A nanofiltration membrane, a preparation method and application thereof

Abstract

The application discloses a nanofiltration membrane and a preparation method and application thereof. The nanofiltration membrane comprises a polyether sulfone / sulfonated polysulfone base film, the base film is in a porous structure, and beta-cyclodextrin and polydopamine are fixed in the pores of the base film. The application utilizes the heat shrinkability of sulfonated polysulfone, mutual recognition of beta-cyclodextrin and dyes, and adsorption of polydopamine to realize excellent permeation flux and dye interception, the permeation flux reaches 225 L·m ‑2 ·h ‑1 ·bar ‑1 , and the interception rates of methyl blue and congo red are more than 96%. In addition, the permeation flux and separation performance of the nanofiltration membrane are still stable in a long-time continuous filtration process, and deionized water at 40 DEG C is more favorable to improve the cycle stability of the membrane.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of membrane separation, in particular to a nanofiltration membrane and a preparation method and application thereof. BACKGROUND

[0002] With the rapid development of various industries, the diversification trend of dye types is becoming more and more significant, resulting in the increasing complexity of dye components in wastewater, which brings great challenges to wastewater treatment. At present, although there are coagulation sedimentation method, advanced oxidation method, chemical precipitation, biofilm method and aerobic / anaerobic sludge treatment for the recovery and utilization of printing and dyeing wastewater, these methods have achieved certain results, but they are still limited by their own limitations and cannot fully meet the requirements of high efficiency and environmental protection of wastewater treatment.

[0003] Membrane separation technology has been widely used in wastewater treatment, seawater desalination, lithium extraction from salt lake and oil-water separation due to its low energy consumption, simple operation and environmental friendliness. As an important part of membrane separation technology, the performance of nanofiltration membrane directly determines the separation effect and application prospect. However, the preparation of most nanofiltration membranes still relies on interfacial polymerization technology, resulting in small pore size and low porosity of the obtained membrane material, which further increases the transmembrane resistance and seriously restricts the separation efficiency and flux.

[0004] More importantly, the existing nanofiltration membranes often face the "Trade-off" effect between selectivity and permeation flux, that is, improving selectivity often comes at the cost of sacrificing permeation flux, and vice versa. This inherent contradiction makes it difficult to make breakthroughs in efficient separation and flux improvement for nanofiltration membranes.

[0005] Therefore, in order to overcome the limitations of existing nanofiltration membranes and improve the efficiency and flux of wastewater treatment, it is particularly urgent to develop a loose nanofiltration membrane with high flux. SUMMARY

[0006] The present application aims to at least solve one of the problems in the prior art. To this end, the present application proposes a nanofiltration membrane, which has a high permeation flux.

[0007] According to one aspect of the present application, a nanofiltration membrane is proposed, which comprises a polyether sulfone / sulfonated polysulfone base film, the base film has a porous structure, and β-cyclodextrin and polydopamine are fixed in the pores of the base film; during the preparation of the nanofiltration membrane, β-cyclodextrin and polydopamine are added in the form of a mixed aqueous solution, the mass concentration of β-cyclodextrin in the mixed aqueous solution is 0.5-2.5wt%, and the mass concentration of polydopamine is 0.5-1wt%.

[0008] According to a preferred embodiment of the present application, at least the following advantages are achieved: (1) The present application ingeniously incorporates β-cyclodextrin macromolecular structure into the pores of polyethersulfone / sulfonated polysulfone-based membrane by taking advantage of the thermal shrinkage of sulfonated polysulfone. On the one hand, the condensation reaction between the amino groups of polydopamine and the hydroxyl groups of β-cyclodextrin enables the successful modification of polydopamine on the inner and outer rings of β-cyclodextrin, further enhancing the hydrophilicity of the membrane and increasing the permeation capacity of the membrane. On the other hand, the presence of polydopamine and β-cyclodextrin increases the overall thickness of the dense layer of the membrane, reduces the porosity of the membrane surface, and improves the retention of dyes.

[0009] In some embodiments of the present application, the β-cyclodextrin has a micro-cone or hollow cylindrical three-dimensional ring structure.

[0010] In some embodiments of the present application, the base membrane fixes the β-cyclodextrin and polydopamine through thermal shrinkage and hydrogen bonding.

[0011] In some embodiments of the present application, the nanofiltration membrane has a permeation flux of 158.9-225 L·m -2 ·h -1 ·bar -1 .

[0012] In some embodiments of the present application, the base membrane has a pore size ranging from 9.58 to 18.89 nm.

[0013] In some embodiments of the present application, the nanofiltration membrane has a pore size ranging from 6.68 to 10.09 nm.

[0014] In another aspect of the present application, a method for preparing a nanofiltration membrane is provided, comprising the following steps:

[0015] immersing a polyethersulfone / sulfonated polysulfone-based membrane in a mixed aqueous solution containing β-cyclodextrin and polydopamine, and heating and curing to obtain the membrane, wherein the mass concentration of β-cyclodextrin in the mixed aqueous solution is 0.5-2.5 wt%, and the mass concentration of polydopamine is 0.5-1 wt%.

[0016] According to a preferred embodiment of the present application, at least the following advantages are achieved: the thermal shrinkage of sulfonated polysulfone is used to incorporate macromolecular structure of β-cyclodextrin into the pores of the polyether sulfone / sulfonated polysulfone-based film, on the one hand, the incorporation of β-cyclodextrin introduces hydroxyl groups on the surface of the film, thereby enhancing the hydrophilicity of the film and improving the permeation amount of the film. On the other hand, the presence of β-cyclodextrin increases the overall thickness of the dense layer of the film, reducing the presence of pores to break through the "Trade-off" effect between flux and dye retention. The preparation method is simple and convenient, and the prepared polyether sulfone / sulfonated polysulfone / polydopamine / β-cyclodextrin high-performance loose nanofiltration membrane has stable circulation performance. The preparation process of the loose nanofiltration membrane of the present application is simple, short in time, easy to control, and low in cost.

[0017] With the increase of the mass concentration of β-cyclodextrin, the water permeation flux and dye retention of the polyether sulfone / sulfonated polysulfone / polydopamine / β-cyclodextrin nanofiltration membrane show a trend of first significantly increasing and then significantly decreasing. When the mass concentration of β-cyclodextrin is low, the embedding process in the pores of the membrane can be effectively stabilized; when the mass concentration of β-cyclodextrin is too large, β-cyclodextrin tends to agglomerate, resulting in uneven distribution of β-cyclodextrin in the pores of the membrane, thereby to a certain extent, leading to a decrease in water permeation flux and retention rate.

[0018] In some embodiments of the present application, the temperature of the heating and curing is 55-75°C, and the time of the heating and curing is 2.5-12.5 min.

[0019] In some embodiments of the present application, the immersion time is 5-8 min.

[0020] In some embodiments of the present application, the preparation process of the polyether sulfone / sulfonated polysulfone-based film comprises the following steps:

[0021] The polyether sulfone, sulfonated polysulfone and polyethylene glycol are mixed, a solvent is added, and heating is performed to obtain a casting solution. After the casting solution is prepared into a nascent film, the polyether sulfone / sulfonated polysulfone-based film is prepared by placing it in water.

[0022] In some embodiments of the present application, the solvent is at least one of N,N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide or N-methyl pyrrolidone.

[0023] In some embodiments of the present application, the temperature of the heating is 65-75°C.

[0024] In some embodiments of the present application, the heating is performed under stirring, and the stirring speed is 250-350 rpm.

[0025] In some embodiments of the present application, the mass ratio of polyether sulfone, sulfonated polysulfone and polyethylene glycol in the casting solution is 15-16:2-4:8-12.

[0026] In some embodiments of the present application, the nascent membrane is placed in water for 10-12 hours to produce the polyether sulfone / sulfonated polysulfone base membrane. The polyether sulfone / sulfonated polysulfone base membrane is prepared by a non-solvent induced phase separation method, and is taken out after being soaked in (deionized) water for 10-12 hours to completely replace the pore-forming agent polyethylene glycol and N,N-dimethylformamide.

[0027] In some embodiments of the present application, the preparation step of the base membrane further comprises a step of static defoaming after the casting solution is heated.

[0028] In some embodiments of the present application, the defoaming time is 10-12 hours.

[0029] In some embodiments of the present application, the preparation step of the base membrane further comprises cutting the base membrane into 5-7 cm square pieces.

[0030] According to another aspect of the present application, the above-mentioned nanofiltration membrane is applied in dye wastewater purification or soil dye removal.

[0031] According to the application of a preferred embodiment of the present application, at least the following beneficial effects are achieved: the nanofiltration membrane of the present application has a permeation flux of 158.9-225 L·m -2 ·h -1 ·bar -1 , and the rejection rate of methyl blue and congo red dyes is more than 96%. However, as the filtration time is prolonged, the pores of the nanofiltration membrane will be contaminated and encapsulated by the dyes, resulting in a slight decrease in rejection and permeate water flux. In the application of deionized water circulation at 35-45℃, the water flux and dye rejection of the membrane hardly decrease after 12-14 hours. The washing of deionized water at 35-45℃ is beneficial to the dissociation of the inclusion complex between β-cyclodextrin and dyes, and improves the stability of the membrane in circulation. The nanofiltration membrane of the present application has a novel structure, which can solve the problems of low porosity, large transmembrane resistance and "Trade-off" effect, thereby realizing more efficient and more environmentally friendly wastewater treatment and resource recovery. This will bring revolutionary changes to the field of dyeing wastewater treatment and promote the further development of wastewater treatment technology.

[0032] The present application utilizes the thermal shrinkage of sulfonated polysulfone, the mutual recognition of β-cyclodextrin and dyes, and the adsorption of polydopamine to achieve excellent permeation flux and dye rejection, with a permeation flux of 225 L·m -2 ·h -1 ·bar -1The rejection rates of methylene blue and Congo red are more than 96%. In addition, the permeation flux and separation performance of the nanofiltration membrane are still stable in a long-time continuous filtration process, and deionized water at 40 DEG C is more conducive to improving the cycle stability of the membrane.

[0033] Other features and advantages of the present application will be set forth in the following description, and in part will become apparent to those skilled in the art upon examination of the following or can be learned by practice of the application. BRIEF DESCRIPTION OF DRAWINGS

[0034] The above and / or additional aspects and advantages of the present application will become apparent and be readily understood from the following description, taken in conjunction with the accompanying drawings, in which:

[0035] Figure 1 is a structural schematic diagram of the nanofiltration membrane prepared in Example 1 of the present application.

[0036] Figure 2 is a scanning electron microscope (SEM) diagram of the surface and cross section of the base membrane prepared in Example 1 of the present application: (a) SEM diagram of the surface of the base membrane; (b) SEM diagram of the cross section of the base membrane.

[0037] Figure 3 is a scanning electron microscope (SEM) diagram of the surface and cross section of the nanofiltration membrane prepared in Example 1 of the present application: (a) SEM diagram of the surface of the nanofiltration membrane; (b) SEM diagram of the cross section of the nanofiltration membrane.

[0038] Reference Signs:

[0039] Polyether sulfone / sulfonated polysulfone base membrane 1, nanofiltration membrane 2. DETAILED DESCRIPTION

[0040] The concept and the technical effects of the present application will be described clearly and completely in combination with the embodiments, so as to fully understand the objects, features and effects of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all the embodiments. Based on the embodiments of the present application, other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the present application. The test methods used in the embodiments are conventional methods without special instructions. The materials, reagents and the like used are commercially available without special instructions. The same parameters are used in the same way in each embodiment without special instructions. The following described embodiments are exemplary and are used only to explain the present application, and cannot be understood as limiting the present application. The main raw materials in the embodiments and comparative examples of the present application are as follows: polyether sulfone (Yuan Ye, item number: S26840-500g), sulfonated polysulfone (BASF, Germany, item number: A-300ANT), β-cyclodextrin (98%, Shanghai Aladdin, item number: C104384-500g), polydopamine (99%, XFNANO, item number: 104550-10ml).

[0041] In the description of the present application, the description of the terms “one embodiment”, “some embodiments”, “illustrative embodiment”, “example”, “specific example” or “some examples” means that the specific features, structures, materials or characteristics described in combination with the embodiment or example are contained in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0042] Embodiment 1

[0043] In this example, a nanofiltration membrane is prepared, and the specific process is as follows:

[0044] Step one, preparation of polyether sulfone casting solution

[0045] First, 15.12wt% of polyether sulfone, 2.88wt% of sulfonated polysulfone and 10wt% of polyethylene glycol 800 are weighed into a conical flask, and then N,N-dimethylformamide solvent is added to prepare 30g of casting solution. Then, it is placed in a water bath, the temperature is adjusted to 75℃, and the stirring speed is 300rpm for 8h, and then it is left still for 12h to degas, and then a uniformly mixed polyether sulfone casting solution is obtained.

[0046] Step two, preparation of polyether sulfone / sulfonated polysulfone base film

[0047] A clean glass plate was taken, and the polyether sulfone casting solution was uniformly cast on one end of the glass plate. Then, a self-made film scraper was used to uniformly scrape a nascent film with a thickness of about 200 μm on the surface of the glass plate. The glass plate was quickly placed in deionized water for phase inversion. After the film was separated from the glass plate, it was taken out and immersed in ionized water for 12 h. Then, a small square of polyether sulfone / sulfonated polysulfone base film 1 with a size of 5 cm was cut and placed in deionized water for cold storage for standby use;

[0048] Step three, preparation of polyether sulfone / sulfonated polysulfone / polydopamine / β-cyclodextrin nanofiltration membrane 2

[0049] A β-cyclodextrin / polydopamine mixed aqueous solution was prepared, in which the mass concentration of β-cyclodextrin was 1.0 wt%, and the mass concentration of polydopamine was 1.0 wt%. Then, the cut polyether sulfone / sulfonated polysulfone base film was inserted into the mold, and the excess water on the surface of the film was removed by compaction with a pressure roller. Then, the polyether sulfone / sulfonated polysulfone base film was immersed in the β-cyclodextrin / polydopamine mixed aqueous solution. After 15 min, the excess β-cyclodextrin / polydopamine mixed aqueous solution was removed, and the film was placed in an oven for heating and curing at 65°C for 6.5 min. Thus, a polyether sulfone / sulfonated polysulfone / polydopamine / β-cyclodextrin nanofiltration membrane was obtained.

[0050] The structure of the polyether sulfone / sulfonated polysulfone / polydopamine / β-cyclodextrin nanofiltration membrane is shown in Figure 1 .

[0051] Example 2

[0052] In this example, a nanofiltration membrane was prepared, which was different from example 1 in that the concentration of β-cyclodextrin used in this example was 0.5 wt%, the heating and curing time was 7.5 min, and the temperature was 65°C.

[0053] Example 3

[0054] In this example, a nanofiltration membrane was prepared, which was different from example 1 in that the concentration of β-cyclodextrin solution used in this example was 1.5 wt%, the heating and curing time was 10 min, and the temperature was 60°C.

[0055] Comparative Example 1

[0056] In this example, a nanofiltration membrane was prepared, which was different from example 1 in that steps one and two were the same as example 1, and step three was as follows:

[0057] A dopamine / polyethylene glycol (200) mixed aqueous solution was prepared: the mass concentration of dopamine was 1.0 wt%, and the mass concentration of polyethylene glycol was 1.0 wt%; then, the polyether sulfone / sulfonated polysulfone base membrane was immersed in the dopamine / polyethylene glycol (200) mixed aqueous solution, and after 15 min, the excess dopamine / polyethylene glycol (200) mixed aqueous solution on the surface of the membrane was removed, and then the membrane was placed in an oven for heating and solidification at 65°C for 7.5 min, to obtain a polyether sulfone / sulfonated polysulfone / dopamine / polyethylene glycol (200) nanofiltration membrane.

[0058] Comparative Example 2

[0059] In this example, a nanofiltration membrane was prepared, which was different from Example 1 in that steps one and two were the same as Example 1, and step three was as follows: a polydopamine / β-cyclodextrin mixed aqueous solution was prepared: the mass concentration of polydopamine was 0.2 wt%, and the mass concentration of β-cyclodextrin was 0.6 wt% added to 100 mL of Tris buffer (pH = 8.5, 50 mM). Then, the porous substrate was immersed in the prepared polydopamine / β-cyclodextrin mixed aqueous solution and gently shaken for a certain time of 3 hours, and finally the membrane was washed with deionized water five times, to obtain a polydopamine / β-cyclodextrin nanofiltration membrane.

[0060] A dopamine / polyethylene glycol (200) mixed aqueous solution was prepared: the mass concentration of dopamine was 1.0 wt%, and the mass concentration of polyethylene glycol was 1.0 wt%; then, the polyether sulfone / sulfonated polysulfone base membrane was immersed in the dopamine / polyethylene glycol (200) mixed aqueous solution, and after 15 min, the excess dopamine / polyethylene glycol (200) mixed aqueous solution on the surface of the membrane was removed, and then the membrane was placed in an oven for heating and solidification at 65°C for 7.5 min, to obtain a polyether sulfone / sulfonated polysulfone / dopamine / polyethylene glycol (200) nanofiltration membrane.

[0061] Comparative Example 3

[0062] In this example, a nanofiltration membrane was prepared, which was different from Example 1 in that steps one and two were the same as Example 1, and step three was as follows: a polydopamine / β-cyclodextrin mixed aqueous solution was prepared: the mass concentration of polydopamine was 0.2 wt%, and the mass concentration of β-cyclodextrin was 0.6 wt% added to 100 mL of Tris buffer (pH = 8.5, 50 mM). Then, the porous substrate was immersed in the prepared polydopamine / β-cyclodextrin mixed aqueous solution and gently shaken for a certain time of 3 hours, and finally the membrane was washed with deionized water five times, to obtain a polydopamine / β-cyclodextrin nanofiltration membrane.

[0063] Test Example

[0064] In this test example, the morphology and performance of the nanofiltration membranes and intermediate products prepared in the examples and comparative examples were tested. Among them:

[0065] 1) Morphology characterization

[0066] The SEM images of the base membrane and nanofiltration membrane prepared in Example 1 are shown in Figure 2 and 3 From the figures, it can be seen that the pore structure distribution of the base membrane and nanofiltration membrane prepared by the scheme of the application is uniform. The test results of other examples are similar to those of Example 1, and to avoid redundancy, they are not shown one by one.

[0067] 2) Separation performance test

[0068] The separation performance of the nanofiltration membrane is analyzed by detecting the water flux and the dye rejection rate of the nanofiltration membrane.

[0069] The test conditions of the dye rejection rate are that the feed liquid is 100 mg / L methylene blue and 150 mg / L congo red, and the liquid temperature is 20-25℃.

[0070] The calculation formula of the rejection rate is R=(1-C p / C r )×100%, wherein R represents the rejection rate, C p and C r are the concentrations of the permeate and the feed liquid respectively.

[0071] The calculation formula of the permeate water flux is F=V / A×t×p, wherein F is the permeation rate of the membrane under a given pressure, L·m -2 ·h -1 ·bar -1 ; V is the volume of the permeate solution, A is the effective area of the membrane, and p is the filtration pressure.

[0072] In addition, the cross-flow filtration device is used, the effective test area is 12.56 cm 2 , the test temperature is 25℃, and the operating pressure is 1 bar. During the test, the nanofiltration membrane is first pre-pressed at 1.5 bar for 0.5 h, after the nanofiltration membrane reaches a stable state, the sample is taken every 0.5 h and the permeate volume is recorded. In order to avoid errors, each sample is tested at least three times.

[0073] Table 1 Separation performance of the nanofiltration membranes prepared in Examples 1-3 and the nanofiltration membranes prepared in Comparative Examples 1-3

[0074]

[0075] It can be seen from the above table that the rejection rate of the nanofiltration membrane prepared by the embodiment scheme of the application to methylene blue is as high as 99.4%, and the permeation amount is as high as 225 L·m -2 ·h -1 ·bar -1 ; the rejection rate to congo red is as high as 96%, and the permeation amount is as high as 158.9 L·m -2 ·h -1 ·bar -1 . When polyethylene glycol with a hydroxyl group or piperazine with an amino group is replaced, the dye rejection amount is significantly reduced, and at the same time, the permeation amount is also significantly reduced, which indicates that the scheme of the application makes the separation performance achieve an unexpected effect by specific fixation of polydopamine / β-cyclodextrin on the base membrane.

[0076] The present application utilizes the thermal shrinkage of sulfonated polysulfone to fix the micro-cone, hollow cylindrical three-dimensional ring structure super-macromolecule β-cyclodextrin in the pores of polyether sulfone / sulfonated polysulfone base film, increase the surface roughness and reduce the pore size; through the mutual recognition of β-cyclodextrin to dyes and the adsorption of polydopamine, the prepared polyether sulfone / sulfonated polysulfone / polydopamine / β-cyclodextrin nanofiltration membrane realizes excellent dye interception and permeation flux function, and overcomes the "Trade-off" effect between dyes and flux. In Examples 1-3, the prepared nanofiltration membrane has a permeation flux of 146-225 L·m -2 ·h -1 ·bar -1 -2 and a dye interception rate of 93.6%-99.4%, thus indicating that the nanofiltration membrane of the present application has a wide application prospect in the field of dye waste liquid treatment.

[0077] The above has made a detailed description on the examples of the present application, but the present application is not limited to the above examples, and various changes can be made within the knowledge range possessed by those skilled in the art without departing from the purpose of the present application.

Claims

1. A nanofiltration membrane, characterized by: The nanofiltration membrane comprises a polyethersulfone / sulfonated polysulfone base film, the base film has a porous structure, and β-cyclodextrin and polydopamine are fixed in the pores of the base film; during preparation of the nanofiltration membrane, the β-cyclodextrin and polydopamine are added in the form of a mixed aqueous solution, the mass concentration of the β-cyclodextrin in the mixed aqueous solution is 0.5-2.5 wt%, and the mass concentration of the polydopamine is 0.5-1 wt%.

2. The nanofiltration membrane according to claim 1, characterized in that: The β-cyclodextrin has a micro-cone or hollow cylindrical three-dimensional ring structure.

3. The nanofiltration membrane according to claim 1, characterized in that: The permeation flux of the nanofiltration membrane is 158.9-225 L·m -2 ·h -1 ·bar -1 .

4. The nanofiltration membrane according to claim 1, characterized in that: The pore size of the base film ranges from 9.58 nm to 18.89 nm; and / or the pore size of the nanofiltration membrane ranges from 6.68 nm to 10.09 nm.

5. A method for producing a nanofiltration membrane as claimed in any one of claims 1-4, characterized in that: The method comprises the following steps: The polyethersulfone / sulfonated polysulfone base film is immersed in a mixed aqueous solution containing β-cyclodextrin and polydopamine, and then heated and cured to obtain the nanofiltration membrane.

6. The method for producing a nanofiltration membrane according to claim 5, characterized in that: The heating and curing temperature is 55-75 ℃, and the heating and curing time is 2.5-12.5 min.

7. The method for preparing a nanofiltration membrane according to claim 5, characterized in that: The immersion time is 5-8 min.

8. The method for preparing a nanofiltration membrane according to claim 5, characterized in that: The preparation process of the polyethersulfone / sulfonated polysulfone base film comprises the following steps: Polyethersulfone, sulfonated polysulfone and polyethylene glycol are mixed, a solvent is added, and then heated to obtain a casting solution; after the casting solution is formed into a nascent membrane, the nascent membrane is placed in water to prepare the polyethersulfone / sulfonated polysulfone base film; the heating temperature is 65-75 ℃, the heating is carried out under stirring, and the stirring speed is 250-350 rpm; the solvent is at least one of N,N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide or N-methyl pyrrolidone.

9. The method for producing a nanofiltration membrane according to claim 8, characterized in that: The mass ratio of polyethersulfone, sulfonated polysulfone and polyethylene glycol in the casting solution is 15-16:2-4:8-12.

10. Application of the nanofiltration membrane according to any one of claims 1-4 in dye waste liquid purification or soil dye removal.

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