BiOCl / CoAl-LDH composite photocatalyst as well as preparation method and application thereof

Abstract

The invention discloses a BiOCl / CoAl-LDH composite photocatalyst as well as a preparation method and application thereof, and belongs to the technical field of photocatalytic semiconductor materials. The BiOCl / CoAl-LDH composite photocatalyst is prepared from BiOCl and CoAl-LDH which is wrapped in the BiOCl. According to the preparation method of the BiOCl / CoAl-LDH composite material, a two-step method is adopted, and the finally synthesized composite material has clear diffraction peaks, can correspond to monomer peaks, and is relatively high in crystallinity and free of generation of impurities. Compared with a monomer, the composite material has a better catalytic effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalytic semiconductor materials, and particularly relates to a BiOCl / CoAl-LDH composite photocatalyst, a preparation method thereof, and an application thereof. Background Art

[0002] With the rapid development of the economy, energy shortage and environmental pollution have become two major problems faced by the world, and environmental governance has always been an issue of concern in our country. Antibiotics have characteristics such as antibacterial, antiviral, and antifungal, and are widely used in the treatment of infectious diseases and agricultural production. Currently, the commonly used types of antibiotics on the market are: sulfonamides, fluoroquinolones, tetracyclines, macrolides, and β-lactams. Among them, tetracycline is widely used due to its low price and characteristics such as anti-inflammatory, inhibiting bacterial reproduction, and high-efficiency sterilization. After being applied in the medical industry, livestock breeding, and agriculture, tetracycline will diffuse in the environment through metabolism. Tetracycline has good hydrophilicity and low volatility, and is difficult to degrade under natural conditions. Therefore, it exists in large quantities in surface water, groundwater, and soil. More seriously, tetracycline has always existed in the environment, which is likely to generate some resistance genes and accelerate the spread and diffusion of antibiotic resistance. These resistant microorganisms can indirectly enhance the drug resistance of the human body after contacting it. This not only causes great harm to human health but also seriously affects the entire ecosystem.

[0003] Bismuth oxychloride (BiOCl) is an inorganic compound belonging to the metal halide class. It is composed of bismuth (Bi), oxygen (O), and chlorine (Cl) atoms, has metal and non-metal properties, and can be used as a semiconductor material. It has characteristics such as easy preparation, low toxicity, high stability, and good electron transport, and has good development prospects and wide applications in the fields of electronics, energy conversion and storage, and environmental remediation. BiOCl has a tetragonal layered structure, excellent optical and electrochemical properties, a strong polar surface, and strong photocatalytic activity. The Cl in BiOCl － and [Bi 2 O 2 2+ form various layered structures and electron transition modes different from other photocatalytic materials. The internal electrostatic field generated between layers can promote the rapid separation and transport of photo-generated carriers. However, due to the relatively wide bandgap of BiOCl and its small response to visible light, it is necessary to modify BiOCl to improve its photocatalytic activity. Summary of the Invention

[0004] ​Aiming at the defects existing in the above-mentioned prior art, in order to solve the problems of the wide band gap of BiOCl, small response to visible light, and low photocatalytic ability, the object of the present invention is to design and provide a bismuth oxychloride composite photocatalyst and its preparation method. By constructing a bismuth oxychloride and CoAl-LDH heterostructure, in the composite material BiOCl / CoAl-LDH, the BiOCl spheres wrap the CoAl-LDH flakes, and the overall contour is spherical. The connection where CoAl-LDH is wrapped by BiOCl shows a net-like filamentous shape. CoAl-LDH is a sheet-like structure, which can effectively separate photo-induced electron-hole pairs, increase the visible light response range of the material, improve the photocatalytic performance, and at the same time improve the treatment and degradation effect of the composite catalyst on antibiotics in water.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] On the one hand, the present invention provides a BiOCl / CoAl-LDH composite photocatalyst, which includes BiOCl and CoAl-LDH wrapped inside it. Among them, BiOCl (bismuth oxychloride) is formed by the accumulation of spheres and presents an irregular spherical shape as a whole; CoAl-LDH (cobalt-aluminum layered double hydroxide) is a sheet-like structure stacked by the superposition of nanosheets, and its size and morphology are inconsistent, with a size of 1-3 μm.

[0007] On the second hand, the present invention provides a preparation method of a BiOCl / CoAl-LDH composite photocatalyst, which includes the following steps:

[0008] (1) Weigh cobalt nitrate hexahydrate and aluminum nitrate nonahydrate, dissolve them in water, stir, add urea and ammonium fluoride, stir strongly, place them in a high-pressure reaction kettle for treatment, cool naturally, wash alternately with water and alcohol, and dry to obtain CoAl-LDH;

[0009] (2) Weigh sodium chloride and dissolve it in a mixed solution of ethanol and water. Slowly add an ethylene glycol solution of bismuth nitrate pentahydrate, dropwise add acetic acid, stir to obtain a suspension solution, add CoAl-LDH, stir until fully mixed, then place it in a high-pressure kettle for reaction, wash with water and alcohol, and dry to obtain a BiOCl / CoAl-LDH composite photocatalyst.

[0010] In the preparation method described above, in step (1), the mass-volume ratio of cobalt nitrate hexahydrate, aluminum nitrate nonahydrate to water is 0.873 g:0.375 g:70 mL;

[0011] The molar ratio of cobalt nitrate hexahydrate, aluminum nitrate nonahydrate, urea and ammonium fluoride is 3-1:1:2:8.

[0012] In the described preparation method, the time of the strong stirring in step (1) is 1 to 2 h.

[0013] In the described preparation method, the conditions for treatment in the high-pressure reactor in step (1) are: temperature 120 to 140 °C, time 10 to 12 h;

[0014] The conditions for the drying are: temperature 60 to 80 °C, time 12 to 18 h.

[0015] In the described preparation method, the mass-volume ratio of sodium chloride, ethanol and water in step (2) is 0.2457 g: 20 mL: 20 to 40 mL;

[0016] The molar ratio of sodium chloride to bismuth nitrate pentahydrate is 1 to 1.5: 2;

[0017] In the ethylene glycol solution of bismuth nitrate pentahydrate, the mass-volume ratio of bismuth nitrate pentahydrate to ethylene glycol is 2.9104 g: 40 to 50 mL;

[0018] The volume ratio of acetic acid to ethylene glycol is 0.5 to 1: 40;

[0019] The mass ratio of CoAl-LDH to bismuth nitrate pentahydrate is 0.23 to 0.5: 2.9104.

[0020] In the described preparation method, the time of the stirring in step (2) is 30 to 60 min;

[0021] The time for stirring until thorough mixing is 3 to 6 h.

[0022] In the described preparation method, the conditions for the reaction in the autoclave in step (2) are: temperature 120 to 150 °C, time 9 to 12 h;

[0023] The conditions for the drying are: temperature 60 to 80 °C, time 12 to 18 h.

[0024] In the third aspect, the present invention provides the application of the described BiOCl / CoAl-LDH composite photocatalyst, or the BiOCl / CoAl-LDH composite photocatalyst prepared by the preparation method described in any one of the above in the photocatalytic degradation of wastewater;

[0025] Preferably, the wastewater is tetracycline-containing wastewater.

[0026] In the fourth aspect, the present invention provides a method for degrading tetracycline-containing wastewater by using a BiOCl / CoAl-LDH composite photocatalyst. Weigh the BiOCl / CoAl-LDH photocatalyst, put it into the tetracycline-containing wastewater, stir under dark conditions until the adsorption / desorption equilibrium is reached, turn on the light source, and stir to obtain the degraded wastewater.

[0027] The degradation principle of the composite material of the present invention is as follows: under the excitation of light, electrons on the conduction band of BiOCl directly transfer to the valence band of CoAl-LDH and combine with the holes of CoAl-LDH. This not only enables the effective separation of electrons and holes, but also accumulates many electrons on the conduction band of CoAl-LDH and many holes on the valence band of BiOCl. A large number of electrons capture O 2 to generate O 2 - , and a large number of holes capture H 2 O to generate OH. Under the combined action of O 2 - , OH and h + three kinds of free radicals, the antibiotic is oxidized and degraded.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The material of the present invention uses solar energy as a clean and renewable energy source, and there is no secondary pollution after treatment. It not only completely degrades organic matter into non-toxic and harmless small molecules, but also can maintain ecological balance.

[0030] 2. The raw materials used in the present invention are bismuth nitrate, sodium chloride, cobalt nitrate, etc., which are all cheap and easily available. The experimental method is simple, feasible, easy to operate, and greatly reduces the cost.

[0031] 3. The materials prepared in the present invention have high purity and can be recycled.

[0032] 4. In the preparation method of the BiOCl / CoAl-LDH composite material of the present invention, a two-step method is adopted. The diffraction peaks of the finally synthesized composite material are clear and can all correspond to the monomer peaks. The crystallinity is relatively high and no impurities are generated. The composite material has a better catalytic effect compared with the monomer. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is the XRD pattern of the photocatalysts prepared in Examples 1-3 of the present invention;

[0034] Figure 2 is the SEM image of the composite BiOCl / CoAl-LDH photocatalytic material prepared in the present invention;

[0035] Figure 3 is the degradation curve obtained by degrading TC with the photocatalysts prepared in Examples 1-3 of the present invention;

[0036] Figure 4 is the kinetic curve of the photocatalysts prepared in Examples 1-3 of the present invention for the degradation of TC. DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention will be further described below in conjunction with the accompanying drawings. The exemplary embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention. In addition, the technical features involved in each embodiment of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0038] Embodiment 1:

[0039] Step 1, preparation of CoAl-LDH: Cobalt nitrate hexahydrate (0.873 g) and aluminum nitrate nonahydrate (0.375 g) were dissolved in 70 mL of deionized water and mechanically stirred for 30 minutes. Then, 1.5 g of urea and 0.296 g of ammonium fluoride were added to the above mixture and kept stirring vigorously for 1 hour. Subsequently, the mixture was placed in a sealed Teflon-lined autoclave at 120°C for 12 hours. After natural cooling, the product was washed alternately with water and alcohol several times and dried at 60°C for 12 hours to obtain CoAl-LDH.

[0040] Step 2, preparation of BiOCl / CoAl-LDH composite photocatalyst: First, 2.9104 g of bismuth nitrate pentahydrate was dissolved in 40 mL of ethylene glycol and mechanically stirred for 40 minutes to form solution A. 0.2457 g of sodium chloride was completely dissolved in 40 mL of a mixed solution containing ethanol (20 mL) and distilled water (20 mL) to prepare solution B, and solution B was slowly poured into solution A. Subsequently, 0.5 mL of acetic acid was added dropwise to the combined solution, and a suspension solution C was obtained by stirring for 30 minutes. After 0.23 g of CoAl-LDH was fully mixed with solution C (stirring for 3 hours), the combined solution was reacted at 120°C in a Teflon-lined autoclave for 12 hours. Finally, the reaction solution was centrifuged and washed alternately with deionized water and alcohol, and dried at 60°C for 12 hours to obtain a composite BiOCl / CoAl-LDH (wherein the mass ratio of BiOCl to CoAl-LDH was 100:10). The composite material was referred to as BCL-10.

[0041] Embodiment 2:

[0042] Step 1, preparation of CoAl-LDH: Cobalt nitrate hexahydrate (0.873 g) and aluminum nitrate nonahydrate (0.375 g) were dissolved in 70 mL of deionized water and mechanically stirred for 30 minutes. Then, 1.5 g of urea and 0.296 g of ammonium fluoride were added to the above mixture and kept stirring vigorously for 1.5 h. Subsequently, the mixture was placed in a sealed Teflon-lined autoclave at 130 ° C for 11 h. After natural cooling, the product was washed alternately with water and alcohol several times and dried at 60 ° C for 12 h to obtain CoAl-LDH.

[0043] Step 2: Preparation of BiOCl / CoAl-LDH composite photocatalyst: First, dissolve 2.9104 g of bismuth nitrate pentahydrate in 40 mL of ethylene glycol and mechanically stir for 40 minutes to form solution A. Completely dissolve 0.2457 g of sodium chloride in a 40 mL mixed solution containing ethanol (20 mL) and distilled water (20 mL) to prepare solution B, and slowly pour solution B into solution A. Subsequently, add 0.5 mL of acetic acid to the combined solution and obtain suspension solution C by stirring for 30 minutes. After fully mixing 0.35 g of CoAl-LDH with solution C (stirring for 3 hours), react the combined solution in a Teflon-lined autoclave at 140 °C for 10 hours. Finally, centrifuge the reaction solution and wash it alternately with deionized water and alcohol, and dry it at 60 °C for 12 h. Finally, a composite BiOCl / CoAl-LDH with a mass ratio of 100:15 is obtained, and this composite material is denoted as BCL-15.

[0044] Example 3:

[0045] Step 1: Preparation of CoAl-LDH: Dissolve cobalt nitrate hexahydrate (0.873 g) and aluminum nitrate nonahydrate (0.375 g) in 70 mL of deionized water and mechanically stir for 30 minutes. Then, add 1.5 g of urea and 0.296 g of ammonium fluoride to the above mixture and maintain strong stirring for 1.5 h. Subsequently, place the mixture in a sealed Teflon-lined autoclave at 140 °C for 10 h. After natural cooling, wash the product alternately with water and alcohol several times and dry it at 60 °C for 12 h to obtain CoAl-LDH.

[0046] Step 2: Preparation of BiOCl / CoAl-LDH composite photocatalyst: First, dissolve 2.9104 g of bismuth nitrate pentahydrate in 40 mL of ethylene glycol and mechanically stir for 40 minutes to form solution A. Completely dissolve 0.2457 g of sodium chloride in a 40 mL mixed solution containing ethanol (20 mL) and distilled water (20 mL) to prepare solution B, and slowly pour solution B into solution A. Subsequently, add 0.5 mL of acetic acid to the combined solution and obtain suspension solution C by stirring for 30 minutes. After fully mixing 0.46 g of CoAl-LDH with solution C (stirring for 3 hours), react the combined solution in a Teflon-lined autoclave at 150 °C for 9 hours. Finally, centrifuge the reaction solution and wash it alternately with deionized water and alcohol, and dry it at 60 °C for 12 h. Finally, a composite BiOCl / CoAl-LDH with a mass ratio of 100:20 is obtained, and this composite material is denoted as BCL-20.

[0047] Perform performance measurements on the BiOCl / CoAl-LDH composite photocatalysts prepared in Examples 1 to 3 respectively:

[0048] 1. The crystal phase structures of the materials prepared in Examples 1-3 and Comparative Example 2 were analyzed by a Rigaku D / max2500PC X-ray diffractometer. Among them, the X-ray was a Cu target Kα, with a voltage of 40 kV, a current of 100 mA, a step size of 0.02°, and a scanning range of 5° to 80°.

[0049] The X-ray diffraction pattern is as Figure 1 shown. The diffraction peaks of BiOCl prepared in Comparative Example 2 were basically consistent with the standard card (JCPDS NO.06-0249), and there were no diffraction peaks of any impurity peaks, indicating that the obtained samples were all BiOCl. The diffraction peaks of CoAl-LDH prepared in Example 1 were basically consistent with the standard card (JCPDS NO.51-0045), and there were no diffraction peaks of any impurity peaks, indicating that the obtained samples were all CoAl-LDH.

[0050] 2. The scanning electron microscope of the BiOCl / CoAl-LDH composite visible light catalysts prepared in Examples 1-3 was analyzed by a Zeiss SUPRA-55 field emission scanning electron microscope. Performance indicators Resolution: 0.8 nm @ 15 kV; 1.6 nm @ 1 kV Acceleration voltage: 0.02 - 30 kV Magnification: 12 - 1,000,000X Probe current: 4 pA to 20 nA Stability: better than 0.2% / h Sample: Sample stage: X = 130 mm Y = 130 mm Z = 50 mm T = -3° to +70° R = 360° continuously adjustable Maximum specimen height: 145 mm Maximum specimen diameter: 250 mm Auxiliary ports: EDS, EBSD, Cryo and others Low electron beam noise: <1%.

[0051] The results of the SEM images are shown by Figure 2 It can be seen that BiOCl prepared in Comparative Example 2 was an irregular spherical shape, CoAl-LDH prepared in Example 1 was an irregular flaky stacked structure, and BiOCl / CoAl-LDH prepared in Example 1 presented an aggregated spherical shape with thin flakes embedded inside. It was proved that the two materials were successfully compounded. The XRD images showed that the bismuth oxychloride and CoAl-LDH with very high purity were prepared by the method of the present invention. The diffraction peaks of the finally synthesized composite material were clear and could all correspond to the monomer peaks, with a relatively high crystallinity and no impurities generated.

[0052] Comparative Example 1:

[0053] First, 2.9104 g of bismuth nitrate pentahydrate was dissolved in 40 mL of ethylene glycol, and mechanically stirred for 40 minutes to form solution A. 0.2457 g of sodium chloride was completely dissolved in a 40 mL mixed solution containing ethanol (20 mL) and distilled water (20 mL) to prepare solution B, and solution B was slowly poured into solution A. Subsequently, 0.5 mL of acetic acid was added dropwise to the combined solution, and a suspension solution C was obtained by stirring for 30 minutes. Solution C was reacted in a Teflon-lined autoclave at 140 °C for 10 hours. Finally, the reaction solution was centrifuged and washed alternately with deionized water and alcohol, and dried at 60 °C for 12 h to obtain BiOCl.

[0054] Comparative Example 2:

[0055] First, 2.9104 g of bismuth nitrate pentahydrate was dissolved in 40 mL of ethylene glycol, and mechanically stirred for 40 minutes to form solution A. 0.2457 g of sodium chloride was completely dissolved in a 40 mL mixed solution containing ethanol (20 mL) and distilled water (20 mL) to prepare solution B, and solution B was slowly poured into solution A. Subsequently, 0.5 mL of acetic acid was added dropwise to the combined solution, and a suspension solution C was obtained by stirring for 30 minutes. Solution C was reacted in a Teflon-lined autoclave at 120 °C for 12 hours. Finally, the reaction solution was centrifuged and washed alternately with deionized water and alcohol, and dried at 60 °C for 12 h to obtain BiOCl.

[0056] Comparative Example 3:

[0057] First, 2.9104 g of bismuth nitrate pentahydrate was dissolved in 40 mL of ethylene glycol, and mechanically stirred for 40 minutes to form solution A. 0.2457 g of sodium chloride was completely dissolved in a 40 mL mixed solution containing ethanol (20 mL) and distilled water (20 mL) to prepare solution B, and solution B was slowly poured into solution A. Subsequently, 0.5 mL of acetic acid was added dropwise to the combined solution, and a suspension solution C was obtained by stirring for 30 minutes. Solution C was reacted in a Teflon-lined autoclave at 150 °C for 9 hours. Finally, the reaction solution was centrifuged and washed alternately with deionized water and alcohol, and dried at 60 °C for 12 h to obtain BiOCl.

[0058] Comparative Example 4:

[0059] Cobalt nitrate hexahydrate (0.873 g) and aluminum nitrate nonahydrate (0.375 g) were dissolved in 70 mL of deionized water and mechanically stirred for 30 minutes. Then, 1.5 g of urea and 0.296 g of ammonium fluoride were added to the above mixture and kept under strong stirring for 1.5 h. Subsequently, the mixture was placed in a sealed Teflon-lined autoclave at 130 °C for 11 h. After natural cooling, the product was washed alternately with water and alcohol several times and dried at 60 °C for 12 h to obtain CoAl-LDH.

[0060] Example 4:

[0061] Step 1: Weigh 20 mg of the BiOCl prepared in Comparative Example 1, the CoAl-LDH prepared in Comparative Example 4, and the BiOCl / CoAl-LDH photocatalyst samples prepared in Examples 1, 2, and 3 respectively, and add them into different colorimetric tubes;

[0062] Step 2: Prepare a 25 mg / L tetracycline solution, and measure 50 mL of it respectively and add it into the above five colorimetric tubes;

[0063] Step 3: Put the colorimetric tubes into a photocatalytic reactor, stir for 30 min under dark conditions to make the system reach the adsorption / desorption equilibrium. Then turn on the light source, take 3 - 4 mL of the mixed solution as a sample every 20 min under magnetic stirring, centrifuge, take the supernatant, and then filter it through a syringe filter to obtain a clear solution; Use a UV-visible spectrophotometer to measure the absorbance of the solution, and calculate the degradation rate of the prepared catalyst on the sample using the following formula (Ⅰ):

[0064] D = (1 - C t / C 0 ) × 100 Formula (Ⅰ)

[0065] where Ct is the absorbance of the TC solution concentration at time t, and C0 is the absorbance of TC at 25 mg / L, Figure 3 The concentration ratio in refers to Ct / C0.

[0066] For example, at the 100th minute, the degradation rate of BCL-15 is: D = (1 - Ct / C0) × 100 = (1 - 107 / 716) × 100 = 0.85.

[0067] The degradation effect diagrams of the BiOCl prepared in Comparative Example 1, the CoAl-LDH prepared in Comparative Example 4, and the BiOCl composite photocatalysts prepared in Examples 1, 2, and 3 are as Figure 3 shown. After 100 min of reaction, the degradation rate of the BiOCl prepared in Comparative Example 1 on tetracycline is 63%; the degradation rate of the CoAl-LDH prepared in Comparative Example 4 on tetracycline is 26%; the degradation rate of the BiOCl / CoAl-LDH composite photocatalyst prepared in Example 1 on tetracycline is 76%; the degradation rate of the BiOCl / CoAl-LDH composite photocatalyst prepared in Example 2 on tetracycline is 85%; the degradation rate of the BiOCl / CoAl-LDH composite photocatalyst prepared in Example 3 on tetracycline is 73%. It can be seen that the composite catalysts prepared in Examples 1 - 3 have high photocatalytic activity, and the composite materials have better catalytic effects than the monomers.

[0068] Example 5:

[0069] A first-order kinetic model was established for the process of degrading TC by the BiOCl prepared in Comparative Example 1, the CoAl-LDH prepared in Comparative Example 4, and the BiOCl / CoAl-LDH photocatalysts prepared in Examples 1, 2, and 3, and its rate constant was calculated using the following formula (II):

[0070] ln(C 0 / C t ) = K t Formula (II)

[0071] where Ct is the solution concentration of TC at time t, C 0 is the concentration of the TC solution when it has not been degraded, K represents the kinetic coefficient, and t represents time.

[0072] As Figure 4 shown, the reaction rate constant of the composite material BCL-15 is 0.019 min -1 which is significantly higher than that of BiOCl (0.0099 min -1 ), CoAl-LDH (0.003 min -1 ), BCL-10 (0.0144 min -1 ), and BCL-20 (0.0132 min -1 ).

[0073] Generally speaking, the photocatalytic effects of the three composite materials are all better than those of the monomers, and among them, BCL-15 has the strongest photocatalytic effect.

[0074] Taking the ideal embodiments of the present invention described above as an inspiration, through the above description, relevant staff can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A BiOCl / CoAl-LDH composite photocatalyst, characterized in that: It contains BiOCl and CoAl-LDH encapsulated inside it.

2. A method for preparing a BiOCl / CoAl-LDH composite photocatalyst, characterized in that: The following steps are involved: (1) Weighing cobalt nitrate hexahydrate and aluminum nitrate nonahydrate, dissolving them in water, stirring, adding urea and ammonium fluoride, vigorously stirring, placing in a high-pressure reactor for treatment, cooling naturally, washing with water and alcohol alternately, and drying to obtain CoAl-LDH; (2) Weighing sodium chloride and dissolving it in a mixed solution of ethanol and water, slowly adding an ethylene glycol solution of bismuth nitrate pentahydrate, dropping acetic acid, stirring to obtain a suspension solution, adding CoAl-LDH, stirring until fully mixed, placing in an autoclave for reaction, washing with water and alcohol, and drying to obtain a BiOCl / CoAl-LDH composite photocatalyst.

3. The preparation method according to claim 2, characterized in that: The mass volume ratio of cobalt nitrate hexahydrate, aluminum nitrate nonahydrate and water in step (1) is 0.873 g:0.375 g:70 mL; The molar ratio of the cobalt nitrate hexahydrate, the aluminum nitrate nonahydrate, the urea and the ammonium fluoride is 3 to 1:1:2:

8.

4. The preparation method according to claim 2, characterized in that: The time of strong stirring in step (1) is 1 to 2 hours.

5. The preparation method according to claim 2, characterized in that: The conditions for the treatment in the high pressure reactor in step (1) are: temperature 120-140° C., time 10-12 h; The drying conditions are: temperature 60-80° C., time 12-18 hours.

6. The preparation method according to claim 2, characterized in that: The mass volume ratio of sodium chloride, ethanol and water in step (2) is 0.2457 g:20 mL:20-40 mL; The molar ratio of sodium chloride to bismuth nitrate pentahydrate is 1 to 1.5:2; The mass volume ratio of bismuth nitrate pentahydrate to ethylene glycol in the ethylene glycol solution of bismuth nitrate pentahydrate is 2.9104 g:40-50 mL; The volume ratio of acetic acid to ethylene glycol is 0.5 to 1:40; The mass ratio of the CoAl-LDH to bismuth nitrate pentahydrate is 0.23-0.5:2.9104.

7. The preparation method according to claim 2, characterized in that: The stirring time in step (2) is 30 to 60 minutes; The stirring time for fully mixing is 3 to 6 hours.

8. The preparation method according to claim 2, characterized in that: The reaction conditions in the autoclave in step (2) are: temperature 120-150° C., time 9-12 h; The drying conditions are: temperature 60-80° C., time 12-18 hours.

9. Use of the BiOCl / CoAl-LDH composite photocatalyst according to claim 1, or the BiOCl / CoAl-LDH composite photocatalyst prepared by the preparation method according to any one of claims 2 to 8 in photocatalytic degradation of wastewater; Preferably, the wastewater is tetracycline-containing wastewater.

10. A method for degrading tetracycline-containing wastewater using a BiOCl / CoAl-LDH composite photocatalyst, characterized in that: Weigh the BiOCl / CoAl-LDH photocatalyst, put it into the wastewater containing tetracycline, stir it in the dark to reach the adsorption / desorption equilibrium, turn on the light source, stir it, and obtain the degraded wastewater.

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