Water desalination performance of h-BN and optimized charged graphene membranes

Abstract

Water desalination using pressure-driven flow in hexagonal boron nitride (h-BN) and charged nanoporous graphene membranes are investigated using molecular dynamics (MD) simulations. Nanoporous h-BN membranes with pore diameters of 10.1 A, 12.2 A, and 14.7 A were selected to compare with the desalination performance of uncharged nanoporous graphene membranes with similar pore diameters. Salt rejection efficiency of uncharged graphene membranes is superior to that of h-BN, and the pressure drop for both materials exhibits the same inverse-cubic dependence on the pore diameter, regardless of the hydrophobic versus hydrophilic nature of graphene and h-BN, respectively. Charged graphene membranes with pore diameters of 15.9 A, 18.9 A, and 20.2 A were also considered, and 15.9 A pore diameter with total fixed charge of 12e was found to be the optimum setting for single-layer graphene membrane, resulting in 100% and 98% rejection efficiencies for Na+ and Cl− ions, respectively. The corresponding pressure drop is 51.8% lower than that obtained with 9.9 A pore diameter uncharged graphene with 100% salt rejection. To maintain perfect salt removal, 15.9 A pore diameter charged bilayer graphene membranes with 12e total charge on the first layer, and − 1e on the second one was simulated at different separation distances between the two membranes. The associated pressure drop is 35.7% lower than that obtained in 9.9 A pore diameter uncharged base-line case. These findings confirm the potential application of using charged bilayer nanoporous graphene membranes in improving the performance of reverse osmosis desalination systems.