Hierarchical Janus membrane with superior fouling and wetting resistance for efficient water recovery from challenging wastewater via membrane distillation

Conventional hydrophobic membranes are susceptible to membrane fouling and pore wetting, impeding the applications of membrane distillation (MD) processes for water recovery from complex wastewater. To address these challenges, we herein developed a novel Janus membrane by a hybrid plant-derived polyphenols coating and polyelectrolytes layer-by-layer assembly strategy. Through alternative deposition of catechol/chitosan and polyethylenimine (PEI) on top of a hydrophobic polyvinylidene fluoride (PVDF) substrate, a Janus membrane with a superhydrophilic and defect-free top layer was achieved. The synergistic effects of covalent bonding, non-covalent interactions, and electrostatic interactions between neighboring coatings endowed the membrane with robust chemical stability in the pH range of 2–12. In comparison with the pristine PVDF membrane, the Janus membrane exhibited remarkably stable water vapor flux and high quality of effluent during long-term treatments of non-ionic/cationic surfactants and Tween® 20-stabilized oil-in-water (O/W) emulsion in the bench-scale direct contact membrane distillation (DCMD) experiments. The excellent anti-fouling and anti-wetting properties of the Janus membrane were attributed to its hierarchical structured superhydrophilic surface, in which a strong hydration layer was formed and served as an energetic barrier to prevent adsorption of surfactants and oil droplets. Moreover, the electrostatic repulsion between the protonated amino groups on the membrane surface and cationic surfactant molecules also contributed to the robust long-term performance (nearly one-week operation) of the Janus membrane in the treatment of saline water containing cationic surfactant. Our findings reveal the significant role of surface functionality in fouling and wetting resistance of MD membranes and the as-developed Janus membrane shows great potential for water recovery from saline wastewater containing challenging contaminants via DCMD.