Polyamide composite nanofiltration membrane, preparation method and application thereof

Abstract

The application belongs to the technical field of nanofiltration, and provides a polyamide composite nanofiltration membrane, a preparation method and application thereof. First, a hydrotalcite colloid is prepared by a nucleation reaction and a crystallization reaction isolation method; then the hydrotalcite colloid is mixed with piperazine, and a precursor solution obtained is coated on a porous base film to form a hydrotalcite intermediate layer containing piperazine; further, a trimesoyl chloride solution is added dropwise to the surface of the intermediate layer to form a polyamide layer through an interfacial polymerization reaction, and finally a polyamide composite nanofiltration membrane is obtained. The precursor solution containing the hydrotalcite colloid and the piperazine is deposited on the porous base film, the hydrogen bond interaction between the uniformly distributed hydroxyl groups of the hydrotalcite main layer and the amine monomer (piperazine) and the diffusion blocking of the two-dimensional layer of the hydrotalcite to the amine monomer can realize the controllable diffusion of the amine monomer, so that an ultrathin and higher performance polyamide layer is prepared, and the final polyamide composite nanofiltration membrane has excellent flux and rejection rate.

Description

Technical Field

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

[0002] Membrane separation technology exhibits significant advantages due to its high efficiency, energy saving, and ease of scalability. Nanofiltration technology, capable of retaining divalent salts and molecules with molecular weights exceeding 200 Daltons, is a key means of achieving goals such as water softening, wastewater treatment, and resource recovery. Polyamide nanofiltration membranes, prepared through interfacial polymerization of amine and acyl chloride monomers at the water-oil interface, dominate the market due to their cost-effectiveness and scalability. However, due to limitations in membrane fabrication technology and materials, a trade-off exists between permeability and selectivity. The key issue lies in the lack of controllability caused by the diffusion behavior of amine monomers during interfacial polymerization, which hinders the optimization of membrane performance at the nanoscale.

[0003] Intermediate layer strategies are crucial for improving membrane performance. Over the past decade, various nanomaterials, such as cadmium hydroxide nanowires, carbon nanotubes, and graphene oxide, have been developed for intermediate layer preparation, enabling the fabrication of high-performance nanofiltration membranes. Exfoliated hydrotalcite nanosheets have also been used as intermediate layer materials. However, the nanofiltration performance of nanofiltration membranes prepared using exfoliated hydrotalcite nanosheets as the intermediate layer needs further improvement, and large-scale fabrication is not easily achieved. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a polyamide composite nanofiltration membrane, its preparation method, and its applications. The polyamide composite nanofiltration membrane prepared by the method provided by this invention exhibits excellent nanofiltration performance and is easier to mass-produce.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides a method for preparing a polyamide composite nanofiltration membrane, comprising the following steps:

[0007] Soluble magnesium salt, soluble aluminum salt, inorganic alkali and water are mixed and subjected to a nucleation reaction to obtain hydrotalcite precipitate;

[0008] The hydrotalcite precipitate was dispersed in water and subjected to a crystallization reaction to obtain hydrotalcite colloid;

[0009] The hydrotalcite colloid and piperazine were mixed to obtain a precursor solution;

[0010] After the precursor liquid is coated onto the porous base membrane, a film is formed to create an intermediate layer-porous base membrane composite membrane.

[0011] A pyromellitic methyl chloride solution was dropped onto the middle layer side of the intermediate layer-porous base membrane composite membrane to carry out an interfacial polymerization reaction, thereby obtaining the polyamide composite nanofiltration membrane.

[0012] Preferably, the soluble magnesium salt includes one or more of magnesium chloride, magnesium nitrate, and magnesium sulfate; the soluble aluminum salt includes one or more of aluminum chloride, aluminum nitrate, and aluminum sulfate; and the inorganic base includes sodium hydroxide and / or sodium carbonate.

[0013] Preferably, the molar ratio of the soluble magnesium salt to the soluble aluminum salt is 2 to 4:1, and the molar amount of the inorganic base is 2 to 2.1 times the total molar amount of the soluble magnesium salt and the soluble aluminum salt.

[0014] Preferably, the nucleation reaction is carried out in a colloid mill, and the nucleation reaction time is 2 to 5 minutes.

[0015] Preferably, the crystallization reaction is carried out at a temperature of 60–120°C for 18–36 h; and the solid content of the hydrotalcite colloid is 0.1–4 wt%.

[0016] Preferably, the precursor solution has a solid content of 0.01–0.8 wt% and a piperazine concentration of 0.01–0.2 wt%.

[0017] Preferably, the coating amount of the precursor solution is 0.1–0.5 mL / cm². 2 .

[0018] Preferably, the concentration of the trimesoyl chloride solution is 0.05-0.5 wt%, and the interfacial polymerization reaction time is 10-20 s.

[0019] The present invention also provides a polyamide composite nanofiltration membrane prepared by the preparation method described above, wherein the polyamide composite nanofiltration membrane comprises a porous base membrane, a hydrotalcite layer coated on the base membrane, and a polyamide layer attached to the hydrotalcite layer.

[0020] The present invention also provides the application of the polyamide composite nanofiltration membrane described above in desalination, wastewater treatment or resource recycling.

[0021] This invention provides a method for preparing a polyamide composite nanofiltration membrane.

[0022] This invention first prepares hydrotalcite colloid through a separation method of nucleation and crystallization reactions (i.e., nucleation and crystallization reactions are carried out in steps); then, the hydrotalcite colloid is mixed with piperazine, and the resulting precursor solution is coated onto a porous base membrane to form a hydrotalcite interlayer containing piperazine molecules; further, a pyromellitic trimethylol chloride solution is dropped onto the surface of the interlayer to undergo an interfacial polymerization reaction to form a polyamide layer, and finally a polyamide composite nanofiltration membrane is obtained.

[0023] This invention utilizes hydrotalcite colloids, resulting in a rougher intermediate layer and a higher specific surface area compared to the direct use of exfoliated hydrotalcite nanosheets in existing technologies. This leads to higher flux in the polyamide composite nanofiltration membrane. Meanwhile, polyamide nanofiltration membranes are the most widely used commercially available nanofiltration membranes; however, their performance improvement is limited by constraints in membrane preparation methods and materials. The applicant discovered that this is because the interfacial polymerization reaction is a diffusion-controlled reaction of amine monomers. However, traditional polyamide nanofiltration membrane preparation methods involve directly loading amine monomers onto a porous base membrane, followed by the addition of acyl chloride monomers to conduct an interfacial polymerization reaction to form a polyamide layer. In these methods, the amine monomers are in a free state, and their diffusion is uncontrollable, leading to uncontrollable polyamide layer formation and preventing the preparation of higher-performance membranes, especially in terms of flux. This invention co-deposits a precursor solution containing hydrotalcite colloid and piperazine onto a porous substrate membrane. The hydrogen-bonding interaction between the uniformly distributed hydroxyl groups of the hydrotalcite matrix and the amine monomer (piperazine), along with the hindrance of diffusion of the amine monomer (piperazine) by the two-dimensional layers, enables controllable diffusion of the amine monomer, thereby producing an ultrathin, higher-performance polyamide layer. This results in a final polyamide composite nanofiltration membrane with excellent flux. Furthermore, the preparation method of this invention, by mixing the hydrotalcite colloid and the amine monomer (piperazine) and co-coating them onto the porous substrate membrane, avoids the erosion of the intermediate layer during the amine monomer adsorption process compared to existing technologies that separately coat the intermediate layer and adsorb the amine monomer, resulting in a higher membrane formation success rate. Moreover, the two-step operation is simpler and easier to scale up and industrialize.

[0024] Data from the embodiments show that the polyamide composite nanofiltration membrane provided by the present invention has excellent nanofiltration performance, that is, it has a high rejection rate for salts (especially sodium sulfate and magnesium sulfate), dye molecules (methylene blue) and antibiotic molecules (clindamycin phosphate and bacitracin) even with high flux, making the polyamide composite nanofiltration membrane a potential application for desalination (especially antibiotic desalination), wastewater treatment or resource recovery. Attached Figure Description

[0025] Figure 1 The image shows the XRD pattern of the hydrotalcite colloid obtained in Example 1.

[0026] Figure 2 This is a SEM image of the intermediate layer in the intermediate layer-porous base membrane composite membrane obtained in Example 1;

[0027] Figure 3 This is a planar SEM image of the polyamide composite nanofiltration membrane obtained in Example 1;

[0028] Figure 4 Here is a cross-sectional SEM image of the polyamide composite nanofiltration membrane obtained in Example 1;

[0029] Figure 5 The nanofiltration performance of the polyamide composite nanofiltration membrane obtained in Example 1 for different salts is shown. Detailed Implementation

[0030] This invention provides a method for preparing a polyamide composite nanofiltration membrane, comprising the following steps:

[0031] Soluble magnesium salt, soluble aluminum salt, inorganic alkali and water are mixed and subjected to a nucleation reaction to obtain hydrotalcite precipitate;

[0032] The hydrotalcite precipitate was dispersed in water and subjected to a crystallization reaction to obtain hydrotalcite colloid;

[0033] The hydrotalcite colloid and piperazine were mixed to obtain a precursor solution;

[0034] After the precursor liquid is coated onto the porous base membrane, a film is formed to create an intermediate layer-porous base membrane composite membrane.

[0035] A pyromellitic methyl chloride solution was dropped onto the middle layer side of the intermediate layer-porous base membrane composite membrane to carry out an interfacial polymerization reaction, thereby obtaining the polyamide composite nanofiltration membrane.

[0036] Unless otherwise specified, the raw materials used in this invention are preferably commercially available products.

[0037] This invention involves mixing soluble magnesium salt, soluble aluminum salt, inorganic alkali, and water to carry out a nucleation reaction, thereby obtaining hydrotalcite precipitate.

[0038] In this invention, the soluble magnesium salt preferably includes one or more of magnesium chloride, magnesium nitrate, and magnesium sulfate, and more preferably magnesium chloride. In this invention, the soluble aluminum salt preferably includes one or more of aluminum chloride, aluminum nitrate, and aluminum sulfate, and more preferably aluminum chloride. In this invention, the inorganic base preferably includes sodium hydroxide and / or sodium carbonate, and more preferably sodium hydroxide. In this invention, the water is preferably deionized water.

[0039] In this invention, the molar ratio of the soluble magnesium salt to the soluble aluminum salt is preferably 2 to 4:1, specifically preferably 2:1, 2.5:1, 3:1, 3.5:1, or 4:1. In this invention, the molar amount of the inorganic base is preferably 2 to 2.1 times the total molar amount of the soluble magnesium salt and the soluble aluminum salt, more preferably twice the total molar amount of the soluble magnesium salt and the soluble aluminum salt.

[0040] In this invention, the nucleation reaction is preferably carried out in a colloid mill, and the rotation speed of the colloid mill is preferably 2000-3000 rpm. In this invention, the nucleation reaction time is preferably 2-5 min, specifically 2 min, 3 min, 4 min, or 5 min.

[0041] In this invention, the nucleation reaction of mixing soluble magnesium salt, soluble aluminum salt, inorganic alkali, and water preferably includes the following steps: dissolving the soluble magnesium salt and soluble aluminum salt in water to obtain a mixed salt solution; dissolving the inorganic alkali in water to obtain an inorganic alkali solution; and simultaneously adding the mixed salt solution and the inorganic alkali solution to a colloid mill to carry out the nucleation reaction. In this invention, the concentration of the soluble magnesium salt in the mixed salt solution is preferably 0.2–0.3 mol / L, and the concentration of the soluble aluminum salt is preferably 0.1–0.15 mol / L. In this invention, the concentration of the inorganic alkali solution is preferably 0.3–0.45 mol / L.

[0042] Following the nucleation reaction, the present invention preferably further includes: centrifuging the obtained nucleation reaction solution, collecting the precipitate, and washing the precipitate to obtain the hydrotalcite precipitate. In the present invention, the washing reagent is preferably water, the water is preferably deionized water, and the washing is preferably performed three times.

[0043] After obtaining the hydrotalcite precipitate, the present invention disperses the hydrotalcite precipitate in water and carries out a crystallization reaction to obtain hydrotalcite colloid.

[0044] In this invention, the temperature of the crystallization reaction is preferably 60–120°C, specifically preferably 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, or 120°C; the time is preferably 18–36 h, more preferably 24 h. In this invention, the solid content of the hydrotalcite colloid is preferably 0.1–4 wt%, specifically preferably 1 wt%. In this invention, the crystallization reaction is preferably carried out under closed conditions.

[0045] In one specific embodiment of the present invention, the crystallization reaction of dispersing the hydrotalcite precipitate in water preferably includes the following steps: after dispersing the hydrotalcite precipitate in water, adding it to a reaction vessel, sealing the reaction vessel, and placing it in an oven for crystallization reaction. In the present invention, the reaction vessel is preferably a polytetrafluoroethylene (PTFE) reaction vessel.

[0046] In this invention, when the hydrotalcite colloid is not in use, it is preferably stored in a sealed container.

[0047] In this invention, the solid phase component of the hydrotalcite colloid is hydrotalcite (LDH), the hydrotalcite is a magnesium aluminum layered double hydroxide, and the interlayer anion of the magnesium aluminum layered double hydroxide is CO3. 2- That is, the solid phase component of the hydrotalcite colloid is interlayer anion CO3. 2- The invention utilizes layered magnesium-aluminum double hydroxides. Compared to the prior art which directly uses exfoliated layered hydrotalcite nanosheets, the resulting intermediate layer is rougher and has a higher specific surface area, resulting in higher flux of the polyamide composite nanofiltration membrane.

[0048] After obtaining the hydrotalcite colloid, the present invention mixes the hydrotalcite colloid with piperazine to obtain a precursor solution.

[0049] In this invention, the solid content of the precursor solution is preferably 0.01 to 0.8 wt%, and the concentration of piperazine is preferably 0.01 to 0.2 wt%.

[0050] In a specific embodiment of the present invention, mixing the hydrotalcite colloid and piperazine to obtain a precursor solution preferably includes the following steps: mixing the hydrotalcite colloid and piperazine to obtain a piperazine-hydrotalcite mixture; diluting the piperazine-hydrotalcite mixture to obtain the precursor solution; the diluent is preferably water, and the dilution factor is preferably 5 to 10 times.

[0051] After obtaining the precursor liquid, the present invention coats the precursor liquid onto a porous base membrane and then forms a film to form an intermediate layer-porous base membrane composite membrane.

[0052] In this invention, the porous base membrane is preferably made of nylon. The pore size of the porous base membrane is preferably 0.22 μm. In one specific embodiment of this invention, the porous base membrane is preferably circular in shape, and the radius of the circle is preferably 1-2 cm, specifically 2 cm.

[0053] In this invention, the coating amount of the precursor solution is preferably 0.1–0.5 mL / cm². 2 Specifically, the preferred value is 0.16 mL / cm 2 Or 0.24 mL / cm 2 In one specific embodiment of the present invention, it is preferable to coat 2-3 mL of precursor solution onto a circular porous substrate membrane with a radius of 2 cm.

[0054] In this invention, the film formation method preferably includes standing or filtration. In this invention, the standing time is preferably 1 min to 3 min, specifically 1 min, 1.5 min, 2 min, 2.5 min, or 3 min.

[0055] This invention co-deposits a precursor solution containing hydrotalcite colloid and piperazine onto a porous substrate membrane. The hydrogen-bonded interaction between the uniformly distributed hydroxyl groups of the hydrotalcite matrix and the amine monomer (piperazine), along with the hindered diffusion of the amine monomer (piperazine) by the two-dimensional layers of hydrotalcite, enables controllable diffusion of the amine monomer, thereby producing an ultrathin, higher-performance polyamide layer. This results in a final polyamide composite nanofiltration membrane with excellent flux and rejection rate. Furthermore, the preparation method of this invention involves mixing the hydrotalcite colloid and the amine monomer (piperazine) and co-coating them onto the porous substrate membrane. Compared to existing technologies that separately coat the intermediate layer and adsorb the amine monomer, this avoids erosion of the intermediate layer during the amine monomer adsorption process, resulting in a higher membrane formation success rate. Moreover, the two-step operation is simpler and easier to scale up and industrialize.

[0056] After forming the intermediate layer-porous base membrane composite membrane, the present invention adds a pyromellitic methyl chloride solution to the intermediate layer side of the intermediate layer-porous base membrane composite membrane to carry out an interfacial polymerization reaction to obtain the polyamide composite nanofiltration membrane.

[0057] In this invention, the concentration of the pyromellitic acid chloride solution is preferably 0.05-0.5 wt%, specifically preferably 0.05 wt% or 0.1 wt%. In this invention, the solvent for the pyromellitic acid chloride solution is preferably n-hexane.

[0058] In this invention, the amount of the pyromellitic chloride solution added is preferably to completely cover the intermediate layer-porous base membrane composite membrane.

[0059] In this invention, the temperature of the interfacial polymerization is preferably room temperature, and the time is preferably 10 to 20 seconds.

[0060] After the interfacial polymerization reaction, the present invention preferably further includes: using an air knife to remove residual liquid, and then washing the membrane surface with n-hexane to remove excess trimesoyl chloride.

[0061] In this invention, the polyamide composite nanofiltration membrane is preferably stored in deionized water when not in use.

[0062] In this invention, piperazine and pyromellitic methyl chloride undergo a polymerization reaction at the interface of the intermediate layer to form a polyamide layer.

[0063] The present invention also provides a polyamide composite nanofiltration membrane prepared by the preparation method described above, wherein the polyamide composite nanofiltration membrane comprises a porous base membrane, a hydrotalcite layer coated on the base membrane, and a polyamide layer attached to the hydrotalcite layer.

[0064] In this invention, the thickness of the intermediate layer is preferably 0.5 μm to 2 μm.

[0065] In this invention, the thickness of the polyamide layer is preferably 8 to 15 nm.

[0066] The present invention also provides the application of the polyamide composite nanofiltration membrane described above in desalination, wastewater treatment or resource recycling.

[0067] In this invention, the desalting is preferably antibiotic desalting.

[0068] The present invention does not impose specific limitations on the application of the polyamide composite nanofiltration membrane; any operation known to those skilled in the art can be used.

[0069] The polyamide composite nanofiltration membrane provided by this invention exhibits excellent nanofiltration performance, meaning it maintains high flux while also exhibiting high rejection rates for salts (especially sodium sulfate and magnesium sulfate), dye molecules (methylene blue), and antibiotic molecules (clindamycin phosphate and bacitracin). Based on the high rejection rate of antibiotic molecules, the polyamide composite nanofiltration membrane has the potential for desalination, particularly antibiotic desalination. The high salt rejection rate also enables its application in wastewater treatment. Furthermore, the high rejection rates of salts, dye molecules, and antibiotic molecules allow for the enrichment of these substances, facilitating resource recovery.

[0070] The following detailed description of the polyamide composite nanofiltration membrane, its preparation method, and its application provided by the present invention, with reference to specific embodiments, should not be construed as limiting the scope of protection of the present invention.

[0071] Example 1

[0072] (1) Magnesium chloride and aluminum chloride were dissolved in 200 mL of deionized water at a molar ratio of 2:1 to obtain a mixed salt solution, wherein the concentration of magnesium chloride was 0.2 mol / L and the concentration of aluminum chloride was 0.1 mol / L. At the same time, sodium hydroxide was dissolved in 400 mL of deionized water to prepare a sodium hydroxide solution with a concentration of 0.3 mol / L. 200 mL of the mixed salt solution and 400 mL of the sodium hydroxide solution were poured into a colloid mill at the same time, stirred at 3000 rpm for 3 min, and the sample was collected. The sample was centrifuged, the precipitate was collected and washed repeatedly 3 times to obtain hydrotalcite precipitate.

[0073] (2) The hydrotalcite precipitate was redispersed in water, the dispersion was placed in a polytetrafluoroethylene reactor, the polytetrafluoroethylene reactor was sealed, and the hydrotalcite was placed in a 100℃ oven for crystallization reaction for 24 hours to obtain hydrotalcite colloid (solid content of 1wt%), and the hydrotalcite colloid was stored in a sealed container.

[0074] (3) Dissolve piperazine in hydrotalcite colloid with a solid content of 1 wt% to obtain a piperazine-hydrotalcite mixture, wherein the concentration of piperazine is 0.5 wt%; dilute the piperazine-hydrotalcite mixture 10 times to obtain a precursor solution (wherein the concentration of piperazine is 0.05 wt% and the solid content is 0.1 wt%); uniformly coat 2 mL of the precursor solution onto a 2 cm radius circular nylon porous filter membrane (pore size 0.22 μm), let it stand for 1 min to filter the water in the precursor solution, and form an intermediate layer-porous base membrane composite membrane.

[0075] (4) Then, a 0.05wt% pyromellitic chloride solution (solvent is n-hexane) is dropped onto the intermediate layer side of the intermediate layer-porous base membrane composite membrane so that the pyromellitic chloride solution completely covers the surface of the intermediate layer-porous base membrane composite membrane and the interfacial polymerization reaction is carried out for 10s.

[0076] (4) After the interfacial polymerization reaction is completed, the residual liquid is removed by air knife, and then the membrane surface is cleaned with n-hexane to remove excess trimesoyl chloride, and polyamide composite nanofiltration membrane is obtained. The polyamide composite nanofiltration membrane is stored in deionized water for use.

[0077] Example 2

[0078] (1) Magnesium chloride and aluminum chloride were dissolved in 200 mL of deionized water at a molar ratio of 2:1 to obtain a mixed salt solution, wherein the concentration of magnesium chloride was 0.3 mol / L and the concentration of aluminum chloride was 0.15 mol / L. At the same time, sodium hydroxide was dissolved in 400 mL of deionized water to prepare a sodium hydroxide solution with a concentration of 0.45 mol / L. 200 mL of the mixed salt solution and 4000 mL of the sodium hydroxide solution were poured into a colloid mill at the same time, stirred at 3000 rpm for 3 min, and the sample was collected. The sample was centrifuged, the precipitate was collected and washed repeatedly 3 times to obtain hydrotalcite precipitate.

[0079] (2) The hydrotalcite precipitate was redispersed in water, the dispersion was placed in a polytetrafluoroethylene reactor, the polytetrafluoroethylene reactor was sealed, and the hydrotalcite was placed in a 100℃ oven for crystallization reaction for 24 hours to obtain hydrotalcite colloid (solid content of 1wt%), and the hydrotalcite colloid was stored in a sealed container.

[0080] (3) Dissolve piperazine in hydrotalcite colloid with a solid content of 1 wt% to obtain a piperazine-hydrotalcite mixture, wherein the concentration of piperazine is 0.75 wt%; dilute the piperazine-hydrotalcite mixture 10 times to obtain a precursor solution (wherein the concentration of piperazine is 0.075 wt% and the solid content is 0.1 wt%); uniformly coat 3 mL of the precursor solution onto a 2 cm radius circular nylon porous filter membrane (pore size 0.22 μm), let it stand for 1.5 min to filter the water in the precursor solution, and form an intermediate layer-porous base membrane composite membrane.

[0081] (4) Then, a 0.1 wt% pyromellitic chloride solution (solvent is n-hexane) is dropped onto the intermediate layer side of the intermediate layer-porous base membrane composite membrane so that the pyromellitic chloride solution completely covers the surface of the intermediate layer-porous base membrane composite membrane and the interfacial polymerization reaction is carried out for 20 s.

[0082] (5) After the interfacial polymerization reaction is completed, the residual liquid is removed by air knife, and then the membrane surface is cleaned with n-hexane solution to remove excess trimesoyl chloride, so as to obtain polyamide composite nanofiltration membrane; and the interfacial polymerization reaction is stored in deionized water for later use.

[0083] Example 3

[0084] (1) Magnesium chloride and aluminum chloride were dissolved in 200 mL of deionized water at a molar ratio of 2:1 to obtain a mixed salt solution, wherein the concentration of magnesium chloride was 0.2 mol / L and the concentration of aluminum chloride was 0.1 mol / L. At the same time, sodium hydroxide was dissolved in 400 mL of deionized water to prepare a sodium hydroxide solution with a concentration of 0.3 mol / L. 200 mL of the mixed salt solution and 400 mL of the sodium hydroxide solution were poured into a colloid mill at the same time, stirred at 3000 rpm for 3 min, and the sample was collected. The sample was centrifuged, the precipitate was collected and washed repeatedly 3 times to obtain hydrotalcite precipitate.

[0085] (2) The hydrotalcite precipitate was redispersed in water, and the dispersion was placed in a polytetrafluoroethylene reactor. After sealing the polytetrafluoroethylene reactor, it was placed in a 100°C oven for crystallization reaction for 24 hours to obtain hydrotalcite colloid (solid content of 1wt%). The hydrotalcite colloid was stored in a sealed container.

[0086] (3) Dissolve piperazine in hydrotalcite colloid with a solid content of 1 wt% to obtain a piperazine-hydrotalcite mixture, wherein the concentration of piperazine is 1 wt%. Dilute the piperazine-hydrotalcite mixture 5 times to obtain a precursor solution (wherein the concentration of piperazine is 0.2 wt% and the solid content is 0.2 wt%). Coat 2 mL of the precursor solution evenly on a 2 cm radius circular nylon porous filter membrane (pore size 0.22 μm), let it stand for 2 min to filter the water in the precursor solution, and form an intermediate layer-porous base membrane composite membrane.

[0087] (4) Then, a 0.05wt% pyromellitic chloride solution (solvent is n-hexane) is dropped onto the intermediate layer side of the intermediate layer-porous base membrane composite membrane so that the pyromellitic chloride solution completely covers the surface of the intermediate layer-porous base membrane composite membrane and the interfacial polymerization reaction is carried out for 10s.

[0088] (5) After the interfacial polymerization reaction is completed, the residual liquid is removed by air knife, and then the membrane surface is cleaned with n-hexane solution to remove excess trimesoyl chloride, and polyamide composite nanofiltration membrane is obtained. The polyamide composite nanofiltration membrane is stored in deionized water for use.

[0089] Comparative Example 1

[0090] The difference from Example 1 is that steps (1) and (2) are omitted.

[0091] (3) Prepare a 0.05wt% piperazine aqueous solution, and then uniformly coat 2mL of the 0.05wt% piperazine aqueous solution onto the surface of the porous base membrane. Let it stand for 1min to ensure that there is no excess water on the surface of the porous base membrane.

[0092] Steps (4) and (5) are the same as in Example 1.

[0093] Comparative Example 2

[0094] The difference from Example 1 is that steps (1) and (2) are omitted.

[0095] Exfoliated hydrotalcite nanosheets were obtained and dispersed in water to obtain a 0.1 wt% exfoliated hydrotalcite nanosheet dispersion. Piperazine was dissolved in water to form a 0.05 wt% piperazine solution. 2 mL of the exfoliated hydrotalcite nanosheet dispersion was uniformly coated onto a 2 cm radius nylon porous filter membrane (pore size 0.22 μm), and allowed to stand for 2 min to allow water in the exfoliated hydrotalcite nanosheet dispersion to filter through, forming an intermediate layer-porous base membrane composite membrane. Then, 2 mL of the 0.05 wt% piperazine solution was uniformly coated onto one side of the intermediate layer of the intermediate layer-porous base membrane composite membrane, and allowed to stand for 2 min to allow water in the piperazine solution to filter through, forming a piperazine-intermediate layer-porous base membrane composite membrane.

[0096] Steps (4) and (5) are the same as in Example 1.

[0097] Comparative Example 3

[0098] The difference from Comparative Example 2 is that the exfoliated hydrotalcite nanosheets were replaced with exfoliated graphene oxide nanosheets, and the other operations were the same as in Comparative Example 2.

[0099] Figure 1 The image shows the XRD pattern of the hydrotalcite colloid obtained in Example 1, where the horizontal axis represents 2Theta in degrees and the vertical axis represents intensity. Figure 1 It can be seen that the peak position of the XRD pattern is typical of hydrotalcite (LDH), where the interlayer anion is carbonate. This is because carbon dioxide in the air dissolves in water, and carbonate has the strongest affinity for LDH, so the interlayer ion is carbonate.

[0100] Figure 2 This is a SEM image of the intermediate layer in the intermediate layer-porous base membrane composite membrane obtained in Example 1. Figure 2 It can be seen that the surface of the Longduo porous filter membrane is completely covered by hydrotalcite nanosheets, and the thickness of the hydrotalcite intermediate layer is 1μm.

[0101] Figure 3 This is a planar SEM image of the polyamide composite nanofiltration membrane obtained in Example 1. Figure 3 It can be seen that a polyamide layer was prepared by interfacial polymerization on the LDH surface. The polyamide layer completely covers the LDH nanosheets and reflects the morphology of the underlying LDH.

[0102] Figure 4 This is a cross-sectional SEM image of the polyamide composite nanofiltration membrane obtained in Example 1. Figure 4 It can be seen that the polyamide layer above the LDH intermediate layer is extremely thin, only about 10nm thick.

[0103] The polyamide composite nanofiltration membranes prepared in the examples and comparative examples were sandwiched in a cross-flow filtration membrane cell for nanofiltration performance testing.

[0104] Four salts—sodium sulfate (Na₂SO₄), magnesium sulfate (Mg₂SO₄), magnesium chloride (MgCl₂), and sodium chloride (NaCl)—and an aqueous solution containing 40 ppm methylene blue, clindamycin phosphate, and bacitracin were selected as feed solutions to evaluate the nanofiltration performance of the membrane. The membrane permeability was calculated by collecting the volume of filtrate per unit time, expressed as flux, using the following formula:

[0105]

[0106] In Formula 1, P is the flux, with units of L / m. 2 •h·bar; V is the filtrate volume in L; M is the effective membrane area in m². 2 H is the filtration time, in hours (h); ΔP is the pressure difference across the membrane, in bars (bar).

[0107] The membrane's retention performance is calculated by measuring changes in the concentration of the filtrate and feed solution, and is expressed as the retention rate (R). The calculation formula is as follows:

[0108]

[0109] In Formula 2, R is the rejection rate, in %; C f and C p These are the concentrations of the raw material solution and the filtrate salt solution, respectively.

[0110] The results are shown in Table 1 and Figure 5 As shown.

[0111] Table 1. Nanofiltration performance of polyamide composite nanofiltration membranes prepared in the examples and comparative examples.

[0112]

[0113]

[0114] Figure 5 To illustrate the nanofiltration performance of the polyamide composite nanofiltration membrane obtained in Example 1 for different salts, from... Figure 5 It can be seen that the polyamide composite nanofiltration membrane obtained in Example 1 has a sodium sulfate rejection rate of 95.7% while maintaining excellent permeability performance, with a flux of 75.42 L / m³. 2 ·h·bar.

[0115] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle 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 polyamide composite nanofiltration membrane, characterized by, The method comprises the following steps: mixing a soluble magnesium salt, a soluble aluminum salt, an inorganic base and water to perform a nucleation reaction to obtain a hydrotalcite precipitate; dispersing the hydrotalcite precipitate in water to perform a crystallization reaction to obtain a hydrotalcite colloid; mixing the hydrotalcite colloid and piperazine to obtain a precursor solution; coating the precursor solution on a porous base film to perform film formation to form an intermediate layer-porous base film composite film; adding a trimesoyl chloride solution to the intermediate layer side of the intermediate layer-porous base film composite film to perform an interfacial polymerization reaction to obtain the polyamide composite nanofiltration membrane.

2. The production method according to claim 1, characterized by, The soluble magnesium salt comprises one or more of magnesium chloride, magnesium nitrate and magnesium sulfate, the soluble aluminum salt comprises one or more of aluminum chloride, aluminum nitrate and aluminum sulfate, and the inorganic base comprises sodium hydroxide and / or sodium carbonate.

3. The production method according to claim 1 or 2, characterized by, The molar ratio of the soluble magnesium salt to the soluble aluminum salt is 2-4:1, and the molar amount of the inorganic base is 2-2.1 times the total molar amount of the soluble magnesium salt and the soluble aluminum salt.

4. The method of claim 1, wherein, The nucleation reaction is performed in a colloid mill, and the time of the nucleation reaction is 2-5 min.

5. The preparation method according to claim 1, characterized in that, The temperature of the crystallization reaction is 60-120℃, and the time is 18-36 h; and the solid content of the hydrotalcite colloid is 0.1-4 wt%.

6. The method of claim 1, wherein, In the precursor solution, the solid content is 0.01-0.8 wt%, and the concentration of piperazine is 0.01-0.2 wt%.

7. The preparation method according to claim 6, characterized in that, The coating amount of the precursor solution is 0.1 to 0.5 mL / cm 2 .

8. The method of claim 1, wherein, The concentration of the trimesoyl chloride solution is 0.05-0.5 wt%, and the time of the interfacial polymerization reaction is 10-20 s.

9. The polyamide composite nanofiltration membrane prepared by the method according to any one of claims 1 to 8, characterized in that, The polyamide composite nanofiltration membrane comprises a porous base film, a hydrotalcite layer coated on the base film, and a polyamide layer attached to the hydrotalcite layer.

10. Use of the polyamide composite nanofiltration membrane of claim 9 in desalination or wastewater treatment.

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