Oxygen Exchange Rates At Aluminum Polynuclear Complexes: Models For Mineral Surfaces

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Casey and PhillipsEAR-0101246The main goal of this project is to determine rate laws for exchange of individual oxygens between structural sites in large clay-like aluminum complexes and water. The aluminum nanoclusters are chosen to have structural elements in common with soil mineral surfaces and a similar surface charge density of protons. Unlike bulk studies of mineral reactions with fluid, our experiments yield information about rate laws for exchange of individual oxygens with the bulk solution at a scale that is suitable for computer simulation. This molecular-scale research requires reconsideration of much current thinking about mechanisms for mineral reaction in water. For example: 1) The important oxygens for mineral dissolution are apparently highly coordinated oxygens deeper in the molecule. 2) Proton transfer to the bridging hydroxyls, which precedes exchange of the oxygens, commonly takes place internally and not by proton uptake from the solution. 3) Dissociation of a surface-bound water molecule controls the rates of many adsorption reactions, yet the rates at individual sites are largely unknown. We find that these solvolysis rates for large polyoxocations fall into the same range as dissolved aluminum monomers, suggesting that the rates for edges of clays and protonated aluminum hydroxide minerals are similar. 4) The rates of oxygen exchange are highly sensitive to metal substitutions, in ways that could not have been anticipated from bulk studies or by changes in from bond lengths determined by X-ray crystallography.