Functionalized nanoporous graphene membrane for water desalination; Effect of feed salinity on permeability and salt rejection, a molecular dynamics study

[Display omitted] •Feed water salinity effects on desalination process was investigated.•Novel membrane materials of nanoporous graphene was studied for water desalination.•Increase in salinity decreases water permeability while enhances salt rejection.•Ion concentration affects the orientation of water molecules near the pore.•Hydroxylated pore proved to be capable of desalination of higher water salinity. Feed water composition are sometimes instable and thus affecting the desalination processes. The effect of feed water salinity on permeability and salt rejection of graphene-based desalination membranes as novel desalination material, was investigated using molecular dynamics simulation to examine the performance of the membrane and permeate water quality. Hydroxylated (OH) pore was tested with pore diameter of 7.4 Å. Three feed water salinity of 33.4 (sea water), 50.1, 66.8, and 83.5 g/L was studied. The result indicated that best water permeability was observed for 33.4 g/L of water salinity while salt rejection of this membrane was lowest for this salinity. Concentration polarization graphs were obtained for desalination process indicating a concentrated area before the membrane as salty as more than 1000 g/L of while the highest off-membrane salinity reached to utmost 2 g/L. Density map plots for water molecules near the nanopores clearly displayed the orientation of water molecules as function of time and coordinate which proves that higher salinity causes lower congestion of water molecules near the pore as result of hydration effects or decreased water permeability as a result of a considerable drop in water entropy. This observation was approved by radial distribution graphs of water molecules around Na and Cl ions that displayed higher bounding at higher salinity. The effect of hydrated ions on water transport and salt rejection revealed the importance of the finely manipulating the feed composition.