The Surface Structure and Reactivity of Mn-Oxides and Their Impact on As Transformation in the Environment: A Multi-Scale Approach

研究在土壤氧化还原化学中起重要作用的环境重要的锰氧化物的化学和物理性质。土壤科学界普遍认为，锰氧化物在土壤中以涂层和离散颗粒的形式存在，部分以纳米尺度存在。其丰富的氧化还原化学影响环境毒素的流动性和生物利用度，包括许多金属和类金属。研究将主要集中在氧化锰的块状水钠锰矿(MnO2)相和纳米MnOOH相在各种环境相关条件下的表面结构和化学反应活性。特别是，将使用一系列先进的表面光谱技术，包括衰减全反射红外光谱和基于同步加速器的光电子能谱，来绘制氧化锰表面反应的图像，例如在确定的土壤pH值范围内确定不同氧化态的相对表面浓度。这些氧化锰表面将通过与含砷氧阴离子的反应进行探测，以建立锰氧化物结构和反应活性的差异对As3向As5转化的控制作用，As3是环境中的主要解毒途径之一。虽然水钠锰矿存在于广泛的土壤环境中，是拟议研究的主要目标，但纳米锰氧化物也存在于环境中，并对当前的研究感兴趣。为了加深对纳米锰氧化物在土壤化学中可能发挥的作用的理解，研究人员将研究纳米MnOOH的反应性和电子结构随尺寸的变化。将制备粒径分布均匀的纳米MnOOH，并在不同pH的溶液中进行研究，AS氧化反应将再次用作反应活性的探针。研究项目的这一特定阶段不仅将发展对氧化锰的大小-反应关系的理解，而且总体上将有助于地质和土壤化学社区评估纳米化学在环境中的重要性的更广泛的努力。拟议活动产生的广泛影响拟议的研究具有重要的教育成分。首先，NSF资金将用于支持和培训特拉华大学的一名博士后助理和坦普尔大学的一名研究生。其次，由于这项研究具有很强的跨学科性质，参与该项目的研究人员的科学广度将受益于在不同土壤和表面化学领域拥有专业知识的小组之间不断交流思想和概念。这种合作符合跨学科研究理解复杂环境化学的更广泛需求。这些研究不仅将推进环境地球化学的前沿，而且还将提供有益于整个社会的污染物转化的重要预测信息。

ABSTRACTResearch is proposed that investigates the chemical and physical properties of the environmentally important oxides of manganese that play an important role in soil redox chemistry. It is well accepted in the soil science community that Mn-oxides exist as coatings and as discrete particles in soils, in part with nano-dimensions. Their rich redox chemistry affects the mobility and bioavailability of environmental toxins including many metals and metaloids. Research will be focused primarily on the surface structure and chemical reactivity of both the bulk birnessite (MnO2) phase of Mn-oxide and also a nano-MnOOH phase under a variety of environmentally relevant conditions. In particular, a selection of advanced surface spectroscopic techniques, including attenuated total reflection infrared and synchrotron-based photoelectron spectroscopy, will be used to develop a picture of the reacting Mn-oxide surface, such as determining the relative surface concentration of different oxidation states, over a range of established soil pH values. These Mn-oxide surfaces will then be probed via reaction with aqueous arsenic oxyanions to establish the control that differences in Mn-oxide structure and reactivity exert on the transformation of As3+ to As5+, one of the central As detoxification pathways in the environment.While birnessite, found in a wide range of soil environments, is a primary target of the proposed research, nanosized Mn-oxides also exist in the environment and are of interest in the current research. Toward developing an understanding of the role that nano-Mn-oxides might play in soil chemistry, research will investigate the reactivity and electronic structure of MnOOH nanoparticles as a function of size. Nano-MnOOH with homogeneous size distributions from 20 to 80 will be prepared and studied in solutions with varying pH and the As oxidation reaction will again be used as a probe for reactivity. This particular phase of the research project will not only develop an understanding of the size-reactivity relationship for Mn-oxide, but will in general contribute to the broader effort in geo- and soil chemical communities to evaluate the importance of nano-chemistry in the environment.Broader Impacts Resulting from the Proposed Activity The proposed study has a significant educational component. First, NSF funds will be used to support and train a postdoctoral associate at the University of Delaware and a graduate student at Temple University. Second, by virtue of this study being strongly interdisciplinary in nature, the scientific breadth of researchers in this project will benefit from the constant exchange of ideas and concepts between groups having expertise in diverse areas of soil and surface chemistry. This collaboration fits into the broader need for interdisciplinary studies to understand complex environmental chemistry. The studies will advance not only the frontiers of environmental geochemistry, but also provide important predictive information on contaminant transformations that will benefit society at large.