Free-standing graphene slit membrane for enhanced desalination

Abstract

This study considers two novel ideas to further explore and enhance the graphene membrane for desalination. Firstly, while earlier molecular dynamics (MD) simulations studies have used frozen membranes, free-standing membrane is considered here. Since 2D membranes are usually embedded on porous support in the experimental reverse osmosis (RO) process, the free-standing membrane can more accurately model the behavior expected during operation. This study showed, using MD simulations, that a free-standing nanoporous graphene membrane can provide a higher salt rejection, but lower water permeability as compared to frozen membrane. Secondly, the performance of a slit membrane as compared to a membrane with circular pore is studied. At a pressure of 268 MPa, the critical diameter of circular pore that can maintain perfect salt rejection is found to be 5 Å and the critical size for a slit is determined to be 2.28 Å. It is shown that a slit membrane at its critical size can achieve water flux 3.5 times higher than a membrane with circular pore at its critical radius. This finding highlights the importance of slits over circular pores which can potentially widen the options for the design and fabrication of 2D graphene membranes for experimental verification of RO.