We will continue our research into the mechanisms by which mineral surfaces interact with an aqueous solution during corrosion. In particular, we will determine the enthalpy contribution to the activation energy (Eexp) for mineral dissolution that arises from the rapid acid-base reactions which precede the rate-controlling step in dissolution. These acid-base reactions impart a distinct pH-dependence of the overall reaction. In research funded under EAR-910583, we showed that the enthalpy arising from acid-base reactions is the largest contribution to the Eexp (Casey and Sposito, 1992). This contribution varies considerably with pH and temperature, and is poorly constrained for silicate minerals. This research is important because atomistic models of the reaction energetics , such as molecular-orbital calculations (e.g., Casey et al., 1990), employ Eexp values as a gauge of the model accuracy. We also will indirectly probe the elementary step reaction controlling dissolution of simple minerals containing first-row transition metals. We predict that the affect of certain rate- enhancing ligands on the dissolution rates of these minerals will differ systematically and predictable with the delectronic structure of bonds to oxygen. This prediction arises from differences in the activation entropies and activation volumes of solvent exchange reactions around corresponding dissolved metal complexes. In previous work (e.g., Casey, 1991; Casey and Westrich, 1992) we found a close correspondence between reactivity trends for solvent-exchange reactions and metal removal from a dissolving oxide or orthosilicate mineral surface.
