A method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil

Abstract

The present application relates to heavy metal contaminated soil remediation technical field, disclose a kind of heavy metal contaminated farmland soil in situ regulation and safe utilization method, the present application is first according to the division of three kinds of safe utilization unit according to soil heavy metal content and crop enrichment characteristics;To high-risk unit of layer in situ passivation is implemented, surface layer is applied to mercapto modified sepiolite-biochar composite passivation agent and inoculated heavy metal tolerant rhizosphere growth promoting bacteria agent, middle layer is applied to iron-based modified bentonite-humic acid composite passivation agent, with spatial isolation to avoid passivation antagonism;Each unit matches different safe utilization mode;Through dynamic monitoring target supplementary passivation agent, and straw is carbonized and returned to field to maintain long-term stability;The present application solves the problems of existing technology, such as anion complex heavy metal passivation antagonism, low control precision, straw returning to field passivation effect attenuation, long-term dynamic regulation mechanism loss.

Description

Technical Field

[0001] This invention relates to the field of heavy metal contaminated land remediation technology, and in particular to a method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil. Background Technology

[0002] With the advancement of industrialization and agricultural intensification in my country, heavy metal accumulation has occurred in agricultural soils in some areas. Combined pollution by heavy metals such as cadmium, lead, arsenic, and chromium has become a significant factor affecting arable land quality and agricultural product safety. In-situ passivation remediation technology, due to its advantages of simple construction, low remediation cost, and no interruption of agricultural production, has become a core technical means for the safe utilization of moderately to lightly polluted agricultural land. Constructing an integrated remediation system with supporting measures such as agronomic regulation and planting low-accumulation crops can reduce the bioavailability of heavy metals in the soil while ensuring agricultural production benefits, meeting my country's strategic needs for arable land protection and food security. However, current in-situ remediation technologies for heavy metals in agricultural land are mostly developed for single heavy metal types, and significant technical bottlenecks remain in areas such as the synergistic remediation of combined pollution, precise control, and long-term effectiveness.

[0003] Chinese invention patent application with publication number CN108774531A discloses a remediation agent for treating alkaline cadmium-contaminated soil and its preparation method. The technology uses sepiolite and bentonite as the main adsorbent carriers, applies quicklime to adjust the soil pH, and achieves the passivation and stabilization of cadmium in the soil by uniformly spreading the agent over the entire area and then rotary tilling to mix it. While this technology has some effect on treating single cadmium pollution, it still has several shortcomings in practical applications: First, it is only applicable to the treatment of cationic heavy metal pollution such as cadmium. When farmland is contaminated with anionic and cationic pollution such as cadmium-arsenic or lead-chromium, the simultaneous addition of iron-based anionic passivating materials can lead to competition and antagonism of reactive sites between the two passivating agents in direct contact in the soil, thus reducing the overall passivation efficiency and failing to achieve simultaneous and efficient treatment of complex pollution. Second, the technology adopts a uniform application mode across the entire area without classifying and controlling the treatment according to the degree of soil pollution and the risk of agricultural products exceeding standards. This can easily lead to excessive application of the agent in low-pollution areas and insufficient treatment in high-pollution areas, resulting in low agent utilization and treatment precision. Third, the technology does not consider the negative impact of conventional straw return to the field on the passivation system. The low molecular weight organic acids released during the natural decomposition of straw can significantly activate the fixed heavy metals, causing the passivation effect to gradually decline with the planting cycle and have a limited duration of effectiveness. Fourth, the technology lacks a long-term dynamic regulation mechanism and cannot target the maintenance based on the activation state of heavy metals in the soil, making it difficult to guarantee long-term treatment effects.

[0004] In summary, existing technologies for in-situ remediation and safe utilization of heavy metals in agricultural land generally suffer from technical deficiencies such as the antagonistic effect of complex pollution, insufficient precision in hierarchical control, poor long-term stability of passivation effects, and a disconnect between remediation measures and agricultural production management. A complete technical system covering risk diagnosis, targeted remediation, safe production, and long-term maintenance has not yet been formed. How to simultaneously address the antagonistic effect of anionic and cationic complex heavy metal passivation in in-situ remediation scenarios, achieving differentiated hierarchical control and long-term steady-state maintenance, while also adapting to routine agricultural production management, is a pressing technical challenge that needs to be solved in this field. Summary of the Invention

[0005] The present invention aims to provide a method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil, in order to solve the problems of easy antagonism in the passivation of anionic and cationic complex heavy metals, low control precision, attenuation of passivation effect due to straw returning to the field, and lack of long-term dynamic regulation mechanism in the existing technology.

[0006] To achieve the above objectives, the present invention provides the following method:

[0007] This invention provides a method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil:

[0008] S1. Division of safe utilization units: Soil samples of farmland to be treated are collected, and the total amount of heavy metals, the content of available heavy metals, and the physicochemical properties of the soil are tested. Based on the heavy metal enrichment characteristics of the target crops and the risk threshold of heavy metal exceedance in agricultural products, the farmland to be treated is divided into low-risk utilization units, medium-risk utilization units, and high-risk utilization units. The dominant heavy metal pollution factors in each safe utilization unit are determined.

[0009] S2. Layered targeted passivation: Layered in-situ passivation treatment is carried out on the soil of the medium-risk utilization unit and the high-risk utilization unit. A mercapto-modified sepiolite-biochar composite passivating agent is applied to the soil surface layer and inoculated with a heavy metal-resistant rhizosphere growth-promoting agent. An iron-based modified bentonite-humic acid composite passivating agent is applied to the soil middle layer.

[0010] S3. Graded safe utilization: Differentiated agricultural production and utilization models are matched for the safe utilization units with different risk levels. For low-risk, medium-risk, and high-risk utilization units, safe utilization methods such as strictly controlling new pollution, passing off low-accumulation crop planting, and passing off intercropping or crop rotation are adopted respectively.

[0011] S4. Long-term steady-state maintenance: Regularly monitor the available content of heavy metals in the soil of each safe utilization unit, and apply corresponding passivating agents according to the activation state of heavy metals; collect the straw produced by crop harvesting, and prepare the straw into biomass carbon-based passivating material by carbonization before returning it to the field for application.

[0012] Preferably, in step S1, the dominant heavy metal pollutants are divided into cationic heavy metals and anionic heavy metals; the available content of cationic heavy metals is determined by the diethylenetriaminepentaacetic acid extraction method, and the available content of anionic heavy metals is determined by the sodium dihydrogen phosphate extraction method; the risk threshold for heavy metal contamination in agricultural products is calculated using the following formula:

[0013] ;

[0014] In the formula, The risk threshold for available heavy metals in soil. These are the limits for heavy metal contaminants in food as specified in national food safety standards. The enrichment coefficient of the target crop for the corresponding heavy metal is preset; the enrichment coefficient is the ratio of the heavy metal content in the edible part of the crop to the content of the available heavy metal in the soil, and the field measurement value of the farmland to be treated is adopted; based on the risk threshold of available heavy metal in the soil, the area with available heavy metal content below 80% of the risk threshold is divided into low-risk utilization units, the area with available heavy metal content between 80% and 120% of the risk threshold is divided into medium-risk utilization units, and the area with available heavy metal content above 120% of the risk threshold is divided into high-risk utilization units.

[0015] Preferably, in step S2, the depth range of the top layer of soil is 0-15cm, and the depth range of the middle layer of soil is 15-30cm; the application rate of the mercapto-modified sepiolite-biochar composite passivating agent is 150-200kg per mu, and after application, it is thoroughly mixed with the top layer of soil by rotary tillage to stabilize the cationic heavy metals; the application rate of the iron-based modified bentonite-humic acid composite passivating agent is 200-250kg per mu, and after application, it is thoroughly mixed with the middle layer of soil by deep plowing to stabilize the anionic heavy metals; the plowing operations of the two soil layers are carried out independently to avoid mixing of the passivating agents in the upper and lower layers.

[0016] Preferably, in step S2, the mercapto-modified sepiolite-biochar composite passivating agent is prepared according to the following steps: First, sepiolite is mixed with hydrochloric acid at a concentration of 1 mol / L at a solid-liquid mass-volume ratio of 1 g: 10 mL, soaked and activated for 12 h, washed until neutral, and dried. Then, it is mixed with corn straw biochar at a mass ratio of 2:1 to obtain a composite carrier. Next, the composite carrier is immersed in a mixed solution of anhydrous ethanol and mercaptopropyltrimethoxysilane, with a solid-liquid mass-volume ratio of 1 g: 10 mL between the composite carrier and the mixed solution. The amount of mercaptopropyltrimethoxysilane used is 5%~8% of the mass of the composite carrier. The reaction is carried out under constant temperature of 60°C for 8 h. After the reaction is completed, the mixture is filtered, washed, and dried to obtain the finished product.

[0017] Preferably, in step S2, the iron-based modified bentonite-humic acid composite passivating agent is prepared according to the following steps: First, calcium-based bentonite is mixed with a 5% sodium carbonate solution at a solid-liquid mass-volume ratio of 1g:10mL, and sodium-modified at room temperature for 24 hours. After drying and grinding, a sodium-based bentonite suspension with a mass concentration of 100g / L is prepared. Then, a 0.5mol / L ferrous sulfate solution is added to the sodium-based bentonite suspension. After nitrogen protection, a solution with a concentration of 100g / L is added dropwise at a rate of 2~3mL / min. A 1 mol / L sodium borohydride solution was used, with a molar ratio of sodium borohydride to ferrous sulfate of 2:1. A liquid-phase reduction reaction was carried out at room temperature to obtain a bentonite system loaded with zero valent iron. Finally, a 10% (w / w) sodium humate solution was added to the system, the pH was adjusted to neutral, and the mixture was stirred for 4 hours. The mixture was then filtered, dried, and ground to obtain the final product. The final product contained 10%–15% zero valent iron and 3%–5% humic acid, with the humic acid loading based on pure humic acid.

[0018] Preferably, in step S2, the heavy metal-resistant rhizosphere growth-promoting bacterial agent uses sterilized peat moss as a carrier and is prepared by mixing cadmium-resistant Pseudomonas aeruginosa and lead-resistant Bacillus at a viable count ratio of 1:2. The effective viable count of the heavy metal-resistant rhizosphere growth-promoting bacterial agent is not less than 2 × 10⁻⁶. 9 CFU / g; The heavy metal resistant rhizosphere growth promoter is applied to the crop rhizosphere area of ​​the soil surface by hole application 7 days after the passivation agent is applied, with an application rate of 5-8g per hole, followed by covering with soil and watering.

[0019] Preferably, in step S3, the safe production control method includes optimizing water and fertilizer management and crop rotation system, and spraying a foliar silicon-selenium inhibitor; the foliar silicon-selenium inhibitor uses sodium silicate and sodium selenite as active ingredients, and adds 0.1% sorbitol polyoxyethylene ether as an adhesion promoter, wherein the silicon element mass concentration is 1000~1500mg / L and the selenium element mass concentration is 20~30mg / L; the foliar silicon-selenium inhibitor is sprayed once each during the crop jointing stage and the grain-filling stage, with a spraying amount of 30~40L per acre, and the spraying time is selected on a sunny evening, and it is evenly sprayed on the front and back of the crop leaves.

[0020] Preferably, in step S3, the passivation-supporting intercropping remediation method adopts a strip intercropping layout, with the width ratio of the low-accumulation main crop planting belt to the heavy metal hyperaccumulating plant planting belt being 3:1, and the interval between adjacent planting belts being 30cm; the heavy metal hyperaccumulating plants are selected according to the dominant heavy metal pollutants of the safe utilization unit, with Sedum aizoon selected for cadmium pollution and Centipede Grass selected for arsenic pollution; the plant spacing of Sedum aizoon is 20cm×20cm, and the plant spacing of Centipede Grass is 30cm×30cm; the heavy metal hyperaccumulating plants are transplanted during the seedling stage of the main crop, and the above-ground parts are harvested every 3-4 months of growth, and the harvested above-ground parts are sent to a high-temperature incinerator above 850℃ for harmless disposal, and the ash residue generated from incineration is treated in accordance with the hazardous waste regulations.

[0021] Preferably, in step S4, the monitoring frequency of the available heavy metal content in the soil is once a year, the sampling time is after the harvest of the current season's crops, the sampling adopts a five-point mixed sampling method, and the sampling depth is 0~15cm; the triggering condition for targeted application of passivating agent is calculated by the following formula:

[0022] ;

[0023] In the formula, The percentage increase in the available form of heavy metals in the soil. This represents the available content of heavy metals in the soil as monitored in the current period. The effective content of heavy metals in the soil after passivation treatment is completed; when the effective content of cationic heavy metals increases by more than 20%, the mercapto-modified sepiolite-biochar-based passivating agent is applied as a supplement, and the supplement is applied by spreading on the surface and then rotary tilling to mix it evenly, and is applied to the soil surface layer at a depth of 0-15cm; when the effective content of anionic heavy metals increases by more than 20%, the iron-based modified bentonite-humic acid composite passivating agent is applied as a supplement, and the supplement is applied by deep trenching to the middle layer of the soil at a depth of 15-30cm; the supplementary application amount is 30%-50% of the initial application amount of the corresponding passivating agent.

[0024] Preferably, in step S4, the carbonization treatment is a low-temperature oxygen-limited carbonization treatment; after the crop straw is collected, it is first naturally dried until the moisture content is less than 15%, crushed into pieces with a length of 2-5cm, and then sent into a closed carbonization furnace. Under the oxygen-limited atmosphere protected by nitrogen, the temperature is raised to 350-450℃ at a rate of 5℃ / min, and kept at the temperature for pyrolysis for 2-3 hours; after carbonization, it is naturally cooled to room temperature and passed through a 10-mesh sieve to obtain the finished biochar product; the biochar is evenly spread on the surface of the safe utilization unit at an application rate of 100-150kg per mu, and then thoroughly mixed with the soil at a depth of 0-15cm by rotary tillage.

[0025] The beneficial effects of this invention are as follows: By adopting a layered in-situ passivation scheme, the passivation components of anionic and cationic heavy metals are spatially isolated, avoiding direct contact between the two and the resulting antagonistic effects, thus achieving simultaneous, efficient, and stable treatment of compound heavy metals in agricultural land; by dividing risk-safe utilization units based on field-measured enrichment coefficients and matching differentiated treatment and utilization modes, the accuracy of treatment and the efficiency of pesticide utilization are effectively improved, avoiding pesticide waste and uneven treatment effects caused by uniform application across the entire area; by returning crop straw to the field after low-temperature oxygen-limited carbonization to prepare biochar, the problem of heavy metal reactivation caused by the production of small-molecule organic acids from direct straw decomposition is avoided, which is conducive to maintaining the long-term stability of the passivation system; with the support of a targeted reapplication mechanism based on dynamic monitoring of the available state of heavy metals in the soil, the passivation efficiency can be replenished in a timely manner and the passivation duration can be extended, forming a complete technical system that connects risk diagnosis, targeted remediation, safe production, and long-term maintenance, achieving long-term in-situ control and safe utilization of heavy metal pollution while ensuring normal agricultural production on agricultural land. Attached Figure Description

[0026] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0027] Figure 1 This is a schematic diagram of a method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil provided in an embodiment of the present invention;

[0028] Figure 2 A detailed process flow diagram of the layered targeted passivation treatment provided in an embodiment of the present invention. Detailed Implementation

[0029] To enable those skilled in the art to better understand the present invention, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] The terms "first," "second," etc., used in the specification, claims, and accompanying drawings of this invention are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, apparatus, product, or end that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or ends.

[0031] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0032] Existing technologies for in-situ remediation and safe utilization of heavy metals in agricultural land generally suffer from technical deficiencies such as the antagonistic effect of complex pollution, insufficient precision in hierarchical control, poor long-term stability of passivation effects, and a disconnect between remediation measures and agricultural production management. A complete technical system covering risk diagnosis, targeted remediation, safe production, and long-term maintenance has not yet been formed. How to simultaneously address the antagonistic effect of anionic and cationic complex heavy metal passivation in in-situ remediation scenarios, achieving differentiated hierarchical control and long-term steady-state maintenance, while also adapting to routine agricultural production management, is a pressing technical challenge that needs to be solved in this field.

[0033] The present invention aims to provide a method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil, in order to solve the problems of easy antagonism in the passivation of anionic and cationic complex heavy metals, low control precision, attenuation of passivation effect due to straw returning to the field, and lack of long-term dynamic regulation mechanism in the existing technology.

[0034] like Figure 1 and Figure 2 As shown in the figure, a specific embodiment of the present invention provides a method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil, the specific steps of which are as follows:

[0035] S1: Division of safe utilization units: Soil samples were collected from the farmland to be treated, and the total amount of heavy metals, the content of available heavy metals, and the physicochemical properties of the soil were tested. Based on the heavy metal enrichment characteristics of the target crops and the risk threshold of heavy metal exceedance in agricultural products, the farmland to be treated was divided into low-risk, medium-risk, and high-risk utilization units, and the dominant heavy metal pollution factors in each safe utilization unit were determined.

[0036] In this embodiment of the invention, soil samples are collected using a five-point mixed sampling method, with a sampling depth of 0-15 cm. At least five sampling points are set up within each safe utilization unit, and the samples are mixed to serve as the test samples for that unit. Specific soil physicochemical parameters include soil pH, cation exchange capacity, organic matter content, and soil texture type. The available content of cationic heavy metals is determined using the diethylenetriaminepentaacetic acid extraction method, and the available content of anionic heavy metals is determined using the sodium dihydrogen phosphate extraction method. The risk threshold for heavy metal contamination in agricultural products is determined using the formula:

[0037] ;

[0038] Calculation, where The risk threshold for available heavy metals in soil. These are the limits for heavy metal contaminants in food as specified in national food safety standards. The enrichment coefficients of target crops for corresponding heavy metals are preset. The enrichment coefficients are obtained by field measurements in the area where the farmland to be treated is located. The target crops are planted in typical fields, and the heavy metal content in the edible parts of the crops and the content of available heavy metals in the corresponding rhizosphere soil are measured simultaneously at maturity. The dominant heavy metal pollutant is determined based on the highest proportion of available content of each heavy metal to the corresponding risk threshold. Based on the risk threshold of available heavy metals in the soil, areas with available content below 80% of the threshold are classified as low-risk utilization units, areas with available content between 80% and 120% of the threshold are classified as medium-risk utilization units, and areas with available content above 120% of the threshold are classified as high-risk utilization units.

[0039] S2: Layered Targeted Passivation: Layered in-situ passivation treatment is carried out on the soil of medium-risk and high-risk utilization units. A mercapto-modified sepiolite-biochar composite passivating agent is applied to the soil surface and inoculated with heavy metal-resistant rhizosphere growth-promoting bacteria. An iron-based modified bentonite-humic acid composite passivating agent is applied to the soil middle layer.

[0040] In this embodiment of the invention, the depth range of the topsoil layer is 0-15 cm, and the depth range of the middle soil layer is 15-30 cm. Construction follows a sequence of deep layer application followed by surface layer application. The passivating agent is first applied to the middle soil layer and then deeply tilled and mixed, before the passivating agent is applied to the topsoil layer and rotary tilled and mixed, avoiding cross-mixing between the two passivating agents. The application rate of the mercapto-modified sepiolite-biochar composite passivating agent is 150-200 kg per acre, and after application, it is thoroughly mixed with the topsoil layer using rotary tillage to stabilize cationic heavy metals. The application rate of the iron-based modified bentonite-humic acid composite passivating agent is 200-250 kg per acre, and after application, it is thoroughly mixed with the middle soil layer using deep tillage. The thiol-modified sepiolite-biochar composite passivating agent is prepared as follows: First, sepiolite is mixed with 1 mol / L hydrochloric acid at a solid-liquid mass-volume ratio of 1 g:10 mL, and soaked for 12 h to expand the porous structure. After washing to neutrality and drying, it is mixed with corn straw biochar at a mass ratio of 2:1 to obtain a composite carrier. Then, the composite carrier is immersed in a mixed solution of anhydrous ethanol and mercaptopropyltrimethoxysilane, with a solid-liquid mass-volume ratio of 1 g:10 mL. The amount of mercaptopropyltrimethoxysilane is 5%~8% of the mass of the composite carrier. The reaction is carried out under constant temperature and sealed conditions at 60℃ for 8 h with stirring. After the reaction is completed... The finished product was obtained by filtration, washing, and drying. The iron-based modified bentonite-humic acid composite passivating agent was prepared as follows: First, calcium-based bentonite was mixed with a 5% sodium carbonate solution at a solid-liquid mass-to-volume ratio of 1g:10mL. Sodium modification was carried out at room temperature for 24 hours. After drying and grinding, a sodium-based bentonite suspension with a mass concentration of 100g / L was prepared. Then, a 0.5mol / L ferrous sulfate solution was added to the sodium-based bentonite suspension. Under nitrogen protection, a 1mol / L sodium borohydride solution was added dropwise at a rate of 2-3mL / min. The molar ratio of sodium borohydride to ferrous sulfate was 2:1. A liquid-phase reduction reaction was carried out at room temperature. After the reaction was completed, the loaded product was obtained. Zero-valent iron (ZVFe) in a bentonite system; finally, a 10% sodium humate solution was added to the system, the pH was adjusted to neutral, and the mixture was stirred for 4 hours. The mixture was then filtered, dried, and ground to obtain the final product. The ZVFe loading fraction in the final product was 10%–15%, and the humic acid loading fraction was 3%–5%, calculated as pure humic acid. The final product was vacuum-sealed and stored in a cool, dry environment at a temperature not exceeding 30°C to prevent the oxidative deactivation of ZVFe and thiol groups. A heavy metal-resistant rhizosphere growth promoter was prepared using sterilized peat moss as a carrier, consisting of a cadmium-resistant Pseudomonas aeruginosa and a lead-resistant Bacillus thuringiensis mixed at a viable count ratio of 1:2, with an effective viable count of not less than 2 × 10⁻⁶. 9CFU / g; The heavy metal resistant rhizosphere growth promoter should be applied to the rhizosphere of the crop by hole application 7 days after the passivation agent is applied. The application rate is 5-8g per hole. After inoculation, cover with soil and water. One day before inoculation, lightly water the rhizosphere area to maintain the soil moisture content at 60%-70% of field capacity to improve the survival rate of the promoter.

[0041] S3. Graded safe utilization: Differentiated agricultural production and utilization models are matched for safe utilization units with different risk levels. For low-risk, medium-risk, and high-risk utilization units, safe utilization methods such as strictly controlling new pollution, passing off low-accumulation crop planting, and passing off intercropping or crop rotation are adopted respectively.

[0042] In this embodiment of the invention, the safety production control methods include optimizing water and fertilizer management and crop rotation, and supplementing it with foliar application of silicon-selenium inhibitors; water and fertilizer management adopts a soil testing and formula fertilization scheme, reducing the application of acidic fertilizers by 20% to 30% compared to conventional fertilization, and increasing the application of 1000 to 1500 kg of decomposed organic fertilizer per mu; paddy field crops use shallow and frequent irrigation during their growth period, while dryland crops use drip or furrow irrigation to avoid the soil being in an extreme reducing or extreme oxidizing state for a long time, thus reducing the risk of heavy metal activation; the crop rotation system adopts a rotation system of one year of grain crops and one year of legume crops to regulate the soil rhizosphere microenvironment; foliar silicon-selenium inhibitors... The control agent uses sodium silicate and sodium selenite as active ingredients, with 0.1% sorbitol polyoxyethylene ether added as an adhesion promoter. The silicon concentration is 1000-1500 mg / L, and the selenium concentration is 20-30 mg / L. The foliar silicon-selenium control agent is sprayed once each during the jointing and grain-filling stages, at a rate of 30-40 L per acre. Spraying should be done on a sunny evening, evenly on both sides of the crop leaves. If rainfall occurs within 24 hours after spraying, a 50% reapplication of the original dosage of foliar silicon-selenium control agent should be applied within 24 hours after the rain stops. This is used in conjunction with low-accumulation crop planting patterns. Based on the dominant heavy metal pollutants in the safe utilization unit, corresponding low-accumulation crop varieties were selected. Low-cadmium rice varieties were chosen for cadmium-contaminated paddy fields, and low-accumulation maize varieties were chosen for lead-contaminated dryland. The passivation and intercropping remediation method adopted a strip intercropping layout, with a width ratio of 3:1 between the low-accumulation main crop planting strip and the heavy metal hyperaccumulating plant planting strip, and a spacing of 30cm between adjacent planting strips. Heavy metal hyperaccumulating plants were selected according to the dominant heavy metal pollutants in the safe utilization unit; *Sedum aizoon* was selected for cadmium pollution, and *Pteris vittata* for arsenic pollution. The plant spacing for *Sedum aizoon* was 20cm × 20cm, and for *Pteris vittata*... The plant spacing is 30cm×30cm. Heavy metal hyperaccumulators are transplanted during the seedling stage of the main crop. The above-ground parts are harvested every 3-4 months. The harvested above-ground parts are sent to a high-temperature incinerator above 850℃ for harmless disposal. The ash produced by incineration is treated in accordance with the regulations for hazardous waste. Differentiated water and fertilizer management is implemented in different zones. During the vigorous growth period, the heavy metal hyperaccumulator planting zone is sprayed with a 0.5% urea solution once a month, with a spraying amount of 20-30L per acre, to promote the growth of above-ground biomass and improve the efficiency of heavy metal extraction. The main crop planting zone is supplied with water and fertilizer according to the conventional farmland management standards.

[0043] S4. Long-term stable maintenance: Regularly monitor the available content of heavy metals in the soil of each safe utilization unit, and apply corresponding passivating agents according to the activation state of heavy metals; collect the straw produced by crop harvesting, and prepare the straw into biomass carbon-based passivating materials after carbonization treatment before returning it to the field for application.

[0044] In this embodiment of the invention, the monitoring frequency of available heavy metal content in the soil is once a year, the sampling time is after the harvest of the current season's crops, the sampling adopts a five-point mixed sampling method, and the sampling depth is 0~15cm; the monitoring points correspond one-to-one with the sampling points in the initial safe utilization unit division stage to ensure the longitudinal comparability of monitoring data; the triggering condition for targeted application of passivating agents is determined by the formula:

[0045] ;

[0046] Calculation, where The percentage increase in the available form of heavy metals in the soil. This represents the available content of heavy metals in the soil as monitored in the current period. The data represents the available heavy metal content in the soil after passivation treatment. When the available content of cationic heavy metals increases by more than 20%, a mercapto-modified sepiolite-biochar-based passivating agent is applied, using a surface application followed by rotary tillage to mix it into the soil surface at a depth of 0-15 cm. When the available content of anionic heavy metals increases by more than 20%, an iron-modified bentonite-humic acid composite passivating agent is applied, using a deep trench application method to mix it into the soil middle layer at a depth of 15-30 cm. The application rate is 30%-50% of the initial application rate of the corresponding passivating agent. The available heavy metal content in the soil is retested 30 days after the retest, using the same sampling method and depth as conventional monitoring. If the increase is still higher than 20%, an additional 20% of the initial application rate of the corresponding passivating agent is applied. The carbonization treatment is a low-temperature, oxygen-limited carbonization treatment. After crop straw is collected, it is first naturally dried until the moisture content is below 15%, then crushed. The biochar is cut into 2-5cm segments and then fed into a closed carbonization furnace. Under a nitrogen-protected, oxygen-limited atmosphere, the temperature is increased to 350-450℃ at a rate of 5℃ / min and held for pyrolysis for 2-3 hours. After carbonization, the biochar is naturally cooled to room temperature and passed through a 10-mesh sieve to obtain the finished product. The biochar is evenly spread on the surface of the safe utilization unit at a rate of 100-150kg per acre and thoroughly mixed with the soil at a depth of 0-15cm by rotary tillage. The application of passivating agent and biochar return to the field are carried out simultaneously. The passivating agent to be applied is premixed with biochar according to the application ratio and then evenly spread, followed by uniform rotary tillage and mixing. Biochar can assist the passivating agent in fixing heavy metals through pore adsorption and complexation, and synergistically improve the long-term stability of the passivation system. After the biochar is returned to the field, it is naturally cured for 7 days. During the curing period, the soil moisture content is maintained at 60%-70% of field capacity to promote the formation of stable aggregate structures between biochar and soil colloids.

[0047] The beneficial effects of this invention are as follows: By adopting a layered in-situ passivation scheme, the passivation components of anionic and cationic heavy metals are spatially isolated, avoiding direct contact between the two and the resulting antagonistic effects, thus achieving simultaneous, efficient, and stable treatment of compound heavy metals in agricultural land; by dividing risk-safe utilization units based on field-measured enrichment coefficients and matching differentiated treatment and utilization modes, the accuracy of treatment and the efficiency of pesticide utilization are effectively improved, avoiding pesticide waste and uneven treatment effects caused by uniform application across the entire area; by returning crop straw to the field after low-temperature oxygen-limited carbonization to prepare biochar, the problem of heavy metal reactivation caused by the production of small-molecule organic acids from direct straw decomposition is avoided, which is conducive to maintaining the long-term stability of the passivation system; with the support of a targeted reapplication mechanism based on dynamic monitoring of the available state of heavy metals in the soil, the passivation efficiency can be replenished in a timely manner and the passivation duration can be extended, forming a complete technical system that connects risk diagnosis, targeted remediation, safe production, and long-term maintenance, achieving long-term in-situ control and safe utilization of heavy metal pollution while ensuring normal agricultural production on agricultural land.

[0048] The above descriptions are merely embodiments of the present invention. Commonly known technical solutions or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil, characterized in that, The method includes: S1. Division of safe utilization units: Soil samples of farmland to be treated are collected, and the total amount of heavy metals, the content of available heavy metals, and the physicochemical properties of the soil are tested. Based on the heavy metal enrichment characteristics of the target crops and the risk threshold of heavy metal exceedance in agricultural products, the farmland to be treated is divided into low-risk utilization units, medium-risk utilization units, and high-risk utilization units. The dominant heavy metal pollution factors in each safe utilization unit are determined. S2. Layered targeted passivation: Layered in-situ passivation treatment is carried out on the soil of the medium-risk utilization unit and the high-risk utilization unit. A mercapto-modified sepiolite-biochar composite passivating agent is applied to the soil surface layer and inoculated with a heavy metal-resistant rhizosphere growth-promoting agent. An iron-based modified bentonite-humic acid composite passivating agent is applied to the soil middle layer. S3. Graded safe utilization: Differentiated agricultural production and utilization models are matched for the safe utilization units with different risk levels. For low-risk, medium-risk, and high-risk utilization units, safe utilization methods such as strictly controlling new pollution, passing off low-accumulation crop planting, and passing off intercropping or crop rotation are adopted respectively. S4. Long-term steady-state maintenance: Regularly monitor the available content of heavy metals in the soil of each safe utilization unit, and apply corresponding passivating agents according to the activation state of heavy metals; collect the straw produced by crop harvesting, and prepare the straw into biomass carbon-based passivating material by carbonization before returning it to the field for application.

2. The method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil according to claim 1, characterized in that: In step S1, the dominant heavy metal pollutants are classified into cationic heavy metals and anionic heavy metals; the available content of cationic heavy metals is determined by the diethylenetriaminepentaacetic acid extraction method, and the available content of anionic heavy metals is determined by the sodium dihydrogen phosphate extraction method; the risk threshold for heavy metal contamination in agricultural products is calculated using the following formula: ； In the formula, The risk threshold for available heavy metals in soil. These are the limits for heavy metal contaminants in food as specified in national food safety standards. The enrichment coefficient of the target crop for the corresponding heavy metal is set; the enrichment coefficient is the ratio of the heavy metal content in the edible part of the crop to the content of available heavy metals in the soil, and the field measurement value of the farmland to be treated is adopted. Based on the risk threshold of available heavy metals in the soil, areas with available heavy metal content below 80% of the risk threshold are classified as low-risk utilization units, areas with available heavy metal content between 80% and 120% of the risk threshold are classified as medium-risk utilization units, and areas with available heavy metal content above 120% of the risk threshold are classified as high-risk utilization units.

3. The method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil according to claim 1, characterized in that: In step S2, the depth range of the soil surface layer is 0~15cm, and the depth range of the soil middle layer is 15~30cm; the application rate of the mercapto-modified sepiolite-biochar composite passivating agent is 150~200kg per mu, and after application, it is thoroughly mixed with the soil surface layer by rotary tillage to stabilize the cationic heavy metals. The application rate of the iron-based modified bentonite-humic acid composite passivating agent is 200-250 kg per mu. After application, it is thoroughly mixed with the soil in the middle layer of the soil by deep plowing to stabilize the anionic heavy metals. The plowing operations of the two soil layers are carried out independently to avoid mixing of the passivating agents in the upper and lower layers.

4. The method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil according to claim 3, characterized in that: In step S2, the mercapto-modified sepiolite-biochar composite passivating agent is prepared according to the following steps: first, sepiolite is mixed with hydrochloric acid with a concentration of 1 mol / L at a solid-liquid mass-volume ratio of 1 g: 10 mL, soaked and activated for 12 h, washed until neutral and dried, and then mixed with corn straw biochar at a mass ratio of 2:1 to obtain a composite carrier. The composite carrier is then immersed in a mixed solution of anhydrous ethanol and mercaptopropyltrimethoxysilane, with a solid-liquid mass-to-volume ratio of 1g:10mL. The amount of mercaptopropyltrimethoxysilane used is 5%~8% of the mass of the composite carrier. The reaction is carried out under constant temperature of 60℃ for 8 hours. After the reaction is completed, the product is obtained by filtration, washing and drying.

5. A method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil according to claim 3, characterized in that: In step S2, the iron-based modified bentonite-humic acid composite passivating agent is prepared according to the following steps: calcium-based bentonite is mixed with sodium carbonate solution with a mass fraction of 5% at a solid-liquid mass-volume ratio of 1g:10mL, sodium-modified at room temperature for 24h, and then dried and ground to prepare a sodium-based bentonite suspension with a mass concentration of 100g / L. Then, a 0.5 mol / L ferrous sulfate solution was added to the sodium-based bentonite suspension. After nitrogen protection, a 1 mol / L sodium borohydride solution was added dropwise at a rate of 2-3 mL / min. The molar ratio of sodium borohydride to ferrous sulfate was 2:

1. The liquid-phase reduction reaction was carried out at room temperature. After the reaction was completed, a bentonite system loaded with zero valent iron was obtained. Finally, a 10% sodium humate solution was added to the system, the pH was adjusted to neutral, and the mixture was stirred for 4 hours. The product was then filtered, dried, and ground to obtain the final product. The finished product contains 10% to 15% zero-valent iron by mass and 3% to 5% humic acid by mass, wherein the humic acid by mass is calculated as pure humic acid.

6. A method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil according to claim 3, characterized in that: In step S2, the heavy metal-resistant rhizosphere growth-promoting bacterial agent uses sterilized peat moss as a carrier and is prepared by mixing cadmium-resistant Pseudomonas aeruginosa and lead-resistant Bacillus at a viable count ratio of 1:

2. The effective viable count of the heavy metal-resistant rhizosphere growth-promoting bacterial agent is not less than 2 × 10⁻⁶. 9 CFU / g; The heavy metal-resistant rhizosphere growth-promoting bacteria agent is applied to the crop rhizosphere area of ​​the soil surface layer by hole application 7 days after the passivation agent is applied. The application amount per hole is 5-8g. After inoculation, the soil is covered and watered.

7. A method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil according to claim 1, characterized in that: In step S3, the safety production control method includes optimizing water and fertilizer management and crop rotation system, and spraying foliar silicon-selenium inhibitor; the foliar silicon-selenium inhibitor uses sodium silicate and sodium selenite as active ingredients, and adds 0.1% sorbitol polyoxyethylene ether as an adhesion promoter, wherein the silicon element mass concentration is 1000~1500mg / L and the selenium element mass concentration is 20~30mg / L; The foliar silicon selenium inhibitor was sprayed once each during the crop's jointing and grain-filling stages, with a spraying rate of 30-40L per acre. The spraying time was chosen to be on a sunny evening, and the agent was sprayed evenly on both sides of the crop leaves.

8. A method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil according to claim 1, characterized in that: In step S3, the passivation-supporting intercropping repair method adopts a strip intercropping layout, with the width ratio of the low-accumulation main crop planting belt to the heavy metal hyperaccumulating plant planting belt being 3:1, and the interval between adjacent planting belts being 30cm. The heavy metal hyperaccumulating plants are selected according to the dominant heavy metal pollutants of the safe utilization unit. For cadmium pollution, Sedum aizoon is selected, and for arsenic pollution, Sedum sarmentosum is selected. The plant spacing of Sedum aizoon is 20cm×20cm, and the plant spacing of Sedum sarmentosum is 30cm×30cm. The heavy metal hyperaccumulating plant is transplanted during the seedling stage of the main crop. The above-ground parts are harvested every 3 to 4 months. The harvested above-ground parts are sent to a high-temperature incinerator above 850°C for harmless disposal. The ash residue produced by incineration is treated in accordance with the regulations for hazardous waste.

9. A method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil according to claim 1, characterized in that: In step S4, the monitoring frequency of the available heavy metal content in the soil is once a year, the sampling time is after the harvest of the current season's crops, the sampling adopts the five-point mixed sampling method, and the sampling depth is 0~15cm; the triggering condition for targeted application of passivating agent is calculated by the following formula: ； In the formula, The percentage increase in the available form of heavy metals in the soil. This represents the available content of heavy metals in the soil as monitored in the current period. The content of available heavy metals in the soil at the time of passivation treatment completion; When the content of cationic heavy metals in the effective form increases by more than 20%, the mercapto-modified sepiolite-biomass carbon-based passivating agent is applied as a supplement. The supplement is applied by spreading on the surface and then rotary tilling to mix it evenly, and is applied to the soil surface layer at a depth of 0-15cm. When the content of anionic heavy metals in the effective form increases by more than 20%, the iron-based modified bentonite-humic acid composite passivating agent is applied as a supplementary agent. The supplementary agent is applied by deep trenching and is applied to the middle layer of the soil at a depth of 15-30 cm. The supplementary application amount is 30%-50% of the amount of the first application of the corresponding passivating agent.

10. A method for in-situ regulation and safe utilization of heavy metal contaminated farmland soil according to claim 1, characterized in that: In step S4, the carbonization treatment is a low-temperature oxygen-limited carbonization treatment; after the crop straw is collected, it is first naturally dried until the moisture content is less than 15%, crushed into pieces with a length of 2-5cm, and then sent into a closed carbonization furnace. Under the oxygen-limited atmosphere protected by nitrogen, the temperature is raised to 350-450℃ at a rate of 5℃ / min, and kept at the temperature for pyrolysis for 2-3 hours. After carbonization, the biochar is naturally cooled to room temperature and passed through a 10-mesh sieve to obtain the finished biochar product. The biochar is evenly spread on the surface of the safe utilization unit at an application rate of 100-150 kg per acre and thoroughly mixed with the soil at a depth of 0-15 cm by rotary tillage.

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