Preparation and characterization of novel triple layer hydrophilic–hydrophobic composite membrane for desalination using air gap membrane distillation

• Novel triple layer nanofiber-based membranes were prepared.
• Membrane were tested in an air gap membrane distillation process.
• Triple layer membranes were compared with dual layer membranes and nanofiber only layers.
• Nanofiber layer significantly improved flux and pore wetting resistance of membrane.

Conventional membrane distillation (MD) membranes typically consist of two layers i.e. a support layer and a microporous hydrophobic layer that is usually casted onto the support layer. In this paper a triple layer configuration, consisting of a thin hydrophobic nanofiber layer over the conventional casted microporous layer in addition to the support layer is prepared. The layers are bound to each other by heat pressing and solvent binding. The unique membrane was characterized using the measurement of the liquid entry pressure of water (LEPw), scanning electron microscopy (SEM), pore size, mat thickness and the surface water contact angle (CA). The triple layer membranes, conventional dual layer membranes and nanofiber only membranes were tested in an Air-Gap Membrane Distillation (AGMD) process using a feed solution containing 3.5 wt.% NaCl. The water contact angle of the nanofiber selective layer, middle micro porous layer and the hydrophilic supportive layers are approximately 145°, 92° and 30° respectively. The LEPw of the triple layer composite membrane was found to be 1.6 times higher than the conventional dual layer membrane (i.e. without the nanofiber selective layer) and more than 8.7 times higher than that of the nanofiber membrane. The fabricated triple layer composite membranes were tested in an AGMD process and the flux was also found to be 1.5 times higher than that for the dual layer membrane (i.e. without nanofiber selective layer). In terms of salt penetration, the triple layer membrane had the lowest salt penetration ⩽0.02% at different feed water temperatures, while the salt penetration of the dual layer membrane was higher at ⩽0.07% and increased with increasing feed water temperature. The nanofiber only membrane’s salt penetration increased dramatically to more than 90% within the first few minutes of filtration indicating severe pore wetting. The triple layer membrane could be operated continuously for more than 40 h without any significant change to permeate quality, whereas the dual layer membrane could only be operated for 10 h before pore wetting occurred. Based on the results it is proposed that the triple layer configuration with nanofiber based selective layer above the conventional dual layer could be a promising membrane for membrane distillation.