Preparation Of Pvdf Superhydrophilic Membrane And Its Oil-water Separation Performance

Poly（vinylidene fluoride）（PVDF）membranes have high mechanical strength,good toughness,creep resistance,weatherability and other advantages to become widely membrane material,while its strong hydrophobic facilitate the adsorption of oil fouling on membrane surface,and then block membrane transport channel,leading to reduce the separation performance and increase maintenance costs.Besides,its chemical inertness also limits the hydrophilic modification methods.Therefore,the development of new materials and facile surface modification strategies are effective strategies to improve the hydrophilicity and fouling resistance of PVDF membrane.We designed to prepare a variety of superhydrophilic PVDF microfiltration membranes（MF）.These studies investigated the effects of different proportions of polyphenols and amino compounds on the chemical composition,surface morphology,hydrophilicity,pollution resistance,separation efficiency and coating stability of PVDF MF and the surface coating mechanism was explored.Firstly,superhydrophilic PVDF MF was prepared by one-step surface coating with procyanidin（PC）andγ-aminopropyl triethoxysilane（APTES）.The nanoparticles coating was constructed according to the cross-linking reaction between phenolic hydroxyl group of PC and amino group of APTES on the membrane surface to increase the surface roughness and further improve the hydrophilicity.The results showed the optimal ratio of PC and APTES coating method can transform PVDF MF with high hydrophobicity into superhydrophilic/underwater super oleophobic.The membrane can achieve the separation of oily wastewater（oil-in-water emulsion）by gravity,and the separation flux was about 180 L m^-2 h^-1 with the oil rejection above99.5%and had excellent anti-oil fouling ability.This modification strategy can be extended to the preparation of anti-fouling clothing and effectively reduce oil fouling.In addition,the results showed that the membrane separation flux at 1 bar pressure was 2.0×10³ L m^-2 h^-1,and the oil rejection rate was only 65.8%.It is almost impossible to realize oil/water separation under pressure drive,which severely restricts the application of this modification strategy in actual sewage treatment.Therefore,it is necessary to develop new superhydrophilic/underwater super hydrophobic membrane materials for accurate separation under pressure drive.In the above study,the separation performance of PC and APTES surface coating to construct superhydrophilic PVDF MF was not applicable to the treatment of large-scale oily wastewater.Moreover,when p H was less than 5 or larger than 13,the surface coating fell off and the loss of superwetting would seriously limit the application of this superhydrophilic PVDF MF.Undoubtedly,the stability of the surface coating is critical to the sustainability and service life of separ ation membrane.Therefore,two-steps composite coating method was proposed to prepare superhydrophilic PVDF MF with high stability coating:1)The cross-linking reaction between phenolic hydroxyl group of PC and amino group of POSS-NH₂ realized the preliminary construction of superhydrophilic PVDF MF.The reasons for the instability of the surface coating under strong acid or alkaline environment were investigated;2)Metal-phenol network structure（FPN）was prepared by Fe³⁺and PC on the initially superhydrophilic PVDF MF,which of the hydrophilicity was further improved.The water contact angle of the obtained PVDF MF was 0°,and the pure water flux was 1.36×10⁴ L m^-2 h^-1 bar,and the separation flux was 3.0×10³ L m^-2h^-1 bar with 99.6%oil rejection for oily wastewater（oil-in-water emulsion）treatment.After exposure to air for 30 d or immersion in strong acid/alkali solution for 24 h,the surface coating maintained high stability,attributing to the synergy of the Michael addition/Schiff base reaction between POSS-NH₂ and PC to form the nanoparticles coating,and the FPN constructed through the coordination of hydroxyl/amino groups with Fe³⁺.The synergistic effect resulted in a complex network structure that enabled the coating to adhere stably to the membrane surface.To further improve the ability to resist crude oil fouling,surface coating with caffeic acid（CA）（Based on above two studies,PC coating has excellent hydrophilicity but still expensive.）and APTES was adopted on membrane surface by constructing the smooth microparticles to achieve preliminary superhydrophilic membrane.Subsequently,Fe³⁺was introduced to construct a wrinkled pattern on the surface of smooth microparticles,and the unique micro-nano structure of dahlia leaf surface was simulated to prepare a superhydrophilic membrane with excellent resistance to viscous crude oil fouling.The underwater oil contact angle was over165°,and the pure water flux of the film reached 1.75×10⁴ L m^-2 h^-1 bar.The separation flux of crude oil/water emulsion was 7.2×10³ L m^-2 h^-1 bar and the separation efficiency was 99.6%.The anti-fouling mechanism of the obtained membrane came from various synergies.Firstly,the introduction of hydrophilic microparticles improved the hydrophilicity of the membrane and increases the surface roughness,thus generating initial anti-oil fouling ability.Then,wrinkled patterns were formed on the surface of the microparticles to appropriately reduce roughness,increase the contact area between the membrane surface and water molecules,and provide a dense and thick hydration layer.The steric hindrance effect between oil and membrane surface can be improved,and this synergistic effect can ignore the viscosity of oil,effectively resist the initial adsorption,migration,diffusion and deposition of oil on membrane surface,and finally achieve excellent anti-viscosity oil fouling.