A magnesium-iron-aluminum ternary layered double hydroxide soil passivator, preparation method and application thereof

Abstract

The present invention relates to a magnesium-iron-aluminum ternary layered double hydroxide soil passivator, its preparation method, and application. The preparation method of the soil passivator comprises: drying, grinding, and sieving red mud in sequence to obtain red mud powder; mixing the red mud powder with an acid solution to obtain a first mixed solution; adding a magnesium source to the first mixed solution, and then adding a sodium hydroxide solution to obtain a second mixed solution; adding hexamethylenetetramine or urea to the second mixed solution to carry out a hydrothermal crystallization reaction, and then completing the reaction. The soil passivator of the present invention can have a good passivation effect on Pb, Cd, and Cu in the soil. After the passivator is added, the migration capacity of various heavy metals in the soil is significantly reduced, thereby reducing the toxicity of heavy metals in the soil.

Description

Technical Field

[0001] The present invention belongs to the technical field of solid waste resource utilization and soil heavy metal pollution control, and particularly relates to a magnesium-iron-aluminum ternary layered double hydroxide soil passivator, a preparation method and application thereof. Background Art

[0002] With the acceleration of industrialization and urbanization, an increasing number of pollutants are entering the soil through human activities, causing serious harm to the soil environment. Heavy metal pollution, in particular, has attracted increasing attention due to its poor mobility, long residence time, and non-degradability. Therefore, finding efficient and sustainable solutions to soil heavy metal pollution is urgently needed.

[0003] Current technologies for remediating heavy metal contaminated soil include physical remediation, bioremediation, and chemical remediation. While physical remediation offers good results, it is expensive, labor-intensive, and inconvenient to operate. While bioremediation is economical, it is significantly affected by environmental factors and has a long remediation cycle. Chemical passivation, a type of chemical remediation technology, combines the advantages of both and has attracted considerable attention due to its high efficiency, wide application, and affordability. Currently, commonly used chemical passivation stabilizers are divided into three categories: inorganic stabilizers, organic stabilizers, and inorganic-organic mixed stabilizers, primarily including lime, metal oxides, phosphate rock powder, and peat. However, most of these materials have limited properties and are only effective against a single heavy metal contaminant element in soil. They are therefore ineffective against soil contaminated by multiple heavy metals. Therefore, the development of new multifunctional soil passivators is of great significance for the prevention and control of soil pollution in my country.

[0004] Therefore, based on this, the technical solution of the present invention is proposed. Summary of the Invention

[0005] In order to solve the problems existing in the prior art, the present invention provides a magnesium-iron-aluminum ternary layered double hydroxide soil passivator, a preparation method and application thereof.

[0006] The present invention provides a method for preparing a magnesium-iron-aluminum ternary layered double hydroxide soil passivator, the preparation method comprising the following steps:

[0007] (1) drying, grinding, and sieving red mud in sequence to obtain red mud powder; wherein the red mud comes from a Bayer red mud dump of Shandong Aluminum Group, and its main components include: ferric oxide (mass percentage ≥35%), aluminum oxide (mass percentage ≥20%), and silicon dioxide (mass percentage ≥10%);

[0008] (2) The red mud powder is mixed with the acid solution and stirred to make the iron and aluminum elements in the red mud 3+ 、Al 3+ dissolving in the form of to obtain a first mixed solution;

[0009] (3) adding a magnesium source to the first mixed solution, and then adding a sodium hydroxide solution to adjust the pH to obtain a second mixed solution;

[0010] (4) adding hexamethylenetetramine or urea to the second mixed solution to carry out a hydrothermal crystallization reaction, and obtaining the magnesium-iron-aluminum ternary layered double hydroxide soil passivator after completion.

[0011] Preferably, in step (1), the drying temperature is 105° C. and the drying time is 8 to 24 hours. The red mud is Bayer red mud, which is a waste of electrolytic aluminum.

[0012] Preferably, in step (1), the particle size of the red mud powder is 100-500 mesh. More preferably, the particle size of the red mud powder is 100 mesh.

[0013] Preferably, in step (2), the acid solution is one of hydrochloric acid solution, nitric acid solution or sulfuric acid solution.

[0014] Preferably, in step (2), the weight ratio of the red mud powder to the acid solution is 1:15-25; the mixing temperature is 70-100°C, the mixing time is 1.5-3 hours; the stirring rate is 480-800 r / min; and the concentration of the solute in the acid solution is ≥1.0 mol / L. More preferably, the weight ratio of the red mud powder to the acid solution is 1:20; the mixing temperature is 90°C, the mixing time is 2 hours; the stirring rate is 600 r / min; and the concentration of the solute in the acid solution is 3.0 mol / L.

[0015] Preferably, the magnesium source is one of magnesium chloride hexahydrate, magnesium nitrate or magnesium sulfate.

[0016] In step (3), the amount of magnesium source added should be based on the amount of trivalent iron and aluminum in the red mud, so that the amount of magnesium source is N Mg , the amount of substance of trivalent iron is N Fe , the amount of aluminum is N Al , then N should be controlled Mg / (N Fe +N Al ) is 3 to 5:1. More preferably, N Mg / (N Fe +N Al ) is 4:1.

[0017] Preferably, in step (3), the concentration of the sodium hydroxide solution is ≥3 mol / L. More preferably, the concentration of the sodium hydroxide solution is 5 mol / L. The pH of the second mixed solution is 10-11.

[0018] In step (4), the amount of hexamethylenetetramine or urea added should be based on the amount of trivalent iron and aluminum in the red mud, so that the amount of hexamethylenetetramine or urea is N N , the amount of substance of trivalent iron is N Fe , the amount of aluminum is N Al , then N should be controlled N / (N Fe +N Al ) is 2 to 4:1. More preferably, N N / (N Fe +N Al ) is 3:1.

[0019] Preferably, in step (4), the temperature of the hydrothermal crystallization reaction is 100 to 140° C., and the time of the hydrothermal crystallization reaction is 12 to 24 hours.

[0020] Based on the same technical concept, another embodiment of the present invention is to provide a magnesium-iron-aluminum ternary layered double hydroxide soil passivator obtained by the above preparation method.

[0021] Based on the same technical concept, another solution of the present invention is to provide an application of the soil passivator, wherein the application method is: uniformly mixing the soil passivator with contaminated soil and water to obtain a mixture, and then curing the mixture; wherein the solid content in the mixture is ≥80%, the soil passivator is 0.5-2.5wt.% of the mixture, and the curing time is ≥1 month.

[0022] The beneficial effects of the present invention are:

[0023] 1. The preparation method of the present invention uses bulk solid waste red mud as raw material and extracts Fe from the red mud through a simple hydrochloric acid leaching process. 3+ 、Al 3+ Valuable elements, followed by magnesium chloride as a supplementary magnesium source, and finally a magnesium-iron-aluminum ternary layered double hydroxide soil passivator was successfully prepared through a one-step hydrothermal method, providing a new high-efficiency and low-cost passivator while solving the problem of large-scale accumulation of red mud, achieving the ideal goal of "treating waste with waste" in environmental governance.

[0024] 2. The soil passivator of the present invention contains some hydroxyl groups, which can interact with heavy metals in the soil to form coordination bonds, providing corresponding reaction and adsorption sites for heavy metal adsorption, so as to achieve the purpose of stabilizing heavy metals in the soil.

[0025] 3. The soil passivator of the present invention can have a good passivation effect on Pb, Cd and Cu in the soil. After the passivator is added, the migration capacity of each heavy metal in the soil is significantly reduced, thereby reducing the toxicity of heavy metals in the soil. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 This is the XRD pattern of the magnesium-iron-aluminum ternary layered double hydroxide soil passivator obtained in Example 1.

[0028] Figure 2 This is the FTIR spectrum of the magnesium-iron-aluminum ternary layered double hydroxide soil passivator obtained in Example 1.

[0029] Figure 3 This is the XRD pattern of the soil passivator material obtained under different pH conditions.

[0030] Figure 4 XRD patterns of soil passivator materials obtained under different magnesium source addition conditions (i.e., the molar ratio of divalent metal elements to trivalent metal elements).

[0031] Figure 5 These are the XRD patterns of soil passivator materials obtained under different hydrothermal crystallization reaction temperature conditions.

[0032] Figure 6 This is the XRD pattern of the soil passivator material obtained under different hydrothermal crystallization reaction time conditions. DETAILED DESCRIPTION

[0033] To make the objectives, technical solutions, and advantages of the present invention more apparent, the technical solutions of the present invention will be described in detail below. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other implementations obtained by those of ordinary skill in the art without inventive effort are within the scope of protection of the present invention.

[0034] Example 1

[0035] This embodiment provides a method for preparing a magnesium-iron-aluminum ternary layered double hydroxide soil passivator, comprising the following steps:

[0036] (1) The electrolytic aluminum waste Bayer red mud was dried in an oven at 105°C for 12 h, and then ground through a 100-mesh sieve to obtain red mud powder;

[0037] (2) Weighing 5 g of the red mud powder and transferring it to a 250 mL beaker, slowly adding 100 mL of a 3 mol / L hydrochloric acid solution thereto, and then acid-leaching for 2 h at a temperature of 90 ° C. and a stirring speed of 600 r / min. After completion, filtering and collecting the filtrate to obtain a first mixed solution;

[0038] (3) Add magnesium chloride hexahydrate to the first mixed solution so that Mg 2+ Elements and Fe 3+ 、Al 3+ The ratio of the amount of substance of the sum of the elements is 4, that is, the amount of substance Mg 2+ / (Fe 3+ 、Al 3+ )=4, then adding 5 mol / L sodium hydroxide solution to adjust the pH of the system to 10 to obtain a second mixed solution;

[0039] (4) Hexamethylenetetramine was added to the second mixed solution at room temperature so that the hexamethylenetetramine and Fe 3+ 、Al 3+ The molar ratio of the sum of the elements is 3. After sufficient stirring, it is transferred to a stainless steel reactor lined with polytetrafluoroethylene and crystallized at 100°C for 12 hours. After the hydrothermal reaction is completed, it is filtered and repeatedly washed with deionized water 6 times. Then, it is dried at 105°C to obtain the magnesium-iron-aluminum ternary layered double hydroxide.

[0040] Example 2

[0041] This embodiment provides a method for preparing a magnesium-iron-aluminum ternary layered double hydroxide soil passivator, comprising the following steps:

[0042] (1) The electrolytic aluminum waste Bayer red mud was dried in an oven at 105°C for 8 h, and then ground through a 300-mesh sieve to obtain red mud powder;

[0043] (2) Weigh 5 g of the red mud powder and transfer it to a 250 mL beaker, slowly add 75 mL of a 3 mol / L hydrochloric acid solution thereto, and then acid-leach for 1.5 h at a temperature of 70° C. and a stirring speed of 480 r / min. After completion, filter and collect the filtrate to obtain a first mixed solution;

[0044] (3) Add magnesium chloride hexahydrate to the first mixed solution so that Mg 2+ Elements and Fe 3+ 、Al 3+ The ratio of the amount of substance of the sum of the elements is 3, that is, the amount of substance Mg 2+ / (Fe 3+ 、Al 3+ )=3, then adding 4 mol / L sodium hydroxide solution to adjust the pH of the system to 10 to obtain a second mixed solution;

[0045] (4) Hexamethylenetetramine was added to the second mixed solution at room temperature so that the hexamethylenetetramine and Fe 3+ 、Al 3+ The molar ratio of the sum of the elements is 2. After sufficient stirring, it is transferred to a stainless steel reactor lined with polytetrafluoroethylene and crystallized at 140°C for 18 hours. After the hydrothermal reaction is completed, it is filtered and repeatedly washed with deionized water 6 times. Then, it is dried at 105°C to obtain the magnesium-iron-aluminum ternary layered double hydroxide.

[0046] Example 3

[0047] This embodiment provides a method for preparing a magnesium-iron-aluminum ternary layered double hydroxide soil passivator, comprising the following steps:

[0048] (1) The electrolytic aluminum waste Bayer red mud was dried in an oven at 105°C for 24 h, and then ground through a 500-mesh sieve to obtain red mud powder;

[0049] (2) Weighing 5 g of the red mud powder and transferring it to a 250 mL beaker, slowly adding 75 mL of a 3 mol / L hydrochloric acid solution thereto, and then acid leaching for 3 h at a temperature of 100 ° C. and a stirring speed of 800 r / min. After completion, filtering and collecting the filtrate to obtain a first mixed solution;

[0050] (3) Add magnesium chloride hexahydrate to the first mixed solution so that Mg 2+ Elements and Fe 3+ 、Al 3+ The ratio of the amount of substance of the sum of the elements is 6, that is, the amount of substance Mg 2+ / (Fe 3+ 、Al 3+ )=5, then adding 5 mol / L sodium hydroxide solution to adjust the pH of the system to 11 to obtain a second mixed solution;

[0051] (4) Hexamethylenetetramine was added to the second mixed solution at room temperature so that the hexamethylenetetramine and Fe 3+ 、Al 3+ The molar ratio of the sum of the elements is 4. After sufficient stirring, it is transferred to a stainless steel reactor lined with polytetrafluoroethylene and crystallized at 120°C for 24 hours. After the hydrothermal reaction is completed, it is filtered and repeatedly washed with deionized water 6 times. Then, it is dried at 105°C to obtain the magnesium-iron-aluminum ternary layered double hydroxide.

[0052] Test example

[0053] The magnesium-iron-aluminum ternary layered double hydroxide soil passivator obtained in Example 1 was subjected to X-ray diffraction test. Figure 1The obtained XRD pattern shows that the main diffraction peaks appear at 2θ = 11.25, 22.17, 34.17, 58.68, and 60.01, corresponding to the (003), (006), (009), (110), and (113) crystal planes, respectively. These peak positions are consistent with the characteristic peaks of layered double hydroxide.

[0054] The product was tested by infrared. Figure 2 The FTIR spectrum obtained is shown below. -1 ~3000cm -1 The broad transmission peak at 1540.87 cm is due to the stretching vibration of hydrogen bonded hydroxyl (-OH). -1 The appearance of the transmission peak at 1374.54 cm is related to the stretching of HOH, which may be due to the bending vibration of hydroxyl (-OH) or the deformation vibration of H2O caused by incomplete dehydration of the sample. -1 The peak of CO3 in the LDH interlayer 2- This is due to the absorption of atmospheric CO2. -1 and 413.18cm -1 The transmission peak at can be attributed to the vibration of OMO or MOM (M is a metal element such as Fe, Al, Mg, etc.) in the LDHs lattice.

[0055] In step (3), the effects of different pH values ​​on the material were tested. Figure 3 shown. Figure 3 The XRD patterns of the soil passivator materials obtained under different pH conditions are shown. The figure shows that at pH = 9 and 12, the characteristic peaks representing layered double hydroxides do not appear, indicating that the material could not be successfully prepared. Layered double hydroxide soil passivators were successfully synthesized at all other pH values. Furthermore, at pH = 10, the diffraction peaks of the layered double hydroxides are stronger than those at pH = 11, making pH = 10 the optimal preparation condition.

[0056] In step (3), different amounts of magnesium source added will have different effects on the resulting soil passivator material. Figure 4 It is known that when the divalent metal element (Mg 2+ ) and trivalent metal elements (Fe 3+ With Al 3+ When the molar ratio of divalent metal element to trivalent metal element was 1:1 and 2:1, the characteristic peak of the layered double hydroxide soil passivator did not appear, indicating that the material could not be successfully prepared. However, the layered double hydroxide soil passivator was successfully synthesized under the other molar ratio conditions. Furthermore, a molar ratio of 4:1 showed a stronger diffraction peak intensity, so a molar ratio of 4:1 of divalent metal element to trivalent metal element was selected as the optimal preparation condition.

[0057] In step (4), different hydrothermal crystallization temperatures will have different effects on the obtained soil passivator material. Figure 5 It can be seen that when the reaction temperature is 160°C, the characteristic peaks representing the layered double hydroxide soil passivator do not appear in the spectrum, indicating that the material cannot be successfully prepared under this temperature condition. Layered double hydroxide soil passivator was successfully synthesized at temperatures of 100°C, 120°C, and 140°C. Because the diffraction peaks at 100°C are more intense and the influence of miscellaneous peaks is completely eliminated, 100°C is the optimal preparation condition.

[0058] In step (4), different hydrothermal crystallization reaction times will have different effects on the obtained soil passivator material. Figure 6 It can be seen that characteristic peaks representing the layered double hydroxide soil passivator appear at all reaction times. Since increasing the reaction time does not enhance the diffraction peaks, and the intensity of the miscellaneous peaks on the XRD spectrum of the layered double hydroxide soil passivator is relatively weak when the reaction time is 12 hours, 12 hours was selected as the hydrothermal crystallization time for the material.

[0059] Comparative Example

[0060] (1) Blank comparison

[0061] The soil passivator described in Example 1 was used for testing, and the specific scheme was as follows:

[0062] Appropriate amounts of Cu(NO₃)₂, Pb(NO₃)₂, and Cd(NO₃)₂·4H₂O were weighed onto a 10kg test soil using an analytical balance. Ultrapure water was added to cover the soil, stirring thoroughly to mix the water and soil, and then the soil was allowed to air dry at room temperature. After air drying and a one-month aging period, the soil was passed through a 20-mesh sieve. 500g portions of soil were weighed onto a balance and placed into small soil basins. Magnesium-iron-aluminum layered double hydroxide passivation agents were added at dry weight ratios of 0.5%, 1%, 1.5%, 2%, and 2.5%, respectively. Ultrapure water was then added to cover the soil and stirring thoroughly. The soil was then air-dried at room temperature, with ultrapure water added regularly to maintain a moisture content of approximately 30%. Three parallel experiments were conducted, each lasting one month. After the passivation period, the available heavy metal contents of Pb, Cd, and Cu in the soil were determined, and speciation analysis of the passivated soil was performed. The results are shown in Tables 1-6.

[0063] Table 1 Available content of heavy metal lead (Pb) after soil passivation

[0064] Passivator addition amount / % 0 0.5 1.0 1.5 2.0 2.5 Effective content / mg / kg 368.1 314.5 307.1 290.5 276 258.9

[0065] Table 2 Heavy metal form of lead (Pb) after soil passivation

[0066] Passivator addition amount / % 0 0.5 1.0 1.5 2.0 2.5 Exchangeable state% 198.5 178.14 218.9 241.02 241.02 214.44 Reducible state% 563.6 540.8 470.8 432.4 496.6 495 Oxidizable state% 8.773 10.81 11.435 14.705 14.373 13.963 Residue % 150.795 192.33 216.365 212.708 144.258 171.181

[0067] Table 3 Available content of heavy metal cadmium (Cd) in soil after passivation

[0068] Passivator addition amount / % 0 0.5 1.0 1.5 2.0 2.5 Effective content / mg / kg 4.052 2.663 2.382 2.017 1.755 1.455

[0069] Table 4 Forms of heavy metal cadmium (Cd) after soil passivation

[0070] Passivator addition amount / % 0 0.5 1.0 1.5 2.0 2.5 Exchangeable state% 4.070 3.813 3.842 3.892 3.818 3.598 Reducible state% 3.527 3.277 2.166 2.040 2.164 2.204 Oxidizable state% 1.485 1.268 1.400 1.105 1.040 1.040 Residue % 2.286 1.747 2.679 2.981 2.895 2.912

[0071] Table 5 Available content of heavy metal cadmium (Cu) after soil passivation

[0072] Passivator addition amount / % 0 0.5 1.0 1.5 2.0 2.5 Effective content / mg / kg 100.12 61.93 51.92 43.55 38.48 34.06

[0073] Table 6 Forms of heavy metal cadmium (Cu) after soil passivation

[0074] Passivator addition amount / % 0 0.5 1.0 1.5 2.0 2.5 Exchangeable state% 73.86 63.58 66.02 67.94 68.4 59.24 Reducible state% 62.48 53.72 42.4 35.92 49.74 52.42 Oxidizable state% 31.0825 35.74 37.335 38 32.6375 35.4425 Residue % 41.0775 47.0017 51.912 53.098 41.2642 38.2725

[0075] The above test results show that magnesium-iron-aluminum ternary layered double hydroxide can reduce the effective content of Cu, Pb, and Cd in soil, and has a certain passivation effect on Cu, Pb, and Cd in contaminated soil. The data show that the effective content of Cu, Pb, and Cd in soil without passivation agent added was 100.12 mg / kg, 368.10 mg / kg, and 4.05 mg / kg, respectively. After adding the passivation agent, the effective content of Cu, Pb, and Cd in the soil decreased significantly, and the degree of decrease was positively correlated with the amount of passivation agent added. As the addition amount increased, the maximum decrease in Cu, Pb, and Cd was: 65.98%, 29.67%, and 64.10%.

[0076] In addition, the results of changes in the occurrence forms of the three heavy metals Cu, Pb, and Cd also show that the addition of the magnesium-iron-aluminum ternary layered double hydroxide passivator reduces the content of the exchangeable state of the three elements and promotes the transformation of the exchangeable state to the other three states (reducible state, oxidizable state, and residual state), thereby effectively enhancing the stability of the three heavy metals in the soil and reducing the migration capacity of Cu, Pb, and Cd.

[0077] (2) Comparison

[0078] (1) The soil passivator described in Example 1 was compared with the zeolite passivator developed by the applicant in the early stage. The results showed that the passivator of the present invention had improved performance in the passivation of soil cadmium (Cd), specifically:

[0079] When the magnesium-iron-aluminum ternary layered double hydroxide soil passivator was added at 0.5%, 1%, 1.5%, and 2%, respectively, the available Cd content in the soil decreased by 34.28%, 41.21%, 50.22%, and 56.69%, respectively, compared to the blank. For the zeolite soil passivator, when the zeolite soil passivator was added at 0.5%, 1%, 1.5%, and 2%, respectively, the available Cd content in the soil decreased by 21.15%, 25.64%, 37.18%, and 39.10%, respectively, compared to the blank. These Cd availability reduction data indicate that the magnesium-iron-aluminum ternary layered double hydroxide soil passivator prepared in this study has a superior passivation effect on Cd contamination in soil.

[0080] (2) The soil passivator described in Example 1 was compared with the nano-biochar (rapeseed straw biochar) of Chengdu Xinchaoyang Crop Science Co., Ltd., the heavy capture agent TMT (trimercapto-s-triazine trisodium salt) and the mixed passivator (containing hydrogen sulfide and sodium sulfide). In order to facilitate the comparison of the passivation effects of different passivators, the data with an addition amount of 1% and a passivation time of 30 days were mainly used.

[0081] When the material addition rate was 1%, the three materials (nano-biochar, heavy metal trapping agent TMT, and mixed passivation agent) reduced the available Cd content by 8.27% to 41.2%. When the magnesium-iron-aluminum ternary layered double hydroxide soil passivation agent was added at a 1% level and passivated for 30 days, the available Cd content in the soil decreased by 41.241% compared to the blank. The magnesium-iron-aluminum ternary layered double hydroxide soil passivation agent was significantly more effective in passivating Cd than the previous three soil passivation agents. (Data from "Effects of Passivation Agent Types on the Remediation of Cadmium and Lead Contaminated Soils" - Soil and Crops)

[0082] (3) The soil passivator described in Example 1 was compared with sepiolite, bentonite, calcium magnesium phosphate fertilizer, and phosphate rock soil passivation materials. The results were:

[0083] The results of the 30-day soil culture experiment showed that the effective Cd content in the sepiolite treatment group decreased by up to 50% compared with the control group; the effective Cd content in the calcium magnesium phosphate treatment group decreased by up to 55.56% compared with the control group; the effective Cd content in the bentonite treatment group decreased by up to 16.67% compared with the control group; and the effective Cd content in the phosphate rock treatment group decreased by up to 16.67% compared with the control group. The soil passivator of the present invention can reduce the effective Cd content in the soil by up to 64.09%. Therefore, compared with the above four soil passivators, the soil passivator of the present invention has a significantly better passivation effect on Cd than the above four soil passivators. (Data from "Screening Study on Passivation Materials for Remediation of Cadmium Pollution in Acidic Agricultural Soils" - Soil Bulletin)

[0084] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for preparing a magnesium-iron-aluminum ternary layered double hydroxide soil passivator, characterized in that: The preparation method comprises the following steps: (1) drying, grinding and sieving the red mud in sequence to obtain red mud powder; (2) mixing and stirring the red mud powder and the acid solution to obtain a first mixed solution; (3) adding a magnesium source to the first mixed solution, and then adding a sodium hydroxide solution to obtain a second mixed solution; the concentration of the sodium hydroxide solution is ≥3 mol / L; and the pH of the second mixed solution is 10-11; (4) Hexamethylenetetramine or urea is added to the second mixed solution to carry out a hydrothermal crystallization reaction, and after completion, the magnesium-iron-aluminum ternary layered double hydroxide soil passivator is obtained.

2. The method for preparing the magnesium-iron-aluminum ternary layered double hydroxide soil passivator according to claim 1, characterized in that: In step (1), the drying temperature is 105° C. and the drying time is 8 to 24 hours.

3. The method for preparing the magnesium-iron-aluminum ternary layered double hydroxide soil passivator according to claim 1, characterized in that: In step (1), the particle size of the red mud powder is 100-500 mesh.

4. The method for preparing the magnesium-iron-aluminum ternary layered double hydroxide soil passivator according to claim 1, characterized in that: In step (2), the acid solution is one of hydrochloric acid solution, nitric acid solution or sulfuric acid solution.

5. The method for preparing the magnesium-iron-aluminum ternary layered double hydroxide soil passivator according to claim 1, characterized in that: In step (2), the weight ratio of the red mud powder to the acid solution is 1:15~25; the mixing temperature is 70~100°C, and the mixing time is 1.5~3h; the stirring rate is 480~800r / min; and the concentration of the solute in the acid solution is ≥1.0mol / L.

6. The method for preparing the magnesium-iron-aluminum ternary layered double hydroxide soil passivator according to claim 1, characterized in that: In step (3), the magnesium source is one of magnesium chloride hexahydrate, magnesium nitrate or magnesium sulfate.

7. The method for preparing the magnesium-iron-aluminum ternary layered double hydroxide soil passivator according to claim 1, characterized in that: In step (4), the temperature of the hydrothermal crystallization reaction is 100-140° C., and the time of the hydrothermal crystallization reaction is 12-24 hours.

8. The magnesium-iron-aluminum ternary layered double hydroxide soil passivator obtained by the preparation method according to any one of claims 1 to 7.

9. The use of the soil passivator according to claim 8, characterized in that: The application method comprises: uniformly mixing the soil passivator, contaminated soil, and water to obtain a mixture, and then curing the mixture; wherein the solid content of the mixture is ≥80%, the soil passivator accounts for 0.5-2.5 wt.% of the mixture, and the curing time is ≥1 month.

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