A nanofiltration membrane and its preparation method

Abstract

This invention belongs to the field of separation membranes, and particularly relates to a nanofiltration membrane and its preparation method. The nanofiltration membrane provided by this invention comprises a nonwoven fabric layer, a porous support layer, and a separation layer in sequential contact. The porous support layer is formed by coating the surface of the nonwoven fabric layer with a casting solution, followed by immersion in an aqueous solution for phase inversion and curing. The casting solution contains sulfonated polysulfone, and the aqueous solution contains branched polyethyleneimine. The separation layer is formed by interfacial polymerization of an aqueous solution and an oil solution on the porous support layer. The aqueous solution contains a polyamine monomer, and the oil solution contains a polysulfinyl chloride monomer. This invention achieves a high-performance nanofiltration membrane integrating acid resistance, oxidation resistance, and high flux by adding branched polyethyleneimine to the water during the phase inversion of the casting solution, thereby allowing the branched polyethyleneimine to deposit into the base membrane during the phase inversion process. Furthermore, by interfacial polymerization of the polysulfinyl chloride monomer with the polyamine, a high-performance nanofiltration membrane integrating acid resistance, oxidation resistance, and high flux is obtained.

Description

Technical Field

[0001] This invention belongs to the field of separation membranes, and particularly relates to a nanofiltration membrane and its preparation method. Background Technology

[0002] Nanofiltration membranes, due to their selective separation properties, can effectively retain large molecules and divalent or higher ions, while allowing most small molecules and monovalent ions to pass through. This property also allows them to operate at lower pressures compared to reverse osmosis membranes, making them more widely used in the fields of material concentration and selective separation.

[0003] Currently, nanofiltration membranes are widely used in the treatment of acidic wastewater and the separation and extraction of heavy metals in certain metal industries. However, since conventional nanofiltration membranes are mainly composed of polyamide structures, when they encounter strong acidic substances, the carbon atoms of the amide groups are easily attacked by nucleophilic water molecules, resulting in hydrolysis. This damages the membrane structure and causes it to lose its function, greatly reducing the service life of the nanofiltration membrane and increasing the application cost.

[0004] To address the weak acid resistance of polyamide nanofiltration membranes, some studies have suggested using polysulfonyl chlorides instead of traditional polyacrylamide monomers, reacting them with polyamines to obtain polysulfonamide polymers. Compared to amide groups, sulfonamides have a larger electron cloud formed by the sulfur atom and two oxygen atoms, and are subject to steric hindrance, making them less susceptible to nucleophilic attack by water molecules in acidic environments, thus exhibiting superior acid resistance. However, precisely because of the sulfonyl structure, during the interfacial polymerization reaction to form sulfonamides, the sulfur atom is also less susceptible to nucleophilic attack by the amino groups of the polyamine. The sulfonation reaction rate is slower than the reaction between polyacrylamide and polyamine, resulting in a looser and thicker sulfonamide structure with relatively low membrane flux. This necessitates increasing pump pressure to achieve a certain water production rate, leading to higher energy consumption. Furthermore, due to the limitations of the polyamide structure, its oxidation resistance is poor, making it easily degraded by oxidizing substances in complex water conditions, compromising membrane stability and causing loss of separation function. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a nanofiltration membrane and a method for preparing the same. The nanofiltration membrane provided by the present invention has good acid resistance, can maintain a high membrane flux, and has good antioxidant properties.

[0006] This invention provides a nanofiltration membrane comprising a nonwoven fabric layer, a porous support layer, and a separation layer in sequential contact;

[0007] The porous support layer is formed by coating the surface of the nonwoven fabric layer with a casting solution and then immersing it in an aqueous solution for phase transformation and curing. The casting solution contains sulfonated polysulfone and the aqueous solution contains branched polyethyleneimine.

[0008] The separation layer is formed by the interfacial polymerization reaction of an aqueous solution and an oil solution on a porous support layer. The aqueous solution contains a polyamine monomer, and the oil solution contains a polysulfinyl chloride monomer. The polysulfinyl chloride monomer is one or more of the following: pyromellitic sulfinyl chloride, isophenylene disulfinyl chloride, terephthalene disulfinyl chloride, 1,6-naphthalene disulfinyl chloride, and 1,3,6-naphthalene trisulfinyl chloride.

[0009] Preferably, the sulfonated polysulfone content in the casting solution is 10-30 wt%.

[0010] Preferably, the branched polyethyleneimine content in the aqueous solution is 0.2–0.5 wt%.

[0011] Preferably, the casting solution further contains a pore-forming agent, which is one or more of polyvinyl alcohol, polyvinylpyrrolidone, glycerol, and dimethoxyethanol.

[0012] This invention provides a method for preparing the nanofiltration membrane described above, comprising the following steps:

[0013] a) The casting solution is coated onto one side of the nonwoven fabric layer, and then phase-inversion curing is performed in an aqueous solution to obtain a base film; the front side of the base film is a porous support layer and the back side is a nonwoven fabric layer.

[0014] b) Immerse the base membrane in an aqueous solution, and remove it to remove excess aqueous solution from the base membrane;

[0015] c) Coat the front side of the base film after step b) with an oil phase solution to carry out interfacial polymerization reaction, and then remove the excess oil phase solution from the front side of the base film;

[0016] d) The base membrane after step c) is dried to obtain a nanofiltration membrane.

[0017] Preferably, in step a), the water temperature for phase transformation and solidification is 10–20°C.

[0018] Preferably, in step b), the soaking temperature is 15–35°C, and the soaking time is 10–30 seconds.

[0019] Preferably, in step c), the temperature of the interfacial polymerization reaction is 15–35°C, and the time of the interfacial polymerization reaction is 5–20 s.

[0020] Preferably, in step d), the drying temperature is 50-60°C.

[0021] Compared with the prior art, the present invention provides a nanofiltration membrane and its preparation method. The nanofiltration membrane provided by the present invention comprises a nonwoven fabric layer, a porous support layer, and a separation layer in sequential contact; the porous support layer is formed by coating the surface of the nonwoven fabric layer with a casting solution and then immersing it in an aqueous solution for phase transformation and curing, wherein the casting solution contains sulfonated polysulfone and the aqueous solution contains branched polyethyleneimine; the separation layer is formed by interfacial polymerization of an aqueous solution and an oil solution on the porous support layer, wherein the aqueous solution contains a polyamine monomer and the oil solution contains a polysulfinyl chloride monomer, wherein the polysulfinyl chloride monomer is one or more selected from pyromellitic sulfinyl chloride, isophenylene disulfinyl chloride, terephthalene disulfinyl chloride, 1,6-naphthalene disulfinyl chloride, and 1,3,6-naphthalene trisulfinyl chloride. This invention introduces branched polyethyleneimine into water undergoing phase transformation in the casting solution, thereby allowing the branched polyethyleneimine to deposit into the base membrane during the phase transformation process. By using a polysulfinyl chloride monomer to carry out an interfacial polymerization reaction with a polyamine, a high-performance nanofiltration membrane integrating acid resistance, oxidation resistance, and high flux is finally obtained, which has good market prospects. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] This invention provides a nanofiltration membrane comprising a nonwoven fabric layer, a porous support layer, and a separation layer in sequential contact;

[0024] The porous support layer is formed by coating the surface of the nonwoven fabric layer with a casting solution and then immersing it in an aqueous solution for phase transformation and curing. The casting solution contains sulfonated polysulfone and the aqueous solution contains branched polyethyleneimine.

[0025] The separation layer is formed by the interfacial polymerization reaction of an aqueous solution and an oil solution on a porous support layer. The aqueous solution contains polyamine monomers, and the oil solution contains polysulfinyl chloride monomers.

[0026] In the nanofiltration membrane provided by this invention, the thickness of the nonwoven fabric layer is preferably 50–200 μm, specifically 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, or 200 μm; the air permeability of the nonwoven fabric layer is preferably 0.5–5 cc / cm². 2 / s, specifically 0.5cc / cm 2 / s, 0.7cc / cm 2 / s、1cc / cm 2 / s, 1.2cc / cm 2 / s, 1.5cc / cm 2 / s, 1.7cc / cm 2 / s、2cc / cm 2 / s, 2.3cc / cm 2 / s, 2.5cc / cm 2 / s, 2.7cc / cm 2 / s、3cc / cm 2 / s, 3.2cc / cm 2 / s, 3.5cc / cm 2 / s, 3.7cc / cm 2 / s、4cc / cm 2 / s, 4.2cc / cm 2 / s, 4.5cc / cm 2 / s, 4.7cc / cm 2 / s or 5cc / cm 2 / s.

[0027] In the nanofiltration membrane provided by the present invention, the casting solution forming the porous support layer comprises sulfonated polysulfone and an organic solvent; wherein, the sulfonated polysulfone is preferably purchased from BASF, with product number S2010G6; the organic solvent includes, but is not limited to, one or more of N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP); the content of the sulfonated polysulfone in the casting solution is preferably 10-30 wt%, specifically 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, 25 wt%, 26 wt%, 27 wt%, 28 wt%, 29 wt%, or 30 wt%.

[0028] In the nanofiltration membrane provided by the present invention, the casting solution forming the porous support layer preferably further includes a pore-forming agent; the pore-forming agent includes, but is not limited to, one or more of polyvinyl alcohol, polyvinylpyrrolidone, glycerol and dimethoxyethanol; the number average molecular weight of the polyvinylpyrrolidone is preferably 24,000 to 130,000; the content of the pore-forming agent in the casting solution is preferably 0 to 15 wt%, specifically 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt% or 15 wt%.

[0029] In the nanofiltration membrane provided by the present invention, the aqueous solution forming the porous support layer comprises branched polyethyleneimine and water; wherein, the number average molecular weight of the branched polyethyleneimine is preferably 5000-50000, specifically 5000, 10000, 15000, 20000, 25000, 30000, 35000, 40000, 45000 or 50000; the content of the branched polyethyleneimine in the aqueous solution is preferably 0.2-0.5 wt%, specifically 0.2 wt%, 0.23 wt%, 0.25 wt%, 0.27 wt%, 0.3 wt%, 0.32 wt%, 0.35 wt%, 0.37 wt%, 0.4 wt%, 0.42 wt%, 0.45 wt%, 0.47 wt% or 0.5 wt%.

[0030] In the nanofiltration membrane provided by the present invention, the thickness of the porous support layer is preferably 20 to 80 μm, specifically 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm or 80 μm.

[0031] In the nanofiltration membrane provided by the present invention, the aqueous solution forming the separation layer comprises a polyamine monomer and water; wherein, the polyamine monomer is preferably piperazine; the content of the polyamine monomer in the aqueous solution is preferably 0.5-3 wt%, specifically 0.5 wt%, 0.7 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, or 3 wt%.

[0032] In the nanofiltration membrane provided by the present invention, the aqueous solution forming the separation layer preferably further includes a surfactant and / or a pH adjuster; wherein the surfactant includes, but is not limited to, one or more of sodium dodecyl sulfate, sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, and sodium lauryl sulfate; the content of the surfactant in the aqueous solution is preferably 0.1-1 wt%, specifically 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, or 1 wt%; the pH adjuster includes, but is not limited to, sodium hydroxide, the content of which in the aqueous solution is preferably 0.5-3 wt%, specifically 0.5 wt%, 0.7 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, or 3 wt%.

[0033] In the nanofiltration membrane provided by the present invention, the oil phase solution forming the separation layer comprises a poly(sulfinyl chloride) monomer and a solvent oil; wherein, the poly(sulfinyl chloride) monomer is one or more selected from pyromellitic sulfinyl chloride, isophthalic acid sulfinyl chloride, terephthalic acid sulfinyl chloride, 1,6-naphthalene disulfinyl chloride, and 1,3,6-naphthalene trisulfinyl chloride; the solvent oil is preferably Isopar G and / or n-hexane; the content of the poly(sulfinyl chloride) monomer in the oil phase solution is preferably 0.05-0.5 wt%, specifically 0.05 wt%, 0.1 wt%, 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%, or 0.5 wt%.

[0034] This invention also provides a method for preparing a nanofiltration membrane, comprising the following steps:

[0035] a) The casting solution is coated onto one side of the nonwoven fabric layer, and then phase-inversion curing is performed in an aqueous solution to obtain a base film; the front side of the base film is a porous support layer and the back side is a nonwoven fabric layer.

[0036] b) Immerse the base membrane in an aqueous solution, and remove it to remove excess aqueous solution from the base membrane;

[0037] c) Coat the front side of the base film after step b) with an oil phase solution to carry out interfacial polymerization reaction, and then remove the excess oil phase solution from the front side of the base film;

[0038] d) The base membrane after step c) is dried to obtain a nanofiltration membrane.

[0039] In the preparation method provided by this invention, in step a), the thickness of the nonwoven fabric layer is preferably 50–200 μm, specifically 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, or 200 μm; the air permeability of the nonwoven fabric layer is preferably 0.5–5 cc / cm. 2 / s, specifically 0.5cc / cm 2 / s, 0.7cc / cm 2 / s、1cc / cm 2 / s, 1.2cc / cm 2 / s, 1.5cc / cm 2 / s, 1.7cc / cm 2 / s、2cc / cm 2 / s, 2.3cc / cm 2 / s, 2.5cc / cm 2 / s, 2.7cc / cm2 / s、3cc / cm 2 / s, 3.2cc / cm 2 / s, 3.5cc / cm 2 / s, 3.7cc / cm 2 / s、4cc / cm 2 / s, 4.2cc / cm 2 / s, 4.5cc / cm 2 / s, 4.7cc / cm 2 / s or 5cc / cm 2 / s.

[0040] In the preparation method provided by the present invention, in step a), the casting solution comprises sulfonated polysulfone and an organic solvent; wherein, the sulfonated polysulfone is preferably purchased from BASF, catalog number S2010G6; the organic solvent includes, but is not limited to, one or more of N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP); the content of the sulfonated polysulfone in the casting solution is preferably 10-30 wt%, specifically 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, 25 wt%, 26 wt%, 27 wt%, 28 wt%, 29 wt%, or 30 wt%.

[0041] In the preparation method provided by the present invention, in step a), the casting solution preferably further includes a pore-forming agent; the pore-forming agent includes, but is not limited to, one or more of polyvinyl alcohol, polyvinylpyrrolidone, glycerol and dimethoxyethanol; the number average molecular weight of the polyvinylpyrrolidone is preferably 24,000 to 130,000; the content of the pore-forming agent in the casting solution is preferably 0 to 15 wt%, specifically 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt% or 15 wt%.

[0042] In the preparation method provided by this invention, in step a), the aqueous solution comprises branched polyethyleneimine and water; wherein, the number average molecular weight of the branched polyethyleneimine is preferably 5000-50000, specifically 5000, 10000, 15000, 20000, 25000, 30000, 35000, 40000, 45000 or 50000; the content of the branched polyethyleneimine in the aqueous solution is preferably 0.2-0.5 wt%, specifically 0.2 wt%, 0.23 wt%, 0.25 wt%, 0.27 wt%, 0.3 wt%, 0.32 wt%, 0.35 wt%, 0.37 wt%, 0.4 wt%, 0.42 wt%, 0.45 wt%, 0.47 wt% or 0.5 wt%.

[0043] In the preparation method provided by the present invention, in step a), the water temperature for phase transformation and curing is preferably 10-20℃, specifically 10℃, 11℃, 12℃, 13℃, 14℃, 15℃, 16℃, 17℃, 18℃, 19℃ or 20℃; the time for phase transformation and curing is not particularly limited, as long as the casting solution is completely cured.

[0044] In the preparation method provided by the present invention, in step a), after the phase transformation and curing are completed, the thickness of the porous support layer formed on the front side of the base film is preferably 20-80 μm, specifically 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm or 80 μm.

[0045] In the preparation method provided by the present invention, in step b), the aqueous solution comprises a polyamine monomer and water; wherein, the polyamine monomer is preferably piperazine; the content of the polyamine monomer in the aqueous solution is preferably 0.5-3 wt%, specifically 0.5 wt%, 0.7 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, or 3 wt%.

[0046] In the preparation method provided by the present invention, in step b), the aqueous solution preferably further includes a surfactant and / or a pH adjuster; wherein the surfactant includes, but is not limited to, one or more of sodium dodecyl sulfate, sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, and sodium lauryl sulfate; the content of the surfactant in the aqueous solution is preferably 0.1-1 wt%, specifically 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, or 1 wt%; the pH adjuster includes, but is not limited to, sodium hydroxide, the content of which in the aqueous solution is preferably 0.5-3 wt%, specifically 0.5 wt%, 0.7 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, or 3 wt%.

[0047] In the preparation method provided by the present invention, in step b), the soaking temperature is preferably 15-35℃, specifically 15℃, 16℃, 17℃, 18℃, 19℃, 20℃, 21℃, 22℃, 23℃, 24℃, 25℃ (room temperature), 26℃, 27℃, 28℃, 29℃, 30℃, 31℃, 32℃, 33℃, 34℃ or 35℃; the soaking time is preferably 10-30s, specifically 10s, 11s, 12s, 13s, 14s, 15s, 16s, 17s, 18s, 19s, 20s, 21s, 22s, 23s, 24s, 25s, 26s, 27s, 28s, 29s or 30s.

[0048] In the preparation method provided by the present invention, in step b), the preferred method for removing excess aqueous solution is to first remove excess aqueous solution from the surface of the base film using a surface roller, and then remove the residual aqueous solution on the front and back sides of the base film by using a hot air knife and vacuum suction, so that there is no visible aqueous solution on the base film.

[0049] In the preparation method provided by the present invention, in step c), the oil phase solution comprises a poly(sulfinyl chloride) monomer and a solvent oil; wherein, the poly(sulfinyl chloride) monomer is preferably one or more selected from pyromellitic sulfinyl chloride, isophenylene disulfinyl chloride, terephthalene disulfinyl chloride, 1,6-naphthalene disulfinyl chloride, and 1,3,6-naphthalene trisulfinyl chloride; the solvent oil is preferably Isopar G and / or n-hexane; the content of the poly(sulfinyl chloride) monomer in the oil phase solution is preferably 0.05-0.5 wt%, specifically 0.05 wt%, 0.1 wt%, 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%, or 0.5 wt%.

[0050] In the preparation method provided by the present invention, in step c), the temperature of the interfacial polymerization reaction is preferably 15-35℃, specifically 15℃, 16℃, 17℃, 18℃, 19℃, 20℃, 21℃, 22℃, 23℃, 24℃, 25℃ (room temperature), 26℃, 27℃, 28℃, 29℃, 30℃, 31℃, 32℃, 33℃, 34℃ or 35℃; the time of the interfacial polymerization reaction is preferably 5-20s, specifically 5s, 6s, 7s, 8s, 9s, 10s, 11s, 12s, 13s, 14s, 15s, 16s, 17s, 18s, 19s or 20s.

[0051] In the preparation method provided by the present invention, in step c), the preferred method for removing excess oil phase solution is to use a front roller and an air knife to remove the excess oil phase solution.

[0052] In the preparation method provided by the present invention, in step d), the drying temperature is preferably 50-60°C, specifically 50°C, 51°C, 52°C, 53°C, 54°C, 55°C, 56°C, 57°C, 58°C, 59°C or 60°C; the drying time is not particularly limited, as long as the film surface is completely dry.

[0053] This invention introduces branched polyethyleneimine during the phase transformation of the base membrane and then uses a polysulfinyl chloride monomer to perform an interfacial polymerization reaction with the polyamine, resulting in a high-performance nanofiltration membrane that integrates acid resistance, oxidation resistance, and high flux. More specifically, the technical solution provided by this invention has at least the following beneficial effects:

[0054] 1) Adding branched polyethyleneimine to the water during the phase transition of the base membrane results in a base membrane containing branched polyethyleneimine molecules. This reduces the negative charge on the base membrane surface, decreases the adsorption of positively charged piperazine in the aqueous phase, thereby weakening the polyamide reaction intensity and making the resulting polyamide layer porous with higher water flux. Simultaneously, the hydrophilicity of branched polyethyleneimine also facilitates water permeation. Furthermore, the terminal amino groups of the branched polyethyleneimine on the base membrane surface can react with sulfinyl chloride groups in subsequent interfacial polymerization reactions, allowing polysulfinamide molecules to form a strong bond with the base membrane through chemical bonds. This strengthens the stability of the functional layer's molecular structure and improves its oxidation and acid resistance.

[0055] 2) Using molecules containing polysulfinyl chloride groups as reactants, after reacting with polyamines to form sulfinamides, because the oxidation state of the S atom is not saturated, when there are oxidizing substances in the water, the S atom can be oxidized, and the sulfinamide is converted into sulfonamide, while the original amide structure is not greatly damaged, so that the molecule has certain antioxidant properties.

[0056] 3) The S atom in the sulfinamide structure has a lone pair of electrons. In acidic water, it is more difficult for water molecules to nucleophilically attack the S atom than for amides to attack the C atom, resulting in lower reactivity and better acid resistance of sulfinamide. When sulfinamide is oxidized to the sulfonamide structure, the S atom is connected to two O atoms, the electron cloud density is larger, and the steric hindrance has a greater impact when attacked by water molecules, resulting in lower reactivity and better acid resistance.

[0057] For clarity, the following examples and comparative examples provide a detailed description. Unless otherwise specified, all operations in the following examples and comparative examples of the present invention are performed at room temperature and normal pressure.

[0058] Example 1

[0059] 1. Base film preparation: 84 parts by weight of N,N-dimethylformamide, 15 parts by weight of sulfonated polysulfone (BASF, Germany, S2010G6), and 1 part by weight of polyvinylpyrrolidone (number average molecular weight 24,000–130,000) were stirred at high speed at 70°C for 4 hours to obtain a uniform and transparent solution; this solution was then uniformly coated onto a nonwoven fabric (100 micrometers thick, air permeability 2.5 cc / cm). 2 One side of the substrate was immersed in an aqueous solution containing 0.5 wt% branched polyethyleneimine (Aladdin, number average molecular weight 25000) at 15°C for phase inversion curing to obtain a polysulfone-based film with branched polyethyleneimine deposited on it (the front side is a polysulfone porous support layer, the back side is a non-woven fabric layer, and the thickness of the polysulfone porous support layer is 50 micrometers).

[0060] 2. Preparation of aqueous solution: 1 part by weight of piperazine, 0.5 parts by weight of sodium dodecyl sulfonate, 1.5 parts by weight of sodium hydroxide, and 97 parts by weight of pure water. Stir for 0.5 hours to obtain a homogeneous and transparent solution.

[0061] 3. Preparation of oil phase solution: 0.1 parts by mass of pyromellitic sulfonyl chloride and 99.9 parts by mass of n-hexane were stirred for 0.5 hours to obtain a homogeneous solution.

[0062] 4. Nanofiltration membrane preparation: The base membrane is immersed in an aqueous solution for 20 seconds. Excess aqueous solution on the surface is removed with a surface roller. Then, residual aqueous solution on the front and back of the membrane is removed by hot air knife and vacuum dewatering, so that there is no visible aqueous solution on the base membrane. After that, an oil phase solution is coated on the front of the base membrane for interfacial polymerization reaction. After the reaction is 10 seconds, excess oil phase solution is removed with a front roller and air knife. Then, the membrane is placed in a 60℃ oven. After the membrane is completely dry, it is removed from the oven to obtain the nanofiltration composite membrane.

[0063] Example 2

[0064] 1. Preparation of base film: The content of branched polyethyleneimine in the aqueous solution is 0.2 wt%, and other conditions are the same as in Example 1.

[0065] 2. Preparation of aqueous solution: Same as in Example 1.

[0066] 3. Preparation of oil phase solution: Same as in Example 1.

[0067] 4. Nanofiltration membrane preparation: Same as in Example 1.

[0068] Comparative Example 1

[0069] 1. Preparation of base film: The aqueous solution was prepared without branched polyethyleneimine, and the rest was the same as in Example 1.

[0070] 2. Preparation of aqueous solution: Same as in Example 1.

[0071] 3. Preparation of oil phase solution: Same as in Example 1.

[0072] 4. Nanofiltration membrane preparation: Same as in Example 1.

[0073] Comparative Example 2

[0074] 1. Preparation of base film: The aqueous solution was prepared without branched polyethyleneimine, and the rest was the same as in Example 1.

[0075] 2. Preparation of aqueous solution: Same as in Example 1.

[0076] 3. Preparation of oil phase solution: 0.1 parts by mass of pyromellitic trisulfonyl chloride and 99.9 parts by mass of n-hexane were stirred for 0.5 hours to obtain a homogeneous solution.

[0077] 4. Nanofiltration membrane preparation: Same as in Example 1.

[0078] Comparative Example 3

[0079] 1. Preparation of base film: The aqueous solution was prepared without branched polyethyleneimine, and the rest was the same as in Example 1.

[0080] 2. Preparation of aqueous solution: Same as in Example 1.

[0081] 3. Preparation of oil phase solution: 0.1 parts by mass of pyromellitic methyl chloride and 99.9 parts by mass of n-hexane were stirred for 0.5 hours to obtain a homogeneous solution.

[0082] 4. Nanofiltration membrane preparation: Same as in Example 1.

[0083] Performance testing

[0084] The nanofiltration membranes prepared in the above examples and comparative examples were subjected to performance tests on a standard membrane testing bench. The results are shown in the table below:

[0085] Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Flux F / GFD 36.3 33.1 29.2 28.5 33.0 Desalination rate R / % 98.7 98.5 99.1 99.3 99.0

[0086] The above membrane performance tests were conducted using a 2000 mg / L MgSO4 aqueous solution at a pressure of 70 psi and a temperature of 25 ± 1 °C.

[0087] in:

[0088] The formula for calculating flux F is as follows:

[0089]

[0090] In the formula, V is the volume of permeate, S is the effective membrane area, and t is time.

[0091] The formula for calculating the desalination rate R is as follows:

[0092]

[0093] In the formula, C f C represents the concentration of MgSO4 in the original solution. p This represents the concentration of MgSO4 in the permeate.

[0094] From the initial performance of the membrane, the changes in membrane flux in Examples 1, 2 and Comparative Example 1 are directly proportional to the content of branched polyethyleneimine. The branched polyethyleneimine structure contains a large number of amino and imino groups, which have good hydrophilicity. At the same time, the branched polyethyleneimine weakens the negative charge on the surface of the base membrane, reduces the adsorption of positively charged piperazine in the aqueous phase, thereby weakening the polyamide reaction intensity and making the generated polyamide layer loose. The two factors work synergistically to effectively improve the membrane flux. The membrane performance of Comparative Example 1 and Comparative Example 2 is similar, which is related to the similar reactivity of sulfinyl chloride and sulfonyl chloride groups with piperazine amino groups. Among them, sulfinyl chloride has slightly higher reactivity and slightly higher flux.

[0095] To further verify the acid resistance of the membrane prepared by this invention, a hydrochloric acid solution with pH=2 was prepared, heated and maintained at 40±0.5℃, and the membrane was immersed for 24 hours. Then, a standard MgSO4 test was performed, and the results are shown in the table below:

[0096] Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Initial flux F / GFD 36.3 29.2 28.5 33.0 Initial desalination rate R / % 98.7 99.1 99.3 99.0 Acid-treated flux F / GFD 38.2 33.5 31.4 60.5 Desalination rate R / % after acid treatment 98.1 97.4 98.2 90.5

[0097] In terms of performance after acid treatment, the membrane with sulfinamide (Example 1, Comparative Example 1) or sulfonamide (Comparative Example 2) structure has better acid resistance than the amide structure of Comparative Example 3. In Example 1, the membrane with branched polyethyleneimine deposited in the base membrane is slightly better than Comparative Example 1, which is related to the fact that the sulfinamide structure can be stably connected to the base membrane through chemical bonds. The acid resistance of the membrane of Comparative Example 1 is slightly lower than that of Comparative Example 2, which is related to the fact that the sulfinamide structure has slightly less steric hindrance and is more easily attacked by water molecules, but it has a significant improvement in acid resistance compared with the amide structure membrane of Comparative Example 3.

[0098] To further verify the antioxidant properties of the membrane prepared by this invention, the prepared membrane was immersed in a 1000 ppm sodium hypochlorite aqueous solution for 24 hours, and then subjected to a MgSO4 standard test. The results are shown in the table below:

[0099] Example 1 Comparative Example 2 Initial flux F / GFD 36.3 28.5 Initial desalination rate R / % 98.7 99.3 Flux of F / GFD after oxidation treatment 38.5 33.2 Desalination rate R / % after oxidation treatment 96.4 95.2

[0100] From the perspective of performance after oxidation treatment, the membrane of Example 1 has a sulfinamide structure, and the S atom has a certain reducing property, which can react with oxidizing substances to obtain a sulfonamide structure. Compared with Comparative Example 2, it exhibits a certain degree of antioxidant performance.

[0101] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A nanofiltration membrane, characterized in that, It includes a nonwoven fabric layer, a porous support layer, and a separation layer that are in contact with each other in sequence; The porous support layer is formed by coating the surface of a nonwoven fabric layer with a casting solution, followed by immersion in an aqueous solution for phase transformation and curing. The casting solution contains sulfonated polysulfone, and the aqueous solution contains branched polyethyleneimine. The number average molecular weight of the branched polyethyleneimine is 5000~50000. The content of the branched polyethyleneimine in the aqueous solution is 0.2~0.5wt%. The casting solution also contains a pore-forming agent. The separation layer is formed by interfacial polymerization of an aqueous solution and an oil solution on a porous support layer. The aqueous solution contains a polyamine monomer, the content of which is 0.5-3 wt%. The oil solution contains a polysulfinyl chloride monomer, the content of which is 0.05-0.5 wt%. The polysulfinyl chloride monomer is one or more of the following: pyromellitic sulfinyl chloride, isophenylene disulfinyl chloride, terephthalene disulfinyl chloride, 1,6-naphthalene disulfinyl chloride, and 1,3,6-naphthalene trisulfinyl chloride.

2. The nanofiltration membrane according to claim 1, characterized in that, The content of the sulfonated polysulfone in the casting solution is 10~30wt%.

3. The nanofiltration membrane according to claim 1, characterized in that, The pore-forming agent is one or more of polyvinyl alcohol, polyvinylpyrrolidone, glycerol, and dimethoxyethanol.

4. A method for preparing a nanofiltration membrane according to any one of claims 1 to 3, characterized in that, Includes the following steps: a) The casting solution is coated onto one side of the nonwoven fabric layer, and then phase-inversion curing is performed in an aqueous solution to obtain a base film; the front side of the base film is a porous support layer and the back side is a nonwoven fabric layer. b) Immerse the base membrane in an aqueous solution, and remove it to remove excess aqueous solution from the base membrane; c) Coat the front side of the base film after step b) with an oil phase solution to carry out interfacial polymerization reaction, and then remove the excess oil phase solution from the front side of the base film; d) The base membrane after step c) is dried to obtain a nanofiltration membrane.

5. The preparation method according to claim 4, characterized in that, In step a), the water temperature for phase transformation and solidification is 10~20℃.

6. The preparation method according to claim 4, characterized in that, In step b), the soaking temperature is 15~35℃; the soaking time is 10~30s.

7. The preparation method according to claim 4, characterized in that, In step c), the temperature of the interfacial polymerization reaction is 15~35℃; the time of the interfacial polymerization reaction is 5~20s.

8. The preparation method according to claim 4, characterized in that, In step d), the drying temperature is 50~60℃.

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