Ion solvation as a predictor of lanthanide adsorption structures and energetics in alumina nanopores

Adsorption reactions at solid-water interfaces define elemental fate and transport and enable contaminant clean-up, water purification, and chemical separations. For nanoparticles and nanopores, nanoconfinement may lead to unexpected and hard-to-predict products and energetics of adsorption, compared to analogous unconfined surfaces. Here we use X-ray absorption fine structure spectroscopy and operando flow microcalorimetry to determine nanoconfinement effects on the energetics and local coordination environment of trivalent lanthanides adsorbed on Al₂O₃ surfaces. We show that the nanoconfinement effects on adsorption become more pronounced as the hydration free energy, ΔG_hydr, of a lanthanide decreases. Neodymium (Nd³⁺) has the least exothermic ΔG_hydr (-3336 kJ·mol^-1) and forms mostly outer-sphere complexes on unconfined Al₂O₃ surfaces but shifts to inner-sphere complexes within the 4 nm Al₂O₃ pores. Lutetium (Lu³⁺) has the most exothermic ΔG_hydr (-3589 kJ·mol^-1) and forms inner-sphere adsorption complexes regardless of whether Al₂O₃ surfaces are nanoconfined. Importantly, the energetics of adsorption is exothermic in nanopores only, and becomes endothermic with increasing surface coverage. Changes to the energetics and products of adsorption in nanopores are ion-specific, even within chemically similar trivalent lanthanide series, and can be predicted by considering the hydration energies of adsorbing ions.