Preparation method of heavy metal polluted farmland soil remediation agent and soil remediation method

Abstract

The invention provides a preparation method of a heavy metal polluted farmland soil remediation agent and a soil remediation method, and relates to the technical field of soil remediation, the soil remediation method comprises the following steps: before soil thawing in spring, freezing and thawing a soil layer from the surface of a heavy metal polluted farmland to the underground by 50cm for 5-20 times; 1-2 weeks before crops are sown, the chitosan-magnetic composite biochar is uniformly spread on the soil surface of the heavy metal polluted farmland, and permeable irrigation is carried out; when crops are sown, the soil remediation agent is evenly applied to a soil layer in the area from the surface of the heavy metal polluted farmland to the underground depth of 15 cm, the chitosan-magnetic composite charcoal is evenly applied to a soil layer in the area from the underground depth of 16 cm to the underground depth of 20 cm in the heavy metal polluted farmland, and conventional field water and fertilizer management is conducted after soil covering. By means of the method, efficient and stable remediation of the heavy metal polluted farmland deep soil in the freezing and thawing cycle environment can be achieved.

Description

Technical Field

[0001] This invention relates to the field of soil remediation technology, and more specifically, to a method for preparing a soil remediation agent for heavy metal-contaminated farmland and a soil remediation method. Background Technology

[0002] Remediation of heavy metal-contaminated farmland soil under freeze-thaw cycles is a key challenge in environmental engineering. The unique physicochemical processes triggered by freeze-thaw cycles profoundly affect the long-term effectiveness of soil remediation materials. Freeze-thaw cycles alter soil physicochemical properties through periodic freezing and thawing processes, thereby significantly regulating heavy metal behavior. Studies have shown that after 30 freeze-thaw cycles, the content of weakly acidic extractable heavy metals in soil can increase by 20% to 40%, and their bioavailability is significantly improved. With the freezing and thawing of water, activated heavy metals readily migrate to deeper soil layers.

[0003] However, most current heavy metal passivation materials are applied to the soil using conventional methods such as mechanical rotary tillage, which struggles to overcome the physical barriers of the underlying undisturbed soil. This results in surface-applied passivation materials failing to effectively reach and immobilize heavy metals that have migrated to deeper layers, creating a technological blind spot where surface soil is over-remediated while deep soil remains contaminated with heavy metals. Furthermore, traditional biochar, as a heavy metal passivation material, faces significant bottlenecks under freeze-thaw cycles. Its structure is easily damaged, its passivation capacity decreases, and its function is limited, severely impacting soil remediation efficiency. Summary of the Invention

[0004] The problem solved by this invention is at least one of the following: (1) how to improve the remediation effect of deep soil for heavy metal contaminated farmland soil; (2) how to obtain heavy metal passivation materials with higher remediation efficiency.

[0005] To address the above problems, this invention provides a method for preparing a soil remediation agent for heavy metal-contaminated farmland, comprising: Step S1: Dissolve biochar, ferric chloride hexahydrate, and ferrous sulfate heptahydrate in water to obtain a first mixture; add alkaline solution to the first mixture to adjust the pH to 10 to 11, carry out the first stirring reaction, separate the magnetic precipitate and wash it until neutral, dry it to obtain magnetic biochar; dissolve chitosan in acetic acid solution, add the magnetic biochar, stir evenly to obtain a second mixture; Step S2: Add glutaraldehyde solution to the second mixture, carry out a second stirring reaction, separate the magnetic products and wash until neutral, dry to obtain chitosan-magnetic composite biochar; Step S3: Mix the composite bacterial solution with sodium alginate solution evenly, then add the chitosan-magnetic composite biochar and stir evenly to obtain a third mixture; add the third mixture to calcium chloride solution, let it stand to solidify, collect the gel microspheres, wash and dry them to obtain the soil remediation agent; wherein, the composite bacterial solution contains Bacillus belyssioides and Pseudomonas fluorescens.

[0006] Compared with related technologies, this invention generates magnetic biochar by loading iron oxides onto the surface of biochar. The abundant hydroxyl functional groups on its surface can rapidly fix Cd through complexation and co-precipitation. 2+ Pb 2+ The chitosan, after being cross-linked with glutaraldehyde, further introduces amino functional groups to enhance its chelating ability and improve its structural stability in the soil, preventing the adsorption performance of biochar from decaying over time. Furthermore, the *Bacillus belye* and *Pseudomonas fluorescens* in the composite bacterial solution can not only alter the form of heavy metals and reduce bioavailability by secreting organic acids and iron oxides, but also produce plant hormones to promote crop growth and enhance stress resistance, and inhibit soil-borne pathogens to improve soil microecology. Finally, the sodium alginate-calcium chloride gel microsphere structure provides a protective barrier for the functional bacteria, reducing their risk of environmental stress or predation by protozoa in the soil, achieving long-term maintenance of microbial activity. Simultaneously, its magnetic properties facilitate precise positioning or recovery in the later stages, further optimizing soil remediation efficiency and sustainability. In summary, the soil remediation agent prepared using the method of this invention has a good remediation effect on farmland soil contaminated with heavy metals, achieving multiple goals of heavy metal passivation, crop safety assurance, and soil health.

[0007] Optionally, in step S1, the mass ratio of the biochar, the ferric chloride hexahydrate, and the ferrous sulfate heptahydrate is (1.8 to 2.2): (3.1 to 3.4): (1.5 to 1.7).

[0008] Optionally, in step S1, the mass ratio of the magnetic biochar to the chitosan is 5:(3.8 to 4.2).

[0009] Optionally, the volume fraction of the glutaraldehyde solution is 1.8% to 2.2%, and the mass ratio of glutaraldehyde in the glutaraldehyde solution to the magnetic biochar is 1:(40 to 60).

[0010] Optionally, in step S3, the concentration of *Bacillus belyssus* in the composite bacterial solution is (2.9 to 3.1) × 10⁻⁶. 9 The concentration of the fluorescent Pseudomonas was (1.9 to 2.1) × 10 CFU / mL. 9 CFU / mL; the mass-to-volume ratio of the chitosan-magnetic composite biochar to the composite bacterial solution is (4 to 6):1.

[0011] Optionally, in step S3, the mass fraction of the sodium alginate solution is 3.5% to 4.5%, and the volume ratio of the composite bacterial solution to the sodium alginate solution is (0.9 to 1.1):(0.9 to 1.1).

[0012] Optionally, in step S3, the mass fraction of the calcium chloride solution is 1.8% to 2.2%, and the volume ratio of the calcium chloride solution to the third mixture is (5 to 10):1.

[0013] This invention provides a soil remediation method, comprising: Step M1: Before the soil thaws in spring, the soil layer in the area from the surface of the heavy metal contaminated farmland to a depth of 50cm underground is subjected to 5 to 20 freeze-thaw cycles. The freeze-thaw cycle includes: lowering the temperature of the soil layer to the freezing temperature and maintaining it for 10 to 14 hours, and then lowering it to the thawing temperature and maintaining it for 10 to 14 hours. Step M2: One to two weeks before crop sowing, apply chitosan-magnetic composite biochar evenly to the soil surface of the heavy metal-contaminated farmland at the first application rate, and irrigate with water; wherein, the first application rate is 30 kg / mu to 50 kg / mu. Step M3: When sowing crops, apply the soil remediation agent evenly to the soil layer from the surface to a depth of 15cm in the heavy metal contaminated farmland according to the second application amount, and apply the chitosan-magnetic composite biochar evenly to the soil layer from a depth of 16cm to a depth of 20cm in the heavy metal contaminated farmland according to the third application amount. After covering with soil, carry out conventional field water and fertilizer management. The second application amount is 60kg / mu to 80kg / mu, and the third application amount is 30kg / mu to 50kg / mu. The chitosan-magnetic composite biochar and the soil remediation agent are prepared using the methods described above.

[0014] Compared with related technologies, the heavy metal contaminated farmland soil remediation method of this invention significantly improves the remediation effect of deep soil through a synergistic mechanism of "physical pretreatment-layered targeted remediation". Specifically, before the spring thaw, the 0 to 50 cm soil layer is subjected to 5 to 20 freeze-thaw cycles. The physical stress of the water-ice phase change disrupts the soil aggregate structure, increases the porosity and permeability of deep soil, and facilitates the migration of remediation materials to the deeper soil. At the same time, the "freeze-concentration" effect activates the heavy metals adsorbed on the surface of soil colloids, breaking their stable binding balance with soil particles and creating favorable conditions for the subsequent action of remediation materials. Subsequently, precise targeted remediation is achieved by applying remediation materials in layers. One to two weeks before sowing, 30 kg / mu to 50 kg / mu of chitosan-magnetic composite biochar is spread on the soil surface. After permeable irrigation, it infiltrates into the deep soil with the water. The abundant hydroxyl and amino functional groups on its surface rapidly adsorb deep free heavy metals through complexation and chelation. At sowing time, apply 60-80 kg / mu of soil remediation agent (charcoal-microbe symbiosis) to the soil layer (0-15 cm) in the densely active root zone of crops. The soil remediation agent not only passivates heavy metals but also leverages the synergistic stress-resistance effect of microorganisms and plant roots. Apply 30-50 kg / mu of chitosan-magnetic composite biochar to the 16-20 cm soil layer. Utilizing the expanded pore network resulting from freeze-thaw cycles and the physical action of surface gravity and water flow, combined with the high specific gravity of chitosan-magnetic composite biochar, it smoothly penetrates and migrates to deeper soil layers. Simultaneously, relying on the efficient aggregation and binding of the magnetic iron oxide components on the surface of the chitosan-magnetic composite biochar with deep soil minerals, it achieves precise anchoring and long-term retention in deep micro-domains, thereby effectively adsorbing and fixing deep heavy metals. Ultimately, this achieves efficient and stable remediation of deep soil in farmland contaminated with heavy metals under freeze-thaw cycles.

[0015] Optionally, the crop is soybean.

[0016] Optionally, in step M1, the freezing temperature is between -10°C and -30°C, and the melting temperature is between 10°C and 25°C. Attached Figure Description

[0017] Figure 1 The image shows a comparison of the XRD curves of biochar, magnetic biochar, and chitosan-magnetic composite biochar in Example 1 of this invention. Detailed Implementation

[0018] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Although some embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the present invention. It should be understood that the accompanying drawings and embodiments of the present invention are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0019] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0020] The term "comprising" and its variations as used herein are open-ended, meaning "including but not limited to"; the term "based on" means "at least partially based on"; the term "one embodiment" means "at least one embodiment"; the term "another embodiment" means "at least one additional embodiment"; the term "some embodiments" means "at least some embodiments"; and the term "optionally" means "optional embodiments". Definitions of other terms will be given in the following description. It should be noted that the concepts of "first," "second," etc., mentioned in this invention are used to distinguish different objects, not to describe a specific order or hierarchy. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0021] It should be noted that, in this invention, deep soil refers to the soil layer in the area 15cm to 30cm below the surface of the farmland.

[0022] To address the problems existing in the aforementioned related technologies, this embodiment provides a method for preparing a heavy metal contaminated farmland soil remediation agent, comprising: Step S1: Dissolve biochar, ferric chloride hexahydrate, and ferrous sulfate heptahydrate in water to obtain a first mixture; add alkali to the first mixture to adjust the pH to 10 to 11, perform a first stirring reaction, separate the magnetic precipitate and wash until neutral, and dry to obtain magnetic biochar; dissolve chitosan in acetic acid solution, add the magnetic biochar, and stir evenly to obtain a second mixture; wherein the volume concentration of the acetic acid solution is 1.9% to 2.1%, and the mass-volume ratio of chitosan to acetic acid solution is 1:(30 to 50) g / mL; Step S2: Add glutaraldehyde solution to the second mixture, carry out a second stirring reaction, separate the magnetic products and wash until neutral, dry to obtain chitosan-magnetic composite biochar; Step S3: Mix the composite bacterial solution with sodium alginate solution evenly, then add the chitosan-magnetic composite biochar and stir evenly to obtain a third mixture; add the third mixture to calcium chloride solution, let it stand to solidify, collect the gel microspheres, wash and dry them to obtain the soil remediation agent; wherein, the composite bacterial solution contains Bacillus belyssioides and Pseudomonas fluorescens.

[0023] This invention generates magnetic biochar by loading iron oxides onto the surface of biochar. The abundant hydroxyl functional groups on its surface can rapidly immobilize Cd through complexation and co-precipitation. 2+ Pb 2+ The chitosan, after being cross-linked with glutaraldehyde, further introduces amino functional groups to enhance its chelating ability and improve its structural stability in the soil, preventing the adsorption performance of biochar from decaying over time. Furthermore, the *Bacillus belye* and *Pseudomonas fluorescens* in the composite bacterial solution can not only alter the form of heavy metals and reduce bioavailability by secreting organic acids and iron oxides, but also produce plant hormones to promote crop growth and enhance stress resistance, and inhibit soil-borne pathogens to improve soil microecology. Finally, the sodium alginate-calcium chloride gel microsphere structure provides a protective barrier for the functional bacteria, reducing their risk of environmental stress or predation by protozoa in the soil, achieving long-term maintenance of microbial activity. Simultaneously, its magnetic properties facilitate precise positioning or recovery in the later stages, further optimizing soil remediation efficiency and sustainability. In summary, the soil remediation agent prepared using the method of this invention has a good remediation effect on heavy metal-contaminated farmland soil, achieving multiple goals of heavy metal passivation, crop safety assurance, and soil health.

[0024] In some embodiments of the present invention, in step S1, the method for preparing the biochar includes: Corn stalks are washed, dried, and crushed, and then pyrolyzed under a nitrogen atmosphere to obtain biochar; wherein the pyrolysis temperature is 450℃ to 550℃ and the time is 2h to 4h.

[0025] In some embodiments of the present invention, in step S1, the mass ratio of the biochar, ferric chloride hexahydrate, and ferrous sulfate heptahydrate is (1.8 to 2.2):(3.1 to 3.4):(1.5 to 1.7). This embodiment, by controlling the mass ratio of biochar, ferric chloride hexahydrate, and ferrous sulfate heptahydrate, ensures that the generated magnetic particles are uniformly loaded on the surface of the biochar, thus maintaining the magnetic properties of the material while maximizing its specific surface area and active site density, thereby significantly improving the adsorption capacity and fixation efficiency for heavy metals.

[0026] In some embodiments of the present invention, in step S1, the mass ratio of the magnetic biochar to the chitosan is 5:(3.8 to 4.2). This embodiment, by controlling the mass ratio of magnetic biochar to chitosan, allows chitosan to form a dense but not excessively cross-linked coating layer on the surface of the magnetic biochar. This ensures that the chitosan amino groups fully complex heavy metals while maintaining the permeability of the material's pore structure, thereby enhancing the stability and long-term effectiveness of the material in acidic or high-ionic-strength soil environments.

[0027] In some embodiments of the present invention, the volume fraction of the glutaraldehyde solution is 1.8% to 2.2%, and the mass ratio of glutaraldehyde in the glutaraldehyde solution to the magnetic biochar is 1:(40 to 60).

[0028] In some embodiments of the present invention, in step S3, the concentration of *Bacillus belye* in the composite bacterial solution is (2.9 to 3.1) × 10⁻⁶. 9 The concentration of the fluorescent Pseudomonas was (1.9 to 2.1) × 10 CFU / mL. 9 CFU / mL; the mass-to-volume ratio of the chitosan-magnetic composite biochar to the composite bacterial solution is (4 to 6):1. This embodiment controls the concentrations of Bacillus belyssus and Pseudomonas fluorescens to enable the bacterial community to form a stable symbiotic community on the carrier surface, avoiding ecological imbalance caused by the dominance of a single species, and significantly improving the colonization rate and functional sustainability of the remediation agent in complex soil environments.

[0029] In some embodiments of the present invention, in step S3, the mass fraction of the sodium alginate solution is 3.5% to 4.5%, and the volume ratio of the composite bacterial solution to the sodium alginate solution is (0.9 to 1.1):(0.9 to 1.1).

[0030] In some embodiments of the present invention, in step S3, the mass fraction of the calcium chloride solution is 1.8% to 2.2%, and the volume ratio of the calcium chloride solution to the third mixture is (5 to 10):1.

[0031] This invention also provides a soil remediation method, comprising: Step M1: Before the soil thaws in spring, the soil layer in the area from the surface of the heavy metal contaminated farmland to a depth of 50cm underground is subjected to 5 to 20 freeze-thaw cycles. The freeze-thaw cycle includes: lowering the temperature of the soil layer to the freezing temperature and maintaining it for 10 to 14 hours, and then lowering it to the thawing temperature and maintaining it for 10 to 14 hours. Step M2: One to two weeks before crop sowing, apply chitosan-magnetic composite biochar evenly to the soil surface of the heavy metal-contaminated farmland at the first application rate, and irrigate with water; wherein, the first application rate is 30 kg / mu to 50 kg / mu. Step M3: When sowing crops, apply the soil remediation agent evenly to the soil layer from the surface to a depth of 15cm in the heavy metal contaminated farmland according to the second application amount, and apply the chitosan-magnetic composite biochar evenly to the soil layer from a depth of 16cm to a depth of 20cm in the heavy metal contaminated farmland according to the third application amount. After covering with soil, carry out conventional field water and fertilizer management. The second application amount is 60kg / mu to 80kg / mu, and the third application amount is 30kg / mu to 50kg / mu. The chitosan-magnetic composite biochar and the soil remediation agent are prepared using the methods described above.

[0032] The heavy metal contaminated farmland soil remediation method of this invention significantly improves the remediation effect of deep soil through a synergistic mechanism of "physical pretreatment-layered targeted remediation". Specifically, before the spring thaw, the 0 to 50 cm soil layer is subjected to 5 to 20 freeze-thaw cycles. The physical stress of the water-ice phase change disrupts the soil aggregate structure, increases the porosity and permeability of deep soil, and facilitates the migration of remediation materials to the deeper soil. At the same time, the "freeze-concentration" effect activates the heavy metals adsorbed on the surface of soil colloids, breaking their stable binding balance with soil particles and creating favorable conditions for the subsequent action of remediation materials. Subsequently, precise targeted remediation is achieved by applying remediation materials in layers. One to two weeks before sowing, 30 kg / mu to 50 kg / mu of chitosan-magnetic composite biochar is spread on the soil surface. After permeable irrigation, it infiltrates into the deep soil with the water. The abundant hydroxyl and amino functional groups on its surface rapidly adsorb deep free heavy metals through complexation and chelation. At sowing time, apply 60-80 kg / mu of soil remediation agent (charcoal-microbe symbiosis) to the soil layer (0-15 cm) in the densely active root zone of crops. The soil remediation agent not only passivates heavy metals but also leverages the synergistic stress-resistance effect of microorganisms and plant roots. Apply 30-50 kg / mu of chitosan-magnetic composite biochar to the 16-20 cm soil layer. Utilizing the expanded pore network resulting from freeze-thaw cycles and the physical action of surface gravity and water flow, combined with the high specific gravity of chitosan-magnetic composite biochar, it smoothly penetrates and migrates to deeper soil layers. Simultaneously, relying on the efficient aggregation and binding of the magnetic iron oxide components on the surface of the chitosan-magnetic composite biochar with deep soil minerals, it achieves precise anchoring and long-term retention in deep micro-domains, thereby effectively adsorbing and fixing deep heavy metals. Ultimately, this achieves efficient and stable remediation of deep soil in farmland contaminated with heavy metals under freeze-thaw cycles.

[0033] In some embodiments of the present invention, the crop is, exemplarily, soybean.

[0034] In some embodiments of the present invention, preferably, in step M1, the freezing temperature is between -10°C and -30°C, and the melting temperature is between 10°C and 25°C.

[0035] The present invention will be further described below with reference to specific embodiments. The experimental farmland in the present invention is a heavy metal contaminated farmland under a typical seasonal freeze-thaw cycle environment. The experimental farmland is a soybean farmland of Qixingpao Farm in Nenjiang City, Heilongjiang Province. The soil type of the farmland is typical black soil, with a texture of clay loam, a pH value of 5.8, a soil organic matter content of 32.5 g / kg, and an initial average total cadmium content of 3.12 mg / kg, of which the available cadmium content is 1.45 mg / kg due to the activation effect of early spring freeze-thaw; the average total lead content is 410.5 mg / kg, and the available lead content is 185.2 mg / kg.

[0036] Example 1 A1. The corn stalks are washed, dried, and crushed, and then pyrolyzed under a nitrogen atmosphere to obtain biochar; wherein the pyrolysis temperature is 500℃ and the time is 3h.

[0037] A2. Biochar, ferric chloride hexahydrate, and ferrous sulfate heptahydrate are dissolved in water to obtain a first mixture. An alkaline solution is added to the first mixture to adjust the pH to 10-11. A first stirring reaction is performed, the magnetic precipitate is separated and washed until neutral, and then dried to obtain magnetic biochar. Chitosan is dissolved in an acetic acid solution (2% by volume), and the magnetic biochar is added and stirred until homogeneous to obtain a second mixture. The mass ratio of the biochar, ferric chloride hexahydrate, and ferrous sulfate heptahydrate is 2:3.25:1.67. The alkaline solution is a 1 mol / L sodium hydroxide solution. The temperature of the first stirring reaction is 70°C, and the time is 1 h. The mass-to-volume ratio of chitosan to acetic acid solution is (1:40) g / mL. The mass ratio of magnetic biochar to chitosan is 5:4.

[0038] A3. Add glutaraldehyde solution to the second mixture, carry out a second stirring reaction, separate the magnetic products and wash until neutral, dry to obtain chitosan-magnetic composite biochar; the volume fraction of the glutaraldehyde solution is 2%, and the mass ratio of glutaraldehyde in the glutaraldehyde solution to the magnetic biochar is 1:50; the temperature of the second stirring reaction is 60℃ and the time is 4h.

[0039] A4. Mix the composite bacterial solution with sodium alginate solution until homogeneous, then add the chitosan-magnetic composite biochar and stir until homogeneous to obtain a third mixture; add the third mixture to calcium chloride solution, allow to stand and solidify, collect the gel microspheres, wash and dry to obtain the soil remediation agent; wherein, the composite bacterial solution contains Bacillus belyssioides and Pseudomonas fluorescens, and the concentration of Bacillus belyssioides in the composite bacterial solution is 3 × 10⁻⁶. 9 CFU / mL, the concentration of the fluorescent Pseudomonas was 2 × 10⁻⁶. 9 CFU / mL; the mass of the chitosan-magnetic composite biochar is m grams, the volume of the composite bacterial solution is v milliliters, and the ratio of m to v is 5:1; the mass fraction of the sodium alginate solution is 4%, and the volume ratio of the composite bacterial solution to the sodium alginate solution is 1:1; the mass fraction of the calcium chloride solution is 2%, and the volume ratio of the calcium chloride solution to the third mixture is 8:1.

[0040] The biochar, magnetic biochar, and chitosan-magnetic composite biochar in Example 1 were characterized by XRD, and the results are shown in the figure. Figure 1 ,from Figure 1 It can be seen that the chitosan-magnetic composite biochar has been successfully loaded with magnetic components. Figure 1 XRD pattern of biochar corresponding to BC in the middle. Figure 1 XRD pattern of magnetic biochar corresponding to MBC. Figure 1 XRD pattern of chitosan-magnetic composite biochar corresponding to CMBC. Experimental example: Divide one experimental field into 20 plots, each with an area of ​​10m². 2 Seven communities were randomly selected and numbered A1, A2, A3, B, C, D, and E.

[0041] Experimental Group 1 (using farmland in Zone A1 as the experimental subject) Before the soil thaws in spring, the soil layer from the farmland surface to a depth of 50cm underground is subjected to 10 freeze-thaw cycles. The freeze-thaw cycle includes: lowering the temperature of the soil layer to the freezing temperature and holding it for 12 hours, then lowering it to the thawing temperature and holding it for 12 hours. The freezing temperature is -20°C and the thawing temperature is 20°C.

[0042] One week before soybean sowing, the chitosan-magnetic composite biochar prepared in Example 1 was evenly spread on the soil surface of the farmland at the first application amount and then irrigated with water; wherein, the first application amount was 40 kg / mu.

[0043] When soybeans are sown, the soil remediation agent prepared in Example 1 is applied evenly to the soil layer from the surface to a depth of 15cm in the heavy metal contaminated farmland at the second application rate, and the chitosan-magnetic composite biochar prepared in Example 1 is applied evenly to the soil layer from a depth of 16cm to a depth of 20cm in the heavy metal contaminated farmland at the third application rate. After covering with soil, conventional field water and fertilizer management is carried out. The second application rate is 70kg / mu, and the third application rate is 40kg / mu.

[0044] Experimental Group 2 (using farmland in Zone A2 as the experimental subject) Before the soil thaws in spring, the soil layer from the farmland surface to a depth of 50cm underground is subjected to 10 freeze-thaw cycles. The freeze-thaw cycle includes: lowering the temperature of the soil layer to the freezing temperature and holding it for 12 hours, then lowering it to the thawing temperature and holding it for 12 hours. The freezing temperature is -20°C and the thawing temperature is 20°C.

[0045] One week before soybean sowing, the chitosan-magnetic composite biochar prepared in Example 1 was evenly spread on the soil surface of the farmland at the first application amount and then irrigated with water; wherein, the first application amount was 40 kg / mu.

[0046] When soybeans are sown, the soil remediation agent prepared in Example 1 is applied evenly to the soil layer from the surface to a depth of 15cm in the heavy metal contaminated farmland at the second application rate, and the chitosan-magnetic composite biochar prepared in Example 1 is applied evenly to the soil layer from a depth of 16cm to a depth of 20cm in the heavy metal contaminated farmland at the third application rate. After covering with soil, conventional field water and fertilizer management is carried out. The second application rate is 70kg / mu, and the third application rate is 40kg / mu.

[0047] Experimental Group 3 (using farmland in Zone A3 as the experimental subject) Before the soil thaws in spring, the soil layer from the farmland surface to a depth of 50cm underground is subjected to 10 freeze-thaw cycles. The freeze-thaw cycle includes: lowering the temperature of the soil layer to the freezing temperature and holding it for 12 hours, then lowering it to the thawing temperature and holding it for 12 hours. The freezing temperature is -20°C and the thawing temperature is 20°C.

[0048] One week before soybean sowing, the chitosan-magnetic composite biochar prepared in Example 1 was evenly spread on the soil surface of the farmland at the first application amount and then irrigated with water; wherein, the first application amount was 40 kg / mu.

[0049] When soybeans are sown, the soil remediation agent prepared in Example 1 is applied evenly to the soil layer from the surface to a depth of 15cm in the heavy metal contaminated farmland at the second application rate, and the chitosan-magnetic composite biochar prepared in Example 1 is applied evenly to the soil layer from a depth of 16cm to a depth of 20cm in the heavy metal contaminated farmland at the third application rate. After covering with soil, conventional field water and fertilizer management is carried out. The second application rate is 70kg / mu, and the third application rate is 40kg / mu.

[0050] Control group 1 (using farmland in area B as the test subject) The difference between this group and the experimental group is that no repair agent was applied one week before soybean planting and at the time of soybean planting.

[0051] Control group 2 (using farmland in area C as the test subject) One week before soybean sowing, the chitosan-magnetic composite biochar prepared in Example 1 was evenly spread on the soil surface of the farmland at the first application amount and then irrigated with water; wherein, the first application amount was 40 kg / mu.

[0052] When soybeans are sown, the soil remediation agent prepared in Example 1 is applied evenly to the soil layer from the surface to a depth of 15cm in the heavy metal contaminated farmland at the second application rate, and the chitosan-magnetic composite biochar prepared in Example 1 is applied evenly to the soil layer from a depth of 16cm to a depth of 20cm in the heavy metal contaminated farmland at the third application rate. After covering with soil, conventional field water and fertilizer management is carried out. The second application rate is 70kg / mu, and the third application rate is 40kg / mu.

[0053] Control group 3 (using farmland in area D as the test subject) Before the soil thaws in spring, the soil layer from the farmland surface to a depth of 50cm underground is subjected to 10 freeze-thaw cycles. The freeze-thaw cycle includes: lowering the temperature of the soil layer to the freezing temperature and holding it for 12 hours, then lowering it to the thawing temperature and holding it for 12 hours. The freezing temperature is -20°C and the thawing temperature is 20°C.

[0054] One week before soybean sowing, the chitosan-magnetic composite biochar prepared in Example 1 was evenly spread on the soil surface of the farmland at the first application amount and then irrigated with water; wherein, the first application amount was 40 kg / mu.

[0055] When sowing soybeans, the chitosan-magnetic composite biochar prepared in Example 1 is uniformly applied to the soil layer from the surface of the heavy metal-contaminated farmland to a depth of 15cm underground at the second application rate. After covering with soil, conventional field water and fertilizer management is carried out. The second application rate is 70kg / mu. Control group 4 (using farmland in zone E as the test subject) Before the soil thaws in spring, the soil layer from the farmland surface to a depth of 50cm underground is subjected to 10 freeze-thaw cycles. The freeze-thaw cycle includes: lowering the temperature of the soil layer to the freezing temperature and holding it for 12 hours, then lowering it to the thawing temperature and holding it for 12 hours. The freezing temperature is -20°C and the thawing temperature is 20°C.

[0056] When sowing soybeans, the compound bacterial solution in Example 1 is evenly applied to the soil layer from the surface of the heavy metal contaminated farmland to a depth of 15cm underground at the second application rate. After covering with soil, conventional field water and fertilizer management is carried out. The second application rate is 70kg / mu.

[0057] Effect Example The root rot disease index and yield of soybean plants in experimental groups 1-3 and control groups 1-4 were statistically analyzed, and the results are shown in Table 1. The contents of available Cd, available Pb, and carbon-based material retention in the deep soil at harvest time in experimental groups 1-3 and control groups 1-4 were also measured, and the results are shown in Table 1. Table 1 shows that compared with control groups 1-4, the contents of available Cd and available Pb in the deep soil at harvest time in experimental groups 1-3 were lower, indicating a better passivation effect of heavy metals in the deep soil of the experimental groups. Compared with control groups 1-4, the retention of carbon-based materials in the deep soil at harvest time in experimental groups 1-3 was higher, confirming that the chitosan-magnetic composite biochar successfully utilized the pore channels generated by freeze-thaw cycles to achieve targeted downward movement, thereby significantly reducing the available heavy metal content in the deep soil. Compared with control groups 1 to 4, soybean plants in experimental groups 1 to 3 showed lower root rot disease indices and higher yields, verifying that the soil remediation agent (charcoal-bacterial symbiosis) applied to areas with dense root activity not only passivated heavy metals but also significantly improved the root microecology, achieving both soil remediation and increased agricultural yield. It should be noted that the available content of heavy metal elements in this invention was determined using the BCR continuous extraction method. The retention of carbon-based materials was measured using thermogravimetric analysis.

[0058] Table 1

[0059] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A method for preparing a soil remediation agent for heavy metal-contaminated farmland, characterized in that, include: Step S1: Dissolve biochar, ferric chloride hexahydrate, and ferrous sulfate heptahydrate in water to obtain a first mixture; add alkaline solution to the first mixture to adjust the pH to 10 to 11, carry out the first stirring reaction, separate the magnetic precipitate and wash it until neutral, dry it to obtain magnetic biochar; dissolve chitosan in acetic acid solution, add the magnetic biochar, stir evenly to obtain a second mixture; Step S2: Add glutaraldehyde solution to the second mixture, carry out a second stirring reaction, separate the magnetic products and wash until neutral, dry to obtain chitosan-magnetic composite biochar; Step S3: Mix the composite bacterial solution with sodium alginate solution evenly, then add the chitosan-magnetic composite biochar and stir evenly to obtain a third mixture; add the third mixture to calcium chloride solution, let it stand to solidify, collect the gel microspheres, wash and dry them to obtain the soil remediation agent; wherein, the composite bacterial solution contains Bacillus belyssioides and Pseudomonas fluorescens.

2. The preparation method of the heavy metal contaminated farmland soil remediation agent according to claim 1, characterized in that, In step S1, the mass ratio of the biochar, the ferric chloride hexahydrate, and the ferrous sulfate heptahydrate is (1.8 to 2.2): (3.1 to 3.4): (1.5 to 1.7).

3. The preparation method of the heavy metal contaminated farmland soil remediation agent according to claim 1, characterized in that, In step S1, the mass ratio of the magnetic biochar to the chitosan is 5:(3.8 to 4.2).

4. The preparation method of the heavy metal contaminated farmland soil remediation agent according to claim 1, characterized in that, The volume fraction of the glutaraldehyde solution is 1.8% to 2.2%, and the mass ratio of glutaraldehyde in the glutaraldehyde solution to the magnetic biochar is 1:(40 to 60).

5. The preparation method of the heavy metal contaminated farmland soil remediation agent according to claim 1, characterized in that, In step S3, the concentration of *Bacillus belyssioides* in the composite bacterial solution is (2.9 to 3.1) × 10⁻⁶. 9 The concentration of the fluorescent Pseudomonas was (1.9 to 2.1) × 10 CFU / mL. 9 CFU / mL; the mass-to-volume ratio of the chitosan-magnetic composite biochar to the composite bacterial solution is (4 to 6):

1.

6. The preparation method of the heavy metal contaminated farmland soil remediation agent according to claim 1, characterized in that, In step S3, the mass fraction of the sodium alginate solution is 3.5% to 4.5%, and the volume ratio of the composite bacterial solution to the sodium alginate solution is (0.9 to 1.1):(0.9 to 1.1).

7. The preparation method of the heavy metal contaminated farmland soil remediation agent according to claim 1, characterized in that, In step S3, the mass fraction of the calcium chloride solution is 1.8% to 2.2%, and the volume ratio of the calcium chloride solution to the third mixture is (5 to 10):

1.

8. A soil remediation method, characterized in that, include: Step M1: Before the soil thaws in spring, the soil layer in the area from the surface of the heavy metal contaminated farmland to a depth of 50cm underground is subjected to 5 to 20 freeze-thaw cycles. The freeze-thaw cycle includes: lowering the temperature of the soil layer to the freezing temperature and maintaining it for 10 to 14 hours, and then lowering it to the thawing temperature and maintaining it for 10 to 14 hours. Step M2: One to two weeks before crop sowing, apply chitosan-magnetic composite biochar evenly to the soil surface of the heavy metal-contaminated farmland at the first application rate, and irrigate with water; wherein, the first application rate is 30 kg / mu to 50 kg / mu. Step M3: When sowing crops, apply the soil remediation agent evenly to the soil layer from the surface to a depth of 15cm in the heavy metal contaminated farmland according to the second application amount, and apply the chitosan-magnetic composite biochar evenly to the soil layer from a depth of 16cm to a depth of 20cm in the heavy metal contaminated farmland according to the third application amount. After covering with soil, carry out conventional field water and fertilizer management. The second application amount is 60kg / mu to 80kg / mu, and the third application amount is 30kg / mu to 50kg / mu. The chitosan-magnetic composite biochar and the soil remediation agent are prepared by the preparation method described in any one of claims 1 to 7.

9. The soil remediation method according to claim 8, characterized in that, The crop in question is soybean.

10. The soil remediation method according to claim 8, characterized in that, In step M1, the freezing temperature is between -10°C and -30°C, and the melting temperature is between 10°C and 25°C.

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