Electron Transfer Mediators at Environmental Interfaces

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With this award, the Environmental Chemical Sciences Program in the Division of Chemistry is funding Professor Paul G. Tratnyek of the Oregon Health & Science University to investigate fundamental and applied aspects of how substances termed "electron shuttles" mediate redox reactions at solid-solution surfaces in the environment. Redox (oxidation-reduction) reactions are integral to many of the most important aspects of environmental chemistry, ranging from elemental cycling, to microbial fuel cells, to contaminant fate and remediation. Many of these redox processes are known to be strongly influenced by natural and artificial electron shuttles, but the role of interfaces in electron shuttle effects is hard to characterize and poorly understood. In additional to the laboratory research performed by graduate students, undergraduates, and high school interns, this project includes significant outreach activity with workshops for water industry professionals run by Clackamas Community College. One goal of these workshops is to better understand and utilize redox shuttle effects in the protection of safe drinking water.The approach of this project is to use electrodes as model systems for environmental interfaces. Electrodes are modified in various ways with electron shuttle compounds followed by measurement of redox processes using electrochemical instrumentation. The project scope includes different types of electrode materials (e.g., noble metals vs. iron oxide minerals), shuttle-electrode interactions (e.g., bonded vs. non-bonded), and shuttle types (quinones, flavins, porphyrins, siderophores, etc.). Interfacial effects on ETM properties will be studied using electrode-based model systems that allow systematic characterization of ETM-interface structure and reactivity. Electrode model systems will be used to represent three types of scenarios: (i) solution phase ETM effects only, vs. ETM-interfaceinteractions that are (ii) non-specific and non-bonded (i.e., ?outer-sphere?) or (iii) specific and bonded (i.e., ?inner sphere?). The surfaces will include electrodes made of glass carbon and gold (to isolate outer and inner-sphere interactions, respectively) and iron oxides grown on metallic iron electrodes (to represent environmental mineral surfaces). ETMs will be selected from four classes of environmentally important redox-active organics: quinones, flavins, porphyrins, and siderophores. Each system will be characterized electrochemically, primarily by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Complementary spectroscopic methods will be applied as needed: including UV-vis, FTIR, surface plasmon resonance (SPR) spectroscopy. Quantitative results (potential, current, impedance) will be used to derive properties of interfacial ETMs and these will be compared to values determined for solution-phase ETMs using various methods (experimental and computational). The overall results will be used to assess the overall significance of interfacial effects on environmental redox properties.