Heavy versus Light Lanthanide Selectivity for Graphene Oxide Films Is Concentration-Dependent

Abstract

Rare earths are important materials in various technologies such as catalysis and optoelectronics. Graphene oxide (GO) is a promising material for separation applications, including the isolation of lanthanides from complex mixtures. Previous works using fatty acid monolayers have demonstrated preferential heavy versus light lanthanide adsorption, which has been attributed to differences in lanthanide ion size. In this work, we used interfacial X-ray fluorescence measurements to reveal that GO thin films at the air/water interface have no lanthanide selectivity for dilute subphases. However, at high subphase concentrations, ∼8 times more Lu is adsorbed than La. By comparing the GO results with an ideal monolayer with a carboxylic acid headgroup, arachidic acid (AA), we demonstrate that the number of Lu ions adsorbed to GO is significantly higher than the number expected to compensate for the surface charge. Vibrational sum frequency generation (SFG) spectroscopy results on both GO thin films and AA monolayers reveal a red-shifted SFG signal in the OH region, which we attribute to partial dehydration of the adsorbed ions and carboxylic acid headgroups. Liquid surface X-ray reflectivity data show that the GO thin film structure does not significantly change between the very dilute and concentrated subphases. We speculate that the functional groups of both GO and AA facilitate cation dehydration, which is essential for ion adsorption. Heavy lanthanide Lu has stronger ion−ion correlations that can overcome the electrostatic repulsion between cations at higher concentrations compared to light lanthanide La, meaning GO and AA can exhibit apparent overcharge with Lu. Lastly, the layered structure of the GO films and reactive chemical nature of GO itself can accommodate ion adsorption.

Ion and water adsorption to graphene and graphene oxide surfaces

“…Our group also considered the impact of trivalent ion charge density on water arrangement near interfacial GO films. 97 SFG analysis of GO films created on dilute subphases of higher and lower charge density ions showed similar 3200 cm −1 and 3400 cm −1 band intensities. SFG studies of GO films on high concentration subphases show a decrease in water signal, consistent with ion adsorption, for both trivalent ions.…”

“…To understand the impact of the ion character on sorption, we studied ion adsorption to GO films as a function of ion charge density and subphase concentration. 97 At low subphase concentrations, XFNTR analysis showed similar adsorption of ions with different charge densities to GO films at the air–water interface. Indeed, XR revealed nearly identical film structures.…”

Aqueous Interfaces in Chemical Separations

“…For instance, recent studies have revealed an anomalous increase in Debye length for highly concentrated solutions . Other observations at charged interfaces, such as an apparent overcharging with rubidium, yttrium, neodymium, lutetium, and thiocyanate, indicate the need for a new understanding beyond classical mean field theories to model ion adsorption at interfaces. Specific ion effects, ion–ion interactions, ion–surfactant chemistry, interfacial ion speciation, and other factors need to be considered in these investigations.…”

“…However, this sharp transition is not observed under arachidic acid (AA) monolayers, which exhibit a smoother adsorption curve . SFG studies show that a new −OH band at ∼3100 cm –1 appears with trivalent ion adsorption to the AA films, suggesting partial dehydration of the metal ion. ,, Considering that MC simulations neglect the ion hydration, it is possible that the dominance of direct electrostatic interactions varies depending on the headgroup–ion interactions, leading to this discrepancy. XFNTR experiments of lanthanum and lutetium adsorption on graphene oxide films demonstrate that between 10 μM and 1 mM bulk concentrations, both ions adsorb equally .…”

“…SFG studies show that a new −OH band at ∼3100 cm –1 appears with trivalent ion adsorption to the AA films, suggesting partial dehydration of the metal ion. ,, Considering that MC simulations neglect the ion hydration, it is possible that the dominance of direct electrostatic interactions varies depending on the headgroup–ion interactions, leading to this discrepancy. XFNTR experiments of lanthanum and lutetium adsorption on graphene oxide films demonstrate that between 10 μM and 1 mM bulk concentrations, both ions adsorb equally . Increasing the bulk ion concentration further does not change lanthanum adsorption, but lutetium adsorption increases dramatically.…”