Photothermal nanofiber membrane capable of trapping volatile organic compounds, preparation method and application thereof

Abstract

This invention relates to the field of separation membrane technology, and particularly to a photothermal nanofiber membrane capable of retaining volatile organic compounds (VOCs), its preparation method, and its applications. This photothermal nanofiber membrane employs a multi-structure synergistic design to comprehensively optimize the overall performance of membrane distillation. In the gradient wettability structure of the photothermal nanofiber membrane, the hydrophobic layer, with its low thermal conductivity, suppresses heat conduction loss and alleviates temperature polarization; the hydrophilic layer, through its hydration effect, suppresses solute enrichment and alleviates concentration polarization; the nano-thin-coated PDA photothermal layer achieves efficient photothermal conversion through the light trapping effect, while simultaneously providing sufficient anchoring points for the loaded defective UiO-66 nanoparticles; the synergistic effect of each functional layer achieves multifunctional integration of thermal management, anti-fouling, photothermal conversion, and VOCs retention, comprehensively improving the operating efficiency and stability of the membrane distillation system. The preparation method provided by this invention offers strong process controllability and possesses good reproducibility and potential for large-scale application.

Description

Technical Field

[0001] This invention relates to the field of separation membrane technology, and in particular to photothermal nanofiber membranes capable of retaining volatile organic compounds, their preparation methods, and applications. Background Technology

[0002] Water treatment is a crucial method for addressing water scarcity and pollution issues. The purification of wastewater containing volatile organic compounds (VOCs) is a key focus and challenge in the current water treatment field. Conventional water treatment processes have significant limitations: biological methods are easily inhibited by VOC toxicity, making efficient degradation difficult; reverse osmosis (RO) is limited by osmotic pressure, limiting its ability to retain small-molecule VOCs; adsorption and stripping methods may suffer from adsorbent saturation and secondary pollution, failing to meet the demands for advanced treatment and resource recovery. Against this backdrop, membrane distillation (MD), a thermally driven hydrophobic membrane separation technology, has become an important method for treating recalcitrant water due to its advantages such as no osmotic pressure limitations, low-temperature operation (40–80℃), and strong turbidity resistance. It can efficiently treat wastewater containing large-molecule organic compounds while leveraging low-grade heat sources such as industrial waste heat to reduce energy consumption, achieving water purification and volume reduction goals. However, MD still faces significant technical bottlenecks in water treatment applications: ordinary hydrophobic membranes are prone to wetting during long-term operation, leading to the penetration of salt and small molecule pollutants, which affects the quality of the produced water; VOCs in the water are easily adsorbed on the membrane surface, causing membrane fouling and significantly reducing the treatment flux; in addition, small molecule VOCs are easy to pass through the membrane with water vapor, causing secondary pollution of the produced water, which requires modification of the membrane material to meet the treatment requirements.

[0003] Photothermal membrane distillation (PMD), as a low-carbon upgrade of MD technology, achieves direct conversion of solar energy into thermal energy by combining photothermal materials with hydrophobic membranes. This eliminates the need for external heat sources and large energy inputs, retaining the advantages of MD technology in water treatment while further improving thermal utilization efficiency. PMD, through its localized interfacial heating characteristics, can reduce VOCs emissions caused by overall temperature rise in the feed solution. Furthermore, it is easily functionalized through composite catalysis and adsorption to achieve in-situ retention and degradation of VOCs and recalcitrant organic matter in water, further expanding its application range in complex wastewater treatment. Traditional PMD mainly suffers from the following technical problems: Firstly, constructing a dense separation layer to intercept VOCs using the molecular sieving effect will significantly increase the water vapor mass transfer resistance, resulting in a sharp decrease in membrane distillation flux, making it impossible to balance the interception efficiency and water production efficiency. Secondly, physically loading traditional adsorbents such as activated carbon and zeolite onto the membrane surface has problems such as weak binding force between the adsorbent and the substrate, easy detachment under water flow, easy saturation of adsorption sites, inability to achieve in-situ regeneration, and poor long-term operational stability. Third, the design of the photothermal functional layer and the VOCs adsorption functional layer lacks synergy. The heat energy generated by photothermal conversion is only used to drive membrane distillation and cannot provide energy for adsorbent regeneration, resulting in low functional integration. Fourth, existing modification schemes are prone to damaging the gradient wettability structure of Janus membranes, and cannot effectively alleviate concentration polarization and temperature polarization during membrane distillation, resulting in a decrease in the basic desalination performance and operational stability of the membrane.

[0004] The aforementioned problems severely restrict the application of PMD technology in complex wastewater systems containing VOCs. Therefore, developing membrane materials that combine high-efficiency photothermal membrane distillation performance with long-term VOCs retention capacity is a technical problem that urgently needs to be solved. Summary of the Invention

[0005] The present invention aims to at least solve one of the technical problems existing in the related art. To this end, the first objective of the present invention is to provide a method for preparing a photothermal nanofiber membrane capable of retaining volatile organic compounds; the second objective of the present invention is to provide a photothermal nanofiber membrane capable of retaining volatile organic compounds; and the third objective of the present invention is to provide applications of the photothermal nanofiber membrane capable of retaining volatile organic compounds.

[0006] To achieve the first objective, the technical solution adopted by this invention is as follows: A method for preparing a photothermal nanofiber membrane capable of retaining volatile organic compounds includes: a. Using hydrophobic spinning solution and hydrophilic spinning solution as raw materials, a hydrophobic layer is first prepared by electrospinning technology, and then a hydrophilic layer is prepared by electrospinning directly on the surface of the hydrophobic layer. The hydrophobic layer and the hydrophilic layer are stacked to form a Janus nanofiber base film. Janus nanofiber-based membranes refer to functional films with hydrophobic and hydrophilic properties on both sides. b. Modify the hydrophilic layer of the Janus nanofiber base film with dopamine to obtain a PDA@Janus photothermal composite film; c. Immerse the PDA@Janus photothermal composite film in a dispersion containing UiO-66 nanoparticles, allow it to stand at 20-30°C for 8-16 hours, then remove and dry it to obtain a photothermal nanofiber film that can retain volatile organic compounds.

[0007] Further, the preparation of the hydrophobic spinning solution includes the following steps: a1. Prepare PS solution and TPU solution respectively; PS stands for polystyrene, and TPU stands for thermoplastic polyurethane. a2. Mix the PS solution and the TPU solution to obtain a mixed solution, wherein the mass ratio of PS to TPU in the mixed solution is 1:5 to 5:1; a3. Add PDMS to the mixed solution and stir until homogeneous to obtain a hydrophobic spinning solution; In the hydrophobic spinning solution, the mass ratio of PDMS to the combined mass of PS and TPU is 1:20 to 1:5. PDMS stands for polydimethylsiloxane.

[0008] Furthermore, the solvents for both the PS solution and the TPU solution are selected from N,N-dimethylformamide.

[0009] Furthermore, the mass concentrations of the PS solution and the TPU solution are 12% to 25%.

[0010] Further, the preparation of the hydrophilic layer spinning solution includes the following steps: a10. Add PMDA and ODA to N,N-dimethylformamide, stir to react, and obtain PAA solution; Among them, PMDA is pyromellitic dianhydride, ODA is 4,4'-diaminodiphenyl ether, and PAA is polyacrylic acid; a20. Dissolve PMIA in DMAC to obtain a PMIA solution; Among them, PMIA is poly(m-phenylene isophthalamide), and DMAC is N,N-dimethylacetamide; a30. Mix PAA solution and PMIA solution evenly to obtain hydrophilic spinning solution, wherein the mass ratio of PMIA to PAA in the hydrophilic spinning solution is 1:3 to 5:1.

[0011] Furthermore, the UiO-66 nanoparticles are defective UiO-66 nanoparticles, and the preparation of the defective UiO-66 nanoparticles includes the following steps: c1. Zirconium chloride and terephthalic acid are added to an organic solvent at a molar ratio of 0.8:1 to 1.2:1 to dissolve them and obtain a precursor base solution. c2. Add a small molecule acid defect modifier to the precursor base solution, stir evenly, react at 140-160℃ for 8-16 hours, cool to 20-30℃, centrifuge, collect the precipitate, wash and dry to obtain defective UiO-66 nanoparticles.

[0012] Furthermore, the organic solvent is N,N-dimethylformamide, and the small molecule acid defect modifier is acetic acid.

[0013] A ligand-deficient defect was constructed in UiO-66 by inducing defects using acetic acid as a regulator. Simultaneously, because acetic acid inhibits the growth of the UiO-66(100) crystal facet, it promotes the formation of a truncated octahedral structure, thereby exposing the crystal facet defects. The core of this defect system is the unsaturated Zr site generated during the ligand deficiency process, which can serve as a Lewis acidic active center and is key to achieving high-efficiency adsorption performance.

[0014] In practical applications, the ligand-deficient defect exposes a large number of unsaturated Zr active sites, which can specifically coordinate with the oxygen-containing functional groups of VOCs (such as phenol), significantly enhancing its selective adsorption capacity for VOCs in water. The unsaturated Zr sites form strong coordination bonds with functional groups in polydopamine (PDA) molecules. This interaction enables uniform and stable loading of UiO-66 on the membrane surface, effectively preventing UiO-66 particle aggregation and ensuring excellent stability and no shedding during long-term operation. The defect structure not only optimizes the adsorption-desorption kinetics of UiO-66 nanoparticles for VOCs but also, in conjunction with the photothermal heating effect, enables rapid thermal desorption of VOCs and in-situ regeneration of adsorption sites. Ultimately, this establishes a highly efficient photothermal cyclic adsorption-desorption-regeneration mechanism, overcoming the technical bottleneck of traditional adsorption materials being easily saturated and difficult to regenerate.

[0015] Furthermore, the washing solution required for washing is ethanol; The drying process is carried out at a temperature of 70–90℃ for 8–16 hours.

[0016] To achieve the second objective, the technical solution adopted by this invention is as follows: A photothermal nanofiber membrane capable of retaining volatile organic compounds is prepared using any of the above-described methods for preparing a photothermal nanofiber membrane capable of retaining volatile organic compounds.

[0017] To achieve the third objective, the technical solution adopted by this invention is as follows: The application of photothermal nanofiber membranes capable of retaining volatile organic compounds involves using these membranes to treat seawater containing volatile organic compounds.

[0018] The above-described one or more technical solutions in the embodiments of the present invention have at least one of the following technical effects: The photothermal nanofiber membrane capable of retaining volatile organic compounds provided by this invention has at least the following advantages: Firstly, it achieves synergy between photothermal membrane distillation and efficient VOCs removal, overcoming the industry pain point that traditional PMD technology cannot effectively remove VOCs from water. The composite membrane of this invention can achieve a removal rate of 99.57% for typical VOCs (phenol), and the phenol removal performance is improved by 81.8% compared with the base membrane without UiO-66 loading; at the same time, it can stably maintain a salt rejection rate of over 99.99% and a stable membrane distillation flux, achieving deep purification treatment of complex VOCs-containing wastewater without sacrificing water production efficiency.

[0019] Secondly, by utilizing a photothermal cyclic adsorption-desorption-regeneration mechanism, the problem of traditional adsorbents being easily saturated and unable to operate stably for long periods is solved. This invention utilizes the interfacial heating effect of the PDA photothermal layer to induce thermal desorption of phenol adsorbed in the UiO-66 channels. The desorbed phenol preferentially dissolves in the aqueous phase at the membrane surface and is carried away with the feed liquid during circulation, thereby achieving in-situ regeneration of the UiO-66 adsorption sites. This mechanism allows the membrane to maintain stable VOCs removal performance during 80 hours of continuous operation, significantly extending the membrane's lifespan and reducing operating costs.

[0020] Thirdly, the controlled synthesis and robust anchoring of defective UiO-66 were achieved, balancing VOCs adsorption capacity and functional layer stability. Through acetic acid-induced defect engineering, a truncated octahedral UiO-66 structure was successfully synthesized, exposing numerous unsaturated Zr active sites on its surface, significantly enhancing the adsorption capacity and selectivity for oxygen-containing VOCs. Simultaneously, by utilizing the coordination between the catechol groups of PDA and the unsaturated Zr sites of UiO-66, robust anchoring of the adsorbent on the fiber surface was achieved, effectively solving the problems of easy adsorbent detachment and poor functional layer stability inherent in traditional physical loading methods.

[0021] Fourth, a multi-structure synergistic design is adopted to comprehensively optimize the overall performance of membrane distillation. In the gradient wettability structure of the photothermal nanofiber membrane, the hydrophobic layer inhibits heat conduction loss and alleviates temperature polarization due to its low thermal conductivity, while the hydrophilic layer inhibits solute enrichment and alleviates concentration polarization through the hydration effect. The PDA photothermal layer coated with a nano-thin layer achieves a high efficiency of 88.7% photothermal conversion through the light trapping effect, while providing sufficient anchoring points for UiO-66. The synergistic effect of each functional layer realizes the multi-functional integration of thermal management, anti-fouling, photothermal conversion, and VOCs interception, comprehensively improving the operating efficiency and stability of the membrane distillation system.

[0022] The preparation method provided by this invention offers strong process controllability, excellent reproducibility, and potential for large-scale application. This method employs a combination of stepwise electrospinning, in-situ polymerization, hydrothermal synthesis, and coordination anchoring, eliminating the need for complex equipment and stringent reaction conditions. The structure and properties of each functional layer can be precisely controlled through process parameters. The laboratory batch preparations exhibit excellent reproducibility, laying a solid technical foundation for subsequent large-scale scaling.

[0023] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0024] Figure 1 The above are the characterization results of the defective UiO-66 nanoparticles provided in Example 1 of the present invention; wherein, Figure A is the SEM image of the defective UiO-66 nanoparticles (scale bar is 1 μm), Figure B is the SEM image of the defective UiO-66 nanoparticles (scale bar is 500 nm), and Figure C is the XRD pattern of the defective UiO-66 nanoparticles.

[0025] Figure 2 The figures show the morphology and structural characterization results of the Janus nanofiber substrate membrane provided in Example 1 of this invention; wherein, Figure A is a SEM image of the hydrophobic layer surface of the Janus nanofiber substrate membrane, Figure B is a SEM image of the hydrophilic layer surface of the Janus nanofiber substrate membrane, and Figure C is a SEM image of the cross-section of the Janus nanofiber substrate membrane.

[0026] Figure 3 Figure A shows the test results of the contact angles of the two sides of the Janus nanofiber base membrane provided in Embodiment 1 of the present invention; wherein, Figure A shows the test results of the contact angles of the hydrophobic side of the Janus nanofiber base membrane, and Figure B shows the test results of the contact angles of the hydrophilic side of the Janus nanofiber base membrane.

[0027] Figure 4 The image shows the X-ray energy dispersive spectroscopy (EDS) surface scan analysis results of the Janus nanofiber substrate membrane provided in Example 1 of this invention.

[0028] Figure 5 SEM images of photothermal nanofiber membranes capable of retaining volatile organic compounds provided in Examples 1 to 4, which are verification examples of the present invention.

[0029] Figure 6 SEM images of photothermal nanofiber membranes capable of retaining volatile organic compounds provided in Examples 1 and 5 to 7, which are verification examples of the present invention.

[0030] Figure 7 The EDS surface scan analysis results are provided as a verification example of the present invention.

[0031] Figure 8The following are the photothermal performance analysis results provided for the verification examples of the present invention; wherein, Figure A is the infrared thermal image of the photothermal nanofiber membrane that can retain volatile organic compounds provided in Examples 1 to 4 at steady-state temperature, Figure B is the infrared thermal image of the photothermal nanofiber membrane that can retain volatile organic compounds provided in Examples 5 to 7 and Example 1, and Figure C is the infrared thermal image of the photothermal nanofiber membrane that can retain volatile organic compounds provided in Example 1 when it reaches a stable temperature under different light intensities.

[0032] Figure 9 The analysis results of the distillation performance of the photothermal nanofiber membrane capable of retaining volatile organic compounds provided as a verification example of the present invention.

[0033] Figure 10 The PMD test results are provided as an application example of the present invention. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. The following embodiments are used to illustrate this invention, but cannot be used to limit the scope of this invention.

[0035] In the following embodiments, unless otherwise specified, the experimental methods used are conventional methods, and the materials and reagents used are commercially available, unless otherwise specified, and are carried out in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions.

[0036] Example 1 The process for preparing a photothermal nanofiber membrane capable of retaining volatile organic compounds is as follows: Step 1: Synthesis of defective UiO-66 nanoparticles.

[0037] Preparation of precursor base solution: Add zirconium chloride (ZrCl4) (1 mmol) and terephthalic acid (H2BDC) (1 mmol) to N,N-dimethylformamide (DMF) (30 mL) and stir magnetically (400 rpm) at room temperature (about 25 °C) until the solid is completely dissolved to obtain a clear and transparent precursor base solution.

[0038] Add acetic acid (1 mL) to the precursor base solution as a defect regulator, and stir magnetically (400 rpm, 30 min) until the system is mixed evenly. Transfer the mixture to a stainless steel high-pressure reactor lined with polytetrafluoroethylene, seal it, and place it in a forced-air drying oven. React at 150℃ for 12 h. After the reaction is completed, let the reactor cool naturally to room temperature (about 25℃), then centrifuge at 8000 rpm for 15 min, collect the white precipitate, and discard the supernatant. The white precipitate was washed three times with DMF, and then three times with anhydrous ethanol (to fully displace residual DMF and unreacted raw materials in the product channels). The washed product was then placed in a vacuum oven and dried at 80°C for 12 hours to obtain defective UiO-66 nanoparticles. The characterization results are as follows: Figure 1 As shown; Figure A shows the SEM image of the defective UiO-66 nanoparticles (scale bar is 1 μm), and Figure B shows the SEM image of the defective UiO-66 nanoparticles (scale bar is 500 nm). From Figures A and B, it can be seen that the defective UiO-66 nanoparticles have a regular truncated octahedral morphology and an average particle size of about 200 nm. Figure C shows the XRD pattern of the defective UiO-66 nanoparticles. From this figure, we can see that characteristic diffraction peaks appear at 2θ = 7.4°, 8.5°, and 25.7°. The slight changes in the peak shape in the low-angle region directly confirm the existence of the defect structure.

[0039] By inducing defect engineering with acetic acid regulator, ligand-deficient defects were constructed in UiO-66. At the same time, due to the growth inhibition of the (100) crystal plane by acetic acid, crystal plane exposure defects of truncated octahedral structure were formed. The core of the defects is the unsaturated Zr site (Lewis acidic active center) defect generated by ligand deficiency.

[0040] Step 2: Janus nanofiber membranes with gradient wettability are prepared using stepwise electrospinning technology, as follows: Preparation of hydrophobic spinning solution: Prepare 18wt% PS solution (solvent: DMF) and 18wt% TPU solution (solvent: DMF) by mass fraction, respectively, and stir at room temperature (about 25℃) for 6 hours until completely dissolved; then mix the PS solution and TPU solution at a mass ratio of 2:1. Then add PDMS to the mixed solution at a mass ratio of 1:10 to the sum of the masses of PS and TPU, and stir at room temperature (about 25℃) for 2 hours until PDMS is evenly dispersed to obtain PDMS@PS / TPU hydrophobic spinning solution.

[0041] Hydrophobic base film was prepared using electrospinning technology: The hydrophobic spinning solution prepared above was loaded into the syringe of the electrospinning equipment. A flat-headed metal needle with a specification of 0.8 mm was connected to the syringe outlet, and the needle was connected to the positive terminal of the high-voltage power supply. A metal roller covered with aluminum foil was used as a grounding receiving device. The spinning environment parameters were controlled as follows: room temperature of about 25℃ (temperature range controlled within 5℃), relative humidity of 45% (humidity range controlled within 5%), spinning voltage of 30kV, and receiving distance of 18cm. Electrospinning was carried out. After spinning, the resulting fiber film was placed in a vacuum oven at 60℃ and dried for 2h to obtain PDMS@PS / TPU hydrophobic base film.

[0042] Preparation of hydrophilic spinning solution: PMDA (6.7g) and 4,4'-diaminodiphenyl ether (ODA) (6.2g) were dissolved in DMF (73g) and magnetically stirred at room temperature for 6h to obtain polyacrylic acid (PAA) solution (15wt%). Mix PMIA (10g) and DMAC (7.2g) and stir until completely homogeneous to obtain a PMIA solution; The PMIA solution and PAA solution were mixed at a mass ratio of 2:1 and stirred until homogeneous to obtain the hydrophilic spinning solution.

[0043] Janus nanofiber base membrane was prepared using electrospinning technology: The previously prepared PDMS@PS / TPU hydrophobic base membrane was fixed on the rotating roller receiving device of the spinning equipment, and electrospinning was performed with a hydrophilic spinning solution. The spinning voltage was set to 25kV and the receiving distance was 18cm. A hydrophilic layer was formed on the surface of the hydrophobic base membrane. After spinning, the composite membrane was dried in a vacuum oven at 60℃ for 2h, and then pressed by a roller press at 2MPa pressure to obtain the Janus nanofiber base membrane.

[0044] Morphological and structural characterization results of Janus nanofiber-based membranes, such as Figure 2 As shown; Figure A shows the SEM image of the hydrophobic layer surface of the Janus nanofiber substrate membrane. From this image, it can be seen that the morphology of the electrospun nanofibers on the hydrophobic side is continuous and complete, without beading or breakage defects. The fiber diameter is uniform, and they overlap to form a three-dimensional interconnected porous network structure. This porous network not only provides a through channel for fluid transport, but also further enhances the intrinsic hydrophobicity of the material by means of surface roughness. It is the structural basis for the hydrophobic function of the hydrophobic side. Figure B shows the SEM image of the hydrophilic layer surface of the Janus nanofiber substrate membrane. From this image, it can be seen that compared with the hydrophobic side, the fiber diameter is finer, the fiber stacking is more dense, and there are no obvious structural defects. Combining Figures A and B, we can see that the Janus nanofiber-based membrane is an asymmetric bilayer composite structure, rather than a homogeneous single-layer membrane, which is a structural prerequisite for achieving asymmetric wettability; thus providing a structural basis for the membrane to achieve asymmetric wettability.

[0045] Figure C shows a SEM image of the cross-section of the Janus nanofiber-based membrane. The image reveals a tight bond between the hydrophobic and hydrophilic layers, without delamination or cracking, ensuring structural stability during application. Furthermore, the cross-section shows interconnected channels formed by fiber stacking, providing an effective transport path for transmembrane mass transfer, thus enabling the membrane to possess both excellent structural stability and mass transfer efficiency.

[0046] To verify the asymmetric wettability of the Janus nanofiber-based membrane, the contact angles on both sides were tested, and the results are as follows: Figure 3 As shown; Figure A shows the contact angle test results of the hydrophobic surface of the Janus nanofiber-based membrane. The contact angle of the hydrophobic side of the membrane is 146.8 ± 3.0°, and the water droplets on the surface are nearly spherical, indicating that this side has excellent hydrophobic properties. This hydrophobic property is a necessary condition for membranes used in membrane distillation, which can effectively prevent the feed liquid from penetrating into the interior of the fiber membrane and avoid membrane wetting failure. Figure B shows the contact angle test results of the hydrophilic side of the Janus nanofiber-based membrane. The contact angle of the hydrophilic side of the membrane is 51.11±3.0°, and water droplets spread rapidly on the surface, exhibiting good hydrophilicity. The wettability of the hydrophilic side is conducive to efficient water vapor transport and improves permeation flux; at the same time, this property can reduce heat loss, alleviate the temperature polarization effect, and inhibit the adhesion of pollutants such as oil droplets, thus reducing the risk of membrane fouling. Combining Figures A and B, we can see the significant difference in contact angles on both sides of the Janus nanofiber base membrane. This result confirms that the base membrane has hydrophobic-hydrophilic asymmetric wettability, which is the core performance basis for achieving unidirectional water transport, anti-wetting, and efficient separation.

[0047] Characteristic X-ray energy dispersive spectroscopy (EDS) surface scanning analysis was performed on the cross-section of the Janus nanofiber-based membrane to obtain the distribution mapping of carbon and silicon elements. The results are as follows: Figure 4 As shown in the figure, carbon is widely distributed in both the hydrophilic and hydrophobic layers, while silicon is concentrated only on the hydrophobic side. This significant difference in elemental distribution confirms that the membrane is a hydrophobic-hydrophilic bilayer composite structure, rather than a homogeneous membrane, providing direct chemical evidence for its asymmetric wettability.

[0048] Step 3: Preparation of dopamine (PDA) modified Janus photothermal composite film, the process is as follows: Preparation of dopamine solution: Prepare a Tris-HCl buffer solution with pH=8.5, add dopamine hydrochloride to it, and stir until completely dissolved to obtain a dopamine solution with a concentration of 2.0 mg / mL; After thoroughly wetting the Janus nanofiber-based membrane prepared above with deionized water, it was immersed in the above dopamine solution with its hydrophilic side facing down. The membrane was allowed to stand for 12 hours at room temperature (about 25°C) in the dark. After the reaction was completed, the membrane sample was taken out and repeatedly rinsed with deionized water to remove the residual free polydopamine particles on the membrane surface. Then it was placed in a 60°C forced-air oven to dry for 8 hours to obtain the PDA-modified Janus photothermal composite membrane, denoted as PDA@Janus (blank control membrane, without UiO-66 nanoparticles).

[0049] Step 4: Prepare a photothermal nanofiber membrane capable of retaining volatile organic compounds, as follows: The defective UiO-66 nanoparticles prepared above were added to a mixed solvent of water and anhydrous ethanol (volume ratio of water to anhydrous ethanol was 1:1), and ultrasonically dispersed at room temperature for 30 min to obtain a uniform dispersion of UiO-66 with a concentration of 3.0 mg / mL. The PDA@Janus photothermal composite film prepared above was cut into 5 cm × 5 cm pieces with the hydrophilic side facing the dispersion and completely immersed in the UiO-66 dispersion. The mixture was allowed to stand at room temperature (about 25 °C) for 12 h for coordination anchoring reaction. After the reaction was completed, the film sample was taken out, rinsed with deionized water to remove the physically adsorbed UiO-66 particles on the surface, and dried in a vacuum oven at 60 °C for 6 h to obtain the PDA@UiO-66 functionalized Janus photothermal composite film, which is the photothermal nanofiber film that can retain volatile organic compounds.

[0050] Example 2 A photothermal nanofiber membrane capable of retaining volatile organic compounds was prepared. The difference between this example and Example 1 is that the concentration of the uniformly dispersed UiO-66 solution in step four was replaced with 1.0 mg / mL. All other preparation steps and process parameters were the same as in Example 1.

[0051] Example 3 A photothermal nanofiber membrane capable of retaining volatile organic compounds was prepared. The difference between this example and Example 1 is that the concentration of the uniformly dispersed UiO-66 solution in step four was replaced with 2.0 mg / mL. All other preparation steps and process parameters were the same as in Example 1.

[0052] Example 4 A photothermal nanofiber membrane capable of retaining volatile organic compounds was prepared. The difference between this example and Example 1 is that the concentration of the uniformly dispersed UiO-66 solution in step four was replaced with 5.0 mg / mL. All other preparation steps and process parameters were the same as in Example 1.

[0053] Example 5 The photothermal nanofiber membrane capable of retaining volatile organic compounds was prepared. The difference between this example and Example 1 is that the coordination anchoring reaction time in step four was replaced with 6 hours, while the remaining preparation steps and process parameters were the same as in Example 1.

[0054] Example 6 The photothermal nanofiber membrane capable of retaining volatile organic compounds was prepared. The difference between this example and Example 1 is that the coordination anchoring reaction time in step four was replaced with 8 hours, while the rest of the preparation steps and process parameters were the same as in Example 1.

[0055] Example 7 The photothermal nanofiber membrane capable of retaining volatile organic compounds was prepared. The difference between this example and Example 1 is that the coordination anchoring reaction time in step four was replaced with 24 hours, while the rest of the preparation steps and process parameters were the same as in Example 1.

[0056] Verification Example The characterization of photothermal nanofiber membranes capable of retaining volatile organic compounds includes the following aspects: I. Morphological Analysis.

[0057] The coordination anchoring reaction lasted 12 hours. SEM images of photothermal nanofiber membranes capable of retaining volatile organic compounds, prepared using uniformly dispersed UiO-66 solutions of different concentrations, are shown below. Figure 5 As shown; When the concentration of UiO-66 is 1 mg / mL (corresponding to Example 2), there are only scattered UiO-66 particles on the fiber, and most of the fiber is exposed. This is because the amount of UiO-66 in the solution is limited, and the probability of the particles contacting the PDA active site is low within a fixed reaction time, so the coordination and anchoring reaction cannot be fully completed. When the concentration of UiO-66 reaches 2 mg / mL (corresponding to Example 3), the number of particles on the fiber surface increases significantly, but there are still some exposed areas. At this time, the particle concentration has not yet reached the saturation load limit of the PDA anchoring point. When the UiO-66 concentration reaches 3 mg / mL (corresponding to Example 1), a uniform and dense particle coating layer forms on the fiber surface, with clear boundaries between particles and no aggregation, exhibiting a relatively ideal distribution state. At this point, the number of UiO-66 particles in the solution reaches the optimal match with the number of PDA active sites, maximizing the coordination and anchoring effect, which is beneficial to the functional synergy between UiO-66 and the fiber substrate, thereby improving the removal performance of VOCs. When the concentration of UiO-66 was further increased to 5 mg / mL (corresponding to Example 4), the UiO-66 particles showed obvious stacking and local aggregation, indicating that the number of particles in the solution exceeded the PDA anchoring capacity. The surface of the already attached particles became a new attachment site, triggering secondary attachment and stacking behavior, which may reduce the membrane flux and weaken the adsorption capacity for VOCs.

[0058] The above results indicate that the concentration of the UiO-66 dispersion has a significant impact on its loading density.

[0059] SEM images of photothermal nanofiber membranes capable of retaining volatile organic compounds prepared with different coordination anchoring reaction times at a UiO-66 uniform dispersion concentration of 3 mg / mL are shown below. Figure 6 As shown; When the anchoring time was 6 hours (corresponding to Example 5), the number of UiO-66 particles on the fiber surface was small and unevenly distributed, with no obvious particle adhesion in most areas. This was not due to insufficient particle quantity in the solution, but because the coordination anchoring reaction had not been fully carried out: short-time soaking only achieved physical adhesion of the particles, lacking sufficient time for the formation of Zr-O coordination bonds, causing some particles to detach during subsequent washing, resulting in an actual load lower than expected. When the anchoring time is extended to 8 hours (corresponding to Example 6), the number of particles on the fiber surface increases, but the overall distribution is still relatively sparse, indicating that the coordination reaction has not yet reached saturation.

[0060] When the anchoring time reaches 12h (corresponding to Example 1), the loading effect of UiO-66 is significantly improved. The particles are regular truncated octahedrons with clear edges and corners, and are tightly bonded to the PDA coating interface, indicating that the coordination anchoring reaction has been fully completed at this time, and a stable Zr-O chemical bond has been formed.

[0061] When the anchoring time was further extended to 24 hours (corresponding to Example 7), two significant changes occurred in the sample: Firstly, the morphology of the particles changed: the UiO-66 particles gradually became blunted from their original angular truncated octahedrons, with the edges becoming blurred, and the overall shape transforming into a near-spherical shape, but the core still retained some angular contours. This may be due to the secondary polymerization or swelling of the PDA thin layer during the long-term immersion process, which gradually encapsulates the UiO-66 particles, forming a core-shell structure.

[0062] Secondly, changes in fibers and pores: the overall fiber profile is significantly thickened, the pores between fibers gradually narrow, and some areas even show signs of fiber fusion. This indicates that in a long-term water environment, the residual catechol groups on the molecular chain of the PDA coating undergo slow cross-linking, resulting in coating thickening and further coating of the loaded UiO-66 particles, making the interface between particles and fibers more blurred.

[0063] The above results indicate that the loading concentration and anchoring time of UiO-66 have multidimensional effects on the surface morphology and function of photothermal nanofiber membranes capable of retaining volatile organic compounds: The loading concentration mainly controls the loading density and dispersion state of UiO-66. At the optimal concentration of 3 mg / mL, a single layer of uniform coverage of particles on the fiber surface can be achieved. If the concentration is too low, the loading will be insufficient, and if it is too high, it will easily cause agglomeration, both of which are not conducive to the performance of the membrane.

[0064] The anchoring time affects both the coordination reaction process and the particle micromorphology. Under the optimal condition of 12 hours, the Zr-O coordination bonds are fully formed, achieving stable anchoring while preserving the complete morphology of the UiO-66 particles. Too short a time will lead to incomplete reaction and easy particle detachment; too long a time will trigger secondary cross-linking and coating behavior of PDA, blocking the adsorption channels of UiO-66 and reducing its effective contact with VOCs.

[0065] II. Chemical composition analysis.

[0066] To further verify the successful construction of the photothermal nanofiber membrane capable of retaining volatile organic compounds and the loading effect of UiO-66, the membrane sample from Example 1 was selected for EDS surface scanning analysis using low-magnification field emission scanning electron microscopy (FE-SEM). The mapping distribution results of carbon (C), oxygen (O), and zirconium (Zr) elements are as follows: Figure 7 As shown in the figure, zirconium was introduced into the UiO-66 metal cluster and is uniformly distributed on the membrane surface. This result indicates that PDA was successfully polymerized on the membrane surface and UiO-66 was successfully anchored on the membrane through its active sites.

[0067] III. Photothermal Performance Analysis.

[0068] Photothermal (irradiance of 1.0 kW·m) -2 Performance analysis results, such as Figure 8 As shown; Figure A shows the infrared thermographic images of the photothermal nanofiber membranes capable of retaining volatile organic compounds provided in Examples 1 to 4 at steady-state temperatures. From this figure, it can be seen that the photothermal nanofiber membranes provided in Examples 2 (corresponding to a UiO-66 dispersion concentration of 1 mg / mL) and 3 (corresponding to a UiO-66 dispersion concentration of 2 mg / mL) exhibit a dominant shading effect on the photothermal layer, with steady-state temperatures of 57.6℃ and 59.7℃, respectively. In contrast, the photothermal nanofiber membrane provided in Example 1 (corresponding to a UiO-66 dispersion concentration of 3 mg / mL) forms a uniform sub-monolayer coverage, and polyhedral scattering compensates for the extended optical path, causing the final steady-state temperature to rise to 63.1℃. The photothermal nanofiber membrane provided in Example 4 (corresponding to a UiO-66 dispersion concentration of 5 mg / mL) experiences increased shading and increased interfacial thermal resistance due to particle aggregation, causing the temperature to drop to 60.9℃.

[0069] Figure B shows the infrared thermal imaging of the photothermal nanofiber membranes capable of retaining volatile organic compounds provided in Examples 5-7 and Example 1. From this figure, it can be seen that: samples with shorter coordination times have insufficient anchoring capacity and a significant shading effect, with steady-state temperatures of 57.2℃ (corresponding to Example 5, anchoring reaction time of 6h) and 59.2℃ (corresponding to Example 6, anchoring reaction time of 8h), respectively; samples with a moderate and uniform loading capacity at 12h have a steady-state temperature of 63.1℃ (corresponding to Example 1, anchoring reaction time of 12h); when the time is extended to 24h (corresponding to Example 7, corresponding to an anchoring reaction time of 24h), due to secondary swelling of the PDA causing blurred particle edges, the thermal resistance increases while losing scattering compensation, and the temperature drops to 59.6℃.

[0070] Figure C shows the infrared thermal imaging of the photothermal nanofiber membrane capable of retaining volatile organic compounds provided in Example 1 when it reaches a stable temperature under different light intensities. From this figure, it can be seen that at 0.5, 1.0, 1.5, and 2.0 kW / m², the volatile organic compounds can be retained at various light intensities. 2 The steady-state temperature under light intensity increases nearly linearly, indicating that the photothermal nanofiber membrane, capable of retaining volatile organic compounds (VOCs) under different light conditions, maintains a stable photothermal response. Comparing the photothermal performance of each sample, the photothermal nanofiber membrane provided in Example 1, while minimizing the shading effect, provides sufficient active sites for the adsorption of volatile organic compounds (such as VOCs), thus achieving a balance between photothermal and adsorption functions.

[0071] IV. Analysis of the distillation performance of photothermal nanofiber membranes capable of retaining volatile organic compounds.

[0072] Under the conditions of a feed solution of NaCl aqueous solution (35 g / L) and a feed flow rate of 0.29 L / min, the performance of the photothermal nanofiber membranes capable of retaining volatile organic compounds provided in Examples 1 to 7 was tested. The membranes achieved a flow rate of 1.0 kW / m³. 2 Performance of photothermal membrane distillation (PMD) under light intensity, such as Figure 9 As shown. The PMD test results over 10 hours indicate that the average permeation flux of the photothermal composite membranes in each embodiment are as follows: Example 2 was 1.32 ± 0.05 L·m -2 ·h -1 Example 3 showed a concentration of 1.49 ± 0.05 L·m. -2 ·h -1 Example 1 showed a concentration of 1.65 ± 0.05 L·m. -2 ·h -1 Example 4 showed a concentration of 1.54 ± 0.05 L·m. -2 ·h -1 Example 5 showed a concentration of 1.35 ± 0.05 L·m. -2 ·h -1 Example 6 showed a concentration of 1.46 ± 0.05 L·m. -2 ·h -1 Example 7 showed a concentration of 1.48 ± 0.05 L·m. -2 ·h -1 Furthermore, the salt rejection rate of all membrane samples in the embodiments remained consistently above 99.99%.

[0073] This superior performance is attributed to the stable support structure provided by the Janus nanofiber base membrane. The PDA@Janus membrane surface is anchored and loaded with UiO-66 nanoparticles, which not only endows the membrane with VOCs adsorption function, but also does not cause significant blockage of the water vapor mass transfer channel, ensuring the smooth progress of the mass transfer process.

[0074] Further analysis revealed that as the UiO-66 loading concentration increased from 1 mg·mL⁻¹, the... -1 Increased to 3 mg / mL -1 UiO-66 particles gradually formed a uniform coating layer on the membrane surface, which improved the membrane's photothermal and temperature performance and kept the water vapor mass transfer channels unobstructed, thus increasing the membrane's permeation flux. When the UiO-66 loading concentration was further increased to 5 mg·mL⁻¹, -1 When excessive UiO-66 particles accumulate and agglomerate, the resistance to water vapor mass transfer increases significantly, which in turn causes the permeation flux to decrease.

[0075] The effect of UiO-66 loading time on membrane performance follows a similar pattern to that of loading concentration: when the loading time is 12 hours, the loading amount of UiO-66 on the membrane surface is moderate and uniformly distributed, and the photothermal performance and mass transfer efficiency achieve the best synergistic effect, at which point the membrane permeation flux is optimal. However, if the loading time is too short, the UiO-66 loading amount will be insufficient, and its photothermal and adsorption synergistic effect cannot be fully utilized. If the loading time is too long, excessive particle accumulation will occur, hindering the mass transfer process. Neither of these factors is conducive to the synergistic improvement of the membrane's photothermal performance and mass transfer efficiency.

[0076] Application examples Phenol is a typical volatile organic pollutant, widely found in industrial wastewater, chemical production waste liquid and domestic sewage. Its molecular dynamic diameter is about 0.69 nm, which can represent other phenols and oxygen-containing organics with similar volatility and molecular size in water bodies. It is often used to reflect the removal performance of membrane materials for VOCs in water bodies.

[0077] To test the ability of the photothermal nanofiber membrane to retain VOCs in water, a PMD test was conducted, and the results are as follows: Figure 10 As shown in the figure, this figure illustrates the phenol concentration in the permeate of a photothermal nanofiber membrane (Example 1) and a PDA@Janus membrane (without UiO-66 nanoparticles) (as a control membrane) capable of retaining volatile organic compounds after 10 hours of continuous PMD operation at different phenol feed concentrations. From this figure, it can be seen that the phenol concentration varies from 5 to 200 mg / L. -1 Within the specified range, the photothermal nanofiber membrane modified with UiO-66 exhibited significantly lower phenol concentrations on the permeate side compared to the control membrane, with this performance advantage being most pronounced at low feed concentrations. While the removal efficiency decreased slightly with increasing feed phenol concentration, it remained at a relatively high retention level. This trend fully demonstrates the highly efficient phenol removal capability of the UiO-66 functional layer: UiO-66 possesses well-defined sub-nanopores and unsaturated Zr active sites, providing ample adsorption sites under low concentration conditions for efficient phenol enrichment and retention. Even as the feed concentration increases and adsorption gradually approaches saturation, its self-regenerating properties effectively control phenol transmembrane penetration, indicating that this functional layer exhibits good adaptability to different phenol concentration loads.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a photothermal nanofiber membrane capable of trapping volatile organic compounds, characterized by, include: a. Using hydrophobic spinning solution and hydrophilic spinning solution as raw materials, a hydrophobic layer is first prepared by electrospinning technology, and then a hydrophilic layer is prepared by electrospinning directly on the surface of the hydrophobic layer. The hydrophobic layer and the hydrophilic layer are stacked to form a Janus nanofiber base film. Janus nanofiber-based membranes refer to functional films with hydrophobic and hydrophilic properties on both sides. b. Modify the hydrophilic layer of the Janus nanofiber base film with dopamine to obtain a PDA@Janus photothermal composite film; c. Immerse the PDA@Janus photothermal composite film in a dispersion containing UiO-66 nanoparticles, allow it to stand at 20-30°C for 8-16 hours, then remove and dry it to obtain a photothermal nanofiber film that can retain volatile organic compounds.

2. The method for preparing the photothermal nanofiber membrane capable of retaining volatile organic compounds as described in claim 1, characterized in that, The preparation of the hydrophobic spinning solution includes the following steps: a1. Prepare PS solution and TPU solution respectively; PS stands for polystyrene, and TPU stands for thermoplastic polyurethane. a2. Mix the PS solution and the TPU solution to obtain a mixed solution, wherein the mass ratio of PS to TPU in the mixed solution is 1:5 to 5:1; a3. Add PDMS to the mixed solution and stir until homogeneous to obtain a hydrophobic spinning solution; In the hydrophobic spinning solution, the mass ratio of PDMS to the combined mass of PS and TPU is 1:20 to 1:

5. PDMS stands for polydimethylsiloxane.

3. The method of claim 2, wherein the photo-thermal nanofiber membrane is prepared by electrospinning a solution of the polymer and the volatile organic compound. The solvents for both the PS solution and the TPU solution are selected from N,N-dimethylformamide.

4. The method for preparing the photothermal nanofiber membrane capable of retaining volatile organic compounds as described in claim 2, characterized in that, The mass concentration of PS solution and TPU solution is 12% to 25%.

5. The method for preparing the photothermal nanofiber membrane capable of retaining volatile organic compounds as described in claim 1, characterized in that, The preparation of the hydrophilic layer spinning solution includes the following steps: a10. Add PMDA and ODA to N,N-dimethylformamide, stir to react, and obtain PAA solution; Among them, PMDA is pyromellitic dianhydride, ODA is 4,4'-diaminodiphenyl ether, and PAA is polyacrylic acid; a20. Dissolve PMIA in DMAC to obtain a PMIA solution; Among them, PMIA is poly(m-phenylene isophthalamide), and DMAC is N,N-dimethylacetamide; a30. Mix PAA solution and PMIA solution evenly to obtain hydrophilic spinning solution, wherein the mass ratio of PMIA to PAA in the hydrophilic spinning solution is 1:3 to 5:

1.

6. The method for preparing the photothermal nanofiber membrane capable of retaining volatile organic compounds as described in claim 1, characterized in that, The UiO-66 nanoparticles are defective UiO-66 nanoparticles, and the preparation of the defective UiO-66 nanoparticles includes the following steps: c1. Zirconium chloride and terephthalic acid are added to an organic solvent at a molar ratio of 0.8:1 to 1.2:1 to dissolve them and obtain a precursor base solution. c2. Add a small molecule acid defect modifier to the precursor base solution, stir evenly, react at 140-160℃ for 8-16 hours, cool to 20-30℃, centrifuge, collect the precipitate, wash and dry to obtain defective UiO-66 nanoparticles.

7. The method for preparing the photothermal nanofiber membrane capable of retaining volatile organic compounds as described in claim 6, characterized in that, The organic solvent is N,N-dimethylformamide, and the small molecule acid defect modifier is acetic acid.

8. The method for preparing the photothermal nanofiber membrane capable of retaining volatile organic compounds as described in claim 6, characterized in that, The washing solution required for washing is ethanol; The drying process is carried out at a temperature of 70–90℃ for 8–16 hours.

9. A photothermal nanofiber membrane capable of retaining volatile organic compounds, characterized in that, It is prepared using the photothermal nanofiber membrane capable of retaining volatile organic compounds as described in any one of claims 1 to 8.

10. The application of photothermal nanofiber membranes capable of retaining volatile organic compounds, characterized in that, Seawater containing volatile organic compounds is treated using the photothermal nanofiber membrane described in claim 9, which can retain volatile organic compounds.

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