Ion-Pairing-Mediated Selective Transport of Rare Earth Elements through Functionalized Graphene Nanopores

Rare earth elements (REEs) are indispensable for modern technologies, including electronics, manufacturing, and renewable energy systems. However, due to the similar physicochemical properties, the separation of REEs remains challenging. Here, we investigate the transport of REE ions through functionalized graphene nanopores using molecular dynamics simulations. Remarkably, -COO^--functionalized nanopores exhibit high selectivity for middle REEs over heavy REEs, with Sm³⁺/Y³⁺ and Eu³⁺/Y³⁺ selectivities of 66 and 75, respectively. Our simulations reveal an ion-pairing-mediated transport mechanism: REEs are initially captured by functional groups and paired with counterions, ultimately forming ion pairs that kinetically govern transmission. The strong affinity of Y³⁺ for the functional groups increases the proportion and lifetime of ion pairs, thereby promoting hindrance to transport. Notably, this effect is augmented by functional group density and nanoconfinement that synergistically dictate selectivity among REEs. Our work provides microscopic insights into the affinity-induced ion-pairing-mediated mechanism with which to conceive graphene-based REE ion separation.