VTechWorks Repository :: Browsing by Author "Hochella, Michael F. Jr."

Browsing by Author "Hochella, Michael F. Jr."

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Abiotic synthesis of graphite in hydrothermal vents
Estes, Emily R.; Berti, Debora; Coffey, Nicole R.; Hochella, Michael F. Jr.; Wozniak, Andrew S.; Luther, George W. III (2019-11-15)
Deciphering the origin, age, and composition of deep marine organic carbon remains a challenge in understanding the dynamics of the marine carbon cycle. In particular, the composition of aged organic carbon and what allows its persistence in the deep ocean and in sediment is unresolved. Here, we observe that both high and low temperature hydrothermal vents at the 9 degrees 50' N; 104 degrees 17.5 W East Pacific Rise (EPR) vent field are a source for (sub) micron-sized graphite particles. We demonstrate that commonly applied analytical techniques for quantification of organic carbon detect graphite. These analyses thereby classify graphite as either dissolved or particulate organic carbon, depending on the particle size and filtration method, and overlook its relevance as a carbon source to the deep ocean. Settling velocity calculations indicate the potential for these (sub)micron particles to become entrained in the buoyant plume and distributed far from the vent fields. Thus, our observations provide direct evidence for hydrothermal vents acting as a source of old carbon to the deep ocean.
Adsorption of Extracellular Polymeric Substances Derived from S. cerevisiae to Ceria Nanoparticles and the Effects on Their Colloidal Stability
Masaki, Shota; Nakano, Yuriko; Ichiyoshi, Kenta; Kawamoto, Keisuke; Takeda, Ayaka; Ohnuki, Toshihiko; Hochella, Michael F. Jr.; Utsunomiya, Satoshi (MDPI, 2017-07-11)
In order to understand the adsorption preferences of extracellular polymeric substances (EPS) components derived from fungus Saccharomyces cerevisiae on sparingly soluble CeO₂ nanoparticles (CeNPs), the adsorption experiments of the EPS including organic matter with low molecular weight have been performed at pH 6.0 at room temperature (25 ± 1 °C). The subsequent effects of the coating on the dispersibility of CeNPs was systematically measured as a function of time and ionic strength ranging from 1 to 1000 mmol L⁻¹. Among the EPS and other components, orthophosphate and saccharides preferentially adsorb onto CeNPs, and proteins are the only major N-compounds adsorbing onto the CeNP surfaces. Adsorption of orthophosphate resulted in a dramatic decrease in ζ potential to −40 mV at pH > 5, whereas the EPS adsorption suppressed the deviation of ζ potential within a narrow range (−20–+20 mV) at pHs ranging from 3 to 11. Critical aggregation concentrations (CAC) of an electrolyte (NaCl), inorganic orthophosphate, and EPS solutions are 0.01, 0.14, and 0.25 mol L⁻¹, respectively, indicating that the EPS adsorption suppresses aggregation of CeNPs by the electrostatic repulsive forces derived from the adsorbed orthophosphate and the steric barrier formed by organic matter on the nanoparticle surfaces. Therefore, the EPS derived from fungus S. cerevisiae can potentially enhance colloidal dispersibility of CeNPs at circumneutral pH.
Aerosolization and Atmospheric Transformation of Engineered Nanoparticles
Tiwari, Andrea Jean (Virginia Tech, 2014-04-04)
While research on the environmental impacts of engineered nanoparticles (ENPs) is growing, the potential for them to be chemically transformed in the atmosphere has been largely ignored. The overall objective of this work was to assess the atmospheric transformation of carbonaceous nanoparticles (CNPs). The research focuses on C₆₀ fullerene because it is an important member of the carbonaceous nanoparticle (CNP) family and is used in a wide variety of applications. The first specific objective was to review the potential of atmospheric transformations to alter the environmental impacts of CNPs. We described atmospheric processes that were likely to physically or chemically alter aerosolized CNPs and demonstrated their relevance to CNP behavior and toxicity in the aqueous and terrestrial environment. In order to investigate the transformations of CNP aerosols under controlled conditions, we developed an aerosolization technique that produces nano-scale aerosols without using solvents, which can alter the surface chemistry of the aerosols. We demonstrated the technique with carbonaceous (C₆₀) and metal oxide (TiO₂, CeO₂) nanoparticle powders. All resulting aerosols exhibited unimodal size distributions and mode particle diameters below 100 nm. We used the new aerosolization technique to investigate the reaction between aerosolized C₆₀ and atmospherically realistic levels of ozone (O₃) in terms of reaction products, reaction rate, and oxidative stress potential. We identified C₆₀O, C₆₀O2, and C₆₀O3 as products of the C₆₀-O3 reaction. We demonstrated that the oxidative stress potential of C₆₀ may be enhanced by exposure to O3. We found the pseudo-first order reaction rate to be 9 x 10⁻⁶ to 2 x 10⁻⁵ s⁻¹, which is several orders of magnitude lower than the rate for several PAH species under comparable conditions. This research has demonstrated that a thorough understanding of atmospheric chemistry of ENPs is critical for accurate prediction of their environmental impacts. It has also enabled future research in that vein by developing a novel technique to produce nanoscale aerosols from nanoparticle powders. Results of this research will help guide the formulation of appropriate environmental policy concerning the regulation of ENPs.
AFM surface force measurements between hydrophobized gold surfaces
Wang, Jialin (Virginia Tech, 2008-09-08)
In 1982, Israelachvili and Pashley reported the first measurements of a hitherto unknown attractive force between two mica surfaces hydrophobized in cetyltrimethylammonium bromide (CTAB) solutions. Follow-up experiments conducted by many investigators confirmed their results, while others suggested that the "hydrophobic force" is an artifact due to nanobubbles (or cavitation). Evidences for the latter included the discontinuities (or steps) in the force versus distance curves and the pancake-shaped nano-bubbles seen in atomic force microscopic (AFM) images. Recent measurements conducted in degassed water showed, however, smooth force versus distance curves, indicating that the hydrophobic force is not an artifact due to nanobubbles.1, 2 Still other investigators3, 4 suggested that the long-range attraction observed between hydrophobic surfaces is due to the correlation between the patches of adsorbed ionic surfactant and the patches of unoccupied surface. For this theory to work, it is necessary that the charged patches be laterally mobile to account for the strong attractive forces observed in experiment. In an effort to test this theory, AFM force measurements were conducted with gold substrates hydrophobized by self-assembly of alkanethiols and xanthates of different chain lengths. The results showed long-range attractions despite the fact that the hydrophobizing agents chemisorb on gold and, hence, the adsorption layer is immobile. When the gold surfaces were hydrophobized in a 1 Ã 10-3 M thiol-in-ethanol solution for an extended period of time, the force curves exhibited steps. These results indicate that the long-range attractions are caused by the coalescence of bubbles, as was also reported by Ederth.5 The steps disappeared, however, when the species adsorbed on top of the chemisorbed monolayer were removed by solvent washing, or when the gold substrates were hydrophobized in a 1 Ã 10-5 M solution for a relatively short period of time. AFM force measurements were also conducted between gold substrates coated with short-chain thiols and xanthates to obtain hydrophobic surfaces with water contact angles (ï ±) of less than 90o. Long-range attractions were still observed despite the fact that cavitation is thermodynamically not possible. Having shown that hydrophobic force is not due to coalescence of pre-existing bubbles, cavitation, or correlation of charged patches, the next set of force measurements was conducted in ethanol-water mixtures. The attractive forces became weaker and shorter-ranged than in pure water and pure ethanol. According to the Derjaguin's approximation6, an attractive force arises from the decrease in the excess free energy (ï §f) of the thin film between two hydrophobic surfaces.7 Thus, the stronger hydrophobic forces observed in pure water and pure ethanol can be attributed to the stronger cohesive energy of the liquid due to stronger H-bonding. Further, the increase in hydrophobic force with decreasing separation between two hydrophobic surfaces indicates that the H-bonded structure becomes stronger in the vicinity of hydrophobic surfaces. The force measurements conducted at different temperatures in the range of 10-40C showed that the hydrophobic attraction between macroscopic surfaces causes a decrease in film entropy (Sf), which confirms that the hydrophobic force is due to the structuring of water in the thin film between two hydrophobic surfaces. The results showed also that the hydrophobic interaction entails a reduction in the excess film enthalpy (Hf), which may be associated with the formation of partial (or full) clathrates formed in the vicinity of hydrophobic surfaces. The presence of the clathrates is supported by the recent finding that the density of water in the vicinity of hydrophobic surfaces is lower than in the bulk.8
Aluminum mobility in mildly acidic mine drainage: Interactions between hydrobasaluminite, silica and trace metals from the nano to the meso-scale
Caraballo, Manuel A.; Wanty, Richard B.; Verplanck, Philip L.; Navarro-Valdivia, Leonardo; Ayora, Carlos; Hochella, Michael F. Jr. (2019-08-05)
Aluminum precipitates control the hydrochemistry and mineralogy of a broad variety of environments on Earth (e.g., acid mine drainage, AMD, coastal wetlands, boreal and alpine streams, tropical acid sulfate soils, laterites and bauxites, ...). However, the geochemical and mineralogical processes controlling Al (and other associated metals and metalloids) transport and removal in those environments are not fully understood. The geochemical system of Paradise Portal (Colorado, USA) comprises sulfate-rich mildly acidic waters, the hydrochemistry of which is directly controlled by the massive precipitation of hydrobasaluminite Al-4(SO4)(OH)(10)center dot 12-36H(2)O. Three connected but discernible aluminum precipitation stages were identified and described: 1) nanoparticle formation and size decrease along the creek, 2) hydrobasaluminite neoformation on the riverbed, and 3) precipitate accretion and accumulation on the riverbed leading to Al and Fe banded formations. The co-occurrence of Al and Si in the system was observed, recording significant amounts of Si accompanying the three different components of the system (i.e., nanoparticles and fresh and aged Al-precipitates). Also, abrupt and minor changes in the sedimentary record were described and proposed to be the response of the system to seasonal and interannual changes in AMD chemistry. Concerning the mobility of other metals and metalloids, P, Th, V, W, Ti and B showed a tendency to be preferentially incorporated into hydrobasaluminite, while others like Be, As, Se or Ba tend to remain dissolved in the water.
Assessing the Reactive Surface Area of Phlogopite during Acid Dissolution: An Atomic Force Microscopy, X-ray Photoelectron Spectroscopy, and Low Energy Electron Diffraction Study
Rufe, Eric (Virginia Tech, 2000-01-14)
The behavior during dissolution of edge and basal surfaces of the mica phlogopite were examined using in situ atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED) in an attempt to characterize the reactive surface area during dissolution. Mica minerals are the ideal material for this study because they offer a high degree of structural anisotropy. Therefore surfaces with different structures are easily identified. Dissolution is shown to proceed preferentially by removal of material from {hk0} edges. Dissolution rates were calculated by measuring the volume of material removed from etch pits, and normalizing to either the "reactive" surface area of {hk0} edges exposed at pit walls, or to a total "BET-equivalent" surface area. Rates normalized to total surface area are in the range of dissolution rates reported in the literature. Edge surface normalized rates are about 100 times faster. Long-term in situ AFM observations of phlogopite dissolution reveal that exposed (001) surfaces also display a distinct reactivity, though it operates on a different time scale. The top layer is shown to expand between 39 and 63 hours in contact with pH 2 HCl solution. Subsequent LEED analysis shows that the (001) surface becomes amorphous upon reacting with pH 2 HCl. Compositional characterization of the phlogopite after reaction shows that for pitted phlogopite surfaces, dissolution is characterized by leaching of octahedral cations and polymerization of the silica-enriched residual layer. No chemical changes or polymerization are observed for freshly cleaved unpitted phlogopite after reaction with pH 2 HCl for 24 hours. This suggests a gallery access mechanism is facilitated by edge attack, and is only significant on exposed (001) surfaces after a certain amount of dissolution by edge attack.
Atomic Force Microscopy Study of Clay Mineral Dissolution
Bickmore, Barry Robert (Virginia Tech, 1999-12-09)
An integrated program has been developed to explore the reactivity of 2:1 phyllosilicates (biotite and the clays montmorillonite, hectorite, and nontronite) with respect to acid dissolution using in situ atomic force microscopy (AFM). Three techniques are described which make it possible to fix these minerals and other small particles to a suitable substrate for examination in the fluid cell of the atomic force microscope. A suite of macros has also been developed for the Image SXM image analysis environment which make possible the accurate and consistent measurement of the dimensions of clay particles in a series of AFM images, so that dissolution rates can be measured during a fluid cell experiment. Particles of biotite and montmorillonite were dissolved, and their dissolution rates normalized to their reactive surface area, which corresponds to the area of their edge surfaces (A_e). The A_e-normalized rates for these minerals between pH 1-2 are all ~10E^-8 mol/m²*s, and compare very well to other A_e-normalized dissolution rates in the literature. Differences between the A_e-normalized rates for biotite and the BET-normalized rates (derived from solution chemical studies) found in the literature can be easily explained in terms of the proportion of edge surface area and the formation of leached layers. However, the differences between the A_e-normalized montmorillonite rates and the literature values cannot be explained the same way. Rather, it is demonstrated that rates derived from solution studies of montmorillonite dissolution have been affected by the colloidal behavior of the mineral particles. Finally, the dissolution behavior of hectorite (a trioctahedral smectite) and nontronite ( a dioctahedral smectite) were compared. Based on the differential reactivity of their crystal faces, a model of their surface atomic structures is formulated using Hartman-Perdock crystal growth theory, which explains the observed data if it is assumed that the rate-determining step of the dissolution mechanism is the breaking of connecting bonds between the octahedral and tetrahedral sheets of the mineral structure.
Biomolecular Controls on Calcium Carbonate Formation by Amorphous and Classical Pathways: Insights from Measurements of Nucleation Rates and Isotope Tracers
Giuffre, Anthony J. (Virginia Tech, 2015-04-26)
Calcified skeletons are produced within complex assemblages of proteins and polysaccharides whose roles in mineralization are not well understood. Researchers have long postulated that living organisms utilize the macromolecules of organic matrices to actively guide the formation of crystal structures. The timing and placement of the subsequent minerals that form are most easily controlled during nucleation; however, a physical and chemical picture of how organic functional group chemistry influences the initial stages of nucleation is not yet established. These processes are further complicated by the realization that carbonate biominerals can form by an amorphous to crystalline transformation process, which has prompted the question of how chemical signatures are recorded during mineralization. Investigations of mineralization processes such as the kinetics of nucleation and the transformation of amorphous calcium carbonate (ACC) to crystalline products are critical to building a better understanding of biomineral formation. Only from that fundamental basis can one begin to decipher changes in climate and seawater chemistry over geologic time and by recent anthropogenic effects. This dissertation presents the findings from experimental studies of the thermodynamics and kinetics of multiple mineral formation processes, including nucleation and transformation from an amorphous phase. The kinetics of calcite nucleation onto a suite of high-purity polysaccharide (PS) substrates were quantified under controlled conditions. Nucleation rates were measured as a function of 1) supersaturation extending above and below ACC solubility and 2) ionic strength extending to seawater salinity. These conditions decipher the chemical interactions between the PS substrate, calcite crystal, and solution. These investigations show the energy barrier to calcite formation is regulated by competing interfacial energies between the substrate, crystal, and liquid. The energy barriers to nucleation are PS-specific by a systematic relationship to PS charge density and substrate structure that is rooted in minimization of the competing substrate-crystal and substrate-liquid interfacial energies. The data also suggest ionic strength regulates nucleation barriers through substrate-liquid and crystal-liquid interfacial energetics. In a second experimental study, stable isotope labeling was used to directly probe the transformation pathway. Four processes were considered: dissolution-reprecipitation, solid-state, or combinations of these end member processes. Isotope measurements of calcite crystals that transform from ACC have signatures that are best explained by dissolution-reprecipitation. The extent of isotopic mixing correlates with the amount of ACC transferred and the time to transformation, suggesting the calcite crystals are recording the changing local solution environment during the transformation. These investigations into different mineralization mechanisms build a framework for how functional group chemistries of organic molecules regulate mineralization and the resulting isotopic and elemental signatures in the calcite. This may provide useful insights to interpreting chemical signatures of carbonate biominerals in fossil record and understanding ocean chemistry changes throughout geologic time.
Bioreduction of Hematite Nanoparticles by Shewanella oneidensis MR-1
Bose, Saumyaditya (Virginia Tech, 2006-12-08)
A dissertation is presented on the bioreduction of hematite (α-Fe2O3) nanoparticles. The study shows that an alternative extracellular electron transfer mechanism other than the classical 'direct-contact' mechanism may be simultaneously employed by Shewanella oneidensis MR-1 during solid-phase metal reduction. This conclusion is supported by analysis of the bioreduction kinetics of hematite nanoparticles coupled with microscopic investigations of cell-mineral interactions. The reduction kinetics of metal-oxide nanoparticles were examined to determine how S. oneidensis utilizes these environmentally-relevant solid-phase electron acceptors. Nanoparticles involved in geochemical reactions show different properties relative to larger particles of the same phase, and their reactivity is predicted to change as a function of size. To demonstrate these size-dependent effects, the surface area normalized reduction rates of hematite nanoparticles by S. oneidensis MR-1 with lactate as the sole electron donor were measured. As evident from whole cell TEM analysis, the mode of nanoparticle adhesion to cells is different between the more aggregated, pseudo-hexagonal to irregular shaped 11 nm, 12 nm, 99 nm and the less aggregated 30 nm and 43 nm rhombohedral particles. The 11 nm, 12 nm and 99 nm particles show less cell contact and coverage than the 30 nm and 43 nm particles but still show significant rates of reduction. This leads to the provisional speculation that S. oneidensis MR-1 employs a pathway of indirect electron transfer in conjunction with the direct-contact pathway, and the relative importance of the mechanism employed depends upon aggregation level and the shape of the particles or crystal faces exposed. In accord with the proposed increase in electronic band-gap for hematite nanoparticles, the smallest particles (11 nm) exhibit one order of magnitude decrease in reduction when compared with larger (99 nm) particles, and the 12 nm rates fall in between these two. This effect may also be due to the passivation of the mineral and cell surfaces by Fe(II), or decreasing solubility due to decrease in size.
Bonding properties of coordinated polyhedra in molecules and crystals
Hill, Frances Cull (Virginia Tech, 1995-12-05)
Force constants and electron density distributions in coordination polyhedra in molecules and crystals are modeled using Hartree-Fock molecular orbital methods. Model bond-stretching force constants calculated for coordination polyhedra in a series of nitride, oxide and sulfide molecules are ~ 10-20% larger than obtained with spectroscopic methods. Well-developed correlations obtain between the force constants and minimum energy bond lengths, effective nuclear charges and polyhedral compressibilities of molecules and crystals. Model electron density distributions calculated for a large number of molecules with MOn (n = 1, 2,3,4 or 6) coordination polyhedra show that the MO bonds of a given type vary in a regular way with the value of the electron density at bond critical points, bonded radii and the curvatures of the electron density. The bonded interactions in the polyhedra are examined in terms of criteria set forth by Bader and Essen (1984) and Cremer and Kraka (1984).
Calcification by amorphous carbonate precursors: Towards a new paradigm for sedimentary and skeletal mineralization
Wang, Dongbo (Virginia Tech, 2010-11-29)
A new paradigm for the formation of calcified skeletons suggests mineralization proceeds through amorphous calcium carbonate (ACC) precursors. The implications of this strategy in carbonate crystallization are widespread, particularly for understanding factors controlling impurity and isotopic signatures in calcium carbonates. The first chapter is a literature review of the biomineralization processes used by two important model organisms: the sea urchin larva and the foraminifera. Sea urchin larvae provide a thoroughly studied example of mineralization by an ACC pathway that is under biological control through regulation of protein chemistry and the local mineralization environment. A review of how foraminifera produce their test structures is also examined to explore the question of how organisms regulate the Mg content in proportion to the temperature their environments of formation. The second chapter demonstrates that acidic biomolecules regulate the composition of ACC for a suite of model carboxylated molecules. The physical basis for the systematic trend in Mg content is related to the ability of the affinity of the biomolecule for binding Ca versus Mg. The third chapter builds on these findings to explore the transformation of Mg-rich ACC precursors to calcites of exceptionally high Mg-contents that could not be produced by classical step-dominated growth processes. The data indicate that these materials are likely a result of a nucleation-dominated pathway. The final, fourth chapter develops Raman spectroscopy-based calibrations for determining Mg contents in ACC. The calibrations are based upon peak position or peak width of the carbonate υ₁ stretch.
Calcium carbonate biomineralization: A theoretical and experimental investigation of biomolecular controls on nucleation and growth
Hamm, Laura Mae (Virginia Tech, 2012-04-30)
Organisms have evolved a remarkable ability to mineralize complex skeletons and functional biomaterials. These structures are nucleated and grown in close associaiton with macromolecular assemblages of proteins and polysaccharides that are implicated in regulating all stagees of mineralization. Because of this intimate association of organic with inorgaic components, many studies have investigated the effects of particular organic species on mineral morphology, phase, and growth rate. However, the diversity and species-specific nature of the organic assemblages associated with biominerals across a wide variety of taxa, has limited our understanding of how organisms use biomolecules to regulate skeletal formation. It is clear that a mechanistic picture of biomolecular controls on mineralization requires molecular-level investigations of the interplay between organic and inorganic components at all stages of crystallizaiton. This dissertation presents the findings from theoretical and experimental studies of the physical mechanisms that underlie biomolecule controls on mineral formation. Molecular dynamics simulations probe the effects of acidic molecules on the hydration of alkaline earth cations. After first calculating baseline hydration properties for magnesium, calcium, strontium, and barium, I determine the effects of carboxylate-containing molecules on cation hydration state as well as the kinetics and thermodynamics of water exchange. Experimental work utilizes self-assembled monolayers as proxies for matrix macromolecules in order to understand their effects on CaCO3 nucleation kinetics and thermodynamics. Estimates of nucleation rates and barriers are made from optical microscopy data and correlated with measurements of crystal – substrate rupture force from dynamic force microscopy. These investigations show that an important function of biomolecules in directing mineralization lies in their ability to modulate cation hydration. Both chemical functionality and molecular conformation are influential in regulating the kinetics and thermodynamics of mineral nucleation, and these effects may be predicted by the strength of interaction between organic and inorganic components. These findings contribute to a mechanistic understanding of how organic matrices act to regulate biomineral formation. They demonstrate a plausible physical basis for how carboxyl-rich biomolecules accelerate the kinetics of biomineral growth and suggest roles for organic species in the nucleation and pre-nucleation stages of mineralization.
Cell adhesion of Shewanella oneidensis to iron oxide minerals: Effect of different single crystal faces
Neal, Andrew L.; Bank, Tracy L.; Hochella, Michael F. Jr.; Rosso, Kevin M. (American Institute of Physics, 2005-12-30)
The results of experiments designed to test the hypothesis that near-surface molecular structure of iron oxide minerals influences adhesion of dissimilatory iron reducing bacteria are presented. These experiments involved the measurement, using atomic force microscopy, of interaction forces generated between Shewanella oneidensis MR-1 cells and single crystal growth faces of iron oxide minerals. Significantly different adhesive force was measured between cells and the (001) face of hematite, and the (100) and (111) faces of magnetite. A role for electrostatic interactions is apparent. The trend in relative forces of adhesion generated at the mineral surfaces is in agreement with predicted ferric site densities published previously. These results suggest that near-surface structure does indeed influence initial cell attachment to iron oxide surfaces; whether this is mediated via specific cell surface-mineral surface interactions or by more general interfacial phenomena remains untested. (C) 2005 American Institute of Physics.
Characterization and modeling of soluble manganese removal from drinking water by oxide-coated filter media
Merkle, Peter B. (Virginia Tech, 1995)
Where Mn²⁺ _(aq) is found in water supplies, filter media may naturally develop surface coatings bearing MnO_x(s). These may absorb Mn²⁺ _(aq), and in the presence of oxidant, sorbed Mn²⁺* is oxidized to MnO_x(s), regenerating sorption capacity. The filter accomplishes Mn²⁺ _(aq) removal, a process called the "natural greensand effect". Characterization of naturally coated media showed variation in coating composition and structure. With thicknesses from 1 - 125 μm, primary coating constituents were Al and Mn, with incorporation of minor amounts of Fe, Cu, and Si and trace elements. "Growth ring" features in coating cross-section corresponding to compositional variation were characterized by SEM, electron microprobe, and energy-dispersive x-ray analysis (EDS). Media surface areas of 2 - 135 m² g⁻¹ land microporosity of 15 - 533 cm³ kg⁻¹ were linearly related to extractable Mn content. Diatom remains found in coatings suggest a key role for coating deposition in filtration phenomena. Atomic force microscopy found surface self-similarity over 10 nm - 10 μm. X-ray photoelectron spectroscopy (XPS) confirmed heterogeneous surface composition including C, Al, Si, and Fe. A method to rapidly deposit up to 4 mg g⁻¹ Mn on media was developed, employing sequential batch and recycle reactors. Mn(IV) was the surface species found by XPS analysis. The Freundlich isotherm described Mn²⁺ sorption on this and the naturally coated media; sorption capacity increased between pH 6.0 and 7.5, and was reduced by [Ca²⁺] = 60 mg L⁻¹. The global Mn²⁺ oxidation rates for all coated media at pH 7.5 were 0.008 - 0.11 mg Mn²⁺ g⁻¹ hr⁻¹: rates increased with flow and decreased with pH. A numerical process model for sorption and oxidation of Mn²⁺ _(aq) was calibrated with short bed absorber and differential reactor columns. The Freundlich isotherm, film transport, internal diffusion, and hydrodynamic dispersion were included, with sorption capacity apportioned into kinetically available and unavailable sites. The model performed well in calibration, predicting dynamic system response across a range of flow, pH, [Ca²⁺], and reactant levels. Model performance in validation was less satisfactory, probably due to experimental difficulties and the sensitivity of process performance on recent coating history and media regeneration status.
Characterization and Reactivity of Mo₂C
St. Clair, Todd P. (Virginia Tech, 1998-06-04)
Two types of Mo₂C have been investigated: polycrystalline β-Mo₂C and single crystal α-Mo₂C. The β-Mo₂C material was synthesized via a temperature-programmed method, and then characterized using x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), CO chemisorption, and N₂ physisorption. The catalytic activity of the β-Mo₂C was tested for cumene hydrogenation under high pressure conditions, and the effect of sulfur and oxygen poisons on cumene hydrogenation was also investigated. As a complement to the work done on polycrystalline β-Mo₂C, UHV studies of single crystal α-Mo₂C were undertaken to provide fundamental information about a well-characterized Mo₂C surface. The (0001) surface of α-Mo₂C was investigated using XPS and low energy electron diffraction (LEED). It was found that an ion-bombarded surface could be prepared as either Mo-terminated or C-terminated by choosing either low annealing temperatures (~1000 K) or high annealing temperatures (~1500 K), respectively. CO and O₂ adsorption was also studied on α-Mo₂C (0001) using thermal desorption spectroscopy (TDS), XPS, Auger electron spectroscopy (AES), and LEED. Finally, thiophene adsorption was investigated on α-Mo₂C (0001).
Comparison of the Reactivity of Various Mn-Oxides With CrIIIaq: Microscopic and Spectroscopic Observations of Dissolution, Cr-sorption and Cr and Mn Redox Interactions
Weaver, Robert M. (Virginia Tech, 2001-09-19)
Chapter 1 Dynamic Processes Occurring at the Cr^III_aq – Manganite (γ-MnOOH) Interface: Simultaneous Adsorption, Microprecipitation, Oxidation/Reduction and Dissolution The complex interaction between Cr^III_aq and manganite (γ-MnOOH) was systematically studied at room temperature over a pH range of 3 to 6, and within a concentration range of 10⁻⁴ to 10⁻² M CrOH²⁺_aq. Solution compositional changes during batch reactions were characterized by ICP and UVvis. The manganites were characterized before and after reaction with XPS, SEM, high-resolution FESEM, and EDS analysis. Fluid-cell AFM was used to follow these metal-mineral interactions in situ. The reactions are characterized by 1) sorption of Cr^III and the surface-catalyzed microprecipitation of Cr^III-hydroxy hydrate on manganite surfaces, 2) the acidic dissolution of the manganite, and 3) the simultaneous reductive dissolution of manganite coupled with the oxidation of Cr^III_aq to highly toxic Cr^VI_aq. Cr^III-hydroxy hydrate was shown to precipitate on the manganite surface while still undersaturated in bulk solution. The rate of manganite dissolution increased with decreasing pH due both to faster acid-promoted and Mn-reduction- promoted dissolution. Due to direct redox coupling with Mn reduction, Cr oxidation was most rapid in the lower pH range. Neither Mn^II nor Cr^VI were ever detected on manganite surfaces, even at the maximum rate of their generation. At the highest pHs of this study, Cr^III_aq was effectively removed from solution to form Cr^III-hydroxy hydrate on manganite surfaces and in the bulk solution, and manganite dissolution and Cr^VI_aq generation were minimized. All interface reactions described above were heterogeneous across the manganite surfaces. This heterogeneity is a direct result of the heterogeneous semiconducting nature of natural manganite crystals, and is also an expression of the proximity effect, whereby redox processes on semiconducting surfaces are not limited to next nearest neighbor sites. Chapter 2 Comparison of the Reactivity of Various Mn-Oxides with Cr^III_aq: Microscopic and Spectroscopic Observations of Dissolution, Cr-sorption and Cr and Mn Redox Interactions The interaction between Cr^III_aq and seven different Mn-oxides (6 monomineralic, 1 synthetic) have been observed in pH ~4.4 HNO₃ and pH ~4.4 ~10⁴ M Cr^III_aq solutions. For each mineral-solution interaction, the aqueous chemical concentrations (e.g. [Mn]_aq, [Cr]_aq, [Cr^VI_aq]) were measured with time. Reacted samples were examined by XPS to determine if, and to what extent, the surface chemical states of Cr, Mn and O had changed. Microscopic observations of the reacted surfaces were obtained using AFM and high-resolution, low-voltage FESEM. The solubility of the Mn-oxides in the acidic, non-Cr bearing solutions varied inversely with the average Mn valence, but did not show systematic behavior with respect to the mineral structure type (e.g. tunnel, layer, framework). This trend was interpreted as resulting from the relative ability of an adsorbed proton to polarize surface Mn-O bonds, with the polarizability being in the order Mn²⁺-O > Mn³⁺-O > Mn⁴⁺-O. For samples reacted with Cr^III_aq, the rate and extent of reductive dissolution was always greater than for acidic dissolution during the initial time period. The measured ratios of the [Mn]_aq : [Cr^VI]_aq were approximately in agreement with the values expected from the proposed stoichiometric reactions. Cr-uptake was observed to occur in undersaturated solutions as a result of adsorption, absorption and surface catalyzed precipitation. The chromium as detected by XPS was predominately Cr^III, however pyrolusite contained both Cr^III and Cr^VI. Previous studies have implicated a chromium surface precipitate to be responsible for the cessation of the Cr^III_aq oxidation reaction. Our surface sensitive FESEM and AFM observations tend to suggest that Cr-uptake is by isolated site binding, very small (<30 nm) surface clusters or monolayer scale films. Cr-uptake was followed by slow Cr-release on several of the solids (particularly the layered solids) after a substantial portion of the total aqueous Cr had been converted to Cr^VI_aq. The oxidizing ability of the different Mn-oxides for Cr^III_aq is evaluated with regards to the energy level of the redox couple (i.e. the redox potential) as compared with the Fermi energy level of the Mn-oxide. Although these energies were calculated rather than directly measured, the results indicate that electrons originating from adsorbed Cr^III ions may be transferred into the conduction band or more likely, into available surface states. The presence of an initial limited quantity of electron accepting surface states likely explains the observation of a rapid initial Cr^III-oxidation followed by much slower oxidation. The Mn-oxides that exhibited the greatest and longest lasting Cr^III-oxidizing power were the Mn-oxides containing Mn⁺, and in particular those containing Mn³⁺ and Mn⁺. It is believed that the combined presence of a reducible Mn ion (e.g. Mn³⁺) and a highly soluble Mn⁺ ion facilitates a sustained Cr^III-oxidation reaction because fresh surface is exposed during the reaction.
The competing effects of microbially derived polymeric and low molecular-weight substances on the dispersibility of CeO2 nanoparticles
Nakano, Yuriko; Ochiai, Asumi; Kawamoto, Keisuke; Takeda, Ayaka; Ichiyoshi, Kenta; Ohnuki, Toshihiko; Hochella, Michael F. Jr.; Utsunomiya, Satoshi (Springer Nature, 2018-02-26)
To understand the competing effects of the components in extracellular substances (ES), polymeric substances (PS) and low-molecular-weight small substances (SS) <1 kDa derived from microorganisms, on the colloidal stability of cerium dioxide nanoparticles (CeNPs), we investigated their adsorption to sparingly soluble CeNPs at room temperature at pH 6.0. The ES was extracted from the fungus S. cerevisiae. The polypeptides and phosphates in all components preferentially adsorbed onto the CeNPs. The zeta potentials of ES+CeNPs, PS+CeNPs, and SS+CeNPs overlapped on the plot of PS itself, indicating the surface charge of the polymeric substances controls the zeta potentials. The sizes of the CeNP aggregates, 100-1300 nm, were constrained by the zeta potentials. The steric barrier derived from the polymers, even in SS, enhanced the CeNP dispersibility at pH 1.5-10. Consequently, the PS and SS had similar effects on modifying the CeNP surfaces. The adsorption of ES, which contains PS+SS, can suppress the aggregation of CeNPs over a wider pH range than that for PS only. The present study addresses the non-negligible effects of small-sized molecules derived from microbial activity on the migration of CeNP in aquatic environments, especially where bacterial consortia prevail.
Detection limits of CO₂in fluid inclusions using microthermometry and Raman spectroscopy and the spectroscopic characterization of CO₂
Rosso, Kevin M. (Virginia Tech, 1994)
In many geologic environments, dominantly aqueous solutions contain low concentrations of CO₂. At ambient temperature, in fluid inclusions which trap these solutions, the typical phase assemblage consists of a CO₂-rich vapor (where P_CO₂ ≈ P_internaI) and an aqueous phase containing dissolved salts and CO₂. In this study, the CO₂ minimum detection limits (MDLs) using microthermometry and laser Raman spectroscopy are established in terms of P_CO₂ using synthetic H₂O-CO₂ inclusions. The purpose of the microthermometric experiments was to examine the diagnostic CO₂ phase changes and determine the quantity of CO₂ necessary to result in observable solid CO₂ melting. The results of these experiments show that an observable solid CO₂ melting event requires P_CO₂ ≥ 45 bar at 25°C. The Raman spectroscopic detection limits were investigated using a multichannel Raman spectrometer. Because the Raman spectroscopic MDLs are a function of counts, the CO₂ MDLs were determined by collecting signal-to-noise ratios for both the upper and lower v₁-2v₂ bands as a function of CO₂ pressure (5-60 bars) and over a range of integration times and incident laser power to predict the optimal instrument settings. The resulting CO₂ MDLs are on the order of 1 bar for our instrument. The band splitting of the v₁-2v₂ diad as a function of CO₂ pressure was measured up to 500 bar at ambient temperature. The CO₂ pressures were converted to ρ_CO₂ and the results are given in terms of the frequency separation between the upper and lower bands. These results are compared to those of previous studies. An analysis of the estimated errors indicates that the technique can be used to determine CO₂ densities in fluid inclusions containing a homogenous, free CO₂ phase to a precision of approximately ± 0.02 g/cm³. The temperature dependence of the intensity ratio of the hot bands to the v₁-2v₂ diad was measured from 270 to 315 K. The close agreement between the calculated and observed results indicate that laser induced sample heating is not significant. The intensity ratio can be used to estimate the CO₂ temperature and, combined with the Raman density determination, allows calculation of the CO₂ pressure.
Discovery and ramifications of incidental Magnéli phase generation and release from industrial coal-burning
Yang, Yi; Chen, Bo; Hower, James C.; Schindler, Michael; Winkler, Christopher; Brandt, Jessica E.; Di Giulio, Richard T.; Ge, Jianping; Liu, Min; Fu, Yuhao; Zhang, Lijun; Chen, Yu-ru; Priya, Shashank; Hochella, Michael F. Jr. (Nature Publishing Group, 2017-01-12)
Coal, as one of the most economic and abundant energy sources, remains the leading fuel for producing electricity worldwide. Yet, burning coal produces more global warming CO2 relative to all other fossil fuels, and it is a major contributor to atmospheric particulate matter known to have a deleterious respiratory and cardiovascular impact in humans, especially in China and India. Here we have discovered that burning coal also produces large quantities of otherwise rare Magneli phases (Ti; x; O2x–1 with 4 ≤ x ≤ 9) from TiO2 minerals naturally present in coal. This provides a new tracer for tracking solid-state emissions worldwide from industrial coal-burning. In its first toxicity testing, we have also shown that nanoscale Magneli phases have potential toxicity pathways that are not photoactive like TiO2 phases, but instead seem to be biologically active without photostimulation. In the future, these phases should be thoroughly tested for their toxicity in the human lung. Solid-state emissions from coal burning remain an environmental concern. Here, the authors have found that TiO2 minerals present in coal are converted into titanium suboxides during burning, and initial biotoxicity screening suggests that further testing is needed to look into human lung consequences.
The dissolution rates of amorphous silica and opal-CT
Gu, Jing (Virginia Tech, 1994-08-05)
Dissolution rates of two different glasses (soda-lime glass and fused quartz) and a natural opal (opal-CT) in distilled and deionized water and two concentrations of NaCI solution from 25°C to 75°C were measured by molybdate blue method and determined by initial rate method. The specific surface area of the samples were determined by N2 BET procedure. XRD patterns were obtained to check the crystallinity of these samples. Dissolution experiments show that soda-lime glass dissolves the fastest and opal-CT dissolve the most slowly. The dissolution rate of each sample is about one order of magnitude higher at 75°C than that at 2S°C. The calculated Ea for soda-lime glass is 32.7 kJ/mole, for fused quartz is 37.S kJ/mole and for opal-CT is 41.7 kJ/mole.

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