Desalination of water by vapor transport through hydrophobic nanopores

Abstract

Although Reverse osmosis (RO) is the state-of-the-art desalination technology, it still suffers from persistent drawbacks including low permeate flux, low selectivity for non-ionic species, and lack of resistance to chlorine. This leaves ample rooms. for further improvement for RO technology by addressing these issues. In this thesis, a new approach is proposed for desalination by vapor-phase transport through hydrophobic nanopores in an isothermal condition. Hydrophobic nanopores flanked by vapor-interfaces with a submicron gap provide a complete barrier for salt while behaving as highly permeable medium for water. We first theoretically explore transport of water through a hydrophobic nanopore using a probabilistic model that incorporates rarefied gas dynamics, ballistic transport, and emission and reflection of water molecules at liquid-vapor interfaces. We then expand the model to transition regime where molecular diffusion coexists with the rarified gas transport. Effect of nanopore geometry, salinity, temperature, applied pressure, and interfacial reflection probability on the transport of water molecules through the nanopore are explored. We further realize membranes consisting of hydrophobic nanopores to experimentally study the transport with the various above-mentioned conditions. We find the existence of two mass transport regimes, i.e., diffusion-governed and interface-governed transport, determined by interplay between transmission across the nanopores and condensation at the interfaces. The condensation resistance, represented by condensation coefficient, was experimentally measured. An accurate value of the condensation coefficient was estimated accordingly, which has been debated more than a century. Based on this finding, the proposed approach is expected to produce up to ~2x higher permeate flux at 50°C and with porosity of 40% than conventional RO. This approach further decouples transport properties from membrane material properties, thereby opening the possibility of engineering membranes with appropriate materials that may lead to reverse osmosis membranes with improved flux, better selectivity, and high chlorine resistance allowing for inexpensive and simple fouling control.