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

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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.