Casey 9626553 No processes are more essential to low-temperature geochemistry than those leading to mass transfer among distinct solute species, and between minerals and adjacent aqueous fluids. We propose to continue research that organizes dissolution reactions into a predictive framework. The approach is to demonstrate quantitative similarity between the rate coefficients for metal removal from a surface and the better-characterized mechanisms of ligand exchange around dissolved complexes (Casey, 1991; Casey and Westrich, 1992; Ludwig et al., 1995a, b). The quantitative similarity between these processes allows establishment of linear-free-energy relations (LFER) to predict rate coefficients. In suitable cases we can identify possible rate-controlling elementary step reactions at mineral surfaces, such as the rates of water movement in the activated complexes, which we now know to be important (e.g., Ludwig et al., 1995a). New research will expand the predictive framework to examine the effect of organic ligands on rates of metal transfer. We test the hypothesis that ligand-promoted rate coefficients (KL) for small-molecular-weight complexes are predictable using either the rates of solvent motion in the corresponding dissolved metal-ligand complex or the equilibrium constants that describe metal-ligand speciation in solution (see Ludwig et al., 1995a,b). Key to success is acquisition of high-quality rate data on oxide minerals that differ considerably in metal-oxygen bond strengths. Also critical are: (I) choice of ligands that form a single dominant surface complex stoichiometry in the dissolution reaction, (ii) in understanding the role of adsorbed protons in reaction, and (iii) in characterizing the degradation products. This research addresses the common criticism of equilibrium models of contaminant geochemistry (e.g., Oreskes et al., 1994) that the important reactions are slow and the rate coefficients unknown. Systematic study of a few ligands may serve to bound the re activities of a wide range of compounds because steric crowding must ultimately limit the size, rigidity, and number of ligands that can coordinate to a surface metal.
