New insights into fabrication of hydrophobic/hydrophilic composite hollow fibers for direct contact membrane distillation

Dual-layer composite membrane is a new design for direct contact membrane distillation (DCMD) with membrane performance potentially superior to that of single layer porous membranes. Using dry-wet phase inversion technology, novel dual-layer hollow fiber membranes were fabricated in current research. The outer layer was made from polyvinylidene fluoride (PVDF) with polyvinylpyrolidone (PVP) or glycerol as non-solvent additive, while the inner layer consists of PVDF and polyvinyl alcohol (PVA) blend. The effect of the nonsolvent additive type in the outer layer and that of PVA/PVDF blending ratio in the inner layer on the morphological, mechanical and separation characteristics of the composite membranes was investigated. Membrane performance was further correlated to the physicochemical and morphological characteristics of the membranes. In particular, a thorough investigation of pore wetting in DCMD was attempted for the first time in this work, observing the cross-sectional distribution of EDX chlorine signals as an indication of the penetration of sodium chloride solution into the pore from the feed.