Time-Resolved Diffraction Studies of Aqueous Cation Exchange and Hydrothermal Synthesis of Metal Oxide Clay Minerals

Manganese oxide minerals exhibit a variety of crystal structures with layer and tunnel topologies that offer enhanced capabilities for ion exchange with ambient water systems. These phases are particularly significant in controlling soil geochemistry for several reasons: They occur abundantly as nanocrystalline coatings on soil and sediment particles in surficial environments. The fine grain sizes of these phases are associated with extremely high surface areas, which enhance the accessibility of the mineral exchange sites. In addition, tetravalent Mn is readily reduced to the trivalent and divalent states, and phyllomanganates and tectomanganates actively engage in oxidation-reduction reactions. As a result, Mn oxides serve as major traps for anions and cations (including hydrogen), and they influence groundwater chemistry far out of proportion to their concentrations in natural environments.

The investigators of this proposal have studied dehydration and cation exchange processes in metal oxyhydroxides using novel high-resolution crystallographic techniques, and they hope to expand on these studies over the next three years. Their work to date has provided solutions to new Mn oxide structures, and the authors have used time-resolved synchrotron X-ray powder diffraction to monitor the structural changes that accompany transformation to high-temperature dehydrated states. In addition, they have documented the continuous atomic displacements that occur during the substitution of one cation for another as aqueous exchange reactions proceed.

In order to model the interactions of Mn oxides with natural fluids more accurately, they will perform a series of competitive experiments involving multiple dissolved cationic species. The P.I.'s also will examine the ability of Mn oxides to incorporate organic molecules from aqueous solutions, and they hope to document the disposition of these molecules in Mn oxide organoclays for the first time. In addition, the investigators will perform hydrothermal synthesis experiments involving Mn, Ti, and Fe oxides using time-resolved XRD. Whereas a conventional static analysis of final run products can easily overlook transient intermediate phases, the P.I.'s already have captured transitional structures not seen before during the hydrothermal reaction of Mn oxide polymorphs from layer to tunnel topologies.

Broader Impacts. The proposed research will elucidate the capacity of Mn oxide layer and tunnel structures to remove undesirable metals from contaminated groundwaters.

The exploration of hydrothermal precipitation of Mn and other metal oxides in aqueous systems may offer new routes to materials synthesis and a deeper understanding of crystallization processes in the natural environment. These benefits will complement the vigorous outreach efforts of the investigators, who are actively involved in museum programs at their respective institutions.