Hydro-thermal synthesis method of magnesium-rich silicate and application of magnesium-rich silicate in acid soil improvement

Abstract

The invention discloses a hydrothermal synthesis method of magnesium-rich silicate and application of the magnesium-rich silicate in acid soil improvement, and belongs to the technical field of soil improvement. According to the method, magnesium chloride, calcium chloride and sodium silicate serve as raw materials, calcium magnesium silicate gel is generated through a hydrothermal reaction, a high-purity product is obtained through freeze drying after impurities are removed through multiple times of washing, and a surfactant can be added to optimize the material morphology and the specific surface area and improve the acid soil effect. The obtained material is hydroxyl calcium magnesium silicate with the molar ratio of calcium to magnesium being (1: 3)-(1: 5), and has a porous nanosheet structure. According to the invention, silicate is released to neutralize the acidity of soil, calcium and magnesium ions are released to fix exchangeable aluminum ions in acid soil, magnesium ions and silicate are released to improve the microbial activity of soil and promote the formation of a soil aggregate structure, and the problems of easy acid regurgitation, soil hardening and the like of a traditional lime acid soil improvement material are solved; the soil conditioner is suitable for rapid acid reduction and long-acting acid control of strongly acidic soil (the pH is more than or equal to 4.0 and less than or equal to 5.0).

Description

Technical Field

[0001] The invention belongs to the technical field of soil improvement, and particularly relates to a hydrothermal synthesis method of magnesium-rich silicate and application thereof in improving acidic soil. Background Art

[0002] In southern my country, high temperatures and heavy rainfall have led to intense natural leaching, resulting in extensive acidic red soils. Acidification of cultivated land is particularly prominent in South China, where 92% of cultivated land is acidic, and 54% is highly acidic (pH 4.0 ≤ pH ≤ 5.0). Acidic soils experience leaching of silicates and base ions, enriching iron and aluminum, reducing base saturation and acid buffering capacity, and increasing the pH. + The saturation increases, producing a large amount of exchangeable acid. The combined effects of acid toxicity and aluminum toxicity, coupled with nutrient deficiencies, severely restrict agricultural production on highly acidic arable land in my country. Traditional methods for improving acidic soils mainly rely on lime-based alkaline materials (CaCO3 or Ca(OH)2) or organic fertilizers, but have the following drawbacks: 1. Low efficiency: Lime-based alkaline materials only pass OH - or CO3 2- Short-term neutralization of acidity, excess OH - After absorbing CO2, it turns into CO3 2- , CO3 2- 1. After CO2 is released into the atmosphere after neutralizing soil acidity, the soil quickly becomes acidic again, so it needs to be applied frequently; 2. High cost: Organic fertilizer amendments need to be applied in large quantities (>10 wt%), the repair cycle is long, and a large amount of organic matter accumulates on the soil surface. Under frequent leaching by rainwater, nutrients are seriously lost, polluting water bodies and the atmosphere; 3. It is easy to cause soil compaction: Traditional lime amendments are highly alkaline and release a large amount of calcium to bind with soil colloids, destroying the soil aggregate structure, blocking soil pores, and causing soil compaction; 4. Residual pollution: Industrial by-products (such as phosphogypsum, etc.) contain heavy metal risks, which limits large-scale application. In response to the above problems, studies in recent years have attempted to regulate the release of silicate and alkaline ions through the artificial synthesis of silicate materials, but the existing technology still has shortcomings: single silicate materials (such as MgSiO3) have low specific surface area (<30 m 2 / g), insufficient reaction activity; the calcium-magnesium ratio is not optimized, resulting in difficulty in balancing neutralization efficiency and structural improvement; the synthesis process relies on high-temperature calcination (>600℃), which has high energy consumption and severe product agglomeration. Summary of the Invention

[0003] In order to solve the technical problems raised in the background technology, the present invention proposes a hydrothermal synthesis method of magnesium-rich silicate and its application in improving acidic soil.

[0004] The technical solution of the present invention is as follows:

[0005] A hydrothermal synthesis method of a magnesium-rich silicate material comprises the following steps:

[0006] Calcium chloride (CaCl2·2H2O) and magnesium chloride (MgCl2·6H2O) were mixed at a molar ratio of 1:3 to 1:5 to obtain a total molar concentration of 0.2-0.35 mol L -1 Dissolve sodium silicate (Na2SiO3·9H2O) in deionized water at a concentration of 0.5-1.0 mol L -1 Dissolve in deionized water; add the calcium magnesium chloride solution dropwise to an equal volume of sodium silicate solution under continuous stirring to form a mixed solution; transfer the mixed solution to a high-pressure reactor and perform a hydrothermal reaction at 160-200°C for 8-16 hours. After the reaction is completed, cool to room temperature to obtain a hydroxy calcium magnesium silicate gel; wash the gel several times to remove residual Na + and Cl - ions, freeze-dried and ground into powder.

[0007] In the above technical solution, the temperature of the hydrothermal reaction is 180° C. and the reaction time is 12 hours.

[0008] In the above technical solution, the water washing times are 3-5 times, and the solid product is separated by centrifugation after each water washing.

[0009] In the above technical solution, a surfactant is added to the mixed solution. The surfactant is a mixture of polyethylene glycol (PEG-4000), dodecylamine and oleylamine. The added amounts are 0.5%-1%, 0.1%-0.15% and 0.05%-0.1% of the total mass of the mixed solution, respectively. The product is first centrifuged and washed 3-5 times with ethanol and then centrifuged and washed 3-5 times with deionized water.

[0010] A magnesium-rich silicate material is prepared by the above method, and its chemical composition is calcium magnesium hydroxysilicate (Ca x Mg y [Si2O5](OH)2), where the calcium-magnesium molar ratio x:y=1:3-1:5, and x+y=2.

[0011] In the above technical solution, the specific surface area of ​​the material is 50-180 m 2 / g, and the morphology is a porous nanosheet structure.

[0012] A magnesium-rich silicate material is used to improve acidic soil. The material is added to strongly acidic soil (4.0≤pH≤5.0) at a ratio of 0.1-0.5 wt%, and its improvement effect is verified through soil column leaching tests.

[0013] In the above technical solution, the improvement effects include: the pH value of the eluent is increased by 0.5-1.4 units, the pH value of the soil is increased by 0.8-1.6 units, the exchangeable aluminum content is reduced by 40%-80%, and the effective silicon content in the soil is increased by 100 mg kg -1 above.

[0014] In the above technical solution, the material releases Mg 2+ and SiO3 2- ions, neutralize soil acidity and promote the formation of soil aggregate structure.

[0015] Beneficial effects:

[0016] 1. Highly efficient acid neutralization and aluminum toxicity relief

[0017] By optimizing the calcium-magnesium ratio (1:3-1:5), the Mg content in the material 2+ and SiO3 2- The synergistic release significantly increases the pH value of acidic soil (increases by 0.8-1.5 units in the surface layer), while reducing the exchangeable aluminum content by 40%-80%, effectively alleviating the damage of acid toxicity and aluminum toxicity to plants.

[0018] 2. Material structure advantages

[0019] Porous nanosheet structure (specific surface area 50-180 m 2 / g) to enhance the contact area with the soil and promote ion exchange efficiency; surfactants (polyethylene glycol, lauryl amine and oleylamine) regulate the morphology and further enhance the reaction activity and stability.

[0020] 3. Process controllability and environmental protection

[0021] Hydrothermal synthesis (160-200℃, 8-16h) combined with multiple water washing processes to ensure high product purity (Na + 、Cl - Low residue), no need for high temperature calcination, low energy consumption and no secondary pollution.

[0022] 4. Soil structure improvement

[0023] Mg 2+ 、SiO3 2- Improve soil microbial activity, promote the formation of soil microaggregates, improve air permeability and water retention, and enhance the sustainability of soil fertility.

[0024] 5. Application effects can be quantified and verified

[0025] Through soil column leaching tests, quantitative improvement indicators were clearly defined: the pH of the elution solution was stabilized to 5.5-6.5, and the aluminum saturation was reduced to 20%-30%, providing reliable data support for practical applications.

[0026] 6. Significant cost-effectiveness

[0027] The raw materials (CaCl2, MgCl2, Na2SiO3) are cheap and easily available, and the incorporation amount is low (0.1-0.5 wt%). It is suitable for large-scale acidic soil remediation, and the overall cost is lower than traditional lime or organic amendments.

[0028] 7. Environmental compatibility

[0029] The material is non-biotoxic, Mg 2+ , Ca 2+ and SiO3 2- It is an essential nutrient element for rice, wheat and other grass plants. Long-term use can replenish the loss of magnesium, calcium and silicon in cultivated land, and it has the dual functions of soil improvement and plant nourishment. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a SEM electron microscope photograph of the magnesium-rich silicate material prepared under hydrothermal conditions at 160°C in Example 2.

[0031] Figure 2 This is a SEM electron microscope photograph of the magnesium-rich silicate material prepared under hydrothermal conditions at 180°C in Example 2.

[0032] Figure 3 This is a SEM electron microscope photograph of the magnesium-rich silicate material prepared under hydrothermal conditions at 200°C in Example 2.

[0033] Figure 4 This is a SEM electron microscope photograph of the magnesium-rich silicate material prepared under hydrothermal conditions at 180°C in Example 3. DETAILED DESCRIPTION

[0034] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. However, the following embodiments are intended only to explain the present invention, and the scope of protection of the present invention should include the entire contents of the claims. Moreover, through the description of the following embodiments, those skilled in the art can fully implement the entire contents of the claims of the present invention.

[0035] Example 1: Preparation of magnesium-rich silicate material (standard conditions)

[0036] Raw material ratio: Dissolve MgCl2·6H2O (4.06 g, 0.02 mol) and CaCl2·2H2O (0.735 g, 0.005 mol) in 100 mL of deionized water and stir for 30 min to obtain calcium-magnesium mixed solution A;

[0037] Dissolve Na2SiO3·9H2O (17.04 g, 0.06 mol) in 100 mL of deionized water and stir for 30 min to obtain solution B;

[0038] Under continuous stirring, the obtained calcium magnesium mixed solution A is added dropwise to the sodium silicate solution B. The addition is completed in about 15 minutes, and stirring is continued for 15 minutes to obtain solution C;

[0039] Hydrothermal reaction: Transfer solution C to a 250 ml autoclave and react at 180 °C for 12 h. After cooling, a white gel was obtained.

[0040] Purification: The gel was separated by centrifugation, washed four times with deionized water (to a conductivity of <50 μS / cm), freeze-dried for 48 h, and ground to obtain a powder.

[0041] Characterization: XRD shows that the main component is Ca 0.4 Mg 1.6 [Si2O5](OH)2, BET specific surface area is 85m² / g, SEM shows a porous nanosheet structure (thickness 20~50nm).

[0042] Example 2: Effect of different hydrothermal temperatures

[0043] Keeping other conditions of Example 1 unchanged, the hydrothermal temperatures were set to 160°C, 180°C, and 200°C respectively. The SEM photos of the prepared magnesium-rich silicate materials are shown in Figure 2. Figure 1 、 Figure 2 ,and Figure 3 shown.

[0044] Results: The product with the highest specific surface area at 180℃ (85 m 2 / g), the product at 160 °C has low crystallinity (specific surface area of ​​62m 2 / g), the product showed particle agglomeration at 200℃ (the specific surface area dropped to 70 m 2 / g).

[0045] Example 3: Surfactant morphology control

[0046] 1.0% polyethylene glycol, 0.1% laurylamine, and 0.05% oleylamine (total weight of the solution) were added to the mixed solution of Example 1. The gel was washed by centrifugation three times with ethanol and then four times with deionized water, with other conditions remaining unchanged. The SEM photograph of the prepared magnesium-rich silicate material is shown in FIG. Figure 4 shown.

[0047] Results: SEM showed that the thickness of the flakes was reduced to 10-30 nm and the specific surface area was increased to 165 m 2 / g.

[0048] Example 4: Calcium-magnesium ratio adjustment and acid soil improvement verification

[0049] Formula adjustment: Fixed the total metal ions (Mg 2+ +Ca 2+ ) concentration is 0.25 mol L -1 , add CaCl2·2H2O and MgCl2·6H2O according to Ca:Mg=1:3 and 1:5;

[0050] Synthesis: The magnesium-rich silicate material was prepared under the same conditions as in Example 1 and Example 3;

[0051] Soil column leaching test: two filter papers were placed at the bottom of a PVC leaching column with a diameter of 6 cm and a height of 35 cm, and then a 1 cm thick quartz sand filter layer was filled. Two more filter papers were placed on the quartz sand layer, and then the soil was filtered according to the soil bulk density of 1.50 g / cm 3 Fill a 10 cm soil column (simulating 10 cm of plowed bottom soil) and add more soil with a bulk density of 1.20 g / cm 3 Fill a 15 cm column of soil mixed with magnesium-rich silicate material (simulating a 15 cm cultivated layer of soil). The test soil has a pH of 4.5 and an exchangeable aluminum content of 2.1 cmol / kg -1 , effective silicon content 37 mg kg -1 The content of microaggregates (<0.25 mm) was 28%, and the amount of magnesium-rich silicate material added was 0.5 wt% of the mass of the 15 cm soil column. Natural rainfall was simulated (leaching amount 100 ml d -1 The leachate solution was collected and mixed for 7 consecutive days, and the pH value of the leachate solution was measured. The pH value, exchangeable aluminum content, available silicon content and microaggregate content of the 15 cm tillage layer soil were also measured.

[0052] The results are shown in Table 1. It can be seen that the addition of surfactants can significantly increase the specific surface area of ​​magnesium-rich silicate materials. After the magnesium-rich silicate materials were added to the strongly acidic soil for testing, the pH value of the mixed leaching solution increased by 0.8-1.1 units after 7 days compared with the control without the addition of materials. For the surface soil, the pH value increased by 1.1-1.4 units, the exchangeable aluminum content decreased by 43%-65%, and the available silicon content increased by 408-452 mg kg -1 , the content of soil microaggregates (<0.25 mm) increased by 8-15 percentage points. Comparing magnesium-rich silicate materials with different calcium-magnesium ratios, the magnesium-rich silicate material with a Ca:Mg ratio of 1:4 and the addition of surfactants was most effective in improving strongly acidic soils.

[0053] Table 1. Effects of each treatment on improving acidic soil

[0054]

[0055] 1)Different letters indicate significant differences (p<0.05).

[0056] The foregoing is merely a list of specific embodiments of the present application, intended to enable those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the broadest scope consistent with the principles and novel features of the present application.

Claims

1. A hydrothermal synthesis method of magnesium-rich silicate, characterized in that: The following steps are involved: Calcium chloride (CaCl2·2H2O) and magnesium chloride (MgCl2·6H2O) were mixed at a molar ratio of 1:3 to 1:5 to obtain a total molar concentration of 0.2-0.35 mol L -1 Dissolve sodium silicate (Na2SiO3·9H2O) in deionized water at a concentration of 0.5-1.0 mol L -1 Dissolve in deionized water; add the calcium magnesium chloride solution dropwise to an equal volume of sodium silicate solution under continuous stirring to form a mixed solution; transfer the mixed solution to a high-pressure reactor and perform a hydrothermal reaction at 160-200°C for 8-16 hours. After the reaction is completed, cool to room temperature to obtain a hydroxy calcium magnesium silicate gel; wash the gel several times to remove residual Na + and Cl - ions, freeze-dried and ground into powder.

2. The method according to claim 1, characterized in that The temperature of the hydrothermal reaction is 180° C., and the reaction time is 12 hours.

3. The method according to claim 1, characterized in that The number of water washings is 3-5 times, and the solid product is separated by centrifugation after each water washing.

4. The method according to claim 1, wherein A surfactant is added to the mixed solution. The surfactant is a mixture of polyethylene glycol (PEG-4000), dodecylamine, and oleylamine. The added amounts are 0.5%-1%, 0.1%-0.15%, and 0.05%-0.1% of the total mass of the mixed solution, respectively. The product is first centrifugally washed 3-5 times with ethanol and then centrifugally washed 3-5 times with deionized water.

5. A magnesium-rich silicate material, characterized in that: Prepared by the method according to any one of claims 1 to 4, its chemical composition is calcium magnesium hydroxysilicate (Ca x Mg y [Si2O5](OH)2), wherein the calcium-magnesium molar ratio x:y=1:3-1:5, and x+y=2.

6. The magnesium-rich silicate material according to claim 5, characterized in that The specific surface area of ​​the material is 50-180m 2 / g, and the morphology is a porous nanosheet structure.

7. Application of a magnesium-rich silicate material in improving acidic soil, characterized in that: The material according to claims 5-6 is added to strongly acidic soil (4.0≤pH≤5.0) at a ratio of 0.1-0.5 wt%, and the improvement effect is verified by a soil column leaching test.

8. The use according to claim 7, characterized in that The improvement effects include: the pH value of the eluent increased by 0.5-1.4 units, the pH value of the soil increased by 0.8-1.6 units, the exchangeable aluminum content decreased by 40%-80%, and the effective silicon content in the soil increased by 100 mg kg -1 above.

9. The use according to claim 7, characterized in that The material releases Mg 2+ and SiO3 2- ions, neutralize soil acidity and promote the formation of soil microaggregate structure.

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