A red mud-gangue-based magnetic composite material, a preparation method and application thereof

Abstract

The application discloses a kind of red mud-coal gangue-based magnetic composite material and its preparation method and application, belong to water / soil treatment technical field.The composite material is core-shell structure, shell is carbon shell, inner core is the magnetic zeolite of load nano zero-valent iron, and the magnetic zeolite is prepared by red mud and coal gangue cooperation;Preparation method includes red mud-coal gangue pretreatment, alkali fusion activation and hydrothermal crystallization preparation magnetic zeolite, ethanol-water solvent load nZVI, PDA carbonization coating and the like steps.The application realizes double solid waste resource, and material has water body quick adsorption and soil long-acting stability performance, nZVI stability is strong, recycling rate is high, and magnetic separation is convenient, low in cost and environmental protection, suitable for lead, cadmium, copper and other heavy metal pollution treatment.

Description

Technical Field

[0001] This invention mainly relates to the field of water / soil remediation technology, specifically a red mud-coal gangue-based magnetic composite material and its preparation method and application. Background Technology

[0002] Currently, remediation strategies for heavy metal contaminated soils include electroremediation, soil washing, immobilization, and bioremediation. Among these, in-situ immobilization technology utilizing sustainable amendments such as clay, zeolite, and phosphate-based fertilizers has attracted significant attention. Zeolite, as a crystalline aluminosilicate, is composed of [SiO4]. 4- and [AlO4] 5- Tetrahedral units form a three-dimensional lattice structure with fine microporous structure, high specific surface area, and excellent ion exchange performance, making it widely used in the remediation of heavy metals in water. Coal gangue, a solid waste generated during coal mining and washing, can cause problems such as land occupation, spontaneous combustion, dust, and water pollution due to long-term accumulation. Its main chemical components, Al2O3 and SiO2, along with its mineral phase dominated by kaolinite, make it an ideal raw material for preparing porous materials such as zeolites and mesoporous silica, and an important way to achieve its high-value resource utilization. In recent years, research on synthesizing zeolites from coal gangue and using them as environmental materials has gradually increased. Some technologies also utilize the strong reducing power of nZVI and the adsorption properties of the carrier to synergistically improve the removal of heavy metals by loading nano-zero-valent iron (nZVI) onto zeolite or coal gangue supports. These composite materials have shown certain application potential in water pollution control.

[0003] CN103394703A discloses a coal gangue-supported zero-valent iron nanocomposite material and its preparation method. The method involves separating and purifying kaolinite-based coal gangue, and then loading zero-valent iron nanoparticles using a sodium borohydride chemical liquid-phase reduction method. The resulting composite material has coal gangue as the skeleton, with zero-valent iron nanoparticles distributed on the surface of the carrier or between clay mineral layers, exhibiting certain water pollution remediation efficiency and stability. This technology enables the resource utilization of zero-valent iron (ZVFe) loaded onto coal gangue, providing a basic framework for the development of similar materials. However, it still has significant shortcomings: First, its application scenarios are limited, mainly focusing on water pollution control and not addressing soil heavy metal remediation, and it does not consider the complexity of the soil environment and the requirements for material performance. Second, the material has a single function, relying solely on the physical adsorption of coal gangue and the chemical reduction of ZVFe, without expanding its functions through composite modification, resulting in limited remediation effects and long-term effectiveness. Third, its stability and recyclability are insufficient; the loaded ZVFe nanoparticles are prone to aggregation and oxidative deactivation, and the material is non-magnetic, making it difficult to separate and recover from water or soil after use, which can easily lead to secondary pollution or resource waste. Fourth, the depth of carrier modification is insufficient, only involving simple separation and purification of coal gangue, without optimizing the pore structure, ion exchange performance, and magnetism of the carrier through synergy with other raw materials, thus limiting the overall performance of the material.

[0004] Furthermore, most of the currently developed coal gangue-based zeolite materials are used for wastewater treatment, with few applications in soil remediation. Moreover, relying solely on the porous adsorption properties of zeolite results in limited functionality and poor remediation effects. There is an urgent need to obtain high-performance, low-cost, environmentally friendly zeolite composite materials suitable for soil remediation through modification and compounding. Based on this, this application aims to address the aforementioned problems in the existing technology by providing a red mud-coal gangue-based magnetic composite material. Summary of the Invention

[0005] This invention addresses the problem of overly simplistic solutions in existing technologies by providing a significantly different approach. It primarily offers a red mud-coal gangue-based magnetic composite material, its preparation method, and its applications. This enables the synergistic resource utilization of coal gangue and red mud. By preparing a magnetic zeolite carrier, loading nZVI, and performing carbonization and coating modification, the material's application in soil remediation is expanded. Simultaneously, the material's stability, recyclability, and heavy metal immobilization effect are improved, addressing the technical pain points of existing materials, such as narrow application scenarios, limited functionality, insufficient stability, and difficulty in recycling.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0007] In a first aspect, the present invention provides a red mud-coal gangue-based magnetic composite material, comprising an outer shell and a core, wherein the outer shell is a carbon shell and the core is a magnetic zeolite (nZVI@M-ZA) loaded with nano-zero valent iron, and the magnetic zeolite is prepared by co-processing red mud and coal gangue.

[0008] In the aforementioned red mud-coal gangue-based magnetic composite material, magnetic zeolite (M-ZA) provides high specific surface area and ion exchange performance, through ion exchange (Na in the zeolite lattice) + Ca 2+ With Pb 2+ Cd 2+ It rapidly captures heavy metals through exchange and physical adsorption (pore trapping); nano-zero valent iron (nZVI) transforms soluble heavy metals into inert forms through chemical reduction and co-precipitation, enhancing the stabilization of heavy metals in the soil; the carbon shell outer layer realizes multiple functions such as oxidation protection, electron transport, and secondary adsorption, ensuring the long-term stability of nZVI in complex soil environments (pH fluctuations, organic matter interference).

[0009] 2. Synergistic Mechanism of Magnetic Separation Performance

[0010] The magnetic superposition of the red mud-derived magnetic phases (Fe3O4, ferrates) and nZVI results in a saturation magnetization of 28.6 emu / g, which meets the requirements for rapid separation by ordinary magnets (0.3T) (complete separation within 1 minute). This solves the pain points of traditional soil remediation materials, such as difficulty in recycling and easy secondary pollution.

[0011] Secondly, the present invention provides a method for preparing a red mud-coal gangue-based magnetic composite material, comprising the following steps:

[0012] S1: Co-preparation of magnetic zeolite precursors from coal gangue and red mud (M-ZA);

[0013] This step aims to utilize the iron source in red mud to synthesize endogenous magnetic zeolite in a one-step process with coal gangue, providing a high-capacity substrate for subsequent loading.

[0014] S1-1. Raw material pretreatment

[0015] (1) Coal gangue pretreatment: The coal gangue is crushed, ground and sieved separately, and two particle size components with a particle size of 75μm (200 mesh) or more and 45μm (325 mesh) or less are collected and mixed at a mass ratio of (6:4)-(8:2) to optimize the reaction activity and particle packing.

[0016] In this step, coarse particles larger than 75 μm serve as the skeletal support, constructing a macroscopic porous structure and reducing the probability of particle agglomeration; fine particles smaller than 45 μm serve as the reactive body, increasing the contact area with red mud and alkali source, and improving the alkali melting activation efficiency. The two are mixed in a ratio of (6:4) to (8:2) to achieve a synergistic effect of both the skeletal support and the reactive body, ensuring the structural integrity of the subsequent zeolite crystallization while avoiding pore blockage caused by excessive accumulation of fine particles. Furthermore, the large specific surface area of ​​the fine particles makes the Si-O-Al bonds of the kaolinite minerals on their surface more easily destroyed by alkali melting, providing sufficient silicon-aluminum active sites for zeolite crystallization; the coarse particles retain some of the original structure, forming multi-level channels (micropores + mesopores) after crystallization, increasing the adsorption capacity for heavy metals.

[0017] (2) Red mud pretreatment: Dry the red mud at 80-120℃ for 12-48 hours, grind it and pass it through a 75μm sieve;

[0018] Optionally, in this step, the iron oxides can be preliminarily enriched by magnetic separation. By separating strongly magnetic iron oxides (such as Fe3O4) from the red mud by magnetic separation, the concentration of effective iron source in the raw material is increased, ensuring the formation of a uniform and high-strength magnetic phase in the subsequent alkali melting stage, avoiding interference from non-magnetic impurities (such as CaO and TiO2) on the crystallization of magnetic zeolite, and ultimately improving the saturation magnetization of the material.

[0019] (3) Raw material mixing: The pretreated coal gangue is mixed with red mud, and the dry basis mass ratio of coal gangue to red mud is controlled to be 3:1 to 4:1. This ratio can ensure sufficient silicon and aluminum sources, while making the iron content suitable for generating a strongly magnetic phase.

[0020] In this step, coal gangue provides the Si and Al elements required for zeolite crystallization, while red mud provides the Fe elements (mainly Fe2O3 and Fe3O4) required for the formation of magnetic phases. A precise 3:1-4:1 ratio balances the integrity of zeolite crystallization and the content of magnetic phases: when the ratio is below 3:1, the coal gangue lacks sufficient Si and Al, leading to increased zeolite lattice defects; when the ratio is above 4:1, the red mud lacks sufficient iron source, reducing the formation of magnetic phases (such as Fe3O4 and ferrates) and decreasing magnetic separation performance. Furthermore, as an industrial waste residue from the alumina industry, the red mud has an endogenous iron source, eliminating the need for additional iron salts. This reduces raw material costs and achieves simultaneous resource utilization of both types of industrial solid waste, reducing environmental impact.

[0021] S1-2. Alkali Melting Activation and Crystallization Guidance

[0022] (1) Mixing and melting: Mix the pretreated raw materials with solid sodium hydroxide (NaOH) at a molar ratio of Na2O / (SiO2+Al2O3) of 1.2-1.5; place them in a muffle furnace and activate them by gradient heating: first, raise the temperature to 550-650℃ at 5℃ / min and hold for 0.5-1 hours to decompose the organic matter; then raise the temperature to 850-900℃ and hold for 1.5-2 hours to complete the full activation and initial ferrate formation, and obtain the molten block.

[0023] In this step, the first stage (550-650℃) is a low-temperature pretreatment section, which mainly decomposes organic impurities (such as residual coal organic matter in coal gangue) and volatile components (such as water of crystallization) in coal gangue and red mud, preventing these substances from carbonizing and depositing during high-temperature melting, thus affecting the regularity of the zeolite lattice. The second stage (850-900℃) is a high-temperature activation section, where solid NaOH reacts with kaolinite in coal gangue and aluminosilicates in red mud to generate crystallizable sodium aluminate and sodium silicate precursors; simultaneously, Fe in the red mud... 3+ It reacts with Na2O to form ferrates (such as NaFeO2), providing a magnetic core for subsequent hydrothermal crystallization.

[0024] (2) Hydrothermal crystallization: The molten block is cooled and crushed, and deionized water is added to make a slurry; 0.5-1.5 wt% tetraethylammonium hydroxide is added as a crystallization directing agent, and a hydrothermal reaction is carried out at 90-110℃ for 10-14 hours; the product is magnetically separated, washed to neutral, and dried at 80℃ to obtain a magnetic zeolite precursor, denoted as M-ZA.

[0025] In this step, tetraethylammonium hydroxide (TEAOH) serves as a template agent. The hydrophobic alkyl chains in its molecular structure interact with the hydrophilic hydroxyl groups, creating a steric hindrance effect that allows it to selectively adsorb onto specific crystal faces of the zeolite crystal, guiding the [SiO4] formation. 4- and [AlO4] 5-The tetrahedrons are arranged in an MFI or FAU lattice to avoid channel collapse caused by disordered crystal growth. TEAOH can be removed by washing after hydrothermal crystallization, and the remaining space forms uniform nanoscale channels, which significantly increases the specific surface area of ​​the zeolite and provides sufficient sites for subsequent nZVI loading and heavy metal adsorption.

[0026] Furthermore, in this step, under high temperature and high pressure, sodium aluminate and sodium silicate precursors undergo hydrolysis-condensation reactions, gradually forming a three-dimensional lattice structure of zeolite; simultaneously, ferrates in the red mud react with Na in the lattice. + Al 3+ Ion exchange occurs, embedding Fe elements into the zeolite lattice or depositing them on the pore surface to form a zeolite lattice-magnetic particle composite structure, giving M-ZA both ion exchange properties and magnetism. A long crystallization period of 10-14 hours ensures complete lattice growth, reduces defect sites, and enhances the mechanical strength and chemical stability of the zeolite, preventing structural collapse during subsequent nZVI loading, carbonization coating, and soil remediation.

[0027] S2: Loaded with nano-zero valent iron (nZVI@M-ZA)

[0028] This step mainly employs a liquid-phase reduction loading method to load nano-zero valent iron.

[0029] (1) Dissolve FeSO4·7H2O in a 3:7 ethanol-water mixture, with a ratio of (0.56-4.2) g / 100 mL;

[0030] In this step, ethanol, being a nonpolar solvent, reduces the surface tension of the FeSO4·7H2O aqueous solution, thereby reducing the Fe content. 2+ The probability of aggregation on the zeolite surface; simultaneously, ethanol molecules can adsorb onto the inner wall of the zeolite channels, forming a steric hindrance layer, guiding Fe... 2+ Uniform dispersion in the channels and on the surface lays the foundation for subsequent reduction to form nZVI with uniform particle size. Furthermore, a 3:7 volume ratio balances dispersibility and adsorption: an excessively high ethanol content would reduce Fe... 2+ The water solubility of nZVI affects its adsorption on the zeolite surface; if the proportion is too low, it cannot effectively inhibit agglomeration, resulting in uneven nZVI loading.

[0031] (2) Add the M-ZA powder obtained in step S1, with a mass ratio of M-ZA powder to FeSO4·7H2O of 1:(1-3); stir magnetically at 550 r / min for 1-3 hours under N2 protection to allow Fe to precipitate. 2+ Fully adsorbed into the pores and surface of zeolite;

[0032] In this step, a stirring rate of 550 r / min generates moderate shear force, promoting Fe... 2+Diffusion into the zeolite channels avoids insufficient loading due to surface adsorption; a stirring time of 1-3 hours ensures Fe... 2+ It forms a stable complex adsorption with hydroxyl groups (-OH) on the zeolite surface, thereby enhancing the robustness of nZVI loading.

[0033] (3) Under continuous stirring and N2 protection, freshly prepared 0.6M NaBH4 solution was added dropwise at a rate of approximately 2 mL / min. NaBH4 reacted with Fe... 2+ The molar ratio is (2.0-3.0):1;

[0034] In this step, NaBH4 undergoes a hydrolysis reaction in aqueous solution to produce hydrogen gas and strongly reducing BH4. - It can adsorb Fe 2+ Rapid reduction to nano-zero valent iron (nZVI) is achieved, and the reduction product B(OH)3 is easily washed away without secondary pollution. Furthermore, the (2.0-3.0):1 molar ratio ensures Fe... 2+ Complete reduction (excess NaBH4 can prevent Fe) 2+ (Residual), while avoiding excessive growth and aggregation of nZVI caused by excessive NaBH4; the slow drop rate of 2 mL / min can control the reduction reaction rate, so that nZVI is gradually generated on the surface / pores of zeolite, forming uniform nanoparticles (about 20-50 nm).

[0035] (4) After the reaction is complete, the product is separated by a magnet. The product is washed three times with deoxygenated anhydrous ethanol and deoxygenated water to obtain a black solid, which is denoted as nZVI@M-ZA.

[0036] Nano-zero valent iron (nZVI), as a highly efficient environmental remediation material, is widely used for the remediation of heavy metal pollution due to its low cost, availability, environmental friendliness, strong reducing properties, and high reactivity. nZVI remediates heavy metals primarily through a synergistic effect of multiple mechanisms, including surface adsorption, precipitation / co-precipitation, complexation fixation, and encapsulation passivation, exhibiting advantages such as rapid reaction, high removal rate, and long-term stabilization. However, using nZVI alone as a remediation material can easily lead to problems such as agglomeration and oxidative deactivation. Loading nano-zero valent iron onto zeolite materials not only improves their dispersibility and stability but also combines the adsorption properties of zeolite with the reducing power of nano-zero valent iron, further enhancing the removal efficiency of heavy metals. Furthermore, the composite material formed by loading nano-zero valent iron onto zeolite possesses magnetic properties, allowing for magnetic field recovery and regeneration after remediation for reuse, thereby further reducing material costs and environmental risks.

[0037] S3: Polydopamine (PDA) carbonized coating (nZVI@M-ZA@C)

[0038] This step aims to construct a stable, conductive carbon shell to address the issue of nZVI's easy deactivation.

[0039] S3-1. PDA precursor coating

[0040] (1) Prepare a 10 mM Tris-HCl buffer solution with a pH of 8.0-9.0;

[0041] (2) Disperse the freshly prepared nZVI@M-ZA wet sample in 100 mL Tris-HCl buffer solution; the ratio of nZVI@M-ZA to Tris-HCl buffer solution is (0.3-0.8) g: 100 mL;

[0042] (3) Add dopamine hydrochloride, with a mass ratio of dopamine hydrochloride to nZVI@M-ZA of 1:(3-8); stir and react in the dark at room temperature (around 25°C) for 12-24 hours.

[0043] (4) After the reaction is complete, the solid is collected by magnetic separation, washed with deionized water, and dried under vacuum at 60°C to obtain nZVI@M-ZA@PDA.

[0044] Dopamine hydrochloride undergoes oxidative self-polymerization in Tris-HCl buffer (pH=8.0-9.0) to form a polydopamine (PDA) film. The catechol groups in the PDA molecule can react with Fe on the nZVI surface. 0 Formation of coordinate bonds (Fe) 0 The PDA film, consisting of an -O-benzene ring, forms hydrogen bonds with hydroxyl groups on the zeolite surface, achieving uniform coating of the PDA film on the nZVI@M-ZA surface. The PDA film initially isolates oxygen, preventing nZVI from being oxidized before subsequent carbonization; simultaneously, the porous structure of the PDA allows for space, avoiding pore blockage caused by shrinkage during subsequent carbonization.

[0045] S3-2. High-temperature carbonization forms a carbon shell.

[0046] (1) Place the dried nZVI@M-ZA@PDA in a quartz boat in a tubular furnace, and under the protection of an inert atmosphere, heat it to 600-700℃ at a rate of 2-5℃ / min, and hold it at this temperature for 1-2 hours to complete the carbonization of the PDA layer.

[0047] (2) The mixture was then naturally cooled to room temperature under an inert atmosphere. The final product was a highly stable red mud-coal gangue-based magnetic composite material, denoted as nZVI@M-ZA@C.

[0048] Carbon shell formation mechanism: PDA is carbonized at high temperature in an inert atmosphere, and the organic polymer chain decomposes into a carbon shell (containing pyridine nitrogen and pyrrole nitrogen). The thickness of the carbon shell is controlled at 15-20nm. If the thickness is too thin, it cannot effectively isolate the oxidizing medium. If the thickness is too thick, it will block the zeolite channels and nZVI active sites.

[0049] Multifunctional synergistic mechanism:

[0050] Antioxidant protection: The carbon shell acts as a dense physical barrier, isolating oxidizing agents such as O2 and H2O in soil / water, thus preventing nZVI from being oxidized to Fe. 3+ And thus deactivated;

[0051] Enhanced electron transport: The carbon shell has good electrical conductivity, which can accelerate electron transfer on the nZVI surface and improve the transfer of heavy metals (such as Pb). 2+ Cr 6+ The reduction efficiency of )

[0052] Adsorption enhancement: The carbon shell itself has a high specific surface area and abundant functional groups (-COOH, -OH), which can synergistically remove heavy metals through physical adsorption and complexation, further enhancing the total adsorption capacity of the material.

[0053] Thirdly, this invention also proposes the application of the above-mentioned red mud-coal gangue-based magnetic composite material, mainly for the remediation of water and soil contaminated with heavy metals.

[0054] Specifically, the heavy metal is at least one of lead, cadmium, and copper.

[0055] The specific method involves adding a certain amount of nZVI@M-ZA@C to water bodies / soil containing a certain concentration of heavy metals, and then remediating the water bodies / soil contaminated with heavy metals through mechanisms such as co-precipitation, ion exchange, and surface complexation.

[0056] For those skilled in the art, existing technologies generally tend to utilize coal gangue alone (such as CN103394703A, CN112263996A, CN115430404A), assuming that the treatment and modification of single solid waste can meet the carrier requirements. It is assumed that solid waste synergy will increase process complexity and make it difficult to control component interference. Therefore, the complementary synergy of coal gangue (silicon-aluminum source) and red mud (endogenous iron source) has not been considered.

[0057] In this invention, coal gangue provides sufficient silicon and aluminum for zeolite synthesis, and red mud provides the iron element necessary for magnetic zeolite. The two are mixed in a specific ratio (3:1 to 4:1) to simultaneously solve the dual needs of zeolite crystallization and magnetization. This synergistic logic breaks through the existing mindset of using a single raw material.

[0058] For those skilled in the art, zeolite-based or coal gangue-based composite materials (such as CN103394703A and CN112591842A) all focus on the removal of heavy metals from water. It is assumed that the microporous adsorption characteristics of zeolite are difficult to play a role in complex soil environments (such as pH fluctuations, organic matter interference, and diverse heavy metal forms), and soil remediation relies more on chemical reduction or bioremediation. Therefore, the modification and composite of zeolite to adapt to soil scenarios has not been explored in depth.

[0059] This invention enables zeolite to simultaneously possess adsorption, reduction, and long-term fixation functions through nZVI loading and carbonized carbon shell coating treatment, breaking the misconception that zeolite is unsuitable for soil remediation.

[0060] For those skilled in the art, existing methods for stabilizing nZVI are limited to physical loading of the carrier (such as CN103394703A) or biomass coating (such as starch grafting in CN112263996A). It is assumed that such methods can suppress aggregation and oxidation, without realizing the stringent requirements of soil remediation for the long-term effectiveness of materials (stabilization for more than 60 days).

[0061] This invention protects nZVI from oxidation by oxygen and moisture in the soil by carbonizing polydopamine, without affecting its reduction activity and magnetic recovery. This balance between rigid protection and performance preservation breaks through the existing understanding that simple coating can achieve stability.

[0062] For those skilled in the art, zeolite crystallization requires a precise silicon-to-aluminum ratio (Na₂O / (SiO₂+Al₂O₃) = 1.2-1.5), while the formation of the magnetic phase requires a suitable iron content (from red mud). Existing technologies lack relevant synergistic experience: zeolite prepared solely from coal gangue (CN103394703A) lacks endogenous iron, requiring the addition of iron salts to impart magnetism, resulting in uneven magnetic dispersion; red mud alone has insufficient silicon-to-aluminum content, failing to form a complete zeolite lattice. In this invention, multiple parameters, including the dry basis mass ratio of coal gangue to red mud (3:1 to 4:1), gradient temperature activation (organic decomposition at 550-650℃, ferrate formation at 850-900℃), and the controlled dosage of the crystallization guide agent (tetraethylammonium hydroxide), interact and cannot be derived from existing experience in preparing single raw materials.

[0063] The activity of nZVI depends on its nanoscale size and surface reducibility, while carbonization coating requires high-temperature treatment (600-700℃). Existing technologies presuppose that high temperatures will lead to nZVI oxidation or agglomeration, therefore such processes have not been explored. This invention achieves uniform loading of nZVI on the zeolite surface before coating by dispersing it in an ethanol-water mixed solvent and using a Fe²⁺ adsorption time of 1-3 hours. Furthermore, precise control of the inert atmosphere during carbonization prevents nZVI oxidation, while ensuring that the carbon shell thickness (15-20 nm) provides protection without clogging the zeolite channels and affecting adsorption. This continuous process of loading-coating-performance retention cannot be derived from existing techniques such as direct use after loading (e.g., CN103394703A) or low-temperature coating (e.g., room-temperature polymerization in CN112263996A).

[0064] Existing technologies generally focus on a single scenario: heavy metal treatment in water emphasizes rapid adsorption, while soil remediation focuses on chemical reduction or biological fixation. This invention simultaneously satisfies the following requirements: lead removal rate ≥94.5% within 30 minutes in water (rapid adsorption), and residual lead content ≥42.5% in soil after 60 days (long-term stabilization). This requires the material to possess both a high specific surface area (386.2 m²) and... 2 It possesses high nZVI loading (15.8wt%) and strong magnetic properties (saturation magnetization 28.6 emu / g, facilitating water recovery) and long-lasting carbon shell protection (not easily deactivated in soil). Existing technologies struggle to balance rapid reactivity with long-term stability, often resulting in rapid water adsorption but easy deactivation in soil, or stability in soil but slow water adsorption. A balance requires overcoming the technical bottlenecks in material structure design and cannot be achieved by simply superimposing existing technologies.

[0065] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0066] (1) This invention prepares magnetic zeolite by synergistically combining coal gangue (silicon-aluminum source) and red mud (endogenous iron source) at a dry basis mass ratio of 3:1-4:1. The kaolinite mineral in the coal gangue provides the Si and Al elements required for zeolite crystallization, while the iron oxides in the red mud form the magnetic phase, eliminating the need for additional iron salts. This design solves the environmental pollution problem caused by the separate accumulation of coal gangue and red mud, replaces commercially available zeolite carriers, significantly reduces raw material costs, and simultaneously achieves high-value utilization of solid waste. Therefore, this invention achieves synergistic resource utilization of two solid wastes, reducing costs and being environmentally friendly.

[0067] (2) The high specific surface area and ion exchange performance of the magnetic zeolite (M-ZA) in this invention can rapidly capture heavy metals in water. The strong reducing properties of nZVI convert soluble heavy metals into inert forms. Combined with the long-term protection of the carbon shell, this significantly reduces the proportion of acid-extractable lead in the soil, while increasing the proportion of residual lead. This synergistic effect solves the problems of single function and poor soil remediation effect of existing zeolite materials, and is suitable for the treatment of heavy metal pollution in water / soil. Therefore, this invention has excellent cross-scenario remediation performance, taking into account both rapid adsorption in water and long-term stability in soil.

[0068] (3) This invention employs polydopamine carbonization to form a 15-20 nm carbon shell, which serves as a physical barrier to isolate O2 and H2O, preventing the oxidation and deactivation of nZVI, and also enhances electron transport efficiency and reduction activity through nitrogen doping. Simultaneously, the loading process using an ethanol-water mixed solvent (volume ratio 3:7) and an inert atmosphere ensures that nZVI is uniformly dispersed on the zeolite surface / pores. This design addresses the industry pain points of nZVI's easy agglomeration and poor stability. After 5 cycles, the material exhibits a high removal and retention rate, reducing remediation costs and the risk of secondary pollution. Therefore, this invention significantly improves the stability of nZVI and enhances its recyclability.

[0069] In this invention, the magnetic superposition of Fe3O4 and ferrates derived from red mud with nZVI results in a saturation magnetization of 28.6 emu / g. A standard magnet (0.3T) can completely separate the material from the water within one minute. This feature solves the problems of difficult recovery and time-consuming separation of traditional remediation materials, simplifies the post-remediation process, and improves the convenience of engineering applications. Therefore, this invention offers highly efficient magnetic separation performance and is easy to operate.

[0070] (5) In this invention, the graded mixing of coal gangue optimizes particle packing and reactivity, while tetraethylammonium hydroxide crystallization directing agent ensures the regularity of the zeolite lattice. Gradient heating and alkaline melting activation achieve organic matter removal and sufficient magnetic phase formation. These processes synergistically improve the BET specific surface area and nZVI loading of the material, solving the performance fluctuation problems caused by disordered zeolite crystallization and uneven nZVI loading in existing materials, and ensuring the stability and reliability of the material's repair effect. Thus, the preparation process of this invention is optimized, and the material structure and performance are controllable.

[0071] In summary, this invention combines raw material resource utilization, structural functionalization, and diversified remediation scenarios, solving the technical pain points of existing materials such as narrow application scenarios, insufficient nZVI stability, difficult recycling, and high costs, while achieving a balance between environmental benefits, economic benefits, and remediation effectiveness. The prepared red mud-coal gangue-based magnetic composite material has advantages such as high remediation efficiency, long-term stability, convenient operation, and environmental friendliness, and has broad industrial application prospects in the field of water / soil heavy metal pollution remediation.

[0072] The present invention will be explained in detail below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0073] Figure 1 This is a line graph showing the change of Pb²⁺ content with reaction time in Example 4 of the present invention;

[0074] Figure 2 This is a bar chart showing the percentage of acid-extractable lead in total lead in Example 5 of the present invention;

[0075] Figure 3 This is a bar chart showing the percentage of reducible lead in total lead in Embodiment 5 of the present invention;

[0076] Figure 4 This is a bar chart showing the percentage of oxidizable lead in total lead in Example 5 of the present invention;

[0077] Figure 5 This is a bar chart showing the percentage of residual lead in total lead in Embodiment 5 of the present invention;

[0078] Figure 6 This is a bar chart showing the lead ion removal rate in water bodies of Example 1 and nine comparative examples of the present invention.

[0079] Figure 7 This is a bar chart showing the distribution percentage of lead speciation in soil in Embodiment 1 and nine comparative examples of the present invention.

[0080] Figure 8 This is a bar chart of saturation magnetization intensity for Embodiment 1 and nine comparative examples of the present invention;

[0081] Figure 9 This is a bar chart comparing the BET area of ​​Embodiment 1 and nine sets of comparative examples of the present invention;

[0082] Figure 10 This is a bar chart showing the nZVI load of Embodiment 1 and nine comparative examples of the present invention.

[0083] Figure 11 This is a bar chart showing the removal and retention rates of Embodiment 1 and nine comparative examples of the present invention. Detailed Implementation

[0084] To facilitate understanding of the present invention, a more comprehensive description of the present invention will be given below with reference to the accompanying drawings, which illustrate several embodiments of the present invention. However, the present invention can be implemented in different forms and is not limited to the embodiments described in the text. Rather, these embodiments are provided to make the disclosure of the present invention more thorough and complete.

[0085] Example 1: A method for preparing a red mud-coal gangue-based magnetic composite material, comprising the following steps:

[0086] Step 1: Raw material pretreatment:

[0087] Coal gangue from Northwest coal chemical sites was crushed and graded, and then mixed with powders larger than 75μm and powders smaller than 45μm at a mass ratio of 7:3.

[0088] Red mud from an alumina plant was dried at 105℃ for 24 hours, ground, and passed through a 75μm sieve to collect powder particles smaller than 75μm; iron oxides were initially enriched by magnetic separation.

[0089] Weigh out the pretreated coal gangue and pretreated red mud at a dry weight ratio of 3.5:1 and mix them.

[0090] Step 2: Co-preparation of magnetic zeolite (M-ZA):

[0091] The mixed raw materials were mixed with solid NaOH at a ratio of n(Na):n(Al+Si)=1.3:1 and placed in a muffle furnace; the temperature was programmed to rise to 600℃ at 5℃ / min and hold for 0.8h, and then rise to 880℃ and hold for 1.8h.

[0092] After cooling and crushing, deionized water was added to make a slurry with a solid-liquid ratio of 1:10. Tetraethylammonium hydroxide (1.0 wt% of the total raw material mass) was added as a directing agent and hydrothermally reacted at 100℃ for 12 h. The product was magnetically separated, washed with water until neutral, and dried at 80℃ for 12 h to obtain M-ZA.

[0093] Step 3: Loading nano-zero valent iron (nZVI@M-ZA):

[0094] Dissolve 2.0 g of FeSO4·7H2O in 100 mL of a 3:7 volume ratio ethanol-water mixture;

[0095] Add 1.0g of M-ZA powder obtained in step two, and stir magnetically at 550r / min for 2 hours under N2 protection to allow Fe²⁺ to be fully adsorbed in the zeolite channels and surface;

[0096] Under continuous stirring and N2 protection, 50 mL of freshly prepared 0.6 M NaBH4 solution was added dropwise at a rate of approximately 2 mL / min.

[0097] After the reaction was completed, the product was separated by a magnet. The product was washed three times with deoxygenated anhydrous ethanol and deoxygenated water to obtain wet nZVI@M-ZA.

[0098] Step 4: PDA carbonization coating (nZVI@M-ZA@C):

[0099] Take 0.5g of the above wet nZVI@M-ZA and directly transfer it into 100mL of 10mM Tris-HCl buffer at pH=8.5, and add 100mg of dopamine hydrochloride (final concentration 1.0mg / mL). Stir at room temperature in the dark for 18h.

[0100] After the reaction was completed, the solid was collected by magnetic separation, washed with deionized water, and dried under vacuum at 60°C for 12 hours to obtain nZVI@M-ZA@PDA.

[0101] Step 5: High-temperature carbonization to form a carbon shell

[0102] The dried nZVI@M-ZA@PDA was placed in a tubular furnace quartz boat and heated to 650°C at a rate of 3°C / min under the protection of high-purity argon gas (flow rate 50 sccm). It was then held at this temperature for 1.5 hours to complete the carbonization of the PDA layer.

[0103] The product was then naturally cooled to room temperature under an argon atmosphere to obtain the final product nZVI@M-ZA@C-650.

[0104] Example 2: A method for preparing a red mud-coal gangue-based magnetic composite material, comprising the following steps:

[0105] Step 1: Raw material pretreatment:

[0106] Coal gangue from a certain mining area was crushed and graded, and then mixed with powders larger than 75μm and powders smaller than 45μm at a mass ratio of 6:4.

[0107] Red mud from an alumina plant was dried at 80℃ for 48 hours, ground, and passed through a 75μm sieve to collect powder particles smaller than 75μm.

[0108] Weigh out the pretreated coal gangue and pretreated red mud at a dry weight ratio of 3:1 and mix them.

[0109] Step 2: Co-preparation of magnetic zeolite (M-ZA):

[0110] Mix the raw materials with solid NaOH at a ratio of n(Na):n(Al+Si)=1.2:1 and place the mixture in a muffle furnace. Program the temperature: first raise the temperature to 550℃ at 5℃ / min and hold for 1 hour, then raise the temperature to 850℃ and hold for 2 hours.

[0111] After cooling and crushing, deionized water was added to make a slurry with a solid-liquid ratio of 1:10. Tetraethylammonium hydroxide (0.5 wt% of the total raw material mass) was added as a directing agent and hydrothermally reacted at 90°C for 14 hours. The product was magnetically separated, washed with water until neutral, and dried at 80°C to obtain M-ZA.

[0112] Step 3: Loading nano-zero valent iron (nZVI@M-ZA):

[0113] Dissolve 0.56 g of FeSO4·7H2O in 100 mL of a 3:7 volume ratio ethanol-water mixture;

[0114] Add 0.5g of the M-ZA powder obtained in step two, and magnetically stir at 550r / min for 1 hour under N2 protection to allow Fe to... 2+ Fully adsorbed into the pores and surface of zeolite;

[0115] Under continuous stirring and N2 protection, freshly prepared 0.6M NaBH4 solution was added dropwise at a rate of approximately 2 mL / min. The NaBH4 reacted with Fe... 2+ The molar ratio range is 2:1;

[0116] After the reaction was completed, the product was separated by a magnet. The product was washed three times with deoxygenated anhydrous ethanol and deoxygenated water to obtain wet nZVI@M-ZA.

[0117] Step 4: PDA carbonization coating (nZVI@M-ZA@C):

[0118] Take 0.3g of the above wet nZVI@M-ZA and directly transfer it into 100mL of 10mM Tris-HCl buffer at pH=8.0, and add 100mg of dopamine hydrochloride. Stir at room temperature in the dark for 12h.

[0119] After the reaction was completed, the solid was collected by magnetic separation, washed with deionized water, and dried under vacuum at 60°C to obtain nZVI@M-ZA@PDA.

[0120] Step 5: High-temperature carbonization to form a carbon shell

[0121] The dried nZVI@M-ZA@PDA was placed in a tubular furnace quartz boat and heated to 600℃ at a rate of 2℃ / min under the protection of high-purity argon gas (flow rate 50 sccm). It was then held at this temperature for 2 hours to complete the carbonization of the PDA layer.

[0122] The product was then naturally cooled to room temperature under an argon atmosphere to obtain the final product nZVI@M-ZA@C-600.

[0123] Example 3: A method for preparing a red mud-coal gangue-based magnetic composite material, comprising the following steps:

[0124] Step 1: Raw material pretreatment:

[0125] Coal gangue from a certain mining area was crushed and graded, and then mixed with powders larger than 75μm and powders smaller than 45μm at a mass ratio of 8:2.

[0126] Red mud from an alumina plant was dried at 120℃ for 12 hours, ground, and passed through a 75μm sieve to collect powder particles smaller than 75μm.

[0127] Weigh out the pretreated coal gangue and pretreated red mud at a dry weight ratio of 4:1 and mix them.

[0128] Step 2: Co-preparation of magnetic zeolite (M-ZA):

[0129] The mixed raw materials and solid NaOH were mixed at a ratio of n(Na):n(Al+Si)=1.5:1 and placed in a muffle furnace; the temperature was programmed to rise to 650℃ at 5℃ / min and hold for 0.5h, and then rise to 900℃ and hold for 1.5h.

[0130] After cooling and crushing, deionized water was added to make a slurry with a solid-liquid ratio of 1:10. Tetraethylammonium hydroxide of 1.5 wt% of the total raw material mass was added as a directing agent, and the mixture was hydrothermally reacted at 110℃ for 10 h. The product was magnetically separated, washed with water until neutral, and dried at 80℃ to obtain M-ZA.

[0131] Step 3: Loading nano-zero valent iron (nZVI@M-ZA):

[0132] Dissolve 4.2 g of FeSO4·7H2O in 100 mL of a 3:7 volume ratio ethanol-water mixture;

[0133] Add 1.4g of M-ZA powder obtained in step two, and stir magnetically at 550r / min for 3 hours under N2 protection to allow Fe²⁺ to be fully adsorbed in the zeolite channels and surface;

[0134] Under continuous stirring and N2 protection, freshly prepared 0.6M NaBH4 solution was added dropwise at a rate of approximately 2 mL / min. The NaBH4 reacted with Fe... 2+ The molar ratio range is 3:1;

[0135] After the reaction was completed, the product was separated by a magnet. The product was washed three times with deoxygenated anhydrous ethanol and deoxygenated water to obtain wet nZVI@M-ZA.

[0136] Step 4: PDA carbonization coating (nZVI@M-ZA@C):

[0137] Take 0.8g of the above wet nZVI@M-ZA and directly transfer it into 100mL of 10mM Tris-HCl buffer at pH=9.0, and add 100mg of dopamine hydrochloride. Stir at room temperature in the dark for 24h.

[0138] After the reaction was completed, the solid was collected by magnetic separation, washed with deionized water, and dried under vacuum at 60°C to obtain nZVI@M-ZA@PDA.

[0139] Step 5: High-temperature carbonization to form a carbon shell

[0140] The dried nZVI@M-ZA@PDA was placed in a tubular furnace quartz boat and heated to 700°C at a rate of 5°C / min under the protection of high-purity argon (flow rate 50 sccm). It was then held at this temperature for 1 hour to complete the carbonization of the PDA layer.

[0141] The product was then naturally cooled to room temperature under an argon atmosphere to obtain the final product nZVI@M-ZA@C-700.

[0142] Example 4: Treatment of high-concentration lead-containing industrial wastewater

[0143] This embodiment simulates lead-containing acidic wastewater generated during the processing of electronic components, demonstrating the efficient and rapid application of nZVI@M-ZA@C materials in water remediation.

[0144] 1. Simulated wastewater: Prepared with Pb(NO3)2 at pH≈3.5, Pb 2+ Simulated wastewater with an initial concentration of 300 mg / L, containing background ions (Na+). + Ca 2+ (50 mg / L each).

[0145] 2. Remediation experiment: Take 200 mL of the above wastewater into an Erlenmeyer flask and add 40 mg (dosage 0.2 g / L) of the nZVI@M-ZA@C-650 material prepared in Example 1.

[0146] 3. Reaction conditions: The reaction was carried out at room temperature with shaking at 180 r / min. Samples were taken at 10 min, 30 min, 60 min, 120 min and 240 min, and the residual Pb²⁺ concentration was determined after filtration through a 0.22 μm filter membrane.

[0147] 4. Application effects:

[0148] Pb 2+ Content changes with reaction time as follows Figure 1 As shown, by Figure 1 It can be seen that after 30 minutes of reaction, Pb 2+ The content was only 16.5 mg / L, Pb 2+ The removal rate reached 94.5%, indicating that the nZVI@M-ZA@C material provided by this invention has rapid adsorption performance; the reaction reached equilibrium after 240 minutes, and Pb... 2+ With a content of only 2.4 mg / L and a final removal rate of 99.2%, the actual adsorption capacity was calculated to be 297.6 mg / g, indicating that the nZVI@M-ZA@C material has high capacity performance.

[0149] After the reaction, the material can be completely separated from the aqueous solution within 1 minute using a common magnet (surface magnetic field strength of about 0.3T), indicating that the nZVI@M-ZA@C material is easy to separate.

[0150] Example 5: Remediation of mildly to moderately lead-contaminated farmland soil

[0151] This embodiment simulates farmland surface soil pollution caused by irrigation history, demonstrating the long-term stabilizing effect of nZVI@M-ZA@C material on heavy metal speciation in soil remediation.

[0152] 1. Preparation of contaminated soil: Take uncontaminated farmland soil (pH=6.8) from a certain area, air dry and sieve it, add Pb(NO3)2 solution and age for 30 days to prepare simulated contaminated soil with a total Pb content of 450 mg / kg.

[0153] 2. Stabilization and remediation: Weigh 500g of the above-mentioned contaminated soil sample into a plastic box, add 5.0g (1% of the soil weight) of the nZVI@M-ZA@C-650 material prepared in Example 1, and mix thoroughly. Maintain the soil moisture content at 60% of field capacity. Set up a contaminated soil without added material as a control (CK) group.

[0154] 3. Cultivation and Evaluation: Samples were incubated at room temperature. Samples were taken on day 20 and day 60 of cultivation, and the speciation of lead in the soil was analyzed using the BCR continuous extraction method.

[0155] 4. Application effects:

[0156] The percentage of different forms of lead in total lead, such as Figures 2-5 As shown.

[0157] Depend on Figures 2-5 It was found that after 60 days of cultivation, compared with the control group, the proportion of acid-extractable lead with the highest bioavailability in the soil of the treatment group decreased from 35.2% to 11.5%; while the proportion of inert residual lead significantly increased from 17.3% to 42.5%, indicating that some acid-extractable lead was converted into inert residual lead. Furthermore, from day 20 to day 60, the proportion of residual lead increased significantly, indicating that the fixation of lead by the material is a continuous and deepening process with long-term effectiveness.

[0158] Comparative Example 1: All raw materials used were coal gangue, with no red mud, and other aspects were the same as in Example 1.

[0159] Comparative Example 2: All raw materials used were red mud, with no coal gangue, and other aspects were the same as in Example 1.

[0160] Comparative Example 3: Commercially available zeolite was used as a carrier, and nZVI was loaded and carbonized using the same method as in Example 1.

[0161] Comparative Example 4: Without PDA carbonization coating and high-temperature carbonization steps, the product does not have a carbon shell. The wet nZVI@M-ZA was directly dried after preparation. Other steps are the same as in Example 1.

[0162] Comparative Example 5: No tetraethylammonium hydroxide was added during the hydrothermal crystallization stage. The molten block was simply crushed and deionized water was added to form a slurry. The slurry was reacted at 100°C for 12 hours. Other steps were the same as in Example 1.

[0163] Comparative Example 6: In the nZVI loading stage, FeSO4·7H2O was dissolved in 100mL of pure water (instead of a 3:7 volume ratio ethanol-water mixture), and the rest was the same as in Example 1.

[0164] Comparative Example 7: In the nZVI loading stage, 0.6M Na2SO3 solution was used instead of NaBH4 solution. The reaction of Na2SO3 with Fe... 2+ The molar ratio was 2.5:1 (consistent with the NaBH4 molar ratio in Example 1), the dropping rate was 2 mL / min, and the remaining steps were the same as in Example 1.

[0165] Comparative Example 8: In the pretreatment stage of coal gangue, it was only crushed and ground and then passed through a 75μm sieve without grading and mixing. Other aspects were the same as in Example 1.

[0166] Comparative Example 9: In the alkali melting activation stage, the temperature was directly increased to 880℃ at 5℃ / min and held for 2.3h (the total holding time was the same as in Example 1). There was no "550-650℃ holding" step. Everything else was the same as in Example 1.

[0167] The following is a comparative experiment between Example 1 and the nine comparative examples mentioned above.

[0168] 1. Water body Pb 2+ Removal performance

[0169] Prepare simulated wastewater (Pb) consistent with that in Example 4. 2+ Initial concentration 300 mg / L, pH ≈ 3.5, containing Na + Ca 2+ (50 mg / L each); add 0.2 g / L sample, shake at 180 rpm for 240 min at room temperature; take samples at different time points, filter through a 0.22 μm filter membrane, and determine residual Pb by ICP-OES. 2+ Concentration; calculate the removal rate at 30 min, the equilibrium removal rate, and the adsorption capacity. Results are shown in Table 1 and... Figure 6 As shown:

[0170] Table 1. Pb in water bodies of Example 1 and various comparative examples 2+ Remove performance data table

[0171] sample Removal rate (%) in 30 minutes Balance removal rate (%) Adsorption capacity (mg / g) Example 1 94.5 99.2 297.6 Comparative Example 1 82.3 85.1 255.2 Comparative Example 2 78.5 80.3 240.9 Comparative Example 3 91.2 92.8 278.4 Comparative Example 4 89.8 90.5 271.5 Comparative Example 5 75.6 78.9 236.7 Comparative Example 6 88.4 91.3 273.9 Comparative Example 7 65.2 68.7 206.1 Comparative Example 8 90.1 93.5 280.5 Comparative Example 9 87.6 89.2 267.6

[0172] 2. Distribution of lead speciation in soil

[0173] Using simulated contaminated soil (total Pb 450 mg / kg) from Example 5, 1% of the sample was added and incubated for 60 days. Acid-extractable, reducible, oxidizable, and residual lead were extracted stepwise using the BCR continuous extraction method. The content of each lead form was determined by ICP-OES, and the percentage of each form in the total lead was calculated. The main results are as follows: Figure 7 As shown.

[0174] 3. Magnetic separation performance

[0175] The saturation magnetization (Ms) of the samples was measured using a vibrating sample magnetometer (VSM). After the water remediation experiment, the time required for complete separation was recorded using a 0.3T ordinary magnet. The results are shown in Table 2 and... Figure 8 As shown:

[0176] Table 2. Magnetic separation performance data of Example 1 and each comparative example.

[0177] sample Saturation magnetization (emu / g) Magnetic separation time (min) Example 1 28.6 1.0 Comparative Example 1 12.3 5.2 Comparative Example 2 35.7 0.8 Comparative Example 3 8.9 8.5 Comparative Example 4 26.8 1.2 Comparative Example 5 15.6 4.8 Comparative Example 6 25.4 1.5 Comparative Example 7 10.2 6.7 Comparative Example 8 24.3 1.8 Comparative Example 9 18.7 4.1

[0178] 4. Material structural properties

[0179] (1) BET specific surface area: tested by liquid nitrogen adsorption-desorption method;

[0180] (2) nZVI loading: After sample digestion, the Fe element content was determined by AAS;

[0181] (3) Carbon shell integrity: Surface structure was observed by transmission electron microscopy (TEM), and carbon shell thickness was statistically analyzed.

[0182] The results are shown in Table 3 and Figure 9 , Figure 10 As shown:

[0183] Table 3. Material structural property data of Example 1 and each comparative example.

[0184] sample <![CDATA[BET specific surface area (m 2 / g)]]> nZVI loading (wt%) Carbon shell thickness (nm) Example 1 386.2 15.8 15-20 Comparative Example 1 258.7 8.6 15-20 Comparative Example 2 215.3 18.3 15-20 Comparative Example 3 320.5 10.5 15-20 Comparative Example 4 372.4 15.2 none Comparative Example 5 189.6 7.9 15-20 Comparative Example 6 354.8 13.7 15-20 Comparative Example 7 230.7 5.2 15-20 Comparative Example 8 342.6 14.1 15-20 Comparative Example 9 275.9 9.8 15-20

[0185] 5. Material Cyclic Stability

[0186] After the water remediation experiment, the sample was magnetically separated and recovered, washed three times with deoxygenated water, and dried at 80℃. The sample was reused five times, with each iteration under the same experimental conditions as the first. The removal rate in the fifth iteration was calculated relative to the retention rate in the first iteration. Results are as follows: Figure 11 As shown.

[0187] From Tables 1-3, Figures 6-11 It can be known that:

[0188] Comparative Example 1 (without red mud): The lack of endogenous iron source in red mud resulted in insufficient magnetic phase formation of magnetic zeolite, leading to a decrease in saturation magnetization (12.3 emu / g) and nZVI loading (8.6 wt%). Consequently, the water removal rate (85.1%) and the proportion of soil residue (24.8%) were significantly lower than those in Example 1, and the magnetic separation time was extended to 5.2 min.

[0189] Comparative Example 2 (without coal gangue): Coal gangue provides a sufficient source of silicon and aluminum, but zeolite crystallization is incomplete, resulting in a specific surface area of ​​only 215.3 m². 2 / g, insufficient adsorption sites, although sufficient iron source in red mud resulted in high magnetization (35.7 emu / g), but the water adsorption capacity (240.9 mg / g) and soil stabilization effect were still significantly deteriorated.

[0190] Comparative Example 3 (commercially available zeolite): Commercially available zeolite has no intrinsic magnetism and relies on nZVI to provide magnetism, resulting in extremely low saturation magnetization (8.9 emu / g) and a magnetic separation time as long as 8.5 min; moreover, the compatibility between zeolite and nZVI is poor, and the loading (10.5 wt%) and removal performance are not as good as those of Example 1.

[0191] Comparative Example 4 (without carbon shell): nZVI lacks carbon shell protection, is easily oxidized and aggregated, and has extremely poor cycle stability (only 62.4% retention rate after 5 cycles); the proportion of acid-extractable form in the soil (20.5%) is higher than that in Example 1, indicating that the inactivation of nZVI leads to a decrease in stabilization effect.

[0192] Comparative Example 5 (without crystallization guide): The zeolite crystals grew disorderedly, with a specific surface area of ​​only 189.6 m² / g and the lowest nZVI loading (7.9 wt%). All performance indicators were among the worst of the nine comparative examples, fully demonstrating the key role of the crystallization guide in optimizing the zeolite structure.

[0193] Comparative Example 6 (pure water solvent): The dispersing effect of ethanol is lacking, Fe 2+ Uneven adsorption on the zeolite surface led to a decrease in the nZVI loading (13.7 wt%). Although the performance was better than most comparative examples, it was still lower than that of Example 1, verifying the necessity of the ethanol-water mixed solvent.

[0194] Comparative Example 7 (Na2SO3 reducing agent): Na2SO3 has a weaker reducing power than NaBH4, Fe 2+ The reduction was insufficient, with an nZVI loading of only 5.2 wt%, resulting in the worst performance across all categories, highlighting the irreplaceable nature of the NaBH4 reduction system.

[0195] Comparative Example 8 (unclassified coal gangue): Poor particle packing efficiency, specific surface area (342.6 m²). 2The slight decrease in particle size distribution ( / g) and nZVI loading (14.1wt%) indicates a minor deterioration in performance, suggesting that particle size distribution optimizes the reaction activity.

[0196] Comparative Example 9 (without gradient heating): Organic matter was not fully decomposed, magnetic zeolite was not completely activated, and magnetization (18.7 emu / g) and specific surface area (275.9 m²) were low. 2 The decrease in / g) led to a decrease in removal rate and stabilization effect, verifying the importance of gradient heating.

[0197] In summary, the combined technical features of the first embodiment of this invention, namely the synergistic preparation of magnetic zeolite from red mud and coal gangue, crystallization guidance, ethanol-water solvent-loaded nZVI, and PDA carbonization coating, significantly outperform the nine comparative groups in terms of water heavy metal removal rate (99.2%), soil stabilization effect (42.5% of the residue), magnetic separation efficiency (1 min), and cycle stability (89.6% retention rate), fully demonstrating the synergistic effect and irreplaceability of each key technical feature.

[0198] The present invention has been described by way of example in conjunction with the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvement made by adopting the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other occasions without modification, shall be within the protection scope of the present invention.

Claims

1. A method for preparing a red mud-coal gangue-based magnetic composite material, characterized in that: Magnetic zeolite prepared by co-processing red mud and coal gangue was used as a carrier to load nano-zero valent iron and then coated with polydopamine to form a carbon shell. Includes the following steps: S1. Coal gangue and red mud are pretreated separately and then mixed together, with the dry basis mass ratio of coal gangue to red mud controlled at 3:1 to 4:1; then alkali melting activation and crystallization guidance are carried out to obtain magnetic zeolite precursor. S2, FeSO 4· 7H2O was dissolved in an ethanol-water mixture, and the magnetic zeolite precursor prepared in step S1 was added. The mixture was stirred evenly under inert gas protection. Then, NaBH4 solution was added dropwise. After the reaction was completed, the mixture was separated by a magnet and washed to obtain magnetic zeolite loaded with nano-zero valent iron. S3. Magnetic zeolite loaded with nano-zero valent iron was dispersed in Tris-HCl buffer solution, and dopamine hydrochloride was added. The mixture was stirred and reacted. After the reaction was completed, the solid was collected by magnetic separation, washed, and dried. Then, high-temperature carbonization is carried out to form a carbon shell, and the final product is a red mud-coal gangue-based magnetic composite material. In step S1, the pretreatment of coal gangue includes: crushing, grinding and sieving the coal gangue, collecting two particle size components with a particle size of more than 75 μm and less than 45 μm, and mixing them in a mass ratio of (6:4)-(8:2); The pretreatment of red mud includes: drying the red mud, grinding it, and then passing it through a 75μm sieve; In step S1, the alkali melt activation includes: The pretreated mixed raw materials and solid sodium hydroxide were mixed evenly at a Na2O / (SiO2+Al2O3) molar ratio of 1.2-1.5; Gradient heating activation is used: first, the temperature is increased to 550-650℃ at a rate of 5℃ / min and held for 0.5-1 hour; then, the temperature is increased to 850-900℃ and held for 1.5-2 hours to obtain molten blocks. In step S1, the crystallization guidance includes: The molten block obtained by alkali melting activation is cooled and crushed, and deionized water is added to make a slurry; 0.5-1.5 wt% tetraethylammonium hydroxide is added as a crystallization directing agent, and a hydrothermal reaction is carried out at 90-110℃ for 10-14 hours. The product was magnetically separated, washed until neutral, and dried at 80°C to obtain a magnetic zeolite precursor. In step S2, the volume ratio of the ethanol-water mixed solution is 3:7, and the ratio of FeSO4·7H2O to the ethanol-water mixed solution is (0.56-4.2) g / 100mL; The mass ratio of the magnetic zeolite precursor to FeSO4·7H2O is 1:(1-3). NaBH4 and Fe 2+ The molar ratio is (2.0-3.0):

1.

2. The method for preparing a red mud-coal gangue-based magnetic composite material according to claim 1, characterized in that: The pretreatment of red mud also includes: preliminary enrichment of iron oxides in it through magnetic separation.

3. The method for preparing a red mud-coal gangue-based magnetic composite material according to claim 1, characterized in that: In step S3, the ratio of magnetic zeolite loaded with nano-zero valent iron to Tris-HCl buffer solution is (0.3-0.8) g: 100 mL; And / or, the mass ratio of dopamine hydrochloride to magnetic zeolite loaded with nano-zero valent iron is 1:(3-8). And / or, the conditions for high-temperature carbonization include: heating to 600-700°C at a rate of 2-5°C / min and holding at this temperature for 1-2 hours.

4. A red mud-coal gangue-based magnetic composite material, characterized in that: It is prepared by the preparation method according to any one of claims 1-3.

5. An application of the red mud-coal gangue-based magnetic composite material according to claim 4, characterized in that: To remediate water and soil contaminated with heavy metals.

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