Interplay of charge composition and nanochannel confinement in ion separation through graphene oxide membranes.

Graphene oxide (GO) membranes, composed of two-dimensional (2D) GO nanosheets, have attracted considerable interests for applications in water desalination and resource recovery. However, inconsistent salt rejection sequences reported across the literatures hinder a unified understanding of ion transport mechanisms in GO nanochannels, primarily due to variations in membrane charge and structural properties. In this study, we elucidate the ion transport behavior in chemically converted GO membranes by precisely tuning their charge type, charge density, and interlayer spacing through chemical reduction and amine functionalization. We demonstrate that charge properties, rather than size exclusion alone, dominate the separation trends of salts such as NaCl and MgCl2. Membranes with only negatively charged groups (NGO) preferentially exclude NaCl, while membranes with both negatively and positively charged groups (NPGO) exhibit reversed trend, favoring MgCl2 rejection. For environmental application-oriented validations, NPGO membranes exhibit enhanced monovalent cation selectivity in mixed-ion systems due to asymmetric electrostatic interactions against divalent cations. Additionally, integrating GO reduction and amination produces membranes with balanced charge and channel size, enabling uniform rejection of salts with different valence. This work provides mechanistic insights into the coupled roles of charge and steric interactions in salt rejection through GO-based nanochannels, offering design principles for developing advanced 2D membranes for ion-specific separations.