Hydrothermal synthesis of magnesium-rich silicates and their use in the improvement of acid soils

Abstract

The application discloses a hydrothermal synthesis method of magnesium-rich silicate and application of the magnesium-rich silicate in improving acid soil and belongs to the technical field of soil improvement. The method uses magnesium chloride, calcium chloride and sodium silicate as raw materials, generates calcium magnesium silicate gel through a hydrothermal reaction, removes impurities through multiple water washing, and obtains a high-purity product through freeze drying, and a surfactant can be added to optimize the material morphology, specific surface area and acid soil improvement effect. The obtained material is calcium magnesium hydroxysilicate with a calcium magnesium molar ratio of 1:3-1:5 and has a porous nanosheet structure. The application neutralizes soil acidity by releasing silicate, fixes exchangeable aluminum ions in acid soil by releasing calcium and magnesium ions, and improves soil microbial activity and promotes the formation of soil aggregate structure by releasing magnesium ions and silicate, solves the problems of traditional lime acid soil improvement materials, such as easy acidification and soil compaction, and is suitable for rapid acid reduction and long-acting acid control of strong acid soil (4.0≤pH≤5.0).

Description

Technical Field

[0001] This invention belongs to the field of soil improvement technology, specifically relating to a hydrothermal synthesis method of magnesium-rich silicates and their application in improving acidic soils. Background Technology

[0002] In southern my country, high temperatures and abundant rainfall have led to intense natural leaching, resulting in vast areas of acidic red soil. Due to the leaching of silicates and basic ions, these acidic soils accumulate iron and aluminum, leading to decreased base saturation and acid buffering capacity. + Increased saturation leads to the production of large amounts of exchangeable acids. The combined effects of acid toxicity and aluminum toxicity, coupled with severe nutrient deficiency, severely restrict agricultural production on strongly acidic arable land in my country. Traditional methods for improving acidic soils mainly rely on alkaline materials such as lime (CaCO3 or Ca(OH)2) or organic fertilizers, but these methods have the following drawbacks: 1. Low efficiency: Alkaline materials such as lime only improve acidity through the exchange of OH-. - or CO3 2- Short-term neutralization of acidity, excess OH- - After absorbing CO2, it is converted into CO3. 2- CO3 2- 1. **Frequency Issues:** 1) After neutralizing soil acidity and releasing CO2 into the atmosphere, the soil quickly re-acidifies, requiring frequent application. 2) **High Cost:** Organic fertilizer amendments require large-scale application (>10 wt%), have long remediation cycles, and accumulate large amounts of organic matter on the soil surface. Frequent rain leaching leads to severe nutrient loss, polluting water and the atmosphere. 3) **Soil Compaction Issues:** Traditional lime-based amendments are highly alkaline and release large amounts of calcium, which binds with soil colloids, disrupting soil aggregate structure, clogging soil pores, and causing soil compaction. 4) **Residual Pollution:** Industrial byproducts (such as phosphogypsum) contain heavy metals, limiting large-scale application. To address these issues, recent research has attempted to regulate the release of silicate and base ions through the artificial synthesis of silicate materials. However, existing technologies still have limitations: single silicate materials (such as MgSiO3) have low specific surface areas (<30 m²). 2 / g), insufficient reactivity; unoptimized calcium-magnesium ratio, making it difficult to balance neutralization efficiency and structural improvement; the synthesis process relies on high-temperature calcination (>600℃), resulting in high energy consumption and severe product agglomeration. Summary of the Invention

[0003] To address the technical problems mentioned in the background art, this invention proposes a hydrothermal synthesis method for magnesium-rich silicates and its application in improving acidic soils.

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

[0005] A hydrothermal synthesis method for magnesium-rich silicate materials includes the following steps:

[0006] Calcium chloride (CaCl₂·2H₂O) and magnesium chloride (MgCl₂·6H₂O) are mixed in a molar ratio of 1:3 to 1:5, with 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; under continuous stirring, add calcium magnesium chloride solution dropwise to an equal volume of sodium silicate solution to form a mixed solution; transfer the mixed solution to a high-pressure reactor and hydrothermally react at 160-200℃ for 8-16 hours; after the reaction, cool to room temperature to obtain calcium magnesium hydroxysilicate gel; wash the gel multiple times with water to remove residual Na. + and Cl - Ions are freeze-dried and then 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 number of water washings is 3-5 times, and the solid products are 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, and the addition 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 washed by centrifugation with ethanol 3-5 times, and then washed by centrifugation with deionized water 3-5 times.

[0010] A magnesium-rich silicate material, prepared by the above method, has the chemical composition of calcium magnesium hydroxysilicate (Ca). x Mg y [Si2O5](OH)2), in which 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 m². 2 / g, with a morphology of porous nanosheet structure.

[0012] The application of a magnesium-rich silicate material in improving acidic soil involves incorporating the material into strongly acidic soil (4.0≤pH≤5.0) at a ratio of 0.1-0.5 wt%, and verifying its improvement effect through soil column leaching tests.

[0013] In the above technical solution, the improvement effects include: increasing the pH value of the leachate by 0.5-1.4 units, increasing the soil pH value by 0.8-1.6 units, reducing the exchangeable aluminum content by 40%-80%, and increasing the available silicon content in the soil 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 aggregates.

[0015] Beneficial effects:

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

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

[0018] 2. Advantages of material structure

[0019] Porous nanosheet structures (specific surface area 50-180 m²) 2 / g) increases the contact area with soil and promotes ion exchange efficiency; surfactants (polyethylene glycol, laurylamine and oleylamine) regulate morphology and further enhance reactivity and stability.

[0020] 3. Process controllability and environmental friendliness

[0021] Hydrothermal synthesis (160-200℃, 8-16h) combined with multiple water washing processes ensures high product purity (Na). + Cl - It has low residue levels and does not require high-temperature calcination, resulting in low energy consumption and no secondary pollution.

[0022] 4. Soil structure improvement

[0023] Mg 2+ SiO3 2- It can enhance soil microbial activity, promote the formation of soil micro-aggregates, improve aeration and water retention, and enhance the sustainability of soil fertility.

[0024] 5. The application effect can be quantified and verified.

[0025] The quantitative improvement indicators were determined through soil column leaching tests: the pH of the leaching solution was stabilized at 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 inexpensive and readily available, and the amount added is low (0.1-0.5 wt%), making it suitable for large-area acidic soil remediation. The overall cost is lower than that of 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 for rice, wheat and other gramineous plants. Long-term use can replenish the loss of magnesium, calcium and silicon in cultivated land, and has the dual functions of soil improvement and plant nutrition. Attached Figure Description

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

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

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

[0033] Figure 4 This is an SEM image of the magnesium-rich silicate material prepared under hydrothermal conditions at 180°C in Example 3. Detailed Implementation

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

[0035] Example 1: Preparation of magnesium-rich silicate materials (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 17.04 g, 0.06 mol of Na2SiO3·9H2O in 100 mL of deionized water and stir for 30 min to obtain solution B;

[0038] With continuous stirring, the calcium-magnesium mixed solution A was added dropwise to the sodium silicate solution B, and the addition was completed in about 15 minutes. Stirring was continued for another 15 minutes to obtain solution C.

[0039] Hydrothermal reaction: Solution C was transferred to a 250 ml high-pressure reactor and reacted at 180 ℃ for 12 h. After cooling, a white gel was obtained.

[0040] Purification: The gel was separated by centrifugation, washed 4 times with deionized water (until conductivity <50 μS / cm), freeze-dried for 48 h, and ground to obtain powder;

[0041] Characterization: XRD shows that the principal component is Ca. 0.4 Mg 1.6 [Si2O5](OH)2 has a BET specific surface area of ​​85 m² / g and is shown by SEM to be a porous nanosheet structure (thickness 20~50 nm).

[0042] Example 2: Effect of different hydrothermal temperatures

[0043] Keeping other conditions unchanged from Example 1, the hydrothermal temperatures were set to 160℃, 180℃, and 200℃, respectively. SEM images of the prepared magnesium-rich silicate materials are shown below. Figure 1 , Figure 2 ,and Figure 3 As shown.

[0044] Result: The product with the largest specific surface area (85 m²) was found at 180℃. 2 / g), the product at 160 ℃ has low crystallinity (specific surface area of ​​62m²). 2 / g), at 200℃, the product showed particle agglomeration (specific surface area decreased to 70 m²). 2 / g).

[0045] Example 3: Morphology Modulation by Surfactants

[0046] In the mixed solution of Example 1, 1.0% polyethylene glycol, 0.1% laurylamine, and 0.05% oleylamine (total mass of solution) were added. The gel was first washed three times by centrifugation with ethanol, then four times by centrifugation with deionized water, while other conditions remained unchanged. SEM images of the prepared magnesium-rich silicate material are shown below. Figure 4 As shown.

[0047] Results: SEM showed that the sheet thickness was reduced to 10–30 nm, and the specific surface area increased to 165 nm. 2 / g.

[0048] Example 4: Validation of calcium-magnesium ratio adjustment and acid soil improvement

[0049] Formula adjustment: Fix the total metal ions (Mg) in calcium-magnesium mixed solution A. 2+ +Ca 2+ The concentration is 0.25 mol L. -1 Add CaCl2·2H2O and MgCl2·6H2O according to Ca:Mg = 1:3 and 1:5;

[0050] Synthesis: Magnesium-rich silicate materials were prepared under the same conditions as in Examples 1 and 3;

[0051] Soil column leaching test: Two sheets of filter paper were placed at the bottom of a PVC leaching column with a diameter of 6 cm and a height of 35 cm. Then, a 1 cm thick layer of quartz sand was filled in, followed by two more sheets of filter paper. The soil density was then adjusted to 1.50 g / cm³. 3 Fill a 10cm soil column (simulating a 10cm plowshare), then add soil on top according to a soil bulk density of 1.20 g / cm³. 3 A 15 cm soil column was filled with a mixture of magnesium-rich silicate material (simulating a 15 cm topsoil layer). The tested soil had a pH of 4.5 and an exchangeable aluminum content of 2.1 cmol / kg. -1 Effective silicon content: 37 mg / kg -1 The microaggregate (<0.25 mm) content was 28%, and the magnesium-rich silicate material addition was 0.5 wt% of the mass of a 15 cm soil column. Simulated natural rainfall (leaching volume 100 ml / day) was used. -1 The leachate was collected and mixed for 7 consecutive days, and the pH value of the leachate was measured. The pH value, exchangeable aluminum content, available silicon content and microaggregate content of the top 15 cm soil layer 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 tested strongly acidic soils, compared to the control without the materials, the pH of the mixed leachate increased by 0.8-1.1 units after 7 days; for the topsoil, the pH 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 micro-aggregates (<0.25 mm) increased by 8-15 percentage points. Comparing magnesium-rich silicate materials with different calcium-to-magnesium ratios, the magnesium-rich silicate material prepared with Ca:Mg=1:4 and the addition of surfactants showed the best effect in improving strongly acidic soils.

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

[0054]

[0055] 1)Different letters represent differences that reach a significant level (p<0.05).

[0056] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this 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 this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A hydrothermal synthesis method for magnesium-rich silicates, characterized in that, Includes the following steps: Calcium chloride and magnesium chloride are mixed in a molar ratio of 1:3 to 1:5, to a total molar concentration of 0.2-0.35 mol / L. -1 Dissolve sodium silicate in deionized water at a concentration of 0.5-1.0 mol / L. -1 Dissolve in deionized water; under continuous stirring, add calcium magnesium chloride solution dropwise to an equal volume of sodium silicate solution to form a mixed solution; transfer the mixed solution to a high-pressure reactor and hydrothermally react at 160-200℃ for 8-16 hours; after the reaction, cool to room temperature to obtain calcium magnesium hydroxysilicate gel; wash the gel multiple times with water to remove residual Na. + and Cl - Ions, freeze-dried and then ground into powder; The hydrothermal reaction was carried out at a temperature of 180°C for 12 hours. The water washing is performed 3-5 times, and the solid product is separated by centrifugation after each water washing. A surfactant is added to the mixed solution. The surfactant is a mixture of PEG-4000, laurylamine and oleylamine, and the addition 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 washed by centrifugation with ethanol 3-5 times, and then washed by centrifugation with deionized water 3-5 times.

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

3. The magnesium-rich silicate material according to claim 2, characterized in that, The material has a specific surface area of ​​50-180 m² / g and a porous nanosheet structure.

4. The application of a magnesium-rich silicate material in improving acidic soil, characterized in that, The materials described in claims 2-3 were incorporated into strongly acidic soil with pH 4.0 ≤ pH ≤ 5.0 at a ratio of 0.1-0.5 wt%, and their improvement effect was verified by soil column leaching test.

5. The application according to claim 4, characterized in that, The improvements include: an increase in leachate pH of 0.5-1.4 units, an increase in soil pH of 0.8-1.6 units, a decrease in exchangeable aluminum content of 40%-80%, and an increase in available silicon content in the soil of 100 mg / kg. -1 above.

6. The application according to claim 4, characterized in that, The material releases Mg 2+ and SiO3 2- Ions neutralize soil acidity and promote the formation of soil micro-aggregates.

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