Reaction Kinetics in Hydrous Environments

The arguments treated in the two preceding sections were developed in terms of simple equilibrium thermodynamics. The weathering of rocks at the earth s surface by the chemical action of aqueous solutions, and the complex water-rock interaction phenomena taking place in the upper crust, are irreversible processes that must be investigated from a kinetic viewpoint. As already outlined in section 2.12, the kinetic and equilibrium approaches are mutually compatible, both being based on firm chemical-physical principles, and have a common boundary represented by the steady state condition (cf eq. 2.111). 

The fundamentals of the kinetic approach to water-rock interaction are constituted by the quantitative assessment of reaction kinetics for mineral hydrolysis and precipitation. These may be expressed in terms of rates of change in molality 

The rate of a generic reaction j is represented by its absolute velocity Vj, which may be obtained from experiments, provided that the mass of solvent is held constant throughout the experiment and no additional homogeneous or heterogeneous reactions concur to modify the molality of the ion in solution (Delany et al., 1986)  

Using a molar concentration notation instead of molahty (c, = nj V/, cf appendix 1), equation 8.264 is translated into 

When more than one reaction affects the mass of the ion i in solution (e.g., dissolution and/or precipitation of heterogeneous solid assemblages homogeneous reactions in solution), the overall change in mass of component i is given by the summation of individual absolute reaction velocities Vj, multiplied by their respective stoichiometric reaction coefficients  

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