High-flux polyamide nanofiltration membrane, preparation method and application thereof

Abstract

This invention provides a high-flux polyamide nanofiltration membrane, its preparation method, and its applications. The preparation method includes: dissolving piperazine and L-ascorbic acid in deionized water to prepare an aqueous phase solution; dissolving trimesoyl chloride in n-hexane to prepare an oil phase solution; sequentially immersing a polyethersulfone-based membrane in the aqueous and oil phases for interfacial polymerization, followed by thermosetting, cleaning, and drying to obtain the final product. This method utilizes the green and non-toxic L-ascorbic acid as an interfacial polymerization regulator, whose polyhydroxy structure slows down the diffusion of piperazine monomers, resulting in a smooth and uniform reaction. The obtained nanofiltration membrane has a uniform nanonodular structure, high pure water flux, high rejection rates for sodium sulfate, magnesium sulfate, and PFAS, and excellent antifouling properties, making it suitable for drinking water purification, surface water treatment, and the removal of PFAS-type pollutants.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane technology, and in particular to a high-flux polyamide nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] Per- and polyfluoroalkyl substances (PFAS) pose a serious threat to aquatic ecosystems and human health due to their extreme chemical stability and bioaccumulation. Polyamide nanofiltration membranes have become an important choice for the adsorption and removal of PFAS pollutants due to their combination of low energy consumption and high selectivity for multivalent ions and organic matter.

[0003] Polyamide nanofiltration membranes prepared using the traditional piperazine (PIP) / trimethylammonium chloride (TMC) system often face a severe trade-off between permeability and selectivity. Due to the violent and uncontrollable interfacial polymerization process, the selective layer thickness often increases and the structure becomes non-uniform, thus limiting further improvements in separation efficiency. In existing technologies, most aqueous additives used to regulate reaction kinetics are biotoxic, which contradicts the principles of green manufacturing.

[0004] Therefore, there is an urgent need for a new green and environmentally friendly method to overcome the constraints of throughput and selectivity without increasing process complexity, and to achieve a synergistic improvement in high throughput, high selectivity and anti-pollution performance. Summary of the Invention

[0005] The purpose of this invention is to provide a high-flux polyamide nanofiltration membrane, its preparation method, and its application. This invention relates to a method for preparing a high-flux, highly selective, and antifouling polyamide nanofiltration membrane based on L-ascorbic acid-assisted interfacial polymerization. L-ascorbic acid, as a green and non-toxic substance, is rich in multiple hydroxyl structures and can effectively slow down the diffusion rate of piperazine monomers into the oil phase through hydrogen bonding, making the interfacial polymerization reaction more gradual and uniform.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a high-flux polyamide nanofiltration membrane includes the following steps: Step S1: Piperazine and L-ascorbic acid are added to deionized water and stirred until homogeneous to obtain an aqueous solution; in the aqueous solution, the mass fraction of piperazine is 0.1-0.5 wt%, and the mass fraction of L-ascorbic acid is 0.1-0.5 wt%. Step S2: Add trimesoyl chloride to n-hexane and stir until completely dissolved to obtain an oil phase solution; the mass fraction of trimesoyl chloride in the oil phase solution is 0.05-0.2 wt%. Step S3: Immerse the polyethersulfone-based film in the aqueous phase solution for 1-5 minutes, remove it and remove excess liquid from the surface, then immerse it in the oil phase solution for 20-60 seconds to undergo interfacial polymerization reaction and obtain the preform film. Step S4: Heat-cur the preform at 60°C for 2-10 minutes; Step S5: Immerse the thermocured membrane sample in deionized water to remove unreacted monomers and residual solvents, then rinse with pure water and dry to obtain a high-flux polyamide nanofiltration membrane.

[0007] As a preferred improvement, in step S1, the mass fraction of L-ascorbic acid is 0.3 wt%; the mass fraction of piperazine is 0.3 wt%.

[0008] As a preferred improvement, in step S2, the mass fraction of the pyromellitic trimethylol chloride is 0.1 wt%.

[0009] As a preferred improvement, in step S3, the molecular weight cutoff of the polyethersulfone-based film is 150 kDa.

[0010] As a preferred improvement, in step S3, the method of "removing excess liquid from the surface" is to use an air knife to blow it.

[0011] A high-flux polyamide nanofiltration membrane is prepared using the above-described preparation method.

[0012] As a preferred improvement, the high-flux polyamide nanofiltration membrane comprises a polyethersulfone base membrane and a polyamide selective layer formed on the surface of the polyethersulfone base membrane. The polyamide selective layer has a thickness of 21.3-28.9 nm, an average pore size of 0.67 nm, a static water contact angle of 23.10°, and a Zeta potential of -31.20 mV at pH 7.

[0013] As a preferred improvement, at 25°C and 4 bar operating pressure, the high-flux polyamide nanofiltration membrane has a pure water flux ≥21.04 LMH / bar, a Na2SO4 rejection rate ≥95.89%, a MgSO4 rejection rate ≥93.28%, and a removal rate of 87.06%-99.87% for various perfluorinated and polyfluoroalkyl substances. After three contamination-cleaning cycles, the flux recovery rate is ≥85%.

[0014] One application of the high-flux polyamide nanofiltration membrane described above is for purifying drinking water, treating natural surface water, or removing perfluorinated and polyfluoroalkyl contaminants.

[0015] The beneficial effects of this invention are as follows: (1) L-ascorbic acid is used as an interfacial polymerization regulator. L-ascorbic acid is rich in multiple hydroxyl groups, which can effectively delay the diffusion rate of piperazine monomer to the oil phase through hydrogen bonding, making the interfacial polymerization reaction more gentle and uniform. Thus, an ultrathin polyamide selective layer with a uniform nano-nodular structure is formed on the polyethersulfone supported membrane. It has an optimized hydrogen bond network and a high negative charge density, so that the pure water flux and salt rejection rate of the membrane reach an excellent balance level. After three fouling-cleaning cycles, the flux recovery rate is still above 85%, showing excellent reusability and long-term operational stability. (2) L-ascorbic acid is green and non-toxic, and has a wide range of sources, which avoids the use of traditional toxic additives and meets the environmental protection requirements of green manufacturing. (3) The nanofiltration membrane prepared by the present invention has high efficiency in removing PFAS and can effectively meet the deep purification needs of PFAS pollutants in water. (4) L-ascorbic acid can be added directly to the aqueous phase of traditional interfacial polymerization without changing the original process flow. The operation is simple and easy to realize industrial production. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the nanofiltration membrane preparation process of the present invention; Figure 2 SEM images of the membrane surface morphology at different L-ascorbic acid concentrations; Figure 3 AFM images of the membrane surface morphology at different L-ascorbic acid concentrations; Figure 4 Figures showing the permeability and salt rejection of nanofiltration membranes prepared at different L-ascorbic acid concentrations; Figure 5 The graph shows the removal performance of the membrane for 17 types of PFAS. Detailed Implementation

[0017] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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.

[0018] This embodiment provides a method for preparing a high-flux polyamide nanofiltration membrane, comprising the following steps: Step S1, Preparation of aqueous solution Piperazine (PIP) was added to deionized water, and then L-ascorbic acid was added and stirred until homogeneous to obtain an aqueous solution with a piperazine mass fraction of 0.1-0.5 wt% and an L-ascorbic acid mass fraction of 0.1-0.5 wt%.

[0019] Preferably, the mass fraction of L-ascorbic acid is 0.3 wt%; the mass fraction of piperazine is 0.3 wt%.

[0020] Step S2, Preparation of oil phase solution Add trimesoyl chloride (TMC) to a hexane solution and stir until completely dissolved to obtain an oil phase solution with a mass fraction of 0.05-0.2 wt% trimesoyl chloride.

[0021] Preferably, the mass fraction of the pyromellitic chloroformyl chloride is 0.1 wt%.

[0022] Step S3, interfacial polymerization reaction The polyethersulfone (PES) base film is immersed in an aqueous solution for 1-5 minutes. After removal, excess liquid on the surface is removed with an air knife. The film is then immersed in an oil solution for 20-60 seconds to undergo interfacial polymerization and obtain a preform film.

[0023] The PES base film has a molecular weight cutoff of 150 kDa, and its surface hydrophilicity and microporous structure contribute to the stable adhesion and progress of the interfacial polymerization reaction.

[0024] Using L-ascorbic acid as an interfacial polymerization regulator, its abundant hydroxyl groups are utilized to slow down the diffusion rate of piperazine monomers, which is beneficial for forming a polyamide film with a uniform nanonodular structure. Preferably, the aqueous phase remains on the base film surface for 4 minutes, and the oil phase covers the reaction for 45 seconds. This reaction time avoids excessively rapid interfacial polymerization, ensuring the uniformity of the selective layer and extremely high water permeability.

[0025] With the appropriate addition of L-ascorbic acid, the hydrogen bond network formed by its hydroxyl groups and piperazine effectively slows down the monomer diffusion rate, making the interfacial polymerization reaction more moderate and controllable. This results in the formation of a polyamide selective layer with a uniform nanonodular structure and high free volume, significantly optimizing the membrane surface structure and improving water permeation flux. However, when L-ascorbic acid is in excess, the over-strengthening of the hydrogen bond network leads to severe obstruction of piperazine diffusion and insufficient interfacial reaction. At the same time, the side reaction of L-ascorbic acid and TMC competes for the consumption of acyl chloride groups, resulting in a significant decrease in crosslinking degree, excessively loose membrane structure, and uncontrolled pore size distribution, ultimately leading to a significant reduction in retention rate.

[0026] Step S4, thermosetting treatment Place the preform at 60℃ for 2-10 minutes for heat curing.

[0027] Preferably, the thermosetting time is 4 minutes. The thermosetting treatment is used to improve the crosslinking degree and structural stability of the polyamide layer. This specific heat treatment time and temperature result in a high crosslinking conversion rate of polyamide molecules, balancing the density of the film layer with long-term operational stability.

[0028] Step S5, washing and drying The membrane sample was immersed in deionized water to remove unreacted monomers and residual solvents, then rinsed with pure water and air-dried to obtain the final polyamide nanofiltration membrane product.

[0029] This embodiment also provides a high-flux polyamide nanofiltration membrane, which is prepared by the above-described preparation method. The high-flux polyamide nanofiltration membrane forms a fine and uniform polyamide selective layer on a PES-supported substrate membrane. The membrane surface exhibits a significant nanonodular structure, with an optimized hydrogen bond network and high negative charge density, enabling the membrane to achieve an excellent balance between pure water flux and salt rejection rate. In the high-flux polyamide nanofiltration membrane, the polyamide selective layer thickness is 21.3-28.9 nm, the average pore size is 0.67 nm, the static water contact angle on the membrane surface is 23.10°, the zeta potential is around -30 mV at pH 7, and the pure water flux of the high-flux polyamide nanofiltration membrane is above 21.04 LMH / bar at 25°C and 4 bar operating pressure. The rejection rate of Na2SO4 is 95.00%, and the rejection rate of MgSO4 is 93.28%. The removal rate of 17 perfluorinated and polyfluoroalkyl substances (PFAS) ranges from 87.06% to 99.87%, and the flux recovery rate (FRR) remains above 85% after three fouling-washing cycles.

[0030] This embodiment also provides an application of the above-mentioned high-flux polyamide nanofiltration membrane as a filter membrane for drinking water purification, natural surface water treatment, and removal of perfluorinated and polyfluorinated alkyl pollutants.

[0031] To further illustrate the present invention, specific embodiments are provided below. It should be noted that these embodiments are for illustrative purposes only and do not constitute a limitation on the scope of the invention. The experimental conditions were as follows: permeation and retention tests of pure water and salt solution (2000 ppm) were conducted at 25 °C and an operating pressure of 4 bar. PFAS retention tests were also performed on some embodiments and comparative examples.

[0032] Comparative Example 1 (corresponding to sample AA-0) Nanofiltration membranes were prepared using a conventional interfacial polymerization method. Specifically, an aqueous solution containing 0.3 wt% PIP was prepared, without the addition of L-ascorbic acid. The PES-based membrane was immersed in the aqueous solution for 4 minutes, dried using an air knife, and then covered with an oil solution containing 0.1 wt% TMC in n-hexane for 45 seconds. Finally, it was thermally cured at 60°C for 5 minutes. Test results showed that the membrane had a pure water flux of 15.21 LMH / bar, a Na₂SO₄ rejection rate of 95.23%, and a removal rate of various PFAS ranging from 92.67% to 99.65%.

[0033] Example 1 (corresponding to sample AA-1) The difference between this embodiment and Comparative Example 1 is that 0.1 wt% L-ascorbic acid was added to the aqueous solution. The introduction of a small amount of L-ascorbic acid began to play a regulatory role, increasing the pure water flux of the resulting membrane to 17.79 LMH / bar, while maintaining the Na2SO4 rejection rate at 95.54%, thus achieving an initial increase in flux.

[0034] Example 2 (corresponding to sample AA-2) The difference between this embodiment and Example 1 is that the concentration of L-ascorbic acid in the aqueous phase is increased to 0.2 wt%. The pure water flux of the resulting nanofiltration membrane is further increased to 18.21 LMH / bar, and the Na2SO4 rejection rate is 92.99%.

[0035] Example 3 (corresponding to sample AA-3) The difference between this embodiment and Example 1 is that the concentration of L-ascorbic acid in the aqueous phase was increased to 0.3 wt%. At this concentration, the multiple hydroxyl groups of L-ascorbic acid most effectively slowed down the diffusion rate of PIP monomers, forming a polyamide layer with a uniform nanonodular structure. Test results show that the membrane achieved an optimal balance between flux and selectivity, with a pure water flux of up to 21.04 LMH / bar, a Na2SO4 rejection rate of 95.89%, and a removal rate of various PFAS ranging from 87.06% to 99.85%.

[0036] Example 4 (corresponding to sample AA-4) The difference between this embodiment and Embodiment 1 is that the concentration of L-ascorbic acid in the aqueous phase is increased to 0.4 wt%. Because monomer diffusion is further suppressed, the membrane layer becomes thinner and the pore size increases. This membrane exhibits extremely high water permeability, with a pure water flux surging to 39.71 LMH / bar, a Na₂SO₄ rejection rate of 89.05%, and removal rates of various PFAS ranging from 77.51% to 99.72%. This embodiment is suitable for treatment scenarios requiring extremely high flux and moderate rejection rates.

[0037] Example 5 (corresponding to sample AA-5) The difference between this embodiment and Example 1 is that the concentration of L-ascorbic acid in the aqueous phase is increased to 0.5 wt%. The excessively high concentration of the regulator leads to an overly porous polyamide network structure, causing the pure water flux of the resulting membrane to drop to 28.36 LMH / bar, and the Na2SO4 rejection rate to be 90.22%.

[0038] To visually demonstrate the regulatory effect of L-ascorbic acid on the performance of nanofiltration membranes, Table 1 summarizes the performance test results of the comparative examples and various embodiments.

[0039] Table 1 Comparison of membrane properties prepared at different L-ascorbic acid concentrations The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims, and all of these forms are within the protection scope of the present invention.

Claims

1. A method for preparing a high-flux polyamide nanofiltration membrane, characterized in that, Includes the following steps: Step S1: Piperazine and L-ascorbic acid are added to deionized water and stirred until homogeneous to obtain an aqueous solution; in the aqueous solution, the mass fraction of piperazine is 0.1-0.5 wt%, and the mass fraction of L-ascorbic acid is 0.1-0.5 wt%. Step S2: Add trimesoyl chloride to n-hexane and stir until completely dissolved to obtain an oil phase solution; the mass fraction of trimesoyl chloride in the oil phase solution is 0.05-0.2 wt%. Step S3: Immerse the polyethersulfone-based film in the aqueous phase solution for 1-5 minutes, remove it and remove excess liquid from the surface, then immerse it in the oil phase solution for 20-60 seconds to undergo interfacial polymerization reaction and obtain the preform film. Step S4: Heat-cur the preform at 60°C for 2-10 minutes; Step S5: Immerse the thermocured membrane sample in deionized water to remove unreacted monomers and residual solvents, then rinse with pure water and dry to obtain a high-flux polyamide nanofiltration membrane.

2. The production method according to claim 1, characterized by, In step S1, the mass fraction of L-ascorbic acid is 0.3 wt%; the mass fraction of piperazine is 0.3 wt%.

3. The preparation method according to claim 1, characterized in that, In step S2, the mass fraction of the pyromellitic acid chloride is 0.1 wt%.

4. The preparation method according to claim 1, characterized in that, In step S3, the molecular weight cutoff of the polyethersulfone-based film is 150 kDa.

5. The preparation method according to claim 1, characterized in that, In step S3, the method for "removing excess liquid from the surface" is to use an air knife to blow it off.

6. A high-flux polyamide nanofiltration membrane, characterized in that, It is prepared by the preparation method according to any one of claims 1-5.

7. The high-flux polyamide nanofiltration membrane according to claim 6, characterized in that, The high-flux polyamide nanofiltration membrane includes a polyethersulfone base membrane and a polyamide selective layer formed on the surface of the polyethersulfone base membrane. The polyamide selective layer has a thickness of 21.3-28.9 nm, an average pore size of 0.67 nm, a static water contact angle of 23.10° on the membrane surface, and a Zeta potential of -31.20 mV at pH 7.

8. The high-flux polyamide nanofiltration membrane according to claim 7, characterized in that, At 25°C and 4 bar operating pressure, the high-flux polyamide nanofiltration membrane has a pure water flux ≥21.04 LMH / bar, a Na2SO4 rejection rate ≥95.89%, a MgSO4 rejection rate ≥93.28%, and a removal rate of 87.06%-99.87% for various perfluorinated and polyfluoroalkyl substances. After three contamination-cleaning cycles, the flux recovery rate is ≥85%.

9. An application of the high-flux polyamide nanofiltration membrane as described in any one of claims 6-8, characterized in that, Used for purifying drinking water, treating natural surface water, or removing perfluorinated and polyfluoroalkyl substances.

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