Synergistic emulsion induced polyamide nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention discloses a synergistic emulsion induced polyamide nanofiltration membrane and a preparation method thereof, which are inspired by a synergistic emulsion effect and utilize an anionic surfactant at a water-oil interface and a nano material with negative charges to jointly influence interface characteristics. The organic phase is stably dispersed on the water surface in the form of oil drops in the instant time when the organic phase is in contact with the water phase through the electrostatic repulsion effect between the organic phase and the water phase, so that an annular interfacial polymerization template is constructed, and finally the polyamide nanofiltration membrane with the coffee annular structure is prepared. The coffee annular structure increases the volume of a nanometer cavity in the separation layer and the effective surface filtering area, and the induced drainage ditch effect shortens the transverse transmission path of water molecules in the separation layer to the maximum extent, so that the funnel effect is weakened; meanwhile, the water permeability of the nanofiltration membrane is further improved by reducing the thickness of the separation layer and increasing the surface wettability. The invention provides a new technology and a new direction for constructing a high-performance nanofiltration membrane with a specific surface structure.

Description

TECHNICAL FIELD

[0001] The application discloses a synergistic emulsion-induced polyamide nanofiltration membrane and belongs to the technical field of membrane separation. BACKGROUND

[0002] Membrane separation technology is an advanced and novel separation technology in the field of water treatment. In particular, nanofiltration membrane, as a pressure-driven membrane, has a separation performance between ultrafiltration membrane and reverse osmosis membrane, can effectively intercept multivalent ions and various organic compounds, and is widely used in fields such as drinking water purification, papermaking and printing, seawater softening and wastewater treatment.

[0003] However, the polyamide nanofiltration membrane prepared by the traditional interfacial polymerization method usually has the problem of low water flux, and there is an inherent 'trade off' effect between water permeability and selectivity. This means that increasing water flux often leads to a decrease in salt rejection rate, and increasing salt rejection rate reduces water flux. Therefore, it is still an important challenge in the field of nanofiltration membrane technology to improve water flux while maintaining high salt rejection rate. Current research mainly realizes the improvement of water flux by regulating the physical and chemical properties of the polyamide layer, and the two main ways are to increase the effective permeation area of the polyamide separation layer and to optimize the internal pore structure. Patent CN118059688A high-performance polyamide nanofiltration membrane and its preparation method and application uses polymerizable amphiphilic molecules and auxiliary ultraviolet initiation reaction to construct a monolayer membrane at the water-oil interface, thereby changing the interface characteristics and promoting the diffusion of monomers, preparing a polyamide layer with rich wrinkle morphology and larger surface area, and effectively improving the water flux. Patent CN105642133B polyamide / COFs hybrid nanofiltration composite membrane and its preparation method hybridizes covalent organic framework into the separation layer of the composite membrane, and uses the unique nanochannel structure in the covalent organic framework to provide more water channels for the nanofiltration membrane. However, these methods for improving water flux usually only rely on a single strategy of increasing the permeation area of the polyamide separation layer or optimizing the internal pore structure, and thus there is still certain limitation in improving water permeability. Forming a coffee ring structure on the surface of the polyamide separation layer is an effective means to improve the water flux of the nanofiltration membrane. For example, patent CN118925519A discloses a preparation method of a protein-induced polyamide nanofiltration membrane, which slows down the release of piperazine in the water phase by adding proteins, forms a special structure of stripes and coffee rings, provides additional water channels, and greatly improves the water flux.

[0004] Traditional emulsions and Pickering emulsions are emulsion systems stabilized by surfactants or nanoparticles as emulsifiers, respectively, which often need high concentrations of emulsifiers for formation and are prone to separation or instability during use. To address the above research deficiencies, researchers have proposed a new type of synergistic emulsion stabilized by nanoparticles and ionic surfactants with the same charge, which includes an aqueous phase, an oil phase, a surfactant and nanoparticles, forming a stable oil-in-water system. In this system, surfactant molecules spontaneously adsorb at the water-oil interface, significantly reducing the interfacial tension and imparting a surface charge to the oil droplets, thereby causing electrostatic repulsion between the oil droplets. At the same time, the charged nanoparticles are dispersed in the aqueous medium between the oil droplets, not only producing electrostatic repulsion with the surfactant, but also forming a physical barrier, which further prevents the aggregation and coalescence of oil droplets. This method of using surfactants and charged nanoparticles to synergistically stabilize the water-oil interface significantly improves the stability of the system. Inspired by the synergistic emulsion system, the present application provides a synergistic emulsion-induced polyamide nanofiltration membrane and a preparation method thereof. SUMMARY

[0005] To improve the permeability and selectivity of the polyamide nanofiltration membrane, inspired by the synergistic emulsion system, the present application uses anionic surfactants and negatively charged nanomaterials at the water-oil interface to affect the interfacial properties, and through the strong electrostatic repulsion between the two, the organic phase is stably dispersed in the form of oil droplets on the water surface within the instant time of contact with the aqueous phase, thereby forming a ring-shaped interfacial polymerization template. Finally, by affecting the growth process of the polyamide, a polyamide nanofiltration membrane with a coffee ring structure separation layer on the surface is prepared. The coffee ring structure not only effectively increases the effective filtration area, but also increases the volume of the nanocavity inside the separation layer, which maximizes the drainage channel effect and shortens the lateral transmission path of water molecules in the polyamide layer, thereby weakening the "funnel effect"; and the negatively charged nanoparticles affect monomer diffusion through electrostatic interaction, steric hindrance effect, etc., further changing the effective thickness, wettability and charge of the polyamide layer, effectively improving the water flux of the nanofiltration membrane while ensuring the salt rejection rate.

[0006] To achieve the above purpose, the present application adopts the following technical solutions:

[0007] A preparation method of a synergistic emulsion-induced polyamide nanofiltration membrane, specifically comprising the following steps:

[0008] (1) Pour the aqueous solution containing negatively charged nanoparticles and anionic surfactants uniformly on the surface of the support layer, immerse for a period of time, and then remove the excess mixed solution on the surface of the support layer.

[0009] (2) The organic phase solution is evenly poured on the surface of the support layer for a period of time, and the polyamine monomer in the aqueous phase solution and the polyacyl chloride monomer in the organic phase solution undergo interfacial polymerization to form a polyamide separation layer, and the polyamide nanofiltration membrane is obtained after post-processing.

[0010] The nanoparticles and surfactants in step (1) can independently and spontaneously adsorb to the water-oil interface and stabilize the interface. Among them, the nanoparticles are any one of solid particles in Pickering emulsion, such as starch nanoparticles, silica nanoparticles, Fe3O4nanoparticles, surface hydrophilic modified graphene / oxidized graphene / carbon nanotubes, titanium dioxide nanoparticles, aluminum oxide nanoparticles, zinc oxide nanoparticles, and layered double hydroxide nanoparticles, preferably starch nanoparticles, and the concentration of the nanoparticles in the aqueous phase solution is 0.005-0.03wt.%, or any two numerical limits in the range of 0.005wt.%, 0.01wt.%, 0.015wt.%, 0.02wt.%, 0.025wt.%, 0.03wt.%. Anionic surfactants are divided into carboxylate, sulfonate, sulfate and phosphate, such as any one of sodium dodecyl sulfate, fatty alcohol polyoxyethylene ether sodium sulfate, sodium dodecyl sulfonate, sodium dodecyl benzene sulfonate, sodium stearate, potassium laurate, sodium lauroyl amino acid, and potassium mono / di-dodecyl phosphate, and the concentration of the surfactants in the aqueous phase solution is 0.02-0.05wt.%.

[0011] The polyamine and polyacyl chloride monomers in steps (1) and (2) are substances capable of interfacial polymerization known to those skilled in the art. The polyamine is any one of ethylenediamine, hexanediamine, piperazine, p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, m-phenylenediamine, diethylenetriamine, triethylenetetramine, N,N-dimethyl-m-phenylenediamine, 2,6-diaminotoluene, and 4-aminomethylpiperazine. The polyacyl chloride is any one of m-phenyltrimethyl chloride, m-phenyldicarboxylic chloride, cyclohexane dicarboxylic chloride, cyclohexane tricarboxylic chloride, cyclobutane dicarboxylic chloride, cyclobutane tetracarboxylic chloride, 5-isocyanate isophthaloyl chloride, 3,4',5-biphenyl tricarboxylic chloride, and 2,4,4',6-biphenyl tetracarboxylic chloride. The concentration of the polyamine in the aqueous phase solution is 0.1-5.0wt.%, preferably 0.5-2.0wt.%, and the concentration of the polyacyl chloride in the organic phase solution is 0.01-0.5wt.%, preferably 0.05-0.2wt.%.

[0012] The time for the aqueous phase solution to soak in step (1) is 1-8min, preferably 2-6min. The time for the organic phase solution to soak in step (2) is 0.1-3min, preferably 0.5-2min. The post-processing temperature is 50-90℃, and the post-processing time is 1-8min, preferably 3-5min.

[0013] The surface active agent at the water phase-organic phase interface in the application quickly coats the organic phase solution and instantaneously forms oil phase droplets (oil droplets) at the interface, the nanoparticles are dispersed between the oil droplets to hinder the fusion and aggregation of the oil droplets, thereby forming a ring-shaped interface polymerization template, and the polybasic amine in the water phase solution diffuses upward to react with the polybasic acid chloride in the oil phase droplets to form a polyamide nanofiltration membrane with a coffee ring structure.

[0014] Compared with the prior art, the application has the following beneficial effects:

[0015] (1) The negatively charged nanoparticles and anionic surfactant used in the application both have the ability to spontaneously adsorb to the water-oil interface and stabilize the water-oil interface by reducing the interfacial tension. The negatively charged nanoparticles and anionic surfactant at the water-oil interface are used to construct an interface polymerization template to prepare a polyamide nanofiltration membrane with a coffee ring structure, thereby improving the permeation and separation performance thereof; (2) Compared with the technology of using a single surfactant to improve the filtration area of the polyamide or using nanoparticles to improve the water channel inside the polyamide, the application constructs a coffee ring Turing structure on the surface of the membrane, which not only increases the water permeation area of the polyamide layer, but also provides a highly ordered nanocavity structure, thereby effectively promoting the rapid transmission of water molecules while maintaining good divalent salt retention rate; (3) The application uses the negatively charged nanoparticles to increase the concentration of the water phase monomer on the surface of the support layer, and the diffusion of the water phase monomer is inhibited by electrostatic interaction and steric hindrance effect, thereby effectively reducing the thickness of the polyamide layer, improving the hydrophilicity and negative charge of the membrane surface, and improving the permeability and separation ability of the nanofiltration membrane for Cl - / SO4 2- . BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 The plane SEM images of the polyamide nanofiltration membranes prepared in Comparative Examples 1-3.

[0017] Figure 2 The plane SEM images of the polyamide nanofiltration membranes prepared in Examples 1-6.

[0018] Figure 3 The plane SEM images of the polyamide nanofiltration membranes prepared in Example 7 and Comparative Examples 4-5.

[0019] Figure 4 The cross-sectional SEM images of the polyamide nanofiltration membranes prepared in Comparative Example 3 and Examples 2, 4 and 6.

[0020] Figure 5 The plane SEM images of the polyamide nanofiltration membranes prepared in Examples 9-12. DETAILED DESCRIPTION

[0021] The application will be further described in conjunction with specific examples, which are only used to explain the application, and are not a limitation of the application.

[0022] Example 1

[0023] A preparation method of a synergistic emulsion-induced polyamide nanofiltration membrane, specifically:

[0024] (1) 20 g of corn starch was added to dilute sulfuric acid with a concentration of 3.16 M to prepare a dispersion (15 wt.%). The dispersion was stirred at 40°C and 200 rpm for 7 days for acid hydrolysis. After acid hydrolysis, the dispersion was washed with deionized water at a speed of 10,000 rpm until it was neutral, then diluted to 2 mg / mL with deionized water, and finally homogenized to obtain a starch nanoparticle / water suspension. The starch nanoparticles had a particle size range of 10-40 nm, and the BET characterization showed that they had a microporous structure inside.

[0025] (2) 1.25 ml of the starch nanoparticle / water suspension prepared in (1) was added to an aqueous solution containing 0.05 wt.% sodium dodecyl sulfate and 1.0 wt.% piperazine monomer to obtain a mixed solution, wherein the starch nanoparticles accounted for 0.005 wt.% of the mass of the mixed solution.

[0026] (3) The mixed solution in (2) was uniformly poured onto the surface of the support layer, and after standing for 5 min, the excess mixed solution on the surface of the support layer was removed, and then an organic phase solution containing 0.2 wt.% trimesoyl chloride-n-hexane was uniformly poured onto the surface of the support layer. After 1 min of interfacial polymerization, the excess organic phase solution was poured out, the membrane surface was washed with n-hexane to remove the unreacted organic phase solution. Then it was placed in an oven at 60°C for 3 min for heat treatment, and a polyamide nanofiltration membrane was obtained, which was stored in deionized water for use.

[0027] Example 2

[0028] (1) The preparation steps of the starch nanoparticle / water suspension were the same as in Example 1.

[0029] (2) 2.5 ml of the suspension in step (1) was added to an aqueous solution containing 1.0 wt.% piperazine monomer and 0.05 wt.% sodium dodecyl sulfate to obtain a mixed solution, wherein the starch nanoparticles accounted for 0.01 wt.% of the mass of the mixed solution. Except for changing the mass concentration of starch nanoparticles in the mixed solution, the other experimental steps were the same as in Example 1.

[0030] Example 3

[0031] (1) The preparation steps of the starch nanoparticle / water suspension were the same as in Example 1.

[0032] (2) Take 5 ml of the suspension from step (1) and add it to an aqueous solution containing 1.0 wt.% piperazine monomer and 0.05 wt.% sodium dodecyl sulfate to obtain a mixed solution in which the starch nanoparticles account for 0.02 wt.% of the mass of the mixed solution. Except for changing the mass concentration of the starch nanoparticles in the mixed solution, other experimental steps are the same as in Example 1.

[0033] Example 4:

[0034] (1) The preparation steps of the starch nanoparticle / water suspension are the same as in Example 1.

[0035] (2) Take 5 ml of the suspension from step (1) and add it to an aqueous solution containing 1.0 wt.% piperazine monomer and 0.05 wt.% sodium dodecyl sulfate to obtain a mixed solution in which the starch nanoparticles account for 0.02 wt.% of the mass of the mixed solution. Except for changing the mass concentration of the starch nanoparticles in the mixed solution, other experimental steps are the same as in Example 1.

[0036] Example 5:

[0037] (1) The preparation steps of the starch nanoparticle / water suspension are the same as in Example 1.

[0038] (2) Take 6.25 ml of the suspension from step (1) and add it to an aqueous solution containing 1 wt.% piperazine monomer and 0.05 wt.% sodium dodecyl sulfate to obtain a mixed solution in which the starch nanoparticles account for 0.025 wt.% of the mass of the mixed solution. Except for changing the mass concentration of the starch nanoparticles in the mixed solution, other experimental steps are the same as in Example 1.

[0039] Example 6:

[0040] (1) The preparation steps of the starch nanoparticle / water suspension are the same as in Example 1.

[0041] (2) Take 7.5 ml of the suspension from step (1) and add it to an aqueous solution containing 1 wt.% piperazine monomer and 0.05 wt.% sodium dodecyl sulfate to obtain a mixed solution in which the starch nanoparticles account for 0.03 wt.% of the mass of the mixed solution. Except for changing the mass concentration of the starch nanoparticles in the mixed solution, other experimental steps are the same as in Example 1.

[0042] Example 7:

[0043] (1) The preparation steps of the starch nanoparticle / water suspension are the same as in Example 1.

[0044] (2) Take 10 ml of the suspension in step (1) and add it to an aqueous solution containing 1 wt.% of piperazine monomer and 0.05 wt.% of sodium dodecyl sulfate to obtain a mixed solution, in which the starch nanoparticles account for 0.04 wt.% of the mass of the mixed solution. Except for changing the mass concentration of the starch nanoparticles in the mixed solution, other experimental steps are the same as those in Example 1.

[0045] Example 8:

[0046] Except that sodium dodecylbenzenesulfonate is used instead of sodium dodecyl sulfate, other experimental steps are the same as those in Example 1.

[0047] Example 9:

[0048] Negatively charged silica nanoparticles with an average size of 80-90 nm and a surface with hydroxyl groups are used instead of starch nanoparticles, which account for 0.01 wt.% of the mass of the mixed solution, and other experimental steps are the same as those in Example 2.

[0049] Example 10:

[0050] Except that the silica nanoparticles account for 0.02 wt.% of the mass of the mixed solution, other experimental steps are the same as those in Example 9.

[0051] Example 11:

[0052] Negatively charged graphene oxide with a lateral size of about 100 nm and a surface with hydroxyl groups is used instead of starch nanoparticles, which account for 0.01 wt.% of the mass of the mixed solution, and other experimental steps are the same as those in Example 2.

[0053] Example 12:

[0054] Except that the graphene oxide accounts for 0.02 wt.% of the mass of the mixed solution, other experimental steps are the same as those in Example 11.

[0055] Comparative Example 1:

[0056] No starch nanoparticles and sodium dodecyl sulfate are doped in the aqueous solution, and other experimental steps are the same as those in Example 1, and finally a polyamide nanofiltration membrane is obtained.

[0057] Comparative Example 2:

[0058] 0.02 wt.% of starch nanoparticles are doped in the aqueous solution, and no sodium dodecyl sulfate is contained, and other experimental steps are the same as those in Example 1, and finally a polyamide nanofiltration membrane is obtained.

[0059] Comparative Example 3:

[0060] The water phase solution contained 0.05 wt.% sodium dodecyl sulfate, and the starch nanoparticles were not doped. The other experimental procedures were the same as those in Example 1, and finally the polyamide nanofiltration membrane was obtained.

[0061] Comparative Example 4:

[0062] The water phase solution contained 0.075 wt.% sodium dodecyl sulfate, and the starch nanoparticles were not doped. The other experimental procedures were the same as those in Example 1, and finally the polyamide nanofiltration membrane was obtained.

[0063] Comparative Example 5:

[0064] The water phase solution contained 0.06 wt.% starch nanoparticles, and no sodium dodecyl sulfate was contained. The other experimental procedures were the same as those in Example 1, and finally the polyamide nanofiltration membrane was obtained.

[0065] Test Example 1:

[0066] The prepared polyamide nanofiltration membrane was subjected to permeation separation performance test:

[0067] The self-made cross-flow filtration equipment was used to evaluate the permeability and selectivity of the membrane. The test conditions were as follows: the membrane filtration area was 12.56 cm 2 , the test pressure was 5 bar, the test temperature was 25±1℃, and the feed liquid was 2000 ppm Na2SO4 aqueous solution. The corresponding calculation formulas of water flux (Flux, L·m -2 ·h -1 ) and salt retention rate (R, %) were as follows. Before measurement, all the membranes were compacted for 1 h to achieve steady state. Wherein, A was the effective area of the solution permeating the membrane (m 2 ), t was the permeation time (h), V was the permeation volume of the solution collected within t h (L), C p and C f were the salt concentrations of the permeation solution and the feed solution, respectively.

[0068]

[0069] Test Example 2:

[0070] The prepared polyamide nanofiltration membrane was subjected to NaCl / Na2SO4 binary mixed salt solution separation performance test to evaluate its ion sieving ability for Cl - / SO4 2- . The feed liquid was a mixed solution of 1000 ppm NaCl and 1000 ppm Na2SO4, and the other test conditions were the same as those in Test Example 1. The calculation formula of Cl - / SO4 2- separation factor was as follows:

[0071]

[0072] By Figure 1 It can be observed that the polyamide nanofiltration membrane surface presents the traditional globular nodule structure when no or only one of the substances (starch nanoparticles or sodium dodecyl sulfate) is added in the aqueous phase solution. Figure 2 For the SEM images of the membrane surface in Examples 1-6, when starch nanoparticles and sodium dodecyl sulfate are introduced in the aqueous phase solution at the same time, a small amount of semi-ring structure appears on the membrane surface. With the increase of the content of starch nanoparticles, the surface microstructure of the membrane has changed significantly, and the coffee ring-like structure in different states appears. This shows that only in the presence of starch nanoparticles and sodium dodecyl sulfate, the formation of the coffee ring-like polyamide structure on the membrane surface is facilitated by the synergistic effect of the two, and the reason may be that there is a water phase liquid layer containing uniformly dispersed negatively charged starch nanoparticles and anionic sodium dodecyl sulfate surfactant on the polysulfone support layer, and the starch nanoparticles and sodium dodecyl sulfate with interfacial adsorption properties are enriched at the air / water interface. When the organic phase solution contacts the aqueous phase solution, surfactant molecules quickly adsorb at the water-oil interface, reducing the interfacial tension and imparting negative charge to the oil droplets. At the same time, the negatively charged starch nanoparticles at the interface are dispersed around the oil droplets, not only generating electrostatic repulsion with the anionic head group of sodium dodecyl sulfate, but also forming a relatively thick physical barrier, which further prevents the aggregation and fusion of oil droplets, thereby promoting the formation of regular ring-like templates at the water-oil interface. When the piperazine molecules in the aqueous phase solution gradually diffuse to the oil droplets, interfacial polymerization occurs, and finally a polyamide separation layer with coffee ring nanostructure characteristics is formed.

[0073] In particular, by separately increasing the concentration of sodium dodecyl sulfate or starch nanoparticles in the aqueous phase solution to prepare a polyamide nanofiltration membrane, the coffee ring-like structure cannot be formed on the surface of the PA layer (Comparative Examples 4-5, Figure 3 ), further proving that the formation of the coffee ring-like polyamide structure requires the joint action of starch nanoparticles and sodium dodecyl sulfate.

[0074] Table 1. Water flux and Na2SO4 rejection rate of polyamide nanofiltration membranes.

[0075]

[0076] From the performance test results in Table 1, the membrane with the coffee ring structure surface has a higher water flux under the joint action of starch nanoparticles and sodium dodecyl sulfate, which may be related to the ring structure and the nanocavity structure below it. The ring structure on the membrane surface significantly increases the permeation area of water molecules, and the nanocavity below it promotes the rapid transmission of water molecules. At the same time, the drainage ditch effect induced by the nanocavity maximally shortens the lateral transmission path of water molecules in the polyamide layer, effectively alleviating the "funnel effect". In addition, the negative electric starch nanoparticles not only delay the diffusion of the monomer by steric hindrance, electrostatic interaction and the like generated by the piperazine monomer, but also increase the storage amount of the water phase monomer on the support layer surface through the hydroxyl group on the surface, promote the rapid formation of a polyamide dense layer in the initial stage of interfacial polymerization, delay the diffusion of the piperazine monomer to the organic phase, and ultimately lead to a decrease in the effective thickness of the polyamide layer Figure 4 ), thereby shortening the water transmission path and being conducive to the improvement of water flux.

[0077] Among them, Example 4 has the highest water flux, which is 29.97 L·m -2 ·h -1 , which is nearly 2 times the water flux of the polyamide nanofiltration membrane in Comparative Example 3. The improvement of water flux is mainly due to the coffee ring Turing structure on the membrane surface, which not only increases the water permeation area of the polyamide layer, but also enriches the nanocavity structure inside the polyamide layer, providing a rapid transmission channel for water molecules. At the same time, the decrease in the thickness of the polyamide layer and the increase in the surface wettability also promote the improvement of water flux. However, the excessive loading of starch nanoparticles in the water phase solution (Examples 5-6, Figure 2 ) may change the interfacial behavior, thereby destroying the interfacial polymerization template, leading to the gradual disappearance of the coffee ring structure on the membrane surface, which further reduces the number of nanocavities below the coffee ring, thereby causing the water flux of Examples 5-6 to gradually decrease. It should be noted that the coffee ring structure is not observed on the membrane surface in Example 7 with the highest loading amount Figure 3 ), indicating that the interfacial polymerization template is completely destroyed, resulting in a significant decrease in the water flux thereof. However, the water flux of Example 7 is still higher than that of Comparative Example 3, which may be due to the decrease in the thickness of the membrane.

[0078] Table 2. Water contact angle, Zeta potential at pH = 7 and Cl - / SO4 2- separation factor of polyamide nanofiltration membranes

[0079]

[0080] From Table 2, it can be concluded that the enhancement of the negative surface charge of the membranes in the examples is not only attributed to the hydroxyl groups carried by the starch nanoparticles. Based on the above analysis, the addition of the negatively charged starch nanoparticles also promotes the initial interfacial polymerization reaction to generate a polyamide layer with high cross-linking degree, which in turn limits the further diffusion of the piperazine monomer. This situation can lead to a large amount of unreacted trimesoyl chloride on the membrane surface to undergo hydrolysis, thereby generating more carboxyl groups and increasing the negative surface charge of the membrane. Although the increase in the negative surface charge of the membrane helps to enhance the electrostatic repulsion of anions (Table 2), the introduction of starch nanoparticles can change the network structure of the polyamide layer. When the content of starch nanoparticles is low (Examples 1-4), it has little effect on the dense structure of the polyamide layer. Under the condition of enhanced negative surface charge of the membrane, the polyamide membrane still maintains a high salt rejection rate. However, when the content of nanoparticles is excessively increased (Examples 5-7), the polyamide layer can generate more non-continuous regions or defects, thereby leading to a decrease in the salt rejection rate. In addition, Table 2 shows the relationship between the addition amount of starch nanoparticles in the aqueous solution and the Cl - / SO4 2- separation factor of the polyamide nanofiltration membrane. It can be concluded that the addition of a low concentration of starch nanoparticles can effectively improve the Cl - / SO4 2- separation factor of the polyamide nanofiltration membrane.

[0081] In addition, the starch nanoparticles are replaced by negatively charged silica or graphene oxide, Figure 5 It is shown that under the synergistic effect of sodium dodecyl sulfate, a coffee ring structure can be formed on the surface of the membrane. Compared with the surface of the membrane with a silica or graphene oxide addition amount of 0.02 wt.%, the coffee ring size on the surface of the membrane with an addition amount of 0.01 wt.% is smaller and the number is more, which is consistent with the trend when the starch nanoparticles are added. It should be noted that under the condition of the same addition amount of nanoparticles, the change in the morphology and size of the material can lead to certain differences in the size and number of the coffee rings on the surface of the membrane. The performance test results show that when the addition amount of nanoparticles is 0.02 wt.%, the water flux of Example 10 and Example 12 is similar to that of Example 4, but the salt retention rate is much lower than that of Example 4 (Table 1 and Table 3). This can be attributed to the difference in the morphology and size of the material. The larger size of the nanoparticles not only affects the formation of the coffee ring-shaped nanoturing structure on the surface of the membrane, but also affects the density of the network structure inside the polyamide layer.

[0082] Table 3. Water flux and Na2SO4 retention rate of the polyamide nanofiltration membrane.

[0083]

Claims

1. A method for the preparation of a synergistic emulsion-induced polyamide nanofiltration membrane, characterized in that, The method comprises the following steps: a water phase solution containing both negatively charged nanoparticles and anionic surfactant is uniformly poured on the surface of the support layer, after a period of immersion, the excess mixed solution is removed, the concentration of the negatively charged nanoparticles in the water phase solution is 0.005 - 0.03 wt.%; an organic phase solution is uniformly poured on the surface of the support layer for a period of time, the polyamine monomer in the water phase solution and the polyacyl chloride monomer in the organic phase solution undergo interfacial polymerization to form a polyamide separation layer, and a nanofiltration membrane is obtained after post-processing.

2. The method for preparing a synergistic emulsion-induced polyamide nanofiltration membrane according to claim 1, characterized in that, The nanoparticles are any one of starch nanoparticles, silica nanoparticles, Fe3O4 nanoparticles, titanium dioxide nanoparticles, aluminum oxide nanoparticles, zinc oxide nanoparticles, layered double hydroxide nanoparticles, and hydrophilic modified graphene / oxidized graphene / carbon nanotubes.

3. The method for preparing a synergistic emulsion-induced polyamide nanofiltration membrane according to claim 1, characterized in that, The anionic surfactant is any one of sodium dodecyl sulfate, sodium fatty alcohol polyoxyethylene ether sulfate, sodium dodecyl sulfonate, sodium dodecyl benzene sulfonate, sodium stearate, potassium laurate, sodium lauroyl sarcosinate, and potassium mono / di-dodecyl phosphate, and the concentration of the anionic surfactant in the water phase solution is 0.02 - 0.05 wt.%.

4. The method for preparing a synergistic emulsion-induced polyamide nanofiltration membrane according to claim 1, characterized in that, The polyamine monomer in the water phase solution is any one of ethylenediamine, hexanediamine, piperazine, p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, murexide, diethylenetriamine, triethylenetetramine, N,N-dimethyl-m-phenylenediamine, 2,6-diaminotoluene, and 4-aminomethylpiperazine, the organic solvent is any one of n-hexane, n-heptane, and isomeric alkanes, and the polyacyl chloride is any one of trimesoyl chloride, isophthaloyl chloride, cyclohexanedicarboxylic acid chloride, cyclohexanetricarboxylic acid chloride, cyclobutane dicarboxylic acid chloride, cyclobutane tetracarboxylic acid chloride, 5-isocyanate isophthaloyl chloride, 3,4',5-biphenyl tricarboxylic acid chloride, and 2,4,4',6-biphenyl tetracarboxylic acid chloride.

5. The method for preparing a synergistic emulsion-induced polyamide nanofiltration membrane according to claim 1, characterized in that, The concentration of the polyamine monomer in the water phase solution is 0.1 - 5.0 wt.%, the concentration of the polyacyl chloride monomer in the organic phase solution is 0.01 - 0.5 wt.%, the water phase solution is immersed for 1 - 8 min, the organic phase solution is immersed for 0.1 - 3 min, the post-processing temperature is 50 - 90℃, and the post-processing time is 1 - 8 min.

6. A polyamide nanofiltration membrane prepared by the method of any one of claims 1 - 5.

7. The use of the polyamide nanofiltration membrane of claim 6 in the field of desalination, in particular in the field of selective separation of mono- / di-valent anions.

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