A F-19 NMR Probe of Mineral Surface Reactivity

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EAR-0310200PhillipsThis research project will study the molecular-level structure of aluminum oxyhydroxide mineral surfaces and their fundamental reactivity with water. A primary goal of this work is to develop a spectroscopic technique for characterizing the mineral surface in terms of the types of reactive surface sites and their concentrations. Aluminum oxyhydroxides are common in soils and acid-rock drainage areas and provide a simplified model system for the reactive sites in clay minerals. The experimental approach is to react mineral surfaces with dissolved fluoride under carefully controlled conditions, and then determine the resulting fluoride distribution on the surface using F-19 NMR spectroscopy. Previous work on large dissolved molecules with similar structure show that fluoride ions substitute for oxygen in both terminal- and bridging-type surface sites, and that F?19 NMR spectroscopy can resolve these distinct coordination environments. If successful, this technique can be applied to study the surfaces of other heterogeneous disordered solids. Further time-resolved experiments will test the hypothesis that initial fluoride substitution occurs at terminal sites, followed by migration to bridging sites on crystal edges and basal surfaces at longer reaction times. The timescale for fluoride migration among these sites will provide estimates for the fundamental reactivity of the surface oxygens. Additional experiments on dissolved Al-oxyhydroxide nanoclusters, using NMR kinetic techniques, will attempt to determine detailed mechanistic pathways for the fluoride substitution reactions. The results of this study will contribute a deeper understanding of fundamental processes governing bulk geochemical behavior of mineral/fluid systems. Reactions at mineral surfaces control the ability of soil minerals to adsorb and sequester contaminants in near-surface geochemical environments. Significant changes in the mineral surface structure and reactivity toward contaminants can occur through surface rearrangement during reaction with water and dissolved constituents. Techniques developed in this study can allow these changes to be quantified at the molecular level.