CN114432914B

CN114432914B - Nanofiber Janus membrane for brine membrane distillation treatment and preparation method thereof

Info

Publication number: CN114432914B
Application number: CN202210364831.1A
Authority: CN
Inventors: 于云江; 李良忠; 刘畅; 马瑞雪; 阳宸煜; 史国峰; 张蕾; 卢伦
Original assignee: South China Institute of Environmental Science of Ministry of Ecology and Environment
Current assignee: South China Institute of Environmental Science of Ministry of Ecology and Environment
Priority date: 2022-04-08
Filing date: 2022-04-08
Publication date: 2022-06-17
Anticipated expiration: 2042-04-08
Also published as: CN114432914A

Abstract

The invention discloses a nanofiber Janus film for brine membrane distillation treatment and a preparation method thereof, wherein the nanofiber Janus film comprises a hydrophilic nanofiber layer, a hydrophobic nanofiber layer and a plurality of spray beads formed by electrically spraying a PS/PDMS mixed solution, wherein the hydrophilic nanofiber layer, the hydrophobic nanofiber layer and the plurality of spray beads are mutually laminated and connected; the hydrophilic nanofiber layer and the hydrophobic nanofiber layer are subjected to hot pressing, so that nanofibers in each fiber layer are locally contacted with spray beads and are connected with the spray beads to form an integrated nanofiber Janus membrane with a large number of air pockets in the interior and on the surface; the hydrophobic nanofibers, the spray beads between the nanofibers in the layer, and the engineering bead string structure on the nanofibers together form a thin hydrophobic layer, and the multistage surface roughness of the outer surface of the nanofiber Janus film is towards. The Janus film material forming the specific multilayer 3D network elastic pore structure is prepared with high comprehensive performance.

Description

Nanofiber Janus membrane for brine membrane distillation treatment and preparation method thereof

Technical Field

The invention relates to the technical field of membrane distillation desalination, and relates to a novel nanofiber Janus membrane for brine membrane distillation treatment and a preparation method thereof.

Background

The problems of water resource scarcity and water pollution are one of the important obstacles for the sustainable development of human society. Therefore, it has been a popular research topic to develop an efficient and convenient water treatment technology. The membrane separation technology is widely applied to the fields of water purification, seawater desalination and the like due to the advantages of low pollution, low energy consumption and the like.

In recent years, three-dimensional porous membrane materials (Janus membranes) have attracted attention because of asymmetric properties on both sides, excellent water-oriented transport performance and great practical application value. The Janus membrane is a novel separation membrane with liquid single-channel performance, and the Janus membrane refers to a separation membrane material with greatly different properties on two sides, and the difference is usually expressed by different chemical wetting properties on two sides of the membrane, so that under the drive of surface chemical potential, liquid can be subjected to anisotropic transportation between cross-sectional layers of the three-dimensional porous membrane material. Janus membranes based on asymmetric surface wettability on opposite sides are attracting increasing attention due to their promising application potential in the DCMD (direct contact membrane distillation) process. The thick hydrophilic layer of the Janus membrane is beneficial for providing additional resistance to thermal conduction and reducing the vapor transport distance in a DCMD process, while the thin hydrophobic layer on the Janus membrane will act as a barrier layer to separate feed and coolant solutions, but allow water vapor to permeate through the membrane. Thus, compared to traditional hydrophobic membranes, Janus membranes are better able to balance water vapor flux and loss of conducted heat, achieving better DCMD performance. In the process of directional permeation, a hydrophobic surface and a hydrophilic surface are indispensable, and the cooperative Janus membrane needs two surfaces to exist simultaneously to play a role. However, because the hydrophilic membrane layer and the hydrophobic membrane layer in the Janus membrane have poor interface compatibility, the current Janus membrane has poor structural stability, short effective service time (the service life of the Janus membrane is only a few hours in many reports), and low water flux, so that the large-scale application of the Janus membrane is greatly limited.

With the development of the interfacial solar steam power generation technology, the solar direct desalination technology is considered to be a potential technology due to the advantages of low cost and environmental friendliness. Membrane Distillation (MD) is considered a viable and economical alternative to traditional desalination technologies (e.g., multi-stage flash evaporation) as a thermally driven membrane process because it is easy to operate and can utilize low grade heat (e.g., waste heat and solar energy), with lower energy consumption and membrane fouling tendencies as compared to reverse osmosis, etc. pressure driven membrane desalination technologies. However, the lack of a highly efficient membrane with excellent vapor permeability, high salt rejection, high thermal stability and good anti-fouling capability remains a major obstacle to the widespread practical application of the MD process in seawater desalination.

Although Janus membranes show good application prospects in many fields, particularly in seawater desalination, drinking water treatment and wastewater treatment. However, the existing Janus membrane and the preparation method thereof still have some defects, and the preparation method of the Janus membrane has complex process and high requirement on process parameter control, and is difficult to realize industrialization. The current preparation methods of janus membranes mainly comprise two main types: first, films with two different properties are bonded together; and secondly, modifying the existing membrane material. However, both methods are difficult to realize stable interface connection structure, good wettability, controllable adjustment of porosity and surface roughness, and the like, so that a new janus structure and a new method for preparing a janus film need to be explored and developed. Among the currently available Janus membrane manufacturing technologies, electrospinning is considered the most promising approach because of its mass production flexibility, precise controllability of the membrane microstructure and properties (e.g., thickness, porosity, and hydrophobicity), and the ease of incorporation of other functional materials. For example, according to the preparation method of the flexible Janus electrostatic spinning fiber membrane with the automatic moisture-transfer function disclosed in the chinese patent application CN201910478828.0, methyl methacrylate MMA, glycidyl methacrylate GMA and butyl acrylate BA are used as monomers, a PGMA-co-PMMA-co-PBA random polymer matrix with chemical reaction activity and a flexible polymer chain is obtained by a solution polymerization method, and then the fiber membrane is obtained by an electrostatic spinning technology; and finally, respectively carrying out hydrophobic and hydrophilic functional modification on the two surfaces of the flexible Janus electrostatic spinning fiber membrane by a chemical modification means to obtain the flexible Janus electrostatic spinning fiber membrane with the automatic moisture-conducting function. However, the preparation process by the method is complex, chemical modification needs to be used, the surface wettability of the material is low, and the application range is limited.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to provide a nanofiber Janus membrane for brine membrane distillation treatment, a preparation method and application, wherein PDMS modification is introduced, mixed electrostatic spinning and spraying are synchronously carried out to form a multilayer elastic 3D network pore structure with an amorphous nanofiber, spray beads and an engineering bead string structure on the surface and inside and stably connected with each other, unique multistage roughness is constructed on a thin hydrophobic layer, the overall stability and surface hydrophobicity of the membrane material are enhanced, and better durability and pollution resistance are obtained; the advantages of the 3D network structure, the inner and outer pores, the multi-level roughness surface and the like of the nanofiber Janus membrane are fully utilized, so that the durability, the alkali resistance and the pollution resistance of the material are improved, and the water flux and the desalination efficiency are improved.

The preparation method of the nanofiber Janus film provided by the invention mainly solves the problems that the existing preparation technology is complex in process, uses more materials and equipment, is not easy to industrialize, and the prepared materials cannot form a specific multilayer 3D network elastic structure, so that the durability of the materials is low, the application is limited and the like.

In order to achieve the purpose, the invention adopts the technical scheme that:

a nanofiber Janus membrane for brine membrane distillation treatment is characterized by comprising a hydrophilic nanofiber layer, a hydrophobic nanofiber layer and a plurality of spray beads formed by electrically spraying a PS/PDMS mixed solution, wherein the hydrophilic nanofiber layer, the hydrophobic nanofiber layer and the plurality of spray beads are mutually laminated and connected;

the hydrophilic nanofiber layer is an electrospun PAN nanofiber layer, the lower surface of the layer, namely the inner side surface of the nanofiber Janus membrane, is a super-hydrophilic surface with super-hydrophilicity and a highly porous structure, and the upper surface of the layer is connected with the hydrophobic nanofiber layer through spray beads;

the hydrophobic nanofiber layer is an electrospun PH nanofiber layer, and the lower surface of the hydrophobic nanofiber layer is connected with the hydrophilic nanofiber layer;

the spray bead is a hollow spherical structure with a large number of nano-scale mesopores on the surface; the spraying beads formed by electrically spraying the PS/PDMS mixed solution are anchored by a 3D network formed by amorphous nanofibers in a PH nanofiber layer and a PAN nanofiber layer respectively in the bead forming process, and are distributed among different nanofibers in a discontinuous way or on the same nanofiber in a continuous way to form an engineering bead string structure;

the spraying beads and the engineering bead strings formed by the spraying beads are randomly distributed among a plurality of amorphous PAN and PH nanofibers to provide elastic support and connection for the plurality of PAN and PH nanofibers, gaps are kept among the PAN and PH nanofibers and among layers formed by the PAN and PH nanofibers, and elastic support, connection and interval structures among the nanofibers of each layer are formed;

the hydrophilic nanofiber layer and the hydrophobic nanofiber layer are subjected to hot pressing, so that nanofibers in each fiber layer are in local contact with and connected with spraying beads, and the integrated nanofiber Janus membrane with a large number of air pockets in the thickness direction and on the surface is formed;

the hydrophobic nanofibers, spray beads among the nanofibers in the layer and an engineering bead string structure on the nanofibers form a thin hydrophobic layer together, and the surface of the thin hydrophobic layer faces the outer side of the Janus membrane of the nanofibers has multi-level surface roughness;

the PAN is polyacrylonitrile, the PS is polystyrene, the PDMS is polydimethylsiloxane, and the PH is poly (vinylidene fluoride-co-hexafluoropropylene);

according to the nanofiber Janus film, in the brine film distillation MD treatment process, a large number of air pockets existing inside and outside in the thickness direction provide a larger water-air interface area for mass transfer, so that more air is captured by the nanofiber Janus film below a liquid film interface, the heat transfer capacity of the air is lower than that of a polymer skeleton, heat loss and temperature polarization are effectively reduced, the driving force is enhanced, and the water flux of the nanofiber Janus film is improved.

The multistage surface roughness of the surface facing to the outer side of the nanofiber Janus membrane is multistage surface roughness of the surface of the outer side of the thin hydrophobic layer, and the multistage laminated surface roughness is jointly constructed by the PH nanofibers of the thin hydrophobic layer, spray beads and an engineering bead string structure: the pH nanofiber and the gaps thereof form primary surface roughness, the engineering bead structure forms secondary surface roughness on the basis of the primary surface roughness, the spray beads and the structures with a plurality of nanopores on the surfaces of the spray beads form tertiary surface roughness which partially covers the surface of the hydrophobic layer and protrudes towards the direction of the outer surface on the basis of the secondary surface roughness, and the outer surface of the nanofiber Janus film has multi-stage laminated surface roughness and super hydrophobicity.

The porosity of the nanofiber Janus film is more than or equal to 77.6 percent; the multi-stage surface roughness Ra of the outer surface of the thin hydrophobic layer is more than or equal to 800 nm, and the Water Contact Angle (WCA) value is more than or equal to 148^o。

A preparation method of nanofiber Janus membrane for brine membrane distillation treatment is characterized by comprising the following steps:

s1: preparing a hydrophilic nanofiber layer: adhering an electrostatic spinning PAN solution to the surface of the aluminum foil to obtain a plurality of amorphous PAN nanofibers, wherein the average diameter of the PAN nanofibers is 266 nm, and a hydrophilic nanofiber layer is obtained after lamination, wherein the surface of the hydrophilic nanofiber layer has super-hydrophilicity and a highly porous structure;

s2: preparing spray beads and engineering bead strings:

electrically spraying a PS/PDMS mixed solution on the upper surface of the hydrophilic nanofiber layer to form a plurality of hollow spherical PS/PDMS spraying beads and engineering bead strings on the upper surface of the hydrophilic nanofiber layer;

s3: preparation of a thin hydrophobic layer: performing electrostatic spinning of a PH solution on the upper surface of the hydrophilic nanofiber layer to prepare a single-layer electrospun PH nanofiber; synchronously carrying out electric spraying on the single-layer electrospun PH nanofiber with the progress of the electrostatic spinning PH solution by using a PS/PDMS mixed solution, and forming a plurality of PS/PDMS spraying beads and engineering bead strings on the single-layer electrospun PH nanofiber; repeatedly carrying out electrostatic spinning on the PH solution and synchronously carrying out electric spraying on the PS/PDMS mixed solution for many times to form a thin hydrophobic layer with a multi-layer structure and multi-stage roughness on the outer surface, wherein the randomly distributed spraying beads carry out elastic support, connection and spacing on each electro-spinning PH nanofiber and each layer of PH nanofiber;

s4: integral hot pressing: and carrying out integral hot pressing on the hydrophilic nanofiber layer, the spray beads, the engineering bead strings and the thin hydrophobic layer, so that the nanofibers and the spray beads in each fiber layer and among the layers are locally contacted, melted and stably connected with each other to form the integrated nanofiber Janus membrane.

Compared with the prior art, the invention has the advantages that:

1. according to the nanofiber Janus membrane for brine membrane distillation treatment, PDMS modification is introduced, mixed electrostatic spinning and spraying are synchronously carried out to form an amorphous nanofiber, spray beads and an engineering bead string structure on the surface and inside, and a multi-layer elastic 3D network pore structure is stably connected with each other, so that unique multi-stage roughness is built on a thin hydrophobic layer, the overall stability and surface hydrophobicity of the membrane material are enhanced in brine MD and other processes, better durability and pollution resistance are obtained, water flux and desalination efficiency are improved, and the Janus membrane material with high yield, high stability and multiple functions is obtained.

Compared with other membrane materials in the prior art, the Janus membrane modified by the PDMS forms a large number of randomly distributed hollow spray beads with mesopores on the surface and an engineering bead string structure on the inner part and the surface of the thin hydrophobic layer, so that the roughness of the outer surface of the thin hydrophobic layer is increased, the maximum average surface roughness (Ra) of the thin hydrophobic layer is 812nm, and the maximum contact angle (WCA) value is 147.2 degrees, which is far higher than that of other membrane materials.

2. The nanofiber Janus membrane for brine membrane distillation treatment provided by the invention is synchronously mixedCombining electrostatic spinning and spraying to form a multi-layer 3D nanofiber network, wherein spray beads (spray beads) widely distributed on each nanofiber and among layers have hollow and elastic structures, and are used as spacers, supports and connectors among the nanofibers and among the layers, so that the distance between adjacent PH nanofibers is potentially enlarged, the stacking density of a hydrophobic layer is reduced, the porosity is increased (a large number of air pockets are formed in the material and on the surface), the elasticity and the thickness of the thin hydrophobic layer are increased, and the connection strength among the nanofibers and among the layers is improved; the nanofiber Janus membrane has a thin hydrophobic membrane with higher porosity, surface roughness and a firm connection structure, so that the membrane material can capture more air below a liquid membrane interface in the MD process, the existence of a large number of air pockets can provide a larger water-air interface area for mass transfer, and the heat transfer capacity of the air is lower than that of a polymer skeleton, so that the heat loss and temperature polarization can be effectively reduced, the driving force is enhanced, the water flux of the Janus membrane can be greatly enhanced, and the water flux can reach 27.7L/m²h。

3. According to the nanofiber Janus membrane for brine membrane distillation treatment, provided by the invention, the overall elasticity and connection stability of the material are improved through the coordination effect of the spray beads on each nanofiber and among layers, so that the recycling performance of the membrane material is greatly improved, and the service life of the material is greatly prolonged. Through practical tests, after the Janus membrane is continuously used for 20 hours, the membrane material has no obvious dirt deposited on the membrane surface or is blocked by holes, the WCA value of the Janus membrane is only reduced by 4.2 degrees, the surface repulsion between pollutants and the membrane surface is enhanced and is always kept at a higher level, compared with other membrane materials, the unique inner and outer 3D structures of the Janus membrane are combined with PDMS (polydimethylsiloxane) spray beads (and engineering bead strings) protruding outwards on the outer surface of the thin hydrophobic membrane, the organic pollution of the membrane can be effectively reduced, the alkalinity resistance is realized, and the stable water flux and desalination rate of the material can be kept in a long-time use process.

4. The preparation method of the nanofiber Janus membrane for brine membrane distillation treatment provided by the invention is characterized in that synchronous electrostatic mixed spinning and spraying are adopted, and PDMS is introduced, so that the Janus membrane has better performance stability and pollution resistance due to enhanced surface hydrophobicity and unique multi-stage roughness on a hydrophobic layer, the types of adopted materials are simplified, the preparation process steps are reduced, the problems that the existing preparation technology is complex in process, more in used materials and equipment and difficult to industrialize are solved, the prepared materials can form a specific internal and external multilayer 3D network elastic structure, and the problem that the industrial application is limited due to low comprehensive performance of the membrane materials is solved. The electrostatic spinning synchronous mixed electrostatic spraying technology adopted by the invention can effectively build multi-level roughness on the hydrophobic layer. The method is characterized in that a secondary structure (hollow spray beads and engineering bead strings) is constructed on a primary structure (nanofibers), a tertiary structure (a large number of nanopores) is constructed on the basis of the secondary structure, and the comprehensive performance of the material is remarkably improved by constructing a unique 3D network structure inside and outside a membrane and multi-level surface roughness. The multi-level surface roughness provided by the invention increases the surface roughness of the thin hydrophobic layer, and in the process of MD and the like, the effective contact area between the matching surfaces is reduced, the pressure intensity is increased, the friction resistance is increased, and the abrasion speed of the membrane material is increased under normal conditions. According to the invention, PDMS modification is introduced into the surface, and meanwhile, the wear resistance and elasticity of the material are improved through a multilayer 3D network elastic structure formed by spraying beads, the film wear speed is effectively reduced, and the effective service life of the material is prolonged.

5. According to the preparation method of the nanofiber Janus membrane for brine membrane distillation treatment, provided by the invention, a novel Janus membrane with a unique 3D network structure and good DCMD (polymer dispersed membrane) performance is prepared by synchronously electrospinning PH and electrospraying a mixed solution of PS and poly (dimethyl siloxane) (PDMS) on the surface of a super-hydrophilic PAN (ens) substrate. According to the invention, the surface energy of the membrane is reduced by adding the low-surface-energy PDMS, and a tertiary structure (a large number of nano micropores) is constructed on the spray beads, so that better antifouling and anti-wetting capabilities are provided for the membrane material. The addition of PDMS allows the Janus membrane to have better performance stability and anti-fouling capability due to enhanced surface hydrophobicity and unique multi-level roughness on the hydrophobic layer, enabling it to meet various integrated requirements of the Direct Contact Membrane Distillation (DCMD) process.

In the preparation process, the microstructure and the comprehensive performance of the thin hydrophobic layer with the bead structure can be optimized by changing the weight ratio and the concentration of the PS to the PDMS. When an electrospray solution with the weight ratio of PS/PDMS being 1:1 is used for constructing a thin hydrophobic layer structure, the obtained Janus film reaches 24.9L/m at the temperature difference of 40 DEG C²The highest water flux of h is 1.38 times that of a membrane incorporating pure PS particles (without PDMS).

6. According to the preparation method provided by the invention, the 3D network structure and the performance of the thin hydrophobic layer can be optimized by changing the concentrations of PS and PDMS, and the membrane with a large number of nano-scale pores and more uniform surface morphology on the outer surface can be obtained. Actual tests show that the Janus film with the distributed PS/PDMS spray bead structure has higher porosity than the pure PH film, and the Janus film with the thin hydrophobic layer of the PH-7PS/7PDMS spray bead structure obtains the firm hydrophobic film with the highest porosity and surface roughness of 77.6%, and the film can capture more air below the liquid film interface in the MD process, and can effectively reduce heat loss and temperature polarization.

7. The nanofiber Janus membrane provided by the invention can be applied to the brine MD process, the 3D network structure, the inner and outer pores, the multi-level roughness surface and other structures of the nanofiber Janus membrane and the performance advantages of the modified PDMS can be fully utilized, so that the durability, the alkali resistance and the pollution resistance of the material are improved, the water flux and the desalination efficiency are improved, the nanofiber Janus membrane can be widely applied to the membrane distillation MD process and the direct contact membrane distillation DCMD process, and the stability of the main performance can be maintained in the long-time use process. Through practical tests, the surface appearance and the WCA value of the thin hydrophobic layer are kept stable after the thin hydrophobic layer is continuously used for 20 hours, the surface appearance of the thin hydrophobic layer is basically not changed, the WCA value is only reduced by 4.2 degrees, and the problem that the effective service life of the existing similar film is short is solved.

8. According to the nanofiber Janus membrane for brine membrane distillation treatment and the preparation method thereof, the material system adopts an ingenious process to construct a unique microstructure, and the adopted 'layer-layer' preparation method is convenient, fast, efficient, easy in process parameter control and easy in industrial manufacturing and application; the raw materials adopted by the material are easy to obtain and few in variety, and the material is environment-friendly, easy to prepare, low in cost, long in service life and beneficial to industrial application.

9. Practical tests show that the Janus membrane prepared by the method has good comprehensive performance in the MD and DCMD processes of seawater desalination, shows excellent durability, thermal stability, structural stability and stronger antifouling capacity, and has wide application potential in the aspect of high-temperature seawater treatment. When the Janus membrane prepared by the invention is used, 3.5 wt% of sodium chloride (NaCl) solution is used as a feeding solution, the Janus membrane realizes 27.7L/m at the temperature difference of 40 DEG C²h water flux and desalination efficiency close to 100%, the water flux is more than three times of that of the similar membrane in the prior art.

Drawings

FIG. 1 is a schematic diagram of a method of making Janus ENs membranes for use in a brine membrane distillation process according to an embodiment of the present invention;

FIG. 2 is a schematic diagram showing a comparison of surface morphology of thin hydrophobic layers prepared by electrospinning pH and electrospray mixed solutions with different weight ratios of PS/PDMS according to an embodiment of the present invention, wherein FIGS. a1-a3 correspond to PH-3PS, FIGS. b1-b3 correspond to PH-3PS/1.5PDMS, and FIGS. c1-c3 correspond to PH-3PS/3 PDMS; FIGS. d1-d3 correspond to PH-3PS/6PDMS, respectively;

FIG. 3 is a schematic comparison of the surface morphology of Janus films used in the brine membrane distillation process of the present invention on hydrophobic layers prepared using electrospray solutions of different PS/PDMS concentrations: wherein, the graphs a1-a3 correspond to PH-3PS/3PDMS, the graphs b1-b3 correspond to PH-5PS/5PDMS, the graphs c1-c3 correspond to PH-7PS/7PDMS, and the graphs d1-d3 correspond to PH-9PS/9 PDMS;

FIG. 4 is a graphical comparison of WCA value (panel a) and DCMD performance (panel b) for thin hydrophobic layers of Janus films prepared from electrospray solutions with different weight ratios of PS/PDMS according to an embodiment of the present invention;

FIG. 5 is a comparison of XPS (panel a) and FTIR (panel b) spectra of pure PH, PH-3PS and PH-3PS/3PDMS hydrophobic layers prepared in examples of the present invention.

FIG. 6 is a schematic comparison of DCMD performance of Janus films with thin hydrophobic layers prepared by electrospray solutions with different PS/PDMS concentrations according to examples of the present invention;

FIG. 7 is a graph comparing normalized flux (FIG. a) and salt rejection (FIG. b) for a Janus membrane with hydrophobic layers of PH-7PS and PH-7PS/7PDMS prepared in accordance with an example of the present invention over an operating duration of 21 hours using deionized water and 3.5 wt% NaCl solution as the coolant and feed solution, respectively, with a 40 deg.C temperature difference.

FIG. 8 is a graphical comparison of normalized flux (panel a) and distillate conductivity (panel b) for Janus membranes with thin hydrophobic layers of PH-7PS and PH-7PS/7PDMS according to an example of the present invention using a 3.5 wt% NaCl solution containing 100 mg/L HA as the feed solution.

FIG. 9 is a comparison graph of the surface topography and WCA value of hydrophobic layers of PH-7PS (FIG. a1-a3) and PH-7PS/7PDMS (FIG. b1-b3) after DCMD operation and DI water rinse in accordance with an embodiment of the present invention.

The specific implementation mode is as follows:

the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

Example (b):

the purpose of this example is to prepare a variety of different components and ratios of nanofiber Janus films for brine membrane distillation treatment.

The nanofiber Janus membrane for the brine membrane distillation treatment provided by the embodiment comprises a hydrophilic nanofiber layer, a hydrophobic nanofiber layer and a plurality of spraying beads formed by electrically spraying a PS/PDMS mixed solution, wherein the hydrophilic nanofiber layer, the hydrophobic nanofiber layer and the plurality of spraying beads are connected with one another in a stacked mode;

the hydrophilic nanofiber layer and the hydrophobic nanofiber layer are subjected to hot pressing, so that nanofibers in each fiber layer are in local contact with spray beads and are connected with each other to form an integrated nanofiber Janus film with a large number of air pockets on the inner part and the surface in the thickness direction;

according to the nanofiber Janus film for brine membrane distillation treatment, in the process of brine membrane distillation MD treatment, a large number of air pockets existing inside and outside in the thickness direction provide a larger water-air interface area for mass transfer, so that more air is captured by the nanofiber Janus film below a liquid film interface, the heat transfer capacity of the air is lower than that of a polymer skeleton, heat loss and temperature polarization are effectively reduced, the driving force is enhanced, and the water flux of the nanofiber Janus film is improved.

The nanofiber Janus membrane for brine membrane distillation treatment provided by the invention takes a hydrophilic nanofiber layer of an electrostatic spinning Polyacrylonitrile (PAN) solution as a base material, and a PH solution is electrostatically spun on the upper surface of the base material to form a hydrophobic nanofiber layer; a plurality of spraying beads which are formed by electrospray PS/PDMS mixed solution, distributed on the surface of the hydrophobic nanofiber layer and in nanofiber gaps of the hydrophobic nanofiber layer and anchored by a 3D network of the hydrophobic nanofiber layer are arranged on the upper surface of the hydrophobic nanofiber layer and in nanofiber gaps; the bead of the spray bead is in a cotton ball shape with a plurality of nano holes, is distributed on the nano fibers of the hydrophobic layer or in gaps of a plurality of nano fibers, and forms a plurality of engineering bead string structures which partially cover the upper surface of the hydrophobic layer and protrude outwards; the hydrophilic nanofiber layer, the hydrophobic nanofiber layer and the spray beads are subjected to hot pressing to enable the local melting and connection of the joint surface to form an integrated nanofiber Janus membrane; the hydrophobic nanofiber, the engineering bead string structure and the beads form a thin hydrophobic layer and the surface roughness of the multi-level lamination of the thin hydrophobic layer: wherein the hydrophobic nanofibers and their gaps form a primary surface roughness, the bead structure of the spray beads forms a secondary surface roughness on the basis of the primary surface roughness, and the plurality of nanoporous structure structures of the beads of the spray beads form a tertiary surface roughness on the basis of the secondary surface roughness; the lower surface of the nanofiber Janus membrane is a super-hydrophilic nanofiber layer, and the upper surface of the nanofiber Janus membrane is a thin hydrophobic layer with multistage lamination surface roughness and super-hydrophobicity.

The spray beads are in a cotton ball shape with a plurality of nano holes on the outer surface, wherein the beads forming the thin hydrophobic layer are distributed on the nano fibers of the hydrophobic layer or in gaps of a plurality of nano fibers to form a plurality of engineering bead string structures which partially cover the surface of the hydrophobic layer and protrude outwards;

A preparation method of the nanofiber Janus film for the brine membrane distillation treatment comprises the following steps:

s1: preparing a hydrophilic nanofiber layer: adhering an electrostatic spinning PAN solution to the surface of the aluminum foil to obtain a plurality of amorphous PAN nanofibers, wherein the average diameter of the PAN nanofibers is 266 nm, and a hydrophilic nanofiber layer is obtained after lamination, wherein the surface of the hydrophilic nanofiber layer has super-hydrophilicity and a highly porous structure; the method specifically comprises the following steps:

s11: preparing electrostatic spinning solution: dissolving polyacrylonitrile PAN powder in dimethylformamide to obtain 10 wt% polyacrylonitrile PAN solution;

s22: and (3) adhering electrostatic spinning on the surface of the aluminum foil to obtain a plurality of PAN nanofibers, and forming a single-layer polyacrylonitrile PAN fiber film, namely the PAN electrospun hydrophilic nanofiber layer.

S2: preparing spray beads and engineering bead strings:

electrically spraying a PS/PDMS mixed solution on the upper surface of the hydrophilic nanofiber layer to form a plurality of hollow spherical PS/PDMS spraying beads and engineering bead strings on the upper surface of the hydrophilic nanofiber layer, and the method specifically comprises the following steps:

s21: preparing a PDMS solution: the dimethyl siloxane prepolymer was first dispersed in tetrahydrofuran under sonication for 30 minutes, and then dimethylformamide was added to the mixture under sonication (12 minutes sonication at 25 ℃, continued stirring at 25 ℃, 600rpm/min for 20 minutes); dropwise adding a curing agent into the homogeneous solution, and continuously stirring for 4 hours at 65 ℃ by using a magnetic force of 600 rpm/min;

s22: preparing a PS solution: dissolving a polystyrene polymer into the solution under stirring to obtain a PS solution;

s23: preparing an electrosprayed PS/PDMS mixed solution: carrying out ultrasonic stirring treatment on the prepared PDMS and PS solution (ultrasonic treatment is carried out for 30min at the temperature of 60 ℃, and stirring is continuously carried out for 30min at the speed of 600rpm/min at the temperature of 25 ℃) to obtain a PS/PDMS mixed solution; adjusting the forms and the scales of the spray beads and the engineering bead strings by adjusting the proportion and the concentration of the PS/PDMS mixed solution;

s24: preparing spray beads and engineering bead strings: and (3) electrically spraying a PS/PDMS mixed solution on the upper surface of the PAN electrospun hydrophilic nanofiber layer according to set process parameters, wherein spray beads generated by electrically spraying PS/PDMS randomly form a plurality of PS/PDMS spray beads or engineering bead strings on the nanofibers of the hydrophilic nanofiber layer or among the nanofibers.

s31: preparation of electrospun PH solutions: a polyvinylidene fluoride-co-hexafluoropropylene powder was dissolved in 80 mL of a mixed solvent (dimethylformamide: acetone = 4: 1) at 50 ℃ to prepare a polyvinylidene fluoride-co-hexafluoropropylene PH solution;

wherein the solvent ratio for dissolving the PH polyvinylidene fluoride-co-hexafluoropropylene powder is as follows: dimethylformamide acetone = 4: 1; in the PS/PDMS mixed solution, the concentration of PS and PDMS is 7 wt%, so as to obtain a nanofiber Janus membrane with high porosity, multiple surface roughness, stable connection, firm structure and high elasticity;

preparing a PDMS solution: dispersing a dimethyl siloxane prepolymer into tetrahydrofuran for 30 minutes under ultrasonic treatment to obtain a mixture, and then adding dimethylformamide into the mixture under ultrasonic treatment (ultrasonic treatment is carried out at 25 ℃ for 12 minutes, stirring is carried out continuously at the speed of 25 ℃ and 600rpm/min for 20 minutes) to obtain a homogeneous solution; dropwise adding a curing agent into the homogeneous solution, and continuously stirring for 4 hours at 65 ℃ by using a magnetic force of 600rpm/min to obtain a PDMS solution;

preparing a PS solution: dissolving a polystyrene polymer into the solution under stirring to obtain a PS solution;

preparing an electrosprayed PS/PDMS mixed solution: carrying out ultrasonic stirring treatment on the prepared PDMS and PS solution (ultrasonic treatment is carried out for 30min at the temperature of 60 ℃, and stirring is continuously carried out for 30min at the speed of 600rpm/min at the temperature of 25 ℃) to obtain a PS/PDMS mixed solution;

s32: preparing a single-layer electrospun PH nanofiber layer: according to the set process parameters, carrying out electrostatic spinning on the upper surface of the hydrophilic nanofiber layer to obtain a plurality of amorphous PH nanofibers and form a single-layer electrospun PH nanofiber, wherein the PH solution is 20 wt%;

s33: synchronously preparing spray beads and engineering bead strings: according to the set process parameters, while the step S32 is carried out, a PS/PDMS mixed solution is electrically sprayed on the upper surface of the single-layer PH electrospun nanofiber, a plurality of beads generated by the electrically sprayed PS/PDMS randomly form a plurality of PS/PDMS spray beads or engineering bead strings on the PH electrospun nanofiber or among the nanofibers, and the spray beads and the engineering bead strings elastically connect, support and space the nanofibers and the nanofibers at each layer; wherein, the forms and the scales of the spray beads and the engineering bead strings are adjusted by adjusting the proportion and the concentration of the PS/PDMS mixed solution; in this embodiment, each spray bead may be attached or coated on a single nanofiber randomly and discontinuously, or attached or coated on two or more nanofibers discontinuously, and a spherical portion thereof mainly exists in pores between each nanofiber to provide spacing, support and connection for each nanofiber; when a plurality of spraying beads are attached or coated on the same single nanofiber randomly, continuously (non-uniformly), or attached or coated on two or more nanofibers continuously, an engineering bead string is formed; multiple engineering bead strings can be crossed and overlapped;

s34: preparation of a thin hydrophobic layer: repeating the steps S32 and S33 for a set number of times, not less than 2 times, in this embodiment, repeating for 3 times, to obtain a multilayer structure (in this embodiment, a 3-layer structure) of nanofiber-intercalated spray beads and a thin hydrophobic layer with multilevel surface roughness, which are stacked one on top of another; the set number of times is more than 2, and the number of times of repetition can be specifically selected according to the required thickness of the film, and the larger the number of times of repetition, the larger the thickness of the obtained film.

In the present example, the microstructure of a thin hydrophobic layer prepared by electrospray of polystyrene/poly (dimethylsiloxane) (PS/PDMS) using different mass parts and concentration ratios is selected as shown in FIG. 2, and the thin hydrophobic layer prepared by electrospray consists of multiple layers of a large number of randomly oriented nanofibers and polymer beads (i.e., spray beads) from electrostatic spinning of PH and electrospraying of PS or PS/PDMS, respectively. As can be seen from FIG. 2 and Table 1, the WCA measurement is 139^oThe pure PH nanofiber mats of (a) exhibited typical nonwoven structures. As shown in fig. 2 (a 1) - (a 3), irregularly shaped particles were formed by electrospraying PS alone, and some particles were even found to be partially fragmented, mainly due to instability of the spray jet at low polymer solution concentrations. In contrast, in the electrospray process, after the PDMS modification is added, PS can be induced to form more regular hollow bead-shaped particles (spray beads) with smaller size, each particle has complete shape, smooth outer surface and no fragmentation, and a large number of nano-scale micropores can be constructed on the surface of the spray beads.

As shown in fig. 4 (a), the WCA value of the thin hydrophobic layer associated with the electrospray PS particles was determined to be 140.1 ° attributable to PH and the inherent hydrophobicity of the PS polymer. The hydrophobicity of each layer of nano-fiber can be enhanced after the PDMS is added for modification, and the maximum WCA value of 147.2 degrees is reached when the weight ratio of PS to PDMS is 1: 1. However, with further increase of PDMS ratio, a decrease of WCA values was observed. Excessive build-up of PDMS may result in the formation of beads with smooth surfaces, but without nano-scale micro-pores. This would be detrimental to the formation of a large number of air pockets at the outer surface of the spray beads to reduce the contact area between the water droplets and the solid surface.

The specific preparation process of the embodiment of the invention is as follows:

5g of polyacrylonitrile powder was dissolved in 50 mL of dimethylformamide, and magnetically stirred at 500 rpm/min at 50 ℃ for 12 hours to prepare a 10 wt% polyacrylonitrile solution. 20 g of polyvinylidene fluoride-co-hexafluoropropylene powder was dissolved in 80 mL of a mixed solvent (dimethylformamide: acetone = 4: 1) at 50 ℃ to prepare a 20 wt% polyvinylidene fluoride-co-hexafluoropropylene solution. Preparation of the electrospray solution, the dimethylsiloxane prepolymer was first dispersed in tetrahydrofuran under sonication for 30 minutes, then dimethylformamide was added to the mixture under sonication (12 minutes sonication at 25 ℃, continued stirring at 25 ℃, 600rpm/min for 20 minutes). The curing agent was added dropwise to the above homogeneous solution, followed by magnetic stirring at 600rpm/min at 65 ℃ for 4 hours. Finally 5g of polystyrene polymer are dissolved into the solution with stirring. As specific embodiments, the specific components and contents of the electrospray solution in the whole reaction system are shown in table 1 below:

TABLE 1

Through experimental performance tests, the membrane material prepared by the component content of 7PS/7PDMS for the brine membrane distillation treatment shows the highest Water Contact Angle (WCA) value of 148.5^oAnd a maximum surface roughness of 812 nm.

As specific embodiments, the electrospinning and electrospray process parameters throughout the reaction system are shown in table 2 below:

TABLE 2

Scanning electron microscope and performance test analysis show that the nanofiber Janus film obtained by using the PS/PDMS process has the best comprehensive performance. In step S1, the optimal ratio of the polyacrylonitrile powder to the dimethylformamide is 10 wt% of the polyacrylonitrile solution.

In step S2, the mixed solvent of polyvinylidene fluoride-co-hexafluoropropylene powder is dissolved, and the components and mass thereof are dimethylformamide: acetone = 4: 1.

The embodiment of the invention provides an engineering bead string structure electro-spinning nanofiber Janus membrane with multi-stage roughness for membrane distillation and the like and a preparation method thereof, and is characterized in that a thin hydrophobic layer with a multi-layer spray bead and an engineering bead string structure is constructed on the surface of a hydrophilic Polyacrylonitrile (PAN) electro-spinning nanofiber (ENs) substrate through a layer-layer structure, synchronous mixed electrostatic spinning and an electro-spray technology, so that a novel Janus membrane with high yield, high stability and multiple functions is obtained.

Electrically sprayed polystyrene/poly (dimethylsiloxane) (PS/PDMS) spray beads in the process flow interconnect the nanofibers and are further anchored by amorphous nanofibers formed from electrospun poly (vinylidene fluoride-co-hexafluoropropylene) PH to form a 3D network; the surface roughness and morphology of the formed spray beads, the engineered bead string structure (hollow spherical bead string) and the related thin hydrophobic layer can be adjusted by the weight ratio and concentration of PS and PDMS.

In this example, a PS/PDMS mixed solution (PH-7PS/7PDMS) with a weight ratio of PS to PDMS of 1:1 and a concentration of 7 wt% in the electrospray solution was used, and the thin hydrophobic layer of the prepared nanofiber Janus film showed the highest Water Contact Angle (WCA) value of 148.5^oAnd a maximum surface roughness of 812 nm.

The nanofiber Janus membrane prepared in PH-7PS/7PDMS of this example achieved 27.7L/m at a temperature difference of 40 ℃ when using 3.5 wt% sodium chloride (NaCl) solution as the feed solution²h water flux and near 100% salt rejection.

Example 2

The key point of the nanofiber Janus membrane for brine membrane distillation treatment and the preparation method thereof provided in this embodiment is to perform a performance test of PDMS modification effect on the Janus membranes of various thin hydrophobic layers prepared in embodiment 1.

This example determines the composition of surface elements and functional groups on pure PH, PH-3PS and PH-3PS/3PDMS hydrophobic layers by XPS and FTIR analysis. As shown in fig. 5 (a), characteristic XPS peaks of F and C elements can be detected in all hydrophobic layers, mainly from PH nanofibers. The F/C ratio on the PH-3PS layer is significantly reduced compared to the pure PH layer. After addition of PDMS, the F/C ratio was further reduced with new peaks at binding energies 103.08, 154.08, and 533.08 eV, assigned to Si 2 p, Si 2 s, and O1 s core-level XPS peaks, respectively, confirming the loading of PDMS on the PH-3PS/3PDMS hydrophobic layer, since the O and Si elements are well-known PDMS characterization elements. In FIG. 4 (b) FTIR spectra of pure PH, PH-3PS and PH-3PS/3PDMS hydrophobic layer are shown. 1398. 1182 and 883 cm^-1The absorption peaks at (A) are respectively attributed to stretching vibration of CH bond, symmetric stretching of CF bond and skeleton vibration of CC bond, which are main chemical groups of PH skeleton. 2916 and 698 cm observed on FTIR Spectroscopy of the PH-3PS layer^-1The two broad absorption peaks at (a) are respectively attributed to stretching vibration and out-of-plane bending vibration of unsaturated CH bonds on the aromatic ring. In addition, the benzene ring vibrates telescopically at 1452 cm^-1The presence of PS was also confirmed by the peaks at (a). After addition of PDMS at 1261, 1086 and 802 cm^-1New peak appears at the position, respectively attributed to Si-CH₃Flexural vibration of the bond, stretching of the Si-O-Si bond and wobbling of Si, Si (CH)₃)₂The bond provides clear evidence for the modification of PDMS.

Example 3

The nanofiber Janus film for brine membrane distillation treatment and the preparation method thereof provided in this embodiment are mainly used for testing the surface wettability of the Janus films with different thin hydrophobic layers prepared in embodiment 1.

The WCA values for hydrophobic layers prepared from electrospray solutions of different PS/PDMS concentrations were determined in this example and are shown in Table 3. The highest WCA value of 148.5 ℃ can be obtained by preparing a hydrophobic layer by an electrospray mixed solution PH-7PS/PDMS, and the concentration of the PS/PDMS is 7 wt% at the moment. The hydrophobicity of the membrane surface depends to a large extent on the surface energy and microstructure. The accumulation of PDMS with low surface energy on the spherical outer surface of the spray beads can enhance the hydrophobicity of the membrane. Meanwhile, according to AFM analysis, the maximum average surface roughness (Ra) of the hydrophobic layer of PH-7PS/7PDMS was 812 nm. The decrease in WCA of the hydrophobic layer of PH-9PS/9PDMS was probably due to the decrease in surface roughness (717 nm), despite the increased addition of PDMS. As shown in table 3, the Janus membrane with PS/PDMS spray beads showed higher porosity than the pure PH membrane, and the Janus membrane with the PH-7PS/7PDMS hydrophobic layer achieved 77.6% maximum porosity. The spray beads in the 3-D nanofiber network can act as spacers, potentially enlarging the distance between adjacent PH nanofibers, weakening the packing density of the hydrophobic layer, and thus increasing porosity. As can be seen from FIG. 3, when the addition amount of PS and PDMS is between 3-7 wt%, the formation of spray beads and engineering bead strings can be significantly promoted, resulting in an increase in the porosity of the film.

The embodiment of the invention uses the electrospun hydrophilic Polymer (PAN) as the hydrophilic nanofiber (ENs) layer porous substrate of the Janus membrane, and can obviously reduce the mass transfer resistance in the MD process by utilizing the high porosity and low tortuosity of the porous substrate. On the basis, a thin hydrophobic layer with a unique 3D structure of the Janus membrane is constructed, and the Janus membrane in the form of hollow fibers is formed. In the preparation process, the total layer thickness of the film can be controlled by adjusting the time or the number of layers of electrostatic spinning and precision spraying, so that the thickness required by the film with the optimal performance can be obtained; the diameter of the spray bead, the appearance of the engineering bead string structure, the surface mesopores and the like can be adjusted by adjusting the concentration of the PS/PDMS mixed solution so as to optimize the surface wettability of the hydrophobic surface.

Wettability is an important property of a solid surface and is the expression of the ability of a liquid to wet the surface of a material. The surface wettability of the film material is determined by the microstructure and the surface chemical composition of the material. The embodiment of the invention constructs a multilayer elastic microstructure of the material by a unique preparation method and modification by introducing the PDMS material, improves the surface wettability of the film material by changing the microstructure and the surface chemical composition, and can keep the surface wettability at a higher level within a longer working time, thereby prolonging the effective service life of the film. The properties of Janus membranes with hydrophobic layers prepared with electrospray solutions of different PS/PDMS concentrations are given in table 3 below:

TABLE 3

Example 4

When the nanofiber Janus membrane prepared in example 1 and used for brine membrane distillation treatment is applied to a membrane distillation MD process, the existence of a large number of air pockets on the inner portion and the outer portion of the membrane material in the thickness direction can provide a larger water-air interface area for mass transfer, so that the nanofiber Janus membrane can capture more air below a liquid membrane interface, the heat transfer capacity of the air is lower than that of a polymer skeleton, heat loss and temperature polarization are effectively reduced, the driving force is enhanced, and the water flux of the nanofiber Janus membrane is improved. The optimal thickness of the separating membrane for MD is 30-60 mu m, and the thickness of the thin hydrophobic layer in the Janus membrane can be controlled by controlling the time and the number of layers of electrospinning and spraying, so that the total thickness of the membrane is adjusted to reach the optimal thickness.

The Janus membrane with the hydrophobic layer of pH-7PS showed a significant decrease in water flux over 21 hours of run time when 3.5 wt% NaCl solution was used as the feed solution in this example. In contrast, no significant water flux drop was observed for the Janus membrane with the hydrophobic layer of PH-7PS/7PDMS during 21 hours of continuous operation. The good stability of the water permeability can be attributed to the unique surface characteristics of the hydrophobic layer anda microscopic 3D network structure. The addition of PDMS with low surface energy can make the film have better anti-wetting ability, and make WCA value from 139.7 of hydrophobic layer of PH-7PS^oIs remarkably improved to 148.5^o. Furthermore, the surface roughness of the thin hydrophobic layer can be increased by a factor of approximately 2 after addition of PDMS. The high surface roughness can generate more air pockets below the liquid film interface, improve the heat conduction resistance of the film, and simultaneously increase the hydrodynamic shear force, thereby effectively preventing the accumulation process of salt crystals on the surface of the film in the MD process. Since the hydrophobic layer of PH-7PS is relatively weak against wetting, more severe salt leakage from the feed side to the coolant side occurs, resulting in lower desalination efficiency of the Janus membrane with the hydrophobic layer of PH-7PS for the duration of operation, as shown in fig. 8 (b).

During MD, natural organics in the stock solution may deposit on hydrophobic surfaces, blocking surface pores, resulting in membrane wetting. HA is used as a representative of the widely used membrane fouling agents to determine the anti-fouling capacity of the prepared Janus membranes. As shown in fig. 9 (a 1) - (a 3), only a slight decrease in water flux of about 5% was observed during the first 4 hours of operation using a Janus membrane with a PH-7PS/7PDMS hydrophobic layer, and then the water flux tended to stabilize. For Janus membranes with a hydrophobic layer of pH-7PS, the water flux dropped dramatically in the initial 14 hour continuous DCMD test, resulting in a 18% drop in total water flux after 21 hours of operation. As shown in fig. 9 (b 1) - (b 3), the distillate conductivity using Janus membranes with hydrophobic layers of PH-7PS/7PDMS remained around 1.6 μ S/cm throughout the test, while the Janus membranes without PDMS modification demonstrated a significant increase in distillate conductivity.

After the end of the run, the contaminated membrane was rinsed with deionized water for 10 minutes to remove the loose HA accumulation layer on the membrane surface and dried in air before morphological observation. As shown in fig. 9, there was no significant fouling deposited on the membrane surface or pore plugging due to the presence of HA in the feed solution. This is due to the lower tendency for membrane fouling in the MD process compared to other pressure driven membrane desalination processes. The degree of membrane fouling was further assessed by WCA measurements. As shown in fig. 9 a3 and b3, the WCA value of Janus film with hydrophobic layer of PH-7PS decreased from 139.7 ° to 118.6 ° after treatment of HA-containing brine. In contrast, contaminating Janus films with a hydrophobic layer of PH-7PS/PDMS had residual WCA values as high as 135.5. Such high surface hydrophobicity is sufficient to maintain good anti-wetting ability of the contaminated Janus membrane. The existence of PDMS on the surface of the membrane can effectively reduce the organic pollution of the membrane. In addition, because more air is trapped on the membrane surface, the effective contact area between the feed solution and the membrane surface having a higher roughness will be smaller, which will help to minimize fouling deposits on the membrane surface and extend the effective useful life of the membrane.

Example 5:

the nanofiber Janus film prepared in example 1 for brine membrane distillation treatment is applied to a Direct Contact Membrane Distillation (DCMD) process, the nanofiber Janus film has multi-stage lamination surface roughness and super hydrophobicity on the outer side surface, and a multi-layer elastic 3D network pore structure is formed by stably connecting amorphous nanofibers, spray beads and engineering bead string structures inside the nanofiber Janus film, so that the nanofiber Janus film has excellent thermal stability, strong antifouling capacity, alkali resistance and durability.

The polyacrylonitrile PAN used in the invention is used as a high polymer compound, has a functional group of-CN, has a pH tolerance range far exceeding the pH condition of seawater, in addition, the Janus membrane is a double-layer membrane, the thin hydrophobic layer of the Janus membrane is composed of PH nanofibers, PS/PDMS spray beads and engineering bead strings, the thin hydrophobic layer directly faces seawater, and the polyacrylonitrile PAN becomes a hydrophilic layer, only contacts desalted fresh water, and PDMS is introduced to carry out hydrophobic side surface modification, so that the alkali resistance of the two side surfaces is improved, the integral alkali resistance of the membrane material is improved, and the membrane material is suitable for industrial seawater desalination.

Practical tests have shown that the Janus membrane with a thin hydrophobic layer prepared with PH-7PS/7PDMS only reduces the water flux by about 8.5% when the feed concentration is increased from 2.5% to 5.5% and the temperature difference is kept at 40 deg.C (high salt and high heat seawater). The slight decrease in water flux is due to the decrease in water activity coefficient and saturation vapor pressure and the presence of concentration polarization effects. The rejection rate can be maintained above 99.9% over the tested feed concentration range, indicating that the Janus membranes provided by the present invention are good candidates for effective treatment of high temperature seawater without affecting rejection rate.

According to the invention, PDMS is introduced for modification, and a spray bead (and an engineering bead string structure) formed by a PS/PDMS mixed solution is subjected to electric spray coating, so that all the nanofibers and all the layers are connected, and finally, integral hot pressing is carried out, so that the problem of interface connection stability of a hydrophilic layer and a hydrophobic layer is solved. The multilayer network three-dimensional elastic structure formed by the engineering bead strings in the embodiment enables the stability after film forming to be remarkably superior to that after film forming of independently distributed and stacked spray beads (when the number of spray beads is small). The engineered bead string structure formed by the randomly distributed mass of spray beads in the present example, in combination with the immobilization (anchoring) of the PH nanofiber network, provides excellent structural stability for the Janus membrane. Experiments show that the weight difference of the Janus membrane prepared by the embodiment of the invention before and after 2 hours of ultrasonic treatment is obviously reduced along with the increase of the concentration of PS and PDMS, and the stability of the Janus membrane structure is increased along with the increase of the number of sprayed beads, the increase of the number of interconnected nano fibers and the increase of the diameter of the sprayed beads and the enhancement of the elastic effect.

The test result of the embodiment shows that the nanofiber Janus membrane for brine membrane distillation treatment provided by the invention has comparable DCMD performance, good thermal and structural stability and low membrane pollution, and particularly the prepared Janus membrane with the PH-7PS/PDMS hydrophobic layer shows a larger comprehensive performance advantage in DCMD application, and has great application prospects in the fields of biological fluid control, wastewater treatment, seawater desalination and the like.

In other embodiments of the present invention, the technical effects described in the present invention can be achieved by specifically selecting different types of Janus membranes for brine membrane distillation treatment within the ranges of the steps, components, ratios and process parameters described in the present invention, and therefore, the present invention is not listed in the present invention.

The foregoing is merely a preferred embodiment of the invention and is not intended to limit the invention in any manner. Those skilled in the art can make numerous possible variations and modifications to the present invention, or modify equivalent embodiments, using the means and techniques disclosed above, without departing from the scope of the invention. All equivalent changes in the components, proportions and processes according to the present invention are intended to be covered by the scope of the present invention.

Claims

1. A nanofiber Janus membrane for brine membrane distillation treatment is characterized by comprising a hydrophilic nanofiber layer, a hydrophobic nanofiber layer and a plurality of spray beads formed by electrically spraying a PS/PDMS mixed solution, wherein the hydrophilic nanofiber layer, the hydrophobic nanofiber layer and the plurality of spray beads are mutually laminated and connected;

the spray beads are hollow spherical structures with a large number of nano-scale mesopores on the surfaces; the spraying beads formed by electrically spraying the PS/PDMS mixed solution are anchored by a 3D network formed by amorphous nanofibers in a PH nanofiber layer and a PAN nanofiber layer respectively in the bead forming process, and are distributed among different nanofibers in a discontinuous way or on the same nanofiber in a continuous way to form an engineering bead string structure;

the hydrophilic nanofiber layer and the hydrophobic nanofiber layer are subjected to hot pressing, so that nanofibers in each fiber layer are locally contacted and connected with spray beads to form an integrated nanofiber Janus membrane with a large number of air pockets in the thickness direction and on the surface;

in the process of MD treatment by brine membrane distillation, a large number of air pockets exist inside and outside the nanofiber Janus membrane in the thickness direction, and a larger water-air interface area is provided for mass transfer, so that the nanofiber Janus membrane captures more air below a liquid membrane interface, the heat transfer capacity of the air is lower than that of a polymer skeleton, the heat loss and temperature polarization are effectively reduced, the driving force is enhanced, and the water flux of the nanofiber Janus membrane is improved.

2. The nanofiber Janus membrane for brine membrane distillation treatment as claimed in claim 1, wherein,

3. The nanofiber Janus membrane for brine membrane distillation treatment as claimed in claim 1 or 2, wherein the porosity of the nanofiber Janus membrane is more than or equal to 77.6%; the multi-stage surface roughness Ra of the outer surface of the thin hydrophobic layer is more than or equal to 800 nm, and the Water Contact Angle (WCA) value is more than or equal to 148^o。

4. The nanofiber Janus membrane for brine membrane distillation treatment as claimed in claim 1, wherein in the direct contact membrane distillation DCMD process, the nanofiber Janus membrane has multi-stage lamination surface roughness and super hydrophobicity on the outer side surface, and a multi-layer elastic 3D network pore structure is formed by stably connecting amorphous nanofibers, spray beads and engineering bead string structures inside, so that the nanofiber Janus membrane has excellent thermal stability, strong antifouling capability, alkali resistance and durability.

5. A method for preparing the nanofiber Janus film for brine membrane distillation treatment according to any one of claims 1-4, which is characterized by comprising the following steps:

s2: preparing spray beads and engineering bead strings:

s3: preparation of a thin hydrophobic layer: performing electrostatic spinning of a PH solution on the upper surface of the hydrophilic nanofiber layer to prepare a single-layer electrospun PH nanofiber; synchronously carrying out electric spraying on the single-layer electrospun PH nanofiber with the progress of the electrostatic spinning PH solution by using a PS/PDMS mixed solution, and forming a plurality of PS/PDMS spraying beads and engineering bead strings on the single-layer electrospun PH nanofiber; repeatedly and repeatedly carrying out electrostatic spinning on the PH solution and synchronously carrying out electric spraying on the PS/PDMS mixed solution for many times to form a thin hydrophobic layer with a multi-layer structure and multi-stage surface roughness on the outer surface, wherein the randomly distributed spray beads carry out elastic support, connection and spacing on each layer of electrospun PH nano-fibers and between each layer of electrospun PH nano-fibers;

6. The preparation method according to claim 5, wherein the step S1 specifically comprises the following steps:

s22: and (3) adhering electrostatic spinning on the surface of the aluminum foil to obtain a plurality of amorphous PAN nanofibers to form a single-layer PAN fiber film, namely a PAN electrospun hydrophilic nanofiber layer.

7. The preparation method according to claim 5, wherein the step S2 specifically comprises the following steps:

s21: preparation of PDMS solution: dispersing the dimethyl siloxane prepolymer into tetrahydrofuran for 30 minutes under ultrasonic treatment, and then adding dimethylformamide into the mixture under ultrasonic treatment to obtain a homogeneous solution; dropwise adding a curing agent into the homogeneous solution, and continuously stirring at 65 ℃ for 4 hours by using a magnetic force of 600 rpm/min;

s22: preparing a PS solution: dissolving a polystyrene polymer into a solvent under stirring to obtain a PS solution;

s23: preparing an electric spraying PS/PDMS mixed solution: carrying out ultrasonic stirring treatment on the prepared PDMS solution and the PS solution to obtain a PS/PDMS mixed solution;

s24: preparing spray beads and engineering bead strings: electrically spraying a PS/PDMS mixed solution on the upper surface of the PAN electrospun hydrophilic nanofiber layer according to set process parameters, wherein spray beads generated by electrically spraying PS/PDMS randomly form a plurality of PS/PDMS spray beads or engineering bead strings on the nanofibers of the hydrophilic nanofiber layer or among the nanofibers; the form and dimensions of the spray beads and the engineering bead strings of step S24 are adjusted by adjusting the ratio and concentration of the PS/PDMS mixed solution of step S23.

8. The preparation method according to claim 5, wherein the step S3 specifically comprises the following steps:

s31: preparation of electrospun PH solutions: dissolving the PH powder in a mixed solvent at 50 ℃ to prepare a PH solution;

preparing a PDMS solution: dispersing a dimethyl siloxane prepolymer into tetrahydrofuran for 30 minutes under ultrasonic treatment to obtain a mixture, and then adding dimethylformamide into the mixture under ultrasonic treatment to obtain a homogeneous solution; dropwise adding a curing agent into the homogeneous solution, and continuously stirring for 4 hours at 65 ℃ by using a magnetic force of 600rpm/min to obtain a PDMS solution;

preparing a PS solution: dissolving a polystyrene polymer into a solvent under stirring to obtain a PS solution;

preparing an electric spraying PS/PDMS mixed solution: carrying out ultrasonic stirring treatment on the prepared PDMS solution and the PS solution to obtain a PS/PDMS mixed solution;

s32: preparing a single-layer electrospun PH nanofiber layer: according to the set process parameters, performing electrostatic spinning on the PH solution on the upper surface of the hydrophilic nanofiber layer to obtain a plurality of amorphous PH nanofibers and form a single-layer electrospun PH nanofiber;

s33: synchronously preparing spray beads and engineering bead strings: according to the set process parameters, while the step S32 is carried out, a PS/PDMS mixed solution is electrically sprayed on the upper surface of the single-layer PH electrospun nanofiber, a plurality of beads generated by the electrically sprayed PS/PDMS randomly form a plurality of PS/PDMS spray beads or engineering bead strings on the PH electrospun nanofiber or among the nanofibers, and the spray beads and the engineering bead strings elastically connect, support and space the nanofibers and the nanofibers at each layer; wherein, the forms and the scales of the spray beads and the engineering bead strings are adjusted by adjusting the proportion and the concentration of the PS/PDMS mixed solution;

s34: preparing a thin hydrophobic layer: repeating the steps S32 and S33 for a set number of times, wherein the number of times is not less than 2, and obtaining the mutually laminated multilayer structure of the nanofiber-mixed spray beads and the thin hydrophobic layer with multi-level surface roughness.

9. The method according to claim 8,

the mixed solvent for dissolving the PH powder in the step S31 comprises the following components in percentage by mass: dimethylformamide acetone = 4: 1; in the PS/PDMS mixed solution, the concentration of PS and PDMS is 7 wt%, so as to obtain the nanofiber Janus membrane with high porosity, multistage surface roughness, stable connection, firm structure and high elasticity.