Preparation And Of Superhydrophobic Electrospinning Fiber Membrane And Its Perfomance In Membrane Distillation

The major obstacles that hinder the practical application of the membrane distillation (MD) are low permeating flux and membrane-wetting. In order to address these issues, in this research we fabricated superhydrophobic membranes via electrospinning method with PVDF as bulk material, and investigated their performance in MD. Firstly, we studied the electrospinning conditions on morphology of PVDF membranes and optimized the conditions of the electrospinning of PVDF. And then we have developed two methods to construct the superhydrophobic composite nanofiber membranes with high porosity for MD as follows:one is co-electrospinning of PVDF/silica and the other is simultaneous electrospraying and electrospinning. Finally, we have employed direct contact membrane distillation (DCMD) to test the MD performance of these prepared membranes. The main research results are listed below:(1) We have studied the effects of electrospinning conditions on the morphology of the membranes, including polymer concentration, constitution of solvent, voltage, distance of tip to collecting drum, and drum rotating speed. The results indicated that both addition of acetone into DMA and increase of PVDF concentration can decrease the structures of "beads of sting" and "spindle", and increase the diameter of the fibers. The diameter of fibers would initially increase and decreased with the increase of the voltage. The increase of the tip-to-drum distance resulted in the decrease of fiber diameter; the fibers were easily merged together with the distance of less than 5 cm and the distance of 15 cm is the optimum condition for membrane morphology. Lower drum rotating speed may causes looser membrane to be loose and lower surface-strength, but the net-structure matrix will reduce due to too high orientation with rotation speed reaching 2200 rpm.indicating moderate speed is optimum condition. The MD-performance evaluation results showed that the flux of the membrane increased with increase of the temperature difference and the flow velocity of the feed solution, the decreased of the membrane thickness, and decayed with the long-term test.(2) To engineer membranes with stable water insistence, sufficient strength and high porosity for membrane distillation, we developed a facile method to construct composite membranes with high MD performance via one-step electrospinning of PVDF solutions blended with silica nanoparticles (NPs). The characterizations revealed that the incorporation of silica NPs altered membrane surface morphology and endowed the membrane with superhydrophobic structure throughout its depth. The hydrophobicity of the membranes can be easily tailored by the dosage of silica NPs. Furthermore, when the dosages of the silica NPs increased to 40 wt%, the incorporation of silica NPs promoted mechanical strength, salt rejection, and water permeation flux of the membranes. In addition, the composite membrane with superhydrophobicity throughout its depth exhibited more stable performance and wetting resistance than the surface superhydrophobic membrane.(3) Highly porous membranes with superhydrophobic structures throughout their depth (three-dimensional porous superhydrophobic membrane) were successfully fabricated via simultaneous electrospraying of silica/DMAc colloids and electrospinning of PVDF/DMAc solutions. The morphology of the membranes could be tuned by adjusting the time of ultrasonication to disperse the silica/DMAc colloids. When the concentration of silica was 10 wt% and the time of ultrasonication was 3 h, the obtained membrane with hierarchical nanoparticle-microbead fiber structures exhibited the best hydrophobic performance. Moreover, the three-dimensional porous superhydrophobic membrane showed more stable performance and wetting-resistance than the surface-superhydrophobic membrane. The hierarchical structures and the entrapped air maintaining in the bulk layers can provide continuous water-air-solid interfaces, thus keeping a stable state of superhydrophobicity, so that even after the surface of the membrane was wet, water can still not infiltrate quickly. The prepared membrane has a high flux about 32 L h-1 m-2, low permeate condition of 4?S cm-1 and stable operation time more than 150 h.