results and Analysis

[0097] (1) Scanning electron microscopy and X-ray diffraction tests of MnOx biochar and unmodified biochar

[0098] The material properties of MnOx biochar cathodes were described. The morphology of two biochar materials (unmodified biochar and MnOx-modified biochar) was characterized using scanning electron microscopy (SEM). Their morphological structures were observed, and the changes in morphology and structure of biochar before and after modification were compared. The modification results of MnOx on biochar were then presented. The results are as follows: Figure 1 As shown in Figure a, which is the SEM image of unmodified biochar, it exhibits a granular structure with a uniform and smooth surface and no obvious macropores. In contrast, MnOx-modified biochar shows irregular granular shapes, which are smaller, and granular substances grow on the surface. The morphological changes in MnOx-modified biochar provide evidence for the modification and loading of MnOx onto biochar; the rough surface is beneficial for increasing the reaction area. Figure 1 Figure b shows that MnOx exists in particulate form on biochar, and is evenly distributed with good loading.

[0099] The XRD patterns of unmodified biochar and MnOx-modified biochar were compared with standard cards to qualitatively analyze their phase composition and structure. Figure 2It is evident that the phase composition of unmodified biochar is mainly carbon, with sharp main peaks and good crystallinity. The XRD pattern of biochar shows two typical diffraction peaks at 2θ = 23.0° and 43.0°, corresponding to the (002) and (110) crystal planes of graphite, respectively, which is consistent with most pyrolytic carbons. The carbon diffraction peaks of MnOx-modified biochar are weakened, indicating that the loading weakens the diffraction peak intensity of its crystals, suggesting that the coverage of MnOx is uniform. In the spectrum of MnOx biochar, the diffraction peaks at 2θ = 18.03°, 36.08°, 36.49°, 44.41°, 58.50°, and 64.61° can be individually indexed as the (101), (211), (202), (220), (321), and (400) crystal plane diffraction peaks, indicating that the phase composition is Mn3O4. The diffraction peaks at 2θ = 23.13°, 35.67°, 47.28°, 58.83°, and 60.61° are the (211), (123), (422), (600), and (611) crystal plane diffraction peaks of Mn2O3. Qualitative characterization using XRD patterns revealed that the MnOx biochar clearly contains both Mn3O4 and Mn2O3 manganese oxides, indicating that MnOx was successfully loaded onto the biochar surface.

[0100] (2) Measurement and analysis of electrochemical parameters of the bioelectrochemical systems in each embodiment and comparative example

[0101] In the remediation of heavy metal contaminated soil using the bioelectrochemical systems of each embodiment and comparative example, the battery reaction voltage was monitored every 10 minutes using a data acquisition system (34970A, Agilent Inc., USA). The power density and polarization curve of the bioelectrochemical remediation system were determined using the steady-state discharge method. By varying the resistance of a rotating resistance box (10000~100 Ω), the battery load resistance and voltage were recorded after stabilizing for approximately 15 minutes at each resistance value. Based on the obtained external resistance values ​​and the corresponding bioelectrochemical system voltages, the corresponding power density and current density were calculated.

[0102]

[0103] Where P is the power density (mW / m 2 J is the current density (mA / m 2 U is the electrode voltage (mV), R is the resistance of the external resistor (Ω), and A is the effective working area of ​​the anode (A = 11.335 cm² in this study). 2 ).

[0104] The polarization curve reflects the relationship between voltage and current density and is an important indicator for evaluating the overall performance of a bioelectrochemical battery system. When the internal resistance of a bioelectrochemical system is predominantly ohmic, the polarization curve will approximate a straight line, meaning that voltage and current density have a linear relationship. The internal resistance Ri of the bioelectrochemical system is obtained by linearly fitting the ohmic polarization region of the polarization curve and calculating the slope.

[0105] The electrochemical parameters monitored and calculated during the repair cycle are shown in Table 2.

[0106] Table 2. Statistical table of electrochemical parameters of the bioelectrochemical systems of each embodiment and comparative example.

[0107]

[0108] The magnitude of the electric field controlling the power output of the bioelectrochemical system has a significant impact on electromigration and electroosmosis in soil remediation. As shown in Table 2, the bioelectrochemical system exhibits different maximum power densities under different soil Cd and Pb pollution conditions. Compared to the Cg group with an unmodified MnOx biochar cathode, the Cs1 group with MnOx biochar as the cathode catalyst shows higher power density (21.534 mW / m³) in terms of power generation performance. 2 Increased to 39.774 mW / m 2 Significant improvements were observed in the maximum voltage (from 336 mV to 466 mV) and total power generation (from 76.712 mW·h to 160.2 mW·h), indicating that the electrochemical performance and cathode catalytic activity of the bioelectrochemical system with MnOx biochar-modified cathode were both improved.

[0109] according to Figure 3 The power density curve data shown indicates that after modifying the cathode with MnOx biochar, the maximum power density of the bioelectrochemical system for treating Cd and Pb pollution increased from 21.534 mW / m². 2 Increased to 39.774 mW / m 2 The increase in actual power density demonstrates the improved electrochemical performance and catalytic activity of the biochar cathode. In the lead and cadmium combined heavy metal pollution test groups Cs1, Cs2, and Cs3, the maximum power density of the bioelectrochemical system did not show a linear change with the increase of lead and cadmium pollution concentration.

[0110] Depend on Figure 4The polarization curves show that when the soil lead content is below 1500 mg / kg, the bioelectrochemical system exhibits better power generation performance and the best heavy metal removal efficiency. During electroremediation, high current levels increase the total ion concentration and improve ion electromigration efficiency. When the soil environment is contaminated with both lead and cadmium, the presence of heavy metal ions improves the soil's conductivity, resulting in the Cs2 (200 mg / kg Cd, 1500 mg / kg Pb) experimental group exhibiting the best power generation performance. Compared to single-factor contaminated soils under the same conditions, its maximum power increased by 8.2% (compared to Ls5) and 43.49% (compared to Ls2), respectively.

[0111] As shown in Table 1, the internal resistance data of the bioelectrochemical systems in each embodiment and comparative example exhibit a clear trend: the internal resistance of the bioelectrochemical system decreases as the heavy metal content in the soil matrix increases. From Cs1 to Cs3, when the cadmium and lead contents in the soil increased to 150 mg / kg and 1000 mg / kg respectively, the internal resistance decreased by 26.72%. The internal resistance of the Cs2 cell was 729.204 Ω, significantly lower than that of the other reactors. Comparing Cs2 and Cs3, it was found that when the Cd and Pb contents in the composite soil increased, the internal resistance of the Cs3 experimental group decreased by 15.15% year-on-year, but its power density decreased by 34.95%. This indicates that the heavy metal concentration in the Cs3 experimental group had a significant adverse impact on the electrochemical performance of the soil bioelectrochemical system. Although the internal resistance of the bioelectrochemical system decreased with the increase of lead and cadmium heavy metal contents, excessively high heavy metal contents are still detrimental to the treatment of heavy metals in the soil by the bioelectrochemical system.

[0112] When treating Cd and Pb contaminated soil, different Cd and Pb concentrations produce different electrochemical performances. Analysis of electrochemical indicators such as output voltage, internal resistance, and power density revealed that the Cs2 experimental group exhibited a higher maximum power density and relatively lower resistance. While improving the internal conductivity and reducing internal mass transfer resistance, it did not exert severe stress on the anodic microorganisms, ultimately achieving the highest power generation among all systems. This indicates that the bioelectrochemical method of this invention has higher power generation performance, higher maximum power density, and lower resistance for soils contaminated with lead and cadmium at the corresponding concentration in the Cs2 experimental group compared to other concentrations of contaminated soils or soils contaminated with lead or cadmium alone, suggesting a better remediation effect for this concentration of lead and cadmium contaminated soil.

[0113] (3) Effects of each embodiment and comparative example on the remediation of heavy metal contaminated soil

[0114] After the remediation cycle was completed, soil sampling and analysis were conducted to determine the heavy metal content and speciation of the obtained samples. Multiple samples were taken from the soil of the bioelectrochemical system at 2 cm from the cathode and 2 cm from the anode. The samples were mixed to serve as the soil samples to be tested in the cathode and anode areas. The soil samples were sealed in bags and stored at -20℃. After freeze-drying, the heavy metal content of the soil was determined.

[0115] Determination of Lead and Cadmium Content in Soil: Take 0.2 g of freeze-dried soil sample that has passed through a 100-mesh sieve and place it in a 100 mL Erlenmeyer flask. Add 6 mL HCl, 2 mL HNO3, and 2 mL HF. Place the Erlenmeyer flask on a constant-temperature heating platform and heat at 210℃ until the solution boils. Continue heating to evaporate the silicon crystals. When the solution has evaporated to the size of a soybean, digestion is complete. Allow the Erlenmeyer flask to cool to room temperature and filter through slow-speed quantitative filter paper. Collect the solution in a 50 mL volumetric flask. After filtration, wash the Erlenmeyer flask, glass funnel, and filter residue at least three times with a small amount of 0.5 mol / L HNO3 solution. Filter the washings together and collect them in a 50 mL volumetric flask. Dilute to volume with deionized water. Pour the diluted sample into 15 mL test tubes, label them, and store them at -5℃ for total heavy metal content determination.

[0116] The experimental groups Cg, RS, and Cs1 all used soil contaminated with the same concentration (100 mg / kg Cd, 1000 mg / kg Pb) as the remediation target. The different remediation effects of these bioelectrochemical systems were used to illustrate the promoting effect of MnOx biochar cathodes on the remediation of heavy metals in soil within the bioelectrochemical system and its practical effectiveness. Figure 5As shown in Figure a, after 40 days of remediation using the bioelectrochemical system, the Cd content in the anolyte areas of RS and Cg groups was 114.428 mg / kg and 101.898 mg / kg, respectively. The Cd content in both RS and Cg groups not only did not decrease after treatment but actually increased to varying degrees, indicating that the bioelectrochemical system cannot remediate Cd in the soil without the addition of MnOx biochar catalyst. In the absence of a pathway in the bioelectrochemical system, the loss of heavy metals in the soil is due to diffusion. The remediation process of the bioelectrochemical system promotes the migration of heavy metals through the electric field, rather than through diffusion or leaching. The Cd content in RS and Cg groups increased by 14.40% and 1.90% after treatment, respectively. RS and Cg failed to inhibit the diffusion of Cd in the soil, which is attributed to the disordered diffusion of Cd in the soil or the bioaccumulation or bioadsorption of Cd by anolyte microorganisms. The Cs1 group showed a certain remediation effect on Cd-contaminated soil near the anode. The Cd content in the soil near the Cs1 anode decreased from 100 mg / kg to 95.563 mg / kg, while the Cd content in the cathode area increased from 100 mg / kg to 102.749 mg / kg. This demonstrates that in Cs1, metallic Cd was subjected to the electric field of the bioelectrochemical system, resulting in a directional transfer from the anode to the cathode. These results indicate that the MnOx-modified biochar cathode bioelectrochemical system can counteract the diffusion of Cd in the soil and exert a directional electric field migration force on Cd in the soil, while the carbon felt cathode cannot form an effective electric field binding effect on Cd in the soil.

[0117] After the repair cycle was completed, the Pb content in the anode region of RS increased from 1000 mg / kg to 1051.741 mg / kg, while the Pb content in the anode region of both Cg and Cs1 groups decreased. Figure 5 (Figure b) The Pb anodic removal rates of Cg and Cs1 were 7.40% and 16.70%, respectively. Because the Cs1 bioelectrochemical system has a higher power output efficiency than Cg, the power density of the Cs1 reactor (39.774 mW / m²) is significantly higher. 2 ) greater than Cg (21.534 mW / m 2 Therefore, Cs1 improves the remediation efficiency of Pb and Cd in soil. This demonstrates that the bioelectrochemical system can effectively combat the disordered diffusion of Pb in soil, and that the MnOx biochar cathode in the bioelectrochemical system can effectively enhance the remediation system's ability to remove Pb and Cd from soil. According to... Figure 5The study investigated the changes in Cd and Pb content in the soil to determine the differences in the remediation efficiency of the bioelectrochemical system for Cd and Pb under different remediation conditions. Based on the actual results, the optimal remediation conditions for the bioelectrochemical system to remediate Cd and Pb combined pollution were analyzed. Different initial soil Cd and Pb contents resulted in varying remediation effects of Cd and Pb in the anodic region of the bioelectrochemical system. According to the actual heavy metal removal rates, Cs2 and Cs3 showed better anodic Cd and Pb remediation effects, with Cd removal rates of 19.20% and 8.30%, and Pb removal rates of 16.50% and 23.80%, respectively. This demonstrates the positive role of the MnOx-modified biochar cathode bioelectrochemical system in soil remediation efficiency.

[0118] After verifying the effectiveness of MnOx biochar in remediating soil Cd and Pb combined pollution and its treatment effect on different concentrations of Cd and Pb pollution, the process of remediating single Cd or Pb pollution by bioelectrochemical system was analyzed.

[0119] Depend on Figure 6 It is evident that, after remediation by the bioelectrochemical system, the MnOx biochar bioelectrochemical system exhibits a more significant heavy metal remediation effect on Cd and Pb combined pollution than on soil contaminated with only Cd or Pb at the same concentration. For soil contaminated with Cd concentration of 200 mg / kg, Ls2 achieved an anodic Cd removal efficiency of 2.2%; for soil contaminated with Pb concentration of 1500 mg / kg, Ls4 achieved an anodic Pb removal efficiency of 7.6%; while the Cs2 reactor, simultaneously treating combined pollution with Cd and Pb concentrations of 150 mg / kg and 2000 mg / kg respectively, achieved Cd and Pb removal rates of 19.3% and 16.5% respectively, significantly superior to the removal efficiency when treating soil contaminated with only one heavy metal. When treating Cd and Pb combined pollution, the MnOx biochar bioelectrochemical system demonstrates a higher efficiency in transferring Cd and Pb from the combined pollution compared to environments contaminated with only Cd or Pb.

[0120] In addition to studying the changes in total Cd and Pb content in heavy metal contaminated soil, this invention also compared the effects of different embodiments on the chemical stability of heavy metals. Comparing the RS, Cg, and Cs1 groups, since the RS group did not complete a closed circuit and was not affected by the bioelectric field, its cathode heavy metal speciation distribution can be considered as the distribution of unremediated soil metal speciation. Compared to RS, the acid-extractable Cd content in the anolyte soil of Cg increased rather than decreased; therefore, the bioelectrochemical system without added MnOx biochar cannot improve the chemical stability of Cd in the soil. In contrast, Figure 7As shown in Figure a, after remediation, the acid-extractable Cd in the anodic region of Cs1 decreased from 68.4 mg / kg to 59.7 mg / kg, while the acid-extractable Cd in the cathode region increased from 61.9 mg / kg to 66.7 mg / kg, indicating a directional migration of acid-extractable Cd from the anode to the cathode. In the RS, Cg, and Cs1 groups, the acid-extractable Pb showed the same trend as acid-extractable Cd, indicating a transfer of acid-extractable Pb from the anode to the cathode. This demonstrates that the addition of MnOx biochar catalyst improved the directional migration ability of acid-extractable Cd and Pb in the soil bioelectrochemical system and enhanced the stability of Cd at the anode.

[0121] Furthermore, after treatment, the proportions of Cd and Pb remaining in the residual state in the Cs2 cathode region were 10.2% and 26.9%, respectively. Compared with soils with other Cd and Pb concentrations, Cs2-remediated Cd and Pb exhibited better soil stability. This part of the study found that, comparing three different Cd and Pb concentrations of composite contaminated soil remediated by the MnOx biochar bioelectrochemical system, the anodic region showed the best Cd and Pb stability under the condition of Cd concentration of 200 mg / kg and Pb concentration of 1500 mg / kg.

[0122] Analysis of the speciation of Cd in the anode and cathode regions after remediation under different Cd concentrations (Ls1, Ls2, Ls3) revealed differences in both the content and speciation of Cd in the anode and cathode soils. Figure 8 As shown in Figure a (where A represents soil samples from the anode region and C represents soil samples from the cathode region), after remediation by Ls1, Ls2, and Ls3, Cd in both the anode and cathode regions was mainly in acid-extractable and reducible forms. The content of acid-soluble Cd in the cathode region was slightly higher than that in the anode region. For example, after treatment with Ls1, the concentration of acid-soluble Cd in the anode region was 54.1 mg / kg, while the concentration in the cathode region was 73.1 mg / kg. After remediation by the bioelectrochemical system, the proportion of reducible Cd in the anode region of Ls1 and Ls3 was higher than that in the cathode region. The proportions of reducible Cd in the cathode region soil of Ls1 and Ls3 were 32.5% and 28.6%, respectively, while the proportions in the anode region were 37.7% and 35.1%, respectively. A similar pattern was observed with oxidizable Cd, where the proportion of oxidizable Cd in the anode region was greater than that in the cathode region. The soil bioelectrochemical system improved the stability of Cd in the anode region by increasing the distribution levels of both reducible and oxidizable Cd forms. However, the total amount of Cd in the anode region did not decrease significantly. Therefore, the overall remediation effect of the MnOx biochar bioelectrochemical system on soil contaminated with only Cd was not good.

[0123] like Figure 8As shown in Figure b (where A represents soil samples from the anode region and C represents soil samples from the cathode region), the distribution of Pb speciation in the anode and cathode regions after remediation of Ls4, Ls5, and Ls6 at different Pb pollution concentrations was analyzed. It was found that Pb in the remediated Pb-contaminated soil mainly exists in a reducible form, accounting for over 60% of the total. Other components mainly exist in acid-extractable and residual forms. After remediation using the bioelectrochemical system, the levels of acid-extractable and reducible Pb in the anode region were lower than those in the cathode region. This reflects that the bioelectric field of the bioelectrochemical system caused the acid-extractable and reducible Pb to migrate from the anode to the cathode, resulting in a decrease in the total Pb content in the anode region and an increase in the total Pb content in the cathode region. For example, in Ls6, the acid-extractable and reducible Pb levels in the anode region were 165.4 mg / kg and 1206.9 mg / kg, respectively, while in the cathode they were 441.6 mg / kg and 1338.2 mg / kg, respectively. Overall, the extractable and reducible Pb content in the anode region is lower than that in the cathode region, while the residual Pb content in the soil in the anode region is higher than that in the cathode region. Therefore, the stability of soil Pb in the anode region is higher than that in the cathode region after remediation of single Pb pollution by the bioelectrochemical system.

[0124] Comparing the distribution of metal speciation in soils contaminated with Cd and Pb respectively after remediation using a MnOx biochar bioelectrochemical system, Cd contaminated soil was mainly found in a more mobile and hazardous acid-extractable form (53.2%~65.3%), while Pb contaminated soil was mainly found in a more stable reducible form (62.2%~68.1%).

[0125] Based on the remediation effects and speciation of the MnOx biochar cathode bioelectrochemical system on Cd- and Pb-contaminated soils, and single-contaminated soils, redundancy analysis (RDA) was used to identify the influencing factors on changes in soil physicochemical properties and system electrochemical performance during the remediation process of the MnOx biochar bioelectrochemical system. For example... Figure 9 As shown in Figures a~c, in both compound and single-contaminated soil remediation systems, a positive correlation was found between the total Cd and total Pb contents and the internal resistance (Ri) of the bioelectrochemical system, and a positive correlation was found between total energy production (GC) and pH value. Therefore, the internal resistance (Ri) of both compound and single-contaminated soil remediation systems is mainly determined by the total Cd and total Pb contents in the soil matrix, and total energy production (GC) is a direct factor affecting soil pH changes. The correlation between total energy production (GC) and the removal rates of total Cd and total Pb in the anolyte region showed different results in different remediation systems. In the remediation of compound contamination, total energy production (GC) was positively correlated with the removal rates of total Cd and total Pb in the anolyte region. Figure 9(See Figure a). At this point, the stronger the driving force of the electric field and the higher the voltage gradient, the faster the dissolution, desorption, and migration rates of metal ions in the soil. In single Cd or Pb pollution remediation systems, the total energy production (GC) is negatively correlated with both Cd and Pb removal rates. Figure 9 Figures b and c show that the total energy generation (GC) of the MnOx biochar cathode bioelectrochemical system is not primarily used to drive the migration of Cd or Pb; a portion of the energy is used to drive the metal speciation transformation of Cd or Pb within the bioelectrochemical system. Therefore, it can be concluded that the migration of heavy metals in soil mainly depends on the internal electric field strength of the bioelectrochemical system, rather than the total energy generation, and the heavy metal removal rate increases with the increase of the internal electric field strength.

[0126] Based on RDA analysis, the correlation between the remediation efficiency of MnOx biochar cathode bioelectrochemical system and the speciation of Pb and Cd under combined and single pollution was determined, and the main metal speciations of directional migration during remediation were identified, thereby elucidating the remediation process of MnOx biochar cathode bioelectrochemical system.

[0127] Figure 9 Figure a shows that when the bioelectrochemical system remediates Pb and Cd co-pollutation, the removal rates of Pb and Cd in the anode region are positively correlated with the concentrations of reducible Pb and Cd, respectively. It can be inferred that the reducible Pb and Cd in the anode region undergo directional migration under the action of the electric field, and the concentrations of Pb and Cd in the anode region decrease. Figure 9 Figure b shows that when the bioelectrochemical system remediates soil contaminated with only Cd, the concentration of extractable acid in the anode region is positively correlated with the total removal rate, while the concentrations of reducible and oxidizable Cd are negatively correlated with the total Cd removal rate. This indicates that under the influence of the electric field, most of the extractable acid Cd in the anode region migrates directionally to the cathode. Furthermore, the power generation (GC) in this system is positively correlated with the concentrations of reducible and oxidizable Cd, suggesting that the greater the power generation, the higher the stability of Cd in the anode region. Figure 9 Figure c shows that when the bioelectrochemical system treats Pb-contaminated soil, the concentrations of acid-extractable and reducible Pb in the anode region are positively correlated with the total Pb removal rate. Therefore, the Pb removal rate in the anode region is mainly contributed by the two forms of Pb: acid-extractable and reducible Pb.

[0128] When MnOx biochar bioelectrochemical systems remediate soils contaminated with different amounts of Cd and Pb, varying Cd and Pb concentrations produce different electrochemical performances, including maximum voltage, energy generation, and internal resistance. These differences in electrochemical performance trigger varying metal migration processes, leading to differences in remediation efficiency. For example, in Cs2, a Cd concentration of 200 mg / kg and a Pb concentration of 1500 mg / kg increases the system's internal conductivity, reduces internal mass transfer resistance, and does not exert severe stress on anodic microorganisms, ultimately achieving the highest energy generation among all systems. Energy generation (GC) is a crucial condition for bioelectrochemical systems to remediate Cd and Pb combined pollution. However, in the remediation of single Cd or Pb pollution, the total GC of the bioelectrochemical system is not primarily used to drive the electric field migration of Cd and Pb under single pollution conditions. Therefore, the soil Cd and Pb concentrations and the resulting differences in the electrochemical performance of the bioelectrochemical system affect the final soil remediation efficiency of the system.

[0129] This study analyzes the reasons for the significant differences in the remediation efficacy of MnOx biochar bioelectrochemical systems for complex and single-metal pollution from the perspectives of heavy metal type and metal existence form in soil. Considering the existence form of heavy metals in soil, Pb or Cd in soil tends to strongly bind with organic functional groups such as hydroxyl or carboxyl groups, forming specific adsorption. In this case, Pb or Cd is subject to strong soil binding forces, making it difficult for them to migrate within the soil. In soils with complex pollution, Pb and Cd compete for specific adsorption sites, resulting in some Pb and Cd existing in a non-specific adsorption external complex state. This leads to an increase in the concentration of acid-extractable or reducible Pb and Cd, making these Pb and Cd more susceptible to directional migration under the influence of an electric field. The different chemical properties of heavy metals themselves result in different existence states of Cd and Pb in soil functional groups. Cd exists more in an acid-extractable state, while Pb exists more in a reducible state. The existence state mainly depends on the soil functional groups' attraction to Cd. 2+ Pb 2+ The adsorption and desorption states. When the MnOx biochar bioelectrochemical system treats single Cd pollution, it can only cause acid-extractable Cd to migrate in a directional manner. However, when remediating complex Cd and Pb pollution, it can simultaneously apply an electric field force to both acid-extractable and reducible Cd, which is why the Cd removal rate is improved during the remediation of complex pollution.

[0130] In summary, the cathode of the bioelectrochemical system of this invention uses Pb with a high reaction kinetic constant. 2+ Cd 2+Electron acceptors replace oxygen as the direct electron acceptor, and the large specific surface area of ​​biochar expands the cathode area, increasing the internal electric field strength of the bioelectrochemical system, thereby promoting the effective remediation of heavy metals in soil. The electron flow and electric field generated by the system itself serve as the driving force, driving the migration, transformation, and concentrated removal of heavy metals in the soil. The generated electricity directly contributes to the removal of heavy metals in the soil, without the need for external energy input, and it also has a good remediation effect on soils contaminated with lead and cadmium.

[0131] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A MnOx biochar cathode bioelectrochemical system for remediating lead-cadmium contaminated soil, characterized in that, The bioelectrochemical system includes an anode (5) and a MnOx-modified biochar cathode (3). The MnOx-modified biochar cathode (3) and the anode (5) are arranged opposite to each other in the heavy metal contaminated soil (2). The MnOx-modified biochar cathode (3) and the anode (5) form a closed current path outside the heavy metal contaminated soil (2). An external resistor (1) is provided on the current path. The MnOx-modified biochar cathode (3) is composed of an MnOx-modified biochar catalyst layer, a current collector, and a diffusion layer. The catalyst layer is loaded on the current collector to form a catalytically active surface. The preparation method of the MnOx modified biochar cathode (3) includes the following steps: S1: Preparation of the current collector: pretreatment of carbon felt, dilution of 50-70% polytetrafluoroethylene emulsion, complete soaking of carbon felt with 4-6% polytetrafluoroethylene emulsion, heating at 350-400℃ for 30-45 min to obtain the current collector. S2: Preparation of the catalytic mixture: MnOx modified biochar and F900 carbon black are weighed and mixed evenly at a mass ratio of 9~11:

1. Isopropanol and anhydrous ethanol are then added, with a volume ratio of 4~6:

1. The mixture is ultrasonically mixed in a beaker to obtain a catalytic mixture, such that the content of MnOx modified biochar in the catalytic mixture is 36~84 mg / mL. S3: Catalytic layer loading: 5-15% polytetrafluoroethylene emulsion is added to the catalytic mixture. The mass ratio of the polytetrafluoroethylene emulsion to the MnOx modified biochar is 2-4:

1. The mixture is heated and stirred at 75-80°C until the mixture separates into layers with isopropanol and anhydrous ethanol and forms micelles. The micelles are uniformly coated and lightly pressed onto one side of the current collector. The mixture is heated in a muffle furnace at 350-400°C for 30-45 min to form a catalytic layer with micropores on the surface. S4: Diffusion layer loading: A 50-70% polytetrafluoroethylene (PTFE) emulsion is uniformly coated on the other side of the current collector, and heated at 350-400℃ for 30-45 min to form a diffusion layer. The PTFE loading in the diffusion layer reaches 80-140 mg / cm³. 2 .

2. The MnOx biochar cathode (3) bioelectrochemical system for remediating lead-cadmium contaminated soil according to claim 1, characterized in that, The anode (5) is loaded with electrochemically active microorganisms.

3. The MnOx biochar cathode bioelectrochemical system for remediating lead-cadmium contaminated soil according to claim 1, characterized in that, The external resistor (1) is 500~1500 Ω.

4. The MnOx biochar cathode bioelectrochemical system for remediating lead-cadmium contaminated soil according to claim 1, characterized in that, The preparation method of the MnOx modified biochar is as follows: potassium permanganate and biochar are dissolved in water at a mass ratio of 1:1~4, and the amount of water is 5~35 times the total mass of potassium permanganate and biochar. The mixture is stirred at 75~85℃ for 1~1.5 h, and the solution is allowed to stand for 2~2.5 h. After solid-liquid separation, the solid is collected, washed with deionized water, and dried at 75~80℃ for 2~3 h. The dried biochar is then activated at 450~500℃ for 1.5~2 h, and after cooling, MnOx modified biochar is obtained.

5. The MnOx biochar cathode bioelectrochemical system for remediating lead-cadmium contaminated soil according to claim 4, characterized in that, The bioelectrochemical system is also equipped with a reactor (4) for loading heavy metal contaminated soil (2), the water content of the heavy metal contaminated soil (2) is maintained at 15~25%, and artificial nutrient solution is added to the soil around the anode (5) daily during the reaction process.

6. The MnOx biochar cathode bioelectrochemical system for remediating lead-cadmium contaminated soil according to claim 5, wherein the soil remediation cycle of the bioelectrochemical system is 35-45 days.

7. The MnOx biochar cathode bioelectrochemical system for remediating lead-cadmium contaminated soil according to claim 5, characterized in that, The artificial nutrient solution is a mixed solution prepared by adding 2 g CH3COONa, 0.31 g NH4Cl, 0.2 g MgSO4, 0.13 g KCl, and 0.008 g CaCl2 to 1 L of water.

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