Design, Preparation And Performance Study Of Ultra-and Micro-filtration Polymer Membrane For Oil-water Separation

With the improvement of human living standards and the development of social industrialization,more and more oily wastewater has been produced,which threatens the safety of water resources and the ecological environment.Membrane separation technology has been regarded as a significant approach to address the issue due to its simple equipment,convenient operation,wide separation range and high separation efficiency.Herein,the permeate flux,separation efficiency and antifouling ability are crucial factors to measure the membrane performance.However,the low separation efficiency and poor antifouling property of conventional membranes hinders the further development of membrane separation technology to a large extent.Therefore,this paper firstly built an ultra-thin and high-porosity separation network on the mesh to simultaneously obtain high flux and high intrusion pressure.Then,an oil-water separation membrane was obtained with high-efficient anti-fouling performance via combining the strong hydrophilic zwitterions and the micro-/nano-structure of the membrane surface.Next,the density of hydrophilic groups in the separation layer was increased by blending so as to reduce the flux decline ratio and achieve the multi-cycle anti-fouling oil-water separation.In the end,based on the theory of demulsifier,oil interception and coalescence sites were designed on the superhydrophilic surface to realize the controllable separation of large pores toward small oil droplets.1.In view of the trade-off problem between membrane permeate flux and intrusion pressure,an ultra-thin PAA-g-PVDF nanofiber layer with the thickness less than 1 μm was constructed on the stainless mesh membrane,which reduced the pore size of the mesh membrane and improved the intrusion pressure and separation efficiency.Meanwhile,the high permeate flux of the prepared membrane was maintained because of the small effective separation area and the thin thickness caused by small fiber diameter.Therefore,the composite membrane exhibited high permeation flux(53574 L m-2 h-1)even if under gravity,and also realized satisfactory permeation flux and separation efficiency in different oil-water separation experiments and multiple separation cycles.2.To address the problems of weaker antifouling property of the oil-water separation membrane surface and poor separation cycle,zwitterionic nanogel microspheres with superhydrophilic properties were prepared and was grafted on the separation membrane surface.In this work,the bottom membrane was prepared by electrospinning of PAA-g-PVDF with a grafting rate of 20.2%provides grafting site,and the prepared zwitterionic nanohydrogel possessed a thicker hydration layer than the traditional method of directly grafting hydrophilic polymers due to the existence of their own spatial structure.Additionally,the improved hydrophilicity produced by microlevel roughness of membrane surface further endowed the modified membranes show with strong antifouling ability,which can resist the adhesion of high viscosity crude oil under water and display the zero crude oil adhesion of underwater.Overall,the separation efficiency under gravity driving can reach 98.7%,and separation flux almost unchanged(all above 600 L m-2 h-1)in several separation cycles.3.As for the problem of the sharp decrease on permeate flux caused by insufficient density of hydrophilic group modified on the membrane surface during the separation process,a large amount of hydrophilic polyethylene imine(PEI)was introduced by blending,where the hydrophilic PEI was segregated to the membrane surface to obtain a high-density hydrophilic separation layer.Then the stability of hydrophilic substances on the membrane surface was further improved by interfacial polymerization to keep them stable in the long-term separation process.Then the hydrophilic and oil-resistant performance of the membrane surface was further improved by zwitterionization of 1,3propanesultone(1,3-PS).The the prepared separation membrane exhibited low flux decline rate(about 7%)and high recovery rate with up to 100%after water washing.4.The small emulsified droplets are easy to block or penetrate the membrane pores during the separation process of oil-water emulsion separation,resulting in the reduction in flux and separation efficiency.Therefore,a hydrophobic and lipophilic network was constructed on the surface of the traditional single superhydrophilic membrane through the asymmetric wettability design of the surface to provide oil rejection and aggregation sites,so as to realize the demulsification of small oil droplets and the water filtration.At the same time,the effective separation of emulsions with different precisive sizes(from hundreds of nanometers to several nanometers)was achieved by accurately controlling the proportion of hydrophilic and hydrophobic micro-area on the membrane surface.In the cross-flow experiment for 10 h,the separation efficiency can be more than 99%with the relatively stable separation flux.