Aqueous Surface Chemistry and Corrosion of Minerals
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Aqueous Surface Chemistry and Corrosion of Minerals
•Be-OH° + H 3 O + = *-Be-OH2+ + H2O. (3) Likewise, adsorption of hydroxyl ions can form negatively charged surface sites: •Be-OH° + O H - = •BeCT + H2O.
W.H. Casey, C. Eggleston, P.A. Johnsson, H.R. Westrich, and M.F. Hochella Jr. Introduction Nature is generous with complexity. The number of thermodynamic variables necessary to describe even simple stream chemistry can easily number to a hundred. Nevertheless, research on Earth materials remains exciting because of the vastness of geologic time and the huge scale of global processes. For example, even simple ion-exchange experiments have profound implications when considered in the context of global cycling of elements. Sodium exchange from seawater onto the 1.83 X 1016 g of river-borne clays removes 2030% of the yearly sodium addition to the ocean. 1 Research on Earth materials, although complex, is rewarding through the scale of the potential result. The surface chemistry of minerals is important for understanding natural mineral transformations and also because surface reactions help control the migration and degradation rates of pollutants in natural waters. These pollutants range from organic herbicides and pesticides, which leak past reactive soil horizons into groundwaters, to acid rain and heavy-metal leaching from mine tailings, sewage sludge, or coal fly ash. The importance of characterizing mineral surface chemistry is clear when one considers that 0.5 to 2% of usable groundwater in the United States is thought to be contaminated.2 This article reviews some simple surface chemistry of oxide and silicate minerals in water. We focus on the kinetics of mineral corrosion because this subject is interesting to both geochemists and materials scientists. The surface properties that make some solid oxides relatively inert to acid corrosion, for example, are also manifested
MRS BULLETIN/MAY 1992
in the rates of natural mineral weathering. The first part of the article discusses the corrosion kinetics of minerals. The second part introduces particularly fruitful methods of analyzing mineral surfaces.
The Surface Chemistry of Oxide and Silicate Minerals Acid-Base Chemistry Discussion of aqueous surface geochemistry usually starts with the acid-base reactions on a mineral, which create a charged surface in water. This article begins at an even more basic level by examining the add-base chemistry of dissolved metals. Consider, for example, the hydrolysis equilibrium between beryllium ions and the beryllium monohydroxide species in solution: Be+2 + H 2 O = BeOH+
(1)
This conventional formalism blurs the generic link between metal hydrolysis and acid-base chemistry. The hydrolysis reaction is more accurately written as competition between solvent waters and hydration waters for a hydrogen ion: Be(H2O)4+2 + H 2 O = Be(H2O)3(OH)+ + H3O+, (2) which proceeds extensively from left to right at pH > 6. An oxide or silicate mineral also has water molecules and hydroxyl ions coordinated to its surface. The hydroxyl gro
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