Method for promoting phytoremediation of heavy metal contaminated soil by modified biochar combined with microorganisms

Abstract

The application discloses a method for promoting plant remediation of heavy metal contaminated soil by using modified biochar and microorganisms, and relates to the technical field of remediation of heavy metal contaminated soil. The application prepares a remediation agent, which comprises an inner layer, a middle layer and an outer layer structure, and is provided with a coating between the layers, and the outer part of the outer layer is further coated with an outer coating. The inner layer comprises modified biochar and a first microbial agent, the middle layer comprises modified biochar and a second microbial agent, and the outer layer comprises dolomite powder and oyster shell powder. The remediation agent is used in combination with heavy metal enrichment plants to co-remediate contaminated soil, so that the enrichment plants can grow well in the environment with high acid and high concentration of heavy metals, the problem of low biomass of the enrichment plants is solved, and efficient remediation of the contaminated soil is finally realized.

Description

Technical Field

[0001] This invention relates to the field of heavy metal contaminated soil remediation technology, and in particular to a method for promoting phytoremediation of heavy metal contaminated soil by combining modified biochar with microorganisms. Background Technology

[0002] Surveys show that the pH value of soils in mining areas with high heavy metal content in Guangxi is generally between 3 and 7. Among them, the proportion of heavy metal-contaminated soil with a pH value less than 6.5 is as high as 89%, indicating that the soil as a whole is acidic. In acidic soils with high heavy metal content, nutrients such as phosphorus, potassium, calcium, and magnesium easily form insoluble compounds, which are not conducive to plant absorption and utilization. High soil acidity also damages plant roots, leading to stunted root growth, dwarfed plants, and low biomass. Simultaneously, high soil acidity reduces the activity of beneficial bacteria for plant growth, which is also detrimental to plant growth. Furthermore, high acidity leads to high heavy metal activity, and in the early stages of growth, accumulating plants are easily poisoned due to limited root growth. Therefore, when using heavy metal accumulating plants for soil remediation, their growth is doubly limited by heavy metals and high acidity, thus restricting their absorption and remediation efficiency.

[0003] Currently, the main remediation methods for heavy metal contaminated soil are physical remediation, chemical remediation, and bioremediation. Physical remediation is costly, chemical remediation is prone to secondary pollution, and bioremediation includes microbial remediation and heavy metal accumulating phytoremediation. Bioremediation is low-cost and does not cause secondary pollution, but the remediation period is long, and microorganisms are easily affected by environmental conditions, resulting in unstable remediation effects. Heavy metal accumulating phytoremediation plants have low biomass and low remediation efficiency when planted in heavy metal contaminated soil, especially in acidic soil.

[0004] Given the characteristics of heavy metal soil pollution in Guangxi, blindly using existing microorganisms and enriching plants for treatment is unlikely to achieve significant remediation effects. Therefore, further improvements to existing bioremediation methods are needed. Summary of the Invention

[0005] To address the above shortcomings, this invention provides a method for promoting phytoremediation of heavy metal-contaminated soil using modified biochar combined with microorganisms. This method involves preparing a novel remediation agent and combining it with heavy metal-accumulating plants for synergistic remediation. This allows the heavy metal-accumulating plants to thrive in high-acid, high-concentration heavy metal environments, resulting in high remediation efficiency. The specific technical solution is as follows:

[0006] A method for promoting phytoremediation of heavy metal contaminated soil by combining modified biochar with microorganisms includes the following steps:

[0007] S1. Detect the types of heavy metals contained in the contaminated soil and determine the species and planting methods of heavy metal-accumulating plants;

[0008] S2. After tilling the contaminated soil, apply the remediation agent, mix it thoroughly, and irrigate with water every 2-3 days. After 7-10 days, open planting trenches, apply the remediation agent and organic fertilizer in sequence, and plant the selected heavy metal accumulating plants, and water thoroughly.

[0009] The repair agent comprises an inner layer, a middle layer, and an outer layer. The inner layer comprises modified biochar and a first microbial agent. The middle layer comprises modified biochar and a second microbial agent. The outer layer comprises dolomite powder and oyster shell powder. An inner membrane is provided between the inner and middle layers, an intermediate membrane is provided between the middle and outer layers, and an outer membrane covers the outer layer.

[0010] The first microbial agent comprises the following parts by weight: 12-18 parts of Lactobacillus plantarum, 5-11 parts of Saccharomyces cerevisiae, and 15-23 parts of Trichoderma harzianum; the second microbial agent comprises the following parts by weight: 10-16 parts of Pseudomonas putida, 6-15 parts of Bacillus licheniformis, 2-7 parts of Azotobacter azoosporus, 5-15 parts of Azotobacter chrysogenum, 10-20 parts of Pseudomonas fluorescens, and 15-25 parts of Streptomyces.

[0011] S3. Harvest the heavy metal accumulating plants after they mature and then carry out harmless treatment;

[0012] S4. Repeat steps S2 and S3 until the soil test results meet national standards, at which point the remediation is complete.

[0013] Preferably, the repair agent in step S2 is a particle with a diameter of 8-10 mm.

[0014] Preferably, in step S2, the amount of repair agent applied after tilling is 20-30 kg / mu; the amount of repair agent applied into the planting furrow is 5-15 kg / mu.

[0015] Preferably, the modified biochar in step S2 is prepared as follows: longan seeds are dried and crushed at 95-105℃, then calcined at 500-600℃ to obtain raw biochar, ground, and passed through a 20-mesh sieve; the raw biochar is mixed with a 0.1-0.2 mol / L NaOH aqueous solution at a solid-liquid ratio of 2-5 g: 100-120 mL, and stirred continuously for 6-8 h; the mixture is dried at 70-80℃ to obtain alkali-modified biochar; the alkali-modified biochar is placed in a ball mill and ground at a speed of 1000-1500 rpm and a centrifugal acceleration of 70-80 G for 1-2 h; the product is naturally cooled to room temperature, washed, dried at 70-80℃ for 10-12 h, and pulverized to obtain the modified biochar.

[0016] Preferably, the method for preparing the repair agent in step S2 is as follows:

[0017] (1) Disperse the first microbial agent in water to prepare the first bacterial solution, mix the modified biochar and the first bacterial solution at a solid-liquid ratio of 10-15:1, let stand for 10-12 hours, and dry at 30-50℃ to obtain the inner layer material;

[0018] (2) Disperse the second microbial agent in water to prepare a second bacterial solution, mix the modified biochar and the second bacterial solution at a solid-liquid ratio of 1:5-10, let stand for 10-12 hours to obtain the middle layer material;

[0019] (3) Mix the dolomite powder, oyster shell powder and water in a weight ratio of 1:1:5-10 to obtain the outer layer material;

[0020] (4) First, dissolve chitosan, carboxymethyl cellulose and water in a weight ratio of 2:2:10-30 by heating, and then cool to 30-40℃ to obtain the inner coating solution;

[0021] (5) Prepare the intermediate coating solution using the same method as in step (4);

[0022] (6) Dissolve sodium alginate and water at a weight ratio of 1:40-80 by heating, and then cool to 30-40°C to obtain the coating solution;

[0023] (7) After the inner layer material is made into granules, the inner coating liquid is uniformly sprayed onto the surface of the granules made from the inner layer material and dried to obtain inner coating granules; the middle layer material is uniformly sprayed onto the inner coating granules and dried to obtain granules containing the middle layer material; the intermediate coating liquid is uniformly sprayed onto the granules containing the middle layer material and dried to obtain intermediate coating granules; the outer layer material is uniformly sprayed onto the intermediate coating granules and dried to obtain granules containing the outer layer material; the outer coating liquid is uniformly sprayed onto the granules containing the outer layer material and dried to obtain the repair agent.

[0024] Preferably, in step (1), the total mass percentage of the first microbial agent in the first bacterial solution is 1-5%.

[0025] Preferably, in step (2), the total mass percentage of the second microbial agent in the second bacterial solution is 5-10%.

[0026] Preferably, in step (7), the drying temperature is 30-50℃.

[0027] Preferably, the harmless treatment in step S3 is as follows: first, the mature plant is crushed into powder, then dried at 90-120℃ until the moisture content is 7-10%, and then carbonized at 500-600℃ for 35-55 minutes.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] 1. The remediation agent of this invention is designed with an outer, middle, and inner layer structure. The outer membrane of the outer layer is easily ruptured upon contact with water. After rupture, dolomite powder and oyster shell powder dissolve, initially adjusting soil acidity and increasing the local pH value of the soil, especially precisely increasing the pH value of the root zone soil, preventing the accumulation of heavy metals and hindering plant root growth. At the same time, the microbial agent (i.e., the second microbial agent) in the middle layer of the remediation agent also gradually and slowly dissolves. The *Pseudomonas putida* and *Bacillus licheniformis* in the second microbial agent can secrete auxin and cytokinin, stimulating plant root growth. The growth and development of long and lateral roots, *Azotrophus* and *Azotobacter graminearum* can fix nitrogen or promote nitrogen fixation in plants, replenishing the nitrogen source in the soil and thus providing nitrogen nutrition for plants. *Pseudomonas fluorescens* and *Streptomyces* can inhibit pathogens and promote the growth of beneficial bacteria, enriching the beneficial microbial community around the roots. Therefore, the second microbial agent promotes the growth of heavy metal accumulating plants by promoting root growth, nutrition, and enriching the beneficial microbial community, allowing the initial growth of heavy metal accumulating plants to be unrestricted, resulting in vigorous growth, robust and tall plants with high biomass, and the ability to accumulate more heavy metals. Under improved soil environmental conditions and fertility, and with good growth of heavy metal accumulating plants, the inner layer raw material of the remediation agent (i.e., the first microbial agent) also gradually and slowly dissolves. *Lactobacillus plantarum*, *Saccharomyces cerevisiae*, and *Trichoderma harzianum* in the first microbial agent can synergistically work with heavy metal accumulating plants to promote the rapid absorption of heavy metals by plants. Therefore, this invention achieves efficient remediation of heavy metals in acidic soils.

[0030] 2. Compared with the effect of applying each layer of raw materials separately in the repair agent, the multi-layer structure design of the repair agent of this invention can better promote plant growth and effectively play a synergistic role in removing heavy metals.

[0031] 3. The modified biochar in this invention not only provides carbon source for microorganisms, but also adsorbs a large number of microorganisms and prevents their rapid release, thus achieving a long-term repair effect. At the same time, dolomite powder can also provide magnesium, which promotes plant growth. Detailed Implementation

[0032] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0033] The following are the sources of microorganisms:

[0034] Lactobacillus plantarum was selected from CICC (China Industrial Microbial Culture Collection Center) No. 6240; Saccharomyces cerevisiae was selected from CICC No. 31298; Trichoderma harzianum was selected from ACCC (China Agricultural Microbial Culture Collection Center) No. 30371; Pseudomonas putida was selected from ACCC No. 10085; Bacillus licheniformis was selected from ACCC No. 11091; Azotobacter azoosporus was selected from ACCC No. 04138; Azotobacter chrysogenum was selected from ACCC No. 10096; Streptomyces was selected from CCTCC (China Center for Type Culture Collection) No. 207201.

[0035] Example 1

[0036] Preparation of modified biochar:

[0037] Longan seeds were dried and crushed at 95℃, then calcined at 500℃ to produce raw biochar. The raw biochar was then ground and passed through a 20-mesh sieve. The raw biochar was mixed with a 0.1 mol / L NaOH aqueous solution at a solid-liquid ratio of 2g:100mL and stirred continuously for 6 hours. The mixture was then dried at 70℃ to obtain alkali-modified biochar. The alkali-modified biochar was placed in a ball mill and ground at 1000 rpm and 70G centrifugal acceleration for 1 hour. The product was allowed to cool naturally to room temperature, washed, dried at 70℃ for 10 hours, and then pulverized to obtain the modified biochar.

[0038] Preparation of repair agent:

[0039] The repair agent comprises an inner layer, a middle layer, and an outer layer. The inner layer comprises modified biochar and a first microbial agent; the middle layer comprises modified biochar and a second microbial agent; the outer layer comprises dolomite powder and oyster shell powder; an inner membrane is provided between the inner and middle layers, an intermediate membrane is provided between the middle and outer layers, and the outer layer is covered with an outer membrane.

[0040] The first microbial inoculant comprises the following parts by weight: 12 parts of Lactobacillus plantarum, 5 parts of Saccharomyces cerevisiae, and 15 parts of Trichoderma harzianum; the second microbial inoculant comprises the following parts by weight: 10 parts of Pseudomonas putida, 6 parts of Bacillus licheniformis, 2 parts of Azotobacter azoosporus, 5 parts of Azotobacter chrysogenum, 10 parts of Pseudomonas fluorescens, and 15 parts of Streptomyces.

[0041] (1) The first microbial agent is dispersed in water to prepare the first bacterial solution. The total mass percentage of the first microbial agent in the first bacterial solution is 1%. The modified biochar and the first bacterial solution are mixed at a solid-liquid ratio of 10:1, allowed to stand for 10 hours, and dried at 30°C to obtain the inner layer material.

[0042] (2) The second microbial agent is dispersed in water to prepare the second bacterial solution. The total mass percentage of the second microbial agent in the second bacterial solution is 5%. The modified biochar and the second bacterial solution are mixed at a solid-liquid ratio of 1:5 and allowed to stand for 10 hours to obtain the middle layer material.

[0043] (3) Mix dolomite powder, oyster shell powder and water in a weight ratio of 1:1:5 to obtain the outer layer material;

[0044] (4) First, dissolve chitosan, carboxymethyl cellulose and water in a weight ratio of 2:2:10 by heating, and then cool to 30°C to obtain the inner coating solution;

[0045] (5) Prepare the intermediate coating solution using the same method as in step (4);

[0046] (6) Dissolve sodium alginate and water at a weight ratio of 1:40 by heating, and then cool to 30°C to obtain the coating solution;

[0047] (7) After the inner layer material is made into granules, the inner coating liquid is uniformly sprayed onto the surface of the granules made from the inner layer material and dried at 45°C to obtain inner-coated granules; the middle layer material is uniformly sprayed onto the inner-coated granules and dried at 45°C to obtain granules containing the middle layer material; the intermediate coating liquid is uniformly sprayed onto the granules containing the middle layer material and dried at 45°C to obtain intermediate-coated granules; the outer layer material is uniformly sprayed onto the intermediate-coated granules and dried at 45°C to obtain granules containing the outer layer material; the outer coating liquid is uniformly sprayed onto the granules containing the outer layer material and dried at 45°C to obtain the repair agent.

[0048] Example 2

[0049] Preparation of modified biochar:

[0050] Longan seeds were dried and crushed at 105℃, then calcined at 600℃ to produce raw biochar. The raw biochar was then ground and passed through a 20-mesh sieve. The raw biochar was mixed with a 0.2 mol / L NaOH aqueous solution at a solid-liquid ratio of 5g:120mL and stirred continuously for 8 hours. The mixture was then dried at 80℃ to obtain alkali-modified biochar. The alkali-modified biochar was placed in a ball mill and ground at 1500rpm and 80G centrifugal acceleration for 2 hours. The product was allowed to cool naturally to room temperature, washed, dried at 80℃ for 12 hours, and then pulverized to obtain the modified biochar.

[0051] Preparation of repair agent:

[0052] The repair agent comprises an inner layer, a middle layer, and an outer layer. The inner layer comprises modified biochar and a first microbial agent; the middle layer comprises modified biochar and a second microbial agent; the outer layer comprises dolomite powder and oyster shell powder; an inner membrane is provided between the inner and middle layers, an intermediate membrane is provided between the middle and outer layers, and the outer layer is covered with an outer membrane.

[0053] The first microbial inoculant comprises the following parts by weight: 18 parts of Lactobacillus plantarum, 11 parts of Saccharomyces cerevisiae, and 23 parts of Trichoderma harzianum; the second microbial inoculant comprises the following parts by weight: 16 parts of Pseudomonas putida, 15 parts of Bacillus licheniformis, 7 parts of Azotobacter azoosporus, 15 parts of Azotobacter chrysogenum, 20 parts of Pseudomonas fluorescens, and 25 parts of Streptomyces.

[0054] (1) The first microbial agent is dispersed in water to prepare the first bacterial solution. The total mass percentage of the first microbial agent in the first bacterial solution is 5%. The modified biochar and the first bacterial solution are mixed at a solid-liquid ratio of 15:1, allowed to stand for 12 hours, and dried at 50°C to obtain the inner layer material.

[0055] (2) The second microbial agent is dispersed in water to prepare the second bacterial solution. The total mass percentage of the second microbial agent in the second bacterial solution is 10%. The modified biochar and the second bacterial solution are mixed at a solid-liquid ratio of 1:10 and allowed to stand for 12 hours to obtain the middle layer material.

[0056] (3) Mix dolomite powder, oyster shell powder and water in a weight ratio of 1:1:10 to obtain the outer layer material;

[0057] (4) First, dissolve chitosan, carboxymethyl cellulose and water in a weight ratio of 2:2:30 by heating, and then cool to 40°C to obtain the inner coating solution;

[0058] (5) Prepare the intermediate coating solution using the same method as in step (4);

[0059] (6) Dissolve sodium alginate and water at a weight ratio of 1:80 by heating, and then cool to 40°C to obtain the coating solution;

[0060] (7) After the inner layer material is made into granules, the inner coating liquid is uniformly sprayed onto the surface of the granules made from the inner layer material and dried at 50°C to obtain inner-coated granules; the middle layer material is uniformly sprayed onto the inner-coated granules and dried at 50°C to obtain granules containing the middle layer material; the intermediate coating liquid is uniformly sprayed onto the granules containing the middle layer material and dried at 50°C to obtain intermediate-coated granules; the outer layer material is uniformly sprayed onto the intermediate-coated granules and dried at 50°C to obtain granules containing the outer layer material; the outer coating liquid is uniformly sprayed onto the granules containing the outer layer material and dried at 50°C to obtain the repair agent.

[0061] Example 3

[0062] A method for promoting phytoremediation of heavy metal contaminated soil by combining modified biochar with microorganisms includes the following steps:

[0063] S1. Detect the types of heavy metals contained in the contaminated soil and determine the varieties and planting methods of heavy metal-accumulating plants.

[0064] S2. After tilling the contaminated soil, apply 10mm diameter remediation agent granules at a rate of 20kg / mu, mix thoroughly, and irrigate with water every 2 days. After 7 days, open planting furrows, apply the remediation agent and organic fertilizer in sequence at a rate of 10kg / mu, and plant the selected heavy metal accumulating plants, and water thoroughly.

[0065] S3. Harvest the heavy metal accumulating plants after they mature and then carry out harmless treatment. The harmless treatment is as follows: first, crush the mature plants into powder, then dry them at 100°C until the moisture content is 7%, and then carbonize them at 500°C for 55 minutes.

[0066] S4. Repeat steps S2 and S3 until the soil test results meet national standards, at which point the remediation is complete.

[0067] The repair agent used in this embodiment is the repair agent prepared in Example 2.

[0068] Example 4

[0069] A method for promoting phytoremediation of heavy metal contaminated soil by combining modified biochar with microorganisms includes the following steps:

[0070] S1. Detect the types of heavy metals contained in the contaminated soil and determine the varieties and planting methods of heavy metal-accumulating plants.

[0071] S2. After tilling the contaminated soil, apply 10mm diameter remediation agent granules at a rate of 30kg / mu, mix thoroughly, and irrigate with water every 3 days. After 10 days, open planting furrows, apply the remediation agent and organic fertilizer in sequence at a rate of 15kg / mu, and plant the selected heavy metal accumulating plants, and water thoroughly.

[0072] S3. Harvest the heavy metal accumulating plants after they mature and then carry out harmless treatment. The harmless treatment is as follows: first, crush the mature plants into powder, then dry them at 120°C until the moisture content is 10%, and then carbonize them at 500°C for 45 minutes.

[0073] S4. Repeat steps S2 and S3 until the soil test results meet national standards, at which point the remediation is complete.

[0074] The repair agent used in this embodiment is the repair agent prepared in Example 2.

[0075] Comparative Example 1

[0076] The repair agent used in this comparative example does not contain the middle layer material, and other repair methods are the same as in Example 4.

[0077] Comparative Example 2

[0078] The repair agent used in this comparative example does not contain inner layer material, and other repair methods are the same as in Example 4.

[0079] Comparative Example 3

[0080] This comparative example describes a method for promoting phytoremediation of heavy metal contaminated soil using modified biochar combined with microorganisms, comprising the following steps: S1. Detecting the types of heavy metals contained in the contaminated soil and determining the species and planting method of heavy metal-accumulating plants.

[0081] S2. After tilling the contaminated soil, limestone is first applied to reduce the soil acidity. Ten days later, planting trenches are dug. Modified biochar and the first microbial agent from Example 2 are first applied into the trenches, followed by modified biochar and the second microbial agent. Finally, organic fertilizer is applied. The amount of biochar, microbial agent, and organic fertilizer applied is the same as in Example 4, with a total amount of 15 kg / mu. The selected heavy metal accumulating plants are then planted and thoroughly watered.

[0082] S3. Harvest the heavy metal accumulating plants after they mature and then carry out harmless treatment. The harmless treatment is as follows: first, crush the mature plants into powder, then dry them at 120°C until the moisture content is 10%, and then carbonize them at 500°C for 45 minutes.

[0083] S4. Repeat steps S2 and S3 until the soil test results meet national standards, at which point the remediation is complete.

[0084] This comparative example involves applying each layer of the repair agent separately.

[0085] The applicant conducted an experiment in a mining area in Nandan County, Hechi City, Guangxi Province. The soil pH value of the experimental area was 4-4.5, which is acidic soil. The main heavy metals detected were cadmium (Cd), lead (Pb) and arsenic (As). The cadmium content was 5.62 mg / kg, the lead content was 79.8 mg / kg, and the arsenic content was 36.9 mg / kg, all of which exceeded the standard.

[0086] The experiment was divided into 5 groups, with each group having an experimental area of ​​30m². 2Group 1 underwent soil remediation using the method of Example 3, Group 2 used the method of Example 4, Group 3 used the method of Comparative Example 1, Group 4 used the method of Comparative Example 2, and Group 5 used the method of Comparative Example 3, with three replicates for each group. Castor beans were the only enrichment plants planted, with a row spacing of 45 cm and a plant spacing of 20 cm. Fertilization and watering were carried out according to conventional castor bean management methods. After the castor beans from the first round of planting matured, their biomass (mean) was calculated, and the removal rates of cadmium, lead, and arsenic in the soil were tested. The results are shown in Table 1. After the castor beans from the second round of planting matured, their biomass was calculated, and the removal rates of cadmium, lead, and arsenic in the soil were tested. The results are shown in Table 2. Biomass refers to the weight of the entire mature castor bean plant after drying to constant weight.

[0087] Table 1. Biomass of castor beans after maturity and removal rate of heavy metals in soil after the first round of planting.

[0088]

[0089] Table 2. Biomass of castor beans after maturity and removal rate of heavy metals in soil after the second round of planting.

[0090]

[0091] The table above shows that the middle and inner layer materials in the remediation agent of this invention have a significant impact on castor biomass. Only when the middle and inner layer materials act simultaneously can the castor biomass be increased, thereby improving the heavy metal removal efficiency. Furthermore, compared to the effects of applying each layer material individually in the remediation agent, the multi-layered structure design of this invention can better promote plant growth and efficiently exert a synergistic effect in removing heavy metals. Using the method of this invention, the heavy metal removal rate in the soil during the second round of planting can reach over 97%, demonstrating high removal efficiency.

[0092] In summary, the remediation agent and method of this invention achieve efficient remediation of heavy metals in acidic soils.

[0093] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A method for promoting phytoremediation of heavy metal contaminated soil by combining modified biochar with microorganisms, characterized in that, Includes the following steps: S1. Detect the types of heavy metals contained in the contaminated soil and determine the species and planting methods of heavy metal-accumulating plants; S2. After tilling the contaminated soil, apply the remediation agent, mix it thoroughly, and irrigate with water every 2-3 days. After 7-10 days, open planting trenches, apply the remediation agent and organic fertilizer in sequence, plant the heavy metal accumulating plants, and water thoroughly. The repair agent comprises an inner layer, a middle layer, and an outer layer. The inner layer comprises modified biochar and a first microbial agent. The middle layer comprises modified biochar and a second microbial agent. The outer layer comprises dolomite powder and oyster shell powder. An inner membrane is provided between the inner and middle layers, an intermediate membrane is provided between the middle and outer layers, and an outer membrane covers the outer layer. The first microbial agent comprises the following parts by weight: 12-18 parts of Lactobacillus plantarum, 5-11 parts of Saccharomyces cerevisiae, and 15-23 parts of Trichoderma harzianum; the second microbial agent comprises the following parts by weight: 10-16 parts of Pseudomonas putida, 6-15 parts of Bacillus licheniformis, 2-7 parts of Azotobacter azoosporus, 5-15 parts of Azotobacter chrysogenum, 10-20 parts of Pseudomonas fluorescens, and 15-25 parts of Streptomyces. The modified biochar is prepared as follows: longan seeds are dried and crushed at 95-105℃, then calcined at 500-600℃ to obtain raw biochar, which is then ground and passed through a 20-mesh sieve; the raw biochar is mixed with a 0.1-0.2 mol / L NaOH aqueous solution at a solid-liquid ratio of 2-5 g: 100-120 mL, and stirred continuously for 6-8 h; the mixture is dried at 70-80℃ to obtain alkali-modified biochar; the alkali-modified biochar is placed in a ball mill and ground at a speed of 1000-1500 rpm and a centrifugal acceleration of 70-80 G for 1-2 h; the product is naturally cooled to room temperature, washed, dried at 70-80℃ for 10-12 h, and pulverized to obtain the modified biochar; The preparation method of the repair agent is as follows: (1) Disperse the first microbial agent in water to prepare the first bacterial solution, mix the modified biochar and the first bacterial solution at a solid-liquid ratio of 10-15:1, let stand for 10-12 hours, and dry at 30-50℃ to obtain the inner layer material; (2) Disperse the second microbial agent in water to prepare a second bacterial solution, mix the modified biochar and the second bacterial solution at a solid-liquid ratio of 1:5-10, let stand for 10-12 hours to obtain the middle layer material; (3) Mix the dolomite powder, oyster shell powder and water in a weight ratio of 1:1:5-10 to obtain the outer layer material; (4) First, dissolve chitosan, carboxymethyl cellulose and water in a weight ratio of 2:2:10-30 by heating, and then cool to 30-40℃ to obtain the inner coating solution; (5) Prepare the intermediate coating solution using the same method as in step (4); (6) Dissolve sodium alginate and water at a weight ratio of 1:40-80 by heating, and then cool to 30-40°C to obtain the coating solution; (7) After the inner layer material is made into granules, the inner coating liquid is uniformly sprayed onto the surface of the granules made from the inner layer material and dried to obtain inner-coated granules; the middle layer material is uniformly sprayed onto the inner-coated granules and dried to obtain granules containing the middle layer material; the intermediate coating liquid is uniformly sprayed onto the granules containing the middle layer material and dried to obtain intermediate-coated granules; the outer layer material is uniformly sprayed onto the intermediate-coated granules and dried to obtain granules containing the outer layer material; the outer coating liquid is uniformly sprayed onto the granules containing the outer layer material and dried to obtain the repair agent; S3. Harvest the heavy metal accumulating plants after they mature and then carry out harmless treatment; S4. Repeat steps S2 and S3 until the soil test results meet national standards, at which point the remediation is complete.

2. The method for promoting phytoremediation of heavy metal contaminated soil by combining modified biochar with microorganisms according to claim 1, characterized in that, The repair agent in step S2 consists of particles with a diameter of 8-10 mm.

3. The method for promoting phytoremediation of heavy metal contaminated soil by combining modified biochar with microorganisms according to claim 1, characterized in that, In step S2, the amount of repair agent applied after tilling is 20-30 kg / mu; the amount of repair agent applied into the planting furrow is 5-15 kg / mu.

4. The method for promoting phytoremediation of heavy metal contaminated soil by combining modified biochar with microorganisms according to claim 1, characterized in that, In step (1), the total mass percentage of the first microbial agent in the first bacterial solution is 1-5%.

5. The method for promoting phytoremediation of heavy metal contaminated soil by combining modified biochar with microorganisms according to claim 1, characterized in that, In step (2), the total mass percentage of the second microbial agent in the second bacterial solution is 5-10%.

6. The method for promoting phytoremediation of heavy metal contaminated soil by combining modified biochar with microorganisms according to claim 1, characterized in that, In step (7), the drying temperature is 30-50℃.

7. The method for promoting phytoremediation of heavy metal contaminated soil by combining modified biochar with microorganisms according to claim 1, characterized in that, The harmless treatment in step S3 is as follows: first, the mature plants are crushed into powder, then dried at 90-120℃ until the moisture content is 7-10%, and then carbonized at 500-600℃ for 35-55 minutes.

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