Barry R. Bickmore

Dinwoody Creek watershed water temperature and specific conductance data summer 2018

The relationship between bond valence and structural energy has never been fully explored, although several predictive models have assumed some simple relationship between the two. Some of these models relate energy only to bond valence, while others also take into account other factors, such as bond character. We examined periodic trends in bond dissociation energies as a function of their ionicity, covalency, and metallicity, defined in terms of the electronegativity values of the atoms involved. A statistical model was optimized to describe these trends, allowing us to generate rough bond energy vs. bond valence curves. The shapes of these curves vary dramatically as a function of bond character, and are strongly influenced by the lone-pair bond-weakening effect. The curve shapes can be used to rationalize several chemical trends, including the preferred structures of compounds with different bond types, the prevalence of peroxide and persulfide minerals, preferred bond lengths in oxides, and the pK_a values of (hydr)oxy-acids. The last is perhaps the most important, because some valence-based acidity models are in current use, despite the fact that some aspects of their rationale are unclear.

The recent controversy about the structure of liquid water pits a new model involving water molecules in relatively stable "rings-and-chains" structures against the standard model that posits water molecules in distorted tetrahedral coordination. Molecular dynamics (MD) simulations, both classical and ab initio, almost uniformly support the standard model, but because none of them can yet reproduce all of the anomalous properties of water, they leave room for doubt. We argue that it is possible to evaluate these simulations by testing them against their adherence to the bond-valence model, a well-known and quantitatively accurate empirical summary of the behavior of atoms in the bonded networks of inorganic solids. Here we use the results of ab initio MD simulations of ice, water, and several solvated aqueous species to show that the valence sum rule (the first axiom of the bond-valence model) is followed in both solid and liquid bond networks. We then test MD simulations of water, employing several popular potential models against this criterion and the experimental O-O RDF. It appears that most of those tested cannot satisfy both criteria well, except TIP4P, TIP4P/2005, and TIP5P. If the valence sum rule really can be applied to simulated liquid structures, then it follows that the bonding behaviors of atoms in liquids are in some ways identical to those in solids. We support this interpretation by showing that the simulations produce O-H center dot center dot center dot O geometries that are completely consistent with the range of geometries available in solids, and the distributions of instantaneous valence sums reaching the atoms in both the ice and liquid water simulations are essentially identical. Furthermore, we show that none of the extant asymmetric water potentials that produce "rings-and-chains" structures can satisfy our geometric criteria. Taken together, this is powerful evidence in favor of the standard distorted tetrahedral model of liquid water structure.

The Earth Science Storybook Project is being conducted with Brigham Young University's (BYU) Physical Science 110B (PS 110B) class, which is introductory geology for Elementary Education majors. Under the project, preservice elementary teachers in PS 110B are mentored by university geology faculty and geology graduate students as they author children's storybooks in earth science. The storybooks are then illustrated and made available to practicing teachers for classroom use. This is done by an interdisciplinary collaborative effort between the BYU Geology Department and the BYU Visual Arts Department, which is providing senior illustration students to illustrate the storybooks at a relatively low cost. The goal is to use the mentored writing experience as a means of improving geology content knowledge for preservice elementary teachers and fostering their positive attitudes and self-efficacy toward learning and teaching science. These goals are being assessed by (1) quantitative analysis of the PS 110B class exam scores of storybook authors versus a control group of PS 110B students in the same class who are not storybook authors; (2) a pre- and post-class Attitude Toward Science Survey; and (3) a geology teaching self-efficacy instrument. The results of the project have been positive and indicate that when preservice teachers in a science class are given an opportunity to experience personal mentoring from the teaching faculty they do experience a shift toward a more positive attitude toward science.

The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was calculated using pseudopotential planewave density functional theory. Bulk structures of pyrophyllite and ferripyrophyllite were optimized using periodic boundary conditions, after which crystal chemical methods were used to obtain initial terminations for ideal (110)- and (010)-type edge surfaces. The edge surfaces were protonated using various schemes to neutralize the surface charge, and total minimized energies were compared to identify which schemes are the most energetically favorable. The calculations show that significant surface relaxation should occur on the (110)-type faces, as well as in response to different protonation schemes on both surface types. This result is consistent with atomic force microscopy observations of phyllosilicate dissolution behavior. Bond-valence methods incorporating bond lengths from calculated structures can be used to predict intrinsic acidity constants for surface functional groups on (110)- and (010)-type edge surfaces. However, the occurrence of surface relaxation poses problems for applying current bond-valence methods. An alternative method is proposed that considers bond relaxation, and accounts for the energetics of various protonation schemes on phyllosilicate edges.

Caustic NaNO₃ solutions containing dissolved Al were reacted with quartz sand at 89 °C to simulate possible reactions between leaked nuclear waste and primary subsurface minerals at the U.S. Department of Energy's Hanford site in Washington. Nitrate-cancrinite began to precipitate onto the quartz after 2−10 days, cementing the grains together. Estimates of the equilibrium constant for the precipitation reaction differ for solutions with 0.1 or 1.0 m OH^- (log K_eq = 30.4 ± 0.8 and 36.2 ± 0.6, respectively). The difference in solubility may be attributable to more perfect crystallinity (i.e., fewer stacking faults) in the higher-pH cancrinite structure. This is supported by electron micrographs of crystal morphology and measured rates of Na volatilization under an electron beam. Precipitate crystallinity may affect radionuclide mobility, because stacking faults in the cancrinite structure can diminish its zeolitic cation exchange properties. The precipitation rate near the onset of nucleation depends on the total Al and Si concentrations in solution. The evolution of experimental Si concentrations was modeled by considering the dependence of quartz dissolution rate on Al(OH)₄^- activity, cancrinite precipitation, and the reduction of reactive surface area of quartz due to coverage by cancrinite.