A composite film and a preparation method and application thereof

Abstract

The application discloses a composite membrane and a preparation method and application thereof, and belongs to the field of membrane separation technology and water treatment. The composite membrane comprises a polyvinylidene fluoride microporous filter membrane substrate, a bonding layer, a hydrophilic layer and a hydrophobic protruding structure distributed on the surface of the hydrophilic layer which are arranged in a stack; the preparation raw material of the bonding layer comprises dopamine and polyethylene imine; the hydrophilic layer comprises alginate and calcium salt; and the preparation raw material of the hydrophobic protruding structure comprises a hydrophobic and oleophilic inorganic filler. The PDA / PEI intermediate bonding layer and the SA hydrophilic layer are used to improve the oil stain resistance of the membrane surface, and through the capture-coalescence effect of the hydrophobic protruding structure and the hydrophilic repulsion effect of the alginate hydrophilic layer, the demulsification and separation of the oil-in-water emulsion which is stable to a surfactant are realized. The composite membrane has the advantages of high separation efficiency, excellent anti-pollution and self-cleaning performance.

Description

Technical Field

[0001] This invention relates to the field of membrane separation and water treatment, and particularly to a composite membrane, its preparation method, and its application. Background Technology

[0002] With the acceleration of industrialization, industries such as petrochemicals, machinery processing, food processing, and transportation generate large amounts of oily wastewater. This wastewater contains not only floating and dispersed oil but also highly kinetically stable emulsified oil. Because emulsified oil droplets typically range in size from nanometers to micrometers, their dispersion stability is extremely high, making traditional separation methods ineffective. Direct discharge of such wastewater would pollute the ecological environment, especially aquatic environments. Therefore, developing efficient and low-energy-consumption oil-water separation technologies has become a major issue urgently needing to be addressed in the field of environmental governance. Traditional methods for treating oily wastewater mainly include gravity separation, flotation, flocculation, adsorption, and centrifugal separation. However, these methods are often inefficient when treating micron-sized emulsified oil and suffer from high energy consumption, large chemical reagent consumption, and the potential for secondary pollution. In contrast, membrane separation technology, due to its significant advantages such as high separation efficiency, low energy consumption, simple operation, and no phase change, is widely recognized as one of the effective means of treating oily wastewater. In particular, "superhydrophilic-underwater superoleophobic" membranes based on special wettability, utilizing the barrier effect of the surface hydration layer, can effectively intercept oil droplets and achieve efficient water permeation, attracting widespread attention in the field of oil-water separation. Although such superhydrophilic membranes have shown good separation effects on oily wastewater, the separation mechanism of this type of membrane for oil-water emulsions is mainly based on sieving and interception by the hydration layer. The repelled emulsion droplets may remain in a dispersed state and are unlikely to spontaneously aggregate to form larger droplets, which may lead to demulsification failure. Summary of the Invention

[0003] This invention addresses the technical problems of existing oil-water separation membranes when processing water-in-oil emulsions containing surfactants, such as low separation efficiency, easy oil contamination of the membrane surface leading to a sharp drop in flux, weak adhesion between the hydrophilic modified coating and the hydrophobic substrate, easy detachment under water flow, and difficulty in bonding the hydrophobic heterostructure with the hydrophilic surface. It provides a biomimetic heterogeneous wettability oil-water separation composite membrane based on 3D printing technology, its preparation method, and its application.

[0004] To achieve the above objectives, the present invention provides a composite membrane, the composite membrane comprising a polyvinylidene fluoride microporous filter membrane substrate, an adhesive layer, a hydrophilic layer, and hydrophobic protrusions distributed on the surface of the hydrophilic layer, which are stacked together. The raw materials for preparing the adhesive layer include dopamine and polyethyleneimine; The hydrophilic layer comprises alginate and calcium salt; The raw materials for preparing the hydrophobic protrusion structure include hydrophobic and oleophilic inorganic fillers.

[0005] The composite membrane proposed in this invention comprises a superimposed polyvinylidene fluoride (PVDF) microporous filter membrane substrate, an adhesive layer, a hydrophilic layer, and hydrophobic protrusions distributed on the surface of the hydrophilic layer. The hydrophilic layer, made from alginate and calcium salts, imparts excellent hydrophilic properties to the membrane surface, forming a stable hydration layer that blocks oil droplet adhesion, thereby reducing membrane fouling. The hydrophobic protrusions, made from hydrophobic and oleophilic inorganic fillers, form a heterogeneous wetting structure on the surface of the hydrophilic layer. Through the synergistic effect of the hydrophobic protrusions capturing and coalescing emulsified oil droplets and the repulsive effect of the hydrophilic layer, efficient demulsification and separation of surfactant-stabilized oil-in-water emulsions are achieved. The PVDF microporous filter membrane itself exhibits weak hydrophobic properties, which allows for better surface activation and firm anchoring of the functional layer during subsequent dopamine and polyethyleneimine co-deposition modification, laying the foundation for constructing a highly stable biomimetic heterogeneous wetting composite membrane.

[0006] According to an embodiment of the present invention, the alginate includes at least one of sodium alginate and potassium alginate.

[0007] According to an embodiment of the present invention, the hydrophobic and oleophilic inorganic filler comprises calcium carbonate and fumed nano-silica.

[0008] According to an embodiment of the present invention, the height of the hydrophobic protrusion structure is 80-200 μm, the width of the hydrophobic protrusion structure is 0.7-1.1 mm, and the spacing between the hydrophobic protrusion structures is 1.0-2.5 mm.

[0009] According to an embodiment of the present invention, the height of the hydrophobic protrusion structure is 80~200μm. The height of the hydrophobic protrusion structure can be, for example, 100μm, 116μm, 128μm, 161μm, 200μm or any value between 80 and 200μm.

[0010] According to an embodiment of the present invention, the width of the hydrophobic protrusion structure is 0.7 to 1.1 mm. For example, the width of the hydrophobic protrusion structure can be 0.9 mm or any value between 0.7 and 1.1 mm.

[0011] According to an embodiment of the present invention, the spacing of the hydrophobic protrusions is 1.0 to 2.5 mm. The spacing of the hydrophobic protrusions can be, for example, 1.1 mm, 1.6 mm, 2.1 mm, or any value between 1.0 and 2.5 mm.

[0012] Within the aforementioned size range, a surface tension gradient is formed on the hydrophilic-hydrophobic heterogeneous surface. This facilitates the synergistic effect of the hydrophobic protrusions in capturing and coalescing emulsified oil droplets, combined with the repulsive effect of the hydrophilic layer. This enables the demulsification and separation of surfactant-stabilized oil-in-water emulsions.

[0013] The present invention also proposes a method for preparing the composite membrane, comprising the following steps: S1. Using polyvinylidene fluoride microporous filter membrane as base membrane, an intermediate adhesive layer is constructed on the surface of the base membrane by co-deposition reaction of dopamine and polyethyleneimine. S2. A hydrophilic layer is constructed on the surface of the intermediate adhesive layer by cross-linking an alginate solution with calcium salts. S3. A slurry containing hydrophobic and oleophilic inorganic fillers is printed on the surface of the hydrophilic layer according to a preset array pattern, and the composite film is obtained after curing.

[0014] According to an embodiment of the present invention, in step S1, the co-deposition reaction is carried out in a Tris-HCl buffer solution with a pH of 8.0 to 9.0.

[0015] According to an embodiment of the present invention, in step S1, the concentrations of dopamine hydrochloride and polyethyleneimine are both 1.5 to 2.5 g / L.

[0016] According to an embodiment of the present invention, in step S1, the co-deposition reaction takes 18 to 30 hours.

[0017] According to an embodiment of the present invention, in step S1, the co-deposition reaction is carried out at 15–35°C.

[0018] According to an embodiment of the present invention, in step S3, the printing method includes ink direct writing 3D printing.

[0019] According to an embodiment of the present invention, the diameter of the 3D printed needle is 300~400μm.

[0020] According to an embodiment of the present invention, the air pressure for 3D printing is 0.3~0.5MPa.

[0021] According to an embodiment of the present invention, the 3D printing speed is 15~25mm / s.

[0022] According to an embodiment of the present invention, the preset array pattern is a millimeter-level stripe pattern with a pattern fill rate of 10-25%.

[0023] This invention first selects a polyvinylidene fluoride (PVDF) microporous filter membrane as the base membrane, immersing it in anhydrous ethanol to achieve surface wetting and purification. Then, the pretreated base membrane is immersed in a weakly alkaline buffer solution containing dopamine hydrochloride and polyethyleneimine, forming a firmly anchored PDA / PEI co-deposited intermediate adhesive layer in situ on the base membrane surface, providing strong interfacial bonding for subsequent functional layers. Next, the resulting modified membrane is sequentially immersed in a sodium alginate solution and a calcium ion crosslinking agent solution. Utilizing the coordination crosslinking effect of calcium ions with the carboxyl groups on the sodium alginate molecular chain, a dense calcium alginate hydrogel hydrophilic coating is constructed in situ on the intermediate layer surface. Finally, using printing technology, a hydrophobic slurry is precisely printed onto the hydrophilic coating surface according to a preset array pattern, followed by natural air drying to evaporate the organic solvent in the slurry. A hydrophobic protrusion structure is successfully constructed on the surface, thereby obtaining a biomimetic oil-water separation composite membrane with a heterogeneous wettable surface of "hydrophilic phase-hydrophobic phase". This progressive reaction principle organically combines biomimetic adhesion, hydrophilic hydration layer construction, and digital hydrophobic patterning. Through the active capture-aggregation of tiny oil droplets by the hydrophobic regions and the strong repulsion synergistic mechanism of the hydrophilic regions, the demulsification efficiency for kinetically stable emulsions is improved, while simultaneously achieving digital customization of structural parameters. In summary, the composite membrane prepared by this invention possesses advantages such as high separation efficiency and excellent antifouling performance, providing a simple and controllable new approach for the efficient treatment of oily wastewater.

[0024] According to an embodiment of the present invention, in step S2, the concentration of the alginate solution is 0.5 to 1.0 g / L.

[0025] The above concentration range ensures that the sodium alginate molecular chains diffuse fully and uniformly cover the surface of the PDA / PEI intermediate adhesive layer, while avoiding the problems of excessively thick gel layer clogging the membrane pores due to excessively high concentration or sparse and non-dense hydrophilic layer due to excessively low concentration. Thus, a dense and uniform calcium cross-linked alginate hydrogel coating is formed after in-situ cross-linking of calcium ions.

[0026] According to an embodiment of the present invention, the slurry further includes a binder and a solvent.

[0027] According to an embodiment of the present invention, the adhesive comprises at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, and polyvinylidene fluoride-trifluorochloroethylene copolymer.

[0028] According to an embodiment of the present invention, the solvent includes at least one selected from N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide.

[0029] According to an embodiment of the present invention, the hydrophobic and oleophilic inorganic filler is 7.1 to 9.3 parts, the binder is 0.8 to 1.2 parts, and the organic solvent is 7 to 8 parts.

[0030] According to an embodiment of the present invention, the hydrophobic and oleophilic inorganic filler is 7.1 to 9.3 parts. The hydrophobic and oleophilic inorganic filler can be, for example, 7.1 parts, 8.0 parts, 8.5 parts, 9.0 parts, 9.3 parts, or any value between 7.1 and 9.3 parts.

[0031] According to an embodiment of the present invention, the adhesive is 0.8 to 1.2 parts. The adhesive can be, for example, 0.8 parts, 1.0 parts, 1.2 parts, or any value between 0.8 and 1.2 parts.

[0032] According to an embodiment of the present invention, the organic solvent is 7 to 8 parts. The organic solvent can be, for example, any value between 7 parts, 7.5 parts, 8 parts, or 7 to 8 parts.

[0033] The present invention also proposes the application of the composite membrane in the treatment of oily wastewater.

[0034] The hydrophilic region of the composite membrane of this invention exhibits a dynamic water contact angle in air that decreases to 0° within 20 seconds, while the underwater oil contact angle is greater than 150°. The hydrophobic protrusion structure in air has a water contact angle greater than 120° and an oil contact angle of 0°. Compared to existing technologies, the advantages and beneficial effects of this invention are: 1. Highly efficient demulsification, solving the problem of stable emulsion separation. This invention mimics the back structure of a desert beetle, utilizing 3D-printed hydrophobic protrusions to actively "capture and aggregate" tiny oil droplets. Combined with the sieving effect of the PVDF substrate and the repulsive effect of the SA hydrogel layer, multiple synergistic separations are achieved. This structure improves the separation efficiency of surfactant-stabilized emulsions (SSE), resulting in high filtrate clarity. 2. Biomimetic adhesion, strong interlayer bonding and stability. Utilizing the universal adhesion of the PDA / PEI adhesive layer, and through hydrogen bonding and electrostatic interactions, the chemically inert PVDF substrate and SA hydrophilic layer are tightly anchored. This adhesion improves the wettability of the substrate. 3. Underwater superoleophobic with excellent antifouling and self-cleaning properties: The calcium alginate hydrophilic layer forms a dense hydration layer underwater, endowing the membrane surface with superhydrophilic / underwater superoleophobic properties (UOCA>150°). This hydration layer effectively blocks oil droplet contact, making oil stains on the membrane surface easily washable with water, thus achieving good reusability. 4. Digital customization for flexible and low-cost fabrication: Compared to traditional complex processes such as photolithography and etching, this invention uses 3D printing technology, allowing for precise digital control of the shape and distribution of hydrophobic patterns. This process is simple, fast, and allows for the design of different hydrophobic patterns, creating a surface tension gradient on the membrane surface, which is beneficial for the demulsification process. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the membrane preparation and modification provided by the present invention.

[0036] Figure 2This is a schematic diagram of oil-water emulsion separation provided by the present invention.

[0037] Figure 3 This is a physical image of the membrane provided by the present invention.

[0038] Figure 4 These are the membrane microstructure images provided by the present invention, wherein (a) are the microstructure images of the unmodified original membrane PVDF (Pristine M) at magnification of 2000, 5000, and 10000 times; (b) are the microstructure images of the modified membrane MP coated only with PDA / PEI at magnification of 2000, 5000, and 10000 times; (c) are the microstructure images of the MPS membrane coated with PDA / PEI and alginate at magnification of 2000, 5000, and 10000 times; (d) are the microstructure images of the hydrophilic region of the 3D printed composite membrane at magnification of 2000, 5000, and 10000 times; and (e) are the microstructure images of the hydrophobic region of the 3D printed composite membrane, i.e., the 3D printed structure, at magnification of 2000, 5000, and 10000 times.

[0039] Figure 5 This is the infrared spectrum analysis diagram of the membrane surface provided by the present invention.

[0040] Figure 6 This is a dynamic water contact angle diagram of the membrane surface provided by the present invention.

[0041] Figure 7 This is the viscoelastic diagram of oil droplets in a membrane-water environment provided by the present invention.

[0042] Figure 8 This is a graph showing the membrane reuse performance data provided by the present invention.

[0043] Figure 9 These are optical microscope images of oil droplets provided by this invention in different environments. Among them, (a) is an optical microscope image of a surfactant-stabilized emulsion (SSE); (b) is an image of the distribution of SSE on the surface of MPS-0.75 film; and (c) is an image of SSE aggregation on the hydrophobic region of MPS-0.75-20%. Detailed Implementation

[0044] The following description, in conjunction with embodiments, clearly and completely illustrates the technical solutions of the present invention, enabling those skilled in the art to fully understand the invention. Obviously, the described embodiments are merely some preferred embodiments of the present invention, and not all embodiments. Any equivalent modifications or substitutions made by those skilled in the art to the following embodiments without creative effort are within the protection scope of the present invention.

[0045] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0046] To address the problems in existing technologies, such as the susceptibility of hydrophilic membranes to contamination by surfactant-stabilized emulsified oils, the complexity of biomimetic structure preparation, and the weak interlayer bonding, the composite membrane of this invention comprises a polyvinylidene fluoride (PVDF) microporous filter membrane substrate, a PDA / PEI co-deposited intermediate layer sequentially attached to the surface of the PVDF substrate, a calcium alginate hydrogel superhydrophilic layer, and a 3D-printed hydrophobic array pattern distributed on the surface of the hydrogel layer.

[0047] It should be noted that the PVDF microporous filter membrane used in this invention is a commercially available membrane. While it possesses excellent chemical stability and mechanical strength, its original surface has poor hydrophilicity, making it prone to oil droplet adhesion underwater. This invention achieves improved oil resistance of the substrate through a combined strategy of "PDA / PEI co-deposition + SA hydrogel coating." The addition of the organic solvent NMP to the hydrophobic slurry not only mixes the slurry but also acts as a binder between the slurry and the membrane surface; the organic solvent will evaporate during natural drying.

[0048] Furthermore, the present invention utilizes a hydrophobic protrusion structure constructed using 3D printing technology to mimic the water-collecting mechanism on the back of a desert beetle, which has a "capture-aggregation" function for tiny oil droplets in the emulsion. Combined with the repulsive effect of the hydrophilic layer, it achieves efficient demulsification and antifouling.

[0049] Example 1 This embodiment provides a composite membrane and its preparation method, the specific steps of which are as follows: S1.1 Using polyvinylidene fluoride microporous filter membrane as base membrane, wet a commercial PVDF microporous filter membrane with a pore size of 0.45 μm with anhydrous ethanol for 1 h. S1.2. Prepare a Tris-HCl buffer solution with pH 8.5, add PDA and PEI (Mw=600) to make their concentrations 2.0 g / L, immerse the cleaned PVDF membrane in the solution, let it stand at room temperature for 24 h, take it out and rinse with deionized water to obtain a brown PDA / PEI modified membrane. S2. Prepare a 0.75 g / L sodium alginate (SA) aqueous solution, immerse the PDA / PEI modified membrane in the SA solution for 5 min, remove and drain, then immerse it in a 2 wt% CaCl2 solution for crosslinking for 10 min, rinse with deionized water, and vacuum dry at 40℃ for 12 h to obtain the PVDF-PDA / PEI-SA membrane. S3. Prepare hydrophobic and oleophilic calcium carbonate / hydrophobic nano silica / PVDF-HFP printing ink. Using a direct-write 3D printer, print strip-shaped hydrophobic patterns on the surface of a PVDF-SA film. The printing parameters are: needle diameter 340μm, air pressure 0.4MPa, printing speed 20mm / s. The printed pattern is a 3.5cm diameter disc with a pattern fill rate of 20% and a height of 100μm. The actual printed lines are approximately 0.9mm wide, 116μm high, and 1.1mm apart. After printing, allow it to air dry for more than 24 hours to obtain a biomimetic heterogeneous wettability composite film (denoted as MPS-0.75-20%).

[0050] The composite membrane was used in the separation of surfactant-stabilized oil-in-water emulsions (SSE). Under constant pressure separation conditions of -0.1 bar, the total organic carbon (TOC) concentration in the filtrate was 54.25 mg / mL, the separation efficiency of the composite membrane was 99.33%, and the permeate flux was 391.72 L·h. -1 ·m -2 ·bar -1 The contact angle of the hydrophilic region of the membrane was tested, and the water contact angle was 74.18°.

[0051] Figure 1 This is a schematic diagram illustrating the preparation and surface modification process of the biomimetic heterogeneous wettability oil-water separation composite membrane of the present invention. It shows the process of constructing a hierarchical structure through continuous modification using a polyvinylidene fluoride (PVDF) microporous filter membrane as a substrate: First, the base membrane is pretreated; second, a PDA / PEI intermediate adhesive layer is formed on the surface of the base membrane through a co-deposition reaction of dopamine and polyethyleneimine; then, the modified membrane is immersed in a sodium alginate solution and cross-linked in situ with a calcium salt cross-linking agent to construct a calcium alginate hydrogel hydrophilic layer; finally, a hydrophobic protrusion array pattern is precisely constructed on the surface of the hydrophilic layer using 3D printing technology, ultimately obtaining a biomimetic composite membrane with a heterogeneous wettability structure of "hydrophilic phase-hydrophobic phase".

[0052] The novel membrane of this invention was tested for its oil-water separation performance using a dead-end filtration device, with a sand core filter plate as support, and an effective filtration area of ​​12.56 cm². 2 A schematic diagram of the device is shown below. Figure 2 As shown, the membrane prepared in this invention is clamped in the device, and a pressure of -0.1 bar is applied to drive the oil-water separation process. The feed liquid is a surfactant-stabilized oil-in-water emulsion (SSE) and a surfactant-free oil-in-water emulsion (SFE), and the volume of each feed liquid is 100 mL.

[0053] The SSE preparation method is as follows: sulfonated kerosene, deionized water, and surfactant Tween 80 are mixed in a certain volume ratio. Generally, the volume ratio of sulfonated kerosene to deionized water is 1:99, and the concentration of surfactant Tween 80 is 0.2 mL / L. The mixture is stirred continuously at 1800 rpm for 2 hours on a magnetic stirrer to ensure thorough mixing.

[0054] The specific preparation method of SFE is as follows: sulfonated kerosene and deionized water are mixed in a certain volume ratio. Generally, the volume ratio of sulfonated kerosene to deionized water is 1:99. The mixture is stirred continuously at 1800 rpm for 2 hours on a magnetic stirrer to ensure thorough mixing.

[0055] The separation process was carried out at room temperature, and an adjustable vacuum pump was used to maintain a constant negative pressure of -0.1 bar. The collected filtrate was weighed every 5 minutes. The membrane separation flux (L·h) was recorded. -1 ·m -2 ·bar -1 Calculate according to the following formula: ; In the formula, J represents the separation flux (L·h) -1 ·m -2 ·bar -1 V represents the filtrate volume (L), and A represents the effective filtration area of ​​the membrane (m²). 2 ), t represents the filtering time (h). P represents the driving pressure (bar).

[0056] The total organic carbon (TOC) content in the filtrate was tested using a total organic carbon analyzer, and the retention rate was calculated using the following formula: ; In the formula, R represents the oil-water separation efficiency of the membrane (%), C0 represents the TOC concentration in the feed liquid (mg / mL), and C1 represents the TOC concentration in the filtrate (mg / mL).

[0057] Example 2 This embodiment provides a composite membrane and its preparation method, the specific steps of which are as follows: S1.1 Using polyvinylidene fluoride microporous filter membrane as base membrane, wet a commercial PVDF microporous filter membrane with a pore size of 0.45 μm with anhydrous ethanol for 1 h. S1.2. Prepare a Tris-HCl buffer solution with pH 8.5, add PDA and PEI (Mw=600) to make their concentrations 2.0 g / L, immerse the cleaned PVDF membrane in the solution, let it stand at room temperature for 24 h, take it out and rinse with deionized water to obtain a brown PDA / PEI modified membrane. S2. Prepare a 0.75 g / L sodium alginate (SA) aqueous solution, immerse the PDA / PEI modified membrane in the SA solution for 5 min, remove and drain, then immerse it in a 2 wt% CaCl2 solution for crosslinking for 10 min, rinse with deionized water, and vacuum dry at 40℃ for 12 h to obtain the PVDF-PDA / PEI-SA membrane. S3. Prepare hydrophobic and oleophilic calcium carbonate / hydrophobic nano silica / PVDF-HFP printing ink. Using a direct-write 3D printer, print strip-shaped hydrophobic patterns on the surface of a PVDF-SA film. The printing parameters are: needle diameter 340μm, air pressure 0.4MPa, printing speed 20mm / s. The printed pattern is a disc with a diameter of 3.5cm. The pattern fill rate is set to 15%, and the height is set to 100μm. The actual printed line width is about 0.9mm, the height is about 161μm, and the spacing between lines is about 1.6mm. After printing, allow it to air dry for more than 24 hours to obtain a biomimetic heterogeneous wettability composite film (denoted as MPS-0.75-15%).

[0058] The composite membrane was used in the separation of surfactant-stabilized oil-in-water emulsions (SSE). Under constant pressure separation conditions of -0.1 bar, the total organic carbon (TOC) concentration in the filtrate was 62.91 mg / mL, the separation efficiency of the composite membrane was 99.23%, and the permeate flux was 455.21 L·h. -1 m -2 ·bar -1 .

[0059] Example 3 Compared with Examples 1 and 2, Example 3 changes the 3D printing specifications of the hydrophobic structure, as follows: S1.1 Wet a PVDF microporous filter membrane with a pore size of 0.45 μm with anhydrous ethanol for 1 h; S1.2, Intermediate Layer Construction: Prepare a Tris-HCl buffer solution at pH 8.5, add PDA and PEI (Mw=600) to a concentration of 2.0 g / L, immerse the washed PVDF membrane in this solution, and allow it to react at room temperature for 24 h. After removal, rinse with deionized water to obtain a brown PDA / PEI modified membrane; S2. Prepare a 0.75 g / L sodium alginate (SA) aqueous solution. Immerse the PDA / PEI modified membrane in the SA solution for 5 min, remove and drain, then immediately immerse it in a 2 wt% CaCl2 solution for crosslinking for 10 min. After rinsing with deionized water, vacuum dry at 40℃ for 12 h to obtain the PVDF-PDA / PEI-SA membrane; S3, 3D Printing Patterning: Hydrophobic and oleophilic calcium carbonate / hydrophobic nano-silica / PVDF-HFP printing ink was formulated, and a strip-shaped hydrophobic pattern was printed on the surface of a PVDF-SA film using a direct-write 3D printer. Printing parameters: needle diameter 340μm, air pressure 0.4MPa, printing speed 20mm / s, printed pattern was a 3.5cm diameter disc, pattern fill rate set to 13%, height set to 100μm. The actual printed lines were approximately 0.9mm wide, 128μm high, and the spacing between lines was approximately 2.1mm. After printing, the film was allowed to air dry for at least 24 hours to obtain a biomimetic heterostructure wettability composite film (denoted as MPS-0.75-13%).

[0060] The novel biomimetic composite membrane for oil-water separation was used in experiments to separate surfactant-stabilized oil-in-water emulsions (SSE). Under constant pressure separation conditions of -0.1 bar, the total organic carbon (TOC) concentration in the filtrate was 72.45 mg / mL. The separation efficiency of the composite membrane was 99.11%, and the permeate flux was 656.05 L·h. -1 ·m -2 ·bar -1 .

[0061] Comparative Example 1 Compared to Example 1, the difference lies in that only the adhesive layer of the membrane is prepared and the sodium alginate is used for hydrophilic modification without 3D printing, and it is used to separate and test the same type of SSE emulsion, as detailed below: S1.1 Wet a PVDF microporous filter membrane with a pore size of 0.45 μm with anhydrous ethanol for 1 h.

[0062] S1.2. Prepare a Tris-HCl buffer solution with pH 8.5, add PDA and PEI (Mw=600) to a concentration of 2.0 g / L, immerse the cleaned PVDF membrane in this solution, and allow it to react at room temperature for 24 h. After removal, rinse with deionized water to obtain a brown PDA / PEI modified membrane; S2. Prepare a 0.75 g / L sodium alginate (SA) aqueous solution, immerse the PDA / PEI modified membrane in the SA solution for 5 min, remove and drain, then immerse it in a 2 wt% CaCl2 solution for crosslinking for 10 min, rinse with deionized water, and vacuum dry at 40℃ for 12 h to obtain a PVDF-PDA / PEI-SA membrane (denoted as MPS-0.75). The novel biomimetic composite membrane for oil-water separation was used in experiments to separate surfactant-stabilized oil-in-water emulsions (SSE). Under constant pressure separation conditions of -0.1 bar, the total organic carbon (TOC) concentration in the filtrate was 74.38 mg / mL. The separation efficiency of the composite membrane was 99.08%, and the permeate flux was 566.88 L·h. -1 ·m-2 ·bar -1 The membrane was tested for contact angle, and the water contact angle was 23.7°.

[0063] Comparative Example 2 Compared to Example 1, the difference lies in that the original membrane is not treated in any way and is used to separate and test the same type of SSE emulsion, as follows: We offer commercial PVDF microporous filter membranes with a pore size of 0.45 μm, without any modification (denoted as Pristine M).

[0064] The novel biomimetic composite membrane for oil-water separation was used in experiments to separate surfactant-stabilized oil-in-water emulsions (SSE). Under constant pressure separation conditions of -0.1 bar, the permeation flux of the composite membrane was 0 L·h. -1 ·m -2 ·bar -1 The membrane was tested for contact angle, and the water contact angle was 100.40°.

[0065] Comparative Example 3 Compared to Example 1, the membrane modification steps are the same, except that this comparative example is used to separate and test oil-in-water emulsions (SFEs) without surfactants, as detailed below: S1.1 Wet a PVDF microporous filter membrane with a pore size of 0.45 μm with anhydrous ethanol for 1 h.

[0066] S1.2. Prepare a Tris-HCl buffer solution with pH 8.5, add PDA and PEI (Mw=600) to make their concentrations 2.0 g / L, immerse the cleaned PVDF membrane in the solution, let it stand at room temperature for 24 h, take it out and rinse with deionized water to obtain a brown PDA / PEI modified membrane. S2. Prepare a 0.75 g / L sodium alginate (SA) aqueous solution. Immerse the PDA / PEI modified membrane in the SA solution for 5 min, remove and drain, then immerse it in a 2 wt% CaCl2 solution for crosslinking for 10 min. After rinsing with deionized water, dry under vacuum at 40 °C for 12 h to obtain the PVDF-PDA / PEI-SA membrane.

[0067] S3. Prepare hydrophobic and oleophilic calcium carbonate / hydrophobic nano silica / PVDF-HFP printing ink, and use a direct-write 3D printer to print strip-shaped hydrophobic patterns on the surface of a PVDF-PDA / PEI-SA film. Printing parameters: needle diameter 340μm, air pressure 0.4MPa, printing speed 20mm / s, printed pattern is a 3.5cm diameter disc, pattern fill rate set to 20%, height set to 100μm. The actual printed lines are approximately 0.9mm wide, approximately 116μm high, and the spacing between lines is approximately 1.1mm. After printing, allow to air dry for at least 24 hours to obtain a biomimetic heterogeneous wettability composite film (denoted as MPS-0.75-20%).

[0068] The novel biomimetic composite membrane for oil-water separation was used in experiments to separate oil-in-water emulsions (SFE) without surfactants. Under constant pressure separation conditions of -0.1 bar, the total organic carbon (TOC) concentration in the filtrate was 2.77 mg / mL, the separation efficiency of the composite membrane was 99.97%, and the permeate flux was 918.26 L·h. -1 ·m -2 ·bar -1 .

[0069] Comparative Example 4 Compared to Example 1, the difference lies in that the modification step only proceeds to the preparation of the adhesive layer, without the sodium alginate coating and 3D printing. This comparative example was used to test the contact angle, as follows: S1.1 Wet a PVDF microporous filter membrane with a pore size of 0.45 μm with anhydrous ethanol for 1 h; S1.2. Prepare a Tris-HCl buffer solution with pH 8.5, add PDA and PEI (Mw=600) to make their concentrations 2.0 g / L, immerse the cleaned PVDF membrane in the solution, let it stand at room temperature for 24 h, take it out and rinse with deionized water to obtain a brown PDA / PEI modified membrane (denoted as MP).

[0070] The membrane was tested for contact angle, and the water contact angle was 92.2°.

[0071] Test Example 1 Macroscopic morphology testing: The composite film (MPS-0.75-20%) prepared in Example 1, the composite film (MPS-0.72-15%) prepared in Example 2, the hydrophilic film (MPS-0.75-13%) prepared in Example 3, the hydrophilic film (MPS-0.75) prepared in Comparative Example 1, the original PVDF film (Pristine M) of Comparative Example 2, and the MP film prepared in Comparative Example 4 were observed under a digital lens to determine their macroscopic surface morphology. Figure 3The original Pristine M membrane was white; after the PDA / PEI adhesive layer was deposited, the surface of the membrane (MP) changed color, turning light brown; after further coating with sodium alginate, the color became lighter than that of the MP membrane. The composite membranes MPS-0.75-20%, MPS-0.72-15%, and MPS-0.75-13% showed differences in 3D printing infill distribution.

[0072] Test Example 2 Microstructure testing: The composite membrane (MPS-0.75-20%) prepared in Example 1, the hydrophilic membrane (MPS-0.75) prepared in Comparative Example 1, the original PVDF membrane (Pristine M) in Comparative Example 2, and the MP membrane prepared in Comparative Example 4 were examined using field emission scanning electron microscopy to observe their surface microstructure. Figure 4 ),in Figure 4 (a) shows the microstructure of the unmodified PVDF (Pristine M) membrane at magnifications of 2000, 5000, and 10000, revealing a porous structure. Figure 4 (b) shows the microstructure of the modified film MP coated with only PDA / PEI at magnification of 2000, 5000, and 10000. Since the amount of PDA / PEI deposited is small, the microstructure is close to that of the original film. Figure 4 (c) shows the microstructure of the MPS membrane coated with PDA / PEI and alginate at magnifications of 2000, 5000, and 10000, respectively. It shows that the hydrophilic coating covers the original pore size, the surface pore coverage is increased, and the coating morphology is relatively uniform. Figure 4 In the figure (d), the microstructure of the hydrophilic region of the 3D printed composite film is shown at magnification of 2000, 5000, and 10000 times, and the hydrophilic coating is uniformly distributed. Figure 4 (e) in the figure shows the microscopic morphology of the hydrophobic region of the 3D printed composite membrane, i.e., the 3D printed structure, at magnifications of 2000, 5000, and 10000. It can be observed that the surface of the protruding structure is covered with hydrophobic and oleophilic inorganic filler particles, exhibiting microscopic roughness.

[0073] Test Example 3 In-situ Fourier Transform Infrared Spectroscopy: The composite membrane (MPS-0.75-20%) prepared in Example 1, the hydrophilic membrane (MPS-0.75) prepared in Comparative Example 1, the original PVDF membrane (Pristine M) in Comparative Example 2, and the MP membrane prepared in Comparative Example 4 were analyzed using in-situ Fourier Transform Infrared Spectroscopy to determine the chemical functional groups on the membrane surface. Figure 5 As shown, each membrane at 1407 cm⁻¹ -1 1180cm -1 874cm -1 The characteristic absorption peak of PVDF appeared at 1407 cm⁻¹.-1 and 874cm -1 The peak is attributed to the stretching vibration of the -CH2 group, 1180 cm⁻¹ -1 The peaks are due to the presence of numerous -CF groups in the membrane's composition. In contrast, these peaks are somewhat attenuated in the MP, MPS-0.75, and MPS-0.75-20% membranes, but remain clearly visible due to the thinner coating. 3400 cm⁻¹ -1 Insignificant broadening and shifting were observed in the MP, MPS, and MPS-20% films, especially for the MPS and MPS-20% films, with spectral peaks in the 3100–3600 cm⁻¹ range. -1 The range is wider, which is caused by the stretching vibration of -OH. This is because the PDA / PEI layer contains -OH and -NH, and the -OH peak is further enhanced due to the presence of -COOH after a thin SA coating is loaded onto the MPS film. The change in peak intensity in the infrared spectrum proves that the film has been successfully modified layer by layer.

[0074] Test Example 4 Air surface wettability test: A contact angle meter was used to test the dynamic wettability of the hydrophilic region of the composite membrane of Example 1 (MPS-0.75-20%), the hydrophilic membrane of Comparative Example 1 (MPS-0.75), the original PVDF membrane (Pristine M) of Comparative Example 2, and the MP membrane of Comparative Example 4 when in contact with water in air. The volume of each water droplet was approximately 1 μL. The change of water contact angle over time is shown in the table below. Figure 6 As shown.

[0075] Pristine M exhibits weak hydrophobicity upon contact with water droplets, with a water contact angle of 92.83°. After 40 seconds, the contact angle decreases slowly, remaining around 70°. The initial contact angle between the MP membrane and water droplets is 92.22°, showing a significant decreasing trend within 35 seconds, reaching 0°, indicating that the deposition of PDA / PEI initially improves the hydrophilicity of the membrane. The initial contact angle between the MPS-0.75 membrane and water droplets is 23.71°, because the sodium alginate coating rich in hydroxyl and carboxyl groups further enhances the hydrophilicity of the membrane, and the surface of this membrane reaches 0° in a shorter time compared to the MP membrane. The initial contact angle of the hydrophilic region of the MPS-0.75-20% membrane is 74.18°, because the 3D-printed strips on both sides of the hydrophilic region have strong hydrophobicity, and subsequently, water droplets rapidly wet the surface within 15 seconds, as shown in Table 1.

[0076] Table 1. Dynamic water contact angle of different membrane samples in air over time. Test Example 5 Viscoelasticity test of oil droplets on membrane surface underwater: The viscoelastic properties of the hydrophobic region of the composite membrane of Example 1 (MPS-0.75-20%), the hydrophilic membrane of Comparative Example 1 (MPS-0.75), the original PVDF membrane of Comparative Example 2 (Pristine M), and the MP membrane of Comparative Example 4 after contact with oil droplets in an underwater environment were observed using a contact angle meter. The underwater oil contact angle of the hydrophilic membrane of Comparative Example 1 (MPS-0.75), the original PVDF membrane of Comparative Example 2 (Pristine M), and the MP membrane of Comparative Example 4 was tested using a contact angle meter.

[0077] Figure 7 The results showed that Pristine M exhibited strong adhesion to oil droplets in the underwater environment, exhibiting significant deformation when the probe pulled the oil droplets downwards; the MP membrane also showed strong adhesion to oil droplets, with obvious deformation during downward pulling; the MPS-0.75 membrane demonstrated improved anti-oil properties, with oil droplets finding it difficult to adhere to the membrane surface and showing almost no deformation during downward pulling; the hydrophobic region of the MPS-0.75-20% membrane exhibited slight viscoelasticity towards oil droplets, showing slight deformation during downward pulling, proving that there is a difference in the adhesion of oil droplets between the hydrophilic and hydrophobic regions.

[0078] The table shows that, in an underwater environment, the contact angle between the Pristine M membrane surface and oil droplets is 131.7°; the MP membrane surface and oil droplets have a contact angle of 136.4°; and the MPS-0.75 membrane surface and oil droplets have a contact angle of 150.8°, achieving underwater superoleophobic properties. This demonstrates that the modification process gradually improved the underwater oil-resistant properties of the membranes, as shown in Table 2.

[0079] Table 2. Oil contact angle test results of different membrane samples in underwater environment Test Example 6 Stability and reusability: The composite membrane MPS-0.75-20% from Example 1 and the MPS-0.75 membrane from Comparative Example 1 were subjected to simple hydroflushing and reused 5 times. The results are as follows: Figure 8 As shown, at the same concentration, both membranes exhibited a decreasing flux trend, while the MPS-0.75-20% membrane showed a slower decreasing trend.

[0080] Test Example 7 Emulsion Distribution and Coagulation: The distribution of surfactant-stabilized emulsion droplets (SSE) on the surface of the MPS-0.75-20% hydrophobic region of the composite membrane in Example 1 and the MPS-0.75 membrane in Comparative Example 1 was observed. Results are as follows... Figure 9 As shown, the droplets in the original SSE are randomly and uniformly dispersed as follows: Figure 9In (a), no obvious aggregation was observed on the surface of the MPS-0.75 hydrophilic film. Figure 9 In (b), larger oil droplet morphologies were observed to form on the hydrophobic structure with MPS-0.75-20%, such as... Figure 9 (c) in the middle.

[0081] The composite membrane of this invention can effectively treat two types of oil-in-water emulsions: those stabilized with surfactants and those without surfactants. Through the membrane separation process, the hydrophobic protrusions on the membrane surface actively capture highly dispersed micro-oil droplets in the emulsion, promoting their aggregation into larger droplets that quickly detach from the surface. Simultaneously, the stable hydration layer formed by the calcium alginate hydrophilic layer effectively prevents oil droplet adhesion to the membrane surface, avoiding pore blockage. This synergistic effect significantly improves the demulsification efficiency and separation effect for kinetically stable emulsions, achieving high separation efficiency, good permeate flux, and excellent antifouling performance.

[0082] In summary, the above-described technical solutions of the present invention are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent structural transformations made using the contents of the present invention's specification and drawings under the technical concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A composite membrane, characterized in that, The composite membrane includes a polyvinylidene fluoride microporous filter membrane substrate, an adhesive layer, a hydrophilic layer, and hydrophobic protrusions distributed on the surface of the hydrophilic layer, all stacked together. The raw materials for preparing the adhesive layer include dopamine and polyethyleneimine; The hydrophilic layer comprises alginate and calcium salt; The raw materials for preparing the hydrophobic protrusion structure include hydrophobic and oleophilic inorganic fillers.

2. The composite membrane according to claim 1, characterized in that, The alginate includes at least one of sodium alginate and potassium alginate.

3. The composite membrane according to claim 1, characterized in that, The hydrophobic and oleophilic inorganic filler includes calcium carbonate and fumed nano-silica.

4. The composite membrane according to claim 1, characterized in that, The height of the hydrophobic protrusion structure is 80–200 μm, the width of the hydrophobic protrusion structure is 0.7–1.1 mm, and the spacing between the hydrophobic protrusion structures is 1.0–2.5 mm.

5. A method for preparing a composite membrane as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1. Using polyvinylidene fluoride microporous filter membrane as base membrane, an intermediate adhesive layer is constructed on the surface of the base membrane by co-deposition reaction of dopamine and polyethyleneimine. S2. A hydrophilic layer is constructed on the surface of the adhesive layer by cross-linking an alginate solution with calcium salts. S3. The slurry containing hydrophobic and oleophilic inorganic fillers is 3D printed on the surface of the hydrophilic layer according to a preset array pattern, and the composite film is obtained after natural drying.

6. The preparation method according to claim 5, characterized in that, In step S2, the concentration of the alginate solution is 0.5–1.0 g / L.

7. The preparation method according to claim 5, characterized in that, The slurry also includes: binder and solvent.

8. The preparation method according to claim 7, characterized in that, The adhesive includes at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, and polyvinylidene fluoride-trifluorochloroethylene copolymer.

9. The preparation method according to claim 7, characterized in that, In the composite slurry, by weight, the hydrophobic and oleophilic inorganic filler is 7.1 to 9.3 parts, the binder is 0.8 to 1.2 parts, and the organic solvent is 7 to 8 parts.

10. The application of a composite membrane as described in any one of claims 1 to 4 in the treatment of oily wastewater.

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