High-water-permeability anti-pollution reverse osmosis membrane and preparation method thereof

Abstract

The invention discloses a high-water-permeability anti-pollution reverse osmosis membrane and a preparation method thereof, and belongs to the technical field of water treatment membranes. The reverse osmosis membrane comprises a porous support layer, a polyamide separation layer and a composite anti-pollution functional layer, and the composite anti-pollution functional layer is composed of a polyvinyl alcohol matrix and a nanoscale water channel material uniformly dispersed in the polyvinyl alcohol matrix. Comprising the steps of coating liquid preparation, base membrane pretreatment, composite functional layer construction, cross-linking and curing, a rapid transmission path is provided for water molecules by introducing a nanoscale water channel material, the water flux of the reverse osmosis membrane is greatly improved on the premise that the excellent pollution resistance of a PVA coating and the high selectivity of the membrane are kept, and the service life of the reverse osmosis membrane is prolonged. The technical problem that the membrane flux is severely reduced due to a traditional PVA coating is solved, and the method has wide application prospects in the fields of seawater desalination, brackish water desalination, ultrapure water preparation, wastewater reuse and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of water treatment membranes, in particular to a composite membrane for reverse osmosis (RO) or nanofiltration (NF) processes and a preparation method thereof, and more particularly to a high water permeability anti-fouling reverse osmosis membrane with a modified selective separation layer and a preparation method thereof. BACKGROUND

[0002] Reverse osmosis membranes are the core components for seawater desalination, brackish water desalination and ultrapure water preparation. Commercial polyamide thin layer composite reverse osmosis membranes usually have a negative charge on the surface and have a rough "peak-valley" structure. When treating water containing organic matter, colloids and other pollutants, the membrane surface is prone to adsorb pollutants, causing membrane fouling, which in turn leads to a decrease in membrane flux, an increase in operating pressure, an increase in cleaning frequency and a shortening of the service life of the membrane, seriously affecting the efficiency and cost of using reverse osmosis membranes.

[0003] To improve the anti-fouling performance of reverse osmosis membranes, the industry generally uses a method of coating a polyvinyl alcohol (PVA) aqueous solution on the surface of the polyamide active layer to form a dense hydrophilic PVA coating. The PVA coating can effectively cover the rough structure of the polyamide layer, making the membrane surface smooth, hydrophilic and electrically neutral, significantly reducing the adsorption between the pollutants and the membrane surface, and thus effectively improving the anti-fouling performance of the reverse osmosis membrane.

[0004] However, the above-mentioned traditional modification method has significant technical drawbacks: the PVA coating is a dense hydrophilic polymer layer with no or very low porosity, and the transport of water molecules in the coating completely depends on the transient free volume formed by the thermal motion of PVA macromolecular segments, and water molecules need to pass through the coating by a tortuous and long solubility-diffusion path, resulting in a serious additional mass transfer resistance. Therefore, the water flux (water production) of the reverse osmosis membrane after PVA coating usually decreases by 20% to 50% or even more, which seriously sacrifices the production efficiency of the membrane and increases the energy consumption per ton of water produced.

[0005] In summary, there is an urgent need in the art to develop a new technical solution that can significantly reduce the hindering effect of the PVA coating on water flux while maintaining or even enhancing the excellent anti-fouling performance of the PVA coating, achieving the balance between high flux and high anti-fouling performance of the reverse osmosis membrane. SUMMARY

[0006] The technical problem solved by the present application is that the PVA anti-pollution coating in the prior art causes a serious decrease in the water flux of the reverse osmosis membrane, in order to improve the shortcomings, the present application provides a high water permeability anti-pollution reverse osmosis membrane and a preparation method thereof, by introducing a regular and rigid nanoscale water channel structure in the PVA matrix, a fast and low resistance preferential transmission path is provided for water molecules, under the premise of basically not affecting the inherent anti-pollution performance of PVA, the water flux of the modified reverse osmosis membrane is significantly improved, and the high selectivity and mechanical stability of the membrane are ensured.

[0007] In order to achieve the above-mentioned purpose, the present application is realized by the following technical scheme: A high water permeability anti-pollution reverse osmosis membrane comprises a porous support layer and a polyamide separation layer located thereon, and a composite anti-pollution functional layer is further provided on the surface of the polyamide separation layer; the composite anti-pollution functional layer is composed of a polyvinyl alcohol (PVA) matrix and nanoscale water channel materials uniformly dispersed therein.

[0008] As a preferred scheme, the nanoscale water channel material has a regular internal pore structure, and the size of the pore is 0.3nm-2.0nm. The nanoscale water channel material is preferably one or more of metal organic framework material (MOF), covalent organic framework material (COF) and molecular sieve (Zeolite), and the pore size of the nanoscale water channel material is preferably 0.4nm to 1.0nm. The pore size is matched with the kinetic diameter of water molecules (about 0.28nm), and is significantly smaller than the size of common pollutants (such as organic matter, protein and colloid), so that the water molecules can pass through quickly and the pollutants can be effectively intercepted.

[0009] As a preferred scheme, the mass fraction of the water channel material in the PVA matrix is 0.01% to 30%, and is preferably 0.1% to 10%. Too low content cannot form an effective water channel network, and cannot play a role in improving the flux. Too high content may damage the continuity and density of the PVA coating, and affect the anti-pollution performance and mechanical stability of the membrane.

[0010] As a preferred scheme, the alcoholysis degree of PVA is not less than 88%, so as to ensure that the PVA has sufficient hydrophilicity and film-forming property, and to ensure the molding effect and hydrophilic property of the composite anti-pollution functional layer.

[0011] A preparation method of a high water permeability anti-pollution reverse osmosis membrane, characterized in that it comprises the following steps: S1: coating liquid preparation: dissolving PVA in deionized water to prepare a PVA aqueous solution with a concentration of 0.01wt%-2.0wt%, and uniformly dispersing nanoscale water channel materials in the PVA aqueous solution to form a stable composite coating liquid; S2: Base membrane pretreatment: providing a reverse osmosis base membrane with a polyamide selective separation layer, and cleaning the surface of the reverse osmosis base membrane with deionized water; S3: Composite functional layer construction: applying the composite coating liquid to the surface of the polyamide selective separation layer of the wet reverse osmosis base membrane by immersion coating, spin coating, spraying or roller coating; S4: Crosslinking and curing: placing the coated reverse osmosis base membrane in air or an inert atmosphere and heat treating at 50-120°C, or simultaneously using a crosslinking agent to chemically crosslink the PVA, forming a stable composite anti-pollution functional layer, and obtaining a high water permeability anti-pollution reverse osmosis membrane.

[0012] As a preferred solution, the dispersion method of the nanoscale water channel material is one or more of ultrasonic dispersion, high-speed shearing dispersion or adding a dispersant.

[0013] As a preferred solution in step S4, the crosslinking agent is glutaraldehyde or maleic acid.

[0014] Compared with the prior art, the present application has the following beneficial effects: 1. High flux: The introduced MOF, COF or molecular sieve has regular, rigid and hydrophilic sub-nanometer channels, providing a "highway" for water molecules to transport at almost no resistance, greatly shortening the path of water through the PVA layer, effectively offsetting the mass transfer resistance brought by the dense PVA layer, and making the water flux of the composite membrane increase by more than 30%-150% compared with the traditional PVA coated membrane, and even close to or reaching the flux level of the original base membrane without PVA coating.

[0015] 2. High anti-pollution property is maintained: PVA serves as a continuous matrix, completely covering the rough surface of the polyamide layer and providing a smooth, hydrophilic and electrically neutral interface; the water channel material is tightly wrapped in PVA, with very little exposed part, so the surface properties of the membrane are still mainly determined by PVA, and the adsorption capacity of the membrane for common pollutants such as proteins, humic acid and oils is very weak, and the anti-pollution performance is comparable to that of pure PVA coated membrane.

[0016] 3. Excellent selectivity: The pore size of the selected water channel material is precisely designed (usually <1 nm), which is much smaller than most salt ions (such as hydrated Na⁺ with a diameter of about 0.72 nm and Cl⁻ with a diameter of about 0.66 nm) and organic pollutant molecules, so water molecules can pass through preferentially, while salts and pollutants are effectively intercepted, and the desalination rate and pollutant rejection rate of the membrane are not significantly affected, and the desalination rate decreases by less than 1% compared with the original base membrane and the pure PVA coated membrane.

[0017] 4. Synergistic enhancement effect: The hydrophilic water channel material itself can further enhance the overall hydrophilicity of the composite functional layer, which is conducive to the formation of a hydration layer on the membrane surface, further blocking hydrophobic pollutants and improving the anti-fouling effect; at the same time, the rigid nano water channel material can serve as a physical cross-linking point, enhancing the mechanical stability and durability of the PVA coating and extending the service life of the membrane.

[0018] 5. The preparation method is simple and easy to implement: The preparation method of this invention is an improvement on the traditional PVA coating process. It does not require the introduction of complex production equipment. The steps of dispersion, coating, cross-linking and curing are all conventional processes in the field of water treatment membranes. It is easy to realize industrial scale-up production and is suitable for the modification treatment of different types of reverse osmosis membranes such as flat sheet membranes and spiral wound membranes. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the composite membrane structure in this invention (in order: porous support layer, polyamide separation layer, and polyvinyl alcohol matrix containing nanoscale water channel material).

[0020] Figure 2 A schematic diagram comparing the water molecule transport paths of a traditional PVA coating (left) and the composite coating of this invention (right).

[0021] Figure 3 The bar chart comparing the flux recovery rate (FRR) of the example and comparative membrane samples demonstrates their antifouling properties.

[0022] Figure 4 Normalized flux versus time curves for membrane samples in the examples and comparative examples during the contamination experiment. Detailed Implementation

[0023] The technical solution of this application will be further described and illustrated below through embodiments. Example

[0024] This embodiment provides a highly permeable, antifouling reverse osmosis membrane. The nanoscale water channel material in its composite antifouling functional layer is hydrophilic ZIF-8 nanoparticles (metal-organic framework material with a pore size of about 0.34 nm). The mass fraction of ZIF-8 in the PVA matrix is ​​1%, and the PVA is a fully hydrolyzed type with a degree of hydrolysis ≥99%.

[0025] Its preparation method includes the following steps: Preparation of S1 coating solution: Completely alcoholyzed PVA was dissolved in deionized water to prepare a PVA aqueous solution with a concentration of 0.5 wt%. Hydrophilic ZIF-8 nanoparticles were weighed and added to the above PVA aqueous solution at a mass ratio of 1:100 with PVA. The solution was dispersed by ultrasonic dispersion for 2 hours to obtain a stable composite coating solution A.

[0026] S2 base film pretreatment: Commercial polyamide reverse osmosis flat sheet membrane (SW30) is provided as the base membrane. The membrane surface is repeatedly cleaned with deionized water to remove surface impurities. After cleaning, the membrane surface is kept moist.

[0027] S3 Composite Functional Layer Construction: The dip coating method is used to immerse the moist base film in the composite coating solution A for 1 minute, and then pull it out of the liquid surface at a constant speed to make the film surface uniformly coated with the composite coating solution A.

[0028] S4 crosslinking and curing: The coated base film is placed in an air atmosphere and then heat-treated in an 80°C oven for 30 minutes to physically cross-link the PVA molecular chains and form a stable composite anti-fouling functional layer, thus obtaining the composite film M1 of the present invention. Example

[0029] This embodiment provides a highly permeable antifouling reverse osmosis membrane. The nanoscale water channel material in its composite antifouling functional layer is hydrophilically modified MIL-101(Fe) nanoparticles (metal-organic framework material with pore sizes of approximately 1.2 nm and 1.6 nm, and the main pores are hydrophilic after modification). The mass fraction of MIL-101(Fe) in the PVA matrix is ​​5%, and the PVA is partially hydrolyzed with a degree of hydrolysis of 88%.

[0030] Its preparation method includes the following steps: Preparation of S1 coating solution: Partially hydrolyzed PVA was dissolved in deionized water to prepare a PVA aqueous solution with a concentration of 0.2 wt%. Hydrophilically modified MIL-101(Fe) nanoparticles were weighed and added at a mass ratio of 5:100 to PVA. The mixture was dispersed by high-speed shearing to obtain a stable composite coating solution B.

[0031] S2 base film pretreatment: A nanofiltration membrane with a polyamide separation layer is provided. The membrane surface is cleaned with deionized water to keep the membrane surface moist.

[0032] S3 Composite Functional Layer Construction: The composite coating liquid B is uniformly sprayed onto the wet polyamide nanofiltration membrane surface using a spraying method to ensure uniform coating on the membrane surface.

[0033] S4 crosslinking and curing: The coated base film was treated in an air atmosphere at 60°C for 1 hour, and then placed in a crosslinking tank containing glutaraldehyde vapor at room temperature for 2 hours to allow the PVA to complete physical and chemical crosslinking, forming a stable composite anti-fouling functional layer, thus obtaining composite film M2. Example

[0034] This embodiment provides a high-permeability, anti-fouling reverse osmosis spiral wound membrane element. The nanoscale water channel material in its composite anti-fouling functional layer is NaA-type molecular sieve nanocrystals (pore size of about 0.4 nm). The mass fraction of NaA-type molecular sieve in the PVA matrix is ​​10%, and the PVA is a fully alcoholyzed type with an alcoholysis degree ≥99%.

[0035] Its preparation method includes the following steps: Preparation of S1 coating solution: Completely alcoholyzed PVA was dissolved in deionized water to prepare a PVA aqueous solution with a concentration of 1.0 wt%. NaA type molecular sieve nanocrystals were weighed and added at a mass ratio of 10:100 to PVA, and a trace amount of nonionic surfactant was added as a dispersant. The solution was dispersed by ultrasonic dispersion to obtain a stable composite coating solution C.

[0036] S2 base film pretreatment: Provide reverse osmosis spiral wound membrane element blades (with polyamide separation layer), clean the blade surface with deionized water to keep the surface moist.

[0037] S3 Composite Functional Layer Construction: The composite coating liquid C is uniformly coated onto the polyamide separation layer surface of the blades of the reverse osmosis spiral wound membrane element using a roller coating method, which is suitable for the industrial production process of spiral wound membranes.

[0038] S4 crosslinking and curing: The coated membrane element blades are placed in an air atmosphere and heat-treated at 100°C for 15 minutes to physically cross-link the PVA molecular chains, forming a stable composite antifouling functional layer, thus obtaining the composite membrane element M3.

[0039] Comparative Example 1: This comparative example provides a pure PVA-coated reverse osmosis flat sheet membrane without the addition of any nanoscale water channel materials. The preparation process is as follows: a 0.5 wt% fully alcoholyzed PVA aqueous solution is prepared, and a commercial polyamide reverse osmosis flat sheet membrane (SW30) is cleaned and coated with the above PVA aqueous solution by dip coating. The heat treatment conditions are the same as in Example 1 (heat treatment in an oven at 80°C for 30 minutes) to obtain a pure PVA-coated membrane D1.

[0040] Comparative Example 2: This comparative example is the original polyamide reverse osmosis base membrane without any coating treatment, namely the commercial polyamide reverse osmosis flat sheet membrane (SW30), denoted as D2.

[0041] Performance testing: The composite membranes M1-M3 prepared in Examples 1-3 and membranes D1 and D2 in Comparative Examples 1-2 were subjected to performance tests. The test conditions and test items are as follows: 1. Flux and desalination rate test: The water flux and desalination rate of the membrane were tested using a 2000 mg / L NaCl aqueous solution at 25°C and 1.6 MPa operating pressure. 2. Anti-pollution test: A fouling experiment was conducted using a solution containing 500 ppm bovine serum albumin (BSF) for 12 hours. The membrane surface was then rinsed with deionized water, and the water flux of the membrane was tested again. The flux recovery rate (FRR) was calculated. The higher the flux recovery rate, the better the membrane's antifouling performance.

[0042] 1. Water flux: The water flux of composite membranes M1, M2, and M3 is 85%, 78%, and 70% of that of the original base membrane D2, respectively. Compared with the pure PVA coated membrane D1, the water flux of M1, M2, and M3 is increased to 165%, 180%, and 155% of that of D1, respectively. This proves that the composite antifouling functional layer of the present invention can significantly restore and improve the water flux of the reverse osmosis membrane, effectively solving the problem of severe flux loss in traditional PVA coatings.

[0043] 2. Desalination rate: Compared with D1 in Comparative Example 1 and D2 in Comparative Example 2, the desalination rates of the composite membranes M1-M3 in Examples 1-3 decreased by less than 1%, indicating that the introduction of nanoscale water channel materials in this invention did not impair the selectivity of the membrane and still maintained excellent desalination effect.

[0044] 3. Pollution Resistance (FRR): The flux recovery rates of composite membranes M1, M2, and M3 are all between 92% and 96%, which is comparable to that of pure PVA-coated membrane D1 (flux recovery rate of about 94%), while the flux recovery rate of the original base membrane D2 is only about 65%. This proves that the reverse osmosis membrane of the present invention can perfectly inherit the excellent antifouling performance of PVA coating while improving water flux.

[0045] Conclusion: This invention creatively solves the technical problem of severe flux loss in traditional PVA coatings by introducing MOF, COF, or molecular sieves with regular sub-nanopores as nanoscale water channel materials into the PVA antifouling coating, thus constructing a composite antifouling functional layer composed of a PVA matrix and nanoscale water channel materials. The preparation method is simple and easily industrialized. The resulting reverse osmosis membrane possesses the advantages of high flux, high antifouling, and high selectivity, showing great application prospects and promotional value in water treatment fields such as seawater desalination, brackish water desalination, ultrapure water preparation, and industrial wastewater and municipal sewage reuse.

[0046] Industrial Applicability: The high water permeability and antifouling reverse osmosis membrane and its preparation method of the present invention have significant industrial applicability. The preparation process is an improvement on the traditional PVA coating modification process, without the need for additional complex production equipment. Ultrasonic dispersion, high-speed shearing, dip coating, spraying, roller coating, heat treatment crosslinking, etc. are all conventional processes in the field of water treatment membrane production. The process has strong compatibility and can be directly adapted to the existing industrial production lines of reverse osmosis membranes (flat sheet membranes, spiral wound membranes) and nanofiltration membranes, achieving low-cost technology upgrade.

[0047] The reverse osmosis membrane prepared by this invention solves the technical contradiction of "improved antifouling properties but significantly reduced flux" in traditional PVA modified membranes. While maintaining excellent antifouling properties and high desalination rate, it significantly increases water flux, effectively reducing operating pressure and energy consumption in water treatment processes, reducing membrane cleaning frequency, extending membrane lifespan, and lowering the cost per ton of water produced. It has a wide range of applications in various water treatment fields such as seawater desalination, brackish water desalination, ultrapure water preparation, industrial wastewater resource utilization, and advanced municipal sewage treatment, and has significant economic value and social benefits.

[0048] Furthermore, the specific structure of the present invention will be described in detail with reference to the accompanying drawings: like Figure 1 As shown, the high water permeability antifouling reverse osmosis membrane of the present invention consists of a porous support layer, a polyamide separation layer, and a composite antifouling functional layer from bottom to top. The composite antifouling functional layer is composed of a polyvinyl alcohol matrix and nanoscale water channel materials uniformly dispersed inside the polyvinyl alcohol matrix. The pores of the nanoscale water channel materials provide a fast transport path for water molecules. The polyvinyl alcohol matrix completely covers the rough surface of the polyamide separation layer, ensuring the hydrophilic and smooth properties of the membrane surface.

[0049] like Figure 2 As shown, the left side represents the water molecule transport path of a traditional pure PVA coating. Water molecules dissolve and diffuse in the dense PVA layer by relying on the instantaneous free volume formed by the thermal motion of chain segments. The transport path is tortuous and long, with high mass transfer resistance. The right side represents the water molecule transport path of the composite anti-fouling functional layer of this invention. Water molecules can pass through the regular hydrophilic channels of the nanoscale water channel material 32 quickly and in a straight line with minimal mass transfer resistance, which greatly improves the transport efficiency of water molecules.

[0050] like Figure 3 As shown, the flux recovery rates of Examples M1, M2, and M3 are close to those of Comparative Example D1 (pure PVA coated membrane), all remaining above 92%, while the flux recovery rate of Comparative Example D2 (original base membrane) is significantly lower, at approximately 65%. This clearly demonstrates that the reverse osmosis membrane of the present invention maintains excellent antifouling performance comparable to that of the pure PVA coated membrane.

[0051] like Figure 4As shown, in the 12-hour bovine serum albumin contamination experiment, the normalized flux of Comparative Example D2 decreased the fastest and the largest magnitude, while the normalized flux of Comparative Example D1 decreased slowly. The normalized flux change trends of Examples M1, M2, and M3 were basically consistent with those of D1, with minimal decreases. This further demonstrates that the reverse osmosis membrane of the present invention has excellent antifouling performance and can effectively resist the adsorption of protein contaminants.

[0052] This invention is not limited to the above embodiments. Based on the technical solutions disclosed in this invention, those skilled in the art can make some substitutions and modifications to some of the technical features without creative effort, and all such substitutions and modifications are within the protection scope of this invention.

Claims

1. A highly permeable, fouling-resistant reverse osmosis membrane, comprising a porous support layer and a polyamide separation layer thereon, characterized in that: The surface of the polyamide separation layer also has a composite anti-fouling functional layer, which is composed of a polyvinyl alcohol matrix and nanoscale water channel material uniformly dispersed therein.

2. The highly permeable, antifouling reverse osmosis membrane according to claim 1, characterized in that: The nanoscale water channel material has an internal regular pore structure with pore sizes ranging from 0.3 nm to 2.0 nm.

3. The highly permeable, antifouling reverse osmosis membrane according to claim 1, characterized in that: The mass fraction of the nanoscale water channel material in the polyvinyl alcohol matrix is ​​0.01%-30%.

4. The highly permeable, antifouling reverse osmosis membrane according to claim 1, characterized in that: The degree of alcoholysis of the PVA is ≥88%.

5. The high-permeability, anti-fouling reverse osmosis membrane according to claim 1, characterized in that, The nanoscale water channel material is selected from one or more of metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and molecular sieves.

6. A method for preparing a highly permeable, antifouling reverse osmosis membrane according to any one of claims 1-5, characterized in that, It includes the following steps: S1: Coating solution preparation: Dissolve PVA in deionized water to prepare a PVA aqueous solution with a concentration of 0.01wt%-2.0wt%. Disperse the nanoscale water channel material uniformly in the PVA aqueous solution to form a stable composite coating solution. S2: Base membrane pretreatment: A reverse osmosis base membrane with a polyamide selective separation layer is provided, and the surface of the reverse osmosis base membrane is cleaned with deionized water; S3: Construction of composite functional layer: The composite coating liquid is applied to the surface of the polyamide selective separation layer of the wet reverse osmosis base membrane by dip coating, spin coating, spray coating or roller coating. S4: Crosslinking and curing: The coated reverse osmosis base membrane is placed in air or an inert atmosphere and heat-treated at 50-120℃, or PVA is chemically crosslinked with a crosslinking agent at the same time to form a stable composite antifouling functional layer, thereby obtaining a highly permeable antifouling reverse osmosis membrane.

7. The method for preparing a highly permeable, antifouling reverse osmosis membrane according to claim 6, characterized in that: The dispersion method of the nanoscale water channel material is one or more of ultrasonic dispersion, high-speed shear dispersion, or dispersion with the addition of a dispersant.

8. The method for preparing a highly permeable, antifouling reverse osmosis membrane according to claim 6, characterized in that: In step S4, the crosslinking agent is glutaraldehyde or maleic acid.

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