A method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent

Abstract

The application relates to a preparation method of a nitrogen-doped calcium nanoporous carbon adsorbent, and belongs to the field of water treatment phosphorus adsorbent materials. The application aims at solving the problem that existing nitrogen doping cannot directly affect the active site Ca, thereby limiting the effective improvement of the phosphorus adsorption capacity of the calcium-based nanoporous carbon adsorbent. The preparation method comprises the following steps: firstly, calcium hydroxide and alpha-methyl acrylic acid solution are added into deionized water and stirred; secondly, the filtrate is mixed with anhydrous ethanol, and then the mixture is left to stand at room temperature; thirdly, calcium-alpha-methyl acrylic acid and urea are blended and ground, and then ethanol solution is added until the mixture is immersed, and the mixture is left to stand at room temperature; and fourthly, pyrolysis is carried out. The application is used for the preparation of the nitrogen-doped calcium nanoporous carbon adsorbent.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of phosphorus adsorbent materials for water treatment. BACKGROUND

[0002] Phosphorus (P) is an important element in the biosphere of the earth, which is widely used in manufacturing industry and agricultural production to meet the needs of human beings. However, the large-scale unregulated discharge of this phosphorus-containing wastewater will lead to the abnormal growth of algae, bacteria and aquatic plants, resulting in water eutrophication, mass death of fish and possibly causing the collapse of the ecosystem. On the other hand, phosphorus is a non-renewable resource. Therefore, as a non-renewable resource that is crucial to the biosphere cycle, it is of great practical significance to reduce the phosphorus concentration in water bodies and phosphorus recovery.

[0003] At present, adsorption method is an environmentally friendly phosphorus removal technology used in advanced treatment process to remove residual phosphate. Calcium (Ca) as a natural mineral of hydrosphere and lithosphere, has high affinity with P (CaP, Ksp: 2.07 x 10 -29 ) and environmental friendly characteristics, and is a very promising adsorbent for phosphate removal. In addition, among numerous adsorbent carriers, nanoporous carbon (NPC) is considered to be one of the most promising candidates for solid-liquid mass transfer and dispersed metals due to its adjustable pore size, strong loading capacity and large pore volume. However, the preparation process of this composite adsorbent often makes the composite adsorbent exhibit low intrinsic activity due to the redistribution of micro-interface electron cloud and the embedding of more stable atomic orbitals, which further leads to limited improvement of adsorption effect. At present, nitrogen doping is considered to be an effective strategy to improve the physical and chemical properties of the material. The addition of nitrogen can increase the active components, including graphitic nitrogen, pyridine nitrogen and pyrrole nitrogen, to adjust the charge distribution on the surface of the carbon matrix and thus improve the electrostatic attraction between the carbon matrix and phosphorus. However, the current problem is that nitrogen doping cannot directly affect the active site Ca, which limits the further improvement of the phosphorus adsorption capacity of calcium-based nanoporous carbon adsorbent. SUMMARY

[0004] The present application aims to solve the problem that existing nitrogen doping cannot directly affect the active site Ca, which limits the effective improvement of the phosphorus adsorption capacity of calcium-based nanoporous carbon adsorbent, and further provides a preparation method of nitrogen-doped calcium nanoporous carbon adsorbent.

[0005] A preparation method of nitrogen-doped calcium nanoporous carbon adsorbent, which is carried out according to the following steps:

[0006] I. Calcium hydroxide and α-methyl acrylic acid solution are added to deionized water and stirred, then solid-liquid separation is carried out to obtain a filtrate;

[0007] II. The filtrate is mixed with anhydrous ethanol, then left to stand at room temperature, and white nanorod clusters are collected by centrifugation, and then vacuum constant temperature drying is carried out to obtain calcium-α-methyl acrylic acid;

[0008] 3. Blending calcium-α-methacrylic acid with urea and grinding to obtain a mixture, then adding ethanol solution until the mixture is submerged, standing at room temperature, and finally drying under vacuum constant temperature to obtain nitrogen-doped calcium-α-methacrylic acid;

[0009] 4. Under nitrogen protection, the nitrogen-doped calcium-α-methacrylic acid is pyrolyzed, and after cooling to room temperature, the product is washed and ground to obtain a nitrogen-doped calcium nanoporous carbon adsorbent.

[0010] The beneficial effects of the present invention are:

[0011] 1. The present invention utilizes self-assembly to ensure uniform distribution of Ca and N on the support. Directed nitrogen doping through secondary self-assembly effectively alters the surface microscopic electronic structure of the composite adsorbent, providing more abundant and effective active sites for phosphorus adsorption. Subsequently, through direct pyrolysis of the self-assembled precursor, calcite is encapsulated within an in situ nitrogen-doped 3D nanoporous carbon framework, forming a stable porous structure that facilitates the capture of phosphorus from solution.

[0012] 2. The equilibrium adsorption capacity of the Ca / N-nanoporous carbon-phosphorus adsorbent prepared by the method of the present invention is as high as 169.06 mg / g, which is 1.5 times that before N doping.

[0013] Figures in the specification

[0014] Figure 1 Graph showing phosphorus adsorption removal rates of adsorbents prepared in Examples 1 to 3 and Comparative Examples 1 to 4;

[0015] Figure 2 Graph showing the phosphorus adsorption capacity of the adsorbents prepared in Examples 1 to 3 and Comparative Example 4;

[0016] Figure 3 Scanning electron microscope (SEM) characterization images of the adsorbents prepared in Examples 1 to 3 and Comparative Example 4;

[0017] Figure 4 Element distribution diagram of the Ca / N-NPC (1:2) adsorbent prepared in Example 2;

[0018] Figure 5 N1s spectrum of the Ca / N-NPC (1:2) adsorbent surface prepared in Example 2 characterized by X-ray photoelectron spectroscopy (XPS);

[0019] Figure 6 The XRD spectra of the adsorbents prepared in Examples 1 to 3 and Comparative Example 4 are shown. DETAILED DESCRIPTION

[0020] Specific embodiment one: the preparation method of the nitrogen-doped calcium nanoporous carbon adsorbent of the embodiment, which is carried out according to the following steps:

[0021] I. Calcium hydroxide and α-methyl acrylic acid solution are added to deionized water and stirred, and then solid-liquid separation is performed to obtain a filtrate;

[0022] II. The filtrate is mixed with anhydrous ethanol, then left to stand at room temperature, and white nanorod clusters are collected by centrifugation, and then constant-temperature drying under vacuum to obtain calcium-α-methyl acrylic acid;

[0023] III. The calcium-α-methyl acrylic acid is blended and ground with urea to obtain a mixture, then ethanol solution is added until the mixture is immersed, left to stand at room temperature, and finally constant-temperature drying under vacuum to obtain nitrogen-doped calcium-α-methyl acrylic acid;

[0024] IV. The nitrogen-doped calcium-α-methyl acrylic acid is pyrolyzed under nitrogen protection, and after cooling to room temperature, the product is washed and ground to obtain a nitrogen-doped calcium nanoporous carbon adsorbent.

[0025] In the second step of the embodiment, the first nanoscale self-assembly is carried out at room temperature, and in the third step, the second nanoscale self-assembly is carried out at room temperature. By using the two-step nanoscale self-assembly method, the catalyst surface structure is reconfigured by forming aperiodic arrangement of the composite catalyst structure and the foreign atom N, which provides more active Ca / N active sites for the target reaction, enhances the intrinsic activity of the composite adsorbent, and promotes the double improvement of the phosphorus adsorption performance of the adsorbent. Based on this, the N-doped adsorbent precursor is obtained by two-step directional self-assembly, and then carbonized to prepare N-doped Ca / N-NPC, so as to effectively improve the phosphorus adsorption effect.

[0026] The embodiment has the following beneficial effects:

[0027] 1. The embodiment uses the self-assembly method to ensure the uniform distribution of Ca and N on the carrier. By using the two-step self-assembly directional N-doping method, the microelectronic structure of the surface of the composite adsorbent is effectively changed, which provides more active active sites for the adsorption of phosphorus. Then, the precursor material formed by direct pyrolysis self-assembly is encapsulated in the 3D nanoporous carbon framework with in-situ nitrogen doping, forming a stable porous structure, which is beneficial to the capture of phosphorus in the solution.

[0028] 2. The equilibrium adsorption capacity of the Ca / N-nanoporous carbon phosphorus adsorbent prepared by the method of the embodiment is as high as 169.06 mg / g, which is 1.5 times that before N doping.

[0029] Specific embodiment 2: This embodiment differs from specific embodiment 1 in that the molar ratio of calcium hydroxide to α-methacrylic acid in the α-methacrylic acid solution in step 1 is (0.3-0.7):1. Other steps are the same as those in specific embodiment 1.

[0030] Specific embodiment 3: This embodiment differs from either specific embodiment 1 or 2 in that the mass percentage of the α-methacrylic acid solution in step 1 is 98% to 99.5%; and the volume ratio of the α-methacrylic acid solution to deionized water in step 1 is 1:(8-10). Other aspects are the same as specific embodiments 1 or 2.

[0031] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that in step 1, calcium hydroxide and α-methacrylic acid solution are added to deionized water and stirred for 6 to 10 hours at a stirring speed of 120 to 180 rpm. Other aspects are the same as specific embodiments 1 to 3.

[0032] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that the volume ratio of the filtrate to anhydrous ethanol in step 2 is 1:(4-7). Other aspects are the same as specific embodiments 1 to 4.

[0033] Specific embodiment 6: This embodiment differs from Specific embodiments 1 to 5 in that: in step 2, the sample is allowed to stand at room temperature for 16 to 20 hours; the centrifugal collection in step 2 is performed at a centrifugal speed of 6000 to 8000 rpm for 5 to 10 minutes; and in step 2, the sample is dried under vacuum at a constant temperature of 40°C to 60°C for 30 to 40 hours. Other aspects are the same as Specific embodiments 1 to 5.

[0034] Specific embodiment 7: This embodiment differs from Specific embodiments 1 to 6 in that the mass ratio of calcium-α-methacrylic acid to urea in step 3 is 1:(1-3). Other aspects are the same as Specific embodiments 1 to 6.

[0035] Specific embodiment 8: This embodiment differs from Specific embodiments 1 to 7 in that the mass percentage of the ethanol solution in step 3 is 50% to 75%; the mass ratio of the mixture in step 3 to the ethanol solution is 1:(0.8 to 1). Other aspects are the same as Specific embodiments 1 to 7.

[0036] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that: in step 3, the mixture is allowed to stand at room temperature for 1 to 4 hours; in step 3, the mixture is dried at a constant temperature of 60°C to 90°C under vacuum for 10 to 16 hours. Other differences are the same as specific embodiments 1 to 8.

[0037] Specific embodiment 10: This embodiment differs from any one of specific embodiments 1 to 9 in that the pyrolysis described in step 4 is to heat the nitrogen-doped calcium-α-methacrylic acid to 500°C to 700°C at a heating rate of 3°C / min to 5°C / min under an N2 atmosphere, and then maintain the temperature at 500°C to 700°C for 1.5 to 3 hours; and after grinding in step 4, the mixture is sieved through an 80-100 mesh screen. Other aspects are the same as specific embodiments 1 to 9.

[0038] The following examples are used to verify the beneficial effects of the present invention:

[0039] Example 1:

[0040] A method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent is carried out according to the following steps:

[0041] 1. Add calcium hydroxide and α-methacrylic acid solution to deionized water at a stirring speed of 150 rpm and stir for 8 hours, then separate the solid and liquid to obtain a filtrate;

[0042] The molar ratio of calcium hydroxide to α-methacrylic acid in the α-methacrylic acid solution is 0.5:1; the mass percentage of the α-methacrylic acid solution is 99%; and the volume ratio of the α-methacrylic acid solution to deionized water is 1:9;

[0043] Second, the filtrate was mixed with anhydrous ethanol, and then allowed to stand at room temperature for 18 hours. The white nanorod-like clusters were collected by centrifugation and dried at a constant temperature under vacuum to obtain calcium-α-methylacrylate, i.e., Ca(MAA)2;

[0044] The volume ratio of the filtrate to anhydrous ethanol is 1:6;

[0045] 3. Calcium-α-methacrylate and urea are blended and ground to obtain a mixture, and then an ethanol solution is added until the mixture is just submerged, and the mixture is allowed to stand at room temperature for 2 hours, and finally vacuum-dried at a constant temperature to obtain nitrogen-doped calcium-α-methacrylate, namely Ca / N(MAA)2;

[0046] The mass ratio of calcium-α-methacrylic acid to urea is 1:1; the mass percentage of the ethanol solution is 75%; the mass ratio of the mixture to the ethanol solution is 1:1;

[0047] 4. Under N2 atmosphere, nitrogen-doped calcium-α-methacrylate was heated to 600℃ at a heating rate of 4℃ / min, and then maintained at 600℃ for 2h. After cooling to room temperature, the product was washed and ground, and passed through an 80-mesh sieve after grinding to obtain nitrogen-doped calcium nanoporous carbon adsorbent, namely Ca / N-NPC (1:1).

[0048] The filtrate in step 1 was stored at low temperature (5°C) until use.

[0049] The centrifugal collection in step 2 is specifically carried out at a centrifugal speed of 8000 rpm for 5 minutes; in step 2, the vacuum constant temperature drying is carried out at a temperature of 50° C. for 36 hours.

[0050] In step 3, the product was dried under vacuum at 80° C. for 12 h.

[0051] Example 2: This example differs from Example 1 in that the mass ratio of calcium-α-methacrylate to urea in step 3 is 1:2, and step 4 yields Ca / N-NPC (1:2). Other steps are the same as in Example 1.

[0052] Example 3: This example differs from Example 1 in that the mass ratio of calcium-α-methacrylate to urea in step 3 is 1:3, and step 4 yields Ca / N-NPC (1:3). Other steps are the same as in Example 1.

[0053] Comparative Example 1: Under N2 atmosphere, the calcium-α-methacrylic acid prepared in step 2 of Example 1 was heated to 600°C at a heating rate of 4°C / min, and then maintained at 600°C for 2 hours, cooled to room temperature, and then blended with urea and ground to obtain a mixture; the mass ratio of calcium-α-methacrylic acid to urea was 1:1; and under N2 atmosphere, the mixture was heated to 600°C at a heating rate of 4°C / min, and then maintained at 600°C for 2 hours. After cooling to room temperature, the product was washed and ground, and after grinding, passed through an 80-mesh sieve to obtain Ca / N-NPC(1:1)-b.

[0054] Comparative Example 2: Under N2 atmosphere, the calcium-α-methacrylic acid prepared in step 2 of Example 1 was heated to 600°C at a heating rate of 4°C / min, and then maintained at 600°C for 2 hours, cooled to room temperature, and then blended with urea and ground to obtain a mixture; the mass ratio of calcium-α-methacrylic acid to urea was 1:2; and under N2 atmosphere, the mixture was heated to 600°C at a heating rate of 4°C / min, and then maintained at 600°C for 2 hours. After cooling to room temperature, the product was washed and ground, and after grinding, passed through an 80-mesh sieve to obtain Ca / N-NPC(1:2)-b.

[0055] Comparative Example 3: Under N2 atmosphere, the calcium-α-methacrylic acid prepared in step 2 of Example 1 was heated to 600°C at a heating rate of 4°C / min, and then maintained at 600°C for 2 hours, cooled to room temperature, and then blended with urea and ground to obtain a mixture; the mass ratio of calcium-α-methacrylic acid to urea was 1:3; and under N2 atmosphere, the mixture was heated to 600°C at a heating rate of 4°C / min, and then maintained at 600°C for 2 hours. After cooling to room temperature, the product was washed and ground, and after grinding, passed through an 80-mesh sieve to obtain Ca / N-NPC(1:3)-b.

[0056] Comparative Example 4: The calcium-α-methacrylic acid prepared in step 2 of Example 1 was vacuum dried at a constant temperature of 80°C for 12 hours, then heated to 600°C at a heating rate of 4°C / min under a N2 atmosphere, and then maintained at 600°C for 2 hours. After cooling to room temperature, the product was washed and ground, and passed through an 80-mesh sieve after grinding to obtain Ca / N-NPC.

[0057] (1) conducting a phosphorus removal test on the adsorbents prepared in Examples 1 to 3 and Comparative Examples 1 to 4;

[0058] Experimental Methods: 100 mL of 50 m / L phosphate solution was placed in a 250 mL Erlenmeyer flask. The pH was adjusted to 7.2 ± 0.3 using 1 mol / L sodium hydroxide and 1 mol / L dilute sulfuric acid. Subsequently, 0.04 g of adsorbent was added to each flask, shaken, sealed with a rubber stopper, and placed in a 25°C water bath shaker at 150 rpm for adsorption. Sampling intervals were set at 10, 20, 30, 40, 60, 90, 120, 150, 240, 360, 480, 600, 720, 840, and 960 min. 1 mL of the solution was drawn up using a 2.5 mL syringe and filtered through a 0.45 μM polyethersulfone membrane (PES, Jin Teng). Phosphate concentration was then determined spectrophotometrically using ammonium molybdate at a wavelength of 700 nm.

[0059] Figure 1 Figure 2 shows the phosphorus adsorption and removal rates of the adsorbents prepared in Examples 1-3 and Comparative Examples 1-4. The figure shows that the Ca / N-NPC (1:1, 1:2, and 1:3) prepared in the examples reached adsorption equilibrium at 960 minutes, achieving phosphorus removal rates of 74.60%, 90.24%, and 92.80%, respectively. Comparative Example Ca / N-NPC (1:1, 1:2, and 1:3)-b achieved phosphorus removal rates of only 63.00%, 79.51%, and 83.60%, respectively. Compared to the control group, the N-doped phosphorus adsorbent prepared using secondary self-assembly exhibited higher phosphorus adsorption efficiency.

[0060] (2) The phosphorus adsorption capacity of the adsorbents prepared in Examples 1 to 3 and Comparative Example 4 was tested to investigate the effect of N doping on the adsorption behavior and mechanism of Ca / N-NPC.

[0061] Experimental Methods: 100 mL of a 150 m / L phosphate solution was placed in a 250 mL Erlenmeyer flask. The pH was adjusted to 7.2 ± 0.3 using 1 mol / L sodium hydroxide and 1 mol / L dilute sulfuric acid. Subsequently, 0.04 g of adsorbent was added to each flask, shaken, sealed with a rubber stopper, and placed in a 25°C water bath shaker at 150 rpm for adsorption. Sampling intervals were set at 10, 20, 30, 40, 60, 90, 120, 150, 240, 360, 480, 600, 720, 840, and 960 min. 1 mL of the solution was drawn up using a 2.5 mL syringe and filtered through a 0.45 μM polyethersulfone membrane (PES, Jin Teng). Phosphate concentration was then determined spectrophotometrically using ammonium molybdate at a wavelength of 700 nm.

[0062] Data processing method: The pseudo-first-order kinetics and pseudo-second-order kinetics equations in Origin Pro 2021 were used to fit the adsorption results. Figure 2 ( Figure 2 The phosphorus adsorption capacity of the adsorbents prepared in Examples 1 to 3 and Comparative Example 4 is shown in Figure 1 and Table 1, and the effect of N doping on the adsorption mechanism is explored. Compared with the pseudo-first-order model, the pseudo-second-order model provides a more accurate adsorption fitting curve, which shows that the adsorption between the three adsorbents and phosphate after N doping is mainly based on chemical adsorption. It is worth noting that with the increase of the amount of N doping, chemical adsorption gradually dominates the surface of Ca / N-NPC. This shows that the addition of N changes the coordination environment between the active center and phosphate on the Ca / N-NPC surface, making it easier for electron transfer or sharing to occur between phosphate and the active site to form a more stable phosphorus complex, thereby promoting the adsorption and accumulation of excess phosphate. In addition, the equilibrium adsorption amounts of phosphate for Ca-NPC and Ca / N-NPC (1:3) are 96.36 mg / g and 169.06 mg / g, respectively, indicating that the adsorption efficiency of phosphate is increased by 1.5 times after N doping.

[0063] Table 1. Kinetic fitting parameters of four adsorbents

[0064]

[0065] (3) The adsorbents prepared in Examples 1 to 3 and Comparative Example 4 were characterized by microscopic morphology.

[0066] Figure 3Scanning electron microscopy (SEM) characterization images of the adsorbents prepared in Examples 1 to 3 and Comparative Example 4; as shown in the figure, compared with Ca-NPC, many structural mismatches and distortions were observed on Ca / N-NC (1:1, 1:2 and 1:3), indicating that obvious defects were generated, which is conducive to the uniform distribution and exposure of Ca.

[0067] Figure 4 This is the element distribution diagram of the Ca / N-NPC (1:2) adsorbent prepared in Example 2; it can be seen from the figure that N and Ca are successfully loaded into the shell skeleton structure composed of C after calcination.

[0068] The adsorbents prepared in Examples 1 to 3 and Comparative Example 4 were subjected to BET testing to analyze the surface area and pore size distribution. As shown in Table 2, the pore size distribution is mainly concentrated in the range of <2 nm and 8-14 nm, with a micropore-mesopore hierarchical system. Compared to Ca-NPC, when the N doping ratio reaches 1:3, the specific surface area and pore volume of the porous carbon increase by 20 and 10 times, respectively. These defects generated within the porous carbon lead to a three-dimensional porous structure with a larger capacity, and the ends are open to the external environment, promoting the formation of 3D pores within the structure, thereby facilitating the diffusion and adsorption of phosphate on the Ca / N-NPC surface.

[0069] Table 2. BET test results of 4 adsorbents

[0070]

[0071] (iv) Studying the N spectra of the surface doped Ca / N-NPC (1:2) adsorbent prepared in Example 2;

[0072] Figure 5 This is the N1s spectrum of the Ca / N-NPC (1:2) adsorbent prepared in Example 2, characterized by X-ray photoelectron spectroscopy (XPS). As shown in the figure, the N1s spectrum reveals the presence of four configurations of nitrogen, including pyridinic-N (398.5±0.2eV), pyrrolic-N (399.99±0.2eV), graphitic-N (400.9±0.2eVs), and nitrogen oxide (404.2±0.2eV.). This indicates that nitrogen doping increases the active components, including graphitic-N, pyridinic-N, and pyrrolic-N, to change the physical and chemical properties of the composite catalyst surface, which is effective in improving the adsorption efficiency of P.

[0073] (5) Study the XRD spectra of the adsorbents prepared in Examples 1 to 3 and Comparative Example 4;

[0074] Figure 6XRD spectra of the adsorbents prepared in Examples 1 to 3 and Comparative Example 4; As shown in the figure, the main peaks of the four samples are consistent with the corresponding crystal planes of calcite. Combined with the distribution of Ca and C elements in the SEM-EDS element spectrum, it shows that the calcite is encapsulated in the 3D nanoporous carbon framework after pyrolysis. In addition, as the N doping amount in Ca-NPC continues to increase, the overall peak shape of calcite shifts slightly to a small angle. Based on the Bragg law, it shows that the larger the in-situ radius N Replaces O in the lattice This results in an increase in the original lattice spacing, thus affecting the crystal structure of calcite. In other words, an appropriate amount of nitrogen doping will increase the unit cell volume of calcite, making it easier to combine with phosphate. In addition, nitrogen doping has a new peak at 22.5°, indicating the presence of sp2 hybridized carbon defects in the material, which is conducive to the exposure of active sites Ca.

Claims

1. A method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent, characterized in that It is carried out in the following steps:

1. adding calcium hydroxide and α-methacrylic acid solution into deionized water and stirring, and then separating the solid and liquid to obtain a filtrate; 2. Mixing the filtrate with anhydrous ethanol, then standing at room temperature, collecting white nanorod-like clusters by centrifugation, and drying them at a constant temperature under vacuum to obtain calcium-α-methacrylic acid; 3. Blending calcium-α-methacrylic acid with urea and grinding to obtain a mixture, then adding ethanol solution until the mixture is submerged, standing at room temperature, and finally drying under vacuum constant temperature to obtain nitrogen-doped calcium-α-methacrylic acid; 4. Under nitrogen protection, the nitrogen-doped calcium-α-methacrylic acid is pyrolyzed, and after cooling to room temperature, the product is washed and ground to obtain a nitrogen-doped calcium nanoporous carbon adsorbent.

2. The method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent according to claim 1, characterized in that The molar ratio of the calcium hydroxide in step 1 to the α-methacrylic acid in the α-methacrylic acid solution is (0.3-0.7):

1.

3. The method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent according to claim 1, characterized in that The mass percentage of the α-methacrylic acid solution in step 1 is 98% to 99.5%; the volume ratio of the α-methacrylic acid solution in step 1 to deionized water is 1:(8 to 10).

4. The method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent according to claim 1, characterized in that In step 1, calcium hydroxide and α-methacrylic acid solution are added to deionized water and stirred for 6 h to 10 h at a stirring speed of 120 rpm to 180 rpm.

5. The method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent according to claim 1, characterized in that The volume ratio of the filtrate to anhydrous ethanol in step 2 is 1:(4-7).

6. The method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent according to claim 1, characterized in that In step 2, the mixture is allowed to stand at room temperature for 16 to 20 hours; the centrifugal collection in step 2 is specifically performed at a centrifugal speed of 6000 to 8000 rpm for 5 to 10 minutes; in step 2, the mixture is dried at a constant temperature in a vacuum chamber for 30 to 40 hours at a temperature of 40°C to 60°C.

7. The method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent according to claim 1, characterized in that The mass ratio of calcium-α-methacrylic acid to urea in step 3 is 1:(1-3).

8. The method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent according to claim 1, characterized in that The mass percentage of the ethanol solution in step 3 is 50% to 75%; the mass ratio of the mixture in step 3 to the ethanol solution is 1:(0.8 to 1).

9. The method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent according to claim 1, characterized in that In step 3, the mixture is allowed to stand at room temperature for 1 hour to 4 hours; in step 3, the mixture is dried under vacuum constant temperature at a temperature of 60° C. to 90° C. for 10 hours to 16 hours.

10. The method for preparing a nitrogen-doped calcium nanoporous carbon adsorbent according to claim 1, characterized in that The pyrolysis described in step 4 is specifically to heat the nitrogen-doped calcium-α-methacrylic acid to 500°C to 700°C at a heating rate of 3°C / min to 5°C / min under N2 atmosphere, and then maintain the temperature at 500°C to 700°C for 1.5h to 3h; after grinding in step 4, pass through an 80-mesh to 100-mesh sieve.

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