Preparation method of composite nanofiltration membrane

Abstract

The invention discloses a preparation method of a composite nanofiltration membrane, which is characterized in that the surface of an ultrafiltration membrane is coated with natural low-cost biological materials carrageenan and sodium carboxymethyl cellulose, so that the environmental friendliness and technical feasibility in the field of water treatment are enhanced; the preparation method comprises the following steps: firstly preparing a composite membrane, then carrying out cross-linking modification on the surface of the membrane through aldol condensation reaction, and constructing a compact and stable three-dimensional network structure selection layer after cross-linking, so that the composite membrane keeps structural stability under extreme pH conditions and in an oxychlorination environment, meanwhile, the composite membrane is endowed with efficient nanofiltration performance, and the composite membrane can be practically applied to industrial chromium-containing wastewater.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of preparation of separation membranes, and particularly relates to a preparation method of an acid-, alkali- and chlorine-resistant composite nanofiltration membrane for removing chromium. BACKGROUND

[0002] Membrane separation technology is a new type of water treatment technology, which has the advantages of simple operation, high treatment efficiency, low cost and easy combination with other separation processes. Nanofiltration (NF) is a membrane separation technology between ultrafiltration and reverse osmosis, and the pore size of NF membrane is between 0.5-2.0 nm, which has a wide application in water softening and seawater desalination. Based on the space steric hindrance effect and electrostatic repulsion effect, nanofiltration membranes can be used for the separation of inorganic salts of different valence states and various organic molecules (molecular weight in the range of 200-1000 Da). At present, nanofiltration technology with nanofiltration membrane as the core has been widely applied in the fields of seawater desalination pretreatment, recovery and removal of small organic molecules, treatment of industrial wastewater, separation and purification of biological products, and extraction and refining of food and petroleum materials.

[0003] Cr(VI) is a typical pollutant of industrial wastewater, which is widely distributed in the fields of electroplating, leather tanning, steel manufacturing, etc. Cr(VI) is a toxic heavy metal with carcinogenic and mutagenic properties, which seriously endangers human health. Therefore, it is crucial to remove Cr(VI) ions from wastewater before it is discharged into water bodies. Based on the unique pore size and charge characteristics of NF membranes, they can be an alternative for removing Cr(VI) from wastewater. Generally, alkaline leaching process is used in the production of chromate (Cr(VI)), so the generated wastewater is strongly alkaline, while a large amount of acidic chromium-containing wastewater is often discharged in the industries of electroplating and leather making. In such cases, NF membranes are often used in extremely harsh conditions, and during operation, chlorine oxidizing agents are used to clean the composite membranes to reduce pollution.

[0004] The most representative NF membrane on the market is a polyamide composite nanofiltration membrane prepared by interfacial polymerization. However, the commercially available polyamide NF membrane is mainly used for drinking water purification, and its structural characteristics limit its application prospect in harsh environments. When exposed to extreme pH conditions, the rearrangement of polyamide segments induced by nucleophilic attack of protons, etc. can cause the separation layer to hydrolyze. At the same time, in the presence of chloroxidizing agents, the amide bond is prone to Hofmann degradation reaction, which worsens its separation performance. At present, researchers have made great progress in the field of acid-resistant nanofiltration membranes. Chinese patent (Structure of a high-performance polysulfonamide acid-resistant nanofiltration membrane and preparation method thereof, CN115608176A) uses a sulfuryl chloride monomer with acid-resistant structure to prepare a high-performance acid-resistant nanofiltration membrane. Chinese patent (Acid-resistant composite nanofiltration membrane and preparation method thereof, CN117839448A) invents a nanofiltration membrane with a triazine ring structure to enhance its acid resistance. However, the alkali resistance and chlorine resistance of such nanofiltration membranes still need to be strengthened. Chinese patent (Preparation method of PPTA / polyelectrolyte composite nanofiltration membrane resistant to temperature, acid and alkali, and solvent, CN117427493A) successfully prepares an acid and alkali resistant high-performance nanofiltration membrane using layer-by-layer self-assembly method. However, the layer-by-layer self-assembly method is complicated and time-consuming. Chinese patent (Preparation method of aromatic polyamide composite membrane grafted with pyridinium ions and salicylaldehyde on the surface, CN103908902A) reports a method for improving the chlorine resistance of the membrane by surface grafting. However, the acid and alkali resistance of the membrane is not evaluated. Therefore, it is challenging to prepare a high-performance composite nanofiltration membrane that is resistant to acid, alkali and chlorine. Other preparation methods can be used to innovate the chemical structure of the NF membrane and improve its stability in harsh chemical environments, filling the application gap of polyamide nanofiltration membranes.

[0005] NF membranes are usually constructed by a porous support layer and a dense functional layer. In addition to interfacial polymerization, nanofiltration membranes can also be prepared by grafting polymerization, layer-by-layer self-assembly and surface coating methods. Among them, the method of using biomaterials for dip coating modification is becoming more and more popular due to its simplicity and environmental friendliness. The use of natural low-cost biopolymers to construct functional layers enhances the environmental friendliness and technical feasibility in the field of water treatment. Carrageenan (κ-CGN) is a natural anionic polysaccharide, which is widely used in various fields such as food, pharmaceuticals and biomedical due to its low cost, green and good film-forming properties. The high sulfate content (20wt%) of κ-CGN promotes the adsorption of metal ions and other salt molecules on the membrane surface. Sodium carboxymethyl cellulose (CMC) has similar chemical properties to κ-CGN and is structurally compatible. The large number of hydroxyl functional groups on the end groups of κ-CGN and CMC can be cross-linked with aldehyde groups to construct a macromolecular network, which can maintain good chemical stability in harsh environments. SUMMARY

[0006] The present application is aimed at the poor chemical stability of traditional commercial polyamide composite nanofiltration membranes in the separation process, and provides a preparation method of an acid and alkali resistant and chlorine resistant composite nanofiltration membrane. The method uses natural low-cost biopolymers to construct a functional layer, and modifies the separation layer by chemical crosslinking, so that the separation layer has a stable chemical structure and can be practically applied to industrial chromium-containing wastewater treatment in harsh environments.

[0007] To this end, the technical scheme provided by the present application is as follows: a preparation method of an acid, alkali and chlorine resistant composite nanofiltration membrane for chromium removal, and the specific steps are as follows:

[0008] Step one, take an ultrafiltration membrane soaked in ultrapure water, remove the surface moisture, and immerse it in a certain amount of mixed solution of carrageenan and sodium carboxymethyl cellulose for 2-15 min, and then dry the membrane naturally;

[0009] Step two, pour the crosslinking agent solution onto the surface of the ultrafiltration membrane treated in step one, immerse it in the crosslinking solution for reaction for 0.5-2 h, and repeatedly rinse the surface with deionized water to remove excess crosslinking agent;

[0010] Step three, place the membrane treated in step two into an oven at 30-80 DEG C for heat treatment for 1-10 min to obtain the prepared composite nanofiltration membrane, and finally soak it in pure water for storage.

[0011] Further, the content of the mixed solution of carrageenan and sodium carboxymethyl cellulose in step one is 0.1wt%-1wt%, the molecular weight of carrageenan is 30000-150000, the molecular weight of sodium carboxymethyl cellulose is 50000-20000, and the degree of substitution is 0.5-1.

[0012] Further, the mass ratio of carrageenan and sodium carboxymethyl cellulose in step one is 2:8-8:2, preferably 6:4.

[0013] Further, the crosslinking agent in step two is one of glutaraldehyde, epichlorohydrin and citric acid, and the content of the crosslinking agent is 0.5-2w / v%.

[0014] Further, the crosslinking solution in step two contains 0.05wt% of a catalyst, and the catalyst is sulfuric acid or hydrochloric acid.

[0015] Further, the ultrafiltration membrane is a hollow fiber membrane, a flat sheet membrane and a tubular membrane, and the material of the ultrafiltration membrane is one of polyether sulfone, polysulfone and polyvinylidene fluoride. Compared with the prior art, the method has the following advantages:

[0016] (1) The natural low-cost biopolymer is used to construct a functional layer, which enhances the environmental friendliness and technical feasibility in the field of water treatment.

[0017] (2) The composite membrane is prepared by coating method and cross-linking method, the membrane preparation process is simple, and can be practically applied to industry.

[0018] (3) The dense three-dimensional network functional layer is formed, which not only enhances the nanofiltration performance of the composite membrane, but also is beneficial to improve the chemical stability of the membrane, and the membrane has excellent stability in acid, alkali and oxidizing solution. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a surface SEM image of the composite nanofiltration membrane in Example 3 of the present application;

[0020] Figure 2 is the membrane flux (Figure a) and Na2SO retention rate (Figure b) of the composite nanofiltration membrane in Example 7 of the present application in different concentrations and different pH chromium solution;

[0021] Figure 3 is a digital photo of the composite nanofiltration membrane in Example 7 of the present application before (left) and after (right) treating chromium-containing wastewater (100ppm, pH = 7); DETAILED DESCRIPTION

[0022] To achieve the above-mentioned technology, the present application is described below in combination with examples and drawings, but the described examples of the present application are only part, not all examples, and the improved technical solutions and methods of the present application all belong to the protection scope of the present application.

[0023] Examples 1-4

[0024] The preparation processes are basically the same, and the only difference is that the concentration of the carrageenan and sodium carboxymethyl cellulose mixed solution in step 1) is changed.

[0025] 1) Take the flat plate ultrafiltration membrane soaked in ultrapure water, remove the surface moisture, and immerse it in a mixed solution of carrageenan and sodium carboxymethyl cellulose with a certain concentration for 2-15 min, wherein the mass ratio of carrageenan and sodium carboxymethyl cellulose is 6:4, and the prepared membrane is naturally air-dried;

[0026] 2) Pour a mixed solution of 1wt% glutaraldehyde and 0.05wt% sulfuric acid onto the surface of the ultrafiltration membrane treated in step 1), immerse it in the cross-linking solution for 1h, and repeatedly rinse the surface with deionized water to remove excess cross-linking agent;

[0027] 3) Put the nanofiltration membrane treated in step 2) into a 50℃ oven for heat treatment for 5 min, take out the prepared composite nanofiltration membrane, and finally soak it in a room temperature environment for 12h before testing the membrane flux and salt retention performance. The composite nanofiltration membrane is tested for performance under 0.6MPa using 1000ppm Na2SO4 aqueous solution.

[0028] Table 1 Specific embodiments of Examples 1-4

[0029]

[0030] From Examples 1-4, it can be seen that when the concentration of the blended solution increases, the flux gradually decreases, and when the concentration is 0.65wt%, the prepared composite nanofiltration salt rejection rate is the highest.

[0031] The surface morphology of the composite nanofiltration membrane prepared in this experiment was detected by SEM on Example 3. Figure 1 It can be seen from the surface graph that the prepared membrane forms a relatively smooth surface, and the membrane surface has no obvious defects and holes.

[0032] Examples 5-8

[0033] The preparation process is basically the same, except that the mass ratio of carrageenan and sodium carboxymethyl cellulose in step 1) is changed.

[0034] 1) Take the hollow fiber ultrafiltration membrane soaked in ultrapure water, remove the surface moisture, and immerse it in a 0.65wt% carrageenan and sodium carboxymethyl cellulose mixed solution for 2-15min, then air dry the membrane naturally;

[0035] 2) Pour 1wt% glutaraldehyde and 0.05wt% sulfuric acid onto the surface of the ultrafiltration membrane treated in step 1), immerse it in the crosslinking solution for reaction for 1h, and repeatedly rinse the surface with deionized water to remove excess crosslinking agent;

[0036] 3) Put the nanofiltration membrane treated in step 2) into a 60℃ oven for heat treatment for 3min, take out the prepared composite nanofiltration membrane, and finally soak it in a room temperature environment for 12h before testing the salt rejection performance. The composite nanofiltration membrane is tested for performance at 25℃, 0.6MPa, with 1000ppm Na2SO4 aqueous solution.

[0037] Table 2 Specific embodiments of Examples 5-8

[0038]

[0039] From Examples 5-8, it can be seen that when the mass ratio of carrageenan and sodium carboxymethyl cellulose is 6:4, the separation performance of the prepared composite nanofiltration membrane is optimal.

[0040] To evaluate the chemical stability of the composite nanofiltration membrane, static immersion experiments were performed on Example 7. The membranes prepared in Example 7 were immersed in 20 wt% H2SO4, 2 M NaOH and 200 ppm NaOCl solutions, respectively, for a certain period of time. 1000 ppm sodium sulfate was used as the feed solution, and the rejection performance of the membranes before and after immersion was tested at 25°C and 0.6 MPa to evaluate their acid, alkali and chlorine resistance. The results, as shown in Table 3, show that the separation performance of the prepared composite membranes remains essentially unchanged under strong acid, strong alkali and chlorine oxidation conditions, with a rejection rate decrease of less than 4%, exhibiting excellent chemical stability.

[0041] Table 3 Chemical stability evaluation of Example 4

[0042]

[0043] The chromium separation performance of the composite nanofiltration membrane was evaluated. Since Cr(VI) exists in different ionic species, it changes with the change of pH and concentration of chromate solution (equations (1)-(3)).

[0044]

[0045]

[0046]

[0047] When the pH is greater than 6, CrO4 2- is dominant, and as the pH decreases, the concentration of HCrO4 - will increase significantly. At the same time, as the concentration of Cr(VI) increases, the content of Cr2O7 2- also increases. Due to the different ionic valence under different solution conditions, the separation performance of the composite membrane is affected. Therefore, by changing the pH environment of the feed solution (pH = 3, 5, 7, 9) and the Cr(VI) concentration (10, 50, 100, 500 ppm), the rejection performance of the membranes prepared in Example 7 was tested at 25°C and 0.6 MPa. The test results, as shown in Table 3, show that the prepared composite nanofiltration membrane has a rejection performance of more than 97% for Cr(VI) solutions of different concentrations under neutral and alkaline conditions, indicating that the prepared composite nanofiltration membrane can be practically applied to industrial chromium-containing wastewater. Figure 2 Figure 3 Digital photos of the composite nanofiltration membrane prepared in Example 7 before and after treatment of Cr(VI)-containing wastewater, the solution changes from yellow to colorless, which can be judged from the appearance that the Cr(VI) ions in the solution are successfully removed.

[0048] ​Although the present application is described above with reference to the drawings and the attached table, the present application is not limited to the above-described mode, and various improvements made by adopting the method concept and technical solutions of the present application are within the protection scope of the present application.

Claims

1. A method for preparing a composite nanofiltration membrane, characterized in that, Includes the following steps: 1) Take an ultrafiltration membrane that has been soaked in ultrapure water, remove the surface moisture, immerse it in a mixed solution of carrageenan and sodium carboxymethyl cellulose with a mass fraction of 0.1wt%-1wt% for 2-15 minutes, and then let the membrane air dry naturally. 2) Pour the crosslinking agent solution onto the surface of the ultrafiltration membrane treated in step 1), immerse it in the crosslinking agent solution and react for 0.5-2 hours, then rinse the surface repeatedly with deionized water to remove excess crosslinking agent; 3) Place the membrane treated in step 2) into an oven at 30-80°C for 1-10 minutes to heat treat it, and finally immerse it in pure water.

2. The preparation method according to claim 1, characterized in that: In step 1), the mass ratio of carrageenan to sodium carboxymethyl cellulose is 2:8-8:2, preferably 6:

4.

3. The preparation method according to claim 1, characterized in that: In step 2), the crosslinking agent is one of glutaraldehyde, epichlorohydrin, or citric acid, and the crosslinking agent content is 0.5–2 w / v.

4. The preparation method according to claim 1, characterized in that: Step 2) The crosslinking solution contains 0.5 wt% of a catalyst, which is sulfuric acid or hydrochloric acid.

5. The preparation method according to claim 1, characterized in that... The ultrafiltration membrane is a hollow fiber membrane, a flat sheet membrane, or a capillary membrane. The ultrafiltration membrane material is one of polyethersulfone, polysulfone, or polyvinylidene fluoride.

6. The preparation method according to claim 1, characterized in that... The prepared composite nanofiltration membrane exhibits excellent structural stability in acidic, alkaline, and oxidizing solutions, with a rejection rate decrease of less than 4%.

7. The preparation method according to claim 1, characterized in that... The prepared composite nanofiltration membrane can efficiently separate chromium (Cr(VI)) from aqueous solution under neutral and alkaline conditions, with a Cr(VI) removal rate of over 97%.

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