On the mechanisms of dissolution of bunsenite [NiO(s)] and other simple oxide minerals

Rates of proton-promoted dissolution of bunsenite [NiO(s)] are coupled to titrations for adsorbed protons at the dissolving surface, The results confirm the finding of G. Furrer and W. Stumm (GEO-chim. Cosmochim. Acta 50, 1847, 1986) that the rate order with respect to protons at pH < ZPC is given by the metal valence: rate orders are dose to 2 for NiO(s) and BeO(s) dissolution even though these minerals are structurally distinct. The rate-controlling step is hydration and detachment of a surface complex, and in acidic solutions this complex resembles the hydrated metal, We extend this model by speculating that the proton stoichiometry reflects the hydrolysis of surface species and varies with solution pH. This hypothesis leads tr, rate orders with respect to protons that are nonintegers and that decrease with increasing pH to the ZPC, This dissolution behavior will be observed for oxide solids containing metals that hydrolyze at pH values significantly below the ZPC, such as Al2O3, and may account for the low rate orders observed for complicated mixed-oxide solids. This model predicts that the reactivities of partly hydrolyzed surface complexes are greater than those of the fully hydrated metals, and therefore establishes close consistency between the processes of metal removal from a dissolving surface and ligand exchange around the corresponding metal in solution. (C) 1996 Academic Press, Inc.