Time-Resolved Diffraction Studies of Aqueous Cation Exchange and Hydrothermal Synthesis of Metal Oxide Clay Minerals

氧化锰矿物具有层状和隧道状的多种晶体结构，可增强与环境水系统进行离子交换的能力。由于以下几个原因，这些相在控制土壤地球化学方面特别重要：它们以纳米晶的形式大量存在于土壤和表层环境中的沉积物颗粒上。这些相的细粒尺寸与极高的比表面积有关，这增加了矿物交换场所的可及性。此外，四价锰很容易被还原为三价和二价态，而锰酸盐和锰酸盐积极地参与氧化还原反应。因此，锰氧化物是阴离子和阳离子(包括氢)的主要陷阱，它们对地下水化学的影响远远超出了它们在自然环境中的浓度。这项提议的研究人员利用新的高分辨率结晶学技术研究了金属氧化物中的脱水和阳离子交换过程，他们希望在未来三年内扩大这些研究的范围。到目前为止，他们的工作已经为新的锰氧化物结构提供了解决方案，并且作者使用了时间分辨的同步辐射X射线粉末衍射来监测伴随着转变到高温脱水状态的结构变化。此外，他们还记录了随着水交换反应的进行，一个阳离子被另一个阳离子取代时发生的连续原子位移。为了更准确地模拟氧化锰与自然流体的相互作用，他们将进行一系列涉及多个溶解阳离子物种的竞争性实验。P.I.S还将研究锰氧化物结合水溶液中有机分子的能力，他们希望首次证明这些分子在锰氧化物有机粘土中的配置。此外，研究人员还将使用时间分辨X射线衍射法进行涉及锰、钛和铁氧化物的水热合成实验。常规的静态分析很容易忽略瞬时中间相，而在氧化锰晶型从层到隧道的水热反应过程中，P.I.S已经捕捉到了前所未见的过渡结构。这项研究将阐明氧化锰层和隧道结构从受污染的地下水中去除有害金属的能力。探索水相体系中锰和其他金属氧化物的水热沉淀可能为材料合成提供新的途径，并加深对自然环境中结晶过程的理解。这些好处将补充调查人员的积极外展努力，他们积极参与各自机构的博物馆项目。

Manganese oxide minerals exhibit a variety of crystal structures with layer and tunnel topologies that offer enhanced capabilities for ion exchange with ambient water systems. These phases are particularly significant in controlling soil geochemistry for several reasons: They occur abundantly as nanocrystalline coatings on soil and sediment particles in surficial environments. The fine grain sizes of these phases are associated with extremely high surface areas, which enhance the accessibility of the mineral exchange sites. In addition, tetravalent Mn is readily reduced to the trivalent and divalent states, and phyllomanganates and tectomanganates actively engage in oxidation-reduction reactions. As a result, Mn oxides serve as major traps for anions and cations (including hydrogen), and they influence groundwater chemistry far out of proportion to their concentrations in natural environments.The investigators of this proposal have studied dehydration and cation exchange processes in metal oxyhydroxides using novel high-resolution crystallographic techniques, and they hope to expand on these studies over the next three years. Their work to date has provided solutions to new Mn oxide structures, and the authors have used time-resolved synchrotron X-ray powder diffraction to monitor the structural changes that accompany transformation to high-temperature dehydrated states. In addition, they have documented the continuous atomic displacements that occur during the substitution of one cation for another as aqueous exchange reactions proceed.In order to model the interactions of Mn oxides with natural fluids more accurately, they will perform a series of competitive experiments involving multiple dissolved cationic species. The P.I.'s also will examine the ability of Mn oxides to incorporate organic molecules from aqueous solutions, and they hope to document the disposition of these molecules in Mn oxide organoclays for the first time. In addition, the investigators will perform hydrothermal synthesis experiments involving Mn, Ti, and Fe oxides using time-resolved XRD. Whereas a conventional static analysis of final run products can easily overlook transient intermediate phases, the P.I.'s already have captured transitional structures not seen before during the hydrothermal reaction of Mn oxide polymorphs from layer to tunnel topologies.Broader Impacts. The proposed research will elucidate the capacity of Mn oxide layer and tunnel structures to remove undesirable metals from contaminated groundwaters.The exploration of hydrothermal precipitation of Mn and other metal oxides in aqueous systems may offer new routes to materials synthesis and a deeper understanding of crystallization processes in the natural environment. These benefits will complement the vigorous outreach efforts of the investigators, who are actively involved in museum programs at their respective institutions.