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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

INDIRECT PHOTOCHEMICAL FORMATION OF COS AND CS2 IN NATURAL WATERS: KINETICS AND REACTION MECHANISMS

Mahsa Modiri-Gharehveran (6594389) 15 August 2019 (has links)
<p></p><p><a>COS and CS<sub>2</sub> are sulfur compounds that are formed in natural waters. These compounds are also volatile, which leads them move into the atmosphere and serve as critical precursors to sulfate aerosols. Sulfate aerosols are known to counteract global warming by reflecting solar radiation. One major source of COS and CS<sub>2</sub> stems from the ocean. While previous studies have linked COS and CS<sub>2</sub> formation in these waters to the indirect photolysis of organic sulfur compounds, much of the chemistry behind how this occurs remains unclear. This study examined this chemistry by evaluating how different organic sulfur precursors, water quality constituents, and temperature affected COS and CS<sub>2</sub> formation in natural waters.</a></p> <p>In the first part of this thesis (chapters 2 and 3), nine natural waters ranging in salinity were spiked with various organic sulfur precursors (e.g. cysteine, cystine, dimethylsulfide (DMS) and methionine) exposed to simulated sunlight over varying exposures. Other water quality conditions including the presence of O<sub>2</sub>, CO and temperature were also varied. Results indicated that COS and CS<sub>2</sub> formation increased up to 11× and 4×, respectively, after 12 h of sunlight while diurnal cycling exhibited varied effects. COS and CS<sub>2</sub> formation were also strongly affected by the DOC concentration, organic sulfur precursor type, O<sub>2</sub> concentration, and temperature while salinity differences and CO addition did not play a significant role.</p> <p>To then specifically evaluate the role of DOM in cleaner matrices, COS and CS<sub>2</sub> formation was examined in synthetic waters (see chapters 4 and 5). In this case, synthetic waters were spiked with different types of DOM isolates ranging from freshwater to ocean water along with either cysteine or DMS and exposed to simulated sunlight for up to 4 h. Surprisingly, CS<sub>2</sub> was not formed under any of the tested conditions, indicating that other water quality constituents, aside from DOM, were responsible for its formation. However, COS formation was observed. Interestingly, COS formation with cysteine was fairly similar for all DOM types, but increasing DOM concentration actually decreased formation. This is likely due to the dual role of DOM on simultaneously forming and quenching the reactive intermediates (RIs). Additional experiments with quenching agents to RIs (e.g. <sup>3</sup>DOM* and ·OH) further indicated that ·OH was not involved in COS formation with cysteine but <sup>3</sup>DOM* was involved. This result differed with DMS in that ·OH and <sup>3</sup>DOM* were both found to be involved. In addition, treating DOM isolates with sodium borohydride (NaBH<sub>4</sub>) to reduce ketone/aldehydes to their corresponding alcohols increased COS formation, which implied that the RIs formed by these functional groups in DOM were not involved. The alcohols formed by this process were not likely to act as quenching agents since they have been shown to low in reactivity. Since ketones are known to form high-energy-triplet-states of DOM while quinones are known to form low-energy-triplet-states of DOM, removing ketones from the system further supported the role of low-energy-triplet-states on COS formation. This was initially hypothesized by findings from the testes on DOM types. In the end there are several major research contributions from this thesis. First, cysteine and DMS have different mechanisms for forming COS. Second, adding O<sub>2</sub> decreased COS formation, but it did not stop it completely, which suggests that further research is required to evaluate the role of RI in the presence of O<sub>2</sub>. Lastly, considering the low formation yields of COS and CS<sub>2</sub> formation from the organic sulfur precursors tested in this study, it is believed that some other organic sulfur precursors are missing which are likely to generate these compounds to higher levels and this needs to be investigated in future research. </p><br><p></p>
2

Transformation of Trace Organic Contaminants Involving Reactive Oxygen Species Driven By Solar Lights

Vo, Hao T.H., Vo, Hao T.H. January 2017 (has links)
The presence of trace organic compounds (TOrCs) in wastewater effluent and surface waters has raised attention due to their health and environmental effects. Some TOrCs are naturally attenuated via biodegradation, photo-degradation and/or adsorption, but some persist in the environment as contaminants in surface and ground waters. Thus, it is crucially important to understand their transformation pathways and their mechanisms following their discharge into the environment. This work presents research in three parts: • The first part represents an investigation of photo-transformation of TOrCs (e.g., furfuryl alcohol, p-cresol, gemfibrozil) under UV254, UVA and natural sunlight, and involving reactive oxygen species including singlet oxygen, hydroxyl radicals, triplet excited states, and specific inorganic radicals that are created by effluent organic matter (EfOM). Singlet oxygen was the only ROS, generated from effluent organic matter (EfOM) that mainly contributed to the photo-transformation of these selected TOrCs. A comprehensive mechanism and complementary kinetic model were proposed to predict the trajectories of TOrC removals via reaction with singlet oxygen. Simulations built on predicted quantum efficiencies accounted for light shading and competitive effects. Agreement between measurements and simulations was excellent. • The second part of the dissertation summarizes expected removal efficiencies for fifty-five TOrCs in alternative engineered and natural treatment processes including conventional biological treatment, advanced oxidation processes (AOP), reverse osmosis (RO), granular activated carbon (GAC), and sunlight photolysis. • The last section of the dissertation follows the trajectories of a series of TOrCs and total estrogenic activity in the Santa Cruz River, following their discharge from a wastewater treatment plant in the Tucson area. The study suggests that some TOrCs tend to persist in the environment while others experience photo (or other) transformations that diminish their concentrations or activities with distance and time of travel in the river. The attenuation of estrogenic activity was dependent on sunlight and the presence of specific (unidentified) wastewater components.

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