Collaborative research: A multi-method approach to determine the role of semiconducting oxide and sulfide surfaces in catalyzing As, Cr, and Se redox reactions

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Technical description.Redox reactions are the driving force behind the mobilization and geochemical (and biogeochemical) cycling of As, Cr, and Se at the Earth's surface. The kinetics of these reactions are known to be catalyzed by sulfide and oxide mineral surfaces based on the development of empirically derived rate laws from macroscale experiments, but the actual reaction mechanisms (occurring at the nanoscale) are far from being understood. Previous studies by the Principle Investigators have demonstrated the importance of processes such as proximity effects, spintransitions, and oxygen dissociation to control the rate of redox reactions. This project will attempt to quantify these atomic scale processes in the context of the known rate laws, thereby linking the macroscale observables with the nanoscale processes that may be rate limiting.The strength of the U. of M. and Dartmouth College approach is their unique combination of molecular simulations (quantum mechanical to account for charge and electron spin transfer), electrochemical methods, and surface probe techniques to describe surface-mediated redox mechanisms and to resolve individual rate-determining steps. Molecular simulations will predict possible activated states that can occur between co-adsorbates on the surface and give insight into the reaction mechanism itself, especially steps that may be rate limiting, such as spin transfer, ligand reorganization, bond breaking, or mass transport (to name a few). Microelectrode techniques that have not traditionally been applied to study environmental reactions will be used to rapidly evaluate surface-specific mechanisms and kinetics in multivariable space. Finally, redox-dependent scanning probe microscopy will allow to observe these processes in situ.This study is a necessary "transition state" for the development of more comprehensive kinetic redox models in the future that will include bacterially-mediated redox processes, biomineralization, and the role of sulfide and oxide mineral surfaces as templates for the formation of complex organic molecules (i.e., precursors for origin of life).Broader significance and importance.Answering the aforementioned questions has profound implications for understanding the geochemical cycling of toxic elements and, maybe most importantly in this regard, provides a necessary foundation for future investigations on microbe-mineral interfaces. The results of this study will be important to the engineer developing permeable reactive barriers to immobilize toxic elements or to the geoscientist developing predictive models to describe the mobility of these species in the near-surface environment. The potential implications for technology development are widespread, including heterogeneous catalysts, chemical sensors, anti-corrosion processes, and photovoltaics, to name a few.