Preparation Method and Application of an Anti-Fouling Hydrogel Composite Membrane for Membrane Distillation

Abstract

The present invention provides a method for preparing an anti-fouling hydrogel composite membrane for membrane distillation, comprising the following steps: subjecting the support layer of a hydrophobic membrane to oxygen plasma treatment to obtain a membrane product after plasma treatment; the hydrophobic membrane is selected from a polytetrafluoroethylene hydrophobic membrane or a polyvinylidene fluoride hydrophobic membrane; grafting and copolymerizing a hydrogel prepolymer solution inside and on the surface of the support layer of the membrane product after plasma treatment to obtain the anti-fouling hydrogel composite membrane. The present invention also provides the application of the anti-fouling hydrogel composite membrane in membrane distillation. The hydrogel composite membrane of the present invention has strong anti-oil pollution ability underwater. Its underwater hydrophilic and super-oleophobic property can effectively prevent oil pollution from blocking the pores of the hydrophobic membrane. The hydrogel composite membrane has excellent ability to prevent the destruction of membrane pores by surfactants, and also has a certain salt rejection effect, can effectively extend the service life of the hydrophobic membrane, has universality, and has the prospect of popularization and application in the fields of seawater desalination, oil and gas field produced water treatment, industrial wastewater treatment, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of membrane distillation wastewater treatment, and specifically relates to a preparation method of an anti-fouling hydrogel composite membrane for membrane distillation, and also relates to the application of an anti-fouling hydrogel composite membrane for membrane distillation. Background Art

[0002] In recent years, under the dual pressures of water resource shortage and water pollution, traditional high-salt wastewater treatment methods mainly based on discharge have been subject to more and more policy restrictions. Exploring zero liquid discharge solutions for high-salt wastewater has gradually received more attention globally and developed rapidly. Electrodialysis, forward osmosis, and membrane distillation, as technologies for achieving zero liquid discharge, further concentrate wastewater after the reverse osmosis stage. Membrane distillation is considered an effective way to solve the problem of high-salt wastewater because of its high inlet salinity limit (>200 g / L), theoretically 100% retention effect on various inorganic salts in the solution (such as chloride salts, calcium salts), and low operating temperature (usually 40 - 80°C), which can effectively utilize low-grade waste heat.

[0003] However, when commercial hydrophobic membranes are used to treat high-salt wastewater with coexisting organic and inorganic pollutants, the strong hydrophobic interaction between organic pollutants such as oil droplets and the membrane surface makes the membrane easily blocked by pollutants such as oil; organic pollutants similar to surfactants will reduce the surface tension of the liquid, greatly increasing the risk of membrane pore wetting; as the salt concentration continues to increase, it may exceed the saturation concentration, and crystallization will occur on the membrane surface (heterogeneous nucleation) or in the nearby liquid phase (homogeneous nucleation), resulting in membrane fouling. The existence of the above problems will seriously affect the stable operation of the membrane distillation system.

[0004] To solve the above problems, the most direct and effective way is to modify the membrane material itself. In recent years, many membrane modification methods have been developed, including layer-by-layer assembly, coating and other methods, but they basically have disadvantages such as poor stability and damage to the membrane structure, and the modification of traditional membranes cannot effectively solve the problem of treating high-salt wastewater with coexisting organic and inorganic pollutants.

[0005] Based on this, providing a composite membrane product with universality and capable of specifically treating high-salt wastewater containing oil and surfactants and its preparation method is a technical problem that urgently needs to be solved. Summary of the Invention

[0006] One of the purposes of the present invention is to provide an anti-fouling hydrogel composite membrane for membrane distillation that can effectively treat high-salt wastewater containing oil and surfactants.

[0007] One of the purposes of the present invention is to provide a preparation method of an anti-fouling hydrogel composite membrane for membrane distillation that can effectively treat high-salt wastewater containing oil and surfactants.

[0008] One of the technical solutions adopted to achieve the object of the present invention is: to provide a method for preparing an anti-fouling hydrogel composite membrane for membrane distillation, comprising the following steps:

[0009] Perform oxygen plasma treatment on the support layer of the hydrophobic membrane to obtain a membrane product after plasma treatment; the hydrophobic membrane is selected from a polytetrafluoroethylene hydrophobic membrane or a polyvinylidene fluoride hydrophobic membrane;

[0010] Graft copolymerize the hydrogel prepolymer solution onto the surface of the support layer of the plasma-treated membrane product to obtain the anti-fouling hydrogel composite membrane.

[0011] The general idea of the present invention is as follows: The key component in the membrane distillation system is the hydrophobic membrane, which has the property of being breathable but impermeable to water and provides a transmission path for water vapor. In the present invention, a hydrogel is graft copolymerized on the surface of the hydrophobic membrane to form a hydrogel composite membrane. During the operation of membrane distillation, the hydrophilic layer of the hydrogel contacts the feed solution, and it has the property of underwater superoleophobicity, which can effectively prevent the attachment of oil stains on the membrane surface; in addition, the hydrophilic-hydrophobic composite structure can also effectively prevent the surfactant from damaging the hydrophobic membrane. Further, the polyelectrolyte hydrogel has the Donnan effect, making the salt ion concentration inside the hydrogel always lower than that of the external salt solution, and inorganic pollutants such as sodium chloride crystallization will not appear inside the gel, thereby reducing membrane fouling and ensuring the stable operation of the membrane distillation system.

[0012] In the present invention, a commercial hydrophobic membrane (polytetrafluoroethylene hydrophobic membrane or polyvinylidene fluoride hydrophobic membrane) with a support layer at the bottom is used, which can not only provide support for the relatively thin hydrophobic layer to ensure the smooth operation of membrane distillation, but also graft copolymerize the hydrogel inside and on the surface of the support layer through oxygen plasma treatment of the support layer, and the preparation process will not change the structure and properties of polytetrafluoroethylene, effectively avoiding the damage to the structure of the hydrophobic layer caused by surface treatment.

[0013] Further, the hydrophobic membrane comprises a hydrophobic layer and a support layer. The hydrophobic layers of the polytetrafluoroethylene hydrophobic membrane and the polyvinylidene fluoride hydrophobic membrane are polytetrafluoroethylene and polyvinylidene fluoride respectively, and the support layer is selected from polypropylene or polyethylene terephthalate. The thickness of the hydrophobic layer is 5-15 μm, and the thickness of the support layer is 100-130 μm.

[0014] In some preferred embodiments, the thickness of the hydrophobic layer of the tetrafluoroethylene hydrophobic membrane is 10 μm, the thickness of the support layer is 120 μm, the pore size of the polytetrafluoroethylene hydrophobic membrane is 100 nm, and the diameter is 47 nm.

[0015] In some preferred embodiments, the thickness of the hydrophobic layer of the polyvinylidene fluoride hydrophobic membrane is 10 μm, the thickness of the support layer is 110 μm, and the pore size of the polytetrafluoroethylene hydrophobic membrane is 220 nm.

[0016] Furthermore, the power of the oxygen plasma treatment is 18 - 250 W, and the time is 1 - 10 min. By adjusting the power and treatment time of the oxygen plasma, the grafting rate of the hydrogel on the surface of the support layer can be increased.

[0017] In the present invention, the method of graft copolymerizing the hydrogel prepolymer solution onto the surface of the treated membrane product can be carried out by thermal polymerization or photopolymerization.

[0018] For graft copolymerization by thermal polymerization, in the hydrogel prepolymer solution, the molar ratio of the monomer to the crosslinking agent is 50 - 1000:1, preferably 50:1. Since the hydrogel layer of the present invention needs to be applied to treat high - salt wastewater in membrane distillation, this has relatively high requirements for the mechanical strength and swelling performance of the hydrogel layer. By controlling the proportion of the monomer, the swelling phenomenon of the hydrogel layer when contacting high - salt wastewater can be inhibited, ensuring relatively high mechanical strength and guaranteeing the long - term stable operation of the membrane distillation system.

[0019] Preferably, the monomer is selected from one or a combination of more of sodium 2 - methyl - 2 - acrylamido - propanesulfonate, sodium acrylate, acrylamide, [2 - (methacrylamidoxy)ethyl]dimethyl - (3 - sulfopropyl) ammonium hydroxide, and sodium 2 - acrylamido - 2 - methyl - 1 - propane sulfonate, and its concentration is 1 - 3 mol / L;

[0020] Preferably, the crosslinking agent is N,N’ - methylene bisacrylamide, and its concentration is 1 - 60 mmol / L;

[0021] The hydrogel prepolymer solution further includes a catalyst, and the catalyst is N,N,N',N' - tetramethylethylenediamine, and its volume concentration is 0.1 vol%.

[0022] Furthermore, the method of graft copolymerization includes: making a mold on the surface of the support layer of the membrane product after plasma treatment, mixing the hydrogel prepolymer solution with an initiator evenly and inverting it in the mold, covering a shaping plate on the surface of the mold, and reacting under vacuum conditions to obtain an anti - fouling hydrogel composite membrane.

[0023] Preferably, the initiator is ammonium persulfate, and its concentration is 3 - 6 mmol / L.

[0024] Preferably, the degree of vacuum of the vacuum condition is 0.05 - 0.08 Mpa; the reaction temperature under the vacuum condition is 30 - 60 °C.

[0025] In the above graft copolymerization method, the polypropylene support layer of the commercial hydrophobic membrane polytetrafluoroethylene is activated by plasma. Through the sulfate anion radical in the thermal initiator to initiate the reaction, hydrogen is extracted from a functional group on the side chain of the substrate (i.e., COOH and C-OH..) to form the corresponding radical, and then the generated radical initiates the graft copolymerization of the hydrogel. It belongs to the thermal polymerization method in the preparation of hydrogels and does not require a hydrophilic polymer compound as a link.

[0026] For the graft copolymerization by the photopolymerization method, in the hydrogel prepolymer solution, the molar ratio of the monomer to the crosslinker is 50 to 1000:1, preferably 50:1.

[0027] The monomer is selected from one or a combination of more than one of sodium 2-methyl-2-acrylamidopropanesulfonate, sodium acrylate, acrylamide, [2-(methacrylamidoxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide, and sodium 2-acrylamido-2-methyl-1-propanesulfonate, and its concentration is 1 to 3 mol / L; the crosslinker is N,N'-methylenebisacrylamide, and its concentration is 1 to 60 mmol / L.

[0028] Further, the graft copolymerization method includes: immersing the surface of the support layer of the plasma-treated membrane product in a silane coupling agent solution for pretreatment, making a mold on the surface of the support layer of the pretreated membrane product, inverting the hydrogel prepolymer solution in the mold, and carrying out a polymerization reaction under nitrogen protection in an ultraviolet light chamber to obtain an anti-pollution hydrogel composite membrane.

[0029] Preferably, the photoinitiator is selected from 2-hydroxy-2-methyl-1-[4-(2-hydroxyethoxy)phenyl]-1-propanone or α-ketoglutaric acid, and its concentration is 1 to 30 mmol / L. The ultraviolet wavelength of the polymerization reaction is 365 nm, and the ultraviolet intensity is 4 mW cm -2 , and the polymerization reaction time is 4 to 8 h. More preferably, the polymerization reaction time is 6 h.

[0030] Further, the preparation method of the silane coupling agent solution includes: adding 3-(isobutenyloxy)propyltrimethoxysilane and glacial acetic acid to deionized water according to the addition amounts of 20 g / L and 0.015 vot%, respectively, and mixing for 4 h to obtain the silane coupling agent solution.

[0031] In the above graft copolymerization method, high-density functional groups are generated on the side of the polyethylene terephthalate support layer by plasma treatment. Using the silane coupling agent as a bridge, and then photopolymerizing the hydrogel.

[0032] Both of the above methods can prepare an anti-pollution hydrogel composite membrane for membrane distillation, the structure of which includes a polytetrafluoroethylene hydrophobic layer and a hydrogel layer embedded in the interior and surface of a support layer. Furthermore, in the present invention, the thickness of the hydrogel can also be controlled to achieve the regulation of heat and mass transfer in membrane distillation. In the present invention, the thickness of the hydrogel layer is 50 μm to 5 mm. Preferably, the thickness of the hydrogel layer is 50 to 100 μm.

[0033] The technical solution adopted by the present invention to achieve the second purpose is: to provide an application of an anti-pollution hydrogel composite membrane for membrane distillation prepared by the preparation method described in one of the purposes of the present invention, comprising: placing the anti-pollution hydrogel composite membrane in a system membrane distillation in a manner such that the hydrogel side is close to the hot end and the polytetrafluoroethylene side is close to the cold end.

[0034] When the hydrogel composite membrane prepared by the present invention is used, the hydrogel side must be kept in contact with the hot end wastewater, and the oil stains are prevented from contacting the hydrophobic membrane and blocking the hydrophobic pores by means of the blocking effect of the hydrogel layer on the oil stains; at the same time, the structure of the hydrogel composite membrane can also prevent the surfactant from damaging the hydrophobic membrane. If the hydrogel layer faces the cold end, the above function cannot be achieved. When in use, the evaporation interface of the hydrogel composite membrane of the present invention is located at the interface between the hydrogel and the hydrophobic membrane.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] (1) The present invention provides a method for preparing an anti-pollution hydrogel composite membrane for membrane distillation, wherein a hydrogel prepolymer is grafted and copolymerized on the surface of the support layer of the membrane product after plasma treatment to obtain an anti-pollution hydrogel composite membrane. With the help of the three-dimensional network structure of the hydrogel, the internal pores can be intelligently adjusted to block macromolecular substances by adjusting the concentration of its monomers and cross-linking agents. The preparation method provided by the present invention has a simple process, readily available materials, and low cost, and has the potential to be applied to large-scale industrial production.

[0037] (2) The anti-pollution hydrogel composite membrane for membrane distillation prepared by the present invention has a strong ability to prevent oil pollution underwater. Compared with commercial polytetrafluoroethylene, polyvinylidene fluoride, and polypropylene hydrophobic membranes, the underwater hydrophilicity and oleophobicity of this composite membrane prevent oil pollution from clogging the hydrophobic membrane pores. In addition, the hydrogel composite membrane has excellent ability to prevent surfactants from damaging the membrane pores. Commercial hydrophobic membranes are very likely to become permanently ineffective due to wetting of the membrane pores when surfactants and salt ions coexist. The hydrogel layer of the composite membrane can prevent low surface tension substances such as surfactants from damaging the membrane pores. Furthermore, the hydrogel composite membrane of the present invention also has a certain salt blocking effect. Based on the Donnan effect, the salt ion concentration inside it is always lower than that of the external solution.

[0038] (3) The anti-fouling hydrogel composite membrane for membrane distillation prepared by the present invention can be applied to other membrane separation technologies such as the combination of hydrogel and nanofibers for oil-water separation, etc., so it has strong universality and can also be applied to multiple fields such as seawater desalination, produced water treatment in oil and gas fields, and industrial wastewater treatment, and has broad promotion and application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 FIG. is a schematic diagram of the operation principle of the anti-fouling hydrogel composite membrane for membrane distillation provided by the present invention in a membrane distillation system;

[0040] Figure 2 FIG. is a flowchart of the preparation method of the hydrogel composite membrane provided in Example 1 of the present invention;

[0041] Figure 3 FIG. is a flowchart of the preparation method of the hydrogel composite membrane provided in Example 5 of the present invention;

[0042] Figure 4 FIG. is a scanning electron microscope image of the hydrogel composite membrane provided in Example 1 of the present invention;

[0043] Figure 5 FIG. is a graph of the surface hydrophilic contact angle and underwater oil contact angle of the hydrogel composite membrane provided in Example 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts shall fall within the protection scope of the present invention.

[0045] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0046] The present invention will be further described below in conjunction with specific embodiments, but it is not limited to the present invention.

[0047] The main raw materials and parameters of Embodiments 1-6 of the present invention are shown in Table 1 below.

[0048] Table 1

[0049]

[0050]

[0051] In the above table,

[0052] The polytetrafluoroethylene hydrophobic membrane used in Examples 1-4 is the PTFE hydrophobic membrane provided by Sterlitech Corporation of the United States. Its PTFE thickness is about 10 μm, the support layer thickness is about 120 μm, the support layer is a polypropylene layer, the pore size is 100 nm, and the diameter is 47 mm.

[0053] The polyvinylidene fluoride hydrophobic membrane used in Examples 5 and 6 is the polytetrafluoroethylene hydrophobic membrane provided by Deli New Materials Technology Co., Ltd. of Haining, China. Its PTFE thickness is 10 μm, the support layer thickness is 110 μm, the pore size is 220 μm, and the diameter is 50 mm.

[0054] The plasma cleaner is provided by HARRICK PLASMA of the United States, and the model is PDC-32G-2.

[0055] Example 1

[0056] Step 1: Add 10 mL of deionized water, 1.8808 g of sodium acrylate, 0.061668 g of N,N'-methylenebisacrylamide, and 10 μL of tetramethylethylenediamine into a container in sequence, ultrasonically dissolve them evenly, and bubble with nitrogen for 10 min to remove dissolved oxygen.

[0057] Step 2: Immerse the polytetrafluoroethylene flat membrane with a polypropylene support layer in an ethanol solution, treat it with ultrasonic waves for 10 min, and place the membrane in a nitrogen environment to dry for 24 h.

[0058] Step 3: Make a circular mold with a diameter of 38 mm and a thickness of 100 μm on the polypropylene side surface of the polytetrafluoroethylene hydrophobic membrane, and treat it with oxygen plasma with a power of 18 W for 4 min. By controlling the plasma discharge, high-density active functional groups are generated in the polypropylene fibers.

[0059] Step 4: Add 40 μL of ammonium persulfate (228 mg / L) to the solution in Step 1 to obtain a hydrogel prepolymer solution.

[0060] Step 5: Under the protection of a nitrogen environment, pour the hydrogel prepolymer solution in Step 4 onto the polypropylene surface treated with oxygen plasma, add a layer of glass upper cover plate for shaping, and then load it with a vacuum negative pressure pump. Control the vacuum degree to be 0.05-0.08 Mpa, and react for 4 h under nitrogen protection to form a hydrogel composite membrane.

[0061] Example 2

[0062] Step 1: Add 10 mL of deionized water, 2.13 g of acrylamide, 0.0046251 g of N,N'-methylenebisacrylamide, and 10 μL of tetramethylethylenediamine into a container in sequence, ultrasonically dissolve them evenly, and bubble with nitrogen for 10 min to remove dissolved oxygen.

[0063] Step 2: Immerse the polytetrafluoroethylene flat membrane with a polypropylene support layer in an ethanol solution, treat it with ultrasonic waves for 10 min, and dry the membrane in a nitrogen environment for 24 h.

[0064] Step 3: Fabricate a circular mold with a diameter of 38 mm and a thickness of 100 μm on the polypropylene side surface of the polytetrafluoroethylene hydrophobic membrane, and treat it with oxygen plasma at a power of 18 W for 4 min to generate high-density active functional groups in the polypropylene fibers by controlling plasma discharge.

[0065] Step 4: Add 60 μL of ammonium persulfate (228 mg / L) to the solution in Step 1 to obtain a hydrogel prepolymer solution.

[0066] Step 5: Under a nitrogen protection environment, pour the hydrogel prepolymer solution in Step 4 onto the polypropylene surface treated with oxygen plasma, add a layer of glass upper cover for shaping, and then load it with a vacuum negative pressure pump. Control the vacuum degree to be 0.05 - 0.08 Mpa, and react for 3 h under nitrogen protection to form a hydrogel composite membrane.

[0067] Example 3

[0068] Step 1: Sequentially add 7.7077 mL of deionized water, 4.5846 g of 2-acrylamido-2-methylpropanesulfonic acid sodium salt solution (50 wt% aqueous solution), 0.030834 g of N,N'-methylenebisacrylamide, and 10 μL of tetramethylethylenediamine into a container, dissolve them evenly by ultrasonic treatment, and bubble with nitrogen for 10 min to remove dissolved oxygen.

[0069] Step 2: Immerse the polytetrafluoroethylene flat membrane with a polypropylene support layer in an ethanol solution, treat it with ultrasonic waves for 10 min, and dry the membrane in a nitrogen environment for 24 h.

[0070] Step 3: Fabricate a circular mold with a diameter of 38 mm and a thickness of 100 μm on the polypropylene side surface of the polytetrafluoroethylene hydrophobic membrane, and treat it with oxygen plasma at a power of 250 W for 1 min to generate high-density active functional groups in the polypropylene fibers by controlling plasma discharge.

[0071] Step 4: Add 50 μL of ammonium persulfate (228 mg / L) to the solution in Step 1 to obtain a hydrogel prepolymer solution.

[0072] Step 5: Under a nitrogen protection environment, pour the hydrogel prepolymer solution in Step 4 onto the polypropylene surface treated with oxygen plasma, add a layer of glass upper cover for shaping, and then load it with a vacuum negative pressure pump. Control the vacuum degree to be 0.05 - 0.08 Mpa, and react for 3.5 h under nitrogen protection to form a hydrogel composite membrane.

[0073] Example 4

[0074] Step 1: Add 3 g of 3-(methacryloyloxy)propyltrimethoxysilane and 22 μL of glacial acetic acid to 150 ml of deionized water, stir at room temperature for 4 h, mix evenly, and set aside.

[0075] Step 2: Wash the commercial polytetrafluoroethylene hydrophobic membrane with ethanol for 10 min, and place it in a vacuum drying oven before use. The polypropylene support layer at the bottom of the polytetrafluoroethylene hydrophobic membrane is treated with oxygen plasma at a power of 250 w for 1 minute to further increase the surface roughness. At the same time, make the surface of polyethylene terephthalate rich in hydroxyl groups.

[0076] Step 3: Immerse the plasma-treated polypropylene surface in the silane coupling agent solution for 12 h. One end of the alkoxy group hydrolyzes into a silanol group in an aqueous environment and condenses with the hydroxyl group on the target surface to form a siloxane bond, forming a strong bond between the target substrate and the bridging molecule.

[0077] Step 4: Add 7.7077 mL of deionized water, 4.5846 g of 2-acrylamido-2-methylpropanesulfonic acid sodium salt solution (50 wt% aqueous solution), 0.030834 g of N,N'-methylenebisacrylamide, and 0.022425 g of 2-hydroxy-2-methyl-1-[4-(2-hydroxyethoxy)phenyl]-1-propanone to a container, dissolve uniformly by ultrasonic treatment, and bubble with nitrogen for 10 min to remove dissolved oxygen.

[0078] Step 5: Make a mold on the surface of the support layer on one side of the membrane product treated in Step 3, then invert the hydrogel prepolymer solution and cover it with a glass plate for qualitative analysis, and irradiate it with ultraviolet light in a polymerization chamber at 365 nm for 4 h. The vinyl group at the other end of 3-(methacryloyloxy)propyltrimethoxysilane can participate in the polymerization of the hydrogel precursor to form a covalent bond, and finally obtain a hydrogel composite membrane.

[0079] Example 5

[0080] Step 1: Add 3 g of 3-(methacryloyloxy)propyltrimethoxysilane and 22 μL of glacial acetic acid to 150 ml of deionized water, stir at room temperature for 4 h, mix evenly, and set aside.

[0081] Step 2: Wash the commercial polyvinylidene fluoride hydrophobic membrane with ethanol for 10 min, and place it in a vacuum drying oven before use. The polyethylene terephthalate support layer at the bottom of the polyvinylidene fluoride hydrophobic membrane is treated with oxygen plasma at a power of 18 w for 10 minutes to further increase the surface roughness. At the same time, make the surface of polyethylene terephthalate rich in hydroxyl groups.

[0082] Step 3: Immerse the surface of the plasma-treated polyethylene terephthalate in the silane coupling agent solution for 12 h. One end of the alkoxy group hydrolyzes into a silanol group in an aqueous environment and condenses with the hydroxyl groups on the target surface to form siloxane bonds, thereby forming strong bonding between the target substrate and the bridging molecule.

[0083] Step 4: Sequentially add 10 mL of deionized water, 2.8212 g of sodium acrylate, 0.0046251 g of N,N’-methylenebisacrylamide, and 10 μL of tetramethylethylenediamine into a container, ultrasonically dissolve them evenly, and bubble with nitrogen for 10 min to remove dissolved oxygen.

[0084] Step 5: Add 50 μL of ammonium persulfate (228 mg / L) to the solution in Step 4 to obtain a hydrogel prepolymer solution.

[0085] Step 6: Under a nitrogen protection environment, pour the hydrogel prepolymer solution in Step 4 onto the surface of the oxygen plasma-treated polyethylene terephthalate, add a layer of glass upper cover plate for shaping, and then apply a vacuum negative pressure pump load. Control the vacuum degree to be 0.05 - 0.08 Mpa, and react for 3.5 h under nitrogen protection to form a hydrogel composite film.

[0086] Example 6

[0087] Step 1: Add 3 g of 3-(methacryloyloxy)propyltrimethoxysilane and 22 μL of glacial acetic acid to 150 ml of deionized water, stir at room temperature for 4 h, and mix evenly for standby.

[0088] Step 2: Wash the commercial polyvinylidene fluoride hydrophobic membrane with ethanol for 10 min and place it in a vacuum drying oven before use. The polyethylene terephthalate support layer at the bottom of the polyvinylidene fluoride hydrophobic membrane is treated with oxygen plasma at a power of 18 w for 10 minutes to further increase the surface roughness. At the same time, make the surface of the polyethylene terephthalate rich in hydroxyl groups.

[0089] Step 3: Immerse the surface of the plasma-treated polyethylene terephthalate in the silane coupling agent solution for 12 h. One end of the alkoxy group hydrolyzes into a silanol group in an aqueous environment and condenses with the hydroxyl groups on the target surface to form siloxane bonds, thereby forming strong bonding between the target substrate and the bridging molecule.

[0090] Step 4: Sequentially add 10 mL of deionized water, 1.4216 g of acrylamide, 0.030834 g of N,N’-methylenebisacrylamide, and 0.00292 g of α-ketoglutaric acid into a container, ultrasonically dissolve them evenly, and bubble with nitrogen for 10 min to remove dissolved oxygen.

[0091] Step 5: Fabricate a mold on the surface of the support layer of the membrane product after being treated in Step 3. Then, invert the hydrogel prepolymer solution and cover it with a glass plate for qualitative analysis. Irradiate it with a 365 nm ultraviolet lamp in a polymerization chamber for 8 h. The vinyl group at the other end of 3-(methacryloyloxy)propyltrimethoxysilane can participate in the polymerization of the hydrogel precursor to form covalent bonds, and finally obtain a hydrogel composite membrane.

[0092] Comparative Example 1

[0093] Use the same polytetrafluoroethylene hydrophobic membrane as in Examples 1-4 (PTFE hydrophobic membrane provided by Sterlitech Corporation, USA).

[0094] Comparative Example 2

[0095] Use the same polyvinylidene fluoride hydrophobic membrane as in Examples 5 and 6 (polyvinylidene fluoride hydrophobic membrane provided by Haining Deli New Materials Technology Co., Ltd., China).

[0096] Performance Test

[0097] (I) Hydrophilicity and Oleophobicity Test

[0098] Figure 4 This is the scanning electron microscope image of the hydrogel composite membrane finally obtained in Example 1 of the present invention; Figure 5 This is the surface hydrophilic contact angle and underwater oil contact angle image of the hydrogel composite membrane provided in Example 1 of the present invention.

[0099] It can be seen from Figure 5 that the water contact angle of the hydrogel composite membrane prepared by the present invention is <5°, indicating that the hydrogel composite membrane has good hydrophilicity. The underwater oil dynamic contact angle is 180°. During the dynamic process of the oil droplet gradually contacting and leaving the membrane surface, the modified membrane shows extremely low adhesion force with the oil droplet and excellent superoleophobic characteristics.

[0100] (II) Wastewater Treatment Capacity Test

[0101] Configure wastewater samples A and B. Among them: Wastewater sample A is an O / W wastewater obtained by mixing 0.1 mM sodium dodecyl sulfate and 1000 ppm mineral oil in a 3.5 wt% NaCl solution; Wastewater sample B is wastewater containing 2000 ppm mineral oil in a 3.5 wt.% NaCl solution.

[0102] Adopt a direct contact membrane distillation test system. At 60 °C at the hot end and 20 °C at the cold end, evaluate the flux and salt rejection rate of the hydrogel composite membranes prepared in Examples 1-6 and the membranes in Comparative Examples 1 and 2 based on the changes in the mass and conductivity of the water on the permeate side. The test results are shown in Table 2 below.

[0103] Table 2:

[0104]

[0105] As can be seen from the above table,

[0106] When the hot-end wastewater is O / W, for the original polytetrafluoroethylene hydrophobic membrane (Comparative Example 1), the normalized steam flux increased significantly and the salt rejection rate decreased significantly within 45 minutes of operation time, indicating that the polytetrafluoroethylene hydrophobic membrane was severely wetted. For the hydrogel composite membranes (Examples 1-4), 100% salt rejection and relatively stable steam flux could be achieved within 10 hours. When the hot-end wastewater is mineral oil with a concentration of 2000 ppm, the steam flux of the original polyvinylidene fluoride hydrophobic membrane (Comparative Example 2) decreased rapidly soon after the start of the experiment, indicating that the membrane was severely contaminated by mineral oil. In contrast, the hydrogel composite membranes prepared in Examples 5-6 could maintain relatively stable steam flux and perfect salt rejection within 10 hours, indicating good fouling resistance.

[0107] In summary, a preparation method of an anti-fouling hydrogel composite membrane for membrane distillation provided by the present invention constructs a hydrogel layer in the middle and on the surface of the support layer of the hydrophobic membrane. The successfully prepared hydrogel composite membrane shows excellent performance in treating high-salt wastewater containing oil and surfactant.

[0108] The above are only the preferred embodiments of the present invention, and do not limit the implementation manners and protection scope of the present invention accordingly. For those skilled in the art, it should be realized that all the equivalent substitutions and obvious changes made by using the content of the specification of the present invention should be included in the protection scope of the present invention.

Claims

1. A preparation method of an anti-fouling hydrogel composite membrane for membrane distillation, characterized in that, It includes the following steps: Perform oxygen plasma treatment on the support layer of the hydrophobic membrane to obtain a membrane product after plasma treatment; the hydrophobic membrane is selected from a polytetrafluoroethylene hydrophobic membrane or a polyvinylidene fluoride hydrophobic membrane, and includes a hydrophobic layer and a support layer; the support layer is selected from polypropylene or polyethylene terephthalate, and the power of the oxygen plasma treatment is 18-250 W, and the time is 1-10 min; Graft copolymerize the hydrogel prepolymer solution inside and on the surface of the support layer of the membrane product after plasma treatment to obtain an anti-pollution hydrogel composite membrane.

2. The preparation method according to claim 1, characterized in that, In the hydrogel prepolymer solution, the molar ratio of the monomer to the crosslinking agent is 50-1000:1; the monomer is selected from one or a combination of more of sodium 2-methyl-2-acrylamidopropanesulfonate, sodium acrylate, acrylamide, [2-(methacrylamidoxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide, and sodium 2-acrylamido-2-methyl-1-propanesulfonate; the crosslinking agent is N,N'-methylenebisacrylamide.

3. The preparation method according to claim 2, wherein, The hydrogel prepolymer solution further includes a catalyst; the catalyst is tetramethylethylenediamine, and its volume concentration is 0.1 vol%.

4. The preparation method according to claim 3, wherein The method of graft copolymerization includes: making a mold on the surface of the support layer of the membrane product after plasma treatment, mixing the hydrogel prepolymer solution with an initiator evenly and inverting it in the mold, covering a shaping plate on the surface of the mold, and reacting under a vacuum condition to obtain an anti-pollution hydrogel composite membrane.

5. The preparation method according to claim 4, wherein, The initiator is ammonium persulfate, and its concentration is 3-6 mmol / L; the degree of vacuum of the vacuum condition is 0.05-0.08 Mpa; the temperature of the reaction under the vacuum condition is 30-60 °C.

6. The preparation method according to claim 2, wherein The method of graft copolymerization includes: immersing the surface of the support layer of the membrane product after plasma treatment in a silane coupling agent solution for pretreatment, making a mold on the surface of the support layer of the membrane product after pretreatment, inverting the hydrogel prepolymer solution in the mold, and performing a polymerization reaction under nitrogen protection in an ultraviolet light chamber to obtain an anti-pollution hydrogel composite membrane.

7. The preparation method according to claim 6, characterized in that, The photoinitiator used in the polymerization reaction is selected from 2-hydroxy-2-methyl-1-[4-(2-hydroxyethoxy)phenyl]-1-propanone or α-ketoglutaric acid, and its concentration is 1-30 mmol / L; the ultraviolet wavelength of the polymerization reaction is 365 nm, and the time of the polymerization reaction is 4-8 h.

8. Use of an anti-fouling hydrogel composite membrane for membrane distillation prepared by the preparation method according to any one of claims 1-7, characterized in that, Place the anti-pollution hydrogel composite membrane in the membrane distillation system with the hydrogel side close to the hot end and the hydrophobic side close to the cold end.

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