High‐Flux and High‐Selectivity Graphene Oxide Membrane Prepared by a Two‐step Reduction Method for Metal Ion Separation

Abstract Aiming at the problems of the graphene oxide (GO) membrane‘s water swelling and low ion selectivity, a two‐step reduction method was designed to prepare the interlayer positive charge modified two‐step reduced graphene oxide (TS‐rGO) membrane. Modified stable polyamino molecules onto GO, such as ethylenediamine (EDA), piperazine (PIP) and melamine (MA), provided the membrane interlayer with positive charge to enhance ion selectivity and maintained the stability of the interlayer channel at the same time. The moderate reduced graphene oxide (RGO) membrane was obtained by two‐step reduction process of hydrothermal reduction and heat treatment reduction, which enhanced the stability in water environment and increased the water flux. The permeability of TS‐rGO membrane, represented by PIP‐rGO, to monovalent metal ions (K + , Na + , Li + ) was similar to that of GO membrane, but its rejection effect on trivalent metal ion (the penetration rate of La 3+ is as low as 4.65×10 −5 mol m −2 h −1 .) was enhanced by nearly two orders of magnitude. The TS‐rGO membrane could have high ion selectivity, high low‐valent ion permeability, and high water flux simultaneously. In the process of ion permeation, due to the successful modification of interlayer positive charge, the charge effect and size effect synergistically improve the ion selectivity of TS‐rGO membrane.