Pressure-driven distillation through air-trapping hydrophobic membranes is proving to be a superior water purification option relative to conventional heat-driven membrane distillation systems. Reliance on a tiny layer of air bubbles to distill rather than sieve water makes for a more permeable treatment system with improved capacity to remove impurities. Schematic diagram of pressure-driven water vapor transport through a nanoporous membrane with an ultrathin air gap. Source: Science Advances (2023). DOI: 10.1126/sciadv.adg6638

In the system designed by researchers from the University of Colorado Boulder and Canada’s University of British Columbia, applied pressure drives evaporation on the feed side of the membrane, gas-phase diffusion through the pore, and condensation on the permeate side of the membrane. Nanoporous membranes with sub-200-nm-thick air layers demonstrated near-complete rejection of low-volatility contaminants, including dissolved salts and micropollutants.

By decreasing the air layer thickness, high water permeabilities were maintained without sacrificing water-salt selectivity as air layer thickness decreased, confirming this treatment approach can avoid the permeability-selectivity trade-off that undermines the performance of conventional membrane-based desalination systems. The use of an air layer as a separation barrier allows desalination performance to be maintained even when membranes are exposed to chlorine and ozone disinfectants.

In addition to seawater desalination, the technology described in Science Advances can also be of value in wastewater reuse applications.