Novel dense Janus membrane with engineered gradient architectures for molecular separation in membrane contactor

Membrane contactor based on porous hydrophobic membrane is a promising approach for precise sieving between (semi-)volatile organic compounds ( e.g. , phenol, C 6 H 5 OH) and salt ions ( e.g. , Na + , Cl - , Ca 2+ , SO 4 2- ). Herein, a novel dense Janus membrane with engineered gradient architectures was proposed via two-step interfacial polymerization-based strategy for molecular separation, aiming to overcome the trade-off in selectivity-permeability. Stevioside (St), a rigid and contorted molecule, was used as alternative to piperazine in loose hydrophilic layer (∼35.8 o WCA) formation on polyvinylidene fluoride (PVDF) substrate, which also promoted phenol accumulation to facilitate volatilization-diffusion. Furthermore, a m-phenylenediamine (MPD) based electro-assisted ultrasonic spray was developed to form dense polyamide layer precisely, and resultant pore size (∼0.99 nm) fell between the hydration diameters of Ca 2+ -SO 4 2- ion pair and phenol molecule for selective separation. Collectively, phenol transmembrane flux increased to 26.3 g m -2 h -1 (MPD-St-PVDF JM, 67.5% higher than PVDF substrate) without sacrificing salt rejection. Meanwhile, membrane scaling was almost alleviated during long-term operation as shown in the smallest electrochemical impedance change (∼5000 Ω R b ) and negligible phenol flux reduction (∼1.0%). Atomic force microscopy (AFM) force-distance measurement, quartz crystal microbalance with dissipation monitoring (QCM-D), and extended Derjaguin-Landau-Verwey-Overbeek (xDLVO) calculation revealed that its scaling resistances mainly arose from kinetic inhibition by hydration layers, structural selectivity from sub-nano network, and unfavorable thermodynamic with high positive interfacial energy. In summary, remarkable molecule selectivity, scaling resistance, and gas permeability were simultaneously achieved on this newly-developed dense Janus membrane, which would provide a feasible approach for molecular separation in high-salinity industrial wastewater treatment.