Graphene oxide membrane regulated by surface charges and interlayer channels for selective transport of monovalent ions over divalent ions

Separation of ions has wide applications both in industry and health care. Among them, the separation of mono-/di-valent ions remains great challenging, even for highly efficient membrane separation technology. The emerging two-dimensional-material (typically, graphene oxide, GO) membranes have shown outstanding molecular separation performance such as for water desalination and gas separation, while unattractive permeance and/or selectivity for separating mono/divalent ions. In this work, we designed a new type of GO membranes to separate mono-/di-valent ions, in which rationally-selected polyelectrolyte coating was introduced to simultaneously tune the surface charge and interlayer channels of assembled GO laminate. The resulting positively-charged membrane surface showed stronger repulsive force towards divalent cations over monovalent cations according to the Coulomb's law; meanwhile, the regulated interlayer channels rejected divalent cations over monovalent cations with smaller size. The ion transport mechanism was understood by mathematic models of ion diffusion and DLVO theory. By utilizing the synergistic effect of electrostatic interaction and size sieving, the designed GO membrane exhibited excellent separation performance for various mono-/di-valent ion pairs, including K+/Mg2+, Na+/Mg2+, Li+/Mg2+. Especially, the K+ transport rate of ∼ 0.29 mol m−2 h−1 and K+/Mg2+ selectivity of ∼ 116 are demonstrated in single-salt solution test, outperforming the-state-of-the-art 2D-material membranes. In mixed-salt solution test, the K+/Mg2+ selectivity reached 45 and good structural stability was displayed during 80 h continuous operation on the GO membrane. This facile polyelectrolyte coating approach provides a new avenue to develop high-performing 2D-material membranes for efficient separation of mono/divalent ions.