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Oil spills and chemical pollution pose a serious threat to the environment and human health. Traditional separation techniques face problems such as low efficiency, high costs, and secondary pollution when treating oil/water emulsions. Electrospun nanofiber membranes are widely used in the treatment of oily wastewater due to their excellent properties, but the insufficient stability of nanoparticles limits their applications. In this study, a PVDF nanofiber membrane with a porous channel structure was prepared via coaxial electrospinning and a sacrificial template strategy. The in - situ growth of iron phytate nanoparticles significantly improved the membrane's stability and separation efficiency, providing new ideas for the development of high - performance oil - water separation membranes.
The materials used in the experiment included chemical reagents such as PVDF, PVP, phytic acid, N,N - dimethylformamide, and ferric chloride, as well as various oils and surfactants. A nanofiber membrane with a porous channel structure was prepared by electrospinning machine coaxial electrospinning. First, outer and inner layer solutions were prepared separately. The outer layer solution was obtained by dissolving different concentrations of PVP in a PVDF/DMF/AC solution, and the inner layer solution was a 40 wt% PVP solution in DMF. The prepared solutions were magnetically stirred at 60°C for 6 h. Then, spinning was carried out under specific electrospinning parameters using an electrospinning device. After the electrospinning process, the membrane was treated successively in ethanol and deionized water at 80°C to remove PVP, and then immersed in a 6% FeCl₃ solution and a 6% PA solution for 8 cycles. Finally, it was washed with deionized water and vacuum - dried to obtain the modified membrane. (See Figure 1)
Figure 1 Preparation and formation mechanism of the modified PVDF - PVP nanofiber membrane with a porous channel structure. (a) Schematic diagram of the nanofiber membrane preparation and modification process. (b) Formation mechanism of iron phytate.
(1) Interlocking Structure Enhances the Bond between Nanoparticles and Fibers
The PVDF nanofiber membrane prepared by coaxial electrospinning and the sacrificial template strategy has a porous channel structure, enabling the iron phytate nanoparticles to form an "interlocking structure" with the fiber pores. As can be clearly seen from Figure 2d1 - d2 and h, the nanoparticles are evenly attached to the fiber surface and interpenetrate with the fiber pores. This microstructure increases the contact points and friction, enhancing the stability and binding force between the nanoparticles and the fiber membrane. Even after 7 h of ultrasonic treatment, the nanoparticles remain firmly attached to the fiber surface, proving that the interlocking structure significantly strengthens the bond between them.
Figure 2 Scanning electron microscope (SEM) images of fiber membranes after different treatments. (a1 - a2) SEM image of PVDF. (b1 - b2) SEM image of PVDF - PVP18 before water etching. (c1 - c2) SEM image of PVDF - PVP18 after water etching. (d1 - d2) SEM image of PVDF - PVP modified with iron phytate nanoparticles. (e - g) SEM images of the porous channel structure of PVDF - PVP18. (h) Cross - sectional SEM image of PVDF - PVP18 modified with iron phytate nanoparticles.
(2) Nanomodified Fibers Have Better Hydrophilicity and Permeability
In the study of membrane wettability, Figure 3a shows the influence of different PVP contents on the wettability of the modified membrane. When the PVP content reaches 18 wt%, more microporous channel structures are formed, significantly enhancing the hydrophilicity of the modified PVDF - PVP18 membrane. Water droplets can be completely absorbed within 10 s. As can be seen from Figure 3b, after modification by in - situ growth of iron phytate nanoparticles, the water contact angle of the membrane drops from the superhydrophobic state (160°) before modification to 0°, achieving a transition from superhydrophobicity to superhydrophilicity. In terms of permeability, the PVDF - PVP18 membrane has enhanced water absorption and transmission capabilities due to its improved hydrophilicity, showing a high flux in the oil - water separation experiment, which proves that nanomodification significantly improves the hydrophilicity and permeability of the fibers.
Figure 3 Analysis of the contact angle, stability of structure and performance, and anti - crude - oil adhesion properties of nanofiber membranes. (a) Comparison of water contact angles in the air for PVDF and different contents of PVDF - PVPx. (b) Changes in the wettability of membranes during the modification process. (c) Underwater oil contact angles of PVDF - PVP18 with different oils. (d) Underwater oil adhesion resistance experiment using oil - red - stained dichloromethane. (e) Comparison of changes in water contact angles in the air between PVDF and PVDF - PVP18 before and after sonication. (f) SEM images of PVDF - PVP18 before and after sonication. (g) Anti - oil adhesion test of PVDF - PVP18 before and after sonication.
(3) Hollow Structure Improves the Membrane's Oil - Adhesion Resistance
The hollow structure of the nanofiber membrane plays an important role in resisting oil adhesion. When water fills the inside of the nanofiber membrane, the capillary - induced hydrodynamic effect changes the distribution of the fiber surface tension. As can be seen from the oil - water separation mechanism schematic diagram in Figure 4b, this effect helps to break the oil layer attached to the fiber surface, promoting the dispersion and detachment of oil droplets, and endowing the membrane with excellent oil - adhesion resistance. In the experiment, when an underwater oil - adhesion resistance test was carried out using oil - stained dichloromethane, the oil droplets did not adhere to the nanofiber membrane surface but could remain for a certain period. When the membrane was gently lifted, the oil droplets rolled off immediately, and the membrane surface was clean without oil adhesion, fully demonstrating the positive effect of the hollow structure on improving the membrane's oil - adhesion resistance.
Figure 4 Emulsion separation performance and dye removal capacity of PVDF - PVP18. (a) Separation efficiency and flux of PVDF - PVP18 for various emulsions. (b) Cyclic stability test of PVDF - PVP18. (c) Microscopic images and separation process of emulsions before and after separation. (d) Separation efficiencies and fluxes of different thicknesses of PVDF - PVP18. (e) Water flux in 5 separation cycles of iso - octane emulsion in stabilized water using PVDF - PVP18 (green squares) and PVDF - PVP0 membrane (red squares). (f - i) Ultraviolet - visible spectra of Rhodamine B (Rh B), Malachite Green (MG), Methylene Blue (MB), and Crystal Violet (CV) before and after separation by PVDF - PVP18.
(4) The Separation Membrane Has High - Efficiency Emulsion Separation Ability
The separation performance of the nanofiber membrane was tested by the gravity - driven method for various oil - in - water emulsions, and the results showed its excellent separation performance. As can be seen from Figure 4a, for emulsions such as epoxidized soybean oil, vacuum pump oil, iso - octane, and dodecane, the separation efficiency of the nanofiber membrane exceeds 99%, and the maximum water flux can reach 1953.61 Lm⁻²h⁻¹. At the same time, dynamic light scattering (DLS) analysis shows that the particle size distribution of oil droplets in the emulsion samples is wide before separation, and it becomes significantly narrower in the filtrate after separation. When observed under a metallographic microscope, the separated solution becomes transparent, and almost no oil droplets can be detected, which proves that the separation membrane has high - efficiency emulsion separation ability.
(5) The Separation Membrane Has Reversible Flux Recovery Ability
The flux recovery of the modified PVDF membrane was tested using the separation of iso - octane emulsion as an example. After 5 separation cycles, Figure 4e shows that the water flux in the first filtration cycle is 2153.2 ± 10 Lm⁻²h⁻¹, and the emulsion filtration flux is 1746.3 Lm⁻²h⁻¹. After each test, the membrane was rinsed with deionized water, and the water flux in the second cycle remained at 2151.6 ± 10 Lm⁻²h⁻¹. The calculated total flux reduction rate (DRt) is 18.89%, the flux recovery ratio (FRR) is approximately 98.61%, the reversible fouling ratio (FRr) is 99.74%, and the irreversible fouling ratio (FRir) is approximately 0.26%. These data indicate that the modified hydrophilic PVDF membrane is almost unaffected by oil pollution. After simply rinsing to remove the attached oil, the membrane flux can return to a level close to the original, showing good reversible flux recovery ability.
In this study, a nanofiber membrane with a porous channel structure was successfully prepared by electrospinning device coaxial electrospinning technology using polyvinylpyrrolidone (PVP) as a sacrificial template. The resulting nanofiber membrane exhibits a porous structure, and the unique "interlocking structure" enhances the bond between iron phytate nanoparticles and the nanofiber membrane, achieving a transition from superhydrophobicity to superhydrophilicity. In conclusion, the preparation of nanofiber membranes with a porous channel structure by coaxial electrospinning technology has great potential in the fields of oil - water separation and wastewater treatment.
Article Source: https://doi.org/10.1016/j.seppur.2024.128835
