Superhydrophobic PVDF nanofibre membranes coated with an organic fouling resistant hydrophilic active layer for direct-contact membrane distillation

Abstract

Although Membrane Distillation (MD) is envisaged as a promising technology for achieving a cost-effective and sustainable recovery of water from saline solutions, membrane fouling and wetting remain the major challenges that compromise its separation efficiency. To mitigate the high fouling propensities of hydrophobic MD membranes while retaining their high salt rejection efficiencies, superhydrophobic poly-vinylidene fluoride (PVDF) nanofibre membranes embedded with silanized silica nanoparticles (f-SiO2NPs) were synthesised and coated with a hydrophilic active layer containing silver nanoparticles and carboxylated multi-walled carbon nanotubes (AgNPs/f-MWCNTs). While the f-SiO2NPs-modified PVDF nanofibres exclusively allow the transport of water vapour, the AgNPs/f-MWCNTs active provides hydrophilic and biocidal (i.e., biofouling control) properties. The morphology, hydrophobicity, thermal and mechanical properties of the PVDF membranes were rigorously characterized. The PVDF nanofibres were investigated for their resistance to flux decline using a solution containing Bovine Serum Albumin and a real complex effluent containing thermophilic bacteria. f-SiO2NPs-modified PVDF nanofibres showed the highest flux decline (82% after 60 h) associated with BSA adsorption induced by favourable hydrophobic-hydrophobic interactions. AgNPs/f-MWCNTs coated PVDF membranes showed a significant decrease in fouling (47% flux decline) and the highest salt rejections (99.8%). Remarkably, the coating layer showed high biocidal properties towards thermophilic bacteria when subjected to effluent. The current approach provides an important step towards a successful implementation of MD processes at a pilot-scale for water desalting.