24 research outputs found
<i>In Situ</i> TEM and AFM Investigation of Morphological Controls during the Growth of Single Crystal BaWO<sub>4</sub>
Barium
tungstate (BaWO<sub>4</sub>) is a widely investigated inorganic
optical material due to its attractive emission properties. Because
those properties strongly depend on crystal structure and morphology,
numerous approaches to controlling growth have been pursued. However,
an understanding of the growth mechanisms that lead to the wide range
of morphologies obtained to date is largely lacking, and most attempts
to develop that understanding have been based on post-growth analyses.
Significantly, such analyses have led to the conclusion that certain
BaWO<sub>4</sub> crystal morphologies result from a nonclassical growth
process of oriented attachment. In this work, we systematically varied
the morphology of BaWO<sub>4</sub> crystals by adjusting the relative
concentrations of solute, water, and ethanol. We then explored the
growth mechanism leading to the observed range of morphologies through <i>in situ</i> TEM and <i>in situ</i> AFM. We find that
even the most complex BaWO<sub>4</sub> morphologies occur through
purely classical growth mechanisms largely controlled by the content
of solute and ethanol. The latter acts as an impurity to poison growth
at low concentrations and low solute levels, but leads to development
of growth instabilities and eventual dendritic growth at high alcohol
and moderate solute concentrations by driving up the supersaturation.
These findings also highlight the necessity of <i>in situ</i> experiments to interpret <i>ex situ</i> observations of
crystal growth and decipher the controlling mechanisms
<i>In Situ</i> TEM and AFM Investigation of Morphological Controls during the Growth of Single Crystal BaWO<sub>4</sub>
Barium
tungstate (BaWO<sub>4</sub>) is a widely investigated inorganic
optical material due to its attractive emission properties. Because
those properties strongly depend on crystal structure and morphology,
numerous approaches to controlling growth have been pursued. However,
an understanding of the growth mechanisms that lead to the wide range
of morphologies obtained to date is largely lacking, and most attempts
to develop that understanding have been based on post-growth analyses.
Significantly, such analyses have led to the conclusion that certain
BaWO<sub>4</sub> crystal morphologies result from a nonclassical growth
process of oriented attachment. In this work, we systematically varied
the morphology of BaWO<sub>4</sub> crystals by adjusting the relative
concentrations of solute, water, and ethanol. We then explored the
growth mechanism leading to the observed range of morphologies through <i>in situ</i> TEM and <i>in situ</i> AFM. We find that
even the most complex BaWO<sub>4</sub> morphologies occur through
purely classical growth mechanisms largely controlled by the content
of solute and ethanol. The latter acts as an impurity to poison growth
at low concentrations and low solute levels, but leads to development
of growth instabilities and eventual dendritic growth at high alcohol
and moderate solute concentrations by driving up the supersaturation.
These findings also highlight the necessity of <i>in situ</i> experiments to interpret <i>ex situ</i> observations of
crystal growth and decipher the controlling mechanisms
Coupled Geochemical Impacts of Leaking CO<sub>2</sub> and Contaminants from Subsurface Storage Reservoirs on Groundwater Quality
The
leakage of CO<sub>2</sub> and the concomitant brine from deep
storage reservoirs to overlying groundwater aquifers is considered
one of the major potential risks associated with geologic CO<sub>2</sub> sequestration (GCS). In this work both batch and column experiments
were conducted to determine the fate of trace metals in groundwater
in the scenarios of CO<sub>2</sub> and metal-contaminated brine leakage.
The sediments for this study were from an unconsolidated sand and
gravel aquifer in Kansas, containing 0–4 wt % carbonates. Cd
(114 μg/L) and As (40 μg/L) were spiked into the reaction
system to represent potential contaminants from the reservoir brine.
Through this research we demonstrated that Cd and As were adsorbed
on the sediments, in spite of the lowered pH due to CO<sub>2</sub> dissolution in the groundwater. Cd concentrations in the effluent
were below the Cd MCL, even for sediments without detectable carbonate
to buffer the pH. Arsenic concentrations in the effluent were also
significantly lower than the influent concentration, suggesting that
the sediments tested have the capacity to mitigate the coupled adverse
effects of CO<sub>2</sub> leakage and brine intrusion. The mitigation
capacity of sediment is a function of its geochemical properties (e.g.,
the presence of carbonate minerals, adsorbed As, and phosphate)
Impacts of Organic Ligands on Forsterite Reactivity in Supercritical CO<sub>2</sub> Fluids
Subsurface
injection of CO<sub>2</sub> for enhanced hydrocarbon
recovery, hydraulic fracturing of unconventional reservoirs, and geologic
carbon sequestration produces a complex geochemical setting in which
CO<sub>2</sub>-dominated fluids containing dissolved water and organic
compounds interact with rocks and minerals. The details of these reactions
are relatively unknown and benefit from additional experimentally
derived data. In this study, we utilized an in situ X-ray diffraction
technique to examine the carbonation reactions of forsterite (Mg<sub>2</sub>SiO<sub>4</sub>) during exposure to supercritical CO<sub>2</sub> (scCO<sub>2</sub>) that had been equilibrated with aqueous solutions
of acetate, oxalate, malonate, or citrate at 50 °C and 90 bar.
The organics affected the relative abundances of the crystalline reaction
products, nesquehonite (MgCO<sub>3</sub>·3H<sub>2</sub>O) and
magnesite (MgCO<sub>3</sub>), likely due to enhanced dehydration of
the Mg<sup>2+</sup> cations by the organic ligands. These results
also indicate that the scCO<sub>2</sub> solvated and transported the
organic ligands to the forsterite surface. This phenomenon has profound
implications for mineral transformations and mass transfer in the
upper crust
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Abiotic Protein Fragmentation by Manganese Oxide: Implications for a Mechanism to Supply Soil Biota with Oligopeptides
The ability of plants and microorganisms
to take up organic nitrogen
in the form of free amino acids and oligopeptides has received increasing
attention over the last two decades, yet the mechanisms for the formation
of such compounds in soil environments remain poorly understood. We
used Nuclear Magnetic Resonance (NMR) and Electron Paramagnetic Resonance
(EPR) spectroscopies to distinguish the reaction of a model protein
with a pedogenic oxide (Birnessite, MnO<sub>2</sub>) from its response
to a phyllosilicate (Kaolinite). Our data demonstrate that birnessite
fragments the model protein while kaolinite does not, resulting in
soluble peptides that would be available to soil biota and confirming
the existence of an abiotic pathway for the formation of organic nitrogen
compounds for direct uptake by plants and microorganisms. The absence
of reduced MnÂ(II) in the solution suggests that birnessite acts as
a catalyst rather than an oxidant in this reaction. NMR and EPR spectroscopies
are shown to be valuable tools to observe these reactions and capture
the extent of protein transformation together with the extent of mineral
response
Tunable Manipulation of Mineral Carbonation Kinetics in Nanoscale Water Films via Citrate Additives
We
explored the influence of a model organic ligand on mineral
carbonation in nanoscale interfacial water films by conducting five
time-resolved in situ X-ray diffraction (XRD) experiments at 50 °C.
Forsterite was exposed to water-saturated supercritical carbon dioxide
(90 bar) that had been equilibrated with 0–0.5 <i>m</i> citrate (C<sub>6</sub>H<sub>5</sub>O<sub>7</sub><sup>–3</sup>) solutions. The experimental results demonstrated that greater concentrations
of citrate in the nanoscale interfacial water film promoted the precipitation
of magnesite (MgCO<sub>3</sub>) relative to nesquehonite (MgCO<sub>3</sub>·3H<sub>2</sub>O). At the highest concentrations tested,
magnesite nucleation and growth were inhibited, lowering the carbonation
rate constant from 9.1 × 10<sup>–6</sup> to 3.6 ×
10<sup>–6</sup> s<sup>–1</sup>. These impacts of citrate
were due to partial dehydration of Mg<sup>2+</sup>(aq) and the adsorption
of citrate onto nuclei and magnesite surfaces. This type of information
may be used to predict and tailor subsurface mineralization rates
and pathways
Organic Matter Remineralization Predominates Phosphorus Cycling in the Mid-Bay Sediments in the Chesapeake Bay
Chesapeake Bay, the
largest and most productive estuary in the
U.S., suffers from varying degrees of water quality issues fueled
by both point and nonpoint nutrient sources. Restoration of the Bay
is complicated by the multitude of nutrient sources, their variable
inputs, and complex interaction between imported and regenerated nutrients.
These complexities not only restrict formulation of effective restoration
plans but also open up debates on accountability issues with nutrient
loading. A detailed understanding of sediment phosphorus (P) dynamics
provides information useful in identifying the exchange of dissolved
constituents across the sediment–water interface as well as
helps to better constrain the mechanisms and processes controlling
the coupling between sediments and the overlying waters. Here we used
phosphate oxygen isotope ratios (δ<sup>18</sup>O<sub>P</sub>) in concert with sediment chemistry, X-ray diffraction, and Mössbauer
spectroscopy on sediments retrieved from an organic rich, sulfidic
site in the mesohaline portion of the mid-Bay to identify sources
and pathway of sedimentary P cycling and to infer potential feedbacks
on bottom water hypoxia and surface water eutrophication. Authigenic
phosphate isotope data suggest that the regeneration of inorganic
P from organic matter degradation (remineralization) is the predominant,
if not sole, pathway for authigenic P precipitation in the mid-Bay
sediments. This indicates that the excess inorganic P generated by
remineralization should have overwhelmed any pore water and/or bottom
water because only a fraction of this precipitates as authigenic P.
This is the first research that identifies the predominance of remineralization
pathway and recycling of P within the Chesapeake Bay. Therefore, these
results have significant implications on the current understanding
of sediment P cycling and P exchange across the sediment–water
interface in the Bay, particularly in terms of the sources and pathways
of P that sustain hypoxia and may potentially support phytoplankton
growth in the surface water
Band-Gap Reduction and Dopant Interaction in Epitaxial La,Cr Co-doped SrTiO<sub>3</sub> Thin Films
We show that by co-doping SrTiO<sub>3</sub> (STO) epitaxial thin
films with equal amounts of La and Cr, it is possible to produce films
with an optical band gap ∼0.9 eV lower than that of undoped
STO. Sr<sub>1–<i>x</i></sub>La<sub><i>x</i></sub>Ti<sub>1–<i>x</i></sub>Cr<sub><i>x</i></sub>O<sub>3</sub> thin films were deposited by molecular beam epitaxy
and characterized using X-ray photoelectron spectroscopy and X-ray
absorption near-edge spectroscopy to show that the Cr dopants are
almost exclusively in the Cr<sup>3+</sup> oxidation state. Extended
X-ray absorption fine structure measurements and theoretical modeling
suggest that it is thermodynamically preferred for La and Cr dopants
to occupy nearest-neighbor A- and B-sites in the lattice. Transport
measurements show that the material exhibits variable-range hopping
conductivity with high resistivity. These results create new opportunities
for the use of doped STO films in photovoltaic and photocatalytic
applications
Microstructure and Cs Behavior of Ba-Doped Aluminosilicate Pollucite Irradiated with F<sup>+</sup> Ions
Radionuclide <sup>137</sup>Cs is one of the major fission products that dominate heat
generation in spent fuels over the first 300 years. A durable waste
form for <sup>137</sup>Cs that decays to <sup>137</sup>Ba is needed
to minimize its environmental impact. Aluminosilicate pollucite CsAlSi<sub>2</sub>O<sub>6</sub> is selected as a model waste form to study the
decay-induced structural effects. Whereas Ba-containing precipitates
are not present in charge-balanced Cs<sub>0.9</sub>Ba<sub>0.05</sub>AlSi<sub>2</sub>O<sub>6</sub>, they are found in Cs<sub>0.9</sub>Ba<sub>0.1</sub>AlSi<sub>2</sub>O<sub>6</sub> and identified as monoclinic
Ba<sub>2</sub>Si<sub>3</sub>O<sub>8</sub>. Pollucite is susceptible
to electron-irradiation-induced amorphization. The threshold density
of electronic energy deposition for amorphization was determined to
be ∼235 keV/nm<sup>3</sup>. Pollucite can be readily amorphized
under F<sup>+</sup> ion irradiation at 673 K. A significant amount
of Cs diffusion and release from the amorphized pollucite occurs during
the irradiation. However, cesium is immobile in the crystalline structure
under He<sup>+</sup> ion irradiation at room temperature. The critical
temperature for amorphization is not higher than 873 K under F<sup>+</sup> ion irradiation. If kept at or above 873 K all the time,
the pollucite structure is unlikely to be amorphized; Cs diffusion
and release are improbable. A general discussion regarding pollucite
as a potential waste form is provided in this report
Dynamics of Magnesite Formation at Low Temperature and High pCO<sub>2</sub> in Aqueous Solution
Magnesite
precipitation from aqueous solution, despite conditions
of supersaturation, is kinetically hindered at low temperatures for
reasons that remain poorly understood. The present study examines
the products of MgÂ(OH)<sub>2</sub> reaction in solutions saturated
with supercritical CO<sub>2</sub> at high pressures (90 and 110 atm)
and low temperatures (35 and 50 °C). Solids characterization
combined with in situ solution analysis reveal that the first reaction
products are the hydrated carbonates hydromagnesite and nesquehonite,
appearing simultaneously with brucite dissolution. Magnesite is not
observed until it comprises a minor product at 7 days reaction at
50 °C. Complete transition to magnesite as the sole product at
35 °C (135 days) and at a faster rate at 50 °C (56 days)
occurs as the hydrated carbonates slowly dissolve under the slightly
acidic conditions generated at high pCO<sub>2</sub>. Such a reaction
progression at high pCO<sub>2</sub> suggests that over long term the
hydrated Mg-carbonates functioned as intermediates in magnesite formation.
These findings highlight the importance of developing a better understanding
of the processes expected to occur during CO<sub>2</sub> storage.
They also support the importance of integrating magnesite as an equilibrium
phase in reactive transport calculations of the effects of CO<sub>2</sub> sequestration on geological formations at long time scale