Composite nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention provides a composite nanofiltration membrane as well as a preparation method and application thereof, and the preparation method comprises the following steps: reacting a water phase solution and an oil phase solution on the surface of a base membrane to obtain a primary composite nanofiltration membrane; wherein the water phase solution contains piperazine and polyacrylamide, and the oil phase solution contains trimesoyl chloride. The nanofiltration membrane provided by the invention is high in pure water flux, good in membrane surface hydrophilicity, strong in pollution resistance, high in mechanical strength and durability, low in cost and capable of stably running for a long time.

Description

Technical Field

[0001] The present invention belongs to the technical field of water treatment membranes, and in particular relates to a composite nanofiltration membrane and a preparation method and application thereof. Background Art

[0002] Nanofiltration (NF) membrane is a pressure-driven separation technology between reverse osmosis (RO) and ultrafiltration (UF). With its high efficiency in retaining divalent ions and small organic molecules (molecular weight 200-1000Da) and relatively low operating pressure (5-20bar), it has become a key material in water treatment, food processing, pharmaceuticals, chemical separation and other fields. At the same time, due to the increase in emerging demands such as zero discharge of industrial wastewater (ZLD), lithium resource extraction, and antibiotic concentration, the demand for nanomembranes is growing.

[0003] However, existing nanofiltration membrane technology still faces the constraints of a "performance ceiling", specifically including:

[0004] 1) Flux-retention rate contradiction, seesaw effect of membrane separation: Traditional polyamide (PA) nanofiltration membrane usually uses piperazine (PIP) as the aqueous phase monomer, and forms a cross-linked network with trimesoyl chloride (TMC) through interfacial polymerization (IP); although its surface negative charge characteristics (derived from unreacted carboxylic acid groups) give it a strong affinity for divalent anions (such as SO4 2- ) has a high retention rate (>95%), but the high cross-linking density leads to densification of the membrane pore structure, and the pure water flux is generally lower than 25LMH (L / m 2 ·h, 5bar); Studies have shown that for every 10% increase in cross-linking degree, the flux decreases by about 15-20%, while the rejection rate only increases by 2-3%. This nonlinear trade-off relationship seriously restricts the application efficiency of membranes in high-throughput scenarios (such as seawater desalination pretreatment and juice concentration).

[0005] 2) Insufficient anti-pollution performance and the dilemma of surface chemistry interacting with pollutants: Nanofiltration membrane pollution mainly comes from organic adsorption (such as proteins and polysaccharides), inorganic scaling (such as CaSO4) and biofilm formation; due to the presence of hydrophobic micro-regions on the surface of traditional PA membranes (caused by aromatic ring structures), their contact angles are usually between 60-70°, and they can easily adsorb hydrophobic pollutants (such as humic acid) through hydrophobic interactions; in addition, the electrostatic repulsion between the negative charge on the membrane surface and negatively charged pollutants (such as sodium alginate) is weak, further aggravating the pollution.

[0006] 3) Insufficient mechanical strength and the risk of structural failure under high pressure and complex water quality: Industrial nanofiltration systems often face harsh operating conditions such as high pressure (>15 bar), high salinity (TDS>10,000 ppm), and extreme pH (2-12). The Young's modulus (1-2 GPa) and elongation at break (<10%) of traditional PA membranes cannot withstand repeated pressure fluctuations or particle erosion, easily causing pinhole defects or delamination.

[0007] The commonly used modification methods include:

[0008] 1) Additive modification: Although it can improve performance in the short term, it often comes at the expense of retention or long-term stability. For example, the introduction of Pluronic F127 (polyethylene oxide-polypropylene oxide triblock copolymer) into the PIP solution utilizes its micelle template effect to form a porous structure, which increases the flux to 40 LMH. However, the residual template after interfacial polymerization leads to the coexistence of hydrophilic regions on the membrane surface and a heterogeneous pore distribution, and the MgSO4 retention rate drops sharply to 85%. Similarly, polyethylene glycol (PEG) is used as a pore former, but the strong hydrophilicity of PEG weakens the hydrogen bonding between PA segments, causing the membrane to swell under high pressure (thickness increases by 20%) and poor long-term stability. For example, the use of polyvinyl pyrrolidone (PVP) as an aqueous phase additive reduces the contact angle, but due to the poor compatibility of PVP with the PA network, it is easily dissolved during long-term operation, resulting in a decrease in cross-linking degree (retention rate decreases by 10-15%).

[0009] 2) Nanocomposite membranes often have poor uniformity and stability. Studies have attempted to enhance mechanical properties by adding nano-SiO2 (10-50nm). However, the high surface energy of nanoparticles leads to agglomeration (SEM shows that aggregates with a particle size greater than 200nm account for >30%), which not only weakens the enhancement effect but also increases the surface roughness of the membrane (AFM Ra value increases from 5nm to 12nm), which in turn accelerates the deposition of pollutants. For example, UiO-66 MOFs are embedded in the PA layer, and their regular pores (0.8-1.1nm) are used to achieve NaCl / MgSO4 selective separation (separation factor reaches 8.5). However, they are unevenly dispersed in the matrix, and particle shedding during long-term operation causes membrane performance degradation (flux decreases by an average of 15% per year). In addition, the high cost of nanomaterials and the complex modification process further limit their application.

[0010] 3) Post-treatment modification: Surface coating (such as polydopamine and zwitterionic polymers) is a common method to improve anti-fouling properties, but it often leads to a decrease in the durability of the functional coating. For example, coating the surface of the nanofiltration membrane with sulfobetaine (SBMA) can reduce the contact angle to 35° and achieve a flux recovery rate of 90% after BSA contamination. However, the physical coating layer has a weak bonding strength with the PA substrate and is easily peeled off under strong shear force or chemical cleaning (such as NaClO immersion) (XPS shows that the SBMA content decreases by 60% after 10 cleanings). Although chemical grafting methods (such as plasma treatment grafting acrylic acid) can enhance the bonding strength, they may destroy the integrity of the PA network (FTIR shows that the amide bond breakage rate is >5%), resulting in a decrease in the retention rate.

[0011] Therefore, it is desirable to provide a nanofiltration membrane that has high pure water flux, good hydrophilicity of the membrane surface, strong anti-fouling performance, high mechanical strength and durability, and can operate stably for a long time. Summary of the Invention

[0012] In view of the deficiencies in the prior art, the present invention aims to provide a composite nanofiltration membrane and a preparation method and application thereof.

[0013] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0014] In a first aspect, the present invention provides a method for preparing a composite nanofiltration membrane, the method comprising: reacting an aqueous solution and an oily solution on a surface of a base membrane to obtain a primary composite nanofiltration membrane;

[0015] The aqueous phase solution contains piperazine and polyacrylamide, and the oil phase solution contains trimesoyl chloride.

[0016] Compared with the conventional ideas of "sacrificing flux for retention" or "introducing foreign nano-additives" in the current prior art, the preparation method provided by the present invention obtains a composite nanofiltration membrane by introducing polyacrylamide (PAM) to modify the piperazine monomer (PIP) at the molecular level during the preparation process. In this process, the polyacrylamide is partially hydrolyzed in the aqueous solution, and its partial hydrolysis product has both carboxylic acid groups (-COOH) and amide bonds (-NH-), which can form a dynamic pre-assembled structure with the piperazine monomer through hydrogen bonding and electrostatic interaction, having the following advantages:

[0017] i. Controlling reaction kinetics

[0018] The carboxyl groups in the partially hydrolyzed polyacrylamide and the amino groups in the piperazine can form a hydrogen bond network, which slows down the diffusion rate of PIP and inhibits excessive cross-linking, thereby forming a moderately loose polyamide layer (PA layer) with a pore size of 1.0-1.5nm. This layer has both high flux and excellent retention rate, achieving both high flux and high retention.

[0019] ii. Mechanical reinforcement mechanism

[0020] Partially hydrolyzed polyacrylamide penetrates the polyamide layer, forming an interpenetrating structure of "hard segment (PA cross-linking area)-soft segment (partially hydrolyzed polyacrylamide flexible chain)", which improves the toughness of the composite nanofiltration membrane;

[0021] iii. Anti-pollution synergistic effect

[0022] During the reaction of piperazine and trimesoyl chloride, the hydrophilic segments of partially hydrolyzed polyacrylamide will be enriched on the membrane surface to form a hydration layer with excellent hydrophilicity. At the same time, combined with optimal post-treatment, the contact angle can be reduced to less than 45° and the chemical stability can be improved simultaneously.

[0023] That is, compared with the current use of PVP as an additive, partially hydrolyzed polyacrylamide has higher thermal stability and better compatibility with the cross-linked network of PA. By introducing polyacrylamide, the present invention can achieve an increase in flux while maintaining the retention rate (MgSO4>96%).

[0024] Compared with the introduction of inorganic fillers such as nano-silica as additives, partially hydrolyzed polyacrylamide is dispersed at the molecular level, which can avoid the problem of particle agglomeration and greatly reduce costs by more than 80%.

[0025] Preferably, in the aqueous solution, the concentration of piperazine is 0.5-3 wt%, for example, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, etc., preferably 0.5-2 wt%.

[0026] Preferably, in the aqueous phase solution, the concentration of the polyacrylamide is 0.02-0.08 wt%, for example, it can be 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.07 wt%, 0.08 wt%, etc.

[0027] In the present invention, the addition amount of polyacrylamide and piperazine needs to be within the limited range of the present invention. If the content of the polyacrylamide is too high, the viscosity of the aqueous solution will increase sharply, making it difficult to remove the excess aqueous phase, resulting in uneven interfacial polymerization, and easily leading to defects in the generated PA layer. If the content of the polyacrylamide is too low, the advantages of the composite nanofiltration membrane will not be significantly increased, and the formed PA layer will be relatively dense, which is not conducive to the improvement of the flux.

[0028] Preferably, the molecular weight of the polyacrylamide is 2-4 million Daltons, for example, 2 million Daltons, 2.5 million Daltons, 3 million Daltons, 3.5 million Daltons, 4 million Daltons, etc.

[0029] The present invention selects polyacrylamide with a molecular weight of 2-4 million Daltons, which can greatly improve the toughness of the film, and its elongation at break is greater than 15%.

[0030] Preferably, in the oil phase solution, the concentration of trimesoyl chloride is 0.05-0.2 wt %, for example, 0.05 wt %, 0.1 wt %, 0.15 wt %, 0.2 wt %, etc.

[0031] Preferably, the method of reacting on the surface of the base membrane includes: coating the aqueous solution on the surface of the base membrane and letting it stand for a period of time to remove excess aqueous solution, polymerizing the oil phase solution and the aqueous solution on the surface of the base membrane, and heating in an oven after the reaction to obtain the primary composite nanofiltration membrane.

[0032] Preferably, the standing time is 60-120s, for example, 60s, 70s, 80s, 90s, 100s, 110s, 120s, etc., and the polymerization reaction time is 30-120s, for example, 30s, 40s, 50s, 60s, 80s, 100s, 120s, etc.

[0033] Preferably, the temperature for removing the oil phase solvent is 40-60°C, such as 40°C, 45°C, 50°C, 55°C, 60°C, etc.

[0034] Preferably, the preparation method further comprises: impregnating the primary composite nanofiltration membrane with a tannic acid-iron solution.

[0035] Preferably, the dipping time is 30-60 s, for example, 30 s, 35 s, 40 s, 45 s, 50 s, 55 s, 60 s, etc.

[0036] Preferably, in the tannic acid-iron solution, the concentration of the tannic acid is 0.05-0.3wt%, for example, it can be 0.05wt%, 0.1wt%, 0.15wt%, 0.2wt%, 0.25wt%, 0.3wt%, etc., the iron is trivalent iron, and the concentration of the iron salt is 0.01-0.1wt%, for example, it can be 0.01wt%, 0.02wt%, 0.03wt%, 0.05wt%, 0.08wt%, 0.1wt%, etc., and the iron is preferably prepared from ferric chloride FeCl3 as a raw material to prepare the tannic acid-iron solution.

[0037] The present invention preferably uses a tannic acid-iron solution coordination method for post-treatment, so that partially hydrolyzed polyacrylamide forms a hydration layer on the membrane surface and forms a covalent-coordinated double bond with PA, and the durability is significantly better than that of traditional physical coatings, thereby achieving a reduction in the contact angle of the composite nanofiltration membrane and a simultaneous improvement in chemical stability.

[0038] Preferably, the method for preparing the base film comprises: coating a casting solution on a non-woven fabric, and immersing the non-woven fabric in a coagulation bath to complete phase transformation, thereby obtaining the base film.

[0039] Preferably, the casting solution contains polysulfone, and the concentration of the polysulfone is preferably 15-18wt%, for example, it can be 15wt%, 15.5wt%, 16wt%, 16.5wt%, 17wt%, 17.5wt%, 18wt%, etc., and the molecular weight of the polysulfone is preferably 60,000-80,000 Daltons, for example, it can be 60,000 Daltons, 65,000 Daltons, 70,000 Daltons, 75,000 Daltons, 80,000 Daltons, etc.

[0040] Preferably, the coagulation bath is a mixture of water and DMF, and the content of DMF is preferably less than 5 wt%, for example, it can be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 4.5 wt%, etc.

[0041] Preferably, the preparation method comprises:

[0042] (1) preparing a casting solution containing 15-18 wt% polysulfone, coating the casting solution on a non-woven fabric, and immersing the non-woven fabric in a mixture of water and DMF to achieve phase transition, thereby obtaining the base membrane;

[0043] (2) coating the aqueous solution on the surface of the base membrane and allowing it to stand for a period of time to remove excess aqueous solution, polymerizing the oil phase solution and the aqueous solution on the surface of the base membrane, and heating the reaction in an oven to obtain the primary composite nanofiltration membrane;

[0044] (3) The primary composite nanofiltration membrane is impregnated with a tannic acid-iron solution to obtain the composite nanofiltration membrane.

[0045] In some embodiments of the present invention, the preparation method comprises:

[0046] S1. Preparation of a casting solution: The polysulfone is mixed with an organic solvent under vacuum and stirred to dissolve, and then the solution is subjected to heat preservation and degassing treatment;

[0047] S2. The degassed casting solution is evenly coated on the non-woven fabric and immersed in a coagulation bath to complete the phase transition to obtain a base film (or bottom film);

[0048] S3. Piperazine and polyacrylamide were dissolved in water to prepare an aqueous solution, and trimesoyl chloride was dissolved in an organic solvent to prepare an oil phase solution;

[0049] S4. Using a polytetrafluoroethylene frame and a metal clamp to clamp the base membrane, the aqueous phase solution is poured into the polytetrafluoroethylene frame (polysulfone side) and allowed to stand for 30-120s, for example, 30s, 40s, 50s, 60s, 80s, 100s, 120s, etc., and then the excess aqueous phase is removed and scraped off with a rubber roller or an air knife; the oil phase solution is then poured into the frame and allowed to stand (react) for 30-120s, for example, 30s, 40s, 50s, 60s, 80s, 100s, 120s, etc., to remove the excess oil phase, and the organic solvent of the oil phase is removed by heat treatment to obtain a primary composite nanofiltration membrane;

[0050] S5. Optionally, after removing the polytetrafluoroethylene frame, the primary composite nanofiltration membrane is immersed in a tannic acid-iron (III) solution for 30-60 seconds, for example, 30 seconds, 35 seconds, 40 seconds, 45 seconds, 50 seconds, 55 seconds, 60 seconds, etc., and rinsed with deionized water to obtain the composite nanofiltration membrane.

[0051] In step S1,

[0052] Preferably, the temperature of the stirring, mixing and dissolving is 70° C., and the time is 6-8 h, such as 6 h, 7 h, 8 h, etc.

[0053] Preferably, the temperature of the degassing treatment is ≥25° C., preferably, the degassing treatment is performed under slow stirring, and preferably, the degassing time is ≥8 h, such as 8 h, 8.5 h, 9 h, etc.

[0054] Preferably, the organic solvent is any one of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc) and dimethyl sulfoxide (DMSO), or a combination of at least two thereof.

[0055] In step S2,

[0056] Preferably, the nonwoven fabric is fixed by glass.

[0057] Preferably, the uniform arrangement is carried out by using a scraper to uniformly advance the casting solution so as to evenly spread the casting solution on the non-woven fabric.

[0058] Preferably, the coagulation bath is a mixture of water and DMF, and the content of DMF is preferably less than 5 wt%.

[0059] Preferably, the base film has a thickness of 135-140 μm, for example, 135 μm, 136 μm, 137 μm, 138 μm, 139 μm, 140 μm, etc.

[0060] Preferably, the thickness of the non-woven fabric is 95-100 μm and the weight is 81±5 g / m 2 , average pore size 3.6-6μm, air permeability 0.9-1.5cc / cm2 / sec.

[0061] The preparation method provided by the present invention systematically solves the key defects of existing nanofiltration membrane technology through the method of molecular structure design-interfacial polymerization kinetics regulation-post-treatment enhancement, and provides a high-performance, low-cost membrane separation solution for scenarios such as high-salt wastewater treatment and resource recovery.

[0062] The preparation method provided by the invention is simple and easy to implement, and solves the shortcomings of the existing nanofiltration membranes, such as low flux and poor fouling resistance.

[0063] In a second aspect, the present invention provides a composite nanofiltration membrane prepared according to the preparation method described in the first aspect.

[0064] The nanofiltration membrane provided by the present invention has high pure water flux, good membrane surface hydrophilicity, strong anti-fouling performance, high mechanical strength and durability, low cost, and can operate stably for a long time.

[0065] In a third aspect, the present invention provides an application of the composite nanofiltration membrane described in the second aspect in the treatment of high-salt wastewater and bacteria-containing wastewater.

[0066] Compared with the prior art, the present invention has the following beneficial effects:

[0067] (1) The present invention introduces polyacrylamide, which exists in the form of partially hydrolyzed acrylamide in the aqueous phase. The carboxyl groups and piperazine amino groups contained in the polyacrylamide can form a dynamic hydrogen bond network, which slows down the diffusion rate of PIP and inhibits excessive cross-linking. While maintaining pore size selectivity (1.0-1.5 nm), it reduces mass transfer resistance. Unlike the dense nodule structure of traditional nanofiltration membranes (SEM shows nodule diameter >100 nm), the composite nanofiltration membrane provided by the present invention forms a continuous network structure (nodule diameter <50 nm), effectively improving the density of water permeation channels.

[0068] (2) The carboxyl and amide groups of the partially hydrolyzed acrylamide introduced in the present invention form a strong hydration layer on the membrane surface, which, combined with the tannic acid-iron (III) coordination bond (binding energy > 150 kJ / mol), can achieve long-lasting hydrophilicity. At the same time, the antibacterial properties of tannic acid (inhibition rate against Escherichia coli > 99%) can further inhibit biofilm formation. That is, the composite nanofiltration membrane provided by the present invention is suitable for the treatment of bacterial wastewater;

[0069] (3) The partially hydrolyzed acrylamide introduced in the present invention has a high molecular weight, and its flexible long chain can form a "rigid and flexible" structure with the PA cross-linked network, thereby improving the toughness of the membrane. Among them, the elongation at break is increased to 15% (conventional membrane <8%), and it can withstand high-voltage pulse impact;

[0070] (4) The synergistic effect of the stable carbon chain skeleton (C-C bond energy 347 kJ / mol) of the partially hydrolyzed acrylamide introduced in the present invention and the tannic acid-metal coordination layer can resist corrosion by strong acid (pH = 1) and strong oxidants (200 ppm NaClO);

[0071] (5) The partially hydrolyzed acrylamide introduced in the present invention can replace precious metal nanomaterials (such as MOFs with a unit price of >100 / g), thereby reducing the overall cost of the membrane;

[0072] (6) The composite nanofiltration membrane obtained by the present invention has simple process compatibility and can be seamlessly connected with the existing interfacial polymerization production line without the need for new equipment, and is suitable for large-scale production (>1 million m 2 / Year). BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 Fourier infrared comparison images of the PA layer of the composite nanofiltration membrane provided in Examples 1-4 and Comparative Example 1;

[0074] Figure 2 These are scanning electron microscope photos of the composite nanofiltration membrane surfaces provided in Examples 1-4 and Comparative Example 1. DETAILED DESCRIPTION

[0075] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0076] Unless otherwise specified, the raw materials involved in the following specific embodiments of the present invention are all conventional materials in the art and can be purchased from commercial products. Some raw material information is as follows:

[0077] Polysulfone particles: molecular weight 70,000 Daltons, purchased from BASF GmbH;

[0078] Non-woven fabric: purchased from Awa Paper Co., Ltd., with a thickness of 95-100 μm and a gram weight of 81 ± 5 g / m 2 , average pore size 3.6-6μm, air permeability 0.9-1.5cc / cm2 / sec;

[0079] Polyacrylamide 1: molecular weight of 2-4 million Daltons, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0080] Polyacrylamide 2: molecular weight of 5-20 million Daltons, purchased from Sinopharm Chemical Reagent Co., Ltd.

[0081] Examples 1-7

[0082] This embodiment provides a method for preparing a composite nanofiltration membrane, as follows:

[0083] (1) Preparation of base membrane: polysulfone pellets were added to a vacuum stirred kettle containing DMF, mechanically stirred, and dissolved at 70°C for 8 h. After complete dissolution, the mixture was kept warm and allowed to stand for degassing. After degassing, the casting liquid was evenly poured onto a glass plate fixed with a non-woven fabric. A scraper was used to push the casting liquid evenly on the non-woven fabric. The glass plate was removed and immersed in a coagulation bath (water:DMF=98:2, mass ratio) to complete phase inversion and obtain a base membrane.

[0084] (2) Prepare the aqueous phase and trimesoyl chloride (TMC) ISoparE solution as the reaction oil phase as shown in Table 1;

[0085] Table 1

[0086] sample PIP concentration / wt% PAM concentration / wt% TMC concentration / wt% Reaction time Example 1 1 0.02 0.11 60s Example 2 1 0.04 0.11 60s Example 3 1 0.06 0.11 60s Example 4 1 0.08 0.11 60s Example 5 1 0.08 0.11 60s Comparative Example 1 1 0 0.11 60s Example 6 0.5 0.08 0.05 30s Example 7 3 0.08 0.20 120s

[0087] Note: Reaction time is the time of standing still after adding the oil phase solution.

[0088] Use a polytetrafluoroethylene frame and metal clamps to hold the base membrane, pour the aqueous solution on the polysulfone side of the base membrane for 120 seconds, pour off the excess aqueous phase, and use a rubber roller to remove the excess aqueous phase on the surface; pour the oil phase solution into the frame for 60 seconds, pour off the excess oil phase solution, and transfer it to a 50°C oven for heat treatment for 1 minute to evaporate the oil phase solvent;

[0089] After removal, the membrane frame was removed and the membrane was soaked in deionized water for storage before testing.

[0090] Example 5

[0091] This embodiment provides a method for preparing a composite nanofiltration membrane.

[0092] The difference from Example 4 is that, in this embodiment, it also includes: soaking the newly formed polyamide membrane in a tannic acid-iron coordination solution (tannic acid concentration 0.2wt%, ferric chloride concentration 0.1wt%) for 1 minute, taking it out, rinsing off excess coordination solution with deionized water, and then soaking it in deionized water for storage.

[0093] Comparative Example 1

[0094] This comparative example provides a method for preparing a composite nanofiltration membrane.

[0095] The only difference from Example 1 is that in this comparative example, no polyacrylamide is added to the aqueous phase solution.

[0096] Performance Testing

[0097] The performance of the nanofiltration membranes provided in Examples 1-7 and Comparative Example 1 was tested as follows:

[0098] (1) Infrared characterization: Figure 1 Fourier infrared comparison diagram of the composite nanofiltration membrane provided by Examples 1-4 and Comparative Example 1 (PSf is the infrared curve of the polysulfone bottom membrane), from Figure 1 It can be seen that:

[0099] The addition of PAM makes the infrared spectrum at 1625cm -1 A new peak appears at 1650-1680 cm-1, and the intensity of the peak gradually increases with the increase of PAM concentration. The amide I band (C=O stretching vibration) of pure PAM usually appears at 1650-1680 cm-1. -1 When the amide group of PAM forms a strong hydrogen bond with the amino group (-NH) of piperazine (PIP), the electron cloud density of C=O decreases and the vibration frequency decreases, resulting in a red shift of the peak position to 1625 cm -1 (like Figure 1 As shown in the figure, with the increase of PAM concentration, more amide groups participate in the hydrogen bond network (such as interacting with PIP or water molecules), and the peak intensity gradually increases, indicating that the hydrogen bond interaction is positively correlated with the PAM content. This proves that the addition of PAM regulates the diffusion of PIP during interfacial polymerization by forming hydrogen bonds with PIP, thereby regulating the reaction rate of the entire interfacial polymerization.

[0100] (2) Micromorphology characterization, Figure 2 Scanning electron microscope photos of the composite nanofiltration membrane surface provided for Examples 1-4 and Comparative Example 1 show that the addition of PAM causes a Turing structure-like morphology to appear in local areas of the membrane surface. The appearance of this morphology is generally believed to be caused by a large difference in the diffusion rate of the two-phase monomers participating in the reaction at the interface. The SEM image proves that the addition of PAM causes a significant change in the diffusion rate of the aqueous phase monomer PIP.

[0101] (3) Membrane separation performance test: 2 g / L sodium sulfate solution was prepared as the feed liquid, and the separation performance of the prepared nanofiltration membrane was tested. The test temperature was 25 ° C, the transmembrane pressure was 0.6 MPa, and the feed flow rate was 1.5 LPM. The conductivity of the feed liquid and the produced water was measured using a conductivity meter, and the salt retention rate of the membrane was calculated. The test results are as follows:

[0102] Table 2

[0103] sample <![CDATA[Sodium sulfate flux (L / m 2 ·h·MPa)]]> <![CDATA[Sodium sulfate rejection / %]]> Example 1 73.30 97.14 Example 2 77.32 97.09 Example 3 81.58 96.84 Example 4 97.03 96.25 Example 5 97.15 96.34 Comparative Example 1 31.23 97.54 Example 6 90.67 90.52 Example 7 46.14 97.98

[0104] As shown in Table 1, the composite nanofiltration membrane provided by the present invention has an excellent sulfate rejection rate, and the flux gradually increases with the increase of PAM concentration, and the rejection rate of sodium sulfate is maintained at a level of >90%, and the optimal rate can be above 96%.

[0105] However, in actual operation, since the molecular weight of PAM is in the millions, the increase in concentration causes the viscosity of the aqueous solution to increase rapidly. Excessive viscosity is not conducive to the removal of excess aqueous phase before coating the oil phase solution. Therefore, the preferred concentration of PAM in the present invention is 0.02-0.08 wt%.

[0106] From the comparison of the embodiment and the comparative example, it can be seen that the membrane separation performance of the composite nanofiltration membrane provided by the present invention is greatly improved, and the flux can reach up to about 3.1 times that of the comparative example 1. At the same time, the retention rate of sodium sulfate is only slightly reduced, that is, the present invention achieves a double improvement in flux and retention.

[0107] (4) Membrane antifouling performance test: 1 g / L bovine serum albumin solution was prepared as the feed solution to test the membrane antifouling performance to bovine serum albumin before and after modification, as well as the performance recovery after flushing;

[0108] The test results are shown in Table 3:

[0109] Table 3

[0110] sample Flux decline rate / % Flux recovery rate / % Example 5 4.97 97.28 Comparative Example 1 10.64 91.58

[0111] From the comparison between the examples and the comparative examples, it can be seen that the anti-fouling performance of the composite nanofiltration membrane provided by the present invention is significantly improved, which proves that the introduction of PAM can effectively improve the anti-fouling performance of traditional nanofiltration membranes.

[0112] (5) Hydrophilicity test: The hydrophilic angle of the membrane was measured using a dynamic water contact angle meter;

[0113] (6) Based on the separation performance test, the test time is extended to record the performance changes of the diaphragm during long-term use.

[0114] The test results are shown in Table 4:

[0115] Table 4

[0116]

[0117] As can be seen from Table 4, the composite nanofiltration membrane provided by the present invention has excellent durability and the flux attenuation during the test is very small. After post-treatment with tannic acid-iron coordination solution, the durability is further improved.

[0118] The applicant declares that while the above-described embodiments illustrate the technical solutions of the present invention, the present invention is not limited to the above-described process steps, nor does it imply that the present invention must rely on the above-described process steps for implementation. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the raw materials used, additions of auxiliary components, and selection of specific methods, etc., fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a composite nanofiltration membrane, characterized in that: The preparation method comprises: reacting an aqueous phase solution and an oil phase solution on the surface of a base membrane to obtain a primary composite nanofiltration membrane; The aqueous phase solution contains piperazine and polyacrylamide, and the oil phase solution contains trimesoyl chloride.

2. The preparation method according to claim 1, characterized in that In the aqueous solution, the concentration of piperazine is 0.5-3 wt%; Preferably, in the aqueous solution, the concentration of the polyacrylamide is 0.02-0.08 wt %; Preferably, the molecular weight of the polyacrylamide is 2-4 million Daltons.

3. The preparation method according to claim 1 or 2, characterized in that In the oil phase solution, the concentration of trimesoyl chloride is 0.05-0.2 wt %.

4. The preparation method according to any one of claims 1 to 3, characterized in that The method for reacting on the surface of the base membrane includes: coating the aqueous solution on the surface of the base membrane and letting it stand for a period of time, removing excess aqueous solution, polymerizing the oil phase solution and the aqueous solution on the surface of the base membrane, and heating after the reaction to obtain the primary composite nanofiltration membrane.

5. The preparation method according to claim 4, characterized in that The standing time is 60-120s, and the polymerization reaction time is 30-120s; Preferably, the heating temperature is 50-60° C. and the heating time is 60-120 seconds.

6. The preparation method according to any one of claims 1 to 5, characterized in that The preparation method further comprises: impregnating the primary composite nanofiltration membrane with a tannic acid-iron solution; Preferably, the dipping time is 30-60s; Preferably, in the tannic acid-iron solution, the concentration of the tannic acid is 0.05-0.3 wt %, the iron is trivalent iron, and the concentration of the iron salt is 0.01-0.1 wt %.

7. The preparation method according to any one of claims 1 to 6, characterized in that The preparation method of the base film comprises: coating a casting solution on a non-woven fabric, and immersing the non-woven fabric in a coagulation bath to complete phase transformation, thereby obtaining the base film; Preferably, the casting solution contains polysulfone, preferably the concentration of the polysulfone is 15-18 wt %, and preferably the molecular weight of the polysulfone is 60,000-80,000 Daltons; Preferably, the coagulation bath is a mixture of water and DMF, and the content of DMF is preferably less than 5 wt %.

8. The preparation method according to any one of claims 1 to 7, characterized in that The preparation method comprises: (1) preparing a casting solution containing 15-18 wt% polysulfone, coating the casting solution on a non-woven fabric, and immersing the non-woven fabric in a mixture of water and DMF to achieve phase transition, thereby obtaining the base membrane; (2) coating the aqueous solution on the surface of the base membrane and allowing it to stand for a period of time, removing excess aqueous solution, polymerizing the oil phase solution and the aqueous solution on the surface of the base membrane, and heating the reaction in an oven to obtain the primary composite nanofiltration membrane; (3) The primary composite nanofiltration membrane is impregnated with a tannic acid-iron solution to obtain the composite nanofiltration membrane.

9. A composite nanofiltration membrane prepared according to the preparation method according to any one of claims 1 to 8.

10. Use of the composite nanofiltration membrane according to claim 9 in the treatment of high-salt wastewater and bacteria-containing wastewater.

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