Preparation method of super-hydrophobic fiber / nanoflower membrane and application of the membrane in membrane distillation

Abstract

The application discloses a preparation method of super-hydrophobic fiber / nano-flower membrane applied to a membrane distillation process. A simple chemical bath deposition method is used to deposit a zinc oxide nano-flower thin layer on a fiber membrane, and a super-hydrophobic / nano-flower membrane is prepared through fluorination. The deposition of the zinc oxide thin layer greatly improves the roughness of the membrane surface, and forms a reentrant rough surface. The prepared super-hydrophobic fiber / nano-flower membrane has a high water contact angle, and has a better membrane distillation performance than the hydrophobic membrane reported in the literature.

Description

Technical Field

[0001] This invention relates to a method for preparing superhydrophobic fiber / nanoflower membranes, belonging to the field of superhydrophobic membrane preparation technology. Background Technology

[0002] In recent years, with rapid industrial development and a continuously rising population, the shortage of freshwater resources such as rivers and lakes, as well as various water pollution problems, have increasingly affected and constrained human survival and development. Seawater, as the largest body of water on Earth, requires desalination as a primary means to address the current freshwater shortage. Membrane distillation (MD), as an emerging desalination technology, works by utilizing the vapor pressure difference generated by the different temperatures on both sides of a membrane, allowing water vapor to pass from the hot side to the cold side. Therefore, theoretically, it can achieve a 100% retention rate for ions, macromolecules, colloids, cells, and other non-volatile substances. Furthermore, membrane distillation requires a low-grade heat source and can fully utilize low-grade thermal energy such as geothermal and solar energy, showing promising application prospects in the treatment of industrial saline wastewater.

[0003] Traditional membrane distillation membranes are made of hydrophobic polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polypropylene (PP), or are prepared by hydrophobic modification of inorganic membranes. However, when the feed solution contains complex components such as organic matter, inorganic particles, and microorganisms, these substances can foul the membrane surface and wet the pores of the hydrophobic membrane, leading to a significant decrease in the membrane's desalination rate and permeate flux, thus reducing its service life. Therefore, preparing membrane materials that can maintain high antifouling and antiwetting properties when used with complex feed solutions is essential for improving the service life of membrane materials.

[0004] Existing technologies employ various methods to hydrophobically treat membranes; however, the superhydrophobic membranes prepared by existing technologies suffer from drawbacks such as poor particle-membrane bonding and poor resistance to wetting by surface-active substances. Summary of the Invention

[0005] This invention enhances the bonding force between nanoparticles and the fiber membrane by coating a dopamine layer onto the fiber-based membrane, while simultaneously depositing nanoflowers on the membrane surface to significantly increase the membrane roughness and form a rough, reentrant structure. Furthermore, a superhydrophobic fiber / nanoflower membrane is prepared by reducing the surface energy of the membrane through fluorination. Droplets on the membrane exhibit a Cassie-Baxter state, demonstrating excellent anti-wetting properties, which can mitigate the contamination of various pollutants in wastewater, thereby improving the stability of membrane distillation wastewater treatment processes. This invention holds significant importance and shows promising prospects for seawater desalination and the treatment of various industrial wastewaters.

[0006] The purpose of this invention is to overcome the problems existing in the prior art. This invention provides a method for preparing superhydrophobic fiber / nanoflower film based on chemical bath deposition.

[0007] The technical solution adopted by this invention to solve the technical problem is: a method for preparing a superhydrophobic fiber / nanoflower membrane: a nanoflower-shaped zinc oxide thin layer is deposited on a fiber membrane pretreated with dopamine by chemical bath deposition, and then the superhydrophobic fiber / nanoflower membrane is prepared by fluorination with fluorosilane. Specifically, it includes the following steps:

[0008] A. Dopamine-coated base membrane: The wetted fiber membrane is immersed in a dopamine solution, so that dopamine can be uniformly coated on the membrane surface;

[0009] B. Depositing zinc oxide nanoflowers on the surface of a dopamine-treated fiber membrane: using a Zn-containing... 2+ The compound was used as a precursor. Ethanolamine (MEA) and ammonia (NH3·H2O) were added and stirred to obtain a homogeneous precursor solution. The membrane treated in step A was then placed in the precursor solution and heated for a certain time to allow a thin layer of ZnO to be deposited on the membrane surface. After the reaction was completed, the membrane was washed with deionized water and then placed in an oven to dry.

[0010] C. The membrane is immersed in an ethanol solution containing a certain volume ratio of fluorosilane and subjected to a fluorination reaction under certain temperature conditions.

[0011] The membrane was then heat-treated to prepare a superhydrophobic fiber / nanoflower membrane.

[0012] Preferably, the wetting in step A involves immersing the fiber membrane in anhydrous ethanol for a certain period of time to fully wet the membrane, and then washing the membrane with deionized water to remove excess ethanol; the time for immersing the base membrane in anhydrous ethanol is 10-20 minutes, and the time for washing with deionized water is 3-10 minutes.

[0013] Preferably, the dopamine solution in step A is prepared by adding dopamine hydrochloride to a Tris buffer solution to form a dopamine solution of a certain concentration; the concentration of the Tris buffer solution is 5-15 mmol / L; the concentration of the dopamine hydrochloride solution prepared in step A is 1-3 g / L.

[0014] Preferably, the dopamine solution in step A is subjected to stirring at 200-300 rpm for 4-24 hours.

[0015] Preferably, in step B, Zn 2+ The precursors are Zn(NO3)2·6H2O, Zn(Ac)2·2H2O or ZnSO4·7H2O.

[0016] Preferably, in step B, when depositing zinc oxide on the film, Zn 2+ The concentration of the precursor solution is 50-75 mmol / L.

[0017] Preferably, the temperature for zinc oxide deposition in step B is 90-96℃, and the zinc oxide deposition time is 0.5-3h.

[0018] Preferably, the fluorosilane ethanol solution prepared in step C has a volume concentration of 1%-2% v / v.

[0019] Preferably, the fluorination reaction in step C is carried out at 60°C for 12-48 hours.

[0020] Preferably, the heat treatment temperature in the step is 90-150℃, and the reaction time is 3-6h.

[0021] The advantages and positive effects of this invention are:

[0022] This invention prepares a superhydrophobic fiber / nanoflower membrane by synergistically enhancing the surface roughness of a fiber membrane through zinc oxide nanoflower deposition and reducing surface energy through fluorosilane fluorination. Compared to the original membrane, the membrane prepared by this method exhibits significantly increased surface roughness and enhanced hydrophobicity, demonstrating excellent repellency to various liquids and superior performance in membrane distillation experiments involving saline wastewater. To address the issue of weak adhesion between nanoparticles and the base membrane, this invention pretreats the fiber membrane with dopamine. Dopamine, containing catechol and ethylamine functional groups, possesses high hydrophilicity and can activate the fiber base membrane. Furthermore, dopamine undergoes self-polymerization under weakly alkaline conditions, synthesizing polydopamine on the membrane surface. Polydopamine is a bio-inspired polymer with a molecular structure similar to mussel adhesive proteins, exhibiting high adhesion. The polydopamine layer, acting as an interlayer, significantly improves the adhesion between nanoparticles and the fiber membrane. This invention utilizes the unique properties of dopamine to achieve a stable bond between nanoparticles and the fiber membrane, ensuring long-term stable operation of membrane distillation. Attached Figure Description

[0023] Figure 1 This is a SEM image of the selected PVDF fiber membrane surface;

[0024] Figure 2 The images show a comparison of SEM images of ZnO deposited directly on a PVDF substrate and the composite film with nano-flower-shaped ZnO deposited in Example 8.

[0025] Figure 3 This is the XRD pattern of the composite membrane prepared in Example 8;

[0026] Figure 4 This is an XPS image of the composite membrane prepared in Example 8.

[0027] Figure 5 This is a comparison diagram of the water contact angles of various membranes prepared in Example 8.

[0028] Figure 6 This is a schematic diagram of the surface roughness of various films prepared in Example 8. Detailed Implementation

[0029] Example 1

[0030] First, a commercial PVDF fiber membrane was thoroughly wetted in anhydrous ethanol, and then the ethanol on the membrane surface was washed with deionized water. The washed membrane was then placed in a prepared 1 g / L dopamine solution, and mechanical stirring was performed at approximately 200 rpm to ensure that the dopamine was uniformly coated on the fiber membrane. After mechanical stirring for 4 hours, the membrane was removed and washed with deionized water. The washed membrane was then placed in an oven at 60°C and dried for 4 hours (referred to as PVDF-D). 0.781 g of zinc nitrate hexahydrate was weighed and added to 35 mL of deionized water. The solution was magnetically stirred to dissolve the zinc nitrate. Then, 4 mL of ethanolamine and 1 mL of ammonia were added to the solution, and the solution was magnetically stirred to prepare a 75 mmol / L zinc nitrate precursor solution. The dopamine-coated membrane was placed in the solution, and the oil bath temperature was set to 90°C for 3 hours. The membrane was then removed and washed with deionized water. The washed membrane was placed in an oven at 60℃ and dried for 6 hours (referred to as PVDF-DZ). The dried membrane was then placed in an ethanol solution of 2% v / v PFDTES and reacted at 60℃ for 36 hours. Afterward, the membrane was removed and heat-treated at 120℃ for 3 hours (referred to as PVDF-DZF). The prepared superhydrophobic membrane had a water contact angle of 174.5°. During membrane distillation performance testing, under the conditions of a 3.5wt% NaCl solution feed, a feed temperature of 60℃, a vacuum of 0.97 MPa, and a feed flow rate of 200 ml / min, the membrane distillation flux was 15.8 kg·m³. -2 ·h -1 .

[0031] Example 2

[0032] First, a commercial fiber membrane was thoroughly wetted by immersing it in anhydrous ethanol. Then, the ethanol on the membrane surface was washed with deionized water. The washed membrane was then placed in a prepared 1 g / L dopamine solution, and mechanical stirring was performed at approximately 200 rpm to ensure uniform coating of the fiber membrane with dopamine. After mechanical stirring for 4 hours, the membrane was removed and washed with deionized water. The washed membrane was then placed in an oven at 60°C and dried for 4 hours. 0.729 g of zinc nitrate hexahydrate was weighed and added to 35 mL of deionized water. The solution was magnetically stirred to dissolve the zinc nitrate. Then, 4 mL of ethanolamine and 1 mL of ammonia were added to the solution, and the solution was magnetically stirred to prepare a 70 mmol / L zinc nitrate precursor solution. The dopamine-coated membrane was placed in the solution, and the oil bath temperature was set to 90°C for 3 hours. The membrane was then removed and washed with deionized water. The washed membrane was placed in an oven at 60℃ and dried for 6 hours. The dried membrane was then placed in a 2% v / v PFDTES ethanol solution and reacted at 60℃ for 36 hours. Afterward, the membrane was removed and heat-treated at 120℃ for 3 hours. The prepared superhydrophobic membrane had a water contact angle of 174.1°. During membrane distillation performance testing, under the conditions of a 3.5 wt% NaCl solution feed at 60℃, a vacuum of 0.97 MPa, and a feed flow rate of 200 ml / min, the membrane distillation flux was 25.5 kg·m³. -2 ·h -1 .

[0033] Example 3

[0034] First, a commercial fiber membrane was thoroughly wetted in anhydrous ethanol, then the ethanol on the membrane surface was washed with deionized water. The washed membrane was then placed in a prepared 1 g / L dopamine solution, and mechanical stirring was performed at approximately 200 rpm to ensure uniform coating of the fiber membrane with dopamine. After mechanical stirring for 4 hours, the membrane was removed and washed with deionized water. The washed membrane was then placed in an oven at 60°C and dried for 4 hours. 0.677 g of zinc nitrate hexahydrate was weighed and added to 35 mL of deionized water, and the solution was magnetically stirred to dissolve the zinc nitrate. Then, 4 mL of ethanolamine and 1 mL of ammonia were added to the solution, and the solution was magnetically stirred to prepare a 65 mmol / L zinc nitrate precursor solution. The dopamine-coated membrane was placed in the solution, and the oil bath temperature was set to 90°C for 3 hours. The membrane was then removed and washed with deionized water. The washed membrane was placed in an oven at 60℃ and dried for 6 hours. The dried membrane was then placed in a 2% v / v PFDTES ethanol solution and reacted at 60℃ for 36 hours. Afterward, the membrane was removed and heat-treated at 120℃ for 3 hours. The prepared superhydrophobic membrane had a water contact angle of 173.8°. During membrane distillation performance testing, under the conditions of a 3.5 wt% NaCl solution feed at 60℃, a vacuum of 0.97 MPa, and a feed flow rate of 200 ml / min, the membrane distillation flux was 33.7 kg·m³. -2 ·h -1 .

[0035] Example 4

[0036] First, a commercial fiber membrane was thoroughly wetted by immersing it in anhydrous ethanol. Then, the ethanol on the membrane surface was washed with deionized water. The washed membrane was then placed in a prepared 1 g / L dopamine solution, and mechanical stirring was performed at approximately 200 rpm to ensure uniform coating of the fiber membrane with dopamine. After mechanical stirring for 4 hours, the membrane was removed and washed with deionized water. The washed membrane was then placed in an oven at 60°C and dried for 4 hours. 0.625 g of zinc nitrate hexahydrate was weighed and added to 35 mL of deionized water. The solution was magnetically stirred to dissolve the zinc nitrate. Then, 4 mL of ethanolamine and 1 mL of ammonia were added to the solution, and the solution was magnetically stirred to prepare a 60 mmol / L zinc nitrate precursor solution. The dopamine-coated membrane was placed in the solution, and the oil bath temperature was set to 90°C for 3 hours. The membrane was then removed and washed with deionized water. The washed membrane was placed in an oven at 60℃ and dried for 6 hours. The dried membrane was then placed in a 2% v / v PFDTES ethanol solution and reacted at 60℃ for 36 hours. Afterward, the membrane was removed and heat-treated at 120℃ for 3 hours. The prepared superhydrophobic membrane had a water contact angle of 173.7°. During membrane distillation performance testing, under the conditions of a 3.5 wt% NaCl solution, a feed temperature of 60℃, a vacuum of 0.97 MPa, and a feed flow rate of 200 ml / min, the membrane distillation flux was 40.8 kg·m³. -2 ·h -1 .

[0037] Example 5

[0038] First, a commercial fiber membrane was thoroughly wetted by immersing it in anhydrous ethanol. Then, the ethanol on the membrane surface was washed with deionized water. The washed membrane was then placed in a prepared 1 g / L dopamine solution, and mechanical stirring was performed at approximately 200 rpm to ensure uniform coating of the fiber membrane with dopamine. After mechanical stirring for 4 hours, the membrane was removed and washed with deionized water. The washed membrane was then placed in an oven at 60°C and dried for 4 hours. 0.521 g of zinc nitrate hexahydrate was weighed and added to 35 mL of deionized water. The solution was magnetically stirred to dissolve the zinc nitrate. Then, 4 mL of ethanolamine and 1 mL of ammonia were added to the solution, and the solution was magnetically stirred to prepare a 50 mmol / L zinc nitrate precursor solution. The dopamine-coated membrane was placed in the solution, and an oil bath temperature of 90°C was set for 3 hours. The membrane was then removed and washed with deionized water. The washed membrane was placed in an oven at 60℃ and dried for 6 hours. The dried membrane was then placed in a 2% v / v PFDTES ethanol solution and reacted at 60℃ for 36 hours. Afterward, the membrane was removed and heat-treated at 120℃ for 3 hours. The prepared superhydrophobic membrane had a water contact angle of 173.5°. During membrane distillation performance testing, under the conditions of a 3.5 wt% NaCl solution, a feed temperature of 60℃, a vacuum of 0.97 MPa, and a feed flow rate of 200 ml / min, the membrane distillation flux was 46.5 kg·m³. -2 ·h -1 .

[0039] Example 6

[0040] First, a commercial fiber membrane was thoroughly wetted by immersing it in anhydrous ethanol. Then, the ethanol on the membrane surface was washed with deionized water. The washed membrane was then placed in a prepared 1 g / L dopamine solution, and mechanical stirring was performed at approximately 200 rpm to ensure uniform coating of the fiber membrane with dopamine. After mechanical stirring for 4 hours, the membrane was removed and washed with deionized water. The washed membrane was then placed in an oven at 60°C and dried for 4 hours. 0.521 g of zinc nitrate hexahydrate was weighed and added to 35 mL of deionized water. The solution was magnetically stirred to dissolve the zinc nitrate. Then, 4 mL of ethanolamine and 1 mL of ammonia were added to the solution, and the solution was magnetically stirred to prepare a 50 mmol / L zinc nitrate precursor solution. The dopamine-coated membrane was placed in the solution, and an oil bath temperature of 90°C was set for 2 hours. The membrane was then removed and washed with deionized water. The washed membrane was placed in an oven at 60℃ and dried for 6 hours. The dried membrane was then placed in a 2% v / v PFDTES ethanol solution and reacted at 60℃ for 36 hours. Afterward, the membrane was removed and heat-treated at 120℃ for 3 hours. The prepared superhydrophobic membrane had a water contact angle of 173.3°. During membrane distillation performance testing, under the conditions of a 3.5 wt% NaCl solution, a feed temperature of 60℃, a vacuum of 0.97 MPa, and a feed flow rate of 200 ml / min, the membrane distillation flux was 50.7 kg·m³. -2 ·h -1 .

[0041] Example 7

[0042] First, a commercial fiber membrane was thoroughly wetted by immersing it in anhydrous ethanol. Then, the ethanol on the membrane surface was washed with deionized water. The washed membrane was then placed in a prepared 1 g / L dopamine solution, and mechanical stirring was performed at approximately 200 rpm to ensure uniform coating of the fiber membrane with dopamine. After mechanical stirring for 4 hours, the membrane was removed and washed with deionized water. The washed membrane was then placed in an oven at 60°C and dried for 4 hours. 0.521 g of zinc nitrate hexahydrate was weighed and added to 35 mL of deionized water. The solution was magnetically stirred to dissolve the zinc nitrate. Then, 4 mL of ethanolamine and 1 mL of ammonia were added to the solution, and the solution was magnetically stirred to prepare a 50 mmol / L zinc nitrate precursor solution. The dopamine-coated membrane was placed in the solution, and the oil bath temperature was set to 90°C for 1 hour. The membrane was then removed and washed with deionized water. The washed membrane was placed in an oven at 60℃ and dried for 6 hours. The dried membrane was then placed in a 2% v / v PFDTES ethanol solution and reacted at 60℃ for 36 hours. Afterward, the membrane was removed and heat-treated at 120℃ for 3 hours. The prepared superhydrophobic membrane had a water contact angle of 173.1°. During membrane distillation performance testing, under the conditions of a 3.5 wt% NaCl solution, a feed temperature of 60℃, a vacuum of 0.97 MPa, and a feed flow rate of 200 ml / min, the membrane distillation flux was 56.5 kg·m³. -2 ·h -1 .

[0043] Example 8

[0044] First, a commercial fiber membrane was thoroughly wetted by immersing it in anhydrous ethanol. Then, the ethanol on the membrane surface was washed with deionized water. The washed membrane was then placed in a prepared 1 g / L dopamine solution, and mechanical stirring was performed at approximately 200 rpm to ensure uniform coating of the fiber membrane with dopamine. After mechanical stirring for 4 hours, the membrane was removed and washed with deionized water. The washed membrane was then placed in an oven at 60°C and dried for 4 hours. 0.521 g of zinc nitrate hexahydrate was weighed and added to 35 mL of deionized water. The solution was magnetically stirred to dissolve the zinc nitrate. Then, 4 mL of ethanolamine and 1 mL of ammonia were added to the solution, and the solution was magnetically stirred to prepare a 50 mmol / L zinc nitrate precursor solution. The dopamine-coated membrane was placed in the solution, and the oil bath temperature was set to 90°C for 0.5 hours. The membrane was then removed and washed with deionized water. The washed membrane was placed in an oven at 60℃ and dried for 6 hours. The dried membrane was then placed in a 2% v / v PFDTES ethanol solution and reacted at 60℃ for 36 hours. Afterward, the membrane was removed and heat-treated at 120℃ for 3 hours. The prepared superhydrophobic membrane had a water contact angle of 172.8°. During membrane distillation performance testing, under the conditions of a 3.5 wt% NaCl solution, a feed temperature of 60℃, a vacuum of 0.97 MPa, and a feed flow rate of 200 ml / min, the membrane distillation flux was 60.5 kg·m³. -2 ·h -1 .

[0045] Comparative Example 1

[0046] The difference from Example 8 is that a PVDF-DF film was prepared without generating a ZnO nanoflower structure layer.

[0047] First, a commercially available fiber membrane was thoroughly wetted in anhydrous ethanol. Then, the ethanol on the membrane surface was washed with deionized water. The washed membrane was then placed in a prepared 1 g / L dopamine solution, and mechanical stirring was performed at approximately 200 rpm to ensure uniform coating of the fiber membrane with dopamine. After stirring for 4 hours, the membrane was removed and washed with deionized water. The washed membrane was then placed in an oven at 60°C and dried for 4 hours. The dried membrane was then placed in a 2% v / v PFDTES ethanol solution and reacted at 60°C for 36 hours. Afterward, the membrane was removed and heat-treated at 120°C for 3 hours. The resulting hydrophobic membrane had a water contact angle of 127.5°.

[0048] In the above preparation process, if dopamine is directly loaded first and then fluorosilane is directly modified, the strong phenolic hydroxyl group effect on the surface of dopamine will prevent the fluorosilane from being effectively modified and grafted, thus failing to achieve surface hydrophobicity. In addition, if zinc oxide nanoflower layer is directly generated without coating dopamine layer, the zinc oxide nanolayer cannot be effectively fixed on the surface of the membrane, which will also prevent the membrane preparation process from obtaining a complete and defect-free superhydrophobic membrane.

[0049] The XRD characterization results of the several films prepared in this embodiment are as follows: Figure 3 As shown, the XPS characterization results are as follows: Figure 4 As shown, the water droplet contact angle test is as follows: Figure 5 As shown, the PVDF-DZF film exhibits a higher contact angle compared to the PVDF-DF film. This is because the formation of the nanoflower structure increases the surface roughness, thereby increasing the contact angle. This can also be seen from... Figure 5 This has been proven in practice.

[0050] Table 1 compares the performance of the superhydrophobic fiber / nanoflower membrane prepared in Example 8 with that of the superhydrophobic membranes in the literature.

[0051]

Claims

1. The application of superhydrophobic fiber / nanoflower membrane in membrane distillation, characterized in that... The preparation method of superhydrophobic fiber / nanoflower film includes the following steps: A. Dopamine-coated base membrane: The wetted fiber membrane is immersed in a dopamine solution, so that dopamine can be uniformly coated on the membrane surface; B. Depositing zinc oxide nanoflowers on the surface of a dopamine-treated fiber membrane: using a Zn-containing... 2+ The compound is used as a precursor. Ethanolamine and ammonia are added and stirred to obtain a homogeneous precursor solution. Then, the membrane treated in step A is placed in the precursor solution and heated for a certain time to allow a thin layer of ZnO to be deposited on the membrane surface. After the reaction is completed, the membrane is washed with deionized water and then placed in an oven to dry. C. The membrane is immersed in an ethanol solution containing a certain volume ratio of fluorosilane and fluorinated under certain temperature conditions. Then the membrane is heat-treated to prepare a superhydrophobic fiber / nanoflower membrane. The dopamine solution in step A is prepared by adding dopamine hydrochloride to a Tris buffer solution to obtain a dopamine solution of a certain concentration. The concentration of the Tris buffer solution is 5-15 mmol / L; the concentration of the dopamine hydrochloride solution prepared in step A is 1-3 g / L; the wetted fiber membrane is immersed in the dopamine solution while stirring at 200-300 rpm for 4-24 hours. In step B, Zn 2+ The precursors are Zn(NO3)2·6H2O, Zn(Ac)2·2H2O, or ZnSO4·7H2O; so that when a ZnO thin layer is deposited on the film surface, Zn 2+ The concentration of the precursor solution was 50-75 mmol / L, the temperature was 90-96℃, and the deposition time was 0.5-3h. The volume concentration of the fluorosilane ethanol solution prepared in step C is 1-2% v / v; the fluorination reaction is carried out at 60°C for 12-48 hours.

2. The application according to claim 1, characterized in that... In step A, wetting involves immersing the fiber membrane in anhydrous ethanol for a certain period of time to fully wet the membrane. After removal, the membrane is washed with deionized water to remove excess ethanol. The time for wetting the base membrane with anhydrous ethanol is 10-20 minutes, and the time for washing with deionized water is 3-10 minutes.

3. The application according to claim 1, characterized in that... In step C, the heat treatment temperature is 90-150℃ and the reaction time is 3-6h.

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