Surface-enhanced tryptophan modified nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention discloses a surface-enhanced tryptophan modified nanofiltration membrane as well as a preparation method and application thereof. Belongs to the field of water treatment membrane preparation. The invention aims to solve the technical problems of non-uniform pore size distribution and surface hydrophobicity of the conventional nanofiltration membrane. The method comprises the following steps: adding tryptophan into a piperazine water-phase solution, and carrying out interfacial polymerization with oil-phase trimesoyl chloride to prepare the tryptophan modified nanofiltration membrane. According to the preparation method, the diffusion rate of the piperazine monomer can be adjusted in the interfacial polymerization process by utilizing a similar surfactant structure of tryptophan, meanwhile, the tryptophan has hydrophilicity and can improve the hydrophilicity of the surface of an active layer, and the prepared modified nanofiltration membrane is reduced in molecular weight cut-off, improved in surface hydrophilicity and good in water resistance. The retention rate of organic micro-pollutants is improved, and meanwhile, good anti-pollution performance is achieved.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of water treatment membranes; specifically, it relates to a surface-enhanced tryptophan-modified nanofiltration membrane, a preparation method thereof, and an application thereof. Background Art

[0002] Organic micropollutants in environmental water bodies may pose a great potential threat to human health through the drinking water route due to their persistence, bioaccumulation, and potential toxicity. As an efficient separation technology, the pore size of nanofiltration membranes is between 0.5 and 2 nm, and the surface is negatively charged, enabling them to remove inorganic salt ions and organic substances in water. Therefore, they can be applied to the advanced treatment of drinking water. However, there are two key technical bottlenecks in the nanofiltration membranes prepared by traditional interfacial polymerization processes: on the one hand, due to the uncontrollable diffusion of reaction monomers during the interfacial polymerization process, the formed membrane pore structure shows non-uniform distribution, allowing some low-molecular-weight organic micropollutants to penetrate the membrane barrier; on the other hand, the inherent hydrophobic characteristics of nanofiltration membrane materials will have a strong affinity with hydrophobic pollutants. This membrane-pollutant interaction will not only exacerbate membrane fouling but also significantly reduce the actual interception efficiency of nanofiltration for organic micropollutants, limiting the application of nanofiltration membranes in drinking water treatment. Therefore, it is crucial to develop a surface-enhanced nanofiltration membrane to improve the interception of organic micropollutants. Summary of the Invention

[0003] The present invention aims to solve the technical problems of uneven pore size distribution and surface hydrophobicity of existing nanofiltration membranes, and provides a surface-enhanced tryptophan-modified nanofiltration membrane, a preparation method thereof, and an application thereof. The present invention greatly improves the uniformity of the pore size distribution and the surface hydrophilicity of the nanofiltration membrane, and improves the removal efficiency of organic micropollutants and the anti-fouling performance of the nanofiltration membrane.

[0004] In order to achieve the above technical problems, the present invention adopts the following technical solutions: The present invention utilizes the surfactant-like structure of tryptophan to regulate the diffusion rate of piperazine monomers during the interfacial polymerization process. At the same time, tryptophan has hydrophilicity, which can improve the hydrophilic property of the active layer surface. The prepared modified nanofiltration membrane has a reduced molecular weight cut-off, improved surface hydrophilicity, increased interception rate of organic micropollutants, and good anti-fouling performance.

[0005] The purpose of the present invention is to provide a preparation method for a surface-enhanced tryptophan-modified nanofiltration membrane, which specifically includes the following steps: Step 1: Dissolve piperazine and NaOH in deionized water, add tryptophan, and stir until completely dissolved to obtain an aqueous solution; Step 2: Place a polyethersulfone ultrafiltration membrane in a reactor, pour the aqueous solution into it, carry out an adsorption reaction, and then remove the excess aqueous solution on the surface; Step 3: completely dissolve trimesoyl chloride in n-hexane to obtain an oil phase, and then immerse the polyethersulfone ultrafiltration membrane treated in step 2 in the oil phase, perform interfacial polymerization for a period of time, take out and heat treat, and obtain the tryptophan modified nanofiltration membrane.

[0006] It is further defined that in the aqueous solution of step 1, the concentration of piperazine is 0.6 wt %.

[0007] It is further defined that in the aqueous solution of step 1, the tryptophan concentration is 0.6-2.0 wt%.

[0008] It is further defined that in the aqueous solution of step 1, the concentration of sodium hydroxide is 0.4 wt%.

[0009] It is further defined that in step 2, the adsorption reaction time is 1 min to 3 min.

[0010] It is further defined that in step three, the concentration of trimesoyl chloride in the oil phase is 0.1 wt %.

[0011] It is further defined that in step three, the interfacial polymerization reaction time is 30 to 60 s.

[0012] It is further defined that in step three, the heat treatment is carried out at a temperature of 50°C to 70°C for 5 min to 10 min.

[0013] Another object of the present invention is to provide a surface-enhanced tryptophan-modified nanofiltration membrane prepared by any of the above-mentioned preparation methods.

[0014] In addition, the invention also provides the use of the surface-enhanced tryptophan-modified nanofiltration membrane in the field of drinking water treatment; the nanofiltration membrane improves the organic micropollutant retention rate and anti-pollution performance in drinking water treatment.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The modified nanofiltration membrane obtained by the tryptophan-mediated interfacial polymerization reaction has good pore size distribution uniformity and surface hydrophilicity.

[0016] The tryptophan modified nanofiltration membrane obtained by the invention achieves a good interception effect on organic micropollutants and exhibits good anti-pollution ability.

[0017] The present invention utilizes tryptophan, which has a structure similar to that of a surfactant. During the interfacial polymerization process, tryptophan tends to stay at the water-oil interface, controlling the diffusion of PIP into the oil phase, so that the interfacial polymerization reaction proceeds in an orderly manner and forms a dense active layer.

[0018] The present invention utilizes the fact that tryptophan has certain hydrophilicity, and tryptophan can be grafted onto the active layer during the interfacial polymerization process, thereby improving the hydrophilicity of the nanofiltration membrane surface.

[0019] The tryptophan used in the present invention is commonly present in plants, is environmentally friendly, and is suitable for drinking water treatment.

[0020] In order to further understand the features and technical content of the present invention, please refer to the following detailed description of the present invention and the accompanying drawings. However, the attached drawings are only for reference and illustration purposes, and are not used to limit the present invention. Description of the Drawings

[0021] Figure 1 It is the ultraviolet-visible absorption spectrogram of the diffusion of piperazine across the water-oil interface in the presence and absence of tryptophan; Figure 2 a and b are respectively the scanning electron microscope images of the surface of the control membrane and the tryptophan-modified nanofiltration membrane; Figure 3 a and b are respectively the water contact angle diagrams and the molecular weight cut-off diagrams of the control membrane and the tryptophan-modified nanofiltration membrane; Figure 4 a and b are respectively the X-ray photoelectron spectroscopy diagrams of the control membrane and the tryptophan-modified nanofiltration membrane; Figure 5 It is the pure water permeation flux and salt rejection rate diagrams of the control membrane and the tryptophan-modified nanofiltration membrane; Figure 6 It is the organic micro-pollutant rejection rate diagram of the control membrane and the tryptophan-modified nanofiltration membrane; Figure 7 It is the fouling curve diagram of the control membrane and the tryptophan-modified nanofiltration membrane. Detailed Embodiments

[0022] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, and at the same time do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made. These all fall within the protection scope of the present invention.

[0023] Example 1: The surface-enhanced tryptophan-modified nanofiltration membrane in this example was completed according to the following steps: Step 1: Dissolve piperazine and NaOH in deionized water, add tryptophan, and stir until completely dissolved to obtain an aqueous solution. The concentration of piperazine in the aqueous solution is 0.6 wt%, the concentration of tryptophan is 1.6 wt%, and the concentration of sodium hydroxide is 0.4 wt%; Step 2: Place the polyethersulfone ultrafiltration membrane in a reactor, pour in the aqueous solution, and conduct an adsorption reaction for 2 min (i.e., the aqueous solution contacts the surface of the polyethersulfone ultrafiltration membrane for 2 min), and then use an air gun to remove the excess aqueous solution on the surface; Step 3: Completely dissolve trimesoyl chloride in n-hexane to obtain an oil phase with a trimesoyl chloride concentration of 0.1 wt%. Then immerse the polyethersulfone ultrafiltration membrane treated in Step 2 in the oil phase. Stop the interfacial polymerization after 30 s of interfacial polymerization, take out the nascent membrane and heat-treat it in a blast drying oven at 60 °C for 5 min to obtain the tryptophan-modified nanofiltration membrane.

[0024] Comparative example: The difference between the control nanofiltration membrane of this example and the specific implementation method 1 is that: directly conduct an interfacial polymerization reaction on the surface of the polyethersulfone ultrafiltration membrane to prepare the obtained nanofiltration membrane, without going through the step of tryptophan-doped aqueous solution.

[0025] The ultraviolet-visible absorption spectrum of piperazine in the oil phase during the diffusion process of piperazine monomer across the water-oil interface is as Figure 1 shown. Compared with the situation without the presence of tryptophan, in the presence of tryptophan, the ultraviolet absorbance of piperazine in the oil phase is significantly reduced, that is, the concentration of piperazine in the oil phase is reduced, indicating that tryptophan hinders the diffusion of piperazine monomer into the oil phase.

[0026] The scanning electron microscope images of the control nanofiltration membrane and the tryptophan-modified nanofiltration membrane are as Figure 2 shown in a and b. By comparison, different from the nodular structure on the surface of the control membrane, the surface of the tryptophan-modified nanofiltration membrane prepared by the method of this example presents a wrinkled structure. This is because the presence of tryptophan hinders the diffusion of PIP, resulting in the instability of the diffusion-driven reaction and generating a specific microtopography.

[0027] The water contact angle diagrams and surface potential diagrams of the control membrane and the tryptophan-modified nanofiltration membrane are as Figure 3 shown. The water contact angle of the control membrane is 52.5°, while the water contact angle of the tryptophan-modified nanofiltration membrane prepared by the method of this example is reduced to 37.6°, showing good surface hydrophilicity. The molecular weight cut-off of the control membrane is 361 Da, while the molecular weight cut-off of the tryptophan-modified nanofiltration membrane prepared by the method of this example is reduced to 309 Da.

[0028] The O 1s fine X-ray photoelectron spectroscopy diagrams of the control membrane and the tryptophan-modified nanofiltration membrane are as Figure 4As shown in the figure. By performing deconvolution analysis on the spectrum of O1s, it was found that two characteristic peaks located at 532.4 eV and 530.9 eV appeared in both the control membrane and the tryptophan-modified nanofiltration membrane prepared by the method of this example, corresponding to the O-C=O bond and the N-C=O bond respectively, which proved the formation of the polyamide active layer. Compared with the control membrane, a characteristic peak located at 532.9 eV appeared in the tryptophan-modified nanofiltration membrane prepared by the method of this example, corresponding to the anhydride functional group, which proved the binding of tryptophan and TMC.

[0029] The pure water permeation flux and salt rejection rate of the control membrane and the tryptophan-modified nanofiltration membrane are as Figure 5 shown. The pure water flux of the control membrane is 8.3 L·m -2 ·h -1 ·bar -1 , and the pure water flux of the tryptophan-modified nanofiltration membrane prepared by the method of this example is 11.9 L·m -2 ·h -1 ·bar -1 ; the rejection rate of the control membrane for Na2SO4 is 97.6%, and the rejection rate of the modified membrane for Na2SO4 is 99.0%; the rejection rate of the control membrane for MgSO4 is 94.8%, and the rejection rate of the modified membrane for MgSO4 is 98.7%; the rejection rate of the control membrane for MgCl2 is 20.3%, and the rejection rate of the modified membrane for MgCl2 is 47.2%; the rejection rate of the control membrane for CaCl2 is 13.0%, and the rejection rate of the modified membrane for CaCl2 is 36.2%; the rejection rate of the control membrane for NaCl is 32.3%, and the rejection rate of the modified membrane for NaCl is 33.7%.

[0030] The rejection rates of organic micro-pollutants of the control membrane and the tryptophan-modified nanofiltration membrane are as Figure 6 shown. The rejection rates of the control membrane for the negatively charged micro-pollutants indomethacin and ibuprofen are 96.0% and 56.7% respectively. The rejection rates of the tryptophan-modified nanofiltration membrane prepared by the method of this example for indomethacin and ibuprofen are slightly increased, being 97.8% and 57.1% respectively. The rejection rates of the control membrane for the neutral micro-pollutants nizatidine and norfloxacin are 33.6% and 77.7% respectively. The rejection rates of the tryptophan-modified nanofiltration membrane prepared by the method of this example for nizatidine and norfloxacin are increased to 70.0% and 85.5% respectively. The rejection rates of the control membrane for the positively charged micro-pollutants trimethoprim and fluoxetine are 35.0% and 85.1% respectively, while the rejection rates of the tryptophan-modified nanofiltration membrane prepared by the method of this example for trimethoprim and fluoxetine are increased to 77.9% and 91.6% respectively.

[0031] The fouling curves of the control membrane and the tryptophan-modified nanofiltration membrane are as Figure 7As shown. After the control membrane experienced four cycles of humic acid pollution - cleaning cycles, the flux decline rate was 30.2%, and the flux decline rate of the tryptophan - modified nanofiltration membrane prepared by the method of this example was 25.1%; after the control membrane experienced four pollution - cleaning cycles, the flux recovery rate was 79.1%, and the flux recovery rate of the tryptophan - modified nanofiltration membrane prepared by the method of this example after humic acid pollution cleaning was 83.8%.

[0032] Example 2: The surface - enhanced tryptophan - modified nanofiltration membrane in this example was completed according to the following steps: Step 1: Dissolve piperazine and NaOH in deionized water, add tryptophan, and stir until completely dissolved to obtain an aqueous solution. In the aqueous solution, the concentration of piperazine is 0.6 wt%, the concentration of tryptophan is 0.6 wt%, and the concentration of sodium hydroxide is 0.4 wt%. Step 2: Place the polyethersulfone ultrafiltration membrane in a reactor, pour in the aqueous solution, carry out an adsorption reaction for 1 min (i.e., the aqueous solution contacts the surface of the polyethersulfone ultrafiltration membrane for 1 min), and then use an air gun to remove the excess aqueous solution on the surface. Step 3: Completely dissolve trimesoyl chloride in n - hexane to obtain an oil phase with a trimesoyl chloride concentration of 0.1 wt%. Then immerse the polyethersulfone ultrafiltration membrane treated in Step 2 in the oil phase, stop the interfacial polymerization after 30 s of interfacial polymerization, take out the nascent membrane, and heat - treat it in a 50°C forced - air drying oven for 5 min to obtain the tryptophan - modified nanofiltration membrane.

[0033] Example 3: The surface - enhanced tryptophan - modified nanofiltration membrane in this example was completed according to the following steps: Step 1: Dissolve piperazine and NaOH in deionized water, add tryptophan, and stir until completely dissolved to obtain an aqueous solution. In the aqueous solution, the concentration of piperazine is 0.6 wt%, the concentration of tryptophan is 2.0 wt%, and the concentration of sodium hydroxide is 0.4 wt%. Step 2: Place the polyethersulfone ultrafiltration membrane in a reactor, pour in the aqueous solution, carry out an adsorption reaction for 5 min (i.e., the aqueous solution contacts the surface of the polyethersulfone ultrafiltration membrane for 5 min), and then use an air gun to remove the excess aqueous solution on the surface. Step 3: Completely dissolve trimesoyl chloride in n - hexane to obtain an oil phase with a trimesoyl chloride concentration of 0.1 wt%. Then immerse the polyethersulfone ultrafiltration membrane treated in Step 2 in the oil phase, stop the interfacial polymerization after 30 s of interfacial polymerization, take out the nascent membrane, and heat - treat it in a 70°C forced - air drying oven for 10 min to obtain the tryptophan - modified nanofiltration membrane.

[0034] The specific embodiments of the present invention have been described above. It should be noted that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A preparation method of a surface-enhanced tryptophan-modified nanofiltration membrane, characterized in that, It includes the following steps: Step 1: Dissolve piperazine and NaOH in deionized water, add tryptophan, and stir until completely dissolved to obtain an aqueous solution; Step 2: Place the polyethersulfone ultrafiltration membrane in a reactor, pour in the aqueous solution, conduct an adsorption reaction, and then remove the excess aqueous solution on the surface; Step 3: Completely dissolve trimesoyl chloride in n-hexane to obtain an oil phase. Then immerse the polyethersulfone ultrafiltration membrane treated in Step 2 in the oil phase, conduct an interfacial polymerization reaction for a period of time, take it out and perform heat treatment to obtain the tryptophan-modified nanofiltration membrane.

2. The method according to claim 1, wherein In the aqueous solution, the concentration of piperazine is 0.6 wt%.

3. The method according to claim 1, wherein In the aqueous solution, the concentration of tryptophan is 0.6 wt% - 2.0 wt%.

4. The method according to claim 1, wherein In the aqueous solution, the concentration of sodium hydroxide is 0.4 wt%.

5. The method according to claim 1, wherein The adsorption reaction time is 1 min - 3 min.

6. The method according to claim 1, wherein The concentration of trimesoyl chloride is 0.1 wt%.

7. The method according to claim 1, wherein The interfacial polymerization reaction time is 30 s - 60 s.

8. The method according to claim 1, wherein The heat treatment time at 50°C - 70°C is 5 min - 10 min.

9. A surface-enhanced tryptophan-modified nanofiltration membrane prepared by the method according to any one of claims 1 - 8.

10. The application of the surface-enhanced tryptophan-modified nanofiltration membrane prepared by the method according to any one of claims 1 - 8 or the surface-enhanced tryptophan-modified nanofiltration membrane according to claim 9 in the field of drinking water treatment.

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