Effects of additives on dual-layer hydrophobic--hydrophilic PVDF hollow fiber membranes for membrane distillation and continuous performance

The advantages of the implementation of dual-layer hydrophobic–hydrophilic hollow fiber membranes for membrane distillation (MD) have been highlighted in this work. The effects of incorporating methanol as a non-solvent additive and self-synthesized fluorinated silica (FSi) particles as a hydrophobic modifier on the resultant membrane morphology and MD performance were investigated. Employing a 3.5 wt% sodium chloride solution at 80 °C, the highest direct contact membrane distillation (DCMD) flux of 83.40±3.66 kg/(m2 h) and separation factor higher than 99.99% were attained for the membrane spun with methanol additive. Moreover, the stability of the dual-layer hydrophobic–hydrophilic hollow fiber membrane has been demonstrated through continuous DCMD experiments for 5 days. The separation factor was maintained higher than 99.99% for the membrane spun with methanol additive, verifying the suitability of the dual-layer hydrophobic–hydrophilic hollow fiber membrane configuration for desalination processes. The morphological transformation of the outer membrane surface from a porous agglomerated globule structure into a denser interconnected globule structure may be accounted for by the stability improvement of the membrane spun with methanol additive. On the other hand, it was found that an enhanced hydrophobicity of the membrane spun with FSi particles did not result in an improvement of the membrane stability. The existence of the hydrophilic hydroxyl group on the FSi particle surface may favor the occurrence of membrane wetting.