ECTFE-based corrosion-resistant nanofiltration membrane, preparation method and application thereof

Abstract

The present invention discloses an ECTFE-based corrosion-resistant nanofiltration membrane, a preparation method and its application, and belongs to the field of membrane separation technology. The present application is based on the preparation of an ECTFE-based nanofiber-based membrane by a polyvinyl alcohol (PVA)-assisted electrospinning method, and the preparation is completed by acid-catalyzed glutaraldehyde (GA) crosslinking and polyethyleneimine (PEI) and trimesoyl chloride (TMC) interfacial polymerization. In this patent, an intertwined corrosion-resistant polymer network is formed by the corrosion-resistant ECTFE polymer, the acid-catalyzed PVA and GA acetal crosslinking network, the aldehyde amine crosslinking network of GA and PEI, the esterification crosslinking of PVA and TMC, and the interfacial polymerization network of PEI and TMC. It shows excellent long-term stability and chemical stability, and can withstand application environments of high temperature, strong acid, strong base and organic solvent. The composite membrane has broad application prospects in treating wastewater in harsh environments.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane separation, and in particular to an ECTFE-based corrosion-resistant nanofiltration membrane, a preparation method and applications thereof. Background Art

[0002] Nanofiltration (NF) membranes feature sub-nanometer pore sizes (0.5-2 nm) and a unique surface charge. Through pore size sieving and the Donan effect, they can effectively separate small molecules with molecular weights ranging from 100 to 2000 g / mol. Consequently, they are widely used in industries such as wastewater treatment, water softening, food processing, and pharmaceuticals. However, harsh separation environments, such as those in the food, pickling, chlor-alkali, and bulk chemical separation and recovery industries, generate large quantities of polar solvents, strong acids, strong bases, and high-temperature wastewater. Highly efficient recovery places high demands on membrane materials. Traditional thin-layer composite NF membrane materials exhibit poor stability in these harsh environments. For example, the polyamide and cellulose acetate selective layer networks are susceptible to acid and alkali attack. Nanoporous polyethylene (EVAL) substrates are unstable in acidic conditions, polyimide (PI) substrates are unstable in alkaline conditions, and substrates such as polyethersulfone and polysulfone easily swell in organic solvents. Polymers are prone to creep at high temperatures, which can damage the membrane structure, leading to performance degradation and reduced membrane life, making them unable to meet the separation requirements in these harsh environments. Therefore, there is an urgent need to develop highly stable support layer and selection layer membrane materials, and then construct high-performance composite nanofiltration membranes suitable for harsh environments.

[0003] Ethylene-chlorotrifluoroethylene (ECTFE) is a copolymer of ethylene chlorotrifluoroethylene (ECTFE), which is rich in high-energy C-C and C-F bonds in its molecular chain. Its main chain skeleton is stable, and it has excellent mechanical strength, thermal stability, and chemical stability. It also exhibits excellent resistance to acids, alkalis, heat, and solvents, making it an ideal material for preparing stable polymer substrates. However, ECTFE is insoluble in any reagent at room temperature, making it impossible to prepare membranes using the non-solvent induced phase separation (NIPS) method. The main method for preparing membranes using ECTFE as a substrate is the thermally induced phase separation (TIPS) method. For example, Chinese patent CN 108057346 B discloses a high-flux polymer separation membrane that utilizes binary diluents tributyl acetyl citrate and dioctyl terephthalate to prepare an ECTFE membrane using TIPS, which increases the membrane's pure water flux by more than 1.5 times. However, the TIPS membrane preparation process is complex and energy-intensive. Furthermore, this method requires high-temperature reaction, where the ECTFE reacts with the diluent, potentially breaking the C-Cl bond and affecting the membrane structure and performance. Therefore, there is an urgent need to explore new membrane production methods to prepare high-performance ECTFE-based membrane materials.

[0004] Electrospinning utilizes the interaction between a polymer solution and an applied electric field to overcome the surface tension of the solution, generating a jet and forming nanofibers. Nanofiber membranes offer significant advantages, such as high porosity, large surface area, low tortuosity, and a through-pore structure, which help reduce concentration polarization and mass transfer resistance. Furthermore, the technology is flexible and highly operational, allowing for the uniform incorporation of well-dispersed insoluble components into nanofibers to form nanofiber polymer membranes, with a simple preparation process. Xiao et al. (Huang Y, Xiao C, Huang Q, et al. Robust preparation of tubular PTFE / FEP ultrafine fibers-covered porous membrane by electrospinning for continuous highly effective oil / water separation [J]. J Membr Sci, 2018, 568: 87-96.) Tubular polytetrafluoroethylene (PTFE) / poly (tetrafluoroethylene-co-hexafluoropropylene) (FEP) porous membranes were prepared by electrospinning-sintering. PTFE and FEP were uniformly dispersed in polyvinyl alcohol (PVA), co-spun with polyacrylonitrile, and fused with PTFE and FEP by sintering. PVA was removed at the same time to prepare an oil / water separation membrane with excellent mechanical properties. However, the post-sintering process is a high energy consumption process, and the temperature is difficult to control, resulting in membrane difficulties. In addition, the surface of the fluoropolymer membrane is highly hydrophobic, with a contact angle range of 93-160°, while the contact angle of the ECTFE membrane is about 130°. Its surface hydrophilic modification usually involves the use of strong oxidants, which can easily lead to the destruction of the molecular chain structure. The strong hydrophobicity and solvent stability make post-modification of this membrane difficult, so the preparation of ECTFE-based composite nanofiltration membranes remains a challenge. Summary of the Invention

[0005] The purpose of the present invention is to solve the problems existing in the prior art and provide an ECTFE-based nanofiltration membrane. The nanofiber composite membrane is prepared based on the strategy of "PVA-assisted ECTFE + glutaraldehyde cross-linking + interfacial polymerization". In addition, the composite membrane exhibits excellent long-term stability and chemical stability, and has broad application prospects in treating wastewater in harsh environments. It is expected to be applied to the treatment of printing and dyeing wastewater, the recovery of high-value-added materials, and the removal of pharmaceutical waste under harsh conditions.

[0006] In order to achieve the above technical objectives, the present invention is implemented through the following technical solutions:

[0007] A method for preparing an ECTFE-based corrosion-resistant nanofiltration membrane comprises the following steps:

[0008] Step 1: prepare a PVA aqueous solution, and then continue to add ECTFE to make it evenly dispersed to obtain a PVA-ECTFE solution;

[0009] Step 2, adding boric acid to the PVA-ECTFE solution, stirring evenly to obtain a spinning solution, and performing electrospinning to obtain a base film;

[0010] Step 3, placing the base film in a cross-linking solution containing a cross-linking agent to undergo a cross-linking reaction to obtain a cross-linked base film;

[0011] Step 4: preparing the selective separation layer of the nanofiltration membrane on the surface of the cross-linked basement membrane by interfacial polymerization.

[0012] In the step 1, when preparing the PVA aqueous solution, the treatment conditions are heating at 70-90° C. for 1-30 hours; and stirring for 5-50 hours during the process of uniform dispersion.

[0013] In the step 1, the weight ratio of PVA, ECTFE, and water in the PVA-ECTFE solution is 5-10:10-20:60-90. In the step 2, the amount of boric acid added is 1-15 mg / 100 g solution, preferably 6-10 mg / 100 g solution.

[0014] The operating parameters of electrospinning are: continuous spinning at a static voltage of 15-25 V, a bolus injection rate of 0.05-0.1 mL / h, a spinning temperature of 10-40 °C, a humidity of 10-50%, a distance between the nozzle and the receiver of 10-30 cm, and a receiver speed of 10-30 rpm·min -1 .

[0015] In step 3, the cross-linking solution comprises, by weight percentage, 4-6% glutaraldehyde, 3-8% HCl, and the balance ethanol; the cross-linking time is 1-10 h.

[0016] In step 4, the interfacial polymerization method includes the following steps: first contacting the base membrane with a PEI aqueous solution, and then contacting it with an organic phase solution containing TMC.

[0017] The concentration of PEI in the aqueous solution is 1-5%, and the contact time between the basement membrane and the PEI aqueous solution is 5-20 minutes; the concentration of TMC in the organic solution is 0.05-0.2%, and the contact time between the basement membrane and the organic solution containing TMC is 1-5 minutes.

[0018] The nanofiltration membrane prepared by the above method.

[0019] A nanofiltration separation method for a solution containing a dye adopts the nanofiltration membrane to intercept the dye.

[0020] A nanofiltration separation method in a high-temperature liquid environment, a strongly acidic aqueous solution, a strongly alkaline aqueous solution or an organic solvent system adopts the above-mentioned nanofiltration membrane for filtration.

[0021] The above-mentioned strongly acidic aqueous solution includes a mixture of one or more of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid with a mass percentage concentration of 1-20% (or a range value consisting of any point values ​​among 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, and 18%); the above-mentioned strongly alkaline aqueous solution includes KOH or NaOH with a concentration of 0.1-5M; the above-mentioned organic solvent system includes a mixture of one or more of alcohol solvents, hydrocarbon solvents, ether solvents, and amide solvents; the high-temperature liquid environment refers to a temperature of 20-85°C (or a range value consisting of any point values ​​including 30°C, 40°C, 50°C, 60°C, 70°C, and 80°C).

[0022] The beneficial effects of the present invention are:

[0023] 1. This application successfully prepared a nanofiber composite membrane by the method of "PVA-assisted ECTFE + glutaraldehyde crosslinking + polyethyleneimine and trimesoyl chloride interfacial polymerization". The resulting composite membrane has a significant pure water permeability (PWP) of up to 32LMH / bar and can efficiently remove various dyes with a molecular weight greater than 658g / mol, with a removal rate of over 99%;

[0024] 2. The composite membrane prepared in this application exhibits excellent long-term stability and chemical stability, and shows a high dye molecule retention rate under different harsh environments (high temperature, strong acid, strong base, polar solvent). The composite membrane is expected to be used in the fields of treatment of printing and dyeing wastewater, recovery of high value-added materials, and removal of pharmaceutical waste under harsh conditions. It has broad application prospects in treating wastewater in harsh environments and provides new ideas for the preparation and application of nanofiltration membranes resistant to harsh conditions. The nanofiltration membrane can show excellent stability under various harsh conditions and has the advantage of high adaptability.

[0025] 3. This application uses ECTFE polymer as the base material and successfully blends and prepares a nanofiber substrate with strong stability using electrospinning technology. The introduction of PVA as a spinning aid solves the problem of material hydrophobicity, improves the hydrophilicity of the substrate, and helps to construct a defect-free polyamide selective layer.

[0026] 4. This application uses PVA to assist in the spinning of ECTFE nanofiber base membranes. ECTFE has excellent chemical stability. The PVA and GA in the substrate undergo a cross-linking reaction under acid-catalyzed conditions, and the structure formed is resistant to chemical corrosion. The cross-linking between the polyamide selective layer and GA forms a Schiff base (imine) with excellent stability at extreme pH, which enhances the acid-base stability of the polyamide layer. The acetal reaction of PVA and GA, the aldehyde-amine condensation reaction of GA and PEI, the amide reaction of PEI and TMC, and the esterification reaction of PVA and TMC form a polymer network that is intertwined to form a synergistically stable cross-linked network, which further strengthens the bonding force between the membrane layers and the chemical stability of the membrane. In particular, the composite membrane exhibits comprehensive corrosion resistance (acid resistance, alkali resistance, solvent resistance, and high temperature resistance) in 10wt% H2SO4, 1M NaOH, ethanol solvent, and high temperature systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the preparation process of TFC-ECTFE membrane;

[0028] Figure 2 Panels (a) to (d) are SEM images of S-ECTFE@0, S-ECTFE@6.5, S-ECTFE@8.5, and S-ECTFE@10.5 nanofiber-based membranes, respectively; panels (e) to (h) are SEM images of the S-ECTFE@8.5 nanofiber-based membrane before immersion (e) and after immersion in alkaline solution (f), acidic solution (g), and ethanol (h), respectively;

[0029] Figure 3 is the SEM image of the TFC-ECTFE@0 nanofiber composite membrane prepared by interfacial polymerization on the SG-ECTFE@0 substrate;

[0030] Figure 4 are the SEM and EDX images of the S-ECTFE@8.5 nanofiber substrate;

[0031] Figure 5 (a) The inset shows the cross-linking reaction between PVA and GA; (b) The inset shows the principle diagram of the preparation of S-ECTFE nanofiber-based membrane; (c) The inset shows the SEM image of the S-ECTFE@8.5 nanofiber-based membrane structure; (d) The inset shows the FTIR spectra of different nanofiber-based membranes S-PVA, S-ECTFE@8.5 and SG-ECTFE; (e) The inset shows the EDX image of the S-ECTFE@8.5 nanofiber-based membrane;

[0032] Figure 6 is the EDX spectrum of S-ECTFE@8.5 nanofiber-based membrane;

[0033] Figure 7 Here are the digital photos and SEM images of the SG-ECTFE membrane before and after immersion in deionized water;

[0034] Figure 8 These are digital photos of the SG-ECTFE substrate before and after being immersed in different solutions;

[0035] Figure 9 Inset (a) shows the surface water contact angle data of S-ECTFE@8.5 and SG-ECTFE; inset (b) shows a schematic diagram of the interfacial polymerization reaction; inset (c) shows the FTIR image of SG-ECTFE and the TFC-ECTFE nanofiber membrane prepared based on it; inset (d) shows the surface and cross-sectional SEM images of the TFC-ECTFE membrane;

[0036] Figure 10 This is the dynamic water contact angle test diagram of SG-ECTFE nanofiber substrate;

[0037] Figure 11 is the FTIR spectra of SG-ECTFE and PEI-SG-ECTFE nanofiber membranes;

[0038] Figure 12 (a) XPS spectrum of TFC-ECTFE membrane and high-resolution spectra of (b) N1s, (c) C1s, and (d) O1s; Figure 13 (a) Inset: Pure water flux of TFC-ECTFE membrane at different pressures; (b) Inset: MWCO and pore size distribution of TFC-ECTFE membrane; (c) Inset: Rejection of different dyes by TFC-ECTFE membrane (feed: 50 mg / L dye solution); (d) Inset: Surface zeta potential of SG-ECTFE and TFC-ECTFE membranes; (e) Performance comparison between the most advanced nanofiltration membranes reported in the literature and the TFC-ECTFE membrane prepared in this application (dye molecular weight range 320-520 g / mol);

[0039] Figure 14 is the ethanol flux of TFC-ECTFE membrane at different pressures;

[0040] Figure 15 Panels (a)-(c) show the change in rejection over time of TFC-ECTFE membrane immersion experiments in 10 wt% H2SO4, 1 M NaOH, and ethanol (feed: 50 mg / LRB19 solution); Panels (d)-(g) show SEM images of TFC-ECTFE membrane before immersion (d) and after immersion in 10 wt% H2SO4 (e), 1 M NaOH (f), and ethanol (g), respectively;

[0041] Figure 16 Figure 1 shows the long-term stability test of TFC-ECTFE membrane using 50 mg / L JGB solution prepared in ethanol at pH values ​​of (a) 1, (b) 13, and (c) 50 mg / L JGB solution as the feed solution; (d) inset shows the stability test data of TFC-ECTFE membrane at high temperature (feed: 50 mg / L JGB solution); (e) inset compares the permeability and MWCO of TFC-ECTFE membrane with those of some previously reported membranes. DETAILED DESCRIPTION

[0042] The following examples further illustrate the present invention, but should not be construed as limiting the present invention. Without departing from the essence of the present invention, modifications and substitutions made to the methods, steps or conditions of the present invention are within the scope of the present invention.

[0043] In some typical embodiments, the solution of the present invention is implemented as follows:

[0044] In order to achieve the above technical objectives, the present invention is implemented by the following technical solutions: a method for preparing an ECTFE-based corrosion-resistant nanofiltration membrane, comprising the following steps:

[0045] 1) Preparing a spinning solution: heating a PVA aqueous solution, adding ECTFE thereto, stirring to obtain a uniformly dispersed ECTFE-PVA mixed solution, and adding boric acid to the mixed solution;

[0046] 2) Electrospinning: Using the spinning solution obtained in step 1), a nanofiber-based membrane S-ECTFE@b was prepared by electrospinning, where b is the amount of boric acid added to the spinning solution / mg;

[0047] 3) Cross-linking: S-ECTFE@b was cross-linked with acid-catalyzed glutaraldehyde to obtain a support layer membrane material, denoted as SG-ECTFE@b;

[0048] 4) Preparation of the selective layer: The selective layer was prepared on the surface of the support layer by interfacial polymerization. Specifically, the support layer membrane material was first reacted with an aqueous solution containing polyethyleneimine, and then with an organic solution containing trimesoyl chloride. The reaction was heated and cured to obtain an ECTFE-based nanofiltration membrane, denoted as TFC-ECTFE@b.

[0049] Furthermore, in step 1), the mass ratio of PVA to ECTFE in the ECTFE-PVA mixed solution is 1:2, and the amount of boric acid added to each 100 g of the ECTFE-PVA mixed solution is 6.5 to 8.5 mg.

[0050] Furthermore, in step 2), the electrospinning parameters were as follows: continuous spinning for 24 h at a static voltage of 19.8 V and a bolus injection rate of 0.067 mL / h, a spinning temperature of 20°C, a humidity of 40%, a translation distance of 20 mm, a distance between the nozzle and the receiver of 18 cm, and a receiver speed of 20 rpm·min -1 .

[0051] Furthermore, in step 3), the cross-linking solution for the cross-linking reaction is a mixture of glutaraldehyde, hydrochloric acid and ethanol, and the mass ratio of glutaraldehyde, hydrochloric acid and ethanol is 5:6:89.

[0052] Furthermore, in step 4), the mass percentage of polyethyleneimine in the aqueous solution is 3wt%, and the mass percentage of trimesoyl chloride in the organic solvent is 0.1wt%. During the reaction, the supporting layer membrane material is first immersed in an aqueous solution containing polyethyleneimine at 60°C for 10 minutes. After removing excess solution on the surface of the base membrane, it is immersed in an organic solvent containing trimesoyl chloride for 2 minutes. The organic solvent is n-hexane (other available solvents can also be listed if they are replaceable).

[0053] Furthermore, in step 4), the heating curing temperature is 60°C and the time is 10 minutes.

[0054] The ECTFE-based nanofiltration membrane prepared using the above method uses a PVA-assisted ECTFE nanofiber base membrane as the support layer. The base membrane is cross-linked with acid-catalyzed glutaraldehyde to enhance the membrane's water stability. A selective layer is then formed on the support layer via interfacial polymerization of polyethyleneimine and trimesoyl chloride. Boric acid is added to the spinning dope to increase its viscosity. This nanofiltration membrane exhibits comprehensive corrosion resistance and stability in 10 wt% H2SO4, 1 M NaOH, ethanol solvents, and high temperatures.

[0055] The ECTFE-based nanofiltration membrane can be used to treat printing and dyeing wastewater. The membrane has a pure water permeability of 32 LMH / bar and retains 99% of dyes with a molecular weight greater than 658 g / mol. The membrane can also be used to recover high-value-added materials from wastewater and remove pharmaceutical waste. Filtering wastewater through the membrane allows for material recovery or waste removal.

[0056] The sources of materials used in the examples are as follows:

[0057] ECTFE was purchased from Halar, boric acid (BA) was purchased from Shanghai Chemical Regeneration Co., and polyvinyl alcohol 1788 type (PVA, 87.0-89.0%) was purchased from Aladdin for the preparation of electrospinning solution.

[0058] Ethanol (EtOH, AR), hydrochloric acid (36.0-38.0%, AR) and glutaraldehyde (GA, 50%) were purchased from Aladdin and used in cross-linking experiments.

[0059] Polyethyleneimine (PEI, Mw = 1800 g / mol, >99%) was purchased from Macklin, trimethylolmethane (TMC, 98%) and n-hexane (>99.9%) were purchased from Aladdin for interfacial polymerization (IP) reaction.

[0060] Polyethylene glycol (PEG, AR) with different molecular weights (200, 400, 800, 1000 and 2000 g / mol) was purchased from MacLean and used to test the molecular weight cutoff (MWCO).

[0061] In addition, N,N-dimethylformamide (DMF, AR), methanol (MeOH, AR), H2SO4 (95.0-98.0%, GR), HCl (36.0-38.0%, AR. Alcian blue 8GX (AGX8 1299 g / mol), reactive black 5 (RB5 1992 g / mol) and methyl blue (BS 799 g / mol), Congo red (CR, 697 g / mol), reactive blue 19 (RB19, 626 g / mol), yellow green B (JGB, 511 g / mol), rhodamine B (RB, 479 g / mol), acridine orange (AO, 438 g / mol), crystal violet (CV, 408 g / mol), methyl orange (MO, 327 g / mol) and methylene blue (MB, 319 g / mol) were purchased from Aladdin and used to measure membrane selectivity.

[0062] Example 1

[0063] 1) Preparation of electrospinning solution

[0064] Prepare a PVA aqueous solution and heat and stir in an 80°C waterbath for at least 6 hours to obtain a homogeneous precursor solution. Add ECTFE to the PVA aqueous solution and stir for 12 hours to obtain 100 g of a uniformly dispersed ECTFE-PVA mixed solution (8.3 wt% PVA, 16.7 wt% ECTFE, 75 wt% H2O). Prepare four portions.

[0065] Different masses of boric acid (0 mg, 6.5 mg, 8.5 mg, 10.5 mg) were added to different mixed solutions, stirred evenly, and allowed to stand at room temperature for degassing.

[0066] A 10 wt % PVA aqueous solution (total mass 100 g) without additives was prepared as a spinning solution to prepare a pure PVA film as a control.

[0067] 2) Preparation of ECTFE nanofiber membrane with PVA-assisted spinning

[0068] Using 20 mL of the uniform ECTFE spinning solution prepared in step (1), the spinning was continued for 24 h at a static voltage of 19.8 V and a feed rate of 0.067 mL / h (spinning temperature 20 ° C, humidity 40%, translation distance 20 mm, distance between nozzle and receiver 18 cm, receiver speed 20 rpm min). -1 ), the nanofibers were collected on a round roller covered with tin foil to form a nanofiber base membrane. Subsequently, the nanofiber base membrane was separated from the tin foil, hot-pressed three times at 80°C, and placed in a 40°C incubator to remove residual solvent and keep warm for later use. The four groups of base membranes prepared by spinning were named S-ECTFE@b, where b represents the amount of boric acid (BA) added to the spinning mixture (mg). The four groups of base membranes prepared were respectively recorded as S-ECTFE@0, S-ECTFE@6.5, S-ECTFE@8.5, and S-ECTFE@10.5.

[0069] At 50°C, the S-ECTFE@b base membrane was placed in a crosslinking solution (5wt% GA + 6wt% HCl + 89wt% ethanol) and reacted for 2 hours. The resulting crosslinked membrane was named SG-ECTFE@b.

[0070] The prepared substrate was named S-PVA based on spinning of 10 wt% PVA aqueous solution.

[0071] 3) Preparation of nanofiber composite membrane

[0072] Composite membranes were prepared by interfacial polymerization on the SG-ECTFE@b series nanofiber base membrane. First, the membrane was immersed in a 3wt% PEI 1800 aqueous solution at 60°C for 10 minutes. The excess solution on the surface of the base membrane was then removed. The membrane was then immersed in a 0.1wt% TMC n-hexane solution for 2 minutes. Finally, it was cured in an oven at 60°C for 10 minutes. The prepared membrane was labeled TFC-ECTFE@b ( Figure 1 ). All membranes were placed in deionized water before performance testing.

[0073] Characterization and properties of corrosion-resistant ECTFE nanofiber membranes prepared by PVA-assisted spinning coupling and post-crosslinking

[0074] 1. Optimization and stability study of SG-ECTFE membrane structure

[0075] In the electrospinning process, the viscosity and conductivity of the spinning solution are very important. The viscosity of the spinning solution prepared by simply dispersing ECTFE particles in PVA solution is low, resulting in insufficient entanglement of the molecular chains, which leads to the instability of the Taylor cone during electrospinning and the formation of a beaded structure ( Figure 2(a) inset). The polyamide selective layer prepared by interfacial polymerization on this substrate has obvious defects ( Figure 3 ). Since BA can react with PVA to form a complex and restrict the movement of PVA molecular chains, this embodiment adds BA to the spinning solution to increase the viscosity of the spinning solution. The viscosity data of the spinning solutions with different BA contents are as follows:

[0076]

[0077] The addition of BA can also improve the conductivity of the spinning solution, and the conductivity of the spinning solution can be determined by EDX ( Figure 4 ) confirmed that BA existed in the nanofiber membrane structure. When the BA addition amount was 8.5 mg, the nanofibers on the surface of the prepared S-ECTFE@8.5 membrane were uniform in thickness and had no beaded structure ( Figure 2 (c)). However, when the BA addition amount increased to 10.5 mg, the spinning solution viscosity was too high, making it difficult to stretch into uniform nanofibers, resulting in the adhesion of large agglomerates on the surface of the S-ECTFE@10.5 membrane. In summary, S-ECTFE@8.5 (directly labeled as S-ECTFE in subsequent experiments) and its cross-linked membrane SG-ECTFE@8.5 (directly labeled as SG-ECTFE in subsequent experiments) have the optimal nanofiber structure and were selected as the optimal substrate for subsequent research. Similarly, the composite membrane prepared based on SG-ECTFE is directly labeled as TFC-ECTFE.

[0078] 2. Characterization of ECTFE-based nanofiber membranes Since PVA is highly spinnable and ECTFE can be uniformly dispersed in PVA solution, S-ECTFE nanofiber membranes were prepared by electrospinning ( Figure 5 ), Figure 5 The small picture (c) shows that the S-ECTFE@8.5 nanofibers are uniform in thickness and arranged regularly, with a flat circular structure without beads; in FTIR ( Figure 5 In (d), compared with the S-PVA substrate without any polymer addition, the S-ECTFE@8.5 (S-ECTFE) substrate has a high -1 The stretching vibration of CF and 740 cm -1 The C-Cl vibration absorption peak at the 10 nm cleavage site indicates the successful preparation of the ECTFE membrane. EDX analysis shows that fluorine and chlorine are evenly distributed on the single nanofiber ( Figure 5 (e) small picture and Figure 6 ), which once again proved the successful preparation of ECTFE membrane.

[0079] The ability of this technology to form a stable film mainly relies on the connection of PVA. However, PVA has a large number of hydroxyl groups and exhibits good water solubility. Therefore, in an aqueous environment, PVA is easily dissolved, leading to the collapse of the membrane structure.

[0080] In order to avoid PVA dissolution, the preliminarily prepared S-ECTFE membrane was cross-linked with GA. Acetalization reaction occurred between GA and PVA, and the hydroxyl groups were consumed, which reduced the polarity of the compound and prepared a water-stable SG-ECTFE nanofiber substrate. -1 The OH stretching vibration absorption peak in the range is greatly weakened, which confirms that GA cross-linking consumes the hydroxyl groups on the PVA chain. -1 The absorption peak at 2910-2950 cm-1 of SG-ECTFE substrate is attributed to the vibration of COC, which further confirms the acetal reaction between PVA and GA. -1 The symmetrical and asymmetrical stretching vibration peaks of the -CH2 group at 2931 cm-1 are split into two absorption peaks after GA cross-linking and acetalization. -1 and 2860cm -1 , which once again proves the acetal reaction between PVA and GA. The cross-linked substrate was immersed in pure water for three days, and the weight loss ratio was only 1.76%. There was no obvious change in the surface morphology before and after immersion ( Figure 7 ), demonstrating the successful preparation of water-stable SG-ECTFE nanofiber substrates.

[0081] 3. Investigate the stability of SG-ECTFE substrate in different acid solutions, alkaline solutions and solvents

[0082] By observing the changes of SG-ECTFE substrate before and after immersion in different solutions and calculating the weight loss ratio of the substrate before and after immersion for three days, the stability of SG-ECTFE substrate in different acid solutions, alkaline solutions and solvents was explored. The results showed that there was no obvious change in the appearance of the membrane before and after immersion ( Figure 8 ), and the SEM images also showed that the morphology of the nanofibers on the membrane surface was almost unchanged ( Figure 2 (e) to (h) panels). The weight loss data after immersion are as follows:

[0083]

[0084] From the above data, it can be seen that after immersion, the weight loss is less than 5%, which shows that the substrate has excellent acid resistance, alkali resistance and solvent resistance.

[0085] 4. Characterization of ECTFE-based nanofiber composite membranes

[0086] PVA not only plays a role in the spinning process of ECTFE nanofiber preparation, but also gives the substrate surface higher hydrophilicity and wettability ( Figure 9 (a) small picture, Figure 10 ), providing an excellent substrate for subsequent interfacial polymerization. Polyethyleneimine (PEI) and trimesoyl chloride (TMC) were selected to carry out interfacial polymerization (IP) on SG-ECTFE substrate to prepare the selective layer ( Figure 9 In this process, when the SG-ECTFE substrate is immersed in a 60°C aqueous phase, the PEI in the aqueous phase reacts with the aldehyde groups remaining in the GA in the substrate to form an aldehyde-amine condensation reaction. Figure 11 Medium 1643cm -1 The absorption peak at 1562 cm-1, which belongs to -C=N, confirms the reaction between PEI and GA. -1 The peak of the primary amine NH single bond bending vibration at ( ) indicates that -NH2 is not completely cross-linked. Afterwards, the membrane is immersed in the oil phase, and PEI and TMC react rapidly to form a polyamide layer. At the same time, the -OH groups on the PVA in the substrate that are not cross-linked by GA react with TMC for esterification. Figure 9 (c) The small figure is attributed to the amide group at 1640 cm -1 C=O at 1546cm -1 The characteristic peak of CN at confirms the formation of the polyamide selective layer.

[0087] The XPS spectrochemical compositions of different films are as follows:

[0088]

[0089]

[0090] The significant increase in N content after IP in XPS element distribution indicates the presence of PEI in the selective layer. Peak fitting was performed on the XPS spectra of C1s, N 1s, and O 1s ( Figure 12 ), the acyl group NC=O at 287.3eV in the C1s spectrum also confirmed the formation of the polyamide network. The presence of OC=O was detected at 289.1eV in the C 1s spectrum and at 532.5eV in the O1s spectrum, which confirmed the esterification reaction between PVA and TMC. The aldehyde amine condensation between GA and PEI, the interfacial polymerization between PEI and TMC, and the esterification reaction between PVA and TMC resulted in the formation of a stable membrane structure between the polymer networks intertwined with each other. SEM images showed that the prepared nanofiber composite membrane TFC-ECTFE had no obvious defects on the surface and had a recognizable nanofiber structure, indicating the formation of a thin (31.3nm) and strong selective layer ( Figure 9 (d) and (e) insets).

[0091] 5. Exploring the separation performance of composite nanofiltration membranes

[0092] The permeability and selectivity of TFC-ECTFE composite membranes were investigated in a homemade cross-flow device.

[0093] The pure water permeability of the nanofiber composite membrane is about 32LMH / bar, and the membrane flux increases linearly with the increase of operating pressure (1 bar to 10 bar) ( Figure 13 (a) small picture, Figure 14 ), indicating that the composite membrane has excellent compressive resistance. In addition, the molecular weight cut-off of the TFC-ECTFE composite membrane is 658g / mol ( Figure 13 (b) small figure), it shows good retention of various dyes ( Figure 13 (c) Inset). The retention rate for dye molecules with a molecular weight greater than 658 g / mol was greater than 99%, while the retention rate for JGB and RB with a molecular weight much less than 658 g / mol was as high as 97.6%. This is mainly attributed to the amino-rich PEI, which makes the composite membrane surface highly positively charged ( Figure 13 (D) inset), which produces a strong charge repulsion effect on the positively charged dye molecules. Compared with similar NF membranes reported before, the permeation selectivity of this work is at a higher level ( Figure 13 (e) Small figure).

[0094] Investigating the static immersion stability of composite membranes in harsh environments

[0095] The membrane was immersed in strong acid (10wt% H2SO4), strong base (1M NaOH) and solvent (ethanol) for 21 days. The stability of the composite membrane in harsh environment was evaluated by measuring the changes in the retention of Reactive Blue 19 before and after immersion. The results of regular tests showed that after immersion in the three solutions, the retention rate of the membrane remained stable at around 98% ( Figure 15 (a)-(c) insets) show that the membrane has excellent acid resistance, alkali resistance and solvent resistance. This is mainly attributed to: (1) As the main component of the substrate, ECTFE has excellent chemical stability; (2) The PVA and GA in the substrate undergo a cross-linking reaction under acid catalysis, and the structure formed is resistant to chemical corrosion; (3) The cross-linking between the polyamide selective layer and GA forms a Schiff base (imine) with excellent stability at extreme pH, which enhances the acid-base stability of the polyamide layer; (4) The polymer networks formed by the acetal reaction of PVA and GA, the aldehyde amine condensation reaction of GA and PEI, the amide reaction of PEI and TMC, and the esterification reaction of PVA and TMC are intertwined to form a synergistically stable cross-linked network. In order to further verify the structural stability of the composite membrane in harsh environments, the composite membrane was characterized before and after immersion in 10wt% sulfuric acid, 1M sodium hydroxide and ethanol solutions ( Figure 15Panels (d)-(g) show that the composite membrane's surface morphology remains largely unchanged before and after immersion, with no noticeable defects. Therefore, the TFC-ECTFE composite membrane exhibits high dye retention and structural stability in diverse harsh environments (strong acid, strong base, and polar solvents), suggesting promising applications in the treatment of printing and dyeing wastewater and the recovery of high-value-added materials.

[0096] 6. Exploring the long-term stability of composite membranes

[0097] In order to further evaluate the separation stability of the membrane under harsh environment, dynamic permeation experiments were carried out in Janus Green B solutions at pH = 1 and pH = 13. Figure 16 As shown in the figure, during the long-term filtration experiment lasting up to 111 hours, although the dye molecules were adsorbed and aggregated on the membrane surface, resulting in a slow decrease in permeability, the retention rate of the dye molecules remained stable at around 98%, once again proving the excellent acid and alkali resistance of the TFC-ECTFE membrane. In organic solvents, the charge effect disappears, resulting in the disappearance of the electrostatic attraction between the polymer chains in the membrane structure, causing the membrane structure to be slightly loose. Therefore, in ethanol solvent, with Alcian Blue as the target test substance, its retention rate remained above 97.8% ( Figure 16 (c) small figure), indicating that the composite membrane has excellent solvent resistance. In addition, the performance of the composite membrane was tested under high temperature system ( Figure 16 (D) As the temperature continues to rise until 80 ° C, the membrane pores expand at high temperatures, the permeability increases, and the retention of bonded green B decreases from 99% to 97.27%. When the temperature is lowered, the permeability and retention performance of the composite membrane recover, indicating that the membrane has a certain degree of high temperature resistance. Compared with the composite membranes resistant to harsh environments reported in the literature, the TFC-ECTFE membrane prepared in this work has a smaller cut-off molecular weight and a higher pure water permeability ( Figure 16 (e) Small picture)

[0098] The above shows and describes the basic principles, main features, and advantages of the present invention. However, the above is only a specific embodiment of the present invention, and the technical features of the present invention are not limited thereto. Any other implementation methods derived by any person skilled in the art without departing from the technical solution of the present invention should be included in the patent scope of the present invention.

Claims

1. A method for preparing an ECTFE-based corrosion-resistant nanofiltration membrane, characterized in that: The steps include: Step 1: prepare a PVA aqueous solution, and then continue to add ECTFE to make it evenly dispersed to obtain a PVA-ECTFE solution; Step 2, adding boric acid to the PVA-ECTFE solution, stirring evenly to obtain a spinning solution, and performing electrospinning to obtain a base film; Step 3, placing the base film in a cross-linking solution containing a cross-linking agent to undergo a cross-linking reaction to obtain a cross-linked base film; Step 4: preparing the selective separation layer of the nanofiltration membrane on the surface of the cross-linked basement membrane by interfacial polymerization.

2. The method for preparing an ECTFE-based corrosion-resistant nanofiltration membrane according to claim 1, wherein In the step 1, when preparing the PVA aqueous solution, the treatment conditions are heating at 70-90° C. for 1-30 hours; and stirring for 5-50 hours during the process of uniform dispersion.

3. The method for preparing the ECTFE-based corrosion-resistant nanofiltration membrane according to claim 1, wherein In the step 1, in the PVA-ECTFE solution, the weight ratio of PVA, ECTFE and water is in the range of 5-10:10-20:60-90.

4. The method for preparing the ECTFE-based corrosion-resistant nanofiltration membrane according to claim 1, wherein In the step 2, the amount of boric acid added is 1-15 mg / 100 g of solution.

5. The method for preparing the ECTFE-based corrosion-resistant nanofiltration membrane according to claim 1, wherein The operating parameters of electrospinning were: continuous spinning at a static voltage of 15–25 V, a bolus injection rate of 0.05–0.1 mL / h, a spinning temperature of 10–40 °C, a humidity of 10–50%, a distance between the nozzle and the receiver of 10–30 cm, and a receiver speed of 10–30 rpm min. -1 In step 3, the cross-linking solution includes, by weight percentage, 4-6% glutaraldehyde, 3-8% HCl, and the remainder is ethanol; the cross-linking time is 1-10h.

6. The method for preparing the ECTFE-based corrosion-resistant nanofiltration membrane according to claim 1, characterized in that: In step 4, the interfacial polymerization method includes the following steps: first contacting the base membrane with a PEI aqueous solution, and then contacting it with an organic solution containing TMC; the concentration of PEI in the aqueous solution is 1-5%, and the base membrane and the PEI aqueous solution are in contact for 5-20 minutes; The concentration of TMC in the organic phase solution is 0.05-0.2%, and the contact time with the organic phase solution containing TMC is 1-5 minutes.

7. A nanofiltration membrane, characterized in that The method is prepared by any one of claims 1 to 6.

8. A nanofiltration separation method for a solution containing a dye, characterized in that: The dye is retained by using the nanofiltration membrane described in claim 7.

9. A nanofiltration separation method in a high-temperature liquid environment, a strongly acidic aqueous solution, a strongly alkaline aqueous solution or an organic solvent system, characterized in that: Filtration is performed using the nanofiltration membrane according to claim 7.

10. The method according to claim 9, characterized in that The strongly acidic aqueous solution includes a mixture of one or more of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid with a mass percentage concentration of 1-20%; the strongly alkaline aqueous solution includes a mixture of one or two of KOH and NaOH with a concentration of 0.1-5M; the organic solvent system includes a mixture of one or more of alcohol solvents, hydrocarbon solvents, ether solvents, and amide solvents; the high-temperature liquid environment refers to a temperature of 20-85°C.

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