NiFe-LDH / PVDF composite membrane and preparation method thereof

Abstract

The present invention belongs to the technical field of oil-water separation photocatalytic membranes, and specifically relates to a NiFe-LDH / PVDF composite membrane and a method for preparing the same. The NiFe-LDH / PVDF composite membrane comprises an SPVDF substrate membrane containing nickel-iron layered double hydroxide (NiFe-LDH) and nickel-iron layered double hydroxide (NiFe-LDH) grown in situ on the surface of the SPVDF substrate membrane. The NiFe-LDH / PVDF composite membrane has excellent separation efficiency and good photocatalytic function, and has great application potential in actual wastewater treatment and purification.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil-water separation photocatalytic membranes, and particularly relates to a NiFe-LDH / PVDF composite membrane and a preparation method thereof. Background Art

[0002] Oily wastewater is a common by-product of industrial activities such as oil extraction, metallurgical processing, textile printing and dyeing, chemical synthesis, pharmaceutical manufacturing, and daily life. Traditional separation technologies such as gravity sedimentation, centrifugal separation, and adsorption have low efficiency, high energy consumption, and the risk of secondary pollution, and can no longer meet modern environmental protection requirements. In contrast, membrane technology, with its excellent separation efficiency, compact modular design, low sludge production, and economic advantages, has become a revolutionary solution for treating emulsified wastewater containing oil droplets less than 20 μm. However, irreversible pollution caused by oil adhesion and membrane pore clogging seriously restricts its long-term flux stability and operational efficiency. In addition, the coexistence of suspended solids, organic / inorganic pollutants, and microorganisms in actual wastewater further challenges separation efficiency, and there is an urgent need to develop new membrane fouling control strategies.

[0003] Therefore, the existing oil-water separation membranes for oily wastewater still need to be improved. Summary of the Invention

[0004] The purpose of the present invention is to provide a NiFe-LDH / PVDF composite membrane and a preparation method thereof. The NiFe-LDH / PVDF composite membrane of the present invention has excellent separation efficiency and good photocatalytic function, and has great application potential in actual wastewater treatment and purification.

[0005] The first aspect of the present invention provides a NiFe-LDH / PVDF composite membrane, which includes an SPVDF substrate membrane containing nickel-iron layered double hydroxide NiFe-LDH, and nickel-iron layered double hydroxide NiFe-LDH in situ grown on the surface of the SPVDF substrate membrane.

[0006] In some embodiments of the present invention, the pure water flux of the NiFe-LDH / PVDF composite membrane is 6220.1±41.9 L·m -2 ·h -1 bar -1 The permeation flux of n-hexane / water emulsion was 3564.7±339.3 L·m -2 ·h -1 bar -1 , the interception rate reached 99.78%.

[0007] The second aspect of the present invention also provides a method for preparing the NiFe-LDH / PVDF composite membrane described in the first aspect, the preparation method comprising the following steps: subjecting a first NiFe-LDH precursor solution containing an iron salt and a nickel salt to a hydrothermal reaction to obtain a NiFe-LDH suspension, and centrifugally drying the NiFe-LDH suspension to obtain NiFe-LDH; mixing the NiFe-LDH with polyvinylidene fluoride, a pore-forming agent, a surfactant, and a solvent to form a casting liquid, coating the casting liquid to form a cast liquid, immersing it in deionized water for phase inversion, and obtaining a PVDF substrate membrane containing NiFe-LDH, recorded as an SPVDF substrate membrane; placing the SPVDF substrate membrane in a second NiFe-LDH precursor solution containing an iron salt and a nickel salt, and in situ growing NiFe-LDH on one side surface of the SPVDF substrate membrane under hydrothermal conditions to obtain the NiFe-LDH / PVDF composite membrane.

[0008] In some embodiments of the present invention, in the first NiFe-LDH precursor solution, the mass percentage of the iron salt is 0.579 wt %, and the mass percentage of the nickel salt is 1.261 wt %.

[0009] In some embodiments of the present invention, the iron salt is selected from Fe(NO3)3·9H2O, and the nickel salt is selected from Ni(NO3)2·6H2O.

[0010] In some embodiments of the present invention, the temperature of the hydrothermal reaction is 120° C., and the reaction time is 8 h.

[0011] In some embodiments of the present invention, in the SPVDF basement membrane, the mass percentage of the NiFe-LDH is 0.14 wt% to 0.55 wt%.

[0012] In some embodiments of the present invention, in the casting solution, the mass percentage of the NiFe-LDH is 0.14 wt%~0.55 wt%, the mass percentage of the polyvinylidene fluoride is 12.47 wt%~12.52 wt%, the mass percentage of the pore former is 1.11 wt%, the mass percentage of the surfactant is 0.28 wt%, and the mass percentage of the solvent is 85.60 wt%~85.95 wt%; and the sum of the mass percentages of each component is 100%.

[0013] In some embodiments of the present invention, in the second NiFe-LDH precursor solution, the mass percentage of the iron salt is 0.295 wt %, and the mass percentage of the nickel salt is 0.644 wt %.

[0014] In some embodiments of the present invention, the iron salt is selected from Fe(NO3)3·9H2O, and the nickel salt is selected from Ni(NO3)2·6H2O.

[0015] In some embodiments of the present invention, the first NiFe-LDH precursor solution and the second NiFe-LDH precursor solution are each independently an aqueous solution formed by mixing iron salt, nickel salt, urea and ammonium fluoride.

[0016] In some embodiments of the present invention, the temperature for the in-situ growth reaction under the hydrothermal conditions is 120° C., and the reaction time is 2 h to 8 h.

[0017] The NiFe-LDH / PVDF composite membrane obtained by in-situ growth of NiFe-LDH on the SPVDF substrate membrane shows excellent performance, with a pure water flux of up to 6220.1 L·m -2 ·h -1 bar -1 In the n-hexane / water emulsion separation experiment, the permeate flux reached 3564.7 L·m -2 ·h -1 bar -1 , while the retention efficiency is as high as 99.78%, showing excellent separation performance.

[0018] The present invention uses in-situ growth technology to directly grow NiFe-LDH on the surface of the SPVDF substrate membrane, thereby effectively modifying the performance of the PVDF membrane. The preparation method of the present invention has the advantages of strong process controllability, simple operation and low cost.

[0019] The present invention grows NiFe-LDH on a PVDF membrane through an in-situ growth method, so that the membrane has excellent photocatalytic performance, is conducive to avoiding secondary pollution, and reduces the application cost of the membrane.

[0020] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are specifically listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0022] Figure 1 Schematic diagram of the preparation process of NiFe-LDH / PVDF composite membrane in an embodiment of the present invention.

[0023] Figure 2 The following are scanning electron microscope (SEM) images, transmission electron microscope (TEM) images, and energy dispersive x-ray spectroscopy (EDX) images of the films and NiFe-LDH seed crystals in the examples of the present invention and the comparative examples. Figure 2 (a) and (b) show the SEM images of NiFe-LDH seeds at different magnifications, respectively; Figure 2 (c) and (d) show TEM images of NiFe-LDH seeds at different magnifications, respectively; Figure 2 (e) shows the SEM cross-sectional image of the original PVDF membrane; Figure 2 (f) shows the SEM cross-sectional image of the 0.42 wt%-SPVDF membrane in Example 1; Figure 2 (g) shows the SEM cross-sectional image of the 0.42 wt%-SPVDF-6 h membrane in Example 1; Figure 2 (h) and (i) show the EDX images of 0.42 wt%-SPVDF-6 h in Example 1.

[0024] Figure 3 These are SEM images of the morphological evolution of the SPVDF substrate membrane surface containing different NiFe-LDH seed crystal contents at different growth times in Examples 1 to 16 of the present invention; wherein, all SEM images have the same scale, as shown in the lower right corner.

[0025] Figure 4 The three-dimensional atomic force microscope (AFM) morphology of the membrane in the comparative example and the embodiment. Figure 4 (a) shows the AFM morphology of the original PVDF membrane in Comparative Example 1. Figure 4 (b) shows the AFM morphology of the 0.42 wt%-SPVDF substrate membrane in Example 1. Figure 4 (c) shows the AFM morphology of the 0.42 wt%-SPVDF-6 h membrane in Example 1.

[0026] Figure 5 (a) is the XRD spectra of NiFe-LDH seed crystals, PVDF original film, 0.42 wt%-SPVDF substrate film and 0.42 wt%-SPVDF-6 h film; Figure 5 Middle (b) is the Fourier transform infrared (FT-IR) spectra of the original PVDF membrane, 0.42 wt%-SPVDF substrate membrane and 0.42 wt%-SPVDF-6 h membrane; Figure 5(c) is the wide scan XPS spectra of the original PVDF membrane, 0.42 wt%-SPVDF substrate membrane, and 0.42 wt%-SPVDF-6 h membrane; Figure 5 Middle (d) shows the C 1s wide scan and high-resolution XPS spectra of the original PVDF membrane; Figure 5 (e) is the C 1s wide scan and high-resolution XPS spectrum of the 0.42 wt%-SPVDF substrate membrane; Figure 5 (f) C 1s wide scan and high-resolution XPS spectra of 0.42 wt%-SPVDF-6 h membrane.

[0027] Figure 6 (a) shows the water contact angles (WCAs) of the original PVDF membrane and the x-SPVDF substrate membrane containing different amounts of NiFe-LDH; Figure 6 Middle (b) is the water contact angle of x-SPVDF-y films containing different amounts of NiFe-LDH and with different growth times; Figure 6 Middle (c) shows the ultrasound-assisted contact angles (UOCAs) of the original PVDF membrane, 0.42 wt%-SPVDF substrate membrane, and 0.42 wt%-SPVDF-6 h membrane; Figure 6 Middle (d) is the digital image of the water contact angle of the original PVDF membrane, 0.42 wt%-SPVDF substrate membrane and 0.42 wt%-SPVDF-6 h membrane surface at 0 seconds and 10 seconds.

[0028] Figure 7 (a) shows the pure water flux, emulsion flux, and oil retention rate of 0.14 wt%-SPVDF substrate membrane for n-hexane / water emulsion; Figure 7 (b) shows the pure water flux, emulsion flux, and oil retention rate of 0.28 wt%-SPVDF substrate membrane for n-hexane / water emulsion; Figure 7 (c) shows the pure water flux, emulsion flux and oil retention rate of 0.42 wt%-SPVDF substrate membrane for n-hexane / water emulsion; Figure 7 (d) is the pure water flux, emulsion flux and oil retention rate of 0.55 wt%-SPVDF substrate membrane for n-hexane / water emulsion; Figure 7 Middle (e) shows the emulsion flux and oil rejection of 0.42 wt%-SPVDF-6 h membrane for different oil-water emulsions; Figure 7 (f) is a digital image of oil-water separation of 0.42 wt%-SPVDF-6 h membrane treating complex oily wastewater.

[0029] Figure 8(a) is the UV-visible diffuse reflectance spectra (UV-vis DRS) of the original PVDF membrane, 0.42 wt%-SPVDF substrate membrane, and 0.42 wt%-SPVDF-6 h membrane; Figure 8 (b) shows DMPO-·OH and ·SO4 of 0.42 wt%-SPVDF-6 h membrane - Electron spin resonance (ESR) spectroscopy; Figure 8 (c) is DMPO-·O2 - Electron spin resonance (ESR) spectroscopy; Figure 8 (d) is TEMP- 1 O2 electron spin resonance (ESR) spectrum.

[0030] Figure 9 (a) and (b) show the normalized water flux and oil rejection of the original PVDF membrane and 0.42 wt%-SPVDF-6 h membrane for n-hexane / water emulsion in six cycles (water flushing was used every week); Figure 9 (c) and (d) show the normalized water flux and oil rejection of the original PVDF membrane and the 0.42 wt%-SPVDF-6 h membrane for n-hexane / water emulsion under the same test conditions (using PMS+visible light as a cleaning method instead of water rinsing); Figure 9 (e) and (f) are the normalized fluxes of 0.42 wt%-SPVDF-6 h membrane treating n-hexane / water emulsion at pH = 3 and pH = 11; Figure 9 (g) and (h) are actual photos of n-hexane / water emulsion before and after filtration through 0.42 wt%-SPVDF-6 h membrane at pH = 3 and pH = 11.

[0031] Figure 10 (a) and (b) show the three-cycle separation performance test of 0.42 wt%-SPVDF-6 h membrane and original PVDF membrane for catering wastewater (20 minutes of visible light irradiation and 0.4 g·L during each week). -1 PMS solution treatment); Figure 10 (c) is a microscope image of food wastewater; Figure 10 (d) and (e) are optical microscope images of catering wastewater before and after filtration by the original PVDF membrane and 0.42 wt%-SPVDF-6 h membrane.

[0032] Figure 11 The SEM images of the original PVDF membrane and the SPVDF substrate membrane containing different amounts of NiFe-LDH seeds are shown in Figure 2. Figure 11 (a) shows the SEM image of the original PVDF membrane surface. Figure 11 (b) shows the SEM image of the surface of the 0.14 wt%-SPVDF substrate membrane. Figure 11 (c) shows the SEM image of the surface of the 0.28 wt%-SPVDF substrate membrane. Figure 11 (d) shows the SEM image of the surface of the 0.42 wt%-SPVDF substrate membrane. Figure 11 Middle (e) shows the SEM image of the surface of the 0.55 wt%-SPVDF substrate membrane.

[0033] Figure 12 Scanning electron microscope images of 0.42 wt%-SPVDF-6 h membrane after immersion in different solutions (i.e., 3.5 wt% NaCl solution, 1 mol / L NaOH solution, and acidic solution with a pH value of 3) for more than 20 h.

[0034] Figure 13 XRD patterns of 0.42 wt%-SPVDF-6 h membrane recorded after soaking in different solutions (i.e., 3.5 wt% NaCl solution, 1 mol / L NaOH solution, and acidic solution with pH 3) for >20 h.

[0035] Figure 14 The three-cycle filtration performance test of 0.42 wt%-SPVDF-6 h membrane after soaking in different solutions (3.5 wt% NaCl solution, 1 mol / LNaOH solution and pH=3 acidic solution) for >20 h is presented.

[0036] Figure 15 Shows a comparison of digital photos of kitchen wastewater before and after agitation treatment.

[0037] Figure 16 A comparison of kitchen wastewater permeate treated with the original PVDF membrane (left) and the 0.42 wt%-SPVDF-6 h membrane (right) is shown. DETAILED DESCRIPTION

[0038] Exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. Rather, these embodiments are provided to enable a more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art.

[0039] It should be understood that the terms used herein are for the purpose of describing specific example embodiments only and are not intended to be limiting. Unless the context clearly indicates otherwise, the singular forms "one", "an" and "said" as used herein may also be meant to include plural forms. The terms "comprise", "include", "contain" and "have" are inclusive and therefore specify the presence of stated features, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, steps, operations, elements, parts, and / or combinations thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring them to be performed in the specific order described or illustrated, unless the order of execution is clearly indicated. It should also be understood that additional or alternative steps may be used.

[0040] In the description of the embodiments of the present invention, technical terms such as "first" and "second" are used solely to distinguish between different objects and should not be understood to indicate or imply relative importance or to implicitly specify the quantity, specific order, or primary and secondary relationship of the technical features indicated. In the description of the embodiments of the present invention, "plurality" means more than two, unless otherwise specifically defined.

[0041] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present invention. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute a separate or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0042] In the description of the embodiments of the present invention, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exists simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0043] In the description of the embodiments of the present invention, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0044] In the description of the embodiments of the present invention, unless otherwise expressly specified or limited, technical terms such as "installed," "connected," "connect," and "fixed" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and can refer to internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in the embodiments of the present invention based on specific circumstances.

[0045] The present invention prepares a NiFe-LDH / PVDF composite membrane with an interwoven lamellar structure by regulating the NiFe-LDH seed crystal loading and in-situ growth time, achieving coordinated regulation of pore structure and surface function. Simultaneously, the surface chemistry and nanostructure characteristics of the material are analyzed using SEM, XPS, XRD, AFM and other characterization methods. Its separation performance for complex oil-water systems is systematically evaluated, and the membrane's cyclic stability and anti-pollution mechanism are investigated through visible light-driven SR-AOP regeneration experiments. This invention innovatively constructs a membrane platform that synergizes high-throughput separation with self-regeneration capabilities, providing a sustainable solution for oily wastewater treatment.

[0046] The present invention provides an oil-water separation membrane with excellent separation efficiency and good photocatalytic function, namely a NiFe-LDH / PVDF composite membrane and a preparation method thereof, which has great potential in actual wastewater treatment and purification.

[0047] The first aspect of the present invention provides a NiFe-LDH / PVDF composite membrane, which includes an SPVDF substrate membrane containing nickel-iron layered double hydroxide NiFe-LDH, and nickel-iron layered double hydroxide NiFe-LDH in situ grown on the surface of the SPVDF substrate membrane.

[0048] In an embodiment of the present invention, NiFe-LDH is directly grown on the surface of the SPVDF membrane using an in-situ growth technology, thereby achieving effective modification of the PVDF membrane performance, so that the NiFe-LDH / PVDF composite membrane has excellent photocatalytic performance, which is beneficial to avoid secondary pollution and reduce the application cost of the membrane.

[0049] In the embodiment of the present invention, the pure water flux of NiFe-LDH / PVDF composite membrane is 6220.1 L·m -2 ·h -1 bar -1 The permeation flux of n-hexane / water emulsion is 3564.7 L·m -2 ·h -1 bar -1 , the interception rate reached 99.78%.

[0050] In some embodiments of the present invention, the pure water flux of the NiFe-LDH / PVDF composite membrane is 5814.1 L·m -2 ·h -1 bar -1 ~6984.1 L·m -2 ·h -1 bar -1 The permeation flux of n-hexane / water emulsion is 3564.7 L·m -2 ·h -1 bar -1 ~4614.5 L·m -2 ·h -1 bar -1 , the retention rate is 99.72%~99.86%.

[0051] The pure water flux of the NiFe-LDH / PVDF composite membrane provided by the present invention can be 5814.1 L·m -2 ·h -1 bar -1 、6014.3 L·m -2 ·h -1 bar -1 、6090.7 L·m -2 ·h -1 bar -1 、6183.7 L·m -2 ·h -1 bar -1 、6220.1 L·m -2 ·h -1 bar -1 、6550.3 L·m -2 ·h -1 bar -1 、6757.0 L·m -2 ·h -1 bar -1 、6797.5 L·m -2 ·h -1 bar -1 、6984.1 L·m -2 ·h -1 bar -1 or any value that satisfies the above range.

[0052] The permeation flux of the NiFe-LDH / PVDF composite membrane provided by the present invention for n-hexane / water emulsion can be 3564.7 L·m -2 ·h -1 bar-1 、3576 L·m -2 ·h -1 bar -1 、3845.3 L·m -2 ·h -1 bar -1 、3849.2L·m -2 ·h -1 bar -1 、4614.5 L·m -2 ·h -1 bar -1 or any value that satisfies the above range.

[0053] The rejection rate of the NiFe-LDH / PVDF composite membrane provided by the present invention can be one of 99.72%, 99.76%, 99.78%, 99.79%, 99.86% or any value within the above range.

[0054] The second aspect of the present invention provides a method for preparing the NiFe-LDH / PVDF composite membrane described in the first aspect. The key to this preparation method is to successfully construct a hierarchical nanosheet structure with optimized surface hydrophilicity and roughness by systematically regulating the NiFe-LDH seed content and in-situ growth time. This preparation method has the advantages of strong process controllability, simple operation, and low cost.

[0055] See also Figure 1 As shown, the preparation method of the NiFe-LDH / PVDF composite membrane in the present invention is specifically carried out according to the following steps.

[0056] Preparation of NiFe-LDH

[0057] In an embodiment of the present invention, a first NiFe-LDH precursor solution containing an iron salt and a nickel salt is subjected to a hydrothermal reaction to obtain a NiFe-LDH suspension, and the NiFe-LDH suspension is successively centrifuged and dried to obtain NiFe-LDH, which may also be referred to as NiFe-LDH seed crystals.

[0058] In some embodiments of the present invention, in the first NiFe-LDH precursor solution, the mass percentage of the iron salt is 0.579 wt %, and the mass percentage of the nickel salt is 1.261 wt %.

[0059] In some embodiments of the present invention, the first NiFe-LDH precursor solution is an aqueous solution formed by mixing iron salt, nickel salt, urea and ammonium fluoride.

[0060] In some embodiments of the present invention, in the first NiFe-LDH precursor solution, the mass percentage of urea is 1.726 wt %, and the mass percentage of ammonium fluoride is 0.532 wt %.

[0061] In some embodiments of the present invention, the iron salt is selected from Fe(NO3)3·9H2O, and the nickel salt is selected from Ni(NO3)2·6H2O.

[0062] In some embodiments of the present invention, the solvent used in the first NiFe-LDH precursor solution is deionized water, and the mass percentage of deionized water is 95.902 wt %.

[0063] In some embodiments of the present invention, the temperature of the hydrothermal reaction is 120° C., and the reaction time is 8 h.

[0064] In some embodiments of the present invention, the centrifugal speed of the NiFe-LDH suspension is 8000 rpm to 10000 rpm. For example, the centrifugal speed can be one of 8000 rpm, 9000 rpm, 10000 rpm, or any value within the above range.

[0065] In some embodiments of the present invention, the yellow precipitate NiFe-LDH obtained after centrifugation of the NiFe-LDH suspension is washed three times with deionized water and three times with anhydrous ethanol, and then dried at 60°C to 80°C to produce NiFe-LDH seed crystals. For example, the drying temperature can be one of 60°C, 65°C, 70°C, 75°C, 80°C, or any value within the above range.

[0066] In some embodiments of the present invention, 1.261 wt% Ni(NO₃)₂·6H₂O, 0.579 wt% Fe(NO₃)₃·9H₂O, 0.532 wt% NH₄F, and 1.726 wt% urea were mixed in 95.902 wt% deionized water (DI water) and magnetically stirred for 15 minutes to form a uniform NiFe-LDH precursor solution. This precursor solution was then hydrothermally reacted at 120°C for 8 hours to form a yellow NiFe-LDH suspension. The yellow precipitate, NiFe-LDH, was then centrifuged at 8,000–10,000 rpm and washed three times with deionized water and three times with anhydrous ethanol. Finally, the solution was dried in a vacuum oven at 60–80°C to yield the NiFe-LDH.

[0067] Preparation of SPVDF basement membrane

[0068] A PVDF substrate membrane containing NiFe-LDH, namely an SPVDF substrate membrane, was prepared by an immersion phase inversion method and was denoted as x-SPVDF substrate membrane, where S represents NiFe-LDH and x represents the mass percentage of NiFe-LDH.

[0069] In an embodiment of the present invention, NiFe-LDH is mixed with polyvinylidene fluoride, a pore-forming agent, a surfactant and a solvent to form a casting solution, and the casting solution is coated to form a casting solution, which is immersed in deionized water for phase inversion to prepare an SPVDF substrate membrane.

[0070] In some embodiments of the present invention, the mass percentage of NiFe-LDH in the SPVDF substrate membrane is 0.14 wt% to 0.55 wt%. The mass percentage of NiFe-LDH provided by the present invention can be a range consisting of any two values ​​within the above range, for example, 0.14 wt% to 0.28 wt%, 0.28 wt% to 0.55 wt%, 0.42 wt% to 0.55 wt%, and so on. Exemplarily, the mass percentage of NiFe-LDH can also be one of 0.14 wt%, 0.15 wt%, 0.18 wt%, 0.20 wt%, 0.21 wt%, 0.25 wt%, 0.28 wt%, 0.30 wt%, 0.32 wt%, 0.35 wt%, 0.38 wt%, 0.40 wt%, 0.42 wt%, 0.45 wt%, 0.48 wt%, 0.50 wt%, 0.52 wt%, 0.55 wt% or any value satisfying the above range.

[0071] In some embodiments of the present invention, in the casting solution, the mass percentage of NiFe-LDH is 0.14 wt%~0.55 wt%, the mass percentage of polyvinylidene fluoride (PVDF) is 12.47 wt%~12.52 wt%, the mass percentage of the pore former is 1.11 wt%, the mass percentage of the surfactant is 0.28 wt%, the mass percentage of the solvent is 85.60 wt%~85.95 wt%, and the sum of the mass percentages of each component is 100%.

[0072] The mass percentage of NiFe-LDH provided by the present invention can be a value of an interval consisting of any two values ​​within the above range, for example, it can be 0.14 wt%~0.28 wt%, it can also be 0.28 wt%~0.55 wt%, it can also be 0.42 wt%~0.55 wt%, and so on. Exemplarily, the mass percentage of NiFe-LDH can also be 0.14 wt%, 0.15 wt%, 0.18 wt%, 0.20 wt%, 0.21 wt%, 0.25 wt%, 0.28 wt%, 0.30 wt%, 0.32 wt%, 0.35 wt%, 0.38 wt%, 0.40 wt%, 0.42 wt%, 0.45 wt%, 0.48 wt%, 0.50 wt%, 0.52 wt%, 0.55 wt% or any numerical value that satisfies the above range value.

[0073] The mass percentage of the polyvinylidene fluoride (PVDF) provided by the present invention can be one of 12.47 wt%, 12.48 wt%, 12.49 wt%, 12.50 wt%, 12.51 wt%, 12.52 wt% or any value within the above range.

[0074] The mass percentage of the solvent provided by the present invention can be one of 85.60 wt%, 85.62 wt%, 85.65 wt%, 85.68 wt%, 85.70 wt%, 85.71 wt%, 85.72 wt%, 85.75 wt%, 85.78 wt%, 85.80 wt%, 85.83wt%, 85.85 wt%, 85.88 wt%, 85.90 wt%, 85.92 wt%, 85.95 wt% or any value that meets the above range.

[0075] In some embodiments of the present invention, in the casting solution, the pore-forming agent is selected from polyvinyl pyrrolidone (PVP), the surfactant is selected from sodium dodecyl sulfate (SDS), and the solvent is selected from N-methylpyrrolidone (NMP).

[0076] In some embodiments of the present invention, 12.47 wt% to 12.52 wt% of PVDF, 1.11 wt% of PVP, 0.28 wt% of SDS, and 0.14 wt% to 0.55 wt% of NiFe-LDH seeds are added to an NMP solution having a mass percentage of 85.60 wt% to 85.95 wt% and stirred for 24 h. Subsequently, the mixture is placed in an oven at 60°C to 65°C to remove bubbles and obtain a casting solution. For example, the temperature for removing bubbles can be one of 60°C, 65°C, or any value within the above range.

[0077] In some embodiments of the present invention, the casting solution is slowly poured onto a clean glass plate and evenly coated with a 200 μm thick scraper to form a cast liquid. Subsequently, the entire glass plate is horizontally immersed in 30°C deionized water for phase inversion. After the membrane is completely peeled off from the glass plate, it is transferred to another container filled with 30°C deionized water and soaked for 12 hours. Finally, the membrane is washed again to remove residual solvent to obtain a composite membrane containing NiFe-LDH, which is labeled as x-SPVDF substrate membrane (wherein x represents the mass percentage of NiFe-LDH seed crystals). The x-SPVDF substrate membrane is stored in deionized water for future use.

[0078] Preparation of NiFe-LDH / PVDF composite membrane

[0079] In an embodiment of the present invention, the x-SPVDF substrate membrane is placed in a second NiFe-LDH precursor solution containing iron salt and nickel salt, and NiFe-LDH is in situ grown on one surface of the x-SPVDF substrate membrane under hydrothermal conditions to obtain a NiFe-LDH / PVDF composite membrane.

[0080] In some embodiments of the present invention, in the second NiFe-LDH precursor solution, the mass percentage of the iron salt is 0.295 wt %, and the mass percentage of the nickel salt is 0.644 wt %.

[0081] In some embodiments of the present invention, the second NiFe-LDH precursor solution is an aqueous solution formed by mixing iron salt, nickel salt, urea and ammonium fluoride.

[0082] In some embodiments of the present invention, in the second NiFe-LDH precursor solution, the mass percentage of NH 4 F is 0.272 wt %, and the mass percentage of urea is 0.881 wt %.

[0083] In some embodiments of the present invention, the solvent used in the second NiFe-LDH precursor solution is deionized water, and the mass percentage of deionized water is 97.908 wt%.

[0084] In some embodiments of the present invention, the iron salt is selected from Fe(NO3)3·9H2O, and the nickel salt is selected from Ni(NO3)2·6H2O.

[0085] In some embodiments of the present invention, 0.644 wt% Ni(NO3)2·6H2O, 0.295 wt% Fe(NO3)3·9H2O, 0.272 wt% NH4F, and 0.881 wt% urea were added to 97.908 wt% deionized water and magnetically stirred for 15 min to prepare a uniform green growth solution, i.e., the second NiFe-LDH precursor solution.

[0086] In some embodiments of the present invention, the in situ growth reaction is carried out under hydrothermal conditions at a temperature of 120° C. and a reaction time of 2 to 8 hours. For example, the reaction time can be one of 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, and 8 hours, or any value within the aforementioned range.

[0087] In some embodiments of the present invention, an x-SPVDF substrate membrane is cut into 5 cm x 5 cm squares and attached vertically to the inner wall of a 60 mL reaction vessel, with the front side facing outward. The reaction vessel is placed in a 120°C oven. After the reaction is complete, a NiFe-LDH / PVDF composite membrane with NiFe-LDH grown on the surface is obtained. This NiFe-LDH / PVDF composite membrane can be designated as x-SPVDF-y (where x represents the mass percentage of NiFe-LDH in the SPVDF substrate membrane; y represents the in-situ growth time in hours).

[0088] In the embodiment of the present invention, the NiFe-LDH / PVDF composite membrane showed strong adaptability to complex oily wastewater such as actual kitchen wastewater containing grease, surfactants and suspended solids. Under visible light irradiation and peroxymonosulfate (PMS) activation conditions, the composite membrane was degraded by free radicals (·OH, SO4 - and· 1 O2) achieved a flux recovery rate (FRR) of 93% to 100% over six cycles, significantly outperforming the original PVDF membrane (81% FRR). This dual-function design effectively addresses the long-standing challenges of membrane fouling and the complexity of actual wastewater, providing a sustainable solution for industrial and domestic oily wastewater treatment.

[0089] Unless otherwise defined, the technical terms used in the following examples have the same meanings as those generally understood by those skilled in the art to which the present invention belongs. The experimental reagents used in the following examples, unless otherwise specified, are all conventional biochemical reagents; the raw materials, instruments and equipment used in the following examples, etc., can all be purchased on the market or can be obtained by existing methods; the amounts of the experimental reagents used, unless otherwise specified, are the amounts of reagents used in conventional experimental operations; the experimental methods, unless otherwise specified, are all conventional methods. It should be further noted that the following description is merely exemplary and does not specifically limit the present invention.

[0090] Preparation of NiFe-LDH seed crystals: 789 mg of Ni(NO₃)₂·6H₂O, 362 mg of Fe(NO₃)₃·9H₂O, 333 mg of NH₄F, and 1080 mg of urea were mixed in 60 mL of deionized water (DI water) and magnetically stirred for 15 minutes to form a homogeneous green solution, the first NiFe-LDH precursor solution. This precursor solution was then hydrothermally reacted at 120°C for 8 hours to form a yellow solution, the NiFe-LDH suspension. The yellow precipitate, NiFe-LDH, obtained by centrifugation at 8000 rpm, was washed three times with deionized water and three times with anhydrous ethanol, then dried in a vacuum oven at 60°C to obtain NiFe-LDH seed crystals. These NiFe-LDH seed crystals were used in the following examples.

[0091] It should be noted that in the embodiment, 0.42 wt%-SPVDF-6 h can also be expressed as 0.15-SPVDF-6 h, 0.42 wt%-SPVDF can also be expressed as 0.15-SPVDF, 0.55 wt%-SPVDF can also be expressed as 0.2-SPVDF, 0.28 wt%-SPVDF can also be expressed as 0.1-SPVDF, and 0.14 wt%-SPVDF can also be expressed as 0.05-SPVDF. One is expressed in mass percentage and the other is expressed in mass, which are essentially the same and are the same membrane.

[0092] Example 1

[0093] A NiFe-LDH / PVDF composite membrane, such as Figure 1 As shown, the preparation steps of the NiFe-LDH / PVDF composite membrane are as follows:

[0094] A PVDF substrate membrane containing NiFe-LDH seeds was prepared using an immersion phase inversion method. Specifically, 12.48 wt% PVDF, 1.11 wt% PVP, 0.28 wt% SDS, and 0.42 wt% NiFe-LDH seeds (corresponding to 0.15 g) were added to an 85.71 wt% NMP solution and stirred for 24 hours. The mixture was then placed in a 60°C oven to remove air bubbles, resulting in a casting solution. The casting solution was then slowly poured onto a clean glass plate and evenly coated using a 200 μm-thick doctor blade to form a casting solution. The glass plate was then horizontally immersed in 30°C deionized water for phase inversion. After the membrane was completely peeled from the glass plate, it was transferred to another container filled with 30°C deionized water and immersed for 12 hours. Finally, the membrane was rinsed again to remove any residual solvent. The membrane sample prepared in Example 1 was labeled as 0.42 wt%-SPVDF (wherein 0.42 wt% represents the mass percentage of NiFe-LDH seeds), and the membrane sample was stored in deionized water for future use.

[0095] Under hydrothermal conditions, NiFe-LDH was in situ grown on one side of a 0.42 wt%-SPVDF substrate membrane to prepare a NiFe-LDH / PVDF composite membrane. Specifically, 789 mg of Ni(NO₃)₂·6H₂O, 362 mg of Fe(NO₃)₃·9H₂O, 333 mg of NH₄F, and 1080 mg of urea were added to 120 mL of deionized water and magnetically stirred for 15 minutes to produce a uniform green growth solution, the second NiFe-LDH precursor solution. The 0.42 wt%-SPVDF composite membrane prepared in the previous step was cut into 5 cm × 5 cm squares and attached vertically to the inner wall of a 60 mL reaction vessel, with the front side facing outward. The reaction vessel was placed in a 120°C oven for 6 h to obtain a composite membrane with NiFe-LDH grown on the surface, the resulting NiFe-LDH / PVDF composite membrane, labeled 0.42 wt%-SPVDF-6 h.

[0096] Example 2

[0097] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 2 is the same as that in Example 1, the only difference is that in Example 2, the PVDF substrate membrane containing NiFe-LDH seeds is prepared by specifically adding 12.47 wt% PVDF by mass, 1.11 wt% PVP by mass, 0.28 wt% SDS by mass, and 0.55 wt% NiFe-LDH seeds by mass (corresponding to a mass of 0.2 g) to an NMP solution with a mass percentage of 85.60 wt%.

[0098] The PVDF substrate membrane containing NiFe-LDH seeds prepared in Example 2 is labeled as 0.55 wt%-SPVDF; the NiFe-LDH / PVDF composite membrane finally prepared is labeled as 0.55 wt%-SPVDF-6.

[0099] Example 3

[0100] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 3 is the same as that in Example 1, the only difference is that in Example 3, the PVDF substrate membrane containing NiFe-LDH seeds is prepared by specifically adding 12.50 wt% PVDF by mass, 1.11 wt% PVP by mass, 0.28 wt% SDS by mass, and 0.28 wt% NiFe-LDH seeds by mass (corresponding to a mass of 0.1 g) to an NMP solution with a mass percentage of 85.83 wt%.

[0101] The PVDF substrate membrane containing NiFe-LDH seeds prepared in Example 3 is labeled as 0.28 wt%-SPVDF; the NiFe-LDH / PVDF composite membrane finally prepared is labeled as 0.28 wt%-SPVDF-6.

[0102] Example 4

[0103] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 4 is the same as that in Example 1, the only difference is that in Example 4, the PVDF substrate membrane containing NiFe-LDH seeds is prepared by specifically adding 12.52 wt% PVDF by mass, 1.11 wt% PVP by mass, 0.28 wt% SDS by mass, and 0.14 wt% NiFe-LDH seeds by mass (corresponding to a mass of 0.05 g) to an 85.95 wt% NMP solution.

[0104] The PVDF substrate membrane containing NiFe-LDH seeds prepared in Example 4 is labeled as 0.14 wt%-SPVDF; the NiFe-LDH / PVDF composite membrane finally prepared is labeled as 0.14 wt%-SPVDF-6.

[0105] Example 5

[0106] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 5 is the same as that in Example 1, except that the in-situ growth time under hydrothermal conditions in Example 5 is 2 h.

[0107] The NiFe-LDH / PVDF composite membrane finally prepared in Example 5 is labeled as 0.42 wt%-SPVDF-2.

[0108] Example 6

[0109] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 6 is the same as that in Example 1, except that the in-situ growth time under hydrothermal conditions in Example 6 is 4 h.

[0110] The NiFe-LDH / PVDF composite membrane finally prepared in Example 6 is labeled as 0.42 wt%-SPVDF-4.

[0111] Example 7

[0112] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 7 is the same as that in Example 1, except that the in-situ growth time under hydrothermal conditions in Example 6 is 8 h.

[0113] The NiFe-LDH / PVDF composite membrane finally prepared in Example 7 is labeled as 0.42 wt%-SPVDF-7.

[0114] Example 8

[0115] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 8 is the same as that in Example 2, except that the in-situ growth time under hydrothermal conditions in Example 8 is 2 h.

[0116] The NiFe-LDH / PVDF composite membrane finally prepared in Example 8 is labeled as 0.55 wt%-SPVDF-2.

[0117] Example 9

[0118] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 9 is the same as that in Example 2, except that the in-situ growth time under hydrothermal conditions in Example 8 is 4 h.

[0119] The NiFe-LDH / PVDF composite membrane finally prepared in Example 9 is labeled as 0.55 wt%-SPVDF-4.

[0120] Example 10

[0121] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 10 is the same as that in Example 2, except that the in-situ growth time under hydrothermal conditions in Example 10 is 8 h.

[0122] The NiFe-LDH / PVDF composite membrane finally prepared in Example 10 is labeled as 0.55 wt%-SPVDF-8.

[0123] Example 11

[0124] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 11 is the same as that in Example 3, except that the in-situ growth time under the hydrothermal conditions in Example 5 is 2 h.

[0125] The NiFe-LDH / PVDF composite membrane finally prepared in Example 11 is labeled as 0.28 wt%-SPVDF-2.

[0126] Example 12

[0127] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 12 is the same as that in Example 3, except that the in-situ growth time under hydrothermal conditions in Example 5 is 4 h.

[0128] The NiFe-LDH / PVDF composite membrane finally prepared in Example 12 is labeled as 0.28 wt%-SPVDF-4.

[0129] Example 13

[0130] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 13 is the same as that in Example 3, except that the in-situ growth time under hydrothermal conditions in Example 5 is 8 h.

[0131] The NiFe-LDH / PVDF composite membrane finally prepared in Example 13 is labeled as 0.28 wt%-SPVDF-8.

[0132] Example 14

[0133] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 14 is the same as that in Example 4, except that the in-situ growth time under hydrothermal conditions in Example 5 is 2 h.

[0134] The NiFe-LDH / PVDF composite membrane finally prepared in Example 14 is labeled as 0.14 wt%-SPVDF-2.

[0135] Example 15

[0136] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 15 is the same as that in Example 4, except that the in-situ growth time under hydrothermal conditions in Example 5 is 4 h.

[0137] The NiFe-LDH / PVDF composite membrane finally prepared in Example 15 is marked as 0.14 wt%-SPVDF-4.

[0138] Example 16

[0139] The preparation method of the NiFe-LDH / PVDF composite membrane in Example 16 is the same as that in Example 4, except that the in-situ growth time under hydrothermal conditions in Example 5 is 8 h.

[0140] The NiFe-LDH / PVDF composite membrane finally prepared in Example 16 is labeled as 0.14 wt%-SPVDF-8.

[0141] Comparative Example 1

[0142] A PVDF membrane without NiFe-LDH seed crystals was used as a control membrane. The preparation steps of the PVDF membrane are as follows:

[0143] 12.53 wt% PVDF, 1.11 wt% PVP, and 0.28 wt% SDS were added to an 86.08 wt% NMP solution and stirred for 24 hours. The mixture was then placed in a 60°C oven to remove bubbles, yielding a casting solution. The casting solution was then slowly poured onto a clean glass plate and evenly coated using a 200 μm-thick doctor blade to form a casting solution. The entire glass plate was then horizontally immersed in 30°C deionized water for phase inversion. After the membrane was completely peeled from the glass plate, it was transferred to another container filled with 30°C deionized water and soaked for 12 hours. Finally, the membrane was rinsed again to remove any residual solvent. The membrane sample prepared in Comparative Example 1 is labeled 0-SPVDF (where 0 represents the absence of NiFe-LDH seed crystals). The final membrane sample was stored in deionized water for subsequent testing.

[0144] Performance Testing

[0145] The NiFe-LDH / PVDF composite membranes prepared in the examples of the present invention were tested in a laboratory-scale dead-end filtration apparatus to evaluate their separation performance and retention of oil-water emulsions. The membranes were also evaluated for their cyclic separation performance under visible light using an n-hexane / water emulsion as a simulated pollutant. The test was provided by a nitrogen cylinder, and the test pressure was set to 0.1 bar. Filtration was initiated and water flux data were recorded. Each test was initially stabilized at 1 bar for 30 minutes before data collection.

[0146] In the present invention, n-hexane / water emulsion was used to evaluate membrane selectivity. The rejection of n-hexane / water emulsion was analyzed using a UV-visible spectrophotometer (Unocal, UV-2800A).

[0147] In the present invention, the DMPO-·OH and ·SO4 of the original PVDF membrane, 0.42 wt%-SPVDF substrate membrane and NiFe-LDH / PVDF composite membrane were recorded on Bruker A300. - DMPO-·O2 - and DMPO- Their ability to collect visible light was investigated and compared by UV-vis diffuse reflectance spectroscopy (DRS, Agilent Cary 5000).

[0148] The present invention evaluates the antifouling performance of the membrane by filtering a simulated fouling model consisting of n-hexane / water emulsion. During the experiment, the changes in membrane flux and the retention performance of the n-hexane / water emulsion were monitored. After 30 minutes of filtration, the fouled membrane was immersed in a solution containing 0.4 g·L -1 The membrane was then irradiated with a 300 W xenon lamp equipped with a cutoff filter (λ > 400 nm) in deionized water containing PMS (monopotassium persulfate) for 10 minutes. Subsequently, the membrane was briefly rinsed with deionized water, and the deionized water flux through the membrane was measured for 30 minutes. Six cycles were performed for each membrane sample.

[0149] Table 1 Summary of membrane properties in Examples and Comparative Examples

[0150]

[0151] The present invention uses an in-situ growth method to grow NiFe-LDH on a PVDF membrane. The prepared NiFe-LDH / PVDF composite membrane shows excellent performance. The pure water flux of the 0.42 wt%-SPVDF-6 h membrane is as high as 6220.1 L·m -2 ·h -1 bar -1 In the n-hexane / water emulsion separation experiment, the permeate flux reached 3564.7 L·m-2 ·h -1 bar -1 , while the retention rate is as high as 99.78%, showing excellent separation performance.

[0152] See also Figure 2 As shown, Figure 2 (a) to (d) show the SEM and TEM images of NiFe-LDH seeds at different magnifications, respectively; Figure 2 (e) shows the SEM cross-sectional image of the original PVDF membrane; Figure 2 (f) shows the SEM cross-sectional image of the 0.42 wt%-SPVDF substrate membrane in Example 1; Figure 2 (g) shows the SEM cross-sectional image of the 0.42 wt%-SPVDF-6 h membrane in Example 1; Figure 2 (h) and (i) show EDX images of the 0.42 wt%-SPVDF-6 h membrane in Example 1.

[0153] The original PVDF membrane exhibits typical finger-like pores and macropore structures, see Figure 2 After adding NiFe-LDH seeds, the finger-like pore structure of the original PVDF membrane is retained, indicating that the introduction of NiFe-LDH seeds does not significantly change the internal overall pore morphology of the original PVDF membrane, see Figure 2 More importantly, the introduction of NiFe-LDH seeds provides nucleation sites on the surface of the original PVDF membrane, which further promotes the uniform growth of subsequent NiFe-LDH crystals.

[0154] During the in situ growth process, a uniform and dense NiFe-LDH crystalline coating was formed on the membrane surface, showing typical layered stacking characteristics, see Figure 2 As shown in (g). Through the in-situ growth method, the crystalline coating is firmly and uniformly attached to the surface of the SPVDF substrate membrane. For the 0.42 wt%-SPVDF-6 h membrane, the thickness of the NiFe-LDH coating is approximately 1 μm. Energy dispersive X-ray spectroscopy (EDX) elemental distribution map further confirms the dense and uniform growth of the NiFe-LDH coating on the surface of the 0.42 wt%-SPVDF substrate membrane, see Figure 2 As shown in (h) and (i).

[0155] It is well known that the in situ growth of target materials inevitably affects the separation performance of the membrane. To this end, we systematically manipulated the growth conditions to minimize the flux impact. Figure 11 The SEM images of the surface of the original PVDF membrane and the SPVDF substrate membrane containing different amounts of NiFe-LDH seeds are shown in FIG. Figure 3The SEM characterization results of the in situ growth of NiFe-LDH on the surface of SPVDF substrate membrane at 120 ℃ with different seed contents and different growth times are presented.

[0156] Because the nucleation and crystallization of NiFe-LDH seeds occur simultaneously in solution and on the substrate film surface, precisely controlling the growth layer is often a significant challenge. Initially, NiFe-LDH crystals randomly nucleate on the substrate film surface. Due to the layered nature of NiFe-LDH, its lateral growth rate is significantly higher than its longitudinal growth rate. Regulated by the van der Drift growth mechanism, the crystals ultimately develop a petal-like or densely stacked morphology.

[0157] At low seed concentrations (0.14 wt%–0.28 wt%), sparse nucleation sites lead to insufficient competition between crystals, resulting in unclear preferred orientation and uneven crystal distribution. At a moderate seed concentration (0.42 wt%), the optimal nucleation density triggers significant crystal competition, promoting oriented growth and forming a uniform layered structure. With increased growth time, the nanosheets further expand and form a denser structure. However, excessive seeding (0.55 wt%) results in overcrowding of nucleation sites. Intense competition between crystals leads to non-preferred growth orientations and stacking defects, ultimately compromising the structural uniformity of the film surface.

[0158] See also Figure 4 As shown in Figure 2, the surface of the original PVDF membrane is relatively smooth and has a low surface roughness (Rq = 25.9 nm, Ra = 20.4 nm). Figure 4 As shown in (a). After the introduction of NiFe-LDH seeds, the surface roughness of the membrane increases slightly (Rq = 57.5 nm, Ra = 40.3 nm), which is attributed to the presence of NiFe-LDH seeds on the surface and the increase in surface porosity caused by the NiFe-LDH seeds promoting the exchange rate between solvent and non-solvent, as shown in Figure 2. Figure 4 As shown in (b). After 6 hours of in-situ growth, a NiFe-LDH layer with an interlocking lamellar structure was formed on the surface of the SPVDF substrate membrane containing 0.42 wt% NiFe-LDH seeds, and its surface roughness increased significantly (Ra = 128 nm, Rq = 97.6 nm). Figure 4 As shown in (c), the surface area also increased from 27.2 μm² to 45.2 μm², nearly doubling.

[0159] like Figure 5As shown in (a), the NiFe-LDH seed crystals show characteristic diffraction peaks at 2θ angles of 11.5°, 23.4°, 33.6°, 34.4°, 39.1°, 46.7°, 59.9° and 61.2°, which correspond to the (003), (006), (101), (012), (015), (018), (110) and (013) crystal planes, respectively, confirming the successful synthesis of NiFe-LDH (JCPDS No. 40-0215). The XRD spectrum of the 0.42wt%-SPVDF-6 h film shows characteristic peaks of both PVDF and NiFe-LDH. According to Figure 5 FT-IR spectrum of (b) 0.42 wt%-SPVDF-6 h membrane at 3200 cm -1 ~3600 cm -1 The broad peak intensity is attributed to the stretching vibration of hydroxyl groups (OH) on the LDH surface, which also indicates that its hydrophilicity is significantly enhanced compared with the original PVDF membrane and SPVDF substrate membrane.

[0160] The surface chemical characteristics of the membrane were further analyzed by XPS spectroscopy. Figure 5 (c) to (f). Figure 5 (d) shows that the full spectrum of the original PVDF membrane is mainly composed of C and F element signals, while the characteristic peaks of Ni and Fe elements are also observed in the 0.42 wt%-SPVDF-6 h membrane. Figure 5 As shown in (d) to (f), the C 1s peaks of the original PVDF membrane correspond to CF2 (290.6 eV), CH2 (286.1 eV) and CC / CH (284.8 eV). The CO bond caused by oxygen-containing polymers (such as PVP) produces a peak at around 287.3 eV. Compared with the original PVDF membrane, the urea added during the preparation of the 0.42 wt%-SPVDF-6 h membrane produces CO2 and OH- through thermal decomposition. These substances can be intercalated into the LDH crystal structure to form carbonate (CO3 2- ) or bicarbonate (HCO3 - ) and other interlayer compensating anions, so in Figure 5 In (f), an O=CO peak (288.9 eV) appears. In addition, CO2 may be adsorbed on the NiFe-LDH surface, resulting in the appearance of a C=O peak (287.9 ​​eV).

[0161] The wettability of filtration membranes is crucial for the phase separation of oil-in-water emulsions, and this property is mainly controlled by the composition of the membrane material, chemical groups, and surface structure (pore size and roughness). Figure 6As shown in (a), for a 0.42 wt%-SPVDF substrate membrane, the introduction of NiFe-LDH seeds reduces the water contact angle (WCA) from 78.3° for pure PVDF to 66.9°, confirming enhanced surface hydrophilicity. Further analysis of the effect of in-situ growth time on hydrophilicity reveals that when the growth time is extended to 4 hours, the WCA decreases monotonically with growth time (e.g., from 80.4° for 2 hours to 57.8° for 4 hours), indicating a continuous improvement in hydrophilicity. However, it is noteworthy that the contact angle of the 0.42 wt%-SPVDF-2 hour membrane increases abnormally compared to the 4 hour sample, indicating a temporary decrease in hydrophilicity.

[0162] This abnormal phenomenon may be closely related to the heterogeneous nucleation growth kinetics of NiFe-LDH. In the initial growth stage (2 h), the NiFe-LDH precursor in the solution preferentially nucleates heterogeneously on the PVDF substrate, and the disordered coverage of the new crystals may mask the original hydrophilic hydroxyl sites on the seed surface. When the growth time is extended to 4 h, the crystals undergo rearrangement and orientation adjustment through the Ostwald ripening process, exposing more hydroxyl-rich active crystal faces, thereby improving the surface hydrophilicity. After 6 hours of growth, the WCA of the 0.42 wt%-SPVDF substrate membrane dropped to 23.2°, which is attributed to the complete coverage of LDH on the substrate surface, see Figure 6 According to the Wenzel equation, higher surface roughness can effectively enhance hydrophilicity, which is highly consistent with the experimental results.

[0163] Figure 6 (c) compares the evolution of membrane hydrophilicity over a 10-second period: the native PVDF membrane remains hydrophobic, with almost no change in WCA; whereas the WCA of the 0.42 wt%-SPVDF substrate membrane decreases modestly from 65.2° to 60.2°; and the 0.42 wt%-SPVDF-6 h membrane decreases dramatically from 25.2° to complete wetting (0°). This significant difference highlights the significantly enhanced surface wettability of the 0.42 wt%-SPVDF-6 h membrane compared to both the native PVDF membrane and the seeded SPVDF substrate membrane. Its superior water affinity promotes the formation of a hydration layer on the membrane surface, effectively preventing oil droplet adhesion.

[0164] Figure 6Panel (d) shows the ultrasound-assisted contact angle (UOCA) measurements of the original PVDF membrane, a 0.42 wt% SPVDF substrate membrane, and a 0.42 wt% SPVDF-6 h membrane. The UOCA of the original PVDF membrane was 137.6±1.2°, while that of the NiFe-LDH seeded SPVDF substrate membrane dropped to 117.1±1.3°. This phenomenon can be explained by the Wenzel state: when an oil droplet wets the membrane surface, the increased surface roughness (increased contact area between the membrane and the oil droplet) significantly enhances the oil droplet's adhesion to the membrane surface. Although SPVDF exhibits improved hydrophilicity compared to the PVDF substrate, the lack of synergistic effects between surface roughness and chemical properties may result in the oil droplet's increased contact with the substrate. In contrast, the 0.42 wt% SPVDF-6 h membrane exhibits superoleophobic properties, with a UOCA as high as 148.5±1.1°. This significant improvement is primarily due to: ① the intrinsic hydrophilicity of NiFe-LDH, which is rich in hydrophilic functional groups; and ② the high roughness generated by its interwoven lamellar structure. More importantly, under the Cassie-Baxter state, water molecules trapped in the grooves of the surface with nano- / micrometer-scale roughness features form a barrier to oil droplet adhesion, while increased roughness further reduces the adhesion of oil droplets to the membrane surface. These synergistic effects significantly enhance the oleophobicity of the membrane surface.

[0165] Figure 7 (a) to (d) show the changes in the flux and retention rate of the membrane under different NiFe-LDH seed crystal contents and in situ growth times. When the in situ growth time is 2 hours, due to the low coverage of NiFe-LDH crystals, the membrane pore structure basically remains in its original state, providing a good channel for water flow. Although the hydrophilicity decreases slightly, the NiFe-LDH crystals themselves have certain wetting properties, and their local hydrophilicity promotes the diffusion of water molecules into the membrane pores, and the water flow resistance is low, so the flux is slightly improved. As the in situ growth time increases, the membrane flux gradually decreases compared with the corresponding SPVDF substrate membrane. This is because the coverage of NiFe-LDH crystals on the membrane surface increases, resulting in a gradual decrease in pore volume and effective pore size, thereby restricting water flow. This trend is Figure 3 This is also intuitively confirmed in the SEM images of the membrane. Taking into account the membrane permeability and selectivity, the 0.42 wt%-SPVDF substrate showed the best performance after 6 hours of in situ growth. Therefore, the 0.42 wt%-SPVDF-6 h membrane was selected as the optimal membrane for subsequent characterization.

[0166] The 0.42 wt%-SPVDF-6 h membrane exhibited high pure water flux (PWP) for emulsions prepared from various oils (n-hexane, n-heptane, cyclohexane, petroleum ether, and gasoline), and the rejection rate for all types of emulsions remained above 99%. Specifically, the water flux of these emulsions was 3564.7 ± 339.3 L·m-2 ·h -1 bar -1 、3692.3±108.5 L·m -2 ·h -1 bar -1 、3830.5±205.6 L·m -2 ·h -1 bar -1 、3168.3±223.3 L·m -2 ·h -1 bar -1 and 2483.0±223.3 L·m -2 ·h -1 bar -1 , the corresponding oil retention rates are 99.78±0.07%, 99.89±0.03%, 99.50±0.18%, 99.40±0.09% and 99.79±0.17%, see Figure 7 The results show that the 0.42 wt%-SPVDF-6 h membrane has both high flux and high separation efficiency when treating a variety of oil-water emulsions, demonstrating excellent comprehensive performance.

[0167] In order to explore the practical application performance of 0.42 wt%-SPVDF-6 h membrane in complex wastewater separation, the present invention obtained oily wastewater containing multiple unknown components from the organic waste liquid collection container and used Oil Red O to mark the organic phase. Figure 7 As shown in (f), despite the complex composition of the wastewater, the 0.42 wt%-SPVDF-6 h membrane successfully achieved complete separation of the organic phase labeled with Oil Red O and the aqueous phase labeled with methylene blue, demonstrating excellent separation efficiency.

[0168] The light absorption capacity of the original PVDF membrane, SPVDF substrate membrane and 0.42wt%-SPVDF-6 h membrane was analyzed by UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). Figure 8 The results show that the light absorption capacity of the original PVDF membrane and the SPVDF substrate membrane is negligible, while the 0.42 wt%-SPVDF-6 h membrane exhibits significantly enhanced light absorption across the entire spectral range (especially in the visible light region of 400 nm to 700 nm) due to the presence of NiFe-LDH on its surface.

[0169] Furthermore, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and 2,2,6,6-tetramethylpiperidinium N-oxide (TEMP) were used as free radical scavengers to identify the active species of the 0.42 wt%-SPVDF-6 h membrane system by electron spin resonance (ESR) spectroscopy. The free radical generation in the presence and absence of PMS and under visible light conditions was systematically investigated. Figure 8 In (b) to (d), ·OH and ·O2 are almost undetectable in the system without PMS and visible light. - and ¹O2 signals; when PMS was added and visible light irradiation was applied, DMPO-·OH, DMPO-·O2 - The characteristic signal intensities of TEMP-¹O2 were significantly enhanced, indicating that the free radicals in the system mainly came from the catalytic activation of PMS by 0.42 wt%-SPVDF-6 h membrane under visible light irradiation.

[0170] According to existing research, Fe in NiFe-LDH 2+ / Fe 3+ and Ni 2+ / Ni 3+ The mutual conversion of redox pairs can promote the activation of PMS to produce reactive oxygen species (ROS). Based on this, the degradation reaction mechanism of 0.42 wt%-SPVDF-6 h film + PMS + visible light system can be described as follows: NiFe-LDH generates photogenerated electrons (e - ) and holes (h + ), photogenerated electrons react with PMS to generate SO4 - , the holes react with H2O or OH ⁻ The reaction produces ·OH, achieving efficient degradation of pollutants.

[0171] To compare the cleaning performance of the original PVDF membrane and the 0.42 wt%-SPVDF-6 h membrane, water rinsing and visible light-driven PMS activation were performed using a n-hexane / water emulsion system, respectively. Figure 9 (a) to (d) in the middle. The results show that during the 30-minute oil-water emulsion filtration process, the permeate flux of the 0.42 wt%-SPVDF-6 h membrane slowly decreased, while the PVDF membrane showed a more significant decrease. This is primarily attributed to the staggered "petal-like" NiFe-LDH nanosheet structure on the surface of the 0.42 wt%-SPVDF-6 h membrane, which effectively inhibits the initial non-migratory adsorption of pollutants on the surface through the traditional "passive antifouling" (PRF) mechanism, thereby ensuring the long-term stable operation of the membrane.

[0172] After 30 min of continuous filtration and water washing, the flux recovery rates (FRR) of the original PVDF membrane and the 0.42 wt%-SPVDF-6 h membrane reached 80% and 76% of the initial values, respectively. Figure 9 (a) to (b) in the figure. This phenomenon is mainly due to the larger pore size and smoother surface of the original PVDF membrane, which means that contaminants are mostly present in the form of physical blockage or surface attachment. In addition, the low pressure conditions (0.1 bar) used in the test make it difficult for contaminants to penetrate the membrane pores and are more easily removed by water flushing. In contrast, the NiFe-LDH-modified 0.42 wt%-SPVDF-6 h membrane has a rough surface and a multi-level pore structure between nanosheet layers. Emulsified oil droplets easily embed into surface cracks or interlayer gaps, hindering the recovery of water flushing flux. After six consecutive filtration cycles, the flux of the original PVDF membrane and the 0.42 wt%-SPVDF-6 h membrane dropped to 60% and 58% of the initial value, respectively. The final oily wastewater filtration flux was maintained at 57% and 64%.

[0173] However, when 0.4 g / L PMS and 10 min of visible light irradiation were added after each filtration cycle, the FRR of the two membranes increased significantly to 90% and 99%, respectively. Figure 9 (c) to (d). It is worth noting that the FRR of the 0.42 wt%-SPVDF-6 h membrane was always ≥93% during the six cycle tests, while the original PVDF membrane gradually dropped to 81%, confirming that the visible light-driven PMS activation endowed the 0.42 wt%-SPVDF-6 h membrane with excellent self-cleaning ability and cyclic stability. Both membranes showed good structural stability during repeated filtration and photocatalytic degradation - the retention rate remained at a high and stable level during oil-water emulsion filtration. Periodic retention rate tests performed every 5 minutes showed that the final value (99.46%) was negligible compared to the initial value (99.62%), indicating that the membrane structural integrity remained good after multiple photocatalytic treatments.

[0174] Oily wastewaters generated by different industrial processes vary significantly in composition and properties. For example, the pH of biodiesel production can fluctuate between 4 and 11, depending on process parameters. To this end, the separation performance and fouling characteristics of a 0.42 wt%-SPVDF-6 h membrane for surfactant-stabilized n-hexane / water emulsions at different pH conditions were systematically investigated. Notably, the membrane exhibited a higher retention rate at pH 3, as shown in Figure 2. Figure 9 (g), while the retention rate decreased slightly at pH = 11, see Figure 9 (h), which may be due to the enhanced water solubility of Oil Red O and its degradation products under strong alkaline conditions. The experimental data also show that after photocatalytic cleaning, whether it is acidic (pH = 3, Figure 9 neutral (e)) or alkaline (pH = 11, Figure 9 The membrane flux recovery rate (FRR) remained good under the conditions of medium (f), confirming its wide pH applicability range.

[0175] To evaluate the stability of the 0.42 wt%-SPVDF-6 h membrane in complex environments, the present invention immersed it in 3.5 wt% NaCl solution, 1 mol / L NaOH solution (pH = 13.89), and pH = 3 acidic solution for 20 hours. SEM analysis showed that the membrane surface structure remained intact after immersion treatment, with no obvious deformation or structural damage. Figure 12 Although the XRD patterns show that the crystal structure of the film under different conditions has slight changes (see Figure 13 ), but these changes have limited impact on overall stability. In the subsequent three-cycle separation test, the membrane FRR always remained above 97% and the retention rate remained high (see Figure 14 ), fully demonstrating its excellent stability in harsh environments.

[0176] In response to actual wastewater treatment needs, this paper collected kitchen wastewater generated during dishwashing. The wastewater has a complex composition, including detergents (water-soluble substances), cooking oil (oily substances), and rice and vegetable residues (solid substances). It has the characteristics of high oil-water miscibility and suspended matter concentration. After stirring for 4 hours, a homogeneous emulsion was obtained and used for separation experiments (see Figure 15 Due to the presence of a large number of unknown components in the wastewater (some of which have large particle sizes or are resistant to photocatalytic degradation), photocleaning cannot completely remove pollutants: the FRR of the original PVDF membrane after the first cycle of photocleaning was only 68%, while the water flux of the 0.42 wt%-SPVDF-6 h membrane recovered to 83% of the initial value, see Figure 10 (a) to (b) show that light cleaning has limited effect on some difficult-to-degrade components, and the oxidation time needs to be extended to improve the cleaning efficiency. Figure 10 In (c) to (e), the filtration performance analysis showed that the filtrate of the modified membrane was clearer than that of the original PVDF membrane (e.g. Figure 16 ), which is mainly attributed to the unique layer stacking structure on the surface of 0.42 wt%-SPVDF-6 h membrane, which can efficiently intercept complex components of wastewater, while the treatment performance of PVDF membrane with a single structure is poor, highlighting the advantages of modified membranes in practical applications.

[0177] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a NiFe-LDH / PVDF composite membrane, characterized in that: The following steps are involved: subjecting a first NiFe-LDH precursor solution containing an iron salt and a nickel salt to a hydrothermal reaction to obtain a NiFe-LDH suspension, and centrifugally drying the NiFe-LDH suspension to obtain NiFe-LDH; The NiFe-LDH is mixed with polyvinylidene fluoride, a pore-forming agent, a surfactant and a solvent to form a casting solution, and the casting solution is coated to form a casting solution, which is immersed in deionized water for phase inversion to prepare a PVDF basement membrane containing NiFe-LDH, which is recorded as an SPVDF basement membrane; The SPVDF substrate membrane is placed in a second NiFe-LDH precursor solution containing iron salt and nickel salt, and NiFe-LDH is in situ grown on one side surface of the SPVDF substrate membrane under hydrothermal conditions to obtain the NiFe-LDH / PVDF composite membrane.

2. The method for preparing the NiFe-LDH / PVDF composite membrane according to claim 1, wherein In the first NiFe-LDH precursor solution, the mass percentage of the iron salt is 0.579 wt %, and the mass percentage of the nickel salt is 1.261 wt %; and / or, The iron salt is selected from Fe(NO3)3·9H2O, and the nickel salt is selected from Ni(NO3)2·6H2O.

3. The method for preparing the NiFe-LDH / PVDF composite membrane according to claim 1, wherein The temperature of the hydrothermal reaction is 120° C., and the reaction time is 8 h.

4. The method for preparing the NiFe-LDH / PVDF composite membrane according to claim 1, wherein In the SPVDF basement membrane, the mass percentage of the NiFe-LDH is 0.14 wt% to 0.55 wt%.

5. The method for preparing the NiFe-LDH / PVDF composite membrane according to claim 1, wherein In the casting solution, the mass percentage of the NiFe-LDH is 0.14 wt%~0.55 wt%, the mass percentage of the polyvinylidene fluoride is 12.47 wt%~12.52 wt%, the mass percentage of the pore former is 1.11 wt%, the mass percentage of the surfactant is 0.28 wt%, and the mass percentage of the solvent is 85.60 wt%~85.95 wt%; and the sum of the mass percentages of each component is 100%.

6. The method for preparing the NiFe-LDH / PVDF composite membrane according to claim 1, wherein In the second NiFe-LDH precursor solution, the mass percentage of the iron salt is 0.295 wt %, and the mass percentage of the nickel salt is 0.644 wt %; and / or, The iron salt is selected from Fe(NO3)3·9H2O, and the nickel salt is selected from Ni(NO3)2·6H2O.

7. The method for preparing the NiFe-LDH / PVDF composite membrane according to claim 1, wherein: The first NiFe-LDH precursor solution and the second NiFe-LDH precursor solution are each independently an aqueous solution formed by mixing iron salt, nickel salt, urea and ammonium fluoride.

8. The method for preparing the NiFe-LDH / PVDF composite membrane according to claim 1, wherein The temperature for the in-situ growth reaction under the hydrothermal conditions is 120° C., and the reaction time is 2 h to 8 h.

9. A NiFe-LDH / PVDF composite membrane prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The NiFe-LDH / PVDF composite membrane comprises an SPVDF substrate membrane containing nickel-iron layered double hydroxide NiFe-LDH, and nickel-iron layered double hydroxide NiFe-LDH in situ grown on the surface of the SPVDF substrate membrane.

10. The NiFe-LDH / PVDF composite membrane according to claim 9, wherein The pure water flux of the NiFe-LDH / PVDF composite membrane reached 6220.1 L·m -2 ·h -1 bar -1 The permeation flux of n-hexane / water emulsion is 3564.7 L·m -2 ·h -1 bar -1 , the interception rate reached 99.78%.

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