A heavy metal passivation remediation method based on soil improvement

Abstract

This invention relates to the field of contaminated soil remediation technology and discloses a method for heavy metal passivation remediation based on soil improvement. The method employs a layered application of core-shell composite passivation particles and a structural modifier. The core-shell composite passivation particles comprise a mineralized core and an active shell. The mineralized core includes a silicon-calcium based modifier and calcium-deficient hydroxyapatite, while the active shell comprises iron-loaded biochar and sodium humate. During in-situ remediation, the core-shell composite passivation particles are mixed into the deep tillage layer, and the structural modifier is mixed into the surface tillage layer, forming functional zones at both the particle and soil layer scales. The active shell is used for initial adsorption and complexation of heavy metals, the mineralized core is used for continuous mineralization and fixation of heavy metals, and the surface structural modifier is used to improve pore structure, water retention capacity, and rhizosphere environment. This method is applicable to in-situ stabilization remediation and soil quality improvement of cadmium, lead, and zinc contaminated farmland.

Description

Technical Field

[0001] This invention relates to the field of contaminated soil remediation technology, specifically to a heavy metal passivation remediation method based on soil improvement. Background Technology

[0002] After cadmium, lead, and zinc enter agricultural soil, they can distribute across different soil components through processes such as ion exchange, surface adsorption, complexation, precipitation, co-precipitation, and lattice substitution. While the total amount of heavy metals reflects the degree of pollutant accumulation, the risks of absorption and migration by crops are also influenced by the heavy metal's form, soil pH, organic matter content, cation exchange capacity, redox conditions, and moisture variations. Heavy metals extracted in weakly acidic environments exchange rapidly with the soil solution and typically exhibit high migration potential and bioavailability. Converting these heavy metals into more reducible, oxidizable, and residual forms with higher binding strength is an important technological direction for in-situ stabilization and remediation of agricultural land.

[0003] Commonly used passivation materials include limestone, calcium silicate, phosphates, iron and manganese oxides, biochar, humates, clay minerals, and zeolites. Calcium silicate materials can increase the pH of acidic soils and provide silicate and calcium ions to the soil solution. Phosphate materials can fix some metal ions through precipitation or lattice substitution. Biochar, with its porous structure, oxygen-containing functional groups, and mineral ash, can adsorb or complex heavy metals. Iron loading treatment can introduce an iron-loaded phase into the surface and pores of biochar, increasing surface hydroxyl groups and metal binding sites. Sodium humate contains carboxyl and phenolic hydroxyl groups, which can complex with metal ions. Different materials can reduce the activity of heavy metals through different reaction pathways.

[0004] Applying a mixture of multiple passivating materials directly as a powder can easily lead to dust, migration with surface runoff, and localized accumulation. While preparing the mixture into homogeneous particles can improve application performance, the contact time between components and moisture within these particles is similar, and there is a lack of spatial separation between adsorbed, complexed, and mineralized components. Some adsorption or complexation reactions can occur relatively quickly, while phosphate and silicate mineralization require longer mass transfer and reaction times. When multiple reaction mechanisms occur in a disordered parallel manner within the same particle, it is difficult to simultaneously regulate the initial fixation rate and the later morphological stability.

[0005] Core-shell particles typically utilize a coating layer to slow the leaching rate of the core, with the coating layer primarily serving mechanical protection or release control functions. For heavy metal contaminated soil remediation, if the coating layer only acts as a barrier material, the outer layer cannot provide sufficient heavy metal adsorption sites in the initial application phase. If highly reactive iron-loaded biochar is entirely placed within the core, external moisture and metal ions must penetrate the coating layer before contacting the iron-loaded sites, potentially delaying early fixation. Conversely, if phosphate and calcium silicate materials are entirely exposed on the particle exterior, mineralized components may dissolve rapidly in localized areas, hindering the formation of a sustained action process.

[0006] Heavy metal contaminated farmland also exhibits differences in soil function across layers. The 25-35 cm soil layer covers the main root distribution areas of most crops and areas disturbed by cultivation, making it suitable for materials that perform heavy metal fixation. The 0-20 cm soil layer is directly affected by rainfall, evaporation, sowing, fertilization, and rhizosphere activity, placing higher demands on aggregate structure, water retention capacity, ion exchange capacity, and microbial colonization environment. Mixing all materials into the same depth makes it difficult to simultaneously meet the needs of deep passivation and surface structure improvement. If sodium polyacrylate superabsorbent polymer (SAP) penetrates excessively into the deep soil layers, it may also reduce the surface water retention improvement efficiency.

[0007] Bioremediation materials can influence the speciation of heavy metals through cell surface adsorption, metabolite complexation, mineralization induction, and soil nutrient cycling. Bacterial inoculants, when directly drip-dried, may migrate with irrigation water, and their rhizosphere colonization is affected by soil porosity, moisture, and nutrient conditions. Natural zeolite, with its pores and ion exchange sites, can serve as a carrier for bacterial inoculants. Preloading concentrated bacterial solutions into the pores of natural zeolite and adding slow-release nutrients within these pores helps maintain a high bacterial count in the surface rhizosphere environment.

[0008] The passivation effect of heavy metals is also affected by acidification, rainfall leaching, organic matter decomposition, and tillage activities. Measuring the total heavy metal content only after construction cannot determine whether the heavy metals extracted by weak acid have rebounded. By using stratified sampling and continuous monitoring of speciation, the activation trend of heavy metals in different soil layers can be identified, and measures such as supplementing with complexing agents or passivating particles can be taken according to the trend. Summary of the Invention

[0009] The purpose of this invention is to provide a heavy metal passivation remediation method based on soil improvement, which solves the technical problems in existing heavy metal contaminated soil remediation methods, such as the difficulty in coordinating the initial fixation rate and long-term morphological stability, and the difficulty in simultaneously achieving deep chemical passivation and surface structure improvement, due to the disordered parallel reaction mechanisms within homogeneous particles and the mixing of functional materials in the same soil layer.

[0010] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for heavy metal passivation remediation based on soil improvement includes the following steps: Collect soil samples contaminated with heavy metals and determine soil pH, organic matter content, cation exchange capacity, and total and speciation content of cadmium, lead, and zinc; Core-shell composite passivation and amendment particles were spread onto the surface of heavy metal contaminated soil. Deep rotary tillage was then used to integrate these particles into a 25-35 cm soil layer, forming a passivation and amendment layer. The core-shell composite passivation and amendment particles comprise a mineralized core, an active shell coating the mineralized core, and a sodium alginate binder phase distributed within both the mineralized core and the active shell. The mineralized core consists of 49.42 parts by weight of a calcium-silica-based amendment and 24.71 parts by weight of calcium-deficient hydroxyapatite. The active shell consists of 19.40 parts by weight of iron-loaded biochar and 6.47 parts by weight of sodium humate. The total dry weight of the calcium-silica-based amendment, calcium-deficient hydroxyapatite, iron-loaded biochar, and sodium humate is 100 parts by weight. The sodium alginate binder phase comprises 0.02 parts by weight. The iron-loaded biochar is obtained by treating rice husk biochar with ferric chloride aqueous solution, followed by solid-liquid separation, washing, and drying. A structural modifier is applied to the soil surface that forms the passivation and improvement layer. The structural modifier is composed of natural zeolite and sodium polyacrylate water-absorbing resin. The structural modifier is mixed into the soil layer of 15 to 20 cm by surface rotary tillage to form the structural modification layer. Apply bacterial inoculants and well-rotted organic fertilizer to the structural improvement layer, and maintain the soil moisture content at 60% to 80% of field capacity; Soil samples were collected at three depths: 0 to 20 cm, 20 to 40 cm, and 40 to 60 cm. Soil physicochemical indicators and the content of cadmium, lead, and zinc in their forms were measured.

[0011] As a preferred embodiment of the present invention, the iron content in the iron-loaded biochar accounts for 4.2% of the total iron content of the iron-loaded biochar.

[0012] As a preferred embodiment of the present invention, the silicon-calcium based modifier contains calcium silicate and wollastonite, wherein the mass fraction of calcium silicate is not less than 85%, the calcium-phosphorus molar ratio of calcium-deficient hydroxyapatite is 1.5 to 1.67, and the total mass fraction of humic acid in sodium humate is not less than 50%.

[0013] As a preferred embodiment of the present invention, the mineralized core has a particle size of 2.2 to 2.4 mm, the core-shell composite passivation improved particles have a particle size of 3.8 to 4.0 mm, and the active shell has a thickness of 0.8 mm.

[0014] As a preferred embodiment of the present invention, the core-shell composite passivation modified particles have a moisture content of no more than 5%, a single particle compressive strength of no less than 15 N, and a bulk density of 800 to 1000 kg / m³. 3 .

[0015] 6. The method for heavy metal passivation remediation based on soil improvement according to claim 1, characterized in that the preparation method of core-shell composite passivation improvement particles includes the following steps: Rice husk biochar was mixed with ferric chloride aqueous solution and subjected to loading treatment under heating and stirring conditions. After loading treatment, solid-liquid separation was performed, and the obtained solid was washed and dried to obtain iron-loaded biochar. A silicon-calcium-based modifier is mixed with calcium-deficient hydroxyapatite, and an aqueous solution of sodium alginate is sprayed on the rolling state to form a mineralized core. Iron-loaded biochar was mixed with sodium humate to obtain activated shell powder. The mineralized core is placed in a rolling state, and active shell powder is added at the same time as the sodium alginate aqueous solution is sprayed, so that the active shell powder coats the mineralized core to obtain wet core-shell particles. The wet core-shell particles were dried and sieved to obtain core-shell type composite passivation modified particles.

[0016] As a preferred embodiment of the present invention, the concentration of the ferric chloride aqueous solution is 0.5 mol / L, the solid-liquid ratio of rice husk biochar to ferric chloride aqueous solution is 1:10, the loading treatment temperature is 70 ℃, the loading treatment time is 2.5 h, the washing endpoint is that the conductivity of the eluent is not higher than 500 μS / cm, and the drying temperature is 105 to 110 ℃.

[0017] As a preferred technical solution of the present invention, the mineralized core is prepared by a disc granulation device with a disc inclination angle of 50° and a disc rotation speed of 18 rpm. The sodium alginate aqueous solution is added by atomization spraying. The spraying is stopped when the mineralized core reaches a particle size of 2.2 to 2.4 mm.

[0018] As a preferred embodiment of the present invention, the spray pressure is controlled at 0.4 MPa during the active shell coating process, and the coating is stopped when the wet core-shell particles reach a particle size of 3.8 to 4.0 mm. The wet core-shell particles are then dried at 70 °C for 2.5 h.

[0019] As a preferred embodiment of the present invention, the application rate of core-shell type composite passivation modified particles is 214 to 220 kg / hm². 2 The application rate of the structural modifier is 150 kg / hm². 2 The structure modifier consists of 65 parts by weight of natural zeolite and 35 parts by weight of sodium polyacrylate superabsorbent polymer. The natural zeolite has a particle size of 0.5 to 2 mm, and the sodium polyacrylate superabsorbent polymer has a water absorption ratio of 200 to 300 times.

[0020] As a preferred technical solution of the present invention, deep rotary tillage adopts an alternating forward and reverse rotation operation mode. When moving forward, the rotary tillage equipment rotates forward, and when returning, the rotary tillage equipment rotates in reverse. The ratio of the forward speed of the rotary tillage equipment to the rotary tillage speed is 0.15 to 0.25 m / r. After deep rotary tillage is completed, the coefficient of variation of the distribution concentration of core-shell type composite passivated improved particles in the 0 to 30 cm soil layer is not higher than 15%.

[0021] As a preferred embodiment of the present invention, the bacterial agent comprises Bacillus laterosporus and Bacillus megaterium, with a mass ratio of Bacillus laterosporus to Bacillus megaterium of 1.5:1, and the total viable count of the two Bacillus species in the bacterial agent is not less than [amount missing]. CFU / g, the application rate of bacterial agent is 52 kg / hm. 2 The well-rotted organic fertilizer consists of well-rotted cow manure and well-rotted chicken manure in a mass ratio of 2.5:1, and the application rate is 3750 kg / hm². 2 .

[0022] As a preferred embodiment of the present invention, the natural zeolite undergoes a bacterial loading treatment before being mixed with sodium polyacrylate superabsorbent resin. The bacterial loading treatment includes placing the natural zeolite in a vacuum container, evacuating the container to an absolute pressure of 0.02 MPa and maintaining this pressure for 10 minutes, and adding a live bacteria count of [missing information]. The bacterial concentrate was prepared at CFU / mL. 0.25mL of bacterial concentrate was added to each gram of natural zeolite, and the mixture was allowed to stand for 30 minutes after restoring normal pressure. The natural zeolite was preloaded with yeast extract at a rate of 0.8% relative to the mass of the natural zeolite.

[0023] As a preferred embodiment of the present invention, the speciation of cadmium, lead, and zinc is determined using a continuous extraction method. This method classifies heavy metals into weakly acid-extractable, reducible, oxidizable, and residual forms. The proportion of weakly acid-extractable heavy metals is measured every 30 days. If the monthly change rate between two consecutive measurements is less than 2%, a 30-day monitoring interval is maintained. When the monthly change rate is between 2% and 5%, a 0.5% sodium humate aqueous solution is sprayed at a spray volume of 40 ml. 3 / hm 2 When the monthly change rate is higher than 5%, supplement with core-shell type composite passivation improved particles, and the supplementary application amount is 40% of the initial application amount.

[0024] Compared with the prior art, the present invention has the following beneficial effects: This invention applies core-shell structured composite passivation and modification particles and structural modifiers in layers to soil layers at different depths, constructing functional zones at both the particle and soil layer scales. This reconciles the contradiction between rapid initial fixation and long-term morphological stability in the remediation of heavy metal-contaminated soils, while simultaneously improving the soil's physicochemical properties. The method involves incorporating iron-loaded biochar and sodium humate into the active shell of the core-shell composite passivation and modification particles. Upon contact with soil moisture, the outer layer rapidly provides pore adsorption, surface coordination, and organic complexation sites, enabling the rapid capture and fixation of heavy metals such as cadmium, lead, and zinc. Simultaneously, a calcium-silica-based modifier and calcium-deficient hydroxyapatite are incorporated into the mineralized core. As moisture gradually penetrates the particles, these modifiers continuously release calcium, silicate, and phosphate-related reactive components. Through precipitation, lattice substitution, and mineralization, the heavy metals are transformed into a more strongly bound residual state, thus maintaining a stable passivation effect throughout the entire period from initial application to long-term maintenance.

[0025] At the soil level, core-shell type composite passivation and improvement particles are applied to the 25 to 35 cm deep tillage layer to stabilize and remediate the root zone contaminated with heavy metals; a structural modifier composed of natural zeolite and sodium polyacrylate water-absorbing resin is applied to the 15 to 20 cm surface tillage layer to improve soil pore structure, enhance water retention capacity, and optimize the rhizosphere microbial colonization environment, thus avoiding functional conflicts and efficiency losses caused by the mixed application of functional materials.

[0026] Furthermore, in terms of particle preparation, rice husk biochar was loaded with ferric chloride aqueous solution, introducing an iron-containing loading phase into the biochar pores and surface. This significantly increased the adsorption sites for heavy metals and surface reactivity, enhancing the material's ability to immobilize weakly acid-extractable heavy metals. Regarding application, a deep rotary tillage system with alternating forward and reverse rotation effectively improved the uniformity of core-shell composite passivated modified particles in the soil layer, reducing the spatial variability of particle concentration. In terms of synergistic microbial remediation, natural zeolite was pre-treated with vacuum-loaded bacteria, and yeast extract was loaded into the zeolite pores as a slow-release nutrient. This provided a stable colonization carrier and initial nutrient supply for the bacterial agent in the surface structure modification layer, thereby increasing the viable bacterial count and urease activity in the surface soil. In terms of long-term risk management, a stratified sampling and monthly change rate response mechanism based on continuous extraction method morphological monitoring has been established. It can trigger differentiated measures such as maintenance monitoring, spraying sodium humate aqueous solution to supplement complexation or supplementing core-shell type composite passivation improvement particles according to the dynamic changes in the proportion of heavy metals extracted in weak acid, thus avoiding repeated remediation effects and rebounds. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0028] Figure 1 This is a flowchart of the heavy metal passivation remediation method based on soil improvement according to the present invention. Detailed Implementation

[0029] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the embodiments of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0030] The following is in conjunction with the appendix Figure 1 The embodiments of the present invention will be described in detail below.

[0031] In this invention, core-shell composite passivation modified particles refer to particulate materials with an internal mineralized core and an external active shell. The active shell is continuously distributed on the outer surface of the mineralized core. Micropores and rough interfaces formed during the granulation process are permitted in some parts of the particles, without affecting the determination of the core-shell structure. The calcium silicate-based modifier refers to an alkaline calcium silicate material with calcium silicate and wollastonite as the main mineral components. In this embodiment, commercially available calcium silicate material with a calcium silicate mass fraction of not less than 85% is used. Calcium-deficient hydroxyapatite refers to calcium phosphate material with a calcium-to-phosphorus molar ratio lower than the theoretical value of stoichiometric hydroxyapatite and within the range of 1.5 to 1.67.

[0032] Iron-loaded biochar refers to biochar that retains an iron-loaded phase on its surface or in its pores after being treated with ferric chloride aqueous solution, undergoing solid-liquid separation, washing, and drying. The iron-loaded phase is not limited to a single iron oxide crystalline phase, iron hydroxide crystalline phase, or amorphous phase. The iron content is determined by inductively coupled plasma after acid digestion.

[0033] The iron loading is calculated using the following formula:

[0034] The iron loading is the percentage of iron mass in the iron-loaded biochar. The percentage of weakly acid-extractable heavy metals refers to the percentage of the mass of weakly acid-extractable heavy metals obtained by continuous extraction relative to the total mass of the four heavy metal forms. The coefficient of variation of particle distribution concentration is the ratio of the standard deviation of particle concentration at multiple sampling points to the average particle concentration multiplied by 100%. The relative rate of change is the difference between the treated and untreated values ​​divided by the untreated value multiplied by 100%.

[0035] Rice husk biochar was prepared from rice husks in an oxygen-limited pyrolysis furnace. The rice husk raw material was purified and dried before being fed into the pyrolysis furnace and pyrolyzed at 450 °C for 5 h. After pyrolysis, the temperature was lowered to room temperature under air-isolated conditions. The resulting rice husk biochar had a carbon content of 52.3%, an ash content of less than 15%, and a specific surface area of ​​320 m². 2 / g. Ferric chloride is analytical grade ferric chloride hexahydrate, prepared as a 0.5 mol / L ferric chloride aqueous solution using deionized water. The silicon-calcium based modifier is a commercially available agricultural silicon-calcium material, with a calcium silicate mass fraction of not less than 85%, a fineness of 200 to 300 mesh, and a specific surface area of ​​15 to 20 m². 2 / g, the pH value of the aqueous extract is 10 to 11.

[0036] The calcium-deficient hydroxyapatite has a purity of not less than 95% and a calcium-to-phosphorus molar ratio of 1.5 to 1.67. The sodium humate is commercially available agricultural-grade sodium humate with a total humic acid content of not less than 50%, a fulvic acid content of not less than 30%, and a water-soluble content of not less than 80%. The sodium alginate is food-grade or industrial-grade; it is dissolved completely in deionized water to prepare a 4 g / L aqueous solution. The natural zeolite is a commercially available zeolite material with a high clinoptilolite content, a particle size of 0.5 to 2 mm, and a cation exchange capacity of 150 mmol / 100 g.

[0037] The number average molecular weight of sodium polyacrylate superabsorbent polymer is The water absorption ratio is 260 times. Bacillus laterosporus and Bacillus megaterium are produced using commercially available agricultural microbial agents, counted separately and mixed according to a mass ratio. Arbuscular mycorrhizal fungi are stored and applied separately and do not participate in the liquid oscillation activation of Bacillus. The disc granulator has a diameter of 2500 mm and an adjustable disc angle; the atomizing nozzle has an adjustable working pressure; the fluidized bed dryer has a temperature control range covering 60 to 110 ℃; the rotary tiller is equipped with L-shaped rotary blades and can perform forward and reverse rotation operations.

[0038] Soil pH was determined using the potentiometric method, with a water-to-soil volume ratio of 5:1. Air-dried soil samples were weighed, added to deionized water, shaken, and allowed to stand before measurement using a calibrated pH meter. Soil organic matter was determined using the potassium dichromate oxidation capacity method. Soil cation exchange capacity was determined using the ammonium acetate exchange method, with results expressed in cmol / kg. Soil bulk density was determined using the ring cutter method, and total soil porosity was calculated based on soil bulk density and particle density. Soil microbial biomass carbon was determined using chloroform fumigation extraction. Soil urease activity was determined colorimetrically after urea substrate culture, with results expressed in mg / (g·h). Total cadmium, lead, and zinc concentrations were determined using inductively coupled plasma mass spectrometry (ICP-MS) after mixed acid digestion. Reagent blanks, parallel samples, and standard substances were included in the detection process, and the calibration curve covered the measured concentrations of the samples.

[0039] Heavy metal speciation was determined using a continuous extraction method. 1.000 g of air-dried soil sample, passed through a 0.149 mm sieve, was placed in a 100 mL polyethylene centrifuge tube. For weak acid extraction, 40 mL of 0.11 mol / L acetic acid aqueous solution was added, and the mixture was shaken at 25 rpm for 16 h at 25 ± 2 ℃, followed by centrifugation at 3000 rpm for 10 min, and the supernatant was collected. For reducible extraction, 40 mL of 0.5 mol / L hydroxylamine hydrochloride aqueous solution was added to the solid residue after weak acid extraction, and the mixture was shaken and centrifuged under the same conditions, and the supernatant was collected. For oxidizable extraction, 10 mL of 30% hydrogen peroxide aqueous solution was added to the solid residue after reducible extraction, and the mixture was incubated at room temperature for 1 h, then heated to 85 ℃ for digestion. A second portion of hydrogen peroxide aqueous solution was added for repeated digestion. After cooling, 50 mL of 1 mol / L ammonium acetate aqueous solution was added, and the mixture was shaken and centrifuged to collect the supernatant. For the determination of residual states, the solid residue after extraction in the oxidizable state was transferred to a polytetrafluoroethylene digestion vessel and digested using a mixed acid mixture of nitric acid, hydrofluoric acid, and perchloric acid. The digestion solution was then diluted to a final volume before measurement. The supernatant from each step was filtered through a 0.45 μm filter membrane before measurement. The sum of the results from the four states was compared with the total soil digestion result for mass balance verification.

[0040] The iron-loaded biochar in the active shell has a porous structure and iron-binding sites, while sodium humate has carboxyl and phenolic hydroxyl groups. After soil moisture comes into contact with the outer surface of the particles, cadmium, lead, and zinc can be adsorbed by the biochar pores and undergo surface coordination or complexation with iron-containing sites and humic acid functional groups. The silicon-calcium-based amendment in the mineralized core can neutralize acidic soil and provide calcium and silicon-related reaction components. Calcium-deficient hydroxyapatite can provide a phosphate mineralization pathway, fixing some heavy metals through surface exchange, precipitation, and lattice substitution. The mineralized core is located inside the particles, and water must pass through the active shell and particle pores to enter. The core reaction has a longer mass transfer path than the outer shell reaction. Both the mineralized core and the active shell are effective passivation regions, without the use of an inert coating. The active shell undertakes early adsorption and complexation, while the mineralized core undertakes subsequent continuous mineralization. Together, these two functional regions constitute a time-continuous fixation process.

[0041] To facilitate a better understanding of the present invention by those skilled in the art, the present invention will be further illustrated below with reference to specific implementation examples.

[0042] Example 1: This example discloses homogeneous composite passivation modified particles and basic layered repair, with a repair area of ​​5.2 hm². 2 The farmland was contaminated with cadmium, lead, and zinc, and the soil type was medium loam. Before remediation, the soil pH was 5.8, organic matter content was 18.5 g / kg, cation exchange capacity was 12.3 cmol / kg, total cadmium content was 1.85 mg / kg, total lead content was 268 mg / kg, and total zinc content was 456 mg / kg. Monitoring points were set up at 3 points per hectare in the remediation area, for a total of 18 points. Soil samples from the 0 to 20 cm layer were collected at each point using a five-point pooling method. The monitoring data was represented by the average of the results from the 18 points.

[0043] Preparation of iron-loaded biochar. Rice husks were pyrolyzed at 450 °C under limited oxygen conditions for 5 h to obtain rice husk biochar. A 0.5 mol / L ferric chloride aqueous solution was added at a solid-liquid ratio of 1:10, and the mixture was stirred at 70 °C for 2.5 h. After stirring, the mixture was filtered, and the solid was washed with deionized water until the conductivity of the eluent reached 420 μS / cm. The washed solid was dried at 108 °C until its mass stabilized, yielding iron-loaded biochar. Acid digestion and inductively coupled plasma atomic emission spectrometry results showed that the iron content in the iron-loaded biochar was 4.2% by mass.

[0044] 470.5 kg of calcium silicate-based modifier, 184.7 kg of iron-loaded biochar, 235.2 kg of calcium-deficient hydroxyapatite, and 61.6 kg of sodium humate were weighed, with a total mass of 952 kg for the four functional solids. The four functional solids were fed into a disc granulator with a disc inclination angle of 50° and a disc rotation speed of 18 rpm. A sodium alginate aqueous solution with a mass concentration of 4 g / L was used as the granulation liquid, and 48 kg was cumulatively sprayed through an atomizing nozzle at a spray pressure of 0.4 MPa. The dry matter mass of sodium alginate was 0.192 kg, representing 0.0202% of the total mass of the four functional solids. Spraying was stopped after the particle size reached 3 to 4 mm, and the granules continued to rotate for 10 min. The wet granules were dried at 70 ℃ for 2.5 h. Particles with a diameter of 2 to 5 mm were collected by sieving, with a moisture content of 4.2% and a single particle compressive strength of 18 N. The same dry basis ratio was used to prepare granules meeting the requirements of 5.2 hm². 2 The amount of homogeneous composite passivation modified particles used in the repair area.

[0045] The deep passivation process is as follows: The application rate of homogeneous composite passivation improved granules is 220 kg / hm². 2 The total application rate was 1.144 t, using a towed pellet spreader with a spreader width of 4.5 m and a working speed of 4 km / h. Adjacent work rows overlapped by 40 cm. After application, a tracked rotary tiller was used for deep mixing at a speed of 250 rpm and a tillage depth of 30 cm. The working width was 2.5 m, and adjacent work rows overlapped by 12 cm.

[0046] The surface structure is improved as follows: The structure modifier consists of 65 parts by weight of natural zeolite and 35 parts by weight of sodium polyacrylate superabsorbent polymer. The natural zeolite has a cation exchange capacity of 150 mmol / 100 g and a particle size of 0.5 to 2 mm. The number average molecular weight of the sodium polyacrylate superabsorbent polymer is... The water absorption ratio is 260 times. The application rate of the structural modifier is 150 kg / hm². 2 The total application rate was 780 kg. After application, a second rotary tillage was performed to a depth of 18 cm at a speed of 200 rpm. Following rotary tillage, a 400 mm diameter twin-shaft roller was used for compaction at a pressure of 0.7 kg / cm². 2 The soil bulk density decreased from 1.45 g / cm³ after treatment. 3 Reduced to 1.28 g / cm³ 3 The total porosity of the soil increased from 45.2% to 51.8%.

[0047] Bioactivity restored. Bacillus laterosporus and Bacillus megaterium were mixed at a mass ratio of 1.5:1, with a total viable count of [missing information]. CFU / g, the bacterial inoculum was mixed with sterile water at a mass ratio of 1:10, and shaken at 28 ℃ and 140 rpm for 15 h. The total viable count of the two Bacillus species in the activated bacterial solution was [value missing]. CFU / mL. The bacterial agent application rate is 52 kg / hm². 2 The bacterial solution was continuously applied for 3 days via a drip irrigation system, with a dripper spacing of 30 cm, a dripper flow rate of 2.5 L / h, and an operating pressure of 0.12 MPa. During application, the soil moisture content was maintained at 60% to 80% of field capacity. The well-rotted organic fertilizer consisted of well-rotted cow manure and well-rotted chicken manure in a mass ratio of 2.5:1, with a total organic matter content of 48%, total nitrogen content of 1.6%, total phosphorus content of 1.1%, and total potassium content of 1.3%. The application rate was 3750 kg / hm². 2 After mixing in a 15 cm soil layer, irrigate with 180 m³ of water. 3 / hm 2 After 30 days of treatment, soil microbial biomass carbon increased from 86.5 mg / kg to 118.2 mg / kg, a relative increase of 36.6%; soil urease activity increased from 0.85 mg / (g·h) to 1.12 mg / (g·h), a relative increase of 31.8%.

[0048] Before treatment, the proportion of cadmium in the weakly acid-extractable form was 47.2%, lead was 38.7%, and zinc was 32.5%. After 30 days of treatment, the proportions of cadmium in the weakly acid-extractable, reducible, oxidizable, and residual forms were 28.6%, 33.2%, 18.5%, and 19.7%, respectively; the corresponding proportions for lead were 22.4%, 41.5%, 20.1%, and 16.0%; and for zinc were 19.3%, 38.6%, 22.7%, and 19.4%. After 180 days of treatment, the proportions of cadmium in the weakly acid-extractable, reducible, oxidizable, and residual forms were 22.1%, 30.8%, 21.3%, and 25.8%, respectively; the proportion of lead in the weakly acid-extractable form was 17.2%, and for zinc it was 14.5%. After 365 days of treatment, the proportion of cadmium in the weakly acid-extractable form was 18.1%.

[0049] Example 2: This example discloses core-shell type composite passivation modified particles and long-term passivation. This example uses the same source and specifications as Example 1 for a calcium-silica-based modifier, iron-supported biochar, calcium-deficient hydroxyapatite, and sodium humate. The total amount of the four functional solids, calculated per 100 parts by mass, is as follows: the mineralized core accounts for 74.13%, and the active shell accounts for 25.87%. The mineralized core is composed of the calcium-silica-based modifier and calcium-deficient hydroxyapatite in a mass ratio of 2:1, and the active shell is composed of the iron-supported biochar and sodium humate in a mass ratio of 3:1. After conversion, the calcium-silica-based modifier is 49.42 parts by mass, the calcium-deficient hydroxyapatite is 24.71 parts by mass, the iron-supported biochar is 19.40 parts by mass, and the sodium humate is 6.47 parts by mass.

[0050] For the preparation of mineralized cores, a silicon-calcium based modifier and calcium-deficient hydroxyapatite are fed into a disc granulation device with a disc inclination angle of 50° and a disc rotation speed of 18 rpm. A sodium alginate aqueous solution with a mass concentration of 4 g / L is sprayed on. After the mineralized core particle size reaches 2.2 to 2.4 mm, the spraying is stopped, and the device continues to roll for 10 min. The cores are then pre-dried at 60 ℃ until there is no free water on the surface.

[0051] For the active shell coating, iron-loaded biochar and sodium humate are mixed at a mass ratio of 3:1. The pre-dried mineralized core is then fed back into a disc granulator. During rotation, an aqueous solution of sodium alginate and active shell powder are sprayed in at a spray pressure of 0.4 MPa. Once the particle size reaches 3.8 to 4.0 mm, the addition of active shell powder is stopped. The wet core-shell particles are dried at 70 ℃ for 2.5 h, resulting in an average active shell thickness of 0.8 mm. The finished particles have a moisture content of less than 5% and a single particle compressive strength of not less than 15 N.

[0052] For field treatment, the application rate of core-shell type composite passivated and modified granules was 220 kg / hm². 2 The deep rotary tillage depth was 30 cm, and the structural modifier, bioactivity recovery, and monitoring conditions were consistent with those in Example 1. After 180 days of treatment, the proportions of cadmium in the weakly acid-extractable, reducible, oxidizable, and residual states were 16.3%, 29.7%, 22.5%, and 31.5%, respectively. After 180 days of treatment with homogeneous particles in Example 1, the proportion of weakly acid-extractable cadmium was 22.1%, a decrease of 5.8 percentage points compared to the core-shell particles; the proportion of residual cadmium was 25.8%, an increase of 5.7 percentage points compared to the core-shell particles. After 365 days of treatment, the proportion of weakly acid-extractable cadmium in the core-shell composite passivated modified particle treatment area was 13.7%, while it was 18.1% in the homogeneous particle treatment area of ​​Example 1 during the same period. The comparison shows that, under the condition of the same types of functional components and maintaining similar proportions within each functional region, configuring iron-loaded biochar and sodium humate in the active shell layer and configuring silicon-calcium-based modifier and calcium-deficient hydroxyapatite in the mineralized core can further reduce the proportion of long-term weakly acid-extractable cadmium.

[0053] Example 3: This example uses the core-shell type composite passivation modified granules prepared in Example 2. The target application rate of the composite passivation modified granules is calculated according to the following formula: ; Where D is the target application rate, kg / hm² 2 D0 is the baseline application rate, 160 kg / hm². 2 C represents the comprehensive pollution index; C_s represents the baseline comprehensive pollution index, 1.0; k1 represents the pollution degree correction factor, 0.08; pH represents the measured soil pH value; pH_s represents the target pH value, 7.0; k2 represents the pH correction factor, 0.2; CEC represents the measured soil cation exchange capacity, cmol / kg; CEC_s represents the baseline cation exchange capacity, 15 cmol / kg; k3 represents the cation exchange capacity correction factor, 0.15.

[0054] The comprehensive pollution index is calculated using the following formula: ; Wherein PI_Cd, PI_Pb, and PI_Zn are the single-factor pollution indices for cadmium, lead, and zinc, respectively. In this embodiment, calculated according to other farmland types, with a soil pH of 5.8, the risk screening values ​​for cadmium, lead, and zinc are 0.3 mg / kg, 90 mg / kg, and 200 mg / kg, respectively. The single-factor pollution index for cadmium is 6.17, for lead it is 2.98, and for zinc it is 2.28, with a comprehensive pollution index of 4.43. Substituting these values ​​into the calculation formula, the target application rate is 216.6 kg / hm². 2 The feed rate for the project is 217 kg / hm. 2 .

[0055] During operation, the rotary tiller moved forward in a forward rotation and reverse rotation in a reverse rotation. The tillage depth was 30 cm, the forward speed was 4 km / h, and the tillage speed was 300 rpm. The ratio of forward speed to tillage speed was 0.222 m / r. After the rotary tillage was completed, 20 sampling points were randomly placed in the 0-30 cm soil layer to determine the number of particles and particle mass concentration per unit mass of soil. The coefficient of variation for particle distribution concentration was 11.8% under alternating forward and reverse rotary tillage conditions, and 18.7% when only forward rotation was used.

[0056] Example 4: Restoration of surface bioactivity in bacterial-loaded natural zeolite. Natural zeolite was placed in a vacuum adsorption tank, and a vacuum was drawn to an absolute pressure of 0.02 MPa and maintained for 10 min. The total viable count of Bacillus laterosporus and Bacillus megaterium in the bacterial concentrate was [missing information]. CFU / mL, add 0.25 mL of bacterial concentrate per gram of natural zeolite, and let stand for 30 min after restoring normal pressure. Before bacterial loading, pre-load the natural zeolite with 0.8% yeast extract relative to the weight of the natural zeolite. After loading, dry under ventilation at 30 ℃ until the moisture content is below 15%. Mix the bacterial-loaded natural zeolite with sodium polyacrylate superabsorbent polymer at a mass ratio of 65:35. The structural modifier application rate is 150 kg / hm². 2 The bacteria were rotary tilled and incorporated into the top 18 cm of soil. After 15 days of treatment, the number of viable bacteria in the surface soil of the treated area with natural zeolite was [number missing]. CFU / g, the treatment area using liquid drip irrigation to apply bacterial agents was at the same time CFU / g; After 30 days of treatment, the soil urease activity in the bacterial-loaded natural zeolite treatment area was 1.31 mg / (g·h), while that in the liquid drip irrigation treatment area was 1.12 mg / (g·h). The bacterial-loaded natural zeolite utilizes negative pressure to expel air from the zeolite pores, allowing the bacterial solution to enter the zeolite pores during the restoration of normal pressure. The yeast extract provides nutrients for the initial colonization of bacteria. The combination of the natural zeolite porous carrier and the surface structure modification layer enhances the retention of bacteria on the surface.

[0057] Comparative Example 1: Biochar without ferric chloride loading treatment. Comparative Example 1 used rice husk biochar prepared under the same batch of rice husks and pyrolysis conditions as Example 1, without ferric chloride aqueous solution loading treatment. Except for omitting the ferric chloride loading step, the homogeneous particle formulation, sodium alginate aqueous solution, particle size, particle application rate, rotary tillage depth, structure modifier, bioactivity restoration method, soil conditions, and monitoring methods were all consistent with Example 1.

[0058] After 30 days of treatment, the proportions of cadmium in the weakly acid-extractable, reducible, oxidizable, and residual forms were 35.7%, 32.4%, 17.2%, and 14.7%, respectively; the proportion of lead in the weakly acid-extractable form was 29.8%; and the proportion of zinc in the weakly acid-extractable form was 25.6%. After 180 days of treatment, the proportions of cadmium in the weakly acid-extractable, reducible, oxidizable, and residual forms were 29.4%, 31.1%, 19.6%, and 19.9%, respectively. In Example 1, when iron-supported biochar was used, after 30 days of treatment, the proportion of cadmium in the weakly acid-extractable form was 28.6%, a decrease of 7.1 percentage points compared to Comparative Example 1; the proportion of lead in the weakly acid-extractable form was 22.4%, a decrease of 7.4 percentage points; the proportion of zinc in the weakly acid-extractable form was 19.3%, a decrease of 6.3 percentage points; and after 180 days of treatment, the proportion of cadmium in the weakly acid-extractable form was 22.1%, a decrease of 7.3 percentage points. The comparative results show that the loading treatment with ferric chloride aqueous solution directly contributes to the heavy metal fixation capacity of biochar.

[0059] Long-term monitoring and risk response. Monitoring will be conducted at fixed locations in the remediation area. Soil samples will be collected from the 0-20cm, 20-40cm, and 40-60cm depths at each location, with a monitoring interval of 30 days and a monitoring period of no less than 180 days. The monthly change rate of the proportion of weakly acidic extractables between two consecutive monitoring periods will be calculated using the following formula:

[0060] The monthly change rate is the percentage change in the proportion of weakly acidic extractables between two consecutive monitoring periods. When the monthly change rate is less than 2%, a 30-day monitoring interval is maintained; when the monthly change rate is between 2% and 5%, a 0.5% sodium humate aqueous solution is sprayed onto the corresponding area at a spraying rate of 40 m³. 3 / hm 2 When the monthly change rate is higher than 5%, supplement with core-shell type composite passivation and improvement particles, with the supplementary application amount being 40% of the initial application amount. After supplementation, use shallow rotary tillage to mix it into the 0 to 20 cm soil layer to avoid damaging the already formed deep passivation and improvement layer.

[0061] Instructions for calculating material application rates. The calculation of the application rate of composite passivation and amendment particles should first determine the farmland type and soil pH range, then select the corresponding heavy metal risk screening value. Screening values ​​for different land use types must not be mixed. Pollution index correction reflects the degree of pollution, pH correction reflects changes in heavy metal activity under acidic conditions, and cation exchange capacity correction reflects the soil's ion retention capacity. The calculation results are used to determine the feed level for the project. If the construction equipment cannot continuously output arbitrary decimal dosages, the calculation results can be adjusted to an adjacent feed level, and the adjusted actual dosage should be recorded.

[0062] This invention utilizes a combination of internal particle functional zoning and soil layer functional zoning: core-shell particles penetrate the 25-35cm soil layer to stabilize heavy metals in the root zone, which is the primary source of pollution; natural zeolite and sodium polyacrylate superabsorbent polymer penetrate the 15-20cm soil layer to facilitate surface ion exchange, water retention, and rhizosphere environment improvement. These two spatial zoning methods address the issues of particle reaction sequence and soil layer functional differences, respectively. The core-shell composite passivation and modification particles are configured with functional components according to the reaction sequence of different fixation mechanisms: iron-loaded biochar and sodium humate are located in the outer layer, providing adsorption and complexation functions when the particles contact soil moisture; calcium silicate-based amendments and calcium-deficient hydroxyapatite are located in the inner layer, continuously providing mineralization and fixation functions as moisture gradually enters the particle interior. Both the active shell and the mineralized core directly participate in heavy metal fixation. Long-term morphological data from Examples 2 and 1 show that the core-shell spatial configuration can further reduce the proportion of cadmium in weakly acid-extractable form. Data from Comparative Example 1 and Example 1 show that iron-containing loading treatment can reduce the proportion of heavy metals in weakly acid-extractable form. Data from alternating forward and reverse rotary tillage show that the construction method can reduce the coefficient of variation of particle distribution concentration. Data from naturally zeolite loaded with bacteria show that loading bacteria into the pores of natural zeolite can increase the number of viable bacteria and urease activity on the surface.

[0063] Furthermore, in some preferred embodiments, the calcium silicate-based amendment can be a steel slag-based calcium silicate material that meets the requirements for calcium silicate content, fineness, and alkalinity. Before use, it undergoes crushing, magnetic separation for iron removal, grinding, and heavy metal leaching safety testing. It cannot directly replace the calcium silicate-based amendment in the examples if the components are missing or leaching tests are not performed. Arbuscular mycorrhizal fungi can be inoculated separately into the crop rhizosphere after the structural amendment is applied. Their effectiveness is evaluated by spore count, hyphal length, root segment infection rate, or number of effective propagules. They are not co-inoculated with *Bacillus laterosporus* and *Bacillus megaterium* for hostless liquid amplification. After deep rotary tillage, water-based maintenance can be carried out, with an irrigation volume of 80 to 120 m³. 3 / hm 2 During the maintenance period, the soil moisture content was maintained at 65% to 75% of field capacity, and the maintenance time was 7 days, to promote particle wetting and reaction initiation. Monitoring data can be stored in a local database, a remote server, or an agricultural environment management platform; the data transmission method does not constitute a necessary technical feature of the remediation method.

[0064] The implementation methods described in this manual are used to illustrate the process of implementing a heavy metal passivation remediation method based on soil improvement. Without altering the functional zoning relationship between the mineralized core and the active shell, or the zoning relationship between deep passivation and surface structure improvement, the equipment specifications and construction organization methods can be adjusted according to the remediation area and soil texture.

[0065] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0066] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. It should be noted that any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for heavy metal passivation remediation based on soil improvement, characterized by comprising the following steps: Collect soil samples contaminated with heavy metals and determine soil pH, organic matter content, cation exchange capacity, and total and speciation content of cadmium, lead, and zinc; Core-shell composite passivation and amendment particles were spread onto the surface of heavy metal contaminated soil. Deep rotary tillage was then used to integrate these particles into a 25-35 cm soil layer, forming a passivation and amendment layer. The core-shell composite passivation and amendment particles comprise a mineralized core, an active shell coating the mineralized core, and a sodium alginate binder phase distributed within both the mineralized core and the active shell. The mineralized core consists of 49.42 parts by weight of a calcium-silica-based amendment and 24.71 parts by weight of calcium-deficient hydroxyapatite. The active shell consists of 19.40 parts by weight of iron-loaded biochar and 6.47 parts by weight of sodium humate. The total dry weight of the calcium-silica-based amendment, calcium-deficient hydroxyapatite, iron-loaded biochar, and sodium humate is 100 parts by weight. The sodium alginate binder phase comprises 0.02 parts by weight. The iron-loaded biochar is obtained by treating rice husk biochar with ferric chloride aqueous solution, followed by solid-liquid separation, washing, and drying. A structural modifier is applied to the soil surface that forms the passivation and improvement layer. The structural modifier is composed of natural zeolite and sodium polyacrylate water-absorbing resin. The structural modifier is mixed into the soil layer of 15 to 20 cm by surface rotary tillage to form the structural modification layer. Apply bacterial inoculants and well-rotted organic fertilizer to the structural improvement layer, and maintain the soil moisture content at 60% to 80% of field capacity; Soil samples were collected at three depths: 0 to 20 cm, 20 to 40 cm, and 40 to 60 cm. Soil physicochemical indicators and the content of cadmium, lead, and zinc in their forms were measured.

2. The heavy metal passivation remediation method based on soil improvement according to claim 1, characterized in that the iron content in the iron-loaded biochar accounts for 4.2% of the total iron-loaded biochar mass.

3. The heavy metal passivation remediation method based on soil improvement according to claim 1, characterized in that the calcium silicate-based amendment contains calcium silicate and wollastonite, the calcium silicate mass fraction is not less than 85%, the calcium-phosphorus molar ratio of the calcium-deficient hydroxyapatite is 1.5 to 1.67, and the total mass fraction of humic acid in sodium humate is not less than 50%.

4. The heavy metal passivation remediation method based on soil improvement according to claim 1, characterized in that the particle size of the mineralized core is 2.2 to 2.4 mm, the particle size of the core-shell composite passivation improvement particles is 3.8 to 4.0 mm, and the thickness of the active shell layer is 0.8 mm.

5. The method for heavy metal passivation remediation based on soil improvement according to claim 1, characterized in that the moisture content of the core-shell composite passivation improvement particles is not higher than 5%, the compressive strength of a single particle is not less than 15 N, and the bulk density is 800 to 1000 kg / m³. 3 .

6. The method for heavy metal passivation remediation based on soil improvement according to claim 1, characterized in that the preparation method of core-shell composite passivation improvement particles includes the following steps: Rice husk biochar was mixed with ferric chloride aqueous solution and subjected to loading treatment under heating and stirring conditions. After loading treatment, solid-liquid separation was performed, and the obtained solid was washed and dried to obtain iron-loaded biochar. A calcium-silicon-based modifier is mixed with calcium-deficient hydroxyapatite, and an aqueous solution of sodium alginate is sprayed on the rolling state to form a mineralized core. Iron-loaded biochar was mixed with sodium humate to obtain activated shell powder. The mineralized core is placed in a rolling state, and active shell powder is added at the same time as the sodium alginate aqueous solution is sprayed, so that the active shell powder coats the mineralized core to obtain wet core-shell particles. The wet core-shell particles were dried and sieved to obtain core-shell type composite passivation modified particles.

7. The heavy metal passivation remediation method based on soil improvement according to claim 6, characterized in that the concentration of ferric chloride aqueous solution is 0.5 mol / L, the solid-liquid ratio of rice husk biochar to ferric chloride aqueous solution is 1:10, the loading treatment temperature is 70 ℃, the loading treatment time is 2.5 h; the washing endpoint is that the conductivity of the eluent is not higher than 500 μS / cm, and the drying temperature is 105 to 110 ℃.

8. The heavy metal passivation remediation method based on soil improvement according to claim 6, characterized in that the mineralized core is prepared by a disc granulation device with a disc inclination angle of 50° and a disc rotation speed of 18 rpm, and the sodium alginate aqueous solution is added by atomization spraying, and the spraying is stopped when the mineralized core reaches a particle size of 2.2 to 2.4 mm.

9. A method for heavy metal passivation remediation based on soil improvement according to claim 6, characterized in that the spray pressure is controlled at 0.4 MPa during the active shell coating process, the coating is stopped when the wet core-shell particles reach a particle size of 3.8 to 4.0 mm, and the wet core-shell particles are dried at 70 ℃ for 2.5 h.

10. The method for heavy metal passivation remediation based on soil improvement according to claim 1, characterized in that the application rate of core-shell composite passivation improvement particles is 214 to 220 kg / hm². 2 The application rate of the structural modifier is 150 kg / hm². 2 The structure modifier consists of 65 parts by weight of natural zeolite and 35 parts by weight of sodium polyacrylate superabsorbent polymer. The natural zeolite has a particle size of 0.5 to 2 mm, and the sodium polyacrylate superabsorbent polymer has a water absorption ratio of 200 to 300 times.

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