Gradient-Pore-Engineered Janus Membranes for Sequential Molecular Sieving in Membrane Desalination

Membrane distillation (MD) holds significant promise for desalinating hypersaline wastewater but faces challenges in simultaneously rejecting both nonvolatile and volatile pollutants. Herein, we introduce a thermo-osmosis-evaporation (TOE) system utilizing asymmetric Janus membranes with gradient pores for sequential molecular sieving, which achieved by sequentially vacuum filtering a mixture of m-phenylenediamine (MPD) and graphene oxide (GO) nanosheets onto a hydrophobic electrospun poly(vinylidene fluoride) (PVDF) fibrous membrane, followed by interfacial polymerization to form an ultrathin polyamide (PA) layer. The role of the MPD-GO interlayer in modulating the structural properties and selective transport behavior of the PA layer in the PA@MPD-GO configuration was systematically investigated. Results reveal that MPD effectively intercalates between the layers of GO nanosheets, significantly reducing the mass transfer resistance of water within the GO membrane. Furthermore, the MPD present on the GO membrane surface acts as a monomer source for interfacial polymerization, enabling the formation of an ultrathin PA layer ∼9 nm thick. The optimized membrane achieved a high water flux of 63 L m^-2 h^-1 at a flow rate of 480 mL min^-1 under a temperature gradient of 40 °C, with 97.55% rejection of volatile phenylamine, while also demonstrating exceptional antifouling, antiwetting, and antiscaling properties. This gradient membrane design offers a promising approach to advancing thermal desalination technologies for treating hypersaline wastewater in complex scenarios.