The Influence Mechanism Of Surface Velocity Slip On Selectivity And Permeability Of Graphene Reverse Osmosis Membranes

Graphene material has proved to be promising for separation membrane because of its single atom thickness and excellent mechanical properties.Recently,the researchers conduct desalination studies using the tangentially moving graphene RO membrane relative to brine,and find that the velocity slip on the surface of the RO membrane greatly improves the ion rejection rate,resulting in both a high water flux and excellent ion rejection under a large pore size.This study provides a new direction for the improvement of RO membrane performance.In this paper,molecular dynamics method is used to study the influence of surface velocity slip on selectivity and permeability of RO membrane during shear motion.The results will provide a clearer understanding of the shear behavior of RO membrane and provide theoretical reference for the design of RO membrane.The main contents of this paper are as follows:As the surface slip of RO membrane is slight,the change of RO properties is analyzed by computing the hydrogen bond and ionic hydration.Firstly,the pillared graphene RO membrane is constructed,and the influences of the pressure within feed solution,temperature and the shearing of membrane on the desalination properties are considered,which proves the great application potential of pillared graphene in water purification.In particular,it is found that the increase of shearing velocity will enhance the stability of hydrogen bond and ion hydration shell,leading the water flux to decrease and the ion rejection rate to increase.Secondly,different functionalized single-layer graphene membranes are constructed,and the influence of pressure and shearing velocity on RO properties are considered.The results verify that the influence of the shear motion of RO membrane on water flux and salt ion rejection rate is universal.Further analysis show that the increase of shearing velocity will enhance the stability of hydrogen bonds and the order of water molecule distribution,thus leads the density of water molecules to increase.At the same time,the increase of shearing velocity will enhance the stability of the ionic hydration shell,which makes it difficult for ions to pass through the nanopore by weakening the hydration shell.As a large slip occurs on the surface of RO membrane,the change of RO properties is analyzed by computing the slip velocity and shear rate.Firstly,the influence of slip velocity on RO properties is studied by changing the potential parameters between the graphene piston and water molecules in the brine zone under the constant shearing velocity.It is found that the increase of slip velocity will increase the ion rejection rate,but have no effect on the water molecule penetration.The increase of shear rate will weaken the stability of hydrogen bond and leads the water flux to increase.Secondly,the influence of porosity variation on RO properties is considered.The results show the increase of porosity will lead the slip velocity and ion rejection rate to decrease,but it also leads the shear rate and the water flux to increase.Finally,the influence of different layers of membrane on the RO properties is considered.The results show that with the same shearing velocity,the rise of the number of layers will leads the salt ion rejection rate to increase,but the water flux will decrease accordingly.