Chemistry & Biochemistry Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2752
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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 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 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.