Positively charged nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention relates to the field of membranes, and discloses a positively charged nanofiltration membrane as well as a preparation method and application thereof. The positively charged nanofiltration membrane comprises a bottom layer, a porous support layer and a polyamide separation layer which are arranged in sequence, wherein the polyamide separation layer comprises a guanidyl structure as shown in a formula I and / or a formula II; and in the imgabs0 #, R1 and R2 are respectively and independently alkyl with 1 to 5 carbon atoms. The polyamide separation layer of the positively charged nanofiltration membrane comprises a guanidyl structure with a specific structure, so that the electropositivity of the surface of the nanofiltration membrane can be remarkably enhanced, the interception effect of the nanofiltration membrane on high-valence ions is improved, and higher selectivity of monovalent and multivalent cations is obtained.

Description

Technical Field

[0001] The present invention relates to the field of membranes, and in particular to a positively charged nanofiltration membrane and a preparation method and application thereof. Background Art

[0002] Lithium and its derivatives are widely used in new energy vehicles, electronics, pharmaceuticals, and other fields. Compared to mining lithium resources from lithium ores such as spodumene and lepidolite, extracting lithium from brine offers a significant cost advantage, at approximately half the cost. Currently, extracting lithium from salt lakes is becoming a leading focus of the domestic lithium extraction industry.

[0003] The lithium extraction process and difficulty of the target salt lake are mainly determined by the specific composition of its brine. The associated magnesium ions in the brine cause great difficulties in lithium extraction from the salt lake. This is mainly because the size of magnesium ions (7.2×10 -13 m) and lithium ions (7.4×10 -13 m) are very close, and their chemical properties are similar, making separation difficult.

[0004] Compared with traditional methods such as precipitation, adsorption, solvent extraction, etc., nanofiltration membrane separation technology has attracted much attention in the field of lithium extraction from salt lakes due to its advantages such as high permeation flux, low cost, simple operation, and green environmental protection. Based on the Donnan effect and size screening mechanism, nanofiltration membranes can selectively separate monovalent and multivalent ions. However, commercial polyamide nanofiltration membranes such as NF270, NF90, and DesalDL membranes are all negatively charged membranes, which have good separation effects on monovalent and divalent anions, but have low retention rates for divalent magnesium ions and poor magnesium-lithium separation performance. Taking into account the separation mechanism of nanofiltration membranes, positively charged nanofiltration membranes are more suitable for practical application scenarios of magnesium-lithium separation. Therefore, there is an urgent need to develop low-cost, positively charged, and high magnesium-lithium separation efficiency nanofiltration membranes. Based on this, the inventors have completed the present invention based on a comprehensive summary of the existing technology and in response to the technical defects of existing membranes. Summary of the Invention

[0005] The purpose of the present invention is to overcome the problems of low surface positive charge density and poor magnesium-lithium separation performance of nanofiltration membranes in the prior art, and to provide a positively charged nanofiltration membrane and its preparation method and application. The polyamide separation layer of the positively charged nanofiltration membrane contains a guanidine structure with a specific structure, which can significantly enhance the positive charge of the nanofiltration membrane surface, improve the nanofiltration membrane's retention effect on divalent / high-valent cations, and obtain higher selectivity for monovalent and divalent / high-valent cations.

[0006] In order to achieve the above object, the first aspect of the present invention provides a positively charged nanofiltration membrane, characterized in that the nanofiltration membrane comprises a bottom layer, a porous support layer and a polyamide separation layer arranged in sequence;

[0007] Wherein, the polyamide separation layer includes a guanidine structure represented by Formula I and / or Formula II;

[0008]

[0009] Among them, R1 and R2 are each independently an alkyl group having 1 to 5 carbon atoms.

[0010] The second aspect of the present invention provides a method for preparing a positively charged nanofiltration membrane, characterized in that the preparation method includes:

[0011] S1. Prepare a porous support layer on the bottom layer;

[0012] S2. The membrane layer obtained in step S1 is successively brought into first contact with an aqueous phase containing a polyamine and a guanidyl compound containing an amino group, and into second contact with an organic phase containing a polyacyl chloride. After heat treatment, the positively charged nanofiltration membrane is obtained.

[0013] The third aspect of the present invention provides a positively charged nanofiltration membrane prepared by the above preparation method.

[0014] The fourth aspect of the present invention provides an application of the above positively charged nanofiltration membrane in the field of water treatment separation.

[0015] Through the above technical solutions, the positively charged nanofiltration membrane, its preparation method and application provided by the present invention obtain the following beneficial effects:

[0016] The polyamide separation layer of the positively charged nanofiltration membrane provided by the present invention contains a guanidyl structure with a specific structure, which can significantly enhance the positive charge on the surface of the nanofiltration membrane, enhance the Donnan effect, improve the retention effect of the nanofiltration membrane on divalent / higher-valent cations, and obtain a higher selectivity of monovalent cations to divalent / higher-valent cations.

[0017] Specifically, the positively charged nanofiltration membrane provided by the present invention has excellent magnesium-lithium separation performance. At 0.6 MPa, the water flux of the membrane is more than 29 LMH, the retention rate of the membrane to magnesium chloride can reach more than 98.9%, the desalination rate of lithium chloride is more than 28.3%, and the magnesium-lithium separation coefficient reaches more than 65.2. In addition, the guanidyl group is stably modified inside the polyamide separation layer through an amide bond, so the membrane structure is stable and shows good long-term working stability in continuous tests.

[0018] In the preparation method of the positively charged nanofiltration membrane provided by the present invention, a guanidyl compound containing an amino group in its structure is added to the aqueous solution of the polyamine, and they jointly participate in the interfacial polymerization reaction, so that the guanidyl group can be uniformly introduced into the polyamide separation layer. The operation of the present invention is simple, the required reagents are easily available, and it does not involve complex synthesis processes. The process is well compatible with the continuous film-making equipment currently used in the industry and is easy to scale up industrially. Description of the Drawings

[0019] Figure 1It is the surface infrared characterization result of the nanofiltration membrane prepared in Example 1 and Comparative Example 1 of the present invention.

[0020] Figure 2 It is the continuous stability test result of the nanofiltration membrane prepared in Example 1 of the present invention. Detailed implementation manners

[0021] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values and individual point values of each range, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0022] The first aspect of the present invention provides a positively charged nanofiltration membrane, wherein the nanofiltration membrane includes a bottom layer, a porous support layer, and a polyamide separation layer arranged in sequence;

[0023] Wherein, the polyamide separation layer includes a guanidine group structure represented by Formula I and / or Formula II;

[0024]

[0025] Wherein, R1 and R2 are each independently an alkyl group having 1 to 5 carbon atoms.

[0026] In the present invention, the polyamide separation layer of the positively charged nanofiltration membrane contains a guanidine group structure with a specific structure, which can significantly enhance the positive charge on the surface of the nanofiltration membrane, enhance the Donnan effect, improve the retention effect of the nanofiltration membrane on divalent / higher-valent cations, and obtain a higher selectivity of monovalent to divalent / higher-valent cations.

[0027] In the present invention, the polyamide separation layer of the nanofiltration membrane contains a guanidine group structure. When the guanidine group is ionized, the positive charge is delocalized on three nitrogen atoms to form a stable planar guanidine cation structure, thereby strengthening the positive charge on the membrane surface. During the nanofiltration separation process, due to the enhancement of the Donnan effect, the retention rate of divalent magnesium ions by the membrane is increased, and at the same time, lithium ions can pass through, thereby obtaining excellent monovalent / divalent ion selective separation performance and showing a high monovalent / divalent separation coefficient.

[0028] In the present invention, * refers to the bonding position where the guanidine group structure is connected to the polyamide separation layer.

[0029] Further, R1 and R2 are each independently an alkyl group having 1 to 3 carbon atoms.

[0030] According to the present invention, the surface Zeta potential of the nanofiltration membrane is 0 mV to 30 mV.

[0031] In the present invention, when the surface Zeta potential of the nanofiltration membrane satisfies the above range, the positive charge on the surface of the nanofiltration membrane is enhanced, and the nanofiltration membrane has a high rejection rate for divalent and higher-valent cations, while having good permeability for monovalent cations, thereby achieving high-selectivity separation.

[0032] Furthermore, the surface Zeta potential of the nanofiltration membrane is 5 mV to 25 mV.

[0033] According to the present invention, the average pore size of the nanofiltration membrane is 0.2 - 0.5 nm.

[0034] In the present invention, when the average pore size of the nanofiltration membrane satisfies the above range, the nanofiltration membrane can achieve a high rejection rate for divalent and higher-valent metal cations, while having good permeability for monovalent cations and a high water flux.

[0035] Furthermore, the average pore size of the nanofiltration membrane is 0.25 - 0.35 nm.

[0036] In the present invention, for the nanofiltration membrane, the content of the guanidine group structure of formula I and / or formula II is not particularly limited, as long as the surface Zeta potential of the nanofiltration membrane meets the requirements of the present invention.

[0037] In the present invention, there are no specific limitations on the bottom layer and the porous support layer, and they can be made of various existing materials with a certain strength and capable of being used for nanofiltration and reverse osmosis membranes.

[0038] In the present invention, the bottom layer is a non-woven fabric material, preferably polyester and / or polyethylene.

[0039] In the present invention, the porous support layer material can be at least one of polyethersulfone, polysulfone, polyarylether, polybenzimidazole, polyetherketone, polyetheretherketone, polyacrylonitrile, polyvinylidene fluoride, and polyaryletherketone.

[0040] According to the present invention, there are no particular limitations on the thicknesses of the bottom layer, the porous support layer, and the polyamide separation layer of the present invention, and they can be conventional selections in the art. However, in order to enable these three layers to play a better synergistic cooperation role and make the obtained composite nanofiltration membrane better combine excellent magnesium-lithium separation coefficient and high water flux, preferably, the thickness of the bottom layer is 30 - 150 μm, preferably 50 - 120 μm; the thickness of the porous support layer is 10 - 100 μm, preferably 30 - 60 μm; the thickness of the polyamide separation layer is 10 - 500 nm, preferably 50 - 300 nm.

[0041] The second aspect of the present invention provides a method for preparing a positively charged nanofiltration membrane, characterized in that the preparation method includes:

[0042] S1. Prepare a porous support layer on the bottom layer;

[0043] S2. First, the film layer obtained in step S1 is brought into first contact with an aqueous phase containing a polyamine and a guanidyl compound containing an amino group, and then into second contact with an organic phase containing a polyacyl chloride. After heat treatment, the positively charged nanofiltration membrane is obtained.

[0044] In the present invention, by adding a guanidyl compound containing an amino group in the aqueous solution of the polyamine and allowing it to participate in the interfacial polymerization reaction together, the guanidyl group can be uniformly introduced into the polyamide separation layer. The operation of the present invention is simple, the required reagents are easily available, and it does not involve a complex synthesis process. The process has good compatibility with the continuous film-making equipment currently used in the industry and is easy to scale up industrially.

[0045] In the present invention, the role of the polyamine in the aqueous phase is to form a dense polyamide separation layer with cross-linked structure through interfacial polymerization with the polyacyl chloride, and the role of the guanidyl compound containing an amino group in the structure is to react with the polyacyl chloride to introduce the guanidyl group into the polyamide separation layer.

[0046] In the present invention, there is no particular limitation on the method for preparing the porous support layer on the bottom layer, and it can be prepared by a conventional method in the art. Preferably, the phase inversion method is adopted. Specifically, a polymer solution of the porous support layer material is coated on one surface of the bottom layer, and the porous support layer is obtained through phase inversion.

[0047] In the present invention, the phase inversion method preferably can be: dissolving the support layer polymer material in a solvent to obtain a polymer solution with a concentration of 10 - 20% by weight, degassing at 20 - 40 °C for 10 - 180 min; then coating the polymer solution on the bottom layer to obtain an initial film, and immediately immersing it in water at a temperature of 10 - 30 °C for 10 - 60 min to obtain the porous membrane of the support layer polymer through the phase inversion layer.

[0048] Among them, the solvent can be N,N - dimethylformamide, N,N - dimethylacetamide, N - methylpyrrolidone, dimethyl sulfoxide, etc.

[0049] According to the present invention, in step S2, the polyamine is selected from at least one of polyethyleneimine, triethylenetetramine, tetraethylenepentamine, diethylenetriamine, and polyethylenepolyamine. Preferably, the polyamine is selected from polyethyleneimine and / or polyethylenepolyamine.

[0050] According to the present invention, the concentration of the polyamine in the aqueous phase is 0.1 - 5 wt%.

[0051] In the present invention, when the concentration of the polyamine in the aqueous phase satisfies the above range, the prepared nanofiltration membrane can have both excellent retention and permeation performance. Specifically, when the concentration of the polyamine is too low, the retention performance of the formed polyamide separation layer is poor; when the concentration of the polyamine is too high, the permeation performance of the formed polyamide separation layer is poor.

[0052] Further, the concentration of the polyamine in the aqueous phase is 0.5-2 wt%.

[0053] According to the present invention, the guanidine compound containing an amino group is the compound shown in Formula 1 and / or Formula 2 and its salts:

[0054]

[0055] Wherein, R1 and R2 are each independently an alkyl group having 1-5 carbon atoms, preferably an alkyl group having 1-3 carbon atoms.

[0056] In the present invention, the guanidine compound containing an amino group is selected from at least one of the compound shown in Formula 1, the salt of the compound shown in Formula 1, the compound shown in Formula 2, and the salt of the compound shown in Formula 2. Preferably, the compound salt refers to the hydrochloride salt of the compound shown in Formula 1 or Formula ......

[0057] In a specific embodiment of the present invention, the guanidine compound containing an amino group is selected from at least one of polyaminopropyl biguanide, 1-(4-aminobutyl)guanidine, 1-amino-4-butylguanidine dihydrochloride, and 1,3-diaminoguanidine hydrochloride, preferably polyaminopropyl biguanide and / or 1-(4-aminobutyl)guanidine

[0058] According to the present invention, the concentration of the guanidine compound containing an amino group in the aqueous phase is 0.05-1 wt%.

[0059] In the present invention, when the concentration of the guanidine compound containing an amino group in the aqueous phase satisfies the above range, the prepared nanofiltration membrane can simultaneously have a high surface Zeta potential and a high crosslinking degree of the polyamide separation layer, ensuring that the nanofiltration membrane has both excellent retention performance and a separation coefficient of monovalent and divalent ions. Specifically, when the concentration of the guanidine compound containing an amino group in the aqueous phase is too low, the enhancement effect of the guanidine structure on the surface positive charge of the nanofiltration membrane is not obvious; if the concentration of the guanidine compound containing an amino group in the aqueous phase is too high, too much guanidine compound will consume too much polyacyl chloride, resulting in insufficient polyacyl chloride for reaction with the polyamine, leading to a lower crosslinking degree of the polyamide separation layer and poor retention performance of the nanofiltration membrane.

[0060] Further, the concentration of the guanidine compound containing an amino group in the aqueous phase is 0.1-0.5 wt%.

[0061] According to the present invention, the polyvalent acyl chloride is selected from at least one of 1,3,5-benzenetricarbonyl chloride, 1,3-benzenedicarbonyl chloride, and 1,4-benzenedicarbonyl chloride; preferably 1,3,5-benzenetricarbonyl chloride and / or 1,4-benzenedicarbonyl chloride.

[0062] According to the present invention, the concentration of the polyvalent acyl chloride in the organic phase containing the polyvalent acyl chloride is 0.01 wt% - 1 wt%; preferably 0.1 - 0.5 wt%.

[0063] According to the present invention, the amounts of the aqueous phase and the organic phase are such that the mass ratio of the polyamine, the guanidyl compound containing an amino group, and the polyvalent acyl chloride is 2 - 50:1 - 20:1.

[0064] In the present invention, when the mass ratio of the polyamine, the guanidyl compound containing an amino group, and the polyvalent acyl chloride is controlled to satisfy the above range, on the one hand, a dense polyamide separation layer can be obtained in the interfacial polymerization process, and on the other hand, a high-density guanidyl group can be controllably introduced into the structure of the separation layer, making the membrane surface positively charged, so that high rejection of divalent and higher-valent cations and high permeation of monovalent cations can be achieved simultaneously, thereby achieving an efficient separation effect.

[0065] Further, the amounts of the aqueous phase and the organic phase are such that the mass ratio of the polyamine, the guanidyl compound containing an amino group, and the polyvalent acyl chloride is 3 - 20:1 - 10:1.

[0066] In the present invention, there is no particular limitation on the type of the solvent in the organic phase containing the polyvalent acyl chloride, as long as it can dissolve the polyvalent acyl chloride. Preferably, the solvent of the organic phase is one or more of n-hexane, dodecane, n-heptane, and paraffin solvent oils (Isopar E, Isopar G, Isopar H, Isopar L, and Isopar M).

[0067] According to the present invention, there are no special limitations on the interfacial polymerization conditions of the polyamine and the polyvalent acyl chloride and the reaction conditions of the polyvalent acyl chloride and the guanidyl compound containing an amino group, and they can be carried out under the conventional conditions in the art. However, in order to enable these three layers to play a better synergistic cooperation role and make the obtained composite nanofiltration membrane better have both excellent monovalent / divalent separation coefficient and higher water flux, preferably, the time of the first contact is 5 - 100 s, preferably 10 - 60 s; the time of the second contact is 10 - 200 s, preferably 20 - 120 s; the heat treatment conditions include: the heat treatment temperature is 40 - 150 °C, preferably 50 - 120 °C; the heat treatment time is 0.5 - 10 min, preferably 1 - 5 min.

[0068] In the present invention, the time of the first contact is 5 - 100 s. If the time of the first contact is too short, the amount of water phase infiltration is insufficient, and the separation layer formed by the subsequent interfacial polymerization reaction may be defective; if the contact time is longer than the preferred value, the amount of water phase infiltration is excessive, and the separation layer formed by the subsequent interfacial polymerization reaction may be too thick and have poor permeability. Preferably, the time of the first contact is 10 - 60 s.

[0069] In the present invention, the time of the second contact is 10 - 200 s. When the time of the second contact is controlled to meet the above range, it can ensure that the polyamine and the guanidine compound containing an amino group can fully contact with the polyacyl chloride and react to form a polyamide separation layer containing a guanidine structure. When the time of the second contact is too short, the two-phase reaction is insufficient, and the separation layer formed by the interfacial polymerization reaction may be defective; if the time of the second contact is too long, the separation layer formed by the interfacial polymerization reaction may be too thick, and the permeability of the nanofiltration membrane decreases. Preferably, the time of the second contact is 20 - 120 s.

[0070] In the present invention, the heat treatment temperature is 40 - 150 °C; the heat treatment time is 0.5 - 10 min. When the heat treatment conditions are controlled to meet the above range, it can ensure that the aqueous phase monomers (polyamine and guanidine compound containing an amino group) and the organic phase monomer (polyacyl chloride) fully react, further improve the crosslinking degree of the separation layer, and at the same time dry the organic solvent. If the heat treatment temperature is too low and the heat treatment time is too short, the organic solvent volatilizes insufficiently; if the heat treatment temperature is too high and the heat treatment time is too long, the permeability of the obtained composite nanofiltration membrane will deteriorate and the separation performance will decrease.

[0071] Further, the heat treatment temperature is 50 - 120 °C; the heat treatment time is 1 - 5 min.

[0072] In the present invention, in step S2, the volume ratio of the aqueous phase containing polyamine and guanidine compound containing an amino group to the membrane area of the membrane layer obtained in step S1 is 0.1 - 1 mL / cm 2 , preferably 0.2 - 0.5 mL / cm 2 .

[0073] In the present invention, in step S2, the volume ratio of the organic phase containing polyacyl chloride to the membrane area of the membrane layer obtained in step S1 is 0.05 - 0.5 mL / cm 2 , preferably 0.1 - 0.3 mL / cm 2 .

[0074] The third aspect of the present invention provides a positively charged nanofiltration membrane prepared by the above preparation method.

[0075] The fourth aspect of the present invention provides an application of the above positively charged nanofiltration membrane in the field of water treatment separation.

[0076] To enable those skilled in the art to better understand the features and effects of the present invention, the technical solutions and effects of the present invention will be described in detail below in conjunction with specific embodiments.

[0077] In this article, for the sake of concise description, all possible combinations of all technical features in the embodiments are not described. The described embodiments are partial embodiments of the present invention, rather than all embodiments, and are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0078] In the following examples and comparative examples, the water flux of the nanofiltration membrane was measured by the following method: The composite nanofiltration membrane was installed in a membrane cell, pre-pressed at 0.6 MPa for 1 h, and then the water permeation amount of the composite nanofiltration membrane within a certain time was measured under the conditions of a pressure of 0.6 MPa and a temperature of 25 °C. The water flux was calculated by the following formula: J = Q / (A·t), where J is the water flux (LMH), Q is the water permeation amount (L), A is the effective filtration area of the composite nanofiltration membrane (m 2 ), and t is the time (h).

[0079] The salt rejection rate of the nanofiltration membrane was measured by the following method: The composite nanofiltration membrane was installed in a membrane cell, the original aqueous solution was magnesium chloride at 2000 ppm or lithium chloride at 2000 ppm. After pre-pressing at 0.6 MPa for 1 h, the permeate was obtained under a pressure of 0.6 MPa. The conductivities of the original solution and the permeate were measured by the conductivity method, and the salt rejection rate was calculated by the following formula: R = (Cf - Cp) / Cf × 100%, where R is the salt rejection rate, Cf is the concentration of magnesium chloride or lithium chloride in the original solution, and Cp is the concentration of magnesium chloride or lithium chloride in the corresponding permeate.

[0080] The magnesium-lithium separation coefficient of the nanofiltration membrane was measured by the following method: The composite nanofiltration membrane was installed in a membrane cell, the raw water was a mixed solution of 2000 ppm magnesium chloride and 100 ppm lithium chloride. After pre-pressing at 0.6 MPa for 1 h, the permeate was obtained under a pressure of 0.6 MPa. The concentrations of magnesium ions and lithium ions in the original solution and the permeate were measured by an ion chromatograph analyzer, and the magnesium-lithium separation coefficient of the membrane was calculated by the following formula:

[0081]

[0082] where S is the magnesium-lithium separation coefficient of the membrane, C Mg,f and C Li,f are the concentrations of magnesium ions and lithium ions in the original aqueous solution respectively, and C Mg,p and C Li,p are the concentrations of magnesium ions and lithium ions in the permeate respectively, both measured by the ion chromatograph.

[0083] Stability test of nanofiltration membrane: The stability test of the nanofiltration membrane was carried out in a mixed solution of 2000 ppm magnesium chloride and 100 ppm lithium chloride. The test conditions were the same as those for the magnesium-lithium separation coefficient test, lasting for 80 hours in total. Samples were taken every hour for the water flux test of the membrane, and samples were taken every 5 hours for the magnesium-lithium separation coefficient test of the membrane.

[0084] The Zeta potential of the nanofiltration membrane was measured based on the streaming potential and streaming current measurement methods. The model of the Zeta potential analyzer used was SurPASS 3, and it was measured according to the streaming potential method in the test method for the surface Zeta potential of nanofiltration membranes (GB / T 37617-2019).

[0085] Thickness measurement: The cross-sectional morphology of the membrane sheet was observed using a Hitachi S-4800 high-resolution field emission scanning electron microscope (FESEM) to obtain the thickness of the membrane.

[0086] The surface structure of the nanofiltration membrane was characterized by infrared spectroscopy.

[0087] In the following examples and comparative examples, branched polyethyleneimine (weight average molecular weight of 25000 g / mol), polyethylenepolyamine, and 1,3,5-benzenetricarbonyl chloride were purchased from J&K Scientific Ltd. Polyaminopropyl biguanide and 1-(4-aminobutyl)guanidine were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyhexamethylene biguanide hydrochloride and 1-amino-4-butylguanidine dihydrochloride were purchased from Xilong Scientific.

[0088] In the following examples and comparative examples, the porous substrate was prepared as follows: A polymer solution of the porous support layer material was coated on the surface of the polyester non-woven fabric, and a porous support layer was obtained through a phase inversion process. More specifically: Polysulfone with a number average molecular weight of 80000 g / mol was dissolved in N,N-dimethylformamide to obtain a polysulfone solution with a concentration of 18 wt%. The above polysulfone solution was degassed at 25 °C for 120 min, and then the above polysulfone solution was coated on a polyester non-woven fabric with a thickness of 75 μm using a doctor blade to obtain an initial membrane. Immediately, it was immersed in water at a temperature of 25 °C for 60 min, so that the polysulfone layer on the surface of the polyester non-woven fabric underwent phase inversion to form a porous structure. Finally, it was washed 3 times with water to obtain a bottom layer - porous support layer with a total thickness of 115 μm.

[0089] Example 1

[0090] Step 1: Prepare the aqueous solution: Polyethyleneimine and polyaminopropyl biguanide (R2 in formula 2 is ethyl) were dissolved in water to obtain a binary blend aqueous phase. The concentration of polyethyleneimine was 0.5 wt%, and the concentration of polyaminopropyl biguanide was 0.1 wt%.

[0091] Step 2: Prepare the organic phase solution: Dissolve 1,3,5-benzenetricarbonyl chloride in IsoparE to obtain the organic phase solution. The concentration of 1,3,5-benzenetricarbonyl chloride is 0.1 wt%.

[0092] Step 3: Contact the upper surface of the polysulfone support layer with 50 mL of the aqueous phase solution prepared in Step 1, drain the liquid after contacting for 60 s at 25°C; then, contact the upper surface of the support layer with 50 mL of the organic phase solution prepared in Step 2, drain the liquid after contacting for 60 s at 25°C; then, put the membrane into an oven and heat it at 70°C for 3 min to obtain the composite nanofiltration membrane N1. Among them, the mass ratio of polyethyleneimine, polyaminopropyl biguanide to 1,3,5-benzenetricarbonyl chloride is 5:1:1. 2 Then, contact the upper surface of the polysulfone support layer with 50 mL of the aqueous phase solution prepared in Step 1, drain the liquid after contacting for 60 s at 25°C; then, contact the upper surface of the support layer with 50 mL of the organic phase solution prepared in Step 2, drain the liquid after contacting for 60 s at 25°C; then, put the membrane into an oven and heat it at 70°C for 3 min to obtain the composite nanofiltration membrane N1. Among them, the mass ratio of polyethyleneimine, polyaminopropyl biguanide to 1,3,5-benzenetricarbonyl chloride is 5:1:1.

[0093] The infrared characterization results of the surface of the composite nanofiltration membrane N1 are as Figure 1 shown.

[0094] It can be seen from the infrared characterization results of the surface of the composite membrane N1 that compared with the case where there is no polyaminopropyl biguanide in the aqueous phase (Comparative Example 1), a new peak appears at 1555 cm -1 This is the stretching vibration peak of the C=N bond in the guanidine group, indicating that polyaminopropyl biguanide participates in the interfacial polymerization reaction and introduces the guanidine group structure into the polyamide separation layer.

[0095] Example 2

[0096] Step 1: Prepare the aqueous phase solution: Dissolve polyethyleneimine and polyaminopropyl biguanide in water to obtain a binary blend aqueous phase. The concentration of polyethyleneimine is 0.5 wt%, and the concentration of polyaminopropyl biguanide is 0.5 wt%.

[0097] Step 2: Prepare the organic phase solution: Dissolve 1,3,5-benzenetricarbonyl chloride in IsoparE to obtain the organic phase solution. The concentration of 1,3,5-benzenetricarbonyl chloride is 0.1 wt%.

[0098] Step 3: Contact the upper surface of the polysulfone support layer with 50 mL of the aqueous phase solution prepared in Step 1, drain the liquid after contacting for 60 s at 25°C; then, contact the upper surface of the support layer with 50 mL of the organic phase solution prepared in Step 2, drain the liquid after contacting for 60 s at 25°C; then, put the membrane into an oven and heat it at 70°C for 3 min to obtain the composite nanofiltration membrane N2. Among them, the mass ratio of polyethyleneimine, polyaminopropyl biguanide to 1,3,5-benzenetricarbonyl chloride is 5:5:1. 2 Then, contact the upper surface of the polysulfone support layer with 50 mL of the aqueous phase solution prepared in Step 1, drain the liquid after contacting for 60 s at 25°C; then, contact the upper surface of the support layer with 50 mL of the organic phase solution prepared in Step 2, drain the liquid after contacting for 60 s at 25°C; then, put the membrane into an oven and heat it at 70°C for 3 min to obtain the composite nanofiltration membrane N2. Among them, the mass ratio of polyethyleneimine, polyaminopropyl biguanide to 1,3,5-benzenetricarbonyl chloride is 5:5:1.

[0099] Example 3

[0100] Step 1: Prepare the aqueous solution: Dissolve polyethyleneimine and polyaminopropyl biguanide in water to obtain a binary blended aqueous phase. The concentration of polyethyleneimine is 0.5 wt%, and the concentration of polyaminopropyl biguanide is 1 wt%.

[0101] Step 2: Prepare the organic phase solution: Dissolve 1,3,5-benzenetricarbonyl chloride in IsoparE to obtain the organic phase solution. The concentration of 1,3,5-benzenetricarbonyl chloride is 0.1 wt%.

[0102] Step 3: Contact the upper surface of the polysulfone support layer with 50 mL of the aqueous phase solution prepared in Step 1, drain the liquid after contacting for 60 s at 25°C; then, contact the upper surface of the support layer with 50 mL of the organic phase solution prepared in Step 2, drain the liquid after contacting for 60 s at 25°C; then, put the membrane into an oven and heat it at 70°C for 3 min to obtain the composite nanofiltration membrane N3. Among them, the mass ratio of polyethyleneimine, polyaminopropyl biguanide to 1,3,5-benzenetricarbonyl chloride is 5:10:1. 2

[0103] Example 4

[0104] Step 1: Prepare the aqueous solution: Dissolve polyethyleneimine and 1-(4-aminobutyl)guanidine (R1 is butyl in Formula 1) in water to obtain a binary blended aqueous phase. The concentration of polyethyleneimine is 1 wt%, and the concentration of 1-(4-aminobutyl)guanidine is 0.3 wt%.

[0105] Step 2: Prepare the organic phase solution: Dissolve 1,3,5-benzenetricarbonyl chloride in IsoparE to obtain the organic phase solution. The concentration of 1,3,5-benzenetricarbonyl chloride is 0.1 wt%.

[0106] Step 3: Contact the upper surface of the polysulfone support layer with 50 mL of the aqueous phase solution prepared in Step 1, drain the liquid after contacting for 60 s at 25°C; then, contact the upper surface of the support layer with 50 mL of the organic phase solution prepared in Step 2, drain the liquid after contacting for 60 s at 25°C; then, put the membrane into an oven and heat it at 70°C for 3 min to obtain the composite nanofiltration membrane N4. Among them, the mass ratio of polyethyleneimine, 1-(4-aminobutyl)guanidine to 1,3,5-benzenetricarbonyl chloride is 10:3:1. 2

[0107] Example 5

[0108] Step 1: Prepare the aqueous solution: Dissolve polyethylenepolyamine and polyaminopropyl biguanide in water to obtain a binary blended aqueous phase. The concentration of polyethylenepolyamine is 0.5 wt%, and the concentration of polyaminopropyl biguanide is 0.2 wt%.

[0109] ​​Step 2: Prepare the organic phase solution: Dissolve 1,3,5-benzenetricarbonyl chloride in IsoparE to obtain the organic phase solution. The concentration of 1,3,5-benzenetricarbonyl chloride is 0.1 wt%.

[0110] Step 3: Contact the upper surface of the polysulfone support layer with 50 mL of the aqueous phase solution prepared in Step 1, drain the liquid after contacting for 60 s at 25 °C; then, contact the upper surface of the support layer with 50 mL of the organic phase solution prepared in Step 2, drain the liquid after contacting for 60 s at 25 °C; then, put the membrane into an oven and heat it at 70 °C for 3 min to obtain the composite nanofiltration membrane N5. Among them, the mass ratio of polyethylenepolyamine, polyaminopropyl biguanide to 1,3,5-benzenetricarbonyl chloride is 5:2:1. 2

[0111] Example 6

[0112] Step 1: Prepare the aqueous phase solution: Dissolve polyethylenepolyamine and 1-(4-aminobutyl)guanidine in water to obtain a binary blended aqueous phase. The concentration of polyethylenepolyamine is 1 wt%, and the concentration of 1-(4-aminobutyl)guanidine is 0.5 wt%.

[0113] Step 2: Prepare the organic phase solution: Dissolve 1,3,5-benzenetricarbonyl chloride in IsoparE to obtain the organic phase solution. The concentration of 1,3,5-benzenetricarbonyl chloride is 0.1 wt%.

[0114] Step 3: Contact the upper surface of the polysulfone support layer with 50 mL of the aqueous phase solution prepared in Step 1, drain the liquid after contacting for 60 s at 25 °C; then, contact the upper surface of the support layer with 50 mL of the organic phase solution prepared in Step 2, drain the liquid after contacting for 60 s at 25 °C; then, put the membrane into an oven and heat it at 70 °C for 3 min to obtain the composite nanofiltration membrane N6. Among them, the mass ratio of polyethylenepolyamine, 1-(4-aminobutyl)guanidine to 1,3,5-benzenetricarbonyl chloride is 10:5:1. 2

[0115] Example 7

[0116] Step 1: Prepare the aqueous phase solution: Dissolve polyethyleneimine and 1-amino-4-butylguanidine dihydrochloride (the hydrochloride of the guanidine compound shown in Formula 1 when R1 is butyl) in water to obtain a binary blended aqueous phase. The concentration of polyethyleneimine is 0.5 wt%, and the concentration of 1-amino-4-butylguanidine dihydrochloride is 0.1 wt%.

[0117] Step 2: Prepare the organic phase solution: Dissolve 1,3,5-benzenetricarbonyl chloride in IsoparE to obtain the organic phase solution. The concentration of 1,3,5-benzenetricarbonyl chloride is 0.1 wt%.

[0118] ​​Step 3: Contact the upper surface of the polysulfone support layer with 50 mL of the aqueous solution prepared in Step 1, drain the liquid after contacting at 25 °C for 60 s; then, contact the upper surface of the support layer with 50 mL of the organic solution prepared in Step 2, drain the liquid after contacting at 25 °C for 60 s; then, put the membrane into an oven and heat it at 70 °C for 3 min to obtain the composite nanofiltration membrane N7. Among them, the mass ratio of polyethyleneimine, 1-amino-4-butylguanidine disalt to 1,3,5-benzenetricarbonyl chloride is 5:1:1. 2

[0119] Comparative Example 1

[0120] Step 1: Prepare an aqueous solution: Dissolve polyethyleneimine in water to obtain an aqueous solution. The concentration of polyethyleneimine is 0.5 wt%.

[0121] Step 2: Prepare an organic solution: Dissolve 1,3,5-benzenetricarbonyl chloride in IsoparE to obtain an organic solution. The concentration of 1,3,5-benzenetricarbonyl chloride is 0.1 wt%.

[0122] Step 3: Contact the upper surface of the polysulfone support layer with 50 mL of the aqueous solution prepared in Step 1, drain the liquid after contacting at 25 °C for 60 s; then, contact the upper surface of the support layer with 50 mL of the organic solution prepared in Step 2, drain the liquid after contacting at 25 °C for 60 s; then, put the membrane into an oven and heat it at 70 °C for 3 min to obtain the composite nanofiltration membrane D1. Among them, the mass ratio of polyethyleneimine to 1,3,5-benzenetricarbonyl chloride is 5:1. 2

[0123] The infrared characterization results of the surface of the composite nanofiltration membrane D1 are as Figure 1 shown.

[0124] Comparative Example 2

[0125] Step 1: Prepare an aqueous solution: Dissolve polyethylenepolyamine in water to obtain an aqueous solution. The concentration of polyethylenepolyamine is 0.5 wt%.

[0126] Step 2: Prepare an organic solution: Dissolve 1,3,5-benzenetricarbonyl chloride in IsoparE to obtain an organic solution. The concentration of 1,3,5-benzenetricarbonyl chloride is 0.1 wt%.

[0127] Step 3: Contact the upper surface of the polysulfone support layer with 50 mL of the aqueous solution prepared in Step 1, drain the liquid after contacting at 25 °C for 60 s; then, contact the upper surface of the support layer with 50 mL of the organic solution prepared in Step 2, drain the liquid after contacting at 25 °C for 60 s; then, put the membrane into an oven and heat it at 70 °C for 3 min to obtain the composite nanofiltration membrane D1. Among them, the mass ratio of polyethyleneimine to 1,3,5-benzenetricarbonyl chloride is 5:1. 2 ​​The upper surface of the polysulfone support layer was contacted with 50 mL of the aqueous solution prepared in Step 1 and drained after contacting for 60 s at 25 °C; then, the upper surface of the support layer was contacted with 50 mL of the organic solution prepared in Step 2 and drained after contacting for 60 s at 25 °C; then, the membrane was placed in an oven and heated at 70 °C for 3 min to obtain the composite nanofiltration membrane D2. Among them, the mass ratio of polyethylenepolyamine to 1,3,5-benzenetricarbonyl chloride was 5:1.

[0128] Comparative Example 3

[0129] Step 1: Prepare an aqueous solution: Dissolve polyaminopropyl biguanide in water to obtain an aqueous solution. The concentration of polyaminopropyl biguanide was 0.5 wt%.

[0130] Step 2: Prepare an organic solution: Dissolve 1,3,5-benzenetricarbonyl chloride in IsoparE to obtain an organic solution. The concentration of 1,3,5-benzenetricarbonyl chloride was 0.1 wt%.

[0131] Step 3: The above-mentioned one with an area of 200 cm 2 The upper surface of the polysulfone support layer was contacted with 50 mL of the aqueous solution prepared in Step 1 and drained after contacting for 60 s at 25 °C; then, the upper surface of the support layer was contacted with 50 mL of the organic solution prepared in Step / / 2 and drained after contacting for 60 s at 25 °C; then, the membrane was placed in an oven and heated at 70 °C for 3 min to obtain the composite nanofiltration membrane D3. Among them, the mass ratio of polyaminopropyl biguanide to 1,3,5-benzenetricarbonyl chloride was 5:1.

[0132] The thickness, surface Zeta potential, and average pore size of each layer of the composite membrane in the examples and comparative examples are shown in Table 1. Table 1

[0133] Bottom layer / μm Porous support layer / μm Separation layer / nm Surface Zeta potential (mV) Average pore size (nm) Example 1 75 40 108 15.3 0.28 Example 2 75 40 115 17.6 0.31 Example 3 75 40 124 19.2 0.34 Example 4 75 40 145 16.9 0.25 Example 5 75 40 132 17.1 0.23 Example 6 75 40 148 18.7 0.2 Example 7 75 40 122 7.4 0.31 Comparative Example 1 75 40 128 -20.3 0.35 Comparative Example 2 75 40 138 -18.4 0.28 Comparative Example 3 75 40 256 21.5 0.62

[0134] The water flux, desalination rate for magnesium chloride, desalination rate for lithium chloride, and magnesium-lithium separation coefficient S of the composite nanofiltration membranes prepared in the examples and comparative examples were tested, and the results are shown in Table 2.

[0135] Table 2

[0136] Membrane number Water flux (LMH) <![CDATA[Desalination rate of MgCl2 (%)]]> LiCl desalination rate (%) Separation coefficient S N1 45.3 99.3 30.3 96.4 N2 40.3 99.5 34.3 93.6 N3 49.6 99.2 29.6 74.3 N4 40.7 99.5 35.4 91.7 N5 30.5 99.6 60.4 67.5 N6 29.4 99.7 63.4 65.2 N7 47.2 98.9 28.3 69.3 D1 50.0 95.6 27.8 23.5 D2 32.4 97.2 56.8 12.2 D3 123.7 52.5 9.3 2.4

[0137] As can be seen from Table 1 and Table 2, by adding a guanidine compound containing an amino group in its structure to the aqueous solution of polyamine and participating in the interfacial polymerization reaction together, the guanidine group can be evenly introduced into the polyamide separation layer. The introduction of the guanidine group enhances the positive charge on the membrane surface, and the enhancement of the Donnan effect can optimize the retention effect of the membrane on divalent Mg 2+ and obtain better magnesium-lithium separation performance.

[0138] Figure 2 This is the result of the continuous stability test of the nanofiltration membrane prepared in Example 1. It can be seen from Figure 2 that during the 80-hour continuous test, the magnesium-lithium separation factor of the membrane remained at about 95 all the time, and the water flux fluctuated only slightly without performance degradation, showing good working stability. The above results indicate that by introducing guanidine into the polyamide separation layer in a chemically bonded manner through amide groups, long-term high-selectivity separation of magnesium and lithium can be achieved.

[0139] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A positively charged nanofiltration membrane, characterized in that, The nanofiltration membrane comprises a bottom layer, a porous support layer and a polyamide separation layer which are arranged in sequence; Wherein, the polyamide separation layer comprises a guanidine group structure shown in Formula I and / or Formula II; Wherein, R1 and R2 are each independently an alkyl group having 1 to 5 carbon atoms.

2. The positively charged nanofiltration membrane according to claim 1, wherein, R1 and R2 are each independently an alkyl group having 1 to 3 carbon atoms.

3. The positively charged nanofiltration membrane according to claim 1 or 2, wherein, The surface Zeta potential of the nanofiltration membrane is 0 mV to 30 mV, preferably 5 mV to 25 mV.

4. The positively charged nanofiltration membrane according to any one of claims 1-3, wherein, The average pore size of the nanofiltration membrane is 0.2 - 0.5 nm, preferably 0.25 - 0.35 nm.

5. The positively charged nanofiltration membrane according to any one of claims 1-4, wherein, The thickness of the bottom layer is 30 - 150 μm, preferably 50 - 120 μm; Preferably, the thickness of the porous support layer is 10 - 100 μm, preferably 30 - 60 μm; Preferably, the thickness of the polyamide separation layer is 10 - 500 nm, preferably 50 - 300 nm.

6. A preparation method of a positively charged nanofiltration membrane, characterized in that, The preparation method comprises: S1. Prepare a porous support layer on the bottom layer; S2. The membrane layer obtained in step S1 is successively contacted with an aqueous phase containing a polyamine and a guanidine group compound containing an amino group for the first time, and contacted with an organic phase containing a polyacyl chloride for the second time, and after heat treatment, the positively charged nanofiltration membrane is obtained.

7. The preparation method according to claim 6, wherein, In step S2, the polyamine is selected from at least one of polyethyleneimine, triethylenetetramine, tetraethylenepentamine, diethylenetriamine and polyethylenepolyamine; Preferably, the concentration of the polyamine in the aqueous phase is 0.1 - 5 wt%; Preferably, the guanidine group compound containing an amino group has a structure shown in Formula 1 and / or Formula 2; Formula 2; wherein, R1 and R2 are each independently an alkyl group having 1 to 5 carbon atoms, preferably an alkyl group having 1 to 3 carbon atoms; Preferably, the concentration of the guanidine group compound containing an amino group in the aqueous phase is 0.05 - 1 wt%; Preferably, the polyacyl chloride is selected from at least one of 1,3,5-benzenetricarbonyl chloride, 1,3-benzenedicarbonyl chloride and 1,4-benzenedicarbonyl chloride; Preferably, the concentration of the polyacyl chloride in the organic phase containing the polyacyl chloride is 0.01 wt% - 1 wt%; Preferably, the amounts of the aqueous phase and the organic phase are such that the mass ratio of the polyamine, the guanidine group compound containing an amino group to the polyacyl chloride is 2 - 50:1 - 20:

1.

8. The positively charged nanofiltration membrane according to claim 6 or 7, wherein, The time of the first contact is 5 - 100 s, preferably 10 - 60 s; Preferably, the time of the second contact is 10 - 200 s, preferably 20 - 120 s; Preferably, the conditions of the heat treatment include: the heat treatment temperature is 40 - 150 °C, preferably 50 - 120 °C; the heat treatment time is 0.5 - 10 min, preferably 1 - 5 min.

9. A positively charged nanofiltration membrane prepared by the preparation method according to any one of claims 6 - 8.

10. Use of the positively charged nanofiltration membrane according to any one of claims 1 - 5 and 9 in the field of water treatment separation.

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