Convenient Confinement: Interplay of Solution Conditions and Graphene Oxide Film Structure on Rare Earth Separations

Graphene oxide (GO) membranes are excellent candidates for a range of applications, including rare earth separations and radionuclide decontamination. Membrane performance varies widely under different conditions, as water and ion interactions with the membrane are strongly affected by small system changes. Feed solution pH is one critical factor, as pH, GO surface functionality, and ion speciation are interconnected parameters that cannot be controlled individually. Understanding nanoscale water and ion behavior near interfacial GO is critical for groundbreaking membrane advances, including improved selectivity and permeability. We experimentally examine the impact of solution conditions on water and lanthanide interactions with self-assembled GO films and connect these results to GO membrane performance. The investigation of the confined films at the air-water interface with a combination of surface-specific spectroscopy and X-ray scattering techniques allows us to understand water and ion behaviors separately. Sum frequency generation spectroscopy reveals a dramatic change in interfacial water organization because of graphene oxide film deprotonation. Interfacial X-ray fluorescence measurements show a 17x increase in adsorbed lanthanide to the GO film from subphase pH 3 to pH 9. Liquid surface X-ray reflectivity data show an additional 2.7 e- per Å2 for GO films at pH 9 versus pH 3 as well. These data are connected to GO membrane performance, which show increased selectivity and decreased flux for membranes filtering pH 9 solutions. We posit insoluble lanthanide hydroxides form at higher pHs. Take together, these results highlight the importance of interfacial experiments on model GO systems.