Robust superhydrophobic electrospun membrane fabricated by combination of electrospinning and electrospraying techniques for air gap membrane distillation

Abstract

Membrane pore wetting is the main problem hindering long term stability of permeate flux quality in membrane distillation (MD) applications. A superhydrophobic membrane with micro and nanostructured surface features can offer a unique solution to resolve this issue. Thus, a modified electrospun membrane was fabricated using a combination of electrospinning and electrospraying. The membrane surface hydrophobicity was enhanced by constructing a beaded structure from spraying a mixture of non-fluorinated alumina (Al2O3) nanoparticles (NPs) mixed with low concentration of PVDF polymer on an electrospun base membrane made from PVDF. The results revealed that a rough surface with a hierarchical structure can be constructed, which could not only enhance the membrane hydrophobicity, but also further enhance the permeate efficiency by improving parameters such as flux and rejection. Additionally, the membrane hydrophobicity could be further tuned by controlling the bead spinning volume. Our study shows that the modified membrane with 7.8 μm beads layer thickness has boosted the liquid entry pressure (LEP) by 61% from 15.5 psi and the water contact angle to 154°. The performance of modified membranes with different spraying volume (1–5 ml) along with the neat electrospun and commercial membranes were examined in an air gap membrane distillation (AGMD) application for 5 h using a 2.5 wt% of synthetic heavy metal solution as a wastewater model. Then, the optimized superhydrophobic membrane with 2 ml spinning volume (ES15–2) was further tested in comparison with the commercial membrane during long-term operations (30 h) using 3.5 wt% of mixed heavy metals. The flux was 18.67 LMH (l m−2 h−1) for modified membrane (ES15–2) compare with 12.62 LMH for commercial PVDF membrane during 30 h of long-term operation with feed and coolant temperature at 60 °C, 20 °C, respectively. The present superhydrophobic membrane fabricated by a combined electrospinning/electrospray method shows high potential for MD applications.