Preparation method of carbon dot-coated calcium-aluminum hydrotalcite composite material suitable for industrial production

Abstract

The invention relates to a preparation method of a carbon dot-coated calcium-aluminum hydrotalcite composite material suitable for industrial production, and belongs to the technical field of composite materials. The preparation method comprises the following steps: placing calcium-aluminum hydrotalcite in a polytetrafluoroethylene lining in a reaction kettle, taking a mixture of citric acid and sodium citrate as a carbon dot precursor, uniformly stirring the substances, and adding a proper amount of water to prepare a mixed solution; placing the reaction kettle in a muffle furnace for reaction, taking out the reaction kettle after a certain time, naturally cooling the reaction kettle to room temperature, drying a product, and grinding the product into powder, so as to obtain the carbon dot-coated calcium-aluminum hydrotalcite composite material. The carbon dot and hydrotalcite composite material is prepared by adopting an in-situ growth method, and the method is simple in process, high in efficiency, low in equipment requirement, high in yield and capable of realizing industrial large-scale production and application.

Description

Technical Field

[0001] The present invention belongs to the technical field of composite materials, and relates to a carbon dot@calcium-aluminum hydrotalcite composite material for treating soil nitrogen pollution, specifically a method for preparing a carbon dot@calcium-aluminum hydrotalcite composite material by an in-situ growth method. Background Art

[0002] Regarding the application of carbon dot@calcium-aluminum hydrotalcite composite material in the adsorption of soil ammonium nitrogen, researchers have determined that it has good adsorption effect. The key difficulty lies in that the synthesis method of carbon dot@calcium-aluminum hydrotalcite composite material does not have the possibility of large-scale production.

[0003] Specifically, the traditional preparation method is mainly a post-modification strategy, namely the colloid deposition method. First, calcium-aluminum hydrotalcite is synthesized by the co-precipitation method; then, a carbon dot solution is synthesized by the microwave method using citric acid as a precursor, and dialysis is carried out for 24 h for separation and purification; finally, the two are compounded by stirring for 24 h using the colloid deposition method. The dialysis step in this method cannot be industrialized, and the process flow is complex, increasing the operation difficulty and time cost, and also having high requirements for equipment and technology. Moreover, the yield is low, which is not suitable for forming an industrial production line and cannot meet the needs of large-scale production and application. Summary of the Invention

[0004] To solve the above technical problems existing in the existing preparation of carbon dot@calcium-aluminum hydrotalcite composite material, the present invention provides a preparation method of a carbon dot@calcium-aluminum hydrotalcite composite material suitable for industrial production. The present invention uses an in-situ growth method to prepare a carbon dot@calcium-aluminum hydrotalcite composite material. This method has a simple process flow, high efficiency, low requirements for equipment, and high yield, and can realize industrial large-scale production and application.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A preparation method of a carbon dot@calcium-aluminum hydrotalcite composite material suitable for industrial production, comprising the following steps:

[0007] Step 1, preparation of calcium-aluminum hydrotalcite

[0008] Step 2, preparation of carbon dot@calcium-aluminum hydrotalcite composite material

[0009] (1) Place the calcium-aluminum hydrotalcite prepared in Step 1 in the polytetrafluoroethylene inner liner of the reaction kettle, use a mixture of citric acid and sodium citrate as the carbon dot precursor, stir the above substances evenly, and add an appropriate amount of water to obtain a mixed solution;

[0010] (2) Place the reaction kettle in a muffle furnace for reaction, take it out after a certain time, naturally cool it to room temperature, dry the product and grind it into powder to obtain a carbon dot@calcium-aluminum hydrotalcite composite material.

[0011] Further, in the second step, the mass ratio of the calcium-aluminum hydrotalcite to the citrate radical is 1:3.0 - 5.5, providing a carbon dot precursor for the system.

[0012] Further, in the second step, the amount of water added is such that the liquid-solid ratio is 0.1 - 0.5.

[0013] Further, in the second step, the reaction temperature is 250 - 300 °C.

[0014] Further, in the second step, the reaction time is 8 - 12 h.

[0015] Further, the preparation process of the calcium-aluminum hydrotalcite in the first step is as follows:

[0016] (1) Weigh Ca(NO3)2·4H2O and Al(NO3)3·9H2O and prepare a mixed solution.

[0017] (2) Prepare a 1.0 - 2.5 mol / L sodium hydroxide or calcium hydroxide solution.

[0018] (3) While stirring the sodium hydroxide or calcium hydroxide solution, slowly drip in the mixed solution, maintaining the pH of the system at 8 - 12.

[0019] (4) React the obtained mixture at 50 - 100 °C for 4 - 10 h.

[0020] (5) Centrifuge the obtained solution for 10 - 30 min, pour off the supernatant, and wash the obtained precipitate several times with deionized water and ethanol until it is neutral.

[0021] (6) Dry in an oven at 60 - 120 °C for 6 - 24 h to obtain the hydrotalcite.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] 1. The preparation method of the carbon dot@calcium-aluminum hydrotalcite composite material of the present invention is simple and time-consuming.

[0024] 2. Each link in the preparation process of the present invention has mature industrial equipment, and industrial preparation can be realized.

[0025] 3. Experiments in soil prove that the carbon dot@calcium-aluminum hydrotalcite composite material prepared by the in-situ growth method does not affect the soil pH and has good nitrogen fixation ability. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a flowchart for the preparation of the calcium-aluminum hydrotalcite and the carbon dot@calcium-aluminum hydrotalcite composite material of the present invention;

[0027] Figure 2SEM images of carbon dots@calcium aluminum hydrotalcite composites prepared by traditional colloid deposition method (a: 1 μm, b: 1 μm) and carbon dots@calcium aluminum hydrotalcite composites prepared by the present invention (c: 1 μm, d: 500 nm);

[0028] Figure 3 XRD patterns of carbon dots, calcined calcium aluminum hydrotalcite and carbon dots@calcium aluminum hydrotalcite composites. Specific Embodiments

[0029] The technical solutions and effects of the present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

[0030] The preparation process of calcium aluminum hydrotalcite used in the following examples is as Figure 1 shown, specifically:

[0031] (1) Weigh 15.74 g of Ca(NO3)2·4H2O and 12.50 g of Al(NO3)3·9H2O, add them to 200 mL of water, and prepare a mixed solution;

[0032] (2) Weigh 8 g of NaOH and prepare 200 mL of sodium hydroxide solution;

[0033] (3) While stirring the sodium hydroxide solution, slowly drip the mixed solution into it, and maintain the pH of the system at 10 ± 1;

[0034] (4) Age the obtained mixture in an oil bath at 60 °C for 10 h;

[0035] (5) Centrifuge the obtained solution at 8000 r / min for 10 min, pour off the supernatant, wash the obtained precipitate with deionized water and ethanol, and centrifuge and wash repeatedly for several times until it is neutral;

[0036] (6) Dry it in an oven at 105 °C for 12 h to obtain calcium aluminum hydrotalcite.

[0037] Example 1

[0038] The preparation method of the carbon dots@calcium aluminum hydrotalcite composite material in this example is as Figure 1 shown, specifically:

[0039] Place 1 g of calcium aluminum hydrotalcite in the polytetrafluoroethylene inner liner of the reaction kettle. Use a mixture of citric acid and sodium citrate as the carbon dot precursor, 0.895 g of citric acid and 5.674 g of sodium citrate. Stir the above substances evenly, add 1.69 g of water to prepare a mixed solution; place the reaction kettle in a muffle furnace for reaction, the reaction temperature is 285 °C, take it out after reacting for 10 h, naturally cool it to room temperature, dry the product and grind it into powder to obtain the carbon dots@calcium aluminum hydrotalcite composite material.

[0040] Under standard temperature conditions, ammonium chloride (NH4Cl) was dissolved in deionized water to prepare a stock solution with a concentration of 100 mg / L. 0.015 g of the carbon dots@calcium-aluminum hydrotalcite composite was accurately weighed and transferred to a reaction vessel, and 10 mL of the stock solution with an initial concentration of 100 mg / L was injected. After sealing, it was placed under the conditions of a temperature of 25 °C, pH = 7, dosage of 1.5 g / L, rotation speed N = 300 rpm, and adsorption time of 12 h for adsorption. After standing and stratifying, the supernatant was collected, filtered through a microporous membrane (0.45 μm), diluted 100 times, and the residual ammonia nitrogen concentration was determined by spectrophotometry. Three parallel samples were set for each experiment to control data deviation. The ammonium nitrogen content in the water sample was accurately determined by the Nessler's reagent photometry method. First, an aliquot of the water sample containing no more than 0.1 mg of ammonia nitrogen was accurately measured and transferred to a 50 mL colorimetric tube and diluted to the mark. Then 1.0 mL of the sodium potassium tartrate solution was added. The measurement process followed the standard curve drawing procedure, and the ammonia nitrogen content (mg) was obtained from the standard curve by subtracting the absorbance of the blank test.

[0041] (1. 1)

[0042] In Equation 1.1: N: ammonia nitrogen concentration, mg / L;

[0043] m: ammonia nitrogen content obtained from the standard curve, mg;

[0044] V: water sample volume, mL.

[0045] According to Equations 1.2 and 1.3, the adsorption capacity and removal rate of ammonia nitrogen per unit mass of the carbon dots@calcium-aluminum hydrotalcite composite were calculated:

[0046] (1. 2)

[0047] (1. 3)

[0048] In Equations 1.2 and 1.3: Q e : adsorption capacity of ammonia nitrogen per unit mass of the carbon dots@calcium-aluminum hydrotalcite composite, mg / g;

[0049] C0: initial ammonia nitrogen concentration, mg / L;

[0050] C e : ammonia nitrogen concentration at adsorption equilibrium, mg / L;

[0051] V: solution volume at adsorption equilibrium, L;

[0052] m: dosage of the carbon dots@calcium-aluminum hydrotalcite composite, g;

[0053] R e : Removal rate of ammonia nitrogen by carbon dots@calcium aluminum hydrotalcite composite per unit mass, %.

[0054] The adsorption capacity of the carbon dots@calcium aluminum hydrotalcite composite in this example after 12 h of adsorption is 7.43 mg / g.

[0055] Example 2

[0056] The preparation method of the carbon dots@calcium aluminum hydrotalcite composite in this example is as Figure 1 shown, specifically:

[0057] Place 1 g of calcium aluminum hydrotalcite in the polytetrafluoroethylene liner in the reaction kettle. Use the mixture of citric acid and sodium citrate as the carbon dot precursor, 0.83 g of citric acid and 4 g of sodium citrate. Stir the above substances evenly, add 2 g of water to obtain a mixed solution; place the reaction kettle in a muffle furnace for reaction, the reaction temperature is 250 °C, take it out after reacting for 10 h, naturally cool to room temperature, dry the product and grind it into powder to obtain the carbon dots@calcium aluminum hydrotalcite composite.

[0058] According to the adsorption experiment in Example 1, the adsorption capacity of the carbon dots@calcium aluminum hydrotalcite composite in this example after 12 h of adsorption is 4.43 mg / g.

[0059] Example 3

[0060] The preparation method of the carbon dots@calcium aluminum hydrotalcite composite in this example is as Figure 1 shown, specifically:

[0061] Place 1 g of calcium aluminum hydrotalcite in the polytetrafluoroethylene liner in the reaction kettle. Use the mixture of citric acid and sodium citrate as the carbon dot precursor, 0.93 g of citric acid and 6 g of sodium citrate. Stir the above substances evenly, add 1.36 g of water to obtain a mixed solution; place the reaction kettle in a muffle furnace for reaction, the reaction temperature is 250 °C, take it out after reacting for 8 h, naturally cool to room temperature, dry the product and grind it into powder to obtain the carbon dots@calcium aluminum hydrotalcite composite.

[0062] According to the adsorption experiment in Example 1, the adsorption capacity of the carbon dots@calcium aluminum hydrotalcite composite in this example after 12 h of adsorption is 5.76 mg / g.

[0063] Example 4

[0064] The preparation method of the carbon dots@calcium aluminum hydrotalcite composite in this example is as Figure 1 shown, specifically:

[0065] Place 1 g of calcium-aluminum hydrotalcite in the PTFE liner inside the reactor. Use a mixture of citric acid and sodium citrate as the carbon dot precursor, with 0.93 g of citric acid and 6 g of sodium citrate. Stir the above substances evenly, add 1.36 g of water to obtain a mixed solution. Place the reactor in a muffle furnace for reaction at a reaction temperature of 300 °C. After reacting for 10 h, take it out and let it cool naturally to room temperature. Dry the product and grind it into powder to obtain the carbon dot@calcium-aluminum hydrotalcite composite material.

[0066] According to the adsorption experiment in Example 1, the adsorption capacity of the carbon dot@calcium-aluminum hydrotalcite composite material in this example after 12 h of adsorption is 4.96 mg / g.

[0067] Example 5

[0068] Under the optimal experimental conditions (pH = 7, 25 °C), put the carbon dot@calcium-aluminum hydrotalcite material (1.5 g / L) prepared in Example 1, the traditional ammonia nitrogen adsorption material zeolite (2 g / L), acidic biochar (2 g / L), and alkaline biochar (2 g / L) materials into 10 mL of an ammonium nitrogen solution with a concentration of 100 mg / L. Carry out adsorption at a fixed rotation speed N = 300 rpm. After the adsorption is completed, measure the adsorption capacity according to the method in Example 1. Compare the adsorption effects of the four adsorption materials on ammonium nitrogen. The measurement of the adsorption capacity is as shown in Example 1, and the experimental results after 36 hours are shown in Table 1.

[0069] Table 1 Comparison of the adsorption effects of different materials on ammonium nitrogen

[0070]

[0071] Among them, the zeolite was purchased from Beijing Panbao Dushikou Zeolite Technology Co., Ltd., 180 - 200 mesh; the acidic biochar was purchased from Fujian Yuanli Activated Carbon Co., Ltd., 80 - 120 mesh; the alkaline biochar was purchased from Fujian Yuanli Activated Carbon Co., Ltd., 80 - 120 mesh.

[0072] Through comparison, it is found that compared with the commonly used ammonium nitrogen adsorption materials (zeolite and biochar), the carbon dot@calcium-aluminum hydrotalcite composite material shows better adsorption performance for ammonium nitrogen.

[0073] Example 6

[0074] The test soil used in this study is the middle Henan fluvo-aquic soil, and the sampling depth is 0 - 20 cm. The collected soil is air-dried, stones and plant roots are removed, and then it is screened through a 2-mm sieve and mixed evenly. The basic physical and chemical properties of the soil are shown in Table 2 in detail.

[0075] Table 2 Basic physical and chemical properties of the soil

[0076]

[0077] A 50 mL beaker was used as the experimental container, which was filled with 60 g of dried soil. The experiment was divided into a blank group and treatment groups with carbon dot@calcium-aluminum hydrotalcite composites at different mass ratios (the addition amounts were 0.10%, 0.15%, and 0.20% of the soil respectively), and each group had three replicates. After the soil and the treatment materials were fully mixed, the water content was adjusted to 65% of the field water holding capacity, and 90 mg of N was applied to make the soil nitrogen addition amount 1.5 mg / g. The experiment was carried out in a constant temperature and humidity chamber at 25 °C, sealed with plastic wrap and punctured with holes, and weighed regularly to supplement water to maintain humidity. The ammonium nitrogen content was compared after 30 days of cultivation.

[0078] The specific detection method for the ammonium nitrogen content was as follows: The ammonium nitrogen in the soil was determined by the potassium chloride solution extraction-spectrophotometry method. Take 0, 0.10, 0.20, 0.50, 1.00, 2.00, 3.50 mL of ammonium chloride standard solution (containing 0, 1.0, 2.0, 5.0, 10.0, 20.0, 35.0 μg of ammonia nitrogen) into 50 mL colorimetric tubes, and add water to 10.0 mL. Add 40 mL of the color reagent, mix well and let stand for 15 minutes, then add 1.0 mL of another color reagent and let stand at room temperature for 5 hours. Using water as the reference, measure the absorbance at 630 nm and draw the standard curve. Take 4.0 g of soil sample, add 20 mL of 1 mol / L potassium chloride solution, and oscillate at 20 ± 2 °C for 1 hour. Take 6 mL of the extract and centrifuge it, and collect 5 mL of the supernatant. Take 1.0 mL of the supernatant to measure the absorbance.

[0079] The calculation of the retention rate is shown in formula 1.4,

[0080] (1.4)

[0081] In the formula: C0: The ammonium nitrogen content of the soil in the blank group after 30 days of cultivation, mg / kg;

[0082] C e : The ammonium nitrogen content of the soil in the control group after 30 days of cultivation, mg / kg;

[0083] The experimental results of the soil with different mass ratios of carbon dot@calcium-aluminum hydrotalcite composites added are shown in Table 3.

[0084] Table 3 Adsorption effects of composites / soil with different mass ratios on ammonium nitrogen

[0085]

[0086] From the experimental results of Examples 5 and 6, it can be concluded that the carbon dot@calcium-aluminum hydrotalcite composite has good adsorption and retention effects on ammonium nitrogen.

[0087] The process of preparing the carbon dots@hydrotalcite composite material by the traditional colloid deposition method is specifically as follows:

[0088] Dissolve 10 grams of citric acid in distilled water, and then heat it in a microwave oven at a power of 700 W in high-fire mode for 6 minutes. Subsequently, add 10 mL of ultrapure water, and after standing, perform centrifugation to collect the supernatant. Dialyze the supernatant for 24 hours using a dialysis bag with a molecular weight cut-off of 200 Dalton (Da). When it shows green fluorescence under ultraviolet light, carbon dots are obtained. To synthesize the carbon dots-hydrotalcite composite material by the colloid deposition method, add 1 g of hydrotalcite to 5 mL of the carbon dot solution, stir it at 300 rpm using a magnetic stirrer at 25 °C for 1 h, then centrifuge to obtain a suspension, wash it with deionized water, and finally dry it at 65 °C to obtain the carbon dots@hydrotalcite composite material.

[0089] From the scanning electron microscope (SEM) characterization and analysis of the carbon dots@calcium-aluminum hydrotalcite composite material prepared in Example 1 of the present invention, it can be seen that the carbon dots@calcium-aluminum hydrotalcite composite material prepared by the traditional method ( Figure 2 a and Figure 2 b) clearly still maintains the size and morphology of calcium-aluminum hydrotalcite and retains its layered structural characteristics. Small granular dot-like attachments can be seen on its surface, forming the carbon dots@calcium-aluminum hydrotalcite composite material. For the carbon dots@calcium-aluminum hydrotalcite composite material prepared by the in-situ growth method of the present invention ( Figure 2 c and Figure 2 d), it is observed that its surface shows non-uniformity and porosity, has significant roughness, and there are many tiny surface features. In addition, aggregates of spherical nanosheets are formed on the material surface, with a small amount of particles and textures attached to the surface. These features together indicate the formation of a flower cluster-like calcium-aluminum hydrotalcite structure. This is because under appropriate heat treatment conditions (usually between 200 - 400 °C), the hydrotalcite will undergo a thermal decomposition and reconstruction process, resulting in the volatilization of intercalated anions and redox reactions of metal cations, ultimately leading to the self-assembly of nanoparticles or nanosheets into a flower cluster-like structure.

[0090] The XRD pattern of the carbon dots@calcium-aluminum hydrotalcite composite material prepared in Example 1 of the present invention is as shown in Figure 3 shown. In Figure 3 , the characteristic peaks of d(003) and (006) of calcium-aluminum hydrotalcite are still visible, indicating that its metal laminate structure is maintained.

[0091] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a carbon dot@calcium aluminum hydrotalcite composite material suitable for industrial production, characterized in that: The preparation method comprises the following steps: Step 1: Preparation of calcium aluminum hydrotalcite Step 2: Preparation of carbon dots@calcium aluminum hydrotalcite composites (1) placing the calcium aluminum hydrotalcite prepared in step 1 in a polytetrafluoroethylene liner in a reaction kettle, using a mixture of citric acid and sodium citrate as a carbon dot precursor, stirring the above substances evenly, and adding an appropriate amount of water to prepare a mixed solution; (2) The reactor is placed in a muffle furnace for reaction, and after a certain period of time, it is taken out and naturally cooled to room temperature. The product is dried and ground into powder to obtain a carbon dot@calcium aluminum hydrotalcite composite material.

2. The method for preparing the carbon dot@calcium aluminum hydrotalcite composite material suitable for industrial production according to claim 1, characterized in that: In the step 2, the mass ratio of calcium aluminum hydrotalcite to citrate is 1:3.0-5.5, providing a carbon dot precursor for the system.

3. The method for preparing the carbon dot@calcium aluminum hydrotalcite composite material suitable for industrial production according to claim 1, characterized in that: In the step 2, the amount of water added is such that the liquid-to-solid ratio is 0.1-0.

5.

4. The method for preparing the carbon dot@calcium aluminum hydrotalcite composite material suitable for industrial production according to claim 1, characterized in that: In the step 2, the reaction temperature is 250-300°C.

5. The method for preparing the carbon dot@calcium aluminum hydrotalcite composite material suitable for industrial production according to claim 1, characterized in that: In the step 2, the reaction time is 8-12 h.

6. The method for preparing the carbon dot@calcium aluminum hydrotalcite composite material suitable for industrial production according to claim 1, characterized in that: The preparation process of calcium aluminum hydrotalcite in step 1 is as follows: (1) Weigh Ca(NO3)2·4H2O and Al(NO3)3·9H2O to prepare a mixed solution; (2) Prepare 1.0-2.5 mol / L sodium hydroxide or calcium hydroxide solution; (3) While stirring the sodium hydroxide or calcium hydroxide solution, add the mixed solution dropwise to maintain the pH of the system at 8-12; (4) reacting the obtained mixture at 50-100° C. for 4-10 h; (5) Centrifuge the resulting solution for 10-30 min, discard the supernatant, and wash the resulting precipitate several times with deionized water and ethanol until it is neutral; (6) Drying in an oven at 60-120°C for 6-24 h to obtain hydrotalcite.

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