FUNDAMENTAL CONTROLS ON THE ENVIRONMENTAL REACTIVITY OF TRANSITION- AND HEAVY-METAL BACTERIOGENIC OXIDES

Transition- and heavy-metal oxides, abundant in natural waters and subsurface sediments, drive rich trace-element chemistries through sorption, dissolution, redox, heterogeneous catalytic, and photochemical reactions. Often, the production of these reactive natural materials is mediated by microorganisms, and in some systems biogenic oxides may dominate over abiotic oxides in volume and overall importance. Biogenic transition/heavy metal oxides are often nanoparticulate and can interact readily with their chemical environments via ongoing synthesis, aging, and dissolution/reprecipitation reactions. Through these processes, co-contaminant cations may be readily sorbed/ incorporated. The compositions and molecular-scale structures of biogenic transition/heavy metal oxides thus are influenced by ground water solute chemistry. As these factors may significantly moderate the Gibbs energy of the materials, this behavior is of significance as a moderator or predictor of the reactivity of the oxide materials. Understanding these structure/function relationships in bacteriogenic oxides is a fundamental prerequisite to understanding the roles of these reactive solid phases in the environment and in harnessing them for engineered applications such as in-situ waste forms for the stabilization of subsurface contaminants. This talk will discuss geochemical factors that control the environmental reactivity of bacteriogenic Mn and U oxides, with emphasis on molecular-scale structures, morphology, and the mechanisms by which they drive or moderate linked elemental cycles.