Superhydrophobic nanofiber membrane containing carbon nanotubes for high-performance direct contact membrane distillation

Abstract

Tailoring the membrane to have superhydrophobicity, coupled with high porosity, adequate pore sizes and narrow pore size distribution, and thin thickness could find potential application for high-performing direct contact membrane distillation (DCMD) process. Electrospinning is an excellent approach in fabricating nanofiber membranes with adequate properties required of an MD membrane. In this study, superhydrophobic, robust, mixed matrix polyvinylidene fluoride-co-hexafluoropropylene (PcH) nanofiber membranes were fabricated incorporating different concentrations (1–5wt%) of carbon nanotubes (CNTs) as nanofillers to impart additional mechanical and hydrophobic properties. The electrospun membrane has been designed to have two cohesive layers, a thin CNT/PcH top layer and a thick neat PcH bottom layer. Through different characterization techniques, CNTs were found to be widely distributed on/in the nanofibers, where more beads-on-string were formed at higher CNT content. However, the beads-on-string did not significantly affect the membrane porosity and pore size, as well as did not degrade the MD performance. Highly-porous structure was observed for all membranes and the nanofiber membrane showed comparable pore sizes with a commercial flat-sheet PVDF membrane but at a much higher porosity (>85%). The contact angle increased to superhydrophobic at 158.5° upon the incorporation of 5wt% CNTs in the nanofiber due to increased roughness and added effect of hydrophobic CNTs. The liquid entry pressure also increased when 5wt% CNT was added compared to the neat PcH nanofiber membrane. The resulting flux of the 5wt% CNT-incorporated nanofiber membrane (24–29.5L/m2h) was consistently higher than the commercial PVDF membrane (18–18.5L/m2h), with an average increase of 33–59% depending on the feed water type (35 or 70g/L NaCl solution) without compromising the salt rejection (>99.99%). The present nanofiber membranes containing CNTs with one-step electrospinning fabrication show high potential for DCMD desalination application.