Nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention provides a nanofiltration membrane and a preparation method and application thereof, and the method comprises the following steps: placing a base membrane infiltrated with a water phase monomer in a first external electric field for pretreatment, then placing an organic phase solution on the surface of the pretreated base membrane, and carrying out interfacial polymerization reaction in a second external electric field to obtain a primary nanofiltration membrane. According to the nanofiltration membrane prepared through assistance of an external electric field, the nanofiltration membrane has certain permeability, meanwhile, the rejection rate of inorganic salt is remarkably increased, chemical reagents do not need to be added, other substances do not need to be introduced, and the preparation method is high in safety and environmentally friendly.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanofiltration membrane preparation, and relates to a nanofiltration membrane, a preparation method and an application thereof, in particular to a nanofiltration membrane and a method for preparing a nanofiltration membrane assisted by an electric field. Background Art

[0002] Nanofiltration membranes are widely used in sewage treatment, seawater desalination, lithium-magnesium separation and other fields based on mechanisms such as pore size screening and electrostatic repulsion. Traditional nanofiltration membranes are prepared by interfacial polymerization technology. Reactant monomers rapidly and violently undergo polycondensation reactions at the two-phase interface to form a network cross-linked structure with nano-scale pores, which is deposited on a non-woven support layer to form a nanofiltration membrane. The violent and uncontrollable interfacial polymerization leads to the heterogeneity of the selective layer structure of the nanofiltration membrane, resulting in the mutual restriction of the performance of traditional nanofiltration membranes between water flux and salt ion retention, that is, the "Trade-off" effect. Therefore, obtaining a high-performance nanofiltration membrane with uniform pores and high water / salt selectivity through controllable interfacial polymerization is beneficial to promoting the wide application of nanofiltration membranes, and at the same time has great practical significance for efficient and low-energy water resource recovery and obtaining clean water resources.

[0003] For controllable interfacial polymerization to obtain a high-performance nanofiltration membrane structure, researchers have optimized the interfacial polymerization process through various means, improving the membrane flux and salt rejection rate of the nanofiltration membrane. At present, the means to obtain a high-performance nanofiltration membrane through a controllable interfacial polymerization process mainly regulate the following three aspects: one is to regulate the monomer diffusion process and delay the interfacial diffusion of amine monomers; the second is to regulate the reaction process of monomers to optimize the membrane structure; the third is to optimize the membrane morphology through a controllable reaction region.

[0004] CN119139924A discloses a method for preparing a highly selective nanofiltration membrane based on functionalized two-dimensional materials. This method improves the unit charge density and electrical properties of the membrane surface by loading hydrophilic modified molybdenum disulfide nanomaterials on the surface of the nanofiltration membrane, thereby enhancing the selective performance of the nanofiltration membrane for salt ions. However, the preparation process of nanomaterials is relatively complex, and the chemical reagents used increase the environmental burden.

[0005] CN118925519A discloses a method for preparing a protein-induced polyamide nanofiltration membrane. This method uses the attraction of proteins and piperazine in the aqueous phase through hydrogen bonding to slow down the release of piperazine; at the same time, since the pores formed by the reaction of proteins and TMC are relatively large, a hierarchical pore structure is formed with the dense pores formed by the reaction of piperazine and TMC, and finally a thinner and defect-free polyamide membrane is formed to improve the water flux and salt rejection rate. However, this method has a cumbersome process and long steps. The conditions for preparing the protein and piperazine mixed solution are harsh, and it is difficult to preserve the protein structure. In addition, the requirements for proteins are relatively high. Proteins with too large molecular weights will cause excessive enlargement of the pores of the nanofiltration membrane, which is not conducive to the retention of salt ions.

[0006] However, existing technical means usually require the addition of environmentally unfriendly chemical reagents (such as surfactants, nanomaterials, etc.), have poor technical stability, or are not conducive to compatibility with the actual nanofiltration membrane production and preparation process. Therefore, providing a new method for preparing a high-performance nanofiltration membrane is an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0007] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a nanofiltration membrane, its preparation method and application. The nanofiltration membrane prepared by the assistance of an external electric field has certain permeability while significantly improving the inorganic salt rejection rate. Moreover, this preparation method does not require the addition of chemical reagents, does not introduce other substances, has high safety, and is environmentally friendly.

[0008] To achieve the purpose of this invention, the following technical solutions are adopted:

[0009] In the first aspect, the present invention provides a method for preparing a nanofiltration membrane assisted by an electric field, and the method includes the following steps:

[0010] Place the substrate membrane infiltrated with the aqueous monomer under the first external electric field for pretreatment, and then place the organic phase solution on the surface of the pretreated substrate membrane, and carry out an interfacial polymerization reaction under the second external electric field to obtain a primary nanofiltration membrane.

[0011] In the present invention, the intensities of the first external electric field and the second external electric field can be the same or different, and those skilled in the art can determine according to actual production needs.

[0012] The method provided by the present invention, the nanofiltration membrane prepared by the assistance of an external electric field has certain permeability while significantly improving the inorganic salt rejection rate. Moreover, this preparation method does not require the addition of chemical reagents, does not introduce other substances, has high safety, and is environmentally friendly.

[0013] It should be noted that within a certain pH value range, conventional aqueous monomers are positively charged due to protonation in water. Therefore, under the action of the first external electric field, the interfacial distribution and diffusion behavior of the charged aqueous monomers are regulated. Then, under the assistance of the second external electric field, an interfacial polymerization reaction is carried out. By controlling the bulk and interfacial diffusion processes of the reaction monomers and removing the by-products of the interfacial polymerization reaction protons from the reaction zone to reduce their hindrance to the reaction, the charge distribution of the nanofiltration membrane can be regulated and the surface charge density can be increased. Finally, the interfacial polymerization reaction can be regulated to synthesize a nanofiltration membrane with more excellent performance, realizing the high-efficiency separation ability of the nanofiltration membrane and further improving the water treatment ability of the nanofiltration membrane.

[0014] As a preferred technical solution of the present invention, the substrate membrane includes any one of a polyethersulfone ultrafiltration membrane, a polyacrylonitrile ultrafiltration membrane, or a polyvinylidene fluoride ultrafiltration membrane.

[0015] Preferably, the aqueous monomer includes any one or a combination of at least two of piperazine, m-phenylenediamine, polyethyleneimine, or 3,5-diaminobenzoic acid.

[0016] As a preferred technical solution of the present invention, the method for infiltrating the base film with the aqueous monomer includes:

[0017] Soak the base film in an aqueous solution, and then take out the base film and let it stand.

[0018] Preferably, the mass concentration of the aqueous solution is 0.1 wt% - 0.5 wt%, for example, it can be 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, or 0.45 wt% etc., but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0019] Preferably, the pH value of the aqueous solution is 5 - 12, for example, it can be 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, or 11.5 etc., but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0020] In the present invention, as long as the pH value of the aqueous solution is controlled within the above range, a nanofiltration membrane with excellent performance can be obtained.

[0021] Preferably, the soaking time is 2 - 5 min, for example, it can be 2.5 min, 3 min, 3.5 min, 4 min, 4.5 min, or 4.8 min etc., but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0022] Preferably, the end point of standing is that there is no moisture on the surface of the base film.

[0023] As a preferred technical solution of the present invention, both the first external electric field and the second external electric field are composed of a DC power supply and titanium electrodes.

[0024] Preferably, the titanium electrode is arranged parallel to the base film, and the distance between the two is 4 - 6 mm, for example, it can be 4.2 mm, 4.4 mm, 4.5 mm, 4.6 mm, 4.8 mm, 5 mm, 5.2 mm, 5.4 mm, 5.5 mm, 5.6 mm, or 5.8 mm etc., but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0025] As a preferred technical solution of the present invention, the intensity of the first externally applied electric field is 500 - 700 V / m. For example, it can be 520 V / m, 540 V / m, 550 V / m, 560 V / m, 580 V / m, 600 V / m, 620 V / m, 640 V / m, 650 V / m, 660 V / m or 680 V / m, etc., but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0026] Preferably, the time of the pretreatment is 50 - 80 s. For example, it can be 52 s, 54 s, 55 s, 56 s, 58 s, 60 s, 62 s, 64 s, 65 s, 66 s, 68 s, 70 s, 72 s, 74 s, 75 s, 76 s or 78 s, etc., but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0027] In the present invention, by controlling the application time of the first externally applied electric field during the pretreatment, the diffusion of the aqueous phase monomer is ensured to be within a controllable range.

[0028] As a preferred technical solution of the present invention, the mass concentration of the organic phase solution is 0.1 wt% - 0.5 wt%. For example, it can be 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt% or 0.45 wt%, etc., but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0029] Preferably, the organic phase solution includes an organic phase monomer and an organic solvent.

[0030] Preferably, the organic phase monomer includes polyacyl chloride, and the polyacyl chloride includes isophthaloyl chloride and / or trimesoyl chloride.

[0031] In the present invention, the organic solvent in the organic phase solution is not specifically limited, and any commonly known solvent in the field of nanofiltration membrane preparation can be selected. For example, the organic solvent can be selected from n - hexane, cyclohexane, toluene, n - heptane or n - octane, etc.

[0032] As a preferred technical solution of the present invention, the intensity of the second externally applied electric field is 500 - 700 V / m. For example, it can be 520 V / m, 540 V / m, 550 V / m, 560 V / m, 580 V / m, 600 V / m, 620 V / m, 640 V / m, 650 V / m, 660 V / m or 680 V / m, etc., but not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0033] It should be noted that by controlling the intensities of the first external electric field and the second external electric field within a specific range, the controllable diffusion of reactant monomers and the removal of reaction by-product protons are achieved, effectively regulating the pore size and surface charge distribution of the membrane to obtain a nanofiltration membrane with excellent structure and performance, thereby enhancing the separation performance of the nanofiltration membrane; in addition, the selectivity of the nanofiltration membrane for specific ions can also be improved, breaking the "trade-off" effect of traditional nanofiltration membranes.

[0034] Preferably, the time of the interfacial polymerization reaction is 20 - 50 s, for example, it can be 22 s, 24 s, 25 s, 26 s, 28 s, 30 s, 32 s, 34 s, 35 s, 36 s, 38 s, 40 s, 42 s, 44 s, 45 s, 46 s or 48 s, etc., but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0035] In the present invention, the thickness of the separation layer of the nanofiltration membrane can be further regulated by controlling the time of the interfacial polymerization reaction.

[0036] Preferably, after the interfacial polymerization reaction, the second external electric field is removed, the excess organic phase solution is poured out, and heat curing is carried out in sequence to obtain the finished nanofiltration membrane.

[0037] Preferably, the temperature of the heat curing is 40 - 60 °C, for example, it can be 42 °C, 44 °C, 45 °C, 46 °C, 48 °C, 50 °C, 52 °C, 54 °C, 55 °C, 56 °C or 58 °C, etc., but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0038] Preferably, the time of the heat curing is 8 - 15 min, for example, it can be 8.5 min, 9 min, 9.5 min, 10 min, 11 min, 12 min, 13 min or 14 min, etc., but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0039] In the present invention, heat curing treatment can further promote the interfacial polymerization reaction of unreacted monomers, thereby enhancing the crosslinking degree of the separation layer of the nanofiltration membrane.

[0040] Preferably, the method includes the following steps:

[0041] (1) Immerse the base membrane in an aqueous solution with a mass concentration of 0.1 wt% - 0.5 wt% and a pH value of 5 - 12 for 2 - 5 min, then take out the base membrane and let it stand until there is no moisture on the surface of the base membrane to obtain a base membrane infiltrated with aqueous monomers;

[0042] The aqueous monomers include any one or a combination of at least two of piperazine, m-phenylenediamine, polyethyleneimine or 3,5-diaminobenzoic acid;

[0043] (2) Place the substrate membrane infiltrated with the aqueous monomer in step (1) under a first external electric field with an intensity of 500 - 700 V / m for pre-treatment for 50 - 80 s. Then, place the organic phase solution with a mass concentration of 0.1 wt% - 0.5 wt% on the surface of the pre-treated substrate membrane, and carry out an interfacial polymerization reaction for 20 - 50 s under a second external electric field with an intensity of 500 - 700 V / m. After the reaction, remove the second external electric field and pour out the excess organic phase solution. Finally, thermally cure at a temperature of 40 - 60 °C for 8 - 15 min to obtain the finished nanofiltration membrane;

[0044] Both the first external electric field and the second external electric field are composed of a DC power supply and titanium electrodes; the titanium electrodes are arranged parallel to the substrate membrane, and the distance between the two is 4 - 6 mm;

[0045] The organic phase solution includes an organic phase monomer and an organic solvent; the organic phase monomer includes polyacyl chloride, and the polyacyl chloride includes isophthaloyl chloride and / or trimesoyl chloride.

[0046] In the present invention, the obtained nanofiltration membrane is washed with deionized water and stored in deionized water.

[0047] In a second aspect, the present invention provides a nanofiltration membrane prepared by the method described in the first aspect.

[0048] Preferably, the surface Zeta potential of the nanofiltration membrane is -24.5 to -26.4 mV, for example, it can be -24.6 mV, -24.8 mV, -25 mV, -25.2 mV, -25.5 mV, -25.6 mV, -25.8 mV, -26 mV or -26.2 mV, etc., but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0049] The nanofiltration membrane prepared by the present invention has a high surface charge density, which significantly improves the inorganic salt rejection rate while having a certain permeation performance.

[0050] In a third aspect, the present invention provides an application of the nanofiltration membrane prepared by the method described in the first aspect or the nanofiltration membrane described in the second aspect in the field of separation and purification.

[0051] In a fourth aspect, the present invention provides an application of the nanofiltration membrane prepared by the method described in the first aspect or the nanofiltration membrane described in the second aspect in the pre-treatment of seawater desalination.

[0052] Compared with the prior art, the present invention has the following beneficial effects:

[0053] (1) The method provided by the present invention pre-treats the substrate membrane infiltrated with the aqueous monomer under the first external electric field to regulate the interfacial distribution and diffusion behavior of the charged aqueous monomer, and then carries out the interfacial polymerization reaction assisted by the second external electric field. By controlling the bulk and interfacial diffusion processes of the reaction monomers and removing the by-product protons in the interfacial polymerization reaction from the reaction zone to reduce its hindrance to the reaction, the charge distribution of the nanofiltration membrane can be regulated and the surface charge density can be increased, preparing a nanofiltration membrane with excellent structure and performance and realizing the high-efficiency separation ability of the nanofiltration membrane;

[0054] (2) The method provided by the present invention prepares a nanofiltration membrane under the condition of an electric field with a direction perpendicular to the surface of the substrate membrane. While the nanofiltration membrane has certain permeability, the inorganic salt rejection rate is significantly improved. Moreover, this preparation method does not require the addition of chemical reagents, does not introduce other substances, has high safety, and is green and environmentally friendly; among them, compared with the nanofiltration membrane prepared by classical interfacial polymerization, the inorganic salt rejection rate is increased by more than 9%. Specific Embodiments

[0055] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0056] Example 1

[0057] This example provides a method for preparing a nanofiltration membrane assisted by an electric field, and the method includes the following steps:

[0058] (1) Immerse a 50000Da polyethersulfone ultrafiltration membrane in an aqueous piperazine solution with a mass concentration of 0.3 wt% and a pH value of 10.0 for 3 minutes, then take out the substrate membrane and let it stand until there is no moisture on the surface of the polyethersulfone ultrafiltration membrane, obtaining a polyethersulfone ultrafiltration membrane infiltrated with piperazine;

[0059] (2) Place the polyethersulfone ultrafiltration membrane infiltrated with piperazine in step (1) under the first external electric field with a strength of 600 V / m for pre-treatment for 60 s, and then place a n-hexane solution of trimesoyl chloride with a mass concentration of 0.3 wt% on the surface of the pre-treated polyethersulfone ultrafiltration membrane, and carry out an interfacial polymerization reaction for 30 s under the second external electric field with a strength of 600 V / m. After the reaction, remove the second external electric field and pour out the excess n-hexane solution of trimesoyl chloride, and finally cure at a temperature of 50 °C for 10 minutes to obtain a finished nanofiltration membrane;

[0060] Both the first external electric field and the second external electric field are composed of a DC power supply and titanium electrodes; the titanium electrodes are arranged parallel to the polyethersulfone ultrafiltration membrane, and the distance between the two is 5 mm.

[0061] In this example, the Zeta potential of the surface of the obtained finished nanofiltration membrane is -24.5 mV

[0062] Example 2

[0063] This example provides a method for preparing a nanofiltration membrane assisted by an electric field. Except that the "50000Da polyethersulfone ultrafiltration membrane" is replaced with a "100000Da polyvinylidene fluoride ultrafiltration membrane", other conditions are the same as those in Example 1.

[0064] Example 3

[0065] This example provides a method for preparing a nanofiltration membrane assisted by an electric field. The method includes the following steps:

[0066] (1) Immerse a 50000Da polyethersulfone ultrafiltration membrane in an aqueous piperazine solution with a mass concentration of 0.3 wt% and a pH value of 10.0 for 5 min, then take out the base membrane and let it stand until there is no moisture on the surface of the polyethersulfone ultrafiltration membrane, obtaining a polyethersulfone ultrafiltration membrane infiltrated with piperazine;

[0067] (2) Place the polyethersulfone ultrafiltration membrane infiltrated with piperazine in step (1) under a first applied electric field with a strength of 500 V / m for pretreatment for 80 s. Then, place a n - hexane solution of trimesoyl chloride with a mass concentration of 0.3 wt% on the surface of the pretreated polyethersulfone ultrafiltration membrane, and carry out an interfacial polymerization reaction for 50 s under a second applied electric field with a strength of 500 V / m. After the reaction, remove the second applied electric field and pour out the excess n - hexane solution of trimesoyl chloride. Finally, thermally cure at a temperature of 50 °C for 10 min to obtain a finished nanofiltration membrane;

[0068] Both the first applied electric field and the second applied electric field are composed of a DC power supply and titanium electrodes; the titanium electrodes are arranged parallel to the polyethersulfone ultrafiltration membrane, and the distance between the two is 4 mm.

[0069] Example 4

[0070] This example provides a method for preparing a nanofiltration membrane assisted by an electric field. The method includes the following steps:

[0071] (1) Immerse a 50000Da polyethersulfone ultrafiltration membrane in an aqueous piperazine solution with a mass concentration of 0.3 wt% and a pH value of 10.0 for 2 min, then take out the base membrane and let it stand until there is no moisture on the surface of the polyethersulfone ultrafiltration membrane, obtaining a polyethersulfone ultrafiltration membrane infiltrated with piperazine;

[0072] (2) Place the polyethersulfone ultrafiltration membrane impregnated with piperazine in step (1) under a first external electric field with an intensity of 700 V / m for pretreatment for 50 s. Then, place a n - hexane solution of trimesoyl chloride with a mass concentration of 0.3 wt% on the surface of the pretreated polyethersulfone ultrafiltration membrane, and carry out an interfacial polymerization reaction for 25 s under a second external electric field with an intensity of 700 V / m. After the reaction, remove the second external electric field and pour out the excess n - hexane solution of trimesoyl chloride. Finally, perform thermal curing at a temperature of 50 °C for 10 min to obtain the finished nanofiltration membrane;

[0073] Both the first external electric field and the second external electric field are composed of a DC power supply and titanium electrodes; the titanium electrodes are arranged parallel to the polyethersulfone ultrafiltration membrane, and the distance between the two is 6 mm.

[0074] Example 5

[0075] This example provides a method for preparing a nanofiltration membrane assisted by an electric field. Except that the intensities of both the first external electric field and the second external electric field are 400 V / m, other conditions are the same as those in Example 1.

[0076] Example 6

[0077] This example provides a method for preparing a nanofiltration membrane assisted by an electric field. Except that the intensities of both the first external electric field and the second external electric field are 800 V / m, other conditions are the same as those in Example 1.

[0078] Example 7

[0079] This example provides a method for preparing a nanofiltration membrane assisted by an electric field. Except that the pretreatment time is 30 s, other conditions are the same as those in Example 1.

[0080] Example 8

[0081] This example provides a method for preparing a nanofiltration membrane assisted by an electric field. Except that the pretreatment time is 120 s, other conditions are the same as those in Example 1.

[0082] Comparative Example 1

[0083] This comparative example provides a method for preparing a nanofiltration membrane. Except that it is not prepared under the assistance of an external electric field, that is, step (2) is adjusted to place a n - hexane solution of trimesoyl chloride with a mass concentration of 0.3 wt% on the surface of the polyethersulfone ultrafiltration membrane impregnated with piperazine, then carry out an interfacial polymerization reaction for 30 s, pour out the excess n - hexane solution of trimesoyl chloride after the reaction, and finally perform thermal curing at a temperature of 50 °C for 10 min. Other conditions are the same as those in Example 1.

[0084] In this comparative example, the surface Zeta potential of the obtained finished nanofiltration membrane was -32.3 mV, indicating that the nanofiltration membrane prepared with the assistance of an external electric field can increase the charge density on the surface of the nanofiltration membrane.

[0085] Comparative Example 2

[0086] This comparative example provides a method for preparing a nanofiltration membrane. Except for not being prepared with the assistance of an external electric field, that is, step (2) is adjusted to place a n-hexane solution of trimellitic acid chloride with a mass concentration of 0.3 wt% on the surface of a polyvinylidene fluoride ultrafiltration membrane infiltrated with piperazine, followed by an interfacial polymerization reaction for 30 s. After the reaction, the excess n-hexane solution of trimellitic acid chloride is poured out, and finally, it is thermally cured at 50 °C for 10 min. Other conditions are the same as those in Example 2.

[0087] Comparative Example 3

[0088] This comparative example provides a method for preparing a nanofiltration membrane with the assistance of an electric field. Except for not being pretreated under the first external electric field, other conditions are the same as those in Example 1.

[0089] The nanofiltration membranes prepared in the above examples and comparative examples were rinsed with deionized water and then subjected to nanofiltration performance tests. A pressure-driven cross-flow nanofiltration instrument was used for the tests. The test conditions were: a 1000 ppm sodium sulfate solution, and the test pressure and temperature were 0.4 MPa and 25 °C, respectively.

[0090] The membrane permeability calculation formula is as follows:

[0091]

[0092] where P m is the membrane permeability (LMH / bar), V is the volume of the permeated liquid (L) within time t, S is the effective test area of the membrane (m 2 ), t is the operation time (s), and P is the test pressure (bar).

[0093] The membrane rejection calculation formula is as follows:

[0094]

[0095] where C0 and C p are the solute concentrations of the feed liquid and the permeated liquid, respectively. The concentration of the sodium sulfate solution was measured by a conductivity meter.

[0096] The test results are shown in Table 1.

[0097] Table 1

[0098] Nanofiltration membrane permeability (LMH / bar) Sodium sulfate rejection rate (%) Example 1 4.35 94.67 Example 2 7.62 85.67 Example 3 4.28 93.31 Example 4 4.89 94.99 Example 5 2.69 84.88 Example 6 4.65 87.01 Example 7 4.20 84.89 Example 8 6.85 85.14 Comparative Example 1 4.31 85.34 Comparative Example 2 7.52 73.31 Comparative Example 3 4.63 83.50

[0099] As can be seen from Table 1:

[0100] (1) The methods provided in Embodiments 1-4 of the present invention, by preparing a nanofiltration membrane assisted by an external electric field, while the nanofiltration membrane has certain permeation performance, significantly improves the inorganic salt rejection rate. Among them, compared with the nanofiltration membrane prepared by classical interfacial polymerization, the inorganic salt rejection rate is increased by more than 9%;

[0101] (2) By comprehensively comparing Embodiment 1 and Embodiments 5-6, it can be seen that when the intensities of the first external electric field and the second external electric field are too low, due to the poor proton action on the monomer and by-products, the monomer is unevenly distributed at the interface, resulting in a thicker membrane formed, reducing the permeation performance of the membrane; when the intensities of the first external electric field and the second external electric field are too high, due to the too strong proton action on the monomer and by-products, it leads to the chaos of monomer interface diffusion, resulting in the non-uniformity of the formed membrane, which is not conducive to improving the inorganic salt rejection rate of the membrane;

[0102] (3) By comprehensively comparing Embodiment 1 and Embodiments 7-8, it can be seen that when the pretreatment time of applying the electric field is too short, since the aqueous monomer fails to be effectively and uniformly distributed at the oil-water phase interface and still needs to be supplemented by bulk diffusion to the reaction interface during subsequent interfacial polymerization, the inorganic salt rejection rate of the nanofiltration membrane is relatively low; when the pretreatment time of applying the electric field is too long, since too many monomers gather at the reaction interface, a thinner membrane is formed, resulting in poor inorganic salt rejection performance of the nanofiltration membrane and a higher water flux;

[0103] (4) By comprehensively comparing Embodiments 1-2 and Comparative Examples 1-2, it can be seen that the nanofiltration membrane synthesized under the condition of an electric field with the application direction perpendicular to the surface of the base membrane, compared with the nanofiltration membrane synthesized by traditional interfacial polymerization, while the nanofiltration membrane has certain permeation performance, greatly improves the inorganic salt rejection rate. Thus, it is shown that the external electric field assistance helps to improve the high-efficiency and precise separation performance of the nanofiltration membrane;

[0104] (5) By comprehensively comparing Embodiment 1 and Comparative Example 3, it can be seen that if the first external electric field is not applied to pretreat the base membrane infiltrated with the aqueous monomer, it is not conducive to the effective diffusion of the aqueous monomer, and further not conducive to the rapid and uniform reaction between the aqueous monomer and the organic monomer, resulting in a significant decrease in the inorganic salt rejection performance of the nanofiltration membrane.

[0105] The applicant declares that the above description is only the specific implementation manners of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A method for preparing a nanofiltration membrane assisted by an electric field, characterized in that, The method includes the following steps: The substrate membrane infiltrated with the aqueous monomer is placed under a first external electric field for pretreatment, and then the organic phase solution is placed on the surface of the pretreated substrate membrane, and an interfacial polymerization reaction is carried out under a second external electric field to obtain a primary nanofiltration membrane.

2. The method according to claim 1, characterized in that, The substrate membrane includes any one of a polyethersulfone ultrafiltration membrane, a polyacrylonitrile ultrafiltration membrane, or a polyvinylidene fluoride ultrafiltration membrane; Preferably, the aqueous monomer includes any one or a combination of at least two of piperazine, m-phenylenediamine, polyethyleneimine, or 3,5-diaminobenzoic acid.

3. The method according to claim 1 or 2, characterized in that, The method for infiltrating the substrate membrane with the aqueous monomer includes: Placing the substrate membrane in an aqueous solution for soaking, and then taking out the substrate membrane and allowing it to stand; Preferably, the mass concentration of the aqueous solution is 0.1 wt% - 0.5 wt%; Preferably, the pH value of the aqueous solution is 5 - 12; Preferably, the soaking time is 2 - 5 min; Preferably, the end point of standing is that there is no moisture on the surface of the substrate membrane.

4. The method according to any one of claims 1-3, characterized in that, Both the first external electric field and the second external electric field are composed of a DC power supply and titanium electrodes; Preferably, the titanium electrode is arranged parallel to the substrate membrane, and the distance between the two is 4 - 6 mm.

5. The method according to any one of claims 1 to 4, characterized in that, The intensity of the first external electric field is 500 - 700 V / m; Preferably, the pretreatment time is 50 - 80 s.

6. The method according to any one of claims 1-5, characterized in that, The mass concentration of the organic phase solution is 0.1 wt% - 0.5 wt%; Preferably, the organic phase solution includes an organic phase monomer and an organic solvent; Preferably, the organic phase monomer includes a polyvalent acyl chloride, and the polyvalent acyl chloride includes isophthaloyl chloride and / or trimellitic chloride.

7. The method according to any one of claims 1-6, characterized in that, The intensity of the second external electric field is 500 - 700 V / m; Preferably, the interfacial polymerization reaction time is 20 - 50 s; Preferably, after the interfacial polymerization reaction, the second external electric field is removed, the excess organic phase solution is poured out, and heat curing is carried out in sequence to obtain a finished nanofiltration membrane; Preferably, the heat curing temperature is 40 - 60 °C; Preferably, the heat curing time is 8 - 15 min.

8. A nanofiltration membrane, characterized in that, The nanofiltration membrane is prepared by the method according to any one of claims 1 - 7; Preferably, the surface Zeta potential of the nanofiltration membrane is -24.5 to -26.4 mV.

9. Application of a nanofiltration membrane prepared by the method according to any one of claims 1 - 7 or the nanofiltration membrane according to claim 8 in the field of separation and purification.

10. Application of a nanofiltration membrane prepared by the method according to any one of claims 1 - 7 or the nanofiltration membrane according to claim 8 in the pretreatment of seawater desalination.

Images