Theses and Dissertations from UMD
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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 give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Influence of Various Wastewater Treatment Processes on Concentrations of Anthropogenic Pollutants and Their Transformation Products(2018) Armstrong, Dana Lynne; Torrents, Alba; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Whether the use be in household, industrial, medicinal, or agricultural situations, modern society relies heavily on the use of chemicals. Unsurprisingly, many of these compounds are washed down the drain and have been detected in the wastewater treatment system. Compounds such as pharmaceuticals and personal care products (PPCPs), flame retardants, surfactants, and plasticizers have all been consistently detected in samples collected from wastewater treatment plants (WWTPs). Wastewater treatment is not designed specifically to remove these pollutants so they are oftentimes released into the environment via the discharge of WWTP effluent to local water bodies or the land application of treated sludge, also known as biosolids. Once released into the environment, chemicals can influence environmental health due to toxicity, bioaccumulation, microbial resistance issues, etc. Additionally, when degradation of these chemicals during treatment does take place, they are often not fully mineralized, leading to concerns regarding the environmental effects of transformation products. This research focuses on the impact that individual treatment systems have on concentrations of the antimicrobials triclosan (TCS) and triclocarban (TCC), 4 phthalate plasticizers, and their transformation products. The primary compounds studied have been shown to possess endocrine disrupting capabilities and to be present in biosolids at high concentrations due to extensive use. Treatment systems studied included activated sludge, nitrification, anaerobic digestion, and Cambi Thermal Hydrolysis Process (CambiTHP) pretreatment. Experiments were carried out in-lab using bioreactors to simulate treatment in a controlled manner as well as on-site at local WWTPs. The final goal of this research was the development of an extraction/analytical method for the detection of 27 compounds of concern in wastewater solids samples. Experimental results indicate that aerobic, anaerobic, and physical treatment processes can have mixed impacts on compound degradation and transformation product formation.Item Development of Approaches to Common Cause Dependencies with Applications to Multi-Unit Nuclear Power Plant(2018) Zhou, Taotao; Modarres, Mohammad; Droguett, Enrique López; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The term “common cause dependencies” encompasses the possible mechanisms that directly compromise components performances and ultimately cause degradation or failure of multiple components, referred to as common cause failure (CCF) events. The CCF events have been a major contributor to the risk posed by the nuclear power plants and considerable research efforts have been devoted to model the impacts of CCF based on historical observations and engineering judgment, referred to as CCF models. However, most current probabilistic risk assessment (PRA) studies are restricted to single reactor units and could not appropriately consider the common cause dependencies across reactor units. Recently, the common cause dependencies across reactor units have attracted a lot of attention, especially following the 2011 Fukushima accident in Japan that involved multiple reactor unit damages and radioactive source term releases. To gain an accurate view of a site's risk profile, a site-based risk metric representing the entire site rather than single reactor unit should be considered and evaluated through a multi-unit PRA (MUPRA). However, the multi-unit risk is neither formally nor adequately addressed in either the regulatory or the commercial nuclear environments and there are still gaps in the PRA methods to model such multi-unit events. In particular, external events, especially seismic events, are expected to be very important in the assessment of risks related to multi-unit nuclear plant sites. The objective of this dissertation is to develop three inter-related approaches to address important issues in both external events and internal events in the MUPRA. 1) Develop a general MUPRA framework to identify and characterize the multi-unit events, and ultimately to assess the risk profile of multi-unit sites. 2) Develop an improved approach to seismic MUPRA through identifying and addressing the issues in the current methods for seismic dependency modeling. The proposed approach can also be extended to address other external events involved in the MUPRA. 3) Develop a novel CCF model for components undergoing age-related degradation by superimposing the maintenance impacts on the component degradation evolutions inferred from condition monitoring data. This approach advances the state-of-the-art CCF analysis in general and assists in the studies of internal events of the MUPRA.Item ADDRESSING THE IMPACT ON SOIL DEGRADATION OF CHANGE FROM GRASSLAND TO CROPLAND: A CASE STUDY IN THE URUGUAYAN GRASSLANDS(2017) Castano-Sanchez, Jose P; Prince, Stephen D; Izaurralde, Roberto C; Geography; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Globally, there has been large-scale conversion of natural grassland to cropland ecosystems which this has led to land degradation that could reduce future food security, other ecosystem services and even climate. Currently, there is a dearth of quantitative information assessing the severity, distribution, and causes of this land degradation. For practical purposes, this information is needed to develop improved methods of land use (LU) conversion. Uruguay, in contrast with many other regions, still has a high proportion of unimproved grasslands but, during the last 15 years, there has been extensive conversion to grow grain crops. The fundamental goal of this dissertation was to quantify soil degradation resulting from this LU change. Two aspects of soil degradation were studied, soil organic carbon (SOC) and erosion by water. The Environmental Policy Integrated Climate biophysical simulation model (EPIC) was used to model the grassland and cropping systems. The study consisted of three steps: (1) calibration and validation of the model for the Uruguayan agroecosystems, and development of a spatial version, (2) identification of the LU change areas, and (3) quantification of soil degradation as a result of the LU changes. The EPIC model adequately reproduced the field-scale SOC dynamics and erosion in field validation sites. Further, the spatial version of the model was found to simulate spatial and temporal performance adequately. LU change areas during 2000-2013 were mapped and found to cover an area of 410,000 ha, about 13% of potential area for commercial agriculture. LU greatly affected soil degradation. It was greatest for continuous Soybean cultivation with no crop rotation, and lowest for grassland (no conversion to cropping). In addition to LU, slope and initial SOC had significant effects on degradation. The main conclusions were that the recent and continuing conversion from grassland to cropland has caused significant soil degradation, but that some modifications of LU can reduce the risk of degradation.Item LAND USE AND LAND COVER CHANGE AS A DRIVER OF ECOSYSTEM DEGRADATION ACROSS BIOMES(2016) Noojipady, Praveen; Prince, Stephen D; Geography; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The expansion and intensification of agricultural production in human-dominated landscapes threaten efforts to sustain natural ecosystems and maintain agricultural production in a changing climate. Long-term use of agricultural lands, combined with conversion of natural ecosystems for agricultural production, can rapidly degrade the health of remaining natural ecosystems. The fundamental goal of this dissertation was to assess the impacts of anthropogenic degradation on stocks and sequestration of carbon. Although degradation alters a range of ecosystem services, case studies of ecosystem degradation in this dissertation focus on reductions in vegetation productivity, carbon stocks, and the extent of natural forest cover as a result of human activity. Time series of satellite remote sensing data were used to track forest and rangeland degradation in the southwestern United States, forest carbon emissions from cropland expansion in the Brazilian Cerrado, and fire-driven forest conversion for oil palm plantations in Southeast Asia. Three major themes link the regional case studies: expansion and intensification of agricultural production, market demand and certification, and agricultural management in response to climate variability. Conclusions from the dissertation underscore the widespread influence of land management on vegetation productivity and forest carbon stocks. In the Southwest United States, reductions in net primary production on managed lands were higher in forested landscapes than other cover types. In contrast, Native American Indian Reservations, often considered to be more degraded, actually had smaller absolute reductions in net primary productivity during 2000-2011. Multi-year droughts in the southwest present new challenges for managing forests and rangelands, and climate projections suggest dry conditions will intensify in the coming century. In Southeast Asia, industry-led efforts to certify sustainable palm oil production were evaluated using satellite data on fires and forest loss. Rates of fire-driven deforestation and total fire activity declined following certification, highlighting the potential for certification to reduce ignitions during El Niño years and protect remaining fragments of lowland and peat forest. Aligning certification criteria for sustainable palm oil with satellite monitoring capabilities may help accelerate compliance with environmental legislation and market demands for deforestation-free products. In Brazil, government and industry actions to limit Amazon deforestation have largely overlooked the neighboring Cerrado biome. Forest carbon emissions from deforestation for soy expansion in the Cerrado increased substantially after the implementation of the Soy Moratorium in the Brazilian Amazon, partially offsetting recent reductions in Amazon deforestation carbon emissions. The success of policies to support sustainable agricultural production therefore depends on efforts to minimize cross-biome leakage and the ability to monitor compliance and unintended consequences. Solutions for management must also confront the growing influence of climate variability. Time series of satellite data may allow early detection of degradation impacts and support efforts to mitigate the influence of sustained agricultural production on natural systems. Changes in vegetation carbon stocks from ecosystem degradation varied across case studies, underscoring the diverse nature of direct and indirect drivers of degradation across different land use systems. Direct human drivers of ecosystem degradation in the southwest United States from management of livestock grazing resulted in gradual changes in vegetation productivity, whereas mining and oil extraction areas showed large and permanent reductions. Forest carbon emissions from agriculture expansion in the Cerrado were a one-time process, as native vegetation is cleared for cropland expansion. In contrast, the carbon emissions from Southeast Asia’s forest and peatland conversion involve both sudden and gradual processes, as carbon accumulation in oil palm plantations partially compensates for emissions from forest conversion. Overall, this research made contributions to understanding of the regional impacts of human activity and the potential for climate change mitigation from sustainable land use practices in human-dominated landscapes.Item FUNDAMENTAL UNDERSTANDING OF SOFC CATHODE DURABILITY; A KINETICS AND CATALYSIS STUDY(2015) Huang, Yi-Lin; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solid oxide fuel cells (SOFC) have been demonstrated as great prospects for electrochemical conversion of fuels, providing both high efficiency and high power density. Understanding the fundamentals of the oxygen reduction reaction (ORR) mechanisms is necessary to further improve cathode performance. Two different testing systems, gas phase isotopic oxygen exchange and electrical conductivity relaxation, were built to study the kinetics of cathode powders and bulk samples, respectively. A robust strategy was established to extract kinetic parameters from transient response curves for a variety of materials and conditions using numerical solutions. In-situ gas phase isotopic oxygen exchange, which provides real-time information about cathode surface kinetics, was used to determine the ORR mechanisms and the interactions of other gaseous species with the solid surface for two cathode materials: La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and (La0.8Sr0.2)0.95MnO3±x (LSM). LSCF has a faster dissociation reaction than LSM, and the limiting step is the surface exchange. Additionally, LSM likely contains different vacancy concentrations in the near surface region and in the bulk. A mathematic model is further established to unify surface exchange rates from different experiments and link solid-state diffusion to surface heterogeneous catalysis. In addition, the long-term durability of these materials is a major challenge. A novel technique called isotope saturated temperature programmed exchange (ISTPX) has been developed to determine the temperature and PO2 range that is preferable for the exchange of water and CO2 on LSM and LSCF. The presence of CO2 and water indicates blocking effects on the LSCF surface from 300°C to 600°C, possibly resulting in two separate degradation mechanisms. On the other hand, CO2 and water exchange with LSM through homoexchange mechanism with a relatively minor impact. Based on isotope exchange results, surface modified LSCF cathodes were fabricated. The surface modification of LSCF through Mn ion implantation enhances the chemical surface exchange coefficient (kchem) from 4.4x10-4 cm/s to 1.9x10-3 cm/s at 800°C. The aims of this study are to increase knowledge and information about the ORR. The results allow us to further investigate the ORR mechanisms as well as to engineer new cathode materials/structures that can improve cathode performance and durability.