Heavy metal ion separation from industrial wastewater using stacked graphene Membranes: A molecular dynamics simulation study

Abstract

In recent years, fluid transport through stacked graphene membranes has gained considerable attention due to its potential applications in water purification and desalination. Here, we are resorting to molecular dynamics (MD) simulations to elucidate the metal ion separation efficiency and mechanism of ion transport in stacked graphene membranes of varying interlayer spacing (d). Our simulation results show that metal ions, including Cd2+, Cu2+, Hg2+, and Pb2+, can permeate through wide membranes (d = 1.0 nm), but metal ion rejection is close to complete for the narrower nanochannels, irrespectively of the metal ion type. The increase of the interlayer spacing positively influences the permeance of the metal ions and water. The impact of the first hydration shell of the metal ions on ion rejection from membranes is significantly high, whereas the impact of the second hydration shell is considerably low. Even if the second hydration radius of the metal ions is greater than the half-width of the channel, the ions can pass through the membranes because of the loosely bonded water molecules in that hydration shell. Moreover, free energy analysis using steered molecular dynamics (SMD) simulation techniques show that free energy profiles of a metal atom in wider channels are almost the same, and free energy for the metal atom within the membrane channels is always higher compared to that for water. The free energy of the metal atom within the narrower channel is noticeably high, and that is the reason for complete ion rejection. To sum up, our results suggest that stacked graphene membranes with an interlayer spacing of 0.8 nm are the best for complete metal ion rejection and considerable permeation of water.