CN115845629A - Preparation method and application of anti-pollution hydrogel composite membrane for membrane distillation - Google Patents

Abstract

The invention provides a preparation method of an anti-pollution hydrogel composite membrane for membrane distillation, which comprises the following steps: carrying out oxygen plasma treatment on the supporting 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 (3) graft copolymerizing the hydrogel prepolymerization solution in the interior and on the surface of the supporting layer of the membrane product after plasma treatment to obtain the anti-pollution hydrogel composite membrane. The invention also provides application of the anti-pollution hydrogel composite membrane in membrane distillation. The hydrogel composite membrane has strong underwater oil stain resistance, can effectively prevent oil stains from blocking hydrophobic membrane holes due to underwater hydrophilic super-oleophobic property, has excellent capability of preventing a surfactant from damaging the membrane holes, also has a certain salt resistance effect, can effectively prolong the service life of the hydrophobic membrane, has universality, and has popularization and application prospects in the fields of seawater desalination, treatment of produced water of oil and gas fields, treatment of industrial wastewater and the like.

Description

Preparation method and application of anti-pollution hydrogel composite membrane for membrane distillation

Technical Field

The invention belongs to the technical field of membrane distillation wastewater treatment, and specifically relates to a method for preparing an anti-pollution hydrogel composite membrane for membrane distillation, and also relates to an application of the anti-pollution hydrogel composite membrane for membrane distillation.

Background Art

In recent years, under the dual pressure of water shortage and water pollution, the traditional high-salinity wastewater treatment method, which is mainly based on discharge, has been subject to more and more policy restrictions. The exploration of zero liquid discharge solutions for high-salinity wastewater has gradually attracted more attention from the world and has developed rapidly. Electrodialysis, forward osmosis and membrane distillation are technologies to achieve zero liquid discharge, which further concentrate the wastewater after the reverse osmosis stage. Membrane distillation has a theoretical 100% interception effect on various inorganic salts in the solution (such as chloride salts and calcium salts) due to its high inlet salinity limit (>200g/L), and low operating temperature (usually 40-80°C), which can effectively utilize low-grade waste heat. It is considered to be an effective way to solve the problem of high-salinity wastewater.

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 oil and other pollutants; organic pollutants similar to surfactants will reduce the surface tension of the liquid and greatly increase the risk of membrane pore wetting; as the salt concentration continues to increase, it may exceed the saturation concentration, and crystals will precipitate on the membrane surface (heterogeneous nucleation) or in the nearby liquid phase (homogeneous nucleation), causing membrane pollution. The existence of the above problems will seriously affect the stable operation of the membrane distillation system.

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 shortcomings such as poor stability and destruction of membrane structure. In addition, the modification of traditional membranes cannot effectively solve the treatment problem of high-salt wastewater where organic and inorganic pollutants coexist.

Based on this, it is a technical problem that needs to be solved urgently to provide a composite membrane product and a preparation method that is universal and can specifically treat high-salt wastewater containing oil and surfactants.

Summary of the invention

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

One of the purposes of the present invention is to provide a method for preparing an anti-pollution hydrogel composite membrane for membrane distillation that can effectively treat high-salt wastewater containing oil and surfactant.

The technical solution adopted by the present invention to achieve one of the purposes is: to provide a method for preparing an anti-pollution hydrogel composite membrane for membrane distillation, comprising the following steps:

The support layer of the hydrophobic membrane is subjected 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;

The hydrogel prepolymer solution is grafted and copolymerized on the support layer surface of the membrane product after the plasma treatment to obtain an anti-pollution hydrogel composite membrane.

The overall idea of the present invention is as follows: The key component in the membrane distillation system is the hydrophobic membrane, which has the characteristics of being breathable and impermeable to water, and provides a transmission path for water vapor. The present invention forms a hydrogel composite membrane by grafting copolymerized hydrogel on the surface of the hydrophobic membrane. During the operation of the membrane distillation, the hydrogel hydrophilic layer contacts the feed solution, and it has the characteristics of underwater superoleophobicity, which can effectively prevent the attachment of oil stains on the membrane surface; in addition, the hydrophilic and hydrophobic composite structure can also effectively prevent the damage of the surfactant to the hydrophobic membrane. Furthermore, the polyion hydrogel has a Donnan effect, so that the salt ion concentration inside the hydrogel is always lower than that of the external salt solution, and inorganic pollutants such as sodium chloride crystals will not appear inside the gel, thereby reducing membrane pollution and ensuring the stable operation of the membrane distillation system.

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 thinner hydrophobic layer to ensure the smooth operation of membrane distillation, but also by subjecting the support layer to oxygen plasma treatment and grafting hydrogel inside and on the surface thereof, the preparation process will not change the structure and properties of polytetrafluoroethylene, effectively avoiding damage to the structure of the hydrophobic layer caused by surface treatment.

Furthermore, the hydrophobic membrane includes a hydrophobic layer and a support layer. The hydrophobic layer of the polytetrafluoroethylene hydrophobic membrane and the polyvinylidene fluoride hydrophobic membrane is divided into polytetrafluoroethylene and polyvinylidene fluoride, and the support layer is selected from polypropylene or polyethylene terephthalate. The thickness of the hydrophobic layer is 5 to 15 μm, and the thickness of the support layer is 100 to 130 μm.

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

In some preferred embodiments, the hydrophobic layer thickness 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.

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

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.

For graft copolymerization by thermal polymerization, the molar ratio of the monomer to the cross-linking agent in the hydrogel prepolymer solution is 50 to 1000: 1, preferably 50: 1. Since the hydrogel layer of the present invention needs to be used in membrane distillation to treat high-salt wastewater, high requirements are placed on the mechanical strength and swelling properties of the hydrogel layer. By controlling the ratio of the monomers, the swelling of the hydrogel layer when in contact with high-salt wastewater can be suppressed, ensuring high mechanical strength and ensuring that the membrane distillation system can operate stably for a long time.

Preferably, the monomer is selected from one or more of sodium 2-methyl-2-acrylamidopropanesulfonate, sodium acrylate, acrylamide, [2-(methacrylamidooxy)ethyl]dimethyl-(3-sulfonic acid propyl)ammonium hydroxide, and sodium 2-acrylamido-2-methyl-1-propanesulfonate, and the concentration thereof is 1 to 3 mol/L;

Preferably, the cross-linking agent is N,N'-methylenebisacrylamide, and its concentration is 1 to 60 mmol/L;

The hydrogel prepolymer solution also includes a catalyst, which is tetramethylethylenediamine with a volume concentration of 0.1 vol%.

Furthermore, the graft copolymerization method includes: making a mold on the support layer surface of the membrane product after plasma treatment, mixing the hydrogel prepolymer liquid and the initiator evenly and inverting them in the mold, covering the mold surface with a shaping plate, reacting under vacuum conditions, and obtaining an anti-pollution hydrogel composite membrane.

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

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

In the above-mentioned graft copolymerization method, plasma activates the polypropylene support layer of commercial hydrophobic membrane polytetrafluoroethylene, and the sulfate anion free radical in the thermal initiator initiates the reaction, extracts hydrogen from a functional group of the substrate side chain (i.e., COOH and C-OH...) to form a corresponding free radical, and then the generated free radical initiates the graft copolymerization of the hydrogel, which belongs to the thermal polymerization in the preparation of hydrogel and does not require a hydrophilic polymer compound as a link.

For the graft copolymerization by photopolymerization, the molar ratio of the monomer to the crosslinking agent in the hydrogel prepolymer solution is 50 to 1000:1, preferably 50:1.

The monomer is selected from one or more combinations of 2-methyl-2-acrylamidopropanesulfonate sodium, sodium acrylate, acrylamide, [2-(methacrylamidooxy)ethyl]dimethyl-(3-sulfonic acid propyl)ammonium hydroxide, and 2-acrylamido-2-methyl-1-propanesulfonate sodium, and the concentration thereof is 1 to 3 mol/L; the cross-linking agent is N,N'-methylenebisacrylamide, and the concentration thereof is 1 to 60 mmol/L.

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

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, the ultraviolet intensity is 4 mW cm ^-2 , and the polymerization reaction time is 4 to 8 hours. More preferably, the polymerization reaction time is 6 hours.

Furthermore, the preparation method of the silane coupling agent solution includes: adding 3-(isobutyleneoxy)propyltrimethoxysilane and glacial acetic acid in amounts of 20 g/L and 0.015 vot% respectively into deionized water, mixing for 4 hours, to obtain a silane coupling agent solution.

In the above graft copolymerization method, high-density functional groups are generated on the polyethylene terephthalate support layer side by plasma treatment, and a silane coupling agent is used as a bridge, and then the hydrogel is photopolymerized.

Both of the above methods can prepare an anti-fouling 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.

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.

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 hydrogel layer's barrier effect 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, and the above function cannot be achieved if the hydrogel layer faces the cold end. 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.

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

(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. By means 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.

(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.

(3) The anti-pollution hydrogel composite membrane for membrane distillation prepared by the present invention can be applied to other membrane separation technologies such as oil-water separation by combining hydrogel and nanofibers. Therefore, it has strong universality and can also be applied to seawater desalination, oil and gas field produced water treatment, industrial wastewater treatment and other fields. It has broad promotion and application prospects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the operating principle of the anti-pollution hydrogel composite membrane for membrane distillation provided by the present invention in a membrane distillation system;

FIG2 is a flow chart of a method for preparing a hydrogel composite film provided in Example 1 of the present invention;

FIG3 is a flow chart of a method for preparing a hydrogel composite membrane provided in Example 5 of the present invention;

FIG4 is a scanning electron microscope image of the hydrogel composite membrane provided in Example 1 of the present invention;

FIG. 5 is a diagram showing 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

The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

The present invention will be further described below in conjunction with specific embodiments, but they are not intended to be limiting of the present invention.

The main raw materials and parameters of Examples 1-6 of the present invention are shown in Table 1 below.

Table 1

In the above table,

The polytetrafluoroethylene hydrophobic membrane used in Examples 1-4 is a PTFE hydrophobic membrane provided by Sterlitech Corporation of the United States, wherein the 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.

The polyvinylidene fluoride hydrophobic membrane used in Examples 5 and 6 is a polytetrafluoroethylene hydrophobic membrane provided by Haining Delv New Material Technology Co., Ltd., China, with a PTFE thickness of 10 μm, a support layer thickness of 110 μm, a pore size of 220 μm, and a diameter of 50 mm.

The plasma cleaning machine is provided by the American HARRICK PLASMA Company, and the model is PDC-32G-2.

Example 1

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, dissolve evenly by ultrasonication, and remove dissolved oxygen by bubbling with nitrogen for 10 minutes.

Step 2: The polytetrafluoroethylene flat membrane with a polypropylene support layer was immersed in an ethanol solution, treated with ultrasound for 10 minutes, and the membrane was placed in a nitrogen environment to dry for 24 hours.

Step 3: A circular mold with a diameter of 38 mm and a thickness of 100 μm was made on the polypropylene side surface of the polytetrafluoroethylene hydrophobic membrane, and treated with oxygen plasma with a power of 18 W for 4 minutes to generate high-density active functional groups in the polypropylene fiber by controlling plasma discharge.

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

Step 5: Under nitrogen protection, pour the hydrogel prepolymer in step 4 onto the surface of polypropylene treated with oxygen plasma, add a layer of glass cover plate, and use a vacuum negative pressure pump to load it after shaping, control the vacuum degree to 0.05-0.08Mpa, and react for 4 hours under nitrogen protection to form a hydrogel composite film.

Example 2

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, dissolve evenly by ultrasonication, and remove dissolved oxygen by bubbling with nitrogen for 10 minutes.

Step 2: The polytetrafluoroethylene flat membrane with a polypropylene support layer was immersed in an ethanol solution, treated with ultrasound for 10 minutes, and the membrane was placed in a nitrogen environment to dry for 24 hours.

Step 3: A circular mold with a diameter of 38 mm and a thickness of 100 μm was made on the polypropylene side surface of the polytetrafluoroethylene hydrophobic membrane, and treated with oxygen plasma with a power of 18 W for 4 minutes to generate high-density active functional groups in the polypropylene fiber by controlling plasma discharge.

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

Step 5: Under nitrogen protection, pour the hydrogel prepolymer in step 4 onto the surface of polypropylene treated with oxygen plasma, add a layer of glass cover plate, and use a vacuum negative pressure pump to load it after shaping, control the vacuum degree to 0.05-0.08Mpa, and react for 3 hours under nitrogen protection to form a hydrogel composite film.

Example 3

Step 1: 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 in sequence, dissolve evenly by ultrasonication, and remove dissolved oxygen by bubbling with nitrogen for 10 minutes.

Step 2: The polytetrafluoroethylene flat membrane with a polypropylene support layer was immersed in an ethanol solution, treated with ultrasound for 10 minutes, and the membrane was placed in a nitrogen environment to dry for 24 hours.

Step 3: A circular mold with a diameter of 38 mm and a thickness of 100 μm was made on the polypropylene side surface of the polytetrafluoroethylene hydrophobic membrane, and treated with oxygen plasma at a power of 250 W for 1 min to generate high-density active functional groups in the polypropylene fiber by controlling plasma discharge.

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

Step 5: Under nitrogen protection, pour the hydrogel prepolymer in step 4 onto the surface of polypropylene treated with oxygen plasma, add a layer of glass cover plate, and use a vacuum negative pressure pump to load after shaping, control the vacuum degree to 0.05-0.08Mpa, react for 3.5 hours under nitrogen protection to form a hydrogel composite film.

Example 4

Step 1: Add 3 g of 3-(isobutyleneoxy)propyltrimethoxysilane and 22 μL of glacial acetic acid to 150 ml of deionized water, stir at room temperature for 4 h, and mix well for later use.

Step 2: The commercial polytetrafluoroethylene hydrophobic membrane was cleaned with ethanol for 10 minutes and placed in a vacuum drying oven before use. The polypropylene support layer at the bottom of the polytetrafluoroethylene hydrophobic membrane was treated with oxygen plasma at 250W for 1 minute to further improve the surface roughness. At the same time, the polyethylene terephthalate surface was enriched with hydroxyl groups.

Step 3: The plasma-treated polypropylene surface is immersed in a silane coupling agent solution for 12 hours. The alkoxy group at one end is hydrolyzed into a silanol group in an aqueous environment, which condenses with the hydroxyl group on the target surface to form a siloxane bond, thereby forming a strong bond between the target substrate and the bridge molecule.

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 into a container, dissolve evenly by ultrasonication, and remove dissolved oxygen by bubbling with nitrogen for 10 minutes.

Step 5: Make a mold on one side of the support layer of the membrane product treated in step 3, then invert the hydrogel prepolymer liquid and cover the glass plate for qualitative analysis, and illuminate in a 365nm ultraviolet lamp polymerization chamber for 4 hours. The vinyl group at the other end of 3-(isobutyleneoxy)propyltrimethoxysilane can participate in the polymerization of the hydrogel precursor to form a covalent bond, and finally a hydrogel composite membrane is obtained.

Example 5

Step 1: Add 3 g of 3-(isobutyleneoxy)propyltrimethoxysilane and 22 μL of glacial acetic acid to 150 ml of deionized water, stir at room temperature for 4 h, and mix well for later use.

Step 2: The commercial polyvinylidene fluoride hydrophobic membrane was cleaned with ethanol for 10 minutes and placed in a vacuum oven before use. The polyethylene terephthalate support layer at the bottom of the polyvinylidene fluoride hydrophobic membrane was treated with oxygen plasma at 18W for 10 minutes to further improve the surface roughness. At the same time, the polyethylene terephthalate surface was enriched with hydroxyl groups.

Step 3: The plasma-treated polyethylene terephthalate surface is immersed in a silane coupling agent solution for 12 hours. The alkoxy group at one end is hydrolyzed into a silanol group in an aqueous environment, which condenses with the hydroxyl group on the target surface to form a siloxane bond, thereby forming a strong bond between the target substrate and the bridge molecule.

Step 4: 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 the container in sequence, dissolve evenly by ultrasonication, and remove dissolved oxygen by bubbling with nitrogen for 10 minutes.

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

Step 6: Under nitrogen protection, pour the hydrogel prepolymer liquid in step 4 onto the surface of the polyethylene terephthalate treated with oxygen plasma, add a layer of glass cover plate, and then use a vacuum negative pressure pump to load it, control the vacuum degree to 0.05-0.08Mpa, and react for 3.5 hours under nitrogen protection to form a hydrogel composite film.

Example 6

Step 1: Add 3 g of 3-(isobutyleneoxy)propyltrimethoxysilane and 22 μL of glacial acetic acid to 150 ml of deionized water, stir at room temperature for 4 h, and mix well for later use.

Step 2: The commercial PVDF hydrophobic membrane was cleaned with ethanol for 10 min and placed in a vacuum oven before use. The polyethylene terephthalate support layer at the bottom of the PVDF hydrophobic membrane was treated with oxygen plasma at 18 W for 10 min to further improve the surface roughness. At the same time, the polyethylene terephthalate surface was enriched with hydroxyl groups.

Step 3: The plasma-treated polyethylene terephthalate surface is immersed in a silane coupling agent solution for 12 hours. The alkoxy group at one end is hydrolyzed into a silanol group in an aqueous environment, which condenses with the hydroxyl group on the target surface to form a siloxane bond, thereby forming a strong bond between the target substrate and the bridge molecule.

Step 4: 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 the container in sequence, dissolve evenly by ultrasonication, and remove dissolved oxygen by bubbling with nitrogen for 10 minutes.

Step 5: Make a mold on one side of the support layer of the membrane product treated in step 3, then invert the hydrogel prepolymer solution and cover the glass plate for qualitative analysis, and illuminate in a 365nm ultraviolet lamp polymerization room for 8 hours. The vinyl group at the other end of 3-(isobutyleneoxy)propyltrimethoxysilane can participate in the polymerization of the hydrogel precursor to form a covalent bond, and finally obtain a hydrogel composite membrane.

Comparative Example 1

The same polytetrafluoroethylene hydrophobic membrane as in Example 1-4 was used (PTFE hydrophobic membrane provided by Sterlitech, USA).

Comparative Example 2

The same polyvinylidene fluoride hydrophobic membrane as in Examples 5 and 6 was used (polyvinylidene fluoride hydrophobic membrane provided by Haining Delv New Material Technology Co., Ltd., China).

Performance Testing

(I) Hydrophilicity and oleophobicity test

FIG. 4 is a scanning electron microscope image of the hydrogel composite membrane finally obtained in Example 1 of the present invention; FIG. 5 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.

As shown in FIG5 , the water contact angle of the hydrogel composite film prepared by the present invention is less than 5°, indicating that the hydrogel composite film has good hydrophilicity, and the underwater oil dynamic contact angle is 180°. In the dynamic process in which the oil droplets gradually contact and leave the membrane surface, the modified membrane exhibits ultra-low adhesion with the oil droplets and high-quality superoleophobic properties.

(II) Wastewater treatment capacity test

Wastewater sample A and wastewater sample B were prepared, wherein: wastewater sample A was 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 was wastewater containing 2000 ppm mineral oil in a 3.5 wt. % NaCl solution.

A direct contact membrane distillation test system was used to evaluate the flux and salt rejection of the hydrogel composite membranes prepared in Examples 1-6 and the membranes in Comparative Examples 1 and 2 by measuring the changes in the mass and conductivity of water on the permeate side at 60°C on the hot end and 20°C on the cold end. The test results are shown in Table 2 below.

Table 2:

From the above table, we can see that

When the hot end wastewater is O/W, the normalized steam flux of the original polytetrafluoroethylene hydrophobic membrane (Comparative Example 1) increased significantly and the salt rejection rate decreased significantly within 45 minutes of operation, indicating that the polytetrafluoroethylene hydrophobic membrane was severely wetted, while the hydrogel composite membrane (Examples 1-4) had 100% salt rejection and relatively stable steam flux within 10 hours. When the hot end wastewater was 2000ppm of mineral oil, the steam flux of the original polyvinylidene fluoride hydrophobic membrane (Comparative Example 2) dropped rapidly shortly after the start of the experiment, indicating that the membrane was seriously contaminated by mineral oil. In contrast, the hydrogel composite membrane prepared in Examples 5-6 was able to maintain a relatively stable steam flux and perfect salt rejection within 10 hours, indicating good stain resistance.

In summary, the present invention provides a method for preparing an anti-pollution hydrogel composite membrane for membrane distillation. By constructing a hydrogel layer in the middle and 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 surfactants.

The above are only preferred embodiments of the present invention, and are not intended to limit the implementation methods and protection scope of the present invention. Those skilled in the art should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the contents of the specification of the present invention should be included in the protection scope of the present invention.

Claims (10)

1. A preparation method of an anti-pollution hydrogel composite membrane for membrane distillation comprises the following steps:

carrying out oxygen plasma treatment on the supporting 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 grafting and copolymerizing the hydrogel prepolymerization solution on the surface of the supporting layer of the membrane product treated by the plasma to obtain the anti-pollution hydrogel composite membrane.

2. The production method according to claim 1, wherein the hydrophobic membrane includes a hydrophobic layer and a support layer; the support layer is selected from polypropylene or polyethylene terephthalate.

3. The method of claim 1, wherein the oxygen plasma treatment is performed at a power of 18 to 250W for a time of 1 to 10min.

4. The production method according to claim 1, wherein the hydrogel prepolymerized liquid has a ratio of the amount of the monomer to the amount of the crosslinking agent of 50 to 1000; the monomer is selected from one or more of 2-methyl-2-acrylamide propyl sodium sulfonate, sodium acrylate, acrylamide, [2- (methacrylamide acyloxy) ethyl ] dimethyl- (3-sulfopropyl) ammonium hydroxide and 2-acrylamide-2-methyl-1-propyl sodium sulfonate; the cross-linking agent is N, N' -methylene bisacrylamide.

5. The method of claim 4, wherein the hydrogel pre-polymerization solution further comprises a catalyst; the catalyst is tetramethylethylenediamine, and the volume concentration of the catalyst is 0.1vol%.

6. The production method according to claim 5, wherein the graft copolymerization method comprises: and (3) manufacturing a mold on the surface of the supporting layer of the membrane product after the plasma treatment, uniformly mixing the hydrogel pre-polymerization liquid and the initiator, inverting the mixture in the mold, covering a shaping plate on the surface of the mold, and reacting under a vacuum condition to obtain the anti-pollution hydrogel composite membrane.

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

8. The production method according to claim 4, wherein the graft copolymerization method comprises: and (3) immersing the surface of the supporting layer of the membrane product subjected to plasma treatment into a silane coupling agent solution for pretreatment, manufacturing a mold on the surface of the supporting layer of the membrane product subjected to pretreatment, inverting the hydrogel prepolymerization solution in the mold, and carrying out polymerization reaction under the protection of nitrogen in an ultraviolet light chamber to obtain the anti-pollution hydrogel composite membrane.

9. The method of claim 8, wherein the photoinitiator is selected from the group consisting of 2-hydroxy-2-methyl-1- [4- (2-hydroxyethoxy) phenyl ] -1-propanone and α -ketoglutaric acid at a concentration of 1 to 30mmol/L; the ultraviolet wavelength of the polymerization reaction is 365nm, and the polymerization reaction time is 4-8 h.

10. Use of an anti-fouling hydrogel composite membrane for membrane distillation prepared by the preparation method according to any one of claims 1 to 9, wherein the anti-fouling hydrogel composite membrane is placed in a 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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