THE MECHANISM OF DISSOLUTION OF OXIDE MINERALS

THE exchange of metal ions between an oxide mineral surface and water occurs in a wide range of processes, including corrosion(1), the break down, of inhaled dusts(2,3), soil formation(4) and the cycling of toxi c substances in the environment(5). In studies of the mechanisms of di ssolution, the measured rate-law order with respect to protons(6-15) c annot be reconciled with the number of protons needed to form any reas onable assumed activated complex. Here we suggest that this discrepanc y can be avoided if one takes into account the number of protonation a nd deprotonation steps leading to detachment of the hydrated metal ion . We show that the experimental proton rate order reflects a net balan ce of protons removed and attached in these steps. Our mechanism expla ins why the rate order generally coincides with the metal valence(8,9, 11,12,16-18), and why there is a similarity between rates of water lig and liability in dissolved complexes and rates of mineral dissolution( 19-22) and metal desorption(23). It eliminates the need to invoke cata lysis by protons, and establishes a close consistency between reaction s at surfaces and (better understood) ligand-exchange reactions in sol ution.