Fabrication And Performance Evaluation Of PVDF Based Electrospun Nanofibrous Membrane With Anti-Wetting And Anti-Fouling In Direct Contact Membrane Distillation

Among various membrane technologies,membrane distillation（MD）with theoretical separation rate of 100%,has been applied in freshwater production,desalination and shale gas produced wastewater treatment.MD is a remarkable desalination process using vapor pressure gradient across membrane to deliver water vapor from a hot salty feed side to a cold fresh permeate side.Non-volatile dissolved solutes cannot pass through the membrane.Compared with traditional pressure-driven membrane process like reverse osmosis（RO）,MD technique has its unique advantages such as low operation pressure and small footprint.Compared with traditional distillation like multi-stage flash（MSF）,the operation temperature of MD is low.Therefore,some low-grade heat such as solar energy,geothermal energy and factorial waste heat can be fully utilized in MD process.MD is expected to be an inexpensive technology for producing clean water.In MD process,membrane is the key factor.Among the various membrane fabrication techniques,as v versatile technique for generating durable and efficient membranes for MD application,the electrospinning technique for fabricating nanofibrous membrane has gained lots of attention.Electrospun membrane possesses several advantages involving high porosity and interconnected open pore structures.Polyvinylidene fluoride（PVDF）is widely used in electrospinning process due to its remarkable hydrophobicity and high solubility in polar solvent.However,large-scale commercialization of MD has not been fully achieved yet because of some technical challenges,such as wetting,fouling and scaling of membrane.In this study,a series of membranes including hydrophobic,superhydrophobic,omniphobic and hydrophobic-hydrophilic membranes were fabricated to mitigate the afore mentioned wetting,fouling and scaling problems.The main achievements of this study are listed as follows:Effectively modeling and optimization of electrospinning parameters and membrane distillation process by response surface methodology.In this chapter,seven variables:concentration of PVDF,concentration of PTFE,mass ratio of DMAc to acetone,tip to collector distance（TCD）,voltage,flow rate and rotation rate were optimized by RSM in consideration of contact angle,liquid entry pressure and thickness.Results showed LEP value was mainly determined by PVDF concentration and its interaction effects with mass ratio of DMAc to acetone and voltage.CA was mostly dependent on the interaction effect of PTFE concentration and flow rate.Thickness was mainly determined by PVDF concentration and TCD.The optimizes electrospinning parameters were:10%PVDF,0.9%PTFE,4 of DMAc/acetone,16 cm TCD,8 k V voltage,0.07 mm min-1 flow rate,140rpm rotation rate.Under optimized condition,the corresponding CA,thickness and LEP value were 146.2°,70μm and 53.5 k Pa,respectively.Furthermore,operation parameters of DCMD process were also optimized by RSM.The optimized parameters were:3.5%Na Cl feed,80°C of feed temperature,1 L min^-1 feed flow rate and 0.5 L min^-1 permeate flow rate.A maximal distillation flux of 67.5 kg m^-2 h^-1 was obtained under the optimum conditions.Successfully fabrication of superhydrophobic membrane（FZP）by fluorinating of zinc oxide（ZnO）blended electrospun PVDF membrane.The FZP membrane showed a stable superhydrophobicity with contact angle of 162.3°and sliding angle of 9.8°When desalinating 35 g L^-1 sodium chloride（Na Cl）feed,the water flux of FZP membrane was found to be stable for 68 h with an average of 14.8 kg m^-2 h^-1 due to its superhydrophobicity,which was more than 2 times longer than neat membrane.For desalination of sodium dodecyl sulfate（SDS）contained feed,neat membrane was easily wetted after SDS concentration reached 0.1 m M,whereas,FZP was not wetted until the concentration of SDS reached 0.2 m M.Mechnisms analysis showed FZP membrane exhibited the highest breakthrough pressure among three membranes.Further,FZP membrane could induce a low transmembrane temperature graduate（23.88°C）that may further postpone the occurrence of fully wetting.An omniphobic FZnO-PVDF membrane was successfully fabricated through fluorinating a ZnO hydrothermally grown membrane.Various membrane characterizations such as SEM,TEM,3D profiles,FTIR,XPS and XRD,all confirm the successful growth of ZnO.Contact angle of 50%ethanol droplet on FZnO-PVDF membrane surface was 121.1°.Na₂SO₄ and Ca Cl₂ were used as feed in DCMD process to evaluate the anti-scaling property of resultant membrane.Results showed flux of PVDF membrane decreased after 350 min operation.Whereas,the flux of FZnO-PVDF membrane kept almost stable during the whole desalination process.Theoretically study showed Gibbs free energy for gypsum depositing on FZnO-PVDF membrane was 34.94m J mol^-1,which was higher than that of PVDF membrane with 29.26 m J mol^-1.Compared with prisinte membrane,it was more difficult for gypsum to deposite on FZnO-PVDF membrane.After fabricating the optimized PVDF-PTFE electrospun membrane in chapter 2,ahydrophobic-hydrophilic composite membrane was further prepared by two-step electrospinning process.The composite membrane was composed of a PVDF-PTFE hydrophobic layer,a PET support layer and a chitosan-polyethylene oxide hydrophilic layer（labeled as TL-M）.Membrane characterizations including SEM,FTIR and contact angle,all confirm the asymmetric structure of TL-M membrane.The contact angle and thickness of hydrophobic layer were 144°and 55μm,respectively.The contact angle and thickness of hydrophilic layer were 20°and 75μm,respectively.TL-M membrane and DL-M membrane without hydrophilic layer were tested in DCMD process using 3.5%Na Cl feed.Results showed TL-M membrane with 19 kg m^-2 h^-1 average flux was higher than that of DL-M membrane(average flux:15 kg m^-2 h^-1).Mean driving force of TL-M was 14.26 k Pa,which was higher than that of DL-M membrane.This indicated the incorporation of hydrophilic layer could increase the driving force,and largely enhance the membrane flux.A novel inner super-hydrophobic and external hydrophilic nanofibrous membrane（labeled as PFZnO-PVDF）was fabricated via fluorination and polydopamine coating to alleviate the wetting and fouling problems.Results of SEM,XPS and contact angle showed super-hydrophobic modification and hydrophilic coating of PFZnO-PVDF were successful.When treating Na Cl feed solution,PFZnO-PVDF membrane had higher average flux(18.6 kg m^-2 h^-1)and longer operation time（for 6 cycles）than PVDF membrane with 10 kg m^-2 h^-1 average flux and 2 cycles.Compared to PVDF membrane,the higher flux of PFZnO-PVDF membrane was ascribed to its higher mass transfer coefficient.Furthermore,when dealing with oil-in-water emulsion,PFZnO-PVDF membrane exhibited robust anti-fouling properties,while the performance of PVDF membrane can be easily deteriorated.The remarkable anti fouling property of PFZnO-PVDF membrane was due to high energy barrier of 0.12 k T for oil deposition.In summary,MD performance of electrospun membrane can be largely enhanced by optimization of electrospinning parameters and preparing membranes with different structure.The main objective of this study was to illustrate the relationship between membrane structure and performance.