Low-pressure high-flux polyamide / polysulfone hollow fiber composite nanofiltration membrane and preparation method thereof

Abstract

The invention belongs to the technical field of membrane separation, and particularly relates to a low-pressure high-flux polyamide / polysulfone hollow fiber composite nanofiltration membrane and a preparation method thereof. The preparation method of the composite nanofiltration membrane comprises the following steps: dissolving sulfonated polysulfone, poly (1-vinyl-3-butylimidazole) bromide and polyvinylpyrrolidone to prepare a spinning solution, and spinning to obtain a base membrane; carrying out hydrophilic modification on the base membrane by using 1, 3-propane sultone; then, an aqueous phase solution containing poly (gamma-glutamic acid) and triethylene tetramine and an organic phase solution containing trimesoyl chloride and 3, 5-diaminobenzoic acid are subjected to interfacial polymerization in the inner cavity of the modified base membrane to form a polyamide separation layer; and finally, carrying out heat treatment to obtain the composite nanofiltration membrane. Through the synergistic effect of zwitterionic ionization of the base membrane and biomacromolecule construction of the separation layer, the mass transfer resistance of water is remarkably reduced, unification of high flux and high selectivity is achieved, and good structural stability is achieved.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology, specifically relating to a low-pressure, high-flux polyamide / polysulfone hollow fiber composite nanofiltration membrane and its preparation method. Background Technology

[0002] Nanofiltration membranes, as a type of separation membrane with performance between ultrafiltration and reverse osmosis, have been widely used in advanced drinking water treatment, industrial wastewater reuse, and material separation and concentration due to their high efficiency in retaining divalent ions and small organic molecules, as well as relatively low operating pressure. Among them, polyamide composite nanofiltration membranes have become the mainstream choice in the market due to their excellent separation performance and mechanical strength. These membranes typically use hollow fiber ultrafiltration membranes made of materials such as polysulfone and polyethersulfone as the base membrane, and form an extremely thin polyamide separation layer on the surface of the base membrane through interfacial polymerization.

[0003] Although polyamide composite nanofiltration membrane technology is relatively mature, several technical bottlenecks still need to be addressed in its practical application. The most prominent issue is the contradiction between membrane flux performance and operating energy consumption. Theoretically, reducing the separation layer thickness or increasing the membrane pore size can increase water flux, but this often comes at the cost of sacrificing retention rate, leading to a decrease in separation accuracy. Therefore, how to effectively increase membrane flux while maintaining a high retention rate, thereby reducing system operating pressure and energy consumption, has become an important direction for technological innovation in this field.

[0004] In existing technologies, researchers generally recognize the importance of improving the hydrophilicity of membrane surfaces to enhance flux. Hydrophilic membrane surfaces can more effectively adsorb water molecules, forming a hydration layer and reducing the mass transfer resistance of water through the membrane, thus potentially achieving higher water flux under the same pressure. Common hydrophilic modification methods include surface coating with hydrophilic polymers, blending hydrophilic nanomaterials into the base membrane or separation layer, and introducing zwitterionic monomers. However, these existing modification methods still have significant drawbacks. Surface-coated or grafted hydrophilic layers are prone to detachment or degradation during long-term operation and chemical cleaning, leading to a decline in hydrophilicity and flux, severely impacting membrane lifespan. Simultaneously, the dispersion of nanomaterials requires complex pretreatment processes and they are prone to agglomeration in the membrane, forming defects; the synthesis and purification of zwitterionic monomers are costly. These factors all limit their large-scale industrial application. More importantly, many modification methods, while introducing hydrophilic groups, may over-crosslink or densify the polyamide separation layer. Although this may improve the rejection rate, it increases the mass transfer resistance of water, which runs counter to the original intention of increasing flux and fails to fundamentally solve the problem of synergy between "low pressure" and "high flux".

[0005] Therefore, there is an urgent need in this field to develop a composite nanofiltration membrane that can achieve high water flux at low operating pressure while maintaining excellent rejection rate and long-term stable operation performance. Summary of the Invention

[0006] The purpose of this invention is to provide a low-pressure, high-flux polyamide / polysulfone hollow fiber composite nanofiltration membrane and its preparation method, to solve the problems of insufficient flux and high energy consumption of existing nanofiltration membranes under low-pressure conditions. The composite nanofiltration membrane obtained by this invention maintains excellent rejection rates while achieving high-flux performance at lower operating pressures, thereby significantly reducing operating energy consumption and improving separation efficiency.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] The first aspect of this invention provides a method for preparing a low-pressure, high-flux polyamide / polysulfone hollow fiber composite nanofiltration membrane, comprising the following steps:

[0009] (1) Dissolve sulfonated polysulfone, poly(1-vinyl-3-butylimidazolium) bromide and polyvinylpyrrolidone in N-methylpyrrolidone solvent, stir to dissolve and let stand to remove bubbles to obtain spinning solution;

[0010] (2) A dry-wet spinning process is used to extrude and solidify the spinning solution to obtain a hollow fiber base membrane;

[0011] (3) The hollow fiber membrane was immersed in an aqueous solution of 1,3-propanesulfonyl lactone for reaction. After the reaction was completed, it was washed to obtain a hydrophilic modified polysulfone membrane.

[0012] (4) Prepare an aqueous phase solution containing poly(γ-glutamic acid) and triethylenetetramine, and an organic phase solution containing pyromellitic acid chloride and 3,5-diaminobenzoic acid;

[0013] (5) Pump the aqueous solution into the inner cavity of the hydrophilic modified polysulfone membrane obtained in step (3) to wet it. After purging the residual liquid, pump in the organic phase solution to carry out the interfacial polymerization reaction and form a polyamide separation layer on the inner surface of the base membrane.

[0014] (6) The membrane after interfacial polymerization in step (5) is heat-treated and then cleaned to obtain the composite nanofiltration membrane.

[0015] Further, in step (1), based on 1 part of the mass of the sulfonated polysulfone, the mass of the poly(1-vinyl-3-butylimidazolium) bromide is 0.07~0.11 parts, and the mass of the polyvinylpyrrolidone is 0.05~0.07 parts.

[0016] Further, in step (1), the total mass of sulfonated polysulfone, poly(1-vinyl-3-butylimidazolium) bromide and polyvinylpyrrolidone in the spinning solution accounts for 15% to 25% of the total mass of the spinning solution.

[0017] Further, in step (3), the concentration of the 1,3-propanesulfonyl lactone aqueous solution is 0.05-0.2 mol / L, the reaction temperature is 40-60℃, and the reaction time is 4-8 hours.

[0018] In the preparation of the base membrane, this invention innovatively introduces poly(1-vinyl-3-butylimidazolium) bromide and utilizes its post-modification reaction with 1,3-propanesulfonyl lactone to generate a stable zwitterionic structure in situ within the polysulfone base membrane framework. This chemically anchored zwitterionic layer, compared to physically blended hydrophilic agents, provides a durable and robust water-binding capacity, significantly reducing the transport resistance of water molecules in the micropores of the base membrane and providing an excellent hydrophilic interface for subsequent interfacial polymerization.

[0019] Further, in step (4), the mass concentration of poly(γ-glutamic acid) in the aqueous solution is 2.0-3.0 wt%, and the mass concentration of triethylenetetramine is 0.1-0.2 wt%.

[0020] Further, in step (4), the organic phase solution contains 0.1-0.3 wt% pyromellitic acid and 0.05-0.15 wt% 3,5-diaminobenzoic acid.

[0021] In constructing the separation layer, this invention abandons traditional small-molecule amine monomers and instead uses the biomacromolecule poly(γ-glutamic acid) as the host for the aqueous phase reaction. The high density of carboxyl groups and the long chain characteristics of its molecular chain tend to form a relatively loosely cross-linked but hydrophilic three-dimensional network structure when polymerizing with trimesoyl chloride at the interface. This unique "hydrogel-polyamide" hybrid separation layer, while maintaining nanoscale pore size for sieving and retention, also greatly promotes the dissolution and diffusion of water molecules through its internal hydrophilic microenvironment, achieving high throughput under low pressure.

[0022] Furthermore, by introducing the carboxyl-containing comonomer 3,5-diaminobenzoic acid into the organic phase, the chemical composition of the separation layer was further finely controlled, enhancing its hydrophilicity and surface electronegativity, and strengthening the ion retention capacity based on the Donnan effect. The addition of triethylenetetramine in the aqueous phase and subsequent specific heat treatment processes ensured the necessary mechanical integrity and chemical stability of this loose hydrophilic network, preventing structural failure during long-term operation. These components work together to ultimately achieve a highly efficient synergy between water flux and selective separation performance under low operating pressure.

[0023] Furthermore, in step (5), the wetting time of the aqueous solution is 60-120 seconds, and the interfacial polymerization reaction time is 30-60 seconds.

[0024] Furthermore, in step (6), the heat treatment temperature is 70-80°C and the time is 3-8 minutes.

[0025] The second aspect of the present invention provides a low-pressure, high-flux polyamide / polysulfone hollow fiber composite nanofiltration membrane, which is obtained by the preparation method of the present invention.

[0026] Furthermore, the composite nanofiltration membrane exhibits a pure water flux exceeding 40 L / (m³) at a pressure of 0.4 MPa. 2 (·h), with a magnesium sulfate rejection rate of over 90%.

[0027] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0028] This invention utilizes the reaction of polyionic liquids with sulfonyl lactones to construct a stable zwitterionic structure in the base membrane, fundamentally solving the problem of short-lasting hydrophilicity in traditional physical modifications and endowing the membrane material with excellent antifouling properties and long-term stability. Simultaneously, this invention employs the biomacromolecule polyγ-glutamic acid to construct the separation layer, forming a "hydrogel-polyamide" hybrid structure with abundant hydrophilic functional groups and a moderately cross-linked network. While maintaining a high rejection rate, it significantly reduces the mass transfer resistance of water, thus achieving a flow rate exceeding 40 L / (m³) at a low operating pressure of 0.4 MPa. 2 The high water flux of ·h) effectively solves the industry problem of balancing high flux and low energy consumption. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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 scope of protection of the present invention.

[0030] Unless otherwise specified, all raw materials used in the embodiments are commercially available products. The following sources are illustrative examples.

[0031] Sulfonated polysulfone was purchased from Beijing Kangbang Technology Co., Ltd., product model SPSf-20; poly(1-vinyl-3-butylimidazolium) bromide was purchased from Beijing Green Process Engineering Research Co., Ltd., product model P[VBIM]Br-100k; polyvinylpyrrolidone was purchased from BASF (China) Co., Ltd., product model PVP K30; and poly(γ-glutamic acid) was purchased from Nanjing Xuankai Biotechnology Co., Ltd., product model γ-PGA-500k.

[0032] Example 1

[0033] This embodiment provides a low-pressure, high-flux polyamide / polysulfone hollow fiber composite nanofiltration membrane, the preparation method of which includes the following steps:

[0034] (1) Weigh 20.0g of sulfonated polysulfone, 1.6g of poly(1-vinyl-3-butylimidazolium) bromide and 1.2g of polyvinylpyrrolidone (PVP K30), and dissolve them together in 100g of N-methylpyrrolidone solvent. Stir mechanically at 60°C for 12 hours until completely dissolved, and then let stand at room temperature for 8 hours to remove bubbles, to obtain a uniform and transparent spinning solution.

[0035] (2) Using a dry-wet spinning process, the spinning solution obtained in step (1) is extruded through a hollow fiber spinning device. After passing through a 10 cm air gap, it enters a pure water coagulation bath at a temperature of 45°C to complete phase separation and obtain the nascent hollow fiber base membrane.

[0036] (3) The hollow fiber membrane obtained in step (2) was immersed in an aqueous solution of 1,3-propanesulfonyl lactone with a concentration of 0.1 mol / L and reacted at 50°C for 6 hours. After the reaction was completed, the membrane was washed with a large amount of deionized water to remove the residual reactants and obtain a hydrophilic modified polysulfone membrane.

[0037] (4) Preparation of aqueous solution: Weigh 2.5g of poly(γ-glutamic acid) and 0.15g of triethylenetetramine, dissolve them in 97.35g of deionized water, stir and mix thoroughly to obtain an aqueous solution.

[0038] Preparation of organic phase solution: Weigh 0.2 g of trimesoyl chloride and 0.08 g of 3,5-diaminobenzoic acid, dissolve them in 99.72 g of n-hexane, and stir thoroughly to obtain organic phase solution.

[0039] (5) Pump the aqueous solution obtained in step (4) into the inner cavity of the hydrophilically modified polysulfone-based membrane obtained in step (3) and allow it to fully wet for 90 seconds. Then, purge the inner cavity of the membrane with high-pressure nitrogen for 30 seconds to remove any residual aqueous droplets on the surface. Immediately afterward, pump the organic solution into the inner cavity of the base membrane and allow it to undergo interfacial polymerization with the aqueous-wetted membrane surface for 45 seconds. After the reaction is complete, purge again with nitrogen to form a polyamide separation layer on the inner surface of the base membrane.

[0040] (6) The membrane after interfacial polymerization in step (5) is transferred into a 75°C forced-air oven and heat-treated for 5 minutes. Then, the membrane is thoroughly cleaned with hexane and deionized water in sequence to finally obtain the low-pressure high-flux polyamide / polysulfone hollow fiber composite nanofiltration membrane.

[0041] Example 2

[0042] This embodiment provides a low-pressure, high-flux polyamide / polysulfone hollow fiber composite nanofiltration membrane, the preparation method of which includes the following steps:

[0043] (1) Weigh 22.0 g of sulfonated polysulfone, 1.8 g of poly(1-vinyl-3-butylimidazolium) bromide and 1.3 g of polyvinylpyrrolidone (PVP K30), and dissolve them together in 100 g of N-methylpyrrolidone solvent. Stir mechanically at 60 °C for 12 hours until completely dissolved, and then let stand at room temperature for 8 hours to remove bubbles, to obtain a uniform and transparent spinning solution.

[0044] (2) Using a dry-wet spinning process, the spinning solution obtained in step (1) is extruded through a hollow fiber spinning device. After passing through a 10 cm air gap, it enters a pure water coagulation bath at a temperature of 50°C to complete phase separation and obtain the nascent hollow fiber base membrane.

[0045] (3) The hollow fiber membrane obtained in step (2) was immersed in an aqueous solution of 1,3-propanesulfonyl lactone with a concentration of 0.15 mol / L and reacted at 55°C for 5 hours. After the reaction was completed, the membrane was washed with a large amount of deionized water to remove the residual reactants and obtain the hydrophilic modified polysulfone membrane.

[0046] (4) Preparation of aqueous solution: Weigh 2.2 g of poly(γ-glutamic acid) and 0.12 g of triethylenetetramine, dissolve them in 97.68 g of deionized water, stir and mix thoroughly to obtain aqueous solution.

[0047] Preparation of organic phase solution: Weigh 0.25 g of trimesoyl chloride and 0.10 g of 3,5-diaminobenzoic acid, dissolve them in 99.65 g of n-hexane, and stir thoroughly to obtain organic phase solution.

[0048] (5) Pump the aqueous solution obtained in step (4) into the inner cavity of the hydrophilically modified polysulfone-based membrane obtained in step (3) and allow it to fully wet for 75 seconds. Then, purge the inner cavity of the membrane with high-pressure nitrogen for 30 seconds to remove any residual aqueous droplets on the surface. Immediately afterward, pump the organic solution into the inner cavity of the base membrane and allow it to undergo interfacial polymerization with the aqueous-wetted membrane surface for 35 seconds. After the reaction is complete, purge again with nitrogen to form a polyamide separation layer on the inner surface of the base membrane.

[0049] (6) The membrane after interfacial polymerization in step (5) is transferred into a 72°C forced-air oven and heat-treated for 4 minutes. Subsequently, the membrane is thoroughly cleaned with hexane and deionized water in sequence to finally obtain the low-pressure high-flux polyamide / polysulfone hollow fiber composite nanofiltration membrane.

[0050] Comparative Example 1

[0051] The difference between this comparative example and Example 1 is that, in step (1) when preparing the spinning solution, poly(1-vinyl-3-butylimidazolium) bromide was replaced with an equal mass of polyvinylpyrrolidone (PVP K30). Specifically, 20.0 g of sulfonated polysulfone and 2.8 g of PVP K30 were weighed and dissolved in 100 g of N-methylpyrrolidone. At the same time, the hydrophilic modification treatment of the base film in step (3) was omitted.

[0052] Comparative Example 2

[0053] The difference between this comparative example and Example 1 is that the 1,3-propanesulfonyl lactone aqueous solution in step (3) is replaced with 1.0 wt% glutaraldehyde aqueous solution, and the reaction is carried out at 50°C for 2 hours.

[0054] Comparative Example 3

[0055] The difference between this comparative example and Example 1 is that in step (4) when preparing the aqueous solution, poly(γ-glutamic acid) is replaced with piperazine, so that the mass concentration of piperazine in the aqueous solution is 1.0 wt%.

[0056] Comparative Example 4

[0057] The difference between this comparative example and Example 1 is that in step (4) when preparing the aqueous solution, triethylenetetramine is not used, but only 2.5 wt% of poly(γ-glutamic acid) is used.

[0058] Comparative Example 5

[0059] The difference between this comparative example and Example 1 is that 3,5-diaminobenzoic acid is not used when preparing the organic phase solution in step (4), only pyromellitic acid chloride is used, and its mass concentration is adjusted to 0.15 wt%.

[0060] Comparative Example 6

[0061] The difference between this comparative example and Example 1 is that in step (6), after the interfacial polymerization reaction, the membrane is dried at room temperature (25°C) for 10 minutes and then cleaned.

[0062] Performance testing

[0063] To verify the performance of the nanofiltration membrane of the present invention, the membrane samples prepared in Examples 1-2 and Comparative Examples 1-6 were subjected to the following tests:

[0064] 1. Pure water flux: Under operating pressure of 0.4 MPa and temperature of 25°C, using pure water as the feed liquid, after the flux stabilizes, record the permeate flow rate per unit time per unit membrane area, expressed in L / (m²). 2 ·h).

[0065] 2. Retention Rate: Under operating pressure of 0.4 MPa and temperature of 25°C, using a 1000 mg / L magnesium sulfate (MgSO4) aqueous solution as the feed solution, the system was run for 30 minutes until performance stabilized. The feed solution and permeate were then collected separately. The conductivity of the solution was measured using a conductivity meter and converted to concentration. The retention rate R was calculated using the following formula:

[0066] R = (1-C) p / C f ) × 100%

[0067] Among them, C p and C f These are the concentrations of the permeate and the feed solution, respectively.

[0068] The test results are shown in Table 1:

[0069] Table 1 Performance Test Results

[0070]

[0071] The above results demonstrate that the nanofiltration membranes prepared in Examples 1 and 2 exhibit excellent overall performance. At a low operating pressure of 0.4 MPa, the pure water flux exceeded 40 L / (m²). 2 ·h), while the retention rate of MgSO4 remained stable at over 92%.

[0072] Comparative Example 1 showed a significant performance decline, especially in flux loss. This indicates that simply physically blending with the conventional hydrophilic agent PVP, without the stable zwitterionic structure constructed through chemical reaction, cannot effectively reduce the mass transfer resistance of water, demonstrating the irreplaceable nature of in-situ zwitterionic modification. Comparative Example 2 showed a decrease in both flux and rejection rate. This indicates that crosslinking with glutaraldehyde cannot introduce strongly hydrophilic zwitterionic groups onto the base membrane, and its modification effect is far inferior to that of 1,3-propanesulfonyl lactone. Comparative Example 3 showed a sharp decrease in flux, demonstrating that using the traditional small-molecule monomer piperazine results in an excessively crosslinked and denser separation layer, severely hindering water molecule transport. Comparative Example 4 showed a significant decline in rejection rate, indicating that without triethylenetetramine as a crosslinking promoter, the reaction of poly(γ-glutamic acid) with TMC alone may not form a sufficiently complete and stable separation layer, leading to a decrease in selectivity. The reduced rejection rate in Comparative Example 5 indicates that the removal of the carboxyl-containing comonomer 3,5-diaminobenzoic acid weakens the hydrophilicity and electronegativity of the separation layer, affecting its screening capacity and resulting in impaired rejection performance. Comparative Example 6 did not achieve optimal flux and rejection rate, demonstrating that specific heat treatment steps are crucial for the final solidification of the separation layer network.

[0073] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1.A method for preparing a low-pressure high-flux polyamide / polysulfone hollow fiber composite nanofiltration membrane, comprising the following steps: (1) dissolving sulfonated polysulfone, poly (1-vinyl-3-butylimidazole) bromide and polyvinylpyrrolidone in N-methylpyrrolidone solvent, stirring and dissolving and standing to degas, to obtain a spinning solution; (2) using dry-wet spinning process, extruding and coagulating the spinning solution to obtain a hollow fiber base membrane; (3) immersing the hollow fiber base membrane in an aqueous solution of 1, 3-propane sultone for reaction, and washing after the reaction is completed to obtain a hydrophilic modified polysulfone base membrane; (4) preparing an aqueous phase solution containing poly (γ-glutamic acid) and triethylenetetramine, and an organic phase solution containing trimesoyl chloride and 3, 5-diaminobenzoic acid; (5) pumping the aqueous phase solution into the lumen of the hydrophilic modified polysulfone base membrane obtained in step (3) for infiltration, blowing off the residual liquid, and then pumping the organic phase solution for interfacial polymerization reaction to form a polyamide separation layer on the inner surface of the base membrane; (6) heat treating the membrane piece after the interfacial polymerization of step (5), and then washing to obtain the composite nanofiltration membrane. In step (1), the mass of poly (1-vinyl-3-butylimidazole) bromide is 0.07-0.11 parts and the mass of polyvinylpyrrolidone is 0.05-0.07 parts based on 1 part of the mass of the sulfonated polysulfone. In step (1), the total mass of sulfonated polysulfone, poly (1-vinyl-3-butylimidazole) bromide and polyvinylpyrrolidone in the spinning solution accounts for 15-25% of the total mass of the spinning solution. In step (3), the concentration of the aqueous solution of 1, 3-propane sultone is 0.05-0.2 mol / L, the reaction temperature is 40-60℃, and the reaction time is 4-8 hours. In step (4), the mass concentration of poly (γ-glutamic acid) in the aqueous phase solution is 2.0-3.0 wt% and the mass concentration of triethylenetetramine is 0.1-0.2 wt%. In step (4), the mass concentration of trimesoyl chloride in the organic phase solution is 0.1-0.3 wt% and the mass concentration of 3, 5-diaminobenzoic acid is 0.05-0.15 wt%. In step (5), the infiltration time of the aqueous phase solution is 60-120 seconds and the interfacial polymerization reaction time is 30-60 seconds. In step (6), the heat treatment temperature is 70-80℃ and the time is 3-8 minutes. The composite nanofiltration membrane is obtained by the method of any one of claims 1-8. ​ ​ ​ ​ ​ ​ 2. The production method according to claim 1, characterized by, ​ 3. The preparation method according to claim 1, characterized in that, ​ 4. The method of claim 1, wherein, ​ 5. The preparation method according to claim 1, characterized in that, ​ 6. The method of claim 1, wherein, ​ 7. The preparation method according to claim 1, characterized in that, ​ 8. The method of claim 1, wherein, ​ 9. A low pressure high flux polyamide / poly sulfone hollow fiber composite nanofiltration membrane, characterized in that, ​ 10. The low pressure high flux polyamide / poly sulfone hollow fiber composite nanofiltration membrane according to claim 9, characterized in that, The composite nanofiltration membrane has a pure water flux higher than 40 L / (m 2 ·h) under a pressure of 0.4 MPa and a rejection rate of magnesium sulfate higher than 90%.

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