Preparation Of Superhydrophobic PVDF Nanofibrous Composite Membrane And Its Application In Solar-driven Membrane Distillation

Fresh water is scarce and seawater is abundant,so obtaining fresh water in the form of seawater desalination can effectively alleviate the shortage of fresh water resources.Membrane distillation is widely used in the field of seawater desalination.It uses hydrophobic microporous membrane and takes the vapor pressure difference on both sides of the membrane as the driving force.Electrospinning nanofibers membrane can improve the closed cell structure of traditional membrane distillation membrane due to the advantages of its interconnecting porous structure,high specific surface area,high porosity and adjustable membrane thickness.Compared with the traditional distillation method,membrane distillation has lower operating pressure and temperature,and higher separation efficiency.In addition,the method can be combined with solar energy,geothermal,waste heat and other low quality heat sources,with lower cost,more efficient access to fresh water.However,in the current membrane distillation process,most of the feed liquid is centrally heated,and the temperature polarization phenomenon is common,which leads to a significant reduction of mass transfer driving force,and then seriously affects the evaporation flux and increases the energy loss in the membrane distillation process.The composite membrane with good photothermal conversion efficiency can be prepared by combining the traditional membrane distillation membrane with photothermal conversion particles.The influence of temperature polarization effect can be effectively reduced,the mass transfer force can be increased,and the evaporation flux can be improved by making full use of solar energy.In this work,the electrospun polyvinylidene fluoride(PVDF)nanofiber membrane was employed as the base,loaded functional particles through the method of vacuum extraction and filtration on its surface,for the fabrication of PVDF based photothermal composite membrane.The structural characteristics and Solar-driven membrane distillation(SDMD)properties of the composite membrane were investigated.The specific research contents are as follows:(1)In order to solve the problems of temperature polarization and low energy utilization in the traditional membrane distillation process,PVDF nanofiber membrane was used as the base membrane,and carbon nanotubes(CNTs)were uniformly loaded on the PVDF base membrane by vacuum extraction and filtration method.In order to bind CNTs firmly to PVDF base membrane,the"binder"polydimethylsiloxane(PDMS)was added during the filtration process to obtain a stable bilayer CNTs-PDMS/PVDF nanofiber composite membrane.The characterization test showed that the composite membrane had a high surface temperature.Under 1kw/m~2 simulated solar light,the surface temperature of the composite film increased rapidly to 107?.This allowed the feed fluid to be heated rapidly as it circulated to the membrane surface,effectively reducing the impact of temperature planning.When the feed liquid and condensate were kept at 20?,the evaporation flux of the composite membrane reached1.06 kg/m~2h when 1 kw/m~2 solar illumination was applied on the membrane surface.(2)In order to reduce the transport path of steam,we prepared"Janus"membrane with hydrophilic surface and hydrophobic base.PVDF nanofiber membrane was also used as the base membrane.By soaking PDMS,the superhydrophobic PDMS@PVDF substrate was obtained.Then,the hydrophilic photothermal conversion particle PANI was uniformly loaded on the substrate through vacuum extraction and filtration,and PVA was introduced to cross-link with GA to obtain stable hydrophilic and hydrophobic PVA-PANI/PDMS@PVDF nanofiber composite membrane.The results show that PANI has a good photothermal conversion effect.Under 1 kw/m~2 simulated solar light,the maximum surface temperature of the composite film could reach about 104?,and it increases slightly with the increase of the load.During the SDMD test,the feed fluid can go directly into the hydrophilic functional zone,which greatly reduced the water vapor transport path.Under the same conditions,the feed liquid and condensate were kept at 20?,and the evaporation flux of the composite membrane was up to 1.22kg/m~2h when 1kw/m~2 solar illumination was applied on the membrane surface.