A method for preparing a hydrophilic composite membrane

Abstract

The application belongs to the field of membrane separation technology, and discloses a preparation method of a hydrophilic composite membrane. The hydrophilic composite membrane is a flat sheet membrane or a hollow fiber membrane, and comprises a support layer and a separation functional layer, wherein the support layer is located below the separation functional layer; the separation functional layer is a polydopamine separation membrane containing uniformly dispersed nanoparticles, and the mass percentage of the nanoparticles in the total solid content of the separation functional layer is 0.01-1%. The organic composite membrane has not only good separation selectivity, but also good anti-pollution performance. In addition, during the membrane preparation process, dopamine hydrochloride and inorganic nanoparticles are added into a gel bath, and the two are uniformly dispersed when ultrasonic mixing is performed, and then the immersion precipitation phase inversion method is adopted to prepare the polydopamine high polymer composite membrane modified by the nanoparticles. The membrane preparation process is simple, the permeation flux is large, the hydrophilicity is high, the service life can be significantly improved, and the cleaning cycle can be prolonged.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of membrane separation technology, and particularly relates to a preparation method of a composite membrane, especially a separation membrane suitable for treatment of wastewater containing organic pollutants. BACKGROUND

[0002] Membrane separation technology is a new energy-saving and environment-friendly separation technology. Pressure-driven separation membranes can be used for heavy metal ion removal, inorganic salt separation, macromolecule separation and dye treatment according to the pore size and surface charge characteristics. Common polymer separation membranes such as polyvinylidene fluoride membranes and polysulfone membranes are prone to membrane fouling during use, which greatly reduces the separation capacity of the membranes. Therefore, improving the anti-fouling property of such membranes and the deep treatment of the concentrate are still important research topics in the field of membrane separation. Blending modification of polymers with hydrophilic polymers and nanoparticles is a commonly used method. Due to poor material compatibility, the addition amount of hydrophilic polymers and nanoparticles in the polymer casting solution is relatively low, and the proportion of hydrophilic materials on the surface of the formed membrane is extremely low, making it difficult to obtain a high hydrophilic polymer membrane. How to further improve the surface hydrophilicity and anti-fouling property of the polymer membrane and simplify the modification process has important research value. Inspired by the adhesion protein of mussels, polydopamine (PDA) coating has become one of the easiest and most common surface modification methods. The simplest surface modification method is to immerse the polymer membrane in an alkaline aqueous solution of dopamine, and a PDA coating with a thickness of usually 1-100 nm is spontaneously formed on the surface of the substrate. Adding a certain amount of inorganic nanoparticles to the polydopamine hydrochloride solution can obtain a polydopamine composite membrane doped with nanoparticles. Shao Bing et al. blended polydopamine powder with polyvinylidene fluoride to prepare a blended membrane. When the PDA addition amount is 0.3% (mass fraction), the pure water flux is increased to 69.63 L / m 2 h from 59.19 L / m 2h. Shao B, Liu LF, Yang FL. Preparation and properties of PVDF / PDA blend membranes. Membrane Science and Technology, 34, 2014: 57-61. CN107486037 A discloses a method for preparing a PVDF / PDA composite membrane with hydrophilic performance. Dopamine is added to a buffer solution, and after magnetic stirring for 24-72 h, PVDF membrane is added, and magnetic stirring is performed for 3-5 days; a large number of PDA microspheres are generated in situ on the upper and lower surfaces of the membrane and the pore walls of the membrane to modify the membrane to obtain a whole hydrophilic structure, wherein the membrane cross section is a sponge-like structure in which the PDA microspheres are laid on the pore walls but the membrane pores are interconnected. Due to the good hydrophilicity of dopamine itself and the existence of the membrane micro-reactor effect in the modification process, the hydrophilic performance and anti-pollution performance of the modified PVDF / PDA composite membrane are greatly improved when used as a water treatment application membrane. In order to improve the hydrophilicity and anti-pollution performance of the PVDF microfiltration membrane, polyvinylidene fluoride (PVDF) microfiltration membranes are prepared by the immersion precipitation phase inversion method, and Feng Xia et al. use the super adhesion of dopamine and the advantage of easy self-polymerization of polydopamine (PDA) to modify the surface of the PVDF microfiltration membrane. The results show that PDA is successfully deposited on the surface of the PVDF microfiltration membrane by the coating method, and the hydrophilicity and anti-protein adsorption performance of the PVDF microfiltration membrane surface are improved. When the modification time is 6 h, the pure water flux increases from 80 L / m 2 h to 107 L / m 2 h, and the water contact angle decreases from 95° of the pure PVDF membrane to 44° of the modified membrane.[Feng Xia, Xia Weiwei, Ma Xiao, Zhao Yiping, Chen Li, Preparation and performance of dopamine biomimetic modified PVDF microfiltration membrane, Journal of Tianjin University of Technology, 37, 2018, 14-19.]

[0003] However, this method is a combination of phase inversion and interfacial coating two-step process, which increases the technical complexity, the process control is more difficult, the coating stage takes a long time, which is not conducive to large-scale application, and the product cost is high. At the same time, the combination of the base film and the functional layer is not tight enough, and the modification effect is poor. SUMMARY

[0004] To solve the above technical problems, the present application provides an easy-to-prepare polydopamine hydrophilic composite membrane modified by nanoparticles and a preparation method thereof. Commercial polymers such as polyvinylidene fluoride, polysulfone, polyacrylonitrile, and cellulose acetate are used as film-forming materials, titanium dioxide and silicon dioxide nanoparticles are used as inorganic additives in the gel bath, and dopamine hydrochloride is used as a modification reagent. The composite membrane is prepared by phase inversion, the nascent membrane is adsorbed and contacted with polydopamine and nanoparticles, the separation membrane skin layer is rapidly modified, a composite membrane with stable and high hydrophilicity is obtained, and membrane pollution is significantly reduced. The present application helps to greatly improve the anti-pollution performance of organic polymer membranes, simplify the preparation process of hydrophilic modified membranes, and thus help to realize the wider application of polymer membranes.

[0005] The technical solution of the present invention is as follows:

[0006] A method for preparing a hydrophilic composite membrane includes the following steps:

[0007] (1) Preparation of casting solution A

[0008] Organic polymer materials are added to an organic solvent and stirred rapidly. After 24 hours, casting solution A is obtained.

[0009] (2) Preparation of gel bath B

[0010] Inorganic nanoparticles were added to deionized water and ultrasonically dispersed. Hydrogen peroxide and dopamine hydrochloride were added sequentially, and ultrasonication was continued to obtain gel bath B.

[0011] (3) Preparation of composite membrane

[0012] The casting solution A is evenly scraped or spun with a scraper and then immersed in a gel bath B. After a period of phase transformation, a polydopamine composite film doped with nanoparticles is obtained.

[0013] Step (1) The mass percentage of organic polymer materials in casting solution A is 12%-25%; the mass percentage of organic solvent is 75-88%.

[0014] In step (1), the organic polymer material is one or more of polyvinylidene fluoride, polyethersulfone, polyacrylonitrile, and cellulose acetate;

[0015] In step (1), the organic solvent is a strongly polar organic solvent. More specifically, the strongly polar organic solvent is one or a combination of two or more of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0016] In step (2), the inorganic nanoparticles in gel bath B have a mass percentage of 0.001-1% and a particle size of 5-50 nm; the dopamine hydrochloride has a mass percentage of 0.01-1%; and the hydrogen peroxide has a mass percentage of 0.001-1%.

[0017] In step (2), the nanoparticles are one or more of titanium dioxide, silicon dioxide, silver, graphene oxide, and nanomolecular sieves.

[0018] The beneficial effects of this invention are as follows: The composite membrane prepared by this invention is a flat sheet membrane or hollow fiber separation membrane. Its separation functional layer uses a hydrophilic polydopamine material and is doped with hydrophilic inorganic nanoparticles. Due to its excellent hydrophilicity and interfacial properties, as well as its extremely thin layer thickness, the prepared composite membrane not only has excellent separation selectivity but also excellent antifouling properties. The membrane preparation process of this invention is simple, has a large permeate flux, and can significantly improve its service life and extend the cleaning cycle, making it very suitable for the treatment of wastewater containing organic pollutants. Detailed Implementation

[0019] The specific embodiments of the present invention will be further described below in conjunction with the technical solution.

[0020] Example 1

[0021] Polyvinylidene fluoride (PVDF) material was uniformly dispersed in N,N-dimethylacetamide solution to obtain 30g of casting solution A, in which the mass fraction of PVDF was 14%. The solution was stirred and dissolved for later use. Nano-titanium dioxide with a particle size of 25nm was dispersed in deionized water to prepare a 3000g gel bath B, in which the mass percentage of titanium dioxide was 0.01%. 1.5g of dopamine hydrochloride and 1g of hydrogen peroxide (30%) were added, and the mixture was ultrasonically dispersed for 10min. The casting solution prepared by scraping with a doctor blade was quickly immersed into the gel bath, where a phase inversion occurred, resulting in a hydrophilically modified composite membrane.

[0022] The prepared hydrophilic composite membrane is a flat sheet membrane or a hollow fiber membrane, comprising a support layer and a separation functional layer, with the support layer located below the separation functional layer; the separation functional layer is a polydopamine separation membrane containing uniformly dispersed inorganic nanoparticles, wherein the inorganic nanoparticles account for 0.01-1% of the total solid content of the separation functional layer by mass.

[0023] The thickness of the separation functional layer is 10-200 nm.

[0024] The nanoparticles have a particle size of 5-50 nm.

[0025] The support layer is a microfiltration or ultrafiltration membrane with a thickness of 50–500 μm and a pore size of 10–1000 nm.

[0026] The prepared composite membrane has a pure water flux of 290 L / m at 0.1 MPa. 2 h, the rejection rate for 50 mg / L Rhodamine B was 99%, and the dye solution flux reached 260 L / m after 1 h of operation. 2 h.

[0027] Example 2

[0028] Polyethersulfone material was added to N-methylpyrrolidone solution and uniformly dispersed to obtain 30g casting solution A, in which the mass fraction of polyethersulfone was 15%, and stirred evenly for later use; 25nm nano-titanium dioxide particles were dispersed in deionized water to prepare 3000g gel bath B, in which the mass percentage of titanium dioxide was 0.01%; 1.5g of dopamine hydrochloride and 1g of hydrogen peroxide (30%) were added, and ultrasonically dispersed for 10min; the casting solution prepared by scraping with a doctor blade was quickly immersed into the gel bath, and a phase inversion occurred to obtain a hydrophilic modified composite membrane.

[0029] The prepared composite membrane has a pure water flux of 278 L / m at 0.1 MPa. 2 h, the rejection rate for 50 mg / L Rhodamine B was 97%, and the dye solution flux reached 260 L / m after 1 h of operation. 2 h.

[0030] Example 3

[0031] Polyacrylonitrile material was added to N,N-dimethylformamide solution and uniformly dispersed to obtain 30g casting solution A, in which the mass fraction of polyethersulfone was 15%, and stirred evenly for later use; 25nm nano-titanium dioxide particles were dispersed in deionized water to prepare 3000g gel bath B, in which the mass percentage of titanium dioxide was 0.01%; 3g of dopamine hydrochloride and 1g of hydrogen peroxide (30%) were added, and ultrasonically dispersed for 10min; the casting solution prepared by scraping with a doctor blade was quickly immersed into the gel bath, and a phase inversion occurred to obtain a hydrophilic modified composite membrane.

[0032] The prepared composite membrane has a pure water flux of 230 L / m at 0.1 MPa. 2 h, the rejection rate for 50 mg / L Rhodamine B was 97%, and the dye solution flux reached 204 L / m after 1 h of operation. 2 h.

[0033] Compare with Example 1

[0034] Polyvinylidene fluoride (PVDF), dopamine hydrochloride, and 25 nm nano-titanium dioxide were added to an N,N-dimethylacetamide solution and uniformly dispersed to obtain 30 g of dispersion A, in which the mass fraction of PVDF was 14%, the mass fraction of dopamine hydrochloride was 0.5%, and the mass fraction of nano-titanium dioxide was 0.5%. The mixture was stirred evenly and set aside. 3000 g of deionized water was used as gel bath B. The casting solution prepared by scraping with a doctor blade was quickly immersed into the gel bath, where a phase inversion occurred to obtain a separation membrane.

[0035] The prepared separation membrane has a pure water flux of 187 L / m at 0.1 MPa. 2 h, the rejection rate for 50 mg / L Rhodamine B was 98%, and the dye solution flux reached 138 L / m after 1 h of operation. 2h.

[0036] Compare with Example 2

[0037] Add hydrogen peroxide to the hydrochloric acid dopamine solution, stir at room temperature for 24 hours, filter, dry, grind and collect polydopamine powder;

[0038] Polyvinylidene fluoride (PVDF) and polydopamine were added to an N,N-dimethylacetamide solution and uniformly dispersed to obtain 30g of dispersion A, in which the mass fraction of PVDF was 14% and the mass fraction of polydopamine was 1%. The mixture was stirred evenly and set aside. 3000g of deionized water was used as a gel bath B. The casting solution prepared by scraping with a doctor blade was quickly immersed into the gel bath, where a phase inversion occurred to obtain a separation membrane.

[0039] The prepared separation membrane has a pure water flux of 166 L / m at 0.1 MPa. 2 h, the rejection rate for 50 mg / L Rhodamine B was 98%, and the dye solution flux reached 134 L / m after 1 h of operation. 2 h.

[0040] Compare with Example 3

[0041] Polyvinylidene fluoride (PVDF) material was added to N,N-dimethylacetamide solution and uniformly dispersed to obtain 30g of casting solution A, in which the mass fraction of PVDF was 14%. The solution was stirred and dissolved for later use. 3000g of gel bath B was prepared using deionized water. The casting solution was scraped with a doctor blade and quickly immersed into the gel bath, where a phase inversion occurred to obtain a PVDF separation membrane.

[0042] Prepare a 3000g solution of dopamine hydrochloride, add 1.5g of dopamine hydrochloride, 1g of hydrogen peroxide (30%), and 25nm titanium dioxide, and ultrasonically disperse for 10min;

[0043] The above-mentioned polyvinylidene fluoride separation membrane was immersed in a hydrochloric acid dopamine solution for 10 minutes, and then removed and dried to obtain a polyvinylidene fluoride / polydopamine composite membrane.

[0044] The prepared composite membrane has a pure water flux of 97 L / m at 0.1 MPa. 2 h, the rejection rate for 50 mg / L Rhodamine B was 99%, and the dye solution flux reached 85 L / m after 1 h of operation. 2 h.

[0045] As can be seen from Comparative Example 1, the separation membrane obtained by adding inorganic nanoparticles and dopamine hydrochloride to the polyvinylidene fluoride casting solution is an organic-inorganic hybrid membrane. Because dopamine hydrochloride and nano-titanium dioxide are highly hydrophilic and have poor compatibility with polyvinylidene fluoride, the resulting membrane is denser and has a lower flux compared to the composite membrane prepared in this invention. Furthermore, the hydrophilic material is encapsulated by polyvinylidene fluoride, resulting in poor antifouling properties.

[0046] As can be seen from Example 2, when polydopamine and polyvinylidene fluoride are directly blended to form a membrane, the pure water flux and dye solution flux of the obtained membrane are both smaller compared with those of the polyvinylidene fluoride membrane. Although polydopamine has good hydrophilicity, when the membrane is blended, the polydopamine nanoparticles are coated by polyvinylidene fluoride, and less polydopamine remains on the membrane surface, thus resulting in less improvement in membrane performance.

[0047] As can be seen from Comparative Example 3, a composite membrane can also be obtained by immersing a polyvinylidene fluoride membrane in a dopamine hydrochloride solution, but this requires two steps, and the resulting composite membrane has a significantly reduced flux due to its thicker skin layer. This invention uses a one-step method to obtain a polydopamine composite membrane with excellent antifouling properties. The polydopamine skin layer is located on the surface of the polyvinylidene fluoride composite membrane, preventing hydrophilic particles from being encapsulated by the polymer. The method is simple and feasible.

Claims

1. A method for preparing a hydrophilic composite membrane, characterized in that, Includes the following steps: (1) Preparation of casting solution A Organic polymer materials are added to an organic solvent and stirred rapidly. After 24 hours, casting solution A is obtained. (2) Preparation of gel bath B Inorganic nanoparticles were added to deionized water and ultrasonically dispersed. Hydrogen peroxide and dopamine hydrochloride were added sequentially, and ultrasonication was continued to obtain gel bath B. (3) Preparation of composite membrane The casting solution A is evenly scraped or spun with a scraper and then immersed in a gel bath B. After a period of phase transformation, a polydopamine composite film doped with nanoparticles is obtained.

2. The preparation method according to claim 1, characterized in that, Step (1) The mass percentage of organic polymer materials in casting solution A is 12%-25%; the mass percentage of organic solvent is 75-88%. In step (1), the organic polymer material is one or more of polyvinylidene fluoride, polysulfone, polyacrylonitrile, and cellulose acetate; The organic solvent in step (1) is a highly polar organic solvent.

3. The preparation method according to claim 2, characterized in that, The highly polar organic solvent is one or a combination of two or more of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide.

4. The preparation method according to any one of claims 1-3, characterized in that, In step (2), the inorganic nanoparticles in gel bath B have a mass percentage of 0.001-1% and a particle size of 5-50 nm; the dopamine hydrochloride has a mass percentage of 0.01-1%; and the hydrogen peroxide has a mass percentage of 0.001-1%.

5. The preparation method according to claim 4, characterized in that, In step (2), the nanoparticles are one or more of titanium dioxide, silicon dioxide, silver, graphene oxide, and nanomolecular sieves.