Interfacial Kinetics Through the Lens of Solution Chemistry: Hydrolytic Processes at Oxide Mineral Surfaces

Abstract

Mineral dissolution and growth can be understood, in part, by drawing analogies with reactions where a dissolved metal exchanges ligands in solution. The rates of dissolution are generally controlled by slow hydrolytic dissociation of surface complexes at monomolecular steps on the mineral surfaces. Rates of oxygen exchange around metals in dissolved metal-ligand complexes also involve hydrolysis. In both environments these rates depend on association to protons, or certain ligands, in the inner-coordination sphere of the metal. An enormous literature of useful data describes microscopic acid-base chemistry of oxide oligomers and the molecular controls on their condensation and dissociation.This literature may be extraordinarily useful in understanding oxide interfacial chemistry. It is presently unknown whether rate coefficients from the solution state can be directly transferred to surface reactions, but useful comparisons can be made to identify reactivity trends.Geochemistry is much simpler if one ignores the shallow Earth. At elevated temperatures and pressures, such as those in the deeper crust of the Earth, equilibrium thermodynamics gives meaningful predictions of solution chemistries and mineral assemblages. Many reactions in the shallow Earth, however, do not proceed to equilibrium and time is a critical variable. Fluids exist in pores that are interstitial to minerals and rates of cycling of organic matter, degradation of pesticides, metal mobilities, and the diagenesis of minerals are influenced by interactions between solutes and surfaces. Reactions at surfaces modulate the flow of toxicants and nutrients to, and from, the biosphere. 244

Oxide– and Silicate–Water Interfaces and Their Roles in Technology and the Environment

“…Kinetic studies are used to infer reaction mechanisms, useful in the geochemical context of oxide– and silicate–water interactions. , Kinetic studies at oxide– and silicate–water interfaces have involved isotope exchange techniques mostly in relation to weathering. ,, In the context of interfacial (de)­protonation and cation/anion adsorption reactions, pressure-jump experiments have been used to infer rate constants,. ,− The adsorbate concentration is typically in the millimolar range, and the method records changes in conductivity following a pressure change. The data is interpreted to derive a mechanistic rate law and includes one or two reaction steps with the appropriate rate constants for adsorption and desorption.…”

“…498,499 Kinetic studies at oxide− and silicate−water interfaces have involved isotope exchange techniques mostly in relation to weathering. 31,500,501 In the context of interfacial (de)protonation and cation/anion adsorption reactions, pressure-jump experiments have been used to infer rate constants,. 499,502−505 The adsorbate concentration is typically in the millimolar range, and the method records changes in conductivity following a pressure change.…”