Preparation method of loose nanofiltration membrane for high-quality water treatment

Abstract

The application relates to a preparation method of a loose nanofiltration membrane for high-quality water treatment, and belongs to the field of membrane separation and drinking water purification. The application aims to solve the problem that an existing method is difficult to prepare a structure-uniform and high-performance loose nanofiltration membrane through interface polymerization. The method comprises the following steps: S1, base membrane pretreatment; S2, preparation of an aqueous solution; S3, preparation of an organic phase solution; S4, interface polymerization reaction; and S5, post-treatment. In the application, a protein is used as the only reaction monomer in the aqueous phase, an interface polymerization reaction of the protein and trimesoyl chloride is utilized to form a separation layer, the use of a small-molecule amine monomer is avoided, and the preparation process is green and safe. By controlling the interface polymerization time, the looseness degree and separation performance of the membrane can be effectively controlled. The prepared membrane shows excellent water flux and adjustable salt rejection characteristics, and is particularly suitable for the field of high-quality water treatment.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation and drinking water purification, and specifically relates to a method for preparing a loose nanofiltration membrane for high-quality water treatment. Background Technology

[0002] With the acceleration of industrialization and population growth, water scarcity and water pollution are becoming increasingly serious problems. Membrane separation technology, as a highly efficient, energy-saving, and environmentally friendly water treatment technology, has been widely used in drinking water purification, advanced wastewater treatment, and reuse. Among them, nanofiltration (NF) is a pressure-driven membrane separation process that lies between ultrafiltration and reverse osmosis. It can effectively retain multivalent ions, organic matter (such as pesticides, pigments, and antibiotics), and microorganisms, while allowing some monovalent salts and small-molecule organic matter to pass through. Therefore, it shows great application potential in hard water softening, drinking water purification, and wastewater treatment.

[0003] Currently, most commercially available nanofiltration membranes are polyamide composite membranes, typically prepared on a porous support layer using interfacial polymerization. Interfacial polymerization is a technique that utilizes a rapid polycondensation reaction between two highly reactive monomers (such as amine monomers in the aqueous phase and acyl chloride monomers in the organic phase) at the interface between two phases to form an ultrathin separation layer. Traditional polyamide nanofiltration membranes usually have a relatively dense structure to achieve efficient retention of divalent salts. However, for certain specific applications, such as the preparation of high-quality drinking water and the separation of specific components, membranes need to allow water molecules and some beneficial mineral elements to pass through rapidly. Existing methods for preparing loose nanofiltration membranes include blending, surface coating, and interfacial polymerization parameter control. Among these, interfacial polymerization parameter control has been widely studied due to its simplicity and ease of control. However, conventional interfacial polymerization reactions are extremely fast, making it difficult to precisely control the polymerization process at the kinetic level, resulting in an uneven polyamide layer structure and difficulty in accurately controlling its thickness. For example, attempting to prepare a loose layer by reducing the monomer concentration or shortening the reaction time often leads to defects on the membrane surface, reducing retention performance. Therefore, developing a method to effectively control the interfacial polymerization process and prepare porous nanofiltration membranes with uniform structure and excellent performance is of great research value and practical significance. Summary of the Invention

[0004] The purpose of this invention is to solve the problem that existing methods are difficult to use for finely controlling the preparation of structurally uniform, high-performance loose nanofiltration membranes through interfacial polymerization, and to provide a method for preparing loose nanofiltration membranes for high-quality water treatment.

[0005] A method for preparing a loose nanofiltration membrane for high-quality water treatment is specifically carried out according to the following steps:

[0006] I. Base film pretreatment:

[0007] The ultrafiltration membrane was immersed in anhydrous ethanol to clean and remove the surface protectant. Then it was taken out and soaked in pure water for later use, thus obtaining the pretreated ultrafiltration membrane.

[0008] II. Preparation of Aqueous Solutions:

[0009] Dissolve the protein in a buffer solution or deionized water to obtain a homogeneous aqueous solution.

[0010] III. Preparation of Organic Phase Solutions:

[0011] Trimethylbenzene chloride was dissolved in an organic solvent to obtain a homogeneous organic phase solution;

[0012] IV. Interfacial polymerization reaction:

[0013] ① Take the pretreated ultrafiltration membrane out of the pure water and dry it. Then immerse it in the aqueous solution and let it stand for soaking. Then take it out of the aqueous solution to obtain the ultrafiltration membrane after soaking in the aqueous solution.

[0014] ② After soaking in the aqueous solution, the ultrafiltration membrane is purged with nitrogen or air until there are no droplets on the membrane surface. Then, it is immersed in the organic solution and allowed to stand for soaking to carry out the interfacial polymerization reaction, so as to obtain the ultrafiltration membrane after soaking in the organic solution.

[0015] ③ Take out the ultrafiltration membrane after soaking in the organic phase solution, rinse the membrane surface with organic solvent to remove unreacted monomers and residual solvent, and obtain the ultrafiltration membrane after interfacial polymerization reaction;

[0016] V. Post-processing:

[0017] The ultrafiltration membrane after interfacial polymerization was placed in an oven and heat-treated at a certain temperature to obtain a loose nanofiltration membrane for high-quality water treatment.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] I. This invention uses protein as the sole reactant in the aqueous phase, and forms a separation layer directly through interfacial polymerization with trimesoyl chloride, without the need to add traditional small molecule amines. This method not only simplifies the reaction system, but also makes the preparation process green and safe, avoiding the risk of residual small molecule monomers. The resulting membrane has good biocompatibility and is particularly suitable for high-quality drinking water treatment.

[0020] Second, by precisely controlling the cross-linking time of interfacial polymerization, this invention can effectively regulate the reaction degree between proteins and trimesoyl chloride, thereby achieving controllable adjustment of the porosity of the polyamide separation layer; by optimizing the reaction time, a porous nanofiltration membrane with both high water flux and ideal retention performance can be obtained to meet the needs of different high-quality water treatment scenarios.

[0021] Third, the loose nanofiltration membrane prepared by the present invention has a separation layer composed of a network structure formed by cross-linking of proteins and trimesoyl chloride. This structure is more porous than the dense polyamide layer formed by traditional small molecule amines, providing more transport channels for water molecules. This is beneficial to significantly increase the water flux of the membrane and reduce operating energy consumption while ensuring effective retention of pollutants (such as organic matter, viruses, and colloids). Attached Figure Description

[0022] Figure 1 The zeta potential curves of the nanofiltration membrane prepared in Example 2 and the conventional nanofiltration membrane prepared in Control Example 1 are shown.

[0023] Figure 2 Atomic force microscopy images of the nanofiltration membrane prepared in Example 2 and the conventional nanofiltration membrane prepared in Control Example 1, and their roughness;

[0024] Figure 3 Nanofiltration performance of nanofiltration membranes prepared in Examples 1-3 and conventional nanofiltration membrane prepared in Control Example 1. Detailed Implementation

[0025] Specific Implementation Method 1: This implementation method is a method for preparing a loose nanofiltration membrane for high-quality water treatment, specifically completed according to the following steps:

[0026] I. Base film pretreatment:

[0027] The ultrafiltration membrane was immersed in anhydrous ethanol to clean and remove the surface protectant. Then it was taken out and soaked in pure water for later use, thus obtaining the pretreated ultrafiltration membrane.

[0028] II. Preparation of Aqueous Solutions:

[0029] Dissolve the protein in a buffer solution or deionized water to obtain a homogeneous aqueous solution.

[0030] III. Preparation of Organic Phase Solutions:

[0031] Trimethylbenzene chloride was dissolved in an organic solvent to obtain a homogeneous organic phase solution;

[0032] IV. Interfacial polymerization reaction:

[0033] ① Take the pretreated ultrafiltration membrane out of the pure water and dry it. Then immerse it in the aqueous solution and let it stand for soaking. Then take it out of the aqueous solution to obtain the ultrafiltration membrane after soaking in the aqueous solution.

[0034] ② After soaking in the aqueous solution, the ultrafiltration membrane is purged with nitrogen or air until there are no droplets on the membrane surface. Then, it is immersed in the organic solution and allowed to stand for soaking to carry out the interfacial polymerization reaction, so as to obtain the ultrafiltration membrane after soaking in the organic solution.

[0035] ③ Take out the ultrafiltration membrane after soaking in the organic phase solution, rinse the membrane surface with organic solvent to remove unreacted monomers and residual solvent, and obtain the ultrafiltration membrane after interfacial polymerization reaction;

[0036] V. Post-processing:

[0037] The ultrafiltration membrane after interfacial polymerization was placed in an oven and heat-treated at a certain temperature to obtain a loose nanofiltration membrane for high-quality water treatment.

[0038] This embodiment uses protein as the sole reactant monomer in the aqueous phase, utilizing its interfacial polymerization with trimesoyl chloride to form a separation layer. This avoids the use of small-molecule amine monomers, making the preparation process green and safe. By controlling the interfacial polymerization time, the membrane porosity and separation performance can be effectively adjusted. The prepared membrane exhibits excellent water flux and adjustable salt rejection characteristics, making it particularly suitable for high-quality water treatment applications.

[0039] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the ultrafiltration base membrane mentioned in step one includes a non-woven fabric support layer and a base membrane layer; the base membrane layer is a polyethersulfone or polysulfone membrane. Other steps are the same as in Specific Implementation Method One.

[0040] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the protein mentioned in step two is bovine serum albumin, lysozyme, or whey protein. The other steps are the same as in Specific Implementation Method One or Two.

[0041] Specific Implementation Method Four: The difference between this implementation method and Specific Implementation Methods One to Three is that the concentration of protein in the aqueous solution described in step two is 0.1wt% to 0.6wt%. The other steps are the same as in Specific Implementation Methods One to Three.

[0042] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that the buffer solution mentioned in step two is Tris-HCl buffer or PBS buffer, with a concentration of 10 mmol / L to 20 mmol / L and a pH value of 6.5 to 8.5. The other steps are the same as in Specific Implementation Methods One to Four.

[0043] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that the organic solvent mentioned in step three is n-hexane or ISOPAR-G. The other steps are the same as in Specific Implementation Methods One to Five.

[0044] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that the concentration of the organic phase solution mentioned in step three is 0.1wt% to 2wt%. The other steps are the same as in Specific Implementation Methods One to Six.

[0045] Specific Implementation Method Eight: The difference between this implementation method and Specific Implementation Methods One to Seven is that the soaking time in step four① is 10 to 15 minutes. The other steps are the same as in Specific Implementation Methods One to Seven.

[0046] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that: the interfacial polymerization reaction time in step four ② is 1 min to 10 min; the organic solvent in step four ③ is n-hexane. Other steps are the same as in Specific Implementation Methods One to Eight.

[0047] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods One to Nine in that the heat treatment temperature in step five is 50℃~80℃, and the heat treatment time is 5min~10min. Other steps are the same as in Specific Implementation Methods One to Nine.

[0048] The beneficial effects of the present invention are verified using the following embodiments:

[0049] Example 1: A method for preparing a loose nanofiltration membrane for high-quality water treatment, specifically comprising the following steps:

[0050] I. Base film pretreatment:

[0051] The ultrafiltration membrane was immersed in anhydrous ethanol to clean and remove the surface protectant. Then it was taken out and soaked in pure water for later use, thus obtaining the pretreated ultrafiltration membrane.

[0052] The ultrafiltration base membrane mentioned in step one includes a non-woven fabric support layer and a base membrane layer; the base membrane layer is polyethersulfone.

[0053] II. Preparation of Aqueous Solutions:

[0054] The protein was dissolved in a buffer solution to obtain a homogeneous aqueous solution;

[0055] The buffer solution mentioned in step two is Tris-HCl buffer solution with a concentration of 10 mmol / L and a pH of 7.4;

[0056] The protein mentioned in step two is bovine serum albumin;

[0057] The concentration of protein in the aqueous solution described in step two is 0.2 wt%.

[0058] III. Preparation of Organic Phase Solutions:

[0059] Trimethylbenzene chloride was dissolved in an organic solvent to obtain a homogeneous organic phase solution;

[0060] The organic solvent mentioned in step three is n-hexane;

[0061] The concentration of pyromellitic acid chloride in the organic phase solution described in step three is 0.1 wt%.

[0062] IV. Interfacial polymerization reaction:

[0063] ① Take the pretreated ultrafiltration membrane out of the pure water and dry it. Then immerse it in the aqueous solution and let it stand for soaking. Then take it out of the aqueous solution to obtain the ultrafiltration membrane after soaking in the aqueous solution.

[0064] The soaking time mentioned in step four① is 10 minutes;

[0065] ② After soaking in the aqueous solution, the ultrafiltration membrane is purged with nitrogen or air until there are no obvious droplets on the membrane surface. Then, it is immersed in the organic solution and allowed to stand for interfacial polymerization reaction to obtain the ultrafiltration membrane after soaking in the organic solution.

[0066] The time for the interfacial polymerization reaction described in step 4② is 1 minute;

[0067] ③ Take out the ultrafiltration membrane after soaking in the organic phase solution, rinse the membrane surface with organic solvent to remove unreacted monomers and residual solvent, and obtain the ultrafiltration membrane after interfacial polymerization reaction;

[0068] The organic solvent mentioned in step four ③ is n-hexane;

[0069] V. Post-processing:

[0070] The ultrafiltration membrane after interfacial polymerization was placed in an oven and heat-treated at a certain temperature to obtain a loose nanofiltration membrane for high-quality water treatment.

[0071] The heat treatment temperature in step five is 60°C, and the heat treatment time is 10 minutes.

[0072] Example 2: The difference between this example and Example 1 is that the interfacial polymerization reaction time in step four ② is 3 minutes. All other steps and parameters are the same as in Example 1.

[0073] Example 3: The difference between this example and Example 1 is that the interfacial polymerization reaction time in step four ② is 5 minutes. All other steps and parameters are the same as in Example 1.

[0074] Comparative Example 1: The difference between this example and Example 1 is that the aqueous solution in step two is replaced with a 0.1 wt% anhydrous piperazine aqueous solution; the interfacial polymerization reaction time in step four ② is 3 min; and the nanofiltration membrane obtained in step five is denoted as PA membrane. All other steps and parameters are the same as in Example 1.

[0075] The loose nanofiltration membranes prepared in Examples 1-3 for high-quality water treatment were named BM-0.1, BM-0.3, and BM-0.5, respectively. These membranes were primarily used to investigate the regulatory effect of crosslinking time on the performance of membranes prepared by crosslinking proteins with trimesoyl chloride. The membrane prepared in Comparative Example 1 was a conventional polyamide membrane, used as a control.

[0076] Zeta potential tests were performed on the BM-0.3 membrane (interfacial polymerization time 3 min) prepared in Example 2 and the PA membrane (conventional PIP-TMC polyamide membrane) prepared in Comparative Example 1 to investigate the change of their surface charge characteristics with pH value. The results are shown in [Figure 1]. Figure 1 ;

[0077] Figure 1 The zeta potential curves of the nanofiltration membrane prepared in Example 2 and the conventional nanofiltration membrane prepared in Control Example 1 are shown.

[0078] Depend on Figure 1 It can be seen that the Zeta potential of the BM-0.3 membrane and the PA membrane exhibits a basically consistent trend with pH. When the Zeta potential is 0 mV (i.e., the isoelectric point), the pH corresponding to the BM-0.3 membrane is 3.54, and the pH corresponding to the PA membrane is 3.69, which are very close. Under neutral conditions (pH=7), the Zeta potential of the BM-0.3 membrane is -32.8 mV, and that of the PA membrane is -31.8 mV, indicating that the protein cross-linked membrane prepared in this invention also carries a strong negative charge under neutral conditions. This surface charge characteristic is beneficial for repelling negatively charged pollutants (such as natural organic matter, sulfate ions, etc.) through the Donnan effect, providing favorable conditions for its application in the field of drinking water purification.

[0079] The surface morphology and roughness of the BM-0.3 film prepared in Example 2 and the PA film prepared in Comparative Example 1 were characterized using atomic force microscopy (AFM). The results are shown in the figure. Figure 2 ;

[0080] Figure 2 Atomic force microscopy images of the nanofiltration membrane prepared in Example 2 and the conventional nanofiltration membrane prepared in Control Example 1, and their roughness;

[0081] Depend on Figure 2 It can be seen that the PA membrane prepared by the traditional interfacial polymerization method exhibits a typical nodular protrusion structure on its surface, which is a characteristic morphology formed by the rapid reaction of PIP and trimesoyl chloride (TMC). In contrast, the BM-0.3 membrane prepared by the method of this invention shows a significantly reduced protrusion structure, exhibiting a smoother morphology. Further analysis of surface roughness parameters reveals that the average roughness (Ra) of the BM-0.3 membrane is 1.20 nm, significantly lower than the 3.78 nm of the PA membrane. The reduced roughness means a smoother membrane surface, which helps reduce membrane fouling and improve the long-term operational stability of the membrane.

[0082] Nanofiltration performance tests were conducted on the BM-0.1, BM-0.3, and BM-0.5 membranes prepared in Examples 1-3, as well as the PA membrane prepared in Comparative Example 1. The pure water flux and the rejection rate for sodium sulfate at a concentration of 1000 ppm were tested. The results are shown in [Figure Number]. Figure 3 ;

[0083] Membrane water flux ( ) and retention rate ( The cross-flow filtration system was used for measurement. Water flux was calculated using the following formula:

[0084] ;

[0085] in, The volume of the permeate is (L). For the effective area of ​​the membrane ( ), P is the filtration time (h) and P is the pressure used during filtration.

[0086] Salt rejection rate is calculated using the following formula:

[0087] ;

[0088] In the formula, and These represent the salt concentrations in the permeate and feed solutions, respectively.

[0089] Figure 3 The nanofiltration performance of the nanofiltration membranes prepared in Examples 1-3 and the conventional nanofiltration membrane prepared in Control Example 1;

[0090] Depend on Figure 3 It can be seen that the traditional PA membrane exhibits a high sodium sulfate rejection rate (97.4%) and a low water flux (10.4 L·m). -2 ·h -1 ·bar -1 The BM series membranes prepared in this invention exhibit tunable separation performance. As the interfacial polymerization time increased from 1 min (BM-0.1) to 5 min (BM-0.5), the water flux of the membrane increased from 15.7 L·m⁻¹. -2 ·h -1 ·bar -1 It gradually decreased to 12.1 L·m -2 ·h -1 ·bar -1 Meanwhile, the sodium sulfate rejection rate gradually increased from 87.1% to 96.3%. This indicates that by simply adjusting the interfacial polymerization time, the degree of cross-linking between proteins and TMC can be effectively controlled, thereby achieving fine regulation of membrane porosity and separation performance.

Claims

1. A method for preparing a loose nanofiltration membrane for high-quality water treatment, characterized in that... The preparation method is specifically carried out according to the following steps: I. Base film pretreatment: The ultrafiltration membrane was immersed in anhydrous ethanol to clean and remove the surface protectant. Then it was taken out and soaked in pure water for later use, thus obtaining the pretreated ultrafiltration membrane. II. Preparation of Aqueous Solutions: Dissolve the protein in a buffer solution or deionized water to obtain a homogeneous aqueous solution. III. Preparation of Organic Phase Solutions: Trimethylbenzene chloride was dissolved in an organic solvent to obtain a homogeneous organic phase solution; IV. Interfacial polymerization reaction: ① Take the pretreated ultrafiltration membrane out of the pure water and dry it. Then immerse it in the aqueous solution and let it stand for soaking. Then take it out of the aqueous solution to obtain the ultrafiltration membrane after soaking in the aqueous solution. ② After soaking in the aqueous solution, the ultrafiltration membrane is purged with nitrogen or air until there are no droplets on the membrane surface. Then, it is immersed in the organic solution and allowed to stand for soaking to carry out the interfacial polymerization reaction, so as to obtain the ultrafiltration membrane after soaking in the organic solution. ③ Take out the ultrafiltration membrane after soaking in the organic phase solution, rinse the membrane surface with organic solvent to remove unreacted monomers and residual solvent, and obtain the ultrafiltration membrane after interfacial polymerization reaction; V. Post-processing: The ultrafiltration membrane after interfacial polymerization is placed in an oven and heat-treated at a certain temperature to obtain a loose nanofiltration membrane for high-quality water treatment.

2. The method for preparing a loose nanofiltration membrane for high-quality water treatment according to claim 1, characterized in that... The ultrafiltration base membrane mentioned in step one includes a nonwoven fabric support layer and a base membrane layer; the base membrane layer is a polyethersulfone or polysulfone membrane.

3. The method for preparing a loose nanofiltration membrane for high-quality water treatment according to claim 1, characterized in that... The protein mentioned in step two is bovine serum albumin, lysozyme, or whey protein.

4. The method for preparing a loose nanofiltration membrane for high-quality water treatment according to claim 1, characterized in that... The concentration of protein in the aqueous solution described in step two is 0.1wt%~0.6wt%.

5. The method for preparing a loose nanofiltration membrane for high-quality water treatment according to claim 1, characterized in that... The buffer solution mentioned in step two is Tris-HCl buffer or PBS buffer, with a concentration of 10 mmol / L to 20 mmol / L and a pH value of 6.5 to 8.

5.

6. The method for preparing a loose nanofiltration membrane for high-quality water treatment according to claim 1, characterized in that... The organic solvent mentioned in step three is n-hexane or ISOPAR-G.

7. The method for preparing a loose nanofiltration membrane for high-quality water treatment according to claim 1, characterized in that... The concentration of the organic phase solution mentioned in step three is 0.1wt%~2wt%.

8. The method for preparing a loose nanofiltration membrane for high-quality water treatment according to claim 1, characterized in that... The soaking time mentioned in step 4① is 10 min to 15 min.

9. The method for preparing a loose nanofiltration membrane for high-quality water treatment according to claim 1, characterized in that... The interfacial polymerization reaction time described in step 4② is 1 min to 10 min; the organic solvent described in step 4③ is n-hexane.

10. The method for preparing a loose nanofiltration membrane for high-quality water treatment according to claim 1, characterized in that... The heat treatment temperature in step five is 50℃~80℃, and the heat treatment time is 5min~10min.

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