Charge-Gated Ion Transport through Polyelectrolyte Intercalated Amine Reduced Graphene Oxide Membranes

Charge-gated channels are nature's solutions for transport of water molecules and ions through aquaporins in biological membranes while excluding undesired substances. The same mechanism has good potentials to be adopted in pressure or electrically driven membrane separation processes. Herein, we report highly charged nanochannels created in polyelectrolyte (PE) intercalated amine reduced graphene oxide membrane (PE@ArGO membrane). The PE@ArGO membrane, with a rejection layer of ∼160 nm in thickness, features a laminate structure and a smooth top surface of a low roughness (typically ∼17.2 nm). Further, a modified PE@ArGO membrane (mPE@ArGO membrane) was developed in situ using free chlorine scavenging post-treatment method, which was designed to alter the charge while keeping alteration to the layered structure minimal. The surface charge of the PE@ArGO and mPE@ArGO membrane was +4.37 and -4.28 mC/m² respectively. In pressure driven processes, the pure water permeability for PE@ArGO and mPE@ArGO was 2.9 and 10.8 L m^-2 h^-1 bar^-1, respectively. Salt rejection is highly dependent on the charge density of the membrane surface, the valence of the co-ions and the size of ions in hydrated form. For example, in the positively charged PE@ArGO membranes, the rejection of the salts follows the order of: R(MgCl₂), 93.0% > R(NaCl), 88.2% ≈ R(MgSO₄), 88.1% > R(Na₂SO₄), 65.1%; while in the negatively charged mPE@ArGO membranes, the rejection of the salts follows the order of: R(Na₂SO₄), 90.3% > R(NaCl), 85.4% > R(MgSO₄), 68.3% > R(MgCl₂), 42.9%. To the best knowledge of the authors, this is the first study to report graphene oxide based membranes (GOBMs) with high density positive/negative charge gated ion transport behavior. What's more, the high rejection rate along with high water permeability of the PE@ArGO and mPE@ArGO membranes has not been achieved by other types of GOBMs.