Chlorine-resistant nanofiltration membrane based on interlayer modification and preparation method thereof

Abstract

The application discloses a kind of based on intermediate layer modification chlorine-resistant nanofiltration membrane and preparation method, including selecting PES ultrafiltration membrane as nanofiltration bottom membrane, and it is pretreated;Prepared lanthanum nitrate metal ion solution, TTSBI and sodium hydroxide are dissolved in deionized water, and polyphenol ligand solution is obtained;Nanofiltration bottom membrane after pretreatment is soaked in metal ion solution, polyphenol ligand solution in sequence and is carried out coordination reaction, to construct MPN intermediate layer on nanofiltration bottom membrane;PIP and mannitol are dissolved in deionized water, and mixed aqueous solution is obtained;TMC is dissolved in n-hexane, and organic phase solution is obtained;Nanofiltration bottom membrane with MPN intermediate layer is soaked in mixed aqueous solution, organic phase solution in sequence and is carried out interfacial polymerization, and chlorine-resistant nanofiltration membrane based on intermediate layer modification is obtained.The nanofiltration membrane of the application has uniform wrinkle, the separation layer of superthin structure, high water flux, excellent divalent salt rejection performance, and also has excellent chlorine resistance.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane wastewater treatment technology, specifically to a chlorine-resistant nanofiltration membrane based on intermediate layer modification and its preparation method. Background Technology

[0002] Nanofiltration membranes have pore sizes between ultrafiltration and reverse osmosis membranes, with a molecular weight cutoff ranging from 200 to 1000 Da. Driven by pressure, nanofiltration membranes remove inorganic salt ions from water through size sieving and Donnan charge effects. Commercially available nanofiltration membranes are typically prepared using piperazine (PIP) and trimesoyl chloride (TMC) via interfacial polymerization (IP). However, these membranes suffer from several problems in practical applications: firstly, the "upper limit balance" effect between permeability and selectivity limits the water flux of high-selectivity nanofiltration membranes; secondly, the amide bonds in the polyamide molecular chain are easily attacked by active chlorine, leading to irreversible chloramine rearrangement reactions, which damage the separation layer structure, reduce desalination rates, and shorten membrane lifespan.

[0003] While some existing modification studies improve membrane chlorine resistance by constructing an intermediate layer on the substrate to block the contact between chlorine and amide bonds, this method often leads to a significant decrease in membrane separation performance. This severely limits the long-term stable operation of nanofiltration membranes in high-chlorine environments and their widespread application in practical water treatment projects. Therefore, providing an effective intermediate layer modification method to prepare nanofiltration membranes with both good separation performance and chlorine resistance is of great research significance and application value. Summary of the Invention

[0004] Based on the above, the present invention provides a chlorine-resistant nanofiltration membrane based on intermediate layer modification and its preparation method. The nanofiltration membrane prepared by the present invention has a separation layer with uniform pleats and an ultra-thin structure, high water flux, excellent retention performance for divalent salts, and excellent chlorine resistance.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention discloses a method for preparing a chloride-resistant nanofiltration membrane based on intermediate layer modification, comprising the following steps: Step 1: Select polyethersulfone (PES) ultrafiltration membrane as nanofiltration substrate membrane, and pretreat the nanofiltration substrate membrane by immersing it in ethanol solution and deionized water in sequence. Step 2: Dissolve lanthanum nitrate in deionized water to obtain a metal ion solution; Step 3: Dissolve 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane (TTSBI) and sodium hydroxide in deionized water to obtain a polyphenol ligand solution; Step 4: Immerse the pretreated nanofiltration membrane in the metal ion solution, remove it and use a rubber roller to remove the residual solution on the surface, then immerse it in the polyphenol ligand solution to carry out the coordination reaction, remove it and use a rubber roller to remove the residual solution on the surface, and construct the MPN intermediate layer on the nanofiltration membrane. Step 5: Dissolve piperazine (PIP) and mannitol in deionized water to obtain a mixed aqueous solution; Step 6: Dissolve trimesoyl chloride (TMC) in n-hexane to obtain an organic phase solution; Step 7: Immerse the nanofiltration membrane with the MPN interlayer in the mixed aqueous solution, remove it and use a rubber roller to remove the residual solution on the surface, then immerse it in the organic phase solution to carry out the interfacial polymerization reaction; Step 8: After the interfacial polymerization reaction is completed, the membrane is removed, air-dried, and then subjected to heat curing treatment to finally obtain a chlorine-resistant nanofiltration membrane based on MPN intermediate layer modification.

[0006] As a further improvement of the present invention, in step 1, the nanofiltration membrane has a molecular weight cutoff of 50 kDa to 100 kDa, a physical size of 7 x 7 cm, an ethanol solution concentration of 2 to 5 wt%, a pretreatment time of 24 to 48 h, and a treatment temperature of 25 to 30 ℃.

[0007] As a further improvement of the present invention, in step 2, the concentration of the lanthanum nitrate solution is 0.05~0.20 wt%.

[0008] As a further improvement of the present invention, in step 3, the concentration of the 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane solution is 0.5~1.0 wt%; and the concentration of the sodium hydroxide solution is 0.4~0.8 wt%.

[0009] As a further improvement of the present invention, in step 4, the time for immersing the pretreated nanofiltration membrane in the metal ion solution is 5.0~10.0 min, and the time for immersing it in the polyphenol ligand solution for coordination reaction is 2.0~6.0 min.

[0010] As a further improvement of the present invention, in step 5, the concentration of piperazine in the mixed aqueous solution is 0.01~0.10 wt%, the concentration of mannitol is 0.10~1.00 wt%, and the concentration ratio of piperazine to mannitol is 1:5~1:20.

[0011] As a further improvement of the present invention, in step 6, the concentration of pyromellitic acid chloride in the organic phase solution is 0.10~0.50 wt%.

[0012] As a further improvement of the present invention, in step 7, the time for immersing the nanofiltration membrane with the intermediate layer in the mixed aqueous solution is 2.0~8.0 min, and the time for the interfacial polymerization reaction is 0.5~3.0 min.

[0013] As a further improvement of the present invention, in step 8, the air drying time is 30~60 s, the curing temperature is 40.0~80.0 ℃, and the curing time is 0.5~5.0 min.

[0014] The present invention also discloses a chlorine-resistant nanofiltration membrane based on intermediate layer modification, which is prepared by the above-described preparation method.

[0015] The beneficial effects of this invention are as follows: This invention utilizes metal ions and polyphenol ligands with rigid twisted structures to construct a stable metal-polyphenol network (MPN) interlayer in situ on the bottom membrane surface through coordination reaction. The porous structure of this interlayer enhances the adsorption of aqueous monomers, effectively slows down the diffusion rate of aqueous monomers during interfacial polymerization, and forms an ultrathin separation layer with a wrinkled structure, further improving the membrane's permeability. The active groups on the surface of the interlayer can interact with aqueous monomer molecules, further regulating the interfacial polymerization reaction.

[0016] Based on this interlayer, this invention employs a mixed aqueous solution of piperazine and mannitol for interfacial polymerization. The polyester component generated by the reaction of mannitol and trimesoyl chloride contains no amide bonds and has no Cl- attack sites, thus enhancing the chlorine resistance of the nanofiltration membrane. Simultaneously, the polyhydroxy structure of mannitol enhances the hydrophilicity of the separation layer, which is beneficial for increasing water flux. The MPN interlayer provides a uniform monomer diffusion environment for the mannitol-involved interfacial polymerization reaction, allowing the polyester component to coexist with the polyamide component in the ultrathin pleated separation layer, thereby preparing a nanofiltration membrane with both excellent separation performance and chlorine resistance. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] The present invention will now be described in further detail: Example 1

[0019] A method for preparing a chloride-resistant nanofiltration membrane based on intermediate layer modification (unless otherwise specified, all steps are performed at room temperature), comprising: S1. A 7 cm × 7 cm polyethersulfone (PES) nanofiltration membrane (molecular weight cutoff of 100 kDa) was soaked in 3.0 wt% ethanol solution for 2.0 h. After being removed, it was rinsed with deionized water and then soaked in deionized water for 48 h. The deionized water was replaced every 24 h to obtain the pretreated nanofiltration membrane.

[0020] S2. Take a certain amount of lanthanum nitrate and mix it with deionized water to prepare a metal ion solution with a lanthanum nitrate concentration of 0.125 wt%.

[0021] S3. Take a certain mass of 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane (TTSBI) and sodium hydroxide in a beaker, add deionized water, and prepare a polyphenol ligand solution containing 0.7 wt% TTSBI and 0.6 wt% sodium hydroxide.

[0022] S4. Immerse the pretreated nanofiltration membrane obtained in S1 in a metal ion solution for 7 min. After removing it, use a rubber roller to remove the residual solution on the surface. Then immerse it in a polyphenol ligand solution for 4 min to carry out the coordination reaction. After removing it, use a rubber roller to remove the residual solution on the surface. Construct an MPN intermediate layer on the nanofiltration membrane.

[0023] S5. Take a certain mass of piperazine (PIP) and mannitol into a beaker, add deionized water, and prepare a mixed aqueous solution containing 0.2 wt% piperazine and 0.5 wt% mannitol.

[0024] S6. Take a certain mass of trimesoyl chloride (TMC) in a beaker, add n-hexane, and prepare an organic phase solution with a concentration of 0.25wt%.

[0025] S7. Immerse the nanofiltration membrane with the MPN interlayer in the mixed aqueous solution for 4 min, remove it and use a rubber roller to remove the residual solution on the surface, then immerse it in the organic phase solution for interfacial polymerization reaction for 1 min. S8. After the interfacial polymerization reaction is complete, the membrane is removed and air-dried for 45 seconds, then placed in a constant temperature drying oven and heat-cured at 50 °C for 3 minutes to finally obtain a chlorine-resistant nanofiltration membrane modified based on the MPN interlayer. The heat-cured chlorine-resistant nanofiltration membrane is sealed in deionized water for storage and subsequent performance testing.

[0026] The prepared nanofiltration membrane was removed and pre-pressed with deionized water at 6.0 bar for 30 min to stabilize its performance. The pressure was then adjusted to 2.0 bar, and the membrane separation performance was tested. The test results showed that the pure water flux of the nanofiltration membrane prepared in Example 1 was 59.1 L·m³. -2 ·h -1 ·bar -1The Na2SO4 rejection rate was 98.2%. Example 2

[0027] The chlorine-resistant nanofiltration membrane was prepared according to the method of Example 1, except that the concentration of piperazine in the aqueous solution of S5 was 0.5 wt%.

[0028] The performance of the chlorine-resistant nanofiltration membrane prepared in Example 2 was tested using the same method as in Example 1. The test results showed that the pure water flux of the nanofiltration membrane prepared in Example 2 was 64.1 L·m⁻¹. -2 ·h -1 ·bar -1 The Na2SO4 rejection rate was 98.5%. Example 3

[0029] The chlorine-resistant nanofiltration membrane was prepared according to the method of Example 1, except that the concentration of piperazine in the aqueous solution of S5 was 0.6 wt%.

[0030] The performance of the chlorine-resistant nanofiltration membrane prepared in Example 3 was tested using the same method as in Example 1. The test results showed that the pure water flux of the nanofiltration membrane prepared in Example 3 was 58.2 L·m⁻¹. -2 ·h -1 ·bar -1 The Na2SO4 rejection rate was 97.6%. Example 4

[0031] The chlorine-resistant nanofiltration membrane prepared in Example 2 was taken out and pre-pressed at 2.0 bar for 30 min. Then, the membrane was immersed in a 2000 ppm NaClO solution for 10 h. After removal, it was thoroughly washed with deionized water, and the performance of the chlorinated nanofiltration membrane was tested. The test results showed that the pure water flux of the chlorine-resistant nanofiltration membrane prepared in Example 4 after chlorination was 63.8 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate of Na2SO4 aqueous solution was 97.3%. Compared with before chlorination, the separation performance of the membrane did not change significantly.

[0032] Comparative Example 1 Nanofiltration membranes were prepared according to the method of Example 1, except that S2 to S4 were not performed, and the concentration of the aqueous solution in S5 was 0.2 wt%, thus obtaining nanofiltration membranes without MPN interlayers.

[0033] The performance of the chlorine-resistant nanofiltration membrane prepared in Comparative Example 1 was tested using the same method as in Example 1. The test results showed that the pure water flux of the nanofiltration membrane prepared in Comparative Example 1 was 36.90 L·m⁻¹. -2 ·h -1 ·bar-1 The Na2SO4 rejection rate was 93.66%.

[0034] Comparative Example 2 Nanofiltration membranes were prepared according to the method of Example 1, except that S2 to S4 were not performed, and the concentration of the aqueous solution in S5 was 0.5 wt%, thus obtaining nanofiltration membranes without MPN interlayers.

[0035] The performance of the chlorine-resistant nanofiltration membrane prepared in Comparative Example 2 was tested using the same method as in Example 1. The test results showed that the pure water flux of the nanofiltration membrane prepared in Comparative Example 1 was 38.55 L·m⁻¹. -2 ·h -1 ·bar -1 The Na2SO4 rejection rate was 95.66%.

[0036] Comparative Example 3 Nanofiltration membranes were prepared according to the method of Example 1, except that S2 to S4 were not performed, and the concentration of the aqueous solution in S5 was 0.6 wt%, thus obtaining nanofiltration membranes without MPN interlayers.

[0037] The performance of the chlorine-resistant nanofiltration membrane prepared in Comparative Example 3 was tested using the same method as in Example 1. The test results showed that the pure water flux of the nanofiltration membrane prepared in Comparative Example 1 was 37.72 L·m⁻¹. -2 ·h -1 ·bar -1 The Na2SO4 rejection rate was 95.42%.

[0038] Comparative Example 4 The chlorine resistance of the nanofiltration membrane prepared in Comparative Example 2 was tested using the same method as in Example 4. The test results showed that the pure water flux after chlorination of the chlorine-resistant nanofiltration membrane prepared in Comparative Example 4 was 52.1 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate of Na2SO4 aqueous solution was 36.5%. Compared with before chlorination, the separation performance of the membrane was significantly reduced, and its chlorine resistance was poor.

[0039] In summary, the chlorine-resistant nanofiltration membrane prepared based on MPN interlayer modification in Example 2 of this invention did not show significant changes in its permeation separation performance after chlorination treatment; while the conventional PIP / TMC polyamide nanofiltration membrane in Comparative Example 2, which did not have an MPN interlayer, showed a significant decrease in membrane performance after chlorination treatment. This invention prepares a chlorine-resistant nanofiltration membrane by constructing an MPN interlayer on the substrate surface. This membrane maintains good separation performance even under high-concentration chlorine environments, while the nanofiltration membrane without an MPN interlayer shows a significant decrease in separation performance under chlorine attack. Therefore, the chlorine-resistant nanofiltration membrane based on interlayer modification and its preparation method involved in this invention have very good effects.

[0040] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a chloride-resistant nanofiltration membrane based on intermediate layer modification, characterized in that, Includes the following steps: Step 1: Select polyethersulfone ultrafiltration membrane as nanofiltration substrate membrane, and pretreat the nanofiltration substrate membrane by immersing it in ethanol solution and deionized water in sequence. Step 2: Dissolve lanthanum nitrate in deionized water to obtain a metal ion solution; Step 3: Dissolve 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane and sodium hydroxide in deionized water to obtain a polyphenol ligand solution; Step 4: Immerse the pretreated nanofiltration membrane in the metal ion solution, remove it and use a rubber roller to remove the residual solution on the surface, then immerse it in the polyphenol ligand solution to carry out the coordination reaction, remove it and use a rubber roller to remove the residual solution on the surface, and construct the MPN intermediate layer on the nanofiltration membrane. Step 5: Dissolve piperazine and mannitol in deionized water to obtain a mixed aqueous solution; Step 6: Dissolve pyromellitic acid chloride in n-hexane to obtain an organic phase solution; Step 7: Immerse the nanofiltration membrane with the MPN interlayer into the mixed aqueous solution, remove it and use a rubber roller to remove the residual solution on the surface, then immerse it in the organic phase solution to carry out the interfacial polymerization reaction; Step 8: After the interfacial polymerization reaction is completed, the membrane is removed, air-dried, and then subjected to heat curing treatment to finally obtain a chlorine-resistant nanofiltration membrane based on MPN intermediate layer modification.

2. The preparation method according to claim 1, characterized in that, In step 1, the nanofiltration membrane has a molecular weight cutoff of 50 kDa to 100 kDa, the ethanol solution concentration is 2 to 5 wt%, the pretreatment time is 24 to 48 h, and the treatment temperature is 25 to 30 ℃.

3. The preparation method according to claim 1, characterized in that, In step 2, the concentration of the lanthanum nitrate solution is 0.05~0.20 wt%.

4. The preparation method according to claim 1, characterized in that, In step 3, the concentration of the 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane solution is 0.5~1.0 wt%; the concentration of the sodium hydroxide solution is 0.4~0.8 wt%.

5. The preparation method according to claim 1, characterized in that, In step 4, the pretreated nanofiltration membrane is immersed in the metal ion solution for 5.0 to 10.0 min, and immersed in the polyphenol ligand solution for coordination reaction for 2.0 to 6.0 min.

6. The preparation method according to claim 1, characterized in that, In step 5, the concentration of piperazine in the mixed aqueous solution is 0.01~0.10 wt%, the concentration of mannitol is 0.10~1.00 wt%, and the concentration ratio of piperazine to mannitol is 1:5~1:

20.

7. The preparation method according to claim 1, characterized in that, In step 6, the concentration of pyromellitic acid chloride in the organic phase solution is 0.10~0.50 wt%.

8. The preparation method according to claim 1, characterized in that, In step 7, the nanofiltration membrane with the intermediate layer is immersed in the mixed aqueous solution for 2.0 to 8.0 minutes, and the interfacial polymerization reaction takes 0.5 to 3.0 minutes.

9. The preparation method according to claim 1, characterized in that, In step 8, the air drying time is 30~60s, the curing temperature is 40.0~80.0℃, and the curing time is 0.5~5.0 min.

10. A chloride-resistant nanofiltration membrane based on intermediate layer modification, characterized in that, It is prepared by any one of claims 1 to 9.