Janus membrane suitable for membrane distillation concentration as well as preparation method and application of Janus membrane

Abstract

The invention discloses a method for preparing a Janus membrane based on a PDA / PEI floating deposition method developed on the basis of a high-hydrophobicity PVDF spray membrane, deposition conditions and a raw material system are synergistically regulated and controlled, hydrophilic functional groups such as hydroxyl and amino are successfully introduced into a modified layer, single-side hydrophilic modification of the high-hydrophobicity PVDF spray membrane is achieved, and the high-hydrophobicity PVDF spray membrane is prepared. The composite Janus membrane with high hydrophilicity and high hydrophobicity is successfully prepared, and the comprehensive performance of the Janus membrane is optimized. The prepared Janus membrane shows obvious asymmetric wetting characteristic difference, in a membrane distillation desalination test, the membrane flux is stably maintained at a relatively high level, extremely high salt rejection rate and separation performance are also realized, and meanwhile, the stability of long-term operation is ensured; therefore, the method has good application value in separation and concentration processes in the fields of food, medicine and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to a membrane and a preparation method thereof, in particular to a Janus membrane suitable for membrane distillation concentration and a preparation method and application thereof, and belongs to the technical field of membrane separation. BACKGROUND

[0002] Membrane distillation, as a new heat-driven separation technology, has unique advantages in the fields of high-salinity wastewater treatment, seawater desalination, solution concentration and purification, etc. The key to maintaining the stability of membrane distillation is the anti-wetting property of hydrophobic membrane materials. However, during the long-term operation of traditional hydrophobic membranes, there are generally problems of flux decay caused by concentration polarization and membrane wetting caused by low-surface-energy pollutants, which seriously limits the practical industrial application of membrane distillation technology.

[0003] To solve the above problems, researchers have proposed a Janus membrane with asymmetric structure. The Janus membrane has opposite properties on both sides and is a functional membrane material with hydrophilic-hydrophobic asymmetric structure, which exhibits significant performance advantages in membrane distillation concentration and other fields:

[0004] (1) In terms of separation performance, through the synergistic effect of the hydrophilic-hydrophobic interface, the hydrophilic side of the Janus membrane can promote the adsorption and rapid transfer of water molecules, and the hydrophobic side can maintain high flux transmission of vapor, reduce mass transfer resistance, and always have high salt rejection, achieving the balance of efficient mass transfer and high rejection. At the same time, the hydrophilic layer can reduce the water evaporation resistance on the membrane surface and reduce the deposition of solutes on the membrane surface, thereby alleviating the flux decay caused by concentration polarization.

[0005] (2) In terms of anti-wetting and anti-pollution performance, the superhydrophilicity of the hydrophilic modification layer can effectively inhibit the adsorption of surfactants on the membrane surface, and the superstrong adhesion of polydopamine can enhance the interfacial bonding force between the hydrophilic layer and the hydrophobic substrate, forming a three-dimensional interlocking structure to avoid delamination and reduce the attachment of pollutants.

[0006] (3) In terms of mechanical properties, the tensile stress and elongation at break of the Janus membrane are greatly improved, showing good strength and stability.

[0007] Therefore, the Janus membrane, with its unique structure and performance, has been widely used in the fields of interfacial mass transfer, controlled ion transport, and one-way oil-water emulsion separation. However, there are still some deficiencies in the practical application of the Janus membrane. For example, the traditional polydopamine deposition method for preparing the hydrophilic modification layer takes a long time. Although the time is shortened by optimizing the process, it still takes a long time to form a stable modification layer under normal temperature deposition conditions, and some oxidant systems are difficult to form a continuous hydrophilic layer, affecting the consistency of the membrane performance.

[0008] For the above reasons, it is necessary to explore the preparation process of Janus membrane to improve the preparation efficiency and further optimize the comprehensive performance of the single-side hydrophilicity, mechanical properties, separation performance, long-term stability and the like of the membrane. SUMMARY

[0009] To solve the problems of the prior art, the purpose of the present application is to provide a preparation process of Janus membrane, which improves the single-side hydrophilicity of the membrane by synergistically controlling the raw materials and process conditions, optimizes the comprehensive performance of the membrane, effectively reduces the risk of membrane wetting and pollution, and ensures the stability of long-term operation.

[0010] In order to achieve the above-mentioned goal, the technical scheme adopted by the present application is as follows:

[0011] The present application first discloses a Janus membrane suitable for membrane distillation concentration, which comprises a highly hydrophobic PVDF base membrane and a hydrophilic modification layer formed on one side of the surface of the base membrane. The hydrophilic modification layer is formed on the surface of the base membrane by a floating deposition method using a deposition solution, and the deposition solution at least comprises DA-HCl and PEI, and then a polymerization reaction is performed to form PDA (polydopamine).

[0012] Preferably, the aforementioned highly hydrophobic PVDF base membrane is prepared according to patent CN 120169184A, which is the previous research result of the inventors' team, and the present application is a further innovation based on the previous research result.

[0013] More preferably, the aforementioned deposition solution further comprises a catalyst / oxidizing agent, the catalyst is preferably CuSO4, and the oxidizing agent is selected from one of FeCl3, NaIO4 or APS, all of which can accelerate the deposition of PDA.

[0014] Further preferably, the aforementioned hydrophilic modification layer not only covers the surface of the membrane, but also partially penetrates into the interstitial space of the spherical crystal to form a three-dimensional interlocking structure.

[0015] The present application also discloses a preparation method of the aforementioned Janus membrane suitable for membrane distillation concentration, which comprises the following steps:

[0016] (1) Preparation of Tris-HCl solution

[0017] Prepare a Tris-HCl buffer solution with a concentration of 0.1 mol / L and a pH of 8.5;

[0018] (2) Preparation of Janus membrane

[0019] Dissolve the modifier in the Tris-HCl buffer solution to prepare a co-deposition solution, then float the highly hydrophobic PVDF base membrane on the surface of the deposition solution and deposit for a period of time;

[0020] After the deposition, the modified membrane was taken out of the deposition solution and rinsed with deionized water, and then dried at room temperature for standby use.

[0021] Preferably, the preparation method of the aforementioned Tris-HCl buffer solution is as follows: 121.14 g of Tris solid is dissolved in 800 mL of ultrapure water, and magnetically stirred at 25℃ until completely dissolved; then, the pH value of the solution is monitored in real time using a calibrated pH meter, and 2 mol / L hydrochloric acid solution is slowly added to adjust the pH value to 8.5; finally, the volume is made up to 1 L with ultrapure water, and stored in the dark for standby use.

[0022] More preferably, the aforementioned modifier includes DA-HCl, PEI and a catalyst / oxidant selected from one of FeCl3, NaIO4, CuSO4 and APS, which can greatly shorten the polymerization / deposition time of dopamine and improve the preparation efficiency.

[0023] Further preferably, the dissolution temperature and the deposition temperature of the aforementioned Tris-HCl buffer solution are both 25-85℃, and the deposition time is 15 min-96 h.

[0024] Still further preferably, the aforementioned co-deposition solution is prepared by mixing DA-HCl, PEI and APS in a mass ratio of 1:1:1 in a Tris-HCl buffer solution at 85℃, and the deposition temperature is 85℃ and the deposition time is 60 min.

[0025] The application also protects the use of the Janus membrane as described above in the separation and concentration process for membrane distillation concentration.

[0026] The application has the following advantages:

[0027] (1) The Janus membrane is first prepared by the PDA / PEI floating deposition method, and the deposition conditions (such as deposition temperature, type of catalyst / oxidant and deposition time) are controlled, and hydrophilic functional groups such as hydroxyl and amino groups are successfully introduced into the modified layer. The water contact angle of the hydrophilic side of the PDA / PEI / APS-85℃-60min composite membrane is 7.2±1.1°, reaching the super-hydrophilic state, realizing the single-sided hydrophilic modification of the high-hydrophobic PVDF spray membrane, and successfully preparing the composite Janus membrane with high hydrophilic-high hydrophobic characteristics.

[0028] (2) The composite Janus membrane prepared by the floating deposition method also has strong mechanical properties, and the deposition layer and the substrate form an interlocking structure, and the tensile stress of the composite membrane reaches 460.0 kPa, which is 23.1% higher than that of the high-hydrophobic PVDF substrate membrane (373.8 kPa), while maintaining a breaking elongation of 26.9%.

[0029] (3) The direct contact membrane distillation (DCMD) separation performance and anti-wetting performance test of the Janus membrane of the application showed that with the extension of deposition time, the membrane flux slightly decreased, but the rejection rate remained 99.99%. Among them, the membrane flux of PDA / PEI / APS-85℃-60min was 29.1±1.4kg·m -2 ·h -1 During the 72h continuous anti-wetting operation, the water flux of the PDA / PEI / APS-85℃-60min modified membrane only decreased from the initial 30.5kg·m -2 ·h -1 to 26.7kg·m -2 ·h -1 , while maintaining a stable rejection rate close to 100%, showing excellent anti-wetting performance and long-term stability.

[0030] (4) In the DCMD concentration test of the Janus membrane of the application on 2.0L erythritol solution (initial concentration 20wt.%), the flux and rejection rate remained relatively stable, and compared with the high-hydrophobic PVDF spray membrane, the flux decline caused by the increase of the feed side concentration was greatly delayed. In addition, the recovery rate of the PDA / PEI / APS-85℃-60min membrane reached 88.4%, which was effectively improved compared with the recovery rate (76.6%) of the high-hydrophobic PVDF substrate membrane. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is a preparation flow chart of the Janus membrane of the application;

[0032] Figure 2 is a polymerization reaction mechanism diagram of the Janus membrane of the application;

[0033] Figure 3 is a surface and cross-section micro-morphology diagram of the Janus membrane prepared in the comparative example and examples 1-4 of the application;

[0034] Figure 4 is a surface and cross-section micro-morphology diagram of the Janus membrane prepared in examples 5-8 of the application;

[0035] Figure 5 is a surface and cross-section micro-morphology diagram of the Janus membrane prepared in examples 9-12 of the application;

[0036] Figure 6 is a surface and cross-section micro-morphology diagram of the Janus membrane prepared in examples 13-16 of the application;

[0037] Figure 7 is a surface water contact angle change diagram of the Janus membrane prepared in the comparative example and examples 1-4 of the application;

[0038] Figure 8 Surface water contact angle change graph of Janus membranes prepared for Examples 5-8 of the present application;

[0039] Figure 9 Surface water contact angle change graph of Janus membranes prepared for Examples 9-12 of the present application;

[0040] Figure 10 Hydrophobic side and hydrophilic layer surface water contact angle change graph of Janus membranes prepared for Examples 13-16 of the present application;

[0041] Figure 11 FTIR graph of base membrane and Examples 13-16 of the present application;

[0042] Figure 12 XPS full spectrum analysis graph of base membrane and PDA / PEI / APS-85℃-60min modified membrane (Example 16);

[0043] Figure 13 EDS analysis graph of PDA / PEI / APS-85℃-60min modified membrane (Example 16);

[0044] Figure 14 Mechanical property comparison test graph of base membrane and PDA / PEI / APS-85℃-60min modified membrane (Example 16);

[0045] Figure 15 Flux and rejection rate test result graph of Janus membranes prepared for Examples 13-16 of the present application;

[0046] Figure 16 Wetting resistance result graph of base membrane and PDA / PEI / APS-85℃-60min modified membrane (Example 16);

[0047] Figure 17 Flux and rejection rate change graph over time of Example 16 of the present application;

[0048] Figure 18 Laboratory scale direct contact membrane distillation device used for detection of the present application;

[0049] Figure 19 DCMD concentration experiment result graph of base membrane and membrane prepared in Example 16 on 2.0L erythritol solution for a long time;

[0050] Figure 20 FTIR and XRD characterization result graph of erythritol before and after concentration and crystallization. DETAILED DESCRIPTION

[0051] The application will be specifically introduced below in combination with the drawings and specific examples.

[0052] Unless otherwise specified, the reagents used in the application (except ionized water) are commercially available, and the preferred routes are shown in Table 1 below:

[0053]

[0054] Table 1 Preferred sources of raw materials used in the application

[0055] The Janus membrane is prepared by the floating deposition method in the application, and the preparation process of the Janus membrane is as shown in Figure 1 The polymerization mechanism is as shown in Figure 2 The specific preparation process of the comparative example and each of the examples is as follows.

[0056] Comparative example and examples 1-4

[0057] (1) Preparation of Tris-HCl solution

[0058] Prepare a 0.1 mol / L Tris-HCl buffer solution (pH 8.5). The specific preparation process is as follows: weigh 121.14 g of Tris solid and dissolve it in 800 mL of ultrapure water, and magnetically stir at 25°C until completely dissolved. Then, use a calibrated pH meter to monitor the pH value of the solution in real time, and slowly add 2 mol / L hydrochloric acid solution to adjust the pH value to 8.5. Finally, dilute to 1 L with ultrapure water, and store in the dark for subsequent use.

[0059] (2) Preparation of Janus membrane

[0060] First, dissolve DA-HCl (0.2 g) and PEI (0.2 g) in 100 ml of Tris buffer solution, and magnetically stir at 25°C for 10 min to fully dissolve, to prepare a DA / PEI co-deposition solution with a concentration of 2 g / L.

[0061] Subsequently, the high-hydrophobic PVDF spray membrane 18% PVD-SiO2-E5 prepared by the application publication No. CN 120169184 A (the applicant's previous research results) is floated on the surface of the deposition solution with the back facing down, and the entire deposition tank is wrapped with tin paper with small holes to ensure the supply of oxygen required for dopamine oxidation and to avoid the influence of light. To prevent the formation of bubbles at the interface between the high-hydrophobic membrane and the deposition solution, slight stirring is required during the initial stage of deposition.

[0062] The deposition time of the comparative example and examples 1-4 is 0 h, 24 h, 48 h, 72 h and 96 h, respectively, and all the deposition processes are carried out in the dark at room temperature of 25±1°C.

[0063] After the deposition, the membrane was taken out and rinsed with deionized water to remove the loose PDA / PEI particles attached to the surface of the modified layer, and finally the membrane was left to dry at room temperature.

[0064] The Janus membranes prepared were named according to the "composition-time" rule, for example: PDA / PEI-24h represents the composition of the deposition solution as PDA / PEI, and the Janus membrane obtained by deposition at room temperature for 24h.

[0065] Examples 5-8

[0066] (1) Preparation of Tris-HCl solution

[0067] Prepare a 0.1 mol / L Tris-HCl buffer solution (pH 8.5), the specific preparation process is the same as above, which is not repeated here.

[0068] (2) Preparation of Janus membrane

[0069] First, the Tris buffer solution (0.1M, pH=8.5) was preheated to the specified temperature in a vacuum drying oven, and the specified temperatures of Examples 5-8 were 25℃, 45℃, 65℃ and 85℃, respectively.

[0070] Next, DA-HCl (0.2g) and PEI (0.2g) were dissolved together in 100ml of preheated Tris buffer solution, and fully dissolved by magnetic stirring for 10min to prepare a 2g / L DA / PEI co-deposition solution.

[0071] Subsequently, the high-hydrophobic PVDF spray membrane 18%PVD-SiO2-E5 prepared by the application publication No. CN 120169184 A (applicant's previous research results) was floated on the surface of the deposition solution with the back facing down, and the entire deposition tank was wrapped with tin paper with small holes pre-punched and placed in the drying oven at the specified temperature for deposition for 6h.

[0072] After the deposition, the modified membrane was taken out from the deposition solution and rinsed with deionized water to remove the loose PDA / PEI particles attached to the surface of the modified layer, and finally the membrane was left to dry at room temperature.

[0073] The Janus membranes prepared were named according to the "composition-time" rule, for example: PDA / PEI-85℃-6h represents the composition of the deposition solution as PDA / PEI, and the Janus membrane obtained by deposition at 85℃ for 24h.

[0074] Examples 9-12

[0075] (1) Preparation of Tris-HCl solution

[0076] A 0.1 mol / L Tris-HCl buffer solution (pH 8.5) was prepared as described above.

[0077] (2) Preparation of Janus membrane

[0078] First, the Tris buffer solution (0.1 M, pH = 8.5) was preheated to 85°C in a vacuum drying oven.

[0079] Next, 0.2 g of DA-HCl, 0.2 g of PEI, and 0.2 g of catalyst / oxidant (FeCl3, NaIO4, CuSO4, APS were used in Examples 9-12, respectively) were dissolved in 100 ml of Tris buffer solution at 85°C. The solution was fully dissolved by magnetic stirring for 10 min, and a DA / PEI / Oxidant co-deposition solution with a concentration of 2 g / L was prepared.

[0080] Subsequently, the high-hydrophobic PVDF spray membrane 18% PVD-SiO2-E5 prepared by the application publication No. CN 120169184 A (the applicant's previous research results) was floated on the surface of the deposition solution with the back facing down, and the entire deposition tank was wrapped with tin paper with small holes pre-punched and placed in a drying oven at 85°C for deposition for 6 h.

[0081] After deposition, the modified membrane was taken out of the deposition solution and rinsed with deionized water to remove loose PDA / PEI / Oxidant particles attached to the surface of the modified layer. Finally, the membrane was placed at room temperature to dry.

[0082] The Janus membrane prepared was named according to the "composition-time" naming rule, for example: PDA / PEI / FeCl3-85°C-6h represents the composition of the deposition solution as PDA / PEI, under the condition of FeCl3 oxidant, the Janus membrane obtained by reacting and depositing at 85°C for 6 h.

[0083] Examples 13-16

[0084] (1) Preparation of Tris-HCl solution

[0085] A 0.1 mol / L Tris-HCl buffer solution (pH 8.5) was prepared as described above.

[0086] (2) Preparation of Janus membrane

[0087] First, the Tris buffer solution (0.1 M, pH = 8.5) was preheated to 85°C in a vacuum drying oven.

[0088] Then, 0.2 g of DA-HCl, 0.2 g of PEI and 0.2 g of APS were dissolved together in 100 ml of Tris buffer at 85°C, and fully dissolved by magnetic stirring for 10 min to prepare a DA / PEI / APS co-deposition solution with a concentration of 2 g / L.

[0089] Subsequently, the high-hydrophobic PVDF spray film 18% PVD-SiO2-E5 prepared by the application publication No. CN 120169184 A (the applicant's previous research results) was floated on the surface of the deposition solution with the back facing down, and the entire deposition tank was wrapped with tin paper with small holes pre-punched and placed in a drying oven at 85°C for a certain time (15 min, 30 min, 45 min and 60 min for Examples 13-16, respectively).

[0090] After the deposition was completed, the modified film was taken out of the deposition solution and rinsed with deionized water to remove loose PDA / PEI / oxidant particles attached to the surface of the modified layer, and finally the film was placed at room temperature to dry.

[0091] The Janus film prepared adopts the naming rule of "composition-time", for example: PDA / PEI / APS-85°C-45 min represents that the composition of the deposition solution is PDA / PEI, the Janus film is obtained under the condition of APS oxidant, at 85°C for 45 min.

[0092] It can be seen that the preparation processes of the above examples and comparative examples are basically the same, and the main difference lies in the process parameters and part of the raw materials, which are shown in Table 2 below.

[0093] Serial number Membrane expression Deposition temperature Deposition time Catalyst / oxidant Comparative example PDA / PEI-0h 25℃ 0h None Example 1 PDA / PEI-24h 25℃ 24h None Example 2 PDA / PEI-48h 25℃ 48h None Example 3 PDA / PEI-72h 25℃ 72h None Example 4 PDA / PEI-96h 25℃ 96h None Example 5 PDA / PEI-25℃-6h 25℃ 6h None Example 6 PDA / PEI-45℃-6h 45℃ 6h None Example 7 PDA / PEI-65℃-6h 65℃ 6h None Example 8 PDA / PEI-85℃-6h 85℃ 6h None Example 9 [PDA / PEI / FeCl3 - 85°C - 6h] 85℃ 6h FeCl3 Example 10 [PDA / PEI / NaIO4 - 85°C - 6h] 85℃ 6h NaIO4 Example 11 [PDA / PEI / CuSO4 - 85°C - 6h] 85℃ 6h [CuSO4] Example 12 PDA / PEI / APS-85℃-6h 85℃ 6h APS Example 13 PDA / PEI / APS-85℃-15min 85℃ 15min APS Example 14 PDA / PEI / APS-85℃-30min 85℃ 30min APS Example 15 PDA / PEI / APS-85℃-45min 85℃ 45min APS Example 16 PDA / PEI / APS-85℃-60min 85℃ 60min APS

[0094] Table 2: Membrane expression and main difference process of each example and comparative example

[0095] Structural characterization

[0096] (1) Membrane surface morphology

[0097] The surface and cross-section morphology of the prepared Janus film were observed by field emission scanning electron microscope (FESEM, SUPRA TM55, Carl Zeiss, Germany).

[0098] The influence of deposition time at room temperature on the structure and surface wettability of the modified film was investigated by comparative example and examples 1-4, and four deposition time gradients of 0 h, 24 h, 48 h, 72 h and 96 h were set. By controlling the deposition time variable, the influence of different deposition times on the microstructure and hydrophilicity of the Janus film was systematically investigated, and the surface and cross-section micro-morphology of the Janus film prepared under different deposition times is shown in Figure 3 .

[0099] Depend on Figure 3 As can be seen, the back surface microstructure of the original highly hydrophobic spray membrane (0h) exhibits a spherulitic structure. With the increase of PDA / PEI deposition time, the spherulitic structure on the membrane surface is gradually covered by the PDA / PEI modified layer. After 24h of deposition, the large pores between the spherulites are partially covered by the PDA / PEI modified layer, and obvious polydopamine particles are generated on the modified layer and the exposed spherulite surface. After 48h of deposition, SEM observation shows that a continuous and dense PDA / PEI thin layer (average thickness of approximately 0.22±0.03μm) is formed on the membrane surface, the spherulitic structure is basically covered by the thin layer, and the surface roughness is significantly reduced. With further extension of deposition time, the surfaces of the PDA / PEI-72h and PDA / PEI-96h membranes exhibit a network stacking structure formed by polydopamine aggregates.

[0100] A longitudinal comparison of the SEM images reveals that the PDA / PEI deposition process follows an evolutionary pattern of "first dense coverage, then particle accumulation and growth." Specifically, a continuous and dense thin layer is preferentially formed on the substrate surface, followed by a network accumulation structure of polydopamine aggregates. This is mainly because the oxygen concentration at the gas / liquid interface is higher than that inside the deposition liquid. The higher dissolved oxygen concentration at the interface effectively promotes the oxidative polymerization reaction of dopamine, leading to the preferential formation of a dense PDA / PEI composite layer in the interfacial region. The cross-sectional images show that the thickness of the PDA / PEI modified thin layer on the surface of the highly hydrophobic spray film increases with deposition time, gradually increasing from 0.19±0.03 μm for the PDA / PEI-24h film to 1.79±0.36 μm for the PDA / PEI-96h film. Furthermore, the film surface and cross-sectional images show that the PDA / PEI modified layer not only covers the film surface but also partially penetrates into the interspherulite gaps to form a three-dimensional interlocking structure. This unique interfacial bonding method is beneficial for enhancing interfacial adhesion.

[0101] Figure 4 SEM images of Janus films deposited at different temperatures for 6 hours (Examples 5-8) show that deposition temperature affects the surface and cross-sectional morphology of the Janus films. The images reveal that the Janus film deposited at 25℃ (PDA / PEI-25℃-6h) has a small amount of network-like PDA / PEI deposited layer in the interspherulite pores, but a continuous deposition layer has not yet formed, and the original spherulite structure is still clearly visible. As the temperature increases to 45℃ and 65℃, the modified layer gradually becomes denser, and the exposed area of ​​the spherulites on the surface decreases accordingly. The film deposited at 85℃ (PDA / PEI-85℃-6h) forms a dense layer with a thickness of 0.21±0.06μm on the surface, and obvious polydopamine particles appear. This indicates that increasing the temperature can accelerate the oxidative self-polymerization reaction rate of dopamine, which is beneficial for the rapid formation of the PDA / PEI modified layer.

[0102] Further, different oxidants or catalysts were introduced into the system to explore their effects on the structure of the modified layer on the film surface and wettability, Figure 5 The micro-morphology of the Janus film surface modified by introducing different catalytic / oxidizing agent systems (Examples 9-12) for 6 h (deposition temperature 85°C) was observed. As can be seen from the SEM surface and cross-sectional images, among the four catalytic / oxidizing agent systems, only the PDA / PEI / FeCl3-85°C-6h film surface still showed the original spray film spherulite structure, and the PDA / PEI deposition layer was thin and cracked, and did not form a continuous hydrophilic modified layer. The PDA / PEI / NaIO4-85°C-6h film surface showed a loose structure formed by the accumulation of polydopamine particles, with large cracks on the surface and no dense layer formed immediately adjacent to the spherulite surface. The other two catalytic / oxidizing systems (CuSO4 and APS) formed a relatively thick PDA / PEI continuous modified layer on the dense layer, and the surface showed a relatively loose polydopamine particle accumulation structure. The presence or absence of a modified PDA / PEI dense layer immediately adjacent to the spherulite surface may be related to the speed of deposition accelerated by different catalytic / oxidizing agents. As can be seen from the cross-sectional images, the thickness of the continuous hydrophilic modified layer formed by the CuSO4 and APS systems was 0.52±0.05 μm and 1.84±0.39 μm, respectively. It can be seen that under the conditions of FeCL3 and NaIO4 as modifiers, the PDA layer formed on the film surface is discontinuous and has cracks; while under the conditions of APS and CuSO4 as modifiers, a dense PDA layer is first formed on the spherulite film surface, and then a loose polydopamine particle accumulation structure is formed on the dense layer. Under the same modification conditions, the APS system has a thicker hydrophilic layer, a smaller contact angle, and is more hydrophilic, indicating that APS as an oxidizing agent is more efficient. This indicates that compared with other oxidizing agents, APS catalyzing the dopamine self-polymerization reaction is more suitable and more conducive to the rapid preparation of a continuous PDA / PEI modified layer.

[0103] The PDA / PEI / APS system was deposited at 85°C for different times (Examples 13-16, deposition times were 15 min, 30 min, 45 min and 60 min, respectively), and the surface and cross-sectional micro-morphology of the series of Janus films prepared under this condition was observed. Figure 6The PDA / PEI / APS-85℃-15min film surface was covered with uniform PDA aggregates after 15 min of deposition. As the deposition time increased, the PDA / PEI aggregates on the film surface increased, and the PDA / PEI / APS-85℃-60min film surface was covered with PDA aggregates after 60 min of deposition. The cross-sectional view also showed that the PDA / PEI deposition layer gradually thickened from 0.26±0.03 μm for the initial PDA / PEI / APS-85℃-15min film to 1.00±0.07 μm for the PDA / PEI / APS-85℃-60min film.

[0104] (2) Water contact angle of the film surface

[0105] The surface properties (including micro-morphology and wettability) of the hydrophilic side of the Janus film are key factors affecting the separation performance of the Janus film. In the present application, the water contact angle (WCA) of the film sample in air was accurately measured using a contact angle meter (OCA-15EC, Dataphysics, Germany). The mechanical strength of the film sample was tested using a tensile strength tester (Dongguang Dongri Instrument Co., Ltd.).

[0106] Figure 7 The surface water contact angle of the Janus film prepared by PDA / PEI deposition at room temperature for different times was shown. The test results showed that the water contact angle of the back of the high-hydrophobic spray film before modification was 125.3±2.7°, and as the PDA / PEI deposition time increased, the water contact angle of the film surface showed a trend of first decreasing, then increasing, and then decreasing. In the initial deposition period (0-48 h), the original film surface was gradually covered with a PDA / PEI hydrophilic layer, but the surface roughness gradually decreased, so the water contact angle first decreased to 62.7±2.4° and then increased to 70.3±3.6°. When the deposition time was more than 48 h, the water contact angle gradually decreased and the hydrophilicity gradually increased as the PDA / PEI layer thickness continued to increase and formed a polydopamine nanoscale rough structure. After 96 h of deposition, the water contact angle of the hydrophilic side of the obtained Janus film was 12.8±2.4°, achieving a super-hydrophilic state. This wettability change rule was consistent with the surface morphology change trend shown in the figure, and was also consistent with the Wenzel hydrophilic surface wetting theory, i.e., the increase in the roughness of the hydrophilic surface was beneficial to further enhancing the wettability. Figure 3

[0107] Figure 8 ​The static water contact angle of Janus membranes modified with PDA / PEI at different deposition temperatures (25–85 °C) for 6 hours was shown. The results indicate that deposition temperature significantly affects the wettability of the PDA / PEI-modified membranes. Compared to the original highly hydrophobic spray membrane (125.3 ± 2.7°), the water contact angle of the PDA / PEI-25℃-6h membrane deposited at 25 °C decreased to 102.0 ± 6.7°, showing a slight increase in hydrophilicity. With further increases in temperature to 45 °C and 65 °C, the water contact angles of the PDA / PEI-45℃-6h and PDA / PEI-65℃-6h membranes decreased significantly. When the temperature was further increased to 85 °C, the membrane surface exhibited near-superhydrophilic properties, with a water contact angle of only 13.3 ± 4.6°. These results demonstrate that, at the same deposition time, the hydrophilicity of the membrane surface increases with increasing temperature. The applicant analyzed that this was mainly due to the uniform deposition of the PDA / PEI modified layer on the high-temperature membrane surface, which provided more hydrophilic groups such as amino and hydroxyl groups. Furthermore, the increase in the thickness of the modified layer (e.g.) Figure 4 As shown, this condition (85℃) is more conducive to the spread of water droplets on the membrane surface, reducing the water contact angle and further improving hydrophilicity. Therefore, the deposition condition of 85℃ is more conducive to the rapid formation of a uniform and more hydrophilic PDA / PEI modified layer on the surface of a highly hydrophobic spray membrane.

[0108] The water contact angles of Janus films obtained by modifying and depositing with different catalyst / oxidant systems for 6 hours (deposition temperature 85℃) are as follows: Figure 9 As shown, the three catalytic / oxidant systems of NaIO4, CuSO4, and APS are more conducive to promoting the formation of the PDA / PEI hydrophilic modified layer. Among them, the Janus membrane surface formed by the APS system has a water contact angle of 6.9±1.4°, reaching a superhydrophilic state. This is mainly attributed to the surface microstructure formed under the oxidation of APS: a dense PDA layer is first formed on the surface of the spherulite membrane, and a loose polydopamine particle stacking structure is formed on the dense layer.

[0109] In comparison, the contact angles of the films prepared by the NaIO4 and CuSO4 systems were 25.3±2.2° and 11.2±2.9°, respectively, which were better than those of the PDA / PEI / FeCl3-85℃-6h film, but still did not reach a superhydrophilic state. Comprehensive analysis indicates that adding APS as an oxidant to the deposition solution is more conducive to the rapid formation of a uniform and more hydrophilic PDA / PEI modified layer on the surface of the highly hydrophobic spray film.

[0110] Figure 10The water contact angles of the hydrophobic side and the hydrophilic layer of the Janus membranes obtained by PDA / PEI / APS system at 85°C for different deposition times are shown. As can be seen from the data in the figure, the hydrophobic side of the modified series of Janus membranes still maintains a high hydrophobic state (about 145°), and the water contact angle of the hydrophilic side gradually decreases with the increase of the deposition time. This confirms the selective single-sided deposition characteristics of PDA / PEI, realizing the asymmetric wettability of the series of membranes. After 60 min of deposition, the water contact angle of the obtained PDA / PEI / APS-85°C-60min membrane is reduced to 7.2±1.1°, reaching the super-hydrophilic level (WCA<10°). The hydrophilicity achieved by this system within 60 min exceeds the effect of 96h deposition at room temperature (12.8±2.4°), confirming that the synergistic effect of APS oxidant and high temperature conditions can significantly shorten the deposition time and improve the deposition efficiency.

[0111] (3) Membrane surface chemical composition

[0112] In order to confirm the successful preparation of the PDA / PEI modified Janus membrane, FTIR was used to test the chemical structure of the PDA / PEI / APS series of membrane samples. Figure 11 The different functional group absorption peaks corresponding to different infrared wave bands of different membrane samples are shown. In the FTIR spectrum of the original high-hydrophobic PVDF membrane, the characteristic peak at 1403cm -1 corresponds to the bending vibration of -CH2 group, and the three characteristic peaks at 1173, 875 and 836cm -1 belong to the asymmetric stretching vibration peak and symmetric stretching vibration peak of C-F2. Compared with the original high-hydrophobic PVDF substrate membrane, the PDA / PEI / APS modified Janus membrane has new characteristic absorption bands in the 3200-3600cm -1 and 1476-1783cm -1 wave number ranges. Specifically, the absorption peak at 1508cm -1 corresponds to the bending vibration of N-H bond, the characteristic peak at 1616cm -1 is derived from the C=C skeleton vibration of aromatic ring, the peak at 1735cm -1 corresponds to the stretching vibration of C=O group generated by oxidation, and the wide peak at 3200-3600cm -1 corresponds to the stretching vibration of O-H bond and N-H bond. The appearance of these characteristic peaks confirms the successful deposition of the PDA / PEI modified layer on the PVDF membrane surface. Compared with the infrared spectrum of the initial PVDF membrane, the characteristic absorption peak intensity of the series of Janus membranes in the range of 780~1450cm -1 belongs to the high-hydrophobic PVDF spray membrane, which is obviously weakened with the increase of the deposition time, which is mainly due to the enhanced shielding effect of the PDA / PEI modified layer thickness on the substrate signal.

[0113] X-ray photoelectron spectroscopy (XPS) was further employed to characterize the surface chemical composition of pristine high hydrophobic PVDF and PDA / PEI / APS-85°C-60min modified membrane. Figure 12 XPS survey spectra of pristine high hydrophobic PVDF and PDA / PEI / APS-85°C-60min modified membrane were compared to reveal the difference in surface chemical composition. The results showed that the spectra of the back surface of modified membrane and pristine high hydrophobic PVDF membrane were completely consistent, both of which appeared characteristic peaks at 683.9 eV (F 1s), 532.1 eV (O 1s), 284.8 eV / 289.3 eV (C 1s) and 101.8 eV (Si 2p). However, the surface chemical composition of modified membrane changed significantly, which confirmed the successful construction of PDA / PEI co-deposition layer on one side of the PVDF membrane. The appearance of a new characteristic peak at 398.2 eV in XPS spectrum belonged to N 1s, which directly proved the existence of PDA / PEI layer. Meanwhile, the O 1s signal peak was observed to be enhanced, while the F 1s signal peak disappeared. These results indicated that PDA / PEI co-deposition layer formed a continuous and complete covering layer on the surface of PVDF substrate membrane, which was consistent with the SEM results. Compared with the pristine PVDF membrane, two new peaks appeared at 284.8 eV and 283.6 eV in PDA / PEI / APS-85°C-60min membrane, which were attributed to C-O / C-N and C=O bonds, respectively. The O 1s spectrum further confirmed the existence of C-O and C=O groups on the surface of modified membrane. The N 1s spectrum could be fitted into two peak components: deprotonated C-N / C= N bond at 398.4 eV, and protonated amino group at 400.2 eV. The XPS analysis verified the deposition modification mechanism assumed in Figure 1 , and also confirmed the FTIR analysis results, which together proved the successful deposition of PDA / PEI and the successful preparation of Janus membrane.

[0114] The above FTIR and XPS analysis results confirmed the successful deposition of PDA / PEI on the surface of high hydrophobic PVDF substrate membrane. To further characterize the surface element distribution of modified membrane, energy dispersive X-ray spectroscopy (EDS) was used to perform surface scanning analysis on PDA / PEI / APS-85°C-60min membrane, as shown in Figure 13 The results showed that N and O elements were uniformly distributed on the membrane surface, and compared with the unmodified PVDF spray membrane, the O element content increased significantly and N element appeared. This change in element distribution was mutually confirmed with the XPS analysis results (increased O 1s peak intensity, new N 1s characteristic peak), which further confirmed that PDA / PEI co-deposition layer formed a continuous and complete covering on the surface of PVDF membrane.

[0115] (4) Membrane pore structure

[0116] The average pore size of the prepared membranes was measured using a pore size analyzer (3H-2000PB, Brookhaven Instruments Corporation).

[0117] The experimental results show that the extension of the deposition time not only improves the hydrophilicity of the membrane surface, but also has a significant impact on the membrane pore structure.

[0118] Membrane expression Average pore size (nm) PDA / PEI / APS-85℃-15min 306.8±3.5 PDA / PEI / APS-85℃-30min 202.2±6.4 PDA / PEI / APS-85℃-45min 142.7±4.0 PDA / PEI / APS-85℃-60min 124.7±10.0

[0119] Table 3 Average pore size of PDA / PEI / APS series Janus membranes

[0120] As shown in Table 3, the average pore size of the PDA / PEI / APS modified membranes showed a significant decreasing trend with the extension of the deposition time. When the deposition time was extended from 15 min to 60 min, the average pore size of the membrane decreased from 306.8 ± 3.5 nm to 124.7 ± 10.0 nm. This phenomenon of pore size reduction is mainly attributed to the dense layer formed by polydopamine on the membrane surface and the physical plugging effect of its aggregated particles on the membrane pores. In addition, longer deposition time promotes the deposition and cross-linking of more polydopamine molecules, forming a more complete coverage layer on the membrane surface, while also causing some particles to deposit within the membrane pores, ultimately resulting in a decrease in the average pore size of the membrane.

[0121] Performance detection

[0122] (1) Membrane mechanical property analysis

[0123] To evaluate the impact of surface modification on the mechanical properties of the membrane, we conducted a mechanical property comparison test between the PDA / PEI / APS-85℃-60min membrane with the optimal hydrophilicity (Example 16) and the original highly hydrophobic PVDF substrate membrane, and the results are shown in Figure 14 The tensile stress of the modified membrane reached 460.0 kPa, which was 23.1% higher than that of the 18% PVDF-SiO2-E5 substrate membrane (373.8 kPa); its elongation at break was 26.9%, which was slightly lower than that of the substrate membrane. The tensile strength of the modified membrane reached 1.23 times that of the original PVDF spray membrane, indicating that the introduction of the PDA / PEI modification layer not only maintained the toughness of the membrane material but also enhanced its mechanical strength. This improvement in mechanical properties is attributed to both the strong structural stability of the composite Janus membrane and the cross-linked network structure of the polydopamine coating and the strong interfacial bonding force between PDA / PEI and the highly hydrophobic PVDF substrate.

[0124] (2) Separation and desalination performance of Janus membranes

[0125] To evaluate the separation and desalination performance of the PDA / PEI / APS modified Janus membranes, we conducted a DCMD test (feed liquid was 3.5 wt.% NaCl solution), and the results are shown inFigure 15 As shown.

[0126] As shown in the figure, the membrane flux exhibited a slight decreasing trend as the deposition time increased from 15 min to 60 min, from the initial 31.6 ± 2.5 kg·m⁻². -2 ·h -1 It decreased to 29.1 ± 1.4 kg·m -2 ·h -1 However, the salt rejection rate of the PDA / PEI / APS series modified Janus membranes remained consistently greater than 99.99%. With prolonged deposition time, the membrane pore size gradually decreased, and the hydrophilic layer thickness increased from 0.26±0.03 μm to 1.00±0.07 μm. These structural changes increased mass transfer resistance, but the flux only showed a slight decreasing trend. This may be because the surface hydrophilic layer provides sufficient intermediate-state water molecules, effectively reducing the evaporation enthalpy and making the liquid water enriched at the hydrophilic layer evaporation interface more easily vaporize into water vapor.

[0127] (3) Antiwetting properties of Janus membrane

[0128] As an anionic surfactant, SDS is widely used in membrane antiwetting tests because its hydrophobic tail easily adsorbs onto the surface of traditional hydrophobic membranes through hydrophobic-hydrophobic interactions, leading to membrane wetting and performance degradation. This invention uses a gradient addition of SDS to a 3.5 wt.% NaCl feed solution to gradually reduce surface tension, thereby testing the antiwetting performance of a pristine highly hydrophobic PVDF substrate membrane and a PDA / PEI / APS membrane at 85℃ for 60 min.

[0129] The results are as follows Figure 16 As shown, F 18％ and F Janus R represents the flux of the highly hydrophobic 18% PVDF-SiO2-E5 membrane and the PDA / PEI / APS-85℃-60min Janus membrane, respectively. 18％ and R Janus The values ​​represent the rejection rates of the two membranes, respectively. When SDS (0.2 mM) was added to the feed solution, the flux of the PVDF membrane increased sharply, while the rejection rate decreased significantly, indicating that the highly hydrophobic PVDF membrane became wetted. In contrast, when the SDS concentration in the feed solution was gradually increased from 0 mM to 0.4 mM, the PDA / PEI / APS-85℃-60min membrane maintained a stable flux and a high salt rejection rate (consistently greater than 99.9%). This indicates that the anti-wetting properties of the PDA / PEI / APS-85℃-60min membrane are significantly better than those of the original highly hydrophobic PVDF membrane.

[0130] To further investigate the long-term anti-wetting performance of the PDA / PEI / APS membrane at 85℃ for 60 min, a NaCl solution containing 0.2 mM MSDS was used as the feed solution, and a membrane distillation test was conducted for up to 72 h. Figure 17 As shown. During a continuous 72-hour operation, the PDA / PEI / APS-85℃-60min modified membrane exhibited excellent anti-wetting and long-term stability, with its flux decreasing only from the initial 30.5 kg·m³. -2 ·h -1 Reduced to 26.7 kg·m -2 ·h -1 The rejection rate remained close to 100%. This result fully demonstrates that the modified Janus membrane has significantly improved anti-wetting properties, and its operational stability is significantly better than that of the highly hydrophobic PVDF membrane. Figure 16 The applicant analyzed that this might be due to the relatively dense PDA / PEI modified layer formed on the membrane surface, which effectively screened the surfactant, thus ensuring stable separation performance during long-term operation.

[0131] (4) Membrane distillation concentration performance test

[0132] Adopting such Figure 18 The laboratory-scale direct contact membrane distillation apparatus shown was used to test the membrane's separation and concentration performance and long-term stability.

[0133] Flux (F) is calculated using formula (1):

[0134]

[0135] Where Δm(kg) is the increase in mass of the osmotic solution within a certain time interval, Δt(h) is the sampling time interval, and A(m 2 ) represents the effective area of ​​the membrane.

[0136] When the feed solution was a 3.5 wt.% NaCl solution, the concentrations of both the feed solution and the permeate were determined using a conductivity meter. When a 10 wt.% erythritol solution was used as the feed solution, the concentrations of both the feed solution and the permeate were tested using a total organic carbon analyzer.

[0137] The retention rate (R) is calculated using formula (2):

[0138]

[0139] Among them, C f (g / L) and C p (g / L) represents the solution concentration on the feed side and the permeate side, respectively.

[0140] In the long-term stability and concentration performance test experiment, the initial feed liquid volume was 5.5 L, and the concentration factor (CF) of the feed tank of erythritol was calculated by formula (3)

[0141]

[0142] Wherein, C1(g / L) and C2(g / L) are the initial concentration of erythritol in the feed tank and the concentration of erythritol in the feed tank after running for 24h, respectively.

[0143] The anti-wetting performance and concentration performance of the membrane were evaluated by the DCMD process. The DCMD concentration crystallization experiment was continuously operated for 13h, and after the test was completed, the concentrated liquid was crystallized at room temperature, and the obtained crystals were vacuum dried and weighed. The mass of the obtained crystals was recorded, and the erythritol recovery rate (Y) was calculated according to formula (4).

[0144]

[0145] Wherein, m' represents the mass of erythritol after concentration crystallization and drying, and m0 represents the mass of erythritol in the initial feed liquid.

[0146] According to the previous systematic characterization of the surface microstructure, chemical composition, separation performance and anti-wetting ability of the composite membrane, the PDA / PEI / APS-85℃-60min membrane has a super-hydrophilic and relatively dense PDA / PEI modified layer (WCA ~ 7.2°) and a high-hydrophobic spherical crystal structure PVDF bottom membrane (WCA ~ 145°), which exhibits excellent separation performance and anti-wetting property. To verify its practical application potential, the PDA / PEI / APS-85℃-60min membrane was selected to perform a long-term DCMD concentration experiment on 2.0L erythritol solution (initial concentration 20wt.%), and compared with the initial high-hydrophobic PVDF membrane, and the results are shown in Figure 19 F 18％ and F Janus represent the flux of high-hydrophobic 18% PVDF-SiO2-E5 membrane and PDA / PEI / APS-85℃-60min Janus membrane, respectively, R 18％ and R Janus represent the rejection rate of the two membranes, respectively.

[0147] From Figure 19It can be seen that the high hydrophobic 18% PVDF-SiO2-E5 membrane and the PDA / PEI / APS-85℃-60min Janus membrane both showed nearly 100% erythritol rejection rate, but the flux change showed obvious difference. Among them, the high hydrophobic 18% PVDF-SiO2-E5 membrane and the PDA / PEI / APS-85℃-60min Janus membrane both showed nearly 100% erythritol rejection rate, but the flux change showed obvious difference. The initial flux of the high hydrophobic PVDF spray membrane was 30.5kg·m -2 ·h -1 The flux decline phenomenon occurred after 530min of operation, which was mainly due to the membrane pore blockage caused by high concentration of feed liquid and the decrease of vapor pressure caused by the increase of erythritol concentration (Raoult's law). In contrast, the initial flux of the PDA / PEI / APS-85℃-60min membrane was 30.8kg·m -2 ·h -1 The initial time of flux decline was delayed to 720min.

[0148] Therefore, compared with the initial high hydrophobic PVDF spray membrane, the Janus membrane after hydrophilic modification still maintained a high permeation flux compared with the salt solution separation test, although the pore size was reduced and the thickness was increased. This phenomenon is different from the observed flux decline trend when the high hydrophobic spray membrane is used as the feed of erythritol solution, and the reason may be that the introduction of the hydrophilic modification layer changes the interaction state of water molecules at the liquid-membrane interface, which is more conducive to the mass transfer process.

[0149] In addition, the recovery rate of erythritol obtained by membrane distillation (MD) concentration crystallization was calculated, and the results showed that the recovery rate of the PDA / PEI / APS-85℃-60min membrane was 88.4%, which was effectively improved compared with the recovery rate of the high hydrophobic PVDF substrate membrane (76.6%). This performance improvement further proves the advantages of the PDA / PEI modified Janus membrane in alleviating membrane fouling and maintaining long-term operation stability, and shows the potential for application in food and pharmaceutical production and other concentration applications.

[0150] (5) Characterization of erythritol before and after concentration crystallization

[0151] To verify whether the properties of erythritol obtained by MD concentration crystallization of PDA / PEI / APS-85℃-60min membrane remain the original physical and chemical properties, the components (Fourier transform infrared spectroscopy, FTIR) and structure (X-ray diffraction, XRD) of the erythritol obtained by concentration crystallization were characterized, and compared with the commercially available erythritol, as shown in Figure 20 The infrared spectrum (a) shows that the erythritol obtained by MD concentration crystallization shows typical characteristic absorption bands: 3200-3400cm-1 The broad peak at 2850–3000 cm⁻¹ corresponds to the OH stretching vibration of intermolecular hydrogen bonding. -1 The characteristic peaks within this range are attributed to the symmetric and asymmetric stretching vibrations of the -CH2 / -CH groups. These peaks are located in the 1300–1450 cm⁻¹ range. -1 The characteristic absorption observed in the region is the in-plane bending vibration of the OH phase, 1000–1100 cm⁻¹. -1 The strong peaks at these locations originate from the stretching vibrations of the COC skeleton. The positions and relative intensities of these characteristic peaks are completely consistent with the infrared spectrum of commercial erythritol, confirming that the concentrated crystallized product maintains the integrity of the chemical structure of erythritol. Figure 20 Figure b shows that the erythritol obtained from MD concentration and crystallization has completely consistent diffraction peak positions with the commercial standard (2θ = 15°, 19.5°, 20.3°, and 24.6°). FTIR and XRD analyses confirm that MD concentration and crystallization did not alter the chemical composition and crystal structure of erythritol.

[0152] In summary, this invention prepared Janus membranes using the PDA / PEI floating deposition method, and controlled the deposition conditions (including deposition temperature, catalyst / oxidant type, and deposition time) to enhance the unilateral hydrophilicity of the Janus membranes. The microstructure, water contact angle, chemical composition, pore size, and mechanical strength of the prepared series of Janus membranes were characterized, and their DCMD separation and concentration and anti-wetting performance were tested. The results showed that the prepared PDA / PEI / APS-85℃-60min membranes exhibited significant differences in asymmetric wetting characteristics (water contact angle: ~7.2° / 145°), and in the membrane distillation desalination test, the membrane flux remained stably maintained at 29.1±1.4 kg·m³. -2 ·h -1 At the same time, it achieved a salt rejection rate of 99.99%. Compared with similar membranes, this Janus membrane not only has outstanding asymmetric wetting characteristics, but also exhibits excellent separation performance. Its comprehensive performance is at a high level among similar membrane materials, while ensuring long-term operational stability, making it a valuable application in separation and concentration processes in the food and pharmaceutical industries.

[0153] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any way, and all technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A Janus membrane suitable for use in membrane distillation concentration, characterized in that, The Janus membrane comprises a high-hydrophobic PVDF base membrane and a hydrophilic modified layer formed on one side surface of the base membrane, wherein the hydrophilic modified layer is formed on the surface of the base membrane by a floating deposition method using a deposition solution containing DA-HCl and PEI.

2. A Janus membrane suitable for concentration by membrane distillation according to claim 1, characterized in that, The high-hydrophobic PVDF base membrane is prepared according to the patent CN 120169184A.

3. The Janus membrane suitable for concentration by membrane distillation according to claim 1, wherein, The deposition solution further contains a catalyst / oxidant selected from one of FeCl3, NaIO4, CuSO4 or APS.

4. The Janus membrane suitable for concentration by membrane distillation according to claim 1, wherein, The hydrophilic modified layer not only covers the surface of the base membrane, but also partially penetrates into the interlamellar space to form a three-dimensional interlocking structure.

5. The method of making a Janus membrane suitable for use in membrane distillation concentration of claim 1, wherein, The Janus membrane comprises: (1) preparing a Tris-HCl solution Prepare a 0.1 mol / L, pH 8.5 Tris-HCl buffer solution; (2) preparing a Janus membrane Dissolve the modifier in the Tris-HCl buffer solution to prepare a co-deposition solution, and then float the high-hydrophobic PVDF base membrane on the surface of the deposition solution for a certain period of time; After the deposition is completed, the modified membrane is taken out of the deposition solution and washed with deionized water, and then dried at room temperature for standby.

6. The method of claim 5, wherein the Janus membrane is prepared by the steps of: The Tris-HCl buffer solution is prepared by weighing 121.14 g of Tris solid and dissolving it in 800 mL of ultrapure water, and then stirring magnetically at 25°C until completely dissolved; then, use a calibrated pH meter to monitor the pH value of the solution in real time, and slowly add 2 mol / L hydrochloric acid solution to adjust the pH value to 8.5; finally, dilute to 1 L with ultrapure water, and store in the dark, ready for use.

7. The method for preparing a Janus membrane suitable for membrane distillation concentration according to claim 5, characterized in that, The modifier includes DA-HCl, PEI and a catalyst / oxidant selected from one of FeCl3, NaIO4, CuSO4 and APS.

8. The method for preparing a Janus membrane suitable for membrane distillation concentration according to claim 5, characterized in that, The dissolution temperature and deposition temperature of the Tris-HCl buffer solution are both 25-85°C, and the deposition time is 15 min-96 h.

9. A method for preparing a Janus membrane suitable for membrane distillation concentration according to claim 5, characterized in that, The co-deposition solution is prepared by mixing DA-HCl, PEI and APS in a mass ratio of 1:1:1 in a 85°C Tris-HCl buffer solution, and the deposition temperature is 85°C and the deposition time is 60 min.

10. The application of a Janus membrane suitable for membrane distillation concentration in a separation and concentration process according to claim 1.

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