Remediation method for arsenic-cadmium combined polluted paddy soil

Abstract

The invention discloses a method for repairing arsenic-cadmium combined polluted rice field soil, which comprises the following steps of: reducing the redox potential Eh value of the soil to-100 to-300mV by controlling the height of a flooded water layer of the polluted soil and adding a reducing agent, reducing arsenic in the soil into trivalent arsenic, and simultaneously constructing an indigenous microbial community with high arsenic methylation capability; oxidizing by draining and drying, methylating trivalent arsenic by indigenous microbial communities, and gradually volatilizing arsenic through methylation; and applying a carbon-based functional biological membrane mainly comprising a cadmium passivation flora to stably passivate cadmium. According to the method, the arsenic methylation capacity of microorganisms is improved by precisely regulating and controlling the Eh value, in-situ passivation of cadmium is enhanced through the synergistic effect of the carbon-based carrier and the composite flora, synchronous enhancement of arsenic methylation volatilization and cadmium biological membrane passivation is achieved, the difficulty that an arsenic and cadmium pollution treatment technology is'helpful 'is converted into an acting point of technical breakthrough, and the method is suitable for large-scale popularization and application. The arsenic and cadmium co-treatment of the combined polluted soil is realized.

Description

Technical Field

[0001] The invention belongs to the technical field of contaminated soil remediation, and in particular relates to a method for remediating paddy field soil contaminated by arsenic and cadmium complex. Background Art

[0002] At present, the remediation technologies for arsenic and cadmium pollution in paddy field soil can be divided into physical remediation, chemical remediation, phytoremediation and microbial remediation. Among them:

[0003] (1) Physical remediation technology: The most direct way to remove arsenic and cadmium contamination in paddy soil is through soil replacement. Use uncontaminated or low-contaminated soil to replace the contaminated soil, and then treat or dispose of the contaminated soil in situ.

[0004] Although this method can permanently remove pollution, it is difficult to implement, and if the contaminated soil is not properly handled, it is easy to cause secondary pollution. In addition, methods such as soil surface covering, deep plowing, and electric repair require a lot of manpower and material resources to effectively control the combined pollution of arsenic and cadmium in farmland soil. These methods are not the best choice because they fail to solve the fundamental problem of arsenic and cadmium in the soil, and the treatment process will have a negative impact on the soil structure and ecosystem.

[0005] (2) Chemical remediation technology is: Chemical remediation methods for arsenic and cadmium contamination in paddy field soil include solidification / stabilization, soil leaching, etc., which act on arsenic and cadmium through precipitation, adsorption, redox, catalytic oxidation, hydrolysis, etc., or adjust the pH value and Eh (redox potential) of the soil environment to directly or indirectly convert arsenic and cadmium from highly toxic pollutants into low-toxic or low-mobility chemical forms. In actual remediation, chemical remediation usually involves multiple chemical mechanisms acting simultaneously.

[0006] At present, the commonly used chemical amendments for arsenic are mainly iron-manganese oxides, aluminum oxides, natural organic acids, artificial chelating agents, etc., and the commonly used chemical amendments for cadmium are lime passivators, biochar, etc. Commonly used passivation materials for cadmium-arsenic complex contaminated soils include biomass charcoal, phosphates, metals and their oxides, silicon-containing materials, clay minerals, organic fertilizers, etc. Exogenously applied amendments may directly or indirectly reduce the bioavailability and mobility of metals, but the application of these exogenous amendments may affect soil ecological processes and functions and thus affect soil health. The use of materials that are originally soil components, either alone or in combination, can avoid these problems.

[0007] (3) Phytoremediation technology is: Phytoremediation technology uses hyperaccumulators to absorb, extract, transfer, and fix specific heavy metals to achieve the purpose of removing heavy metal pollutants. However, after the hyperaccumulators are harvested, they are usually incinerated or landfilled, which can easily cause secondary pollution to the environment. In recent years, how to carry out subsequent resource processing of mature plants has gradually become one of the hot topics in research and application.

[0008] In addition, the screening and application of low arsenic accumulation crop varieties is another effective means of safe utilization. By screening and planting crop varieties with extremely low accumulation capacity of arsenic and cadmium, the arsenic and cadmium content of crops can be controlled within a safe range, thereby achieving safe production of arsenic-contaminated soil.

[0009] (4) Microbial remediation technology: Microbial remediation technology has attracted much attention in the field of heavy metal contaminated soil remediation in recent years due to its advantages such as low cost, simple operation and no secondary pollution. For arsenic-contaminated soil, trivalent arsenic can be oxidized to pentavalent arsenic by arsenic-oxidizing bacteria, or iron-oxidizing bacteria can be used to increase the adsorption of arsenic by iron oxides, thereby reducing the bioavailability of arsenic. Autotrophic arsenic-oxidizing bacteria and heterotrophic arsenic-oxidizing bacteria convert As(III) in the soil into As(V) on the one hand, and drive ferrous oxidation and secondary mineralization of Fe(III) on the other hand, thereby achieving the fixation of As(III) and As(V) on the surface of secondary minerals.

[0010] However, in practical applications, due to the adaptability of these special microorganisms to the environment, the microbial passivation of arsenic is subject to many restrictions. For cadmium-contaminated soils, the addition of microbial agents with high cadmium passivation ability is currently a widely used bioremediation method at home and abroad. However, the growth metabolism of functional bacteria and the efficiency of cadmium passivation are limited by environmental conditions such as temperature, moisture, pH, and redox potential, resulting in obvious limitations in the use of microbial agents alone. Soil microorganisms profoundly affect the form and fate of pollutants in the soil through biological transformation.

[0011] At present, there have been many theoretical studies on the biogeochemical cycle of arsenic and cadmium in paddy soil, but the technical research and development of arsenic-cadmium co-management is not yet mature. In the special habitat of flooded and dried paddy fields, the behavior of arsenic / cadmium is almost completely opposite to that of Eh-pH. As the soil Eh decreases, the bioavailability of cadmium decreases, while the bioavailability of arsenic increases. This feature is the difficulty in the remediation of arsenic-cadmium compound contaminated paddy fields. It is difficult to reduce the effectiveness of cadmium and arsenic in soil at the same time with a single agent or a single measure. Commonly used arsenic passivators, iron salts, easily lead to a decrease in soil pH and increase the bioavailability of other cationic heavy metals. Measures such as water management can effectively reduce the bioavailability of cadmium in soil, but increase the migration of arsenic to rice grains. Even after implementing cadmium passivation technology on soils where arsenic does not exceed the standard, rice arsenic may exceed the standard.

[0012] Arsenic-cadmium compound pollution in paddy soil is a hot issue of global concern. At present, the arsenic and cadmium exceeding the standard rates in paddy soil in 19 major rice-producing provinces in my country have reached 6.2% and 33.6%, respectively. Among them, arsenic-cadmium compound pollution in some areas of paddy fields accounts for 16.7% of the rice planting area. Rice is the main food crop in South Asia and Southeast Asia, and the safety of rice is particularly prominent. The development of composite passivators and the coordinated use of multiple safe utilization modes can achieve risk control of composite polluted soil to a certain extent, but there are still unstable passivation efficiency and difficulty in arsenic-cadmium synchronization. The existing remediation methods have low control accuracy. Cadmium and arsenic pollution in paddy fields mostly use water management and the application of chemical passivators to reduce the effectiveness of cadmium and arsenic in paddy fields. However, the physical and chemical properties of soil in paddy fields will change with flooding and drainage, making it difficult to accurately control the existence forms of cadmium and arsenic, and there is a risk of secondary pollution. In the process of removing cadmium and arsenic pollution in paddy fields by plant remediation and chemical passivation, cadmium and arsenic pollution may become effective again due to environmental changes. On the one hand, during the post-treatment of plants, cadmium and arsenic in plants will be released into the environment again, increasing the risk of soil pollution. Moreover, most remediation strategies for cadmium and arsenic pollution are carried out at the laboratory level. A certain removal effect can be achieved at the laboratory level, but when transferred to the field, the level of effect may drop sharply. In addition, cadmium and arsenic pollution in rice field soil is widespread, the cost of remediation is high, and there is a lack of support from a healthy industrial chain and technical guidance. Summary of the invention

[0013] In view of the shortcomings of the prior art, the present invention provides a method for repairing paddy field soil contaminated with arsenic and cadmium. The method achieves the simultaneous enhancement of arsenic methylation volatilization and cadmium biofilm passivation processes, solving the problem of "focusing on one while neglecting the other" in arsenic and cadmium pollution control technology.

[0014] To achieve the purpose of the present invention, the present invention adopts the following technical solutions:

[0015] The present invention provides a method for repairing paddy field soil contaminated by arsenic and cadmium, the method comprising the following steps:

[0016] Reduction cycle: By controlling the height of the flooded water layer of the contaminated soil and adding reducing agents, the redox potential Eh value of the soil is reduced to -100 to -300 mV. Under low redox potential Eh conditions, the arsenic in the soil is reduced to trivalent arsenic. At the same time, an indigenous microbial community with high arsenic methylation ability is constructed in the soil, and part of the cadmium in the soil is converted into a stable form of cadmium sulfide.

[0017] Oxidation cycle: Oxidation is carried out through drainage, and the indigenous microbial community with high arsenic methylation ability in the soil methylates trivalent arsenic, and the arsenic in the soil gradually volatilizes through the methylation process;

[0018] Cadmium stabilization and passivation: Apply a carbon-based functional biofilm composed mainly of cadmium passivating bacteria to chelate or precipitate cadmium into cadmium-extracellular polymers and cadmium phosphate, thereby stabilizing and passivating the cadmium;

[0019] Wherein, in the cadmium stabilization passivation step, the carbon-based functional biofilm mainly composed of cadmium passivation bacteria is prepared by culturing 70-90% of biochar and 10-30% of composite bacterial solution under the condition of adding a carbon source, calculated by mass percentage;

[0020] The composite bacterial solution comprises the following components by mass percentage: 30-50% of cadmium-resistant bacteria, 10-20% of potassium-solubilizing bacteria, 10-20% of phosphate-solubilizing bacteria, 10-20% of nitrogen-fixing bacteria, 10-20% of Pseudomonas aeruginosa, and 5-10% of Bacillus thuringiensis;

[0021] The biochar is prepared by the following method: placing rice straw powder in a pyrolysis furnace, passing a nitrogen atmosphere, and slowly heating the furnace to 450-550°C after about 0.5-1.5 hours, maintaining the temperature for 1.5-2.5 hours, and then slowly lowering the temperature to room temperature, and then washing the obtained biochar with ethanol and ultrapure water respectively, and drying it for use.

[0022] In a preferred embodiment, during the reduction period, the height of the flooded water layer is above 3 cm; preferably, the height of the flooded water layer is 4 to 10 cm; most preferably, the height of the flooded water layer is 5 to 8 cm.

[0023] In a preferred embodiment, during the reduction period, the reducing agent is a 5-15 mmol / L sodium acetate aqueous solution, and the amount of the reducing agent is 0.5-1% of the soil mass.

[0024] In a preferred embodiment, the reduction period is 12 to 20 days; preferably, the reduction period is 13 to 18 days; most preferably, the reduction period is 15 days.

[0025] In a preferred embodiment, during the reduction period, the redox potential Eh value of the soil is reduced to -150 to -250 mV; preferably, the redox potential Eh value of the soil is reduced to -200 mV.

[0026] In a preferred embodiment, the oxidation period is 10 to 30 days; preferably, the oxidation period is 15 to 25 days; most preferably, the oxidation period is 20 days.

[0027] In a preferred embodiment, in the cadmium stabilization passivation step, the amount of the carbon-based functional biofilm mainly composed of cadmium passivating bacteria is 0.5-1% of the soil mass.

[0028] In a preferred embodiment, between the implementation of a reduction-oxidation cycle and the cadmium stabilization passivation step, the arsenic content in the soil is further tested;

[0029] And / or, further testing of the cadmium content in the soil after the addition of the carbon-based functional biofilm for the cadmium stabilization passivation step.

[0030] Compared with the prior art, the present invention has at least the following technical effects:

[0031] (1) The present invention aims to solve the challenge of "co-removal" or "co-passivation" of arsenic and cadmium in composite contaminated soils due to the "increase and decrease" relationship between the bioavailability of arsenic and cadmium. The present invention transforms the challenge into a breakthrough, adopts the idea of ​​"arsenic removal + cadmium passivation", and constructs a technical model for the co-management of arsenic and cadmium pollution in soils, especially in southern rice fields.

[0032] (2) The present invention utilizes the characteristic that the bioavailability of arsenic and cadmium responds rapidly to soil Eh, improves the arsenic methylation ability of microorganisms by precisely controlling the Eh value, and enhances the in-situ passivation of cadmium through the synergistic effect of carbon-based carriers and composite bacterial communities, thereby achieving the simultaneous enhancement of arsenic methylation volatilization and cadmium biofilm passivation processes, converting the difficulty of arsenic and cadmium pollution control technology of "giving priority to one thing and losing another" into the focus of technological breakthroughs, thereby achieving the co-control of arsenic and cadmium in composite contaminated soils.

[0033] (3) The method for repairing rice field soil contaminated by arsenic and cadmium of the present invention realizes production and repair at the same time, and only needs to complete the arsenic methylation removal process during the winter idle field before rice planting, which does not affect agricultural production. The passivation of cadmium only requires the addition of a carbon-based functional biofilm once, and based on the self-reproduction and stable growth of the biofilm, the safe production of rice can be guaranteed. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0035] Figure 1 It is a technical principle diagram of the method for repairing paddy field soil contaminated by arsenic and cadmium compound of the present invention;

[0036] Figure 2 It is a diagram of the implementation process of the method for repairing paddy field soil contaminated by arsenic and cadmium of the present invention;

[0037] Figure 3 It is a schematic diagram of the residual amount of arsenic in the soil after one reduction-oxidation cycle of the present invention;

[0038] Figure 4 It is a schematic diagram of the residual amount of arsenic in the soil after two reduction-oxidation cycles of the present invention;

[0039] Figure 5 It is a schematic diagram of the bioavailable cadmium residue in the soil after the cadmium stabilization passivation is implemented in the present invention. DETAILED DESCRIPTION

[0040] The embodiments of the present invention are described in detail below. The embodiments are given to better illustrate the contents of the present invention and are only used to explain the present invention, but should not be understood as limiting the present invention.

[0041] If no specific techniques or conditions are specified in the examples, the techniques or conditions described in the literature in the field or the product instructions are used. If no manufacturer is specified for the reagents or instruments used, they are all conventional products that can be purchased through regular channels.

[0042] according to Figure 1 The technical schematic diagram of the remediation method for paddy field soil contaminated by arsenic and cadmium is shown. The present invention utilizes the characteristic that the bioavailability of arsenic and cadmium responds rapidly to the redox potential Eh of the soil to accurately control the soil environment Eh. During the flooding period, the arsenic in the soil is reduced to trivalent arsenic by controlling the height of the flooded water layer and adding a reducing agent. At the same time, an indigenous microbial community with high arsenic methylation ability is constructed in the soil, and part of the cadmium in the soil is converted into a stable form of cadmium sulfide. During the drying period, the arsenic gradually volatilizes through the methylation process, and then the cadmium is chelated or precipitated through a carbon-based functional biofilm to form stable forms such as cadmium-extracellular polymers and cadmium phosphate, thereby achieving the simultaneous enhancement of arsenic methylation volatilization and cadmium biofilm passivation, solving the problem of "losing one while focusing on the other" in arsenic and cadmium pollution control technology.

[0043] Example 1: A method for repairing paddy field soil contaminated by arsenic and cadmium, comprising the following specific steps:

[0044] Reduction cycle: The height of the flooded water layer of the contaminated soil is controlled to 5 cm, and a 10 mmol / L sodium acetate aqueous solution of 0.8% of the soil mass is applied to the contaminated soil. The redox potential Eh value of the soil is reduced to -200 mV within 2 days. Under this redox potential Eh condition, the arsenic in the soil is reduced to trivalent arsenic, and an indigenous microbial community with high arsenic methylation ability is constructed in the soil; the reduction cycle is to keep the flooding for 15 days to ensure that the methylation ability of the microorganisms is fully improved; during this reduction cycle, part of the cadmium is converted into stable forms such as cadmium sulfide under this redox potential Eh condition;

[0045] Oxidation cycle: Oxidation is carried out through drainage. The indigenous microbial community with high arsenic methylation ability in the soil methylates trivalent arsenic. The arsenic in the soil gradually volatilizes through the methylation process. The oxidation cycle lasts for about 20 days.

[0046] After a reduction-oxidation cycle is completed, the arsenic content in the soil is tested to determine whether the arsenic pollution removal requirements are met; if the requirements are met, the next stage of cadmium stabilization passivation step is carried out; if not, the above reduction-oxidation cycle is repeated;

[0047] Cadmium stabilization and passivation: When the arsenic removal reaches the target value, a carbon-based functional biofilm composed mainly of cadmium passivating bacteria with a soil mass of 0.8% is applied to stabilize and passivate the cadmium by chelating or precipitating the cadmium into cadmium-extracellular polymers and cadmium phosphate;

[0048] After the cadmium stabilization passivation step, the soil was tested for cadmium levels;

[0049] When the arsenic and cadmium contents in the soil meet the requirements of safe production, double-season rice planting is carried out, and the cadmium and arsenic contents in the soil-rice are regularly monitored during the planting process to ensure the safe production of rice.

[0050] The carbon-based functional biofilm mainly composed of cadmium passivating bacteria is prepared by culturing 80% of biochar and 20% of composite bacterial solution with the addition of a carbon source;

[0051] The composite bacterial solution comprises the following components by mass percentage: 40% of cadmium-resistant bacteria, 15% of potassium-solubilizing bacteria, 15% of phosphate-solubilizing bacteria, 10% of nitrogen-fixing bacteria, 15% of Pseudomonas aeruginosa, and 5% of Bacillus thuringiensis;

[0052] The biochar is prepared by the following method: placing rice straw powder in a pyrolysis furnace, passing a nitrogen atmosphere, and slowly heating it up to 500°C after about 1 hour, maintaining the temperature for 2 hours, and then slowly lowering the temperature to room temperature, and then washing the obtained biochar with ethanol and ultrapure water respectively, and drying at 70°C for standby use.

[0053] Example 2: Field test of the method for remediating rice field soil contaminated by arsenic and cadmium according to Example 1

[0054] Taking the double-season rice planting mode as an example, the implementation process of the technical solution of the present invention is explained (eg Figure 2 The planting time for double-season rice is from the end of March to mid-July and from mid-July to the end of October.

[0055] The test site is located in a rice field contaminated by arsenic and cadmium in Hengyang City, Hunan Province. The soil is mainly paddy soil developed from granite parent material, with a pH value of 5.1-6.1, and the main farming methods are double-season rice and rice-oil rotation.

[0056] The winter idle field period from early November to late March is used as the starting stage of the restoration method of the present invention. Rice fields with arsenic-cadmium compound pollution in the soil of 47.3 mg / kg and cadmium content of 0.3 mg / kg, and rice fields with arsenic-cadmium compound pollution in the soil of 104.2 mg / kg and cadmium content of 0.6 mg / kg are selected as test fields, and then the method of Example 1 is used to repair the arsenic-cadmium compound pollution of rice fields. When the arsenic and cadmium contents in the paddy field soil meet the requirements of safe production, double-season rice is planted.

[0057] Determination of arsenic and cadmium content in soil

[0058] Weigh 0.3g of sample into a crucible, drip experimental water along the inner wall to wet the sample, add 10ml of hydrochloric acid, and heat at 90℃~100℃ on a hot plate in a fume hood to decompose the sample preliminarily. When the contents evaporate to about 5ml remaining, add 15ml of nitric acid, heat at 120℃~140℃ until there are no obvious particles, add 5ml of hydrofluoric acid, and heat at 120℃~140℃ until the contents are in a non-flowing viscous state. In order to achieve a good flying silicon effect, the crucible should be shaken frequently during heating. Remove the crucible, cool to room temperature, add 1ml of perchloric acid, and continue to heat at 160℃~180℃ until the white smoke is almost gone and the contents are in a viscous state. Remove the crucible, cool it slightly, add a small amount of nitric acid solution to rinse the inner wall of the crucible, dissolve the contents by warming, cool to room temperature, transfer to a 50 ml volumetric flask, wash the inner wall of the crucible repeatedly with a small amount of nitric acid solution, transfer the washing liquid to the volumetric flask, make up to the mark with nitric acid solution, shake well, store in a polyethylene bottle, and use an inductively coupled plasma optical emission spectrometer (ICPOES) to determine the cadmium and arsenic contents in the solution.

[0059] Experimental Results

[0060] (1) Arsenic removal rate in soil after different times of reduction-oxidation cycles

[0061] After one reduction-oxidation cycle, the arsenic removal rate in the contaminated paddy soil was 12.2% ( Figure 3 ); after two reduction-oxidation cycles, the arsenic removal rate in the contaminated paddy soil was 25.7% (as shown in Figure 4 After 6 reduction-oxidation cycles, the arsenic removal rate in the contaminated paddy soil was 71.6%.

[0062] (2) Cadmium passivation efficiency

[0063] The carbon-based functional biofilm containing cadmium-passivating bacteria prepared in Example 1 was used for treatment, and 73.1% of the cadmium (such as Figure 5 shown).

[0064] In summary, the remediation method of the present invention can ensure that the arsenic content and cadmium content in the arsenic-cadmium composite-contaminated paddy field soil meet the requirements for safe rice production.

[0065] Obviously, the above embodiments are only examples for clearly explaining the present invention, and are not intended to limit the implementation methods of the present invention. Those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for repairing paddy field soil contaminated by arsenic and cadmium, characterized in that: The repair method comprises the following steps: Reduction cycle: By controlling the height of the flooded water layer of the contaminated soil and adding reducing agents, the redox potential Eh value of the soil is reduced to -100 to -300 mV. Under low redox potential Eh conditions, the arsenic in the soil is reduced to trivalent arsenic. At the same time, an indigenous microbial community with high arsenic methylation ability is constructed in the soil, and part of the cadmium in the soil is converted into a stable form of cadmium sulfide. Oxidation cycle: Oxidation is carried out through drainage, and the indigenous microbial community with high arsenic methylation ability in the soil methylates trivalent arsenic, and the arsenic in the soil gradually volatilizes through the methylation process; Cadmium stabilization and passivation: Apply a carbon-based functional biofilm composed mainly of cadmium passivating bacteria to chelate or precipitate cadmium into cadmium-extracellular polymers and cadmium phosphate, thereby stabilizing and passivating the cadmium; Wherein, in the cadmium stabilization passivation step, the carbon-based functional biofilm mainly composed of cadmium passivation bacteria is prepared by culturing 70-90% of biochar and 10-30% of composite bacterial solution under the condition of adding a carbon source, calculated by mass percentage; The composite bacterial solution comprises the following components by mass percentage: 30-50% of cadmium-resistant bacteria, 10-20% of potassium-solubilizing bacteria, 10-20% of phosphate-solubilizing bacteria, 10-20% of nitrogen-fixing bacteria, 10-20% of Pseudomonas aeruginosa, and 5-10% of Bacillus thuringiensis; The biochar is prepared by the following method: placing rice straw powder in a pyrolysis furnace, passing a nitrogen atmosphere, and slowly heating the furnace to 450-550°C after about 0.5-1.5 hours, maintaining the temperature for 1.5-2.5 hours, and then slowly lowering the temperature to room temperature, and then washing the obtained biochar with ethanol and ultrapure water respectively, and drying it for use.

2. The repair method according to claim 1, characterized in that: During the reduction period, the height of the flooded water layer is above 3 cm; preferably, the height of the flooded water layer is 4 to 10 cm; most preferably, the height of the flooded water layer is 5 to 8 cm.

3. The repair method according to claim 1, characterized in that: In the reduction cycle, the reducing agent is a 5-15 mmol / L sodium acetate aqueous solution, and the amount of the reducing agent used is 0.5-1% of the soil mass.

4. The repair method according to claim 1, characterized in that: The reduction period is 12 to 20 days; preferably, the reduction period is 13 to 18 days; most preferably, the reduction period is 15 days.

5. The repair method according to claim 1, characterized in that: During the reduction period, the redox potential Eh value of the soil is reduced to -150 to -250 mV; preferably, the redox potential Eh value of the soil is reduced to -200 mV.

6. The repair method according to claim 1, characterized in that: The oxidation cycle is 10 to 30 days; preferably, the oxidation cycle is 15 to 25 days; most preferably, the oxidation cycle is 20 days.

7. The repair method according to claim 1, characterized in that: In the cadmium stabilization passivation step, the amount of the carbon-based functional biofilm mainly composed of cadmium passivation bacteria is 0.5-1% of the soil mass.

8. The repair method according to claim 1, characterized in that: After one reduction-oxidation cycle and between the cadmium stabilization passivation steps, the soil was further tested for arsenic content; And / or, further testing of the cadmium content in the soil after the addition of the carbon-based functional biofilm for the cadmium stabilization passivation step.

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