Vapor-Gap Membranes for Highly Selective Osmotically Driven Desalination

In this study, we demonstrate nanostructured osmosis membranes that employ vapor-phase water transport to simultaneously achieve high rejection of solutes and a high permeability. The developed membranes consist of a hydrophobic, thermally conductive silica nanoparticle (SiNP) layer with tunable thickness supported by a hydrophilic porous substrate modified with positively charged poly(ethyleneimine). We demonstrate osmotically driven, vapor-phase flow through the developed membrane enabling high solute rejection. The membrane permeability for water vapor is 1 – 2 orders of magnitude higher than hydrophobic microporous membranes typically used for osmotic distillation in the literature. This permeability is only mildly lower (~3 times) than the equivalent water permeability of typical forward osmosis (FO) membranes. The high selectivity of the developed membrane is highlighted by permeation tests using urea as a representative small and neutrally charged contaminant that is poorly rejected by conventional FO membranes. The developed membrane exhibits 2 – 3 orders of magnitude lower urea permeation than a thin-film composite (TFC) FO membrane while maintaining reasonable water flux. Further measurements of water vapor flux using different SiNP layer thicknesses, as well as theoretical predictions based on the dusty-gas model (DGM), show that membranes with a smaller SiNP thickness are capable of having comparable fluxes to TFC FO membranes while maintaining higher selectivity. We conclude that thin, hydrophobic nanostructured membranes composed of thermally conductive materials have a great potential to significantly extend the applications of osmosis-driven processes to treat challenging water sources.