Engineering topics

Stack, Andrew G.

Publications and source records attributed to Stack, Andrew G..

62 records · Page 4

DOE OSTI2019

Resolving Local Configurational Contributions to X-ray and Neutron Radial Distribution Functions Within Solutions of Concentrated Electrolytes – A Case Study of Concentrated NaOH

Extreme conditions of complex materials often lead to a manifold of local environments that challenge characterization and require new advances at the intersection of modern experimental and theoretical techniques. In this contribution, highly caustic and viscous aqueous NaOD solutions were characterized with a combination of X-ray and neutron radial distribution function (RDF) analyses, molecular dynamics simulations and sub-ensemble analysis. The interpretation of the experimental RDFs extends beyond the traditional understanding of atom-pair interactions and to the individual contributions of specific molecular-scale configurations (below 3.5 Å). This integrated approach yields unique insight into the complex and significant breath of ion solvation and pairing structural motifs present, the experimental sensitivity of RDFs to changes in local geometries, and the challenge of experimentally differentiating the ensemble of all superimposed local environments — a feature of increasing importance within the extreme condition of high ionic strength. This study was facilitated by the development of a chemically resistant sample cell for in-situ neutron total scattering measurements of highly caustic materials at elevated temperature and pressure conditions. Advancing these integrated capabilities is critical for ongoing model validation efforts and for extending the predictive capabilities needed for the extremes of chemical composition, aiding a variety of industrial processes that include advanced nuclear waste remediation and metal refining strategies.

Semrouni, David↗

DOE OSTI2018

Simultaneous Adsorption and Incorporation of Sr 2+ at the Barite (001)–Water Interface

Ionically-bonded minerals are ubiquitous and play a determinative role in controlling the mobility of toxic metals in natural environments. However, little is known about the mechanism of ion uptake by these mineral surfaces. In this study, the sorption of strontium ions (Sr 2+ ) to the barite (001) – water interface was studied using a combination of synchrotron x-ray scattering and three types of computational simulations (density functional theory, classical molecular dynamics (CMD), and CMD-metadynamics). In situ resonant anomalous X-ray reflectivity (RAXR) revealed that Sr 2+ adsorbed on the barite surface as inner-sphere surface complexes and was incorporated within the outermost barite atomic layers. Density functional theory combined with classical molecular dynamics simulations confirmed the thermodynamic stability of these species, demonstrating almost equal magnitudes in the free energy of sorption between these species. Metadynamics simulations showed a more detailed feature in the free energy landscape for metal adsorption where adsorbed Sr 2+ are stabilized in as many as four distinct inner-sphere sites and additional outer-sphere sites that are more diffuse and less energetically favorable than the inner-sphere sites. Additionally, the energy barriers among those inner-sphere sites were significantly lower compared with those for constituent cation Ba 2+ , implying fast exchange among adsorbed Sr 2+ species. The Sr 2+ uptake measured by RAXR followed a Frumkin isotherm defined with an apparent free energy of sorption, Δ≈ G Sr -22 kJ/mol, and an effective attractive interaction constant, γ ≈ -4.5 kJ/mol, between sorbed Sr 2+ . While the observed free energy can be mostly explained by the Helmholtz free energy of adsorption for Sr 2+ , ΔF Sr = -15.3 kJ/mol, the origin of the sorbate – sorbate correlation could not be fully described by our computational work. Together, these experimental and computational results demonstrate the complexity of Sr 2+ adsorption behavior at the barite (001) surface.

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DOE OSTI2018

Coupled Multimodal Dynamics of Hydrogen-Containing Ion Networks in Water-Deficient, Sodium Hydroxide-Aluminate Solutions

The (meta)stability of low water activity sodium hydroxide/aluminate (Na + OH–/Al(OH) 4 –) electrolytes dictates kinetics in the Bayer process for aluminum refining and high-level nuclear waste processing. We utilized quasi-elastic neutron scattering (QENS) and proton nuclear magnetic resonance spectroscopy ( 1 H NMR) in extremely concentrated sodium aluminate solutions to investigate the picosecond (ps) to microsecond (ms) timescale motions of H-bearing species (Al(OH) 4 – monomers/clusters, OH– and H 2 O). In the QENS data, in contrast to typical liquids, no short-time translational diffusion was observed at 293 K, but two types of localized motions were found: (i) local backbone tumbling or a formation of large hydrated ion clusters on the order of 40–60 ps; and (ii) much slower, complex, and collective dynamics of the ensemble of H-bearing species on the order of 350–750 ps. Variable temperature, pulsed field gradient, diffusion-ordered 1H NMR was used to determine the ensemble translational motion along with relaxometry to calculate rotational correlation coefficients. The ensemble rotational correlation times were on the order of 184–300 ps from 1 H NMR, which is consistent with the timescale of the QENS components. Complementary molecular dynamics simulation of NaOH solutions exhibit extensive ion networks potentially responsible for the observed dynamical coupling of water with the motion of large hydrated ion clusters. Understanding these collective motions will aid in predicting the behavior of complex solutions during aluminum production and during nuclear waste processing.

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DOE OSTI2018

Solvent-pore interactions in the Eagle Ford shale formation

In this work, the effect of solvent extraction on pore space was examined on a suite of samples from the Eagle Ford Shale Formation with varying lithologies and maturities. Several solvents -toluene, cyclohexane, methanol, dichloromethane, and hydrochloric acid- were contacted with shale samples, extracting the compatible organic matter. The porosity in these extracted shale samples was compared to unmodified samples. The amount and type of organic matter extracted were determined using Gas Chromatography - Mass Spectrometry, and the porosity was determined by (Ultra) Small Angle Neutron Scattering. Mostly alkanes and aromatics were detected in the extracts, but other portions of bitumen may also have been present. Only higher molecular weight alkanes were extracted with hydrochloric acid, suggesting that physical dissolution of carbonate minerals may have liberated this portion of organic matter and the solvent was not able to penetrate the bitumen and kerogen to extract the lower molecular weight alkanes. Lastly, a decrease in porosity with extraction was observed and attributed to a dominant mechanism of kerogen swelling due to kerogen-solvent interaction.

02 PETROLEUM↗

DOE OSTI2018

In-Situ 27Al NMR Spectroscopy of Aluminate in Sodium Hydroxide Solutions Above and Below Saturation with Respect to Gibbsite

Aluminum hydroxide (Al(OH)3, gibbsite) dissolution and precipitation processes in alkaline environments play a commanding role in aluminum refining and nuclear waste processing, yet mechanistic aspects underlying sluggish kinetics during crystallization have remained obscured due to a lack of in situ probes capable of isolating incipient ion pairs. At a molecular level Al is cycling between tetrahedral (Td) coordination in solution to octahedral (Oh) in the solid. We explored dissolution of Al(OH)3 that prepared variably saturated aluminate (Al(OH)4 -)-containing solutions under alkaline conditions (pH > 13) with in-situ 27Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopy, and interrogated the results with ab initio molecular dynamics (AIMD) simulations complemented with chemical shift calculations based on high level theory. The collective results highlight the overall stability of the solvation structure for Td Al in the Al(OH)4 - oxyanion as a function of both temperature and Al concentration. The observed chemical shift did not change significantly even when the Al concentration in solution became supersaturated upon cooling and limited precipitation of the octahedral Al(OH)3 phase occurred. However, subtle changes in Al(OH)4 - speciation correlated with the dissolution/precipitation reaction were found. AIMDinformed chemical shift calculations indicate that measurable perturbations should begin when the Al(OH)4 -?Na+ distance is less than 6 Å, increasing dramatically at shorter distances, coinciding with appreciable changes to the electrostatic interaction and reorganization of the Al(OH)4 - solvation shell. The integrated findings thus suggest that, at conditions incipient to and concurrent with gibbsite crystallization, nominally expected contact ion pairs are insignificant and instead medium-range (4 – 6 Å) solvent-separated Al(OH)4 -?Na+ pairs predominate. Moreover, the fact that these medium-range interactions bear directly on resulting gibbsite characteristics was demonstrated by detailed microscopic and X-ray diffraction analysis, and by progressive changes in the FWHM of the Oh resonance, as measured by in-situ NMR. Sluggish gibbsite crystallization may arise from the activation energy associated with disrupting this robust medium-range ion pair interaction.

Graham, Trenton R.↗

DOE OSTI2018

Mineral–Water Interface Structure of Xenotime (YPO 4 ) {100}

Crystal truncation rod (CTR) measurements and density functional theory (DFT) calculations were performed to determine the atomic structure of the mineral–water interface of the {100} surface of xenotime (nominally YPO 4 ). This mineral is important, because it incorporates a variety of rare earth elements (REEs) in its crystal structure. REEs are critical materials necessary for a variety of renewable and energy efficient technologies. Current beneficiation techniques are not highly selective for REE ore minerals, and large amounts go to waste; this is a first step toward designing more efficient beneficiation. Evidence is found for minor relaxation of the surface within the topmost monolayer with little or no relaxation in subsurface layers. Justification for ordered water at the interface is found, where water binds to surface cations and donates hydrogen bonds to surface phosphates. The average bond lengths between cations and oxygens on water are 228 pm in the best fit to the CTR data, versus 243 and 251 pm for the DFT. No agreement on water positions bound to surface phosphates is obtained. Altogether, the findings suggest that ligands used in beneficiation with a single anionic headgroup, such as fatty acids, will have limited selectivity for xenotime relative to undesirable minerals.

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DOE OSTI2018

Unraveling the Effects of Strontium Incorporation on Barite Growth—In Situ and Ex Situ Observations Using Multiscale Chemical Imaging

Impurity ions influence mineral growth rates through a variety of kinetic and thermodynamic processes that also affect partitioning of the impurity ion between the solid and solution. Here in this paper, the effect of an impurity ion, strontium, on Barite (BaSO 4 ) (001) growth rates was studied using a combination of high-resolution in situ microscopy with ex situ chemical imaging techniques. In the presence of strontium, $\langle$120$\rangle$ steps roughened and bifurcated. The overall Barite growth rate also decreased with increasing aqueous strontium-to-barium ratio ([Sr]/[Ba]aq) < 1. Analysis of the reacted solids using chemical imaging techniques indicated strontium incorporated uniformly across all step orientations into the Barite growth hillock for [Sr]/[Ba] aq < 1. However, at [Sr]/[Ba] aq > 5, steps with an apparent [010] orientation were expressed and growth in the [010] step direction led to an increase in the overall growth rate of the surface. Strontium became preferentially incorporated into the [010] step direction, rather than being homogeneously distributed. The [Sr]/[Ba] s in the newly grown solid was found to correlate directly with that of solutions at [Sr]/[Ba] aq < 5, but not for higher [Sr]/[Ba] aq . Solid composition analyses indicate that thermodynamic equilibrium was not achieved. However, kinetic transport modeling successfully reproduces the shift in growth mechanism.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Decoding Oxyanion Aqueous Solvation Structure: A Potassium Nitrate Example at Saturation

We present that the ability to probe the structure of a salt solution at the atomic scale is fundamentally important for our understanding of many chemical reactions and their mechanisms. The capability of neutron diffraction to “see” hydrogen (or deuterium) and other light isotopes is exceptional for resolving the structural complexity around the dissolved solutes in aqueous electrolytes. We have made measurements using oxygen isotopes on aqueous nitrate to reveal a small hydrogen-bonded water coordination number (3.9 ± 1.2) around a nitrate oxyanion. This is compared to estimates made using the existing method of nitrogen isotope substitution and those of computational simulations (>5–6 water molecules). The low water coordination number, combined with a comparison to classical molecular dynamics simulations, suggests that ion-pair formation is significant. Finally, this insight demonstrates the utility of experimental diffraction data for benchmarking atomistic computer simulations, enabling the development of more accurate intermolecular potentials.

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