A method for remediation of heavy metal contaminated soil by composting with recyclable magnetic layered double oxide pellets

Abstract

The application discloses a method for remediation of heavy metal contaminated soil by composting with recyclable magnetic layered double oxide small balls, which comprises adding the magnetic layered double oxide small balls to the heavy metal contaminated soil for composting treatment, wherein the magnetic layered double oxide small balls are prepared by calcining a skeleton material, a layered double hydroxide and nano ferroferric oxide. The magnetic layered double oxide small balls have the advantages of large specific surface area, rich porous structure, many heavy metal adsorption sites, strong magnetism, easy recycling, good structural stability, green environmental protection and the like, can realize the soil remediation target of high efficiency, green and low cost, have the advantages of simple process, convenient operation, low cost, good remediation effect, high remediation efficiency, no secondary pollution and the like, solve the problems of humification deficiency and heavy metal reactivation of traditional composting technology, overcome the problems of adsorbent loss and secondary pollution, and have high use value and good application prospect.

Description

Technical Field

[0001] This invention belongs to the field of soil pollution remediation technology in environmental engineering, specifically relating to a method for remediating heavy metal contaminated soil by enhancing composting using recyclable magnetic layered double oxide microspheres. Background Technology

[0002] With the acceleration of industrialization and agricultural intensification, soil heavy metal pollution has become increasingly serious. Heavy metals (such as cadmium, lead, and arsenic) accumulate through the food chain, directly threatening human health and ecological security, necessitating the development of efficient, economical, and sustainable soil remediation technologies. Currently, physical / chemical remediation methods (such as leaching and solidification) suffer from high costs and damage to soil structure, while traditional bioremediation technologies (such as phytoremediation and microbial remediation) are inefficient and time-consuming, making it difficult to meet the needs of large-scale pollution control.

[0003] In recent years, composting remediation technology has attracted much attention due to its environmental friendliness and resource potential. By co-compiling organic waste (such as agricultural waste and fruit and vegetable residues) with contaminated soil, the metabolic activity of microorganisms can promote the transformation of heavy metal forms (such as passivation or fixation). However, existing composting technologies still face the following bottlenecks: (1.1) Insufficient humification, resulting in limited complexation capacity between organic matter and heavy metals; (1.2) Heavy metals are only temporarily passivated, resulting in poor long-term stability, and the concentration increases due to the concentration effect, posing a risk of secondary release; (1.3) The addition of functional materials helps promote the humification process of composting, but there is a lack of recycling mechanisms for functional materials, leading to high remediation costs and a high risk of secondary pollution. In addition, existing recyclable functional materials also have the following defects: (2.1) Small specific surface area and few adsorption sites, making it difficult to effectively adsorb multiple heavy metals; (2.2) Poor stability, making them unsuitable for complex applications and posing a high risk of secondary pollution; (2.3) Poor catalyst regeneration and reuse.

[0004] Therefore, obtaining a magnetic layered double oxide microsphere with a large specific surface area, abundant porous structure, multiple heavy metal adsorption sites, strong magnetism, easy recycling, good structural stability, and environmental friendliness is of great significance for the effective treatment of heavy metal contaminated soil. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for remediating heavy metal contaminated soil by using recyclable magnetic layered dioxide microspheres for enhanced composting, which is simple in process, convenient in operation, low in cost, good in remediation effect, high in remediation efficiency, and without secondary pollution.

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

[0007] A method for enhancing composting remediation of heavy metal contaminated soil using recyclable magnetic layered double oxide microspheres, wherein the method involves adding magnetic layered double oxide microspheres to the heavy metal contaminated soil for composting treatment; the magnetic layered double oxide microspheres are prepared by sintering a framework material, layered double hydroxides, and nano-ferric oxide.

[0008] A further improvement to the above method is the preparation method of the magnetic layered double oxide microspheres, which includes the following steps:

[0009] (1) Mix the framework material, layered double hydroxide, nano-iron oxide and organic solvent, and shape them to form spheres;

[0010] (2) The spheres are sintered to obtain magnetic layered double oxide microspheres.

[0011] In a further improvement to the above method, in step (1), the mass ratio of the skeleton material, the layered double hydroxide, and the nano-iron oxide is 6:4:1 to 4:6:1; the skeleton material is at least one of zeolite, diatomite, and bentonite; the cation in the layered double hydroxide is two of magnesium, aluminum, calcium, and iron; the amount of the organic solvent is 55% to 65% of the total mass of all raw materials; and the organic solvent is at least one of methanol, ethanol, formamide, and glycerol.

[0012] In a further improvement to the above method, in step (2), the firing is carried out at a temperature of 450℃~550℃; the firing time is 4h~5h; and the particle size of the magnetic layered double oxide microspheres is 0.5cm~1.0cm.

[0013] A further improvement to the above method involves adding magnetic layered double oxide microspheres to heavy metal-contaminated soil for composting, including the following steps:

[0014] S1. Mix agricultural waste, fruit and vegetable residues, conditioning agents, heavy metal contaminated soil, and magnetic layered double oxide microspheres to obtain a mixture.

[0015] S2. Perform aerobic composting on the mixture to complete the remediation of soil contaminated with heavy metals.

[0016] In a further improvement to the above method, in step S1, the amount of magnetic layered double oxide microspheres added is 3% to 6% of the total dry weight of the agricultural waste, fruit and vegetable residues, conditioner, and heavy metal contaminated soil.

[0017] In a further improvement to the above method, in step S1, the mass ratio of agricultural waste, fruit and vegetable residues, conditioner, and heavy metal contaminated soil is 4–6:4–6:2.5–3.5:6–8; the C / N mass ratio in the mixture of agricultural waste, fruit and vegetable residues, conditioner, and heavy metal contaminated soil is controlled to be 25–30, and the moisture content is 55%–65%; the agricultural waste is crop straw; the crop straw is rice straw; the conditioner is wheat bran; and the heavy metal in the heavy metal contaminated soil is at least one of cadmium, lead, copper, and zinc.

[0018] In a further improvement to the above method, in step S2, the aerobic composting time is 55 to 65 days; the frequency of turning the pile during the aerobic composting process is controlled as follows: once a day for the first 10 days, once every three days from the 11th to the 22nd day, and once a week thereafter; the aerobic composting process also includes heat preservation treatment of the pile to maintain the temperature of the pile at 50℃ to 70℃.

[0019] In a further improvement to the above method, step S2, after the aerobic composting is completed, includes the following treatment: recovering magnetic layered double oxide microspheres from the aerobic compost pile, desorbing the recovered magnetic layered double oxide microspheres, and calcining the desorbed magnetic layered double oxide microspheres to obtain regenerated magnetic layered double oxide microspheres; the regenerated magnetic layered double oxide microspheres are then used to remediate heavy metal contaminated soil.

[0020] A further improvement to the above method involves using a magnetic attraction device to recover magnetic layered double oxide microspheres from the aerobic compost pile; the magnetic attraction device is a magnetic rod or a magnetic separator; a desorption reagent is used to desorb the recovered magnetic layered double oxide microspheres; the desorption reagent is a mixed solution of sodium carbonate and sodium hydroxide; the concentration of sodium carbonate in the mixed solution is 0.2 mol / L to 0.4 mol / L, and the concentration of sodium hydroxide is 0.1 mol / L to 0.2 mol / L; the desorption time is 6 h to 12 h; and the desorbed magnetic layered double oxide microspheres are calcined at a temperature of 450℃ to 550℃ for 4 h to 5 h.

[0021] Compared with the prior art, the advantages of the present invention are as follows:

[0022] To address the problems of high total heavy metal concentration, low humic acid content, and easy loss of adsorbent materials leading to secondary pollution in existing heavy metal contaminated soil remediation technologies, this invention innovatively introduces recyclable magnetic layered double oxide microspheres combined with composting technology, achieving efficient, green, and low-cost soil remediation. Its core advantages are reflected in the following aspects:

[0023] (1) Synergistic effect of multiple materials to enhance the adsorption and fixation of heavy metals: The magnetic layered double oxide microspheres are made of nano-iron oxide, layered double hydroxide and framework materials (such as zeolite, diatomite and bentonite, which have high temperature resistance and porous adsorption characteristics, especially zeolite with superior performance). Under their joint action, the magnetic layered double oxide microspheres have high specific surface area, porous structure and rich heavy metal adsorption sites (such as the interlayer anion exchange capacity of layered double hydroxide and the ion sieving effect of zeolite), which can significantly improve the adsorption capacity of heavy metals such as cadmium and lead. Specifically, the framework material (such as zeolite powder) adsorbs cations through surface complexation and also provides a porous framework to enhance mechanical strength and adsorption stability. Moreover, the layered double hydroxide can efficiently fix a variety of heavy metals through interlayer ion exchange and surface coordination. In particular, the nano-sized iron oxide imparts strong magnetism to the material, enabling rapid recovery of the spheres when used with magnetic adsorption devices (such as magnetic rods or magnetic separators), thus completely separating heavy metals from the soil and avoiding secondary pollution caused by residues of traditional adsorbent materials. Furthermore, compared to micron-sized iron oxide, the nano-sized iron oxide used in this invention exhibits superior adsorption performance, and its response speed is faster and recovery efficiency is higher during magnetic separation and recovery. Therefore, by combining the characteristics of multiple components, this invention solves the problems of low adsorption capacity and difficult recovery associated with traditional adsorbents, while also being adaptable to complex polluted environments.

[0024] (2) Significantly enhanced humification and improved long-term stability of heavy metals: By optimizing the co-composting ratio of agricultural waste, fruit and vegetable residues, conditioners, and heavy metal-contaminated soil (e.g., a mass ratio of 4-6:4-6:2.5-3.5:6-8), combined with the regulatory effect of magnetic layered double oxide microspheres on the microbial community, the synthesis of high-molecular-weight organic matter such as humic acid is promoted, the stability of heavy metal-organic complexes is enhanced, and the secondary release of heavy metals is effectively inhibited. Specifically, the addition of magnetic layered double oxide microspheres to the composting system can increase the aeration effect and porosity of the compost, which is conducive to the supply of oxygen and provides more attachment sites for microorganisms, which helps the growth and reproduction of microorganisms, thereby improving the microbial community structure and increasing the degradation of organic matter (Firmicutes, Actinobacteriota, and Bacteroidota) and the fixation of heavy metals (Bacillus and The increased abundance of Pseudomonas-related microorganisms, and the improved microbial community structure, further promoted organic matter degradation and provided more humic acid precursors, thus accelerating the humification of compost. The increase in humic acid provided more functional groups (ketones, quinones, amides, and carboxyl groups) for heavy metal fixation. Furthermore, precise control of the C / N ratio (25–30), moisture content (55%–65%), and turning frequency (stage-wise adjustment) during composting ensured maximum microbial activity and accelerated the humification process of organic matter.

[0025] (3) Material recycling significantly reduces repair costs: After being regenerated by desorption from a mixed solution of sodium carbonate and sodium hydroxide, the magnetic layered double oxide microspheres can be repeatedly calcined (450-550℃) and used multiple times. The material loss rate is low, and the repair cost is lower than that of traditional disposable adsorbents. Specifically, high-temperature calcination not only removes residual organic matter from the composting process, but also fully utilizes the memory effect of layered double hydroxides to reconstruct the layered structure of the layered double hydroxides, restore the adsorption performance, and the adsorption capacity recovery rate of the regenerated microspheres is ≥80%. At the same time, the magnetic recycling technology simplifies the material separation process, avoids high-energy-consuming operations such as manual screening or centrifugation, and saves time and economic costs.

[0026] (4) The process is green and efficient, with no risk of secondary pollution: Agricultural waste (such as rice straw) and fruit and vegetable residues are used as composting raw materials to realize the resource utilization of waste, reduce the environmental burden caused by landfill or incineration, and the composting process is naturally heated through heat preservation and aerobic fermentation (55-65 days), without the need for additional energy input, which is in line with the concept of low carbon and environmental protection.

[0027] (5) Simple operation and strong adaptability: The preparation process of magnetic layered double oxide microspheres is simple (e.g., ethanol shaping + high-temperature calcination), and the particle size (0.5-1.0 cm) and dosage (3%-6%) are easy to control, making it suitable for soil remediation scenarios with different levels of heavy metal pollution. Furthermore, by optimizing the particle size of the magnetic layered double oxide microspheres to 0.5-1.0 cm, large-scale adsorbents can be prepared, ensuring structural uniformity and high recovery rate. In particular, calcination at a high temperature of 450-550℃ helps enhance the structural stability of the material. Calcination also converts layered double hydroxides into layered double oxides, which is beneficial for increasing the porosity of the material, increasing adsorption active sites, and improving adsorption capacity. In addition, the organic solvent used is at least one of methanol, ethanol, formamide, and glycerol. Ethanol, in particular, not only dissolves layered double hydroxides but also has advantages in performance, environmental protection, safety, and economy. In addition, the desorption and regeneration conditions are mild (0.2–0.4 mol / L sodium carbonate / sodium hydroxide, 6–12 h), and the operating parameters are clear, making it easy to promote industrialization.

[0028] Overall Advantages: This invention organically combines functional material innovation, bio-enhanced remediation, and resource recycling. It not only solves the problems of insufficient humification and heavy metal reactivation in traditional composting technology, but also overcomes the problems of adsorbent loss and secondary pollution through the recyclability of magnetic materials. It has outstanding features such as high remediation efficiency (high heavy metal removal rate), low cost (material recycling), and environmental friendliness (zero residue), providing an efficient and feasible technical solution for large-scale heavy metal contaminated soil remediation. Attached Figure Description

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0030] Figure 1 This is a diagram showing the changes in the speciation of heavy metals during the process of enhanced composting remediation of heavy metal-contaminated soil using recyclable magnetic layered double oxide microspheres in Example 1 of the present invention. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention. All materials and instruments used in the following embodiments are commercially available.

[0032] To address the problems of high total heavy metal concentration, low humic acid content, and easy loss of adsorbent materials leading to secondary pollution in existing heavy metal contaminated soil remediation technologies, this invention proposes a method for enhancing composting remediation of heavy metal contaminated soil using recyclable magnetic layered double oxide microspheres. This method involves adding magnetic layered double oxide microspheres to the heavy metal contaminated soil for composting treatment. The magnetic layered double oxide microspheres are prepared by sintering a framework material, layered double hydroxides, and nano-ferric oxide.

[0033] In some embodiments of the present invention, the method for preparing magnetic layered double oxide microspheres includes the following steps:

[0034] (1) Mix the framework material, layered double hydroxide, nano-iron oxide and organic solvent, and shape them to form spheres;

[0035] (2) The spheres are sintered to obtain magnetic layered double oxide microspheres.

[0036] More preferably, in step (1), the mass ratio of the skeleton material, the layered double hydroxide and the nano-iron oxide is 6:4:1 to 4:6:1, the skeleton material is at least one of zeolite, diatomite and bentonite, the cation in the layered double hydroxide is two of magnesium, aluminum, calcium and iron, the amount of organic solvent is 55% to 65% of the total mass of all raw materials, and the organic solvent is at least one of methanol, ethanol, formamide and glycerol.

[0037] In a further preferred embodiment, the firing is carried out at a temperature of 450℃~550℃ for a firing time of 4h~5h.

[0038] More preferably, the magnetic layered double oxide microspheres have a particle size of 0.5 cm to 1.0 cm.

[0039] In some embodiments of the present invention, the addition of magnetic layered double oxide microspheres to heavy metal contaminated soil for composting treatment includes the following steps:

[0040] S1. Mix agricultural waste, fruit and vegetable residues, conditioning agents, heavy metal contaminated soil, and magnetic layered double oxide microspheres to obtain a mixture.

[0041] S2. Perform aerobic composting on the mixture to complete the remediation of soil contaminated with heavy metals.

[0042] In a further preferred embodiment, in step S1, the mass ratio of agricultural waste, fruit and vegetable residues, conditioner, and heavy metal contaminated soil is 4–6:4–6:2.5–3.5:6–8.

[0043] More preferably, in step S1, the C / N mass ratio of the mixture of agricultural waste, fruit and vegetable residues, conditioner and heavy metal contaminated soil is controlled to be 25-30, and the moisture content is 55%-65%.

[0044] In a further preferred embodiment, in step S1, the agricultural waste is crop straw, the crop straw is rice straw, and the conditioner is wheat bran.

[0045] More preferably, in step S1, the heavy metal in the heavy metal contaminated soil is at least one of cadmium, lead, copper, and zinc.

[0046] In a further preferred embodiment, the aerobic composting time in step S2 is 55 to 65 days.

[0047] Further preferably, in step S2, the frequency of turning the compost pile during the aerobic composting process is controlled as follows: once a day for the first 10 days, once every three days from day 11 to day 22, and once a week thereafter.

[0048] In a further preferred embodiment, step S2, the aerobic composting process also includes heat preservation treatment of the compost pile to maintain the temperature of the compost pile at 50℃~70℃.

[0049] In some embodiments of the present invention, step S2, after aerobic composting is completed, further includes the following treatment: recovering magnetic layered double oxide microspheres from the aerobic compost pile, desorbing the recovered magnetic layered double oxide microspheres, and calcining the desorbed magnetic layered double oxide microspheres to obtain regenerated magnetic layered double oxide microspheres.

[0050] Further optimized, the regenerated magnetic layered dioxide microspheres can be used to remediate heavy metal contaminated soil.

[0051] More preferably, a magnetic suction device is used to recover magnetic layered double oxide microspheres from the aerobic compost pile, wherein the magnetic suction device is a magnetic rod or a magnetic separator.

[0052] In a further preferred embodiment, the recovered magnetic layered dioxide microspheres are desorbed using a desorption reagent, wherein the desorption reagent is a mixed solution of sodium carbonate and sodium hydroxide; the concentration of sodium carbonate in the mixed solution is 0.2 mol / L to 0.4 mol / L, and the concentration of sodium hydroxide is 0.1 mol / L to 0.2 mol / L; the desorption time is 6 h to 12 h.

[0053] In a further preferred embodiment, the desorbed magnetic layered double oxide microspheres are sintered at a temperature of 450℃~550℃ for a sintering time of 4h~5h.

[0054] Example 1:

[0055] A method for enhancing composting remediation of heavy metal contaminated soil using recyclable magnetic layered double oxide microspheres includes the following steps:

[0056] S1. Prepare the mixture according to the mass ratio of zeolite powder (commercially available), magnesium aluminum layered double hydroxide (commercially available), and nano iron oxide (commercially available) of 6:4:1. Add ethanol at 60% of the total mass and mix well. Shape the mixture into 0.5cm spheres in a mold. After placing it at room temperature for 6 hours, transfer it to a muffle furnace and fire it at 450℃ for 4 hours to obtain magnetic layered double oxide spheres.

[0057] S2. The composting raw materials, agricultural waste, heavy metal contaminated soil, fruit and vegetable residues and conditioner, are mixed in a mass ratio of 5:7:5:3 to obtain compost body one with a C / N mass ratio of 25 and a moisture content of 60%.

[0058] S3. Add magnetic layered double oxide spheres to pile one at a mass of 5% of the total dry weight of pile one, mix evenly, and obtain pile two.

[0059] S4. Perform aerobic composting on pile 2 for 60 days to remediate heavy metal contaminated soil. Turn the pile once a day for the first ten days of composting, turn it once every three days from day 11 to day 22, and then turn it once a week thereafter.

[0060] Among them, agricultural waste mainly provides organic matter and acts as a leavening agent. The agricultural waste is rice straw, which is relatively easy to obtain. The rice straw used was tested and found to have a total nitrogen (TN) of 8.79 g / kg and a total organic matter (OM) of 898.26 g / kg. Fruit and vegetable residues are easily degradable organic matter, and the fruit and vegetable residues are withered and yellowed vegetables. They are mainly used to provide nutrients in the early stage and promote composting. The conditioner is wheat bran, which is mainly used to increase the contact area and adjust the nitrogen content of the compost pile. The wheat bran used was tested and found to have a TN of 24.21 g / kg and a total organic matter (OM) of 941.48 g / kg. The selected heavy metal contaminated soil was taken from the vicinity of an abandoned factory in a certain city. The background values ​​of heavy metal content in the soil were 8.23 ​​mg / kg (Cd) and 1051.35 mg / kg (Pb).

[0061] During aerobic composting, the compost pile is insulated by wrapping it with insulating materials to maintain its temperature between 50°C and 70°C.

[0062] Control group 1: Without adding magnetic layered double oxide microspheres, the pile obtained in step S2 was directly subjected to aerobic composting, with other conditions remaining the same.

[0063] Control group 2: Magnetic layered double oxide microspheres with a total dry weight of 2% were added to the pile, and other conditions were the same.

[0064] Samples were taken on days 0, 5, 10, 15, 25, 40, and 60 of composting, air-dried under natural conditions, thoroughly ground, and sieved through a 1mm sieve for later testing.

[0065] After 60 days of composting, the entire pile is spread out to air dry. Magnetic layered double oxide microspheres are then recovered by stirring with a magnetic rod, achieving a recovery rate of up to 75%. The recovered magnetic layered double oxide microspheres are also desorbed using a mixture of 0.2 mol / L sodium carbonate and 0.1 mol / L sodium hydroxide for 6 hours. After desorption, the microspheres are transferred to a muffle furnace and fired at 450°C for 4 hours for reuse. The recovered magnetic layered double oxide microspheres still exhibit an adsorption rate of 70%–80% for Cd and Pb.

[0066] Humic acid content was determined by extracting the dried sample with sodium pyrophosphate, separating it with 0.5 mol / L sulfuric acid, dissolving it with sodium bicarbonate, and then measuring it using a total organic carbon analyzer. Total heavy metal content was determined by atomic absorption spectrometry after strong acid digestion of the sample. Heavy metal speciation was analyzed using a modified BCR method. Each sample was measured three times, and the average value was taken as the result. The humic acid production, total Cd and Pb content, and Cd and Pb passivation rates are shown in Table 1. The changes in heavy metal speciation are shown in... Figure 1 As shown.

[0067] From Table 1 and Figure 1 The results showed that the method of enhancing composting remediation of heavy metal contaminated soil using recyclable magnetic layered double oxide microspheres in Example 1 significantly promoted the production of humic acid and the passivation of Cd and Pb, and significantly reduced the total amount of heavy metals Cd and Pb. Compared with control group 1, the production of humic acid increased by 36.98%, the passivation rate of Cd and Pb increased by 48.04% and 11.57% respectively, and the total amount of Cd and Pb decreased by 18.31% and 21.93% respectively, all of which were lower than the risk control values ​​for agricultural land soil pollution in China's "Soil Environmental Quality" (GB15618-2018) (Cd: 4 mg / kg, Pb: 1000 mg / kg). The above results indicate that the method of the present invention can effectively increase the humic acid content of compost, promote the passivation of Cd and Pb, significantly reduce the Cd and Pb content in the soil, and ultimately achieve the purification of heavy metal contaminated soil. It has the advantages of simple process, convenient operation, low cost, good remediation effect, and high remediation efficiency, and has high application value and good application prospects.

[0068] Table 1. Effects of different addition amounts of magnetic layered double oxide microspheres on the generation of humic acid, passivation rate of heavy metals, and content of heavy metals in soil.

[0069] Serial Number Added amount Humic acid production Cd passivation rate Pb passivation rate Cd content Pb content Control group 1 none 21.54g / kg 10.14％ 2.28％ 4.37 mg / kg 664.51 mg / kg Control group 2 2％ 27.86g / kg 18.13％ 9.18％ 3.99 mg / kg 567.73 mg / kg Example 1 5％ 29.51g / kg 58.22％ 13.85％ 3.57 mg / kg 518.76 mg / kg

[0070] In addition, this embodiment also investigated the removal effect of magnetic layered double oxide microspheres prepared under different raw material mass ratios on heavy metals in water, as shown in Table 2.

[0071] Table 2. Removal rate of heavy metals in water by magnetic layered double oxide microspheres prepared under different raw material mass ratios:

[0072] Mass ratio (zeolite powder : layered double hydroxide : nano iron tetroxide) Cd removal rate Pb removal rate 4∶6∶1 78.10％ 77.53％ 5∶5∶1 62.62％ 65.87％ 6∶4∶1 58.30％ 57.17％ 7∶3∶1 22.47％ 37.17％

[0073] Generally, when the removal rate of heavy metals from water by the microspheres is greater than 50%, they can be used in composting. However, when the proportion of zeolite powder is less than 3 / 11, the microspheres cannot maintain their spherical shape after contact with water and disperse into powder. Therefore, this invention optimizes the mass ratio of raw materials to achieve the best material stability and heavy metal remediation for the microspheres. As shown in Table 2, when the mass ratio of the framework material, layered double hydroxide, and nano-Fe3O4 is 6:4:1 to 4:6:1, the magnetic layered double oxide microspheres prepared achieve a removal rate of heavy metals from water greater than 50%, making them suitable for composting. Furthermore, when the zeolite powder content is too high, the adsorption effect of the microspheres on heavy metals decreases, while when the zeolite powder content is too low, the microspheres become too loose and cannot be formed, turning into powder upon contact with water. Additionally, when the content of layered double hydroxide is too high, the microspheres become loose and cannot be stably formed, while when the content of layered double hydroxide is too low, the adsorption effect of the microspheres on heavy metals is poor.

[0074] In addition, this embodiment also investigated the removal effect of magnetic layered double oxide microspheres prepared under different firing conditions on heavy metals in water, as shown in Table 3.

[0075] Table 3. Removal rate of heavy metals from water by magnetic layered dioxide microspheres prepared under different calcination conditions.

[0076] Temperature (°C) Time (h) Cd removal rate Pb removal rate 250 4 42.76％ 41.85％ 350 4 46.93％ 45.62％ 450 4 58.30％ 57.17％ 550 4 56.32％ 54.48％ 650 4 23.03％ 26.55％ 450 2 40.64％ 42.37％ 450 3 45.72％ 43.95％ 450 5 56.82％ 53.16％ 450 6 39.54％ 37.71％

[0077] As shown in Table 3, optimizing the firing conditions improves the removal efficiency of the microspheres from heavy metals in water. Specifically, firing at 450℃~550℃ for 4h~5h resulted in magnetic layered double oxide microspheres with a heavy metal removal rate greater than 50% in water, making them suitable for composting. However, excessively high firing temperatures or prolonged firing times damage the structure of the layered double oxides and zeolite powder, weakening their adsorption effect on heavy metals. Conversely, excessively low temperatures or short firing times prevent the complete conversion of layered double hydroxides into layered double oxides, leading to reduced heavy metal remediation.

[0078] Meanwhile, in this invention, by optimizing the addition amount of magnetic layered double oxide microspheres to 3%–6% of the total dry weight of the agricultural waste, fruit and vegetable residues, conditioner, and heavy metal-contaminated soil, the magnetic layered double oxide microspheres exhibit the best remediation effect on heavy metals and the highest humic acid generation. Furthermore, if the addition amount of magnetic layered double oxide microspheres is too low, it will lead to a low removal rate of heavy metals and poor passivation effect; while if the addition amount is too high, it will increase economic costs and also lead to excessively high compost porosity, which may activate heavy metals and result in excessively high levels of trace elements such as iron and aluminum in the compost, thereby increasing phytotoxicity.

[0079] The results above show that, compared with existing composting remediation methods, this invention, by creatively utilizing recyclable magnetic layered double oxide microspheres to enhance composting remediation of heavy metal contaminated soil, can achieve the goals of efficient, green, and low-cost soil remediation. It boasts advantages such as simple process, convenient operation, low cost, good remediation effect, high remediation efficiency, and no secondary pollution. It not only solves the problems of insufficient humification and heavy metal reactivation in traditional composting technologies, but also overcomes the challenges of adsorbent loss and secondary pollution through the recyclability of magnetic materials. It features high remediation efficiency (high heavy metal removal rate), low cost (material recycling), and environmental friendliness (zero residue), providing an efficient and feasible technical solution for large-scale heavy metal contaminated soil remediation.

[0080] The above embodiments are merely preferred embodiments of the present invention, and the scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for enhancing composting remediation of heavy metal contaminated soil using recyclable magnetic layered double oxide microspheres, characterized in that, The method involves adding magnetic layered double oxide microspheres to heavy metal-contaminated soil for composting. The magnetic layered double oxide microspheres are prepared by sintering a framework material, layered double hydroxides, and nano-ferric oxide. The mass ratio of the framework material, layered double hydroxides, and nano-ferric oxide is 6:4:1 to 4:6:

1. The sintering is carried out at a temperature of 450℃ to 550℃ for 4 to 5 hours. The particle size of the magnetic layered double oxide microspheres is 0.5 cm to 1.0 cm. The preparation method of the magnetic layered double oxide microspheres includes the following steps: (1) The framework material, layered double hydroxide, nano-iron oxide and organic solvent are mixed, shaped and formed into spheres; the framework material is at least one of zeolite, diatomite and bentonite; the cation in the layered double hydroxide is two of magnesium, aluminum, calcium and iron; the amount of organic solvent is 55% to 65% of the total mass of all raw materials; (2) The spheres are sintered to obtain magnetic layered double oxide microspheres.

2. The method according to claim 1, characterized in that, In step (1), the organic solvent is at least one of methanol, ethanol, formamide, and glycerol.

3. The method according to claim 1 or 2, characterized in that, Adding magnetic layered double oxide microspheres to heavy metal contaminated soil for composting treatment includes the following steps: S1. Mix agricultural waste, fruit and vegetable residues, conditioning agents, heavy metal contaminated soil, and magnetic layered double oxide microspheres to obtain a mixture. S2. Perform aerobic composting on the mixture to complete the remediation of soil contaminated with heavy metals.

4. The method according to claim 3, characterized in that, In step S1, the amount of magnetic layered double oxide microspheres added is 3% to 6% of the total dry weight of the agricultural waste, fruit and vegetable residues, conditioner and heavy metal contaminated soil.

5. The method according to claim 4, characterized in that, In step S1, the mass ratio of agricultural waste, fruit and vegetable residues, conditioner, and heavy metal contaminated soil is 4–6:4–6:2.5–3.5:6–8; the C / N mass ratio in the mixture of agricultural waste, fruit and vegetable residues, conditioner, and heavy metal contaminated soil is controlled to be 25–30, and the moisture content is 55%–65%; the agricultural waste is crop straw; the crop straw is rice straw; the conditioner is wheat bran; and the heavy metal in the heavy metal contaminated soil is at least one of cadmium, lead, copper, and zinc.

6. The method according to claim 5, characterized in that, In step S2, the aerobic composting time is 55 to 65 days; the frequency of turning the pile during the aerobic composting process is controlled as follows: once a day for the first 10 days, once every three days from the 11th to the 22nd day, and once a week thereafter; the aerobic composting process also includes heat preservation treatment of the pile to maintain the temperature of the pile at 50℃ to 70℃.

7. The method according to claim 3, characterized in that, In step S2, after the aerobic composting is completed, the following treatment is also included: recovering magnetic layered double oxide microspheres from the aerobic compost pile, desorbing the recovered magnetic layered double oxide microspheres, and burning the desorbed magnetic layered double oxide microspheres to obtain regenerated magnetic layered double oxide microspheres; the regenerated magnetic layered double oxide microspheres are then used to remediate heavy metal contaminated soil.

8. The method according to claim 7, characterized in that, Magnetic layered double oxide microspheres are recovered from the aerobic compost pile using a magnetic attraction device, which can be a magnetic rod or a magnetic separator. The recovered magnetic layered double oxide microspheres are desorbed using a desorption reagent, which is a mixed solution of sodium carbonate and sodium hydroxide. The concentration of sodium carbonate in the mixed solution is 0.2 mol / L to 0.4 mol / L, and the concentration of sodium hydroxide is 0.1 mol / L to 0.2 mol / L. The desorption time is 6 h to 12 h. The desorbed magnetic layered double oxide microspheres are then calcined at a temperature of 450℃ to 550℃ for 4 h to 5 h.

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