Construction Of Selective Superwetting Membrane Interface For Treatment Of Oily High Salinity Wastewater

The oily high salinity wastewater is difficult to be treated by traditional techniques because of its high oil content,high salt concentration and extremely complex component.Membrane technology is now widely used in oily high salinity wastewater treatment,but the recovery rate of pure water is only 50-60%and the high operation pressure generally causes severe membrane fouling.The selective superwetting membrane interface can effectively separate the oils from salinity wastewater under gravity or operated at low driving pressure and separate the salts from water operated under the temperature difference,which can be concluded as two separation techniques:oil/high salinity water separation and membrane distillation.However,the key points for a selective superwetting interface for membrane separation technology are its stability,anti-wetting as well as anti-fouling performances.Inspired by the natural plants and animals such as lotus leaf?superhydrophobicity?,scales?anti-oil fouling?and mussels?high adhesion property?,the high porosity and low tortuosity electrospinning fibrous membrane in combination with bionic and surface modification techniques were developed to fabricate the 3-dimensional interconnected fibrous membrane.A two-step membrane separation process was used to separate oils from high salinity wastewater and salts from water.The results of the current research are summarized as follows:Given the characteristics of oils in high salinity wastewater,two different selective superwetting porous membranes were designed to efficiently separate oils from high salinity wastewater.Owing to the thermal stability and acid/alkali resistance of polysulfonamide?PSA?and polyacrylonitrile?PAN?,an ultrafine composite PSA/PAN fibrous membrane with a uniform diameter,narrow pore size distribution and high mechanical strength was prepared by adjusting the mass ratio of PSA and PAN and the weight concentration of the hybrid PSA/PAN solutions.The formation mechanism of the adhesive PSA/PAN fiber was analyzed by the in situ free-scanning electron microscopy.According to the characteristic of the hydrophobic oils in the high salinity wastewater,the hydrothermal method was used to construct the hierarchically micro/nanostructured roughness on the fiber surface.After that,low surface energy molecular was selectively grafted onto the micro/nanostructured rough membrane surface aimed at obtaining a superhydrophobic/superoleophilic?156°/0°?and a superhydrophilic and underwater superoleophobic?0°/155°?fibrous membrane for separation of different characterized oils from high salinity wastewater.The separation flux of sodium dodecyl benzene sulfonate?SDBS?stabilized water-in-oil microemulsion and oil-in-water microemulsion reached⁷300 L m^-2 h^-1 and³200 L m^-2 h^-1,respectively and both the separation efficiency reached about 99.90%.The residual concentration of hydrophobic oils in the high salinity wastewater treated with the above superwettable fibrous membrane is generally from several to dozens of mg L^-1.In the process of membrane distillation,the residual oils in high salinity wastewater can easily adhere to the hydrophobic membrane surface due to the strong hydrophobic-hydrophobic interactions underwater.In order to solve the above-mentioned solution,electrospinning and electrospraying methods were used to fabricate a breathable and an asymmetrically superwettable fibrous membrane which consisted of a superhydrophilic protective layer and a superhydrophobic layer integrated with a hydrophobic polyvinylidene fluoride?PVDF?matrix.The superhydrophilic silica@polyacrylonitrile?SiO₂@/PAN?protective layer exhibits excellent underwater oleophobicity of 166°that can resist the residual oils.The silica@polyvinylidene fluoride-hexafluoropropylene/polystyrene?SiO₂@PVDF-HFP/PS?layer shows excellent superhydrophobicity of 164°that the saline water can vaporize at the superhydrophobic membrane surface while preventing the penetration of saline water across the hydrophobic membrane.During the direct contact membrane distillation,the superhydrophilic/superhydrophobic fibrous membrane not only exhibited a high cross-membrane flux of²8 L m^-2 h^-1 but also showed a high separation efficiency of¹00%and kept robust stability.In addition to oily substances,the oil-in-salinity wastewater generally contains a small amount of hydrophilic organic pollutants.Due to the surface energy of hydrophilic organic pollutants is much lower than that of hydrophobic distillation membrane,the hydrophilic organic pollutants can easily adhere to the surface of the hydrophobic distillation membrane and make the membrane distillation invalid.Therefore,it is of great significance to fabricate a high-flux distillation membrane with both anti-wetting and anti-fouling properties to treat the complex and changeable oil-in-salinity wastewater.First,a dual bionic method was used to prepare the omniphobic fibrous membrane via the assembly of micro/nano multi-level structures on the fibrous membrane in combination with fluorination technology by utilizing the fluorosilane?17-FAS?to lower the membrane surface energy.The omniphobic membrane shows robust repulsive interaction toward different-property liquids.Second,an electrospinning and electrospraying method was also used to fabricate a superhydrophilic and an omniphobic fibrous membrane.By regulating the composition and concentration of electrospinning solution,a self-roughened microstructured polyvinylidene fluoride?PVDF?membrane was obtained via one-step electrospinning without inorganic nanoparticles.The PVDF fibrous membrane displays an omniphobic surface with a water contact angle of 159°and an oil contact angle of 145°,respectively.Then a self-roughened beads-on-string structured polyvinyl alcohol/polyacrylic acid?PVA/PAA?oil protection layer showed superoleophobicity with an oil contact angle of 156°underwater,which was electrosprayed onto the omniphobic PVDF fibrous membrane surface.Finally,the superhydrophilic/omniphobic composite membrane was heated at 140?for 1 h to make the stable fibrous membrane that can effectively treat the high salinity wastewater with a high water flux of²7 L m^-2 h^-1 and high separation efficiency of¹00%.The superhydrophilic/omniphobic composite fibrous membrane can treat the high salinity wastewater with more complex components.In summary,the electrospinning technique in combination with the biometric methods seems to be a promising method to prepare low-twist three-dimensional pores and asymmetrically superwettable fibrous membrane,realizing the effective separation of oil from salinity wastewater and water from salts.Meanwhile,during the process of membrane distillation,the membrane water flux and the wetting and fouling problems were simultaneously solved via the construction of asymmetrically superwettble membrane.In the next process,by regulating the fibrous membrane structures such as membrane pore structure,membrane thickness and the macro/nanostructures and so on to realize the composite distillation membrane with higher water flux and better anti-wetting and anti-fouling properties.