A novel Cu-BTC@PVA/PVDF Janus membrane with underwater-oleophobic/hydrophobic asymmetric wettability for anti-fouling membrane distillation

• The Cu-BTC@PVA/PVDF membrane with asymmetric wettability was facilely prepared. • Cu-BTC had porous structure and high surface area, benefit the mass transfer in MD. • The Janus membrane had robust anti-fouling and wetting- retardation property. • Hydration layer, slip effect, electrostatic repulsion were main antifouling mechanisms. • Fouling mechanism turned from hydrophobic interaction and CP to H-bond for Janus membrane. In this study, a novel Janus membrane was developed by constructing Cu-1,3,5-benzenetricarboxylic acid (Cu-BTC) metal–organic frameworks (MOFs) hydrophilic coating on polyvinylidene fluoride (PVDF) membrane. The Cu-BTC@PVA/PVDF Janus membrane showed a rougher surface with an in-air water contact angle of 27.4˚ and an underwater oil contact angle of 152.8°. The Cu-BTC had minimal impact on mass transfer efficiency in MD process, and the Janus membrane exhibited excellent anti-oil-fouling and anti-NOM-fouling properties. The formation of the hydration layer, the slip boundary effect, and the electrostatic repulsion by the hydrophilic serrated interface were the main antifouling mechanisms. The hydrophobic-hydrophobic interaction was the main fouling mechanism of PVDF membrane for both oil and NOM. Sodium alginate (SA) displayed severe fouling with a compacted fouling layer in PVDF membrane due to the concentration polarization (CP) and gel layer formation. Inorganic fouling was the predominant fouling for the high CP and inorganic–organic coupling. While humic acid (HA) surpassed SA in Cu-BTC@PVA/PVDF Janus membrane for the stronger hydrogen bond, hydrophobic-hydrophobic interaction was significantly alleviated. Inorganic fouling was also significantly alleviated due to the strong electrostatic repulsion effect. The prepared Janus membrane has great potential to promote MD dealing with hypersaline industrial wastewaters containing complex organic matters.