Comparison between dual-layer (superhydrophobic-hydrophobic) and single superhydrophobic layer electrospun membranes for heavy metal recovery by air-gap membrane distillation

A novel approach to enhance membrane performance using electrospinning fabrication technique for recovery of heavy metals using air-gap membrane distillation is described. Accordingly, a comprehensive study was accomplished to fabricate a unique electrospun dual-layer membrane (ESD) with an upper superhydrophobic layer and hydrophobic electrospun support layer and compare with a superhydrophobic electrospun single layer membrane (ESS). Superhydrophobic alumina nanoparticles (Al2O3) were embedded in a low polymer concentration of polyvinylidene fluoride (PVDF) to produce superhydrophobic ESS and top layer of ESD using electrospinning technique, while a mat with different concentrations of PVDF were used as hydrophobic electrospun support layer. In this study, dual layer membranes were fabricated in two sets. In the first set, layer thickness was varied by changing the spinning volume of the top and support layer with maintain total spinning volume, while in the second set the fibre diameter of the support layer was varied by changing the polymer concentration. Moreover, the electrospun membranes were characterized in terms of membrane performance such as: permeate flux, heavy metal rejection and energy consumption; wettability performance such as liquid entry pressure (LEP), and water contact angle (WCA); membrane structure such as mean with maximum pore size and porosity; membrane integrity such as mechanical and thermal integrity. The heavy metal rejection was >99% for all single and dual layer membranes when filtering artificial wastewater (Pb, Cd, Cr, Cu, Ni). When compared with single layer electrospun membrane made from spinning 16 ml PVDF, dual layer membrane made from the same spinning volume exhibited some improvement, such as higher permeate flux above 23 l/m2∙h (LMH) when filtering 2500 ppm concentration heavy metal feed water. Additionally, both sets of dual layer membrane demonstrated better mechanical performance and slight reduction of LEP compared with single layer electrospun membrane.