A simple preparation method of high-performance thin film composite forward osmosis membrane

Abstract

The application discloses a simple preparation method of a high-performance thin film composite forward osmosis membrane and belongs to the technical field of water treatment. First, a cationic surfactant is introduced into an aqueous solution to promote the improvement of membrane flux. Then, the modified membrane is soaked in an alkaline solution for a period of time to obtain a TFC-NF membrane with high performance. The TFC-NF membrane prepared through the method has excellent water flux and rejection performance.

Description

Technical Field

[0001] This invention relates to a simple method for preparing a high-performance thin-film composite forward osmosis membrane, belonging to the field of water treatment technology. Background Technology

[0002] Thin-film composite polyamide nanofiltration (TFC-NF) membranes, relying on precise molecular sieving and the synergistic separation mechanism of Dornan repulsion, have significant application value in water treatment fields such as dye desalination, trace pollutant removal, and reclaimed water reuse. Traditional interfacial polymerization reactions are driven by diffusion mass transfer, making it difficult to precisely control the monomer ratio between the water and oil phases. This easily leads to the formation of a polyamide separation skin with uneven cross-linking and high internal hydraulic mass transfer resistance, resulting in a common flux-retention trade-off problem where pure water flux and salt retention performance are mutually constrained. Currently, the industry's modification technologies are mainly divided into two categories: in-situ modification of the aqueous phase with additives during interfacial polymerization and post-film formation chemical activation treatment. Both single modification methods have significant drawbacks. Existing surfactant modification-related patent systems have poor adaptability, and the improvement in overall membrane separation performance is limited, making it difficult to simultaneously meet the application requirements of high flux and high salt retention.

[0003] Patent CN119548986A discloses a technical solution for preparing TFC-FO forward osmosis membranes by modifying the large-pore support layer with anionic surfactants such as sodium dodecylbenzenesulfonate and sodium dodecyl sulfate. This modification system is only suitable for forward osmosis membranes without chemical post-treatment. Directly applying it to nanofiltration membrane preparation processes would result in serious performance defects: electrostatic repulsion exists between the anionic surfactant and the m-phenylenediamine monomer, disrupting the interfacial polymerization kinetics and causing numerous microscopic pore defects in the separation layer; furthermore, if a subsequent alkaline soaking post-treatment process is added, alkaline hydrolysis would further amplify the skin layer defects, significantly reducing the membrane's ability to retain divalent salts and failing to meet the high-precision desalination requirements of nanofiltration. Therefore, this solution relies solely on anionic surfactant modification, resulting in poor membrane pore uniformity and limited improvement in pure water flux.

[0004] Patent CN115888418A uses hexadecyltrimethylammonium bromide cationic surfactant to modify and prepare lithium-charged positively charged membranes for salt lake extraction. This invention uses polyethyleneimine as an aqueous monomer, which is completely different from the reaction system and applicable working conditions of the m-phenylenediamine nanofiltration of this invention. Moreover, this process requires the compounding of anionic and cationic surfactants, carbonate foaming templates and a high-temperature oil phase system. It involves many types of additives, complex preparation processes, and high industrial production costs.

[0005] Patent CN112755817A uses a dicarbon-chain phosphate ester oil-soluble anionic surfactant for modification in the oil phase, without introducing a cationic surfactant in the aqueous phase or setting up an alkaline post-treatment process, thus lacking an integrated control approach that combines in-situ regulation with alkaline bubble coupling; the optimal pure water flux of the modified membrane in this patent is only 16 L·m. -2·h -1 ·bar -1 The throughput increase has a relatively low upper limit, and the separation performance has obvious shortcomings.

[0006] In addition, existing conventional modification technologies still have many shortcomings: modification processes that only use alkali immersion are prone to excessive damage to the polyamide cross-linking structure by alkali etching, which will irreversibly lose salt retention performance while increasing throughput; schemes that only implement in-situ aqueous phase additive modification cannot fully extend the polyamide polymer chains, and the mass transfer channels between the support layer and the separation layer cannot be effectively connected, resulting in a significant ceiling on throughput improvement; while modification methods such as nanofiller doping and intermediate layer modification require expensive raw materials and have lengthy preparation processes, which are not suitable for large-scale mass production.

[0007] In summary, existing publicly available modification patents and conventional modification technologies all have limitations. Some use anionic surfactants that are incompatible with alkaline bath post-treatment processes, while others employ cationic surfactants with completely incompatible systems and complex preparation processes. Furthermore, none of these single modification methods can simultaneously achieve a synergistic improvement in pure water flux and salt rejection performance. Therefore, the industry urgently needs an integrated TFC-NF membrane preparation solution that is simple to implement, uses inexpensive raw materials, is suitable for large-scale production, and offers excellent modification results. Summary of the Invention

[0008] Technical issues How to simplify the preparation process of TFC-NF membranes and achieve a synergistic improvement in pure water flux and salt rejection performance.

[0009] Technical solution To address the aforementioned issues, this invention couples in-situ modification of cationic surfactants in the aqueous phase with alkaline immersion post-treatment to construct an integrated control strategy: First, cationic surfactants are introduced into the aqueous system for interfacial polymerization. Their regulatory effect accelerates amine monomer diffusion, induces Malagoni convection, optimizes interfacial polymerization kinetics, and pre-constructs a uniformly ordered cross-linked polyamide framework, increasing the effective water transport area of ​​the membrane. After completing interfacial polymerization and thermal cross-linking to obtain a nascent TFC membrane, the modified nascent membrane is then immersed in an alkaline solution. Alkali-induced physical reconstruction of the polyamide polymer chains is achieved, opening up mass transfer channels between the support layer and the separation layer and reducing internal hydraulic resistance. This invention, through the synergistic effect of in-situ control and mild post-treatment, can significantly increase the pure water permeation flux of the membrane without damaging the polyamide cross-linked structure or reducing the ion and neutral solute retention performance. It effectively overcomes the inherent permeate-selectivity trade-off bottleneck of traditional polyamide nanofiltration membranes. The entire preparation process is mild, simple to operate, and inexpensive, suitable for large-scale mass production, providing an efficient and integrated preparation solution for high-performance TFC-NF nanofiltration membranes.

[0010] The first objective of this invention is to provide a method for preparing a high-performance TFC-NF membrane, comprising the following steps: (1) Modification of aqueous solution in the TFC membrane interfacial polymerization process: The surfactant and m-phenylenediamine are dissolved in water to form an aqueous solution; (2) Preparation of high-performance TFC-NF membranes: The base membrane was immersed in the aqueous solution prepared in step (1), and after the base membrane was removed and dried, it was immersed in a pyromellitic chloride solution for interfacial polymerization. Finally, after the interfacial polymerization was completed, the base membrane was removed and thermal crosslinking was carried out to obtain a high-performance TFC-NF membrane.

[0011] In one embodiment of the present invention, the surfactant in step (1) is a cationic surfactant.

[0012] In one embodiment of the present invention, the cationic surfactant is dodecylpyridine bromide (DPB), hexadecyltrimethylammonium bromide (CTAB), or hexadecylpyridine bromide (CPB).

[0013] Preferably, the cationic surfactant is dodecylpyridine bromide (DPB); the use of DPB can stabilize the retention rate of the TFC-NF membrane at over 99%.

[0014] In one embodiment of the present invention, the concentration of the surfactant in the aqueous solution in step (1) is 0.25 to 3 times the critical micelle concentration (CMC) of the surfactant; preferably, the concentration is 0.5 to 1.5 times the critical micelle concentration of the surfactant.

[0015] More preferably, the concentration of the surfactant in the aqueous solution in step (1) is 0.75 to 1.25 times the critical micelle concentration of the surfactant.

[0016] The critical micelle concentration refers to the minimum concentration at which surfactant molecules associate to form micelles in a solution. When the surfactant concentration reaches the CMC, the surface tension of the solution drops to its minimum value. If the concentration is increased further, the surface tension of the solution will no longer decrease, but a large number of micelles will form.

[0017] The critical micelle concentration of the DPB is 11.5 mM / L.

[0018] The critical micelle concentration of the CTAB is 9.2 mM / L.

[0019] The critical micelle concentration of the CPB is 8.5 mM / L.

[0020] In one embodiment of the present invention, the concentration of intermediate phenylenediamine in the aqueous solution in step (1) is 0.1~5wt%.

[0021] In one embodiment of the present invention, the concentration of intermediate phenylenediamine in the aqueous solution in step (1) is 0.1~3wt%.

[0022] In one embodiment of the present invention, the concentration of intermediate phenylenediamine in the aqueous solution in step (1) is 0.1~1wt%.

[0023] In one embodiment of the present invention, the average surface pore size of the base film in step (2) is 0.01~2.00 µm.

[0024] In one embodiment of the present invention, the average surface pore size of the base membrane in step (2) is 0.01~1.50 µm.

[0025] In one embodiment of the present invention, the average surface pore size of the base film in step (2) is 0.01~1.00 µm.

[0026] In one embodiment of the present invention, the average surface pore size of the base film in step (2) is 0.01~0.50 µm.

[0027] In one embodiment of the present invention, the average surface pore size of the base film in step (2) is 0.01~0.10 µm.

[0028] In one embodiment of the present invention, the immersion time in step (2) is 1~10 min and the immersion temperature is 15~30 ℃.

[0029] In one embodiment of the present invention, the solvent of the benzotriformyl chloride solution in step (2) is a non-polar organic solvent; the non-polar organic solvent includes n-hexane or n-heptane.

[0030] In one embodiment of the present invention, the concentration of benzotricarboxylic acid chloride in the benzotricarboxylic acid chloride solution in step (2) is 0.01~1wt%.

[0031] In one embodiment of the present invention, the concentration of benzotricarboxylic acid chloride in the benzotricarboxylic acid chloride solution in step (2) is 0.01~0.5wt%.

[0032] In one embodiment of the present invention, the concentration of benzotricarboxylic acid chloride in the benzotricarboxylic acid chloride solution in step (2) is 0.1~0.5wt%.

[0033] In one embodiment of the present invention, the time for the interfacial polymerization reaction in step (2) is 1~10 min, and the temperature for the interfacial polymerization reaction is 15~30 ℃.

[0034] In one embodiment of the present invention, the thermal crosslinking reaction in step (2) is carried out in a water bath or air bath atmosphere; the temperature of the thermal crosslinking reaction is 50~100 ℃, and the time of the thermal crosslinking reaction is 1~20 min.

[0035] In one embodiment of the present invention, the pH of the alkaline solution used for soaking in step (2) is between 8 and 12.

[0036] In one embodiment of the present invention, the pH of the alkaline solution used for soaking in step (2) is 10 to 12.

[0037] In one embodiment of the present invention, the alkali in the alkaline solution is sodium hydroxide and / or potassium hydroxide.

[0038] In one embodiment of the present invention, the soaking time in the alkaline solution in step (2) is 1 to 60 hours.

[0039] In one embodiment of the present invention, the soaking time in the alkaline solution in step (2) is 10~60h.

[0040] A second objective of this invention is to provide a high-performance TFC-NF membrane prepared according to the above method.

[0041] The high-performance TFC-NF membrane provided by this invention can be applied in the field of water treatment.

[0042] Beneficial effects (1) This invention achieves a significant improvement in membrane separation performance through an integrated process that couples in-situ modification of cationic surfactants in aqueous phase with post-treatment after alkali immersion. This process can significantly increase the pure water permeate flux (20 L·m³) without damaging the polyamide crosslinking structure or reducing the sodium sulfate rejection rate (which is stably maintained above 99%). -2 ·h -1 ·bar -1 (Above), the retention performance does not significantly decrease under long-term cross-flow filtration conditions, and the operation stability is excellent.

[0043] (2) The process of the present invention is simple and mild, requiring only in-situ modification of interface polymerization followed by one-step alkaline soaking and post-treatment. Compared with modification schemes such as nanofiller doping and multilayer intermediate layer modification, the process is greatly simplified and no complex high temperature and high pressure equipment is required. The reagents used are all conventional and inexpensive raw materials, with low dosage and low production cost, which is suitable for industrial mass production. In addition, the high-performance TFC-NF nanofiltration membrane prepared by the present invention has a wide range of applications and can be used in various water treatment fields such as dye desalination, removal of trace pollutants, and reuse of wastewater. Attached Figure Description

[0044] Figure 1A schematic diagram of a method for preparing high-performance TFC-NF membranes.

[0045] Figure 2 The graph shows a comparison of the pure water flux of the TFC-NF membranes prepared in Example 1 and Comparative Examples 1-3.

[0046] Figure 3 The graph shows a comparison of the salt rejection rates of the TFC-NF membranes prepared in Example 1 and Comparative Examples 1-3.

[0047] Figure 4 The effect of DPB mixing concentration on the performance of alkaline foam modified membrane.

[0048] Figure 5 The effects of different ionic surfactants on the properties of alkaline foam modified films. Detailed Implementation

[0049] The preferred embodiments of the present invention are described below. It should be understood that the embodiments are for better explanation of the present invention and are not intended to limit the present invention.

[0050] Test methods 1. Water flux: The volume of pure water is recorded at regular intervals, and the water flux is obtained using formula (1): (1) In equation (1), J W The water flux of the membrane is expressed in L·m. -2 ·h -1 ; ΔV The volume of the feed liquid permeating through the membrane is expressed in L; Δt Indicates the duration of the test experiment, in hours (h). A m The effective area of ​​the membrane is expressed in m. 2 P represents pressure, in bar.

[0051] 2. Salt retention rate: By testing the conductivity of the feed solution before and after operation, its ion concentration can be calculated, and then the salt rejection rate can be calculated. (2) In equation (2), R Retention rate, % C p and C f These are the electrical conductivity of the feed liquid at the beginning and end, respectively.

[0052] Raw materials used in the examples The support layer is a polyethersulfone (PES) substrate membrane with an average surface pore size of 0.05 µm and a diameter of 47 mm, all purchased from Haining Yibo Filter Material Factory in China. Unless otherwise specified, all reagents or instruments used are commercially available products. Unless otherwise specified, the procedure shall be carried out in accordance with common knowledge in the field or in accordance with the product manual.

[0053] Example 1 A simplified method for preparing a high-performance thin-film composite nanofiltration membrane includes the following steps: (1) Modification of aqueous solution in the interfacial polymerization process of thin film composite membranes: The cationic surfactant dodecylpyridine bromide (DPB) was dissolved in water and ultrasonically vibrated to ensure homogeneity, forming a 5.75 mM / L (0.50 CMC) solution of dodecylpyridine bromide (DPB has a critical micelle concentration of 11.5 mM / L). Then, m-phenylenediamine (MPD) was added to this aqueous solution to obtain an MPD aqueous solution, wherein the amount of MPD added was 0.2% of the mass of pure water.

[0054] (2) Preparation of high-performance thin-film composite nanofiltration membranes: First, a polyethersulfone-based membrane with an average surface pore size of 0.05 µm was placed on a glass plate and fixed using a circular polytetrafluoroethylene frame with an inner diameter of 4 cm, rubber pads, and clamps. 7.00 mL of MPD aqueous solution was poured onto the membrane surface and incubated at 25 °C for 2 min. The aqueous solution was then discarded, and excess solution was quickly removed from the membrane surface using a rubber roller. Next, 7.00 mL of a 0.15% (w / w) TMC / n-hexane solution was poured onto the membrane surface and reacted at 25 °C for 1 min. The reacted membrane was then drained for 1 min and placed in a 98 °C water bath for 2 min for thermal crosslinking. After removal and cooling for 2 min, the membrane was immersed in a NaOH solution with pH 11.5 for 36 h. After the reaction, a high-performance TFC-NF membrane was obtained, named TFC-gp.

[0055] Comparative Example 1 A method for preparing a conventional TFC-NF membrane includes the following steps: First, a polyethersulfone-based membrane with an average surface pore size of 0.05 µm was placed on a glass plate and fixed using a circular polytetrafluoroethylene frame with an inner diameter of 4 cm, rubber pads, and clamps. 7.00 mL of a 0.2% (w / w) MPD aqueous solution (without surfactant) was poured onto the membrane surface and incubated at 20 °C for 2 min. The aqueous solution was then poured off, and excess solution was quickly removed from the membrane surface using a rubber roller. Next, 7.00 mL of a 0.15% (w / w) TMC / n-hexane solution was poured onto the membrane surface and reacted at 20 °C for 1 min. The reacted membrane was then drained for 1 min and placed in a 98 °C water bath for 2 min for thermal crosslinking. After cooling for 2 min, it was immersed in a solution with pH=11.5 for 36 h. The resulting conventional TFC-NF membrane was then obtained and named Control-1.

[0056] Comparative Example 2 The procedure was performed in Comparative Example 1, except that the soaking in alkaline solution was omitted after cooling. The resulting sample was named Control-2.

[0057] Comparative Example 3 The procedure was performed as described in Example 1, except that the soaking in alkaline solution was omitted after cooling, and the resulting sample was named TFC-g-bp.

[0058] Example 2 The procedure was performed in accordance with Example 1, wherein the DPB concentrations of the aqueous solution in Example 1 were adjusted to 2.875 mM / L (0.25 CMC), 8.625 mM / L (0.75 CMC), 11.5 mM / L (1.00 CMC), and 17.25 mM / L (1.50 CMC).

[0059] Example 3 The procedure was performed as described in Example 1, except that the DPB in Example 1 was adjusted to be hexadecyltrimethylammonium bromide (CTAB), hexadecylpyridine bromide (CPB), sodium dodecyl sulfate (SDS), and sodium n-hexadecyl sulfate (SHS), all with a concentration of 1.00 CMC; the critical micelle concentration of CTAB was 9.2 mM / L, the critical micelle concentration of CPB was 8.5 mM / L, the critical micelle concentration of SDS was 8.3 mM / L, and the critical micelle concentration of SHS was 5.3 mM / L.

[0060] The nanofiltration membranes of Examples 1-3 and Comparative Examples 1-2 were placed in a cross-flow filtration device for performance testing. Specifically, the membranes were placed in a membrane tank, and pure water was passed through them to pre-pressurize them at a pressure of 6 bar for 30 minutes. Then, the pressure was adjusted to 4 bar, and the test tube was used to connect the flow for 5 minutes. The mass of the pure water was measured and substituted into formula (1) to calculate the pure water flux. The conductivity of a 1 g / L sodium sulfate solution was measured using a conductivity meter. Then, the pure water was replaced with a 1 g / L sodium sulfate solution, and the solution was run at a pressure of 4 bar for 30 minutes. The test tube was used to connect the flow for 5 minutes, and the conductivity of the effluent was measured and substituted into formula (2) to calculate the salt rejection capacity. The test results are shown in the figure.

[0061] from Figure 2 It can be seen that the TFC-gp flux of the membrane modified with DPB and post-treated in alkaline solution is significantly higher than that of the control group, indicating that DPB and alkaline foam simultaneously promote the flux of nanofiltration membrane.

[0062] Figure 3 The results showed that the DPB-modified and alkaline-soaked membrane exhibited high rejection rate and stability in a 2-hour cross-flow filtration experiment with sodium sulfate solution. This indicates that alkaline soaking did not increase membrane flux by enlarging the membrane pore size, but rather increased membrane flux while maintaining a high salt rejection rate.

[0063] Figure 4 It can be seen that the DPB blending concentration affects the performance of the alkaline foam modified membrane. The membrane flux first increases and then decreases with the increase of DPB blending concentration, while the rejection rate changes little. Among them, the optimal addition concentration of DPB is 1.00 CMC, at which the water flux and salt rejection rate are the highest.

[0064] Figure 5 It can be seen that the water flux of the alkaline bubble membrane is significantly improved after mixing DPB, CTAB, and CPB, while the water flux of the alkaline bubble membrane is not significantly improved after mixing SDS and SHS, two anionic surfactants. Furthermore, among DPB, CTAB, and CPB, DPB exhibits the highest and most stable retention rate.

[0065] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A method for preparing a high-performance TFC-NF membrane, characterized in that, The preparation method includes the following steps: (1) Modification of aqueous solution in the TFC membrane interfacial polymerization process: A cationic surfactant and m-phenylenediamine are dissolved in water to form an aqueous solution; the concentration of the surfactant in the aqueous solution is 0.25 to 3 times the critical micelle concentration of the surfactant. (2) Preparation of high-performance TFC-NF membranes: The base membrane was immersed in the aqueous solution prepared in step (1), and after the base membrane was removed and dried, it was immersed in a pyromellitic chloride solution for interfacial polymerization. Finally, after the interfacial polymerization was completed, the base membrane was removed and thermal crosslinking was carried out to obtain a high-performance TFC-NF membrane.

2. The preparation method according to claim 1, characterized in that, The cationic surfactant is dodecylpyridine bromide, hexadecyltrimethylammonium bromide, or hexadecylpyridine bromide.

3. The preparation method according to claim 1, characterized in that, In step (1), the concentration of the surfactant in the aqueous solution is 0.5 to 1.5 times the critical micelle concentration of the surfactant.

4. The preparation method according to claim 1, characterized in that, In step (1), the concentration of intermediate phenylenediamine in the aqueous solution is 0.1~5wt%.

5. The preparation method according to claim 1, characterized in that, In step (2), the average surface pore size of the base membrane is 0.01~2.00 µm.

6. The preparation method according to claim 1, characterized in that, In step (2), the solvent of the benzotriacyl chloride solution is a non-polar organic solvent; the non-polar organic solvent includes n-hexane or n-heptane; the concentration of benzotriacyl chloride in the benzotriacyl chloride solution is 0.01~1wt%.

7. The preparation method according to claim 1, characterized in that, In step (2), the interfacial polymerization reaction takes 1 to 10 minutes and the temperature is 15 to 30 °C. The thermal crosslinking reaction is carried out in a water bath or air bath atmosphere. The temperature of the thermal crosslinking reaction is 50 to 100 °C and the time is 1 to 20 minutes.

8. The preparation method according to claim 1, characterized in that, In step (2), the pH of the alkaline solution used for soaking is 8 to 12; the soaking time in the alkaline solution is 1 to 60 hours.

9. A high-performance TFC-NF membrane, characterized in that, The high-performance TFC-NF membrane is prepared by the method according to any one of claims 1 to 8.

10. The application of the high-performance TFC-NF membrane as described in claim 9 in the field of water treatment.

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