Preparation Of Structurally Integrated Janus Membrane And Its Performance Study In Oil-water Emulsion Separation

Oily wastewater is a kind of water pollution with complex composition,huge yield and difficult treatment,which is the key and difficult point of industrial pollution prevention and control.In many new treatment technologies of oily wastewater,membrane separation technology has become one of the most commonly used methods for the separation of oily wastewater because of its characteristics of high efficiency,energy saving,no phase transformation,less investment and less pollution.Janus membrane is a kind of higher efficient separation membrane with asymmetric wetting property,which can be used for switching separation of oil-water emulsion.It is usually prepared as double or multilayer composite membrane.The structurally integrated Janus preparation method is novel.The hydrophobic polyvinylidene fluoride（PVDF）membrane with dense structure or macro-porous structure was used as the matrix membrane,and the hydrophilic polymer（PVP-VTES）was hydrolyzed,unidirectional migration and thermal crosslinking polymerization in the matrix membrane.The effective hydrophilic layer in the hydrophobic matrix membrane was constructed and structurally integrated Janus membrane with asymmetric wettability was prepared.Regulation of the modifier ratio and thermal crosslinking time in the process of modification,by comparing the morphology and structure,pore size,chemical properties and wettability of the both surfaces of the membranes,it was found that the thermal crosslinking time was the dominant factor affecting the wettability and thickness regulation of the hydrophilic layer of Janus membrane,but had no effect on the morphology and pore size.A series of characterization of Janus membrane under different thermal crosslinking time were compared,and the best Janus membrane and their hydrophilic modification conditions were selected.The experimental results showed that the Janus membrane with dense pore structure with optimal asymmetric wettability was M1-2h,that was,the optimal preparation condition was the thermal crosslinking time of 2h.The Janus membrane with the finger-like pore structure with the best asymmetric wettability was M1-1-2h,that was,the best preparation conditions were the modifier ratio of 1:1 and the thermal crosslinking time of 2h.The Janus membrane with dense pore structure,optimal asymmetric wettability and integration of hydrophilic and hydrophobic layers was used for the switchable separation experiments of oil-in-water（O/W）and water-in-oil（W/O）emulsions in the microfiltration process.The separation results were as follows:for the oil-water emulsions with soybean oil as oil body,the separation flux of O/W emulsions on the hydrophilic surface of Janus membrane was 22.88 L/m~2·h,and the separation efficiency was 93.35%.The separation flux of W/O emulsions on the hydrophobic surface of Janus membrane was 17.45 L/m~2·h,and the separation efficiency was 91.47%.For the O/W emulsions with chloroform as oil body,the water flux on the hydrophilic surface of Janus membrane was 358.81 L/m~2·h,and the separation efficiency was 95.54%,while for the O/W emulsions with toluene as oil body,the oil flux on the hydrophobic surface of Janus membrane was 269.21 L/m~2·h,and the separation efficiency was 91.87%.Compared with the low water flux or low oil flux and low separation efficiency of pure hydrophobic and pure hydrophilic membranes,Janus membrane showed better separation effect for several oil-water stable emulsions.The Janus membrane with finger-like pore structure,optimal asymmetric wettability,and an integrated hydrophilic/hydrophobic layer was used for the separation of brine and O/W emulsions systems in membrane distillation.The results showed that the water flux of Janus membrane separation brine system was 27.2 kg/m~2·h,and that of O/W emulsions was 22.5 kg/m~2·h.Compared with the conductivity values of brine and O/W emulsions（285 and 248μS/cm）,the conductivity values of the osmotic fluid in both systems were lower（<10μS/cm）and remained stable.The experimental results showed that Janus membrane had a good retention effect on salt and oil droplets,which proved that the main role of Janus membrane hydrophilic layer in membrane distillation separation was to reduce membrane surface contamination,and was synergistic effect with the hydrophobic layer on the bottom of the membrane to improve membrane distillation osmotic flux.