Composite nanofiltration membrane and preparation method and application thereofTechnical Field
The invention relates to the field of separation membranes, in particular to a composite nanofiltration membrane with a high-penetration porous structure supporting layer, a preparation method thereof and application of the nanofiltration membrane in a water treatment process.
Background
Nanofiltration is a pressure-driven membrane separation process between reverse osmosis and ultrafiltration, the pore size of the nanofiltration membrane is about a few nanometers, the removal of monovalent ions and organic matters with the molecular weight smaller than 200 is poor, and the removal rate of divalent or multivalent ions and organic matters with the molecular weight between 200 and 500 is high. Can be widely used in the fields of fresh water softening, seawater softening, drinking water purifying, water quality improvement, oil-water separation, wastewater treatment and recycling, and classification, purification, concentration and the like of chemical products such as dye, antibiotics, polypeptide, polysaccharide and the like.
The composite membrane is a novel separation membrane developed in recent years, and most of commercial nanofiltration membranes are composite membranes at present, and the composite membrane is formed by compounding a very thin and compact functional separation layer and a microporous support layer. Typically, a porous support membrane is prepared and then a very thin dense separation layer is formed on its surface. The comprehensive performance of the composite membrane is influenced by the separation layer and the supporting layer, the separation surface layer plays a leading role in separation, the supporting layer is beneficial to reducing the fluid resistance and increasing the supporting strength of the membrane, the important role is played in the whole composite membrane, and the morphology structure and the permeability of the composite membrane directly determine the permeability of the whole composite membrane. The existing polymer composite membrane supporting layer is an asymmetric porous membrane prepared by adopting a solution phase inversion method, the supporting layer prepared by the method is generally large in thickness, small in porosity and in closed pore structure among micropores, and the water flux is low during composite membrane filtration. In order to increase the porosity of the support layer, in recent years, the flux of a composite membrane prepared by using a base membrane with high porosity and high through porous structure as the support layer is remarkably improved. Document Environmental Science & Technology,2005 (39): 7 684-7 691) reports: compared with the traditional commercial composite nanofiltration membrane, the ultrathin nanofiber-based composite filtration membrane composed of the electrostatic spinning nanofiber porous membrane and the functional coating can improve the water permeation flux by as much as 10 times on the premise of ensuring high retention rate. Chinese patent (CN 106582297A) takes a cuboid steel wire mesh with a porous carrier layer coated on the surface as a nanofiltration membrane for preparation, and the prepared nanofiltration membrane has good permeability and retention, and the membrane interlayer filtration is more efficient. However, the high porosity and the interpenetrating macroporous structure of the nanofiber and metal skeleton porous base membrane (CN 110280222A) can obviously improve the fluxion, and simultaneously easily cause the technical problems of difficult infiltration of interfacial polymerization monomer solution or difficult control of the thickness of a coating in the process of preparing a composite membrane, and the like, so that a complete separation layer structure cannot be formed, and large-scale industrial application is difficult to realize. In addition, the preparation of the nanofiber-based support layer still faces a plurality of problems to be solved, including film-making cost and film-making efficiency, large-scale preparation of high-quality nanofibers and nanofiber-based composite materials, suitability for selection of electrostatic spinning materials, physical and chemical means required by functionalization, perfection and improvement of a fine preparation method and the like.
Therefore, the nano-filtration composite membrane with high porosity and high through structure supporting layer prepared by developing a simple and efficient method has very important practical significance and commercial value.
Disclosure of Invention
The invention aims to overcome the problems in the prior art and provide a composite nanofiltration membrane with a high-penetration porous structure supporting layer, a preparation method thereof and application of the composite nanofiltration membrane in the field of water treatment.
The invention aims to provide a composite nanofiltration membrane which sequentially comprises a bottom layer, a porous supporting layer and an active separating layer, wherein the active separating layer is a polyamide layer, the porous supporting layer is divided into a sub-layer and a surface layer, the surface layer is of a small-hole structure with narrow pore diameter distribution, and the sub-layer is attached to the bottom layer and is of a highly-communicated three-dimensional network porous structure.
The composite nanofiltration membrane comprises a three-layer structure of a bottom layer, a porous supporting layer and an active separation layer, wherein the porous supporting layer is positioned between the bottom layer and the active separation layer and mainly provides mechanical strength and a fluid transmission channel; the active separation layer mainly plays a role in separation and screening.
The materials of the bottom layer may include, but are not limited to: nonwoven fabrics, woven fabrics, polyester screens, electrostatic spinning films and other porous support materials.
The active separation layer is formed by interfacial polymerization of an amine compound containing two or more amino groups and an acid chloride compound containing two or more acid chloride groups on the porous support layer.
Preferably, the active separation layer is prepared by performing interfacial polymerization reaction on an aliphatic polyfunctional amine compound and an aromatic polyfunctional acyl chloride compound, wherein the aliphatic polyfunctional amine compound is preferably at least one of polyethyleneimine, ethylenediamine, piperazine and 4-aminomethylpiperazine, and the aromatic polyfunctional acyl chloride compound is preferably at least one of terephthaloyl chloride, isophthaloyl chloride, phthaloyl chloride, biphenylyl chloride, phthaloyl chloride and trimesoyl chloride.
The porous supporting layer is prepared by adopting an atomization pretreatment auxiliary non-solvent induced phase separation method, and is characterized in that the process of preparing the supporting layer by inducing phase separation is divided into two steps, and the atomization pretreatment process is combined with non-solvent induced phase separation, namely, film forming is firstly stopped in an atomization liquid drop bath for partial induced phase separation, and then enters a non-solvent coagulation bath for complete phase separation.
The polymer for preparing the porous support layer is at least one selected from polyvinyl chloride, polysulfone, polyethersulfone, sulfonated polyethersulfone, polyacrylonitrile, cellulose acetate, polyvinylidene fluoride, polyimide, polyacrylic acid, polylactic acid, polyamide, chitosan, polyetherimide, polystyrene, polyolefin, polyester, polytrifluoroethylene, silicone resin, acrylonitrile-styrene copolymer and modified polymers thereof.
The porous supporting layer is of an asymmetric structure and is provided with a thin surface layer and a sub-layer attached to the bottom layer, and the sub-layer is of a double-continuous highly-communicated three-dimensional network porous structure, so that water transmission resistance can be reduced, and water flux can be increased; the surface layer is attached to the active separation layer.
The average pore diameter of the surface layer of the supporting layer is 10-50 nm.
The sublayers of the supporting layer are mutually communicated three-dimensional network porous structures, and the pore structures are highly communicated and have larger porosity. The section of the sub-layer of the supporting layer is a structure of a polymer fiber skeleton and holes with basically consistent appearance along the thickness direction of the film, namely the section of the sub-layer is a structure of the polymer fiber skeleton and the holes of the same type distributed along the thickness direction of the film. Ultrafiltration membranes, such as those obtained by conventional non-solvent phase separation methods, often have different types of pore structures present in the cross-section of the sub-layer at the same time, typically comprising a sponge-like pore structure and a large finger-like pore structure.
The porosity of the porous support layer is 40 to 90%, preferably 60 to 90%, more preferably 70 to 80%.
In the composite nanofiltration membrane, the thickness of the bottom layer is 50-300 mu m, the thickness of the porous support layer sub-layer is 10-60 mu m, the thickness of the surface layer of the porous support layer is 0.5-5 mu m, and the thickness of the active separation layer is 5-100 nm.
The second purpose of the invention is to provide a preparation method of the composite nanofiltration membrane, which comprises the following steps:
1) Dissolving components including a polymer of the porous support layer in a solvent to prepare a casting solution;
2) Scraping and casting the casting solution on the bottom layer;
3) Carrying out atomization pretreatment, wherein the atomization pretreatment stays in an atomized liquid drop bath, the bottom surface faces to atomized liquid drops, and the surface of the protective film coated with the casting solution is not contacted with the atomized liquid drops;
4) Immersing in a coagulating bath to obtain the porous support layer;
5) Contacting the porous support layer sequentially with an aqueous phase solution containing an aliphatic polyfunctional amine compound and an organic phase solution containing an aromatic polyfunctional acyl chloride compound;
6) And drying and heat treating to obtain the composite nanofiltration membrane.
In step 1), the polymer may be selected from polymer materials for filtration membranes as usual in the art, and usable polymer materials may include, but are not limited to: polyvinyl chloride, polysulfone, polyethersulfone, sulfonated polyethersulfone, polyacrylonitrile, cellulose acetate, polyvinylidene fluoride, polyimide, polyacrylic acid, polylactic acid, polyamide, chitosan, polyetherimide, polystyrene, polyolefin, polyester, polytrifluoroethylene, silicone resin, acrylonitrile-styrene copolymer, and the like, and at least one of the polymers modified by them.
In the step 1), the solid content of the polymer in the casting film liquid is 6-20 wt%, preferably 8-18 wt%.
The casting solution can also contain common additives and the like.
The film-forming additive may be a polymer material which is miscible in a film-forming polymer good solvent and has hydrophilicity, and may include, but is not limited to: at least one of chitosan, polyvinylpyrrolidone, polyethylene glycol, polyvinyl alcohol, glycerol, propylene glycol, acetone, polyoxyethylene polyoxypropylene ether block copolymer, etc. The membrane-forming additives may also include conventional inorganic salt porogens, poor solvents, and/or various inorganic nanoparticles such as nanoscale inorganic fillers required in typical filtration membrane preparation processes, including but not limited to: zinc chloride, lithium chloride, magnesium chloride, lithium bromide, water, various small molecule alcohols, and the like; the inorganic filler is manganese dioxide, silicon dioxide, zinc oxide, etc.
The amount of the film-forming additive used is a conventional amount, and in the present invention, it is preferable that: the concentration of the polymer additive is 1-200 g/L; the concentration of the small molecule additive is 0.5-50 g/L.
In the step 1), the solvent is a good solvent capable of dissolving the film-forming polymer and the film-forming additive, and the solvent includes at least one of N, N-dimethylformamide, N-dimethylacetamide, acetone, N-methyl-2-pyrrolidone, dimethyl sulfoxide, tetramethyl sulfoxide, tetrahydrofuran, dioxane, acetonitrile, chloroform, polarclean solvent, triethyl phosphate, trimethyl phosphate, hexamethyl ammonium phosphate, tetramethyl urea, acetonitrile, toluene, hexane, octane and the like, and the preparation time and the preparation temperature of the casting solution can be determined according to the casting material.
In step 2), the primer material required for coating the casting solution may be a support layer material or a base material used as a polymer solution in the prior art, and may include, but is not limited to: nonwoven fabrics, woven fabrics, polyester screens, electrostatic spinning films and other porous support materials.
In the step 2), the casting solution is uniformly coated on the bottom layer for film scraping.
In the step 2), the wet film is coated with the casting solution, and the thickness is not particularly limited, but the thickness of the scratch film is preferably 50 to 500. Mu.m, more preferably 75 to 300. Mu.m.
The porous supporting layer used in the invention is prepared by adopting an atomization pretreatment auxiliary non-solvent induced phase separation method. I.e. first stay in the atomized droplet bath for partially induced phase separation, and then enter the non-solvent coagulation bath for complete phase separation.
The present invention provides a great distinction from Vapor Induced Phase Separation (VIPS), which refers to the phase separation that occurs under certain high humidity (or saturation humidity) conditions, without involving an atomized droplet bath.
In the step 3), the atomization pretreatment is that after the casting solution is coated, the bottom layer of the film faces to atomized liquid drops, the atomized liquid drops stay in contact with the atomized liquid drops for a certain time, and the surface of the film coated with the casting solution is protected from contacting with the atomized liquid drops. The method of obtaining the atomized liquid droplet bath is not particularly limited, and various conventional methods of liquid atomization, such as pressure atomization, rotary disk atomization, high-pressure air stream atomization, ultrasonic atomization, and the like, may be employed.
The atomization pretreatment time is preferably 1s to 60s, more preferably 2s to 40s.
The size of the droplets in the droplet bath is preferably 1 to 50. Mu.m, more preferably 5 to 20. Mu.m.
The required atomization amount per unit membrane area is 2-50L/m2 H is preferably 10 to 20L/m2 ·h。
The liquid drops in the atomization pretreatment are poor solvents of the casting film polymer, and can be single components such as water, ethanol, glycol and the like, can also be composed of water, polar aprotic solvents, surfactants or other solvents, and can also be solution of salt, acid and alkali.
The mode that the surface of the protective film coated with the casting film liquid does not contact with atomized liquid drops can adopt the conventional methods such as shielding protection, blowing protection and the like.
In the step 4), the coagulating bath is a poor solvent of the casting film polymer, and can be single components such as water, ethanol, glycol and the like, or can be mixed by water and a polar aprotic solvent or other solvents, such as sodium hydroxide aqueous solution.
In step 5), the porous support layer is sequentially contacted with an aqueous phase solution of a compound containing two or more amino groups and an organic phase solution containing two or more acid chloride compounds to perform interfacial polymerization.
The compound containing two or more amino groups is one or more of aliphatic polyfunctional amine compounds. The aliphatic polyfunctional amine compound is preferably at least one of polyethylenimine, ethylenediamine, piperazine, and 4-aminomethylpiperazine.
The concentration of the aliphatic polyfunctional amine compound in the aqueous phase solution of the porous support layer is 0.05-2.5 w/v%.
The contact time with the aqueous solution containing the aliphatic polyfunctional amine compound is 5 to 150 seconds.
The acyl chloride compound containing two or more acyl chloride groups is one or more of aromatic polyfunctional acyl chloride compounds. The aromatic polyfunctional acyl chloride compound is preferably at least one of terephthaloyl chloride, isophthaloyl chloride, phthaloyl chloride, biphenyldicarboxylic acid chloride, benzenedisulfonyl chloride and trimesoyl chloride.
The organic solvent of the organic phase solution is preferably one or more of n-hexane, cyclohexane, trifluorotrichloroethane, n-heptane, n-octane, toluene, ethylbenzene and ISOPAR solvent oil.
The concentration of the aromatic polyfunctional acyl chloride compound in the organic phase solution is 0.02-0.15 w/v%.
The porous support layer is contacted with the organic phase solution containing the aromatic polyfunctional acyl chloride compound for 5 to 150 seconds.
In the step 6), the temperature of the heat treatment is 25-70 ℃ and the time is 1-5 minutes.
The process for preparing the active separation layer in the present invention may be preferably performed as follows:
a) Contacting the porous support layer with an aqueous solution of an aliphatic compound containing two or more amino groups;
b) Removing excessive aqueous solution on the surface of the porous support layer after being infiltrated by the aqueous solution, wherein the method for removing the excessive solution can be selected from but not limited to a wind spraying method, a rolling method and the like;
c) The treated porous support layer is contacted with an organic phase solution containing two or more acyl chloride compounds;
d) Drying, heat treatment and water washing.
The invention further provides the composite nanofiltration membrane obtained by the preparation method.
The fourth purpose of the invention is to provide the application of the composite nanofiltration membrane or the composite nanofiltration membrane obtained by the preparation method in the fields of water treatment, biology, medicine, energy and the like.
Compared with the prior art, the invention is characterized in that:
the nanofiltration membrane support layer prepared by the method disclosed by the invention has a special structure, the porous support layer is provided with a small pore separation surface layer with narrow pore size distribution and a sublayer with a bicontinuous high through hole structure, and has larger porosity, so that the mass transfer resistance of the nanofiltration membrane can be effectively reduced, the mass transfer channel is increased, and the permeation flux of the membrane is greatly improved. The invention only needs to add an atomization pretreatment process on the basis of the traditional process for preparing the composite nanofiltration membrane supporting layer by a non-solvent phase inversion method. The method has the characteristics of simple preparation process, easily available raw materials, low cost and the like, can be used for continuously preparing nanofiltration membrane materials on a large scale, and is easy for industrial application. The method has wide application space in the fields of water treatment, biology, medicine, energy sources and the like, and has good application prospect.
Additional features and advantages of the invention will be set forth in the detailed description which follows.
Drawings
FIG. 1 is a cross-sectional view of a porous support layer of a nanofiltration membrane obtained in example 3.
FIG. 2 is a surface topography of the nanofiltration membrane obtained in example 3.
FIG. 3 is a cross-sectional view showing the morphology of the support layer of the nanofiltration membrane obtained in comparative example 1.
Detailed Description
The present invention is described in detail below with reference to specific embodiments, and it should be noted that the following embodiments are only for further description of the present invention and should not be construed as limiting the scope of the present invention, and some insubstantial modifications and adjustments of the present invention by those skilled in the art from the present disclosure are still within the scope of the present invention.
In addition, the specific features described in the following embodiments may be combined in any suitable manner without contradiction. The various possible combinations of the invention are not described in detail in order to avoid unnecessary repetition.
In addition, any combination of the various embodiments of the present invention can be made, so long as the concept of the present invention is not deviated, and the technical solution formed thereby is a part of the original disclosure of the present specification, and also falls within the protection scope of the present invention.
According to a preferred embodiment of the present invention, the preparation method of the composite nanofiltration membrane may be performed according to the following steps:
1') dissolving a component containing a polymer in a solvent to prepare a casting solution;
2') scraping and casting the casting solution on the supporting layer to form a film;
3') carrying out atomization pretreatment, namely, staying in an atomized liquid drop bath for a certain time, wherein the bottom surface faces to atomized liquid drops, and the surface of the protective film coated with the casting film liquid does not contact with the atomized liquid drops;
4') immersing in a coagulation bath to obtain the polymer support layer;
5') contacting the porous support layer with an aqueous solution of an aliphatic compound containing two or more amino groups after dilution for a period of 5 to 150 seconds;
6') rolling the porous support layer which is infiltrated by the aqueous phase solution by using a rubber roller, and removing redundant aqueous phase solution;
7') contacting the porous support layer infiltrated by the aqueous phase solution with an organic phase solution containing two or more acyl chloride compounds of acyl chloride groups for 5-150 seconds, and generating a compact functional layer on the surface of the porous support layer through interfacial polymerization reaction;
8') finally, naturally drying the porous support layer immersed in the organic phase solution in air, performing post-treatment for 1-5 minutes at a certain temperature, and washing to obtain the nano-filtration composite membrane.
In the following examples and comparative examples:
(1) The water flux of the composite nanofiltration membrane is tested by the following method: loading the composite nanofiltration membrane into a membrane pool, prepressing for 0.5 hours under 0.5MPa, measuring the water permeability of the composite nanofiltration membrane within 1h under the conditions of the pressure of 0.5MPa and the temperature of 25 ℃, and calculating by the following formula:
J＝Q/A·t，
wherein J is water flux, Q is water transmission amount (L), A is effective membrane area (m2 ) T is time (h);
(2) The desalination rate of the composite nanofiltration membrane is tested by the following method: loading the composite nanofiltration membrane into a membrane tank, pre-pressing for 0.5h under 0.5MPa, and measuring initial concentration of 2g.L in 1h under the condition that the pressure is 0.5MPa and the temperature is 25 DEG C-1 MgSO4 MgSO in the raw water solution and the permeate4 Is calculated by the following formula:
R＝(Cp -Cf )/Cp ×100％，
wherein R is desalination rate (%), Cp As MgSO in stock solution4 Concentration (g/L or mol/L), Cf As MgSO in the permeate4 Concentration (g/L or mol/L);
in the examples of the present invention, the chemical reagents used were all commercially available products, and unless individually indicated, no particular purification treatment was carried out.
Spraying equipment: the ultrasonic humidifier is Haoqi HQ-JS130H.
Example 1
(1) Preparing a porous supporting layer: 12g of polyacrylonitrile is dissolved in 88g of DMF solvent, heated and stirred at 50 ℃ to form uniform solution, and vacuumized and defoamed; then a continuous film scraping machine is adopted to scrape and coat the film on non-woven fabrics, the thickness of a feeler gauge is controlled to be 200 mu m during coating, then the back surface of the film after coating faces to a liquid drop bath obtained by ultrasonic atomization of deionized water, the film stays for 20s in the liquid drop bath, the surface of the film coated with casting solution is protected from contacting atomized liquid drops, and the atomization amount per unit film area is 6.2L/m2 H; immersing the film into deionized water coagulation bath to completely phase separate; and washing with water to obtain the porous support layer.
(2) Preparing a composite film: a certain amount of aqueous phase monomer piperazine (PIP) is weighed in a volumetric flask, deionized water is used for fixing the volume to the scale, and ultrasonic dissolution is carried out to obtain a uniform aqueous phase solution containing 0.5w/v% PIP. A certain amount of organic phase monomer trimesoyl chloride (TMC) is weighed into a volumetric flask, the volume is fixed to the scale by using ISOPAR solvent oil, and the organic phase monomer trimesoyl chloride (TMC) is dissolved into a uniform organic phase solution containing 0.1w/v% TMC by ultrasonic. Using the polyacrylonitrile porous support membrane prepared in the step (1), and immersing and contacting with the aqueous phase solution for 30 seconds. Then the excess aqueous phase solution is poured off, the surface of the film is dried by a clean rubber roller, and then the film is soaked and contacted with the trimesoyl chloride organic phase solution for 30 seconds. And then pouring out the redundant organic phase solution, airing the formed polyamide layer in the air, and carrying out aftertreatment for 4min at normal temperature to obtain the composite nanofiltration membrane. The prepared composite nanofiltration membrane is stored in deionized water and tested for standby.
In the composite nanofiltration membrane obtained in example 1, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 44 μm, the thickness of the surface layer of the porous support layer was 2.1 μm, and the thickness of the active separation layer was 46nm. The average pore diameter of the surface layer of the supporting layer is 17nm.
Example 2
Nanofiltration membrane was produced in the same manner as in example 1, except that in the nanofiltration membrane support layer production process of step (1), the atomization amount of atomized droplets used in the atomization pretreatment stage was 10L/m2 ·h。
In the composite nanofiltration membrane obtained in example 2, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 45 μm, the thickness of the surface layer of the porous support layer was 2.0 μm, and the thickness of the active separation layer was 43nm. The average pore diameter of the surface layer of the supporting layer is 18nm.
Example 3
Nanofiltration membrane was produced in the same manner as in example 1, except that in the nanofiltration membrane support layer production process of step (1), the atomization amount of atomized droplets used in the atomization pretreatment stage was 17L/m2 H. The section morphology of the porous supporting layer of the nanofiltration membrane is shown in figure 1, and the surface morphology of the nanofiltration membrane is shown in figure 2.
In the composite nanofiltration membrane obtained in example 3, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 43 μm, the thickness of the surface layer of the porous support layer was 1.9 μm, and the thickness of the active separation layer was 43nm. The average pore diameter of the surface layer of the supporting layer is 19nm.
Example 4
Nanofiltration membranes were prepared as in example 3, except that during the preparation of the nanofiltration membrane support layer of step (1), the back side of the coated membrane was subjected to ultrasonic atomization towards deionized water in an atomization pretreatment stage to obtain a droplet bath, and the droplet bath was left for 10s.
In the composite nanofiltration membrane obtained in example 4, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 44 μm, the thickness of the surface layer of the porous support layer was 2.2 μm, and the thickness of the active separation layer was 45nm. The average pore diameter of the surface layer of the supporting layer is 17nm.
Example 5
Nanofiltration membranes were prepared as in example 3, except that during the preparation of the nanofiltration membrane support layer of step (1), the back side of the coated membrane was subjected to ultrasonic atomization towards deionized water in an atomization pretreatment stage to obtain a droplet bath, and the droplet bath was left for 30s.
In the composite nanofiltration membrane obtained in example 5, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 48 μm, the thickness of the surface layer of the porous support layer was 1.7 μm, and the thickness of the active separation layer was 41nm. The average pore diameter of the surface layer of the supporting layer is 23nm.
Example 6
Nanofiltration membranes were prepared as in example 3, except that during the preparation of the nanofiltration membrane support layer of step (1), the back side of the coated membrane was subjected to ultrasonic atomization towards deionized water in an atomization pretreatment stage to obtain a droplet bath, and the droplet bath was left for 40s.
In the composite nanofiltration membrane obtained in example 6, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 51 μm, the thickness of the surface layer of the porous support layer was 1.7 μm, and the thickness of the active separation layer was 38nm. The average pore diameter of the surface layer of the supporting layer is 57nm.
Example 7
Nanofiltration membranes were prepared as in example 3, except that during the preparation of the nanofiltration membrane support layer of step (1), the back side of the coated membrane was subjected to ultrasonic atomization towards deionized water in an atomization pretreatment stage to obtain a droplet bath, and the droplet bath was left for 50s.
In the composite nanofiltration membrane obtained in example 7, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 54 μm, the thickness of the surface layer of the porous support layer was 1.5 μm, and the thickness of the active separation layer was 36nm. The average pore diameter of the surface layer of the supporting layer is 223nm.
Comparative example 1
Nanofiltration membranes were prepared as in example 3, except that during the preparation of the nanofiltration membrane support layer, the ultrafiltration membrane was immersed directly in a solvent coagulation bath for complete phase separation without an atomization pretreatment stage, and the support layer was obtained after washing with water. The cross-sectional morphology of the support layer is shown in figure 3.
In the composite nanofiltration membrane obtained in comparative example 1, the thickness of the active separation layer was 46nm. The average pore diameter of the support layer was 14nm.
Under the test conditions of an operating pressure of 0.5MPa and a temperature of 25 ℃, 2g.L was used-1 MgSO4 Aqueous solution the nanofiltration membranes prepared in examples 1 to 7 and comparative example 1 above were tested for water flux and retention rate, and the results obtained from the tests are shown in table 1.
TABLE 1
From the results of examples 3 to 5 and comparative example 1 above, it can be seen that the composite nanofiltration membrane prepared by using the support layer after the atomization pretreatment has excellent water flux and salt rejection rate, and the nanofiltration membrane water flux increases with the increase of the atomization time on the premise that the salt rejection rate is kept constant. It can be seen from examples 6 to 7 that with further increase of the atomization time, the surface pore size of the support layer becomes larger, which is unfavorable for forming a complete polyamide separation layer, resulting in a significant decrease of the salt rejection rate of the nanofiltration membrane. It can be seen from examples 1-3 that the permeation flux of the nanofiltration membrane increased with increasing atomization.
Example 8
(1) Preparing a porous supporting layer: 12g polysulfone and 0.5g polyvinylpyrrolidone (PVP) are dissolved in 87.5g polarclean solvent, heated and stirred into uniform solution at 100 ℃, and vacuumized and defoamed; then a continuous film scraping machine is adopted to scrape and coat the film on non-woven fabrics, the thickness of a feeler gauge is controlled to be 200 mu m during coating, then the back surface (non-woven fabrics side) of the film after coating is subjected to ultrasonic atomization towards deionized water to obtain a liquid drop bath, the liquid drop bath stays for 4s, the surface of the protective film coated with casting solution is not contacted with atomized liquid drops, and the atomization amount per unit film area is 17L/m2 H; immersing the film into deionized water coagulation bath to completely phase separate; and washing with water to obtain the porous support layer.
(2) Preparing a composite film: a certain amount of aqueous phase monomer piperazine (PIP) is weighed in a volumetric flask, deionized water is used for fixing the volume to the scale, and ultrasonic dissolution is carried out to obtain a uniform aqueous phase solution containing 0.5w/v% PIP. A certain amount of organic phase monomer trimesoyl chloride (TMC) is weighed into a volumetric flask, the volume is fixed to the scale by using ISOPAR solvent oil, and the organic phase monomer trimesoyl chloride (TMC) is dissolved into a uniform organic phase solution containing 0.1w/v% TMC by ultrasonic. Using the polysulfone porous support membrane prepared in step (1), it was immersed in contact with the above aqueous solution for 30 seconds. Then the excess aqueous phase solution is poured off, the surface of the film is dried by a clean rubber roller, and then the film is soaked and contacted with the trimesoyl chloride organic phase solution for 30 seconds. And then pouring out the redundant organic phase solution, airing the formed polyamide layer in the air, and carrying out aftertreatment for 4min at normal temperature to obtain the composite nanofiltration membrane. The prepared composite nanofiltration membrane is stored in deionized water and tested for standby.
In the composite nanofiltration membrane obtained in example 8, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 47 μm, the thickness of the surface layer of the porous support layer was 1.5 μm, and the thickness of the active separation layer was 46nm. The average pore diameter of the surface layer of the supporting layer is 21nm.
Example 9
Nanofiltration membranes were prepared as in example 8, except that during the preparation of the nanofiltration membrane support layer of step (1), the back side of the coated membrane was subjected to ultrasonic atomization towards deionized water in an atomization pretreatment stage to obtain a droplet bath, and the droplet bath was left for 6s.
In the composite nanofiltration membrane obtained in example 9, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 51 μm, the thickness of the surface layer of the porous support layer was 1.4 μm, and the thickness of the active separation layer was 42nm. The average pore diameter of the surface layer of the supporting layer is 24nm.
Example 10
Nanofiltration membranes were prepared as in example 8, except that during the preparation of the nanofiltration membrane support layer of step (1), the back side of the coated membrane was subjected to ultrasonic atomization towards deionized water in an atomization pretreatment stage to obtain a droplet bath, and the droplet bath was left for 8s.
In the composite nanofiltration membrane obtained in example 10, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 53 μm, the thickness of the surface layer of the porous support layer was 1.3 μm, and the thickness of the active separation layer was 39nm. The average pore diameter of the surface layer of the supporting layer is 167nm.
Example 11
Nanofiltration membranes were prepared as in example 8, except that during the nanofiltration membrane support layer preparation in step (1), the back side of the coated membrane was subjected to ultrasonic atomization towards deionized water in an atomization pretreatment stage to obtain a droplet bath, and the droplet bath was left for 12s.
In the composite nanofiltration membrane obtained in example 11, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 55 μm, the thickness of the surface layer of the porous support layer was 1.1 μm, and the thickness of the active separation layer was 35nm. The average pore diameter of the surface layer of the supporting layer is 289nm.
Example 12
Nanofiltration membranes were prepared as in example 8, except that during the preparation of the nanofiltration membrane support layer of step (1), the back side of the coated membrane was subjected to ultrasonic atomization towards deionized water in an atomization pretreatment stage to obtain a droplet bath, and the droplet bath was left for 24s.
In the composite nanofiltration membrane obtained in example 12, the thickness of the bottom layer was 85 μm, the thickness of the sub-layer of the porous support layer was 57 μm, the thickness of the surface layer of the porous support layer was 1.0 μm, and the thickness of the active separation layer was 34nm. The average pore diameter of the surface layer of the supporting layer is 402nm.
Comparative example 2
Nanofiltration membranes were prepared according to the method of example 8, except that during the preparation of the nanofiltration membrane support layer, the ultrafiltration membrane was directly immersed in a solvent coagulation bath for complete phase separation without an atomization pretreatment stage, and the support layer was obtained after washing with water.
In the composite nanofiltration membrane obtained in comparative example 2, the thickness of the active separation layer was 49nm. The average pore diameter of the support layer was 16nm.
Under the test conditions of an operating pressure of 0.5MPa and a temperature of 25 ℃, 2g.L was used-1 MgSO4 Aqueous solution the nanofiltration membranes prepared in examples 8-12 and comparative example 2 above were tested for water flux and retention, and the results obtained from the tests are shown in table 2.
TABLE 2
From the results of the above examples and comparative examples, it can be seen that nanofiltration membranes prepared by using the support layer after the atomization pretreatment have excellent water flux and salt rejection rate. The atomization time has a large influence on the permeability of the nanofiltration membrane, when the atomization pretreatment time is within a certain range, the surface pore diameter of the supporting layer is small, a complete polyamide active layer is easy to generate in the interfacial polymerization process, the separation permeability of the membrane is improved, and the water flux and the salt interception rate of the nanofiltration membrane are obviously reduced along with the continuous increase of the atomization pretreatment time.