Nanofibrous Membranes With High Flux And Scaling-Resistance For Membrane Distillation:Fabrication And Desalination Performances

In recent years,developments of social and economic have not only increased the demand for energy and water resources,but also intensified the pollution of water environment.Membrane distillation(MD)can use low-grade heat and waste heat to treat high salinity water and obtain high-quality fresh water,which can reduce environmental pollution and alleviate the shortage of water and energy resources.The key problems should to be solved for the industrial application of membrane distillation for treatment of high salinity water are to improve the membrane flux and to resistant the membrane wetting and membrane scaling.From the perspective of material innovation,this study prepared nanofibrous membranes with high flux,excellent scaling and wetting resistance for high energy efficiency and robust MD process.The main results and conclusions of this paper are as follows:(1)A novel hollow fiber membrane based on nanofibers(N-HFM)was designed by combining the advantages of high porosity of nanofiber membrane and high filling density of hollow fiber membrane.A new technique for continuous and large-scale preparation of N-HFM was developed by using a non-rotating linear electrode as the collector,and the mechanism of the formation of N-HFM was discussed by investigating the motion trails of nanofibers during electrospinning process.Through the systematic study of the parameters of electrospinning process,it was found that the the positive and negative voltage,and the feed rate of electrospinning solution were important parameters for the fabrication of N-HFM with ideal structure morphology Due to the high porosity,the flux of the membrane prepared in this study was much higher than that of hollow fiber membrane and plate membrane prepared by other traditional methods.(2)A quick method was developed to enhance the mechanical strength and control pore-size-distribution of nanofibers:dilute solvent welding.The mechanism of this method is to dilute the solvent of a certain polymer by using its non-solvent to dilute its solvent and make the polymer cannot be dissolved by the mixed solvent(welding solution)at room temperature while can be partially dissolved after heating.Effects of the composition of welding solution,welding temperature and welding time were investigated by comparing the morphology,pore size distribution,porosity,mechanical strength,thermal decomposition curve and crystallinity of membranes before and after the welding.It was proved that the dilute solvent welding is a quick method to increase membrane mechanical strength and narrow the pore-size-distribution.It was found that the flux of the welded membrane decreased slightly,but the stability of the membrane was improved greatly after welding.(3)N-HFM membrane composed of pure polytetrafluorothylene(PTFE)nanofibers was prepared via emulsion electrospinning and sintering using water-soluble polyethylene oxide(PEO)as the carrier of the insoluble PTFE particles.The optimal mass ratio between PEO and PTFE was 8/92.The effects of different sintering temperatures on the morphologies,chemical properties,hydrophobicity,pore size distributions and mechanical strength of the membranes were investigated.Due to the high porosity and through-hole structure,the MD flux of the prepared N-HFM were 4.6?8.8 and 3.6?11.6 times of that of the commercial and reported PTFE hollow fiber membrane,respectively.In addition,the prepared N-HFM also showed very stable performance in MD experiments with different temperature,feed flow rate and salt concentration over a long period of time,which proved that this novel N-HFM composed of PTFE is promising for practical application.(4)Aiming at the problem of membrane scaling in the treatment of high salinity water by MD,a high-flux and anti-scaling membrane was prepared based on electrospinning and electrospray technology(ES2)and welding.ES2 technique constructed unique nanofiber-binding-microsphere structure through the simultaneous deposition of nanofibers and microspheres layer by layer,and the bonds between the nanofibers and microbeads were formed after welding which can enhance the strength of the membrane and robustness of the superhydrophobicity.The superhydrophobic membrane showed a significant scaling-resistance than the hydrophobic membrane during brine-concentrating experiment with MD.Based on the bounded gas layer on the surface of superhydrophobic membrane under water,three mechanisms for scaling-mitigation of the superhydrophobic membrane(also been named as slippery membrane)with Cassie-Baxter state(low rolling angle)were proposed:reducing the liquid-solid contact area,decreasing the propensity of heterogeneous crystallization,and reducing the interaction time between mineral ions and the membrane surface.(5)To solve the problems of membrane wetting and scaling of the hydrophobic membrane in the treatment of high salinity water which containing low surface energy materials,a membrane with slippery and omniphobic surface was proposed to treat this kind of complex waste water.The slippery and omniphobic membrane was fabricated via coating the mixture solution of fluoride-propyltriethoxysilane(F-POSS)and polydimethylsiloxane(PDMS)on the surface of the nanofibrous membrane which are showing the "re-entrant structure".The fabricated membrane exhibited excellent anti-wetting and anti-scaling properties in treating of high saline water containing low surface energy materials with MD process.