Fabrication of polyvinylidene fluoride (PVDF) nanofiber membranes by electro-spinning for direct contact membrane distillation

The main challenges for membranes used in membrane distillation (MD) are well-designed porous structure and hydrophobic surface property. Hydrophobic nanofiber membranes possess high hydrophobicity, high surface porosity and adjustable pore sizes and membrane thickness, which make them an attractive candidate as MD membrane. The current study aims to fabricate and optimize polyvinylidene fluoride (PVDF) nanofiber membranes for MD application. Scanning electron microscopy (SEM), capillary flow porometer, geniometer, a homemade setup for liquid entry pressure (LEP) measurement and direct contact MD (DCMD) setup were used to characterize the resultant nanofiber PVDF membranes. Polymer dope compositions, spinning parameters such as sprayer moving speed and chamber moisture were optimized to examine their effects on pore size distribution of the membranes. Inorganic additives were also added into the dope to improve the electro-spinability of diluted polymer dopes in order to further decrease membrane pore size. Meanwhile, a heat-press post-treatment was considered as a necessary step to improve fresh nanofiber membrane integrity, enhance water permeation flux and help prevent membrane pores from wetting in DCMD operation. The experiment confirmed that all the electrospun membranes exhibit a rough surface with high hydrophobicity (>135° water contact angle). The post-treated PVDF nanofiber membranes were able to present a steady water permeation flux of about 21 kg m−2 h−1 throughout the entire testing period of 15 h, using a 3.5 wt% NaCl solution as the feed under the feed and permeate inlet temperatures of 323 K and 293 K, respectively. This result was better than these of commercial PVDF membranes and the nanofiber PVDF-clay membrane reported in literature, suggesting the competency of PVDF nanofiber membranes for DCMD applications.