Structure and thermal behavior of nanocrystalline boehmite

First, the structural features of nanocrystalline boehmite synthesized by hydrolysis of aluminum sec-butoxide according to the Yoldas method are reported. The nanosized boehmite consists of rectangular platelets averaging 8 by 9 nm and 2–3 nm in thickness which contain a large excess of water. Dehydration by heating under vacuum induced an increase in the specific surface area, down to a minimum water content ( 0.2 H2O per Al2O3); values up to 470 m2/g can be reached. However this enlargement of specific surface area only results from water loss, the surface area remaining constant. The particle morphology, the excess of water, as well as the specific surface area, depend on the amount of acid used for the peptization during the synthesis. Second, a comprehensive investigation of the dehydration kinetics is presented. The simulations of the non-isothermal experiments at constant heating rates show that thermally stimulated transformation of nanocrystalline boehmite into alumina can be accurately modeled by a 4-reaction mechanism involving: (I) the loss of physisorbed water, (II) the loss of chemisorbed water, (III) the conversion of boehmite into transition alumina, (IV) the dehydration of transition alumina (loss of residual hydroxyl groups). The activation energy of each step is found to be very similar for experiments done in various conditions (heating rate, atmosphere, kind of sample,…)

Tracking the weathering of basalts on Mars using lithium isotope fractionation models

An edited version of this paper was published by AGU. Copyright (2015) American Geophysical UnionLithium (Li), the lightest of the alkali elements, has geochemical properties that include high aqueous solubility (Li is the most fluid mobile element) and high relative abundance in basalt-forming minerals (values ranking between 0.2 and 12 ppm). Li isotopes are particularly subject to fractionation because the two stable isotopes of lithium - 7Li and 6Li - have a large relative mass difference (∼15%) that results in significant fractionation between water and solid phases. The extent of Li isotope fractionation during aqueous alteration of basalt depends on the dissolution rate of primary minerals - the source of Li - and on the precipitation kinetics, leading to formation of secondary phases. Consequently, a detailed analysis of Li isotopic ratios in both solution and secondary mineral lattices could provide clues about past Martian weathering conditions, including weathering extent, temperature, pH, supersaturation, and evaporation rate of the initial solutions in contact with basalt rocks. In this paper, we discuss ways in which Martian aqueous processes could have lead to Li isotope fractionation. We show that Li isotopic data obtained by future exploration of Mars could be relevant to highlighting different processes of Li isotopic fractionation in the past, and therefore to understanding basalt weathering and environmental conditions early in the planet's historyData supporting our models and calculations are available as supporting information. The research leading to these results is a contribution from the Project ‘icyMARS’’, funded by the European Research Council, Starting Grant no 307496. This work was also partially supported by the European FEDER program and the Spanish Ministry of Science (MICINN) through the project CGL2011–30079. Comments by R. James and four anonymous reviewers helped us to clarify and strengthen our wor

Interlaboratory Comparison of Extractable Organofluorine Measurements in Groundwater and Eel (Anguilla rostrata): Recommendations for Methods Standardization

Research on per- and polyfluoroalkyl substances (PFAS) frequently incorporates organofluorine measurements, particularly because they could support a class-based approach to regulation. However, standardized methods for organofluorine analysis in a broad suite of matrices are currently unavailable, including a method for extractable organofluorine (EOF) measured using combustion ion chromatography (CIC). Here, we report the results of an international interlaboratory comparison. Seven laboratories representing academia, government, and the private sector measured paired EOF and PFAS concentrations in groundwater and eel (Anguilla rostrata) from a site contaminated by aqueous film-forming foam. Among all laboratories, targeted PFAS could not explain all EOF in groundwater but accounted for most EOF in eel. EOF results from all laboratories for at least one replicate extract fell within one standard deviation of the interlaboratory mean for groundwater and five out of seven laboratories for eel. PFAS spike mixture recoveries for EOF measurements in groundwater and eel were close to the criterion (±30%) for standardized targeted PFAS methods. Instrumental operation of the CIC such as replicate sample injections was a major source of measurement uncertainty. Blank contamination and incomplete inorganic fluorine removal may introduce additional uncertainties. To elucidate the presence of unknown organofluorine using paired EOF and PFAS measurements, we recommend that analysts carefully consider confounding methodological uncertainties such as differences in precision between measurements, data processing steps such as blank subtraction and replicate analyses, and the relative recoveries of PFAS and other fluorine compounds.publishedVersio

Chemical controls on incipient Mg-silicate crystallisation at 25oC : implications for early and late diagenesis

Mg-silicate minerals (e.g., stevensite, kerolite, talc, sepiolite) play an important role in the construction of facies models in lacustrine and peri-marine environments because they are sensitive to changes in solution chemistry. However, the response of Mg-silicate mineralogy to changing aqueous chemistry is only broadly understood because the mechanisms underpinning the co-precipitation of Mg2+ and SiO2(aq) from surface water, and subsequent Mg-silicate crystallization, are unclear. Here we describe the results of experiments designed to systematically examine the effects of pH, Mg/Si and salinity of the parent solution on the nature of initially precipitated products. Structural interrogation of the products with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and thermal analysis (TGA/DTA) allow comparison of synthetic products with naturally occurring crystalline counterparts. In general, Mg2+ and SiO2(aq) co-precipitation and nucleation of Mg-silicate layer structures first involves the rapid formation of 2:1 layers with trioctahedral occupancy and a mean coherent X-ray scattering domain between 1–2 unit cells with respect to the c axis. Well defined but diffuse hk reflections indicate two-dimensional growth, turbostratic stacking and highly variable interlayer hydration. Diffuse reflectance FTIR shows numerous structural similarities with stevensite, kerolite and sepiolite. However, TGA/DTA analysis indicates the presence of variable kerolite/stevensite interstratification not readily detectable through XRD analyses, as well as a significant degree of surface and interlayer hydration (e.g. 15–20 wt.%). We observe a number of clear trends in the products with respect to solution chemistry. For example, at low salinity, kerolite-like products dominate at high Mg/Si and high pH, whereas sepiolite-like products are formed at lower pH and lower Mg/Si. At high salinity and high Mg/Si, stevensite-like products are favoured at high pH and kerolite-like products dominate at lower pH, whereas a decrease in Mg/Si of the solution leads to sepiolite-like products at low pH and only stevensite-like products at high pH. Higher pH leads to an increase in octahedral vacancies which favour stevensite-like products; this may result from a higher rate of two-dimensional tetrahedral sheet expansion relative to the octahedral sheet, as inferred from studies of silica oligomerization and brucite growth kinetics. Together, our results indicate that the neoformation of Mg-rich silicates from solution may often begin with the rapid nucleation of hydrated 2:1 layers. Subsequent dehydration leads to progressive layer stacking order and could occur in response to wetting/drying cycles, prolonged exposure to high salinity solutions, or burial and heating. The surface and interlayer water associated with these products is undoubtedly an important source of diagenetic water in Mg-silicate-bearing successions, and the chemistry of this water upon later diagenesis should be a focus of future investigation.Publisher PDFPeer reviewe

Hodge Decomposition and the Shapley Value of a Cooperative Game

From the Washington University Senior Honors Thesis Abstracts (WUSHTA), Spring 2016. Published by the Office of Undergraduate Research. Joy Zalis Kiefer, Director of Undergraduate Research and Associate Dean in the College of Arts & Sciences; Lindsey Paunovich, Editor; Kristin G. Sobotka, Undergraduate Research Coordinator; Jennifer Kohl. Mentor: Ari Ster