Developing high-flux dense Janus membrane with robust wetting and scaling resistance: A systematic comparison with Omniphobic membranes

Membrane wetting, scaling and low flux are challenging obstacles for membrane distillation (MD). However, the lack of advanced membranes capable of achieving both high flux and high wetting/scaling resistance constrains MD potential applications. Herein, a dense Janus membrane (JM) was facilely developed by co-depositing a dense PVA/PDA layer on a highly porous PTFE substrate to overcome this dilemma and systematically compared with omniphobic PTFE substrates with varied pore sizes for the first time. Results showed that, unlike flux- resistance "trade-off" observed for PTFE substrates, dense JM effectively resisted 0.4 mM SDS at a high flux of 56.8 LMH and retarded membrane scaling by 20 mM oversaturated gypsum. Robust wetting resistance was primarily attributed to the sieving effect of the sub-nanometer surface layer, which greatly protected substrate from SDS adsorption and feed intrusion, and was less affected by severe surface concentration polarization and relatively low substrate LEP than PTFE substrates. Strongly polar PVA/PDA layer provided a much more thermodynamically unfavorable continuous interface for gypsum adhesion than PTFE substrates, which played a key role in determining anti-scaling behavior. In contrast, gypsum still tended to rapidly cause membrane scaling in stagnant zones within PTFE substrates when flux was increased. This study shall provide new insights in designing high-performance membranes for robust hypersaline wastewater treatment, and highlighting dense JMs with a macroporous substrate and a highly hydrophilic dense surface layer as ideal alternatives.