New molecular understanding of hydrated ion trapping mechanism during thermally-driven desalination by pervaporation using GO membrane

The graphene oxide (GO)-based membranes have shown effective salt separation from the brine solution. Although several experimental works have been done to study the salt rejection in the thermal-driven desalination system, the behavior of ions inside the GO nanochannels during the pervaporation process remains mostly unelucidated. Moreover, the previous theoretical studies on the driving force for ion transport within the GO membrane only focused on pressure, voltage, or concentration gradient. Here, we investigated the transport of water molecules and hydrated cations through the GO membrane in a temperature-assisted system by using dispersion-corrected density functional theory (DFT) calculations at PBE/Grimme with the ions under extreme confinement, reactive molecular dynamics (MD) simulations as well as validated by experimental methods. The water permeation increases with temperature, while the transport of cations remains minimal, which was not observed in other non-thermal desalination approaches. We have shown that high temperature eases the binding between the hydrated divalent cations and the oxygen functional groups on the GO nanosheets by reducing the hydration shell. Furthermore, the cross-linked cations inside the GO nanochannel create an accessible corridor for water molecules and block other cations such as Na+. Our simulation results provide a new mechanism of ion transport in the GO membranes by the thermal-driven process, which can be tailored by using other cations with higher charge density and high temperature to speed up the process.