Micron bubble space template strategy assisted ultrathin polyamide composite membrane as well as preparation method and application thereof

Abstract

The invention discloses an ultrathin polyamide composite membrane assisted by a micron bubble space template strategy as well as a preparation method and application of the ultrathin polyamide composite membrane. The method comprises the following steps: firstly, adding a micron bubble precursor into a neutral / acidic aqueous solution, and generating micron bubbles by utilizing reaction between the precursor and the neutral / acidic aqueous solution or self-decomposition of the precursor; then, covering the water solution with an organic phase solution, and promoting the transmission of the micron bubbles from the water solution to the organic phase solution under the assistance of ultrasonic waves; and finally, under a free interface polymerization process, the diffusion path and the diffusion rate of a water-phase monomer in an interface reaction area are effectively limited by utilizing the steric hindrance effect of micron bubbles on the interface organic phase side, a polyamide separation layer with an ultrathin structure is constructed, the transmembrane transmission resistance of water molecules is effectively reduced, the mass transfer path of the water molecules is shortened, and the separation efficiency is improved. The water flux of the composite membrane is obviously improved.

Description

Technical Field

[0001] The invention discloses an ultra-thin polyamide composite membrane prepared with the assistance of a micron bubble space template strategy, and belongs to the technical field of separation membrane preparation and application. Background Art

[0002] For thin composite membranes, the ideal polyamide separation layer should be as thin as possible and maintain high-precision selectivity. The preparation of such ultrathin polyamide layers typically involves the introduction of substances such as nanomaterials or regulators, or even the construction of an intermediate layer. Zhang et al. introduced sodium 4-hydroxybenzenesulfonic acid into an aqueous solution, utilizing the electrostatic interaction between the negatively charged sulfonic acid groups of the organic salt and the protonated amine monomers, as well as the hydrogen bonding between the hydroxyl groups of the organic salt and the amine monomers, to synergistically slow the diffusion rate of the aqueous monomers, thereby forming an ultrathin polyamide separation layer (Journal of Membrane Science 680 (2023) 121739). Patent CN 116371222 B discloses an ultrathin composite polyamide nanofiltration membrane, its preparation method, and application. A super-hydrophilic, amino-rich intermediate layer is constructed using three-dimensional porous melamine particles. The hydrogen bonding between the aqueous monomers and the porous melamine intermediate layer inhibits the diffusion of the aqueous monomers, thereby preparing an ultrathin polyamide separation layer and significantly improving the water flux of the nanofiltration membrane. The successful implementation of these strategies is mainly attributed to their modification of the diffusion state of monomers in the entire aqueous system, rather than fine-tuning the diffusion of aqueous monomers in the interfacial reaction zone.

[0003] Because aqueous monomers typically have high partition coefficients in organic solutions, this property drives interfacial polymerization reactions to occur primarily on the organic side of the interface. Therefore, the key to the rational design of ultrathin polyamide separation layers lies in specifically regulating the diffusion rate of aqueous monomers on the organic side of the interface, thereby achieving fine-tuned control of interfacial polymerization. Template-based approaches, with their spatially confined effects, offer a viable path to this goal. However, given the nanoscale dimensions of the polyamide separation layer, the distance between the template material and the interfacial reaction zone must be strictly controlled, and structural defects in the polyamide separation layer caused by template self-deposition must be avoided. Complexities between the template material and the interface make this approach difficult to achieve. Given these limitations, bubbles, commonly used as soft templates for functional nanomaterials, have attracted considerable attention. Studies have reported on the structural manipulation of polyamide separation layers using bubbles. These studies typically involve pre-treating the aqueous solution physically or chemically to create nanobubbles in the aqueous solution, or generating nanobubbles in situ during interfacial polymerization, thereby controlling the structure of the polyamide separation layer. However, nanobubbles in the aqueous solution or at the interface easily generate nanocavities in the polyamide separation layer during upward transmission, thereby increasing the thickness of the separation layer, which is in sharp conflict with the idea of ​​using the template method to design ultra-thin polyamide separation layers. Summary of the Invention

[0004] In view of this, the present invention proposes a method for preparing ultrathin polyamide composite membranes assisted by a micron bubble space template strategy. First, a micron bubble precursor is added to a neutral or acidic aqueous solution, and micron bubbles are generated by the reaction between the precursor and the neutral / acidic aqueous solution, or by the self-decomposition of the precursor; then, an organic phase solution is covered on the surface of the above aqueous solution, and with the assistance of ultrasound, the intense transmission of micron bubbles from the aqueous solution to the organic phase solution is promoted, thereby achieving the loading of micron bubbles in the organic phase solution; finally, under the free interface polymerization process, the steric hindrance of the micron bubbles on the organic phase side is utilized to effectively limit the diffusion path and diffusion rate of the aqueous phase monomers in the interfacial reaction zone, thereby constructing a polyamide separation layer with an ultrathin structure, effectively reducing the transmembrane transmission resistance of water molecules and shortening the mass transfer path, significantly enhancing the permeation separation performance of the polyamide composite membrane.

[0005] To achieve the above-mentioned purpose, the present invention adopts the following technical solution: an ultra-thin polyamide composite membrane prepared with the assistance of a micron bubble space template strategy, comprising a porous support layer and an ultra-thin polyamide separation layer on the porous support layer, the ultra-thin polyamide separation layer being prepared by interfacial polymerization reaction of an aqueous solution containing polyamines and an organic solution containing polyacyl chlorides and micron bubbles, the surface of the ultra-thin polyamide separation layer containing a collapsed bubble-like structure.

[0006] Preferably, the ultrathin polyamide separation layer has a thickness of 24-35 nm.

[0007] A method for preparing an ultra-thin polyamide composite membrane assisted by a micron bubble space template strategy specifically comprises the following steps: (1) Preparation of ultrathin polyamide composite membrane: a porous support layer is placed at the bottom of a reaction vessel. Under a free interfacial polymerization strategy, an aqueous solution containing polyamine, an organic buffer solution, and an organic solution containing polyacyl chloride and microbubbles are sequentially added to the surface of the support layer to carry out an interfacial polymerization reaction. The height of the aqueous solution is 2-10 mm, preferably 3-7 mm, the height of the organic buffer solution is 0.05-0.3 mm, preferably 0.1-0.2 mm, and the height of the organic solution is 1-10 mm, preferably 1-5 mm. (2) Post-treatment of the polyamide composite membrane: After the interfacial polymerization reaction is completed, the solution between the polyamide separation layer and the support layer is extracted through the support layer through the bottom outlet of the reaction vessel, so that the polyamide separation layer is deposited on the surface of the support layer to form an initial composite membrane. Subsequently, an organic solvent, which is the same as the organic phase solvent in the organic phase solution, is used to rinse the surface of the initial composite membrane, and the membrane is heat-treated to form a stable polyamide composite membrane.

[0008] Adding an organic buffer above the aqueous solution not only helps prevent the impact of direct diffusion of the organic phase on the uniformity of the separation layer during interfacial polymerization, but also promotes the rapid and uniform dispersion of microbubbles at the aqueous-organic interface, thereby fully utilizing their spatial template effect. The organic buffer is the same as the organic solvent in the organic solution.

[0009] The specific preparation method of the organic phase solution containing polyacid chloride and micron bubbles in step (1) is as follows: first, different types of micron bubble precursors are added to a neutral or acidic aqueous solution, wherein the precursor concentration in the neutral or acidic aqueous solution is 0.5-1.5 wt.%, preferably 0.6-1.0 wt.%; then, the organic phase solution is covered on the surface of the neutral or acidic aqueous solution, and the volume ratio of the aqueous solution to the organic phase solution is 1:1-1:5; then, ultrasonic treatment is performed 2-6 times, each time for 1-5 s, with an ultrasonic power of 200-400 W and a frequency of 20-50 kHz, to promote the violent transmission of micron bubbles in the neutral or acidic aqueous solution to the organic phase solution, thereby achieving the loading of micron bubbles in the organic phase solution. Of course, micron bubbles can also be loaded in the organic phase solution by other methods, such as aerating the organic phase solution using a micron bubble generator.

[0010] In a first implementation, a microbubble precursor reacts with a neutral aqueous solution to form microbubbles. The microbubble precursor includes, but is not limited to, any one or more of borohydride salts (e.g., lithium borohydride, sodium borohydride, potassium borohydride), hydride salts (e.g., lithium hydride, sodium hydride, potassium hydride, calcium hydride, magnesium hydride, aluminum hydride), carbide salts (e.g., sodium carbide, potassium carbide, calcium carbide, aluminum carbide), sodium superoxide, and potassium superoxide. After the precursor is added to the neutral aqueous solution, an organic phase solution is immediately added to the surface of the aqueous solution, followed by ultrasonic treatment to intensify the reaction between the precursor and the neutral aqueous solution, promote the transfer of microbubbles from the neutral aqueous solution to the organic phase solution, and achieve microbubble loading in the organic phase solution, thereby utilizing the microbubbles in the organic phase solution to control interfacial polymerization reactions.

[0011] As a second implementation, a microbubble precursor reacts with an acidic aqueous solution to form microbubbles. The microbubble precursor includes, but is not limited to, any one or more of carbonates, bicarbonates, percarbonates, sulfites, bisulfites, sodium hydrosulfide, potassium hydrosulfide, and pure metals (such as zero-valent copper and zero-valent iron). After the precursor is added to the acidic aqueous solution, an organic phase solution is immediately added to the surface of the aqueous solution, followed by ultrasonic treatment. This intensifies the reaction between the precursor and the acidic aqueous solution, promotes the transfer of microbubbles from the acidic aqueous solution to the organic phase solution, and achieves microbubble loading in the organic phase solution, thereby utilizing the microbubbles in the organic phase solution to control interfacial polymerization reactions.

[0012] As a third implementation method, a microbubble precursor, including but not limited to one or more of hydrogen peroxide, sodium peroxide, calcium peroxide, and sodium percarbonate, self-decomposes in a neutral aqueous solution to produce microbubbles. After the precursor is added to the neutral aqueous solution, an organic phase solution is immediately added to the surface of the aqueous solution, followed by ultrasonic treatment. This accelerates the self-decomposition of the precursor in the neutral aqueous solution and promotes the transfer of microbubbles from the neutral aqueous solution to the organic phase solution, achieving microbubble loading in the organic phase solution and utilizing the microbubbles in the organic phase solution to regulate interfacial polymerization reactions.

[0013] The pH value of the acidic aqueous solution is 1-6, and the adjustment method includes but is not limited to adding any one or more of hydrochloric acid, acetic acid, phosphoric acid, nitric acid, sulfuric acid, citric acid, and perchloric acid.

[0014] The porous support layer in step (1) is a support layer material commonly used in preparing organic composite membranes, including but not limited to any one of polysulfone, polyester, polycarbonate, polyacrylonitrile, and polyvinylidene fluoride.

[0015] In step (1), the polyamine includes but is not limited to any one or more of piperazine, 4-aminopiperazine, p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, triethylenetetramine, adamantyl diamine, 2,6-diaminotoluene, polyethyleneimine, ethylenediamine, β-cyclodextrin, diethylenetriamine, 3,5-dihydroxybenzoic acid, N,N-dimethylm-phenylenediamine, 3,5-dihydroxy-4-methylbenzoic acid, 4,4',4'',4'',4'''-methyltetraethyltetra(benzene-1,2-diamine), 4,6-diaminoresorcinol dihydrochloride, diethylamino-β-cyclodextrin, and 3,5-diamino-N-(4-aminophenyl)benzamide, and the concentration of the polyamine in the aqueous solution is 0.1-5.0 wt.%, preferably 0.3-3.0 wt.%; The organic phase solvent is a normal alkane, including but not limited to any one of n-pentane, n-hexane, n-heptane, n-octane, n-decane, n-undecane and n-dodecane; the polyacyl chlorides include but not limited to any one of isophthaloyl chloride, naphthalene dicarboxylic acid chloride, phthaloyl chloride, terephthaloyl chloride, biphenyl dicarboxylic acid chloride, trimesoyl chloride, cyclohexanetricarboxylic acid chloride, trimellitic anhydride chloride, 3,5-bis(sulfoxide amino)benzoyl chloride, cyclopentanetetracarboxylic acid chloride, 1,2,3,4-cyclobutanetetracarboxylic acid chloride, 2,4,4',6-biphenyltetracarboxylic acid chloride, 2,2',4,4',6,6'-biphenylhexacarboxylic acid chloride, 5-isocyanate isopeptide chloride, 5-(1-pyrrolidinyl)-1,3-isophthaloyl chloride and 5-(dichlorophosphoryl)isophthaloyl chloride, and the concentration of the polyacyl chloride in the organic phase solution is 0.01-0.4 wt.%, preferably 0.05-0.2 wt.%.

[0016] The interfacial polymerization time in step (1) is 10-180 s, preferably 45-90 s.

[0017] The polyamide composite membrane in step (1) includes but is not limited to any one of a nanofiltration membrane, a forward osmosis membrane, and a reverse osmosis membrane.

[0018] In step (2), the heat treatment temperature is 50-90 °C and the time is 1-5 min.

[0019] Furthermore, the ultrathin polyamide composite membrane prepared with the aid of the micron bubble space template strategy is used in separation processes suitable for nanofiltration membranes, forward osmosis membranes, and reverse osmosis membranes, such as liquid-liquid separation and gas-gas separation.

[0020] Compared with the prior art, the present application has the following beneficial effects: (1) Compared with the method of adding a regulator or a nanomaterial, the microbubble space template strategy of the present application is simple and low in cost, avoids rough adjustment of the monomer diffusion rate in the whole aqueous phase system, and realizes fine regulation of the diffusion path and diffusion rate of the monomer in the aqueous phase in the interface reaction zone through the steric hindrance effect of the microbubbles on the interface organic phase side; (2) Compared with the sacrificial intermediate layer strategy, the present application effectively avoids damage to the fine structure of the polyamide separation layer that may be caused by the etching process, and ensures the compactness of the separation layer; (3) Compared with the general bubble regulation strategy, the present application realizes the loading of microbubbles in the organic phase solution through a pretreatment method, effectively limits the thickness of the polyamide separation layer by using the microbubble space template, breaks the general cognition that the nanocavity structure in the separation layer is regulated by bubbles, and opens up a new path for the combination of bubbles and the polyamide separation layer; (4) The microbubble space template strategy of the present application has strong universality, can be applied to microbubble precursors of various reaction types, and can be flexibly applied to the preparation of various ultra-thin polyamide composite membranes, and has high practical application value. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 For Comparative Example 1, the surface and cross-sectional SEM images of the polyamide reverse osmosis (RO) membrane prepared without introducing microbubbles.

[0022] Figure 2 For Example 2, the surface and cross-sectional SEM images of the ultra-thin polyamide RO membrane prepared by H2 microbubbles.

[0023] Figure 3 For Examples 1 and 3, the cross-sectional SEM images of the ultra-thin polyamide RO membrane prepared by H2 microbubbles.

[0024] Figure 4 For Comparative Example 2, the surface and cross-sectional SEM images of the polyamide RO membrane prepared by H2 nanobubbles.

[0025] Figure 5 For Example 4, the surface and cross-sectional SEM images of the ultra-thin polyamide RO membrane prepared by CO2 microbubbles.

[0026] Figure 6 For Example 5, the surface and cross-sectional SEM images of the ultra-thin polyamide RO membrane prepared by O2 microbubbles.

[0027] Figure 7 For Comparative Example 3, the cross-sectional SEM images of the polyamide nanofiltration (NF) membrane prepared without introducing microbubbles and Example 6 prepared by H2 microbubbles. DETAILED DESCRIPTION

[0028] The application provides a preparation method of a micro-bubble spatial template strategy assisted ultra-thin polyamide composite membrane. First, a micro-bubble precursor is added to a neutral or acidic aqueous solution, and micro-bubbles are generated through the reaction between the precursor and the neutral / acidic aqueous solution or the self-decomposition of the precursor. Then, an organic phase solution is covered on the surface of the aqueous solution, and the transmission of the micro-bubbles from the aqueous solution to the organic phase solution is promoted under the assistance of ultrasonic waves, so that the micro-bubbles are loaded in the organic phase solution. Finally, under the free interface polymerization process, the spatial steric hindrance of the micro-bubbles on the organic phase side effectively limits the diffusion path and diffusion rate of the water phase monomer in the interface reaction zone, promotes the formation of an ultra-thin polyamide separation layer (thickness: 24-35 nm), and significantly enhances the permeation and separation performance of the polyamide composite membrane. The technical solutions will be described in detail through the embodiments and the drawings. Obviously, the described embodiments are only part of the embodiments of the application, and do not represent all the embodiments.

[0029] Comparative Example 1 A preparation method of a polyamide RO membrane, specifically: First, the outlet at the bottom of the reaction container is connected with a vacuum pump; then, a polysulfone support layer is placed in the reaction container (the radius is about 40 mm); subsequently, an aqueous solution containing 2.0 wt.% m-phenylenediamine (MPD) (the height is about 2.4 mm), 0.5 mL n-hexane buffer (the height is about 0.1 mm), and an n-hexane solution containing 0.1 wt.% trimesoyl chloride (TMC) (the height is about 1 mm) are poured into the device in sequence; the interfacial polymerization reaction time is about 60 s; subsequently, the reaction container is filtered by the vacuum pump to discharge the solution between the polysulfone support layer and the polyamide separation layer, so that the prepared polyamide separation layer is deposited on the surface of the polysulfone support layer; finally, the reverse osmosis composite membrane is taken out by the pulling method, washed with n-hexane, and heat treated in an 80 ℃ oven for 3 min.

[0030] Example 1 A preparation method of an H2 micro-bubble spatial template strategy assisted ultra-thin polyamide RO membrane, specifically: First, 0.6 wt.% sodium borohydride was added to the neutral aqueous solution, and a n-hexane solution containing 0.1 wt.% TMC was transferred to the surface of the above-mentioned neutral aqueous solution. Ultrasonication was performed for 5 s and repeated twice to intensify the reaction between sodium borohydride and the aqueous solution, promote the transfer of H2 micron bubbles from the neutral aqueous solution to the organic phase solution (n-hexane solution containing 0.1 wt.% TMC), and realize the loading of H2 micron bubbles in the organic phase solution. The ultrasonic power was 300 W and the frequency was 40 kHz. The n-hexane solution containing H2 micron bubbles and 0.1 wt.% TMC above the reaction solution was drawn and poured onto 0.5 mL of n-hexane buffer (height of about 0.1 mm) according to the method of Comparative Example 1; except that the n-hexane solution containing 0.1 wt.% TMC was replaced by the n-hexane solution containing H2 micron bubbles and 0.1 wt.% TMC, the subsequent experimental steps and conditions were the same as those in Comparative Example 1.

[0031] Examples 2 and 3: In Examples 2 and 3, the concentrations of sodium borohydride in the neutral aqueous solution were 0.8 wt.% and 1.0 wt.%, respectively, and the subsequent experimental steps and conditions were the same as those in Example 1.

[0032] Comparative Example 2: A method for preparing a polyamide RO membrane assisted by a H2 nanobubble space template strategy, specifically: First, 0.4 wt.% sodium borohydride was added to a neutral aqueous solution, and a n-hexane solution containing 0.1 wt.% TMC was transferred to the surface of the neutral aqueous solution. Ultrasonication was performed for 5 s and repeated twice to intensify the reaction between sodium borohydride and the aqueous solution, promote the transfer of H2 nanobubbles from the neutral aqueous solution to the organic phase solution, and realize the loading of H2 nanobubbles in the organic phase solution. The ultrasonic power was 300 W and the frequency was 40 kHz to obtain a n-hexane solution containing H2 nanobubbles and 0.1 wt.% TMC. Except for replacing the n-hexane solution containing 0.1 wt.% TMC with a n-hexane solution containing H2 nanobubbles and 0.1 wt.% TMC, the subsequent experimental steps and conditions were the same as in Example 1.

[0033] Example 4: A method for preparing an ultra-thin polyamide RO membrane assisted by a CO2 micron bubble space template strategy, specifically: First, the pH value of the pure aqueous solution was adjusted to about 2 using hydrochloric acid, 0.8 wt.% sodium bicarbonate was added to the above acidic aqueous solution, and a n-hexane solution containing 0.1 wt.% TMC was transferred to the surface of the above aqueous solution, and ultrasonication was performed for 5 s and repeated twice to intensify the reaction between sodium bicarbonate and the acidic aqueous solution, promote the transmission of CO2 micron bubbles from the acidic aqueous solution to the organic phase solution, realize the loading of CO2 micron bubbles in the organic phase solution, and obtain a n-hexane solution containing CO2 micron bubbles and 0.1 wt.% TMC; except that the n-hexane solution containing 0.1 wt.% TMC was replaced with a n-hexane solution containing CO2 micron bubbles and 0.1 wt.% TMC, the subsequent experimental steps and conditions were the same as in Example 1.

[0034] Example 5: A method for preparing an ultra-thin polyamide RO membrane assisted by an O2 micron bubble space template strategy, specifically: First, 0.8 wt.% sodium percarbonate was added to a neutral aqueous solution, and a n-hexane solution containing 0.1 wt.% TMC was transferred to the surface of the above aqueous solution. Ultrasonication was performed for 5 s and repeated twice to intensify the decomposition of sodium percarbonate, promote the transmission of O2 micron bubbles from the neutral aqueous solution to the organic phase solution, and realize the loading of O2 micron bubbles in the organic phase solution to obtain a n-hexane solution containing O2 micron bubbles and 0.1 wt.% TMC; except that the n-hexane solution containing 0.1 wt.% TMC was replaced by a n-hexane solution containing O2 micron bubbles and 0.1 wt.% TMC, the subsequent experimental steps and conditions were the same as in Example 1.

[0035] Comparative Example 3: A method for preparing a polyamide NF membrane: Compared with Comparative Example 1, the aqueous solution of this comparative example does not contain MPD but contains 0.4 wt.% piperazine, and all other conditions are the same.

[0036] Example 6: A method for preparing ultrathin polyamide NF membrane assisted by H2 micron bubble space template strategy: Compared with Example 1, the aqueous solution of this example does not contain MPD but contains 0.4 wt.% piperazine, and all other conditions are the same.

[0037] Test Example 1: The performance test process of polyamide RO membrane is as follows: The water flux and salt rejection of the RO membrane were tested in a cross-flow filtration device. The operating temperature was 25 °C, the operating pressure was 2 MPa, the feed liquid was 2000 ppm NaCl solution, and the effective filtration area of ​​the RO membrane was 12.56 cm 2, the operation time should be maintained for at least 1 h to keep the water flux stable.

[0038] Test Example 2: The performance test process of the polyamide NF membrane is as follows: the water flux and salt rejection of the NF membrane are tested in a cross-flow filtration device. The operating temperature is 25 °C, the operating pressure is 0.5 MPa, the feed liquid is 1000 ppm Na2SO4 solution, and the effective filtration area of ​​the NF membrane is 12.56 cm 2 , the operation time should be maintained for at least 1 h to keep the water flux stable.

[0039] Table 1 is a data sheet of water flux and salt rejection of ultra-thin polyamide RO membranes prepared by the auxiliary preparation of H2, CO2, and O2 micron bubble space template strategies. The results of Comparative Example 1 and Examples 1-3 show that under the steric hindrance of the H2 micron bubble template, the prepared RO membrane water flux is significantly improved. As the concentration of sodium borohydride increases, the RO membrane water flux shows a trend of first increasing and then decreasing, and this change may be related to the subtle difference in the thickness of the polyamide separation layer. Among them, Example 2 shows the highest water flux, which is 2.22 times that of Comparative Example 1. By careful comparison, it can be found that the polyamide RO membrane salt rejection prepared by the H2 micron bubble space template strategy increases slightly, which may be related to the diffusion of amine monomers to the organic phase restricted by H2 micron bubbles. Amine monomers accumulate in the interfacial reaction zone below the micron bubbles, causing the polyamide separation layer structure to be more dense.

[0040] In addition, the micron bubble space template strategy proposed in the present invention has strong universality. By adjusting the way in which the precursor produces micron bubbles, the structure and performance of the RO membrane can also be improved. In Examples 4 and 5, the micron bubble precursors are replaced with sodium bicarbonate and sodium percarbonate, respectively. The CO2 micron bubbles formed by the reaction of sodium bicarbonate with an acidic aqueous solution, and the O2 micron bubbles generated by the self-decomposition of sodium percarbonate in a neutral aqueous solution, are used to load two different types of gas micron bubbles in the organic phase solution, respectively, effectively limiting the diffusion path and diffusion rate of the amine monomer on the organic phase side of the interface. Compared with Comparative Example 1, the RO membrane prepared accordingly also exhibits high water flux and maintains a relatively reasonable salt rejection rate. Table 2 is a data correspondence table of water flux and salt rejection of ultra-thin polyamide NF membranes assisted by the H2 micron bubble space template strategy. It can be seen from Table 2 that the H2 micron bubble space template strategy proposed in the present invention is also suitable for the preparation of high-performance NF membranes, wherein Example 6 exhibits a higher water flux, which is about 3 times that of Comparative Example 3.

[0041] Table 1 Water flux and salt rejection of RO membranes prepared in Comparative Examples 1-2 and Examples 1-5 Case Micron Bubble-Concentration (wt.%) <![CDATA[水通量(L·m −2 ·h −1 )]]> Salt rejection rate (%) Polyamide layer thickness (nm) Comparative Example 1 <![CDATA[H2-0]]> 41.8 98.0 128 ± 11 Comparative Example 2 <![CDATA[H2-0.4]]> 71.7 98.7 292 ± 22 Example 1 <![CDATA[H2-0.6]]> 80.8 98.5 31 ± 1 Example 2 <![CDATA[H2-0.8]]> 92.9 98.4 27 ± 1 Example 3 [H2-1.0] 77.6 98.4 33 ± 2 Example 4 <![CDATA[CO2-0.8]]> 83.5 98.3 28 ± 1 Example 5 <![CDATA[O2-0.8]]> 94.9 98.2 25 ± 1 Table 2 Water flux and salt rejection of NF membranes prepared in Comparative Example 3 and Example 6 Case Micron Bubble-Concentration (wt.%) <![CDATA[水通量(L·m −2 ·h −1 )]]> Salt rejection rate (%) Polyamide layer thickness (nm) Comparative Example 3 <![CDATA[H2-0]]> 18.7 97.7 70 ± 5 Example 6 [H2-0.6] 55.3 98.0 31 ± 5 Figure 1 The surface of the RO membrane in Comparative Example 1 is relatively smooth, and the thickness of the polyamide separation layer is about 128 nm. After loading micro-bubbles in the organic phase by pre-treatment, with the assistance of the H2 micro-bubble space template, Figure 2 The RO membrane surface shows a collapsed, bubble-like polyamide structure. Compared with Comparative Example 1, the thickness of the polyamide separation layer in Example 2 was reduced by approximately 80%, to only 26 nm. The formation of this ultra-thin polyamide separation layer significantly reduced the transmembrane transport resistance of water molecules and shortened the mass transfer path, which is beneficial for improving the water flux of the RO membrane. As the sodium borohydride concentration decreases (Example 1) or increases (Example 3), the size of the H2 micron bubbles on the organic phase side of the interface changes, thereby affecting the cross-interface diffusion path and diffusion rate of the amine monomer, resulting in slight differences in the thickness of the polyamide separation layer. Figure 3 The thickness of the polyamide separation layer of Examples 1 and 3 is significantly lower than that of Comparative Example 1. When sodium borohydride is excessively reduced to generate nanobubbles, Figure 4 The separation layer thickness of Comparative Example 2 increases by approximately an order of magnitude compared to Examples 1-3, and the separation layer contains numerous nanocavities, primarily due to the weak diffusion restriction of the amine monomer by the nanobubbles. Although the discrete nanocavities within the separation layer of Comparative Example 2 facilitate rapid water transport, the thickness of the separation layer affects the water flux, as shown in Table 1. The ultrathin separation layer structures of Examples 1-3 still yield lower water flux.

[0042] In addition, the strategy of regulating the structure of the polyamide separation layer by micron bubbles is independent of the type of gas. The precursors are replaced with sodium bicarbonate and sodium percarbonate, respectively. With the assistance of CO2 and O2 micron bubbles, Figure 5 and 6 The polyamide separation layers in Examples 4 and 5 exhibited similar morphological features to those in Example 2: collapsed, bubble-like polyamide structures. Furthermore, the thicknesses of both polyamide separation layers were relatively low, approximately 28 and 25 nm, respectively, demonstrating the universal applicability of the microbubble spatial template strategy.

[0043] More importantly, the micron bubble space template strategy proposed in the present invention is also applicable to the preparation of ultrathin NF membranes. Figure 7 a shows that the thickness of the polyamide separation layer of the NF membrane in Comparative Example 3 is about 70 nm. On the contrary, under the limitation of the H2 micron bubble space template on the organic phase side of the interface, Figure 7 b shows that the thickness of the polyamide separation layer of Example 6 is significantly reduced to about 31 nm.

[0044] The above results show that by utilizing the reaction between the precursor and the neutral / acidic aqueous solution, or the self-decomposition of the precursor to produce micron bubbles, and promoting the loading of the micron bubbles in the organic phase solution through a pretreatment process, the thickness of the polyamide separation layer can be effectively limited, and the preparation of ultra-thin polyamide composite membranes can be achieved, which is beneficial to improving the water flux of the composite membrane and maintaining its salt rejection rate. This regulation strategy is applicable to micron bubbles of various gas properties and various types of polyamide composite membranes.

Claims

1. An ultra-thin polyamide composite membrane assisted by a micron bubble space template strategy, characterized in that: It includes a porous support layer and an ultra-thin polyamide separation layer on the porous support layer. The ultra-thin polyamide separation layer is prepared by interfacial polymerization reaction between an aqueous solution containing polyamine and an organic solution containing polyacyl chloride and micron bubbles. The surface of the ultra-thin polyamide separation layer contains a collapsed bubble-like structure.

2. The ultra-thin polyamide composite membrane assisted by the micron bubble space template strategy according to claim 1, characterized in that: The thickness of the ultrathin polyamide separation layer is 24-35 nm.

3. A method for preparing an ultra-thin polyamide composite membrane assisted by the micron bubble space template strategy according to claim 1, characterized in that: The following steps are involved: (1) Preparation of ultrathin polyamide composite membranes: A porous support layer is placed at the bottom of a reaction vessel. Under a free interfacial polymerization strategy, an aqueous solution containing polyamine, an organic buffer solution, and an organic solution containing polyacyl chloride and microbubbles are sequentially added to the surface of the support layer to carry out interfacial polymerization. (2) Post-treatment of polyamide composite membrane: After the interfacial polymerization reaction is completed, the solution between the polyamide separation layer and the support layer is extracted through the support layer through the bottom outlet of the reaction vessel, so that the polyamide separation layer is deposited on the surface of the support layer to form an initial composite membrane. Subsequently, the surface of the initial composite membrane is rinsed with an organic solvent, and the membrane is heat-treated to form a stable polyamide composite membrane.

4. The method for preparing an ultra-thin polyamide composite membrane assisted by a micron bubble space template strategy according to claim 3, characterized in that: The height of the aqueous phase solution is 2-10 mm, the height of the organic buffer solution is 0.05-0.3 mm, and the height of the organic phase solution is 1-10 mm.

5. The method for preparing an ultra-thin polyamide composite membrane assisted by a micron bubble space template strategy according to claim 3, characterized in that: The polyamine includes any one or more of piperazine, 4-aminopiperazine, p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, triethylenetetramine, adamantyl diamine, 2,6-diaminotoluene, polyethyleneimine, ethylenediamine, β-cyclodextrin, diethylenetriamine, 3,5-dihydroxybenzoic acid, N,N-dimethylm-phenylenediamine, 3,5-dihydroxy-4-methylbenzoic acid, 4,4',4'',4'',4'''-methyltetraethyltetra(benzene-1,2-diamine), 4,6-diaminoresorcinol dihydrochloride, diethylamino-β-cyclodextrin, and 3,5-diamino-N-(4-aminophenyl)benzamide, and its concentration is 0.1-5.0 wt.%; The polyvalent acid chloride includes any one or more of isophthaloyl chloride, naphthalene dicarbonyl chloride, phthaloyl chloride, terephthaloyl chloride, biphenyl dicarbonyl chloride, trimesoyl chloride, cyclohexane trimesoyl chloride, trimellitic anhydride chloride, 3,5-bis(sulfoxide amino)benzoyl chloride, cyclopentanetetracarbonyl chloride, 1,2,3,4-cyclobutanetetracarbonyl chloride, 2,4,4',6-biphenyltetracarbonyl chloride, 2,2',4,4',6,6'-biphenylhexacarbonyl chloride, 5-isocyanate isopeptide chloride, 5-(1-pyrrolidinyl)-1,3-isophthaloyl chloride, and 5-(dichlorophosphoryl)isophthaloyl chloride, and the concentration thereof is 0.01-0.4 wt.%.

6. The method for preparing an ultra-thin polyamide composite membrane assisted by a micron bubble space template strategy according to claim 3, characterized in that: The organic buffer solution in step (1) and the organic solvent in step (2) are the same as the organic phase solvent in the organic phase solution, and the organic phase solvent is a normal alkane, including any one of n-pentane, n-hexane, n-heptane, n-octane, n-decane, n-undecane, and n-dodecane.

7. The method for preparing an ultra-thin polyamide composite membrane assisted by a micron bubble space template strategy according to claim 3, characterized in that: The specific preparation method of the organic phase solution containing polyacid chloride and micron bubbles in step (1) is as follows: first, different types of micron bubble precursors are added to a neutral or acidic aqueous solution, and the precursor concentration in the neutral or acidic aqueous solution is 0.5-1.5 wt.%; then, the organic phase solution is covered on the surface of the above neutral or acidic aqueous solution, and the volume ratio of the aqueous solution to the organic phase solution is 1:1-1:5; then, ultrasonic treatment is performed 2-6 times, each time for 1-5 s, the ultrasonic power is 200-400 W, and the frequency is 20-50 kHz.

8. The method for preparing an ultra-thin polyamide composite membrane assisted by a micron bubble space template strategy according to claim 7, characterized in that: A micron bubble precursor reacts with a neutral aqueous solution to form micron bubbles, wherein the micron bubble precursor comprises any one or more of borohydride salt, hydrogenated salt, carbonate salt, sodium superoxide, and potassium superoxide; Alternatively, a micron bubble precursor reacts with an acidic aqueous solution to form micron bubbles, wherein the micron bubble precursor comprises any one or more of carbonate, bicarbonate, percarbonate, sulfite, bisulfite, sodium hydrosulfide, potassium hydrosulfide, and pure metal; Alternatively, a micron bubble precursor self-decomposes in a neutral aqueous solution to generate micron bubbles, and the micron bubble precursor includes any one or more of hydrogen peroxide, sodium peroxide, calcium peroxide, and sodium percarbonate.

9. The method for preparing an ultra-thin polyamide composite membrane assisted by a micron bubble space template strategy according to claim 3, characterized in that: The porous support layer material includes any one of polysulfone, polyester, polycarbonate, polyacrylonitrile and polyvinylidene fluoride; the polyamide composite membrane includes any one of nanofiltration membrane, forward osmosis membrane and reverse osmosis membrane.

10. Use of the polyamide composite membrane according to claim 1 or 2, or the polyamide composite membrane prepared by the method according to any one of claims 2 to 9 in a separation process.

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