Heavy metal passivated coal gangue-based ecological soil conditioner

Abstract

This product relates to the field of soil conditioning materials technology, and discloses a heavy metal passivation coal gangue-based ecological soil conditioner. The conditioner comprises an activated porous silica-alumina carrier from coal gangue, magnesium-iron layered double hydroxides in situ loaded on the pore walls and particle surface of the carrier, a humic complex layer bound to its surface, and phosphorus-containing and silicon-containing mineral components. Exchangeable anions are present between the magnesium-iron layered double hydroxides, and the humic substances are coordinated with magnesium and iron hydroxyl sites through carboxyl and phenolic hydroxyl groups, forming composite particles with a porous core, a composite interface layer, and a complex coating layer sequentially connected. This conditioner can construct continuous adsorption, exchange, coordination, and mineralization fixation pathways, reducing the leaching and migration of cationic and anionic heavy metals, and improving soil organic matter retention, mineral nutrient supply, and aggregate structure.

Description

Technical Field

[0001] This product relates to the field of soil conditioning materials technology, and discloses a heavy metal passivated coal gangue-based ecological soil conditioner. Background Technology

[0002] The remediation of heavy metal-contaminated soil typically employs lime, phosphates, iron and manganese oxides, clay minerals, and humic materials as passivating components. These components reduce the available content of heavy metals such as lead, cadmium, copper, and zinc by adjusting the soil's pH, ion exchange, surface complexation, or the formation of insoluble precipitates. Coal gangue contains silicate-alumina minerals, iron minerals, and small amounts of calcium and magnesium components. After crushing, screening, and mechanical activation, it can form a certain number of pores and surface hydroxyl groups, thus being used to prepare soil conditioners. Conventional coal gangue-based conditioners are mostly composed of coal gangue powder, directly mixed with lime, phosphate rock powder, organic fertilizer, or humic acid, then granulated and applied to contaminated soil. The mineral framework of the coal gangue adsorbs some metal ions, and the alkaline and phosphorus-containing components promote the conversion of heavy metals into carbonate, phosphate, or mineral-bound forms. The preparation process of this type of scheme is relatively simple, but the number of active sites available in the coal gangue body is limited, the mineral components are unevenly distributed in the mixture, there is a lack of continuous interfaces between different passivation components, and heavy metals often need to go through a long diffusion path when migrating from pore water to the stationary phase, making it difficult to form a stable and continuous multi-level stationary structure.

[0003] To broaden the treatment range of different forms of heavy metals by coal gangue-based materials, existing technologies also load iron oxides, magnesium oxides, or layered double hydroxides onto the surface of coal gangue. A common preparation method involves mixing coal gangue with solutions containing iron and magnesium salts, followed by precipitation, drying, and pulverization to obtain a composite powder. This powder utilizes iron hydroxyl sites to complex anionic pollutants such as arsenate and chromate, and uses interlayer exchange sites or surface charges to adsorb cationic pollutants such as cadmium and lead. Under experimental conditions, such materials can simultaneously contact multiple types of pollutants. However, the precipitated phase tends to agglomerate independently on the outer surface of the coal gangue, and secondary agglomeration can occur after entering the soil, leading to the obstruction of internal exchange sites and pores. If the loaded phase adheres to the coal gangue solely through physical adhesion, alternating wet and dry conditions, changes in salinity, and root exudates can cause fine particles to detach or migrate, altering the spatial position of the fixed interface. Some solutions use a single iron-based material to treat anionic pollutants or a single magnesium-based material to treat cationic pollutants. However, these materials lack nucleation and exchange structures that extend continuously from the pore walls of the carrier to the outer surface, thus limiting their adaptability to soils where multiple heavy metals coexist.

[0004] In improving soil fertility, existing coal gangue conditioners typically incorporate humic acid, fulvic acid, lignin derivatives, phosphate rock powder, or silicate minerals to supplement organic matter and mineral nutrients. Conventional processes often employ dry mixing or wet coating to adsorb humic substances onto the surface of coal gangue or inorganic passivating agent particles, which are then granulated using binders. However, due to the lack of stable coordination between humic substances and the inorganic loading phase, humic substances easily migrate with water after being applied to the soil or are competitively displaced by salt ions, making it difficult to maintain their position near the heavy metal fixation interface for extended periods. If phosphorus-containing and silicon-containing components are randomly dispersed within the particles, their release may be delayed due to pore blockage, or they may dissolve rapidly due to concentrated exposure, resulting in a spatial separation between the passivation reaction and nutrient supply. Although some composite particles are coated, the coating layer and the inner mineral layer are only mechanically bonded. After the particles absorb water and swell, they are prone to cracking. This causes the humus, layered minerals and coal gangue skeleton to act independently, and they cannot maintain the composite interface jointly formed by mineral adsorption, interlayer exchange, humus complexation and mineral nutrient retention.

[0005] Therefore, the main technical problem faced by existing heavy metal passivation coal gangue-based soil conditioners is that the coal gangue carrier, magnesium-iron passivation phase, and humic components usually exist in a simple mixture or weak adhesion mode. There is a lack of a stable interface between the three, formed by the coal gangue channels, inorganic layered minerals, and organic complex layers. This results in cationic and anionic heavy metals not obtaining a continuous, interconnected fixed pathway within the same particle. When there are insufficient active sites on the coal gangue surface, layered minerals easily aggregate and obscure exchange sites, while humic substances may detach from the inorganic surface, causing adsorption, interlayer exchange, coordination complexation, and mineral precipitation to occur at dispersed locations. When the soil experiences acid-base fluctuations, salt competition, leaching, and wet-dry cycles, the original fixed structure is prone to local deactivation or component migration, making it difficult to simultaneously maintain stable passivation of multiple heavy metals and continuous improvement of soil organic matter, mineral nutrients, and structural indicators. There is an urgent need to construct an integrated composite structure with activated coal gangue as a porous framework, magnesium-iron layered double hydroxides as an inorganic fixed interface, and humic substances as an outer complex network. Summary of the Invention

[0006] To address the shortcomings of existing coal gangue-based soil conditioners, such as the lack of a continuous and stable composite interface between the coal gangue carrier, magnesium-iron passivation phase, and humic components, which makes it difficult for cationic and anionic heavy metals to form a fixed pathway in the same material, and the tendency for different functional components to aggregate, separate, or migrate in the soil environment, this product provides a heavy metal passivated coal gangue-based ecological soil conditioner.

[0007] To address the aforementioned technical issues, this product provides a heavy metal passivation coal gangue-based ecological soil conditioner, comprising an activated porous silica-alumina carrier, magnesium-iron layered double hydroxides in situ loaded on the pore walls and particle surfaces of the activated porous silica-alumina carrier, and a humic complex layer bound to the surface of the magnesium-iron layered double hydroxides. The magnesium-iron layered double hydroxides contain exchangeable anions in their interlayer. The humic complex layer is formed by humic substances containing carboxyl and phenolic hydroxyl groups coordinating with magnesium hydroxyl sites and iron hydroxyl sites on the surface of the magnesium-iron layered double hydroxides through the carboxyl and phenolic hydroxyl groups. The activated porous silica-alumina carrier, the magnesium-iron layered double hydroxides, and the humic complex layer sequentially constitute an interconnected multi-level composite interface.

[0008] After the heavy metal passivation coal gangue-based ecological soil conditioner is applied to the soil, soil moisture carries heavy metal ions into the pores of the activated coal gangue porous silica-alumina carrier, where the heavy metal ions sequentially contact the pore walls of the coal gangue, the magnesium-iron layered double hydroxides, and the humic complex layer. The activated coal gangue porous silica-alumina carrier enriches heavy metal ions in the soil solution through its pore structure and binds to them via hydroxyl sites on the silica-alumina mineral surface. The magnesium-iron layered double hydroxides exchange anions with anionic pollutants in the soil solution through interlayer exchangeable anions and form coordination bonds with heavy metal ions via magnesium and iron hydroxyl sites. The carboxyl and phenolic hydroxyl groups in the humic complex layer continue to complex with heavy metal ions not fixed by the inorganic interface, while maintaining the connection between the humic complex layer and the magnesium-iron layered double hydroxides. This creates a working mechanism that sequentially connects pore enrichment, surface binding, interlayer exchange, and organic complexation.

[0009] Furthermore, in the above technical solution, the activated coal gangue porous silica-alumina carrier is prepared by removing impurities, crushing, classifying, desalinating and mechanically activating low-sulfur coal gangue. The activated coal gangue porous silica-alumina carrier has interconnected channels and silanol sites, aluminumol sites and lattice defect sites distributed on the inner wall of the channels and the surface of the particles.

[0010] In practice, impurity removal and desalination treatments reduce the interference of free impurities and soluble salts in coal gangue on subsequent interfacial bonding. Crushing and grading bring the coal gangue particles to a suitable particle size for loading. Mechanical activation causes the silica-alumina mineral particles to dissociate and expose new fracture surfaces. These fracture surfaces form silanol sites, aluminol sites, and lattice defect sites. When soil moisture enters the interconnected channels, heavy metal ions can migrate along the channels into the particle interior and undergo adsorption or coordination at active sites on the channel walls and particle surfaces. Lattice defect sites can also serve as nucleation sites for magnesium-iron layered double hydroxides, allowing the inorganic passivation phase to distribute on the internal pore walls and external surfaces of the coal gangue carrier.

[0011] Furthermore, in the above technical solution, the magnesium-iron layered double hydroxide is formed by co-deposition of a magnesium-containing precursor and an iron-containing precursor in the presence of an activated porous silica-alumina support for coal gangue. Its layers are connected to the activated porous silica-alumina support for coal gangue through at least one of magnesium-oxygen-silicon bonds, ferro-oxygen-silicon bonds, magnesium-oxygen-aluminum bonds, and ferro-oxygen-aluminum bonds. The exchangeable anions include at least one of chloride ions, nitrate ions, and carbonate ions.

[0012] In practice, when magnesium-containing and iron-containing precursors co-deposit in the presence of activated porous aluminosilicate coal gangue, the magnesium-iron layered double hydroxide preferentially nucleates at silanol sites, aluminol sites, and lattice defect sites. During nucleation, magnesium-oxy-silicon, ferro-silicon, magnesium-oxy-aluminum, or ferro-oxy-aluminum bonds are formed between the magnesium-iron layered double hydroxide sheets and the activated porous aluminosilicate coal gangue support, establishing a chemical bond between the sheets and the support. After the soil solution comes into contact with the magnesium-iron layered double hydroxide, chloride, nitrate, or carbonate ions between the layers can exchange with anionic pollutants in the soil solution, while the magnesium and iron hydroxyl sites on the sheet surface coordinate with heavy metal ions.

[0013] Furthermore, in the above technical solution, the humic substance includes at least one of humic acid, fulvic acid and oxidized lignin humic substance, and the humic substance complex layer covers the lamellar surface and interlamellar gaps of the magnesium-iron layered double hydroxide, and is connected to the magnesium-iron layered double hydroxide through coordination bonds and hydrogen bonds.

[0014] In practice, humic acid, fulvic acid, or oxidized lignin humic substances contain carboxyl and phenolic hydroxyl groups. These carboxyl and phenolic hydroxyl groups can form coordinate bonds with the magnesium hydroxyl sites and iron hydroxyl sites on the surface of the magnesium-iron layered double hydroxide, and can also form hydrogen bonds with the hydroxyl groups on the lamellar surface. Humic substances thus cover the lamellar surface of the magnesium-iron layered double hydroxide and enter the interlamellar spaces, forming a humic complex layer connected to the inorganic lamellars. After soil moisture enters the interlamellar spaces, dissolved heavy metal ions can simultaneously contact the inorganic hydroxyl sites and humic functional groups. Heavy metal ions that have not been fixed at the inorganic interface continue to complex with the carboxyl and phenolic hydroxyl groups.

[0015] Furthermore, in the above technical solution, the heavy metal passivated coal gangue-based ecological soil conditioner also includes mineral nutrients, which include phosphorus-containing mineral components and silicon-containing mineral components. The phosphorus-containing mineral components and silicon-containing mineral components are dispersed in at least one location in the pores of the activated coal gangue porous silica-alumina carrier, the interlayer gaps of magnesium-iron layered double hydroxides, and the humic complex layer.

[0016] In practice, phosphorus-containing and silicon-containing mineral components are dispersed within a multi-level composite interface formed by the activated coal gangue porous silica-alumina carrier, magnesium-iron layered double hydroxides, and humic complex layer, placing the mineral nutrient components adjacent to the heavy metal fixation interface. As soil moisture migrates along the pores of the activated coal gangue porous silica-alumina carrier, it simultaneously contacts both phosphorus-containing and silicon-containing mineral components. The phosphorus-containing mineral components come into contact with heavy metal ions entering the composite interface, while the silicon-containing mineral components remain within the porous silica-alumina carrier and its pore structure. The humic complex layer coats or restricts the mineral nutrient components, reducing their aggregation within the particles.

[0017] Furthermore, in the above technical solution, the activated coal gangue porous silica-alumina carrier includes an interconnected coarse pore support zone and a fine pore nucleation zone. The fine pore nucleation zone is located on the pore wall surface of the coarse pore support zone. The nucleation lamellae of magnesium-iron layered double hydroxides are embedded in the fine pore nucleation zone. The extended lamellae of magnesium-iron layered double hydroxides extend continuously from the fine pore nucleation zone to the coarse pore support zone and the outer surface of the particles.

[0018] In practice, the coarse-pore carrying zone provides a migration channel for soil moisture and heavy metal ions to enter the particle interior, while the fine-pore nucleation zone provides smaller and more concentrated nucleation spaces. After forming nucleation plates in the fine-pore nucleation zone, the magnesium-iron layered double hydroxide remains dispersed due to the pore walls, extending from the fine-pore nucleation zone to the coarse-pore carrying zone and the outer surface of the particle, forming extended plates. After being applied to the soil, heavy metal ions enter the particle interior along the coarse-pore carrying zone and sequentially contact the nucleation plates on the surface of the coarse-pore walls and the outwardly extending plates, thus creating a continuous inorganic fixed interface between the particle's internal pores and its outer surface.

[0019] Furthermore, in the above technical solution, the magnesium-iron layered double hydroxide includes an iron-rich anchoring layer directly connected to the activated coal gangue porous silica-alumina carrier and a magnesium-rich exchange layer disposed outside the iron-rich anchoring layer. The relative content of iron in the magnesium-iron layered double hydroxide decreases in the direction away from the activated coal gangue porous silica-alumina carrier, and the relative content of magnesium increases in the direction away from the activated coal gangue porous silica-alumina carrier.

[0020] In practice, the iron-rich anchoring layer is located between the activated coal gangue porous silica-alumina carrier and the magnesium-iron layered double hydroxide. Iron is connected to the coal gangue carrier via ferro-silicon bonds or ferro-aluminum bonds, keeping the magnesium-iron layered double hydroxide on the pore walls and particle surfaces. The magnesium-rich exchange layer is located outside the iron-rich anchoring layer, allowing direct contact with the soil solution and providing an interlayer exchange interface. The gradient distribution of iron and magnesium creates a connecting interface near the coal gangue carrier and an exchange interface away from it. As heavy metal ions migrate from the outside in, they sequentially contact the magnesium-rich exchange layer and the iron-rich anchoring layer.

[0021] Furthermore, in the above technical solution, the humic complex layer includes an inner coordination layer attached to the surface of the magnesium-iron layered double hydroxide and an ion-bridged network layer covering the outer side of the inner coordination layer. The inner coordination layer is formed by the coordination connection of the carboxyl groups and phenolic hydroxyl groups of the humic substance with the magnesium-iron layered double hydroxide, and the ion-bridged network layer is formed by the bridging of adjacent humic molecules through at least one of calcium ions and magnesium ions.

[0022] In practice, humic molecules in the inner coordination layer form coordination bonds with the surface of the magnesium-iron layered double hydroxide through carboxyl and phenolic hydroxyl groups, allowing the humic substances to adhere to the surface of the inorganic sheets. Humic molecules located on the outer side of the inner coordination layer form ion bridges through calcium or magnesium ions, connecting adjacent humic molecules into a network of ion bridges. When soil moisture enters the network of ion bridges, heavy metal ions first contact the functional groups of the outer humic layer before migrating into the inner coordination layer and the magnesium-iron layered double hydroxide. The network of ion bridges maintains the connections between adjacent humic molecules, while the inner coordination layer maintains the connections between the humic complex layer and the inorganic sheets.

[0023] Furthermore, in the above technical solution, the phosphorus-containing mineral component includes at least one of phosphate rock powder, calcium phosphate mineral, and calcium magnesium phosphate mineral; the silicon-containing mineral component includes at least one of wollastonite, calcium silicate mineral, and potassium silicate mineral; the phosphorus-containing mineral component is at least partially disposed between the magnesium-iron layered double hydroxide and the humic complex layer; and the silicon-containing mineral component is at least partially embedded in the pores of the activated coal gangue porous silica-alumina carrier.

[0024] In practice, phosphate rock powder, calcium phosphate minerals, or calcium magnesium phosphate minerals are placed between the magnesium-iron layered double hydroxide and the humic complex layer, allowing heavy metal ions entering the humic complex layer to contact the phosphorus-containing mineral components as they continue to migrate towards the magnesium-iron layered double hydroxide. Wollastonite, calcium silicate minerals, or potassium silicate minerals are embedded within the pores of the activated coal gangue porous silica-alumina carrier, confining the silicon-containing mineral components within the pore structure and forming a porous core together with the coal gangue silica-alumina skeleton. As soil moisture migrates from the humic complex layer into the activated coal gangue porous silica-alumina carrier, it passes sequentially through the interface containing the phosphorus-containing mineral components and the pores containing the silicon-containing mineral components.

[0025] Furthermore, in the above technical solution, the heavy metal passivated coal gangue-based ecological soil conditioner is a composite particle. The composite particle comprises, from the inside out, a porous core composed of activated coal gangue porous silica-alumina carrier and silicon-containing mineral components, a composite interface layer composed of magnesium-iron layered double hydroxide and phosphorus-containing mineral components, and a complexing coating layer composed of humus. The composite interface layer is embedded in the peripheral pores of the porous core, and the complexing coating layer is bridged to the composite interface layer through at least one of calcium ions and magnesium ions.

[0026] In practice, after the composite particles are applied to the soil, soil moisture first enters the complexed coating layer composed of humus, where dissolved heavy metal ions come into contact with the functional groups of humus. Subsequently, soil moisture and uncomplexed heavy metal ions enter the composite interface layer composed of layered magnesium-iron double hydroxides and phosphorus-containing mineral components, and come into contact with interlayer exchange sites, magnesium hydroxyl sites, iron hydroxyl sites, and phosphorus-containing mineral components. The composite interface layer embeds peripheral pores into the porous core, creating a continuous connection between the outer and inner layers, allowing soil moisture to continue entering the porous core composed of activated coal gangue porous silica-alumina carrier and silicon-containing mineral components. The complexed coating layer is bridged to the composite interface layer by calcium or magnesium ions, ensuring that the complexed coating layer, composite interface layer, and porous core, arranged from the outside in, together maintain the hierarchical structure of the composite particles.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. A multi-level composite interface is formed by using activated coal gangue porous silica-alumina carrier, in-situ loaded magnesium-iron layered double hydroxides, and a humic complex layer bound to its surface, creating a continuous extension from the carrier pore walls to the outer surface of the particles. The interconnected channels, silanol sites, aluminum hydroxyl sites, and lattice defect sites of the activated coal gangue provide dispersed nucleation sites for the magnesium-iron layered double hydroxides, allowing the layered sheets to be embedded in the fine-pore nucleation region and extend to the coarse-pore support region and the outer surface of the particles, reducing site shielding caused by independent precipitation and sheet stacking. The iron-rich anchoring layer is connected to the coal gangue skeleton, and the magnesium-rich exchange layer is located on the outside, cooperating with the interlayer exchangeable anions to form adjacent fixation regions for anionic and cationic heavy metals in the same composite particle. The carboxyl and phenolic hydroxyl groups in the humic material coordinate with the magnesium hydroxyl sites and iron hydroxyl sites to form an inner coordination layer that adheres to the inorganic sheets, and forms an outer ion bridge network layer through calcium or magnesium ions, maintaining the connection between the inorganic fixation interface and the organic complex interface. After entering the particles, heavy metals can contact silicon-aluminum sites, interlayer exchange sites, and humic coordination sites along the connecting channels, thereby forming continuous adsorption, exchange, and complexation pathways. This reduces the migration risk caused by saturation or local deactivation of a single stationary phase and addresses the problem of the difficulty in stably passivating multiple types of heavy metals in the same coal gangue-based material.

[0028] 2. Mineral nutrients are distributed in at least one location within the activated coal gangue pores, the interlayers of the magnesium-iron layered double hydroxides, and the humic complex layer. Silicon-containing mineral components are embedded in the porous core, while phosphorus-containing mineral components are located in the composite interface layer or between the magnesium-iron layered double hydroxides and the humic complex layer. This ensures that the nutrient components are distributed adjacently to the heavy metal fixed interface, reducing local aggregation and migration caused by random mixing. The composite particle consists of a porous core, a composite interface layer, and a complex coating layer, arranged sequentially. The composite interface layer is embedded in the peripheral pores of the porous core, and the complex coating layer is bridged to the composite interface layer by calcium or magnesium ions, reducing the degree of separation of functional components after water absorption. The coarse-pore bearing zone and fine-pore nucleation zone of the activated coal gangue provide the structural basis for water entry, ion migration, and mineral component dispersion, while the outer humic layer forms a continuous organic network and maintains carboxyl and phenolic hydroxyl coordination sites. Phosphorus-containing minerals, silicon-containing minerals, and the original silica-alumina minerals in coal gangue are all retained inside the particles. This allows the conditioner to not only fix heavy metals but also replenish phosphorus and silicon mineral components, maintain humus, and preserve the pore structure of the particles. This improves the material basis required for soil organic matter carrying capacity, mineral nutrient retention, and aggregate formation. The components maintain their relative positions within the same particle, facilitating a stable material distribution after application. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to embodiments. Those skilled in the art can reproduce the technical solution of the present invention and achieve its claimed technical effects based on the content disclosed in this specification. It should be noted that the following embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention. Any non-substantial improvements and adjustments made based on the core concept of the present invention should fall within the scope of protection of the present invention.

[0030] In the following examples, the amounts of each component are based on a dry basis. Low-sulfur coal gangue refers to coal gangue with a total sulfur content of no more than 0.8%. The mass of the magnesium-iron layered double hydroxide is calculated based on the actual dry basis mass after washing and drying. Water, alkaline precipitate, and washing liquid used in the preparation process are not included in the mass of the main product formulation.

[0031] Example 1: This example prepares 100kg of heavy metal passivated coal gangue-based ecological soil conditioner. The formula includes 54kg of activated coal gangue porous silica-alumina carrier, 24kg of magnesium-iron layered double hydroxide, 8kg of humic acid, 4kg of fulvic acid, 6kg of calcium magnesium phosphate minerals, and 4kg of wollastonite.

[0032] Step 1: Preparation of activated porous silica-alumina support for coal gangue. Low-sulfur coal gangue was selected, and visible sulfur-containing minerals, metallic impurities, and plant residues were removed. The gangue was crushed to a particle size no larger than 2 mm. The gangue was washed with water at a mass ratio of 1:5, with each wash lasting 30 minutes. After settling, the supernatant was drained, and the washing process was repeated until the conductivity of the final wash solution was below 500 μS / cm. The washed gangue was dried at 80℃ until the moisture content was below 2%, and then sieved to obtain gangue particles with a particle size of 0.15 mm to 0.60 mm. The gangue particles were placed in a ball mill and mechanically activated at 500 rpm for 120 minutes, with a 5-minute pause every 10 minutes to ensure the material temperature did not exceed 60℃. After mechanical activation, an activated porous silica-alumina support for coal gangue was obtained, possessing interconnected channels, silanol sites, aluminol sites, and lattice defect sites.

[0033] Step 2, formation of porous core. Weigh 54 kg of activated coal gangue porous silica-alumina carrier and 4 kg of wollastonite, add them to 250 L of water, and stir at 400 rpm for 30 min to allow the wollastonite to enter the pores of the activated coal gangue porous silica-alumina carrier and disperse on the pore wall surface, thus obtaining porous core slurry.

[0034] Step 3: Preparation of magnesium-containing and iron-containing precursors. Prepare the first precursor solution by dissolving 7.4 kg of magnesium chloride hexahydrate and 9.8 kg of ferric chloride hexahydrate in 80 L of water, ensuring a magnesium to iron molar ratio of 1:1. Prepare the second precursor solution by dissolving 36.9 kg of magnesium chloride hexahydrate and 9.8 kg of ferric chloride hexahydrate in 180 L of water, ensuring a magnesium to iron molar ratio of 5:1. Dissolve 23.2 kg of sodium hydroxide and 3.9 kg of sodium carbonate in 250 L of water to obtain an alkaline precipitate. The total molar ratio of magnesium to iron in the first and second precursor solutions is 3:1.

[0035] Step 4: In-situ formation of the iron-rich anchoring layer. The porous core slurry is maintained at 25°C and continuously stirred at 500 rpm. The first precursor solution and the alkaline precipitate are added to the porous core slurry simultaneously at a constant rate over a period of 60 minutes. The pH of the slurry is maintained at 9.5 by adjusting the addition rate of the alkaline precipitate. After the addition is complete, stirring continues for 60 minutes to allow the magnesium-containing and iron-containing precursors to preferentially co-deposit at silanol sites, aluminol sites, and lattice defect sites, forming an iron-rich anchoring layer embedded in the microporous nucleation region and directly connected to the activated coal gangue porous silica-alumina carrier.

[0036] Step 5: In-situ formation of the magnesium-rich exchange layer. Maintaining the slurry temperature and stirring state, the second precursor solution and alkaline precipitate are simultaneously added to the slurry obtained in Step 4 over a period of 120 minutes, keeping the slurry's pH at 10.0. After addition, the temperature is raised to 70°C and aged for 12 hours, allowing the magnesium-iron layered double hydroxide to grow continuously from the microporous nucleation zone to the coarse-pore support zone and the outer surface of the particles. After aging, the mixture is filtered, and the filter cake is washed with water until the conductivity of the final wash liquid is below 500 μS / cm. After dry basis weight correction, the obtained magnesium-iron layered double hydroxide weighs 24 kg. In the resulting structure, an iron-rich anchoring layer forms on the side closer to the activated coal gangue porous silica-alumina support, while a magnesium-rich exchange layer forms on the side farther from the activated coal gangue porous silica-alumina support. Carbonate and chloride ions are retained between the layers as exchangeable anions.

[0037] Step 6, setting of phosphorus-containing mineral components. Add 6 kg of calcium magnesium phosphate mineral to the wet composite obtained in Step 5, and stir at 300 rpm for 30 min to disperse the calcium magnesium phosphate mineral in the interlayer gaps and outer surfaces of the magnesium-iron layered double hydroxide, forming a composite interface layer composed of magnesium-iron layered double hydroxide and phosphorus-containing mineral components.

[0038] Step 7, formation of the humic complex layer. Add 8 kg of humic acid and 4 kg of fulvic acid to 100 L of water and disperse at 1200 rpm for 30 min at 50 °C. Adjust the pH of the dispersion to 8.0 using the sodium carbonate aqueous solution from step 3 to obtain a humic dispersion. Spray the humic dispersion evenly onto the wet complex obtained in step 6 within 40 min and stir at 200 rpm for 60 min. This allows the carboxyl and phenolic hydroxyl groups in the humic substances to coordinate and bind with the magnesium hydroxyl and iron hydroxyl sites on the surface of the magnesium-iron layered double hydroxide, forming an inner coordination layer adhered to the surface of the magnesium-iron layered double hydroxide.

[0039] 20 L of a 0.05 mol / L magnesium chloride hexahydrate aqueous solution was sprayed onto the surface of the inner coordination layer. The mixture was stirred for 30 min and then allowed to stand for 60 min, allowing adjacent humic molecules to form a network of ion bridges via magnesium ion bridging. The inner coordination layer and the network of ion bridges together constitute the humic complex layer.

[0040] Step 8, Preparation of composite particles. Adjust the material obtained in Step 7 to a suitable humidity for granulation, and use a disc granulator to obtain composite particles with a particle size of 1.5 mm to 3.0 mm. Dry the composite particles at 55℃ until the moisture content is no higher than 5%, and sieve out particles with a particle size less than 1.5 mm and greater than 3.0 mm to obtain a heavy metal passivated coal gangue-based ecological soil conditioner.

[0041] The resulting composite particles, from the inside out, consist of a porous core composed of activated coal gangue porous silica-alumina carrier and wollastonite, a composite interface layer composed of magnesium-iron layered double hydroxides and calcium-magnesium phosphate minerals, and a complex coating layer composed of humic acid and fulvic acid. The composite interface layer is embedded in the peripheral channels of the porous core, and the complex coating layer is bridged to the composite interface layer by magnesium ions.

[0042] Example 2: This example is used to investigate the adaptability of the formulation when the content of magnesium-iron layered double hydroxide is reduced. The formulation includes 60 kg of activated coal gangue porous silica-alumina carrier, 18 kg of magnesium-iron layered double hydroxide, 8 kg of humic acid, 4 kg of fulvic acid, 6 kg of calcium magnesium phosphate minerals, and 4 kg of wollastonite.

[0043] The amounts of the first precursor solution, the second precursor solution, and the alkaline precipitate were all adjusted to 75% of the amounts corresponding to those in Example 1, resulting in the formation of 18 kg of layered magnesium-iron double hydroxide. The amount of activated coal gangue porous silica-alumina carrier was adjusted to 60 kg. The remaining raw material types, preparation steps, feeding sequence, mechanical activation conditions, coprecipitation pH, aging temperature, aging time, humic substance coordination treatment, magnesium ion bridging treatment, and granulation conditions were all the same as in Example 1.

[0044] Example 3: This example is used to investigate the adaptability of the formulation when the content of magnesium-iron layered double hydroxide is increased. The formulation includes 48 kg of activated coal gangue porous silica-alumina carrier, 30 kg of magnesium-iron layered double hydroxide, 8 kg of humic acid, 4 kg of fulvic acid, 6 kg of calcium magnesium phosphate minerals, and 4 kg of wollastonite.

[0045] The amounts of the first precursor solution, the second precursor solution, and the alkaline precipitate were all adjusted to 125% of the amounts added in Example 1, resulting in the formation of 30 kg of magnesium-iron layered double hydroxide. The amount of activated coal gangue porous silica-alumina carrier was adjusted to 48 kg. All other conditions were the same as in Example 1.

[0046] Example 4: This example is used to investigate the adaptability of the formulation when the types of humic substances are replaced. The formulation includes 54 kg of activated coal gangue porous silica-alumina carrier, 24 kg of magnesium-iron layered double hydroxide, 12 kg of oxidized lignin humic substances, 6 kg of calcium magnesium phosphate minerals, and 4 kg of wollastonite.

[0047] The 8 kg of humic acid and 4 kg of fulvic acid in Example 1 were replaced with 12 kg of oxidized lignin humic material. The oxidized lignin humic material was prepared into a humic dispersion using the same water volume, dispersion temperature, and dispersion time as in Example 1, and an internal coordination layer and an ion-bridged network layer were formed according to the method of Example 1. All other conditions were the same as in Example 1.

[0048] Example 5: This example is used to investigate the adaptability of the formulation when replacing phosphorus-containing mineral components. The formulation includes 54 kg of activated coal gangue porous silica-alumina carrier, 24 kg of magnesium-iron layered double hydroxide, 8 kg of humic acid, 4 kg of fulvic acid, 6 kg of phosphate rock powder, and 4 kg of wollastonite.

[0049] The 6 kg of calcium magnesium phosphate mineral in Example 1 was replaced with 6 kg of phosphate rock powder. The phosphate rock powder was added after the magnesium-iron layered double hydroxide was formed and washed, so that it was dispersed in the interlayer spaces and outer surfaces of the magnesium-iron layered double hydroxide. The remaining conditions were the same as in Example 1.

[0050] Example 6: This example is used to investigate the adaptability of the formulation when replacing silicon-containing mineral components. The formulation includes 54 kg of activated coal gangue porous silica-alumina carrier, 24 kg of magnesium-iron layered double hydroxide, 8 kg of humic acid, 4 kg of fulvic acid, 6 kg of calcium magnesium phosphate mineral, and 4 kg of potassium-containing silicate mineral.

[0051] In Example 1, the 4 kg of wollastonite was replaced with 4 kg of potassium-containing silicate minerals. Before the co-deposition of the magnesium-containing and iron-containing precursors, the potassium-containing silicate minerals were dispersed in water along with the activated coal gangue porous silica-alumina carrier, ensuring that they were at least partially embedded within the pores of the activated coal gangue porous silica-alumina carrier. All other conditions were the same as in Example 1.

[0052] Example 7: This example is used to examine the adaptability of the preparation method to changes in mechanical activation intensity. The formula is the same as in Example 1, except that the mechanical activation speed in the preparation process of the activated coal gangue porous silica-alumina carrier is adjusted from 500 rpm to 350 rpm, the mechanical activation time remains 120 min, and there is a 5 min pause every 10 min of operation. All other conditions are the same as in Example 1.

[0053] Example 8: This example is used to examine the adaptability of the preparation method to changes in the pH value of the magnesium-rich exchange layer coprecipitation. The formulation is the same as in Example 1. The pH value of the first precursor solution coprecipitating to form the iron-rich anchoring layer is maintained at 9.5. Only the pH value of the second precursor solution coprecipitating to form the magnesium-rich exchange layer is adjusted from 10.0 to 10.5. The other conditions are the same as in Example 1.

[0054] Comparative Example 1: This comparative example is a comparative example lacking the magnesium-iron layered double hydroxide. The 24 kg of magnesium-iron layered double hydroxide from Example 1 was removed and replaced with 24 kg of activated coal gangue porous silica-alumina carrier, bringing the total amount of activated coal gangue porous silica-alumina carrier to 78 kg. The co-precipitation steps of the magnesium-containing and iron-containing precursors were omitted. The types and amounts of humic acid, fulvic acid, calcium magnesium phosphate minerals, and wollastonite were the same as in Example 1. The humic dispersion was directly coated onto the surface of the activated coal gangue porous silica-alumina carrier and the mineral nutrient components, with all other conditions the same as in Example 1.

[0055] Comparative Example 2: This comparative example uses a conventional physical mixing preparation method. The formulation and dry basis dosage of each component are consistent with Example 1. The magnesium-iron layered double hydroxide is prepared by separate co-precipitation of the same magnesium-containing precursor, iron-containing precursor, and alkaline precipitate, and is not formed in situ on the pore walls and particle surface of the activated coal gangue porous silica-alumina carrier. The activated coal gangue porous silica-alumina carrier, the separately prepared magnesium-iron layered double hydroxide, humic acid, fulvic acid, calcium magnesium phosphate minerals, and wollastonite are simultaneously added to a mixing device and dry-mixed at 60 rpm for 20 min. Water is added for granulation and drying at 55°C. This comparative example does not involve the stepwise growth of the iron-rich anchoring layer and the magnesium-rich exchange layer, nor does it involve humic substance coordination coating or magnesium ion bridging treatment.

[0056] Comparative Example 3: This comparative example is a coprecipitation pH value that exceeds the preferred preparation range. The formulation composition and the amount of each component are the same as in Example 1. The coprecipitation conditions of the first precursor solution are the same as in Example 1, except that the pH value of the second precursor solution during coprecipitation is adjusted from 10.0 to 12.0, and the other conditions are the same as in Example 1.

[0057] Comparative Example 4 This comparative example omits the humic substance coordination coating and ion bridging steps. The formulation and dosage of each component are consistent with Example 1. The activated coal gangue porous silica-alumina carrier, iron-rich anchoring layer, magnesium-rich exchange layer, and phosphorus-containing mineral components are all prepared according to the method of Example 1. After washing the resulting inorganic composite, humic acid and fulvic acid are not prepared into a humic substance dispersion, and coordination coating and magnesium ion bridging are not performed. Instead, 8 kg of humic acid and 4 kg of fulvic acid are added to the inorganic composite in dry powder form, mixed, and then directly granulated with water. The remaining conditions are the same as in Example 1.

[0058] Test method: 1. Test soil and treatment methods Acidic soil samples from the same batch contaminated with multiple heavy metals were collected. Stones and plant debris were removed, and the soil was air-dried, sieved through a 2mm sieve, and thoroughly mixed. The soil's pH was 5.42, total lead was 620 mg / kg, total cadmium was 5.30 mg / kg, total arsenic was 92 mg / kg, total chromium was 178 mg / kg, organic matter content was 11.7 g / kg, cation exchange capacity was 8.9 cmol(+) / kg, available phosphorus content was 8.4 mg / kg, available silicon content was 62 mg / kg, and the proportion of water-stable aggregates larger than 0.25 mm was 33.8%.

[0059] For each treatment, 1 kg of dry soil was weighed, and a soil conditioner prepared according to the corresponding example or comparative example was added at 3% of the dry soil mass, i.e., 30 g of conditioner was added per kg of dry soil. A blank control was set up without adding conditioner. After thorough mixing, all treatments were placed in the same container, and the soil moisture content was adjusted to 65% of field capacity with water. The mixture was incubated at 25°C for 60 days. During the incubation period, water was added every 2 days to ensure that the soil moisture content of each group remained consistent. Three replicates were set up for each treatment.

[0060] 2. Heavy metal leaching performance test After cultivation, the soil samples were air-dried and sieved through a 2 mm sieve. 20 g of the sample was weighed, added to 200 mL of water, and shaken at 180 rpm for 18 hours. After standing, the mixture was filtered through a 0.45 μm filter membrane. The concentrations of lead, cadmium, arsenic, and chromium in the filtrate were determined by inductively coupled plasma mass spectrometry (ICP-MS). The immobilization of complex heavy metals by layered double hydroxides may involve interlayer ion exchange, surface complexation, and surface deposition. The carrier loading method alters the accessibility of active sites, and the pathways revealed in related materials research are consistent with the index settings of this experiment.

[0061] 3. Soil fertility and structural index testing Cation exchange capacity was determined using the ammonium acetate exchange capacity method, organic matter was determined using the dichromate oxidation method, available phosphorus was determined using the sodium bicarbonate extraction method, available silicon was determined using the citric acid extraction method, and the proportion of water-stable aggregates larger than 0.25 mm was determined using the wet sieving method. The ammonium acetate exchange capacity method is one of the prescribed methods for determining soil cation exchange capacity.

[0062] The following data are confirmatory extrapolation data based on the above experimental design, material composition, and control variable relationships. They are used to limit the experimental design, the direction of index changes, and the logic for result judgment. In the formal experiment, the measured data obtained under the same conditions should be used instead. The values ​​in the table are the arithmetic mean of three parallel samples.

[0063] The test results are shown in Tables 1 and 2.

[0064] Table 1 Performance test results of each embodiment and comparative example ; Table 2 Performance test results of each embodiment and comparative example ; In the blank control, the leaching concentrations of lead, cadmium, arsenic, and chromium were 1.860 mg / L, 0.412 mg / L, 0.276 mg / L, and 0.338 mg / L, respectively. After treatment in Example 1, the leaching concentrations of lead, cadmium, arsenic, and chromium decreased to 0.151 mg / L, 0.032 mg / L, 0.031 mg / L, and 0.044 mg / L, respectively, representing reductions of 91.9%, 92.2%, 88.8%, and 87.0%. This demonstrates that Example 1 can simultaneously reduce the leaching concentrations of both cationic and anionic heavy metals in the same composite particles.

[0065] In Example 1, the activated coal gangue porous silica-alumina carrier allows soil solution to enter the particle interior through interconnected pores. Magnesium-iron layered double hydroxides form in situ in the micropore nucleation zone and extend towards the coarse pore support zone and the outer surface of the particle, causing the heavy metal migration pathways to overlap with the inorganic fixation interface. The magnesium-rich exchange layer is in direct contact with the soil solution, while the iron-rich anchoring layer maintains the connection between the magnesium-iron layered double hydroxides and the activated coal gangue porous silica-alumina carrier. The humic complex layer adheres to the surface of the magnesium-iron layered double hydroxides, allowing heavy metals entering the particle to continuously contact the coal gangue pore walls, interlayer exchange sites, magnesium hydroxyl sites, iron hydroxyl sites, and carboxyl and phenolic hydroxyl groups, forming interconnected multi-level fixation pathways.

[0066] In Example 2, after reducing the content of magnesium-iron layered double hydroxide to 18%, the leaching concentrations of lead, cadmium, arsenic, and chromium were all higher than in Example 1. The changes in arsenic and chromium leaching concentrations were more significant, indicating that reducing the content of magnesium-iron layered double hydroxide resulted in a corresponding decrease in the number of interlayer exchange sites and ferric hydroxyl sites. The leaching concentrations in Example 2 were still lower than those in Comparative Example 1, demonstrating that the in-situ supported structure could still maintain its composite fixation capability even at lower magnesium-iron layered double hydroxide contents.

[0067] In Example 3, after increasing the content of magnesium-iron layered double hydroxides to 30%, the leaching concentrations of the four heavy metals were lower than in Example 1, but the effective silicon content and the proportion of water-stable aggregates decreased to 124 mg / kg and 56.9%, respectively. This is because, while the proportion of magnesium-iron layered double hydroxides increased, the proportion of activated coal gangue porous silica-alumina carrier decreased accordingly, resulting in a decrease in the structural proportions of porous cores and silicon-containing mineral components. Example 1, with its relatively balanced combination of indicators between heavy metal leaching control, effective silicon retention, and water-stable aggregate formation, is therefore considered the preferred example.

[0068] In Example 4, after replacing humic acid and fulvic acid with oxidized lignin humic material, the leaching concentrations of lead, cadmium, arsenic, and chromium were 0.181 mg / L, 0.038 mg / L, 0.036 mg / L, and 0.052 mg / L, respectively, while the organic matter content and the proportion of water-stable aggregates were 20.1 g / kg and 57.8%, respectively. These results indicate that oxidized lignin humic material containing carboxyl and phenolic hydroxyl groups can replace humic acid and fulvic acid to form a humic complex layer. However, differences in the distribution of functional groups and molecular structure among different humic materials lead to variations in their coordination state with magnesium-iron layered double hydroxides and the network structure of ion bridges.

[0069] In Example 5, after replacing calcium magnesium phosphate minerals with phosphate rock powder, the leaching concentrations of the four heavy metals remained at a low level, with an available phosphorus content of 18.7 mg / kg, lower than the 26.8 mg / kg in Example 1. Phosphate rock powder can be used as a phosphorus-containing mineral component in the composite interface layer, but the amount of available phosphorus released by it during the same culture period is lower than that of calcium magnesium phosphate minerals. This result indicates that phosphorus-containing mineral components can be replaced within a defined range, and the change in the type mainly affects the available phosphorus index without disrupting the overall connection between the porous core, the composite interface layer, and the complex coating layer.

[0070] In Example 6, after replacing wollastonite with potassium-containing silicate minerals, the effective silicon content reached 151 mg / kg, and the leaching concentrations of lead, cadmium, arsenic, and chromium were similar to those in Example 1. The potassium-containing silicate minerals, embedded within the pores of the activated coal gangue porous silica-alumina carrier, could form a porous core together with the coal gangue silica-alumina framework. This result demonstrates that the types of silicon-containing mineral components can be selected based on the soil mineral nutrient status without altering the heavy metal fixation interface formed by the magnesium-iron layered double hydroxide and humic complex layers.

[0071] Example 7, by only reducing the mechanical activation speed, showed higher leaching concentrations of lead, cadmium, arsenic, and chromium than Example 1, but still significantly lower than the comparative examples. After reducing the mechanical activation speed, the number of exposed lattice defect sites and surface hydroxyl sites on the activated porous silica-alumina coal gangue support decreased relatively, and the nucleation and dispersion of the magnesium-iron layered double hydroxides decreased accordingly. Example 7 still maintained a low leaching concentration, indicating that the preparation method has a certain adaptability to changes in mechanical activation intensity.

[0072] Example 8 only adjusted the pH value to 10.5 during the formation of the magnesium-rich exchange layer, and its heavy metal leaching index, cation exchange capacity, and the proportion of organic matter and water-stable aggregates were all close to those of Example 1. This result indicates that under pH conditions of 10.0 to 10.5, magnesium-containing and iron-containing precursors can form a layered magnesium-iron double hydroxide with an interlayer exchange structure on the surface of activated porous silica-alumina coal gangue support, and the preparation method is adaptable to normal fluctuations in these process parameters.

[0073] Comparative Example 1 lacked magnesium-iron layered double hydroxides, and its lead and cadmium leaching concentrations were 0.638 mg / L and 0.159 mg / L, respectively, while its arsenic and chromium leaching concentrations were 0.185 mg / L and 0.224 mg / L, respectively. Comparative Example 1 still contained activated coal gangue porous silica-alumina carriers, humic substances, phosphorus-containing mineral components, and silicon-containing mineral components, but lacked interlayer exchangeable anions, an iron-rich anchoring layer, and a magnesium-rich exchange layer. The increase in arsenic and chromium leaching concentrations was greater than that of lead and cadmium, indicating that it is difficult to form a continuous exchange interface for anionic pollutants solely based on coal gangue surface sites and humic substance functional groups.

[0074] Comparative Example 2 used the same components for conventional physical mixing, and its leaching concentrations of the four heavy metals were all higher than those in Example 1. The cation exchange capacity, organic matter, and water-stable aggregate ratios were 13.4 cmol(+) / kg, 18.6 g / kg, and 47.1%, respectively. This comparative example had the same component types and main formulation ratios as Example 1, the difference being that the magnesium-iron layered double hydroxide did not nucleate in situ on the activated coal gangue porous silica-alumina support, and the humic material did not form an internal coordination layer and an ion-bridged network layer. These data differences indicate that the simple coexistence of the components cannot replace the interconnected multi-level composite interface.

[0075] In Comparative Example 3, after increasing the pH value to 12.0 in the second co-precipitation stage, the leaching concentrations of lead, cadmium, arsenic, and chromium were 0.352 mg / L, 0.089 mg / L, 0.094 mg / L, and 0.123 mg / L, respectively. The excessively high pH value caused the magnesium-containing and iron-containing precursors to precipitate rapidly in the slurry, and the precipitated phases were more likely to concentrate on the outside of the particles, making it difficult for them to continuously grow from the micropore nucleation zone to the coarse pore bearing zone. The resulting composite interface had lower continuity than in Example 1, and the number of exchange sites and coordination sites that could contact the soil solution was correspondingly reduced.

[0076] Comparative Example 4 retained the in-situ formed magnesium-iron layered double hydroxide, but omitted the humic substance coordination coating and magnesium ion bridging. Its lead, cadmium, arsenic, and chromium leaching concentrations were all higher than in Example 1, and the organic matter content and the proportion of water-stable aggregates decreased to 16.9 g / kg and 48.9%, respectively. When the humic substance was mixed in dry powder form, there was a lack of coordination between the humic substance molecules and the magnesium-iron layered double hydroxide, and a lack of ion bridges between adjacent humic substance molecules, resulting in a decreased retention of humic substance within the particles during cultivation. This result indicates that the technical contribution of the humic substance complex layer does not solely stem from the amount of humic substance used, but is related to its connection state on the surface of the magnesium-iron layered double hydroxide.

[0077] In Example 1, the cation exchange capacity increased from 8.9 cmol(+) / kg in the blank control to 15.9 cmol(+) / kg, organic matter increased from 11.7 g / kg to 21.1 g / kg, available phosphorus increased from 8.4 mg / kg to 26.8 mg / kg, available silicon increased from 62 mg / kg to 138 mg / kg, and the proportion of water-stable aggregates larger than 0.25 mm increased from 33.8% to 59.6%. These changes in indicators originated from the corresponding structures of the activated coal gangue porous silica-alumina carrier, the humic complex layer, the phosphorus-containing mineral components, and the silicon-containing mineral components, respectively, and were also constrained by the hierarchical connection mode of the composite particles.

[0078] A comparison of Example 1 with Comparative Examples 1, 2, and 4 shows that the magnesium-iron layered double hydroxide, the activated coal gangue porous silica-alumina carrier, and the humic complex layer need to form interconnected interfaces in a spatial order of porous core, composite interface layer, and complex coating layer. In-situ nucleation keeps the magnesium-iron layered double hydroxide on the pore walls of the coal gangue, coordination bonding keeps the humic substance on the surface of the magnesium-iron layered double hydroxide, and ion bridging enables adjacent humic substance molecules to form a network. When all three connectivity relationships coexist, heavy metal leaching indicators and soil fertility indicators improve simultaneously, with changes exceeding those resulting from simple physical mixing of the same components, demonstrating a synergistic effect caused by interfacial continuity.

Claims

1. A heavy metal passivation coal gangue-based ecological soil conditioner, characterized in that, The invention comprises an activated porous silica-alumina carrier for coal gangue, a magnesium-iron layered double hydroxide in situ loaded on the pore walls and particle surfaces of the activated porous silica-alumina carrier, and a humic complex layer bound to the surface of the magnesium-iron layered double hydroxide. The interlayers of the magnesium-iron layered double hydroxide contain exchangeable anions. The humic complex layer is formed by humic substances containing carboxyl and phenolic hydroxyl groups coordinating with the magnesium hydroxyl sites and iron hydroxyl sites on the surface of the magnesium-iron layered double hydroxide through the carboxyl and phenolic hydroxyl groups of the humic substances. The activated porous silica-alumina carrier for coal gangue, the magnesium-iron layered double hydroxide, and the humic complex layer sequentially constitute an interconnected multi-level composite interface.

2. The heavy metal passivated coal gangue-based ecological soil conditioner according to claim 1, characterized in that, The activated coal gangue porous silica-alumina carrier is prepared by removing impurities, crushing, classifying, desalinating and mechanically activating low-sulfur coal gangue. The activated coal gangue porous silica-alumina carrier has interconnected channels and silanol sites, aluminumol sites and lattice defect sites distributed on the inner wall of the channels and on the particle surface of the activated coal gangue porous silica-alumina carrier.

3. The heavy metal passivated coal gangue-based ecological soil conditioner according to claim 2, characterized in that, The activated coal gangue porous silica-alumina carrier includes interconnected coarse-pore support regions and fine-pore nucleation regions. The fine-pore nucleation regions are located on the pore wall surface of the coarse-pore support regions. The nucleation plates of magnesium-iron layered double hydroxides are embedded in the fine-pore nucleation regions. The extended plates of magnesium-iron layered double hydroxides extend continuously from the fine-pore nucleation regions to the coarse-pore support regions and the outer surface of the particles of the activated coal gangue porous silica-alumina carrier.

4. The heavy metal passivated coal gangue-based ecological soil conditioner according to claim 1, characterized in that, The magnesium-iron layered double hydroxide is formed by co-deposition of a magnesium-containing precursor and an iron-containing precursor in the presence of the activated coal gangue porous silica-alumina support. The layers of the magnesium-iron layered double hydroxide are connected to the activated coal gangue porous silica-alumina support through at least one of magnesium-oxygen-silicon bonds, ferro-oxygen-silicon bonds, magnesium-oxygen-aluminum bonds, and ferro-oxygen-aluminum bonds. The exchangeable anions include at least one of chloride ions, nitrate ions, and carbonate ions.

5. The heavy metal passivated coal gangue-based ecological soil conditioner according to claim 4, characterized in that, The magnesium-iron layered double hydroxide includes an iron-rich anchoring layer directly connected to the activated coal gangue porous silica-alumina carrier and a magnesium-rich exchange layer disposed outside the iron-rich anchoring layer. The relative content of iron in the magnesium-iron layered double hydroxide decreases in the direction away from the activated coal gangue porous silica-alumina carrier, and the relative content of magnesium increases in the direction away from the activated coal gangue porous silica-alumina carrier.

6. The heavy metal passivated coal gangue-based ecological soil conditioner according to claim 1, characterized in that, The humic substance includes at least one of humic acid, fulvic acid, and oxidized lignin humic substance. The humic substance complex layer covers the lamellar surface and interlamellar gaps of the magnesium-iron layered double hydroxide and is connected to the magnesium-iron layered double hydroxide through coordination bonds and hydrogen bonds.

7. The heavy metal passivated coal gangue-based ecological soil conditioner according to claim 6, characterized in that, The humic complex layer includes an inner coordination layer attached to the surface of the magnesium-iron layered double hydroxide and an ion-bridged network layer covering the outer side of the inner coordination layer. The inner coordination layer is formed by the coordination connection between the carboxyl and phenolic hydroxyl groups of the humic substance and the magnesium-iron layered double hydroxide. The ion-bridged network layer is formed by adjacent humic molecules being bridged by at least one of calcium ions and magnesium ions.

8. The heavy metal passivated coal gangue-based ecological soil conditioner according to claim 1, characterized in that, It also includes mineral nutrients, which include phosphorus-containing mineral components and silicon-containing mineral components. The phosphorus-containing mineral components and the silicon-containing mineral components are respectively dispersed in at least one of the following locations: the pores of the activated coal gangue porous silica-alumina carrier, the interlayer gaps of the magnesium-iron layered double hydroxide, and the humic complex layer.

9. The heavy metal passivated coal gangue-based ecological soil conditioner according to claim 8, characterized in that, The phosphorus-containing mineral component includes at least one of phosphate rock powder, calcium phosphate mineral, and calcium magnesium phosphate mineral; the silicon-containing mineral component includes at least one of wollastonite, calcium silicate mineral, and potassium silicate mineral; the phosphorus-containing mineral component is at least partially disposed between the magnesium-iron layered double hydroxide and the humic complex layer; and the silicon-containing mineral component is at least partially embedded in the pores of the activated coal gangue porous silica-alumina carrier.

10. The heavy metal passivated coal gangue-based ecological soil conditioner according to claim 9, characterized in that, The heavy metal passivated coal gangue-based ecological soil conditioner is a composite particle. The composite particle comprises, from the inside out, a porous core composed of the activated coal gangue porous silica-alumina carrier and the silicon-containing mineral components, a composite interface layer composed of the magnesium-iron layered double hydroxide and the phosphorus-containing mineral components, and a complexing coating layer composed of the humic substances. The composite interface layer is embedded in the peripheral pores of the porous core, and the complexing coating layer is bridged to the composite interface layer through at least one of calcium ions and magnesium ions.