CN115364680B - Alkali-resistant nanofiltration membrane and preparation method and application thereof - Google Patents

Abstract

The invention discloses an alkali-resistant nanofiltration membrane, a preparation method and application thereof, wherein aminated lignin is added in a water phase, and the lignin contains a large amount of hydroxyl groups, so that hydrogen bonds are easily formed between the lignin and polyamide, amino groups and the like or between the lignin and the polyamide, thereby improving the heat stability of the integral polyamide, increasing the energy barrier of alkali hydrolysis and improving the alkali resistance of the integral polyamide.

Description

Alkali-resistant nanofiltration membrane and preparation method and application thereof

Technical Field

The invention belongs to the technical field of membrane separation, and in particular relates to an alkali-resistant nanofiltration membrane and a preparation method thereof.

Background Art

The continuous development of the modern chemical industry, food industry, beverage industry and pharmaceutical industry has generated a large amount of wastewater, which needs to be further treated before it can be discharged. At the same time, the wastewater contains some nutrients or metal ions, which need to be recycled and reused. Membrane separation technology is a modern separation technology based on pore size screening, dielectric exclusion, steric hindrance or one or more of the main separation mechanisms. It has received widespread attention due to its low cost, high efficiency and environmental friendliness.

Nanofiltration membrane is an important separation membrane in water treatment. Due to its excellent separation performance, it is widely used in seawater desalination, dye industry, and polysalt wastewater. In many production processes, a certain amount of alkali needs to be added to adjust the alkalinity, so the alkalinity of the discharged wastewater is extremely high, which requires the separation membrane to have a certain alkali resistance. The traditional nanofiltration membrane selection layer is mainly composed of polyamide, and the operating pH is generally below 12. It is susceptible to alkaline hydrolysis under highly alkaline conditions, destroying the carbon-based skeleton structure and reducing the separation performance. Therefore, the research and development of alkali-resistant polyamide nanofiltration membranes is of great significance and has broad application prospects.

Since the pure polyamide structure is not alkali-resistant enough, its application in complex alkaline wastewater has the problem of low treatment efficiency. Therefore, it is necessary to use other alkali-resistant materials to prepare the nanofiltration membrane selection layer for application in complex alkaline water systems. In recent years, researchers have prepared acid- and alkali-resistant nanofiltration membranes by using substances such as cyanate esters, melamine, and polyurea, and they can still maintain excellent stability under long-term operation. However, in the process of using these highly toxic materials, the problem of incomplete reaction is inevitable. They are embedded in the membrane and may fall off over time and flow into the water, causing secondary pollution to the water quality.

Summary of the invention

The purpose of this section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

In view of the above problems and/or the problems existing in the prior art, the present invention is proposed.

Therefore, the purpose of the present invention is to overcome the deficiencies in the prior art and provide an alkali-resistant nanofiltration membrane.

To solve the above technical problems, the present invention provides the following technical solution: the separation layer of the nanofiltration membrane has a bubble structure and long-term alkali resistance, and the supporting layer is a pure high molecular porous polymer membrane, or a high molecular porous polymer membrane with a non-woven fabric as a lining.

As a preferred embodiment of the alkali-resistant nanofiltration membrane of the present invention, the nanofiltration membrane is mainly composed of nano-aminated lignin particles, and the nano-aminated lignin particles are cross-linked with a dense polyamide layer, so that the separation layer structure is more dense and stable. The pore size is less than 100nm, preferably less than 50nm, and more preferably less than 10nm.

As a preferred embodiment of the alkali-resistant nanofiltration membrane described in the present invention, the alkali-resistant nanofiltration membrane can stably operate for more than 168 hours under the condition of pH 13.

Another object of the present invention is to overcome the deficiencies in the prior art and provide a method for preparing an alkali-resistant nanofiltration membrane.

In order to solve the above technical problems, the present invention provides the following technical solutions: a high molecular polymer and a pore-forming additive are dissolved in an organic solvent according to a certain temperature and heated and stirred to form a homogeneous mixed solution, namely, a casting solution; after the casting solution is statically degassed (12 to 24 hours), it is scraped on a glass plate or a non-woven fabric with a micron-grade scraper. The non-woven fabric needs to be flattened and fixed on the glass plate first, and then immersed in deionized water for 3 to 5 minutes after staying in the air for 0 to 30 seconds to form a membrane, thereby forming a support layer; an amino group is added to the solution to form a film; the film is then immersed in deionized water for 3 to 5 minutes to form a film; the film is then immersed in deionized water for 3 to 5 minutes to form a support layer ... subjected to static degassing (12 to 24 hours). A certain amount of piperazine is added to the alkaline solution of wood (pH is 10-12) to form a homogeneous solution, which is called the water phase; a certain amount of trimesoyl chloride (TMC) is dissolved in n-hexane to form a homogeneous solution, which is called the oil phase; first, the support layer wiped dry with dust-free paper is immersed in the water phase for a certain period of time, the support layer is taken out, and the excess water droplets on the surface are blown dry with a hair dryer; finally, a certain amount of oil phase is poured into the support layer immersed in the water phase, and after a certain period of time, the membrane is taken out, and after the organic solvent on the surface is evaporated, it is immersed in deionized water for storage.

As a preferred embodiment of the method for preparing an alkali-resistant nanofiltration membrane described in the present invention, the casting liquid is a homogeneous solution formed by a polymer, a pore-forming additive and an organic solvent; or a homogeneous solution formed by a polymer and an organic solvent, and the dissolution temperature is 60 to 90°C and the dissolution time is 12 to 24 hours.

As a preferred embodiment of the method for preparing an alkali-resistant nanofiltration membrane described in the present invention, the high molecular polymer is one or more of polyethersulfone, polysulfone, polyacrylonitrile, polyimide, polybenzimidazole, polyphenylene ether, aromatic polyamide, polyvinylidene fluoride, etc.; the pore-forming additive is one or more of polyvinyl pyrrolidone, polyethylene glycol, polyvinyl alcohol, propylene glycol, etc.; the organic solvent is one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), etc.

As a preferred embodiment of the method for preparing an alkali-resistant nanofiltration membrane according to the present invention, the thickness of the scraper is one of 50 μm, 100 μm, 150 μm and 250 μm; the support layer has a certain retention capacity for some macromolecular substances.

As a preferred embodiment of the method for preparing an alkali-resistant nanofiltration membrane of the present invention, the aqueous phase is prepared by dissolving aminated lignin and piperazine in an alkaline solution of pH 12, stirring with a magnetic force for 1 to 3 hours, and then ultrasonically treating for 1 to 2 hours;

Preparation of oil phase: Dissolve trimesoyl chloride in n-hexane and stir with magnetic force for 1-3h to form a homogeneous solution; pour the water phase onto the support layer and keep it there for 2min; pour the oil phase onto the support layer and keep it there for 1min

Another object of the present invention is to overcome the deficiencies in the prior art and provide an application of an alkali-resistant nanofiltration membrane.

In order to solve the above technical problems, the present invention provides the following technical solutions: The nanofiltration membrane is used in the food industry, chemical industry, sewage treatment or biochemical industry.

Beneficial effects of the present invention:

(1) The present invention adds aminated lignin to the aqueous phase. Lignin contains a large number of hydroxyl groups, which can easily form hydrogen bonds with polyamide, amino groups, etc. or with itself, thereby improving the thermal stability of the overall polyamide, increasing the energy barrier for alkaline hydrolysis, and improving its alkali resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

FIG. 1 is an X-ray photoelectron spectrum of the aminated lignin prepared in Example 3 of the present invention and the raw material lignin.

FIG. 2 is an infrared total reflection spectra of the films prepared in Examples 1, 2, and 3 of the present invention and a pure PES film.

FIG3 is a graph showing the flux and sodium sulfate retention changes of the nanofiltration membranes prepared in Examples 1 and 3 of the present invention after being immersed in pH 13 for eight days.

FIG4 is a front scanning electron microscope image of the nanofiltration membrane prepared in Example 3 of the present invention.

DETAILED DESCRIPTION

In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the embodiments of the specification.

In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

Example 1

Weigh 0.5 g of piperazine and dissolve it in 50 g of alkaline solution (pH 12), stir it with magnetic force for 1 hour, and then ultrasonicate it for 1.5 hours to prepare the water phase; weigh 0.05 g of TMC and dissolve it in 50 g of n-hexane, stir it for 1 hour until it is completely dissolved, and then prepare the oil phase for later use;

First, pour the water phase onto the support layer, let it stand for 2 minutes, then take out the membrane and absorb the excess water phase with dust-free paper. Then pour a certain amount of oil phase onto the support layer, let it stand for 1 minute, take out the membrane and let it dry naturally. The alkali-resistant nanofiltration preparation is completed.

The prepared nanofiltration membrane was subjected to an alkali resistance test under pH 13. As shown in Figure 3, M0 showed that its flux was 6.2 and 8.0 LMH/bar on the first and second days, respectively, and reached 35 LMH/bar on the third day. The corresponding sodium sulfate retention dropped from 95% and 92% to 8%, indicating that the surface polyamide structure was severely damaged.

Example 2

Weigh 0.5 g of amino lignin and dissolve it in 50 g of alkaline solution (pH 12), stir it with magnetic force for 1 hour, and then ultrasonicate it for 1.5 hours to prepare the water phase; weigh 0.05 g of TMC and dissolve it in 50 g of n-hexane, stir it for 1 hour until it is completely dissolved, and prepare the oil phase for later use;

First, pour the water phase onto the support layer, let it stand for 2 minutes, then take out the membrane and absorb the excess water phase with dust-free paper. Then pour a certain amount of oil phase onto the support layer, let it stand for 1 minute, take out the membrane and let it dry naturally. The alkali-resistant nanofiltration preparation is completed.

The prepared nanofiltration membrane was tested for sodium sulfate flux and retention, and it was found that its flux was as high as 353LMH/bar, and the retention of sodium sulfate was 0%. Compared with the pure PES-based membrane, the flux only changed slightly, indicating that there was not enough polyamide on the surface to form a dense separation layer.

Example 3

The present invention provides a method for preparing an alkali-resistant nanofiltration membrane, comprising:

(1) Preparation of aminated lignin: Weigh 5 g of dealkalized lignin and dissolve it in 50 ml of 0.4 mol/l sodium hydroxide. Heat it in an oil bath and dissolve it until the solution is dark black and has no precipitation. Then add 2 ml of diethylenetriamine (DETA) and continue to dissolve for 30 min. After the oil bath temperature stabilizes at 90 °C for 10 min, use a syringe to gradually inject 2 ml of acetaldehyde solution (37%) by dropwise addition. After reacting for 4 to 6 h, take out the reaction solution, cool it to room temperature, adjust the pH with HCl until the solution turns brown (pH is 3 to 5), let the reaction solution stand and precipitate until the upper clear liquid appears, and then perform vacuum filtration. The pore size of the filter paper is 0.4 μm. Wash the filter cake layer three times with deionized water and ethanol to remove excess DETA and other soluble impurities. After the vacuum filtration is completed, take out the filter cake and dry it (60 °C) for 12 h for standby use.

(2) Weigh 0.25 g of aminated lignin and 0.75 g of piperazine and dissolve them in 50 g of alkaline solution (pH 12), stir them with a magnetic force for 1 h, and then ultrasonically treat them for 1.5 h until no more particles precipitate, thereby forming an aqueous phase; weigh 0.05 g of TMC and dissolve them in 50 g of n-hexane, stir them for 1 h until they are completely dissolved, and then prepare an oil phase for later use;

(3) First, pour the water phase onto the support layer, let it stand for 2 minutes, then take out the membrane and absorb the excess water phase with dust-free paper. Then, pour a certain amount of oil phase onto the support layer, let it stand for 1 minute, then take out the membrane and let it dry naturally. The alkali-resistant nanofiltration preparation is completed.

The calculation formula of membrane flux (J) is: J = V/(T×A); where: J--membrane flux (ml/cm ² ·s); V--sampling volume (ml); T--sampling time (s); A--membrane effective area (cm ² );

As shown in FIG1 , the prepared aminated lignin powder and dealkalized lignin were subjected to X-ray photoelectron spectroscopy (XPS) analysis, and it was found that there was a peak of nitrogen element (at 400 eV) with a content of 5%; the prepared nanofiltration membrane was subjected to infrared spectroscopy (model Nicolet iS50) analysis, and it was found that it had a characteristic peak at 3670 cm ^-1 , which was the characteristic peak of hydroxyl, and the peak at 1640 cm ^-1 was the characteristic peak of polyamide, indicating the existence of polyamide structure and successful cross-linking to aminated lignin ( FIG2 ); the prepared nanofiltration membrane was subjected to alkali resistance test, as shown in M1 in FIG3 , and it was found that after being immersed in pH 13 for eight days, the flux was 7.0 LMH/bar, and the retention of sodium sulfate was maintained at 90%, indicating that the cross-linked polyamide structure can still remain relatively intact without being seriously damaged by alkaline hydrolysis.

Example 4

The preparation of aminated lignin is the same as that in Example 3.

Weigh 0.125 g of aminated aminated lignin and 0.5 g of piperazine and dissolve them in 50 g of alkaline solution (pH 12), stir them with magnetic force for 1 h, and then ultrasonically treat them for 1.5 h to make the mixed solution completely homogeneous and configure it into an aqueous phase; weigh 0.05 g of TMC and dissolve it in 50 g of n-hexane, stir it for 1 h, configure it into an oil phase, and set it aside;

First, pour the water phase onto the support layer, let it stand for 2 minutes, then take out the membrane and absorb the excess water phase with dust-free paper. Then pour a certain amount of oil phase onto the support layer, let it stand for 1 minute, then take out the membrane and dry it naturally to prepare an alkali-resistant nanofiltration membrane.

The prepared nanofiltration membrane was immersed in an alkaline solution of pH 13 for 7 days, and the retention and flux of the test membrane for 1000 ppm sodium sulfate were observed every day. The test results showed that the flux of the membrane was 9.2LMH/bar on the seventh day, and the retention of sodium sulfate was 89%.

Example 5

The preparation of aminated lignin is the same as in Example 3.

Weigh 0.25g of aminated lignin and 0.5g of piperazine and dissolve them in 50g of alkaline solution (pH 12), stir them with magnetic force for 1h, and then ultrasonically treat them for 1.5h to make the mixed solution completely homogeneous to form an aqueous phase; weigh 0.05g of TMC and dissolve it in 50g of n-hexane, stir for 1h, and form an oil phase for standby use;

First, pour the water phase onto the support layer, let it stand for 2 minutes, then take out the membrane and absorb the excess water phase with dust-free paper. Then pour a certain amount of oil phase onto the support layer, let it stand for 1 minute, take out the membrane and dry it naturally to prepare alkali-resistant nanofiltration.

The prepared nanofiltration membrane was placed in an alkaline solution of pH 13, and the flux and retention of the membrane for 1000 ppm sodium sulfate were tested daily. The results showed that the flux changed from 6.7 LMH/bar on the first day to 7.2 LMH/bar on the seventh day, and the retention changed from 95% to 90%.

Example 6

The preparation of aminated lignin is the same as in Example 3.

0.375 g of aminated lignin and 0.5 g of piperazine were weighed and dissolved in 50 g of alkaline solution (pH 12), stirred with magnetic force for 1 h, and then ultrasonically treated for 1.5 h to make the mixed solution completely homogeneous to form an aqueous phase; 0.05 g of TMC was weighed and dissolved in n-hexane, stirred for 1 h, and prepared into an oil phase for standby use;

First, pour the water phase onto the support layer, let it stand for 2 minutes, take out the membrane, and use dust-free paper to absorb the excess water phase, then pour a certain amount of oil phase onto the support layer, let it stand for 1 minute, take out the membrane and dry it naturally to prepare alkali-resistant nanofiltration. After soaking in an alkaline solution of pH 13 for seven days, the flux of the nanofiltration membrane was measured to change from 5.8LMH/bar to 6.7LMH/bar on the seventh day, and the retention of 1000ppm sodium sulfate changed from 92% to 89%.

Example 7

The preparation of aminated lignin is the same as in Example 3.

Weigh 0.5 g of aminated lignin and 0.5 g of piperazine and dissolve them in 50 g of alkaline solution (pH 12), stir them with magnetic force for 1 h, and then ultrasonically treat them for 1.5 h to make the mixed solution completely homogeneous to form an aqueous phase; weigh 0.05 g of TMC and dissolve it in 50 g of n-hexane, stir for 1 h, and prepare an oil phase for standby use;

First, pour the water phase onto the support layer, let it stand for 2 minutes, take out the membrane, and use dust-free paper to absorb the excess water phase, then pour a certain amount of oil phase onto the support layer, let it stand for 1 minute, take out the membrane and dry it naturally to prepare alkali-resistant nanofiltration. After soaking in an alkaline solution of pH 13 for seven days, the flux of the nanofiltration membrane was measured to change from 6.7LMH/bar to 6.9LMH/bar on the seventh day, and the retention of 1000ppm sodium sulfate changed from 93% to 88%.

It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims (3)

1. A method for preparing an alkali-resistant nanofiltration membrane, comprising: A certain amount of high molecular polymer and pore-forming additive are dissolved in an organic solvent and heated and stirred to form a homogeneous mixed solution, which is called a casting solution; After the casting solution is statically degassed for 12 to 24 hours, it is scraped on a glass plate or non-woven fabric with a micron-grade scraper. The non-woven fabric needs to be laid flat and fixed on the glass plate first, and then immersed in deionized water for 0 to 30 seconds to form a phase-transformed membrane for 3 to 5 minutes to form a support layer. 0.25 g of aminated lignin and 0.75 g of piperazine were weighed and dissolved in 50 g of an alkaline solution with a pH of 12, and magnetically stirred for 1 h, and then ultrasonically treated for 1.5 h until no more particles precipitated to form an aqueous phase; Weigh 0.05g of trimesoyl chloride and dissolve it in 50g of n-hexane. Stir for 1h until it is completely dissolved, and prepare the oil phase for later use. First, immerse the support layer wiped dry with dust-free paper into the water phase for a certain period of time, take out the support layer, and use a hair dryer to dry the excess water droplets on the surface; Finally, a certain amount of oil phase is poured into the support layer soaked in water phase. After a certain period of time, the membrane is taken out and immersed in deionized water for storage after the organic solvent on the surface evaporates, and an alkali-resistant nanofiltration membrane is obtained. The alkali-resistant nanofiltration membrane can stably operate for more than 168 hours under the condition of pH 13, and the retention rate of sodium sulfate is maintained at 90%. 2. The method for preparing an alkali-resistant nanofiltration membrane according to claim 1 is characterized in that it includes: the high molecular polymer is one or more of polyethersulfone, polysulfone, polyacrylonitrile, polyimide, polybenzimidazole, polyphenylene ether, aromatic polyamide, and polyvinylidene fluoride; the pore-forming additive is one or more of polyvinyl pyrrolidone, polyethylene glycol, polyvinyl alcohol, and glycerol; the organic solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide. 3. The method for preparing the alkali-resistant nanofiltration membrane according to claim 1 is characterized in that: the thickness of the scraper is one of 50 μm, 100 μm, 150 μm, and 250 μm.