Comparison of the effects of edge functionalized graphene oxide membranes on monovalent cation selectivity

Layer-by-layer graphene oxide (GO) membrane in principle has great potential in separating Li⁺ from monovalent cations, which is achieved by their functionalized interlayer entrance. The edge effects on different monovalent ions, however, are not fully understood. Therefore, molecular dynamics simulations were utilized in this study to separately elucidate the filtration effects of three typical edge functional groups, which were carboxyl (COOH), hydroxyl (OH), and hydrogen (H), on the LiCl, NaCl, and KCl solutions. The results revealed that the water permeance was dominantly influenced by the steric size of edge functional groups. It could also be affected by the ions blocked at the entrance. The drastic dehydration of the hydrated Na⁺ and K⁺ caused by the OH edge required more energy that led to higher ion rejection. The compressed-dehydrated hydration shell, which was tuned by the edge functional groups, introduced repulsion from Na⁺ and attraction from Li⁺ on Cl⁻ when they were 3–5 Å away from each other. It would be strategic to use all three edge functional groups to retain NaCl in the retentate stream while allowing selective permeance of LiCl and the OH edge could additionally retain KCl.