Branched aminated polyacrylonitrile-based compact ultrafiltration membrane as well as preparation method and application thereof

Abstract

The invention provides a branched aminated polyacrylonitrile-based compact ultrafiltration membrane as well as a preparation method and application thereof, belongs to the technical field of membrane separation, and is used for solving the technical problem that the rejection rate and the membrane flux of a polyacrylonitrile-based membrane are difficult to consider at the same time. A branched aminated polyacrylonitrile solution is used as a membrane casting solution, an aqueous solution of organic acid is used as a coagulating bath, a branched aminated polyacrylonitrile base membrane is prepared through a non-solvent induced phase inversion method, and then the base membrane is soaked in a metal salt solution for cross-linking to obtain the compact ultrafiltration membrane with an ultrathin and compact separation skin layer. The compact ultrafiltration membrane prepared by the invention can effectively reduce the thickness of a membrane separation skin layer, and greatly improves the retention rate of dye while reducing a little flux. The performance of the aminated polyacrylonitrile-based compact ultrafiltration membrane is remarkably improved compared with that of a polypropylene membrane, and the aminated polyacrylonitrile-based compact ultrafiltration membrane particularly has excellent performance in the aspect of dye interception and is expected to play an important role in multiple fields of water treatment, biological pharmacy, food processing and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of membrane separation, and particularly relates to a polyacrylonitrile ultrafiltration membrane. Background Art

[0002] As a semi-crystalline polymer, polyacrylonitrile has polar groups cyano (-CN) attached to the main chain carbon atoms. This cyano group gives polyacrylonitrile certain reactivity, and it can be used to prepare hydrolyzed polyacrylonitrile through alkali hydrolysis reaction, or prepare branched aminated polyacrylonitrile through alkali-catalyzed amidation reaction. A large number of studies have shown that the water treatment membranes directly prepared from polyacrylonitrile by non-solvent induced phase inversion method are usually ultrafiltration membranes, and it is very difficult to prepare dense ultrafiltration membranes with excellent performance through regulating the phase inversion process for the fractional separation of organic dyes and inorganic salts. This phenomenon limits the application of polyacrylonitrile as a membrane-making material.

[0003] In order to obtain polyacrylonitrile-based loose nanofiltration membranes or dense ultrafiltration membranes with better separation performance, researchers generally prepare membranes by the method of post-film modification. First, polyacrylonitrile-based ultrafiltration membranes are prepared, and then polyacrylonitrile-based loose nanofiltration membranes or dense ultrafiltration membranes are obtained through surface grafting and interfacial polymerization methods. In the literature Journal of Membrane Science 643 (2022) 120035, using a polyacrylonitrile ultrafiltration membrane as the base membrane, after treating its surface by grafting diethylenetriamine in DETA and sodium carbonate solution, a polyamide separation layer is prepared by interfacial polymerization method. The polyamide separation layer is prepared by layer-by-layer polymerization method. When the membrane surface undergoes 5 interfacial polymerization reactions, the obtained loose nanofiltration membrane has a high rejection rate for organic dyes. In the literature Journal of Hazardous Materials 186 (2011) 182 - 189, branched amination modification is carried out on polyacrylonitrile nanofibers (PAN-nFS), which improves the strength of PAN-nFS nanofibers and also realizes the efficient adsorption of Cu2+. In the literature Polymer Chemistry 7.23 (2016): 3874 - 3883, using polyacrylonitrile fibers (PANF) as the matrix, N,N-bis(2-pyridylmethyl)ethylenediamine (BPEN) is used to functionalize it to prepare a polyacrylonitrile-based heavy metal ion chelating fiber (BPEN-PANF), and this fiber shows excellent affinity for common toxic heavy metal ions such as 2+ Pb 2+ Hg 2+ Cd + Ag, etc., and shows excellent affinity at a 1:1 stoichiometric ratio and can remove toxic heavy metal ions in water.

[0004] The above modification methods are all surface grafting modifications of polyacrylonitrile-based membranes or fiber materials. The surface grafting modification method is carried out after the material is formed, and can only change the surface properties of the material, such as hydrophilicity, reactivity, adsorption performance, etc. This method has limited ability to regulate the cross-sectional structure of the membrane, will make the membrane cortex become dense, and will cause the membrane flux to decrease when the rejection rate is increased. Therefore, the ability to improve the comprehensive performance of the membrane is limited. Summary of the Invention

[0005] Aiming at the technical problem that it is difficult to balance the rejection rate and membrane flux of polyacrylonitrile-based membranes, the present invention proposes a branched amine polyacrylonitrile-based dense ultrafiltration membrane, its preparation method and application. This membrane preparation method is easy to operate, has simple steps, greatly improves the membrane preparation efficiency, and can quickly prepare a high-performance polyacrylonitrile-based dense ultrafiltration membrane.

[0006] In order to achieve the above object, the technical solution of the present invention is realized as follows:

[0007] A preparation method of a branched amine polyacrylonitrile-based dense ultrafiltration membrane, comprising the following steps:

[0008] (1) Dissolve polyacrylonitrile in N,N-dimethylformamide to prepare a polyacrylonitrile solution; add an amine-containing monomer and a catalyst to the polyacrylonitrile solution for a branched amine reaction to obtain a polymer solution of branched amine polyacrylonitrile, and then centrifuge and age to obtain a branched amine polyacrylonitrile casting solution;

[0009] (2) Dissolve an organic acid in deionized water to obtain a coagulation bath of the organic acid;

[0010] (3) Cast the branched amine polyacrylonitrile casting solution to prepare a primary membrane, and immerse the primary membrane in the coagulation bath to cure to obtain a branched amine polyacrylonitrile-based dense ultrafiltration membrane;

[0011] The concentration of the polyacrylonitrile solution is 15-18 wt%; in the step (1), the molar ratio of polyacrylonitrile, amine-containing monomer and catalyst is 10:1-5:1-5.

[0012] The amine-containing monomer is diethylenetriamine, triethylenetetramine, tetraethylenepentamine, etc.; the catalyst is sodium carbonate.

[0013] The temperature of the branched amine reaction is 60-80 °C, and the time is 6-12 h.

[0014] The solid content of the branched amine polyacrylonitrile casting solution is 16-20 wt%, and the amino grafting rate of the branched amine polyacrylonitrile is 10-40%.

[0015] The organic acid is any one or two or more of citric acid, phytic acid and polyacrylic acid; the concentration of the organic acid in the coagulation bath is 0.05-1.0 wt%.

[0016] Immerse the branched aminated polyacrylonitrile-based dense ultrafiltration membrane obtained in step (3) in a metal salt solution.

[0017] The metal salt in the metal salt solution is ferric chloride, zinc chloride, or calcium chloride; the concentration of the metal salt solution is 0.01 - 0.03 mol / L.

[0018] Advantages of the present invention:

[0019] (1) In the present invention, a solution of branched aminated polyacrylonitrile (APAN) obtained by grafting and modifying the polyacrylonitrile body is used as the casting solution. The APAN nascent membrane is immersed in an organic acid solution, and the acid-base reaction between the organic acid and the branched aminated polyacrylonitrile is introduced into the non-solvent induced phase separation process. This acid-base reaction occurs concomitantly with the solidification and precipitation process of the branched aminated polyacrylonitrile, and a dense ultrafiltration membrane with a dense separation skin layer is prepared. Compared with the traditional phase separation method, this membrane preparation method can achieve synchronous regulation of the membrane structure. The dense ultrafiltration membrane prepared by this method can effectively reduce the thickness of the membrane separation skin layer. Although the flux is slightly reduced, the rejection rate of the membrane for dyes is greatly improved. The performance of the branched aminated polyacrylonitrile-based dense ultrafiltration membrane is significantly improved compared with the branched aminated polyacrylonitrile membrane prepared using water as the coagulation bath, especially showing excellent performance in dye rejection, and is expected to play an important role in multiple fields such as water treatment, biopharmaceuticals, and food processing.

[0020] (2) In the present invention, metal ion crosslinking is used to improve the stability of the dense ultrafiltration membrane. The chelation effect of metal ions such as calcium and iron with organic acids is utilized to enhance the stability of the separation membrane. Furthermore, the flux and rejection rate of the dense ultrafiltration membrane are further improved, realizing the synchronous optimization of the flux and rejection rate, thereby enhancing the comprehensive performance of the membrane. Description of the drawings

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0022] Figure 1 It is a flowchart for preparing a dense ultrafiltration membrane using branched aminated polyacrylonitrile as the base membrane material in the present invention.

[0023] Figure 2 It is an ATR-FTIR spectrum of the branched aminated polyacrylonitrile-based dense ultrafiltration membrane prepared in the present invention. Detailed implementation manners

[0024] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0025] Example 1

[0026] A method for preparing a branched aminated polyacrylonitrile-based dense ultrafiltration membrane includes the following steps:

[0027] (1) Add 16 g of PAN powder to 84 g of DMF, mechanically stir and heat to 70 °C, and dissolve for 2 h to obtain a transparent and uniform PAN solution; then add 9.3 g of diethylenetriamine and 9.6 g of sodium carbonate to the PAN solution, and mechanically stir and react at 70 °C for 8 h to obtain a polymer solution of branched aminated polyacrylonitrile (APAN) modified by bulk grafting. Centrifuge this solution once to remove sodium carbonate, and then centrifuge it a second time to remove air bubbles, and then cure at 30 °C for 12 h to obtain a branched aminated polyacrylonitrile casting solution;

[0028] (2) Dissolve phytic acid in deionized water to obtain a phytic acid aqueous solution with a concentration of 0.05 wt%, and this aqueous solution is the coagulation bath used for film preparation;

[0029] (3) Cast a primary film on the non-woven fabric with the above casting solution, the knife gap of the doctor blade is 150 μm, the film casting environment temperature is 30 °C, and the coagulation bath temperature is 30 °C; immediately immerse the primary film in the coagulation bath, then take it out of the coagulation bath and put it into deionized water to rinse off the residual organic solvent, and obtain a branched aminated polyacrylonitrile-based membrane by non-solvent induced phase separation method for film preparation;

[0030] (4) Immerse the branched aminated polyacrylonitrile-based membrane in a 0.02 M iron chloride solution for 12 h to obtain a dense ultrafiltration membrane with an ultra-thin and dense separation skin layer.

[0031] Example 2

[0032] A method for preparing a branched aminated polyacrylonitrile-based dense ultrafiltration membrane includes the following steps:

[0033] (1) Add 16 g of PAN powder to 84 g of DMF, mechanically stir and heat to 70 °C, and dissolve for 2 h to obtain a transparent and uniform PAN solution; then add 9.3 g of diethylenetriamine and 9.6 g of sodium carbonate to the PAN solution, and mechanically stir and react at 70 °C for 8 h to obtain a polymer solution of branched aminated polyacrylonitrile (APAN) modified by bulk grafting. Centrifuge this solution once to remove sodium carbonate, and then centrifuge it a second time to remove air bubbles, and then cure at 30 °C for 12 h to obtain a branched aminated polyacrylonitrile casting solution;

[0034] (2) Dissolve phytic acid in deionized water to obtain an aqueous phytic acid solution with a concentration of 0.2 wt%, which is the coagulation bath used for film preparation.

[0035] (3) Use the above-mentioned casting solution to scrape a film on the non-woven fabric to prepare a primary film. The blade gap of the scraper is 150 μm, the film scraping environment temperature is 30 °C, and the coagulation bath temperature is 30 °C; immediately immerse the primary film in the coagulation bath, then take it out from the coagulation bath and put it into deionized water to rinse off the residual organic solvent, and obtain a branched aminated polyacrylonitrile-based membrane through non-solvent induced phase separation film formation.

[0036] (4) Immerse the branched aminated polyacrylonitrile-based membrane in a ferric chloride solution with a concentration of 0.02 M for 12 h to obtain a dense ultrafiltration membrane with an ultra-thin and dense separation skin layer.

[0037] Example 3

[0038] A preparation method of a branched aminated polyacrylonitrile-based dense ultrafiltration membrane, comprising the following steps:

[0039] (1) Add 16 g of PAN powder to 84 g of DMF, mechanically stir and heat to 70 °C, and dissolve for 2 h to obtain a transparent and uniform PAN solution; then add 9.3 g of diethylenetriamine and 9.6 g of sodium carbonate to the PAN solution, and mechanically stir and react at 70 °C for 8 h to obtain a polymer solution of branched aminated polyacrylonitrile (APAN) by bulk graft modification. Centrifuge this solution once to remove sodium carbonate, and then centrifuge it a second time to remove air bubbles, and then age it at 30 °C for 12 h to obtain a branched aminated polyacrylonitrile casting solution.

[0040] (2) Dissolve phytic acid in deionized water to obtain an aqueous phytic acid solution with a concentration of 0.6 wt%, which is the coagulation bath used for film preparation.

[0041] (3) Use the above-mentioned casting solution to scrape a film on the non-woven fabric to prepare a primary film. The blade gap of the scraper is 150 μm, the film scraping environment temperature is 30 °C, and the coagulation bath temperature is 30 °C; immediately immerse the primary film in the coagulation bath, then take it out from the coagulation bath and put it into deionized water to rinse off the residual organic solvent, and obtain a branched aminated polyacrylonitrile-based membrane through non-solvent induced phase separation film formation.

[0042] (4) Immerse the branched aminated polyacrylonitrile-based membrane in a ferric chloride solution with a concentration of 0.02 M for 12 h to obtain a dense ultrafiltration membrane with an ultra-thin and dense separation skin layer.

[0043] Example 4

[0044] A preparation method of a branched aminated polyacrylonitrile-based dense ultrafiltration membrane, comprising the following steps:

[0045] (1) Add 16 g of PAN powder to 84 g of DMF, stir mechanically and heat to 70 °C. After 2 h of dissolution, a transparent and uniform PAN solution is obtained. Then add 9.3 g of diethylenetriamine and 9.6 g of sodium carbonate to the PAN solution, and react under mechanical stirring at 70 °C for 8 h to obtain a polymer solution of graft-modified polyacrylonitrile with pendant amine groups (APAN). Centrifuge this solution once to remove sodium carbonate, then centrifuge a second time to remove air bubbles, and then age at 30 °C for 12 h to obtain a casting solution of polyacrylonitrile with pendant amine groups;

[0046] (2) Dissolve phytic acid in deionized water to obtain an aqueous phytic acid solution with a concentration of 1.0 wt%, and this aqueous solution is the coagulation bath used for film preparation;

[0047] (3) Cast a primary film on the non-woven fabric with the above casting solution. The gap of the doctor blade is 150 μm, the temperature of the casting environment is 30 °C, and the temperature of the coagulation bath is 30 °C. Immediately immerse the primary film in the coagulation bath, then take it out of the coagulation bath and put it into deionized water to rinse off the residual organic solvent, and obtain a polyacrylonitrile-based membrane with pendant amine groups by non-solvent induced phase separation method;

[0048] (4) Immerse the polyacrylonitrile-based membrane with pendant amine groups in a 0.02 M iron chloride solution for 12 h to obtain a dense ultrafiltration membrane with an ultrathin and dense separation skin layer.

[0049] Example 5

[0050] A method for preparing a dense ultrafiltration membrane based on polyacrylonitrile with pendant amine groups, comprising the following steps:

[0051] (1) Add 16 g of PAN powder to 84 g of DMF, stir mechanically and heat to 70 °C. After 2 h of dissolution, a transparent and uniform PAN solution is obtained. Then add 9.3 g of diethylenetriamine and 9.6 g of sodium carbonate to the PAN solution, and react under mechanical stirring at 70 °C for 8 h to obtain a polymer solution of graft-modified polyacrylonitrile with pendant amine groups (APAN). Centrifuge this solution once to remove sodium carbonate, then centrifuge a second time to remove air bubbles, and then age at 30 °C for 12 h to obtain a casting solution of polyacrylonitrile with pendant amine groups;

[0052] (2) Dissolve phytic acid in deionized water to obtain an aqueous phytic acid solution with a concentration of 0.2 wt%, and this aqueous solution is the coagulation bath used for film preparation;

[0053] (3) Cast a primary film on the non-woven fabric with the above casting solution. The gap of the doctor blade is 150 μm, the temperature of the casting environment is 30 °C, and the temperature of the coagulation bath is 30 °C. Immediately immerse the primary film in the coagulation bath, then take it out of the coagulation bath and put it into deionized water to rinse off the residual organic solvent, and obtain a polyacrylonitrile-based membrane with pendant amine groups by non-solvent induced phase separation method;

[0054] (4) Immerse the branched aminated polyacrylonitrile-based membrane in a zinc chloride solution with a concentration of 0.02 M for 12 h to obtain a dense ultrafiltration membrane with an ultrathin and dense separation skin layer.

[0055] Example 6

[0056] A method for preparing a branched aminated polyacrylonitrile-based dense ultrafiltration membrane, comprising the following steps:

[0057] (1) Add 16 g of PAN powder to 84 g of DMF, mechanically stir and heat to 70 °C, and dissolve for 2 h to obtain a transparent and uniform PAN solution; then add 9.3 g of diethylenetriamine and 9.6 g of sodium carbonate to the PAN solution, and mechanically stir and react at 70 °C for 8 h to obtain a polymer solution of branched aminated polyacrylonitrile (APAN) with bulk graft modification. Centrifuge this solution once to remove sodium carbonate, then centrifuge a second time to remove air bubbles, and then age at 30 °C for 12 h to obtain a branched aminated polyacrylonitrile casting solution;

[0058] (2) Dissolve phytic acid in deionized water to obtain a phytic acid aqueous solution with a concentration of 0.2 wt%, and this aqueous solution is the coagulation bath used for membrane preparation;

[0059] (3) Cast a primary membrane on non-woven fabric with the above casting solution, the knife gap of the blade is 150 μm, the casting environment temperature is 30 °C, and the coagulation bath temperature is 30 °C; immediately immerse the primary membrane in the coagulation bath, then take it out from the coagulation bath and put it into deionized water to rinse off the residual organic solvent, and obtain a branched aminated polyacrylonitrile-based membrane by non-solvent induced phase separation membrane preparation;

[0060] (4) Immerse the branched aminated polyacrylonitrile-based membrane in a calcium chloride solution with a concentration of 0.02 M for 12 h to obtain a dense ultrafiltration membrane with an ultrathin and dense separation skin layer.

[0061] Example 7

[0062] A method for preparing a branched aminated polyacrylonitrile-based dense ultrafiltration membrane, comprising the following steps:

[0063] (1) Add 16 g of PAN powder to 84 g of DMF, mechanically stir and heat to 70 °C, and dissolve for 2 h to obtain a transparent and uniform PAN solution; then add 9.3 g of diethylenetriamine and 9.6 g of sodium carbonate to the PAN solution, and mechanically stir and react at 70 °C for 8 h to obtain a polymer solution of branched aminated polyacrylonitrile (APAN) with bulk graft modification. Centrifuge this solution once to remove sodium carbonate, then centrifuge a second time to remove air bubbles, and then age at 30 °C for 12 h to obtain a branched aminated polyacrylonitrile casting solution;

[0064] (2) Dissolve polyacrylic acid (average molecular weight M.W 3000) in deionized water to obtain an aqueous polyacrylic acid solution with a concentration of 0.05 wt%, which is the coagulation bath used for film formation;

[0065] (3) Use the above casting solution to scrape a film on the non-woven fabric to prepare a nascent film. The blade gap of the scraper is 150 μm, the film scraping environment temperature is 30 °C, and the coagulation bath temperature is 30 °C; Immediately immerse the nascent film in the coagulation bath, then take it out from the coagulation bath and put it into deionized water to wash away the residual organic solvents, and obtain a branched aminated polyacrylonitrile-based membrane by non-solvent induced phase separation method for film formation;

[0066] (4) Immerse the branched aminated polyacrylonitrile-based membrane in a ferric chloride solution with a concentration of 0.02 M for 12 h to obtain a dense ultrafiltration membrane with an ultra-thin and dense separation skin layer.

[0067] Example 8

[0068] A preparation method of a branched aminated polyacrylonitrile-based dense ultrafiltration membrane, comprising the following steps:

[0069] (1) Add 16 g of PAN powder to 84 g of DMF, mechanically stir and heat to 70 °C, and dissolve for 2 h to obtain a transparent and uniform PAN solution; Then add 9.3 g of diethylenetriamine and 9.6 g of sodium carbonate to the PAN solution, and mechanically stir and react at 70 °C for 8 h to obtain a polymer solution of branched aminated polyacrylonitrile (APAN) with bulk graft modification. Centrifuge this solution once to remove sodium carbonate, and then centrifuge it a second time to remove air bubbles, and then cure it at 30 °C for 12 h to obtain a branched aminated polyacrylonitrile casting solution;

[0070] (2) Dissolve polyacrylic acid (average molecular weight M.W 3000) in deionized water to obtain an aqueous polyacrylic acid solution with a concentration of 0.6 wt%, which is the coagulation bath used for film formation;

[0071] (3) Use the above casting solution to scrape a film on the non-woven fabric to prepare a nascent film. The blade gap of the scraper is 150 μm, the film scraping environment temperature is 30 °C, and the coagulation bath temperature is 30 °C; Immediately immerse the nascent film in the coagulation bath, then take it out from the coagulation bath and put it into deionized water to wash away the residual organic solvents, and obtain a branched aminated polyacrylonitrile-based membrane by non-solvent induced phase separation method for film formation;

[0072] (4) Immerse the branched aminated polyacrylonitrile-based membrane in a zinc chloride solution with a concentration of 0.02 M for 12 h to obtain a dense ultrafiltration membrane with an ultra-thin and dense separation skin layer.

[0073] Example 9

[0074] A preparation method of a branched aminated polyacrylonitrile-based dense ultrafiltration membrane, comprising the following steps:

[0075] (1) Add 16 g of PAN powder to 84 g of DMF, stir mechanically and heat to 70 °C. After a dissolution time of 2 h, a transparent and homogeneous PAN solution is obtained. Then add 9.3 g of diethylenetriamine and 9.6 g of sodium carbonate to the PAN solution, and react under mechanical stirring at 70 °C for 8 h to obtain a polymer solution of graft-modified branched polyacrylonitrile (APAN) by bulk grafting. Centrifuge this solution once to remove sodium carbonate, then centrifuge it a second time to remove air bubbles, and then age it at 30 °C for 12 h to obtain a branched polyacrylonitrile casting solution;

[0076] (2) Dissolve polyacrylic acid (average molecular weight M.W 3000) in deionized water to obtain an aqueous polyacrylic acid solution with a concentration of 1.0 wt%, and this aqueous solution is the coagulation bath used for film preparation;

[0077] (3) Use the above casting solution to scrape a film on the non-woven fabric to prepare a nascent film. The gap of the doctor blade is 150 μm, the temperature of the film scraping environment is 30 °C, and the temperature of the coagulation bath is 30 °C; Immediately immerse the nascent film in the coagulation bath, then take it out from the coagulation bath and put it into deionized water to rinse off the residual organic solvent, and obtain a branched polyacrylonitrile base film by non-solvent induced phase separation film formation;

[0078] (4) Immerse the branched polyacrylonitrile base film in a 0.02 M iron chloride solution for 12 h to obtain a dense ultrafiltration membrane with an ultrathin and dense separation skin layer.

[0079] Example 10

[0080] A method for preparing a dense ultrafiltration membrane based on branched polyacrylonitrile, comprising the following steps:

[0081] (1) Add 15 g of PAN powder to 88 g of DMF, stir mechanically and heat to 60 °C. After a dissolution time of 2 h, a transparent and homogeneous PAN solution is obtained. Then add 4.5 g of triethylenetetramine and 15 g of sodium carbonate to the PAN solution, and react under mechanical stirring at 60 °C for 12 h to obtain a polymer solution of graft-modified branched polyacrylonitrile (APAN) by bulk grafting. Centrifuge this solution once to remove sodium carbonate, then centrifuge it a second time to remove air bubbles, and then age it at 30 °C for 12 h to obtain a branched polyacrylonitrile casting solution;

[0082] (2) Dissolve citric acid in deionized water to obtain an aqueous citric acid solution with a concentration of 0.05 wt%, and this aqueous solution is the coagulation bath used for film preparation;

[0083] (3) Using the above casting solution to cast a film on non-woven fabric to prepare a nascent film. The knife gap of the scraper is 150 μm, the film casting environment temperature is 30 °C, and the coagulation bath temperature is 30 °C. Immediately immerse the nascent film in the coagulation bath, then take it out from the coagulation bath and put it into deionized water for rinsing to remove the residual organic solvent. A branched aminated polyacrylonitrile-based membrane is obtained by non-solvent induced phase separation membrane formation method;

[0084] (4) Immerse the branched aminated polyacrylonitrile-based membrane in a ferric chloride solution with a concentration of 0.02 M for 12 h to obtain a dense ultrafiltration membrane with an ultrathin and dense separation skin layer.

[0085] Example 11

[0086] A preparation method of a branched aminated polyacrylonitrile-based dense ultrafiltration membrane, comprising the following steps:

[0087] (1) Add 18 g of PAN powder to 82 g of DMF, mechanically stir and heat to 70 °C, and dissolve for 2 h to obtain a transparent and uniform PAN solution. Then add 9.3 g of tetraethylenepentamine and 9.6 g of sodium carbonate to the PAN solution, and mechanically stir and react at 80 °C for 6 h to obtain a polymer solution of branched aminated polyacrylonitrile (APAN) with bulk graft modification. Centrifuge this solution once to remove sodium carbonate, then centrifuge it a second time to remove air bubbles, and then age it at 30 °C for 12 h to obtain a branched aminated polyacrylonitrile casting solution;

[0088] (2) Dissolve citric acid in deionized water to obtain a citric acid aqueous solution with a concentration of 0.6 wt%, and this aqueous solution is the coagulation bath used for membrane preparation;

[0089] (3) Using the above casting solution to cast a film on non-woven fabric to prepare a nascent film. The knife gap of the scraper is 150 μm, the film casting environment temperature is 30 °C, and the coagulation bath temperature is 30 °C. Immediately immerse the nascent film in the coagulation bath, then take it out from the coagulation bath and put it into deionized water for rinsing to remove the residual organic solvent. A branched aminated polyacrylonitrile-based membrane is obtained by non-solvent induced phase separation membrane formation method;

[0090] (4) Immerse the branched aminated polyacrylonitrile-based membrane in a zinc chloride solution with a concentration of 0.02 M for 12 h to obtain a dense ultrafiltration membrane with an ultrathin and dense separation skin layer.

[0091] Example 12

[0092] A preparation method of a branched aminated polyacrylonitrile-based dense ultrafiltration membrane, comprising the following steps:

[0093] (1) Add 16 g of PAN powder to 84 g of DMF, stir mechanically and heat to 70 °C, and dissolve for 2 h to obtain a transparent and uniform PAN solution; then add 9.3 g of diethylenetriamine and 3.6 g of sodium carbonate to the PAN solution, and react with mechanical stirring at 70 °C for 8 h to obtain a polymer solution of amine-grafted polyacrylonitrile (APAN) by bulk graft modification. Centrifuge the solution once to remove sodium carbonate, then centrifuge a second time to remove air bubbles, and then age at 30 °C for 12 h to obtain an amine-grafted polyacrylonitrile casting solution;

[0094] (2) Dissolve citric acid in deionized water to obtain a citric acid aqueous solution with a concentration of 1.0 wt%, which is the coagulation bath used for film preparation;

[0095] (3) Use the above casting solution to scrape a film on non-woven fabric to prepare a nascent film. The gap of the doctor blade is 150 μm, the film scraping environment temperature is 30 °C, and the coagulation bath temperature is 30 °C; immediately immerse the nascent film in the coagulation bath, then take it out of the coagulation bath and put it into deionized water to rinse off the residual organic solvent, and obtain an amine-grafted polyacrylonitrile substrate membrane by non-solvent induced phase separation method for film preparation;

[0096] (4) Immerse the amine-grafted polyacrylonitrile substrate membrane in a ferric chloride solution with a concentration of 0.02 M for 12 h to obtain a dense ultrafiltration membrane with an ultrathin and dense separation skin layer.

[0097] Comparative Example 1

[0098] A preparation method of an amine-grafted polyacrylonitrile-based dense ultrafiltration membrane, the preparation process is as Figure 1 shown, including the following steps:

[0099] (1) Add 16 g of PAN powder to 84 g of DMF, stir mechanically and heat to 70 °C, and dissolve for 2 h to obtain a transparent and uniform PAN solution; then add 9.3 g of diethylenetriamine and 9.6 g of sodium carbonate to the PAN solution, and react with mechanical stirring at 70 °C for 8 h to obtain a polymer solution of amine-grafted polyacrylonitrile (APAN) by bulk graft modification. Centrifuge the solution once to remove sodium carbonate, then centrifuge a second time to remove air bubbles, and then age at 30 °C for 12 h to obtain an amine-grafted polyacrylonitrile casting solution;

[0100] (2) Use the above casting solution to scrape a film on non-woven fabric to prepare a nascent film. The gap of the doctor blade is 150 μm, the film scraping environment temperature is 30 °C, and the coagulation bath temperature is 30 °C; immediately immerse the nascent film in water, then take it out of the water, and obtain an amine-grafted polyacrylonitrile substrate membrane by non-solvent induced phase separation method for film preparation;

[0101] Test Example

[0102] Figure 2ATR-FTIR spectra of branched amine-modified polyacrylonitrile-based dense ultrafiltration membranes, including polyacrylonitrile, polyacrylonitrile grafted with diethylenetriamine in Comparative Example 1, branched amine-modified polyacrylonitrile-based dense ultrafiltration membranes prepared with phytic acid as the coagulation bath in Example 2, polyacrylic acid as the coagulation bath in Example 7, and citric acid as the coagulation bath in Example 10.

[0103] Separation performance test of dense ultrafiltration membranes: Cut the membranes into circles and install them on a cross-flow test cell with an effective test area of 3.14 cm 2 . The test temperature is 25 °C and the operating test pressure is 0.4 MPa. The membranes need to be pre-pressed for 1 h under the above pressure before the formal test. The performance is mainly investigated in terms of pure water flux, dye solution flux, and dye rejection rate.

[0104] Dye solution flux: The dye solution permeation flux (Flux, F) of the membrane can be measured by calculating the volume of water passing through the effective area of the membrane per unit time, with the unit of L·m -2 ·h -1 . The formula is as follows:

[0105]

[0106] where V is the volume of water passing through the effective area of the membrane per unit time, A is the effective area of the membrane, and t is the time.

[0107] Dye rejection rate: Under the condition of an operating pressure of 0.4 MPa, when the concentration of Congo red is 50 mg·L -1 , the dye rejection of the membrane is tested. The rejection rate (Rejection, R) is calculated by the following formula:

[0108]

[0109] where C p and C f are the dye concentrations in the permeate and the mother liquor, respectively. Each set of separation performance data is the average data of three simultaneously tested membrane samples. The dye concentration is measured with an ultraviolet spectrophotometer. The dye is mainly Congo red (CR). Then, by collecting the dye solution that has permeated through the membrane and calculating the dye flux and rejection rate of the membrane according to the formula, the dense ultrafiltration membranes prepared in Examples 1-12 and Comparative Example 1 are tested and denoted as S1-S12 and D1 in sequence. The test data results are shown in Table 1.

[0110] Table 1 Rejection rates of branched amine-modified polyacrylonitrile-based dense ultrafiltration membranes

[0111]

[0112] As can be seen from Comparative Example 1, a dense ultrafiltration membrane was prepared using APAN as the membrane material and deionized water as the coagulation bath. The water flux of this membrane was 699 L·m -2 ·h -2 , and the dye rejection was 40.8%.

[0113] As can be seen from the comparison between Example 2 and Comparative Example 1, when a 0.2 wt% aqueous phytic acid solution was used as the coagulation bath to prepare the branched aminated polyacrylonitrile-based dense ultrafiltration membrane, the flux of the membrane decreased by 411 L·m -2 ·h -2 , and the rejection increased by 58.74%;

[0114] As can be seen from the comparison between Example 7 and Comparative Example 1, when a 0.05 wt% aqueous polyacrylic acid solution was used as the coagulation bath to prepare the branched aminated polyacrylonitrile-based dense ultrafiltration membrane, the flux of the membrane decreased by 271 L·m -2 ·h -2 , and the rejection increased by 53.51%;

[0115] As can be seen from the comparison between Example 11 and Comparative Example 1, when a 0.6 wt% aqueous citric acid solution was used as the coagulation bath to prepare the branched aminated polyacrylonitrile-based dense ultrafiltration membrane, the flux of the membrane decreased by 395 L·m -2 ·h -2 , and the rejection increased by 57%;

[0116] As can be seen from the comparison between Example 2, 5, 6, 8, 11 and Comparative Example 1, when the prepared dense ultrafiltration membrane was immersed in different metal salt solutions for 12 h, the flux of the membrane slightly decreased compared with the dense ultrafiltration membranes prepared by other coagulation baths, but the rejection rate of Congo red also reached a maximum of 99.5%;

[0117] Through further optimization of the membrane preparation conditions, when the prepared dense ultrafiltration membrane was immersed in different metal salt solutions for 12 h, it was significantly found that due to the chelation of phytic acid and metal ions, both the maximum flux and the highest rejection rate of the dense ultrafiltration membrane were improved.

[0118] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a branched aminated polyacrylonitrile-based dense ultrafiltration membrane, characterized in that: The following steps are involved: (1) dissolving polyacrylonitrile in N,N-dimethylformamide to prepare a polyacrylonitrile solution; Amine-containing monomers and catalysts are added to a polyacrylonitrile solution to carry out a branched amination reaction to obtain a branched aminated polyacrylonitrile polymer solution, which is then centrifuged and aged to obtain a branched aminated polyacrylonitrile casting solution; (2) dissolving the organic acid in deionized water to obtain a coagulation bath of the organic acid; (3) The branched aminated polyacrylonitrile casting solution is scraped to prepare a primary membrane, and the primary membrane is immersed in a coagulation bath to solidify to obtain a branched aminated polyacrylonitrile-based dense ultrafiltration membrane.

2. The method for preparing the branched aminated polyacrylonitrile-based dense ultrafiltration membrane according to claim 1, characterized in that: The concentration of the polyacrylonitrile solution is 15-18 wt %; the molar ratio of polyacrylonitrile, amine-containing monomer and catalyst in step (1) is 10:1-5:1-5.

3. The method for preparing the branched aminated polyacrylonitrile-based dense ultrafiltration membrane according to claim 2, characterized in that: The amine-containing monomer is diethylenetriamine, triethylenetetramine or tetraethylenepentamine; and the catalyst is sodium carbonate.

4. The method for preparing the branched aminated polyacrylonitrile-based dense ultrafiltration membrane according to claim 3, characterized in that: The branched amination reaction is carried out at a temperature of 60-80° C. and for a time of 6-12 hours.

5. The method for preparing the branched aminated polyacrylonitrile-based dense ultrafiltration membrane according to claim 1, characterized in that: The solid content of the branched aminated polyacrylonitrile casting solution is 16-20wt%, and the amino grafting rate of the branched aminated polyacrylonitrile is 10-40%.

6. The method for preparing the branched aminated polyacrylonitrile-based dense ultrafiltration membrane according to claim 1, characterized in that: The organic acid is any one or more of citric acid, phytic acid and polyacrylic acid; the concentration of the organic acid in the coagulation bath is 0.05-1.0 wt %.

7. The method for preparing the branched aminated polyacrylonitrile-based dense ultrafiltration membrane according to any one of claims 1 to 6, characterized in that: The branched aminated polyacrylonitrile-based dense ultrafiltration membrane obtained in step (3) is immersed in a metal salt solution.

8. The method for preparing the branched aminated polyacrylonitrile-based dense ultrafiltration membrane according to claim 7, characterized in that: The metal salt in the metal salt solution is ferric chloride, zinc chloride and calcium chloride; the concentration of the metal salt solution is 0.01-0.03 mol / L.

9. A branched aminated polyacrylonitrile-based dense ultrafiltration membrane prepared according to the preparation method according to any one of claims 1 to 8.

10. Use of the branched aminated polyacrylonitrile-based dense ultrafiltration membrane according to claim 9 in wastewater treatment.

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