Janus nanofiber membrane and preparation method and application thereof

Abstract

This invention belongs to the field of energy and chemical engineering, specifically relating to a customized solar-driven interface evaporator based on a Janus PAN / PVDFC nanofiber membrane for seawater desalination, wastewater purification, and the separation and collection of salt resources. The Janus PAN / PVDFC nanofiber membrane provided by this invention is composed of a blend of polyacrylonitrile and polyvinylidene fluoride carbon black fibers. The PAN layer of the Janus PAN / PVDFC nanofiber membrane prepared by this invention provides a hydrophilic fiber surface and the ability to transport water, while the PVDFC nanofiber layer provides resistance to salt precipitation and light absorption and heat conversion capabilities, thus ensuring that the heating process only heats a thin layer of water on the surface of the photothermal fiber membrane. The evaporator based on the Janus PAN / PVDFC nanofiber membrane exhibits stable adaptability to organic dye wastewater, high-salinity brine, acidic and alkaline wastewater. When used for seawater desalination, the photothermal fiber membrane of this invention features simple equipment, minimal human intervention, and strong photothermal conversion capabilities. Furthermore, its adjustable structural mechanism enables the simultaneous purification of wastewater, recovery of freshwater, and salt resources. This invention provides a promising approach for customizing solar-driven interfacial evaporative fiber membranes to simultaneously collect pure water and salt resources.

Description

Technical Field

[0001] This invention belongs to the field of energy and chemical engineering, specifically relating to a nanofiber membrane based on Janus double-layer PAN / polyvinylidene fluoride carbon black with different wetting properties on the upper and lower surfaces, its preparation method, and its application. Background Technology

[0002] Compared to traditional high-energy-consuming water purification technologies, solar-driven interfacial desalination technology provides a sustainable, decentralized freshwater production solution for water-scarce regions. Based on efficient photothermal conversion, this technology confines energy input to the vicinity of the phase interface by setting a functionalized photothermal layer at the gas-liquid interface, thereby achieving localized heating and water evaporation and significantly improving energy utilization efficiency.

[0003] However, during long-term operation, the salinity at the evaporation interface continuously increases, easily leading to salt crystallization. This not only reduces evaporation efficiency but may also clog water delivery channels and shorten equipment lifespan. Therefore, recent research has focused on reducing system heat loss and suppressing salt crystallization through structural design, with non-contact suspended evaporators receiving widespread attention. For example, Dong's team successfully fabricated a Janus-structured fiber mat, whose top hydrophobic layer combines water-blocking and light-absorbing functions, while the bottom is a hydrophilic polyacrylonitrile layer. This allows for directional water delivery and guides the selective crystallization of salt in the hydrophilic region, exhibiting excellent salt resistance. Wang et al. reported an inverted conical solar evaporator where the non-contact cone apex absorbs light energy and generates heat, while the hydrophilic bottom structure is used for water delivery and thermal regulation. Gravity allows crystallized salt to automatically detach from the rear surface, achieving synergistic desalination and salt recovery. Zhang et al. constructed a superhydrophilic polydopamine-modified layer on the surface of an industrial-grade carbon black nanofiber membrane using a dopamine self-polymerization method, thereby building a suspended evaporator. This structure maintained stable operation even in a high-concentration NaCl solution of 21.0 wt% without salt deposition. Although the above progress has significantly improved the salt tolerance and sustainability of interfacial evaporation systems, how to further synergistically optimize the freshwater production and salt resource recovery process remains a topic that urgently needs in-depth exploration in this field. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a nanofibrous membrane (NFM) based on Janus bilayer PAN / polyvinylidene fluoride carbon black (PAN / PVDFC). This nanofibrous membrane synergistically optimizes high light absorption and hydraulic transport performance, and can be used for solar-driven interfacial evaporation seawater desalination, synergistically achieving efficient freshwater production and salt resource collection.

[0005] The present invention also provides a method for preparing the above-mentioned Janus bilayer PAN / PAN / PVDFC nanofiber membrane.

[0006] Another objective of this invention is to provide the application of the aforementioned Janus bilayer PAN / PAN / PVDFC nanofiber membrane in a customized suspended solar-driven interfacial evaporator for photothermal seawater desalination. Its evaporation rate can reach 1.85 kg / m³ under 1-sun irradiation. -2 h - ¹ It has a photothermal efficiency of 97.15% and exhibits stable adaptability to organic dye wastewater, high salinity brine and acidic / alkaline wastewater. At the same time, its adjustable structural mechanism can effectively synergistically recover freshwater and salt resources.

[0007] The technical solution adopted by the present invention to achieve the above objectives is as follows: This invention provides a Janus-based bilayer PAN / polyvinylidene fluoride (PVDF) carbon black nanofiber membrane, which is composed of a blend of polyacrylonitrile (PAI) and PVDF carbon black fibers; the carbon black content in the PVDF carbon black fiber membrane is 0.17–0.70 mg / cm³. 2 Polyacrylonitrile fibers have a diameter of 200-400 nm; polyvinylidene fluoride carbon black fibers exhibit a beaded structure with a fiber diameter of 50-100 nm and a bead diameter distribution range of 0.5 μm to 2.0 μm.

[0008] The present invention further provides a method for preparing the above-mentioned Janus bilayer PAN / polyvinylidene fluoride carbon black nanofiber membrane, comprising the following steps: 1) Carbon black pretreatment: First, carbon black (CB) is added to dilute hydrochloric acid and ultrasonically treated; then, it is filtered, a mixture of H2O2 and H2NO3 is added, and ultrasonically dispersed again; after washing and drying, pretreated CB particles are obtained. 2) Preparation of PAN electrospun membrane: Dissolve PAN in DMF and stir until completely dissolved to obtain PAN electrospun solution. Perform electrospunting. After spinning, obtain PAN fiber membrane. 3) Preparation of Janus PAN / PVDFC nanofiber membrane: PVDF was dissolved in DMF solution to obtain PVDF solution; pretreated CB particles were ultrasonically dispersed in DMF solution to obtain CB particle dispersion; under stirring conditions, PVDF solution and CB particle dispersion were mixed and stirred to obtain PVDF-CB solution; PVDF-CB solution was electrospun on the surface of a receiver covered with PAN fiber membrane, and dried to obtain Janus bilayer PAN / PVDFCNFM nanofiber membrane.

[0009] Preferably, in step 1), the concentration of carbon black in dilute hydrochloric acid is 0.1~10 g / mL; the ultrasonication time is 1.0-1.5 h; the concentration of dilute hydrochloric acid is 10-15%; the mixture is composed of H2O2 and HNO3 in a volume ratio of 1:1; the volume concentration of HNO3 is 10-15% and the volume concentration of H2O2 is 10-20%; the time for re-ultrasonic dispersion is 1.0-1.5 h.

[0010] Preferably, the carbon black is carbon nanotubes or biochar.

[0011] Preferably, in step 2), the mass ratio of PAN to DMF is 1:8-10.

[0012] Preferably, in steps 2) and 3), the parameters for electrospinning are: adjusting the distance between the electrospinning needle and the receiving device to 10~20 cm, the needle height to 30~40 cm, the injection speed to 0.5~2.0 mL / s, and the voltage to 8~15 kV.

[0013] Preferably, in step 3), the concentration of the PVDF solution is 10-20 wt%; the content of CB in the CB particle dispersion is 1-10 wt%; the mass ratio of CB particles to PVDF in the PVDF-CB solution is 0.05-0.20; and the stirring time is 3-5 h.

[0014] The nanofiber membrane prepared by this invention may also use polyacrylonitrile as a natural hydrophilic polymer or polyvinyl alcohol; natural hydrophilic polymers such as cellulose and chitosan, or polyvinyl alcohol; and polyvinylidene fluoride may also be replaced with polyvinyl chloride, polytetrafluoroethylene, polypropylene, or polyurethane.

[0015] The present invention also provides the application of the above-mentioned Janus bilayer PAN / polyvinylidene fluoride carbon black nanofiber membrane in water treatment.

[0016] Preferably, a solar-driven interfacial evaporator is prepared using a Janus double-layer PAN / polyvinylidene fluoride carbon black nanofiber membrane for seawater desalination, wastewater purification, and the separation and collection of salt resources.

[0017] Preferably, the aforementioned natural hydrophilic polymers include, but are not limited to, cellulose and chitosan.

[0018] In the application test of the Janus PAN / PVDFC photothermal nanofiber membrane prepared by electrospinning technology provided by this invention, the simulated seawater used was a sodium chloride aqueous solution with a concentration of 3.5 wt% to 20 wt%; the seawater desalination device was a self-built suspended interfacial evaporation system, including a simulated seawater storage tank, a nanofiber membrane evaporator with a Janus structure, and a condensation collection unit for collecting fresh water and regulating the water level; the light source used was simulated sunlight, with a standard light intensity density of 1 kW / m² (denoted as 1-sun).

[0019] The application of the fiber membrane provided by this invention includes the following steps: Solar Evaporation Performance Test: A PAN / PVDFC NFM of a certain size was vertically suspended between two tanks filled with simulated seawater, exposing the central region of the fiber membrane to sunlight. A solar simulator equipped with an AM 1.5 filter was used as the light source, and the system's mass change was monitored in real time using a precision electronic balance. The water evaporation rate and photothermal conversion efficiency of the system were measured under 1-sun, 3-sun, and 5-sun light intensities, respectively. As a control, the baseline water evaporation of the same system under dark conditions was measured simultaneously.

[0020] Salt tolerance and salt collection experiments: NaCl solutions of different concentrations (3.5 wt%, 7 wt%, 10 wt%, 15 wt%, 20 wt%) were prepared to simulate a high-salinity environment, and the evaporation stability and salt crystallization behavior of the evaporator during long-term operation were evaluated. By adjusting the suspension structure and water level difference of the evaporator, salt ions were guided to migrate and crystallize towards the membrane edge and bottom, achieving directional deposition and recovery of salt.

[0021] Complex wastewater purification experiment: The evaporator was applied to the purification of organic dye wastewater containing methyl orange (MO), methylene blue (MB), and rhodamine B (RhB), as well as strong acid (0.01 M HCl) and strong alkali (0.01 M NaOH) solutions. Its purification efficiency and material stability were evaluated by ultraviolet-visible spectroscopy and ion concentration analysis (ICP-MS).

[0022] In the aforementioned applications, under 1-sun standard illumination, the photothermal evaporation rate of the evaporator based on Janus PAN / PVDFC NFM can reach 1.85 kg·m⁻. 2 ·h⁻ 1The photothermal conversion efficiency reaches as high as 97.15%. The Janus PAN / PVDFC nanofiber membrane provided by this invention is composed of blended fibers of polyacrylonitrile and polyvinylidene fluoride carbon black. The PAN layer of the Janus PAN / PVDFC nanofiber membrane prepared by this invention provides a hydrophilic fiber surface and the ability to transport water, while the PVDFC nanofiber layer provides resistance to salt precipitation and light absorption and heat conversion capabilities, thereby ensuring that the heating process only heats a thin layer of water on the surface of the photothermal fiber membrane. Evaporators based on the Janus PAN / PVDFC nanofiber membrane exhibit stable adaptability to organic dye wastewater, high-salinity brine, acidic and alkaline wastewater. When the photothermal fiber membrane of this invention is used for seawater desalination, the equipment is simple, requires less human operation, and has a strong photothermal conversion capacity. In addition, its adjustable structural mechanism can simultaneously achieve the purification of wastewater, freshwater, and salt resources. This invention provides a promising approach for customizing solar-driven interfacial evaporation fiber membranes to simultaneously collect pure water and salt resources.

[0023] This invention constructs a Janus bilayer membrane structure with asymmetric wettability using sequential electrospinning technology. The upper hydrophobic and photothermal PVDF / CB layer effectively absorbs and converts solar energy while inhibiting salt surface crystallization; the lower hydrophilic PAN network achieves continuous and rapid water transport through capillary action. This design enables the separation and synergistic enhancement of photothermal conversion and hydraulic management functions within a single membrane system.

[0024] The beneficial effects of the technical solution of this invention are as follows: (1) The Janus PAN / PVDFC nanofiber membrane prepared by the present invention has a unique bilayer structure and beaded morphology. The uniformly dispersed carbon black particles and the micro-nano rough structure work together to enable the membrane to achieve a light absorption rate of 91.4% in a wide spectral range of 250-2500 nm, thus realizing efficient photothermal conversion.

[0025] (2) The evaporator provided by this invention adopts a non-contact suspension structure, which can localize heat at the gas-liquid interface and effectively suppress heat transfer loss to the bulk water phase. At the same time, the radial ion concentration gradient induced by the Janus structure can drive salt ions to migrate and crystallize towards the membrane edge, realizing the directional recovery of salt resources while producing high-efficiency freshwater. The cumulative salt collection in 15wt% high-salinity brine can reach 3.05 kg·m⁻ 2 .

[0026] (3) The nanofiber membrane described in this invention exhibits excellent mechanical flexibility, chemical stability, and ultralight properties (area density of approximately 33.81 g·m⁻²). Its performance remains stable in strong acid, strong alkali, high salt, and organic dye wastewater, demonstrating broad application potential in the treatment of complex water environments.

[0027] (4) The system described in this invention has a simple structure and is easy to operate. It does not require external power and can be easily scaled up by adjusting the spinning parameters (a membrane with a size of 20.0 cm × 38.0 cm has been successfully prepared). This provides a promising technical path for developing portable, distributed freshwater-salt cogeneration devices suitable for areas with power shortages. Attached Figure Description

[0028] Figure 1 (a) Schematic diagram of the PAN / PVDFC NFM preparation process; (b) Photographs of the PAN layer and PVDFC layer; (c) SEM image of the bilayer membrane cross section; (d) SEM image of PAN nanofibers and (e) their magnified image; (f) SEM image of PVDFC beaded nanofibers and (g) their magnified image; (h) TEM image of CB nanoparticles; Figure 2 Schematic diagram of a solar interface evaporator based on Janus PAN / PVDFC NFM; Figure 3 (a) Water contact angle of PAN layer and (b) PVDFC layer over time; (c) Tensile stress-strain curve of PAN / PVDFC NFM; (d) Comparison of spectral absorption of PAN film and PAN / PVDFC film; (e) Surface temperature change curve of PAN film and PAN / PVDFC film under 1-sun illumination; and (f) Infrared thermal image. Figure 4 (a) Schematic diagram of the evaporator experimental setup; Effects of PAN layer thickness (b) and PVDFC layer thickness (c) on water evaporation mass loss in the evaporator; Effects of different CB contents on water evaporation mass loss (d) and evaporation rate (e) in the evaporator; (f) Water evaporation mass loss under different light intensities, (g) evaporation rate and photothermal efficiency; (h) Stability test of cyclic evaporation in simulated seawater; (i) Performance comparison with previously reported NFM-based evaporators; Figure 5 The effects of different initial water level differences on (a) water evaporation mass loss and (b) evaporation rate of the evaporator; (c) water evaporation mass loss and (d) evaporation rate of the evaporator under different salt concentrations; (e) salt crystallization photographs after 48 hours of natural evaporation and (f) after 10 hours of 1-sun illumination; (g) schematic diagram of the synergistic process of salt ion directional migration and water molecule evaporation in the evaporator; (h) changes in salt collection rate in 15 wt% brine; (i) stability of evaporation performance in long-term salt-tolerant environments; Figure 6 The center of the PAN / PVDFC evaporator is lower than the water surface on both sides: a 0.5 cm, b 1.0 cm; Figure 7(a) Schematic diagram of the designed solar interface evaporation system; (b) Photograph of the self-made outdoor evaporator; (c) Dynamic changes in solar radiation intensity, ambient temperature, and membrane surface temperature during outdoor experiments; (d) Real-time outdoor evaporation rate and cumulative evaporation volume; Figure 8 Comparison of actual photos and UV-Vis absorption spectra of MO (a), MB (b), and RhB (c) dye wastewater before and after purification; (d) Comparison of the concentration of major ions in the water before and after purification; (e) Changes in pH value of strong acid and strong alkali wastewater before and after evaporation; Figure 9 Preparation diagram of PAN / PVDFC NFM extended process. Detailed Implementation

[0029] The present invention will be described in detail below with reference to specific embodiments and accompanying drawings, but the scope of protection of the present invention is not limited thereto. Any improvements and substitutions made by those skilled in the art without departing from the principles of the present invention shall fall within the scope of protection of the present invention.

[0030] Example 1: Preparation of Janus PAN / PVDFC nanofiber membrane 1) Carbon black pretreatment: First, 2.0 g of CB was added to 60.0 mL of dilute hydrochloric acid and sonicated for 1 h. Then, it was filtered under vacuum using a Buchner funnel, and 60 mL of a H2O2 / H2NO3 mixture (composed of a 12% H2NO3 solution and a 15% H2O2 solution in a 1:1 volume ratio) was added, and it was sonicated again for 1 h. Then, the CB was washed with deionized water and dried under vacuum.

[0031] 2) Preparation of PAN electrospun membrane: Dissolve 1 g of PAN in 9 g of DMF and stir at room temperature until completely dissolved to obtain PAN electrospun solution. Load the solution into a syringe, adjust the distance between the electrospun needle and the receiving device to 15 cm, the needle height to 35 cm, the injection speed to 1.3 mL / s, and the voltage to 15 kV, and perform electrospunting. After spinning, do not remove the electrospun membrane to obtain PAN fiber membrane receiver. 3) Preparation of Janus PAN / PVDFC nanofiber membrane: Under gentle stirring, 0.9 g of PVDF was dissolved in 5.4 g of DMF solution, and pretreated CB particles were ultrasonically dispersed in 2.7 g of DMF solution, with a CB content of 4.76 wt%. The two solutions were thoroughly mixed under magnetic stirring and stirred until homogeneous. 4.0 mL of the PVDF-CB solution was transferred to a 10 mL syringe, and spinning was performed on the surface of a PAN fiber membrane receiver with a fixed area (14 cm x 15 cm) (conditions as in step 2). The distance between the electrospinning needle and the receiver was adjusted to 15 cm, the needle height to 35 cm, the injection speed to 0.65 mL / s, and the voltage to 10.5 kV. Finally, the obtained PAN / PVDFC NFM was dried at room temperature.

[0032] PAN / PVDFC NFM with Janus structure was prepared by stepwise electrospinning process. Figure 1 a). First, a white hydrophilic PAN layer was prepared by electrospinning; then, PVDFC fibers containing uniformly dispersed CB material were spun onto the PAN substrate to form a functionalized bilayer Janus PAN / PVDFC NFM ( Figure 1 b). The microstructure of JanusPAN / PVDFC NFM was systematically characterized using field emission scanning electron microscopy. For example... Figure 1 As shown in c, the PAN / PVDFC NFM exhibits a layered structure, with one side being PAN material in the form of nanofibers and the other side being PVDFC material in the form of bead-like nanofibers. Figure 1 Images d and 1e show that PANNFM is composed of randomly arranged nanofibers with a diameter of approximately 330 nm and a smooth surface. In contrast, the microstructure of the PVDFC photothermal layer is significantly different. Figure 1 f). The PVDFC composite NFM exhibits a bead dispersion mode, where the PVDF fiber diameter is 75 nm and the bead diameter distribution ranges from 0.5 μm to 2.0 μm. Figure 1 g). CB particles are uniformly dispersed in the PVDF fiber matrix through an electrospinning process, forming a tightly entangled structure with the polymer matrix. This unique Janus topology not only provides a continuous channel for liquid water transport but also constructs an efficient water vapor escape path. Figure 1 Transmission electron microscopy images showed that the CB nanoparticles were approximately 50 nm in size. Furthermore, the electrospinning strategy using organic-inorganic composite materials significantly improved the interfacial bonding strength between PVDF and CB particles, effectively preventing the loss of inorganic fillers due to dissolution during evaporation.

[0033] Comparative Example 1 1) Preparation of PAN electrospun film: The preparation steps are the same as in Example 1; 2) Preparation of Janus PAN / PVDFC nanofiber membrane: Under gentle stirring, 0.9 g of PVDF was dissolved in 5.4 g of DMF solution, and CB particles (without pretreatment) were ultrasonically dispersed in 2.7 g of DMF solution, with a CB content of 4.76 wt%. The two solutions were thoroughly mixed under magnetic stirring and stirred until homogeneous. 4.0 mL of the PVDF-CB solution was transferred to a 10 mL syringe.

[0034] The mixed spinning solution prepared by this method has poor dispersibility, making it impossible to spin and causing clogging of the needle.

[0035] Example 2: Evaporator Assembly and Solar Desalination Performance Testing A schematic diagram of a solar interfacial evaporator based on Janus PAN / PVDFC NFM is shown below. Figure 2 As shown, a PAN / PVDFC NFM measuring 2.1 cm × 10 cm was cut and its two ends were immersed in two tanks containing simulated seawater (3.5 wt% NaCl), leaving the central 2.1 cm × 2.1 cm area of ​​the membrane suspended as the evaporation surface. A solar simulator (CEL-PE300L-3A) was used to provide 1-sun standard illumination, and a precision electronic balance (accuracy 0.001 g) was used to record system mass changes in real time to calculate the evaporation rate. Simultaneously, an infrared thermal imager was used to monitor the temperature changes of the evaporation surface.

[0036] The wettability of the upper and lower surfaces of the PAN / PVDFC NFM was evaluated using a contact angle meter. The results showed that the contact angle of the hydrophilic PAN layer was close to 0. o ( Figure 3 a) This characteristic is attributed to its three-dimensional through-pore microporous structure and the presence of hydrophilic groups. In contrast, the PVDFC layer exhibits hydrophobicity, with a contact angle of 131.5°. o ( Figure 3 b). The hydrophobic PVDFC layer effectively blocks salt ions, while the hydrophilic layer enables continuous water transport through capillary effects. The mechanical properties of the NFMs were evaluated using uniaxial tensile testing. Figure 3 c), the results showed that its ultimate tensile strength was 2.7 MPa and its elongation at break was 21.6%, which is better than that of fiber membranes reported in the literature (Table 1). Figure 3As shown in Figure d, the spectral response of Janus NFM was tested using a UV-Vis-NIR spectrophotometer equipped with an integrating sphere attachment. The PAN / PVDFC NFM exhibits an absorbance of approximately 91.4% in the 250 nm to 2500 nm spectral range, representing a significant improvement in light collection efficiency compared to the blank PAN film. This enhancement is primarily attributed to the unique surface micro / nano structure of the PVDFC fibers. Specifically, the formation of micron-nano-scale roughness on the fiber surface significantly increases the specific surface area available for light interaction; simultaneously, the protruding CB particles not only act as local photothermal conversion sites but also synergize with the three-dimensional porous network to induce multiple light scattering, thereby reducing reflectivity. To verify the photothermal conversion performance of the material, a suspended solar evaporation device was constructed, and infrared thermal imaging technology was used to monitor the temperature evolution of different evaporator surfaces. Figure 3 As shown in e / f, under dark conditions, the surface temperatures of PAN NFM and PAN / PVDFC NFM are 19.2°C and 19.2°C, respectively. o C and 19.3 o C. Under solar irradiation, the surface temperature of the PAN / PVDFC NFM rapidly rises to 47.6°C within 5 minutes. o C, while PANNFM showed no significant temperature increase. After 1 hour of continuous irradiation, the temperature of the PAN / PVDFC NFM photothermal layer stabilized at 49.5°C. o C, PAN NFM remained at only 22.7. o C indicates that PAN / PVDFC NFM has significant photothermal response characteristics.

[0037] Table 1 Under a fixed spinning area (14 cm x 15 cm), the effect of PAN / PVDFC NFM thickness on its evaporation performance was systematically studied by adjusting the volume of the electrospinning precursor solution. The CB content was fixed at 12.5 wt%, and the PVDFC layer precursor solution volume was set to 4.0 mL. Samples with PAN layer precursor solution volumes of 2.5, 5.0, and 7.5 mL were designated PAN-1, PAN-2, and PAN-3, respectively. Furthermore, while maintaining a CB content of 12.5 wt% and a PAN layer precursor solution volume of 5.0 mL, a series of fiber membranes were prepared with PVDFC layer precursor solution volumes of 2.0, 3.0, 4.0, and 5.0 mL, and their evaporation rates were compared. These samples were named PVDFC-1, PVDFC-2, PVDFC-3, and PVDFC-4, respectively. Next, samples were prepared based on the final CB content (0, 5.0, 12.5, 15.0, 20.0 wt%) and labeled as PAN / PVDFC-x (x = 1, 2, 3, 4, 5). Unless otherwise specified, all experiments used PAN / PVDFC samples with a CB content of 12.5 wt%.

[0038] like Figure 4 As shown in Figure a, the Janus NFM is vertically suspended between two water-filled containers, with the top of the device sealed with PS foam to eliminate interference from direct water evaporation. The effect of the PAN / PVDFC NFM thickness on its evaporation performance was systematically investigated by controlling the volume of the electrospinning precursor solution within a fixed spinning region. The results show that the NFM composed of PAN-1 and PVDFC-3 exhibits the highest evaporation rate. Figure 4 b / c). For example Figure 4 As shown in d / e, under 1-sun irradiation, the evaporation rate first increases and then decreases with increasing CB content. The evaporation rate reaches its peak at a CB content of 12.5 wt%, which, after removing the dark evaporation rate, is 1.46 kg m³. -2 h -1 (The actual evaporation rate is 1.85 kg m) -2 h -1 Unless otherwise stated, all subsequent evaporation rates are calculated after deducting the 0.39 kg m³ dark evaporation rate. -2 h -1 The values ​​after that. When the CB content increases to 20 wt%, the evaporation rate decreases to 1.00 kg m -2 h -1This is mainly attributed to the shrinkage of the photothermal layer pore structure caused by excessive CB particles, which in turn hinders the escape of water vapor. A suitable CB loading not only significantly increases the evaporation rate of the evaporator but also maintains high thermal efficiency. Based on the above optimization results, the PAN layer thickness, PVDFC layer thickness, and carbon black content were optimized and then processed using process parameters corresponding to PAN-2, PVDFC-3, and PAN / PVDFC-3, respectively, for subsequent preparation.

[0039] Furthermore, when the solar radiation intensity is controlled at 1 kg m -2 h -1 (1-sun), 3 kg m -2 h -1 (3-sun) and 5 kgm -2 h -1 At (5-sun), the evaporation rate is 1.46 kg m³. -2 h -1 3.61 kg m -2 h -1 and 5.80 kg m -2 h -1 ( Figure 4 f). Meanwhile, the surface temperature of the PAN / PVDFC NFM rose to 49.5°C. o C(1-sun), 98.4 o C(3-sun) and 120 o C(5-sun) indicates that it has excellent photothermal conversion capability.

[0040] Solar energy conversion efficiency (η) is calculated by the following formula: η=mH LV / E in ,in, m The steady-state evaporation rate (excluding the dark evaporation rate) is the evaporation rate. H LV The total latent heat of liquid-gas phase transition, E in Let be the incident solar power. The system energy conversion efficiency (η) calculated according to this formula is 97.15% under 1-sun irradiation, decreasing to 89.32% and 88.40% under 3-sun and 5-sun irradiation, respectively. Figure 4 g). The efficiency decrease was primarily attributed to increased heat exchange losses between the system and the environment. Next, the operational stability of the PAN / PVDFC NFM-based evaporator was further evaluated. For example... Figure 4 As shown in h, the evaporator maintained approximately 1.46 kg m³ during 10 cycles of evaporation testing. -2 h -1The stable evaporation rate and the absence of significant performance degradation indicate reliable stability for seawater desalination applications. Compared with other nanofiber-based evaporation materials reported in the literature ( Figure 4 (i, Table 2) The PAN / PVDFC NFM prepared in Example 1 of this invention shows significant advantages in terms of solar evaporation rate and energy conversion efficiency.

[0041] Table 2 Example 3: High-salinity brine treatment and salt collection experiment The effect of water level difference on the evaporation performance of the PAN / PVDFC NFM-based evaporator was further investigated. The water supply could be easily controlled by adjusting the initial water level difference (0-2.5 cm) between the two sides of the main water supply. As the water level difference increased, the evaporation rate decreased from 1.46 kg m³ / h. -2 h -1 Reduced to 1.12 kg m -2 h -1 ( Figure 5 (a / b). The decrease in this rate is attributed to the significant increase in flow rate due to the increased water level difference, leading to increased heat loss. To evaluate the evaporative stability of the PAN / PVDFC NFM, different concentrations (3.5 wt%, 7 wt%, 10 wt%, 15 wt%, 20 wt%) of NaCl solution were used to simulate seawater environments with different salinities. Figure 5 As shown in Figure c, there is a good linear relationship between the amount of water evaporated and the duration of light exposure. When the NaCl concentration increases from 3.5 wt% to 20 wt%, the evaporation rate decreases slightly. Figure 5 d), This phenomenon may be attributed to the decrease in solution vapor pressure and the reduced salt backflow rate caused by high salinity brine.

[0042] This invention investigates a salt control strategy based on a flexible PAN / PVDFC NFM structure evaporator to achieve salt crystal collection during evaporation. Figure 5 As shown in e and 5f, under natural environmental conditions or continuous illumination, salt crystals preferentially deposit in the lower and side layers of the NFM, while the top layer remains salt-free. This selective enrichment is mainly attributed to the following synergistic mechanisms: First, the hydrophobic photothermal layer effectively inhibits nucleation on the salt crystal surface and promotes the migration of salt ions to the hydrophilic region; second, under illumination, the photothermal layer locally concentrates heat in the central region, triggering high-flux evaporation, leading to a significant increase in local salt ion concentration, thus forming a radial concentration gradient extending from the center to the edge and from the upper surface to the lower layer. Figure 5Driven by this gradient, Na and Cl ions continuously migrate towards the low-concentration regions at the edges and bottom. The solution at the membrane edges and bottom gradually reaches critical supersaturation, causing salt crystals to precipitate preferentially there. Gravity causes the high-concentration brine to sink, further intensifying bottom salting out, while the restricted flow and slow evaporation at the edges provide favorable conditions for crystal growth and adhesion. Further experiments show that after running for 720 minutes in 15 wt% high-concentration brine, the system accumulated 3.05 kg of salt. -2 h -1 ( Figure 5 (h) Salt particles deposited during evaporation can be easily removed by immersion in water without damaging the membrane structure. No significant surface detachment was observed in the irradiated area after prolonged continuous irradiation. Furthermore, even with the precipitation of large amounts of salt crystals, the evaporator maintained a pressure of 1.28 kg m³. -2 h -1 The evaporation rate and energy conversion efficiency of 78.3% Figure 5 i). The above results demonstrate that this material exhibits excellent mechanical property retention and evaporation stability under long-term brine exposure conditions. Furthermore, under conditions where the distance between the two sides of the water storage container and the liquid level remain constant, and the center point of the PAN / PVDFC NFM is lower than the liquid levels on both sides, this NFM can form stable droplet aggregation in the central region (i). Figure 6 Therefore, by adjusting the suspension configuration of NFM in the evaporation system, synergistic output of freshwater, recovery of concentrated brine, and collection of salt crystals can be achieved.

[0043] Example 4: Complex Wastewater Purification Test The practical application performance of PAN / PVDFC NFM was systematically evaluated through outdoor demonstrations and industrial wastewater treatment experiments. For example... Figure 7 As shown in Figures a and b, the independently designed outdoor evaporation system integrates a seawater storage tank, a condensation module, and a freshwater collection device. The condenser achieves synchronized and precise water level regulation across multiple evaporators through a hydrodynamic structure connected to the storage tank and hoses. This experiment was conducted on November 1, 2024, in Jinan, China (36.7°N). o ) is conducted throughout the day, and real-time monitoring data shows ( Figure 7 c) The NFM surface temperature exhibits a significant dynamic response to environmental parameters. Notably, the evaporation rate is significantly positively correlated with solar radiation intensity. At 12:30 PM, when the radiation intensity reaches 0.5028 kW / m², the evaporation rate... -2 At its peak, the daily evaporation rate reached a maximum of 0.78 kgm³. -2 h -1 ( Figure 7 d). Next, preparations containing methyl orange (MO, Figure 8 a) Methylene Blue (MB, Figure 8b) Rhodamine B (RhB, Figure 8 c) Simulated dye wastewater (concentration 10 mg / L) was purified using a PAN / PVDFC NFM evaporator. After evaporation and condensation, the collected freshwater was clear and colorless, and the UV-Vis spectrum showed that the characteristic absorption peaks of the dye had completely disappeared. ICP-MS analysis of the simulated seawater desalination water was performed to determine the levels of Na⁺, K⁺, and Ca⁺. 2 ⁺, Mg 2 The concentration of ⁺ plasma was significantly reduced to below the World Health Organization (WHO) drinking water standards, demonstrating the evaporator's highly efficient purification capability for various complex water bodies. Figure 8 d). Furthermore, the pH of wastewater containing strong acid (0.01 mol / L HCl) and strong alkali (0.01 mol / L NaOH) can be maintained at neutral after evaporation. Figure 8 This demonstrates the durability and desalination capability of the obtained NFM-based evaporator in harsh environments. Furthermore, PAN / PVDFC NFM evaporators with dimensions of 20.0 cm × 38.0 cm and 26.0 cm × 28.5 cm were successfully fabricated. Figure 9 This demonstrates its applicability and size flexibility in large-scale seawater desalination production.

[0044] This invention constructs a Janus PAN / PVDFC nanofiber membrane with photothermal-water transport separation function through sequential electrospinning, and combines it with a suspended evaporation structure to successfully achieve synergistic desalination and salt resource recovery during solar interfacial evaporation. This material exhibits high light absorption, good hydrophilicity-hydrophobicity synergy, strong mechanical and chemical stability, and high system energy utilization efficiency, providing an effective material basis and design concept for developing sustainable, distributed water-salt co-production technologies.

Claims

1. A Janus bilayer PAN / polyvinylidene fluoride carbon black nanofiber membrane, characterized in that: The nanofiber membrane is composed of polyacrylonitrile and polyvinylidene fluoride carbon black blended fibers; the carbon black content in the polyvinylidene fluoride carbon black fiber membrane is 0.17-0.70 mg / cm 2 ; the polyacrylonitrile fiber diameter is 200-400 nm; the polyvinylidene fluoride carbon black fiber presents a beaded structure, the fiber diameter is 50-100 nm, and the bead diameter distribution range is 0.5 μm-2.0 μm.

2. A method for preparing a Janus bilayer PAN / polyvinylidene fluoride carbon black nanofiber membrane as described in claim 1, characterized in that, Includes the following steps: 1) Carbon black pretreatment: First, add carbon black (CB) to dilute hydrochloric acid and treat with ultrasound; Then, filter, add a mixture of H2O2 and H2NO3, and disperse again by ultrasonication; Wash and dry to obtain pretreated CB particles; 2) Preparation of PAN electrospun membrane: Dissolve PAN in DMF and stir until completely dissolved to obtain PAN electrospun solution. Perform electrospunting. After spinning, obtain PAN fiber membrane. 3) Preparation of Janus PAN / PVDFC nanofiber membrane: PVDF was dissolved in DMF solution to obtain PVDF solution; pretreated CB particles were ultrasonically dispersed in DMF solution to obtain CB particle dispersion; Under stirring conditions, PVDF solution and CB particle dispersion are mixed and stirred to obtain PVDF-CB solution; PVDF-CB solution is electrospun on the surface of receiver covered with PAN fiber membrane, and after drying, PAN / PVDFC NFM based on Janus bilayer PAN / polyvinylidene fluoride carbon black nanofiber membrane is obtained.

3. The preparation method according to claim 2, characterized in that, In step 1), the concentration of carbon black in dilute hydrochloric acid is 0.1~10 g / mL; the ultrasonication time is 1.0-1.5 h; the concentration of dilute hydrochloric acid is 10-15%; the mixture is composed of H2O2 and HNO3 in a volume ratio of 1:1; the volume concentration of HNO3 is 10-15% and the volume concentration of H2O2 is 10-20%; the time for re-ultrasonic dispersion is 1.0-1.5 h.

4. The preparation method according to claim 3, characterized in that, The carbon black is carbon nanotubes or biochar.

5. The preparation method according to claim 2, characterized in that, In step 2), the mass ratio of PAN to DMF is 1:8-10.

6. The preparation method according to claim 2 or 5, characterized in that, In steps 2) and 3), the parameters for electrospinning are as follows: the distance between the electrospinning needle and the receiving device is adjusted to 10~20 cm, the needle height is 30~40 cm, the injection speed is 0.5~2.0 mL / s, and the voltage is 8~15 kV.

7. The preparation method according to any one of claims 2-6, characterized in that, In step 3), the concentration of the PVDF solution is 10-20 wt%; the content of CB in the CB particle dispersion is 1-10 wt%; the mass ratio of CB particles to PVDF in the PVDF-CB solution is 0.05-0.20; and the stirring time is 3-5 h.

8. The preparation method according to any one of claims 2-6, characterized in that, The polyacrylonitrile can also be replaced with natural hydrophilic polymers or polyvinyl alcohol; natural hydrophilic polymers such as cellulose and chitosan, and polyvinyl alcohol; the polyvinylidene fluoride can also be replaced with polyvinyl chloride, polytetrafluoroethylene, polypropylene, and polyurethane.

9. The application of a Janus bilayer PAN / polyvinylidene fluoride carbon black nanofiber membrane prepared by the preparation method according to claim 1 or any one of claims 2-8 in water treatment.

10. The application according to claim 9, characterized in that, A solar-driven interfacial evaporator was prepared using Janus double-layer PAN / polyvinylidene fluoride carbon black nanofiber membrane for seawater desalination, wastewater purification, and salt resource separation and collection.

Images