Quantum chemical study of inner-sphere complexes of trivalent lanthanide and actinide ions on the corundum (0001) surface

Sorption of trivalent metal ions onto mineral surfaces is of special relevance in the safety assessment of nuclear waste disposal. In the present quantum chemical study we mainly focused on understanding the interaction of trivalent metal ions (La3+, Eu3+ and Cm3+) with the corundum (0001) the surface. We studied how the structure of the inner-sphere complex at the corundum (0001) surface depends on the deprotonation of the surface and give a prediction for the most likely structure of the inner-sphere complex (bi-, tri- or tetradentate). We approached this question using a cluster model for the surface. By deprotonating the cluster we mimicked a chemical environment at pH values above the point of zero charge. In a first step, we tested the accuracy of Density Functional Theory calculations with the BP86 functional and various basis sets by comparing them with Møller-Plesset perturbation theory of second order on a small chemically similar test system. This is followed by a series of calculations on a large and realistic cluster which is an extended model for the formation of the inner-sphere complex at the corundum (0001) surface. Our calculations predict the highest stability for a species with six water molecules remaining in the first coordination sphere of the metal ions and forming an inner-sphere surface complex attached to three surface oxygen atoms. The formation of the inner-sphere complexes is even more favoured when the coordination takes place via one or two deprotonated surface oxygen atoms.