The coordination chemistry of weathering: III. A generalization on the dissolution rates of minerals

Details The coordination chemistry of weathering: III. A generalization on the dissolution rates of minerals The coordination chemistry of weathering: III. A generalization on the dissolution rates of minerals

Aug 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52.1969w&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 52, Issue 8, pp.1969-1981

Mathematics

Probability

24

Scientific paper

A general rate law on the surface-controlled dissolution of oxides and silicates is discussed. Combining concepts of surface coordination chemistry with established models of lattice statistics and activated complex theory we propose a general rate law for the acid- and ligand-promoted dissolution of minerals: R = kx a , P j S where R is the proton- or ligand-promoted dissolution rate [moles m -2 s -1 ], k stands for the appropriate rate constant [s -1 ], x a denotes the mole fraction of dissolution active sites [-], P j represents the probability to find a specific site in the coordinative arrangement of the precursor complex [-] and S is the surface concentration of sites [mol m -2 ]. Surface complexes (surface chelates and metal proton complexes) are precursors in the rate-limiting detachment of a central metal ion from the surface into the solution. We develop a mechanistic model, which clarifies the pH-dependence of dissolution rates. First appropriate surface protonation isotherms are derived. The surface protonation equilibria of different minerals at constant ionic strength match a single Freundlich isotherm with a slope of ca . 0.2. This result explains the frequently reported fractional pH-dependence of dissolution rat`es. Based on the Bragg-Williams approximation a lattice statistical procedure is outlined which permits the calculation of the probability P j of the precursor complex. The consistency of reported experimental dissolution rates and activation energy is tested and, as a consequence, different possibilities to correlate reaction rates (kinetic rate constants [s -1 ] are not reported) or apparent activation energies with thermodynamic data in the form of free linear energy relations are explored. We postulate that the site energy (Madelung energy) of the most stable lattice constituent (generally a metal cation at a surface site) or the ion formation energy of the solid--characteristic of the free energy needed to break the essential bonds in the lattice--are suitable free energy parameters to be correlated with the dissolution rates (log R H at pH 5).

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