Calcium-doped magnesium hydroxide with sheet structure as well as preparation method and application of calcium-doped magnesium hydroxide

Abstract

The invention provides calcium-doped magnesium hydroxide with a sheet structure as well as a preparation method and application of the calcium-doped magnesium hydroxide, and belongs to the technical field of inorganic material synthesis. According to the method, ammonium ions are doped in the calcium-magnesium solution and can serve as a pore-enlarging agent in the roasting stage to reinforce formation of calcium-magnesium composite oxide nanoparticles, the calcium-magnesium composite oxide obtained after roasting is small in particle size and high in activity, ultrathin nanosheets can be obtained through phase reconstruction in the next step, and the cadmium and arsenic fixing capacity of the ultrathin nanosheets is enhanced. The calcium-magnesium composite oxide is used as a transition intermediate, the calcium-magnesium composite oxide is subjected to topological transformation to form calcium-doped magnesium hydroxide with a single phase in the hydration process of saccharide molecules and water, the surface of the obtained calcium-doped magnesium hydroxide has rich hydroxyl groups, and the surface of the calcium-doped magnesium hydroxide has a high molecular weight. The soil conditioner can effectively improve the pH value of acid soil and resist acid erosion, realizes efficient mineralization of cadmium and arsenic pollution, inhibits enrichment of cadmium and arsenic pollution in crops, provides calcium and magnesium elements required by crop growth, and improves the yield.

Description

Technical Field

[0001] This invention relates to the field of inorganic material synthesis technology, specifically to a calcium-doped magnesium hydroxide with a thin-film structure, its preparation method, and its applications. Background Technology

[0002] Heavy metal pollution of arable land in my country is becoming increasingly prominent, posing a significant ecological threat to high-quality agricultural development. Surveys show that the main pollutants in Chinese soil include cadmium (Cd), mercury (Pb), and arsenic (As).

[0003] The serious issue is the intensified acidification trend in southern acidic soils, which activates heavy metals that were originally in a precipitated or bound state into a free state, significantly increasing their bioavailability. Data shows that for every unit decrease in soil pH, Cd... 2+ Pb 2 + The effectiveness of isocations can be increased by 1 to 2 times. This chain effect of "acidification-activation-enrichment" makes pollution control more complex and long-term. Traditional passivation remediation methods, such as limestone and clay minerals, can reduce the activity of heavy metals in the short term, but they are easily eroded by acid rain and fail, have poor stability, and pose a risk of secondary release.

[0004] Furthermore, magnesium deficiency is a widespread problem in my country's arable land. Survey data shows that over 63% of arable land nationwide suffers from varying degrees of magnesium deficiency, with 45.3% of the soil having magnesium content below the severe deficiency threshold of 60 mg / kg. This is particularly pronounced in acidic soil regions of southern China. In Hunan, Jiangxi, Fujian, Guangdong, and Guangxi Zhuang Autonomous Region, the area with magnesium deficiency exceeds 70%, and in some areas even reaches over 80%. Magnesium is one of the essential medium-level nutrients for plants, a core component of chlorophyll, and directly participates in photosynthesis, energy metabolism, and carbon and nitrogen assimilation. Magnesium deficiency leads to chlorosis and yellowing of leaves, premature aging of plants, reduced yield, and weakened crop resistance to stress and disease.

[0005] Therefore, developing remediation materials that simultaneously provide heavy metal remediation, magnesium and calcium supply, and have good long-lasting effects has become crucial for ensuring the ecological security of arable land. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a calcium-doped magnesium hydroxide with a thin-film structure, its preparation method, and its application. The calcium-doped magnesium hydroxide with a thin-film structure provided by this invention can effectively treat cadmium and arsenic pollution in acidic soils for a long time, and has a good effect on supplying magnesium and calcium to the soil.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing calcium-doped magnesium hydroxide with a sheet-like structure, comprising the following steps: Soluble calcium salts and soluble magnesium salts are mixed with water to obtain a calcium-magnesium solution; A soluble ammonium salt is mixed with water to obtain an ammonium salt solution; The calcium-magnesium solution and the ammonium salt solution were mixed at high speed and then dried and calcined sequentially to obtain a transition intermediate. The transition intermediate was mixed with a sugar solution to undergo a topological transformation reaction, yielding calcium-doped magnesium hydroxide with a thin-film structure.

[0008] Preferably, the soluble calcium salt includes calcium chloride and / or calcium nitrate; The soluble magnesium salt includes one or more of magnesium chloride, magnesium acetate, and magnesium nitrate; The soluble ammonium salt includes one or more of ammonium carbonate, ammonium bicarbonate, and ammonium oxalate.

[0009] Preferably, the molar ratio of calcium ions to magnesium ions in the calcium-magnesium solution is 1:3~10; The concentration of magnesium ions in the calcium-magnesium solution is 0.2~0.6 mol / L.

[0010] Preferably, the amount of ammonium ions in the ammonium salt solution is 2 to 5 times the sum of the amounts of calcium and magnesium ions. The concentration of ammonium ions in the ammonium salt solution is 0.4~4 mol / L.

[0011] Preferably, the high-speed mixing rate is ≥5000 r / min, and the time is 0.3~3 min; The drying process is a vacuum spiral drying.

[0012] Preferably, the roasting includes a first stage roasting and a second stage roasting; The temperature of the first stage of roasting is 350~450℃, the holding time is 2~4h, and the heating rate to the first stage roasting temperature is 5~10℃ / min. The second stage of roasting is at a temperature of 650~750℃, the holding time is 2~4h, and the heating rate to the second stage roasting temperature is 1~3℃ / min.

[0013] Preferably, the sugar raw materials in the sugar solution include one or more of glucose, fructose and mannose; The mass fraction of sugar raw materials in the sugar solution is ≤30%; The mass ratio of the transition intermediate to the sugar raw material is 1:5~10.

[0014] Preferably, the temperature of the topological transformation reaction is 85~125℃ and the time is 2~6h.

[0015] This invention provides calcium-doped magnesium hydroxide with a thin-film structure prepared by the above preparation method.

[0016] This invention provides the application of the above-mentioned calcium-doped magnesium hydroxide with a sheet-like structure in the remediation of acidic heavy metal contaminated soil.

[0017] This invention provides a method for preparing calcium-doped magnesium hydroxide with a sheet-like structure, comprising the following steps: mixing soluble calcium salt, soluble magnesium salt, and water to obtain a calcium-magnesium solution; mixing a soluble ammonium salt with water to obtain an ammonium salt solution; mixing the calcium-magnesium solution and the ammonium salt solution at high speed, and sequentially drying and calcining to obtain a transition intermediate; mixing the transition intermediate with a sugar solution to undergo a topological transformation reaction to obtain calcium-doped magnesium hydroxide with a sheet-like structure. This invention dops the calcium-magnesium solution with ammonium ions, which act as a pore-expanding agent during the calcination stage, enhancing the formation of calcium-magnesium composite oxide nanoparticles. The resulting calcium-magnesium composite oxide has a small particle size and high activity, which is beneficial for the subsequent phase reconstruction to obtain ultrathin nanosheets, thus enhancing its ability to immobilize cadmium and arsenic.

[0018] In the synthesis of calcium-magnesium complex bimetallic hydroxides, due to the coordination number of calcium (usually 7 or 8 coordination) and ionic radius (0.99 nm), while the coordination number of magnesium is usually 6 coordination and the ionic radius is 0.65 nm, it is difficult to obtain a single phase of calcium-doped magnesium hydroxide by conventional methods. Usually, it is a mixed phase of calcium hydroxide and magnesium hydroxide. This invention utilizes calcium-magnesium composite oxides as transition intermediates. During the hydration process of sugar molecules (such as glucose) and water, calcium-magnesium oxides undergo a topological transformation to form calcium-doped magnesium hydroxide with a single phase. Simultaneously, in solutions containing sugars, the strong alkalinity of calcium-magnesium oxides, acting as solid base catalysts, enables the isomerization of sugars, such as the isomerization of glucose to fructose. The resulting fructose molecules induce the formation of an ultrathin structure in the calcium-magnesium composite oxides during the topological transformation. The resulting calcium-doped magnesium hydroxide has abundant hydroxyl groups on its surface, effectively increasing the pH of acidic soils and resisting acid erosion, exhibiting excellent stability. More importantly, this material can achieve efficient mineralization of cadmium and arsenic pollution, inhibiting their accumulation in crops, while simultaneously providing the calcium and magnesium elements needed for crop growth, thus increasing yield.

[0019] This invention provides the application of the aforementioned thin-film structured calcium-doped magnesium hydroxide in the remediation of acidic heavy metal contaminated soil. Compared with strongly alkaline calcium-magnesium composite oxides, the thin-film structured calcium-doped magnesium hydroxide of this invention releases less heat in solution and is less alkaline, thus having a weaker impact on the soil microenvironment. In the remediation of acidic, calcium- and magnesium-deficient arsenic- and cadmium-contaminated farmland, calcium in the calcium-magnesium composite hydroxide, due to its higher solubility, preferentially releases from the composite hydroxide according to the principle of dissolution-precipitation equilibrium, forming metal vacancies. Free cadmium ions in the soil have the same coordination number and almost equal ionic radius (1.0 nm) as calcium. Therefore, cadmium ions occupy the original calcium vacancies, are anchored in the crystal lattice, and are difficult for plants to absorb. Simultaneously, the release of calcium enhances the activity of magnesium, thus significantly enhancing the adsorption of arsenic, achieving co-fixation of cadmium and arsenic, while the dissolution of some magnesium ions serves to supply magnesium. Therefore, the calcium-doped magnesium hydroxide with a thin sheet structure provided by the present invention can effectively treat cadmium and arsenic pollution in acidic soils for a long time, and has a good effect on supplying magnesium and calcium to the soil. Attached Figure Description

[0020] Figure 1 The image shows the XRD pattern of calcium-doped magnesium hydroxide obtained in Example 1. Figure 2 The image shows the Ca2P XPS diagram of calcium-doped magnesium hydroxide obtained in Example 1. Figure 3 The image shows the Mg2P XPS diagram of calcium-doped magnesium hydroxide obtained in Example 1. Figure 4 The elemental scan of the calcium-doped magnesium hydroxide obtained in Example 1; Figure 5 This is a transmission electron microscope (TEM) image of the calcium-doped magnesium hydroxide obtained in Example 1. Figure 6 The image shows the XRD pattern of calcium-doped magnesium hydroxide obtained in Example 2. Figure 7 This is a scanning electron microscope image of the calcium-doped magnesium hydroxide obtained in Example 2; Figure 8 The XRD pattern of the material obtained in Comparative Example 1; Figure 9 SEM images of the materials obtained in Comparative Example 2; Figure 10 SEM images of the materials obtained in Comparative Example 3; Figure 11 The image shows the XRD pattern of the material obtained in Comparative Example 4. Detailed Implementation

[0021] This invention provides a method for preparing calcium-doped magnesium hydroxide with a sheet-like structure, comprising the following steps: Soluble calcium salts and soluble magnesium salts are mixed with water to obtain a calcium-magnesium solution; A soluble ammonium salt is mixed with water to obtain an ammonium salt solution; The calcium-magnesium solution and the ammonium salt solution were mixed at high speed and then dried and calcined sequentially to obtain a transition intermediate. The transition intermediate was mixed with a sugar solution to undergo a topological transformation reaction, yielding calcium-doped magnesium hydroxide with a thin-film structure.

[0022] Unless otherwise specified, all raw materials used in this invention are commercially available.

[0023] This invention involves mixing soluble calcium salts and soluble magnesium salts with water to obtain a calcium-magnesium solution. In this invention, the soluble calcium salts preferably include calcium chloride and / or calcium nitrate, and the soluble magnesium salts include one or more of magnesium chloride, magnesium acetate, and magnesium nitrate. This invention does not impose any special requirements on the mixing method; any mixing method well-known to those skilled in the art can be used, such as stirring. In this invention, the molar ratio of calcium ions to magnesium ions in the calcium-magnesium solution is preferably 1:3 to 10, more preferably 1:5 to 8; the concentration of magnesium ions in the calcium-magnesium solution is preferably 0.2 to 0.6 mol / L, more preferably 0.3 to 0.5 mol / L.

[0024] This invention involves mixing a soluble ammonium salt with water to obtain an ammonium salt solution. In this invention, the soluble ammonium salt preferably includes one or more of ammonium carbonate, ammonium bicarbonate, and ammonium oxalate. This invention does not impose special requirements on the mixing method; any mixing method well-known to those skilled in the art can be used, such as stirring. In this invention, the amount of ammonium ions in the ammonium salt solution is preferably 2 to 5 times the sum of the amounts of calcium and magnesium ions; the concentration of ammonium ions in the ammonium salt solution is preferably 0.4 to 4 mol / L, more preferably 0.9 to 3 mol / L, and even more preferably 1 to 2 mol / L.

[0025] This invention involves high-speed mixing of a calcium-magnesium solution and an ammonium salt solution, followed by sequential drying and calcination to obtain a transition intermediate. Preferably, the high-speed mixing is achieved by simultaneously adding the calcium-magnesium solution and the ammonium salt solution to a high-speed rotating reactor. The mixing rate is preferably ≥5000 r / min, more preferably 5000~8000 r / min, and the mixing time is preferably 0.3~3 min, more preferably 1~2 min. This invention, through high-speed mixing, enhances centrifugal shear force and promotes rapid reaction of the reactants.

[0026] In this invention, the drying process is preferably vacuum spiral drying. This invention employs vacuum spiral drying, which increases the drying rate while allowing the obtained water to be reused. Furthermore, the ammonium salts, a reaction byproduct in the aqueous phase, do not require treatment, thus reducing the pressure on environmental wastewater treatment.

[0027] In this invention, the roasting is preferably carried out in a tube furnace, and the roasting atmosphere is preferably air. In this invention, the roasting includes a first-stage roasting and a second-stage roasting; the temperature of the first-stage roasting is preferably 350~450℃, more preferably 380~420℃, and even more preferably 400℃; the holding time is preferably 2~4h, more preferably 3h; the heating rate to the first-stage roasting temperature is preferably 5~10℃ / min, more preferably 6~8℃ / min; in this invention, the temperature of the second-stage roasting is preferably 650~750℃, more preferably 680~720℃, and even more preferably 700℃; the holding time is preferably 2~4h, more preferably 3h, and the heating rate to the second-stage roasting temperature is preferably 1~3℃ / min, more preferably 2℃ / min. This invention controls the rapid heating rate during the first-stage calcination process, primarily to remove surface water and water of crystallization; and controls the slower heating rate in the second stage, allowing the phase transformation to occur more completely while preserving the original morphology. After calcination, the invention preferably allows natural cooling to room temperature. In this invention, the main component of the transition intermediate is a calcium-magnesium complex bimetallic hydroxide.

[0028] This invention involves mixing the transition intermediate with a sugar solution to undergo a topological transformation reaction, yielding calcium-doped magnesium hydroxide with a lamellar structure. In this invention, the sugar raw material in the sugar solution preferably includes one or more of glucose, fructose, and mannose, more preferably glucose. In this invention, the mass fraction of the sugar raw material in the sugar solution is preferably ≤30%, more preferably 10-30%, and even more preferably 20%. In this invention, the mass ratio of the transition intermediate to the sugar raw material is preferably 1:5-10, more preferably 1:6-8. By controlling the mass ratio of the transition intermediate to the sugar raw material, this invention enables the intermediate to better transform into a complex hydroxide.

[0029] This invention does not impose any special requirements on the mixing method; any mixing method well-known to those skilled in the art, such as stirring, is acceptable. In this invention, the temperature of the topological transformation reaction is preferably 85-125°C, more preferably 90-120°C, and even more preferably 100-110°C; the time of the topological transformation reaction is preferably 2-6 hours, more preferably 3-5 hours. In this invention, during the topological transformation reaction, the transition intermediate calcium-magnesium composite oxide undergoes a topological transformation through a hydration process to form calcium-doped magnesium hydroxide with a single phase.

[0030] Following the topological transformation reaction, the present invention preferably involves cooling, solid-liquid separation, washing, and drying of the resulting reaction solution. In this invention, the solid-liquid separation is preferably centrifugation, the washing is preferably with deionized water, and the drying temperature is preferably 60°C.

[0031] This invention provides calcium-doped magnesium hydroxide with a thin-film structure prepared by the above preparation method.

[0032] This invention provides the application of the aforementioned calcium-doped magnesium hydroxide with a sheet-like structure in the remediation of acidic heavy metal contaminated soil. In this invention, the heavy metal is preferably cadmium and / or arsenic. Arsenic is scientifically classified as a "heavy metal-like substance," but due to its high toxicity and persistent degradation, it is categorized as heavy metal pollution in the field of soil remediation. In this invention, the pH value of the acidic heavy metal contaminated soil is preferably 5.0–6.5, more preferably 5.5–6.0.

[0033] In this invention, when applied, the amount of calcium-doped magnesium hydroxide with a sheet-like structure added to the soil is preferably 0.3-2 wt%, more preferably 0.5-1.5 wt%, and even more preferably 1 wt%.

[0034] The following detailed description, in conjunction with embodiments, illustrates the calcium-doped magnesium hydroxide with a thin-film structure, its preparation method, and its applications provided by the present invention. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0035] Example 1 A method for preparing calcium-doped magnesium hydroxide with a thin-film structure comprises the following steps: Weigh 2.19 g (0.01 mol) calcium chloride hexahydrate and 6.09 g (0.03 mol) magnesium chloride hexahydrate and place them in a 200 mL beaker. Add 100 mL of deionized water and stir to form a clear solution A. Weigh 7.11 g (0.09 mol) of ammonium bicarbonate and place it in a 200 mL beaker. Add 100 mL of deionized water and stir to obtain a clear solution B. A and B were simultaneously and rapidly poured into a reactor at 5000 r / min to react rapidly. The resulting white slurry was dried in a vacuum spiral dryer to obtain substance C. C was placed in a tube furnace and heated from room temperature to 400℃ at a heating rate of 5℃ / min. After holding at 400℃ for 2 hours, the temperature was increased to 700℃ at a heating rate of 2℃ / min and held for 2 hours before being naturally cooled to obtain substance D. Prepare 100 mL of a 20% glucose solution, add 3 g of the obtained substance D, stir thoroughly, react at 100℃ for 3 hours, cool to room temperature, centrifuge, wash three times with deionized water, and dry at 60℃ to obtain calcium-doped magnesium hydroxide with an ultrathin structure.

[0036] Figure 1 The XRD pattern of the obtained ultrathin calcium-doped magnesium hydroxide is shown below. Figure 1 It can be seen that the diffraction peaks of the obtained product are consistent with those of magnesium hydroxide, and no diffraction peaks of calcium hydroxide are observed, indicating that the obtained product is a single pure phase.

[0037] Figure 2 The Ca 2p XPS spectrum of the obtained ultrathin calcium-doped magnesium hydroxide shows characteristic peaks of calcium around 347 and 350.5 eV, indicating that calcium exists in the +2 valence form.

[0038] Figure 3 The Mg 2p XPS spectrum of the obtained ultrathin calcium-magnesium composite hydroxide shows that a characteristic peak of magnesium appears near 50.3 eV, indicating that magnesium exists in the +2 valence form. Combined with XRD and XPS data, it can be seen that calcium in the obtained calcium-doped magnesium hydroxide has entered the lattice of magnesium hydroxide, forming a uniform pure phase of calcium doping, rather than forming a mixed phase of calcium hydroxide and magnesium hydroxide.

[0039] Figure 4 The elemental scan of the obtained calcium-doped magnesium hydroxide shows that calcium and magnesium are uniformly distributed in the prepared material.

[0040] Figure 5 The image shows a scanning transmission electron microscope (STEM) image of the calcium-doped magnesium hydroxide. It can be seen that the prepared calcium-doped magnesium hydroxide exhibits an extremely thin sheet-like structure.

[0041] Example 2 A method for preparing calcium-doped magnesium hydroxide with a thin-film structure comprises the following steps: Weigh 2.36 g (0.01 mol) calcium nitrate tetrahydrate and 10.26 g (0.04 mol) magnesium nitrate hexahydrate and place them in a 200 mL beaker. Add 100 mL of deionized water and stir to form a clear solution A. Weigh 7.9 g (0.1 mol) of ammonium bicarbonate and place it in a 200 mL beaker. Add 100 mL of deionized water and stir to obtain a clear solution B. A and B were simultaneously and rapidly poured into a reactor at 5000 r / min to react rapidly. The resulting white slurry was dried in a vacuum spiral dryer to obtain substance C. C was placed in a tube furnace and heated from room temperature to 400℃ at a heating rate of 5℃ / min. After holding at 400℃ for 2 hours, the temperature was increased to 700℃ at a heating rate of 2℃ / min and held for 2 hours before being naturally cooled to obtain substance D. Prepare 100 mL of a 30% glucose solution, add 3 g of the obtained substance D, stir thoroughly, react at 120°C for 3 hours, cool to room temperature, centrifuge, wash three times with deionized water, and dry at 60°C to obtain calcium-doped magnesium hydroxide with an ultrathin structure.

[0042] Figure 6 The XRD pattern of the obtained sample shows that the diffraction peaks of the obtained product are consistent with those of magnesium hydroxide, and no diffraction peaks of calcium hydroxide are observed, indicating that the obtained product is a single pure phase. Figure 7 The scanning electron microscope image of the obtained sample shows that the material exhibits a distorted sheet structure.

[0043] Comparative Example 1 In this comparative example, all steps are the same as in Example 1, except that a sugar solution is not used. The specific methods are as follows: Weigh 2.19 g (0.01 mol) calcium chloride hexahydrate and 6.09 g (0.03 mol) magnesium chloride hexahydrate and place them in a 200 mL beaker. Add 100 mL of deionized water and stir to form a clear solution A. Weigh 7.11 g (0.09 mol) of ammonium bicarbonate and place it in a 200 mL beaker. Add 100 mL of deionized water and stir to obtain a clear solution B. A and B were simultaneously and rapidly poured into a reactor at 5000 r / min to react rapidly. The resulting white slurry was dried in a vacuum spiral dryer to obtain substance C. C was placed in a tube furnace and heated from room temperature to 400℃ at a heating rate of 5℃ / min. After holding at 400℃ for 2 hours, the temperature was increased to 700℃ at a heating rate of 2℃ / min and held for 2 hours before being naturally cooled to obtain substance D. Add 3 g of the obtained substance D to 100 mL of deionized water, stir thoroughly, react at 100℃ for 3 hours, let it cool to room temperature, centrifuge, wash 3 times with deionized water, and dry at 60℃ to obtain substance E.

[0044] Figure 8 The XRD pattern of the obtained substance E shows obvious diffraction peaks of calcium hydroxide and magnesium hydroxide, indicating that the obtained material is a mixed phase rather than a single pure phase.

[0045] Comparative Example 2 To demonstrate the importance of the multi-step reaction used in this invention to the material structure, this comparative example uses sodium hydroxide instead of ammonium bicarbonate for direct synthesis, omitting the calcination and hydration steps in the sugar solution. All other steps are consistent with Example 2, and the specific method is as follows: Weigh 2.36 g (0.01 mol) calcium nitrate tetrahydrate and 10.26 g (0.04 mol) magnesium nitrate hexahydrate and place them in a 200 mL beaker. Add 100 mL of deionized water and stir to form a clear solution A. Weigh 4.0 g (0.1 mol) sodium hydroxide and place it in a 200 mL beaker. Add 100 mL of deionized water and stir to obtain a clear solution B. Pour A and B simultaneously and quickly into a reactor at 5000 r / min for rapid reaction. The resulting white slurry is dried in a vacuum spiral dryer to obtain substance F.

[0046] Figure 9 The SEM image of the obtained substance F shows that the material has an aggregated spherical structure rather than a sheet-like structure.

[0047] Application Example 1 Take 100 g of cadmium-contaminated soil, weigh 1 g of calcium-doped magnesium hydroxide prepared in Example 1, add it to the soil, add 40 mL of deionized water, stir thoroughly for 2 hours, and let stand for 7 days. Afterward, air-dry the soil and measure its pH, exchangeable calcium, exchangeable magnesium, and available cadmium. Relevant indicators of the soil before and after remediation are shown in Table 1.

[0048] Table 1 Relevant indicators of soil before and after remediation

[0049] It can be seen that the pH of the soil with added calcium-doped magnesium hydroxide increased from 5.22 to 6.37, effectively improving the acidic soil and benefiting plant growth. Before remediation, the contents of exchangeable calcium and magnesium in the soil were 4.7 cmol / kg and 0.5 cmol / kg, respectively. After remediation, the contents of exchangeable calcium and magnesium in the soil were 16.3 cmol / kg and 8 cmol / kg, respectively, showing a significant increase and effectively supplementing calcium and magnesium. Before remediation, the available cadmium content in the soil was 0.286 mg / kg, and after remediation, the available cadmium content decreased to 0.015 mg / kg, a reduction of 94.8%. Actual field experiments conducted in Anxi, Fujian Province, showed that compared with the control plots, the remediated plots with added calcium-doped magnesium hydroxide showed a 3-14% increase in rice yield, demonstrating a significant effect.

[0050] Application Example 2 Take 100 g of arsenic-contaminated soil, and weigh out 1 g of the calcium-doped magnesium hydroxide prepared in Example 2. Add the mixture to the soil, add 40 mL of deionized water, stir thoroughly for 2 hours, and let stand for 7 days. Then, air-dry the soil and measure its pH, exchangeable calcium, exchangeable magnesium, and available arsenic. The relevant indicators of the soil before and after remediation are shown in Table 2.

[0051] Table 2 Relevant indicators of soil before and after remediation

[0052] It can be seen that the pH of the soil containing calcium-doped magnesium hydroxide (as described in Example 2 of this invention) increased from 5.44 to 6.25, effectively improving the acidic soil and promoting plant growth. Before remediation, the contents of exchangeable calcium and magnesium in the soil were 2.4 cmol / kg and 0.4 cmol / kg, respectively. After remediation, the contents of exchangeable calcium and magnesium in the soil were 6.3 cmol / kg and 11.2 cmol / kg, respectively, showing a significant increase and effectively supplementing calcium and magnesium. Before remediation, the available arsenic content in the soil was 0.364 mg / kg, and after remediation, the available arsenic content decreased to 0.125 mg / kg, a reduction of 65.7%.

[0053] Comparing the remediation effects of Comparative Example 2, it can be seen that the pH of the soil with added calcium-magnesium composite hydroxide increased from 5.44 to 6.14, also achieving the effect of increasing soil pH, but the increase was slightly lower. Before remediation, the contents of exchangeable calcium and magnesium in the soil were 2.4 cmol / kg and 0.4 cmol / kg, respectively; after remediation, the contents of exchangeable calcium and magnesium in the soil were 4.7 cmol / kg and 7.4 cmol / kg, respectively, with the increase being lower than that in Example 2. Before remediation, the available arsenic content in the soil was 0.364 mg / kg; after remediation, the available arsenic content decreased to 0.237 mg / kg, a decrease of 34.9%, indicating that the thin-film structure of calcium-doped magnesium hydroxide provided by this invention has better performance.

[0054] Comparative Example 3 In this comparative example, all steps are the same as in Example 1, except for the roasting treatment method. The specific method is as follows: Weigh 2.19 g (0.01 mol) calcium chloride hexahydrate and 6.09 g (0.03 mol) magnesium chloride hexahydrate and place them in a 200 mL beaker. Add 100 mL of deionized water and stir to form a clear solution A. Weigh 7.11 g (0.09 mol) of ammonium bicarbonate and place it in a 200 mL beaker. Add 100 mL of deionized water and stir to obtain a clear solution B. A and B were simultaneously and rapidly poured into a reactor at 5000 r / min. The resulting white slurry was dried in a vacuum spiral dryer to obtain substance C. C was placed in a tube furnace and heated from room temperature to 700℃ at a rate of 5℃ / min. After being held at 700℃ for 4 hours, it was naturally cooled to obtain substance D. Prepare 100 mL of a 20% glucose solution, add 3 g of the obtained substance D, stir thoroughly, react at 100℃ for 3 hours, cool to room temperature, centrifuge, wash 3 times with deionized water, and dry at 60℃ to obtain calcium-doped magnesium hydroxide.

[0055] Figure 10 The SEM image of the sample obtained in Comparative Example 3 shows that the sample is a particle agglomerate rather than nanosheets, indicating that controlling the calcination process has a significant impact on the material structure.

[0056] 1 g of calcium-doped magnesium hydroxide prepared in Comparative Example 3 was weighed and added to the contaminated soil used in Application Example 1. 40 mL of deionized water was added and stirred thoroughly for 2 hours, then allowed to stand for 7 days. After the soil was air-dried, the content of available cadmium was measured to be 0.087 mg / kg, which was significantly lower than that in Example 1 (0.015 mg / kg), indicating a substantial difference in mineralization efficiency.

[0057] Comparative Example 4 In this comparative example, except for the different molar ratio of calcium and magnesium, all steps are the same as in Example 1, and the specific methods are as follows: Weigh 6.57 g (0.03 mol) calcium chloride hexahydrate and 2.03 g (0.01 mol) magnesium chloride hexahydrate and place them in a 200 mL beaker. Add 100 mL of deionized water and stir to form a clear solution A. Weigh 7.11 g (0.09 mol) of ammonium bicarbonate and place it in a 200 mL beaker. Add 100 mL of deionized water and stir to obtain a clear solution B. A and B were simultaneously and rapidly poured into a reactor at 5000 r / min to react rapidly. The resulting white slurry was dried in a vacuum spiral dryer to obtain substance C. C was placed in a tube furnace and heated from room temperature to 400℃ at a heating rate of 5℃ / min. After holding at 400℃ for 2 hours, the temperature was increased to 700℃ at a heating rate of 2℃ / min and held for 2 hours before being naturally cooled to obtain substance D. Prepare 100 mL of a 20% glucose solution, add 3 g of the obtained substance D, stir thoroughly, react at 100℃ for 3 hours, cool to room temperature, centrifuge, wash 3 times with deionized water, and dry at 60℃ to obtain calcium-doped magnesium hydroxide.

[0058] Figure 11The XRD pattern of the obtained product is significantly different from that of the product obtained in Example 1. The product obtained in the comparative example has obvious diffraction peaks of calcium hydroxide (27°, 47°, 54°, etc.), indicating that it is difficult to form a pure phase product outside the proportions of this invention.

[0059] 1 g of the product prepared in Comparative Example 4 was weighed and added to the contaminated soil used in Application Example 1. 40 mL of deionized water was added and stirred thoroughly for 2 hours, then allowed to stand for 7 days. After the soil was air-dried, the content of available cadmium was measured to be 0.107 mg / kg, which was significantly lower than that in Example 1 (0.015 mg / kg), indicating a substantial difference in mineralization efficiency.

[0060] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing calcium-doped magnesium hydroxide with a thin-film structure, characterized in that, Includes the following steps: Soluble calcium salts and soluble magnesium salts are mixed with water to obtain a calcium-magnesium solution; A soluble ammonium salt is mixed with water to obtain an ammonium salt solution; The calcium-magnesium solution and the ammonium salt solution were mixed at high speed and then dried and calcined sequentially to obtain a transition intermediate. The transition intermediate was mixed with a sugar solution to undergo a topological transformation reaction, yielding calcium-doped magnesium hydroxide with a thin-film structure.

2. The preparation method according to claim 1, characterized in that, The soluble calcium salts include calcium chloride and / or calcium nitrate; The soluble magnesium salt includes one or more of magnesium chloride, magnesium acetate, and magnesium nitrate; The soluble ammonium salt includes one or more of ammonium carbonate, ammonium bicarbonate, and ammonium oxalate.

3. The preparation method according to claim 1, characterized in that, The molar ratio of calcium ions to magnesium ions in the calcium-magnesium solution is 1:3~10. The concentration of magnesium ions in the calcium-magnesium solution is 0.2~0.6 mol / L.

4. The preparation method according to claim 1 or 3, characterized in that, The amount of ammonium ions in the ammonium salt solution is 2 to 5 times the sum of the amounts of calcium and magnesium ions. The concentration of ammonium ions in the ammonium salt solution is 0.4~4 mol / L.

5. The preparation method according to claim 1, characterized in that, The high-speed mixing rate is ≥5000 r / min, and the time is 0.3~3 min; The drying process is a vacuum spiral drying.

6. The preparation method according to claim 1 or 5, characterized in that, The roasting includes a first stage roasting and a second stage roasting; The temperature of the first stage of roasting is 350~450℃, the holding time is 2~4h, and the heating rate to the first stage roasting temperature is 5~10℃ / min. The second stage of roasting is at a temperature of 650~750℃, the holding time is 2~4h, and the heating rate to the second stage roasting temperature is 1~3℃ / min.

7. The preparation method according to claim 1, characterized in that, The sugar raw materials in the sugar solution include one or more of glucose, fructose and mannose; The mass fraction of sugar raw materials in the sugar solution is ≤30%; The mass ratio of the transition intermediate to the sugar raw material is 1:5~10.

8. The preparation method according to claim 1 or 7, characterized in that, The topological transformation reaction is carried out at a temperature of 85~125℃ for 2~6 hours.

9. Calcium-doped magnesium hydroxide with a thin-film structure prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the calcium-doped magnesium hydroxide with a sheet-like structure as described in claim 9 in the remediation of acidic heavy metal contaminated soil.

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