Preparation method of high-performance loose nanofiltration membrane

Abstract

The application relates to a preparation method of high-performance loose nanofiltration membranes, and steps are as follows: 1) a film-forming polymer is dissolved in a solvent, a foaming agent is added, a homogeneous solution is prepared by stirring at 20-70 DEG C, and then vacuum degassing is carried out at 20-70 DEG C to prepare a casting solution; 2) the casting solution obtained in the step 1) is coated on the surface of a supporting film to prepare a solution-state membrane, and the membrane is optionally naturally evaporated in an air atmosphere; 3) the solution-state membrane in the step 2) is soaked in a coagulation bath, the foaming agent is contacted with the coagulation bath to perform phase inversion, and nanobubbles are generated by reaction at the same time, and after the phase inversion is completed, the loose nanofiltration membrane is obtained; 4) the obtained membrane is stored in distilled water or glycerol. The application also relates to the high-performance loose nanofiltration membranes prepared by the method.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of membrane separation technology, and relates to a preparation method of a high-performance loose nanofiltration membrane. BACKGROUND

[0002] The loose nanofiltration membrane is a separation membrane with a pore size between ultrafiltration membranes and nanofiltration membranes, and is generally considered to have a molecular weight cut-off (MWCO) of polyethylene glycol between 500-2000 daltons (Da). Compared with traditional polyamide nanofiltration membranes, the loose nanofiltration membrane has a larger pore size and lower operating pressure, and has obvious advantages in low-molecular-weight organic molecules, polypeptides, biomass and wastewater resource recovery, etc., and has attracted more and more attention. At present, commercial loose nanofiltration membrane products include NP010 (Microdyn-Nadir) and NTR7450 (Hydranautics), etc., but due to the wide pore size distribution, large thickness and poor hydrophilicity of the separation layer, the loose nanofiltration membranes have a series of problems such as poor separation precision, low permeability (all less than 10 L·m -2 ·h -1 ·bar -1 ) and easy fouling, which seriously hinder their application. Therefore, it is urgent and meaningful to improve the comprehensive performance of the loose nanofiltration membrane to meet the increasing practical needs.

[0003] The methods for preparing the loose nanofiltration membrane mainly include non-solvent phase inversion (NIPS), interfacial polymerization, dip coating, chemical crosslinking and self-assembly, etc. Among them, the NIPS method has the advantages of simple process, low cost and high separation efficiency, and the prepared membrane has good chlorine resistance, so it is the most favorable method for preparing the loose nanofiltration membrane. However, the separation membrane prepared by the NIPS method has a wide pore size distribution and low surface porosity, resulting in very low membrane flux and separation precision.

[0004] CN110201546A discloses a pressure-resistant hollow fiber polyether sulfone loose nanofiltration membrane prepared by the NIPS method. The method uses a godet traction coating technology to uniformly coat the membrane-forming system on the outer surface of the fiber strand, and after air bathing, the membrane is drawn into a coagulation bath for cooling and forming. When separating dye salts, the sodium sulfate permeability can reach 99%, but only the Congo red (M w = 696.7 Da) with a larger molecular weight has a better rejection rate (99%).

[0005] CN113769586A discloses a double-layer polyvinylidene fluoride hollow fiber loose nanofiltration membrane for dye desalination. The neutral molecule MWCO of the loose nanofiltration membrane is 1530 Da, and the average pore size is 1.21 nm. Although the dye chrome black T (M w = 461 Da) with a smaller molecular weight has a high rejection rate of nearly 100% and a high NaCl permeability of 96.8%, the pure water permeability of the membrane is very low, only 14.2 L / (m2 ·h·bar).

[0006] CN112108018A discloses a high-flux polyimide composite nanofiltration membrane and a preparation method thereof. The loose nanofiltration has an ultrathin selective layer (30±2nm) and a three-dimensional sponge structure. Although the average pore size of the membrane surface is reduced to 1.05nm, the separation performance shows that it has low rejection of higher molecular weight dye molecules such as Rose Bengal (M w =1018Da), Coomassie Brilliant Blue (M w =854Da) and Amido Black (M w =617Da) (93%, 92% and 90% respectively), and the pure water permeability is also low, only 7.6L / (m 2 ·h·bar). The low rejection performance and low pure water permeation flux are mainly related to the too wide pore size distribution and too low surface porosity of the membrane surface.

[0007] CN113244778A discloses a high-performance ultrafiltration membrane preparation method, called nano-bubble assisted phase inversion method (BNIPS method), in which non-dissolved inorganic nano-ions are used as foaming agents. However, due to the non-uniform size distribution and easy agglomeration of inorganic nano-particles, they are not uniformly distributed in the casting solution, resulting in different sizes of nano-bubbles generated when reacting with acid in the coagulation bath. These nano-bubbles of different sizes affect the pore structure of the membrane, increasing the pore size distribution index of the membrane. At the same time, due to the large size of the nano-bubbles, the separation membrane has a large pore size (about 10nm), which makes it difficult to be used for the preparation of high-performance loose nanofiltration membranes.

[0008] In summary, it is difficult to achieve the preparation of loose nanofiltration membranes with high flux and high rejection performance in the prior art, and it is difficult to achieve efficient separation of small molecule dyes, especially dye molecules with a molecular weight of 400Da and below, and inorganic salts.

[0009] In view of the problems existing in the prior art, the present application provides a new method for preparing a loose nanofiltration membrane. The surface pore structure of the obtained membrane is uniform, the pore size distribution is narrow, and the surface porosity is high. The membrane has low rejection rate of NaCl while ensuring efficient rejection of small molecule dyes, and the selective separation effect is significantly better than that of existing loose nanofiltration membranes. At the same time, the pure water permeability is almost 3-7 times the flux of the existing loose nanofiltration membranes of the same size level. SUMMARY

[0010] The application discloses a preparation method of a new high-performance loose nanofiltration membrane, characterized by adding a gas-generating substance (referred to as a "foaming agent") with good solubility and dispersibility into a casting solution, wherein the foaming agent reacts with liquid in a coagulation bath to generate nanobubbles of "molecular scale" during phase inversion of the casting solution, the bubbles can form pores by themselves, and can also control the phase inversion process in situ; the obtained loose nanofiltration membrane has uniform surface pore structure, narrow pore size distribution and high surface porosity, and the separation performance of the membrane is obviously improved.

[0011] A preparation method of a high-performance loose nanofiltration membrane, steps are as follows:

[0012] 1) dissolve a film-forming polymer into a solvent, then add a foaming agent into the solution, stir to form a homogeneous solution at 20-70 DEG C, and then vacuum degas at 20-70 DEG C to prepare a casting solution;

[0013] 2) coat the casting solution obtained in step 1) onto the surface of a support membrane to form a solution membrane with a thickness of 50-300 microns, and optionally let the solution membrane evaporate naturally in an air atmosphere;

[0014] 3) immerse the solution membrane in step 2) into a coagulation bath, the casting solution contacts the coagulation bath to perform phase inversion, and the foaming agent contacts the coagulation bath to generate nanobubbles by chemical reaction, and after the phase inversion is completed, a loose nanofiltration membrane is obtained;

[0015] 4) store the obtained loose nanofiltration membrane in distilled water or glycerol.

[0016] In the above step 1), the concentration of the film-forming polymer in the casting solution is 12%-30%, and the concentration of the foaming agent is 1%-20%; the concentration is a mass percentage concentration, based on the total mass of the solvent and the film-forming polymer in the casting solution.

[0017] The film-forming polymer is any organic polymer material that can form a film, and is preferably selected from one or more of polysulfone (PSf), polyethersulfone (PES), polyvinylidene fluoride (PVDF) and modified polymers thereof, and the modified polymers are preferably sulfonated polysulfone (SPSf) and sulfonated polyethersulfone (SPES).

[0018] The foaming agent used refers to a foaming agent that can be dissolved in the casting solution and can generate gas under acidic conditions, and preferably one or more of tert-butyl carbonate, di-tert-butyl carbonate, di-tert-butyl dicarbonate, tri-tert-butyl carbonate and isocyanate is used as the foaming agent.

[0019] The solvent of step 1) is an organic solvent that can dissolve the film-forming polymer, and is preferably one or more of N,N-dimethylpyrrolidone (NMP), N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAC).

[0020] The defoaming time in step 1) is usually not less than 8 hours, preferably 8-24 hours.

[0021] In step 2), the solution state film coating method can be carried out by any conventional suitable method, including but not limited to, for example, blade coating, brush coating, spray coating, etc. The natural evaporation time of the solution state film is usually not more than 60 seconds.

[0022] In step 3), the coagulation bath is an acidic solution, and the acid used can be an inorganic acid or an organic acid for adjusting the pH value of the coagulation bath. The acid used is one of hydrochloric acid, sulfuric acid, acetic acid, citric acid or oxalic acid. The pH value of the coagulation bath is 0.1-6. The immersion time of the solution state film in the coagulation bath is about 1-30 minutes.

[0023] The present application further relates to a high-performance loose nanofiltration membrane prepared by the method of the present application.

[0024] The loose nanofiltration membrane obtained by the method of the present application has excellent separation performance, the membrane surface has small pore size and narrow distribution, the average pore diameter is 0.7-5 nm, the membrane skin layer thickness is 40-220 nm, the pure water permeability reaches 40-120 L / (m 2 ·h·bar), the sodium chloride solution rejection rate is less than 5%, the pure water permeation flux is significantly improved compared with the existing same level pore size membrane product, and the separation precision is higher.

[0025] The membrane prepared by the method of the present application can be a flat membrane, or a hollow fiber membrane, an inner lining membrane and a tubular membrane, etc.

[0026] Compared with the prior art, the present application has the following advantages:

[0027] (1) The foaming agent added in the present application has good solubility in the casting solution, so there is no problem of particle agglomeration and size non-uniformity caused by using nanoparticles as foaming agent in the membrane preparation process. Moreover, since the foaming agent is fully dissolved in the high-viscosity casting solution and can generate smaller and more uniform-sized nanobubbles by chemical reaction with the coagulation bath, it is beneficial to prepare small-pore-size separation membranes with uniform size. The soluble foaming agent used in the present application is superior to the insoluble nanoparticle foaming agent in the prior art, because the nanoparticle foaming agent has non-uniform and larger size, and the gas bubbles generated by chemical reaction with the coagulation bath have larger size and wider size distribution, which is not conducive to the preparation of small-pore-size separation membranes with uniform size.

[0028] (2) The nanobubbles produced in the method of the present application are more uniform in size and smaller, which is conducive to the preparation of high-performance loose nanofiltration membranes with high separation precision and high flux. Because the production of nanobubbles can change the liquid-liquid exchange process of the solvent in the solution state membrane and the solvent in the coagulation bath during the phase inversion process, and regulate the diffusion process of the two, the pore-forming mechanism of the traditional NIPS method is changed to some extent, so the surface pore structure and distribution can be independently regulated, and loose nanofiltration membranes with high separation precision and high flux can be prepared. At the same time, the membrane body structure is more uniform and pressure-resistant, and the mechanical strength is improved, so that the separation performance and mechanical properties of the final membrane are significantly improved. The membranes obtained by the existing method usually have large surface pore size and wide pore size distribution, which is not conducive to the precise separation of small-sized substances, and the mechanical strength is low.

[0029] (3) No other conventional organic pore-forming agent is added in the present application except the foaming agent, which can reduce the content of organic matter in the coagulation bath during the film-forming process, simplify the organic solvent recovery process in the coagulation bath, and reduce the cost of organic solvent recovery. Moreover, there is no residual organic pore-forming agent in the obtained separation membrane, which avoids the problem that the residual organic pore-forming agent molecules in the conventional separation membrane are slowly released into the water, causing long-term harm to human health and the environment. DETAILED DESCRIPTION

[0030] In order to further understand the present application, the present application will be described below in conjunction with specific examples. The examples are only used to illustrate the present application and do not constitute a limitation. Any variations and improvements made by those skilled in the art without departing from the technical concept involved in the present application shall fall within the protection scope of the present application.

[0031] The reagents and raw materials used in the embodiments of the present application can be commercially available.

[0032] Example 1

[0033] 4.2 g of PES and 0.8 g of SPSf were dissolved in 20 g of DMAC solvent to prepare a casting solution (i.e. the film-forming polymer accounts for 20% of the total mass of the casting solution), and then 0.25 g of tert-butyl carbonate (1% of the total mass of the casting solution) was added to the casting solution. After being uniformly mixed at 70°C, the mechanical stirring was carried out for 8 h and the static standing was carried out at 70°C for 24 h, and then vacuum degassing was carried out, to obtain a homogeneous casting solution. A 300 μm solution state membrane was scraped, which was immediately immersed in a hydrochloric acid-modified water solution coagulation bath with a pH value of 0.1 for phase inversion, and then taken out after 30 minutes to separate the membrane, which was stored in deionized water. The pure water flux, the rejection rates of Na2SO4 and NaCl solutions with a concentration of 1000 ppm, and the rejection rate of 100 ppm crystal violet dye were tested at 0.1 MPa; after the membrane was freeze-dried, the pore size test was carried out, and the test process is shown in Example 5.

[0034] Example 2

[0035] A casting solution was prepared by dissolving 3.0 g of PVDF in 22.0 g of NMP solvent (i.e. the film-forming polymer accounted for 12% of the total mass of the casting solution), and then adding 3 g of di-tert-butyl carbonate (accounting for 12% of the total mass of the casting solution) to the casting solution. The mixture was uniformly mixed at 40°C, mechanically stirred for 8 h and left to stand at 40°C for 8 h to remove bubbles, to obtain a homogeneous casting solution. A 200 μm solution-state membrane was prepared by blade coating, and then immersed in a citric acid-modified water solution coagulation bath with a pH of 6 after natural evaporation in air for 30 s, to perform phase inversion. The membrane was separated after 30 min, and then stored in deionized water. The pure water flux, the rejection rates of Na2SO4 and NaCl solutions with a concentration of 1000 ppm, and the rejection rate of 100 ppm crystal violet dye were tested at 0.1 MPa. The membrane was freeze-dried, and then tested for pore size, with the testing process being as described in Example 5.

[0036] Example 3

[0037] A casting solution was prepared by dissolving 5 g of PES in 20 g of DMAC solvent (i.e. the film-forming polymer accounted for 20% of the total mass of the casting solution), and then adding 4 g of di-tert-butyl dicarbonate (i.e. accounting for 16% of the total mass of the casting solution) to the casting solution. The mixture was uniformly mixed at 25°C, mechanically stirred for 8 h and left to stand at 25°C for 24 h to remove bubbles under vacuum, to obtain a homogeneous casting solution. A 100 μm membrane was prepared by blade coating, and then immersed in a hydrochloric acid-modified water solution coagulation bath with a pH of 1 after natural evaporation in air for 10 s, to perform phase inversion. The membrane was separated after 5 min, and then stored in deionized water. The pure water flux, the rejection rates of Na2SO4 and NaCl solutions with a concentration of 1000 ppm, and the rejection rate of 100 ppm crystal violet dye were tested at 0.1 MPa. The membrane was freeze-dried, and then tested for pore size, with the testing process being as described in Example 5.

[0038] Example 4

[0039] A casting solution was prepared by dissolving 7.5 g of PSf in 17.5 g of DMF solvent (i.e. the film-forming polymer accounted for 30% of the total mass of the casting solution), and then adding 5 g of di-tert-butyl tricarbonate (accounting for 20 wt% of the total mass of the casting solution) to the casting solution. The mixture was uniformly mixed at 20°C, mechanically stirred for 8 h and left to stand for 8 h to remove bubbles, to obtain a homogeneous casting solution. A 50 μm membrane was prepared by blade coating, and then immersed in an acetic acid-modified water solution coagulation bath with a pH of 2 after natural evaporation in air for 60 s, to perform phase inversion. The membrane was separated after 1 min, and then stored in deionized water. The pure water flux, the rejection rates of Na2SO4 and NaCl solutions with a concentration of 1000 ppm, and the rejection rate of 100 ppm crystal violet dye were tested at 0.1 MPa. The membrane was freeze-dried, and then tested for pore size, with the testing process being as described in Example 5.

[0040] Example 5

[0041] Membrane permeation separation performance test was carried out by cross-flow filtration device. First, the membrane was installed in the membrane cell, and then pre-pressurized for 30 min at 0.2 MPa with deionized water, slowly adjusted to 0.1 MPa, and stabilized for 10 min. The permeate was collected for 20 min to calculate the pure water flux. Then, Na2SO4 (concentration of 1000 ppm) was used to test the separation performance at 0.1 MPa, and the average value was recorded after repeated determination for more than three times.

[0042] The water flux (Flux, F) test formula is as follows:

[0043] F = V / (At) (1)

[0044] In the formula, F is the water flux of the ultrafiltration membrane, unit L / (m 2 ·h); V is the water volume permeating through the membrane in t time, unit L; A is the effective membrane area through which water passes, unit m 2 ; t is the running time, unit h.

[0045] The Na2SO4, NaCl and crystal violet rejection rate (Rejection, R) test formula is as follows:

[0046] R = (1-C p / C f )*100% (2)

[0047] In the formula, R is the rejection rate of the loose nanofiltration membrane, unit %; when testing salt solution, C p is the permeate Na2SO4 and NaCl conductivity value, and when testing dye, C p is the absorbance value of the permeate crystal violet (maximum absorption peak wavelength is 584 nm); when testing salt solution, C f is the initial Na2SO4 and NaCl conductivity value, and when testing dye, C f is the initial crystal violet absorbance value.

[0048] The BET was used to test and analyze the surface pore size and distribution of the loose nanofiltration membrane, and the surface pore size and distribution data of the loose nanofiltration membrane were obtained.

[0049] Comparative Example 1

[0050] The process of Example 1 was repeated, except that no foaming agent was added. Specifically as follows:

[0051] A casting solution was prepared by dissolving 4.2 g of PES and 0.8 g of sulfonated polysulfone (SPSf) in 20 g of DMAC solvent (i.e. the film-forming polymer accounted for 20% of the total mass of the casting solution). The solution was mixed uniformly at 70°C, mechanically stirred for 8 h and left standing at 70°C for 24 h and vacuum degassing to obtain a homogeneous casting solution. A 300 μm solution-state membrane was prepared by doctor blading, which was immediately immersed in a hydrochloric acid-modified aqueous solution with a pH of 0.1 to perform phase inversion, and taken out after 30 min to separate the membrane, which was stored in deionized water. The pure water flux, the rejection rate of Na2SO4 and NaCl solutions with a concentration of 1000 ppm, and the rejection rate of 100 ppm of crystal violet dye were tested at 0.1 MPa; the membrane was freeze-dried for pore size testing, and the testing process is shown in Example 5.

[0052] Comparative Example 2

[0053] Example 3 was repeated, except that no acid coagulation bath was added. The details are as follows:

[0054] A casting solution was prepared by dissolving 5 g of PES in 20 g of DMAC solvent (i.e. the film-forming polymer accounted for 20% of the total mass of the casting solution), and 4 g of di-tert-butyl dicarbonate (i.e. 16% of the total mass of the casting solution) was added to the casting solution. The solution was mixed uniformly at 25°C, mechanically stirred for 8 h and left standing at 25°C for 24 h and vacuum degassing to obtain a homogeneous casting solution. A 100 μm membrane was prepared by doctor blading, which was immersed in a pure water coagulation bath after natural evaporation in air for 10 s to perform phase inversion, and taken out after 5 min to separate the membrane, which was stored in deionized water. The pure water flux, the rejection rate of Na2SO4 and NaCl solutions with a concentration of 1000 ppm, and the rejection rate of 100 ppm of crystal violet dye were tested at 0.1 MPa; the membrane was freeze-dried for pore size testing, and the testing process is shown in Example 5.

[0055] The performance of the loose nanofiltration membranes prepared according to Examples 1-4 and Comparative Examples 1-2 is shown in Table 1. It can be seen that the loose nanofiltration membrane prepared by the method of the present application has a small surface pore size, which is distributed in the range of 1.1-3.4 nm, and has a good flux and a certain rejection of Na2SO4, and has a very good rejection of dye molecules. The membranes prepared in Comparative Example 1 (without adding a foaming agent) and Comparative Example 2 (adding a foaming agent but not adding an acid in the coagulation bath, so that the foaming agent does not produce foaming) have a significantly increased surface pore size and poor rejection of Na2SO4 and dye molecules. It can be seen that a high-performance loose nanofiltration membrane is obtained by the method of the present application.

[0056] Table 1

[0057]

Claims

1. A method for preparing high performance loose nanofiltration membrane, comprising the following steps: 1) dissolving a film-forming polymer in a solvent, adding a foaming agent to the solution, stirring to form a homogeneous solution at 20-70℃, vacuum degassing at 20-70℃, and preparing a casting solution; the foaming agent is selected from one or more of tert-butyl carbonate, di-tert-butyl carbonate, di-tert-butyl dicarbonate, and tri-tert-butyl carbonate; the concentration of the foaming agent is 1-20%, and the concentration is a mass percentage concentration based on the total mass of the solvent and the film-forming polymer in the casting solution; 2) coating the casting solution obtained in step 1) onto the surface of a support membrane to form a solution state membrane, and allowing the solution state membrane to evaporate naturally in an air atmosphere; 3) immersing the solution state membrane of step 2) in a coagulation bath, allowing the casting solution to contact the coagulation bath to perform phase inversion, and allowing the foaming agent in the casting solution to react with the coagulation bath to generate nanobubbles, and obtaining a loose nanofiltration membrane after phase inversion is completed; the coagulation bath is an acidic solution; 4) storing the obtained loose nanofiltration membrane in distilled water or glycerol.

2. The method of claim 1, wherein: In step 1), the film-forming polymer is selected from one or more of polysulfone, polyethersulfone, polyvinylidene fluoride, and modified polymers thereof, and the modified polymers are sulfonated polysulfone and sulfonated polyethersulfone; the solvent is one or more of N,N-dimethylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide.

3. The method of claim 2, wherein: In step 1), the concentration of the film-forming polymer in the casting solution is 12-30%, and the concentration is a mass percentage concentration based on the total mass of the solvent and the film-forming polymer in the casting solution.

4. The method of claim 1, wherein: In step 2), the thickness of the solution state membrane is 50-300 microns, and the time for natural evaporation is not more than 60 seconds.

5. The method of claim 1, wherein: In step 3), the immersion time of the solution state membrane in the coagulation bath is 1-30 minutes.

6. The method of claim 5, wherein: The pH value of the coagulation bath is 0.1-6.

7. The production method according to claim 5 or 6, characterized by: The acid used to adjust the pH value of the coagulation bath is one or more of hydrochloric acid, sulfuric acid, acetic acid, citric acid, or oxalic acid.

8. A high performance loose nanofiltration membrane prepared by the method of any one of the preceding claims 1-7.

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