Asymmetric selective nanofiltration membrane and preparation method thereof

Abstract

The present invention provides a method for preparing an asymmetric selective nanofiltration membrane, comprising preparing an aqueous phase monomer solution and an oil phase monomer solution, wherein the aqueous phase monomer solution is an aqueous solution obtained by mixing an amine monomer, an aqueous phase additive and deionized water, and the aqueous phase additive is an inorganic base; a base membrane is sequentially immersed in the aqueous phase monomer solution and the oil phase monomer solution to undergo interfacial polymerization reaction, and then subjected to heat treatment. The asymmetric selective nanofiltration membrane prepared by the present invention is prepared by adding an alkaline compound during the interfacial polymerization process, and the interaction between the added alkali amount and the interfacial polymerization monomer itself is regulated to regulate the reaction between the alkali and the monomer during the polymerization process, thereby promoting the formation of polyamide and effectively reducing the formation of nano-scale defects in the polyamide layer. At the same time, the added alkali can partially hydrolyze the polyamide layer to produce a negative charge repulsion layer with a larger effective pore size, thereby achieving asymmetric selectivity and enhanced permeability.

Description

Technical Field

[0001] The present application belongs to the technical field of polymer composite membranes, and specifically relates to an asymmetric selective nanofiltration membrane and a preparation method thereof. Background Art

[0002] The separation capacity of nanofiltration composite membrane is between that of ultrafiltration membrane and reverse osmosis membrane. It is generally defined as being able to effectively separate small organic molecules with a molecular weight of 200 to 1000 and divalent ions. It has the characteristics of high separation accuracy, low energy consumption and environmental friendliness, and has great application prospects in fields such as water treatment.

[0003] At present, commercial nanofiltration composite membranes are mainly composite membranes prepared by interfacial polymerization technology, and their materials are mainly polyamide. The denser nanofiltration composite membrane can achieve a lower molecular weight cutoff, remove small molecular organic matter, and have a high affinity for Ca 2+ Mg 2+ and Na + The removal rate of ions is also high. However, at high water recovery rates, the accumulation and supersaturation of these ions in the brine will be aggravated, leading to the enrichment and scaling of insoluble salts on the membrane surface (for example, calcium sulfate dihydrate, gypsum), increasing the operating cost of the membrane system.

[0004] In addition, in the field of water purification, too high an inorganic salt retention rate will lead to low ion concentrations in nanofiltration water, which is not conducive to drinking water health. At the same time, widespread and persistent emerging pollutants perfluoroalkyl substances (PFASs) have been found in groundwater systems. They have high chemical resistance to environmental degradation processes, so it is also extremely important to maintain a high retention rate for persistent emerging pollutants in water treatment. Summary of the invention

[0005] In order to solve the deficiencies in the prior art, the purpose of the present invention is to provide an asymmetric selective nanofiltration membrane and a preparation method thereof, so that calcium ions can pass freely through the membrane, while effectively removing sulfate and perfluoroalkyl substances, which can effectively solve the problem of selective retention in existing nanofiltration composite membranes.

[0006] The first aspect of the present invention provides a method for preparing an asymmetric selective nanofiltration membrane, comprising:

[0007] preparing an aqueous monomer solution, wherein the aqueous monomer solution is an aqueous solution obtained by mixing an amine monomer, an aqueous phase additive and deionized water, and the aqueous phase additive is an inorganic base;

[0008] preparing an oil phase monomer solution, wherein the oil phase monomer solution comprises an acyl chloride monomer and an oil phase solvent;

[0009] After the base membrane is immersed in the aqueous monomer solution, the base membrane immersed in the aqueous monomer solution is immersed in the oil phase monomer solution to perform an interfacial polymerization reaction; the base membrane immersed in the oil phase monomer solution is heat treated to obtain the asymmetric selective nanofiltration membrane.

[0010] In combination with the first aspect, in an implementation of the first aspect, the water phase additive includes at least one of sodium hydroxide and potassium hydroxide.

[0011] In combination with the first aspect, in an implementation of the first aspect, the amine monomer includes at least one of piperazine, m-phenylenediamine, ethylenediamine, polyethyleneimine 600, polyethyleneimine 1800, polyethyleneimine 10000, and polyethyleneimine 70000.

[0012] In combination with the first aspect, in an implementation of the first aspect, the mass percentage concentration of the amine monomer in the aqueous monomer solution is 0.02-5%; the mass percentage concentration of the aqueous phase additive in the aqueous monomer solution is 0.25-5%.

[0013] In combination with the first aspect, in an implementation of the first aspect, the base film is a porous film;

[0014] The base membrane component forms include flat membrane, spiral membrane and hollow fiber membrane;

[0015] The material of the base film includes at least one of polysulfone, polyethersulfone, polyvinylidene fluoride, polyacrylonitrile and sulfonated polyethersulfone.

[0016] In combination with the first aspect, in an implementation of the first aspect, the mass percentage concentration of the acyl chloride monomer in the oil phase monomer solution is 0.1-2%.

[0017] In combination with the first aspect, in an implementation of the first aspect, the oil phase solvent of the oil phase monomer solution includes at least one of n-hexane, cyclohexane, n-heptane, n-decane, Isopar-E, Isopar-G and Isopar-L.

[0018] In combination with the first aspect, in an implementation method of the first aspect, the time for immersing the base film in the aqueous monomer solution is 0.5 to 5 minutes, wherein it is better to immerse the base film in the aqueous monomer solution for 3 minutes; the time for immersing the base film soaked in the aqueous monomer solution in the oily monomer solution is 0.5 to 3 minutes, wherein it is better to immerse the base film in the aqueous monomer solution for 1 minute; the treatment temperature of the heat treatment is 40°C to 80°C, and the treatment time is 5 to 30 minutes.

[0019] The second aspect of the present invention provides an asymmetric selective nanofiltration membrane prepared by the preparation method provided by the first aspect of the present invention.

[0020] The third aspect of the present invention provides an application of an asymmetric selective nanofiltration membrane prepared by the preparation method provided by the first aspect of the present invention in water treatment.

[0021] The beneficial effect of the present invention is that, compared with the prior art, the asymmetric selective nanofiltration membrane prepared by the present invention promotes the formation of polyamide and effectively reduces the formation of nano-scale defects in the polyamide layer by adding NaOH and KOH compounds during the interfacial polymerization process and the interaction between the added alkali and the interfacial polymerization monomers, and regulates the reaction between the alkali and the monomers during the polymerization process, and the added alkali can partially hydrolyze the polyamide layer to produce a negative charge repulsive layer with a larger effective pore size, thereby achieving asymmetric selectivity and enhanced permeability. The obtained selective composite membrane reduces gypsum scaling while maintaining the mineral content in the treated water, reducing the scaling risk, improving the permeability of the membrane and the retention rate of perfluoroalkyl substances and sulfate anions, and overcoming the long-term permeability-selectivity trade-off. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A comparison chart of the pure water flux and asymmetric retention index of the nanofiltration membrane provided by the present invention;

[0023] Figure 2 A comparison diagram of contact angles of the nanofiltration membrane provided by the present invention;

[0024] Figure 3 A comparison chart of molecular weight cut-offs of the nanofiltration membranes provided by the present invention;

[0025] Figure 4 This is a comparison chart of the retention performance of the nanofiltration membrane provided by the present invention for organic micropollutants. DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0027] Unless otherwise specified, the materials, reagents, etc. used in the examples of the present invention can be obtained through commercial channels.

[0028] A first aspect of an embodiment of the present invention provides a method for preparing an asymmetric selective nanofiltration membrane, comprising the following steps:

[0029] Step 1: prepare aqueous monomer solution.

[0030] The aqueous phase monomer solution is an aqueous solution obtained by mixing an amine monomer, an aqueous phase additive and deionized water, wherein the aqueous phase additive is an inorganic base.

[0031] The amine monomer includes at least one of piperazine, m-phenylenediamine, ethylenediamine, polyethyleneimine 600, polyethyleneimine 1800, polyethyleneimine 10000 and polyethyleneimine 70000, and the aqueous phase additive is an inorganic base, preferably, includes at least one of sodium hydroxide and potassium hydroxide.

[0032] Preferably, the mass percentage concentration of the amine monomer in the aqueous monomer solution is 0.02-5%, and the mass percentage concentration of the aqueous phase additive in the aqueous monomer solution is 0.25-5%.

[0033] Step 2: prepare oil phase monomer solution.

[0034] The oil phase monomer solution comprises an acyl chloride monomer and an oil phase solvent, wherein the mass percentage concentration of the acyl chloride monomer in the oil phase monomer solution is 0.1-2%, and the oil phase solvent of the oil phase monomer solution comprises at least one of n-hexane, cyclohexane, n-heptane, n-decane, Isopar-E, Isopar-G and Isopar-L.

[0035] Step 3, after immersing the base membrane in the aqueous monomer solution, immersing the base membrane soaked in the aqueous monomer solution in the oil phase monomer solution to perform an interfacial polymerization reaction; heat-treating the base membrane soaked in the oil phase monomer solution to obtain the asymmetric selective nanofiltration membrane.

[0036] The base membrane is a porous membrane, and the membrane material includes but is not limited to polysulfone, polyethersulfone, polyvinylidene fluoride, polyacrylonitrile, sulfonated polyethersulfone membrane and other base membranes, and the component form includes but is not limited to flat membrane, rolled membrane, hollow fiber membrane and the like.

[0037] The time for immersing the base membrane in the aqueous monomer solution is 0.5 to 5 minutes, and preferably, the base membrane is immersed in the aqueous monomer solution for 3 minutes; the time for immersing the base membrane after being immersed in the aqueous monomer solution in the oily monomer solution is 0.5 to 3 minutes, and preferably, the time for immersing the base membrane after being immersed in the aqueous monomer solution in the oily monomer solution is 1 minute, that is, the reaction time of the aqueous monomer is preferably 3 minutes, and the reaction time of the oily solution is preferably 1 minute.

[0038] Preferably, the heat treatment temperature is 40°C to 80°C, and the treatment time is 5 to 30 minutes. More preferably, the heat treatment is performed at 60°C for 10 minutes.

[0039] A second aspect of the embodiment of the present invention provides an asymmetric selective nanofiltration membrane prepared by the preparation method provided by the first aspect of the embodiment of the present invention.

[0040] A third aspect of the embodiments of the present invention provides an application of an asymmetric selective nanofiltration membrane prepared by the preparation method provided by the first aspect of the embodiments of the present invention in water treatment.

[0041] The following examples will further illustrate the present invention.

[0042] Example 1

[0043] Step 1: dissolving piperazine (PIP) and sodium hydroxide (NaOH) in deionized water to prepare an aqueous monomer solution containing 0.2 wt% piperazine and 0.5 wt% sodium hydroxide.

[0044] Step 2: using a hexane solution to prepare a 0.1 wt % trimesoyl chloride (TMC) solution as an oil phase monomer solution.

[0045] Step 3: Use a polyethersulfone (PES) ultrafiltration membrane (Microdyn Nadir, Germany) with a molecular weight cutoff of 150,000 Da as a base membrane, and wash it in a 25% (v / v) isopropanol / water solution for 1 hour to remove impurities to obtain a polyethersulfone base membrane, and then store the polyethersulfone base membrane in deionized water.

[0046] Step 4: Immerse the polyethersulfone-based membrane in the aqueous monomer solution for 3 minutes and then remove excess solution.

[0047] Step 5: Pour the oil phase monomer solution onto the polyethersulfone-based membrane and react for 1 minute, then rinse with hexane to remove excess oil phase monomer solution on the membrane surface.

[0048] Step 6: Dry the product obtained in step 5 in an oven at 60° C. for 10 minutes to obtain an asymmetric selective nanofiltration membrane, which is then stored in deionized water before application testing.

[0049] Example 2

[0050] The steps of Example 1 were repeated, including the following differences: the mass percentage concentration of piperazine in the aqueous monomer solution was 0.1%.

[0051] Example 3

[0052] The steps of Example 1 were repeated, with the following differences: the mass percentage concentration of piperazine in the aqueous monomer solution was 0.02%.

[0053] Example 4

[0054] The steps of Example 2 were repeated, with the following differences: the mass percentage concentration of NaOH in the aqueous monomer solution was 0.25%.

[0055] Example 5

[0056] The steps of Example 2 were repeated, including the following differences: the mass percentage concentration of NaOH in the aqueous monomer solution was 0.1%.

[0057] Comparative Example 1

[0058] The steps of Example 1 were repeated, including the following differences: NaOH was not added to the aqueous monomer solution.

[0059] Comparative Example 2

[0060] A commercial nanofiltration membrane NF270 (FilmTecCorp, Minneapolis, MN) was used.

[0061] Test Example 1

[0062] The pure water flux, maximum water recovery rate and contact angle of the nanofiltration membrane in the examples and comparative examples were measured.

[0063] The pure water flux test was carried out in a constant temperature cross-flow filtration device at 25±1°C and an operating pressure of 3.5 bar.

[0064] The maximum water recovery experiment used 10mM NaCl, 20mM CaCl 2 and 10 mM Na 2 SO 4 The solution has a pH value of 7.0±0.1, a corresponding gypsum saturation index of 0.7, and an operating pressure of 3.5 bar to maintain about 80 Lm –2 h –1 When the water flux drops to 20Lm –2 h –1 In the following, water recovery was calculated by the mass ratio of permeate to initial feed.

[0065] The test results of pure water flux and water recovery rate are shown in Table 1 and Figure 1 As shown, the film contact angle is Figure 2 shown.

[0066] Table 1 Nanofiltration membrane pure water flux and water recovery rate of different embodiments and comparative examples

[0067]

[0068] Table 1 and Figure 1 The pure water flux data show that NaOH can be used to improve the membrane permeability with lower PIP concentrations without significantly reducing the rejection rate. The pure water permeabilities of TFC-0.1 and TFC-0.02 are 2.1 and 3.9 times that of commercial NF270, respectively, showing great potential for practical applications. The water recovery rate data in Table 1 show that the threshold water recovery rate of TFC-0.1 membrane is about 60%, which is much higher than the commercial NF270 of about 40%. This is because the TFC-0.1 membrane is sensitive to Ca 2+The selectivity of the membrane effectively increases the maximum water recovery under scaling conditions by enabling more permeate water to be produced before reaching critical supersaturation relative to gypsum. In addition to higher water recovery, the TFC-0.1 membrane can have comparable water flux at a lower pressure (4.4 bar) to the NF270 at a higher pressure (6.7 bar), which helps reduce energy consumption.

[0069] Figure 2 The contact angle data show that the contact angle of the TFC-0.2 membrane with 0.5wt% NaOH added is lower than that of the TFC* membrane, which indicates that the TFC-0.2 membrane has better hydrophilicity. As the PIP concentration decreases from 0.2wt% to 0.02wt%, the contact angle decreases and the hydrophilicity increases. This change may be due to the gradual loosening of the polyamide network and the strong hydrolysis of TMC.

[0070] Test Example 2

[0071] The molecular weight cut-off of the nanofiltration membrane in Example 1 and Comparative Example 1 was determined.

[0072] According to the conditions similar to those of test example 1, 0.2 g L –1 The membrane separation performance was evaluated by adding PEG samples (molecular weight of 200, 600, 1000, 2000, 4000 and 8000 Da). Figure 3 The PEG rejection of TFC-0.2 was slightly lower than that of the control TFC* membrane, indicating that the pore size of the TFC-0.2 membrane was enlarged.

[0073] Test Example 3

[0074] The salt separation performance of the nanofiltration membranes in the examples and comparative examples was determined.

[0075] 1gL was used in the test –1 NaCl, CaCl 2 、Na 2 SO 4 The salt rejection of the membrane was determined, and the other conditions were similar to those in Test Example 1. The asymmetric rejection index (defined as Na 2 SO 4 For CaCl 2 The rejection rate of the membrane is used to reflect the asymmetric separation ability of the membrane. The experimental results are shown in Table 2.

[0076] Table 2 Nanofiltration membrane retention rates of different embodiments and comparative examples

[0077]

[0078] Compared with NF270 CaCl 2The retention rate (41.1±0.3%), TFC-0.1 can achieve lower CaCl 2 The retention rate was 11.4 ± 0.6%, but at the same time, the Na 2 SO 4 The retention rate was (96.1±0.6%), which was due to the fact that the addition of NaOH generated a looser polyamide network and increased the electrostatic repulsion between the membrane and the salt ions. NaOH-assisted interfacial polymerization also achieved Ca 2+ and SO 4 2- For TFC-0.1, the asymmetric interception index is 8.5±0.6, which means that SO 4 2- Ca 2+ The rejection rate of Ca was 8 times higher. In contrast, the asymmetric inhibition index of conventional TFC* and commercial NF270 was 2-2.5, indicating that Ca 2+ and SO 4 2- The separation efficiency is low.

[0079] Test Example 4

[0080] The retention performance of the nanofiltration membranes for organic micropollutants in the examples and comparative examples was determined. –1 The test was carried out on solutions of PFOS, PFOA, GenX, PFBS or PFBA. The experimental results are as follows Figure 4 As shown, the rejection rates of TFC-0.2 and TFC-0.1 membranes for all organic micropollutants remained above 90% compared with the TFC* membrane.

[0081] The beneficial effect of the present invention is that, compared with the prior art, the asymmetric selective nanofiltration membrane prepared by the present invention promotes the formation of polyamide and effectively reduces the formation of nano-scale defects in the polyamide layer by adding NaOH and KOH compounds during the interfacial polymerization process, and the interaction between the added alkali amount and the interfacial polymerization monomer itself, and regulates the reaction between the alkali and the monomer during the polymerization process, and the added alkali can partially hydrolyze the polyamide layer to produce a negative charge repulsive layer with a larger effective pore size, thereby achieving asymmetric selectivity and enhanced permeability. The obtained selective composite membrane reduces gypsum scaling while maintaining the mineral content in the treated water, reducing the scaling risk, improving the permeability of the membrane and the retention rate of perfluoroalkyl substances and sulfate anions, and overcoming the long-term permeability-selectivity trade-off.

[0082] The applicant of the present invention has made a detailed explanation and description of the implementation examples of the present invention in conjunction with the drawings in the specification. However, those skilled in the art should understand that the above implementation examples are only preferred implementation schemes of the present invention, and the detailed description is only to help readers better understand the spirit of the present invention, but not to limit the scope of protection of the present invention. On the contrary, any improvements or modifications based on the inventive spirit of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for preparing an asymmetric selective nanofiltration membrane, It is characterized in that include: preparing an aqueous monomer solution, wherein the aqueous monomer solution is an aqueous solution obtained by mixing an amine monomer, an aqueous phase additive and deionized water, and the aqueous phase additive is an inorganic base; The water phase additive includes at least one of sodium hydroxide and potassium hydroxide; The mass percentage concentration of the amine monomer in the aqueous monomer solution is 0.02-5%; the mass percentage concentration of the aqueous additive in the aqueous monomer solution is 0.25-5%; preparing an oil phase monomer solution, wherein the oil phase monomer solution comprises an acyl chloride monomer and an oil phase solvent; After immersing the base membrane in an aqueous monomer solution, immersing the base membrane soaked in the aqueous monomer solution in an oily monomer solution to perform an interfacial polymerization reaction; heat-treating the base membrane soaked in the oily monomer solution to obtain the asymmetric selective nanofiltration membrane; The base film is a porous film; The base membrane component forms include flat membrane, spiral membrane and hollow fiber membrane; The material of the base film includes at least one of polysulfone, polyethersulfone, polyvinylidene fluoride, polyacrylonitrile and sulfonated polyethersulfone; The time for immersing the base film in the aqueous monomer solution is 0.5 to 5 min, and the time for immersing the base film after being immersed in the aqueous monomer solution in the oily monomer solution is 0.5 to 3 min; The heat treatment temperature is 40°C to 80°C, and the treatment time is 5 to 30 min.

2. The method for preparing an asymmetric selective nanofiltration membrane according to claim 1, It is characterized in that The amine monomer includes at least one of piperazine, m-phenylenediamine, ethylenediamine, polyethyleneimine 600, polyethyleneimine 1800, polyethyleneimine 10000, and polyethyleneimine 70000.

3. The method for preparing an asymmetric selective nanofiltration membrane according to claim 1, It is characterized in that The mass percentage concentration of the acyl chloride monomer in the oil phase monomer solution is 0.1-2%.

4. The method for preparing an asymmetric selective nanofiltration membrane according to claim 1, It is characterized in that The oil phase solvent of the oil phase monomer solution includes at least one of n-hexane, cyclohexane, n-heptane, n-decane, Isopar-E, Isopar-G and Isopar-L.

5. An asymmetric selective nanofiltration membrane obtained by the preparation method according to any one of claims 1 to 4.

6. Use of the asymmetric selective nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 4 in water treatment.

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