Method for double enrichment of heavy metal cadmium

Abstract

The invention discloses a method for double enrichment of heavy metal cadmium, and belongs to the technical field of agriculture. Farmland soil polluted by heavy metal cadmium is sampled and analyzed, the soil is turned over, and the application amount of fertilizer is determined according to the cadmium pollution degree in the soil and the fertility condition of the soil; the method comprises the following steps: uniformly applying a fertilizer to a farmland in a broadcast application manner, performing shallow ploughing after the fertilizer is applied, and adding a composition of humic acid, clay minerals and carbonate into the fertilizer; plants rich in heavy metal cadmium are planted in the farmland, and the heavy metal cadmium is doubly enriched by utilizing the enrichment capacity of the plants and calcium carbonate in the fertilizer; collecting soil samples and plant samples in different stages of a plant growth cycle; measuring the content of cadmium, and analyzing the enrichment effect of the plants on cadmium; the content of cadmium in soil and plants is accurately measured by using the atomic absorption spectrometry, and the method is relatively low in cost and environment-friendly, can improve the soil structure and fertility to a certain extent, and provides an effective way for remediation of cadmium-polluted soil and agricultural sustainable development.

Description

Technical Field

[0001] The present invention relates to the technical field of agriculture, and particularly relates to a method for double enrichment of heavy metal cadmium. Background Art

[0002] With the rapid development of industry, heavy metal cadmium pollution has become a severe environmental problem globally. Cadmium is highly toxic, bioaccumulative, and persistent, and can enter the human body through the food chain, causing various serious diseases such as kidney function damage, bone lesions, and cancer, posing a huge threat to the ecological environment and human health. In the agricultural field, cadmium-polluted soil leads to reduced crop yields and deteriorated quality, further endangering food security. Therefore, efficient treatment of cadmium pollution and reduction of its mobility and bioavailability in the environment have become the focus of current research.

[0003] Currently, there are numerous methods for treating cadmium pollution, but each has certain limitations: Traditional physical remediation methods, such as soil replacement and soil washing, can significantly reduce the cadmium content in soil in the short term, but they involve huge engineering quantities, high costs, and cause serious damage to the original soil ecosystem, making it difficult to be widely applied on a large scale. For example, in large-scale farmland remediation, the soil replacement method requires a large amount of high-quality soil resources, not only with high transportation costs but also causing ecological damage to the soil-taking areas.

[0004] In chemical remediation methods, the chemical leaching method uses chemical reagents to dissolve and elute cadmium in the soil, but it is prone to causing soil nutrient loss and damaging the soil structure. Moreover, if the leaching solution is not properly treated, it will cause secondary pollution. For example, some acidic leaching solutions will reduce the soil pH value, leading to soil compaction and affecting soil microbial activity and plant growth. The chemical precipitation method can form cadmium precipitates and reduce its bioavailability, but it will produce a large amount of sludge that is difficult to handle, with high subsequent disposal costs and a high risk of secondary pollution. For instance, using the lime precipitation method to treat cadmium-polluted soil will produce a large amount of calcium- and cadmium-containing precipitates, and the treatment and disposal of these sludges become a problem.

[0005] In bioremediation methods, although microbial remediation has the advantages of environmental friendliness and low cost, its remediation cycle is long, and microorganisms have strict requirements for environmental conditions. For example, small changes in temperature, pH value, etc. may affect their activity, resulting in unstable remediation effects. For example, the ability of some microorganisms to transform and immobilize cadmium will decrease significantly in low-temperature or high-pH environments.

[0006] In phytoremediation, there are problems with low efficiency in single-plant phytoremediation: Some hyperaccumulator plants have small biomass, slow growth, and long remediation times; some plants grow rapidly but have limited cadmium enrichment ability. Moreover, single-plant phytoremediation is difficult to cope with complex polluted soil environments and cannot fully exert the comprehensive remediation effect. Summary of the Invention

[0007] The object of the present invention is to address the deficiencies in the prior art, such as huge engineering quantities, high costs, serious damage to the original soil ecosystem, difficulty in large-scale popularization and application, high transportation costs, ecological damage in the soil extraction area, loss of soil nutrients, destruction of soil structure, and if the leaching solution is not properly treated, it will cause secondary pollution. Acidic leaching solutions will reduce the soil pH value, leading to soil compaction, affecting soil microbial activity and plant growth, generating a large amount of sludge that is difficult to handle, with high subsequent disposal costs and easy to cause secondary pollution risks, generating a large amount of calcium- and cadmium-containing precipitates, and the treatment and disposal of these sludges becoming a problem, unstable remediation effects, small biomass and slow growth of hyperaccumulator plants, long remediation times, single-plant remediation being difficult to cope with complex polluted soil environments, and unable to fully exert the comprehensive remediation effect. The present application provides a method for dual enrichment of heavy metal cadmium, which reduces the pollution of heavy metal cadmium in the environment through the physical adsorption and chemical reaction of cadmium by planting plants that enrich heavy metal cadmium and calcium carbonate in the fertilizer.

[0008] In order to achieve the above-mentioned invention object, the present invention adopts the following technical solutions to achieve:

[0009] A method for dual enrichment of heavy metal cadmium, comprising the following steps:

[0010] First step: Sampling and analyzing the farmland soil polluted by heavy metal cadmium. Use the grid method to set a group of sampling points in the farmland, with a sampling depth of 10 cm. Use a soil sampler to collect soil samples, mix the collected soil samples evenly, and use atomic absorption spectrometry to measure the initial content of cadmium in the soil to determine the pollution degree and distribution of cadmium in the soil. According to the initial pH value of the soil, add lime to the soil, and the addition amount is calculated according to the buffer capacity and initial pH value of the soil. The specific calculation formula: Lime addition amount = [(target pH - initial pH) × buffer capacity × soil bulk density × plow layer depth] / (lime neutralization value × 1000), and adjust the soil pH value to 6.5 - 7.5;

[0011] Second step: Plow the soil, with a plowing depth of 10 - 15 cm, to loosen the soil and improve soil air permeability and water permeability;

[0012] Third step: Prepare a fertilizer containing calcium carbonate component, and control the mass content of calcium carbonate in the fertilizer at 5% - 10%;

[0013] Step 4. Determine the application rate of the fertilizer according to the cadmium pollution level and soil fertility status in the soil: When the cadmium content in the soil is at a low pollution level of 0.3 mg / kg - 1.0 mg / kg, the fertilizer application rate is 2.5 g per square meter; when the cadmium content in the soil is at a medium pollution level of 1 mg / kg - 3 mg / kg, the fertilizer application rate is 5.0 g per square meter; when the cadmium content in the soil is at a high pollution level above 3 mg / kg, the fertilizer application rate is 7.5 g per square meter and above;

[0014] Step 5: Apply the fertilizer evenly to the farmland by broadcasting, and then conduct shallow tillage after application. The shallow tillage depth is 10 - 15 cm to fully mix the fertilizer with the soil;

[0015] Step 6: During the fertilizer application process, add a composition of humic acid, clay minerals, and carbonates and the fertilizer according to the mass ratio of humic acid: clay minerals: carbonates: fertilizer = 1:4:8:195 to enhance the overall performance of the fertilizer and enhance the adsorption and transformation of cadmium by carbonates;

[0016] Step 7: Plant plants that can accumulate heavy metal cadmium in the farmland, and the selected plant is corn;

[0017] Step 8: Utilize the accumulation ability of plants and calcium carbonate in the fertilizer to achieve double accumulation of heavy metal cadmium;

[0018] Step 9: At different stages of the plant growth cycle, namely 30 days, 60 days, and 90 days after planting, collect soil samples and plant samples from the set areas respectively;

[0019] Step 10: After collecting the plant samples, rinse the plant samples with deionized water to remove the soil and impurities on the surface. Dry the washed plant samples in an oven at a temperature of 70°C until the plant samples reach a constant weight, and then grind them into fine powder using a mortar to measure the cadmium content and analyze the cadmium accumulation effect of the plants;

[0020] The eleventh step: precisely determine the cadmium content in soil and plants using atomic absorption spectrometry; if the cadmium content in the treated soil is significantly lower than before treatment, such as a reduction of 30% or more, or the cadmium content in the treated soil is lower than the content before treatment minus three times the standard deviation of the cadmium content before treatment, it indicates that this method is effective in reducing the cadmium content in soil; if the cadmium enrichment in the plants in the treated area is higher than that in the plant control area by a certain amount, such as the cadmium content in each kilogram of plant dry weight is more than 1 mg / kg higher than that in the plant control area, and at the same time, the reduction rate of the cadmium content in the treated soil is greater than that in the fertilizer control area, such as the reduction rate is more than 10% higher than that in the fertilizer control area, it can also reflect the effectiveness of this method; if after multiple maize growth cycles of treatment, the cadmium content in the soil continues to decrease and finally reaches or is lower than the environmental standard value, and the cadmium content in the subsequent non-enriched plants is also within the safe range, it shows that this method has long-term effectiveness and stability and can achieve the sustainable remediation of cadmium-contaminated farmland and the effective enrichment of cadmium.

[0021] Furthermore, the planting density of the plants in the seventh step is adjusted according to the cadmium pollution degree of the soil. When the cadmium content in the soil is at a low pollution level of 0.3 mg / kg - 1.0 mg / kg, the plant planting density is 4 plants per square meter; when the cadmium content in the soil is at a medium pollution level of 1 mg / kg to 3 mg / kg, the plant planting density is 5 plants per square meter; when the cadmium content in the soil is at a high pollution level above 3 mg / kg, the plant planting density is 6 plants per square meter; after planting maize, irrigation and fertilization management are carried out on the farmland. Drip irrigation is used for irrigation, and the irrigation amount per time is 30 liters per square meter, and the fertilization cycle is once every 10 days.

[0022] In the eighth step, a promoter is also added to the soil, such as a microbial inoculant, to promote the adsorption and transformation of cadmium by calcium carbonate. The microbial inoculant is a strain with the ability to secrete organic acids, produce siderophores or have cadmium ion reduction ability, such as Bacillus, which can secrete organic acids such as citric acid and malic acid, reduce the local pH value of the soil, enhance the solubility of calcium carbonate, and release more carbonate ions to combine with cadmium; the addition dosage needs to be determined according to the soil pollution degree and the activity of the microbial inoculant: for slightly cadmium-polluted soil with a cadmium content of 0.3 mg / kg - 1.0 mg / kg, the microbial inoculant with an effective viable count of 1×10 8 -5×10 8 CFU can be added per square meter; for moderately polluted soil with a cadmium content of 1 mg / kg - 3 mg / kg, 5×10 8 -1×10 9 CFU is added per square meter; for severely polluted soil with a cadmium content above 3 mg / kg, 1×10 9 -5×10 9CFU, Method for adding microbial inoculant: After uniformly mixing the microbial inoculant and the carrier at a mass ratio of 1:7, where the carrier is vermiculite and / or peat soil, it is then spread together with the fertilizer onto the farmland, followed by shallow tillage to ensure full contact among the inoculant, the fertilizer, and the soil.

[0023] Furthermore, in the eighth step, heavy metal cadmium exists in the form of cadmium ions, which are active and migratory. Specifically, it comes from water-soluble cadmium and / or exchangeable cadmium in the soil. Cadmium ions will be released into the soil solution and then react with calcium in the form of solid particles in the soil. The calcium in the form of solid particles is calcium carbonate compound.

[0024] Furthermore, in the eighth step, the redox potential of the soil is controlled between -100 mV and 300 mV. A surface regulator with a mass of 0.1% - 0.5% of the fertilizer mass is added to the fertilizer. The surface regulator is a compound of iron ions and aluminum ions, which enhances the adsorption stability and chemical conversion efficiency of calcium carbonate to cadmium, making it easier for cadmium ions to undergo a displacement reaction with calcium carbonate. According to the chemical reaction formula A more stable CdCO3 compound is formed, and a new phase of (Ca,Cd)CO3 is formed on the surface of calcium carbonate. This new phase is a solid solution phase, in which cadmium atoms replace some calcium atoms to form a carbonate structure. Moreover, this new phase passivates the surface of calcium carbonate, reducing the dissolution and migration of cadmium, solidifying cadmium, and preventing it from re-entering the environment.

[0025] Furthermore, the measurement of the redox potential must use a redox potentiometer. Insert its electrode into the soil at a depth of 10 - 15 cm, and regularly measure the soil redox potential every 2 - 3 days to obtain real-time data. If the soil redox potential exceeds 300 mV, it is not conducive to the reaction between calcium carbonate and cadmium. The potential can be reduced by adding organic materials such as adding straw and / or green manure, with a application rate of 1 - 2 kg per square meter. Organic materials consume oxygen during decomposition, making the soil tend to a reducing state. If the potential is lower than -100 mV, tillage can be carried out to increase soil aeration and raise the redox potential. The tillage depth is controlled at 10 - 15 cm, and tillage is carried out every 7 - 10 days until the potential reaches -100 - 300 mV.

[0026] Further, the method for collecting plant samples in the ninth step is to sample different parts of maize, including roots, stems, leaves, and grains, during the filling stage and maturity stage of maize growth; record the growth indicators of the plants in detail. The growth indicators are plant height, stem diameter, leaf area, and biomass. Analyze the change trend of cadmium enrichment in the plants as the plant growth indicators change. At the same time, analyze data such as the change in cadmium content in the soil and the cadmium content in the leaching solution, and evaluate the effect of this method on reducing soil cadmium content, reducing cadmium leaching loss, and the cadmium enrichment effect of plants; as the plant growth indicators develop well, the cadmium enrichment in the plants also shows a stable increasing trend. During the growth of maize, for every 10 cm increase in plant height, the cadmium enrichment in the above-ground part increases by an average of 0.2 mg / kg.

[0027] Further, the sampling and sample preparation process of atomic absorption spectrometry in the eleventh step is as follows:

[0028] Step a: Collect soil samples in different regions according to the sampling method in the first step, with a sampling depth of 10 cm;

[0029] Step b: After mixing a group of collected soil samples evenly, air-dry them in a well-ventilated room, avoiding direct sunlight and dust pollution. The air-drying time is 5 days;

[0030] Step c: Grind the air-dried soil samples in a mortar to a fine powder, and then sieve them using a sieve with a pore size of 100 mesh;

[0031] Step d: Put 0.9 - 1.1 g of soil samples into a polytetrafluoroethylene digestion tank, add mixed acid A composed of 5 - 7 ml of nitric acid, 3 - 5 ml of hydrochloric acid, and 1 - 2 ml of perchloric acid, and then digest them on a hot plate. The heating temperature is controlled at 150 - 200 °C, and the digestion time is 4 - 5 hours until the samples become clear and transparent solutions;

[0032] Step e: After the digested soil sample solution cools to room temperature, transfer it to a volumetric flask and make up the volume to 100 ml with deionized water to obtain the test solution of the soil sample;

[0033] Step f: Collect plant samples, including above-ground and underground parts, 30 days, 60 days, and 90 days after planting the plants. After collection, rinse the plant samples with deionized water to remove the soil and impurities on the surface;

[0034] Step g: Dry the washed plant samples in an oven at a temperature of 70 °C until the plant samples reach a constant weight;

[0035] Step h: Grind the dried plant samples in a mortar to a fine powder, and then sieve them using a sieve with a pore size of 100 mesh;

[0036] Step i: Put 0.9 - 1.1 g of sample powder into a digestion container, add mixed acid B composed of 5 - 7 ml of nitric acid and 2 - 3 ml of sulfuric acid, digest on a heating device at a heating temperature of 120 - 180 °C for 2 - 4 hours until the sample is completely digested and the solution is clear and transparent;

[0037] Step j: After the digested plant sample solution is cooled, transfer it to a volumetric flask and make up the volume to 100 ml with deionized water to prepare a test solution of the plant sample;

[0038] Step k: Aspirate the prepared test solution of the soil or plant sample into the nebulizer of an atomic absorption spectrometer;

[0039] Step l: The solution is converted into an aerosol in the nebulizer and then enters the acetylene-air flame;

[0040] Step m: In the flame, the atoms of cadmium element are excited to produce an absorption spectrum, and the instrument determines the content of cadmium element by detecting the absorbance at a specific wavelength of 228.8 nm;

[0041] Step n: Calculate the content of cadmium in the sample according to the calibration curve and the absorbance value of the sample;

[0042] Step o: Record the absorbance value, measurement time, and sample number information of each sample;

[0043] Step p: According to the regression equation of the calibration curve, calculate the content of cadmium in the soil or plant sample. The calculation formula is: C = (A - b) / a, where C is the concentration of cadmium in the sample, in mg / L, A is the absorbance of the sample, a is the slope of the calibration curve, and b is the intercept of the calibration curve. Then, according to the fixed volume of the sample and the weighed sample amount, calculate the mass fraction of cadmium in the soil or plant, in mg / kg.

[0044] Further, in the said Step p, the derivation steps of the calculation formula are as follows:

[0045] Step 1: In atomic absorption spectrometry, the absorbance A and the concentration C of the element to be measured in the sample conform to the Lambert-Beer Law; the expression of the Lambert-Beer Law is A = εlC, where ε is the molar absorptivity, l is the optical path length, and C is the concentration of the element in the sample;

[0046] Step 2: For the drawing of the calibration curve, by measuring the absorbances A of a series of standard solutions with known concentrations C, the linear relationship between the absorbance and the concentration can be obtained. Usually, the equation of the calibration curve can be expressed as A = aC + b, where a is equivalent to εl and b is the intercept;

[0047] Step 3: After measuring the absorbance of the sample, to calculate the concentration of cadmium in the sample, it is necessary to solve the calibration curve equation A = aC + b. By transposing, we can get C = (A - b) / a.

[0048] Further, in Step 2, a cadmium standard solution with a concentration of 1000 mg / L is prepared, and then diluted with deionized water into standard working solutions with different concentrations of 0.1 mg / L, 0.2 mg / L, 0.5 mg / L, and 1 mg / L. The standard working solutions are successively aspirated into the atomic absorption spectrometer, and the corresponding absorbance values are recorded; with the concentration of the cadmium standard solution as the abscissa and the absorbance value as the ordinate, a calibration curve is plotted. The calibration curve should be a straight line passing through the origin, and its linear correlation coefficient is required to reach above 0.999.

[0049] Compared with the prior art, the beneficial effects of the present invention include:

[0050] 1. The method for double enrichment of heavy metal cadmium in the present invention has significant innovative points. This method innovatively combines plants for enriching heavy metal cadmium with fertilizers containing calcium carbonate to achieve double enrichment of cadmium.

[0051] 2. On the one hand, using the natural enrichment ability of plants, cadmium in the soil is transferred into the plants; on the other hand, through the physical adsorption and chemical reaction between calcium carbonate in the fertilizer and cadmium, cadmium forms more stable compounds, reducing its dissolution and migration.

[0052] 3. This dual action mechanism greatly improves the enrichment efficiency of cadmium and reduces the content of cadmium in the soil.

[0053] 4. This method has a low cost, is environmentally friendly, and can improve the soil structure and fertility to a certain extent, providing a new effective way for the remediation of cadmium - polluted soil and the sustainable development of agriculture.

[0054] 5. Comparison of experimental data with the existing steel slag remediation method:

[0055] Short - term remediation efficiency: After one cycle of steel slag treatment, the average cadmium content in the soil is reduced by 20% - 25% (such as the experimental data of a cadmium - polluted paddy field in Hunan), while in Example 1 of the present invention, a cadmium removal rate of 35% is achieved, which is 75% higher than that of the steel slag method.

[0056] Plant enrichment ability: The cadmium enrichment amount in the above - ground part of plants in the steel slag treatment area is usually 0.8 - 1.0 mg / kg (dry weight), while in Example 1 of the present invention, it reaches 1.5 mg / kg, with an increase of more than 50%.

[0057] Cost - benefit: The cost of steel slag treatment is about 1200 - 1500 yuan per mu (including transportation and tillage costs). By reducing the use of chemical reagents, the cost of the present invention is controlled within 800 yuan per mu, a reduction of 33%.

[0058] Environmental risk: The treatment of steel slag easily causes the soil pH value to rise above 8.0, leading to soil compaction. However, through the regulation of humic acid and microorganisms, the present invention stabilizes the pH value at 6.5 - 7.5;

[0059] Leaching loss: The cadmium leaching loss rate in the steel slag treatment area reaches 15% - 20%. Through the formation of carbonate solid solution, the present invention controls the leaching loss below 5%. Detailed implementation mode

[0060] The present invention will be further elaborated below in combination with specific embodiments.

[0061] Example 1

[0062] A method for double enrichment of heavy metal cadmium includes the following steps:

[0063] First step: In the farmland, divide multiple experimental areas with an area of 25 square meters each. Among them, set a treatment area, a plant control area (only planting plants), and a fertilizer control area (only applying fertilizers), with 3 replicates in each area; sample and analyze the farmland soil polluted by heavy metal cadmium. Use the grid method to set 25 sampling points in the farmland, with a sampling depth of 10 cm. Use a soil sampler to collect soil samples, mix the collected soil samples evenly, and divide them into two parts: one part is used to determine the initial cadmium content in the soil by atomic absorption spectrometry to determine the cadmium pollution degree and distribution in the soil, and the other part is used to detect the initial pH value of the soil. After measurement, the average initial cadmium content in this farmland soil is 1.8 mg / kg, belonging to the medium pollution level, and the initial pH value is 6.0. According to the initial pH value of the soil, add 0.5 kg of lime to the soil. The addition amount = [(target pH - initial pH) × buffer capacity × soil bulk density × plow layer depth] / (lime neutralization value × 1000), and adjust the soil pH value to 6.5 - 7.5;

[0064] Second step: Use a plow attached to a tractor to plow the entire farmland soil to a depth of 12 cm, making the soil loose, improving soil air permeability and water permeability, ensuring that subsequent fertilizers and plant roots can better contact the soil, and at the same time ensuring that the migration and reaction of calcium carbonate and cadmium ions in the soil are more sufficient;

[0065] Third step: Prepare a fertilizer containing calcium carbonate, and control the calcium carbonate mass content in the fertilizer at 7%;

[0066] Fourth step: According to the cadmium pollution degree and soil fertility status in the soil, determine the fertilizer application amount: when the cadmium content in the soil is at the medium pollution level of 1 mg / kg - 3 mg / kg, the fertilizer application amount is 5.0 g per square meter;

[0067] Step 5: Manually apply the fertilizer evenly to the farmland in the treatment area and the fertilizer control area, and then use a small rotary tiller for shallow tillage with a depth of 12 cm to fully mix the fertilizer with the soil;

[0068] Step 6: During the fertilizer application process, add a composition of humic acid, clay minerals, and carbonate to the fertilizer according to the mass ratio of humic acid:clay minerals:carbonate:fertilizer = 1:4:8:195 to enhance the overall performance of the fertilizer and enhance the adsorption and transformation of cadmium by the carbonate;

[0069] Step 7: Plant plants that can enrich heavy metal cadmium in the farmland. The selected plants are required to have a certain cadmium enrichment ability, can accumulate more absorbed cadmium in the roots, stems and leaves, transfer less to the fruits, and have low requirements for soil fertility. The selected plant is corn. Since the cadmium content in the soil is at a medium pollution level, the plant planting density is determined to be 5 plants per square meter; Drip irrigation is used for irrigation, with an irrigation amount of 30 liters per square meter each time, and irrigation is carried out every 3 days to ensure uniform water supply; The fertilization cycle is once every 10 days, and the application amount per square meter each time is 5 g;

[0070] Step 8: Utilize the enrichment ability of plants and calcium carbonate in the fertilizer to achieve double enrichment of heavy metal cadmium; Select Bacillus strains with the ability to secrete organic acids as microbial inoculants. According to the soil pollution degree, add 8×10 8 CFU of the inoculant per square meter. After fully mixing the microbial inoculant and vermiculite evenly according to the mass ratio of 1:7, then spread them together with the fertilizer on the farmland in the treatment area, and then carry out shallow tillage to make the inoculant, fertilizer and soil fully contact; Use an oxidation-reduction potential meter, insert its electrode into the soil at a depth of 12 cm every 2 days to measure the soil oxidation-reduction potential. In the initial stage of the experiment, the soil oxidation-reduction potential in some areas exceeded 300 mV, so 1.5 kg of straw was added per square meter to reduce the potential. After a period of monitoring and adjustment, the soil oxidation-reduction potential gradually stabilized within the suitable range of -100 - 300 mV; Add a compound of iron ions with a mass of 0.1% - 0.5% of the fertilizer mass as a surface regulator to the fertilizer;

[0071] Step 9: At different stages of the growth cycle of the plant corn, namely 30 days, 60 days, and 90 days after planting, collect soil samples and plant samples from the treatment area, plant control area, and fertilizer control area respectively. Samples of the roots, stems, leaves, and grains of corn are taken during the filling stage and the maturity stage respectively;

[0072] Step 10: After collecting the plant samples, rinse the plant samples with deionized water to remove the soil and impurities on the surface. Dry the rinsed plant samples in an oven at a temperature of 70 °C until the plant samples reach a constant weight. Then, use a mortar to grind them into fine powder for the determination of cadmium content and analyze the cadmium enrichment effect of the plants.

[0073] Step 11: Use atomic absorption spectrometry to accurately determine the cadmium content in soil and plants:

[0074] Step a: Collect soil samples in different regions according to the sampling method in Step 1, with a sampling depth of 10 cm.

[0075] Step b: After mixing multiple collected soil samples evenly, air-dry them in a well-ventilated room, avoiding direct sunlight and dust pollution. The air-drying time is 5 days.

[0076] Step c: Grind the air-dried soil samples with a mortar into fine powder, and then sieve them using a sieve with a pore size of 100 mesh.

[0077] Step d: Put 1 g of soil sample into a polytetrafluoroethylene digestion tank, add mixed acid A composed of 6 ml of nitric acid, 4 ml of hydrochloric acid, and 1.5 ml of perchloric acid, and then digest it on a hot plate. The heating temperature is controlled at 180 °C, and the digestion time is 4.5 hours until the sample becomes a clear and transparent solution.

[0078] Step e: After the digested solution cools to room temperature, transfer it to a volumetric flask and dilute it to 100 ml with deionized water to obtain the test solution of the soil sample.

[0079] Step f: Collect plant samples at 30 days, 60 days, and 90 days after planting, including the above-ground part and the underground part. After collection, rinse the plant samples with deionized water to remove the soil and impurities on the surface.

[0080] Step g: Dry the rinsed plant samples in an oven at a temperature of 70 °C until the plant samples reach a constant weight.

[0081] Step h: Grind the dried plant samples with a mortar into fine powder, and then sieve them using a sieve with a pore size of 100 mesh.

[0082] Step i: Put 1 g of sample powder into a digestion container, add mixed acid B composed of 6 ml of nitric acid and 2.5 ml of sulfuric acid, and digest it on a heating device. The heating temperature is 120 - 180 °C, and the time is 2 - 4 hours until the sample is completely digested and the solution is clear and transparent.

[0083] Step j: After the digested plant sample solution cools, transfer it to a volumetric flask and dilute it to 100 ml with deionized water to prepare the test solution of the plant sample.

[0084] Step k: Before measurement, carefully check whether all components of the atomic absorption spectrometer are working properly. Set the working wavelength of the instrument according to the characteristic wavelength of cadmium element, 228.8 nm. Prepare a cadmium standard solution with a concentration of 1000 mg / L and dilute it with deionized water into standard working solutions with concentrations of 0.1 mg / L, 0.2 mg / L, 0.5 mg / L, and 1 mg / L. Sequentially aspirate the standard working solutions into the atomic absorption spectrometer, record the corresponding absorbance values, plot a calibration curve, and its linear correlation coefficient reaches 0.9995. Aspirate the test solution of the prepared soil or plant sample into the nebulizer of the atomic absorption spectrometer;

[0085] Step l: The solution is converted into aerosol in the nebulizer and then enters the acetylene-air flame;

[0086] Step m: In the flame, the atoms of cadmium element are excited to produce an absorption spectrum, and the instrument determines the content of cadmium element by detecting the absorbance at a specific wavelength of 228.8 nm;

[0087] Step n: Calculate the content of cadmium in the sample according to the calibration curve and the absorbance value of the sample;

[0088] Step o: Record the absorbance value, measurement time, and sample number information of each sample;

[0089] Step p: Calculate the content of cadmium in the soil or plant sample according to the regression equation of the calibration curve. The calculation formula is: C = (A - b) / a, where C is the concentration of cadmium in the sample, in mg / L, A is the absorbance of the sample, a is the slope of the calibration curve, and b is the intercept of the calibration curve. Then, according to the constant volume of the sample and the weighed sample amount, calculate the mass fraction of cadmium in the soil or plant. The cadmium content in the soil sample is 1.17 mg / kg, and the cadmium content in the plant sample is 1.8 mg / kg.

[0090] After one maize growth cycle of treatment, the cadmium content in the soil of the treatment area decreased significantly compared with that before treatment, and the decrease amplitude reached 35%. The cadmium content in the treated soil was lower than the content of the blank control area minus 3 times the standard deviation of the cadmium content in the blank control area, indicating that this method has a significant effect on reducing the cadmium content in the soil;

[0091] The cadmium enrichment amount in the maize plants in the treatment area was significantly higher than that in the plant control area. The cadmium content per kilogram of plant dry weight was 1.5 mg / kg higher than that in the plant control area. At the same time, the decrease amplitude of the cadmium content in the soil of the treatment area was 12% more than that in the fertilizer control area, fully demonstrating that the combination of plants and fertilizers in this method has a significant effect on cadmium enrichment.

[0092] Example 2

[0093] A method for double enrichment of heavy metal cadmium, comprising the following steps:

[0094] First step: In the farmland, divide it into multiple experimental areas each with an area of 25 square meters. Among them, set up a treatment area, a plant control area (only planting plants), and a fertilizer control area (only applying fertilizers), and set 3 replicates for each area; sample and analyze the farmland soil polluted by heavy metal cadmium. Use the grid method to set 25 sampling points in the farmland, with a sampling depth of 10 cm. Use a soil sampler to collect soil samples, mix the collected soil samples evenly, and divide them into two parts: one part uses atomic absorption spectrometry to determine the initial cadmium content in the soil to determine the cadmium pollution degree and distribution in the soil, and the other part detects the initial pH value of the soil. After measurement, the average initial cadmium content in this farmland soil is 1.4 mg / kg, belonging to the medium pollution level, and the initial pH value is 6.1. According to the initial pH value of the soil, add 0.5 kg of lime to the soil. The addition amount = [(target pH - initial pH) × buffering capacity × soil bulk density × plough layer depth] / (lime neutralization value × 1000), and adjust the soil pH value to 6.5 - 7.5;

[0095] Second step: Use a plough attached to a tractor to plough the entire farmland soil, with a ploughing depth of 12 cm, to loosen the soil, improve soil aeration and water permeability, ensure that subsequent fertilizers and plant roots can better contact the soil, and at the same time ensure that calcium carbonate and cadmium ions can migrate and react more fully in the soil;

[0096] Third step: Prepare a fertilizer containing calcium carbonate, and control the mass content of calcium carbonate in the fertilizer at 6%;

[0097] Fourth step: According to the cadmium pollution degree and soil fertility status in the soil, determine the application amount of the fertilizer: when the cadmium content in the soil is at the medium pollution level of 1 mg / kg - 3 mg / kg, the fertilizer application amount is 5.0 g per square meter;

[0098] Fifth step: Use the method of manual spreading to evenly apply the fertilizer to the farmland in the treatment area and the fertilizer control area, and then use a small rotary tiller for shallow tillage, with a shallow tillage depth of 12 cm, to make the fertilizer fully mixed with the soil;

[0099] Sixth step: During the fertilizer application process, add a composition of humic acid, clay mineral, carbonate and fertilizer according to the mass ratio of humic acid: clay mineral: carbonate: fertilizer = 1:4:8:195 to enhance the overall performance of the fertilizer and enhance the adsorption and transformation effect of carbonate on cadmium;

[0100] Step 7: Plant cadmium-enriched plants in the farmland. The selected plants are required to have a certain cadmium enrichment ability, be able to accumulate more absorbed cadmium in the roots, stems and leaves, transfer less to the fruits, and have low requirements for soil fertility. Corn is selected as the plant. Since the cadmium content in the soil is at a medium pollution level, the plant planting density is determined to be 5 plants per square meter; drip irrigation is used for irrigation, with each irrigation amount of 30 liters per square meter, and irrigation is carried out every 3 days to ensure uniform water supply; the fertilization cycle is once every 10 days, and the application amount per square meter each time is 5 g;

[0101] Step 8: Utilize the enrichment ability of plants and calcium carbonate in fertilizers to achieve dual enrichment of heavy metal cadmium; select Bacillus strains with the ability to secrete organic acids as microbial inoculants. According to the soil pollution degree, add 8×10 8 CFU of the inoculant per square meter. After fully mixing the microbial inoculant and vermiculite evenly at a mass ratio of 1:7, then spread them together with the fertilizer on the farmland in the treatment area, and then carry out shallow tillage to make the inoculant, fertilizer and soil fully contact; use an oxidation-reduction potential meter, insert its electrode into the soil at a depth of 12 cm every 2 days to measure the soil oxidation-reduction potential. In the initial stage of the experiment, the soil oxidation-reduction potential in some areas exceeded 300 mV, so 1.5 kg of straw was added per square meter to reduce the potential. After a period of monitoring and adjustment, the soil oxidation-reduction potential gradually stabilized within the suitable range of -100 - 300 mV; add a compound of iron ions with a mass of 0.1% - 0.5% of the fertilizer mass as a surface regulator in the fertilizer;

[0102] Step 9: At different stages of the growth cycle of the plant corn, namely 30 days, 60 days, and 90 days after planting, collect soil samples and plant samples from the treatment area, plant control area and fertilizer control area respectively, and sample the roots, stems, leaves and grains of corn during the filling stage and the maturity stage respectively;

[0103] Step 10: After collecting the plant samples, rinse the plant samples with deionized water to remove the soil and impurities on the surface. Dry the washed plant samples in an oven at a temperature of 70°C until the plant samples reach a constant weight, and then grind them into fine powder with a mortar for cadmium content determination to analyze the cadmium enrichment effect of the plants;

[0104] Step 11: Accurately determine the cadmium content in the soil and plants by atomic absorption spectrometry:

[0105] Step a: Collect soil samples in different areas according to the sampling method in Step 1, with a sampling depth of 10 cm;

[0106] Step b: After mixing multiple collected soil samples evenly, carry out air-drying treatment in a well-ventilated room, avoiding direct sunlight and dust pollution. The air-drying time is 5 days;

[0107] Step c: The air-dried soil sample is ground into fine powder using a mortar, and then sieved through a sieve with a pore size of 100 mesh.

[0108] Step d: Put 1 g of the soil sample into a polytetrafluoroethylene digestion vessel, add mixed acid A composed of 6 ml of nitric acid, 4 ml of hydrochloric acid, and 1.5 ml of perchloric acid, and then carry out digestion on a hot plate. The heating temperature is controlled at 180 °C, and the digestion time is 4.5 hours until the sample becomes a clear and transparent solution.

[0109] Step e: After the digested soil sample solution is cooled to room temperature, it is transferred to a volumetric flask and made up to 100 ml with deionized water to obtain the test solution of the soil sample.

[0110] Step f: Collect plant samples, including above-ground and underground parts, 30 days, 60 days, and 90 days after plant planting. After collection, rinse the plant samples with deionized water to remove the soil and impurities on the surface.

[0111] Step g: Dry the washed plant samples in an oven at a temperature of 70 °C until the plant samples reach a constant weight.

[0112] Step h: Grind the dried plant samples into fine powder using a mortar, and then sieve through a sieve with a pore size of 100 mesh.

[0113] Step i: Put 1 g of the sample powder into a digestion container, add mixed acid B composed of 6 ml of nitric acid and 2.5 ml of sulfuric acid, and carry out digestion on a heating device. The heating temperature is 120 - 180 °C, and the time is 2 - 4 hours until the sample is completely digested and the solution is clear and transparent.

[0114] Step j: After the digested plant sample solution is cooled, it is transferred to a volumetric flask and made up to 100 ml with deionized water to prepare the test solution of the plant sample.

[0115] Step k: Before measurement, carefully check whether all components of the atomic absorption spectrometer are working properly. Set the working wavelength of the instrument according to the characteristic wavelength of cadmium element, 228.8 nm. Prepare a cadmium standard solution with a concentration of 1000 mg / L, and dilute it with deionized water to standard working solutions with concentrations of 0.1 mg / L, 0.2 mg / L, 0.5 mg / L, and 1 mg / L. Sequentially aspirate the standard working solutions into the atomic absorption spectrometer, record the corresponding absorbance values, and draw a calibration curve with a linear correlation coefficient reaching 0.9996. Aspirate the prepared test solution of the soil or plant sample into the nebulizer of the atomic absorption spectrometer.

[0116] Step l: The solution is converted into aerosol in the nebulizer and then enters the acetylene-air flame.

[0117] Step m: In the flame, the atoms of cadmium element are excited to produce an absorption spectrum, and the instrument determines the content of cadmium element by detecting the absorbance at a specific wavelength of 228.8 nm.

[0118] Step n: Calculate the content of cadmium in the sample according to the calibration curve and the absorbance value of the sample.

[0119] Step o: Record the absorbance value, measurement time, and sample number information of each sample.

[0120] Step p: Calculate the content of cadmium in the soil or plant sample according to the regression equation of the calibration curve. The calculation formula is: C = (A - b) / a, where C is the concentration of cadmium in the sample, in mg / L, A is the absorbance of the sample, a is the slope of the calibration curve, and b is the intercept of the calibration curve. Then, according to the constant volume of the sample and the weighed sample amount, calculate the mass fraction of cadmium in the soil or plant. The cadmium content in the soil sample is 0.94 mg / kg, and the cadmium content in the plant sample is 1.7 mg / kg.

[0121] After one corn growth cycle of treatment, the cadmium content in the soil of the treatment area decreased significantly compared with that before treatment, and the reduction rate reached 33%. The cadmium content in the treated soil was lower than the content of the blank control area minus three times the standard deviation of the cadmium content in the blank control area, indicating that this method has a significant effect on reducing the cadmium content in the soil.

[0122] The cadmium enrichment amount in the corn plants in the treatment area was significantly higher than that in the plant control area. The cadmium content per kilogram of plant dry weight was 1.4 mg / kg higher than that in the plant control area. At the same time, the reduction rate of the cadmium content in the soil of the treatment area was 12% more than that in the fertilizer control area, fully demonstrating that the combination of plants and fertilizers in this method has a significant effect on cadmium enrichment.

[0123] Example 3

[0124] A method for double enrichment of heavy metal cadmium, comprising the following steps:

[0125] Step 1: In the farmland, divide it into multiple experimental areas each with an area of 25 square meters. Among them, set up a treatment area, a plant control area (only planting plants), and a fertilizer control area (only applying fertilizers), with 3 replicates for each area; sample and analyze the farmland soil polluted by heavy metal cadmium. Use the grid method to set 25 sampling points in the farmland, with a sampling depth of 10 cm. Use a soil sampler to collect soil samples, and mix the collected soil samples evenly and divide them into two parts: one part is used to determine the initial cadmium content in the soil by atomic absorption spectrometry to determine the cadmium pollution degree and distribution in the soil, and the other part is used to detect the initial pH value of the soil. After measurement, the average initial cadmium content in this farmland soil is 0.8 mg / kg, belonging to the low pollution level, and the initial pH value is 6.0. According to the initial pH value of the soil, add 0.5 kg of lime, and the addition amount =

[0126] [(Target pH - Initial pH) × Buffer capacity × Soil bulk density × Plow layer depth] / (Lime neutralization value × 1000), adjust the soil pH value to 6.5 - 7.5;

[0127] Step 2: Use the plow attached to the tractor to plow the entire farmland soil, with a plowing depth of 12 cm, to loosen the soil, improve soil aeration and water permeability, ensure that subsequent fertilizers and plant roots can better contact the soil, and at the same time ensure that the migration and reaction of calcium carbonate and cadmium ions in the soil are more sufficient;

[0128] Step 3: Prepare a fertilizer containing calcium carbonate component, and control the mass content of calcium carbonate in the fertilizer at 7%;

[0129] Step 4: Determine the fertilizer application rate according to the cadmium pollution degree and soil fertility status in the soil: When the cadmium content in the soil is at the low pollution level, the fertilizer application rate is 2.5 g per square meter;

[0130] Step 5: Use the method of manual spreading to evenly apply the fertilizer to the farmland in the treatment area and the fertilizer control area, and then use a small rotary tiller for shallow tillage, with a shallow tillage depth of 12 cm, to make the fertilizer fully mixed with the soil;

[0131] Step 6: During the fertilizer application process, add a composition of humic acid, clay minerals, and carbonates and the fertilizer according to the mass ratio of humic acid: clay minerals: carbonates: fertilizer = 1:4:8:195 to enhance the overall performance of the fertilizer and enhance the adsorption and transformation effect of carbonates on cadmium;

[0132] Step 7: Plant plants that can accumulate heavy metal cadmium in the farmland. The selected plants are required to have a certain ability to accumulate cadmium, and can accumulate more absorbed cadmium in the roots, stems and leaves, and transport less to the fruits, and have low requirements for soil fertility. The selected plant is corn. Since the cadmium content in the soil is at a low pollution level, the plant planting density is determined to be 4 plants per square meter; drip irrigation is used for irrigation, and the irrigation amount per time is 30 liters per square meter, and irrigation is carried out every 3 days to ensure uniform water supply; the fertilization cycle is once every 10 days, and the application amount per square meter each time is 5g;

[0133] Step 8: Utilize the enrichment ability of plants and calcium carbonate in fertilizers to achieve double enrichment of heavy metal cadmium; select Bacillus strains with the ability to secrete organic acids as microbial inoculants. According to the soil pollution degree, add 4×10 8 CFU of inoculant per square meter. After fully mixing the microbial inoculant and vermiculite evenly at a mass ratio of 1:7, then sprinkle them together with the fertilizer on the farmland in the treatment area, and then carry out shallow tillage to make the inoculant, fertilizer and soil fully contact; use an oxidation-reduction potential meter, and insert its electrode into the soil at a depth of 12 cm every 2 days to measure the soil oxidation-reduction potential. In the initial stage of the experiment, the soil oxidation-reduction potential in some areas exceeded 300 mV, so 1.5 kg of straw was added per square meter to reduce the potential. After a period of monitoring and adjustment, the soil oxidation-reduction potential gradually stabilized within the suitable range of -100 - 300 mV; add a compound of iron ions with a mass of 0.1% - 0.5% of the fertilizer mass as a surface regulator in the fertilizer;

[0134] Step 9: At different stages of the growth cycle of the plant corn, namely 30 days, 60 days, and 90 days after planting, collect soil samples and plant samples from the treatment area, plant control area and fertilizer control area respectively, and sample the roots, stems, leaves and grains of corn at the filling stage and maturity stage respectively;

[0135] Step 10: After collecting the plant samples, rinse the plant samples with deionized water to remove the soil and impurities on the surface. Dry the cleaned plant samples in an oven at a temperature of 70°C until the plant samples reach a constant weight, and then use a mortar to grind them into fine powder for the determination of cadmium content to analyze the enrichment effect of plants on cadmium;

[0136] Step 11: Use atomic absorption spectrometry to accurately determine the cadmium content in soil and plants:

[0137] Step a: Collect soil samples in different areas according to the sampling method in Step 1, and the sampling depth is 10 cm;

[0138] Step b: After mixing multiple collected soil samples evenly, carry out air-drying treatment in a well-ventilated room, avoiding direct sunlight and dust pollution, and the air-drying time is 5 days;

[0139] Step c: The air-dried soil sample is ground into fine powder using a mortar, and then sieved through a sieve with a pore size of 100 mesh.

[0140] Step d: Put 1 g of the soil sample into a polytetrafluoroethylene digestion vessel, add mixed acid A composed of 6 ml of nitric acid, 4 ml of hydrochloric acid, and 1.5 ml of perchloric acid, and then carry out digestion on a hot plate. The heating temperature is controlled at 180 °C, and the digestion time is 4.5 hours until the sample becomes a clear and transparent solution.

[0141] Step e: After the digested soil sample solution is cooled to room temperature, it is transferred to a volumetric flask and made up to 100 ml with deionized water to obtain the test solution of the soil sample.

[0142] Step f: Collect plant samples, including above-ground and underground parts, 30 days, 60 days, and 90 days after plant planting. After collection, rinse the plant samples with deionized water to remove the soil and impurities on the surface.

[0143] Step g: Dry the washed plant samples in an oven at a temperature of 70 °C until the plant samples reach a constant weight.

[0144] Step h: The dried plant samples are ground into fine powder using a mortar, and then sieved through a sieve with a pore size of 100 mesh.

[0145] Step i: Put 1 g of the sample powder into a digestion container, add mixed acid B composed of 6 ml of nitric acid and 2.5 ml of sulfuric acid, and carry out digestion on a heating device. The heating temperature is 120 - 180 °C, and the time is 2 - 4 hours until the sample is completely digested and the solution is clear and transparent.

[0146] Step j: After the digested plant sample solution is cooled, it is transferred to a volumetric flask and made up to 100 ml with deionized water to prepare the test solution of the plant sample.

[0147] Step k: Before measurement, carefully check whether all components of the atomic absorption spectrometer are working properly. Set the working wavelength of the instrument according to the characteristic wavelength of cadmium element, 228.8 nm. Prepare a cadmium standard solution with a concentration of 1000 mg / L and dilute it with deionized water to standard working solutions with concentrations of 0.1 mg / L, 0.2 mg / L, 0.5 mg / L, and 1 mg / L. Inhale the standard working solutions into the atomic absorption spectrometer in turn, record the corresponding absorbance values, and draw a calibration curve with a linear correlation coefficient reaching 0.9995. Inhale the prepared test solution of the soil or plant sample into the nebulizer of the atomic absorption spectrometer.

[0148] Step l: The solution is converted into aerosol in the nebulizer and then enters the acetylene-air flame.

[0149] Step m: In the flame, the atoms of cadmium element are excited to produce an absorption spectrum, and the instrument determines the content of cadmium element by detecting the absorbance at a specific wavelength of 228.8 nm.

[0150] Step n: Calculate the content of cadmium in the sample according to the calibration curve and the absorbance value of the sample.

[0151] Step o: Record the absorbance value, measurement time, and sample number information of each sample.

[0152] Step p: Calculate the content of cadmium in the soil or plant sample according to the regression equation of the calibration curve. The calculation formula is: C = (A - b) / a, where C is the concentration of cadmium in the sample, in mg / L, A is the absorbance of the sample, a is the slope of the calibration curve, and b is the intercept of the calibration curve. Then, according to the constant volume of the sample and the weighed sample amount, calculate the mass fraction of cadmium in the soil or plant. The cadmium content in the soil sample is 0.56 mg / kg, and the cadmium content in the plant sample is 1.3 mg / kg.

[0153] After one maize growth cycle of treatment, the cadmium content in the soil of the treatment area decreased significantly compared with that before treatment, and the reduction rate reached 30%. The cadmium content in the treated soil was lower than the content of the blank control area minus three times the standard deviation of the cadmium content in the blank control area, indicating that this method has a significant effect on reducing the cadmium content in the soil.

[0154] The cadmium enrichment amount in the maize plants in the treatment area was significantly higher than that in the plant control area. The cadmium content per kilogram of plant dry weight was 1 mg / kg higher than that in the plant control area. At the same time, the reduction rate of the cadmium content in the soil of the treatment area was 11% more than that in the fertilizer control area, fully demonstrating that the combination of plants and fertilizers in this method has a significant effect on cadmium enrichment.

[0155] The above is only the preferred embodiment of the present invention. The protection scope of the present invention is not limited to the above embodiments. All technical solutions within the idea of the present invention belong to the protection scope of the present invention.

Claims

1. A method for double enrichment of heavy metal cadmium, characterized in that: The steps include: Step 1: Sampling and analyzing farmland soil contaminated by heavy metal cadmium. A set of sampling points were set up in the farmland using the grid method. The sampling depth was 10 cm. Soil samples were collected using a soil sampler. The collected soil samples were mixed evenly. The initial cadmium content in the soil was determined using atomic absorption spectrometry to determine the degree of cadmium contamination and distribution in the soil. The amount of lime added to the soil was calculated based on the soil's buffering capacity and initial pH value according to the initial pH value of the soil. The specific calculation formula was: lime addition amount = [(target pH-initial pH) × buffering capacity × soil bulk density × tillage layer depth] / (lime neutralization value × 1000). The soil pH value was adjusted to 6.5-7.

5. Step 2: Plough the soil to a depth of 10-15 cm to loosen the soil and improve soil aeration and water permeability; Step 3: Prepare fertilizer containing calcium carbonate, and the calcium carbonate content in the fertilizer is controlled at 5%-10%; The fourth step is to determine the amount of fertilizer to be applied based on the degree of cadmium pollution in the soil and the soil fertility: when the cadmium content in the soil is at a low pollution level of 0.3mg / kg-1.0mg / kg, the amount of fertilizer to be applied is 2.5g per square meter; when the cadmium content in the soil is at a medium pollution level of 1mg / kg-3mg / kg, the amount of fertilizer to be applied is 5.0g per square meter; when the cadmium content in the soil is at a high pollution level of more than 3mg / kg, the amount of fertilizer to be applied is 7.5g per square meter and above; Step 5: Apply the fertilizer evenly to the farmland by broadcasting, and then perform shallow tillage with a depth of 10-15 cm to fully mix the fertilizer with the soil; Step 6: During the fertilizer application process, add a combination of humic acid, clay minerals, and carbonates to the fertilizer in a mass ratio of humic acid: clay minerals: carbonates: fertilizer = 1:4:8:195 to enhance the overall performance of the fertilizer and enhance the adsorption and conversion of cadmium by carbonates; Step 7: Planting plants that are rich in heavy metal cadmium in the farmland, and the plants selected are corn; Step 8: Use the enrichment ability of plants and calcium carbonate in fertilizers to achieve dual enrichment of heavy metal cadmium; Step 9: Collect soil samples and plant samples from the designated areas at different stages of the plant growth cycle, i.e., 30 days, 60 days, and 90 days after planting; Step 10: After collecting the plant samples, rinse the plant samples with deionized water to remove the soil and impurities on the surface. Dry the washed plant samples in an oven at 70°C until the plant samples reach a constant weight. Then grind them into fine powder using a mortar and pestle to determine the cadmium content and analyze the cadmium enrichment effect of the plants. Step 11: Use atomic absorption spectrometry to accurately determine the cadmium content in soil and plants; if the cadmium content in the soil of the treated area is significantly reduced compared with that before treatment, such as a reduction of 30% or more, or the cadmium content in the soil after treatment is lower than the content before treatment minus 3 times the standard deviation of the cadmium content before treatment, it indicates that this method is effective in reducing the cadmium content in the soil; if the cadmium enrichment in the plants in the treated area is higher than that in the plant control area by a certain amount, such as the cadmium content per kilogram of plant dry weight is more than 1 mg / kg higher than that in the plant control area, and at the same time, the reduction in cadmium content in the soil of the treated area is greater than that in the fertilizer control area, such as a reduction of more than 10% more than that in the fertilizer control area, it can also reflect the effectiveness of this method; if after multiple corn growth cycles, the cadmium content in the soil continues to decrease and eventually reaches or is lower than the environmental standard value, and the cadmium content in the subsequently planted non-enriched plants is also within a safe range, it indicates that this method has long-term effectiveness and stability, and can achieve sustainable remediation of cadmium-contaminated farmland and effective enrichment of cadmium.

2. The method for dual enrichment of heavy metal cadmium according to claim 1, characterized in that: In the seventh step, the planting density of plants is adjusted according to the degree of cadmium pollution in the soil. When the cadmium content in the soil is at a low pollution level of 0.3mg / kg-1.0mg / kg, the plant planting density is 4 plants per square meter; when the cadmium content in the soil is at a medium pollution level of 1mg / kg to 3mg / kg, the plant planting density is 5 plants per square meter; when the cadmium content in the soil is at a high pollution level above 3mg / kg, the plant planting density is 6 plants per square meter; after planting corn, the farmland is irrigated and fertilized. Drip irrigation is used for irrigation. The irrigation volume is 30 liters per square meter each time, and the fertilization cycle is once every 10 days.

3. The method for dual enrichment of heavy metal cadmium according to claim 1, characterized in that: In the eighth step, promoters, such as microbial agents, are added to the soil to promote the adsorption and conversion of cadmium by calcium carbonate. Microbial agents are strains that have the ability to secrete organic acids, produce iron carriers, or have the ability to reduce cadmium ions, such as Bacillus, which can secrete organic acids such as citric acid and malic acid, reduce the local pH value of the soil, enhance the solubility of calcium carbonate, and release more carbonate ions to combine with cadmium. The dosage of the added agent should be determined according to the degree of soil pollution and the activity of the microbial agent: for soil with a cadmium content of 0.3mg / kg-1.0mg / kg and mild cadmium pollution, the number of effective live bacteria that can be added per square meter is 1×10 8 -5×10 8 CFU of bacteria; cadmium content 1mg / kg-3mg / kg moderate pollution, add 5×10 per square meter 8 -1×10 9 CFU; cadmium content is higher than 3mg / kg severe pollution, add 1×10 per square meter 9 -5×10 9 CFU, method of adding microbial agents: mix the microbial agent and the carrier in a mass ratio of 1:7, the carrier is vermiculite and / or peat soil, and then spread it on the farmland together with fertilizer, followed by shallow tillage to allow full contact between the agent, fertilizer and soil.

4. The method for dual enrichment of heavy metal cadmium according to claim 1, characterized in that: In the eighth step, the heavy metal cadmium exists in the form of cadmium ions, which are active and mobile. Specifically, it comes from water-soluble cadmium and / or exchangeable cadmium in the soil. The cadmium ions will be released into the soil solution, thereby reacting with calcium in the form of solid particles in the soil. The calcium in the form of solid particles is a calcium carbonate compound.

5. The method for dual enrichment of heavy metal cadmium according to claim 1, characterized in that: In the eighth step, the redox potential of the soil is controlled between -100mV and 300mV, and a surface regulator of 0.1% to 0.5% of the fertilizer mass is added to the fertilizer. The surface regulator is a compound of iron ions and aluminum ions, which enhances the adsorption stability and chemical conversion efficiency of calcium carbonate on cadmium, making it easier for cadmium ions to undergo a replacement reaction with calcium carbonate. According to the chemical reaction formula A more stable CdCO3 compound is formed, and a new (Ca, Cd)CO3 phase is formed on the surface of calcium carbonate. This new phase is a solid solution phase, in which cadmium atoms replace part of the calcium atoms to form a carbonate structure. This new phase passivates the surface of calcium carbonate, reduces the dissolution and migration of cadmium, solidifies the cadmium, and prevents it from re-entering the environment.

6. The method for dual enrichment of heavy metal cadmium according to claim 5, characterized in that: The oxidation-reduction potential is measured using an oxidation-reduction potential meter, whose electrodes are inserted into the soil to a depth of 10-15 cm, and the soil oxidation-reduction potential is measured regularly every 2-3 days to obtain real-time data. If the soil oxidation-reduction potential exceeds 300 mV, it is not conducive to the reaction of calcium carbonate and cadmium. The potential can be reduced by adding organic materials, such as adding straw and / or green manure, with an application rate of 1-2 kg per square meter. Organic materials consume oxygen during decomposition, causing the soil to tend to a reduced state; If the potential is lower than -100mV, tillage can be carried out to increase soil aeration and improve redox potential. The tillage depth should be controlled at 10-15 cm, and tillage should be carried out every 7-10 days until the potential reaches -100-300mV.

7. The method for dual enrichment of heavy metal cadmium according to claim 1, characterized in that: The method for collecting plant samples in the ninth step is to sample different parts of corn, including roots, stems, leaves, and grains, during the filling and maturity stages of corn growth; to record plant growth indicators in detail, such as plant height, stem diameter, leaf area, and biomass, and to analyze the changing trend of cadmium enrichment in plants as plant growth indicators change. At the same time, the changing conditions of cadmium content in soil, cadmium content in leaching solution and other data are analyzed to evaluate the effect of this method on reducing soil cadmium content, reducing cadmium leaching losses, and enriching cadmium in plants; with the good development of plant growth indicators, cadmium enrichment in plants also shows a steady increasing trend. During the growth of corn, for every 10 cm increase in plant height, cadmium enrichment in the aboveground part increases by an average of 0.2 mg / kg.

8. The method for dual enrichment of heavy metal cadmium according to claim 1, characterized in that: The sampling and sample preparation process of atomic absorption spectrometry in step 11 is as follows: Step a: Collect soil samples in different areas according to the sampling method in the first step, with a sampling depth of 10 cm; Step b: After mixing the collected soil samples evenly, air-dry them in a well-ventilated room, avoiding direct sunlight and dust pollution. The air-drying time is 5 days; Step c: The air-dried soil sample was ground into a fine powder using a mortar and pestle, and then sieved using a sieve with a pore size of 100 mesh; Step d: 0.9-1.1 g of soil sample is placed in a polytetrafluoroethylene digestion tank, mixed acid A consisting of 5-7 ml nitric acid, 3-5 ml hydrochloric acid, and 1-2 ml perchloric acid is added, and then digested on a hot plate, the heating temperature is controlled at 150-200°C, and the digestion time is 4-5 hours, until the sample becomes a clear and transparent solution; Step e: After the digested soil sample solution is cooled to room temperature, it is transferred to a volumetric flask and fixed to 100 ml with deionized water to obtain a test solution of the soil sample; Step f: collecting plant samples, including above-ground and underground parts, 30 days, 60 days, and 90 days after the plants are planted, and washing the plant samples with deionized water to remove surface soil and impurities; Step g: drying the cleaned plant sample in an oven at 70° C. until the plant sample reaches a constant weight; Step h: The dried plant sample is ground into a fine powder using a mortar and pestle, and then sieved using a sieve with a pore size of 100 mesh; Step i: Place 0.9-1.1 g of sample powder into a digestion container, add a mixed acid B consisting of 5-7 ml nitric acid and 2-3 ml sulfuric acid, and digest on a heating device at a temperature of 120-180°C for 2-4 hours until the sample is completely digested and the solution is clear and transparent; Step j: After the digested plant sample solution is cooled, it is transferred to a volumetric flask and fixed to 100 ml with deionized water to prepare a plant sample test solution; Step k: aspirating the prepared test solution of the soil or plant sample into the atomizer of the atomic absorption spectrometer; Step 1: The solution is converted into an aerosol in an atomizer and then enters an acetylene-air flame; Step m: In the flame, the atoms of the cadmium element are excited to produce an absorption spectrum, and the instrument determines the content of the cadmium element by detecting the absorbance at a specific wavelength of 228.8nm; Step n: Calculate the cadmium content in the sample based on the calibration curve and the absorbance value of the sample; Step o: Record the absorbance value, measurement time, and sample number information of each sample; Step p: Calculate the cadmium content in the soil or plant sample according to the regression equation of the calibration curve, the calculation formula is: C = (Ab) / a, wherein C is the concentration of cadmium in the sample, in mg / L, A is the absorbance of the sample, a is the slope of the calibration curve, b is the intercept of the calibration curve, and then calculate the mass fraction of cadmium in the soil or plant according to the fixed volume and sample weight of the sample, in mg / kg.

9. The method for dual enrichment of heavy metal cadmium according to claim 8, characterized in that: In step p, the derivation steps of the calculation formula are as follows: Step 1: In atomic absorption spectroscopy, the absorbance A and the concentration C of the element to be measured in the sample conform to the Lambert-Beer Law; the expression of the Lambert-Beer Law is A=εlC, where ε is the molar absorption coefficient, l is the optical path length, and C is the concentration of the element in the sample; Step 2: For the drawing of the calibration curve, by measuring the absorbance A of a series of standard solutions with known concentrations C, the linear relationship between absorbance and concentration can be obtained. Usually, the equation of the calibration curve can be expressed as A=aC+b, where a is equivalent to εl and b is the intercept; Step 3: After measuring the absorbance of the sample, to calculate the concentration of cadmium in the sample, it is necessary to solve the calibration curve equation A=aC+b. By shifting the terms, we can get C=(Ab) / a.

10. The method for dual enrichment of heavy metal cadmium according to claim 9, characterized in that: In the step 2, a cadmium standard solution with a concentration of 1000 mg / L is prepared, and then diluted with deionized water to standard working solutions of different concentrations of 0.1 mg / L, 0.2 mg / L, 0.5 mg / L, and 1 mg / L, and the standard working solutions are sucked into the atomic absorption spectrometer in sequence, and the corresponding absorbance values ​​are recorded; a calibration curve is drawn with the concentration of the cadmium standard solution as the abscissa and the absorbance value as the ordinate, and the calibration curve should be a straight line passing through the origin, and its linear correlation coefficient is required to reach 0.999 or above.