Dual-layer membranes with a thin film hydrophilic MOF/PVA nanocomposite for enhanced antiwetting property in membrane distillation

Abstract Membrane distillation (MD) has been considered as a promising separation technology for desalination due to its ultrahigh rejection towards nonvolatile salts. However, pore wetting is a long-standing problem for the practical deployment of conventional hydrophobic MD membranes. Herein, hydrophilic/hydrophobic dual-layer membrane consisting of a thin hydrophilic, aluminum fumarate (AlFu) metal−organic framework (MOF) doped poly (vinyl alcohol) (PVA) dense layer on top of a hydrophobic microporous polytetrafluoroethylene (PTFE) substrate layer was prepared via a facial solution casting procedure. Adjusted by the nanoporous AlFu MOF, the thin PVA based hydrophilic layer (~800 nm) exhibited increased water vapor flux, reaching 45.1 kg/m2 h while maintaining nearly complete salt rejection (99.9%) in desalting NaCl (35,000 ppm) solution (50 °C and 10 °C in the feed side and permeate side, respectively) via direct contact membrane distillation (DCMD) process. Compared with the PTFE membrane, the dual-layer membranes possessed enhanced wetting resistance towards low-surface-tension solution by exhibiting stable throughput and salt rejection until 65% water recovery whereas the permeate conductivity of PTFE membrane increased substantially to 145 μS/cm at only 20% water recovery. Furthermore, the stability of the dual-layer membrane was assessed using real seawater in a 48 h DCMD test, showing no decline in water vapor flux and salt rejection while the permeate conductivity increased to 100 μS/cm for PTFE membrane. This study demonstrated the feasibility of using mixed matrix material as the top layer for designing high-performance MD membranes with outstanding antiwetting property.