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

Abstract

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.

Monovalent ion–graphene oxide interactions are controlled by carboxylic acid groups: Sum frequency generation spectroscopy studies

“…A relatively weaker but distinct band around 3600 cm -1 was also observed. We attribute this band to the water molecules intercalated between GO layers, as discussed below and in previous studies [29,30]. The free -OH peak at 3700 cm -1 was not observed indicating that the GO film covers the surface completely [35].…”

“…The peak width, frequency and 𝜒 𝑁𝑅 (2) were fitted as global variables and the amplitudes were fitted individually for each spectrum [28,30]. We studied the GO/aqueous interface with four alkali metal ions over a wide range of ionic strengths using SFG spectroscopy, under SSP polarization configuration (Figure 1).…”

“…Understanding the various factors governing ion adsorption is crucial toward developing GO as a practical separation material. GO thin films formed at the air/water interface are ideal model systems to study ion-water-GO interactions [28][29][30].…”

“…We recently reported Lu 3+ , Y 3+ , Sr 2+ , Cs + ion behaviors near GO thin films by combining synchrotron X-ray and SFG techniques [28,30]. Hong et al studied water structure with SFG near GO thin films at different ionic strengths using NaCl [35].…”

Elucidating Trivalent Ion Adsorption at Floating Carboxylic Acid Monolayers: Charge Reversal or Water Reorganization?

“…As we focused only on AA/Nd 3+ systems here, we note that the surface structure and chemistry plays an important role, and on mineral, 21 graphene, 22 and graphene oxide [41][42] surfaces significantly different ion adsorption trends had been reported. Even the headgroup chemistry may lead to significant differences.…”