Molecular dynamics study on applying layered graphene oxide membranes for separating cadmium ions from water

Because cadmium can seriously damage human organs, we explored the prospect of using layered graphene oxide membranes to separate cadmium ions from water by molecular dynamics. The effects of three geometric parameters on membrane performance were investigated. The results indicate that the effects of the interlayer spacing and offset value on the salt rejection rate are much greater than the corresponding effects on water permeability. By contrast, for the gap size, comparable effects on the salt rejection rate and water permeability were observed. Then, the mechanism of water transport and ion migration were investigated. The number-density map revealed why the gap size has a more significant effect on water permeability. The radial distribution function and coordinate number indicate that Cd2+ is surrounded by two hydration shells. The second shell is more flexible so that it allows the loss of several water molecules to enable Cd2+ to pass through the membrane. Potential of mean force indicates the energy barriers for the migration of Cd2+ and Cl- on the surface of nanosheets are much larger than that for water molecules. The water permeability can reach up to 458 L/(m2⋅h⋅bar), which is much higher than the value for commercial membranes.