A F-19 NMR Probe of Mineral Surface Reactivity

该研究项目将研究羟基氧化铝矿物表面的分子水平结构及其与水的基本反应性。 这项工作的主要目标是开发一种光谱技术，用于表征矿物表面的活性表面位点的类型及其浓度。 羟基氧化铝在土壤和酸性岩石排水区中很常见，并为粘土矿物中的反应位点提供了一个简化的模型系统。 实验方法是在仔细控制的条件下使矿物表面与溶解的氟化物反应，然后使用F-19 NMR光谱法确定表面上产生的氟化物分布。 以前的工作具有类似的结构的大溶解分子表明，氟离子取代氧在终端和桥接型表面网站，和F？19 NMR光谱可以分辨这些不同的配位环境。 如果成功的话，这种技术可以应用于研究其他非均匀无序固体的表面。 进一步的时间分辨实验将测试的假设，即初始氟化物取代发生在终端网站，然后迁移到晶体边缘和基底表面上的桥接网站在较长的反应时间。 这些站点之间的氟化物迁移的时间尺度将提供估计的基本反应的表面氧。 使用NMR动力学技术对溶解的Al-羟基氧化物纳米团簇进行的其他实验将试图确定氟化物取代反应的详细机理途径。 本研究结果将有助于更深入地了解矿物/流体系统的体相地球化学行为的基本过程。 在近地表地球化学环境中，矿物表面的反应控制着土壤矿物吸附和隔离污染物的能力。 矿物表面结构和对污染物的反应性的显著变化可以通过与水和溶解成分反应期间的表面重排而发生。 本研究中开发的技术可以在分子水平上量化这些变化。

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.