Dual-Layer PVA-HNT/PTFE Membranes for Boosted Antiwettability and Stability in Membrane Distillation

Separation membranes with inherent antiwettability and stability are highly desirable for membrane distillation (MD) in practical applications. In this study, hydrophilic-hydrophobic dual-layer membranes composed of a dense poly (vinyl alcohol)/halloysite nanotube (PVA-HNT) layer and a microporous polytetrafluoroethylene (PTFE) layer were fabricated to improve wetting and fouling resistance during the MD process. The incorporation of the HNT manipulated the crystallization and chain mobility of PVA, endowing the PVA-HNT layer with tunable water transport properties by adjusting the level of HNT loading. Benefiting from the hydrophilic top layer on PTFE, the dual-layer membrane with an optimal HNT loading of 5 wt% showed stable water vapor flux (7.6 kg/m²·h) while maintaining salt rejection above 99.95%. This performance was achieved using a 3.5 wt% NaCl feed solution with 0.4 mM sodium dodecyl sulfate at a feed temperature of 50 °C and permeate temperature of 10 °C. In contrast, the pristine PTFE membrane suffered from severe pore wetting, with its salt selectivity dropping from 99.5% to 91.5%. Antifouling performance was further evaluated using real landfill leachate in a 50 h treatment. The dual-layer membrane with a 5 wt% HNT maintained stable separation behavior with a 15.3% decrease in water flux, whereas the flux of the PTFE membrane declined by 70.5% in 30 h of operation. A distinct fouling layer was observed on the PTFE membrane surface after the operation, while no obvious fouling was identified on the dual-layer membrane, confirming its superior antifouling properties.