Study On Surface & Interface Regulation And Antifouling Mechanism Of Nanofibrous Membrane For Oil/water Separation

Membrane separation technology is one of the mainstream methods for treating oily wastewater.However,membrane fouling seriously hinders its practical application,resulting in shortened service life and increased operating cost.Therefore,developing novel membrane with excellent antifouling performance and sustainable separation technology is of great significance to the practical application of oily wastewater treatment.The properties of membrane surface interface and antifouling mechanism are the key factors determining the long-term separation performance of membrane.In this paper,the antifouling membrane surface interface is constructed based on the platform technology of non-covalent grafting,blending,in-situ growth and counter jet spinning.Combined with multiple antifouling mechanisms to synergistically strengthen the antifouling performance of membrane and realize the long-term stable operation of oil-water separation.The main research contents are as follows:（1）A superhydrophilic,high-strength polyvinylidene fluoride（PVDF）stabilized hydrated antifouling surface interface was designed to solve the problems of easy fouling and poor durability of conventional hydrophobic nanofibrous membrane during the separation and cleaning process.Benefit from the self-assembly of the amphiphilic molecule Span 80 on the surface of PVDF nanofibers,a superhydrophilic,high-strength PVDF nanofibrous membranes was achieved in one step by simple gravity filtration of Span 80/hexane solution on PVDF nanofibrous membrane.Effects of Span 80concentration,filtration time and filtration amount on the morphology,structure,mechanical strength and hydrophilicity of PVDF nanofibrous membrane were systematically studied.Separation and antifouling properties of the nanofibrous membrane were evaluated.Assembly mechanism and stability of Span 80 on PVDF nanofiber membrane were analyzed.Results show that the mechanical strength of Span80 weld PVDF nanofibrous membranes increased significantly（from 1.7 MPa±0.3 MPa to 8.8 MPa±0.6 MPa）.Water contact angle（WCA）decreased from 141^o±2^o to 0^o.Permeability of the membrane to 1,2-dichloroethane-in-water emulsion was 18482.7±287.3 L m^-2h^-1bar^-1 with a separation efficiency of 99.6%.Span 80 weld PVDF nanofibrous membrane has excellent cycle stability and durability,which resisting to long-term washing of water,oil-in-water emulsion,and 8 cycles of oil/water separation.（2）Although the hydrophilic PVDF membrane has a high antifouling ability,it is easy to be polluted or even blocked by oil during the long-term operation,resulting in the decline of the separation performance of the membrane.In this study,an environmentally responsive antifouling surface interface was designed to realize efficient oil-water separation and intelligent antifouling.Thermosensitive copolymer（P（AN-co-AM））with the highest critical solution temperature（UCST）was synthesized by radical polymerization,and blended with PVDF and p H-responsive polymer poly（4-vinyl pyridine）（P4VP）to prepare therm/p H-responsive PVDF nanofibrous membrane with micro/nano structure in one step by electrospinning and phase separation method.Effects of the blending ratio of P（AN-co-AM）and P4VP on the morphology,structure,mechanical strength and wettability transition of PVDF nanofibrous membrane were studied.Separation performance of environment responsive membrane and the self-cleaning performance of contaminated membrane were evaluated,and the mechanism of wettability transition was clarified.The results showed that the membrane exhibited highly hydrophobicity（136.4^o）at room temperature（25℃p H=7）,and exhibited high permeability(60528.76 L m^-2h^-1 bar^-1)and separation efficiency（99.5%）for water-in-D5（water-in-Decamethylcyclopentasiloxane）emulsion.As the hydrogen bond of the thermosensitive copolymer P（AN-co-AM）was broken and the protonation of P4VP in acidic hot water（55℃,p H=3）,the wettability of membrane changes from highly hydrophobic to superhydrophilic,which can release oil from the membrane surface and achieve intelligent cleaning.The fabricated membrane still maintains high separation performance after 6 cycles,which indicates that its potential application in oilywaster purification and provides a new method for the construction of membrane with hierarchical structure.Meanwhile,it also provides a new idea for the design of recyclable oil-water separation membrane.（3）PVDF nanofibrous membrane modified with surfactant Span 80 and environment responsive polymer has excellent antifouling performance and good cycle stability.However,based on the pore size screening mechanism,the pollutants can not be removed from the membrane surface in time.After 5 min of separation,they need to be removed from the separation device for cleaning.To address the continuity and long-term stability of the separation process,an antifouling surface interface was designed based on the mechanism of hydration layer cooperating with in-situ microbubble decontamination.Carbon microspheres were grown on the surface of polyacrylonitrile（PAN）nanofibrous membrane by hydrothermal method,and silver nanoparticles were in situ loaded to obtain superhydrophilic carbon-silver nanofibrous membranes with hydrogel-like properties.Effects of sucrose concentration on the morphology,structure,hydrophilicity and mechanical strength of carbon microspheres/PAN nanofibrous membrane were systematically studied.Long-term stable oil-water separation performance,dye degradation performance,heavy metal ion removal performance and bactericidal performance of c PAN-Ag membrane were evaluated and analyzed,and the antifouling mechanism of hydration layer cooperating with in-situ microbubbles was clarified.The results showed that the membrane had high permeability(45612±430 L m^-2 h^-1 bar^-1)and separation efficiency（99%）for toluene-in-water emulsion.Thanks to the hydration layer,which hinders the contact between oil and membrane surface and the hydrophobic interaction between microbubbles and oil droplets,the oil droplets were removed from the membrane surface in real time.The membrane shows stable permeability and high separation efficiency in 100 min of continuous dead-end filtrate without cleaning,triggered by hydrogen peroxide（H₂O₂）.Meanwhile,the membrane can also realize heavy metal ions removal,dye degradation,and sterilization.The reasonable design of nanofibrous membrane to reduce pollution provides more opportunities for the practical separation of a complex oil-in-water emulsion.（4）Although the long-term stable separation performance of nanofibrous membrane based on the mechanism of hydration layer cooperating with in-situ microbubble decontamination is greatly improved in the process of oil-water separation,H₂O₂ needs to be added to trigger the reaction,and the antifouling process is a passive process,so the application system is limited.To realize an active long-term stable separation process,CPVDF/CPAN nanofibrous membrane with interpenetrating network structure was constructed by conjugate spinning and Michael addition reaction.Effect of CPVDF/CPAN nanofiber ratio on oil-water separation performance was systematically investigated,movement and aggregation pattern of oil droplets on CPVDF and CPAN fibres was verified.Mechanism of hydrophilic/hydrophobic fibers coalescence demulsification was proposed.Breaks through the discontinuity of the oil-water separation process by preventing a continuous oil film from forming on the membrane surface or inside the membrane.In this process,CPVDF nanofibers with a cross-linked structure were produced by defluorination and Michael addition reaction,which enhanced the mechanical properties of the nanofibers,realized the long-term cross-flow filtration separation of nanofiber membrane and provided a channel for coalescing oil phase.At the same time,the CPAN nanofiber was hydrolyzed to further increased its hydrophilicity,providing a water channel for separation and facilitating the transfer of the oil phase to the hydrophobic CPVDF nanofibers.The two-phase permeation strategy enables stable operation of 4 h oil-water separation without additional cleaning under cross-flow filtration,providing a new idea for continuous oil-water separation.