A reverse osmosis membrane and its preparation method

Abstract

This invention provides a method for preparing a reverse osmosis membrane, comprising: preparing a lipid organic solution from LNP lipid raw material; preparing a cellulose acetate aqueous suspension from a small amount of cellulose acetate; synthesizing LNP from the lipid organic solution and the cellulose acetate aqueous suspension using microfluidic technology; diluting and purifying the LNP solution by ultrafiltration to obtain an LNP solution; preparing a cellulose acetate solution, adding a crosslinking agent and a catalyst, and mixing and crosslinking it with the LNP solution to obtain a casting solution; and preparing a reverse osmosis membrane from the casting solution. This invention is the first to prepare a reverse osmosis composite membrane using LNP and cellulose acetate. LNP has a structure similar to that of a biological semi-permeable membrane, thus obtaining a biomimetic biological semi-permeable reverse osmosis membrane. The stability of LNP is maintained through crosslinking. The cellulose acetate composite LNP reverse osmosis membrane can accelerate water permeation and significantly increase water permeation flux while maintaining a high desalination rate.

Description

Technical Field

[0001] This invention relates to the field of filtration, and more specifically to a reverse osmosis membrane and its preparation method. Background Technology

[0002] A reverse osmosis membrane is an artificial semi-permeable membrane with specific characteristics, designed to mimic biological semi-permeable membranes. The principle of reverse osmosis technology is based on the principle that, under pressure higher than the osmotic pressure of the solution, other substances cannot pass through the semi-permeable membrane, thus separating them from water. Reverse osmosis membranes have extremely small pore sizes, enabling them to effectively remove dissolved salts, colloids, microorganisms, organic matter, and other substances from water.

[0003] Currently, the main membrane materials used in reverse osmosis membranes are cellulose acetate and aromatic polyamides. Three performance indicators are primarily considered: desalination rate, permeate flow rate, and recovery rate. Desalination rate and permeate velocity are mutually restrictive; it is difficult for a single material to simultaneously excel in both desalination and support. Therefore, composite membranes and composite materials are the main directions for improving reverse osmosis membranes. Composite membranes are mainly made of the above two materials, consisting of a very thin dense layer and a porous support layer. CN114259885A discloses a double-sided polyamide reverse osmosis composite membrane, composed of a permeate flow cloth in the middle and porous polyethylene layers and polyamide desalination layers on both sides. The resulting composite reverse osmosis membrane is wound with the concentrate flow cloth to form a structurally improved membrane element. Composite materials are generally prepared by chemically modifying materials or by compounding multiple components to prepare reverse osmosis membranes. CN107138052B discloses an antibacterial cellulose acetate reverse osmosis membrane containing bromine alkane, with bromine-containing organic compounds grafted onto its surface. This not only effectively improves the antibacterial properties of the cellulose acetate reverse osmosis membrane, but also ensures that the antibacterial groups are chemically bonded to the membrane surface and are not easily detached. CN105126643B discloses a hollow fiber reverse osmosis membrane. The hollow fiber reverse osmosis membrane is formed by a closed-loop blend of cellulose triacetate, acyl chloride graphene oxide, solvent, and additives. The good dispersion phase and hydrophilicity of graphene oxide endow the resulting hollow fiber reverse osmosis membrane with good permeability.

[0004] The working principle of reverse osmosis membranes is to mimic biological semi-permeable membranes. However, the materials currently used differ significantly in composition from biological semi-permeable membranes. One example of a biological semi-permeable membrane is the phospholipid bilayer, which is a type of semi-permeable membrane with very unique permeability. The hydrophilic phosphate head is located in the outer layer and is exposed to both the water content and the cell interior. The hydrophobic tail is a layer hidden inside the membrane. Phospholipid bilayers readily permeate small, uncharged solutes. However, due to the poor mechanical strength of phospholipid bilayers, they cannot be directly used to design reverse osmosis membranes. Furthermore, the phospholipid bilayer structure is easily damaged, making it difficult to maintain a stable structure when combined with reverse osmosis membrane materials.

[0005] Lipid nanoparticles (LNPs) are nucleic acid delivery carriers composed of neutral lipids, ionizable lipid molecules, cholesterol, and PEG-lipids. The hydrophobic ends of the PEG-lipids bind to the hydrophobic ends of the ionizable lipid molecules, while the hydrophilic ends (attached to PEG) of the PEG-lipids form the outer shell of the nucleic acid-lipid nanoparticles. Cholesterol and neutral lipids stabilize the LNP structure, allowing for a tighter binding between the hydrophobic ends of the PEG-lipids and the hydrophobic ends of the ionizable lipid molecules. Compared to phospholipid bilayers, LNPs offer higher stability, can mimic the structure of cell membranes, and have a diameter of only about 100 nm, making them suitable for fabricating composite membrane structures with reverse osmosis membrane materials.

[0006] There are currently no literature reports on how to prepare biomimetic composite reverse osmosis membranes. In view of this, the present invention is proposed. Summary of the Invention

[0007] To address the aforementioned technical problems, a first aspect of the present invention provides a method for preparing a reverse osmosis membrane, comprising the following steps:

[0008] (1) Prepare a lipid organic solution from LNP lipid raw material and prepare a small amount of cellulose acetate into an aqueous suspension of cellulose acetate;

[0009] (2) LNP was synthesized from lipid organic solution and cellulose acetate aqueous suspension using microfluidic technology, and then diluted and purified by ultrafiltration to obtain LNP solution;

[0010] (3) Prepare a cellulose acetate solution, add a crosslinking agent and a catalyst, and mix it with an LNP solution to obtain a casting solution;

[0011] (4) The casting solution is used to make a reverse osmosis membrane.

[0012] Furthermore, in step (1), the LNP lipid raw material includes ionizable lipid molecules, cholesterol, phospholipids and PEG lipids.

[0013] The method for preparing the lipid organic solution is as follows: ionizable lipid molecules, cholesterol, phospholipids, and PEG lipids are dissolved in an organic solvent at a molar ratio of 50-65%:30-40%:4-10%:0.5-2% to obtain a lipid organic solution with a concentration of 5-20 mg / mL. Preferably, the organic solvent is selected from anhydrous ethanol, methanol, or acetone. The ionizable lipid molecules are selected from ALC-0315 (CAS No.: 2036272-55-4), DLin-MC3-DMA (CAS No.: 1224606-06-7), or SM-102 (CAS No.: 2089251-47-6).

[0014] The phospholipid is selected from one or more of 1,2-distearyl-sn-glycerol-3-phosphocholine (DSPC), 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycerol-3-phosphocholine (DMPC), 1,2-dioleoyl-sn-glycerol-3-phosphocholine (DOPC), and 1,2-dioleoyl-sn-glycerol-3-phosphoethanolamine (DOPE); the PEG lipid is DMG-PEG 2000.

[0015] The volume ratio of the lipid organic solution to the cellulose acetate suspension is 1:3-5, and the concentration of the cellulose acetate suspension is 0.5-4 mg / mL.

[0016] Further, in step (2), the specific operation of synthesizing LNP is as follows: LNP is prepared using a microfluidic synthesis device and a microfluidic chip, the right injection solution is loaded into a lipid organic solution, the left injection solution is loaded into a cellulose acetate suspension, the total liquid flow rate and the left and right side flow rates are set, the product LNP solution is collected, buffer solution is added for dilution, and then LNP is purified by ultrafiltration.

[0017] The total liquid flow rate is 10-20 mL / min, the ratio of the flow rate on the left to the flow rate on the right is 3-4:1, and the buffer solution is selected from PBS buffer or citrate buffer.

[0018] Further, in step (3), the cellulose acetate solution is a mixed solution of cellulose acetate and 1,4-dioxane / acetone, wherein the mass ratio of 1,4-dioxane to acetone is 2-3:1.

[0019] The crosslinking agent is selected from at least one of formaldehyde, glyoxal, glutaraldehyde, diphenylmethane diisocyanate, and hexamethylene diisocyanate.

[0020] The catalyst is selected from at least one of cerium ammonium nitrate, dibutyltin diacetate, and dibutyltin dilaurate.

[0021] The mass ratio of the cellulose acetate, the mixed solution, the catalyst, and the crosslinking agent is 6–18:30–70:1–3:0.5–8.

[0022] The mass ratio of LNP to cellulose acetate in the cellulose acetate solution is 0.5-2:100.

[0023] The specific crosslinking operation is as follows: maintain crosslinking at 30-40℃ for 6-12 hours, and then raise the temperature to 50-60℃ and maintain it for 12-24 hours.

[0024] Further, in step (4), the specific operation of making the casting solution into a reverse osmosis membrane is as follows: the casting solution is scraped into a flat sheet membrane, left to stand, cured in a low-temperature water bath, heat-treated in a water bath, and cleaned to obtain a reverse osmosis membrane.

[0025] Furthermore, the thickness of the flat sheet film can be 100–300 μm.

[0026] Further, in step (4), the specific operation of making the casting solution into a reverse osmosis membrane is as follows: the casting solution is scraped into a flat sheet membrane with a thickness of 100-300μm, and after standing in the air for 5-30 minutes, it is cured in a water bath at 0-10℃ for 5-20 minutes to obtain a nascent flat sheet membrane; the nascent flat sheet membrane is treated in a water bath at 60-70℃ for 5-15 minutes, taken out, and cleaned with distilled water to obtain the reverse osmosis membrane.

[0027] The second aspect of the present invention provides a reverse osmosis membrane prepared by any of the preparation methods described herein.

[0028] Compared with the prior art, the beneficial effects of the present invention include at least the following:

[0029] (1) This invention is the first to prepare a reverse osmosis composite membrane by combining LNP with cellulose acetate. LNP has a similar structure to a biological semipermeable membrane, thus obtaining a reverse osmosis membrane that mimics a biomimetic biological semipermeable membrane.

[0030] (2) In this invention, cellulose acetate is selected as the reverse osmosis membrane material. It carries a negative charge and can bind with ionizable lipids in LNP. During the preparation of LNP, a small amount of cellulose acetate is added. Through cross-linking, LNP and cellulose acetate inside and outside are cross-linked, which improves the stability of LNP and thus maintains its morphology in the reverse osmosis membrane.

[0031] (3) During the crosslinking process, crosslinking is first performed at a lower temperature to prevent the destruction of the LNP structure in the early stage of crosslinking;

[0032] (4) The PEG lipids on the outer layer of LNP can prevent charged ions or molecules from passing through. While maintaining a high desalination rate, it can accelerate water permeation and greatly increase water permeation flux. Detailed Implementation

[0033] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Examples of the embodiments are shown in the accompanying drawings. It should be understood that the specific embodiments described in the following embodiments of the invention are merely illustrative examples of specific implementations of the invention and are intended to explain the invention, but do not constitute a limitation thereof.

[0034] The endpoints of the ranges and any values ​​disclosed herein are not limited to the exact ranges or values, which should be understood to include those close to them.

[0035] Example 1

[0036] A method for preparing a reverse osmosis membrane includes the following steps:

[0037] (1) Ionizable lipid molecules ALC-0315, cholesterol, DSPC and DMG-PEG 2000 were dissolved in anhydrous ethanol at a molar ratio of 50%:38.5%:10%:1.5% to obtain 200 mL of lipid ethanol solution with a concentration of 10 mg / mL; cellulose acetate was weighed and a suspension of 800 mL was formed in water with a concentration of 1 mg / mL.

[0038] (2) LNPs were prepared using the Ignite microfluidic nanoparticle preparation system from Precision Nanosystems Inc. 8 mL of cellulose acetate suspension diluent was added to the left syringe, and 2 mL of lipid ethanol solution was added to the right syringe. The instrument control software parameters were set as follows: total flow rate 16 mL / min, left-right flow rate ratio 4:1, initial discard volume 0.150 mL, and final discard volume 0.050 mL. LNPs were prepared multiple times, and the LNP solutions were collected until the final LNP product mass was 1 g. The collected LNP solution was diluted with 2 volumes of PBS buffer and purified by ultrafiltration.

[0039] (3) Prepare cellulose acetate solution by dissolving 100g of cellulose acetate in 500mL of a mixed solution of 1,4-dioxane / acetone, wherein the mass ratio of 1,4-dioxane to acetone is 2:1; add 2g of cerium ammonium nitrate and 6g of glyoxal, stir evenly, slowly add an LNP solution containing about 1g of LNP, stir slowly evenly, maintain crosslinking at 35℃ for 6h, and then raise the temperature to 60℃ to continue crosslinking for 16h to obtain casting solution.

[0040] (4) The casting solution is scraped into a flat sheet membrane with a thickness of about 200 μm. After standing in the air for 20 min, it is cured in a 0℃ water bath for 10 min to obtain a nascent flat sheet membrane. The nascent flat sheet membrane is treated in a 60℃ water bath for 10 min, taken out, and cleaned with distilled water to obtain the reverse osmosis membrane.

[0041] Example 2

[0042] A method for preparing a reverse osmosis membrane includes the following steps:

[0043] (1) Ionizable lipid molecules DLin-MC3-DMA, cholesterol, DSPC and DMG-PEG 2000 were dissolved in anhydrous ethanol at a molar ratio of 50%:38.5%:10%:1.5% to obtain 200 mL of lipid ethanol solution with a concentration of 20 mg / mL; cellulose acetate was weighed and a suspension of 800 mL was formed in water with a concentration of 2 mg / mL.

[0044] (2) LNPs were prepared using the Ignite microfluidic nanoparticle preparation system from Precision Nanosystems Inc. 6 mL of cellulose acetate suspension diluent was loaded into the left syringe, and 2 mL of lipid ethanol solution was loaded into the right syringe. The instrument control software parameters were set as follows: total flow rate 12 mL / min, left-right flow rate ratio 3:1, initial discard volume 0.150 mL, and final discard volume 0.050 mL. LNPs were prepared multiple times, and the LNP solutions were collected until the final LNP product mass was 2 g. The collected LNP solution was diluted with 2 volumes of citrate buffer and purified by ultrafiltration.

[0045] (3) Prepare cellulose acetate solution by dissolving 100g of cellulose acetate in 500mL of a mixed solution of 1,4-dioxane / acetone, wherein the mass ratio of 1,4-dioxane to acetone is 2:1; add 3g of dibutyltin dilaurate and 8g of diphenylmethane diisocyanate, stir evenly, slowly add an LNP solution containing about 2g of LNP, stir slowly evenly, maintain crosslinking at 40℃ for 10h, and then raise the temperature to 50℃ to continue crosslinking for 24h to obtain casting solution.

[0046] (4) The casting solution is scraped into a flat sheet membrane with a thickness of about 300 μm. After standing in the air for 20 min, it is cured in a water bath at 10°C for 10 min to obtain a nascent flat sheet membrane. The nascent flat sheet membrane is treated in a water bath at 70°C for 15 min, taken out, and cleaned with distilled water to obtain the reverse osmosis membrane.

[0047] Example 3

[0048] A method for preparing a reverse osmosis membrane includes the following steps:

[0049] (1) Ionizable lipid molecules SM-102, cholesterol, DOPE and DMG-PEG 2000 were dissolved in anhydrous acetone at a molar ratio of 50%:38.5%:10%:1.5% to obtain 200 mL of lipid acetone solution with a concentration of 15 mg / mL; cellulose acetate was weighed and suspended in water to form 800 mL of suspension with a concentration of 1.5 mg / mL.

[0050] (2) LNPs were prepared using the Ignite microfluidic nanoparticle preparation system from Precision Nanosystems Inc. 8 mL of cellulose acetate suspension diluent was added to the left syringe, and 2 mL of lipid acetone solution was added to the right syringe. The instrument control software parameters were set as follows: total flow rate 16 mL / min, left-right flow rate ratio 4:1, initial discard volume 0.150 mL, and final discard volume 0.050 mL. LNPs were prepared multiple times, and the LNP solutions were collected until the final LNP product mass was 1 g. The collected LNP solution was diluted with 2 volumes of citrate buffer and purified by ultrafiltration.

[0051] (3) Prepare cellulose acetate solution by dissolving 80g of cellulose acetate in 400mL of a mixed solution of 1,4-dioxane / acetone, wherein the mass ratio of 1,4-dioxane to acetone is 2:1; add 1g of dibutyltin dilaurate and 4g of diphenylmethane diisocyanate, stir evenly, slowly add an LNP solution containing about 1g of LNP, stir slowly evenly, maintain crosslinking at 40℃ for 12h, and then raise the temperature to 60℃ to continue crosslinking for 16h to obtain casting solution.

[0052] (4) The casting solution is scraped into a flat sheet membrane with a thickness of about 100 μm. After standing in the air for 30 min, it is cured in a water bath at 10°C for 20 min to obtain a nascent flat sheet membrane. The nascent flat sheet membrane is treated in a water bath at 70°C for 5 min, taken out, and cleaned with distilled water to obtain the reverse osmosis membrane.

[0053] Comparative Example 1

[0054] A method for preparing a reverse osmosis membrane includes the following steps:

[0055] (1) Prepare a cellulose acetate solution by dissolving 100g of cellulose acetate in a 500mL mixed solution of 1,4-dioxane / acetone, wherein the mass ratio of 1,4-dioxane to acetone is 2:1; add 2g of cerium ammonium nitrate and 6g of glyoxal, stir evenly, maintain cross-linking at 35℃ for 6h, and then raise the temperature to 60℃ to continue cross-linking for 16h to obtain a casting solution.

[0056] (2) The casting solution is scraped into a flat sheet membrane with a thickness of about 200 μm. After standing in the air for 20 min, it is cured in a 0℃ water bath for 10 min to obtain a nascent flat sheet membrane. The nascent flat sheet membrane is treated in a 60℃ water bath for 10 min, taken out, and cleaned with distilled water to obtain the reverse osmosis membrane.

[0057] Performance test results:

[0058] The reverse osmosis membranes of Examples 1-3 and Comparative Example 1 were subjected to performance tests. The test method was as follows: 0.5 mol / L NaCl aqueous solution was used as feed liquid, the operating pressure was 225 psi, the temperature was 25℃, and the pH was 7.0. The water permeation flux and salt rejection rate of the reverse osmosis membranes were tested.

[0059] The test results are shown in the table below:

[0060]

[0061] As shown in the table above, the membrane thickness and composition of Examples 1-3 differ, but the thickness of the reverse osmosis membrane is the main factor affecting water permeate flux and salt rejection rate, and there is a certain negative correlation between water permeate flux and salt rejection rate; however, Examples 1-3 all have good water permeate flux and salt rejection rate. Compared with Comparative Document 1, the use of cellulose acetate combined with LNP significantly improved the water permeate flux of the reverse osmosis membrane, while slightly improving the salt rejection rate. Therefore, LNP can improve the water permeate flux of the reverse osmosis membrane without reducing the salt rejection rate.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and do not constitute a limitation on the content of the present invention. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including combining various technical features in any other suitable manner. These simple modifications and combinations should also be regarded as the content disclosed in the present invention and all fall within the protection scope of the present invention.

Claims

1. A method for preparing a reverse osmosis membrane, characterized in that, Includes the following steps: (1) Prepare LNP lipid raw material into lipid organic solution, and prepare a small amount of cellulose acetate into cellulose acetate aqueous suspension; (2) LNP was synthesized from lipid organic solution and cellulose acetate aqueous suspension using microfluidic technology, and then diluted and purified by ultrafiltration to obtain LNP solution; (3) Prepare a cellulose acetate solution, add a crosslinking agent and a catalyst, and mix it with an LNP solution to obtain a casting solution; (4) The casting solution is used to make a reverse osmosis membrane; The LNP lipid raw material includes ionizable lipid molecules, cholesterol, phospholipids, and PEG lipids; the ionizable lipid molecules are selected from ALC-0315, DLin-MC3-DMA, or SM-102; the phospholipids are selected from one or more of 1,2-distearyl-sn-glycerol-3-phosphocholine (DSPC), 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycerol-3-phosphocholine (DMPC), 1,2-dioleoyl-sn-glycerol-3-phosphocholine (DOPC), and 1,2-dioleoyl-sn-glycerol-3-phosphoethanolamine (DOPE); and the PEG lipid is DMG-PEG 2000.

2. The preparation method according to claim 1, characterized in that, The method for preparing the lipid organic solution is as follows: Ionizable lipid molecules, cholesterol, phospholipids and PEG lipids are dissolved in an organic solvent at a molar ratio of 50-65%:30-40%:4-10%:0.5-2% to obtain the lipid organic solution with a concentration of 5-20 mg / mL.

3. The preparation method according to claim 2, characterized in that, In step (1), the organic solvent is selected from anhydrous ethanol, methanol or acetone.

4. The preparation method according to claim 1, characterized in that, In step (1), the volume ratio of the lipid organic solution to the cellulose acetate suspension is 1:3-5, and the concentration of the cellulose acetate suspension is 0.5-4 mg / mL.

5. The preparation method according to claim 1, characterized in that, In step (2), the specific operation for synthesizing LNP is as follows: LNP is prepared using a microfluidic synthesis device and a microfluidic chip. The right injection solution is loaded with a lipid organic solution, and the left injection solution is loaded with a cellulose acetate suspension. The total liquid flow rate and the flow rates on the left and right sides are set. The product LNP solution is collected, diluted with buffer solution, and then purified by ultrafiltration.

6. The preparation method according to claim 5, characterized in that, In step (2), the total liquid flow rate is 10-20 mL / min, the ratio of the flow rate on the left to the flow rate on the right is 3-4:1, and the buffer solution is selected from PBS buffer or citrate buffer.

7. The preparation method according to claim 1, characterized in that, In step (3), the cellulose acetate solution is a mixed solution of cellulose acetate and 1,4-dioxane / acetone, wherein the mass ratio of 1,4-dioxane to acetone is 2-3:1; the crosslinking agent is selected from at least one of formaldehyde, glyoxal, glutaraldehyde, diphenylmethane diisocyanate, and hexamethylene diisocyanate; the catalyst is selected from at least one of cerium ammonium nitrate, dibutyltin diacetate, and dibutyltin dilaurate; the mass ratio of cellulose acetate, the mixed solution, the catalyst, and the crosslinking agent is 6-18:30-70:1-3:0.5-8; and the mass ratio of LNP to cellulose acetate in the cellulose acetate solution is 0.5-2:

100.

8. The preparation method according to claim 1, characterized in that, In step (3), the crosslinking operation is as follows: maintain crosslinking at 30-40℃ for 6-12h, and then raise the temperature to 50-60℃ and maintain it for 12-24h.

9. The preparation method according to claim 1, characterized in that, In step (4), the specific operation of making the casting solution into a reverse osmosis membrane is as follows: the casting solution is scraped into a flat sheet membrane, left to stand, cured in a low-temperature water bath, heat-treated in a water bath, and cleaned to obtain a reverse osmosis membrane; the thickness of the flat sheet membrane is 100-300 μm.

10. The preparation method according to claim 1, characterized in that, In step (4), the specific operation of making the casting solution into a reverse osmosis membrane is as follows: the casting solution is scraped into a flat sheet membrane with a thickness of 100-300μm, left to stand in the air for 5-30 minutes, and then cured in a water bath at 0-10℃ for 5-20 minutes to obtain a nascent flat sheet membrane; the nascent flat sheet membrane is treated in a water bath at 60-70℃ for 5-15 minutes, taken out, and cleaned with distilled water to obtain the reverse osmosis membrane.

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