Eggleston 9527031 Mineral dissolution and growth are basic forms of chemical communication between rocks and the hydro-, and bio-spheres (i.e., environments at the Earth's surface). This proposal describes a study that uses rate-relaxation kinetics and non steady-state experiments as a powerful new probe of surface chemical processes operating during dissolution and growth of oxide minerals. Conditions at the Earth's surface are variable. Chemical processes in this environment are subject to frequent transient and periodic perturbations and fluctuations of conditions; steady-state is rarely achieved. Our understanding of mineral dissolution and growth kinetics, in contrast is largely limited to steady-state. We know very little about rate-relaxation times (i.e., the time needed for transition from one steady-state to another), despite strong evidence that they exist. This imposes a fundamental limit on our ability to model reaction rates in nature. This study will test and constrain a theory of non-steady-state (NSS) dissolution and growth kinetics that addresses this limitation. The theoretical framework developed here to systematize NSS data combines surface complexation models (SCMs), absorption kinetics, and Burton-Cabrera-Frank (BCF) theory. This approach has direct bearing on several other long-standing problems: 1) relationship(s) between overall thermodynamic driving force and rate; 2) structure-reactivity correlation's for mineral surfaces; and 3) the role of rate-affecting adsorbed species. Importantly, it also creates a way to use new data on kinematics (step and kink motion and interaction) from in-situ scanning probe microscopy to constrain theory. NSS kinetics thus provides a unique opportunity to establish connections between microscopic surface processes and macroscopic kinetic behavior at both laboratory and field scales. Specific hypotheses tested, using simple (single component) Al, Fe, and Si oxide minerals (plus a few structural analogs), and later usi ng albite, are: Rate-relaxation times exist; i.e., in response to an abrupt change (e.g., in pH), dissolution or growth rates are expected to approach a new steady-state slowly relative to the abruptness of the change. This will be tested wet-chemically using specially designed flow-through reactors. The system response to square-wave and sine-wave variations of , e.g., pH, will be recorded. This "frequency-response" technique from chemical engineering provides valuable data on intermediate steps in reaction mechanisms. Rate-relaxation times are expected to correlate with H2O-replacement rates for corresponding aqueous ions (as do dissolution and adsorption rates). Rate-relaxation results from changes in the concentration and/or structure of adsorbed "nutrient" species (e.g., of Al on Al203). Equilibrium and kinetic data on such adsorption will be gained using : a) isotopic dilution, b) a novel use of FTIR spectroscopy, c) a new, high-resolution electrostatic atomic force microscopy (AFM) technique, and d) comparison of AFM-derived step kinematics data to the predictions of BCF-SCM theory discussed above. Iso-structural (corundum and goethite-structure oxides are used so that linear-free energy relationships can be employed to systematize kinetic data.
