Salt Rejection using Conically Shaped Pores with Patterned Surface Charges

Abstract

In light of global potable water scarcity, whether it be due to increasing agricultural demands, higher standards of living, or overdrawing from underground sources, the need for efficient desalination techniques is of high importance. Reverse osmosis membranes offer excellent rejection of salt but the throughput of the technology is very low due to the sub-1nm effective size of the membrane voids. We propose a different approach to achieve desalination using membranes containing pores with opening diameter of few nm, and walls with high density of surface charges and surface charge patterns. We found that even 5 nm in diameter nanopores and containing a junction between a zone with positive surface charges and a zone with negative surface charges could offer rejection of salt up to 80% from 100 mM KCl. Increasing the pore diameter can increase the flux of water even two orders of magnitude compared to fluxes achievable with reverse osmosis membranes. Experiments were performed with track-etch membranes with densities of pores up 109 per cm2. Membranes in polyethylene terephthalate and polyimide were tested. Experimental observations were supported by the numerical modeling performed by solving Poisson-Nernst-Planck and Navier-Stokes equations. Dependence of salt rejection on salt concentration, applied pressure and surface charge pattern is investigated in detail. Discussion on the range of electrostatic interactions in aqueous solutions as a function of surface charge density and surface charge arrangement will also be provided.