Composite nanofiltration membrane, method for preparing the same, and use thereof

Abstract

The application relates to a composite nanofiltration membrane and a preparation method and application thereof, wherein the preparation method of the composite nanofiltration membrane comprises the following steps: preparing a pretreatment solution by mixing a quaternary ammonium salt modified water-soluble polymer, a crosslinking agent and water; preparing an organic acid phase solution by mixing a polyamine, an anionic surfactant and an organic acid, wherein the organic acid is a saturated aliphatic acid with 5-7 carbon atoms; placing the pretreatment solution on the surface of a support membrane to form a positively charged intermediate layer through a crosslinking reaction; and placing the organic acid phase solution and an oil phase solution containing a polyacyl chloride on the surface of the positively charged intermediate layer away from the support membrane in sequence to form a separation layer through heat treatment, thereby obtaining the composite nanofiltration membrane. When the composite nanofiltration membrane prepared by the preparation method is applied to water treatment, the composite nanofiltration membrane can simultaneously have the performances of high water flux and high rejection rate.

Description

Technical Field

[0001] This invention relates to the field of water treatment membrane technology, and in particular to composite nanofiltration membranes, their preparation methods, and applications. Background Technology

[0002] Nanofiltration membranes, as highly efficient pressure-driven membranes, have been widely used in drinking water softening, seawater desalination, wastewater reuse, material separation, and concentration due to their unique separation performance for divalent ions and small organic molecules. Currently, most nanofiltration membranes are prepared using interfacial polymerization. Traditional interfacial polymerization strictly relies on an "aqueous-oil phase" system. However, in the preparation of nanofiltration membranes using this system, the rapid diffusion of amine monomers in the aqueous solution often makes it difficult to precisely control the interfacial polymerization. Simultaneously, the high surface tension of the aqueous solution results in poor spreadability on hydrophobic or charged support membrane surfaces, leading to uneven thickness and structural defects in the formed polyamide layer. Ultimately, this results in a significant trade-off between retention rate and water flux in the prepared nanofiltration membrane, making it difficult to simultaneously achieve high water flux and high retention rate. Summary of the Invention

[0003] Therefore, it is necessary to provide a composite nanofiltration membrane, its preparation method, and its application to address the above problems. When the composite nanofiltration membrane prepared by this method is applied to water treatment, it can simultaneously exhibit high water flux and high rejection rate.

[0004] A method for preparing a composite nanofiltration membrane includes the following steps:

[0005] A pretreatment solution was prepared by combining a water-soluble polymer modified with a quaternary ammonium salt, a crosslinking agent, and water.

[0006] A polyamine, anionic surfactant, and organic acid are formulated into an organic acid phase solution, wherein the organic acid is a saturated fatty acid with 5-7 carbon atoms;

[0007] The pretreatment liquid is placed on the surface of the support membrane, and a positively charged intermediate layer is formed by a cross-linking reaction.

[0008] The organic acid phase solution and the oil phase solution containing polyacryl chloride are sequentially placed on the surface of the positively charged intermediate layer away from the supporting membrane, and a separation layer is formed by heat treatment to obtain a composite nanofiltration membrane.

[0009] In one embodiment, the mass fraction of the quaternary ammonium salt-modified water-soluble polymer in the pretreatment solution is 0.1%-2%;

[0010] And / or, the quaternary ammonium salt modified water-soluble polymer is selected from at least one of chitosan quaternary ammonium salt, quaternized cellulose, and quaternized polyethyleneimine.

[0011] In one embodiment, the mass fraction of the crosslinking agent in the pretreatment solution is 0.1%-2%;

[0012] And / or, the crosslinking agent is selected from dialdehydes, wherein the dialdehyde is selected from at least one of glutaraldehyde, glyoxal, malondialdehyde, or butanedialdehyde.

[0013] In one embodiment, the mass fraction of the anionic surfactant in the organic acid phase solution is 0.02%-0.5%;

[0014] And / or, the anionic surfactant is selected from at least one of sodium dodecyl sulfate, sodium tetradecyl sulfate, sodium dodecylbenzene sulfonate, or sodium dioctyl sulfosuccinate.

[0015] In one embodiment, the organic acid is selected from at least one of valeric acid, hexanoic acid, isohexanoic acid, or heptanoic acid.

[0016] In one embodiment, the mass fraction of the polyamine in the organic acid phase solution is 0.1%-2%;

[0017] And / or, the polyamine is selected from at least one of piperazine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, N,N-dimethylm-phenylenediamine, pyromellitic triamine, cyclohexanediamine, ethylenediamine, triethanolamine, polyethyleneimine, polyetheramine, aniline, m-toluidine, o-toluidine, p-nitroaniline, or m-fluoroaniline.

[0018] In one embodiment, the mass fraction of the polyacrylamide chloride in the oil phase solution containing polyacrylamide chloride is 0.05%-0.2%;

[0019] And / or, the polyacryl chloride is selected from at least one of pyromellitic chloroformyl chloride, terephthaloyl chloride, or isophthaloyl chloride.

[0020] In one embodiment, in the step of forming a positively charged intermediate layer by cross-linking reaction, the cross-linking reaction temperature is 25°C-40°C and the cross-linking reaction time is 1 min-10 min.

[0021] And / or, the specific steps for sequentially placing the organic acid phase solution and the oil phase solution containing polyacryl chloride on the surface of the positively charged intermediate layer away from the supporting film are as follows: First, place the organic acid phase solution on the surface of the positively charged intermediate layer away from the supporting film, let it stand for 30s-180s, and then remove the excess organic acid phase solution; then place the oil phase solution containing polyacryl chloride on the surface of the positively charged intermediate layer away from the supporting film, let it stand for 20s-60s, and then remove the excess oil phase solution containing polyacryl chloride.

[0022] And / or, in the step of forming a separation layer by heat treatment, the heat treatment temperature is 60℃-110℃ and the heat treatment time is 3min-10min.

[0023] A composite nanofiltration membrane prepared using the aforementioned method.

[0024] Application of the aforementioned composite nanofiltration membrane in a water treatment device.

[0025] In the preparation method of the composite nanofiltration membrane of the present invention, a water-soluble polymer containing quaternary ammonium salt and a crosslinking agent are first reacted on the surface of a supporting membrane to form a positively charged intermediate layer with a three-dimensional crosslinking network. Subsequently, an organic acid phase solution with a specific organic acid as a solvent is introduced. On the one hand, the low surface tension of the organic acid enables the organic acid phase solution to achieve super-spreading and wetting on the surface of the positively charged intermediate layer, forming an ultra-thin and uniform monomer pre-placed layer. On the other hand, the organic acid molecules penetrate into the interior of the positively charged intermediate layer network, triggering a "controllable swelling" effect, which changes its microstructure from dense to moderately loose and porous. This not only increases the mass transfer channels of water molecules to improve water flux, but also exposes a large number of masked quaternary ammonium salt groups and active sites, enhancing the efficient anchoring of polyamines and anionic surfactants. Meanwhile, the anionic surfactant in the organic acid phase solution guides the organic acid to penetrate into the pores of the positively charged intermediate layer through electrostatic interaction, enhancing the swelling effect. It then self-assembles into an ordered monolayer on the surface of the positively charged intermediate layer, achieving in-situ modification and activation of the reaction interface. Furthermore, this monolayer structure further optimizes the uniformity of the organic acid phase solution's spread in the positively charged intermediate layer, allowing the polyamine to effectively accumulate at the interface and act as a soft template to guide the subsequent ordered cross-linking and stacking of the polyamide. Finally, when the oil phase solution is introduced, the "compatible but not completely miscible" property of the organic acid and oil phase solution is utilized to form a diffusion region with a viscosity gradient at the liquid-liquid interface. This diffusion region effectively slows down the diffusion rate of the polyamine into the oil phase solution, producing a "slow-release" effect, making the interfacial polymerization reaction more mild and controllable. Ultimately, a polyamide separation layer with nanoscale wrinkles, a large specific surface area, and a moderate degree of cross-linking is successfully constructed, achieving a synergistic improvement in the water flux and rejection rate of the composite nanofiltration membrane.

[0026] Therefore, the composite nanofiltration membrane prepared by this invention can simultaneously exhibit high water flux and high rejection rate when applied to water treatment. Detailed Implementation

[0027] To facilitate understanding of the present invention, it will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. Rather, these embodiments or examples are provided to make the disclosure of the present invention more thorough and complete.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments or examples only and is not intended to limit the invention. The optional range of the term "and / or" as used herein includes any one of two or more of the related listed items, as well as any and all combinations of the related listed items, including any two related listed items, any more related listed items, or a combination of all related listed items.

[0029] The method for preparing the composite nanofiltration membrane provided by the present invention includes the following steps:

[0030] A pretreatment solution was prepared by combining a water-soluble polymer modified with a quaternary ammonium salt, a crosslinking agent, and water.

[0031] A polyamine, anionic surfactant, and organic acid are formulated into an organic acid phase solution, wherein the organic acid is a saturated fatty acid with 5-7 carbon atoms;

[0032] The pretreatment liquid is placed on the surface of the support membrane, and a positively charged intermediate layer is formed by a cross-linking reaction.

[0033] The organic acid phase solution and the oil phase solution containing polyacryl chloride are sequentially placed on the surface of the positively charged intermediate layer away from the supporting membrane, and a separation layer is formed by heat treatment to obtain a composite nanofiltration membrane.

[0034] Specifically, when the pretreatment solution is placed on the surface of the support membrane, the quaternary ammonium salt-modified water-soluble polymer itself has positive charge and abundant hydrophilic groups such as hydroxyl or amino groups, causing it to undergo a cross-linking reaction with the cross-linking agent to form a positively charged intermediate layer with a three-dimensional cross-linked network structure. Moreover, the polymer segments of the positively charged intermediate layer formed by the cross-linking reaction interpenetrate on the surface of the support membrane and extend into the pores of the support membrane, improving the bonding strength between the positively charged intermediate layer and the support membrane.

[0035] When the organic acid phase solution is placed on the surface of the positively charged intermediate layer away from the supporting membrane, the use of a specific organic acid (saturated fatty acid with 5-7 carbon atoms) as a solvent allows for the following benefits: Firstly, the low surface tension of the organic acid phase solution enables super-spreading and wetting on the surface of the positively charged intermediate layer, forming an ultra-thin and uniform monomer pre-placed layer. This provides a foundation for achieving a low-defect separation layer structure. Secondly, organic acid molecules can penetrate into the positively charged intermediate layer network, generating a unique "controllable swelling" effect. This makes the polymer chains of the positively charged intermediate layer more extended, and its microstructure changes from dense to moderately loose and porous. This not only increases the water molecule mass transfer channels to improve water flux but also exposes a large number of masked quaternary ammonium salt positively charged groups and active sites. This enhances the adsorption capacity and strength of polyamines and anionic surfactants in the organic acid phase solution, i.e., it strengthens the efficient anchoring of polyamines and anionic surfactants, providing a foundation for forming a uniform and defect-free separation layer. Meanwhile, the anionic surfactant, through its electrostatic interaction with the positively charged interlayer, can, on the one hand, further guide the organic acid to penetrate into the pores of the positively charged interlayer, thereby enhancing the swelling effect of the organic acid on the positively charged interlayer. This also facilitates a tighter bond between the polyamide separation layer formed by subsequent interfacial polymerization and the positively charged interlayer. On the other hand, it can rapidly anchor the surfactant on the surface of the positively charged interlayer, self-assembling to form an ordered monolayer, achieving in-situ modification and activation of the reaction interface. Moreover, this monolayer structure can further optimize the spreading uniformity of the organic acid phase solution on the surface of the positively charged interlayer, allowing the polyamine to be effectively enriched at the interface. Furthermore, it can serve as a soft template for subsequent interfacial reactions, guiding the orderly cross-linking and stacking of the polyamide to form an ultrathin separation layer with a better charge distribution.

[0036] When the oil phase solution containing polyacrylamide chloride is placed on the surface of the positively charged intermediate layer away from the supporting membrane, the organic acid and the oil phase solution have the characteristics of being "compatible but not completely miscible". This results in the formation of a diffusion zone with a viscosity gradient at the interface between the organic acid phase solution and the oil phase solution. This diffusion zone can effectively slow down the diffusion rate of polyamine into the oil phase solution, producing a "slow-release" effect. This makes the interfacial polymerization reaction more mild and controllable, and ultimately successfully constructs a polyamide separation layer with nanoscale wrinkles, a large specific surface area and a moderate degree of crosslinking, achieving a synergistic improvement in the water flux and rejection rate of the composite nanofiltration membrane.

[0037] It is understood that in this invention, an organic acid phase prepared by using saturated fatty acids with a specific carbon chain length as a solvent is used to replace the traditional aqueous phase, forming an organic acid phase-oil phase system. This system, together with a carefully designed positively charged intermediate layer and anionic surfactant, forms multiple synergistic effects, thereby achieving precise control of interfacial polymerization kinetics and preparing a composite nanofiltration membrane with both high water flux and high rejection rate.

[0038] Therefore, the composite nanofiltration membrane prepared by this invention can simultaneously exhibit high water flux and high rejection rate when applied to water treatment.

[0039] In addition, the preparation method of the composite nanofiltration membrane of the present invention is simple, easy to control, and has good reproducibility, which is conducive to realizing industrial production.

[0040] Optionally, the mass fraction of the quaternary ammonium salt-modified water-soluble polymer in the pretreatment solution is 0.1%-2%; the mass fraction of the crosslinking agent in the pretreatment solution is 0.1%-2%. With this setting, the crosslinking density of the quaternary ammonium salt-modified water-soluble polymer and the crosslinking agent can be controlled by adjusting the mass fraction of the quaternary ammonium salt-modified water-soluble polymer and the crosslinking agent in the pretreatment solution. This allows the polymer chain segments formed to be firmly inserted into the support membrane and form a continuous and complete three-dimensional crosslinked network structure with a positively charged intermediate layer. This results in a positively charged intermediate layer with a stable, permanent, and uniform positive charge, providing a better interface for the subsequent organic acid phase solution. This effectively avoids excessive penetration of the organic acid phase solution and promotes the formation of a better polyamide structure.

[0041] Optionally, the quaternary ammonium salt modified water-soluble polymer is selected from at least one of chitosan quaternary ammonium salt, quaternized cellulose, and quaternized polyethyleneimine, preferably chitosan quaternary ammonium salt.

[0042] Optionally, the crosslinking agent is selected from dialdehydes, wherein the dialdehyde is selected from at least one of glutaraldehyde, glyoxal, malondialdehyde or butanedialdehyde, preferably glutaraldehyde.

[0043] Optionally, the mass fraction of the anionic surfactant in the organic acid phase solution is 0.02%-0.5%. By controlling the mass fraction of the anionic surfactant in the organic acid phase solution, the organic acid can be better guided to penetrate into the network structure of the positively charged intermediate layer through electrostatic interaction and undergo controlled swelling. At the same time, a dense and ordered monolayer is formed on the surface of the positively charged intermediate layer through self-assembly. This not only enables better in-situ modification and activation of the reaction interface, but also better enriches polyamine monomers and guides the orderly cross-linking and stacking of polyamides. This is conducive to the formation of an ultrathin, uniform, and low-defect separation layer, thereby improving the water flux and rejection rate of the composite nanofiltration membrane.

[0044] Furthermore, the anionic surfactant is selected from at least one of sodium dodecyl sulfate, sodium tetradecyl sulfate, sodium dodecylbenzene sulfonate, or sodium dioctyl sulfosuccinate, preferably sodium dodecyl sulfate.

[0045] Optionally, the organic acid is selected from at least one of valeric acid, hexanoic acid, isohexanoic acid, or heptanoic acid, preferably hexanoic acid. This configuration better ensures that the organic acid phase solution and the oil phase solution have the characteristics of being "compatible but not completely miscible," thereby better forming a diffusion layer with a viscosity gradient at the interface between the two phases. This effectively slows down the diffusion rate of the polyamine monomer, producing a "slow-release" effect, and thus better achieves precise control of the interfacial polymerization kinetics. It promotes the formation of a polyamide separation layer with more nanoscale wrinkles, a larger specific surface area, and a moderate degree of crosslinking, thereby better improving the water flux and retention rate of the composite nanofiltration membrane.

[0046] Optionally, the mass fraction of the polyamine in the organic acid phase solution is 0.1%-2%; the mass fraction of the polyacrylamide in the oil phase solution containing polyacrylamide is 0.05%-0.2%. This arrangement facilitates a full reaction between the polyamine and the polyacrylamide to form a dense, uniform, and structurally complete polyamide separation layer, which helps to better ensure that the composite nanofiltration membrane has both high water flux and high rejection rate.

[0047] Further, the polyamine is selected from at least one of piperazine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, N,N-dimethylm-phenylenediamine, pyromellitic triamine, cyclohexanediamine, ethylenediamine, triethanolamine, polyethyleneimine, polyetheramine, aniline, m-toluidine, o-toluidine, p-nitroaniline, or m-fluoroaniline, preferably piperazine; the polyacrylamide is selected from at least one of pyromellitic trimethylbenzene chloride, terephthaloyl chloride, or isophthaloyl chloride, preferably pyromellitic trimethylbenzene chloride.

[0048] In one embodiment, the solvent in the oil phase solution containing polyacrylamide chloride is selected from at least one of Isopar-G, Isopar-L, n-hexane, and cyclohexane, preferably Isopar-L.

[0049] It should be noted that, in the pretreatment solution of the present invention, although the water-soluble polymer modified with quaternary ammonium salt and the crosslinking agent can spontaneously proceed at room temperature, there is a certain reaction window period. That is, in the prepared pretreatment solution, the two do not undergo significant crosslinking reaction, and their crosslinking reaction mainly takes place on the support membrane.

[0050] Optionally, in the step of forming a positively charged intermediate layer through a cross-linking reaction, the cross-linking reaction temperature is 25℃-40℃, preferably 35℃-40℃, and the cross-linking reaction time is 1min-10min, preferably 1min-6min; such settings can further ensure the integrity and uniformity of the cross-linking of the positively charged intermediate layer.

[0051] In this invention, the specific steps for sequentially placing the organic acid phase solution and the oil phase solution containing polyacrylamide chloride on the surface of the positively charged intermediate layer away from the supporting membrane are as follows: First, place the organic acid phase solution on the surface of the positively charged intermediate layer away from the supporting membrane, let it stand for 30s-180s, and then remove the excess organic acid phase solution. Then, place the oil phase solution containing polyacrylamide chloride on the surface of the positively charged intermediate layer away from the supporting membrane, let it stand for 20s-60s, and then remove the excess oil phase solution containing polyacrylamide chloride. This arrangement allows the organic acid to better penetrate and swell the intermediate layer structure, while ensuring the effective adsorption of polyamines and anionic surfactants. On the other hand, it allows for better utilization of the "slow-release" effect at the two-phase interface to achieve controllable interfacial polymerization, ultimately forming a polyamide separation layer with an ideal structure. Therefore, by precisely controlling the action time of each step, this invention ensures the full enrichment of polyamines at the interface and their orderly interfacial reaction with polyacrylamide chlorides. This facilitates the better acquisition of a polyamide separation layer with nanoscale wrinkles, a large specific surface area, and a moderate degree of crosslinking, thereby achieving a synergistic improvement in the water flux and retention rate of the composite nanofiltration membrane.

[0052] In one embodiment, the organic acid phase solution and the oil phase solution containing polyacrylamide chloride are placed sequentially on the surface of the positively charged intermediate layer away from the support film by means of immersion, soaking, or coating.

[0053] The present invention does not specifically limit the type of the supporting membrane. Specifically, in one embodiment, the supporting membrane is selected from polymer ultrafiltration membranes, preferably any one of polysulfone ultrafiltration membranes, polyethersulfone ultrafiltration membranes, polyvinylidene fluoride ultrafiltration membranes, polyethylene ultrafiltration membranes, polypropylene ultrafiltration membranes, or polyacrylonitrile ultrafiltration membranes, and more preferably polysulfone ultrafiltration membranes or polyethersulfone ultrafiltration membranes.

[0054] Optionally, in the step of forming the separation layer by heat treatment, the heat treatment temperature is 60℃-110℃ and the heat treatment time is 3min-10min; this setting can further ensure the integrity and uniformity of the cross-linking of the separation layer, further improve the rejection rate and water flux of the composite nanofiltration membrane, and remove residual solvent at the same time.

[0055] Furthermore, this invention also provides a composite nanofiltration membrane prepared using the aforementioned method. When applied to water treatment, this composite nanofiltration membrane exhibits both high water flux and high rejection rate.

[0056] Furthermore, the present invention also provides an application of the aforementioned composite nanofiltration membrane in a water treatment device.

[0057] In one embodiment, the water treatment device can be a hard water softening device. When the composite nanofiltration membrane is applied in the hard water softening device, the raw water to be softened permeates through the separation layer side of the composite nanofiltration membrane under pressure. In this process, hardening ions such as calcium ions and magnesium ions in the hard water can be efficiently retained, achieving an excellent softening effect.

[0058] In one embodiment, the water treatment device may also be a water purifier or a seawater desalination device.

[0059] The composite nanofiltration membrane, its preparation method, and its applications will be further described below through specific embodiments. However, those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified, specific conditions in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0060] Example 1

[0061] Chitosan quaternary ammonium salt, glutaraldehyde, and water were mixed evenly to prepare a pretreatment solution, wherein the mass fraction of chitosan quaternary ammonium salt and glutaraldehyde in the pretreatment solution was 1%; piperazine, sodium dodecyl sulfate, and hexanoic acid were mixed evenly to prepare an organic acid phase solution, wherein the mass fraction of piperazine and sodium dodecyl sulfate in the organic acid phase solution was 0.2%; and pyromellitic acid chloride and Isopar L solvent were mixed evenly to prepare an oil phase solution, wherein the mass fraction of pyromellitic acid chloride in the oil phase solution was 0.1%.

[0062] The above pretreatment solution was applied to the polysulfone ultrafiltration membrane and allowed to stand at 35°C for 5 minutes for crosslinking reaction. Then, the excess pretreatment solution was removed to form a positively charged intermediate layer. The above organic acid phase solution was then applied to the surface of the positively charged intermediate layer and allowed to stand for 60 seconds before the excess organic acid phase solution was discarded. Next, the above oil phase solution was applied to the surface of the positively charged intermediate layer and allowed to stand for 30 seconds before the excess oil phase solution was discarded. Finally, the membrane was placed in a 100°C oven for 4 minutes for heat treatment to form a separation layer, thus obtaining a composite nanofiltration membrane.

[0063] Example 2

[0064] Quaternized polyethyleneimine, glyoxal, and water were mixed evenly to prepare a pretreatment solution, wherein the mass fraction of quaternized polyethyleneimine and glyoxal in the pretreatment solution was 2%; piperazine, sodium dodecylbenzenesulfonate, and n-valeric acid were mixed evenly to prepare an organic acid phase solution, wherein the mass fraction of piperazine and sodium dodecylbenzenesulfonate in the organic acid phase solution was 1%; and pyromellitic acid chloride and n-hexane solvent were mixed evenly to prepare an oil phase solution, wherein the mass fraction of pyromellitic acid chloride in the oil phase solution was 0.15%.

[0065] The above pretreatment solution was coated onto the polysulfone ultrafiltration membrane. After standing at room temperature for 10 minutes for crosslinking reaction, the excess pretreatment solution was removed to form a positively charged intermediate layer. Then, the above organic acid phase solution was coated onto the surface of the positively charged intermediate layer. After standing for 180 seconds, the excess organic acid phase solution was poured off. Next, the above oil phase solution was coated onto the surface of the positively charged intermediate layer. After standing for 60 seconds, the excess oil phase solution was poured off. Finally, the membrane was placed in a 60°C oven for 10 minutes for heat treatment to form a separation layer, thus obtaining a composite nanofiltration membrane.

[0066] Example 3

[0067] Quaternized cellulose, malondialdehyde, and water were mixed evenly to prepare a pretreatment solution, wherein the mass fraction of quaternized cellulose was 0.1% and the mass fraction of malondialdehyde was 0.1%. Piperazine, sodium tetradecyl sulfate, and heptanoic acid were mixed evenly to prepare an organic acid phase solution, wherein the mass fraction of piperazine was 2% and the mass fraction of sodium tetradecyl sulfate was 0.25%. Trimethylbenzene chloride and Isopar L solvent were mixed evenly to prepare an oil phase solution, wherein the mass fraction of trimethylbenzene chloride in the oil phase solution was 0.2%.

[0068] The above pretreatment solution was applied to the polysulfone ultrafiltration membrane and allowed to stand at 40°C for 2 minutes for crosslinking reaction. Then, the excess pretreatment solution was removed to form a positively charged intermediate layer. The above organic acid phase solution was then applied to the surface of the positively charged intermediate layer and allowed to stand for 30 seconds before the excess organic acid phase solution was discarded. Next, the above oil phase solution was applied to the surface of the positively charged intermediate layer and allowed to stand for 20 seconds before the excess oil phase solution was discarded. Finally, the membrane was placed in an 80°C oven for 5 minutes for heat treatment to form a separation layer, thus obtaining a composite nanofiltration membrane.

[0069] Example 4

[0070] Compared with Example 1, Example 4 differs only in that the mass fraction of sodium dodecyl sulfate in the organic acid phase solution is 0.15% in the step of preparing the organic acid phase solution; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0071] Example 5

[0072] Compared with Example 1, Example 5 differs only in that the mass fraction of chitosan quaternary ammonium salt in the pretreatment solution is 0.05% in the pretreatment solution preparation step; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0073] Example 6

[0074] Compared with Example 1, Example 6 differs only in that the mass fraction of chitosan quaternary ammonium salt in the pretreatment solution is 2.5% in the pretreatment solution preparation step; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0075] Example 7

[0076] Compared with Example 1, Example 7 differs only in that the mass fraction of glutaraldehyde in the pretreatment solution is 0.05% in the pretreatment solution; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0077] Example 8

[0078] Compared with Example 1, Example 8 differs only in that the mass fraction of glutaraldehyde in the pretreatment solution is 2.5% in the pretreatment solution; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0079] Example 9

[0080] Compared with Example 1, Example 9 differs only in that the mass fraction of sodium dodecyl sulfate in the organic acid phase solution is 0.01% in the step of preparing the organic acid phase solution; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0081] Example 10

[0082] Compared with Example 1, Example 10 differs only in that the mass fraction of sodium dodecyl sulfate in the organic acid phase solution is 1% in the step of preparing the organic acid phase solution; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0083] Example 11

[0084] Compared with Example 1, Example 11 differs only in that the above pretreatment solution is applied to the polysulfone ultrafiltration membrane, and after standing at 35°C for 30 seconds for crosslinking reaction, the excess pretreatment solution is removed to form a positively charged intermediate layer; all other conditions are the same to obtain a composite nanofiltration membrane.

[0085] Example 12

[0086] Example 12 differs from Example 1 only in that the above organic acid phase solution is coated on the surface of the positively charged intermediate layer, and after standing for 20 seconds, the excess organic acid phase solution is poured off; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0087] Comparative Example 1

[0088] Compared with Example 1, Comparative Example 1 differs only in that an aqueous solution is used instead of an organic acid solution. Specifically, piperazine, sodium dodecyl sulfate, and water are mixed evenly to prepare an aqueous solution. In the aqueous solution, the mass fraction of piperazine is 0.2%, and the mass fraction of sodium dodecyl sulfate is 0.05%. All other conditions are the same, and a composite nanofiltration membrane is obtained.

[0089] Comparative Example 2

[0090] Compared with Example 1, Comparative Example 2 differs only in that sodium dodecyl sulfate is not added in the step of preparing the organic acid phase solution; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0091] Comparative Example 3

[0092] Comparative Example 3 differs from Example 1 only in that it does not include the steps of preparing a pretreatment solution and forming a positively charged intermediate layer; that is, the above-mentioned organic acid phase solution is directly coated onto the polysulfone ultrafiltration membrane, and after standing for 60 seconds, the excess organic acid phase solution is poured off. Then, the above-mentioned oil phase solution is coated onto the surface of the polysulfone ultrafiltration membrane containing the organic acid phase solution, and after standing for 30 seconds, the excess oil phase solution is poured off. Finally, it is placed in a 100°C oven for 4 minutes for heat treatment to form a separation layer; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0093] Comparative Example 4

[0094] Compared with Example 1, Comparative Example 4 differs only in that chitosan is used instead of chitosan quaternary ammonium salt in the step of preparing the pretreatment solution; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0095] Comparative Example 5

[0096] Compared with Example 1, Comparative Example 5 differs only in that acetic acid is used instead of hexanoic acid in the step of preparing the organic acid phase solution; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0097] Comparative Example 6

[0098] Compared with Example 1, Comparative Example 6 differs only in that octanoic acid is used instead of hexanoic acid in the step of preparing the organic acid phase solution; all other conditions are the same, and a composite nanofiltration membrane is obtained.

[0099] The composite nanofiltration membranes prepared in Examples 1 to 12 and Comparative Examples 1 to 6 were subjected to performance tests. The test conditions were as follows: test pressure was 0.5 MPa, concentrate flow rate was 1.0 GPM, ambient temperature was 25°C, concentrate pH was 6.5-7.5, and concentrate was 2000 mg of magnesium chloride aqueous solution. The test results are shown in Table 1.

[0100] Table 1

[0101]

[0102] It should be noted that in Table 1, the membrane water flux (F) is calculated from the volume of water passing through a unit area of ​​the composite nanofiltration membrane per unit time, using the following formula: Where V is the volume of water permeated, A is the effective membrane area, and T is time.

[0103] The rejection rate (R) is defined as the percentage value of the concentration difference between the feed liquid and the permeate, relative to the feed liquid concentration. The calculation formula is as follows: Where C1 is the concentration of the permeate and C0 is the concentration of the feed liquid.

[0104] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0105] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for preparing a composite nanofiltration membrane, characterized in that, Includes the following steps: A pretreatment solution was prepared by combining a water-soluble polymer modified with a quaternary ammonium salt, a crosslinking agent, and water. A polyamine, anionic surfactant, and organic acid are formulated into an organic acid phase solution, wherein the organic acid is a saturated fatty acid with 5-7 carbon atoms; The pretreatment liquid is placed on the surface of the support membrane, and a positively charged intermediate layer is formed by a cross-linking reaction. The organic acid phase solution and the oil phase solution containing polyacryl chloride are sequentially placed on the surface of the positively charged intermediate layer away from the supporting membrane, and a separation layer is formed by heat treatment to obtain a composite nanofiltration membrane.

2. The method for preparing the composite nanofiltration membrane according to claim 1, characterized in that, The mass fraction of the quaternary ammonium salt-modified water-soluble polymer in the pretreatment solution is 0.1%-2%; And / or, the quaternary ammonium salt modified water-soluble polymer is selected from at least one of chitosan quaternary ammonium salt, quaternized cellulose, or quaternized polyethyleneimine.

3. The method for preparing the composite nanofiltration membrane according to claim 1, characterized in that, The mass fraction of the crosslinking agent in the pretreatment solution is 0.1%-2%; And / or, the crosslinking agent is selected from dialdehydes, wherein the dialdehyde is selected from at least one of glutaraldehyde, glyoxal, malondialdehyde, or butanedialdehyde.

4. The method for preparing the composite nanofiltration membrane according to claim 1, characterized in that, The mass fraction of the anionic surfactant in the organic acid phase solution is 0.02%-0.5%; And / or, the anionic surfactant is selected from at least one of sodium dodecyl sulfate, sodium tetradecyl sulfate, sodium dodecylbenzene sulfonate, or sodium dioctyl sulfosuccinate.

5. The method for preparing the composite nanofiltration membrane according to claim 1, characterized in that, The organic acid is selected from at least one of valeric acid, hexanoic acid, isohexanoic acid, or heptanoic acid.

6. The method for preparing the composite nanofiltration membrane according to claim 1, characterized in that, The mass fraction of the polyamine in the organic acid phase solution is 0.1%-2%; And / or, the polyamine is selected from at least one of piperazine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, N,N-dimethylm-phenylenediamine, pyromellitic triamine, cyclohexanediamine, ethylenediamine, triethanolamine, polyethyleneimine, polyetheramine, aniline, m-toluidine, o-toluidine, p-nitroaniline, or m-fluoroaniline.

7. The method for preparing the composite nanofiltration membrane according to claim 1, characterized in that, The mass fraction of the polyacrylamide chloride in the oil phase solution containing polyacrylamide chloride is 0.05%-0.2%; And / or, the polyacryl chloride is selected from at least one of pyromellitic chloroformyl chloride, terephthaloyl chloride, or isophthaloyl chloride.

8. The method for preparing the composite nanofiltration membrane according to any one of claims 1 to 7, characterized in that, In the step of forming a positively charged intermediate layer via cross-linking reaction, the cross-linking reaction temperature is 25℃-40℃ and the cross-linking reaction time is 1min-10min. And / or, the specific steps for sequentially placing the organic acid phase solution and the oil phase solution containing polyacryl chloride on the surface of the positively charged intermediate layer away from the supporting film are as follows: First, place the organic acid phase solution on the surface of the positively charged intermediate layer away from the supporting film, let it stand for 30s-180s, and then remove the excess organic acid phase solution; then place the oil phase solution containing polyacryl chloride on the surface of the positively charged intermediate layer away from the supporting film, let it stand for 20s-60s, and then remove the excess oil phase solution containing polyacryl chloride. And / or, in the step of forming a separation layer by heat treatment, the heat treatment temperature is 60℃-110℃ and the heat treatment time is 3min-10min.

9. A composite nanofiltration membrane prepared by the method described in any one of claims 1 to 8.

10. The application of the composite nanofiltration membrane as described in claim 9 in a water treatment device.

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