Theses and Dissertations from UMD
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Item AUTOMATION MODEL FOR pH AND CONDUCTIVITY ADJUSTMENT FOR A CATION EXCHANGE COLUMN ELUATE BUFFER(2010) Parmar, Anupreet; Bentley, William E; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A comprehensive model for calculating pH and conductivity values for a buffer solution added to the eluate of a cation exchange column at Bayer Healthcare has been developed. Bayer Healthcare's Berkeley facility manufacturess Kogenate - a drug for Hemophilia. This project was carried out at one of the intermediate purification steps of Kogenate, where the pH and conductivity of a buffer solution added to a chromatographic column's eluate has to be critically adjusted. Currently this process is done manually and requires two operators. In order to reduce the manual labor as well as improve accuracy, an automation model was developed using feedback control and Bayer's pH and conductivity predicting empirical correlations.Item Investigation of Rhenium's Biogeochemistry(2009) Dolor, Marvourneen; Helz, George R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The form of the paleoredox indicator, rhenium, that is sequestered in reducing sediments is not known. To probe this question, I used Laser Ablation ICP-MS to look for correlations between Re signals and those of other elements. My results point to the possibility that Re is incorporated into multiple host phases: sulfides and organic material. Laser Ablation ICP-MS also provided a new way to analyze a large suite of elements in Chesapeake Bay sediments simultaneously. A number of rarely determined elements (Ag, Sn, Sb, Te and Bi) were discovered to exhibit profiles similar to known industrial pollutants such as Cu, Zn, Cd and Pb. This is one of the first demonstrations of LA-ICP-MS as a tool to perform a survey of anthropogenic impacts on modern sediments and suggests that the suite of elements monitored by environmental agencies should be broadened. Rhenium is scavenged between iron and sulfate reduction, in the sequence of microbial terminal electron accepting processes, so I investigated whether Re could be reduced directly or indirectly by iron and/or sulfate reducing bacteria. This hypothesis was also based on the fact that Tc (also Group VIIB), is directly and indirectly reduced by the iron and sulfate reducers used in my experiments: Geobacter metallireducens GS-15, Shewanella oneidensis €MR-1, Desulfovibrio desulfuricans subsp. desulfuricans and Desulfovibrio desulfuricans ND132. I concluded that neither direct nor short-term indirect microbial processes are likely to explain Re fixation in sediments. In order to test the hypothesis that thioperrhenates play a role in Re fixation, experimentally determined stability constants, (K (4-x)(5-x) ) for the following types of reactions are needed. ReO x S 4-x 2+ H2S = ReOx-1S5-x- + H2O where 1 ≤ x ≤ 4 These compounds are analogous to thiomolybdates, which are the particle reactive form of Mo fixed under reducing conditions. The di- and tri- thioperrhenates were never observed, by UV-Vis spectrophotometry. The stability constant for mono-thioperrhenate, K01 = 104, while the stability constant for tetra-thioperrhenate, K04 = 1019. Below pH 8, the formation of a rhenium-sulfur nano-phase competed with the formation of thioperrhenates. The formation of this nano-phase is most likely an important step in rhenium fixation. Future work should involve exploring the sorption characteristics of this material.Item MOLECULAR DYNAMICS STUDIES OF METALLIC NANOPARTICLES(2009) Henz, Brian John; Zachariah, Michael R; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metal nanoparticles have many desirable electrical, magnetic, optical, chemi-cal, and physical properties. In order to utilize these properties effectively it is neces-sary to be able to accurately predict their size-dependent properties. One common method used to predict these properties is with numerical simulation. The numerical simulation technique used throughout this effort is the molecular dynamics (MD) si-mulation method. Using MD simulations I have investigated various metallic nano-particle systems including gold nanoparticles coated with an organic self-assembled monolayer (SAM), the self-propagating high-temperature synthesis (SHS) reaction of nickel and aluminum nanoparticles, and the mechano-chemical behavior of oxide coated aluminum nanoparticles. The model definition, boundary conditions, and re-sults of these simulations are presented in the following dissertation. In the first material system investigated MD simulations are used to probe the structure and stability of alkanethiolate self-assembled monolayers (SAMs) on gold nanoparticles. Numerous results and observations from this parametric study are pre-sented here. By analyzing the mechanical and chemical properties of gold nanopar-ticles at temperatures below the melting point of gold, with different SAM chain lengths and surface coverage properties, we have determined that the material system is metastable. The model and computational results that provide support for this hy-pothesis are presented. The second material system investigated, namely sintering of aluminum and nickel, is explored in chapter 4. In this chapter MD simulations are used to simulate the kinetic reaction of Ni and Al particles at the nanometer scale. The affect of par-ticle size on reaction time and temperature for separate nanoparticles has been consi-dered as a model system for a powder metallurgy process. Coated nanoparticles in the form of Ni-coated Al nanoparticles and Al-coated Ni nanoparticles are also analyzed as a model for nanoparticles of one material embedded within a matrix of the second. Simulation results show that the sintering time for separate and coated nanoparticles is dependent upon the number of atoms or volume of the sintering nanoparticles and their surface area. We have also found that nanoparticle size and surface energy is an important factor in determining the adiabatic reaction temperature for both systems, coated and separate, at nanoparticle sizes of less than 10nm in diameter. The final material system investigated in chapters 5 and 6 is the oxide coated aluminum nanoparticle. This material system is simulated using the reactive force field (ReaxFF) potential which is capable of considering the charge transfer that occurs during oxidation. The oxidation process of oxide coated aluminum nanoparticles has been observed to occur at a lower temperature and a faster rate than micron sized nanoparticles, suggesting a different oxidation mechanism. From this effort we have discovered that the oxidation process for nanometer sized oxide coated aluminum particles is the result of an enhanced transport due to a built-in electric field induced by the oxide shell. In contrast to the currently assumed pressure driven diffusion process the results presented here demonstrate that the high temperature oxidation process is driven by the electric field present in the oxide layer. This electric field ac-counts for over 90% of the mass flux of aluminum ions through the oxide shell. The computed electric fields show good agreement with published theoretical and experi-mental results. The final chapter includes some important conclusions from this work and highlights some future work in these areas. Future work that is outlined includes ef-forts that are currently underway to analyze the interactions of multiple alkanethiolate coated gold nanoparticles in vacuum and in solvent. Other future efforts are farther out over the horizon and include using advanced computing techniques such as gen-eral purpose graphical processing units (GPGPU) to expand simulation sizes and physical details over what it is currently possible to simulate.Item OMI Tropospheric Sulfur Dioxide Retreival: Validation and Analysis(2007-08-28) McClure, Brittany; Dickerson, Russell R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)SO2 impacts the radiative balance of the Earth and is the precursor to the major acid and much of the particulate matter in the atmosphere. Improved spectrometer resolution of the Ozone Monitoring Instrument (OMI) enables SO2 retrieval in the planetary boundary layer. OMI has a small spatial resolution of 13 km x 24 km and daily near-global coverage. I have evaluated the accuracy of the OMI by comparing aircraft measurements in Northeast China to the OMI retrieval of three different algorithms: the Band Residual Difference (BRD), the Spectral Fit (SF), and a combination of the two (SF & BRD). The SF algorithm shows the best agreement with a less than 15% difference for high SO2 loading (greater than 1 DU). The SF & BRD has a ~ -0.25 DU bias, the BRD and SF a ~ -0.1 DU bias. The noise of the OMI is reduced to ~0.2 DU by averaging over 100 days and is not improved by increasing the averaging time. The OMI is also able to track SO2 as it moves away from its source region in the PBL and once it is lofted above this layer.Item THE OPTICAL KERR EFFECT OF LIQUIDS(2006-12-19) Zhu, Xiang; Mullin, Amy; Fourkas, John T; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Optical Kerr effect (OKE) spectroscopy has found broad use in monitoring ultrafast dynamics in transparent media. I demonstrated that by using two pump pulses with independently-controllable polarizations, intensity and timing, different contributions to the OKE signal in liquids can be enhanced and suppressed, and I characterize in detail perpendicularly-polarized pulses used for the excitation step in OKE spectroscopy. The results indicate that the signal can be described well as arising from the sum of independent third-order responses initiated by each pump pulse. OKE spectroscopy has been used to study the orientational dynamics of benzene and benzene-d6 confined in nanoporous sol-gel glass monoliths with a range of average pore sizes. The orientational dynamics are described well by the sum of two exponentials, one of which depends on pore size. Comparision to Raman linewidth data suggests that the liquid exhibits significant structuring at the pore walls, with the benzene molecules lying flat on the surfaces of unmodified pores. OKE spectroscopy has also been used to study the temperature-dependent orientational dynamics of a series of nitriles with n-alkyl chains ranging from one to 11 carbons in length. In all cases the orientational diffusion is found to be described by a single-exponential decay. Analysis of the orientational correlation times using the Debye-Stokes-Einstein equation suggests that the molecules adopt extended configurations and reorient as rigid rods. The liquids with shorter alkyl chains undergo an apparent ordering transition as they are cooled.Item CHARACTERIZATION OF CARBONACEOUS AEROSOL: IMPROVED METHODS, SOURCES AND SIZE DISTRIBUTIONS(2006-11-30) Crimmins, Bernard Shawn; Baker, Joel E; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A highly sensitive method was developed for measuring polycyclic aromatic hydrocarbons and nitro-substituted polycyclic aromatic hydrocarbons in ambient aerosol. Using large volume injection, this technique provided an order of magnitude increase in sensitivity compare to conventional injection techniques. This method facilitated the measurement of the first reported diurnal size distribution of NPAHs. Size resolved samples were collected using a Berner low-pressure impactor deployed at the Baltimore PM2.5 Supersite in April 2002. Both classes of compounds were found predominantly on particles less than 0.49μm with similar size distributions among samples for most of the 12 hr periods. A linear relationship between compound geometric mass median aerodynamic diameter (GMMAD) and log sub-cooled vapor pressures (pl°) was observed for PAHs and NPAHs, respectively, during each sampling period. The inter-relationhips between the slopes and y-intercepts from the GMMAD/log vapor pressure correlations suggest the source of PAHs to the Baltimore atmosphere reside on particles with GMMADs equal to 0.18 μm, consistent with vehicle emissions. Bulk organic aerosol was collected in Baltimore, MD during the spring, summer and winter of 2002-2003. Concentrations of n-alkanes, hopanes, polycyclic aromatic hydrocarbons (PAH), and nitro-substituted polycyclic aromatic hydrocarbons (NPAH) were measured in the gas and particle phase. The organic compounds varied little, with seasonal concentrations typical of North American urban atmospheres. Principal Components Analysis/Multiple Linear Regression (PCA/MLR) and Positive Matrix Factorization (PMF) were used to determine the sources of individual compound classes (PAHs, NPAHs, hopanes and alkanes) and total particulate carbon and PM2.5 to the Baltimore atmosphere for during 2002-2003. PMF was used to determine the total carbon and PM2.5 source estimates to the Baltimore atmosphere. The sources identified included tire wear/road dust, gasoline and diesel exhaust, oil combustion, biogenic, secondary organic aerosol, incineration, and coal explaining 64% of the variability in the total carbon and PM2.5 concentrations.Item THE ROLE OF ORGANIC MATTER IN THE DISSOLVED PHASE SPECIATION AND SOLID PHASE PARTITIONING OF MERCURY(2006-01-24) Miller, Carrie Lynn; Mason, Robert; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The interaction of mercury (Hg) and methylmercury (MeHg) with organic matter is extremely important in the dissolved phase speciation and solid phase partitioning of Hg and MeHg in aquatic systems. This study shows, that under oxic conditions Hg and MeHg will likely associated with Fe oxides through an indirect association with organic matter, while under sulfidic conditions, solid phase Fe sulfide will dominate the complexation of Hg to the solid phase. As a result of the association of Hg with Fe solids, which undergo dynamic changes at redox interfaces in aquatic systems, the distribution of Hg on particles is likely changing at redox boundries, areas that have been shown as active zones of methylation. Redox zones are also going to be important in controlling the mobility of MeHg from the site of production to areas in aquatic systems in which uptake by biota occurs. Although the dissolved phase speciation of Hg has been shown as an important factor in Hg methylation, as a result of the diffusive uptake of neutral Hg-sulfide into bacterial cells, this speciation had previously not been measured. Hg forms stronger bonds with reduced sulfide relative to dissolved organic matter (DOM), therefore, it was not previously thought that DOM was important in the speciation of Hg under sulfidic conditions. Using modified octanol-water partitioning extractions and centrifugal ultrafiltration, the speciation of Hg in sulfidic natural samples and laboratory solutions was examined. It was shown that the concentration of neutral Hg-sulfide complexes are lower than predicted by thermodynamic models, as a result of an interaction of these species with DOM. It is proposed that the interaction of Hg with DOM is not a complexation, but rather, a partitioning of neutral Hg-sulfide complexes into hydrophobic portion of the DOM. Thermodynamic constants were calculated for this interaction and applied to model the speciation of Hg in natural samples. The concentration of neutral Hg-sulfide is lower than models previously predicted, as a result of the DOM interaction. Since the concentration of neutral Hg-sulfide affects methylation, DOM could impact the rate of Hg methylation in aquatic systems.Item AQUEOUS PHOTOCHEMISTRY OF 1,4-BENZOQUINONES AND THEIR POSSIBLE ROLE IN THE PHOTOCHEMISTRY OF NATURAL ORGANIC MATTER(2005-01-13) Gan, Daqing; Blough, Neil V; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Past research implicated the presence of an organic species as a photochemical source of OH radical in natural waters. Quinones were postulated to be one of the possible OH sources due to 1) the ubiquitous occurence of quinonoid compounds in natural systems, and 2) prior work indicating that OH was produced in the photolysis of benzoquinones. However, more recent work indicated that photolysis of 1,4-benzoquinones does not give rise to OH, but instead proceeds through an oxidizing intermediate. To further examine these two possibilities, radical trapping experiments, electron paramagnetic resonance measurements, product analysis of the reactions of benzoic acid, and optical studies were performed for a series of quinones with differing substituents. OH was shown not to be a major product in the photolysis of methyl-1,4-benzoquinone in aqueous solution. Instead, an oxidizing intermediate arising from, but distinct from the triplet quinone was implicated in the photolysis of low concentrations of mBQ. This intermediate is believed to be a triplet quinone-H2O exciplex. In the absence of electron donors, the intermediate collapses to benzene-1,2,4-triol, ultimately to form hydroquinone and hydroxybenzoquinone. At high concentrations of mBQ, however, these products are formed through a reaction of triplet quinone with ground state quinone. A complete kinetic scheme consistent with experimental results is presented. The formation of an intermediate exciplex between triplet state quinone and water was consistent with the results obtained in the photolysis of aqueous solutions of dimethyl- and dichloro-1,4-benzoquinones. Leakage of small amounts of OH from this intermediate was observed for dichloro-1,4-benzoquinones, which are much better electron acceptors than dimethyl-1,4-benzoquinones. In the case of tetrachloro-1,4-benzoquinone, which is an extremely good electron acceptor, a substantial amount of OH was produced. The investigation of isolated natural organic matter (Suwannee River fulvic acid) revealed that either OH radical or a strong oxidant was produced upon UV irradiation. Several natural waters collected from Atlantic Ocean and Chesapeake Bay were studied. In these waters, nitrite and nitrate photolysis appears to be significant source of OH.Item Intercalated MoS2 Nanoparticles for Enhanced Dispersion in Smokes and Obscurants(2003-12-05) Spence, D'Anne Emmett; Eichhorn, Bryan W; ChemistryThe intercalation of MoS2 with diethyl oxalate (DEO) and Meldrum's Acid (MA) has been achieved via an exfoliation and reflocculation process. Ethyl diazoacetate (EDA) did not intercalate under identical conditions. The resulting compounds, Li0.1MoS2(DEO)0.10 and Li0.1MoS2(MA)0.14, have the metastable 1T-MoS2 crystal structure and have been characterized by XRD, TGA, NMR, and DSC. Based on XRD analysis, the intercalated compounds are trigonal with P3 crystal symmetry where a = b = 3.36 Å and c, which varies with the intercalate, is 10.20 Å and 9.97 Å for Li0.1MoS2(DEO)0.10 and Li0.1MoS2(MA)0.14, respectively. The concentration of the DEO and MA in the intercalated compounds, 0.10 and 0.14, respectively, was calculated using air TGA data and supported by nitrogen TGA data and NMR. The structural models of the new compounds are described within.