CN112646069B - A kind of carbon nitride composite hydrogel and its preparation method and application - Google Patents

Abstract

The invention provides a carbon nitride composite hydrogel and a preparation method and application thereof, belonging to the technical field of composite materials. The method comprises the steps of calcining urea to obtain carbon nitride powder; mixing the carbon nitride powder, concentrated nitric acid and concentrated sulfuric acid, stripping, adjusting the pH of the obtained stripping system to be neutral, standing, and taking the supernatant as carbon nitride dispersion liquid; mixing the carbon nitride dispersion liquid with a polymer monomer, and carrying out xenon lamp irradiation to obtain a hydrogel precursor; and soaking the hydrogel precursor into an ethanol water solution to obtain the carbon nitride composite hydrogel. Under the condition of illumination, the carbon nitride dispersion liquid induces polymer monomers to generate free radical polymerization so as to form hydrogel; meanwhile, the obtained hydrogel precursor is soaked in an ethanol aqueous solution, so that the mechanical property of the carbon nitride composite hydrogel is improved. Meanwhile, the carbon nitride composite hydrogel prepared by the invention can be used for treating Al in water ³⁺ Has good detection effect, and the detection limit is 0.26 mu M.

Description

A kind of carbon nitride composite hydrogel and its preparation method and application

technical field

The invention relates to the technical field of composite materials, in particular to a carbon nitride composite hydrogel and a preparation method and application thereof.

Background technique

A hydrogel is a "soft and wet" polymer with a three-dimensional network structure, formed from hydrophilic homopolymers or copolymers. Since hydrogels have both solid and liquid properties, they have been used in many fields. However, traditional chemically cross-linked hydrogels have poor mechanical strength and narrow application scope due to the uneven distribution of cross-linking points and the lack of an effective energy dissipation mechanism.

SUMMARY OF THE INVENTION

In view of this, the purpose of the present invention is to provide a carbon nitride composite hydrogel and a preparation method and application thereof. The carbon nitride composite hydrogel provided by the present invention has excellent mechanical properties.

In order to achieve the above-mentioned purpose of the invention, the present invention provides the following technical solutions:

The invention provides a preparation method of carbon nitride composite hydrogel, comprising the following steps:

calcining urea to obtain carbon nitride powder;

Mixing the carbon nitride powder, concentrated nitric acid and concentrated sulfuric acid for stripping, adjusting the pH of the obtained stripping system to be neutral, after standing, taking the supernatant as the carbon nitride dispersion;

Mixing the carbon nitride dispersion with the polymer monomer and irradiating with a xenon lamp to obtain a hydrogel precursor;

The hydrogel precursor is soaked in an aqueous ethanol solution to obtain the carbon nitride composite hydrogel.

Preferably, the concentration of the concentrated nitric acid is 10-16 mol/L, the concentration of the concentrated sulfuric acid is 10.4-18.4 mol/L, and the dosage ratio of the carbon nitride powder, the concentrated nitric acid and the concentrated sulfuric acid is 0.1-2.0 g: 5 to 15 mL: 5 to 15 mL.

Preferably, the concentration of the carbon nitride dispersion liquid is 0.02-0.15 mg/mL.

Preferably, the mass ratio of carbon nitride to polymer monomer in the carbon nitride dispersion liquid is (1.5-6.0):10000.

Preferably, the mass percentage content of the polymer monomer in the mixed solution obtained by mixing the carbon nitride dispersion liquid with the polymer monomer is 10-35%.

Preferably, the polymer monomer is acrylamide, N-isopropylacrylamide, acrylic acid, vinyl acetate, methyl acrylate, methyl methacrylate, 2-(dimethylamino)ethyl methacrylic acid One or more of ester, 2-hydroxyethyl methacrylate, n-butyl acrylate, styrene and 4-vinylpyridine.

Preferably, the irradiation time of the xenon lamp is 2.5-4.5 hours.

Preferably, the volume concentration of the ethanol aqueous solution is 10-90%.

The present invention also provides the carbon nitride composite hydrogel obtained by the preparation method described in the above technical solution, wherein the solid content of the carbon nitride composite hydrogel is 1-10%.

The present invention also provides the application of the carbon nitride composite hydrogel described in the above technical solution in detecting Al ³⁺ in water.

The invention provides a preparation method of carbon nitride composite hydrogel, which comprises the following steps: calcining urea to obtain carbon nitride powder; mixing the carbon nitride powder, concentrated nitric acid and concentrated sulfuric acid to peel off, The pH of the obtained peeling system is adjusted to be neutral, and after standing, the supernatant is taken as a carbon nitride dispersion; the carbon nitride dispersion is mixed with a polymer monomer, and irradiated with a xenon lamp to obtain a hydrogel precursor ; Immerse the hydrogel precursor in an aqueous ethanol solution to obtain the carbon nitride composite hydrogel. Under the condition of xenon lamp irradiation, the carbon nitride dispersion liquid of the present invention induces free radical polymerization of polymer monomers to form hydrogels; meanwhile, the obtained hydrogel precursors are infiltrated in ethanol aqueous solution, so that the carbon nitride composite is improved. Tensile properties of hydrogels. At the same time, the carbon nitride composite hydrogel obtained by the invention has a good detection effect on Al ³⁺ in water, and the detection limit is 0.26 μM.

Description of drawings

Fig. 1 is the tensile property test chart of the carbon nitride composite hydrogel obtained in Example 2;

Fig. 2 is the tensile property test chart of the carbon nitride composite hydrogel obtained in Comparative Example 1;

Fig. 3 is the rheological frequency scanning test chart of the carbon nitride composite hydrogel obtained in Example 2 and Comparative Example 1;

Fig. 4 is the rheological constant frequency strain sweep test chart of the carbon nitride composite hydrogels obtained in Example 2 and Comparative Example 1;

Fig. 5 is the fluorescence spectrogram of the carbon nitride composite hydrogel obtained in Example 2 to the detection of different concentrations of ^Al in the aqueous solution;

Fig. 6 is the linear regression curve diagram of the carbon nitride composite hydrogel obtained in Example 2 to the detection of different concentrations of ^Al in the aqueous solution;

Fig. 7 is the fluorescence spectrum of the carbonitride-acrylic acid composite hydrogel obtained in Example 3 to the detection of Al ^{3 +} with a concentration of 5.00×10 ^-5 M in the aqueous solution;

FIG. 8 is a fluorescence spectrum of the carbon nitride N-isopropylacrylamide composite hydrogel obtained in Example 4 for detection of Al ³⁺ with a concentration of 5.00×10 ⁻⁵ M in an aqueous solution.

Detailed ways

The invention provides a preparation method of carbon nitride hydrogel, comprising the following steps:

calcining urea to obtain carbon nitride powder;

Mixing the carbon nitride powder, concentrated nitric acid and concentrated sulfuric acid for stripping, adjusting the pH of the obtained stripping system to be neutral, after standing, taking the supernatant as the carbon nitride dispersion;

Mixing the carbon nitride dispersion with the polymer monomer and irradiating with a xenon lamp to obtain a hydrogel precursor;

The hydrogel precursor is soaked in an aqueous ethanol solution to obtain the carbon nitride composite hydrogel.

In the present invention, urea is calcined to obtain carbon nitride powder. In the present invention, the calcination temperature is preferably 400-600°C, more preferably 550°C; the time is preferably 0.5-4.0h, more preferably 2h.

After the carbon nitride powder is obtained, the present invention mixes the carbon nitride powder, concentrated nitric acid and concentrated sulfuric acid for exfoliation, and adjusts the pH of the obtained exfoliation system to be neutral. After standing, the supernatant is taken as the carbon nitride dispersion. liquid. In the present invention, the concentration of the concentrated nitric acid is preferably 10-16 mol/L, more preferably 14.8 mol/L, and the concentration of the concentrated sulfuric acid is preferably 10.4-18.4 mol/L, more preferably 18.4 mol/L; The dosage ratio of the carbon nitride powder, concentrated nitric acid and concentrated sulfuric acid is preferably 0.1-2.0 g: 5-15 mL: 5-15 mL, more preferably 1.0 g: 10 mL: 10 mL.

In the present invention, the peeling time is preferably 0.5 to 3 hours, more preferably 1 hour. The present invention does not specifically limit the reagent for adjusting the pH value of the obtained peeling system to be neutral. In the present invention, the standing time is preferably 12 to 48 hours, more preferably 24 hours. In the present invention, the concentration of the obtained carbon nitride dispersion is preferably 0.02 to 0.15 mg/mL, and more preferably 0.15 mg/mL.

After the carbon nitride dispersion liquid is obtained, the present invention mixes the carbon nitride dispersion liquid with a polymer monomer, and irradiates with a xenon lamp to obtain a hydrogel precursor. In the present invention, the polymer monomer is preferably acrylamide, N-isopropylacrylamide, acrylic acid, vinyl acetate, methyl acrylate, methyl methacrylate, 2-(dimethylamino)ethyl One or more of methacrylate, 2-hydroxyethyl methacrylate, n-butyl acrylate, styrene and 4-vinylpyridine, more preferably acrylamide, N-isopropylacrylamide or acrylic acid . In the present invention, the mass ratio of carbon nitride in the carbon nitride dispersion to the polymer monomer is preferably (1.5-6.0); 10000. In the present invention, the mixing of the carbon nitride dispersion liquid and the polymer monomer is preferably carried out under the conditions of stirring and flowing nitrogen. The present invention does not specifically limit the rotational speed of the stirring. The flow rate is not specifically limited, as long as the oxygen in the solution system can be removed. In the present invention, the mass percentage content of the polymer monomer in the mixed solution obtained by mixing the carbon nitride dispersion with the polymer monomer is preferably 10-35%, specifically 20%, 27% or 33% .

In the present invention, the irradiation time of the xenon lamp is preferably 2.5-4.5 hours. In the present invention, when the mixed solution of the carbon nitride dispersion and the polymer monomer is irradiated with a xenon lamp, the mixed solution is preferably placed in a glass tube or a mold and sealed, and then irradiated with a xenon lamp.

In the present invention, the carbon nitride can induce the polymer monomer to form a hydrogel under the irradiation of a xenon lamp.

After the hydrogel precursor is obtained, the present invention soaks the hydrogel precursor in an aqueous ethanol solution to obtain the carbon nitride composite hydrogel. In the present invention, the solid content of the hydrogel precursor is preferably 1-10%; the volume concentration of the ethanol aqueous solution is preferably 10-90%, more preferably 50%; The volume ratio of the precursor to the aqueous ethanol solution is not particularly limited, as long as the hydrogel precursor can be completely infiltrated with the aqueous ethanol solution. In the present invention, the soaking time is preferably 30-300 min, more preferably 60 min.

The present invention also provides the carbon nitride composite hydrogel obtained by the preparation method described in the above technical solution, wherein the solid content of the carbon nitride composite hydrogel is 1-10%. The carbon nitride composite hydrogel provided by the present invention has excellent mechanical properties.

The present invention also provides the application of the carbon nitride composite hydrogel described in the above technical solution in detecting Al ³⁺ in water. In the present invention, when the carbon nitride composite hydrogel is used to detect Al ³⁺ in water, it preferably includes the following steps:

Measure the fluorescence intensity I ₀ of the blank carbon nitride composite hydrogel;

The carbon nitride composite hydrogel is put into the liquid to be detected, and after soaking for 30 minutes, the composite hydrogel is taken out, and the fluorescence intensity I of the composite hydrogel is measured;

Based on the fluorescence ratio-concentration standard curve, the concentration of Al ³⁺ in the liquid to be detected is obtained.

The invention measures the fluorescence intensity I ₀ of blank carbon nitride composite hydrogel. In the present invention, the excitation wavelength when measuring the fluorescence intensity of the blank carbon nitride composite hydrogel is preferably 320 nm; the fluorescence intensity is preferably the intensity at the emission wavelength of 430 nm.

In the present invention, the carbon nitride composite hydrogel is put into the liquid to be detected, and after soaking for 30 minutes, the composite hydrogel is taken out, and the fluorescence intensity I of the composite hydrogel is measured. In the present invention, the parameters for measuring the fluorescence intensity of the composite hydrogel are preferably consistent with the above technical solutions, and are not repeated here.

After obtaining the fluorescence intensity I ₀ of the blank carbon nitride composite hydrogel and the fluorescence intensity I of the composite hydrogel, the present invention obtains the concentration of Al ³⁺ in the liquid to be detected based on the fluorescence ratio-concentration standard curve.

In the present invention, the fluorescence ratio-concentration standard curve is preferably obtained by a method comprising the following steps:

Measure the fluorescence spectrum I ₀ of blank carbon nitride composite hydrogel;

The Al ³⁺ aqueous solution of gradient concentration was prepared, and the carbon nitride composite hydrogel was added to the Al ³⁺ aqueous solution of gradient concentration respectively, and after soaking for 30 min, the composite hydrogel was taken out; the fluorescence intensity I of the composite hydrogel was measured. ₁ , I _{2 ......In} ;

Calculate the ratio of I ₁ , I ₂ . . . In and I _{0 respectively} as the fluorescence ratio, and perform linear fitting on the fluorescence ratio and Al ³⁺ concentration to obtain the fluorescence ratio-concentration standard curve.

In the present invention, the parameters for measuring the fluorescence spectrum I ₀ of the blank carbon nitride composite hydrogel are preferably consistent with the above technical solutions, and are not repeated here.

In the present invention, the parameters for measuring the fluorescence intensity of the composite hydrogel are preferably consistent with the above technical solutions, and are not repeated here.

The present invention does not specifically limit the method of the linear fitting, and a linear fitting method well known to those skilled in the art may be used.

In a specific embodiment of the present invention, the fluorescence ratio-concentration standard curve is preferably: y=0.1764x+1.2568; wherein, x is the concentration of Al ³⁺ , and y is the concentration of Al ³⁺ after and before the addition of Al ³⁺ The ratio I/I ₀ of the fluorescence emission intensity of the composite hydrogel.

The carbon nitride composite hydrogel provided by the present invention and its preparation method and application are described in detail below with reference to the examples, but they should not be construed as limiting the protection scope of the present invention.

Example 1

10g of urea was placed in a muffle furnace and calcined at 550°C for 2h to obtain carbon nitride powder;

Strip 1g of carbon nitride powder in a mixed solution of 10mL of concentrated sulfuric acid (concentration of 18.4mol/L) and 10mL of concentrated nitric acid (concentration of 14.8mol/L) for 1h, diluted to a neutral solution, let stand for 24h, and take the The clear liquid was used as a carbon nitride dispersion with a concentration of 0.15 mg/mL;

1.0g, 1.5g and 2.0g of acrylamide monomer were respectively added to 4mL of the above carbon nitride dispersion liquid, and in the obtained mixed solution, the concentrations of acrylamide monomer were 20wt%, 27wt% and 33wt% respectively, The concentration was 0.01%; nitrogen was passed to remove oxygen for 30 min; the mixed solution was injected into a glass test tube and sealed, and the sealed reaction solution was placed under a xenon lamp light source to react for 2.5 h to obtain a hydrogel precursor;

The hydrogel precursor was cut into a size of 15×5×2 mm, immersed in 30 mL of ethanol aqueous solution (volume concentration of 50%), and soaked for 60 min to obtain a carbon nitride composite hydrogel.

Example 2

10g of urea was placed in a muffle furnace and calcined at 550°C for 2h to obtain carbon nitride powder;

Strip 1g of carbon nitride powder in a mixed solution of 10mL of concentrated sulfuric acid (concentration of 18.4mol/L) and 10mL of concentrated nitric acid (concentration of 14.8mol/L) for 1h, diluted to a neutral solution, let stand for 24h, and take the The clear liquid was used as a carbon nitride dispersion with a concentration of 0.15 mg/mL;

1.0 g of acrylamide monomer was added to 4 mL of the above-mentioned carbon nitride dispersion liquid, and in the obtained mixed solution, the concentration of acrylamide monomer was 20 wt %, and the concentration of carbon nitride was 0.01 wt %; The mixed solution is injected into a glass test tube or mold and sealed, and the sealed reaction solution is placed under a xenon lamp light source to react for 2.5 hours to obtain a hydrogel precursor;

The hydrogel precursor was cut into a size of 15×5×2 mm, immersed in 30 mL of ethanol aqueous solution (volume concentration of 70%) in turn, and soaked for 60 min to obtain a carbon nitride composite hydrogel.

Comparative Example 1

The difference from Example 2 is that the ethanol aqueous solution was not infiltrated.

The carbon nitride composite hydrogels obtained in Example 2 and Comparative Example 1 were respectively subjected to tensile tests. Figure 1 is the tensile property test diagram of the carbon nitride composite hydrogels obtained in Example 2; Figure 2 is the results obtained in Comparative Example 1. Test chart of tensile properties of carbon nitride composite hydrogels. It can be seen from Figure 1 and Figure 2 that the carbon nitride composite hydrogel obtained in Example 2 can return to the initial state after releasing the tension; the carbon nitride composite hydrogel obtained in Comparative Example 1 cannot return to the initial state after releasing the tension state, indicating that the infiltration of ethanol aqueous solution can improve the tensile properties of carbon nitride composite hydrogels.

The carbon nitride composite hydrogels obtained in Example 2 and Comparative Example 1 were respectively subjected to a rheological frequency sweep test. FIG. 3 is a diagram of the rheological frequency sweep test of the carbon nitride composite hydrogels obtained in Example 2 and Comparative Example 1. It can be seen from Figure 3 that the ratio of the loss modulus (G") to the elastic modulus (G') of the carbon nitride composite hydrogel obtained in Example 2 is smaller than that of the carbon nitride composite hydrogel obtained in Comparative Example 1, indicating that : Infiltration of ethanol aqueous solution can improve the mechanical properties of carbon nitride composite hydrogels.

The carbon nitride composite hydrogels obtained in Example 2 and Comparative Example 1 were respectively subjected to a rheological constant-frequency strain sweep test. Figure 4 shows the rheological constant-frequency strain of the carbon nitride composite hydrogels obtained in Example 2 and Comparative Example 1. Scan the test chart. It can be seen from Figure 4 that the yield strain value of the carbon nitride composite hydrogel obtained in Example 2 (the value of the intersection of G' and G" corresponding to the X axis) is higher than that of the carbon nitride composite hydrogel obtained in Comparative Example 1, Description: The infiltration of ethanol aqueous solution can improve the mechanical properties of carbon nitride composite hydrogels.

The carbon nitride composite hydrogel obtained in the above Example 2 was placed in 20 mL of an aqueous solution with an Al ³⁺ concentration of 0.00 × 10 ^-5 to 9.00 × 10 ^-5 M, and after standing for 30 minutes, the carbon nitride composite hydrogel was taken out. The gel was placed on a fluorescence spectrometer for fluorescence spectrum measurement. Under the excitation of 320 nm wavelength light, the fluorescence emission spectrum of the composite hydrogel was obtained at 430 nm. The results are shown in Figure 5. It can be seen from Figure 5 that with the increase of Al ³⁺ concentration in the solution, the fluorescence emission intensity of the composite hydrogel gradually increases, indicating that the above hydrogel has fluorescence responsiveness to Al ³⁺ and can be used for the detection of Al ³⁺ , and the fluorescence intensity of the above hydrogel is linearly related to the concentration of Al ³⁺ .

Origin software was used to analyze the relationship between the ratio of the fluorescence emission intensity of the composite hydrogel after the addition of Al ³⁺ and before the addition of Al ^{3+ and} the concentration of Al ³⁺ , and the graph was obtained, as shown in Figure 6. It can be seen from Figure 6 that the ratio of the fluorescence emission intensity of the composite hydrogel after adding Al ³⁺ and before adding Al ³⁺ is proportional to the Al ³⁺ concentration. According to the graph in Figure 6, the following equation is obtained:

y=0.1764x+1.2568;

Among them, x is the concentration of Al ³⁺ , and y is the ratio of the fluorescence emission intensity of the composite hydrogel after adding Al ³⁺ and before adding Al ³⁺ .

Then according to formula 1, the detection limit of carbon nitride composite gel for Al ³⁺ ions was determined;

LOD=S/N formula 1;

where S is the standard deviation measured in the absence of ^Al , and N is the slope of the above equation.

The result is that the detection limit of carbon nitride composite hydrogel for Al ³⁺ in aqueous solution is 0.26 μM.

Example 3

10g of urea was placed in a muffle furnace and calcined at 550°C for 2h to obtain carbon nitride powder;

Strip 1g of carbon nitride powder in a mixed solution of 10mL of concentrated sulfuric acid (concentration of 18.4mol/L) and 10mL of concentrated nitric acid (concentration of 14.8mol/L) for 1h, diluted to a neutral solution, let stand for 24h, and take the The clear liquid was used as a carbon nitride dispersion with a concentration of 0.15 mg/mL;

1 g of acrylic monomer was added to 4 mL of the above carbon nitride dispersion liquid, and in the obtained mixed solution, the concentration of acrylic monomer was 20 wt %, and the concentration of carbon nitride was 0.01 wt %; The glass test tube or mold is sealed, and the sealed reaction solution is placed under a xenon lamp light source to react for 2.5 hours to obtain a hydrogel precursor;

The hydrogel precursor was cut into a size of 15 × 5 × 2 mm, immersed in 30 mL of ethanol aqueous solution (volume concentration of 50%) in turn, and soaked for 60 min to obtain a carbon nitride composite hydrogel.

Al ³⁺ detection

The above carbon nitride composite acrylic hydrogel was placed in 20 mL of an aqueous solution with an Al ³⁺ concentration of 5.00 × 10 ^-5 M, and after standing for 30 min, the carbon nitride composite acrylic hydrogel was taken out and placed on a fluorescence spectrometer for fluorescence spectroscopy. Measured, under the excitation of 320 nm wavelength light, the fluorescence emission spectrum of the composite hydrogel was obtained at 430 nm, and the result is shown in FIG. 7 . It can be seen from Figure 7 that the carbonitride acrylic composite hydrogel also has fluorescence responsiveness to Al ³⁺ and can be used for the detection of Al ³⁺ .

Example 4

10g of urea was placed in a muffle furnace and calcined at 550°C for 2h to obtain carbon nitride powder;

Strip 1g of carbon nitride powder in a mixed solution of 10mL of concentrated sulfuric acid (concentration of 18.4mol/L) and 10mL of concentrated nitric acid (concentration of 14.8mol/L) for 1h, diluted to a neutral solution, let stand for 24h, and take the The clear liquid was used as a carbon nitride dispersion with a concentration of 0.15 mg/mL;

1g of N-isopropylacrylamide monomer was added to 4mL of the above carbon nitride dispersion, in the obtained mixed solution, the concentration of N-isopropylacrylamide monomer was 20wt%, and the concentration of carbon nitride was 0.01wt% %; deoxygenate with nitrogen for 30 min; inject the mixed solution into a glass test tube or mold and seal, and place the sealed reaction solution under a xenon lamp light source to react for 2.5 h to obtain a hydrogel precursor;

The hydrogel precursor was cut into a size of 15 × 5 × 2 mm, immersed in 30 mL of ethanol aqueous solution (volume concentration of 50%) in turn, and soaked for 60 min to obtain a carbon nitride composite hydrogel.

Al ³⁺ detection

The above carbon nitride composite N-isopropylacrylamide hydrogel was placed in 20 mL of an aqueous solution with an Al ³⁺ concentration of 5.00 × 10 ^-5 M, and after standing for 30 min, the carbon nitride composite N-isopropyl propylene was taken out. The amide hydrogel was placed on a fluorescence spectrometer for fluorescence spectrum measurement. Under the excitation of light with a wavelength of 320 nm, the fluorescence emission spectrum of the composite hydrogel was obtained at 430 nm. The results are shown in Figure 8. It can be seen from Fig. 8 that the carbon nitride N-isopropylacrylamide composite hydrogel also has fluorescence response to Al ³⁺ and can be used for the detection of Al ³⁺ .

The test results of carbon nitride composite hydrogels prepared with other polymer monomers are consistent with the above-mentioned test results of acrylamide, acrylic acid and N-isopropylacrylamide.

The above are only the preferred embodiments of the present invention. It should be pointed out that for those skilled in the art, without departing from the principles of the present invention, several improvements and modifications can be made. It should be regarded as the protection scope of the present invention.

Claims (9)

1. A preparation method of carbon nitride composite hydrogel is characterized by comprising the following steps:

calcining urea to obtain carbon nitride powder;

mixing the carbon nitride powder, concentrated nitric acid and concentrated sulfuric acid, stripping, adjusting the pH of the obtained stripping system to be neutral, standing, and taking the supernatant as carbon nitride dispersion liquid;

mixing the carbon nitride dispersion liquid with a polymer monomer, and carrying out xenon lamp irradiation to obtain a hydrogel precursor;

soaking the hydrogel precursor into an ethanol aqueous solution to obtain the carbon nitride composite hydrogel;

the polymer monomer is one or more of acrylamide, N-isopropyl acrylamide and acrylic acid.

2. The method according to claim 1, wherein the concentration of the concentrated nitric acid is 10 to 16mol/L, the concentration of the concentrated sulfuric acid is 10.4 to 18.4mol/L, and the ratio of the carbon nitride powder, the concentrated nitric acid, and the concentrated sulfuric acid is 0.1 to 2.0g: 5-15 mL:5 to 15mL.

3. The method according to claim 1 or 2, wherein the concentration of the carbon nitride dispersion is 0.02 to 0.15mg/mL.

4. The production method according to claim 1, wherein the mass ratio of carbon nitride to the polymer monomer in the carbon nitride dispersion liquid is (1.5 to 6.0): 10000.

5. the method according to claim 1, wherein the mixed solution obtained by mixing the carbon nitride dispersion liquid and the polymer monomer contains the polymer monomer in an amount of 10 to 35% by mass.

6. The production method according to claim 1, 4 or 5, characterized in that the xenon lamp irradiation time is 2.5 to 4.5 hours.

7. The method according to claim 1, wherein the ethanol aqueous solution has a volume concentration of 10 to 90%.

8. The carbon nitride composite hydrogel obtained by the production method according to any one of claims 1 to 7, wherein the solid content of the carbon nitride composite hydrogel is 1 to 10%.

9. The carbon nitride composite hydrogel of claim 8 for detecting Al in water ³⁺ The use of (1).

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