A nanofiltration membrane and its preparation method

Abstract

This application discloses a nanofiltration membrane and its preparation method. The nanofiltration membrane consists of a porous polysulfone-based ultrafiltration membrane, an organic ligand-metal coordination interlayer, and a polysulfonamide separation layer. The organic ligand-metal coordination interlayer is disposed on the surface of the porous polysulfone-based ultrafiltration membrane, and the polysulfonamide separation layer is disposed on the surface of the organic ligand-metal coordination interlayer. The nanofiltration membrane prepared in this application exhibits excellent acid resistance, high water flux, high rejection rate, and long-term stability. After operating in an H₂SO₄ solution at pH=2 for 500 hours, the rejection rate remains above 95%, and the flux decline is less than 5%, making it particularly suitable for separation processes in strongly acidic environments.

Description

Technical Field

[0001] This application belongs to the field of membrane separation technology, specifically relating to a nanofiltration membrane and its preparation method. Background Technology

[0002] Nanofiltration membranes, as a membrane separation technology with separation precision between ultrafiltration and reverse osmosis, have been widely used in water treatment, material separation and concentration, and resource recovery. However, in separation processes under acidic environments, such as the treatment of acidic wastewater from metal smelting, concentration of acidic dyes, and extraction of rare earth elements, traditional polyamide nanofiltration membranes suffer from a sharp decline in membrane performance due to the easy hydrolysis of amide bonds under acidic conditions, severely limiting their application in acidic environments. Summary of the Invention

[0003] This application aims to provide a nanofiltration membrane and its preparation method. The prepared nanofiltration membrane has excellent acid resistance, high water flux, high rejection rate and long-term stability, and is particularly suitable for separation processes in strongly acidic environments, thus solving many problems existing in traditional polyamide nanofiltration membranes.

[0004] To solve the above-mentioned technical problems, this application is implemented as follows: In a first aspect, embodiments of this application provide a nanofiltration membrane, which is composed of a porous polysulfone-based ultrafiltration membrane, an organic ligand-metal coordination intermediate layer, and a polysulfonamide separation layer; The organic ligand-metal coordination intermediate layer is disposed on the surface of the porous polysulfone ultrafiltration membrane, and the polysulfonamide separation layer is disposed on the surface of the organic ligand-metal coordination intermediate layer.

[0005] Optionally, the porous polysulfone-based ultrafiltration membrane includes a porous polysulfone ultrafiltration membrane and a porous polyethersulfone ultrafiltration membrane.

[0006] Optionally, in the organic ligand-metal coordination intermediate layer, the organic ligand is selected from one of the aromatic polyphenol compounds, and the metal is selected from at least one of iron, titanium, and zirconium.

[0007] Secondly, embodiments of this application provide a method for preparing the nanofiltration membrane, the method comprising: A porous polysulfone-based ultrafiltration membrane that has undergone hydrophilization treatment is immersed in an organic ligand solution. The resulting impregnated body is then immersed in a metal salt solution. After the organic ligand undergoes a coordination reaction with the metal ions, an organic ligand-metal coordination intermediate layer is formed on the surface of the porous polysulfone-based ultrafiltration membrane, thus obtaining a first composite membrane. The first composite membrane is immersed in an aqueous solution, then removed and immersed in an organic solution, wherein the aqueous phase and the oil phase undergo an interfacial polymerization reaction to form a polysulfonamide separation layer on the surface of the organic ligand-metal coordination intermediate layer. The obtained second composite membrane is then heat-treated to obtain the nanofiltration membrane.

[0008] Optionally, in the organic ligand solution, the organic ligand is selected from at least one of aromatic polyphenolic compounds; The organic ligand has a mass fraction of 0.5%-2.0%.

[0009] Optionally, the organic ligand is selected from at least one of catechol, pyrogallol, tannic acid, gallic acid, gallic acid-modified cyclodextrin, and pyrogallol.

[0010] Optionally, in the metal salt solution, the metal salt is selected from at least one of ferric chloride, zirconium oxychloride, and titanium tetrachloride; The mass fraction of the metal salt is 0.1%-0.5%.

[0011] Optionally, the aqueous solution contains 0.5%-2.0% by mass of a polyamine monomer; The polyamine monomer is selected from at least one of triethylenetetramine, polyethyleneimine, and piperazine.

[0012] Optionally, the organic phase solution is composed of a poly(sulfonyl chloride) monomer and an organic solvent; The polysulfonyl chloride monomer is selected from 1,3,6-naphthalenetrisulfonyl chloride or 1,3,5-benzenetrisulfonyl chloride; The organic solvent is selected from at least one of n-hexane, cyclohexane, and Isopar G; The mass fraction of the polysulfonyl chloride monomer is 0.01%-0.3%.

[0013] Optionally, the aqueous solution further comprises terephthalic acid; The mass fraction of the terephthalic acid is 0.1%-0.2%.

[0014] Beneficial technical effects: In the embodiments of this application, by using polysulfonamide instead of traditional polyamide as the separation layer, the chemical fragility of amide bonds, which are prone to hydrolysis and breakage in acidic environments, is avoided, giving the nanofiltration membrane better acid resistance. At the same time, an intermediate layer formed by the coordination of organic ligands and metal ions is introduced between the polysulfone-based ultrafiltration membrane and the polysulfonamide separation layer. This intermediate layer has a highly cross-linked coordination network structure, which can enhance the interfacial bonding force between the separation layer and the polysulfone-based ultrafiltration membrane, preventing interlayer delamination under long-term acidic liquid scouring. Moreover, its rich hydrophilic groups can improve the wettability of the membrane surface, allowing the acidic liquid to spread evenly on the membrane surface, thereby reducing the risk of concentration polarization and local acid corrosion. The nanofiltration membrane provided in this application can operate stably for a long time under harsh acidic conditions such as acidic industrial wastewater treatment, acidic dye concentration, and rare earth element recovery. While maintaining high separation accuracy, the nanofiltration membrane also has good acid resistance and durability, overcoming the defect of traditional polyamide nanofiltration membranes whose performance drops sharply when exposed to acid. Furthermore, through interface regulation of the intermediate layer, the overall structure of the nanofiltration membrane is made more compact and complete, resulting in a longer service life and a wider range of applications.

[0015] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0016] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 This is a schematic diagram of the nanofiltration membrane structure provided in the embodiments of this application; Figure 2 This is a flowchart of the nanofiltration membrane preparation method proposed in the embodiments of this application.

[0017] Figure label: 11. Polysulfonamide separation layer; 12. Organic ligand-metal coordination intermediate layer; 13. Porous polysulfone ultrafiltration membrane. Detailed Implementation

[0018] The embodiments of this application will now be described in detail. Examples of these embodiments are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0019] The terms "first" and "second" in the specification and claims of this application may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise stated, "multiple" means two or more. Furthermore, "and / or" in the specification and claims indicates at least one of the connected objects, and the character " / " generally indicates that the preceding and following objects are in an "or" relationship.

[0020] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0021] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0022] In related technologies, various attempts have been made to improve the acid resistance of nanofiltration membranes. Polysulfonamide (PSA) is a common acid-resistant membrane material because its sulfonamide bonds (-SO2-NH-) have better chemical stability than the amide bonds (-CO-NH-) in traditional polyamides, resulting in stronger resistance to acid-catalyzed hydrolysis. However, the flux of PSA thin-layer composite membranes prepared by traditional interfacial polymerization methods is low, limiting their practical application.

[0023] A study used porous polysulfone (PSF) ultrafiltration membranes as the base membrane and triethylenetetramine, terephthalic acid (TPA), and 1,3,6-naphthalenetrisulfonyl chloride (NTSC) as raw materials to prepare a composite nanofiltration membrane via interfacial polymerization. The membrane exhibited optimal performance with a TPA content of 0.15% by mass, achieving a pure water flux of 17.0 L / (m³). 2The rejection rate of MgSO4 can reach 91.3%, but the long-term stability of the membrane in a strong acid environment still needs to be improved. In addition, due to its asymmetric structure, NTSC has strong polarity and low solubility in non-polar or weakly polar solvents, which also brings certain difficulties to the interfacial polymerization of NTSC as an organic phase monomer to prepare membranes.

[0024] Other studies have used UV grafting and interfacial polymerization to prepare polyvinylidene fluoride / polysulfonamide (PVDF / PSA) composite nanofiltration membranes. After being soaked in an HCl aqueous solution at pH=1 for 35 days, the membrane can maintain a rejection rate of more than 70% for divalent salts. However, the cost of PVDF-based membranes is significantly higher than that of PSF-based membranes, and PVDF-based membranes have poorer hydrophilicity compared to PSF-based membranes.

[0025] Polysulfone ultrafiltration membranes, as a commonly used base membrane material, possess characteristics such as high chemical stability, acid and alkali resistance, high temperature resistance, and excellent mechanical strength. However, their inherent hydrophilicity is relatively poor, requiring the addition of hydrophilic substances or surface modification to improve hydrophilicity and antifouling properties. Existing modification methods for polysulfone ultrafiltration membranes include sulfonyl methylation and the addition of polyvinylpyrrolidone (PVP), but while these methods improve hydrophilicity, they often affect the membrane's acid resistance or mechanical strength.

[0026] Given the shortcomings of existing technologies, there is an urgent need to develop a composite nanofiltration membrane that combines excellent acid resistance, high permeation flux, good selectivity, and long-term stability to meet the separation requirements in acidic environments.

[0027] This application provides a nanofiltration membrane. Figure 1 This is a schematic diagram of the nanofiltration membrane structure proposed in the embodiments of this application. The nanofiltration membrane is composed of a porous polysulfone ultrafiltration membrane 13, an organic ligand-metal coordination intermediate layer 12, and a polysulfonamide separation layer 11. The organic ligand-metal coordination intermediate layer is disposed on the surface of the porous polysulfone ultrafiltration membrane, and the polysulfonamide separation layer is disposed on the surface of the organic ligand-metal coordination intermediate layer.

[0028] It should be noted that the nanofiltration membrane in this embodiment has a three-layer composite structure, wherein a porous polysulfone ultrafiltration membrane is used as the base membrane, and an organic ligand-metal coordination intermediate layer and a polysulfonamide separation layer are sequentially constructed on the surface of the polysulfone ultrafiltration membrane, thereby forming a structurally stable and functionally synergistic separation barrier. In this embodiment, the organic ligand-metal coordination interlayer is a dense network structure formed in situ on the surface of the polysulfone-based ultrafiltration membrane through the coordination reaction between the organic ligand and the metal ion. The interlayer has abundant polar groups, which can enhance the hydrophilicity and wettability of the polysulfone-based ultrafiltration membrane surface. The interlayer has strong interfacial bonding with the polysulfone-based ultrafiltration membrane and the separation layer, which can effectively avoid the risk of separation layer detachment of nanofiltration membrane under long-term immersion in acidic solution or high-pressure operation conditions, thereby ensuring the overall integrity and durability of the membrane. In the embodiments of this application, the outermost polysulfonamide separation layer uses sulfonamide bonds as the connecting units of the polymer backbone. Compared with the amide bonds in the traditional polyamide separation layer, the sulfonamide bonds have intrinsic chemical inertness to catalytic hydrolysis under acidic conditions. Therefore, no chain segment breakage or performance degradation will occur when treating acidic industrial wastewater, ensuring the separation accuracy and operational stability of the nanofiltration membrane under acidic conditions.

[0029] In some embodiments of this application, the porous polysulfone-based ultrafiltration membrane includes a porous polysulfone ultrafiltration membrane and a porous polyethersulfone ultrafiltration membrane.

[0030] Porous polysulfone ultrafiltration membranes and porous polyethersulfone ultrafiltration membranes have high chemical stability, acid and alkali resistance, and high mechanical strength. They also have good hydrophilicity and oxidation resistance, which can further improve the interfacial bonding effect between the intermediate layer and the polysulfone-based ultrafiltration membrane, and enhance the membrane's tolerance to acidic feed solutions containing oxidizing substances. This broadens the application scenarios of the nanofiltration membrane under different acidic separation conditions, and balances manufacturing cost and performance.

[0031] In some embodiments of this application, the organic ligand in the organic ligand-metal coordination intermediate is selected from one of aromatic polyphenolic compounds, and the metal is selected from at least one of iron, titanium, and zirconium.

[0032] In the embodiments of this application, during the formation of the intermediate layer, the rapid and stable coordination and cross-linking between the ortho-phenolic hydroxyl groups rich in the molecular structure of aromatic polyphenol compounds and high-charge metal ions can be utilized to construct a dense, strongly adhesive and chemically stable hydrophilic network in situ on the surface of the polysulfone ultrafiltration membrane. Iron, titanium, and zirconium ions have high coordination numbers and strong coordination bond energies, which can enhance the hydrolysis resistance and structural retention of the intermediate layer in acidic media. This allows the nanofiltration membrane to maintain the integrity of the separation layer and the interfacial bonding strength even when in contact with acidic feed solutions for a long time.

[0033] This application embodiment achieves a breakthrough in the comprehensive performance of nanofiltration membranes under acidic environments by constructing a three-layer composite structure consisting of a porous polysulfone-based ultrafiltration membrane, an organic ligand-metal coordination intermediate layer, and a polysulfonamide separation layer. Regarding acid resistance, polysulfonamide is used as the separation layer material. Its sulfonamide bonds (-SO2-NH-) exhibit stronger acid resistance than the amide bonds (-CO-NH-) of traditional polyamides, and it is less prone to hydrolysis in strong acid environments. Experiments show that after 500 hours of operation in H2SO4 solution at pH=2, the nanofiltration membrane still maintains a rejection rate of over 95% and a flux decline of less than 5%, fundamentally solving the problem of the rapid performance degradation of traditional polyamide nanofiltration membranes under acidic conditions.

[0034] Regarding permeation flux and rejection rate, by introducing an organic ligand-metal coordination interlayer and a polysulfonamide separation layer, the pure water flux can reach (20-25) L / (m²) under optimal conditions. 2 The MgSO4 rejection rate exceeds 96%, breaking through the application bottleneck of low flux of traditional polysulfonamide nanofiltration membranes.

[0035] This application also provides a method for preparing the nanofiltration membrane. Figure 2 This is a flowchart of the nanofiltration membrane preparation method proposed in the embodiments of this application, which specifically includes: Step S1: The hydrophilicized porous polysulfone ultrafiltration membrane is immersed in an organic ligand solution, and the resulting impregnated body is immersed in a metal salt solution. After the organic ligand undergoes a coordination reaction with the metal ions, an organic ligand-metal coordination intermediate layer is formed on the surface of the porous polysulfone ultrafiltration membrane to obtain the first composite membrane. It should be noted that in step S1, the porous polysulfone ultrafiltration membrane needs to be cleaned before undergoing hydrophilization treatment; In some embodiments of this application, in step S1, the hydrophilization treatment is either sulfonylation or ultraviolet grafting. When sulfonylation is used, HOCH2SO3Na is used as the sulfonating agent, and the modified ratio is n(HOCH2SO3Na):n(porous polysulfone ultrafiltration membrane) = (1-3):1, with a pH of 7.0-9.0; preferably, the modified ratio is n(HOCH2SO3Na):n(porous polysulfone ultrafiltration membrane) = 1.5:1, with a pH of 8.0; The specific steps of sulfonylation treatment include: The porous polysulfone ultrafiltration membrane was ultrasonically cleaned in deionized water to remove residual impurities and preservation solution from the membrane surface. The cleaned membrane is then immersed in a sulfonation treatment solution prepared from sodium hydroxymethylsulfonate and deionized water, wherein the molar ratio of sodium hydroxymethylsulfonate to the repeating structural units in the polysulfone ultrafiltration membrane is 1.5:1. The pH of the treatment solution is adjusted to 8.0 with a buffer solution. The reaction temperature and time are controlled under constant temperature water bath conditions to allow the sodium hydroxymethylsulfonate and polysulfone molecular chains to undergo a sulfonation grafting reaction. After the reaction is completed, the membrane is removed and repeatedly rinsed with deionized water to remove unreacted monomers and byproducts, finally obtaining a hydrophilic modified polysulfone ultrafiltration membrane with a surface rich in sulfonic acid groups.

[0036] When using the ultraviolet grafting method, acrylic acid is used as the grafting monomer, and grafting is performed under ultraviolet light irradiation for 15 minutes.

[0037] The specific steps of ultraviolet grafting include: The porous polysulfone ultrafiltration membrane was ultrasonically cleaned in deionized water to remove residual impurities and preservation solution from the membrane surface. After removal, it was dried with nitrogen or air-dried naturally. The cleaned membrane is then immersed in a grafting solution prepared with acrylic acid, deionized water, and an appropriate amount of photoinitiator or polymerization inhibitor to uniformly adsorb grafted monomers onto the membrane surface. The membrane impregnated with the grafting solution is placed in an ultraviolet irradiation device and irradiated under ultraviolet light of a specific wavelength for 15 minutes to initiate a free radical grafting polymerization reaction of acrylic acid on the surface of the polysulfone ultrafiltration membrane, resulting in covalent bonding of hydrophilic polyacrylic acid segments to the membrane surface. After the reaction is complete, the membrane is removed and repeatedly rinsed with deionized water to remove unreacted monomers and homopolymer byproducts, ultimately obtaining a hydrophilic modified polysulfone ultrafiltration membrane with a surface rich in carboxyl groups.

[0038] In some embodiments of this application, the porous polysulfone ultrafiltration membrane is a polysulfone ultrafiltration membrane or a polyethersulfone ultrafiltration membrane; Polysulfone ultrafiltration membranes have high mechanical strength and chemical stability, which can ensure that the structure of polysulfone ultrafiltration membranes does not swell or break during hydrophilization treatment and subsequent coordination reactions, providing reliable support for the uniform adhesion of the intermediate layer. Polyethersulfone ultrafiltration membranes have relatively high hydrophilicity and stronger oxidation resistance. Their surface is more likely to adsorb organic ligand molecules, thereby promoting the dense film formation and firm anchoring of the coordination intermediate layer on the membrane surface.

[0039] In this embodiment, step S1 involves stepwise impregnation to construct a coordination crosslinking network in situ on the surface of the hydrophilically treated porous polysulfone ultrafiltration membrane; the hydrophilically modified polysulfone ultrafiltration membrane is then immersed in an organic ligand solution, utilizing the hydrogen bonding and affinity adsorption between the abundant sulfonic acid groups on the surface of the polysulfone ultrafiltration membrane and the ligand molecules to uniformly attach and anchor the ligand molecules to the membrane surface and pore surface layer. The surface-adsorbed ligands are then transferred to a metal salt solution containing metal ions. The ligand molecules undergo a coordination reaction with the metal ions, transforming the originally loosely attached ligand molecules into a dense and highly cross-linked organic ligand-metal coordination network intermediate layer under the cross-linking effect of the metal ions, thus obtaining the first composite film.

[0040] In some embodiments of this application, in step S1, the organic ligand in the organic ligand solution is selected from at least one of aromatic polyphenolic compounds; The organic ligand has a mass fraction of 0.5%-2.0%.

[0041] It should be noted that the mass fraction of the organic ligand is a range of one or any two of the following: 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, and 2.0%.

[0042] In the embodiments of this application, the aromatic polyphenol compound is rich in ortho-phenolic hydroxyl groups in its molecular structure. During the impregnation process, it can generate a strong adsorption effect on the surface of the hydrophilically modified polysulfone ultrafiltration membrane through hydrogen bonds, ensuring that the ligand molecules are evenly spread and fully cover the surface of the membrane pores, providing sufficient and evenly distributed reaction sites for subsequent coordination and crosslinking with metal ions. In the embodiments of this application, the mass fraction of organic ligands is controlled between 0.5% and 2.0% to avoid problems caused by excessively low or high concentrations. Organic ligands within this range can form a coordination intermediate layer with moderate thickness, dense uniformity, and significantly enhanced hydrophilicity on the surface of polysulfone-based ultrafiltration membranes.

[0043] In some embodiments of this application, the organic ligand is selected from at least one of catechol, pyrogallol, tannic acid, gallic acid, gallic acid-modified cyclodextrin, and pyrogallol.

[0044] In the embodiments of this application, catechol and pyrogallol are small molecule polyphenol ligands with small molecular size and high phenolic hydroxyl density. They can be uniformly adsorbed on the surface of polysulfone ultrafiltration membrane. After cross-linking with metal ions, they can form a thin-layer coordination network with high coverage of polysulfone ultrafiltration membrane and low mass transfer resistance. Tannic acid and gallic acid are natural polyphenol compounds. Tannic acid has a large molecular weight and multi-branched structure, which can form a continuous adsorption layer rich in reaction sites on the surface of polysulfone ultrafiltration membrane. Gallic acid molecules contain both phenolic hydroxyl and carboxyl groups, which have stronger coordination cross-linking ability and hydrophilic enhancement effect. Gallic acid modified cyclodextrin, by grafting gallic acid onto the outer cavity of cyclodextrin, not only retains the unique cavity structure of cyclodextrin to provide additional selective mass transfer pathways, but also endows it with abundant surface phenolic hydroxyl groups to enhance coordination film-forming properties. Due to the synergistic coordination effect of three adjacent phenolic hydroxyl groups on the benzene ring, pyrogallol has extremely high reactivity with metal ions, and can instantly complete cross-linking and form an extremely dense coordination network.

[0045] In some embodiments of this application, the metal salt in the metal salt solution is selected from at least one of ferric chloride, zirconium oxychloride, and titanium tetrachloride; The mass fraction of the metal salt is 0.1%-0.5%.

[0046] In the embodiments of this application, the iron ions, zirconium oxide ions and titanium ions provided by ferric chloride, zirconium oxychloride and titanium tetrachloride all have high charge density and abundant empty orbitals. They can quickly undergo coordination reactions with the ortho-phenolic hydroxyl groups in the aromatic polyphenol ligand molecules to generate a coordination cross-linking structure with high bond energy and resistance to acid hydrolysis, thereby ensuring that the intermediate layer can still maintain the complete network morphology and interface anchoring effect under strong acidic operating environment. By controlling the mass fraction of metal salt between 0.1% and 0.5%, the ligand molecules adsorbed on the surface of the polysulfone ultrafiltration membrane can be fully solidified and form a continuous and dense intermediate layer, preventing local defects or weak interlayer bonding caused by incomplete cross-linking.

[0047] In some embodiments of this application, in step S1, the hydrophilicated porous polysulfone ultrafiltration membrane needs to be immersed in an organic ligand solution for 1 min to 5 min. Within this range, the sulfonic acid groups on the surface of the hydrophilicated polysulfone ultrafiltration membrane can fully undergo hydrogen bonding and affinity adsorption with the phenolic hydroxyl groups in the aromatic polyphenol ligand molecules, so that the ligand molecules can be evenly spread and firmly anchored on the membrane surface, forming a continuous, complete ligand adsorption layer with a suitable thickness.

[0048] Step S2: The first composite membrane is immersed in an aqueous solution, then removed and immersed in an organic solution. In this process, the aqueous and oil phases undergo interfacial polymerization to form a polysulfonamide separation layer on the surface of the organic ligand-metal coordination intermediate layer. The obtained second composite membrane is then heat-treated to obtain the nanofiltration membrane.

[0049] It should be noted that in step S2, the first composite membrane, which has formed an organic ligand-metal coordination interlayer, is immersed in an aqueous solution containing polyamine monomers. By utilizing the abundant hydrophilic groups and uniform wettability on the surface of the interlayer, the aqueous solution is fully spread on the membrane surface and penetrates into the pores of the interlayer network. The amine monomer molecules are effectively adsorbed and enriched by the interlayer through hydrogen bonding and electrostatic interactions. The membrane, which has fully adsorbed the aqueous monomers, is then removed and immersed in an organic phase solution containing polysulfonyl chloride monomers. The organic solvent is immiscible with water, and the sulfonyl chloride monomers are hydrolyzed and deactivated upon contact with water. The two phases form a clear liquid-liquid interface only at the membrane surface. At this time, the amine monomers adsorbed in the intermediate layer diffuse from the aqueous side to the interface and undergo a condensation reaction with the monomers on the organic side at the interface. The active hydrogen on the amine group and the sulfonyl chloride group remove hydrogen chloride to form a sulfonamide bond, thereby generating a thin and densely cross-linked polysulfonamide separation layer in situ on the surface of the intermediate layer, thus obtaining the second composite membrane. In step S2, the presence of the organic ligand-metal coordination interlayer serves two purposes. First, it acts as a hydrophilic reservoir to uniformly accommodate and slowly release amine monomers, regulating their diffusion rate to the interface to optimize polymerization kinetics and promote the formation of a thinner, more uniformly crosslinked, and less defective separation layer. Second, the appropriate coverage of the interlayer on the pores of the polysulfone-based ultrafiltration membrane can prevent the aqueous monomers from penetrating deep into the support layer, thus avoiding thickening of the separation layer and decrease in flux due to pore permeation. In some embodiments of this application, the aqueous solution contains 0.5%-2.0% by mass of polyamine monomers; The polyamine monomer is selected from at least one of triethylenetetramine, polyethyleneimine, and piperazine.

[0050] It should be noted that triethylenetetramine, polyethyleneimine, and piperazine are all soluble in water, therefore the aqueous solutions of these three monomers are referred to as aqueous solutions. The mass fractions of the polyamine monomers are 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, and 2.0%. In practical implementation, triethylenetetramine molecules contain multiple primary and secondary amine groups, exhibiting high reactivity and moderate diffusion rate. This allows it to rapidly form a highly cross-linked sulfonamide network with polysulfonyl chloride monomers at the interface. Polyethyleneimine's macromolecular chain structure not only participates in cross-linking reactions during interfacial polymerization, enhancing the continuity and flexibility of the separation layer, but also retains a large number of amine groups on its molecular chain that did not participate in the interfacial reaction, further improving the membrane's hydrophilicity and antifouling ability. Piperazine, with its rigid molecular structure and easily controlled diffusion behavior, can form a thin, uniform, and smooth polysulfonamide active layer during interfacial polymerization, which helps reduce mass transfer resistance and increase water flux. In some embodiments of this application, the organic phase solution is composed of a poly(sulfonyl chloride) monomer and an organic solvent; The polysulfonyl chloride monomer is 1,3,6-naphthalenetrisulfonyl chloride or 1,3,5-benzenetrisulfonyl chloride; The organic solvent is selected from at least one of n-hexane, cyclohexane, and Isopar G; The mass fraction of the polysulfonyl chloride monomer is 0.01%-0.3%.

[0051] In this application, n-hexane, cyclohexane, or Isopar G are used as organic solvents. These saturated alkanes are immiscible with water and chemically inert, and can form a clear and stable liquid-liquid interface with the aqueous phase.

[0052] In some embodiments of this application, a co-solvent is also added to the organic phase solution; The co-solvent is dimethylformamide, dimethylacetamide, or ethylene glycol monomethyl ether; The mass fraction of the cosolvent is 1%-10%.

[0053] In this embodiment, a co-solvent is introduced into the organic phase solution. By utilizing the dipole interaction between the carbonyl or etheroxy group and the polar group of sulfonyl chloride in its molecular structure, the solubility and dispersion uniformity of the oil phase monomer in the organic phase are improved. This allows the difficult-to-dissolve naphthalenetrisulfonyl chloride or benzenetrisulfonyl chloride to be evenly distributed, thereby ensuring a sufficient supply and consistent concentration distribution of sulfonyl chloride monomer at the interface. This avoids differences in the thickness of the separation layer and structural defects caused by local monomer precipitation or uneven concentration.

[0054] In some embodiments of this application, the aqueous solution further comprises terephthalic acid; The mass fraction of the terephthalic acid is 0.1%-0.2%.

[0055] It should be noted that the mass fraction of terephthalic acid is one or any two of the following: 0.1%, 0.12%, 0.15%, 0.17%, and 0.2%. In the embodiments of this application, terephthalic acid is added to the aqueous solution to significantly improve the reactivity of the aqueous phase and the degree of crosslinking of the polysulfonamide separation layer during the interfacial polymerization process, so as to achieve synergistic optimization of the densification of the separation layer structure and the reaction efficiency, thereby enhancing the retention accuracy and operational stability of the nanofiltration membrane in an acidic environment.

[0056] In some embodiments of this application, in step S2, triethylamine (TEA) is also added to the aqueous solution, with a mass fraction of 0.1%-1.0%. Triethylamine is used to absorb the interfacial polymerization byproduct HCl, thereby stabilizing and maintaining a suitable weakly alkaline reaction environment at the interface, ensuring a continuous and effective supply of active groups of amine monomers, and promoting the full formation of sulfonamide bonds and the complete growth of polymer chains.

[0057] In some embodiments of this application, in step S2, the heat treatment temperature is 60℃-90℃ and the heat treatment time is 5min-10min.

[0058] It should be noted that the heat treatment is to allow the unreacted functional groups remaining in the separation layer to undergo further condensation and cross-linking, and to eliminate the internal stress generated during the polymerization process, so as to make the polysulfonamide network structure more compact and stable. The heat treatment temperatures are 60℃, 65℃, 70℃, 75℃, 80℃, 85℃ and 90℃; the heat treatment times are 5min, 6min, 7min, 8min, 9min and 10min.

[0059] In summary, the nanofiltration membrane preparation method provided in this application, through a synergistic combination of stepwise impregnation coordination and interfacial polymerization, sequentially constructs an organic ligand-metal coordination interlayer and a polysulfonamide separation layer on the surface of a hydrophilically modified porous polysulfone-based ultrafiltration membrane, thereby obtaining the nanofiltration membrane. Regarding long-term stability, the multilayer composite structure improves the overall stability of the nanofiltration membrane. The organic ligand-metal coordination interlayer, with its highly cross-linked coordination network, enhances the interfacial bonding between the polysulfone-based ultrafiltration membrane and the separation layer, preventing interlayer delamination during long-term operation and ensuring the structural integrity of the membrane under continuous scouring by acidic solutions. In terms of the preparation process, this method uses commercially available polysulfone-based ultrafiltration membranes as the base membrane, and membrane fabrication can be completed through a simple impregnation coordination and interfacial polymerization process. The process conditions are easy to control and do not require expensive equipment investment, making it suitable for large-scale production while maintaining low manufacturing costs.

[0060] To enable those skilled in the art to better understand this application, the following embodiments will be used to provide a detailed description of a nanofiltration membrane and its preparation method provided in this application.

[0061] Example 1 The preparation of a nanofiltration membrane specifically includes: (1) Take a commercial polysulfone ultrafiltration membrane (molecular weight cutoff of 50kDa) as the polysulfone ultrafiltration membrane, rinse the surface with deionized water, then immerse it in 0.1mol / L NaOH solution for 30min, take it out and wash it with deionized water until neutral; (2) The polysulfone ultrafiltration membrane washed to neutral in step (1) is subjected to sulfonation hydrophilization treatment. The polysulfone ultrafiltration membrane is immersed in a solution prepared with sodium hydroxymethylsulfonate (the molar ratio of HOCH2SO3Na to polysulfone ultrafiltration membrane is 1.5:1, pH=8.0) and reacted at 50℃ for 2h. After the reaction is completed, it is thoroughly rinsed with deionized water to remove unreacted sulfonating agent. The resulting hydrophilized membrane is stored in a humid environment for later use. (3) Prepare a tannic acid aqueous solution with a mass fraction of 1.0% and a ferric chloride aqueous solution with a mass fraction of 0.2%. Immerse the hydrophilicized polysulfone ultrafiltration membrane obtained in step (2) in the tannic acid solution for 2 minutes. After taking it out, gently remove the excess droplets on the surface with a rubber roller. Then immerse it in the ferric chloride solution for 2 minutes. The organic ligands and metal ions undergo coordination reaction to form a tannic acid-iron coordination intermediate layer, thus obtaining the first composite membrane. After completion, gently rinse with deionized water to remove unreacted substances. (4) Prepare an aqueous solution by mixing triethylenetetramine (TETA), polyethyleneimine (PEI, molecular weight 70000), 0.15% terephthalic acid (TPA), 0.5% triethylamine (TEA) and water (by mass percentage, TETA is 2.0%, PEI is 1.0%, TPA is 0.15%, TEA is 0.5%, and the remainder is water). Prepare an organic solution by mixing 1,3,6-naphthalenetrisulfonyl chloride (NTSC) and n-hexane (by mass percentage, NTSC is 0.3%, and the remainder is n-hexane). (5) Immerse the first composite membrane in an aqueous solution for 2 minutes to ensure that the surface is fully adsorbed with aqueous monomers. After taking it out, remove excess droplets from the surface with a rubber roller, and then immerse it in an organic solution for 60 seconds. An interfacial polymerization reaction occurs on the surface of the intermediate layer of the first composite membrane to form a polysulfonamide separation layer, thus obtaining the second composite membrane. The interfacial polymerization reaction is carried out in a constant temperature and humidity environment (25±1℃, relative humidity 60±5%). (6) Transfer the second composite membrane obtained in step (5) to an oven at 60°C for heat treatment for 5 minutes to further crosslink the unreacted groups and improve the stability and density of the separation layer. After the heat treatment is completed, rinse the membrane surface thoroughly with deionized water to remove unreacted monomers and byproducts, and obtain a nanofiltration membrane, which is stored in pure water for later use.

[0062] Example 2 The difference between Example 2 and Example 1 is as follows: In Example 2, the aqueous solution contained 1.5% TETA, 1.5% PEI, 0.1% TPA, and 0.3% TEA by mass. The organic phase solution uses 0.2% by mass of 1,3,5-benzenetrisulfonyl chloride (BTSC); In the metal salt solution, the metal salt is zirconium oxychloride, and the mass fraction of tannic acid is 1.5%, while the mass fraction of zirconium oxychloride is 0.3%. The heat treatment conditions were 70℃ for 5 minutes.

[0063] Example 3 The difference between Example 3 and Example 1 is as follows: The polysulfone ultrafiltration membrane was hydrophilicated by ultraviolet grafting: acrylic acid was used as the grafting monomer, and the membrane was obtained after 15 min of ultraviolet light irradiation. The aqueous solution contains 1.5% piperazine, 0.5% PEI, and 0.5% TEA, with no TPA added. The organic phase solution was prepared by combining 0.1% by mass of 1,3,6-naphthalenetrisulfonyl chloride and Isopar G solution.

[0064] The intermediate layer was constructed using a gallic acid-titanium coordination layer, with the gallic acid mass fraction in the organic ligand solution being 1.5% and the titanium tetrachloride mass fraction in the metal salt solution being 0.2%. The heat treatment conditions were 80℃ for 5 minutes.

[0065] Example 4 The difference between Example 4 and Example 1 is as follows: The aqueous solution contains 1.0% PIP, 1.0% PEI, and 0.2% TPA, with no added TEA. The organic phase solution is composed of NTSC, the organic solvent n-hexane, and the flux dimethylformamide, wherein the mass fraction of NTSC is 0.1%, and the mass ratio of n-hexane to dimethylformamide is 98:2.

[0066] Example 5 The difference between Example 5 and Example 1 is: The organic phase solution was prepared by compounding NTSC, organic solvent cyclohexane, and co-solvent ethylene glycol monomethyl ether, wherein the mass fraction of NTSC was 0.1% and the mass ratio of cyclohexane to ethylene glycol monomethyl ether was 95:5. The intermediate layer was constructed using a pyrogallol-zirconium coordination layer. The pyrogallol mass fraction in the organic ligand solution was 2.0%, and the zirconium oxychloride mass fraction in the metal salt solution was 0.4%.

[0067] Comparative Examples 1-4 are set up based on Examples 1-5, specifically including: Comparative Example 1 Comparative Example 1 is a polysulfonamide nanofiltration membrane.

[0068] Comparative Example 2 Comparative Example 2 is a polysulfone ultrafiltration membrane.

[0069] Comparative Example 3 The preparation of a nanofiltration membrane specifically includes: (1) Take a commercial polysulfone ultrafiltration membrane (molecular weight cutoff of 50kDa) as the polysulfone ultrafiltration membrane, rinse the surface with deionized water, then immerse it in 0.1mol / L NaOH solution for 30min, take it out and wash it with deionized water until neutral; (2) Perform sulfonyl methylation hydrophilization treatment on the polysulfone ultrafiltration membrane washed to neutral in step (1): Immerse the polysulfone ultrafiltration membrane in a solution prepared with sodium hydroxymethylsulfonate (the molar ratio of HOCH2SO3Na to the polysulfone ultrafiltration membrane is 1.5:1, pH==8.0), react at 50℃ for 2h, after the reaction is completed, rinse thoroughly with deionized water to remove unreacted sulfonating agent, and store the hydrophilized membrane in a humid environment for later use; (3) Prepare an aqueous solution by mixing triethylenetetramine (TETA), polyethyleneimine (PEI, molecular weight 70000), 0.15% terephthalic acid (TPA), 0.5% triethylamine (TEA) and water (by mass percentage, TETA is 2.0%, PEI is 1.0%, TPA is 0.15%, TEA is 0.5%, and the remainder is water). Prepare an organic solution by mixing 1,3,6-naphthalenetrisulfonyl chloride (NTSC) and n-hexane (by mass percentage, NTSC is 0.3%, and the remainder is n-hexane). (4) Immerse the hydrophilic membrane in an aqueous solution for 2 minutes to ensure that the surface is fully adsorbed with aqueous monomers. After taking it out, remove excess droplets from the surface with a rubber roller, and then immerse it in an organic solution for 60 seconds. An interfacial polymerization reaction occurs on the surface of the intermediate layer of the first composite membrane to form a polysulfonamide separation layer, thus obtaining a composite membrane. The interfacial polymerization reaction is carried out in a constant temperature and humidity environment (25±1℃, relative humidity 60±5%). (5) Transfer the composite membrane obtained in step (4) to an oven at 60°C for heat treatment for 5 minutes to further crosslink the unreacted groups and improve the stability and density of the separation layer. After the heat treatment is completed, rinse the membrane surface thoroughly with deionized water to remove unreacted monomers and byproducts, and obtain a nanofiltration membrane, which is stored in pure water for later use.

[0070] Comparative Example 4 Comparative Example 4 describes the preparation of a polyvinylidene fluoride / polysulfonamide (PVDF / PSA) composite nanofiltration membrane using ultraviolet grafting and interfacial polymerization.

[0071] Performance testing The nanofiltration membranes prepared in Examples 1-5 and Comparative Examples 1-4 were subjected to performance tests, including pure water flux (L / (m²)). 2 •h)), initial MgSO4 rejection rate (%), MgSO4 rejection rate after acid treatment (%), and flux decline after acid treatment (%).

[0072] The test conditions included: operating pressure of 0.6 MPa, test solution of 1000 mg / L MgSO4 aqueous solution, and acidic environment of H2SO4 solution with pH=2. The test results are shown in Table 1.

[0073] Table 1. Performance test results of nanofiltration membranes prepared in Examples 1-5 and Comparative Examples 1-4

[0074] Note: The acid treatment conditions were 500 hours of dynamic operation in an H2SO4 solution with pH=2.

[0075] As shown in Table 1, the nanofiltration membrane prepared in Example 1 exhibits the best overall performance, with the highest water flux and rejection rate, while also demonstrating excellent stability in a strongly acidic environment. The nanofiltration membranes prepared in Examples 2-5 also exhibit good performance and can meet the requirements for use in acidic environments.

[0076] According to Table 1, Comparative Example 1 uses a pure polysulfonamide nanofiltration membrane without an intermediate layer. After acid treatment, its flux decreased by 4.3%, and the rejection rate remained at 96.2%. Its acid resistance was lower than that of Example 1, which proves that the intermediate layer is indispensable for improving interfacial bonding and acid corrosion resistance. Comparative Example 2, using a pure polysulfone ultrafiltration membrane, showed a high pure water flux of 520.2, but almost no retention capacity for MgSO4 (2.5%), and the flux decreased by 12.6% after acid treatment. This clearly reveals that the polysulfonamide separation layer is a crucial structure for achieving nanofiltration-level separation precision, and the polysulfone ultrafiltration membrane itself cannot meet the requirements. Comparative Example 3 (without an intermediate layer, direct interfacial polymerization) had an initial rejection rate of 96.8%, but after acid treatment, the rejection rate plummeted to 88.4%, with a flux decline of 8.6%, approximately twice that of Example 1. This demonstrates that the separation layer is rapidly damaged in an acidic environment when the protective effect of the intermediate layer is absent, verifying the necessity of the intermediate layer as an interfacial barrier.

[0077] Comparative Example 4 (PVDF base film + UV grafting) had the worst performance, with a rejection rate of only 82.5% and a flux attenuation of up to 21.2% after acid treatment, indicating that the structure underwent severe structural damage or interlayer delamination under acidic conditions.

[0078] In summary, the embodiments of this application solve the problem of the rapid decline in performance of traditional polyamide nanofiltration membranes when exposed to acid by synergistically configuring a porous polysulfone-based ultrafiltration membrane, an organic ligand-metal coordination intermediate layer, and a polysulfonamide separation layer. While maintaining a high MgSO4 rejection rate, it achieves excellent acid resistance and durability, providing a reliable technical solution for harsh conditions such as acidic industrial wastewater treatment and rare earth recovery.

[0079] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0080] Although embodiments of this application have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of this application, the scope of which is defined by the claims and their equivalents.

Claims

1. A nanofiltration membrane, characterized in that, The nanofiltration membrane is composed of a porous polysulfone-based ultrafiltration membrane, an organic ligand-metal coordination intermediate layer, and a polysulfonamide separation layer. The organic ligand-metal coordination intermediate layer is disposed on the surface of the porous polysulfone ultrafiltration membrane, and the polysulfonamide separation layer is disposed on the surface of the organic ligand-metal coordination intermediate layer.

2. The nanofiltration membrane according to claim 1, characterized in that, The porous polysulfone-based ultrafiltration membrane includes porous polysulfone ultrafiltration membrane and porous polyethersulfone ultrafiltration membrane.

3. The nanofiltration membrane according to claim 1, characterized in that, In the organic ligand-metal coordination intermediate, the organic ligand is selected from one of the aromatic polyphenol compounds, and the metal is selected from at least one of iron, titanium, and zirconium.

4. A method for preparing the nanofiltration membrane according to any one of claims 1-3, characterized in that, The preparation method includes: A porous polysulfone-based ultrafiltration membrane that has undergone hydrophilization treatment is immersed in an organic ligand solution. The resulting impregnated body is then immersed in a metal salt solution. After the organic ligand undergoes a coordination reaction with the metal ions, an organic ligand-metal coordination intermediate layer is formed on the surface of the porous polysulfone-based ultrafiltration membrane, thus obtaining a first composite membrane. The first composite membrane is immersed in an aqueous solution, then removed and immersed in an organic solution, wherein the aqueous phase and the oil phase undergo an interfacial polymerization reaction to form a polysulfonamide separation layer on the surface of the organic ligand-metal coordination intermediate layer. The obtained second composite membrane is then heat-treated to obtain the nanofiltration membrane.

5. The method for preparing the nanofiltration membrane according to claim 4, characterized in that, In the organic ligand solution, the organic ligand is selected from at least one of aromatic polyphenolic compounds; The organic ligand has a mass fraction of 0.5%-2.0%.

6. The method for preparing the nanofiltration membrane according to claim 5, characterized in that, The organic ligand is selected from at least one of catechol, pyrogallol, tannic acid, gallic acid, gallic acid-modified cyclodextrin, and pyrogallol.

7. The method for preparing the nanofiltration membrane according to claim 4, characterized in that, In the metal salt solution, the metal salt is selected from at least one of ferric chloride, zirconium oxychloride and titanium tetrachloride; The mass fraction of the metal salt is 0.1%-0.5%.

8. The method for preparing the nanofiltration membrane according to claim 4, characterized in that, The aqueous solution contains 0.5%-2.0% by mass of polyamine monomers; The polyamine monomer is selected from at least one of triethylenetetramine, polyethyleneimine, and piperazine.

9. The method for preparing the nanofiltration membrane according to claim 4, characterized in that, The organic phase solution is composed of a poly(sulfonyl chloride) monomer and an organic solvent; The polysulfonyl chloride monomer is selected from 1,3,6-naphthalenetrisulfonyl chloride or 1,3,5-benzenetrisulfonyl chloride; The organic solvent is selected from at least one of n-hexane, cyclohexane, and Isopar G; The mass fraction of the polysulfonyl chloride monomer is 0.01%-0.3%.

10. The method for preparing the nanofiltration membrane according to claim 4 or 8, characterized in that, The aqueous solution also contains terephthalic acid; The mass fraction of the terephthalic acid is 0.1%-0.2%.

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