Porous composite hydrophobic membrane for membrane distillation and preparation method thereof

Abstract

The invention discloses a preparation method of a porous composite hydrophobic membrane for membrane distillation, and relates to the technical field of membrane separation.The preparation method comprises the steps that a waste egg membrane is taken, the egg membrane is dissolved through an enzymolysis method to obtain a keratin fiber solution, then a cross-linking agent with an aldehyde group and a hydrophobic nano additive are added to obtain a spinning solution in a uniform sol state, and the spinning solution is subjected to ultrasonic treatment; the preparation method comprises the following steps: preparing a nanofiber thin film by using an electrostatic spinning technology, carrying out hydrophobic modification treatment on the nanofiber thin film, and carrying out hot-pressing fusion or chemical crosslinking on the nanofiber thin film and a microporous hydrophobic film to prepare the composite hydrophobic film with a hierarchical pore structure. The prepared porous composite hydrophobic membrane for membrane distillation can realize synchronous improvement of permeation flux, infiltration resistance and heat energy utilization efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane separation, and in particular to a porous composite hydrophobic membrane for membrane distillation and a preparation method thereof. Background Art

[0002] According to statistics, the annual egg production in China is about more than 28 million tons. If the eggshell accounts for 10% - 12% of the total egg weight and the inner shell membrane accounts for 4% - 5% of the eggshell weight, it can be obtained that there are about 3.09 million tons of eggshells in China, which is converted into about 0.14 million tons of egg membrane. In addition, the eggshell is composed of the eggshell and the eggshell membrane. The calcium carbonate in the eggshell is a good calcium source, mainly used as a feed additive after being crushed. However, most of the egg membranes are disposed of as attachments to the eggshells. Therefore, if the eggshells are comprehensively utilized, while solving the environmental problems caused by them, their economic benefits can be increased.

[0003] As a new type of membrane separation process, membrane distillation technology physically separates a high-temperature chamber containing a high-temperature feed liquid from a low-temperature cooling chamber containing a liquid or gas by using a porous hydrophobic membrane. Since this process is a non-isothermal process, vapor molecules migrate from the high-vapor-pressure membrane side to the low-vapor-pressure membrane side through the membrane pores, and certain requirements are imposed on the mechanical strength, temperature resistance performance, and filtration performance of the membrane material. The membrane materials for membrane distillation are mostly hydrophobic polymer materials such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polypropylene (PP). Currently, the problem restricting the development of membrane distillation technology is the balance relationship between the hydrophobic membrane flux and membrane wetting under the existing membrane materials. That is, although an increase in pore size can reduce the mass transfer resistance and cause an increase in water flux, the liquid infiltration pressure also decreases, which easily leads to an increase in the risk of membrane wetting; or, if the membrane thickness is increased, although the heat loss caused by heat conduction can be effectively reduced and the membrane anti-infiltration ability can be improved, the membrane filtration resistance is increased, resulting in limited increase in membrane flux. Summary of the Invention

[0004] The object of the present invention is to provide a porous composite hydrophobic membrane for membrane distillation with high flux, high anti-infiltration ability, and low thermal conductivity, and a preparation method thereof.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: On the one hand, the present invention discloses a preparation method of a porous composite hydrophobic membrane for membrane distillation, comprising the following steps: (1) After cleaning, drying, and mechanically crushing the eggshells, sieving to obtain egg membranes; (2) Adding the egg membranes and serine protease into distilled water, heating and stirring to dissolve, and adding an acid solution to adjust the pH value to neutral or weakly alkaline to obtain a mixed solution; (3) After heating and inactivating the enzyme of the mixed solution, performing suction filtration and concentration to obtain a keratin fiber solution; (4) Add the crosslinker with aldehyde groups and the hydrophobic nano-additive to the keratin fiber solution, stir and dissolve it under a nitrogen atmosphere, and perform ultrasonic oscillation to obtain a sol-like spinning solution; (5) Perform electrospinning on the spinning solution to obtain a nanofiber film, and perform surface hydrophobization treatment; (6) Thermally press and fuse or chemically crosslink the nanofiber film after surface hydrophobization treatment with a microporous hydrophobic membrane to obtain a porous composite hydrophobic membrane for membrane distillation.

[0006] Preferably, in the step (1), the particle size of the egg membrane is controlled to be 60-80 mesh to ensure better solubility and dissolution rate in the subsequent steps. If the particle size of the egg membrane is too small, it will be prone to agglomeration; on the contrary, if the particle size is too large, it will delay the subsequent dissolution rate and enzymatic hydrolysis rate of the egg membrane.

[0007] Preferably, in the step (2), the acid solution is any one of dilute hydrochloric acid, dilute sulfuric acid, dilute phosphoric acid, and dilute acetic acid; the mass ratio of the egg membrane to serine protease is 90-100:3; the heating and stirring temperature is 65-70 °C; the pH value of the mixed solution is 7-9. Control the optimal keratin enzymatic hydrolysis conditions to ensure moderate enzymatic hydrolysis of keratin. If the ratio of the egg membrane to serine protease is too high, it will lead to insufficient enzymatic hydrolysis, affecting the solubility of the egg membrane and making it difficult to form a spinning solution. However, if the ratio of the egg membrane to serine protease is too low, it will lead to a higher degree of enzymatic hydrolysis, hydrolyzing macromolecular keratin into small molecule amino acids; after exceeding the optimal temperature, the protease will undergo irreversible denaturation due to high temperature, and its activity will be significantly reduced or even inactivated. If the temperature is too low, it will affect the enzymatic reaction rate; an overly acidic or alkaline environment may directly damage the secondary or tertiary structure of the enzyme protein, rendering it completely inactivated, thus affecting the enzymatic reaction rate.

[0008] Preferably, in the step (3), heating to inactivate the enzyme is to heat the mixed solution to 90-100 °C and keep it warm for 10-15 min, controlling the mass concentration of keratin in the keratin fiber solution to be 9-15% to form a spinning solution with moderate viscosity and ensure the subsequent spinning effect. If the concentration of the spinning solution is too low, the solution viscosity will be too low, and the jet is prone to breakage or droplet separation during the stretching process, forming a bead-like structure; if the concentration of the spinning solution is too high, resulting in too high a solution viscosity, the solution cannot be smoothly extruded from the spinneret, forming a discontinuous or broken jet, or even completely unable to be ejected.

[0009] Preferably, in the step (4), the crosslinking agent with aldehyde groups is any one of glyoxal, glutaraldehyde, and oxidized sugar; the hydrophobic nano-additive is any one of poly(propylene carbonate), polypropylene, poly(vinylidene fluoride), polytetrafluoroethylene, hydrophobic metal-organic framework materials, hydrophobic covalent organic framework materials, hydrophobic supramolecular organic framework materials, hydrophobic hydrogen-bonded organic framework materials, hydrophobic SiO2, and hydrophobic TiO2 nanoparticles; the size of the hydrophobic nano-additive is 30-200 nm; the molar concentration of the crosslinking agent with aldehyde groups added to the keratin fiber solution is 0.1-0.2 mol / L, and the mass concentration of the hydrophobic nano-additive is 3-10 wt%. By reasonably controlling the ratio of the crosslinking agent to the additive and the size of the nanoparticles, the effective fusion of the nanofibers and the nanoparticles can be ensured. If the ratio of the additive is mismatched, abnormal changes in core parameters such as the solution viscosity and conductivity will occur, directly affecting the fiber morphology, resulting in the inability of the nanoparticles to be coated inside the nanofiber filaments, affecting the separation stability and even causing nanoparticle leakage.

[0010] Preferably, in the step (5), the electrospinning process includes the following steps: suck the spinning solution with a syringe, place it on and clamp it on a constant flow pump, connect the positive electrode of the high-voltage electrostatic generator to the needle head, and divide the negative electrode into four parts and connect them to the receiving plate. A layer of aluminum foil is laid on the receiving plate, and electrospinning is carried out to obtain a nanofiber film; the advancing speed is controlled at 0.2-0.8 mL / h, the voltage is controlled at 9-20 kV, the receiving distance is controlled at 12-15 cm, the temperature is controlled at 20-30 °C, and the relative humidity is controlled at 60-70%. This is to effectively regulate the nanofiber filament diameter and the nanofiber stacking density, and ensure the most suitable porosity of the nanofiber film. If the advancing speed is too high, the jet will be extruded before being fully stretched, the fiber diameter will increase, and solvent residues are likely to cause adhesion; conversely, if it is too low, insufficient liquid supply will cause the jet to break, the fiber continuity will be poor, and the nozzle is likely to be blocked. If the operating voltage is too high, the jet will be oversplit or corona discharge will occur, and problems such as fiber breakpoints and uneven thickness are likely to occur; conversely, if the operating voltage is too low, the electric field force will not be sufficient to stretch the jet, resulting in the inability to stably form fibers, and droplets or beaded structures are likely to form. If the receiving distance is too short, the fibers will be deposited before being fully cured, and it is easy to adhere into a film; conversely, if the receiving distance is too long, the jet will break after being overstretched, the fiber diameter distribution will become wider or a brittle structure will be formed. If the temperature is too high, the solvent will volatilize too fast, the jet will be cured in advance, resulting in a rough fiber surface or the formation of holes; conversely, if the temperature is too low, the solution viscosity will increase, the jet fluidity will be poor, the fiber diameter will increase or even no fiber can be formed. If the humidity is too high, moisture will mix into the spinning solution or interfere with the solvent volatilization, resulting in fiber delamination, adhesion, or abnormal surface morphology.

[0011] Preferably, in the step (6), the microporous hydrophobic membrane is any one of polytetrafluoroethylene hydrophobic membranes, polypropylene hydrophobic membranes, and poly(vinylidene fluoride) hydrophobic membranes.

[0012] Preferably, in the step (6), the hot pressing and fusing method is to place the nanofiber film after surface hydrophobization treatment and the microporous hydrophobic membrane on the hot stage from top to bottom and apply a heavy object, adjust the temperature of the hot stage to 60-120 °C, control the hot pressing time to 10-30 min, and obtain a porous composite hydrophobic membrane for membrane distillation. Through the physical and mechanical interlocking method, the mechanical strength between the membrane layers is ensured. If the temperature is too high, the molecular chains will break, the composite peeling strength will decrease, and the original nanofiber structure will be damaged. The fusion of nanofibers will lead to a decrease in porosity. On the contrary, if the temperature is too low, the heat-sealing layer or matrix material of the hydrophobic membrane cannot be fully melted, resulting in hindered rearrangement of molecular segments and difficulty in forming effective physical adsorption and chemical bonding, and the interfacial bonding strength will be significantly reduced; if the time is too short, the bonding interface may not be fully melted and the composite strength is insufficient. On the contrary, if the time is too long, the material will be overly softened or thermally degraded, resulting in attenuation of the interfacial bonding force and then affecting the membrane performance.

[0013] Preferably, in the step (6), the covalent cross-linking method is to stack the microporous hydrophobic membrane and the nanofiber film after surface hydrophobization treatment up and down in a plasma treatment container, use argon plasma to clean its surface and form active sites, control the power to 50-100 W, the treatment time to 5-10 min, and the vacuum degree to be controlled at 10 -2 ~10 -1Pa; switch the atmosphere to fluorocarbon gas, control the power at 200~300 W, and treat for 3~5 minutes to form covalent bonds at the membrane interface; alternately use argon and fluorocarbon gas plasma pulse treatment 2-4 times to enhance the interlayer covalent bond cross-linking density; low-temperature annealing, control the temperature at 60~100℃, and treat for 10~30 minutes to form a stable covalent bond structure, thereby obtaining a porous composite hydrophobic membrane for membrane distillation. If the plasma power is too high, the plasma energy density will increase, promoting the breaking of surface chemical bonds and the introduction of polar groups (such as hydroxyl and carboxyl groups), significantly improving hydrophilicity and enhancing the bonding force between molecular chains, reducing interfacial thermal resistance, and may cause excessive etching or destruction of the microporous structure, reducing the mechanical strength and integrity of the membrane. If the treatment time is too short, it will be difficult to effectively introduce groups. If the treatment time is too long, it will lead to excessive oxidation or group inversion, and even blockage of membrane pores. Therefore, choosing an appropriate treatment time is to achieve a balance between superhydrophobic and hydrophilic surfaces. If the number of ion pulse treatments is too small, the covalent bond cross-linking density will decrease, while too many times may cause the surface polar groups to penetrate into the material or undergo cross-linking reactions. If the annealing temperature is increased, the mechanical strength will be improved, but exceeding the critical value may cause thermal decomposition of organic components and weaken the chemical bonding effect at the composite interface. If the annealing time is insufficient, the interfacial cross-linking reaction is not fully completed, and the composite strength improvement is limited. If the annealing time is too long, it will lead to excessive oxidation or grain coarsening of the hydrophobic membrane surface, reducing the uniformity of the interface bonding. The covalent cross-linking method strengthens the cross-linking strength between membrane layers through high-strength covalent bonds, which can ensure the strength of the porous composite hydrophobic membrane. However, compared with the hot pressing fusion method, its operation steps are relatively cumbersome and the equipment investment and operating costs are higher.

[0014] On the other hand, the present invention discloses a porous composite hydrophobic membrane for membrane distillation prepared by the above preparation method.

[0015] Compared with the prior art, the technical solution of the present invention has the following beneficial effects: The present invention uses waste egg membranes as raw materials. After dissolving the egg membranes by enzymatic hydrolysis, keratin fiber solution is obtained. Then, a cross-linking agent with aldehyde groups and a hydrophobic nano-additive are added to obtain a spinning solution in a homogeneous sol state. The electrospinning technology is used to prepare a nanofiber film, and after hydrophobic modification, it is combined with a microporous hydrophobic membrane through hot pressing fusion or chemical cross-linking to prepare a composite hydrophobic membrane with a hierarchical pore structure; the porosity of the nanofiber thin layer is significantly higher than that of the microporous hydrophobic membrane. After compounding, the increase in the membrane filtration resistance is limited, but the thermal conductivity coefficient of the nanofiber thin layer is significantly lower than that of the microporous hydrophobic membrane, effectively alleviating the heat loss caused by heat conduction, thereby greatly increasing the temperature difference driving force on both sides of the membrane. According to the fact that the permeation flux is the ratio of the driving force to the resistance, it can be known that the permeation flux of the composite membrane increases; after compounding, the membrane thickness increases, and according to the Kelvin formula, the anti-wetting ability is improved; the porosity of the nanofiber thin layer is high and the thermal conductivity coefficient of keratin is significantly lower than that of conventional hydrophobic membrane materials. According to the heat conduction transfer formula, the thermal conductivity coefficient of the composite membrane can be greatly reduced. Therefore, compared with the prior art, the present invention can simultaneously and greatly improve the permeation flux, anti-wetting ability and thermal energy utilization efficiency of the porous composite hydrophobic membrane for membrane distillation. Description of the Drawings

[0016] Figure 1 It is the Fourier transform infrared spectrum of the keratin fiber solution obtained in Example 1; Figure 2 It is the scanning electron microscope photograph of the nanofiber film obtained in Example 1; Figure 3 It is the scanning electron microscope photograph of the nanofiber film obtained in Comparative Example 2; Figure 4 It is the scanning electron microscope photograph of the nanofiber film obtained in Comparative Example 3; Figure 5 It is the scanning electron microscope photograph of the nanofiber film obtained in Comparative Example 4. Detailed Embodiments

[0017] The embodiments of the present invention will be described below in conjunction with specific embodiments of the present invention.

[0018] All the drugs used in the following specific embodiments and comparative examples can be obtained through commercial channels, and the eggshells are collected from the staff canteen. Parameters such as permeation flux, desalination rate, porosity, liquid infiltration pressure, and thermal conductivity coefficient in the test method refer to the article "Engineering carbon nanotubes enhanced hydrophobic membranes with high performance in membrane distillation by spray coating" on page 118978, Volume 625 of Journal of Membrane Science in 2020. Example 1

[0019] (1) The eggshells were washed with distilled water, dried at 40 °C, mechanically crushed and sieved through a 60-mesh sieve. The egg membranes retained on the sieve were used as experimental materials; (2) Weighed 15.00 g of egg membrane, added 200 mL of distilled water, added serine protease at 3% of the weight of the egg membrane, heated to 65 °C, and simultaneously added dilute hydrochloric acid to adjust the pH to 7, and stirred to dissolve to obtain a mixed solution; (3) The mixed solution was heated to 100 °C and kept for 15 min to inactivate the enzyme, and the filtrate was obtained by hot filtration while it was hot; the obtained filtrate was concentrated at 38 °C to obtain a keratin fiber solution, and the mass concentration of keratin in the keratin fiber solution was 9%. The Fourier transform infrared spectrum was measured as Figure 1 shown; (4) Added 0.10 mol / L glutaraldehyde and 3.0 wt% hydrophobic nano-silica (size 30 nm) to the keratin fiber solution, stirred and dissolved in a nitrogen atmosphere, and carried out ultrasonic oscillation until the solution showed a uniform sol state to obtain a spinning solution; (5) Drew 5 mL of the spinning solution with a syringe, placed it on and clamped it on a constant flow pump. The positive electrode of the high-voltage electrostatic generator was connected to the needle tip, and the negative electrode was divided into four parts and connected to the receiving plate. A layer of aluminum foil was laid on the receiving plate for receiving electrospun fibers. The voltage was set at 9 kV, the propulsion speed was 0.2 mL / h, the distance was 12 cm, the temperature in the spinning machine was set at a constant 20 °C, and the relative humidity was set at 60%. Spinning operation was carried out to obtain a nanofiber film; the nanofiber film was placed in a 1.5 wt% n-hexane solution of 3-mercaptopropylsiloxane and reacted for 3 h for surface hydrophobization treatment to obtain a nanofiber film with thiol groups on the surface. Its porosity was measured to reach 80%. The scanning electron microscope photograph of the surface of the nanofiber film was as Figure 2 shown; (6) The polyvinylidene fluoride hydrophobic membrane (pore size 0.45 μm, thickness 167 μm, porosity 65.71%) was hot-pressed with the above-mentioned nanofiber film with thiol groups on the surface on a 60 °C hot stage for 30 min to prepare a porous composite hydrophobic membrane for membrane distillation.

[0020] Test the porous composite hydrophobic membrane. The contact angle on one side of the nanofiber thin film is 140.6°, and the liquid infiltration pressure increases from 1.32 bar to 1.64 bar compared with the original commercial polyvinylidene fluoride hydrophobic membrane. The membrane was tested during the vacuum membrane distillation test (feed side temperature 318.15 K, feed side concentration 0.6 M NaCl solution, feed side cross-flow velocity 0.36 m / s, permeate side vacuum 90 kPa, test time 24 h). The permeation flux of the porous composite membrane hydrophobic membrane increased from 10.15 LMH to 30.21 LMH compared with the original commercial polyvinylidene fluoride hydrophobic membrane, and the desalination rate decreased slightly from 99.80% to 99.72%, but still maintained a relatively high desalination rate level. The thermal conductivity coefficient of the porous composite membrane hydrophobic membrane decreased from 0.10 W / m·K to 0.02 W / m·K compared with the original commercial polyvinylidene fluoride hydrophobic membrane. Example 2

[0021] (1) Wash the eggshells with distilled water, dry them at 30 °C, mechanically crush them through an 80-mesh sieve, and retain the egg membrane on the sieve as the experimental material; (2) Weigh 9.00 g of the egg membrane, add 100 ml of a serine protease solution with a mass fraction of 0.3%, stir and react at 70 °C, and adjust the pH to 9 with acetic acid until the reaction system just becomes a homogeneous mixture and there are no visible fine filamentous egg membrane fragments, that is, the egg membrane is completely dissolved, and stop heating; (3) Heat the mixture to 90 °C and keep it for 10 min to inactivate the enzyme, and filter while it is hot to obtain the filtrate; concentrate the obtained filtrate at 40 °C to obtain a keratin fiber solution, and the mass concentration of keratin in the keratin fiber solution is 15%; (4) Add 0.2 mol / L glyoxal and 10 wt% polypropylene carbonate (size 200 nm) to the keratin fiber solution, and stir and dissolve it under sealed conditions. To ensure complete dissolution of the solution, ultrasonic oscillation is required. If the solution is in a homogeneous sol state after ultrasonic oscillation, it is completely dissolved. Stir for 5 h, and set aside after the solution is stirred evenly; (5) Use a syringe to extract 5 mL of the prepared mixed solution, place it on and clamp it on a constant flow pump. Select a propulsion speed of 0.8 mL / h. Connect the positive electrode on the high-voltage electrostatic generator to the needle tip, and divide the negative electrode into four parts and connect them to the receiving plate. A layer of aluminum foil is laid on the receiving plate for receiving electrospun fibers. Set the voltage to 20 kV, the propulsion speed to 0.8 mL / h, the distance to 15 cm, the temperature inside the spinning machine to be constant at 30 °C, and the relative humidity to be controlled at 70%. After spinning, remove the aluminum foil. Place the nanofiber film in a solution containing dodecafluoroheptyl methacrylate (DFMA) monomer and 2,2-dimethoxy-phenylacetophenone (DMPA) initiator, and react under ultraviolet light. After reacting for 45 min, a nanofiber film with surface hydrophobization treatment is obtained. Wash it with ethanol and place it in an oven at 80 °C for drying. Measure its porosity to reach 78%; (6) Thermocompress a polytetrafluoroethylene hydrophobic membrane (pore size 0.45 μm, thickness 286 μm, porosity 66.61%) with the above-mentioned nanofiber thin layer on a hot stage at 120 °C for 10 min to prepare a porous composite hydrophobic membrane for membrane distillation.

[0022] Test the porous composite hydrophobic membrane. The contact angle on the side of the nanofiber film is 123.7°, and the liquid infiltration pressure increases from 1.64 bar to 2.03 bar compared with the original commercial polytetrafluoroethylene hydrophobic membrane. Test the membrane during the vacuum membrane distillation test (feed side temperature 318.15 K, feed side concentration 0.6 M NaCl solution, feed side cross-flow velocity 0.36 m / s, permeate side vacuum 90 kPa, test time 24 h). The permeation flux of the porous composite membrane hydrophobic membrane increases from 19.05 LMH to 35.42 LMH compared with the original commercial polytetrafluoroethylene hydrophobic membrane, and the desalination rate slightly decreases from 99.91% to 99.83%, but still maintains a relatively high desalination rate level. The thermal conductivity coefficient of the porous composite membrane hydrophobic membrane decreases from 0.11 W / m·K to 0.02 W / m·K compared with the original commercial polytetrafluoroethylene hydrophobic membrane. Example 3

[0023] (1) Wash the eggshells with distilled water, dry them at 35 °C, and mechanically crush them through a 70-mesh sieve. Retain the egg membrane on the sieve as the experimental material; (2) Weigh 9.5 g of egg membrane and 0.30 g of serine protease, add them to 100 ml of distilled water, and adjust the pH to 8 with dilute phosphoric acid. Stir and react at 70 °C until the reaction system just becomes a homogeneous mixture, and then stop heating and stirring; (3) Heat the mixture to 95 °C and keep it for 12 min to inactivate the enzyme. Filter while it is hot to obtain the filtrate; Concentrate the obtained filtrate at 40 °C to obtain a keratin fiber solution, and the mass concentration of keratin in the keratin fiber solution is 12%; (4) Add 0.15 mol / L oxidized sugar and 8 wt% hydrophobic ZIF-8 nanoparticles (size 100 nm) to the keratin fiber solution, stir and dissolve it under sealed conditions. To ensure complete dissolution of the solution, ultrasonic oscillation is required. If the solution becomes a homogeneous sol state after ultrasonic oscillation, it is considered completely dissolved. Stir for 5 h, and set aside the well-stirred solution for later use; (5) Use a syringe to draw 5 mL of the prepared mixed solution, place it on and clamp it to a peristaltic pump, select a flow rate of 0.5 mL / h, connect the positive electrode of the high-voltage electrostatic generator to the needle tip, and divide the negative electrode into four parts and connect them to the receiving plate. A layer of aluminum foil is laid on the receiving plate for receiving electrospun fibers. Set the voltage to 15 kV, the feeding speed to 0.6 mL / h, the distance to 15 cm, set the temperature inside the spinning machine to a constant 30 °C, and control the relative humidity at 70%. After spinning, remove the aluminum foil, place the nanofiber film in a plasma treatment container under a carbon tetrafluoride gas atmosphere, control the vacuum degree at 10 -1 Pa, the high-frequency power supply at 13.56 MHz, the power at 200 W, and the treatment time at 5 min, and measure that its surface hydrophobic contact angle is increased to 126.5° and the porosity is 82%; (6) Stack the polypropylene hydrophobic membrane (pore size 0.45 μm, thickness 120 μm, porosity 40.12%) and the prepared nanofiber film on top of each other in a plasma treatment container, use argon plasma to clean its surface and form active sites, control the power at 50 W, the treatment time at 5 min, and the vacuum degree at 10 -2 Pa; switch the atmosphere to carbon tetrafluoride gas, control the power at 200 W, and the treatment time at 3 min to form covalent bonds at the membrane interface; alternately use argon and fluorocarbon gas plasma pulse treatment 2 times to enhance the covalent bond cross-linking density between layers; perform low-temperature annealing, control the temperature at 60 °C, and the treatment time at 10 min to obtain a porous composite hydrophobic membrane for membrane distillation.

[0024] Test the porous composite hydrophobic membrane. The contact angle on the side of the nanofiber film is 126.5°, and the liquid infiltration pressure increases from 1.22 bar to 1.51 bar compared with the original polypropylene hydrophobic membrane. Test the membrane during the vacuum membrane distillation test (feed side temperature 318.15 K, feed side concentration 0.6 M NaCl solution, feed side cross-flow velocity 0.36 m / s, permeate side vacuum degree 90 kPa, test time 24 h). The permeation flux of the porous composite membrane hydrophobic membrane increases from 12.05 LMH to 25.13 LMH compared with the original polypropylene hydrophobic membrane, and the desalination rate slightly decreases from 99.41% to 99.32%, but still maintains a relatively high desalination rate level. The thermal conductivity coefficient of the porous composite membrane hydrophobic membrane decreases from 0.09 W / m·K to 0.02 W / m·K compared with the original polypropylene hydrophobic membrane. Example 4

[0025] This example is different from Example 3 in that dilute sulfuric acid is used in step (2) and the plasma treatment conditions in (6) are changed.

[0026] (1) The same as Example 3; Weigh 9.5 g of egg membrane and 0.30 g of serine protease, add them to 100 ml of distilled water, and adjust the pH to 8 with dilute sulfuric acid. Stir and react at 70 °C until the reaction system just becomes a homogeneous mixture, then stop heating and stirring; (3)-(5) The same as Example 3; (6) Stack the polypropylene hydrophobic membrane (pore size 0.45 μm, thickness 120 μm, porosity 40.12%) and the prepared nanofiber thin film on top of each other in a plasma treatment container, use argon plasma to clean its surface and form active sites, control the power at 100 W, the treatment time at 10 min, and the vacuum degree at 10 -1 Pa; switch the atmosphere to carbon tetrafluoride gas, control the power at 300 W, and the treatment time at 5 min to form covalent bonds at the membrane interface; alternately use argon and fluorocarbon gas plasma pulse treatment 4 times to enhance the cross-linking density of the interlayer covalent bonds; perform low-temperature annealing, control the temperature at 100 °C, and the treatment time at 30 min to obtain a porous composite hydrophobic membrane for membrane distillation.

[0027] Test the porous composite hydrophobic membrane. The contact angle on the side of the nanofiber thin film is 135.5°, and the liquid infiltration pressure increases from 1.22 bar of the original polypropylene hydrophobic membrane to 1.71 bar. Test the membrane during the vacuum membrane distillation test (feed side temperature 318.15 K, feed side concentration 0.6 M NaCl solution, feed side cross-flow velocity 0.36 m / s, permeate side vacuum degree 90 kPa, test time 24 h). The permeation flux of the porous composite hydrophobic membrane increases from 12.05 LMH of the original polypropylene hydrophobic membrane to 27.43 LMH, and the desalination rate slightly decreases from 99.41% to 99.37%, but still remains at a relatively high desalination rate level. The thermal conductivity coefficient of the porous composite hydrophobic membrane decreases from 0.09 W / m·K of the original polypropylene hydrophobic membrane to 0.02 W / m·K.

[0028] Comparative Example 1 In this example, the ratio of serine protease to egg membrane is increased compared to Example 1.

[0029] (1) Wash the eggshell with distilled water, dry it at 40 °C, mechanically crush it through a 60-mesh sieve, and retain the egg membrane on the sieve as the experimental material; (2) Weigh 15.00 g of egg membrane, add 200 mL of distilled water, add 10% (w / w) serine protease of the egg membrane weight, heat to 65°C, add dilute hydrochloric acid to adjust the pH to 7, and stir to dissolve to obtain a mixed solution.

[0030] (3) Heat the mixture to 100°C and maintain for 15 minutes to inactivate the enzyme, and filter the mixture while hot to obtain the filtrate; concentrate the obtained filtrate at 38°C to obtain the keratin fiber liquid.

[0031] The infrared spectrum of keratin fiber liquid prepared under the current conditions did not appear Figure 1 The characteristic peaks of amide I-III bands of keratin shown in the figure indicate that excessive protease input leads to excessive enzymatic degradation of keratin to form small molecular amino acids.

[0032] Comparative Example 2 Compared with Example 1, this example reduces the mass concentration of keratin in the keratin fiber liquid.

[0033] (1) The egg shells were washed with distilled water, dried at 40°C, and mechanically crushed through a 60-mesh sieve. The egg membranes on the sieve were retained as experimental materials. (2) Weigh 15.00 g of egg membrane, add 200 mL of distilled water, add 3% (w / w) serine protease of the egg membrane weight, heat to 65°C, add dilute hydrochloric acid to adjust the pH to 7, and stir to dissolve to obtain a mixed solution; (3) heating the mixed solution to 100°C and maintaining it for 15 minutes to inactivate the enzyme, and filtering the mixture while hot to obtain a filtrate; concentrating the obtained filtrate at 38°C to obtain a keratin fiber liquid, wherein the mass concentration of keratin in the keratin fiber liquid is 5%; (4) Adding 0.10 mol / L glutaraldehyde and 3.0 wt% hydrophobic nano-silica (size 30 nm) to the keratin fiber solution, stirring and dissolving the solution under a nitrogen atmosphere, and ultrasonically vibrating the solution until the solution presents a uniform sol state, thereby obtaining a spinning solution; (5) Use a syringe to extract 5 mL of spinning solution and place it on the constant flow pump. Connect the positive electrode of the high-voltage electrostatic generator to the needle, and the negative electrode is divided into four parts and connected to the receiving plate. The receiving plate is covered with a layer of aluminum foil for receiving electrospun fibers. Set the voltage to 9 kV, the propulsion speed to 0.2 mL / h, the distance to 12 cm, the temperature in the spinning machine to a constant 20 ° C, and the relative humidity to 60% to perform spinning operations to obtain a nanofiber film; the nanofiber film is placed in a 1.5 wt% 3-mercaptopropylsiloxane n-hexane solution for 3 hours to perform surface hydrophobic treatment to obtain a nanofiber film containing mercapto groups on the surface. The porosity is tested to reach 90%. The scanning electron microscope photo of the nanofiber surface is shown in the figure. Figure 3 shown.

[0034] By comparison Figure 2 and Figure 3 it can be seen that: due to the too low concentration of keratin, the jet is prone to breakage or droplet separation during the stretching process, forming a beaded structure as shown in Figure 3 and a good nanofiber structure is not formed.

[0035] Comparative Example 3 In this example compared with Example 1, the addition amount of the hydrophobic nano-additive is increased.

[0036] (1) The eggshells are washed with distilled water, dried at 40 °C, mechanically crushed and sieved through a 60-mesh sieve, and the egg membranes retained on the sieve are used as experimental materials; (2) Weigh 15.00 g of egg membrane, add 200 mL of distilled water, add serine protease at 3% (w / w) of the weight of the egg membrane, heat to 65 °C, and at the same time add dilute hydrochloric acid to adjust the pH to 7, and stir to dissolve to obtain a mixed solution; (3) Heat the mixed solution to 100 °C and keep it for 15 min to inactivate the enzyme, and filter while it is hot to obtain a filtrate; the obtained filtrate is concentrated at 38 °C to obtain a keratin fiber solution, and the mass concentration of keratin in the keratin fiber solution is 9%, and the Fourier transform infrared spectrum is measured as shown in Figure 1 shown; (4) Add 0.10 mol / L glutaraldehyde and 15.0 wt% hydrophobic nano-silica (size 30 nm) to the keratin fiber solution, stir and dissolve in a nitrogen atmosphere, and perform ultrasonic oscillation until the solution shows a uniform sol state to obtain a spinning solution; (5) Draw 5 mL of the spinning solution with a syringe, place it and clamp it on a constant flow pump, connect the positive electrode of the high-voltage electrostatic generator to the needle tip, and divide the negative electrode into four parts and connect them to the receiving plate. There is a layer of aluminum foil on the receiving plate for receiving electrospun fibers. Set the voltage to 9 kV, the feeding speed to 0.2 mL / h, the distance to 12 cm, the temperature in the spinning machine to be constant at 20 °C, and the relative humidity to be set at 60%, and perform spinning operation to obtain a nanofiber film; place the nanofiber film in a 1.5 wt% n-hexane solution of 3-mercaptopropylsiloxane and react for 3 h for surface hydrophobization treatment to obtain a nanofiber film containing mercapto groups on the surface, and test its porosity to reach 78%. The scanning electron microscope photo of the surface of the nanofiber film is as shown in Figure 4 shown; By comparison Figure 2 and Figure 4 it can be seen that: due to the increase in the addition amount of the hydrophobic nano-additive, excessive nanoparticles are deposited on the surface of the nanofibers, affecting the nanofiber structure and easily causing nanoparticle leakage.

[0037] Comparative Example 4 Compared with Example 1, the spinning voltage in this example is increased, the propulsion speed is decreased, and the receiving distance is shortened.

[0038] (1) The egg shells were washed with distilled water, dried at 40°C, and mechanically crushed through a 60-mesh sieve. The egg membranes on the sieve were retained as experimental materials. (2) Weigh 15.00 g of egg membrane, add 200 mL of distilled water, add 3% (w / w) serine protease of the egg membrane weight, heat to 65°C, add dilute hydrochloric acid to adjust the pH to 7, and stir to dissolve to obtain a mixed solution; (3) heating the mixed solution to 100°C and maintaining the temperature for 15 minutes to inactivate the enzyme, and filtering the mixture while hot to obtain a filtrate; concentrating the obtained filtrate at 38°C to obtain a keratin fiber liquid, wherein the mass concentration of keratin in the keratin fiber liquid is 9%; (4) Adding 0.10 mol / L glutaraldehyde and 3.0 wt% hydrophobic nano-silica (size 30 nm) to the keratin fiber solution, stirring and dissolving the solution under a nitrogen atmosphere, and ultrasonically vibrating the solution until the solution presents a uniform sol state, thereby obtaining a spinning solution; (5) Use a syringe to extract 5 mL of spinning solution and place it on the constant flow pump. Connect the positive electrode of the high-voltage electrostatic generator to the needle, and the negative electrode is divided into four parts and connected to the receiving plate. The receiving plate is covered with a layer of aluminum foil for receiving electrospun fibers. Set the voltage to 25 kV, the propulsion speed to 0.1 mL / h, the distance to 10 cm, the temperature in the spinning machine to a constant 20 ° C, and the relative humidity to 60% to perform spinning operations to obtain a nanofiber film; the nanofiber film is placed in a 1.5 wt% hexane solution of 3-mercaptopropylsiloxane for 3 hours to perform surface hydrophobic treatment to obtain a nanofiber film containing mercapto groups on the surface. The porosity is tested to be 71%. The scanning electron microscope photo of the nanofiber surface is shown in the figure. Figure 5 As shown; (6) A polyvinylidene fluoride hydrophobic membrane (pore size of 0.45 μm, thickness of 167 μm, porosity of 65.71%) and the above-mentioned nanofiber film containing thiol groups on the surface were hot-pressed into one on a hot plate at 60°C for 30 min to obtain a porous composite hydrophobic membrane for membrane distillation.

[0039] By comparing Figure 1 and Figure 5After comparison and analysis by the filament diameter statistical software, it can be known that the diameter of the nanofiber filaments decreases from 437.07 nm to 110.04 nm. Although the hydrophobic nanoparticles are still coated inside the nanofibers, the contact angle on the surface of the nanofiber film decreases from 140.6° to 120.2°, and the liquid infiltration pressure decreases from 1.64 bar to 1.43 bar. The membrane was tested during the vacuum membrane distillation test (the temperature on the feed side is 318.15 K, the concentration of the feed side is 0.6 M NaCl solution, the cross-flow velocity on the feed side is 0.36 m / s, the vacuum degree on the permeate side is 90 kPa, and the test time is 24 h). The average water flux decreases from 30.21 LMH to 20.13 LMH. The thermal conductivity of the porous composite hydrophobic membrane increases from 0.02 W / m·K to 0.03 W / m·K. Compared with Example 1, the porous composite hydrophobic membrane prepared shows a downward trend in both the permeation flux and the liquid infiltration pressure, and at the same time, an upward trend in the thermal conductivity.

[0040] Comparative Example 5 In this example, compared with Example 1, the hot pressing temperature and time are increased.

[0041] (1) The eggshells are washed with distilled water, dried at 40 °C, mechanically crushed and sieved through a 60-mesh sieve, and the egg membrane on the sieve is retained as the experimental material; (2) Weigh 15.00 g of the egg membrane, add 200 mL of distilled water, add serine protease at 3% (w / w) of the weight of the egg membrane, heat to 65 °C, and at the same time add dilute hydrochloric acid to adjust the pH to 7, and stir to dissolve to obtain a mixed solution; (3) Heat the mixed solution to 100 °C and keep it for 15 min to inactivate the enzyme, and filter while it is hot to obtain a filtrate; the obtained filtrate is concentrated at 38 °C to obtain a keratin fiber solution, and the mass concentration of keratin in the keratin fiber solution is 9%; (4) Add 0.10 mol / L glutaraldehyde and 3.5 wt% hydrophobic nano-silica (size 30 nm) to the keratin fiber solution, stir and dissolve in a nitrogen atmosphere, and perform ultrasonic oscillation until the solution presents a uniform sol state to obtain a spinning solution; (5) Draw 5 mL of the spinning solution with a syringe, place it on and clamp it on a constant flow pump, connect the positive electrode of the high-voltage electrostatic generator to the needle tip, and connect the negative electrode to the receiving plate in four parts. There is a layer of aluminum foil on the receiving plate for receiving electrospun fibers. Set the voltage to 9 kV, the feeding speed to 0.2 mL / h, the distance to 12 cm, the temperature in the spinning machine to be kept constant at 20 °C, and the relative humidity to be set at 60%, and perform the spinning operation to obtain a nanofiber film; place the nanofiber film in a 1.5 wt% n-hexane solution of 3-mercaptopropylsiloxane and react for 3 h for surface hydrophobization treatment to obtain a nanofiber film with sulfhydryl groups on the surface, and test its porosity to reach 80%; (6) The polyvinylidene fluoride hydrophobic membrane (pore size 0.45 μm, thickness 167 μm, porosity 65.71%) was hot-pressed with the above-mentioned nanofiber film containing mercapto groups on the surface on a hot stage at 150 °C for 60 min to obtain a porous composite hydrophobic membrane for membrane distillation.

[0042] Compared with the porous composite membrane hydrophobic membrane obtained in Example 1, under the same test conditions, the average water flux decreased from 30.21 LMH to 21.78 LMH, the liquid infiltration pressure decreased slightly from 1.64 bar to 1.58 bar, but the thermal conductivity coefficient increased from 0.02 W / m·K to 0.03 W / m·K. Compared with Example 1, the porous composite membrane hydrophobic membrane prepared had a significant downward trend in the permeation flux, a slight decrease in the liquid infiltration pressure, and an upward trend in the thermal conductivity coefficient at the same time.

[0043] Comparative Example 6 In this comparative example, compared with Example 3, the plasma power and treatment time were increased.

[0044] (1) The eggshells were washed with distilled water, dried at 35 °C, mechanically pulverized and sieved through a 70-mesh sieve, and the egg membranes retained on the sieve were used as experimental materials; (2) 9.50 g of egg membrane and 0.30 g of serine protease were weighed, added to 100 ml of distilled water, and the pH was adjusted to 8 with dilute phosphoric acid and acetic acid, and the mixture was stirred and reacted at 70 °C until the reaction system just became a homogeneous solution mixture, and then the heating and stirring were stopped; (3) The mixed solution was heated to 95 °C and kept for 12 min to inactivate the enzyme, and the filtrate was obtained by hot filtration while it was hot; the obtained filtrate was concentrated at 40 °C to obtain a keratin fiber solution, and the mass concentration of keratin in the keratin fiber solution was 12%; (4) 0.15 mol / L of oxidized sugar and 10 wt% of hydrophobic ZIF-8 nanoparticles (size 100 nm) were added to the keratin fiber solution, and stirring and dissolution were carried out under sealed conditions. To ensure complete dissolution of the solution, ultrasonic oscillation was required. If the solution became a homogeneous sol state after ultrasonic oscillation, it was completely dissolved. Stir for 5 h, and the solution was stirred evenly and then reserved; (5) 5 mL of the prepared mixed solution was drawn with a syringe, placed and clamped on a constant flow pump, the flow rate was selected as 0.5 mL / h, the positive electrode of the high-voltage electrostatic generator was connected to the needle tip, the negative electrode was divided into four and connected to the receiving plate, and a layer of aluminum foil was laid on the receiving plate for receiving electrospun fibers. The voltage was set at 20 kV, the advancing speed was 0.8 mL / h, the distance was 15 cm, the temperature in the spinning machine was set to be constant at 30 °C, and the relative humidity was controlled at 70%. After electrospinning was completed, the aluminum foil was removed, and the nanofiber film was placed in a plasma treatment container under a carbon tetrafluoride gas atmosphere, and the vacuum degree was controlled at 10 -1Pa, high-frequency power supply at 13.56 MHz, power 200 W, treatment time 5 min, and its surface hydrophobic contact angle was measured to increase to 126.5° and porosity to 82%; (6) Stack the polypropylene hydrophobic membrane (pore size 0.45 μm, thickness 120 μm, porosity 40.12%) and the prepared nanofiber film on top of each other in a plasma treatment container, use argon plasma to clean its surface and form active sites, control the power at 50 W, treatment time 5 min, and vacuum degree at 10 -2 Pa; switch the atmosphere to carbon tetrafluoride gas, control the power at 500 W, treatment time 30 min, to form covalent bonds at the membrane interface; alternately use argon and fluorocarbon gas plasma pulse treatment 6 times to enhance the cross-linking density of the interlayer covalent bonds; perform low-temperature annealing, control the temperature at 60 °C, treatment time 10 min, to obtain a porous composite hydrophobic membrane for membrane distillation.

[0045] Compared with the porous composite membrane hydrophobic membrane prepared in Example 3, the contact angle on the nanofiber film side decreased from 126.5° to 105.6°, the liquid infiltration pressure decreased from 1.51 bar to 1.33 bar. Under the same membrane distillation test conditions, the average water flux decreased from 25.13 LMH to 18.77 LMH, and the thermal conductivity coefficient increased from 0.02 W / m·K to 0.03 W / m·K. Compared with Example 3, the obtained porous composite membrane hydrophobic membrane showed a downward trend in both permeation flux and liquid wetting pressure, and an upward trend in the thermal conductivity coefficient.

[0046] Comprehensive comparison of data from examples and comparative examples Summarize the important parameters of the samples in the examples and comparative examples, as shown in Table 1; and characterize the chemical composition and surface morphology of the relevant samples, such as Figures 1 - 5 .

[0047] Table 1 Comparison table of important parameters in examples and comparative examples

[0048] Through Figure 1 It can be seen that except for Comparative Example 1, keratin fiber solution was formed. This is because in Comparative Example 1, an excessive amount of serine protease was added, resulting in excessive enzymatic hydrolysis of keratin to form small molecule amino acids, making it difficult to perform subsequent spinning operations. Therefore, Comparative Example 1 mainly reflects the influence of enzymatic hydrolysis conditions on the key spinning raw materials, and the enzymatic hydrolysis conditions need to be strictly controlled to ensure moderate enzymatic hydrolysis of keratin.

[0049] By comparing Figure 2 and Figure 3 It can be seen that in Comparative Example 2, the mass concentration of keratin in the keratin fiber solution was reduced. Due to the too low keratin concentration, the jet was prone to breakage or droplet separation during the stretching process, forming as Figure 3The bead-like structure shown leads to changes in the morphology of the nanofibers, forming columnar structures and making it difficult to form nanofibers with regular morphology. Therefore, Comparative Example 2 mainly reflects the influence of the mass concentration of keratin in the keratin fiber solution on the morphology of the nanofiber film, and the mass concentration of keratin needs to be strictly controlled.

[0050] By comparing Figure 2 and Figure 4 it can be seen that in Comparative Example 3, an excessive amount of hydrophobic nanoparticle additive was added, resulting in uneven distribution of the nanoparticle additive in the spinning solution, which in turn affected core parameters such as the viscosity and conductivity of the spinning solution, directly affecting the fiber morphology, causing the nanoparticles to fail to be coated inside the nanofiber filaments and deposit on the fiber surface, thereby affecting the separation stability and even leading to nanoparticle leakage. Therefore, Comparative Example 3 mainly reflects the influence of the spinning solution formulation on the morphology of the nanofiber film, and the amount of hydrophobic nanoparticle additive needs to be strictly controlled.

[0051] By comparing Figure 2 and Figure 5 it can be seen that in Comparative Example 4, when spinning conditions such as the spinning voltage were increased, the diameter of the nanofiber filaments decreased from 437.07 nm to 110.04 nm. Although the hydrophobic nanoparticles were still coated inside the nanofibers, the contact angle of the nanofiber film surface decreased from 140.6° to 120.2°. Therefore, Comparative Example 4 mainly reflects the influence of the spinning operation conditions on the morphology of the nanofiber film, and the spinning conditions need to be strictly controlled.

[0052] Compared with Comparative Example 5, Examples 1-2 also used the hot pressing and fusing method to composite the nanofiber film after surface hydrophobization treatment with the microporous hydrophobic membrane to obtain a porous composite hydrophobic membrane for membrane distillation. However, the key parameters (permeation flux, liquid infiltration pressure, and thermal conductivity coefficient) of the porous composite hydrophobic membranes obtained in Examples 1-2 were significantly better than those in Comparative Example 5. This is because in Comparative Example 5, both the hot pressing temperature and the hot pressing time exceeded the limit values, damaging the original nanofiber and microporous hydrophobic membrane pore structures, causing the nanofibers to fuse and interlace or the pore structures to collapse, which in turn affected the membrane permeation performance. Therefore, Comparative Example 5 mainly reflects the influence of the hot pressing and fusing conditions on the performance of the prepared porous composite hydrophobic membrane, and the hot pressing and fusing conditions need to be strictly controlled.

[0053] Compared with Comparative Example 6, Examples 3-4 also used a covalent cross-linking method to composite the surface-hydrophobized nanofiber membrane with the microporous hydrophobic membrane to obtain a porous composite hydrophobic membrane for membrane distillation. However, the key parameters (permeation flux, liquid infiltration pressure, and thermal conductivity coefficient) of the porous composite hydrophobic membranes obtained in Examples 3-4 were all significantly better than those of Comparative Example 6. This is because the plasma treatment power, time, and number of times in Comparative Example 6 all exceeded the limit values. High power would accelerate the surface chemical reaction, resulting in an excessive introduction of polar hydrophilic groups (such as hydroxyl and carboxyl groups) on the surface of the hydrophobic membrane. These groups could enhance the binding force between molecular chains through hydrogen bonds or polar interactions, reducing the interfacial thermal resistance and improving the thermal conductivity. At the same time, the increase in hydrophilic groups significantly enhanced its hydrophilicity. In addition, too long a treatment time or too many treatment times would form a relatively thick oxide layer on the surface, which might change the permeability and selectivity of the membrane and affect its separation performance. Therefore, Comparative Example 6 mainly reflects the influence of covalent cross-linking conditions on the performance of the prepared porous composite hydrophobic membrane, and the covalent cross-linking conditions need to be strictly controlled.

[0054] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should all be covered within the protection scope of the present invention; without conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A preparation method of a porous composite hydrophobic membrane for membrane distillation, characterized in that, It includes the following steps: (1) After washing, drying, and mechanically crushing eggshells, sieve them to obtain egg membranes; (2) Add the egg membranes and serine protease to distilled water, heat and stir to dissolve, and add an acid solution to adjust the pH value to neutral or weakly alkaline to obtain a mixed solution; (3) After heating and inactivating the enzyme in the mixed solution, filter it by suction and concentrate it to obtain a keratin fiber solution; (4) Add a cross-linking agent with aldehyde groups and a hydrophobic nano additive to the keratin fiber solution, stir and dissolve it under a nitrogen atmosphere, and perform ultrasonic oscillation to obtain a sol-like spinning solution; (5) Perform electrospinning on the spinning solution to obtain a nanofiber film, and perform surface hydrophobization treatment; (6) Thermally press and fuse or covalently cross-link the nanofiber film after surface hydrophobization treatment with a microporous hydrophobic membrane to obtain a porous composite hydrophobic membrane for membrane distillation.

2. The preparation method according to claim 1, characterized in that, In the step (1), the particle size of the egg membrane is controlled to be 60 - 80 mesh.

3. The preparation method according to claim 1, characterized in that, In the step (2), the acid solution is any one of dilute hydrochloric acid, dilute sulfuric acid, dilute phosphoric acid, and dilute acetic acid; the mass ratio of the egg membrane to serine protease is 90 - 100:3; the heating and stirring temperature is 65 - 70 °C; the pH value of the mixed solution is 7 - 9.

4. The preparation method according to claim 1, characterized in that, In the step (3), heating and inactivating the enzyme means heating the mixed solution to 90 - 100 °C and keeping it warm for 10 - 15 min; the mass concentration of keratin in the keratin fiber solution is 9 - 15%.

5. The preparation method according to claim 1, characterized in that, In the step (4), the cross-linking agent with aldehyde groups is any one of glyoxal, glutaraldehyde, and oxidized sugar; the hydrophobic nano additive is any one of poly(propylene carbonate), polypropylene, poly(vinylidene fluoride), polytetrafluoroethylene, hydrophobic metal-organic framework materials, hydrophobic covalent organic framework materials, hydrophobic supramolecular organic framework materials, hydrophobic hydrogen-bonded organic framework materials, hydrophobic SiO2 nanoparticles, and hydrophobic TiO2 nanoparticles; the size of the hydrophobic nano additive is 30 - 200 nm; the molar concentration of the cross-linking agent with aldehyde groups added to the keratin fiber solution is 0.1 - 0.2 mol / L, and the mass concentration of the hydrophobic nano additive is 3 - 10 wt%.

6. The preparation method according to claim 1, wherein In the step (5), the electrospinning treatment includes the following steps: Draw the spinning solution with a syringe, place and clamp it on a constant flow pump, connect the positive electrode of the high-voltage electrostatic generator to the needle tip, divide the negative electrode into four items and connect them to the receiving plate, and lay a layer of aluminum foil on the receiving plate to perform electrospinning operation to obtain a nanofiber film; the advancing speed is controlled at 0.2 - 0.8 mL / h, the voltage is controlled at 9 - 20 kV, the receiving distance is controlled at 12 - 15 cm, the temperature is controlled at 20 - 30 °C, and the relative humidity is controlled at 60 - 70%.

7. The preparation method according to claim 1, wherein In the step (6), the microporous hydrophobic membrane is any one of polytetrafluoroethylene hydrophobic membrane, polypropylene hydrophobic membrane, and poly(vinylidene fluoride) hydrophobic membrane.

8. The preparation method according to claim 1, characterized in that, In the step (6), the thermal pressing and fusing method is to place the nanofiber film after surface hydrophobization treatment and the microporous hydrophobic membrane on the hot stage from top to bottom and apply a heavy object, adjust the temperature of the hot stage to 60 - 120 °C, and control the thermal pressing time to 10 - 30 min to obtain a porous composite hydrophobic membrane for membrane distillation.

9. The preparation method according to claim 1, characterized in that, In the step (6), the covalent cross-linking method is to stack the microporous hydrophobic membrane and the nanofiber membrane after surface hydrophobization treatment up and down in a plasma treatment container, use argon plasma to clean its surface and form active sites, control the power at 50-100 W, the treatment time at 5-10 min, and the vacuum degree at 10 -2 ~10 -1 Pa; switch the atmosphere to a fluorocarbon gas, control the power at 200-300 W, and the treatment time at 3-5 min to form covalent bonds at the membrane interface; alternately use argon and fluorocarbon gas plasma pulse treatments 2-4 times to enhance the covalent bond cross-linking density between layers; Low-temperature annealing is carried out at a temperature of 60-100 °C for a treatment time of 10-30 min to form a stable covalent bond structure, and a porous composite hydrophobic membrane for membrane distillation is obtained.

10. A porous composite hydrophobic membrane for membrane distillation prepared by the method according to any one of claims 1 to 9.

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