Metal Detachments from (Hydr)Oxide Mineral Surfaces. A Molecular View

Geochemists strive to understand the reactions that occur in mineral micropores at a molecular scale so that predictions about rates and extents of reactions are possible over long periods of time. Our approach to understanding reactions at surfaces and micropores is to use the reactivity of dissolved species as a guide because analogues of many structural groups at the mineral surface can be found in well-chosen dissolved complexes. In this contribution, we discuss the ways in which the rates of dissociation of metal-oxygen bonds in these dissolved complexes are enhanced by protonations, ligand substitutions and deprotonations. Our approach is multifaceted in that we employ molecular-orbital calculations, NMR spectroscopy, and bulk dissolution experiments to try and constrain these classes of reactions and to understand how they can affect mineral surfaces. This work is relevant to microporous solids because so many of the geochemically important reactions that affect the rates of radioactive waste migration come down to a series of ligand exchanges where molecules in the innercoordination sphere of the dissolved contaminant are replaced by functional groups at the mineral surface. Even the dissolution of an oxide mineral can be viewed as a series of ligand exchanges where bridging oxygens and hydroxides that link a metal to the bulk mineral structure are replaced by terminal ligands, such as water molecules, at migrating monomolecular steps. In this chapter we examine how ligand substitutions can affect these rates and how the surface of a mineral can act as a ligand.