PVA composite membrane based on PMIA porous substrate and preparation method and application thereof

Abstract

The invention discloses a PVA composite membrane based on a PMIA porous substrate and a preparation method and application of the PVA composite membrane based on the PMIA porous substrate, an SE / PMIA membrane is used as a supporting layer, a P (AA-AMPS) cross-linked PVA solution is used as a separation layer, and the PVA composite desalination membrane is successfully prepared through an optimized membrane preparation process. The supporting layer of the membrane has high hydrophilicity, a multi-surface crack pore structure and a proper pore size, so that the mass transfer resistance of water vapor molecules at a composite membrane interface and in a substrate pore channel can be effectively reduced, the transmission distance of the water vapor molecules is reduced, and the permeation efficiency of the water vapor molecules is improved; the ultra-thin compact PVA separation layer can enhance the mass transfer process, breakthrough is achieved from the separation layer and the supporting layer, then the pervaporation flux of the composite membrane is effectively improved, the membrane separation effect is optimized, and the composite membrane has wide application prospects in the saline water treatment fields such as brackish water desalination, seawater desalination and industrial wastewater resource treatment.

Description

Technical Field

[0001] The present invention relates to a PVA composite membrane, and particularly to a PVA composite membrane based on a PMIA porous substrate, its preparation method and application; it belongs to the technical field of preparation of novel membrane materials. Background Art

[0002] In recent years, the pervaporation (PV) desalination technology has gradually come into view and has been widely used in organic solvent dehydration, volatile compound removal, and separation technology of organic mixtures. In the desalination system, the feed liquid is regarded as a water / salt mixture, and water molecules pass through the PV membrane in the form of water vapor. Due to the non-volatility of salts and impurities and the high denseness of hydrophilic polymer membranes or the adjustable pore size of inorganic membranes, the PV membrane can intercept 99% of inorganic salts and impurities in high-salt water or seawater without the need for pretreatment operations such as sedimentation of the feed liquid. Since PV is a phase change process and does not have to overcome the continuously increasing osmotic pressure during the concentration of salt water like reverse osmosis (RO), the PV technology can treat high-concentration salt water and is an efficient desalination technology that helps to develop high-quality water, has simple pretreatment requirements, and is suitable for various energy supplies.

[0003] The key to membrane technology lies in membrane materials. As a molecular-level selective barrier between the feed solution and the permeate, the PV membrane participates in the mass transfer and separation processes. PV desalination membrane materials are mainly divided into three types: inorganic membranes, organic membranes, and mixed matrix membranes. Inorganic membranes have the characteristics of high thermal stability, good chemical stability, high mechanical strength, and low fouling rate. Their rigid structure and precise pore size dimensions help to improve selectivity. Widely used ones include Al 2 O 3 membranes, graphene membranes, etc. However, the forming process of inorganic membranes is relatively complex, resulting in high costs and difficulty in large-scale preparation, which limits their application and promotion. Organic membrane materials are currently the most widely used PV membrane materials and have the advantages of wide material sources, low costs, good processability, and strong scalability. However, since the PV membrane needs to be used in a high-temperature environment, the organic membrane has poor thermal stability and low mechanical strength, which limits its application and development. To address the above bottlenecks, it is usually necessary to modify it, such as grafting, cross-linking, etc.

[0004] Among numerous PV organic membrane materials, polyvinyl alcohol (PVA) is an ideal membrane material for pervaporation desalination because of its good hydrophilicity and film-forming property. It can not only promote the transport of water molecules in the membrane but also effectively reduce membrane fouling. Compared with the currently well-studied high-performance inorganic pervaporation desalination membrane materials, such as covalent-organic frameworks (COFs) and carbon nanotubes (CNTs), etc., PVA materials have the significant advantages of simple preparation methods and low costs. PVA itself contains a large number of hydroxyl groups and has the advantages of good water solubility, excellent film-forming performance, and stable physical and chemical properties. Currently, the vast majority of PV desalination membranes use composite membranes composed of PVA as the separation layer and a support substrate. Taking the PVA composite membrane as the research object, from the perspective of mass transfer, water molecules preferentially adsorb and diffuse through the free volume of the dense hydrophilic PVA membrane. The water permeability depends to a large extent on the thickness of the separation layer, the affinity of water molecules on the membrane surface, and the diffusion coefficient of water molecules in the separation layer. At the same time, the thickness of the support substrate, the internal pore structure (pore size, porosity, pore connectivity), and the surface pore structure (pore size, pore size distribution, surface porosity), etc. will also affect the transport of water molecules.

[0005] In summary, the structure and chemical properties of the PVA composite membrane play a decisive role in the separation effect of the pervaporation process. In order to overcome the disadvantage of poor permeation performance of the PVA composite membrane in the prior art, it is necessary to overcome the technical bottleneck problem of its low flux by regulating the structure and performance of the composite membrane, so as to further develop its great application potential in desalination of high-salt aqueous solution environments such as brackish water and seawater. Summary of the Invention

[0006] To solve the deficiencies of the prior art, the purpose of the present invention is to provide a PVA composite membrane based on a PMIA porous substrate, its preparation method and application. The surface of this composite membrane is smooth, dense, and defect-free. The PMIA substrate with high porosity and appropriate pore size can effectively reduce the transport resistance of water molecules at the interface of the composite membrane and broaden the transport path; the ultra-thin PVA separation layer helps to further improve the flux and separation performance of the composite membrane in pervaporation.

[0007] To achieve the above goals, the present invention adopts the following technical solutions:

[0008] The present invention first discloses a preparation method of a PVA composite membrane based on a PMIA porous substrate, including the following steps:

[0009] S1. Prepare the PMIA substrate

[0010] Mix lithium chloride and DMAc solvent and stir at high speed to dissolve, forming a transparent homogeneous solution. Then add modifier SE and stir to form a uniform and stable solution. Next, add a certain amount of fully dried PMIA fibers into the above solution, heat up to 70 - 100 °C and dissolve thoroughly to form a uniform, homogeneous and stable casting solution. After cooling to room temperature in air and standing for degassing, a PMIA substrate membrane is formed by non-solvent induced phase separation method;

[0011] S2. Prepare PVA / P(AA - AMPS) separation layer solution

[0012] Dissolve polyvinyl alcohol in deionized water, heat and stir in an oil bath to obtain a transparent and homogeneous PVA solution. After the solution cools to room temperature, add a certain mass of P(AA / AMPS) crosslinking agent, which can effectively prevent PVA from swelling. Then use H 2 SO 4 to adjust the pH of the casting solution to 1. H 2 SO 4 acts as a catalyst to promote the crosslinking of PVA and P(AA / AMPS). Finally, stir magnetically at room temperature for a period of time to obtain a transparent and homogeneous PVA / P(AA / AMPS) separation layer solution, and stand for degassing for standby;

[0013] S3. Prepare PVA / PMIA composite membrane

[0014] Prepare the PVA / PMIA composite membrane by using a knife coating or spraying process.

[0015] Preferably, the mass percentage content of the aforementioned modifier SE is 0.2 wt.%.

[0016] More preferably, the mass percentage content of the aforementioned PMIA is not less than 10 wt.%.

[0017] Even more preferably, in the aforementioned step S2, the mass ratio of PVA to the crosslinking agent P(AA - AMPS) is 7:3.

[0018] As a preference, in the aforementioned step S3, the process of preparing the composite membrane by using the knife coating process is as follows: coat the PVA / P(AA - AMPS) separation layer solution on the dried PMIA porous substrate membrane with a 10 - μm wire bar or an automatic film - coating machine, and then heat - treat the as - cast membrane in an oven at 100 °C for 20 - 60 min to obtain the PVA composite membrane, and store it in deionized water.

[0019] As a further preference, in the aforementioned step S3, the process of preparing the composite membrane by the spraying process is as follows: paste the dried PMIA porous substrate membrane obtained in step S1 onto a glass plate, spray the PVA / P(AA-AMPS) separation layer solution onto its surface with a spray gun, and finally transfer the sprayed membrane to an oven at 100 °C for thermal cross-linking for 15 - 60 min.

[0020] More preferably, the nozzle of the aforementioned spray gun is 15 cm away from the membrane surface on the glass plate, the horizontal moving speed of the glass plate is 6 cm·s -1 , the oscillation distance perpendicular to the horizontal direction is 3 cm, and the spray pressure is 0.5 - 4.5 bar.

[0021] Even more preferably, the aforementioned thermal cross-linking time is 45 min and the spray pressure is 2.5 bar.

[0022] The present invention also claims to protect a PVA composite membrane based on a PMIA porous substrate prepared by the aforementioned method. The surface of the composite membrane is dense and defect-free, and the PVA ultra-thin separation layer and the PMIA substrate have an integral membrane cross-section structure.

[0023] The present invention also claims to protect the application of the aforementioned PVA composite membrane based on a PMIA porous substrate in the treatment of saline water, including brackish water desalination, seawater desalination, and the resource treatment of industrial wastewater, etc.

[0024] The advantages of the present invention are as follows:

[0025] (1) By adding the amphiphilic additive SE, the present invention realizes the regulation of the pore structure of the PMIA substrate, effectively reduces the mass transfer resistance of water molecules at the composite membrane interface, provides a shorter mass transfer path, and lays a foundation for the successful preparation of a high-performance pervaporation desalination composite membrane.

[0026] (2) The PVA composite membrane prepared by the present invention has a smooth, dense and defect-free surface. The ultra-thin PVA separation layer is beneficial to further improve the flux of the composite membrane during the pervaporation process, achieving good separation performance. In the long-term operation experiment of more than 1000 hours, the performance of the membrane basically remains unchanged, showing excellent stability.

[0027] (3) By the spraying method and optimizing the spraying process, an ultra-thin PVA pervaporation composite membrane with the best performance is prepared. When treating 3.5 wt.% NaCl solution at 75 °C, the pervaporation flux of the PVA / SE-11 / PMIA composite membrane is as high as 127.62 ± 2.82 Kg·m -2 ·h -1 , and the rejection rate is 99.98%. It has good application prospects in actual seawater separation, brackish water desalination, the resource treatment of industrial wastewater, etc. Description of the Drawings

[0028] Figure 1 Structural diagrams of the surfaces and cross-sections of substrates S1 to S3 and M1 to M5;

[0029] Figure 2 Flow pressure test curves and pore size distribution diagrams of each substrate membrane;

[0030] Figure 3 FTIR characterization diagrams of the PVA / PMIA composite membrane (cast composite membrane: Comparative Example 9, Comparative Example 10, Example 1, and Example 2);

[0031] Figure 4 FTIR characterization diagrams of the PVA / PMIA composite membrane (sprayed composite membrane: Example 9);

[0032] Figure 5 Structural diagrams of the surfaces and cross-sections of the PVA / PMIA composite membranes of each example and comparative example;

[0033] Figure 6 Diagrams of the detection results of the permeation flux and rejection rate of the composite membranes of Examples 1 to 2;

[0034] Figure 7 Diagrams of the detection results of the permeation flux and rejection rate of the composite membranes of Examples 3 to 6;

[0035] Figure 8 Diagrams of the detection results of the permeation flux and rejection rate of the composite membranes of Examples 7 to 11;

[0036] Figure 9 Diagrams of the detection results of the permeation flux and rejection rate of the composite membranes of Comparative Example 11 and Examples 12 to 14;

[0037] Figure 10 Diagrams of the detection results of the permeation flux and rejection rate of the PVA / PMIA composite membrane with the optimal performance for real Bohai Sea water;

[0038] Figure 11 Diagrams of the detection results of the salt ion removal rate of the PVA / PMIA composite membrane with the optimal performance for real Bohai Sea water. Detailed Description of the Invention

[0039] The present invention will be specifically introduced below in conjunction with the drawings and specific examples.

[0040] Unless otherwise specified in the present invention, all raw materials used are commercially available, and the preferred commercial purchase channels are as follows:

[0041]

[0042] Table 1 List of Preferred Commercial Purchase Channels for Each Raw Material

[0043] The present invention first prepares a PMIA substrate membrane: a pure PMIA porous membrane or an SE / PMIA porous membrane. Then, using polyvinyl alcohol (PVA) as the material for the separation layer membrane and acrylate-2-acrylamide-2-methylpropanesulfonic acid copolymer (P(AA-AMPS)) as the crosslinking agent, a PVA casting solution is prepared, and finally an ultrathin PVA pervaporation composite membrane is obtained.

[0044] The specific preparation process includes the following three major steps:

[0045] (I) Preparation of the PMIA substrate

[0046] At room temperature, a certain concentration of lithium chloride and DMAc solvent are mixed and stirred at high speed to dissolve, forming a transparent homogeneous solution. Depending on the situation, SE is added to the solution and stirred continuously until it is dissolved into a uniform and stable solution. Then, a certain amount of fully dried PMIA fibers are added to the above solution, and the temperature is raised to 70-100 °C for full dissolution. After forming a uniform and stable casting solution, it is cooled to room temperature in the air and left to stand for defoaming. After the defoaming is completed, a PMIA substrate membrane or an SE / PMIA substrate membrane is formed by the non-solvent induced phase separation method.

[0047] The codes of each prepared membrane and the proportion of raw materials used are shown in Table 2 below.

[0048] Membrane code PMIA (wt%) LiCl (wt%) DMAc (wt%) SE (wt%) S1 10 3.5 86.5 0 S2 12 3.5 84.5 0 S3 12 4 84 0 M1 12 3.5 84.3 0.2 M2 12 3.5 84.1 0.4 M3 12 4 83.8 0.2 (SE-3) M4 12 4 83.8 0.2 (SE-11) M5 12 4 83.8 0.2 (SE-15)

[0049] Table 2 Membrane codes and raw material proportions of each PMIA substrate

[0050] (II) Preparation of the PVA / P(AA-AMPS) separation layer solution

[0051] Polyvinyl alcohol (PVA) is dissolved in deionized water and stirred in an oil bath at 95 °C for 2 h to obtain a transparent and homogeneous 0.5 wt.% PVA solution. After the solution is cooled to room temperature, a certain mass of P(AA / AMPS) crosslinking agent is added (to prevent PVA from swelling), and the mass ratio of PVA to the crosslinking agent is 7:3. Then, H 2 SO 4 is used to adjust the pH of the casting solution to 1 (H 2 SO 4 is used as a catalyst to promote the crosslinking of PVA and P(AA / AMPS)). Finally, it is magnetically stirred at room temperature for a period of time to obtain a transparent and homogeneous PVA / P(AA / AMPS) separation layer solution, which is left to stand for defoaming and used as a standby.

[0052] (III) Preparation of the PVA / PMIA composite membrane

[0053] There are the following two film-forming methods in the present invention:

[0054] (1) Film formation by scraping: Coat the PVA / P(AA-AMPS) separation layer solution on the dried PMIA porous substrate film with a 10-μm wire bar or an automatic film scraping machine. Then, heat-treat the as-prepared film in an oven at 100 °C for 20 - 60 min to obtain the PVA composite film, and store it in deionized water.

[0055] (2) Film formation by spraying: Paste the dried PMIA porous substrate film prepared above onto a glass plate, and spray the PVA / P(AA-AMPS) separation layer solution onto its surface with a spray gun. The nozzle is 15 cm away from the film surface on the glass plate, the horizontal moving speed of the glass plate is 6 cm·s -1 , the oscillating distance perpendicular to the horizontal direction is 3 cm, and the spray pressures are 0.5, 1.5, 2.5, 3.5, and 4.5 bar respectively. Finally, transfer the sprayed film to an oven at 100 °C for thermal cross-linking for 15, 30, 45, and 60 min respectively.

[0056] The present invention has 11 comparative examples and 14 examples. The preparation steps are the same as above. The main differences among the comparative examples and examples lie in the use of different substrates, film formation processes, film formation parameters, etc. See Table 3 below for details.

[0057]

[0058] Table 3 Specific process parameters of the composite films in each example and comparative example

[0059] Structure characterization and performance detection

[0060] (1) Micro-morphology and structure of the substrate

[0061] The surface and cross-sectional morphology structures of substrates S1 - S3 and M1 - M5 are as Figure 1 shown. Among them, S1 - S3 are pure PMIA substrates. Their surfaces have uniform round holes and long crack holes. As the PMIA content increases from 10 wt.% to 12 wt.%, the viscosity of the casting solution continuously increases, and at the same time, the average pore diameter on the substrate surface decreases from 0.1655 μm to 0.0918 μm. Due to the low solid content of the S1 substrate, the cell structure at the bottom of the film occupies most of the volume. However, for the S2 - S3 substrates, due to the increase in solid content and the increase in the viscosity of the casting solution, high viscosity often reduces the solvent / nonsolvent exchange rate and delays the growth of large pores, and the film structure gradually transforms from a cell structure into a finger-like pore structure, and the pore connectivity weakens, which is consistent with the change trend of the film permeation flux to be characterized later.

[0062] M1 - M5 are SE / PMIA substrates added with amphiphilic surfactants. This substrate has long crack pores. The cross-sectional structure shows that the substrate is supported by a top epidermal layer and a porous layer with a bottom cellular structure or finger-like structure. The pore channels have strong connectivity and do not cause the mechanical properties of the substrate to deteriorate like the cellular structure, and there will be no collapse during the application process. From Figure 1 it can be seen that compared with the pure PMIA membrane, the crack pores on the surface of the PMIA membrane added with SE are significantly increased. This may be because the addition of SE couples the phase inversion process and the self-assembly process of the amphiphilic surfactant, making the surface porosity and regularity of the membrane gradually improve. Among them, local small holes appear on the surface of the SE3 / PMIA (M3) membrane, which may be caused by the microphase separation of SE-3 in the casting solution.

[0063] The PWF of each substrate membrane was detected, and the results are shown in Table 4 below. This shows that the crack pores with high porosity are beneficial to improving the permeability of the support layer in the composite membrane and thus improving the overall pervaporation performance of the composite membrane.

[0064] Membrane number Pure water permeation flux (PWF) S1 <![CDATA[648.41 L·m -2 ·h -1 > S2 <![CDATA[502.85L·m -2 ·h -1 > S3 <![CDATA[511.27L·m -2 ·h -1 > M1 <![CDATA[966.25L·m -2 ·h -1 > M2 <![CDATA[735.17L·m -2 ·h -1 > M3 <![CDATA[1023.64L·m -2 ·h -1 > M4 <![CDATA[750.89L·m -2 ·h -1 > M5 <![CDATA[729.14L·m -2 ·h -1 >

[0065] Table 4 Detection results of PWF of each substrate membrane

[0066] (2) Detection of substrate pore size and pore size distribution

[0067] Research shows that the pore size distribution of the substrate is crucial for the preparation and performance of the composite membrane. In this invention, the bubble pressure method is used to test the pore size and pore size distribution of the substrate. The flow pressure test curves and pore size distributions of each substrate membrane are as Figure 2 shown. Wet Line: The wet line represents the relationship between pressure and flow rate when the pores are filled with liquid (such as mercury). The starting point of the wet line usually corresponds to the bubble point pressure of the largest pore size in the material, that is, the lowest pressure at which the liquid begins to enter the pores. As the pressure increases, the liquid gradually fills the pores, and the shape of the wet line can reflect the characteristics of the pore size distribution. Dry Line: The dry line represents the relationship between pressure and flow rate when there is no liquid in the pores at all. It is usually a straight line passing through the origin, reflecting the permeation characteristics of the material when there is no liquid filling. The difference between the wet line and the dry line can be used to analyze the process of pore filling and emptying. Semi-Dry Line: The semi-dry line is a state between the wet line and the dry line, representing the relationship between pressure and flow rate when the pores are partially filled with liquid. It is usually used to analyze the complexity of the pore structure, especially the permeation behavior when the pores are partially filled.

[0068] The test results show that all PMIA substrates have an ideal pore structure, proving that the PMIA substrates have a good film-forming effect and no defects. At the same time, the pore size distribution of all substrates is unimodal and narrow, indicating that the pore size distribution of the substrates is relatively uniform, which is beneficial to the preparation of composite membranes.

[0069] As the PMIA content increases, the pore size of the substrate continuously decreases. This is because when the PMIA content is high, the polymer chains have a more tightly coiled conformation, and the polymer concentration near the outer surface of the substrate increases, resulting in an increase in the viscosity of the casting solution. Therefore, a denser upper skin layer can be formed, and the average pore size is smaller.

[0070] The average pore sizes of M1 and M2 are increased to 0.1136 μm, and the average pore sizes of M3, M4, and M5 are 0.1166, 0.1526, and 0.1511 μm respectively, which are significantly higher than 0.0918 μm of the pure PMIA substrates S1 - S3. This shows that the pore sizes of the PMIA membranes added with SE modifier have all increased. This may be because SE, as a surfactant with hydrophilic hydroxyl groups, will play a more efficient role in opening pores at the two-phase interface during the film-forming process, increasing the interaction between the membrane surface and water molecules. During the phase inversion process, more water molecules can be attracted into the membrane matrix, thereby expanding the membrane pores and causing large pores to appear on the surface of the cast film. On the other hand, due to the self-assembly phenomenon of SE, micelles are formed in the casting solution, affecting the affinity between the solvent and the surfactant, and then leading to changes in the phase inversion mechanism and kinetics, promoting the formation of more crack pore structures, thus greatly increasing the pore size and porosity of the membrane. All the finally prepared support layers have an ideal pore structure, with a uniform pore size distribution, a very narrow pore size distribution and no defects, indicating that the prepared support layers have a uniform pore structure, which will be beneficial to the subsequent preparation of composite membranes.

[0071] (3) Surface chemical properties of the composite membrane

[0072] The surface chemical properties of some PVA / PMIA composite membranes prepared by the present invention are characterized by FTIR, and the results are as Figure 3 and Figure 4 shown. All the characterized composite membranes have obvious absorption bands at 1250 cm -1 , which is attributed to the stretching vibration of the C - S bond in the characteristic peak of the cross-linking agent P(AA / AMPS) molecules. The absorption peaks in the regions of 1060 cm -1 and 1120 cm -1 correspond to the stretching vibrations of the S - O bond and S=O bond of the sulfonic acid group in the P(AA / AMPS) molecules. The presence of the characteristic peak of the sulfonic acid group indicates that the sulfonic acid group with the function of "promoting mass transfer" has been successfully added to the PVA membrane matrix. 1690 - 1745 cm -1The characteristic peaks within the region are attributed to the stretching vibration peaks of the ester group C=O double bond. The presence of the ester group in the composite membrane proves that PVA has successfully cross-linked with the cross-linking agent, forming a PVA cross-linked network. The peak intensities of M1 / PVA and M2 / PVA membranes in the 1690 - 1745 cm -1 region are enhanced. The applicant analyzes that this may be because the PMIA substrate of SE also has a part of C=O bonds. The broad peak at 3350 cm -1 is the stretching vibration peak of the -OH bond. According to the determination of the surface chemical properties of the composite membrane by FTIR, it shows that the PVA dense layer has been successfully cross-linked and the sulfonic acid group has been successfully introduced.

[0073] (4) Microscopic Morphology and Structure of the Composite Membrane

[0074] Figure 5 The surface and cross-section morphology structure diagrams of the PVA / PMIA composite membranes of each example and comparative example are shown. From the surface SEM images, the surfaces of all composite membranes are dense and have no obvious pore structure, indicating that the surface of the PVA membrane is relatively flat, dense and defect-free. Combining with the cross-section SEM structure of the composite membrane, there is no clear and obvious phase interface between the PVA membrane and the PMIA substrate, but an integral membrane cross-section structure, which is attributed to the fact that the PMIA substrate with good hydrophilicity is conducive to the close combination with the separation layer.

[0075] In addition, compared with the scraping example, in the example using the spraying method, the PVA droplets have a downward acting force, which further enhances the binding force between the PVA layer and the PMIA substrate. During the preparation process, part of the PVA casting solution penetrates into the large pores on the surface of the support layer and forms a "mechanical interlock" structure with high binding force with the PMIA membrane. This structure is beneficial to the composite membrane maintaining stability during long-term operation. Therefore, the improvement of the spraying process can endow the composite membrane of the present invention with better performance and effects, which will be described in detail in the subsequent tests.

[0076] (5) Permeation and Separation Performance of the Composite Membrane

[0077] The composite membrane PVA / PMIA was applied to pervaporation desalination, and a 3.5 wt.% NaCl solution was used as the feed solution to test its separation performance at 75°C.

[0078] Figure 6 Shown are the pervaporation fluxes and rejection rates of the composite membranes (Example 1 and Example 2) prepared with bottom membranes with different SE contents under the scraping process. It can be seen that the differences between them are not significant.

[0079] After spraying to form a film, the PVA / PMIA composite membrane needs to be placed in an oven at 100°C for thermal cross-linking for 15 - 60 min (Example 3 - Example 6). The influence of the thermal cross-linking time on the pervaporation performance of the membrane is as Figure 7As shown, it can be seen that with the increase of crosslinking time, when treating 3.5 wt.% NaCl solution at 75 °C, the pervaporation flux of the composite membrane decreases from 62.61 ± 2.52 Kg·m -2 ·h -1 to 36.27 ± 2.262 Kg·m -2 ·h -1 , and the salt rejection rate increases from 99.66 ± 0.06% to 99.86 ± 0.05%. This is because during the crosslinking process, the -OH of PVA reacts with the -COOH of P(AA / AMPS) to form an esterification reaction. As the crosslinking time prolongs, the crosslinking density increases, so the permeation flux gradually decreases and the salt rejection rate gradually increases.

[0080] The spray pressure also affects the performance of the PVA composite membrane. The pervaporation tests were carried out on the composite membranes prepared under five spray pressures of 0.5 - 4.5 bar (Examples 7 - 11), and the results are as Figure 8 shown. With the increase of spray pressure, the pervaporation performance of the PVA composite membrane first increases and then decreases. The PVA composite membrane prepared under a spray pressure of 2.5 bar (Example 9) has the best performance, with a permeation flux as high as 103.27 ± 8.06 Kg@m -2 @h -1 , and the rejection rate is 99.99 ± 0.01%.

[0081] Meanwhile, we believe that the size of the pervaporation flux of the PVA composite membrane is related to the thickness of the PVA layer. The thicknesses of the PVA layers of the PVA composite membranes prepared under different spray pressures are shown in Table 5. Generally, reducing the thickness of the separation layer will reduce the mass transfer resistance of the membrane. However, for the PVA / P(AA-AMPS) system, in addition to the large number of hydroxyl groups in PVA being beneficial to the transport of water molecules, due to the incorporation of the crosslinking agent P(AA-AMPS), the sulfonic acid groups it contains are hydrophilic and provide additional transport sites for water molecules, constructing a water molecule transport channel, which plays a role in promoting mass transfer during the pervaporation process, and this will greatly increase the pervaporation flux of the composite membrane. Therefore, when the PVA layer is too thin, although the mass transfer resistance will decrease, the water molecule transport channels will be greatly reduced, which will lead to a decrease in the pervaporation flux. Therefore, choosing a medium pressure of 2.5 bar is beneficial to form a defect-free and flat PVA layer with a thickness of 471.60 ± 5.95 nm.

[0082] Spraying pressure (bar) Thickness (nm) 0.5 - 1.5 90.72±5.35 2.5 471.60±5.95 3.5 236.33±7.98 4.5 135.50±7.01

[0083] Table 5 Thicknesses of the PVA layers of the PVA composite membranes prepared under different spray pressures

[0084] Furthermore, at 75 °C, the pervaporation tests were carried out on four PVA composite membranes (Comparative Example 11, Examples 12 - 14) using a 3.5 wt.% NaCl solution as the feed solution. The results are as Figure 9 shown. It can be seen from the test results that the rejection rates of the composite membranes for the 3.5 wt.% NaCl solution are all above 99.9%, indicating that the dense PVA layer crosslinked with P(AA - AMPS) is intact and defect - free. Among them, for the composite membrane with the PMIA membrane as the support layer, its pervaporation flux is 65.37 ± 4.83 Kg·m -2 @h -1 , while the pervaporation fluxes of the composite membranes with the SE / PMIA membrane as the support layer are all significantly improved. Among them, the pervaporation flux of the PVA / SE - 3 / PMIA composite membrane is 132.04 ± 3.98 Kg@m -2 @h -1 , the pervaporation flux of the PVA / SE - 11 / PMIA composite membrane is 127.62 ± 2.82 Kg@m -2 @h -1 , and the pervaporation flux of the PVA / SE - 15 / PMIA composite membrane is 116.11 ± 1.43 Kg@m -2 @h -1 . This is because, on the one hand, the SE / PMIA membrane has better hydrophilicity, so that more water molecules pass through the support layer, thus improving the mass transfer efficiency of water vapor in the support layer; on the other hand, the surface pore size of the SE / PMIA membrane is larger than that of the pure PMIA membrane and there are more crack pores on the surface of the SE / PMIA membrane, thus reducing the transmission path of water vapor molecules on the surface of the support layer membrane, and then improving the pervaporation flux of the composite membrane.

[0085] Table 6 below is a summary table of the pervaporation fluxes and rejection rates of each composite membrane. It can be seen from Table 6 that the support layer with high hydrophilicity, many surface crack pore structures and appropriate pore size can effectively reduce the mass transfer resistance of water vapor molecules at the composite membrane interface, reduce the transmission distance of water vapor molecules, and improve the permeation efficiency of water vapor molecules, thus effectively improving the pervaporation flux of the composite membrane. Therefore, by regulating the surface properties and pore structure of the support layer, high - performance pervaporation desalination composite membranes can be prepared.

[0086]

[0087]

[0088] Table 6 Summary of the pervaporation fluxes and rejection rates of each composite membrane

[0089] (6) Desalination performance of the optimal PVA composite membrane for real Bohai seawater

[0090] To verify the actual applicability of the PVA composite membrane of the present invention, the PVA / PMIA composite membrane with the best performance (i.e., Example 9 or Example 13) was subjected to pervaporation testing using real Bohai Sea water at 75°C. The results are as Figure 10 shown: The permeation flux of the PVA / PMIA composite membrane for Bohai Sea water reached 133.33 ± 2.74 Kg@m -2 ·h -1 , showing a slight increase, and the ion rejection reached 98.32 ± 0.60%.

[0091] At the same time, the contents of various ions in the raw liquid and permeate of Bohai Sea water were detected. The results are as Figure 11 shown: The boron removal rate of the PVA / PMIA composite membrane was 99.31%, and the removal rates of the other salts were Cl - : 97.66%; Mg 2+ : 99.74%; Ca 2+ : 99.64%; SO 4 2- : 99.99%; Na + : 97.36%; K + : 98.95%. This indicates that the PVA / PMIA composite membrane has excellent seawater desalination ability.

[0092] In summary, the surface properties and pore structure of the PMIA support layer are important factors affecting the permeation performance of the composite membrane. The support layer with high hydrophilicity, many surface crack pore structures, and appropriate pore size can effectively reduce the mass transfer resistance of water vapor molecules at the interface of the composite membrane, reduce the transmission distance of water vapor molecules, and improve the permeation efficiency of water vapor molecules, thereby effectively increasing the pervaporation flux of the composite membrane. In the present invention, by comprehensively regulating the pore structure of the substrate, the construction method of the PVA separation layer, and the PVA cross-linking reaction process, a high-performance pervaporation desalination composite membrane can be prepared. The pore size of the substrate affects the separation effect of the composite membrane. Therefore, using a PMIA substrate with high porosity and large pore size can effectively reduce the mass transfer resistance of water molecules at the interface of the composite membrane and provide a wider mass transfer path. At the same time, the pore structure of the substrate also affects the transmission of water molecules in the substrate, and through-hole channels are beneficial to reducing the transmission distance of water molecules, thereby improving the permeation efficiency of water molecules. Therefore, the present invention actually achieves a synergistic breakthrough from two levels of the separation layer and the support layer, improving the flux of the composite membrane during pervaporation, while optimizing the ion rejection performance, making it have good industrial application prospects in actual seawater separation, brackish water desalination, industrial wastewater resource treatment and other scenarios.

[0093] The basic principles, main features and advantages of the present invention have been shown and described above. Those skilled in the art should understand that the above embodiments do not limit the present invention in any form, and all technical solutions obtained by means of equivalent replacement or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A method for preparing a PVA composite membrane based on a PMIA porous substrate, characterized in that: The steps include: S1. Preparation of PMIA substrate Lithium chloride and DMAc solvent are mixed and stirred at high speed to dissolve to form a transparent homogeneous solution, and then a modifier SE is added and stirred to form a uniform and stable solution, and then a certain amount of fully dried PMIA fibers are added to the above solution, and the temperature is raised to 70-100°C to fully dissolve to form a uniform and stable casting solution, and then cooled to room temperature in the air, and after standing and degassing, a PMIA basement membrane is formed by a non-solvent induced phase separation method; S2. Preparation of PVA / P(AA-AMPS) separation layer solution Dissolve polyvinyl alcohol (PVA) in deionized water, heat and stir in an oil bath to obtain a transparent and uniform PVA solution, add a certain amount of P (AA / AMPS) cross-linking agent after the solution is cooled to room temperature, and then adjust the pH of the casting solution to 1 with H2SO4, and finally stir magnetically at room temperature for a period of time to obtain a transparent and uniform PVA / P (AA / AMPS) separation layer solution, and let it stand for degassing for later use; S3. Preparation of PVA / PMIA composite film The PVA composite membrane of the PMIA porous substrate is prepared by blade coating or spray coating process.

2. The method for preparing a PVA composite membrane based on a PMIA porous substrate according to claim 1, characterized in that: The mass percentage of the modifier SE is 0.2 wt.%.

3. The method for preparing a PVA composite membrane based on a PMIA porous substrate according to claim 1, characterized in that: The mass percentage of the PMIA is not less than 10 wt.%.

4. The method for preparing a PVA composite membrane based on a PMIA porous substrate according to claim 1, characterized in that: In the step S2, the mass ratio of PVA to the cross-linking agent P (AA / AMPS) is 7:

3.

5. The method for preparing a PVA composite membrane based on a PMIA porous substrate according to claim 1, characterized in that: In step S3, the process of preparing the composite membrane by using a scraping process is as follows: the PVA / P (AA-AMPS) separation layer solution is coated on the dry PMIA porous substrate membrane using a 10 μm wire rod or an automatic scraping machine, and then the primary membrane is heat-treated in a 100° C. oven for 20 to 60 minutes to obtain a PVA composite membrane, which is then stored in deionized water.

6. The method for preparing a PVA composite membrane based on a PMIA porous substrate according to claim 1, characterized in that: In step S3, the process of preparing the composite membrane by spraying process is as follows: the dried PMIA porous substrate membrane obtained in step S1 is pasted onto a glass plate, the PVA / P (AA-AMPS) separation layer solution is sprayed onto its surface with a spray gun, and finally the sprayed membrane is transferred to an oven at 100° C. for thermal crosslinking for 15 to 60 minutes.

7. The method for preparing a PVA composite membrane based on a PMIA porous substrate according to claim 6, characterized in that: The nozzle of the spray gun was 15 cm away from the film surface on the glass plate, and the horizontal movement speed of the glass plate was 6 cm·s -1 , the oscillation distance perpendicular to the horizontal direction is 3 cm, and the spray pressure is 0.5-4.5 bar.

8. The method for preparing a PVA composite membrane based on a PMIA porous substrate according to claim 7, characterized in that: The thermal crosslinking time was 45 min and the spraying pressure was 2.5 bar.

9. A PVA composite membrane based on a PMIA porous substrate, characterized in that: The composite membrane is prepared by the preparation method according to any one of claims 1 to 8, the surface of the composite membrane is dense and defect-free, and the PVA ultra-thin separation layer and the PMIA substrate form an integrated membrane cross-section structure.

10. Application of a PVA composite membrane based on a PMIA porous substrate as claimed in claim 9 in salt water treatment, including brackish water desalination, seawater desalination and industrial wastewater resource management.

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