<i>In Situ</i> TEM and AFM Investigation of Morphological Controls during the Growth of Single Crystal BaWO₄

Barium tungstate (BaWO₄) 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 BaWO₄ crystal morphologies result from a nonclassical growth process of oriented attachment. In this work, we systematically varied the morphology of BaWO₄ 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 BaWO₄ 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 BaWO₄

Barium tungstate (BaWO₄) 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 BaWO₄ crystal morphologies result from a nonclassical growth process of oriented attachment. In this work, we systematically varied the morphology of BaWO₄ 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 BaWO₄ 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 CO₂ and Contaminants from Subsurface Storage Reservoirs on Groundwater Quality

The leakage of CO₂ and the concomitant brine from deep storage reservoirs to overlying groundwater aquifers is considered one of the major potential risks associated with geologic CO₂ 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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂ Fluids

Subsurface injection of CO₂ for enhanced hydrocarbon recovery, hydraulic fracturing of unconventional reservoirs, and geologic carbon sequestration produces a complex geochemical setting in which CO₂-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 (Mg₂SiO₄) during exposure to supercritical CO₂ (scCO₂) 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 (MgCO₃·3H₂O) and magnesite (MgCO₃), likely due to enhanced dehydration of the Mg²⁺ cations by the organic ligands. These results also indicate that the scCO₂ 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

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, MnO₂) 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 Mn­(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 (C₆H₅O₇^–3) solutions. The experimental results demonstrated that greater concentrations of citrate in the nanoscale interfacial water film promoted the precipitation of magnesite (MgCO₃) relative to nesquehonite (MgCO₃·3H₂O). At the highest concentrations tested, magnesite nucleation and growth were inhibited, lowering the carbonation rate constant from 9.1 × 10^–6 to 3.6 × 10^–6 s^–1. These impacts of citrate were due to partial dehydration of Mg²⁺(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 (δ¹⁸O_P) 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 SrTiO₃ Thin Films

We show that by co-doping SrTiO₃ (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_1–<i>x</i>La_<i>x</i>Ti_1–<i>x</i>Cr_<i>x</i>O₃ 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 Cr³⁺ 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⁺ Ions

Radionuclide ¹³⁷Cs is one of the major fission products that dominate heat generation in spent fuels over the first 300 years. A durable waste form for ¹³⁷Cs that decays to ¹³⁷Ba is needed to minimize its environmental impact. Aluminosilicate pollucite CsAlSi₂O₆ 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_0.9Ba_0.05AlSi₂O₆, they are found in Cs_0.9Ba_0.1AlSi₂O₆ and identified as monoclinic Ba₂Si₃O₈. Pollucite is susceptible to electron-irradiation-induced amorphization. The threshold density of electronic energy deposition for amorphization was determined to be ∼235 keV/nm³. Pollucite can be readily amorphized under F⁺ 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⁺ ion irradiation at room temperature. The critical temperature for amorphization is not higher than 873 K under F⁺ 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 pCO₂ 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 Mg­(OH)₂ reaction in solutions saturated with supercritical CO₂ 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 pCO₂. Such a reaction progression at high pCO₂ 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 CO₂ storage. They also support the importance of integrating magnesite as an equilibrium phase in reactive transport calculations of the effects of CO₂ sequestration on geological formations at long time scale