Novel PVDF membrane with sandwich structure for enhanced membrane distillation

Membrane distillation (MD) holds great potential in the desalination of high concentration brines but need improvement to achieve sustained permeate flux and durable performance in long-term MD operation. In this study, a novel Janus membrane with sandwich structure was fabricated. Detailly, the microporous PVDF membrane was single-side modified with a bio-inspired hydrophilic polydopamine/polyethylenimine (PDA/PEI) coating, then a hydrophobic nanofiber layer was constructed via forcespinning on the other side as an anti-wetting layer. As the PDA/PEI deposition time increases from 0 to 1.5 h, the contact angle of the modified PVDF membrane surface decreases gradually from 124.6° to 31.3° with little change in membrane morphology and pore structures, resulting in an upward trend of permeate flux up to 50.9 LMH at a feed temperature of 70 °C. The rough forcespun nanofiber layer endows the membrane surface with good hydrophobicity, which brings improved LEP (136.4 kPa) and operation durability for the membrane during MD process. Moreover, numerical simulations were conducted to verify and predict the permeate flux variation of MD membranes under different operation conditions, revealing the superiority of the sandwich-structured Janus membrane combined with the experimental results. Finally, the optimized PDA-PVDF-NF membrane was used for the desalination of model inland brine, exhibiting stable MD performance with high permeate flux of 25 LMH, ~100% salt rejection, and excellent water recovery during a continuous operation as long as 96 h.