Electrospun dual-layer nonwoven membrane for desalination by air gap membrane distillation

In the present study, dual-layer nanofiber nonwoven membranes were prepared by a facile electrospinning technique and applied for desalination by air gap membrane distillation (AGMD). Neat single and dual-layer nanofiber membranes composed of a hydrophobic polyvinylidene fluoride-co-hexafluoropropylene (PH) top layer with different supporting hydrophilic layer made of either polyvinyl alcohol (PVA), nylon-6 (N6), or polyacrylonitrile (PAN) nanofibers were fabricated with and without heat-press post-treatment. Surface characterization showed that the active layer (i.e., PH) of all electrospun nanofiber membranes (ENMs) exhibited a rough, highly porous (> 80% porosity), and hydrophobic surface (CA > 140°), while the other side was hydrophilic (CA <90°) with varying porosity. Heat-pressing the membrane resulted to thinner thickness (from > 129 μm to <100 μm) and smaller pore sizes (<0.27 μm). The AGMD experiments in a co-current flow set-up were carried out with constant inlet temperatures at the feed and permeate streams of 60 ± 1.5 and 20 ± 1.5° C, respectively. The AGMD module had a membrane area of 21 cm² and the thickness of the air gap was 3 mm. The neat single and dual-layer ENMs showed a water permeate flux of about 10.9–15.5 L/m² h (LMH) using 3.5 wt.% NaCl solution as feed, which was much higher than that of a commercial PVDF membrane (~ 5 LMH). The provision of a hydrophilic layer at the bottom layer enhanced the AGMD performance depending on the wettability and characteristics of the support layer. The PH/N6 dual-layer nanofiber membrane prepared under the optimum condition showed flux and salt rejection of 15.5 LMH and 99.2%, respectively, which has good potential for AGMD application.