Reactivity at the mineral-water interface: dissolution and inhibition

Abstract

The reactivity of the mineral-water interface is often interpreted with the help of models used in electrochemistry, in solution coordination chemistry and in crystallography. Progress in understanding mechanisms of growth and dissolution of crystals and the inhibition of these processes depends on a better integration of these models. It is shown that dissolution can be explained in terms of a ligand exchange process; simplified rate laws for proton- and ligand-protonated dissolution rates, being related to surface bound protons and ligands, respectively, can be derived. Further refinement in interpreting surface reactivity comes from an appreciation of the molecular structures at the mineral water interface; here significant advances have been made by X-ray absorption spectroscopy, especially EXAFS (Extended X-ray Absorption Fine Structure spectroscopy), which permit distinction to be made between outer-sphere and inner-sphere surface complexes, and in many cases to determine the structure of the surface species at different crystallographic planes (e.g., bi-nuclear or mono-nuclear linkage of ligands on metal ions to surface metal centers). Such information coupled with solution-chemical studies on the extent of adsorption can provide new insight into the mechanisms of dissolution reactions and their inhibition and surface poisoning. A few experimental results are given to exemplify the factors that enhance and inhibit the non-reductive (EDTA) and reductive dissolution (by H 2S) of Fe(III)(hydr)oxides. Binuclear surface complexes by multivalent cations and by oxoanions, such as phosphate, arsenate and borate, are believed to be efficient inhibitors for oxide dissolution because they form bi- or multinuclear innersphere surface complexes that can bridge two or more metal centers in the surface lattice; the simultaneous removal of such bi- or multinuclear surface complexes from the surface is energetically unfavorable. Proton and ligand promoted dissolution reactions and their inhibition by oxoanions and bi- or multinuclear surface complexes are not only relevant in geochemistry (weathering, soil-formation, transfer of elements and pollutants) but also in metallic corrosion, formation and breakdown of passive oxide films.