Uranyl (UO²⁺₂) structuring and dynamics at graphene/electrolyte interface

The physicochemical phenomena at the solid/electrolyte interfaces govern various industrial processes ranging from energy generation, storage, and catalysis to chemical separations and purification. Adsorption-based solid/liquid extraction methods are promising for the selective and rapid separation of nuclear (such as uranium) and other critical materials. In this study, we quantified the adsorption, complexation, and dynamics of UO²⁺₂ ions on the graphene surface in various electrolyte media (LiNO₃, NaNO₃ and CsNO₃) using all-atom molecular dynamics simulations, in combination with network theory based subensemble analysis, enhanced sampling, and temporal analysis. We observe that the choice of background electrolyte impacts the propensity of UO²⁺₂ adsorption on the graphene surface, with LiNO₃ being the most favorable at both low and high uranyl-nitrate concentrations. Even though UO²⁺₂ primarily retained its coordination with water and interacted via the outer-sphere mechanism with graphene, the interfacial segregation of NO₃⁻ increased the number of contact ion pairs (CIPs) between UO²⁺₂ and NO₃⁻ ions, and the residence times of UO²⁺₂ within the interfacial region. This study provides a fundamental understanding of the structure and dynamics of UO²⁺₂ on the solid surface necessary to design advanced adsorption-based separation methods for energy-relevant materials.