Antonio C. Lasaga

A model for crystal dissolution

A comprehensive dissolution rate theory that integrates individual surface reactions into an overall rate is developed. The dissolution theory is based on the movement of dissolution stepwaves stemming from surface defects. The net bulk rate associated with dissolution stepwaves arises quite naturally from the equations describing the spreading of the train of steps from surface defects. The overall rate can be shown to approach a simple linear rate or transition-state theory-like equation far from equilibrium. However, one of the most important results is the strong nonlinear decrease in the rate as equilibrium conditions are approached, as is the case in most natural processes. The model is validated by extensive Monte Carlo simulations of crystal dissolution, which include a detailed treatment of surface defect energetics, adsorption, surface diffusion, transport of elements from solution, and the bonding dependence of detachment processes from the surface. Monte Carlo results show the generation of dissolution stepwaves and the nonlinear dependence of the overall rate on the saturation state. The final rate equations are consistent with both the far-from-equilibrium experimental work and several recent studies that approached equilibrium. The decrease in the rate as equilibrium is approached has far-reaching implications for both man-made problems ( e.g. , radioactive waste disposal, pollution, etc .) and natural processes from ground-water to metamorphic systems.

Mineralogical approaches to fundamental crystal dissolution kinetics

Abstract We introduce a general kinetic model for crystal dissolution that explicitly tracks all the various atoms in the crystal structure as part of the reaction mechanism. This model will be used in this and subsequent articles to develop a theory for the treatment of experimental and field water-rock kinetic data. The model is based on a many-body reaction mechanism. It is built from both elementary reactions, i.e., bond-breaking and bond-forming, and basic reactions, i.e., dissolution of surface units, adsorption and incorporation of solution units, and mobility of units at the crystal surface. The full crystal structure is used to calculate the interactions of neighboring atoms as well as possible defects of the crystal lattice in the model. This approach is different from models based on either molecular precursor complexes or adsorption. We analyze several fundamental concepts such as activation energy, surface free energy, the solubility product, inhibition/catalysis, and saturation-state dependence using our approach. In addition, surface features such as nucleation, steps, and defects are presented and put in a quantitative basis in this paper. The resulting kinetic framework can handle explicitly any crystal structure, treating the actual bonding and position of all atoms within a given surface orientation in the structure. Investigation of the properties of such a general kinetic model leads to new relations between the activation energy and the net energy changes in the hydrolyses reactions, between surface free energy and activation energies and between inhibition and the statistical mechanics of kink sites. The kinetic model can actually account for the emergence of a solubility product from a reaction mechanism involving independent kinetics for the different species using steady-state concepts on the behavior of surface sites. The possible ΔG dependence of the overall rate is studied with the general approach. Isotachs are used to exhibit the interplay of ΔG and inhibition within a simple AB mineral structure. The crystal-based reaction mechanism not only leads to a unified explanation of many observed water-rock features but also produce a series of modifications of kinetic results not fully understood before.

Mineralogical approaches to fundamental crystal dissolution kinetics - Dissolution of an A3B structure

Incorporating mineral structures and interpreting the wide array of water-rock kinetic field and experimental data, requires abandoning molecule-based adsorption phenomenological models and the development and analysis of a general kinetic theory that incorporates all the basic processes occurring in the destruction of a crystal structure. The current paper extends our prior work to include an analysis of the kinetic behavior of an A 3 B-type crystal structure. More complex kink types and two different bonding energetics for A-A and A-B bonds add new kinetic phenomena during dissolution. The rate laws for both perfect and stepped surfaces are contrasted and compared to simple TST-like rate laws. Unexpected changes in mechanism as a function of saturation state are found and studied. Incongruent dissolution is analyzed as a function of time and saturation state. The appropriateness of additive rate laws for various reaction mechanisms is investigated using the mechanism change from step-control to hole-nucleation control for A 3 B structures. The onset of inhibition, predicted by the statistical mechanical treatment of the kinetics, is observed as conditions far from equilibrium are reached. Inhibition is quantified with the use of isotachs and it is seen to depend systematically on the average surface bonding properties. In particular, a quantification of the number of B vacancies with saturation state is carried out and compared with actual results from the full kinetic model. The analysis of the inhibition mechanism itself and its onset are then applied to analyze recent feldspar dissolution data.