UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
7 results
Search Results
Item Marsh-ing Through Time: Resolving the temporal and spatial variability of tidal marsh sediment dissolved organic carbon sorption(2021) Morrissette, Hannah; Hood, Raleigh; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tidal marsh ecosystems are among the most economically and ecologically valuable environments in the world, providing critical ecosystem services and a continuous exchange of carbon between these systems and their surrounding environments. Tidal marshes are an important overall net carbon sink, while simultaneously being a substantial source of dissolved organic carbon (DOC) to estuaries and the coastal ocean. The temporal and spatial variability in these carbon fluxes is large, difficult to measure, and currently considered to be one of the most daunting challenges to carbon exchange quantification. Sorption, despite being known as a dominant DOC exchange process at the sediment-water interface, is still understudied in tidal marsh ecosystems, with exchange kinetics largely unquantified. This research combined observational data with sediment flux modeling to answer a suite of questions addressing sorption speed, its variability, and its impacts to DOC fluxes between sediments and adjacent waters. Sediment flux models must incorporate sorption processes to more accurately simulate DOC fluxes between tidal marsh sediments and adjacent waters. Kinetics of these processes were quantified for the first time through a set of 24 hour sorption laboratory experiments, from which results showed that the majority of sorption processes occur rapidly, within 15 minutes of sediment exposure to water. Sorption rate parameters were determined through a numerical modeling study that simulated the laboratory experiments. These rates were used to parameterize a sediment flux model that included sorption processes formulated with varying degrees of complexity. The sorption kinetics of individual pools of DOC (colored and non-colored) were also measured, revealing that these separate pools sorb quickly but independently of one another, with preferential adsorption of humic colored DOC over time, and preferential desorption of native non-colored DOC over time. Sorption kinetics were also shown to be spatially variable within a marsh site, with adsorption decreasing with sediment depth and distance from the creek edge. This research provided important new information on sorption in tidal marsh sediments that allows these processes to be incorporated into models, which will, ultimately, facilitate efforts to simulate and quantify coastal carbon fluxes.Item INVESTIGATING SOURCES OF AGE-RELATED DIFFERENCES IN WALKING MECHANICS(2019) Krupenevich, Rebecca Lynn; Miller, Ross H.; Kinesiology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Walking is one of the most common activities of daily living and represents independence and improved quality of life, particularly among older adults. However, many older adults report substantial mobility challenges, which may be associated with age-related differences in lower-extremity gait kinetics. These differences are summarily referred to as a ‘distal to proximal shift’ of joint moments and powers, and are characterized by smaller ankle kinetics and larger hip kinetics in older vs. young adults. Although age-related differences in walking mechanics are well-documented, there is little consensus about which biomechanical factors contribute to these differences. Addressing this gap in knowledge is an important step in determining if this shift is preventable, or rather, an unavoidable part of healthy aging. Therefore, the overarching goal of this dissertation was to investigate sources of the age-related distal to proximal shift in gait kinetics. Specifically, this dissertation determined the extent to which the shift in kinetics is explained by age-related differences in (i) step length and trunk kinematics, (ii) years of endurance running (i.e., habitual physical activity), and (iii) gastrocnemius muscle architecture and individual lower-extremity muscle forces. In study 1, step length and trunk position did not reverse or reduce the age-related distal to proximal shift. Similarly, in study 2, a history of habitual endurance running did not reduce or reverse the shift. The third study confirmed the distal to proximal shift at the muscle level, suggesting that gastrocnemius may be a primary site of age-related differences in plantarflexor force, due to the shorter gastrocnemius muscle fascicles and smaller gastrocnemius force production in older adults vs. young adults. The present findings support the notion that the age-related distal to proximal shift of kinetics in active older adults is due primarily to differences at the muscle level and do not support previous speculations that this shift is due to spatiotemporal factors such as step length, joint kinematics, or physical activity. Further, these results suggest that age-related differences in lower-extremity joint kinetics are an unavoidable part of natural aging even in the absence of mobility limitations and the presence of a lifelong history of endurance running.Item Oxygen Exchange Mechanisms on Solid Oxide Fuel Cell Cathodes in the Presence of Gas Phase Contaminants(2016) Pellegrinelli, Christopher; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polarization losses associated with the oxygen reduction reaction (ORR) at the cathode and degradation of cathode materials remain as hurdles for high performance solid oxide fuel cells (SOFC). Rates of degradation depend significantly on the operating temperature and gas conditions, such as the presence of unwanted oxygen-containing compounds, namely H2O and CO2. In this study ORR fundamentals for the common cathode materials, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and (La0.8Sr0.2)0.95MnO3±δ (LSM), as well as their composites with Gd0.10Ce0.90O1.95 (GDC) and (Y2O3)0.08(ZrO2)0.92 (YSZ), respectively, are evaluated as a function of operating environment. A combination of electrochemical impedance spectroscopy (EIS) and gas phase oxygen isotope exchange is used to probe the kinetics of heterogeneous gas-solid reactions and electrode performance under a wide range of conditions. The results suggest that CO2 and H2O actively participate in the ORR and that the level of participation and governing mechanisms are dependent on the specific conditions. It was found that CO2 adsorbs readily on the surface of LSCF and leads to significant performance loss, while the affect of CO2 on LSM, an arguably similar material, is minimal. We propose that intrinsic material properties, such as vacancy concentration, will alter the contaminant interactions significantly, leading to specific contaminant-material relationships. The ORR on LSM is compared to that on LSM-YSZ composite, where the triple phase boundary (TPB), gas-electrode-electrolyte interface, plays a vital role. Further, the role of water in both the single phase, and composite materials is explored. A new in operando isotope exchange technique that couples electrochemical polarization with gas phase isotopic transient and steady state results is proposed, and initial results discussed. The development of in operando experiments is crucial to gain a full understanding of electrodes under real operating conditions, where chemical species are being driven by an electrochemical potential. The results contribute to the understanding of gas-solid reaction kinetics on ion conducting catalysts, and provide a basis for future experimental investigations.Item Multiple sulfur isotope fractionations in inorganic aqueous systems(2016) Eldridge, Daniel Lee; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)New constraints on isotope fractionation factors in inorganic aqueous sulfur systems based on theoretical and experimental techniques relevant to studies of the sulfur cycle in modern environments and the geologic rock record are presented in this dissertation. These include theoretical estimations of equilibrium isotope fractionation factors utilizing quantum mechanical software and a water cluster model approach for aqueous sulfur compounds that span the entire range of oxidation state for sulfur. These theoretical calculations generally reproduce the available experimental determinations from the literature and provide new constraints where no others are available. These theoretical calculations illustrate in detail the relationship between sulfur bonding environment and the mass dependence associated with equilibrium isotope exchange reactions involving all four isotopes of sulfur. I additionally highlight the effect of isomers of protonated compounds (compounds with the same chemical formula but different structure, where protons are bound to either sulfur or oxygen atoms) on isotope partitioning in the sulfite (S4+) and sulfoxylate (S2+) systems, both of which are key intermediates in oxidation-reduction processes in the sulfur cycle. I demonstrate that isomers containing the highest degree of coordination around sulfur (where protonation occurs on the sulfur atom) have a strong influence on isotopic fractionation factors, and argue that isomerization phenomenon should be considered in models of the sulfur cycle. Additionally, experimental results of the reaction rates and isotope fractionations associated with the chemical oxidation of aqueous sulfide are presented. Sulfide oxidation is a major process in the global sulfur cycle due largely to the sulfide-producing activity of anaerobic microorganisms in organic-rich marine sediments. These experiments reveal relationships between isotope fractionations and reaction rate as a function of both temperature and trace metal (ferrous iron) catalysis that I interpret in the context of the complex mechanism of sulfide oxidation. I also demonstrate that sulfide oxidation is a process associated with a mass dependence that can be described as not conforming to the mass dependence typically associated with equilibrium isotope exchange. This observation has implications for the inclusion of oxidative processes in environmental- and global-scale models of the sulfur cycle based on the mass balance of all four isotopes of sulfur. The contents of this dissertation provide key reference information on isotopic fractionation factors in aqueous sulfur systems that will have far-reaching applicability to studies of the sulfur cycle in a wide variety of natural settings.Item Model Development for Gadolinia-doped Ceria-based Anodes in Solid Oxide Fuel Cells(2014) Wang, Lei; Jackson, Greg S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Intermediate temperature (500 &ndash 700 °C) solid oxide fuel cells (IT&ndashSOFCs) with gadolinia&ndashdoped ceria (GDC) electrolytes have significant commercial potential due to reduced materials costs for seals and interconnect and improved performance with high oxide&ndashion conductivity at these temperatures. As an SOFC anode component in the reducing anode environments, GDC offers enhanced catalytic activity and tends to suppress carbon deposition in composite Ni/GDC anodes. The current study investigates relevant kinetics on GDC anodes for IT&ndashSOFC applications. Simultaneous electrochemical characterization and X&ndashray photoelectron spectroscopy of thin&ndashfilm Ni/GDC and Au/GDC electrochemical cells provide a basis for understanding pathways for H2 and CO electrochemical oxidation as well as H2O splitting on GDC and GDC composite electrodes. Differences in electrochemical performance of Ni/GDC and Au/GDC electrodes at temperatures below 650 °C reveal limitations of GDC surfaces in promoting electrooxidation under conditions of low polaron (electron) mobility. These results also suggest the role of the metal in promoting hydrogen spillover to facilitate change transfer reactions at the Ni/GDC interface. Variation in OH- concentration at the metal/GDC interface with operating temperature, effective oxygen partial pressure, and electric bias provides valuable insight into the nature of electrochemical and other heterogeneous reactions in IT&ndashSOFC anodes. A detailed kinetic model for the GDC surface reactions and Ni/GDC charge&ndashtransfer reactions of H2 oxidation and H2O electrolysis is developed based on electrochemical characterization and spectroscopic analysis of GDC surface electrochemistry. The thermodynamically consistent kinetic model is able to capture the observed chemical and electrochemical processes on the thin&ndashfilm Ni/GDC electrode. A full three&ndashdimensional IT&ndashSOFC stack model is developed with simplified kinetics to evaluate GDC&ndashbased anode performance with H2 and methane&ndashderived fuels. The stack model explores the effects of operating condition on performance of stacks with GDC electrolytes and Ni/GDC anodes. The parametric study results of stack model provide essential information for optimizing performance of IT&ndashSOFCs stack and guiding IT&ndashSOFC design. Temperature distribution in non&ndashisothermal model result suggests that internal CH4 reforming can be used as an effective thermal management strategy to maintain high current densities and cell voltages and to lower risk to thermo&ndashmechanical degradation.Item Binding Interactions in the Bacterial Chemotaxis Signal Transduction Pathway(2008-12-08) Eaton, Anna Kolesar; Stewart, Richard C; Cell Biology & Molecular Genetics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The investigation of signal transduction pathways is critical to the basic understanding of cellular processes as these pathways function to regulate diverse processes in both eukaryotes and prokaryotes. This dissertation focuses on understanding some of the biochemical events that take place in the chemotaxis signal transduction pathway of bacteria. In this system, cell-surface receptor proteins regulate a histidine protein kinase, CheA, that autophosphorylates and then transfers its phosphate to an effector protein, CheY. Phospho-CheY, in turn, influences the direction of flagellar rotation. This sequence of biochemical events establishes a chain of communication that ultimately allows the chemotaxis receptor proteins to regulate the swimming pattern of the bacterial cell when it encounters gradients of attractant and repellent chemicals in its environment. The three projects presented in this dissertation sought to fill basic gaps in our current understanding of CheA and CheY function. In the first project, I examined the nucleotide binding reaction of CheA using the fluorescent nucleotide analogue, TNP-ATP [2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate]. TNP-ATP is an effective inhibitor for CheA. By monitoring the fluorescence of TNP-ATP when it bound to CheA, I examined the affinity of the binding interaction and discovered that the two ATP binding sites of each CheA dimer exhibited negative cooperativity in their interactions with TNP-ATP. This is the first evidence of cooperativity in the histidine protein kinase superfamily. In the second project, I focused on elucidating the binding mechanism that underlies formation of the CheA:TNP-ATP complex. My results indicated a three-step mechanism, including rapid formation of a low-affinity complex, followed by two steps during which conformational changes give rise to the final high-affinity complex. This same basic mechanism applied to CheA from Escherichia coli and from Thermotoga maritima. In the third project, I turned my attention to studying the CheY phosphorylation and binding reactions using fluorescently labeled versions of CheY. The results of this final study indicated that CheY proteins labeled with the fluorophore Badan [6-bromoacetyl-2-(dimethylamino)naphthalene] could be useful tools for investigating CheY biochemistry. However my results also brought to light some of the limitations and difficulties of this approach.Item Kinetics and morphology of metallocene catalyzed syndiospecific polymerization of styrene in homogeneous and heterogeneous reaction systems(2008-11-21) Han, Joong Jin; Choi, Kyu Y; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Syndiotactic polystyrene (sPS) is a semicrystalline thermoplastic polymer with many advantageous properties such as excellent heat resistance with a high melting point of 270-272oC, strong chemical resistance against acids, bases, oils and water, and low dielectric constant. The relatively fast crystallization rate makes sPS a promising material for a large number of applications in the automotive, electrical and packaging industries. In this study, the kinetics of syndiospecific polymerization of styrene is investigated through experimentation and theoretical modeling using homogeneous and heterogeneous Cp*Ti(OCH3)3/MAO catalysts. During sPS slurry polymerization, the physical phase changes of reaction mixture occur. With an increase in total solid content, sPS slurry undergoes a series of physical changes from clear liquid to a wet cake or paste-like material. A detailed reaction kinetic model based on a two-site kinetic mechanism has been developed to predict the polymerization rate and polymer molecular weight distribution. The monomer partition effect is incorporated into kinetic models to account for the nonlinear dependence of polymerization rate on the bulk phase monomer concentration. Quite satisfactory agreement between the model simulation results and experimental data has been obtained. The morphological development of nascent sPS particles during the polymerization has also been investigated. Most notably, it was found that sPS particles grow with the nanofibrillar morphology with either homogeneous or silica-supported metallocene catalyst. The analysis of nascent morphology of sPS using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDS/EDX) analysis, revealed that there is a strong correlation between the formation of sPS nanofibrillar structure and sPS crystallization. A mechanism for the growth of sPS particles is also proposed based on the experimental observations and analysis. Ultrahigh molecular weight sPS has also been synthesized in silica nanotube reactors (SNTRs) and the morphological characteristics of sPS produced in the nanotube reactors have been analyzed. A new mechanism is proposed for the formation and growth of sPS nanofibrils extruding out from the nanotube reactors. Also, a kinetic analysis is presented to interpret the observed molecular weight enhancement effect that is believed to be caused by the constrained reaction environment inside the nanotubes.