Collaborative Research: Effects of Iron Redox Processes on Smectite Crystal Structures and Surface Chemistry

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StuckiEAR-0126308 Clays, which are ubiquitous in natural sediments, soils, and other geologic formations, affect the fate of such chemicals as pesticides, organic contaminants, heavy metals, and plant nutrients in the environment. This effect is strongly correlated with the surface chemistry of the clay minerals, which is influenced by the charge or oxidation state of iron (Fe) in their crystal structures. The long-range goal of our work is to understand the role of iron on these surface reactions, and the specific objective of this proposed study is to identify the underlying cause(s) for the effects of structural Fe oxidation state on clay surface properties, and to characterize the nature of the resulting interactions with surface species such as water, metals, and organic compounds. The central hypothesis for the proposed research is that reduction of structural Fe by either biotic (bacterial reduction) or abiotic (chemical reduction) means, not only yields a new redox potential and electrostatic charge at the clay surface, but invokes in situ changes in crystal site occupancy of structural Fe and introduces structural defects, which, in turn, further alter the clay surface chemistry. We plan to test our central hypothesis and accomplish the overall objective of this proposed study by pursuing the following four specific aims: (a) Determine the site occupancy of structural Fe in mixed Al-Fe-Mg dioctahedral smectite clay minerals before and after Fe reduction; (b) Identify the causal relationship between changes in Fe oxidation state and surface chemistry; (c) Determine the reversibility of redox processes; and (d) Characterize the relative effects of bacteria versus inorganic reducing agents on clay properties. The fundamental understanding to be gained during this study will provide a powerful tool for using redox behavior to predict and control chemical reactions at clay surfaces.