Intellectual Merit. The principal investigators (PIs) of this proposal are seeking funds to extend the thrust of time‐resolved diffraction to three areas of critical importance to the cycling of metals in soils: 1) biomineralization; 2) nucleation and growth of oxyhydroxides; and 3) stable isotope fractionation during redox reactions. Recent developments in the design of environmental reaction cells and in the collection of X‐ray diffraction (XRD) data are proving transformative in allowing us to couple rate laws with the crystallographic mechanisms that govern the interactions of minerals with fluids and gases. Intensive Rietveld analyses of time‐resolved (TR) XRD data by the PIs have provided dynamic, atomic‐scale representations of the structural changes that occur when a layered Mn oxide (birnessite) exchanges one cationic species for another. This work confirms the necessity of imaging mineral reactions with high time resolution in order to capture intermediate reaction products that are overlooked by traditional approaches, particularly at the low temperatures and pressures that characterize the ?critical zone? enveloping Earth?s surface.
Over the next 3 years, the PIs propose to explore issues that have long lain outside the boundaries of static X‐ray diffraction. Following much experimentation, they have succeeded in designing a reaction cell in which the total membrane fraction of a common soil bacterium, Shewanella oneidensis, can catalyze the reduction of Mn in birnessite and bioprecipitate rhodochrosite under anoxic conditions. They will explore dissolution and precipitation using TR XRD to generate rate laws with respect to enzyme and electron donor concentrations. In addition, the PIs are using high‐temperature reaction cells to examine structural transitions in the TiO2 system. By following anatase and rutile nucleation and growth from nanoparticles to macroscopic crystals with TR XRD, they are extracting structural variations as a function of particle size. In combination with molecular modeling, these studies will test hypotheses that explain polymorphic stability reversals during crystal growth in terms of free energies of surface structures. Third, the PIs will correlate Cu isotopic fractionation with structural transitions that accompany Cu oxidation during the sequential transformation of chalcocite (Cu2S) to covellite (CuS). They hope to demonstrate that TR XRD is uniquely poised to tie fractionation processes to solid‐state transformations.
Broader Impacts. The PIs are translating the temporal element of their TR XRD studies into 3‐dimensional animations that dynamically illustrate the changes in atomic structure that occur when soil minerals react with synthetic groundwaters. These graphics will be incorporated into a Penn State museum exhibit sponsored by the Center for Environmental Kinetics Analysis. We are attempting to frame molecular scale chemistry as a solution to acid mine waste and contaminant metal migration ‐‐ problems that are well known to residents of Pennsylvania.
