Interplay of physically different properties leading to challenges in separating lanthanide cations – an ab initio molecular dynamics and experimental study

Abstract

Lanthanide elements have well-documented similarities in their chemical behavior, which make the valuable trivalent lanthanide cations (Ln³⁺) particularly difficult to separate from each other in water. In this work, we apply ab initio molecular dynamics simulations to compare the free energies (ΔG_ads) associated with the adsorption of lanthanide cations to silica surfaces at a pH condition where SiO⁻ groups are present. The predicted ΔG_ads for lutetium (Lu³⁺) and europium (Eu³⁺) are similar within statistical uncertainties; this is in qualitative agreement with our batch adsorption measurements on silica. This finding is remarkable because the two cations exhibit hydration free energies (ΔG_hyd) that differ by >2 eV, different hydration numbers, and different hydrolysis behavior far from silica surfaces. We observe that the similarity in Lu³⁺ and Eu³⁺ ΔG_ads is the result of a delicate cancellation between the difference in Eu³⁺ and Lu³⁺ hydration (ΔG_hyd), and their difference in binding energies to silica. We propose that disrupting this cancellation at the two end points, either for adsorbed or completely desorbed lanthanides (e.g., via nanoconfinment or mixed solvents), will lead to effective Ln³⁺ separation.