Separation of mono-/di-valent ions via charged interlayer channels of graphene oxide membranes

Separation of mono-/di-valent ions is widely used in the fields of energy security and environmental protection. Compared with traditional separation methods, membrane separation could achieve higher efficiency with less energy. In this work, we reported graphene oxide (GO) membrane with charged interlayer channels for separation of mono-/di-valent ions by utilizing the synergistic effect of size-sieving and electrostatic repulsion. Positively-charged polyethyleneimine (PEI) were introduced into the interlayer of negatively charged GO layers via layer-by-layer assembly. Characterizations indicated that the introduction of PEI not only controlled the charge property of the interlayer channels, but also increased the hydrophilicity of the membrane surface. The effects of the number of GO-PEI bi-layer, PEI concentration and salt concentration on the membrane performance for mono-/di-valent ions were systematically investigated. The optimal GO-PEI membrane with ∼100 nm thick exhibited K⁺, Na⁺, and Li⁺ permeation rates of 0.474, 0.378, and 0.306 mol m⁻² h⁻¹, as well as the selectivity for K⁺/Mg²⁺, Na⁺/Mg²⁺, and Li⁺/Mg²⁺ of 33.8, 27.0, and 21.9, respectively. Density functional theory (DFT) was employed to understand the transport mechanism of charged interlayer channels for selective transport of monovalent ions over divalent ions.