Near-complete blocking of multivalent anions in graphene oxide membranes with tunable interlayer spacing from 3.7 to 8.0 angstrom

Graphene oxide (GO) membranes have attracted attention as a promising candidate for selective ion permeation due to narrow interlayer spacing. Among several methods to control the interlayer spacing, the reduction of GO membranes has great potential for desalination and water purification. To apply GO membranes for ion separation, investigating ion transport of sub-nanometer channels of reduced GO membranes is needed. In this study, thermally and chemically reduced GO membranes were fabricated with different interlayer spacing ranging from 3.7 to 8.0 Å in a dry state. It was experimentally determined that the minimum spacing allowing water permeation was 5.7 Å (9.6 Å in water), and no water permeated at 5.0 Å or less. For ion permeation, a pure GO membrane exhibited similar permeability for most ions, while the permeability of GO membrane with 5.7 Å spacing was strongly dependent of the size and charge of ions. The permeability of cations significantly changed along with the ionic radius, while there was a noticeable cutoff for permeability of multivalent anions such as SO₄ ²⁻ and Fe(CN)₆ ³⁻. Our reduced GO membrane could be used to effectively block the permeation of large ions and multivalent anions.