Polymer ultrafiltration membrane for printing and dyeing wastewater treatment and preparation method thereof

Abstract

The application relates to the field of printing and dyeing wastewater treatment membrane materials, in particular to a polymer ultrafiltration membrane for printing and dyeing wastewater treatment and a preparation method thereof. The preparation method is as follows: first, a meta-aramid or polybenzimidazole dispersion liquid is prepared; the microfluidic method is adopted to inject the dispersion liquid into a coagulation bath; the raw material composition of the coagulation bath comprises N,N-dimethylacetamide and deionized water; the mass of the N,N-dimethylacetamide is 0-30% of the total mass of the N,N-dimethylacetamide and the deionized water; the cross-section structure of the obtained polymer ultrafiltration membrane presents a double-layer hole which is different from the single-layer hole characteristics of a conventional membrane, specifically, the double-layer hole is a double-layer symmetrical finger-shaped hole; the filter membrane has stronger mechanical strength and structural stability; and the interception capacity of the filter membrane for dyes in water can be maintained at a very high level for a long time.

Description

Technical Field

[0001] This invention relates to the field of membrane materials for treating dyeing and printing wastewater, and specifically to a polymer ultrafiltration membrane for treating dyeing and printing wastewater and its preparation method. Background Technology

[0002] The treatment of dyeing and printing wastewater is an important aspect of wastewater treatment. Traditional dyeing and printing wastewater treatment technologies, such as chemical oxidation, adsorption, and coagulation, generally suffer from low separation efficiency, high resource consumption, and secondary pollution. Membrane separation technology, with its high efficiency, low energy consumption, simple operation, and continuous processing capabilities, has shown significant advantages over traditional technologies. One of the main reasons for these significant advantages is the unique morphological and structural characteristics of membrane materials. Based on different membrane morphologies, filter membranes can generally be divided into ultrafiltration (UF) membranes and microfiltration (MF) membranes. Further, based on the degree of looseness of the membrane structure, they can be divided into tight ultrafiltration (Tight UF) membranes and loose nanofiltration (Loose NF) membranes. Compared to UF membranes and microfiltration (MF) membranes, Tight UF and Loose NF have higher retention rates for dye molecules, higher permeate flux, can operate at lower transmembrane pressures, and have lower energy consumption. Therefore, Tight UF and Loose NF have high application value in the field of membrane separation.

[0003] Currently, the preparation of TUF and LNF mainly relies on phase inversion methods, especially non-solvent-induced phase separation (NIPS) technology. For example, in their published paper (DOI: 10.1016 / j.cej.2018.11.033), Zhang et al. used the NIPS method to pre-prepare polyethersulfone (PES) ultrafiltration substrates, and then deposited epigallocatechin gallate (EGCG) and polyethyleneimine (PEI) with different molecular weights on the substrate surface by co-deposition to form an EGCG / PEI composite coating. This EGCG / PEI composite coating optimized the surface pore structure of the PES membrane and improved the mechanical strength of the final ultrafiltration membrane. In addition, in their published paper (DOI: 10.1016 / j.seppur.2025.133935), Cui et al. used a simple phase inversion method to regulate carboxyl groups... The carboxylate content of acid-functionalized polyarylene ether sulfone (PAES) improved the compatibility of PAES and PES. Further optimization of the total polymer concentration and blending ratio controlled the pore structure of the PES / PAES composite membrane, thus attempting to optimize its mechanical properties. Sun et al., in their published paper (DOI: 10.1016 / j.memsci.2024.123340), investigated the effect of PHI addition on the pore structure of the PMIA / PHI composite membrane by introducing highly crystalline polyheptamethinimide (PHI) nanosheets into a poly(m-phenylene isophthalamide) casting solution. Their study demonstrated that changes in pore structure affect the structural stability of this type of composite membrane, thereby influencing its separation performance. In summary, existing technologies primarily employ strategies based on the introduction of additives and surface modification to control the pore structure of composite membranes, thereby regulating their separation performance.

[0004] However, while the methods employed in the aforementioned existing technologies can improve membrane separation performance, they do not overcome the inherent defects of the traditional NIPS method. The traditional NIPS method generally employs a blade coating process: after the casting solution is coated onto the surface of a solid substrate, only one side is exposed to the coagulation bath for solvent exchange. This results in a highly asymmetric phase separation process, leading to membranes that typically possess an asymmetric structure of a single macroporous support layer and a dense skin layer. Although this structure imparts a high initial porosity to the membrane, the single macroporous support layer inherently suffers from insufficient mechanical strength. During ultrafiltration, the fluid pressure gradually increases, easily causing the cavity walls of the single macroporous support layer to collapse and deform, resulting in membrane pore blockage and consequently, a decline in membrane separation performance.

[0005] Therefore, there is an urgent need to develop a novel composite membrane preparation method that can regulate phase separation symmetry and construct a multi-layer support structure to enhance membrane mechanical strength and avoid membrane pore blockage and performance degradation during the filtration process. Summary of the Invention

[0006] In view of this, the present invention proposes a polymer ultrafiltration membrane for treating dyeing and printing wastewater and its preparation method. The preparation method specifically involves: first, preparing a dispersion of meta-aramid (PMIA) or polybenzimidazole (PBI), and then injecting it into a coagulation bath using a microfluidic method. The mass fraction of N,N-dimethylacetamide (DMAC) in the coagulation bath is 0-30%. The resulting membrane exhibits a bilayer pore structure, unlike the single-layer pore characteristics of conventional membranes, demonstrating stronger mechanical strength and maintaining an extremely high level of dye retention in the water. The technical solution of the present invention is achieved as follows:

[0007] In a first aspect, the present invention provides a method for preparing a polymer ultrafiltration membrane for treating dyeing and printing wastewater, comprising the following steps:

[0008] S1. Vacuum dry the meta-aramid fiber or polybenzimidazole to obtain a dry polymer powder;

[0009] S2. Dissolve the dried meta-aramid fiber or polybenzimidazole in a solvent and stir to obtain a dispersion.

[0010] S3. Load the above dispersion into a syringe and connect it to a microfluidic device;

[0011] S4. Inject the dispersion into the microfluidic device so that the dispersion flows into the area containing the coagulation bath to form a filter membrane;

[0012] S5. Collect the filter membrane, soak it in deionized water and then dry it to obtain the polymer ultrafiltration membrane.

[0013] Preferably, in step S4, the raw material composition of the coagulation bath includes N,N-dimethylacetamide and deionized water; the mass of N,N-dimethylacetamide is 0-30% of the total mass of N,N-dimethylacetamide and deionized water.

[0014] More preferably, in step S4, the mass of the N,N-dimethylacetamide is 10% of the total mass of N,N-dimethylacetamide and deionized water.

[0015] Preferably, in step S2, the concentration of the polymer in the dispersion is 9 wt.% to 12.5 wt.%.

[0016] Specifically, the concentration of the polymer in the dispersion is set within this range, and the viscosity of the polymer is most suitable for the flow of the liquid in the microfluidic device.

[0017] Preferably, when the polymer is meta-aramid fiber, step S2 further includes: first, vacuum drying lithium chloride (co-solvent) and dissolving it in a solvent, and then adding meta-aramid fiber.

[0018] More preferably, the concentration of lithium chloride is 4.5 wt.%.

[0019] Specifically, if a low-concentration lithium chloride casting solution is used to prepare the film, PMIA will be unevenly dispersed; a high concentration of lithium chloride will cause the ion concentration in the solvent system to be too high, the intermolecular forces to be enhanced, and the viscosity of the casting solution to increase significantly, which will increase the difficulty of microfluidic preparation.

[0020] Preferably, when the polymer is meta-aramid fiber, the stirring temperature in step S2 is 80~90℃ and the stirring time is 10h; when the polymer is polybenzimidazole, the stirring temperature in step S2 is 25~30℃ and the stirring time is 10h.

[0021] Specifically, when dissolving meta-aramid fibers, if the temperature is too low, even prolonged stirring will not completely dissolve them. However, if the temperature exceeds 90°C, although it will not directly cause the solvent to boil, it will accelerate the evaporation of the solvent, causing the solution concentration to gradually increase. The rigid polybenzimidazole ring has much weaker intermolecular hydrogen bonding than meta-aramid, and the protonation dissolution mechanism that can occur quickly at room temperature does not require heating.

[0022] Preferably, in step S4, the flow rate of the microfluidic device is 800~1000μL / min.

[0023] Specifically, an appropriate flow rate ensures that the thickness of the solution is within a suitable range. If the flow rate is too high, the film will be too thick; if the flow rate is too low, the film will be too thin.

[0024] Preferably, in step S4, the volume ratio of the dispersion to the coagulation bath is 1:25.

[0025] Specifically, the liquid level of the coagulation bath is higher than that of the dispersion liquid film layer, which allows for sufficient phase exchange between the two sides of the film.

[0026] Preferably, in step S5, the filter membrane is soaked in deionized water for 6 hours and dried at 30°C for 12 hours to obtain the polymer ultrafiltration membrane.

[0027] In a second aspect, the present invention provides a polymer ultrafiltration membrane obtained by the preparation method described in the first aspect.

[0028] Preferably, the polymer ultrafiltration membrane has a bilayer symmetrical finger-like pore structure.

[0029] Thirdly, the present invention provides the application of the polymer ultrafiltration membrane described in the second aspect in the field of dyeing and printing wastewater treatment.

[0030] Compared with the prior art, the advantages of the present invention are as follows:

[0031] (1) This invention uses microfluidic technology to achieve solvent exchange between the polymer solution and the coagulation bath on both sides, and prepares a membrane with a symmetrical double-layer pore structure, avoiding the defects of the single-layer macroporous support layer in the traditional blade coating method; and by adjusting the DMAC concentration in the coagulation bath, the membrane pore structure can be refined and the regularity can be adjusted, forming finger-shaped pores with double-layer symmetry.

[0032] (2) Compared with traditional membranes, the membrane prepared by the present invention has better structural stability and mechanical strength; at the same time, it has high water permeability and high dye rejection rate, higher pure water flux, and can effectively filter dyes such as Congo Red and Coomassie Brilliant Blue.

[0033] (3) In the long-term dye separation experiment of the present invention, the membrane flux decreased by only 17%, which is much lower than the 57% of the traditional membrane, and the structural stability is outstanding. Moreover, the preparation method can be extended to polymers with similar meta-aramid hydrogen bonds and rigidity characteristics (such as polybenzimidazole membrane), providing a general path for the preparation of similar high-performance membranes. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 Scanning electron microscope images of M-PMIA and C-PMIA films;

[0036] Figure 2 A comparison of the tensile strength of M-PMIA and C-PMIA films;

[0037] Figure 3 A comparison chart of pure water flux between M-PMIA membrane and C-PMIA membrane;

[0038] Figure 4 (a) shows the separation performance of Congo red dye solution between M-PMIA membrane and C-PMIA membrane; (b) shows the separation performance of Coomassie brilliant blue dye solution between M-PMIA membrane and C-PMIA membrane.

[0039] Figure 5 Scanning electron microscope images of M-PMIA films prepared under different coagulation bath concentrations;

[0040] Figure 6 The porosity of M-PMIA membranes prepared under different coagulation bath concentrations;

[0041] Figure 7Tensile strength of M-PMIA membranes prepared under different coagulation bath concentrations;

[0042] Figure 8 Pure water flux of M-PMIA membranes prepared under different coagulation bath concentrations;

[0043] Figure 9 (a) shows the separation performance of M-PMIA membranes prepared under different coagulation bath concentrations for Congo red dye solution; (b) shows the separation performance of M-PMIA membranes prepared under different coagulation bath concentrations for Coomassie brilliant blue dye solution.

[0044] Figure 10 The long-term dye separation stability of M-PMIA and C-PMIA membranes at room temperature and 2 bar pressure was evaluated.

[0045] Figure 11 (a) shows a comparison of M-PMIA membrane filtration before and after Coomassie Brilliant Blue dye; (b) shows a comparison of M-PMIA membrane filtration before and after Congo Red dye.

[0046] Figure 12 Schematic diagram and optical photograph of M-PMIA membrane for microfluidic control;

[0047] Figure 13 (a) is a scanning electron microscope image of a para-aramid film prepared by microfluidic technology; (b) is a scanning electron microscope image of a PI film prepared by microfluidic technology; and (c) is a scanning electron microscope image of a PVDF film prepared by microfluidic technology.

[0048] Figure 14 Cross-sectional scanning electron microscope image of a PBI membrane prepared using microfluidic technology. Detailed Implementation

[0049] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0050] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0051] In this document, the terms “containing,” “comprising,” or “including” are open-ended expressions, meaning they include the contents specified in this invention but do not exclude other aspects.

[0052] In this document, the terms “optional,” “optionally,” or “optional” generally refer to an event or condition that may, but may not, occur, and the description includes both cases in which the event or condition occurs and cases in which the event or condition does not occur.

[0053] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0054] All materials used in this invention were purchased commercially, specifically: meta-aramid fiber (PMIA) was purchased from Yantai Taihe New Material Co., Ltd.; polybenzimidazole (PBI) powder was purchased from Shengjun Plastics Co., Ltd.; anhydrous lithium chloride (LiCl), N,N-dimethylacetamide (DMAC), and Congo dye (CR) were all purchased from Maclean's; N-methylpyrrolidone (NMP) was purchased from Shanghai Testing & Consulting Co., Ltd.; Coomassie Brilliant Blue G250 (CCB) was purchased from Aladdin Co., Ltd.; the Aquaphor ultrapure water system was used to generate deionized water for the experiment; all reagents were used as indicated upon receipt without any additional purification.

[0055] The specific method for evaluating the ultrafiltration performance of the membrane in this invention is as follows: The performance of the ultrafiltration membrane is tested using a flat-panel membrane flux analyzer. The ultrafiltration membrane sample to be tested is installed in a mold. First, the membrane sample is pre-pressed at 0.2 MPa for one and a half hours. After pre-pressing, the operating pressure is maintained at 0.2 MPa for half an hour, and the permeate volume is recorded. The filtrate flux is calculated according to formula (1): (1)

[0056] In the formula: Permeation flux (L·m) -2 ·h -1 ·bar -1 V is the volume of solution permeated through the membrane (L), and A is the effective area of ​​the membrane (m²). 2 p represents the test pressure (bar), and t represents the test time (h).

[0057] The UV absorption spectra of the feed and filtrate were obtained using UV-Vis absorption spectroscopy to characterize the retention of dye molecules in each membrane sample; according to Beer-Lambert's law, the concentration of the dye solution is proportional to its UV-Vis absorbance; the retention performance of the membrane for dye molecules was calculated by formula (2):

[0058] (2)

[0059] Where R (%) is the retention rate, and The concentrations of the filtrate and feed solution are replaced by the ultraviolet absorption intensity, respectively.

[0060] Example 1

[0061] This embodiment provides a method for preparing a polymer ultrafiltration membrane for treating dyeing and printing wastewater, including the following steps:

[0062] S1. Thoroughly vacuum dry meta-aramid (PMIA) fibers and lithium chloride (LiCl). Then, in a clean and dry reaction vessel, first dissolve 8g of dried LiCl in 176g of N,N-dimethylacetamide (DMAC) solvent, sonicate until LiCl is completely dissolved, add 16g of dried PMIA fibers to the solvent system and mechanically stir at 800 rpm for 10 hours at 80°C to finally obtain a PMIA dispersion.

[0063] S2. Load the PMIA dispersion into two 5ml syringes and connect the microfluidic device. Set the flow rate of the microfluidic device to 800μL / min. Inject the PMIA dispersion into the microfluidic device from the inlet at a flow rate of 800μL / min. When the PMIA solution flows into and fills the 250mL coagulation bath (100% deionized water + 0% DMAC), the phase inversion process occurs immediately, forming a filter membrane.

[0064] S3. After collecting the above filter membrane using a roll, soak it in deionized water for 6 hours, take the membrane out, and dry it at 30°C for 12 hours to obtain the polymer ultrafiltration membrane, denoted as M-PMIA membrane, and further denoted as M-PMIA-0.

[0065] Example 2

[0066] This embodiment provides a method for preparing a polymer ultrafiltration membrane for treating dyeing and printing wastewater, including the following steps:

[0067] S1. Thoroughly vacuum dry meta-aramid (PMIA) fibers and lithium chloride (LiCl). Then, in a clean and dry reaction vessel, first dissolve 8g of dried LiCl in 176g of N,N-dimethylacetamide (DMAC) solvent, sonicate until LiCl is completely dissolved, add 16g of dried PMIA fibers to the solvent system and mechanically stir at 800 rpm for 10 hours at 80°C to finally obtain a PMIA dispersion.

[0068] S2. Load the PMIA dispersion into two 5ml syringes and connect the microfluidic device. Set the flow rate of the microfluidic device to 800μL / min. Inject the PMIA dispersion into the microfluidic device from the inlet at a flow rate of 800μL / min. When the PMIA solution flows into and fills the 250mL coagulation bath (90% deionized water + 10% DMAC), the phase inversion process occurs immediately, forming a filter membrane.

[0069] S3. After collecting the above filter membrane using a roll, soak it in deionized water for 6 hours, take the membrane out, and dry it at 30°C for 12 hours to obtain the polymer ultrafiltration membrane, denoted as M-PMIA-1.

[0070] Example 3

[0071] This embodiment provides a method for preparing a polymer ultrafiltration membrane for treating dyeing and printing wastewater, including the following steps:

[0072] S1. Thoroughly vacuum dry meta-aramid (PMIA) fibers and lithium chloride (LiCl). Then, in a clean and dry reaction vessel, first dissolve 8g of dried LiCl in 176g of N,N-dimethylacetamide (DMAC) solvent, sonicate until LiCl is completely dissolved, add 16g of dried PMIA fibers to the solvent system and mechanically stir at 800 rpm for 10 hours at 80°C to finally obtain a PMIA dispersion.

[0073] S2. Load the PMIA dispersion into two 5ml syringes and connect the microfluidic device. Set the flow rate of the microfluidic device to 800μL / min. Inject the PMIA dispersion into the microfluidic device from the inlet at a flow rate of 800μL / min. When the PMIA solution flows into and fills the 250mL coagulation bath (80% deionized water + 20% DMAC), the phase inversion process occurs immediately, forming a filter membrane.

[0074] S3. After collecting the above filter membrane using a roll, soak it in deionized water for 6 hours, take the membrane out, and dry it at 30°C for 12 hours to obtain the polymer ultrafiltration membrane, denoted as M-PMIA-2.

[0075] Example 4

[0076] This embodiment provides a method for preparing a polymer ultrafiltration membrane for treating dyeing and printing wastewater, including the following steps:

[0077] S1. Thoroughly vacuum dry meta-aramid (PMIA) fibers and lithium chloride (LiCl). Then, in a clean and dry reaction vessel, first dissolve 8g of dried LiCl in 176g of N,N-dimethylacetamide (DMAC) solvent, sonicate until LiCl is completely dissolved, add 16g of dried PMIA fibers to the solvent system and mechanically stir at 800rpm at 90℃ for 10h to finally obtain PMIA dispersion.

[0078] S2. Load the PMIA dispersion into two 5ml syringes and connect the microfluidic device. Set the flow rate of the microfluidic device to 1000μL / min. Inject the PMIA dispersion into the microfluidic device from the inlet at a flow rate of 1000μL / min. When the PMIA solution flows into and fills the 250mL coagulation bath (80% deionized water + 30% DMAC), the phase inversion process occurs immediately, forming a filter membrane.

[0079] S3. After collecting the above filter membrane using a roll, soak it in deionized water for 6 hours, take the membrane out, and dry it at 30°C for 12 hours to obtain the polymer ultrafiltration membrane, denoted as M-PMIA-3.

[0080] Example 5

[0081] This embodiment provides a method for preparing a polymer ultrafiltration membrane for treating dyeing and printing wastewater, wherein the polymer is replaced with polybenzimidazole, and includes the following steps:

[0082] S1. The polybenzimidazole (PBI) powder was thoroughly vacuum dried. Then, in a clean and dry reaction vessel, 1.3 g of PBI and 10 mL (10.3 g) of N-methylpyrrolidone (NMP) were added and mechanically stirred at 800 rpm for 10 h at 25 °C to finally obtain a PBI dispersion.

[0083] S2. Load the PBI dispersion into two 5ml syringes and connect the microfluidic device. Set the flow rate of the microfluidic device to 800μL / min. Inject the PBI dispersion into the microfluidic device from the inlet at a flow rate of 800μL / min. When the PBI solution flows into and fills the 250mL coagulation bath (100% deionized water), the phase inversion process occurs immediately, forming a filter membrane.

[0084] S3. After collecting the above filter membranes on a roll, soak them in deionized water for 6 hours, take the membranes out, and dry them at 30°C for 12 hours to obtain the polymer ultrafiltration membrane, denoted as PBI membrane.

[0085] Comparative Example 1

[0086] This comparative example provides a traditional comparative ultrafiltration membrane preparation method (scalpel coating method), including the following steps:

[0087] S1. Thoroughly vacuum dry meta-aramid (PMIA) fibers and lithium chloride (LiCl). Then, in a clean and dry reaction vessel, first dissolve 8g of dried LiCl in 176g of N,N-dimethylacetamide (DMAC) solvent, sonicate until LiCl is completely dissolved, add 16g of dried PMIA fibers to the solvent system and mechanically stir at 800 rpm for 10 hours at 80°C to finally obtain a PMIA dispersion.

[0088] S2. Use a scraper to scrape the membrane onto a clean glass plate, then immerse the glass plate in a coagulation bath (deionized water) for 6 hours. Remove the membrane and dry it at 30°C for 12 hours to obtain the comparative ultrafiltration membrane, denoted as C-PMIA membrane.

[0089] Comparative Example 2

[0090] This comparative example provides a method for preparing a comparative polymer ultrafiltration membrane, wherein the mass fraction of DMAC in the coagulation bath is 40%, comprising the following steps:

[0091] S1. Thoroughly vacuum dry meta-aramid (PMIA) fibers and lithium chloride (LiCl). Then, in a clean and dry reaction vessel, first dissolve 8g of dried LiCl in 176g of N,N-dimethylacetamide (DMAC) solvent, sonicate until LiCl is completely dissolved, add 16g of dried PMIA fibers to the solvent system and mechanically stir at 800 rpm for 10 hours at 80°C to finally obtain a PMIA dispersion.

[0092] S2. Load the PMIA dispersion into two 5ml syringes and connect the microfluidic device. Set the flow rate of the microfluidic device to 800μL / min. Inject the PMIA dispersion into the microfluidic device from the inlet at a flow rate of 800μL / min. When the PMIA solution flows into and fills the 250mL coagulation bath (80% deionized water + 40% DMAC), the phase inversion process occurs immediately, forming a filter membrane.

[0093] S3. After collecting the above filter membranes on a roll, soak them in deionized water for 6 hours, take the membranes out, and dry them at 30°C for 12 hours to obtain the comparative polymer ultrafiltration membrane, denoted as M-PMIA-4.

[0094] Comparative Example 3

[0095] This comparative example provides a method for preparing a comparative polymer ultrafiltration membrane, in which the polymer is replaced with polyvinylidene fluoride, including the following steps:

[0096] S1. The polyvinylidene fluoride (PVDF) powder was thoroughly vacuum dried. Then, in a clean and dry reaction vessel, 13g of PVDF was dissolved in 100mL of N,N-dimethylformamide (DMF) and mechanically stirred at 800rpm for 10h at 80℃ to finally obtain a PVDF dispersion.

[0097] S2. Load the PVDF dispersion into two 5ml syringes and connect the microfluidic device. Set the flow rate of the microfluidic device to 800μL / min. Inject the PVDF dispersion into the microfluidic device from the inlet at a flow rate of 800μL / min. When the PVDF solution flows into and fills the 250mL coagulation bath (100% deionized water), the phase inversion process occurs immediately, forming a filter membrane.

[0098] S3. After collecting the above filter membranes on a roll, soak them in deionized water for 6 hours, take the membranes out, and dry them at 30°C for 12 hours to obtain the comparative polymer ultrafiltration membrane, denoted as PVDF membrane.

[0099] Comparative Example 4

[0100] This comparative example provides a method for preparing a comparative polymer ultrafiltration membrane, in which the polymer is replaced with polyimide, and includes the following steps:

[0101] S1. The polyimide (PI) powder was thoroughly vacuum dried. Then, in a clean and dry reaction vessel, 6g of PI was dissolved in 100mL of N-methylpyrrolidone (NMP) and mechanically stirred at 800rpm for 10h at 80℃ to finally obtain a PI dispersion.

[0102] S2. Load the PI dispersion into two 5ml syringes and connect the microfluidic device. Set the flow rate of the microfluidic device to 800μL / min. Inject the PI dispersion into the microfluidic device from the inlet at a flow rate of 800μL / min. When the PI solution flows into and fills the 250mL coagulation bath (100% deionized water), the phase inversion process occurs immediately, forming a filter membrane.

[0103] S3. After collecting the above filter membranes on a roll, soak them in deionized water for 6 hours, take the membranes out, and dry them at 30°C for 12 hours to obtain the comparative polymer ultrafiltration membrane, denoted as PI membrane.

[0104] Comparative Example 5

[0105] This comparative example provides a method for preparing a comparative polymer ultrafiltration membrane, in which the polymer is replaced with para-aramid fiber, including the following steps:

[0106] S1. The para-aramid fiber was thoroughly vacuum dried. Then, 196g of dimethyl sulfoxide (DMSO) solvent, 6g of potassium hydroxide (KOH) and 4g of para-aramid fiber were added to a clean and dry reaction vessel. The mixture was mechanically stirred at 1200rpm for 3 days at 25℃ to finally obtain a para-aramid fiber dispersion.

[0107] S2. Load the para-aramid fiber dispersion into two 5ml syringes and connect them to the microfluidic device. Set the flow rate of the microfluidic device to 800μL / min. Inject the para-aramid fiber dispersion into the microfluidic device from the inlet at a flow rate of 800μL / min. When the para-aramid fiber solution flows into and fills the 250mL coagulation bath (100% deionized water), the phase inversion process occurs immediately, forming a filter membrane.

[0108] S3. After collecting the above filter membranes on a roll, soak them in deionized water for 6 hours, take the membranes out, and dry them at 30°C for 12 hours to obtain the comparative polymer ultrafiltration membrane, denoted as para-aramid fiber membrane.

[0109] Figure 1 The images show scanning electron microscope (SEM) images of the M-PMIA membrane prepared in Example 1 and the C-PMIA membrane prepared in Comparative Example 1. Traditional blade coating relies on a unidirectional solvent exchange process, leading to the formation of typical asymmetric membrane structures. As shown in the figures, the SEM cross-sectional images clearly reveal the asymmetric structure of the C-PMIA membrane—a large through-cavity exists beneath the skin layer. In contrast, when using the microfluidic technology employed in this study, the PMIA solution is injected into the microfluidic device from the inlet and flows into the coagulation bath. Upon contact with the coagulation bath, solvent exchange occurs simultaneously on both sides of the liquid flow at the outlet, forming an M-PMIA membrane with a bilayer porous structure. This is due to the phase exchange on both sides of the liquid layer, resulting in symmetrical finger-like pores in the M-PMIA membrane. The SEM cross-sectional images visually demonstrate this feature, and the top and bottom surfaces of the membrane ultimately exhibit highly similar pore structures.

[0110] Figure 2 The tensile strengths of the M-PMIA membrane (Example 1) and the C-PMIA membrane (Comparative Example 1) are shown in the figure. As can be seen from the figure, the stress-strain curve of the M-PMIA membrane shows a higher resistance to deformation: its elastic modulus is twice that of the C-PMIA membrane, and its elongation at break reaches 18%, indicating that the M-PMIA membrane can better resist structural deformation when subjected to external stress.

[0111] Figure 3This is a comparison chart of the pure water flux of the M-PMIA membrane (Example 1) and the C-PMIA membrane (Comparative Example 1). To compare the separation performance of these two PMIA membrane structures, this application first tested the pure water flux of both membranes. Pure water flux reflects the basic water permeability of the membrane. The test results show that the pure water flux of the M-PMIA membrane is significantly higher than that of the C-PMIA membrane. This indicates that the water permeability of the M-PMIA membrane is higher than that of the C-PMIA membrane. This advantage in water permeability mainly stems from the more stable double-layer pore structure, larger pore size, and higher porosity of the M-PMIA membrane, allowing water to pass through the M-PMIA membrane more quickly and in larger quantities.

[0112] Figure 4 (a) shows the separation performance of Congo red dye solution using M-PMIA (Example 1) and C-PMIA membrane (Comparative Example 1); (b) shows the separation performance of Coomassie Brilliant Blue dye solution using M-PMIA and C-PMIA membranes. Under conditions of 25°C and 100 ppm dye concentration, the separation performance of these two membranes for CBB and CR dye solutions was tested. The comparison shows that the M-PMIA membrane has higher rejection rates and fluxes for both dyes than the C-PMIA membrane. This indicates that the unique double-layer pore structure of the M-PMIA membrane significantly improves both dye rejection and solution permeability compared to traditional single-layer pore structure membranes.

[0113] Figure 5 Scanning electron microscope (SEM) images of M-PMIA membranes (Examples 1-4, Comparative Example 2) prepared at different coagulation bath concentrations are shown. As can be seen from the figures, when the PMIA solution is injected into the coagulation bath using a microfluidic method, PMIA membranes with a bilayer pore structure can be prepared at DMAC mass percentages of 0%, 10%, 20%, and 30%. Furthermore, as the DMAC concentration increases, the bilayer pore structure of the membrane tends to become more refined, and the surface pore structure gradually becomes larger and more regular. However, when the DMAC concentration is too high, reaching 40%, the bilayer pore structure of the membrane disappears, becoming sponge-like, and the pore structure on the membrane surface becomes irregular. Further… Figure 6 The porosity of M-PMIA membranes prepared under different coagulation bath concentrations; Figure 7 Figure 1 shows the tensile strength data of M-PMIA membranes prepared under different coagulation bath concentrations.

[0114] Specifically, from Figures 5-7 As can be seen, the membrane structure can be controlled by adjusting the concentration of the coagulation bath. Figure 5As the DMAC concentration in the coagulation bath increases, the surface pores of the PMIA membrane become larger. Cross-sectional SEM images show that the proportion of sponge pores within the membrane increases with increasing coagulation bath concentration. This is because at lower DMAC concentrations in the coagulation bath, the solvent-non-solvent concentration gradient between the casting solution and the coagulation bath is large, causing rapid solvent diffusion into the coagulation bath and prompting rapid solidification of the membrane surface, forming a membrane with smaller surface pores and easily forming unidirectionally growing finger-like pores internally. In higher concentration coagulation baths, the phase transition rate decreases, and polymer segments can rearrange themselves through intermolecular interactions during solvent removal, gradually accumulating to form interconnected sponge-like channels. At a DMAC concentration of 10% in the coagulation bath (M-PMIA-1 membrane), the membrane porosity reaches its maximum; thereafter, as the DMAC concentration in the coagulation bath increases, the porosity gradually decreases. Figure 6 As the concentration of DMAC in the coagulation bath increases, the proportion of sponge pores in the M-PMIA membrane increases, and the mechanical strength of the membrane gradually increases. Figure 7 The higher the proportion of sponge-like pores, the stronger the resistance to structural deformation caused by external stress. However, an increase in the proportion of sponge-like pores leads to a decrease in the pure water flux of the M-PMIA membrane, see [reference needed]. Figure 8 The test results.

[0115] Figure 8 The pure water flux of M-PMIA membranes (Examples 1-4, Comparative Example 2) prepared under different coagulation bath concentrations is shown in the figure. As can be seen from the figure, changes in membrane structure significantly affect its separation performance. Figure 8 As shown, the pure water flux initially increases and then decreases with increasing DMAC concentration in the coagulation bath. When the DMAC concentration in the coagulation bath reaches 10% (M-PMIA-1 membrane), the porosity reaches its maximum, and the pure water flux reaches its peak. When the coagulation bath concentration further increases, although the surface pore size continues to increase, the porosity decreases significantly, the water transport resistance increases, and the pure water flux decreases significantly with increasing coagulation bath concentration.

[0116] Figure 9(a) shows the separation performance of M-PMIA membranes (Examples 1-4, Comparative Example 2) prepared under different coagulation bath concentrations for Congo red dye solution separation, and (b) shows the separation performance of M-PMIA membranes prepared under different coagulation bath concentrations for Coomassie brilliant blue dye solution separation. As shown in the figure, in the dye separation experiment, the membranes under these conditions, while maintaining the retention effect for CBB and CR dyes, achieved the maximum flux of the M-PMIA-1 membrane, consistent with the results obtained using pure water. Specifically, the highest porosity and optimized surface pore size of the M-PMIA-1 membrane provide it with the largest effective water transport channel and the lowest transport resistance. Although higher coagulation bath concentrations lead to a further increase in membrane surface pore size, its internal sponge-like structure theoretically provides finer sieving capabilities (helping to maintain high retention). However, the significantly reduced porosity of these membranes greatly limits the overall passage capacity of water molecules and solvated dye molecules, resulting in a flux much lower than that of the M-PMIA-1 membrane. Therefore, considering both the structural characteristics and separation performance of the membrane, a DMAC concentration of 10% in the coagulation bath (corresponding to the M-PMIA-1 membrane) is the optimal condition for preparing a high-performance PMIA separation membrane.

[0117] Figure 10 The long-term dye separation stability of M-PMIA (Example 1) and C-PMIA membranes (Comparative Example 1) at 25 °C and 2 bar pressure was assessed. To evaluate the long-term stability of the membranes, a continuous separation experiment of Congo red dye solution was conducted under these conditions for 48 hours. The experimental results showed that the M-PMIA membrane prepared by the microfluidic method significantly outperformed the C-PMIA membrane prepared by the conventional blade coating method in terms of flux retention. After 48 hours of continuous experiment, although the Congo red rejection rate of both membranes remained high, the flux reduction rate of M-PMIA was 17%, far lower than the 57% reduction rate of C-PMIA. This indicates that the PMIA membrane prepared by microfluidic method has better structural stability than the PMIA membrane prepared by the conventional blade coating method.

[0118] Figure 11 (a) shows a comparison of the M-PMIA membrane (Example 1) before and after filtering Coomassie Brilliant Blue dye, and (b) shows a comparison of the M-PMIA membrane (Example 1) before and after filtering Congo Red dye. As can be seen from the figures, after the blue Coomassie Brilliant Blue dye and Congo Red solution in the bottle were filtered through the M-PMIA membrane, the solution in the vial became colorless and transparent, indicating that the dye was effectively filtered out by the M-PMIA membrane.

[0119] Figure 12This diagram and optical photograph illustrate the continuous microfluidic fabrication of PMIA membranes, depicting the process of preparing polymer membranes in a coagulation bath using the microfluidic device employed in this invention. As shown in the figure, the PMIA solution is injected into the microfluidic device through the inlet and flows into the coagulation bath. Upon contact with the coagulation bath, solvent exchange occurs simultaneously on both sides of the outlet liquid flow, forming an M-PMIA membrane. After drying... Figure 12 As shown in the middle illustration, the membrane is white and opaque.

[0120] Figure 13 In the figures, (a) is a scanning electron microscope (SEM) image of a para-aramid membrane (Comparative Example 5) prepared using microfluidic technology; (b) is a scanning electron microscope (SEM) image of a PI membrane (Comparative Example 4) prepared using microfluidic technology; and (c) is a scanning electron microscope (SEM) image of a PVDF membrane (Comparative Example 3) prepared using microfluidic technology. As can be seen from the figures, the para-aramid and polyimide membranes prepared using the microfluidic technology of this invention have dense structures. This is because the molecular chains of para-aramid and polyimide are relatively rigid, and during rapid phase inversion, the rigid molecular chains are difficult to fold to form a loose structure, ultimately resulting in a dense structure. In contrast, the flexible segments of polyvinylidene fluoride allow the molecular chains to slowly adjust their conformation during phase inversion, preventing the formation of a "finger-like pore" structure. This indicates that not all polymers can be used to prepare the bilayer porous membranes in the examples prepared using this microfluidic technology of this invention.

[0121] Figure 14 The image shows a cross-sectional scanning electron microscope image of the PBI membrane prepared using the microfluidic technology in Example 5 of this invention. It can be seen that since the hydrogen bonding and rigidity characteristics of polybenzimidazole at the molecular level are similar to those of meta-aramid, polybenzimidazole can also be used to prepare a bilayer porous membrane structure using the microfluidic technology in this invention.

[0122] In summary, through comparison of Comparative Examples 3-5 and Example 5 with other examples, it is known that the range of polymers that can be prepared using the method of the present invention for bilayer porous polymer membranes can be further expanded to polymers with hydrogen bonding and rigidity characteristics similar to meta-aramids, such as polybenzimidazole membranes. When such polymer solutions come into contact with the coagulation bath, solvent exchange occurs simultaneously on both sides of the outlet liquid flow, and the solvent exchange rate is very fast, much faster than that of the polymer solutions in the comparative examples. This enables specific types of polymers to form the same pore structure on the top and bottom surfaces of the membrane through this method, and to generate a bilayer porous structure inside the membrane.

[0123] The embodiments described above are some, but not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing a polymer ultrafiltration flat sheet membrane for treating dyeing and printing wastewater, characterized in that, Includes the following steps: S1. Vacuum dry the meta-aramid fiber or polybenzimidazole to obtain a dry polymer powder; S2. Dissolve the dried meta-aramid fiber or polybenzimidazole in a solvent and stir to obtain a dispersion. S3. Load the above dispersion into a syringe and connect it to a microfluidic device; S4. Inject the dispersion into the microfluidic device so that the dispersion flows into the area containing the coagulation bath to form a filter membrane; S5. Collect the filter membrane, soak it in deionized water and then dry it to obtain the polymer ultrafiltration flat sheet membrane; In step S4, the liquid level of the coagulation bath is higher than the dispersion liquid film layer, so that the two sides of the film can fully exchange phases. In step S5, the polymer ultrafiltration flat sheet membrane has a bilayer symmetrical finger-shaped pore structure.

2. The preparation method according to claim 1, characterized in that, In step S4, the raw material composition of the coagulation bath includes N,N-dimethylacetamide and deionized water; the mass of N,N-dimethylacetamide is 0-30% of the total mass of N,N-dimethylacetamide and deionized water.

3. The preparation method according to claim 2, characterized in that, In step S4, the mass of the N,N-dimethylacetamide is 10% of the total mass of N,N-dimethylacetamide and deionized water.

4. The preparation method according to claim 1, characterized in that, In step S2, the concentration of the polymer in the dispersion is 9 wt.% to 12.5 wt.%.

5. The preparation method according to claim 1, characterized in that, When the polymer is meta-aramid fiber, step S2 further includes: first, vacuum drying lithium chloride and dissolving it in a solvent, and then adding meta-aramid fiber.

6. The preparation method according to claim 1, characterized in that, When the polymer is meta-aramid fiber, the stirring temperature in step S2 is 80~90℃ and the stirring time is 10h; when the polymer is polybenzimidazole, the stirring temperature in step S2 is 25~30℃ and the stirring time is 10h.

7. The preparation method according to claim 1, characterized in that, In step S4, the flow rate of the microfluidic device is 800~1000μL / min.

8. A polymer ultrafiltration flat sheet membrane obtained by the preparation method according to any one of claims 1 to 7.

9. The application of the polymer ultrafiltration flat sheet membrane as described in claim 8 in the field of dyeing and printing wastewater treatment.

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