Copper oxide / polyvinylidene fluoride Janus photothermal film and preparation method and application thereof
By constructing a copper oxide hydrophilic photothermal layer on a PVDF substrate, the Janus photothermal membrane solves the problems of photothermal material shedding and membrane wetting, achieving efficient photothermal membrane distillation wastewater treatment, improving permeate flux and removal rate, and reducing energy consumption.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- CHANGCHUN UNIV OF TECH
- Filing Date
- 2023-10-12
- Publication Date
- 2026-05-19
AI Technical Summary
The existing Janus photothermal composite membrane is prone to photothermal material detachment during recycling, leading to secondary pollution. Furthermore, it is limited by membrane wetting when treating high-concentration, multi-component organic industrial wastewater, which affects its application in the treatment of photothermal membrane distillation wastewater.
A hydrophobic microporous structure on a PVDF substrate was prepared using a non-solvent phase separation method. A hydrophilic photothermal layer of copper oxide was then constructed on the PVDF surface using chemical plating and in-situ oxidation methods, forming an interconnected pore structure. Photothermal performance was improved through biomimetic light traps, and the adhesion of the film was enhanced through the anchoring effect.
This technology enables efficient local heating of the photothermal film, reduces energy input, improves the film's anti-wetting properties and stability, enhances the permeation flux and removal rate of the photothermal film distillation, and reduces the risk of photothermal material detachment.
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Abstract
Description
Technical Field
[0001] This invention belongs to the field of photothermal film distillation technology, specifically relating to a copper oxide / polyvinylidene fluoride Janus photothermal film, its preparation method, and its application. Background Technology
[0002] Recently, membrane distillation (MD) has been used for wastewater treatment, but the large amount of material heating required in practical applications leads to temperature polarization, and the high investment required still limits its industrial application. Therefore, developing green treatment technologies with low carbon footprints is particularly important. Solar-powered photothermal membrane distillation (PMD) technology is rapidly emerging in the field of seawater desalination. Under sunlight, thermal localization maintains a high temperature on the membrane surface, and the local temperature gradient drives the evaporation of the feed liquid at the boundary layer of the membrane surface. The resulting water vapor passes through the microporous hydrophobic membrane and condenses into pure water. However, due to limitations imposed by membrane wetting when treating high-concentration, multi-component organic industrial wastewater, its application in wastewater treatment is rarely reported.
[0003] Previous studies have proposed methods to mitigate membrane wetting by constructing a hydrophilic layer on a hydrophobic substrate to prepare Janus composite membranes. The hydrophilic membrane adsorbs organic solvents, thereby reducing its hydrophilicity and preventing organic solvents or surfactants from reaching the hydrophobic layer. Furthermore, the hydrophilic layer in the Janus membrane structure can reduce mass transfer resistance between evaporation and condensation processes. The structure of the Janus membrane shortens the water vapor transport path, accelerating water vapor condensation, and also blocks heat transfer, increasing the transmembrane temperature difference and reducing temperature polarization, which is beneficial for improving flux. The photothermal performance of the Janus membrane is the core of photothermal membrane distillation; controlling the microstructure of the photothermal material to create biomimetic light traps is beneficial for improving photothermal performance.
[0004] However, Janus photothermal composite membranes prepared by methods such as surface coating, vacuum filtration, and electrospinning still suffer from secondary pollution caused by photothermal material shedding during recycling. Therefore, constructing a stable and recyclable Janus photothermal membrane with both excellent photothermal performance and anti-wetting properties is crucial for the treatment of photothermal membrane distillation (PMD) wastewater. Summary of the Invention
[0005] This invention relates to a copper oxide / polyvinylidene fluoride (PVDF) Janus photothermal film, its preparation method, and its applications, aiming to solve the problems existing in the background art. By introducing solar energy into the photothermal layer on the film surface, efficient local heating replaces the need for heat pump heating to supply the entire feed system, eliminating the inherent temperature polarization limitations of traditional MD (distillation membrane) and reducing energy input. The hydrophobic microporous structure of the PVDF substrate has excellent separation performance. By controlling the preparation conditions, a hydrophilic photothermal layer of copper oxide (CuO) with biomimetic light traps is constructed, which plays a photothermal anti-wetting role. The photothermal film has an interconnected pore structure, and the anchoring effect of in-situ oxidation after chemical plating gives the CuO photothermal layer excellent bonding force with the PVDF substrate, achieving efficient local heating and long-term stable operation. The copper oxide / PVDF Janus photothermal film prepared by this invention has broad application prospects in the field of photothermal membrane distillation wastewater treatment.
[0006] To achieve the above objectives, the present invention provides a method for preparing a copper oxide / polyvinylidene fluoride Janus photothermal film, comprising the following steps:
[0007] (1) Preparation of polyvinylidene fluoride membrane (PVDF) by non-solvent phase separation (NIPS): A certain amount of polyvinylidene fluoride (PVDF) powder and polyvinylpyrrolidone (PVP) powder were dissolved in N,N-dimethylacetamide (DMAC) and triethyl phosphate (TEP), mechanically stirred at 60°C for 5 h, and allowed to stand for degassing for 24 h to obtain PVDF casting solution;
[0008] (2) Preparation of electroless copper / polyvinylidene fluoride (Cu / PVDF) membrane: First, the polyvinylidene fluoride (PVDF) membrane was sensitized in a sensitizing solution, then repeatedly rinsed with hot water, and finally activated in an activation solution. After activating, it was soaked in deionized water. Subsequently, electroless copper plating solution A, electroless copper plating solution B, and deionized water were mixed in a certain proportion, stirred evenly, and poured into a petri dish. After electroless plating at 35℃ for 2 hours, it was removed, rinsed with plenty of deionized water, and then dried in the air.
[0009] (3) Preparation of in-situ oxidation of copper oxide / polyvinylidene fluoride Janus photothermal film (CuO / PVDF): Different concentrations of NaOH and (NH4)2S2O8 were dissolved in 50 mL of deionized water. After stirring and mixing, the mixture was poured into a petri dish and oxidized in situ for a certain period of time. The mixture was then rinsed with deionized water and dried in an oven at 60 °C for 1 h.
[0010] Preferably, in step (1), the casting solution contains 15% polyvinylidene fluoride, 1% polyvinylpyrrolidone, and 34% and 50% N,N-dimethylacetamide and triethyl phosphate mixed solvent, respectively.
[0011] Preferably, in steps (2) and (3), the preparation process involves floating the membrane on the liquid surface for sensitization, activation, chemical plating, and in-situ oxidation.
[0012] Preferably, in step (2), the sensitization time is 15 min, the activation time is 10 min, and the ratio of the electroless copper plating solution A, electroless copper plating solution B, and deionized water is 1:2:3.
[0013] Preferably, in step (3), the concentration of NaOH is 1-5M, the concentration of (NH4)2S2O8 is 0.13M, and the oxidation time is 2h.
[0014] The present invention also provides a copper oxide / polyvinylidene fluoride Janus photothermal film with biomimetic light traps and its preparation method.
[0015] This invention also provides the application of copper oxide / polyvinylidene fluoride Janus photothermal film in the photothermal film distillation treatment of automotive paint industry wastewater.
[0016] Compared with the shortcomings and deficiencies of existing technologies, this invention has the following advantages: (1) Polyvinylidene fluoride is used as a substrate, and chemical plating is performed on its surface, followed by in-situ oxidation to construct a Janus structure; (2) The copper oxide photothermal layer on the membrane surface has a biomimetic light trap, realizing high-efficiency photothermal conversion of the photothermal membrane; (3) The excellent hydrophilicity of copper oxide provides an anti-wetting effect; (4) The addition of the copper oxide hydrophilic photothermal layer improves the membrane's permeation flux, and the anchoring effect generated by in-situ oxidation can reduce concerns about photothermal materials falling off the membrane surface. The raw material cost for preparing the copper oxide / polyvinylidene fluoride Janus photothermal membrane is low, the preparation process is simple, and it is easy to operate and promote. Attached Figure Description
[0017] Figure 1 This is an electron microscope image of the surface of the copper oxide / polyvinylidene fluoride Janus photothermal film provided in Embodiment 1 of the present invention.
[0018] Figure 2 This is an electron microscope image of the surface of the copper oxide / polyvinylidene fluoride Janus photothermal film provided in Embodiment 2 of the present invention.
[0019] Figure 3 This is an electron microscope image of the surface of the copper oxide / polyvinylidene fluoride Janus photothermal film provided in Embodiment 3 of the present invention.
[0020] Figure 4 This is the XRD pattern of the copper oxide / polyvinylidene fluoride Janus photothermal film provided by the present invention.
[0021] Figure 5 This is a comparison chart of the photothermal performance of the copper oxide / polyvinylidene fluoride Janus photothermal film provided by this invention.
[0022] Figure 6 This is a comparison chart of the application performance of the copper oxide / polyvinylidene fluoride Janus photothermal film provided by this invention in the treatment of distilled wastewater. Detailed Implementation
[0023] The following specific examples will further illustrate the solution, advantages, and significance of this invention.
[0024] Preparations before the experiment:
[0025] 1. Preparation of copper oxide / polyvinylidene fluoride Janus photothermal film (CuO / PVDF):
[0026] (1) Preparation of polyvinylidene fluoride membrane (PVDF) by non-solvent phase separation (NIPS): 15% PVDF powder and 1% PVP powder were dissolved in a mixed solution of 34% DMAC and 50% TEP. The mixture was mechanically stirred at 60°C for 5 hours. After the powder was completely dissolved, it was placed in an oven for 24 hours to remove air bubbles and obtain the casting solution.
[0027] (2) Preparation of electroless copper / polyvinylidene fluoride (Cu / PVDF) membrane: First, pour the sensitizing solution into a petri dish, and float the cut PVDF membrane on the surface of the sensitizing solution, ensuring the rough side is in complete contact with the liquid surface for sensitization. After 15 minutes, remove the membrane and rinse it repeatedly with hot water. Then, using the same floating method, bring the sensitized membrane into contact with the surface of the activation solution for activation. After 10 minutes, remove the membrane and immerse it in deionized water. Finally, mix electroless copper plating solution A, electroless copper plating solution B, and deionized water in a 1:2:3 ratio, stir well, and pour the mixture into a petri dish. Float the treated membrane on the surface of the plating solution, electroless plate at 35°C for 2 hours, remove the membrane, rinse thoroughly with deionized water, and air dry.
[0028] (3) Preparation of in-situ oxidation of copper oxide / polyvinylidene fluoride Janus photothermal film (CuO / PVDF): NaOH and (NH4)2S2O8 were dissolved in 50 mL of deionized water. After stirring and mixing thoroughly, the mixture was poured into a petri dish. The Cu / PVDF film was floated on the oxidation solution and oxidized in situ for 2 h. After rinsing with deionized water, it was dried in an oven at 60 °C for 1 h.
[0029] Figure 1 , Figure 2 and Figure 3 SEM images of the copper oxide / polyvinylidene fluoride Janus photothermal films in Examples 1, 2, and 3 are shown respectively. The SEM images clearly show the presence of copper oxide with different microstructures on the surface of the polyvinylidene fluoride film. Figure 4 As shown in the XRD pattern, the characteristic peaks of copper oxide can be seen, proving that the copper oxide / polyvinylidene fluoride Janus photothermal film was effectively prepared.
[0030] Experimental procedure:
[0031] Simulated solar energy is provided by xenon lamps, and the illumination condition is maintained at 1 sun (1 kW / m²) using a light power density meter. 2 Simulating sunlight perpendicularly illuminating the surface of the photothermal film, the photothermal conversion capability of the copper oxide / polyvinylidene fluoride Janus photothermal film was measured by recording the surface temperature change using a thermal infrared imager. The prepared copper oxide / polyvinylidene fluoride Janus photothermal film was then placed in a self-made film module for experiments, with a light intensity of 1 kW / m². 2 Under conditions of maintaining room temperature of 25-30℃, actual wastewater from FAW-Volkswagen's painting workshop was pumped into the membrane module, and steam was drawn into the condenser at a vacuum pressure of 0.095MPa. The temperature of the condensate was 5℃. The mass of the condensate was measured and recorded using an electronic balance. The flux was calculated using the following formula:
[0032] J = Δm / (A×t), where: J is the flux (kg / (m³)). 2 ·h), where m is the mass of the condensate (kg), t is the time (h), and A is the effective area of the membrane (m²). 2 ).
[0033] The chemical oxygen content of the feed liquid and permeate in the photothermal film distillation experiment was determined using a COD analyzer to assess their organic matter content. The removal rate R was calculated using the following formula:
[0034] R = [(COD) F -COD P COD F [×100%, where: R is the removal rate, COD] F The organic matter concentration (mg / L) and COD of the feed liquid are given. P This represents the concentration of organic matter in the condensate (mg / L).
[0035] Example 1:
[0036] (1) Preparation of polyvinylidene fluoride membrane (PVDF) by non-solvent phase separation (NIPS): 15% PVDF powder and 1% PVP powder were dissolved in a mixed solution of 34% DMAC and 50% TEP. The mixture was mechanically stirred at 60°C for 5 hours. After the powder was completely dissolved, it was placed in an oven for 24 hours to remove air bubbles and obtain the casting solution.
[0037] (2) Preparation of electroless copper / polyvinylidene fluoride (Cu / PVDF) membrane: First, pour the sensitizing solution into a petri dish, and float the cut PVDF membrane on the surface of the sensitizing solution, ensuring the rough side is in complete contact with the liquid surface for sensitization. After 15 minutes, remove the membrane and rinse it repeatedly with hot water. Then, using the same floating method, bring the sensitized membrane into contact with the surface of the activation solution for activation. After 10 minutes, remove the membrane and immerse it in deionized water. Finally, mix electroless copper plating solution A, electroless copper plating solution B, and deionized water in a 1:2:3 ratio, stir well, and pour the mixture into a petri dish. Float the treated membrane on the surface of the plating solution, electroless plate at 35°C for 2 hours, remove the membrane, rinse thoroughly with deionized water, and air dry.
[0038] (3) Preparation of in-situ oxidation of copper oxide / polyvinylidene fluoride Janus photothermal film (CuO / PVDF): Dissolve 1M NaOH and 0.13M (NH4)2S2O8 in 50 mL of deionized water. After stirring thoroughly, pour the mixture into a petri dish and float the Cu / PVDF film on the surface of the oxidation solution for in-situ oxidation for 2 h. Remove the film, rinse it with deionized water, and then dry it in an oven at 60 °C for 1 h.
[0039] The prepared hydrangea-like copper oxide / polyvinylidene fluoride Janus photothermal film and its performance test results are as follows: Figure 1 ,like Figure 5 and Figure 6 As shown, under simulated sunlight conditions, after 10 minutes of illumination, the membrane surface temperature can reach a maximum of 46.8℃, while the wastewater membrane distillation permeate flux under no-light conditions is 3.52 kg / (m²). 2 ·h), while under 1 Sun illumination, the permeate flux of wastewater membrane distillation can reach 4.37 kg / (m²). 2 •h), the removal rate was 61.36%.
[0040] Example 2:
[0041] (1) Preparation of polyvinylidene fluoride membrane (PVDF) by non-solvent phase separation (NIPS): 15% PVDF powder and 1% PVP powder were dissolved in a mixed solution of 34% DMAC and 50% TEP. The mixture was mechanically stirred at 60°C for 5 hours. After the powder was completely dissolved, it was placed in an oven for 24 hours to remove air bubbles and obtain the casting solution.
[0042] (2) Preparation of electroless copper / polyvinylidene fluoride (Cu / PVDF) membrane: First, pour the sensitizing solution into a petri dish, and float the cut PVDF membrane on the surface of the sensitizing solution, ensuring the rough side is in complete contact with the liquid surface for sensitization. After 15 minutes, remove the membrane and rinse it repeatedly with hot water. Then, using the same floating method, bring the sensitized membrane into contact with the surface of the activation solution for activation. After 10 minutes, remove the membrane and immerse it in deionized water. Finally, mix electroless copper plating solution A, electroless copper plating solution B, and deionized water in a 1:2:3 ratio, stir well, and pour the mixture into a petri dish. Float the treated membrane on the surface of the plating solution, electroless plate at 35°C for 2 hours, remove the membrane, rinse thoroughly with deionized water, and air dry.
[0043] (3) Preparation of in-situ oxidation of copper oxide / polyvinylidene fluoride Janus photothermal film (CuO / PVDF): 2.5M NaOH and 0.13M (NH4)2S2O8 were dissolved in 50 mL of deionized water. After thorough mixing, the mixture was poured into a petri dish, and the Cu / PVDF film was floated on the surface of the oxidation solution for in-situ oxidation for 2 h. The film was then removed, rinsed with deionized water, and dried in an oven at 60 °C for 1 h.
[0044] The prepared rose-shaped copper oxide / polyvinylidene fluoride Janus photothermal film and its performance test results are as follows: Figure 2 ,like Figure 5 and Figure 6 As shown, under simulated sunlight conditions, after 10 minutes of illumination, the membrane surface temperature can reach a maximum of 66.2℃, while the wastewater membrane distillation permeate flux under no-light conditions is 3.75 kg / (m²). 2 ·h), while under 1 Sun illumination, the permeate flux of wastewater membrane distillation can reach 5.13 kg / (m²). 2 •h), the removal rate was 61.56%.
[0045] Example 3:
[0046] (1) Preparation of polyvinylidene fluoride membrane (PVDF) by non-solvent phase separation (NIPS): 15% PVDF powder and 1% PVP powder were dissolved in a mixed solution of 34% DMAC and 50% TEP. The mixture was mechanically stirred at 60°C for 5 hours. After the powder was completely dissolved, it was placed in an oven for 24 hours to remove air bubbles and obtain the casting solution.
[0047] (2) Preparation of electroless copper / polyvinylidene fluoride (Cu / PVDF) membrane: First, pour the sensitizing solution into a petri dish, and float the cut PVDF membrane on the surface of the sensitizing solution, ensuring the rough side is in complete contact with the liquid surface for sensitization. After 15 minutes, remove the membrane and rinse it repeatedly with hot water. Then, using the same floating method, bring the sensitized membrane into contact with the surface of the activation solution for activation. After 10 minutes, remove the membrane and immerse it in deionized water. Finally, mix electroless copper plating solution A, electroless copper plating solution B, and deionized water in a 1:2:3 ratio, stir well, and pour the mixture into a petri dish. Float the treated membrane on the surface of the plating solution, electroless plate at 35°C for 2 hours, remove the membrane, rinse thoroughly with deionized water, and air dry.
[0048] (3) Preparation of in-situ oxidation of copper oxide / polyvinylidene fluoride Janus photothermal film (CuO / PVDF): Dissolve 5M NaOH and 0.13M (NH4)2S2O8 in 50 mL of deionized water. After stirring thoroughly, pour the mixture into a petri dish and float the Cu / PVDF film on the oxidation solution for in-situ oxidation for 2 h. Remove the film, rinse it with deionized water, and then dry it in an oven at 60 °C for 1 h.
[0049] The prepared maple leaf-shaped copper oxide / polyvinylidene fluoride Janus photothermal film and its performance test results are as follows: Figure 3 ,like Figure 5 and Figure 6 As shown, under simulated sunlight conditions, after 10 minutes of illumination, the membrane surface temperature can reach a maximum of 63.9℃, while the wastewater membrane distillation permeate flux under no-light conditions is 3.16 kg / (m²). 2 ·h), while under 1 Sun illumination, the permeate flux of wastewater membrane distillation can reach 4.43 kg / (m²). 2 •h), the removal rate was 62.37%.
[0050] Comparative Example 1:
[0051] Preparation of electroless copper / polyvinylidene fluoride photothermal film (Cu / PVDF): The steps (1) and (2) in Example 1 are exactly the same, and a copper / polyvinylidene fluoride photothermal film (Cu-PVDF) is obtained.
[0052] The performance test results of the prepared copper / polyvinylidene fluoride membrane are as follows: Figure 5 and Figure 6 As shown, under simulated sunlight conditions, after 10 minutes of illumination, the membrane surface temperature can reach a maximum of 40.1℃, while the wastewater membrane distillation permeate flux under no-light conditions is 4.66 kg / (m²). 2 ·h), while under 1 Sun illumination, the permeate flux of wastewater membrane distillation can reach 5.00 kg / (m²). 2 •h), the removal rate was 62.46%.
[0053] Comparative Example 2:
[0054] Preparation of polyvinylidene fluoride membrane: The steps are exactly the same as in Example 1 (1), and a PVDF membrane is obtained.
[0055] The performance test results of the prepared polyvinylidene fluoride membrane are as follows: Figure 5 and Figure 6 As shown, under simulated sunlight conditions, after 10 minutes of illumination, the membrane surface temperature can reach a maximum of 37.4℃, while the wastewater membrane distillation permeate flux under no-light conditions is 28.32 kg / (m²). 2 ·h), while under 1 Sun illumination, the permeate flux of wastewater membrane distillation can reach 29.63 kg / (m²). 2 During the treatment of wastewater from photothermal membrane distillation, the PVDF membrane experiences membrane wetting and loses its separation performance, resulting in a removal rate of 0%.
[0056] Conclusion: Through Figure 5 , Figure 6 It can be seen that, compared with polyvinylidene fluoride (PVDF) membranes, copper / PVDF membranes and copper oxide / PVDF Janus photothermal membranes have better anti-wetting effects. Compared with PVDF membranes and copper / PVDF membranes, copper oxide / PVDF Janus photothermal membranes exhibit superior photothermal performance. With changes in preparation conditions, the photothermal conversion capacity increases with increasing alkali concentration. Under the same illumination conditions, the membrane with better photothermal performance has a higher permeation flux when treating wastewater by photothermal membrane distillation. Under the preparation conditions of Example 2, the copper oxide / PVDF Janus photothermal membrane exhibits the best photothermal performance and the highest permeation flux. The copper oxide / PVDF Janus membrane structure with photo-trapping properties combines anti-wetting and photothermal conversion properties. Therefore, copper oxide / PVDF Janus photothermal membranes can be used for photothermal membrane distillation in the treatment of automotive painting wastewater, demonstrating advantages in maximizing the use of renewable energy to produce freshwater.
Claims
1. A copper oxide / polyvinylidene fluoride Janus photothermal film, its preparation method and application, characterized in that, Includes the following steps: (1) Preparation of polyvinylidene fluoride membrane (PVDF) by non-solvent phase separation (NIPS): A certain amount of polyvinylidene fluoride (PVDF) powder and polyvinylpyrrolidone (PVP) powder were dissolved in N,N-dimethylacetamide (DMAC) and triethyl phosphate (TEP), mechanically stirred at 60°C for 5 h, and allowed to stand for degassing for 24 h to obtain PVDF casting solution; (2) Preparation of electroless copper / polyvinylidene fluoride (Cu / PVDF) membrane: First, the polyvinylidene fluoride (PVDF) membrane was sensitized in a sensitizing solution, then rinsed repeatedly with hot water, and then activated in an activation solution. After immersion in deionized water, the membrane was then mixed with electroless copper plating solution A, electroless copper plating solution B and deionized water in a certain proportion, stirred evenly and poured into a petri dish. After electroless plating at 35°C for 2 hours, the membrane was removed, rinsed with plenty of deionized water and then dried in the air. (3) Preparation of in-situ oxidation of copper oxide / polyvinylidene fluoride photothermal film (CuO / PVDF): NaOH and (NH4)2S2O8 of different concentrations were dissolved in 50 ml of deionized water; after stirring and mixing, the mixture was poured into a petri dish and oxidized in situ for a certain time. The mixture was then rinsed with deionized water and dried in an oven at 60℃ for 1 h.
2. The preparation method according to claim 1, characterized in that, In step (1), the casting solution contains 15% polyvinylidene fluoride, 1% polyvinylpyrrolidone, and 34% and 50% N,N-dimethylacetamide and triethyl phosphate mixed solvent, respectively.
3. The preparation method according to claim 1, characterized in that, In steps (2) and (3), in order to construct the Janus structure, the preparation process involves floating the polyvinylidene fluoride membrane on the liquid surface for sensitization, activation, chemical plating, and in-situ oxidation.
4. The preparation method according to claim 1, characterized in that, In step (2), the sensitization time is 15 min, the activation time is 10 min, and the ratio of the electroless copper plating solution A, electroless copper plating solution B and deionized water is 1:2:
3.
5. The preparation method according to claim 1, characterized in that, In step (3), the concentrations of NaOH are 1M, 2.5M and 5M, the concentration of (NH4)2S2O8 is 0.13M, and the oxidation time is 2h.
6. A copper oxide / polyvinylidene fluoride Janus photothermal film with biomimetic light trapping prepared by the preparation method according to any one of claims 1-5.
7. A copper oxide / polyvinylidene fluoride Janus photothermal film as described in claim 6, applied to the photothermal film distillation treatment of automotive paint spraying industrial wastewater.