High-Flux Direct-Contact Pervaporation Membranes for Desalination.

To solve the pore-wetting problems of membrane distillation, we developed a series of three-layer composite pervaporation (PV) membranes that could be applied in direct contact mode. Specifically, a porous poly(vinylidene fluoride) (PVDF) layer was placed on top of a polytetrafluoroethylene (PTFE) microfiltration membrane using the nonsolvent-induced phase inversion method, and then a poly(vinyl alcohol) (PVA) dense layer was coated on the PVDF/PTFE substrate. The hydrophobic PTFE layer prevented the permeate side of the PVA/PVDF/PTFE membrane from being wetted with cooling water. The PVDF intermediate layer acted as a glue to the PVA and PTFE layers. Also, the PVA dense layer provided a high water flux, salt rejection, and antifouling property. As a result, a high water flux of 44.5 ± 3.0 kg/(m2 h) with the NaCl rejection of >99.99% were achieved for the direct-contact pervaporation (DCPV) membranes when separating a 3.5 wt % NaCl solution at 75.0 ± 0.9 °C using a cooling water stream of 20.6 ± 0.3 °C. Moreover, when the NaCl solution contained 10 mg/L Tween20 (acting as a contaminant), a stable water flux of 45.8 ± 0.6 kg/(m2 h) was maintained for at least 24 h, indicating excellent antifouling property. Furthermore, when the permeate side was under vacuum, the water flux increased to 83.4 ± 6.5 kg/(m2 h), similar to the highest reported data of all the existing PV desalination membranes. More importantly, the easy-to-scale-up fabrication method indicated great potential of DCPV membranes for commercialization.