Hydrophilic/hydrophobic Janus membranes with a dual-function surface coating for rapid and robust membrane distillation desalination

Abstract

Membrane distillation (MD) is a promising technique for desalinating brine and purifying wastewater but low water flux and severe membrane fouling always plague its practical application. Developing new membranes with both high flux and fouling resistance is highly desirable. Here, a hydrophilic/hydrophobic Janus membrane that can simultaneously achieve rapid and robust desalination via vacuum MD (VMD) was designed and fabricated by architecting a polydopamine-based hydrophilic surface coating on a commercial polytetrafluoroethylene (PTFE) membrane. Results demonstrated that such hydrophilic coating had the unique dual function: (1) boosting water evaporation via tuning the state of confined water and timely water replenishment and (2) providing a hierarchical layer with highly underwater oleophobicity. Due to the synergistic effect, the Janus membrane flux soared to 84.3 ± 3.4 kg m−2 h−1 from 19.8 ± 1.5 kg m−2 h−1 of the commercial PTFE membrane when treating a 3.5 wt% NaCl solution (70 °C, −90 kPa), achieving an unprecedented 325.7% of flux enhancement, while also exhibiting long-term stability when desalinating oil-containing brine. Conversely, severe oil fouling occurred on the pristine PTFE membrane, quickly leading to near-zero flux. This work may provide a new platform and methodology for designing high-performance membranes for MD desalination.