Preparation Of Superhydrophobic PVDF Nanofiber Membranes And Their Application In Membrane Distillation

Rising demands of sanitary water caused by increasingly serious water shortages and water pollution drives the investigation of innovative water treatment technologies mainly focusing on the recycle of waste water.Additionally,the most widely employed“Ultrafiltration+Reverse Osmosis”double membrane treatment method was confused by the low water production(50%-70%)and the secondary RO concentrated water which could not be drained directly.As an efficient and effective non-isothermal separation process,membrane distillation is applied to treat RO concentrated water for improving the water production rate,aiming at the enhancement of the enterprises'capability of environmental protection and the promotion of energy saving and emission reduction.In this study,electrospinning method was selected for the preparation of PVDF nanofiber membranes to clarify the effects of PVDF concentration,voltage,tip-to-collector distance,rotational speed of collector and speed of injection on the morphology of nanofibers.A further research emphasizing on the functions of collector rotational speed and injection speed on the pore size of PVDF nanofiber membranes was also processed under the condition of no visible beads.The final optimum parameters for electrospinning were found to be the PVDF solution of12wt%concentration,the voltage of 12kv,the tip-to-collector distance of22cm,the collector rotational speed of 50rpm,the injection speed of0.05mm/min.A corresponding membrane with 89%porosity,0.85?m pore size and 139~oC contact angle was fabricated under these parameters.To satisfy the superhydrophobic performance requirements of membranes applied in membrane distillation,a membrane surface modification was operated by depositing TiO2 nanoparticles and FTCS successively on selected electrospun PVDF nanofiber membranes and HVHP4700 membranes.A modified PVDF nanofiber membrane was obtained with a water contact angle of 157°,while the contact angle of modified HVHP4700 was 160°.The stable thermal and mechanical stabilities of coating layers were also evaluated by measuring the relevant contact angles.Furthermore,the influences of a series of operational parameters,such as feed inlet temperature,feed inlet velocity,and permeate inlet velocity,on the performance of DCMD process were studied to compare the flux and the wetting resistance of modified and virgin membranes.Results showed that modified PVDF nanofiber membranes had higher fluxes and better wetting resistance abilities as compared to the modified HVHP700 membranes.Finally,a 21h practical application of the modified superhydrophobic PVDF nanofiber membrane was designed using RO concentrated water as the feed(70~oC,while the permeate inlet temperature was set to 25~oC)to achieve a high membrane flux,a low permeate conductivity under 10?s/cm and a high NaCl rejections above 99.9%.In conclusion,this paper would provide a novel practical,green low-carbon and energy-efficient path to solve RO concentrated water,as well as offering theoretical reference and technological support to the wide application of membrane distillation process.