Browsing by Author "Uyguner, Ceyda Senem."

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    Elucidation of the photocatalytic removal pathways of humic substances: progress towards mechanistic explanations
    (Thesis (Ph.D.)-Bogazici University. Institute of Environmental Sciences, 2005., 2005.) Uyguner, Ceyda Senem.; Bekbölet, Miray.
    The objective of this research was to characterize the spectroscopic properties ofhumic and fulvic acids of different origins as model compounds to represent the naturalorganic matter in aquatic systems. The photocatalytic oxidation of model humic and fulvic acids was carried out using TiO2 Degussa P-25 as the photocatalyst. The degradationkinetics was assessed based on pseudo first order and Langmuir Hinshelwood (L-H)kinetic models. The related data for aquatic and terrestrial humic substances werecomparatively presented in terms of UV-vis parameters such as Color436, Color400, UV365, UV300, UV280, UV254 and total organic carbon (TOC) removal. Furthermore, the molecularand structural characteristics of the humic acid molecule relative to changes duringphotocatalytic oxidation were monitored by spectroscopic techniques.On the basis of their diverse chemical and physical properties such as molecular weight, molecular size, elemental composition and source of origin, substantial differenceswere observed in photocatalytic removal efficiencies of humic and fulvic acids. For all ofthe humic substances, higher removal rates were achieved in terms of UV254 valuescompared to that of Color436. Moreover, humic acids exhibited higher pseudo first order removal rates with respect to that for fulvic acid. The declining trend of the specified UVvisparameters, the related changes in the fluorescence spectra (initial increase of thefluorescence intensities, formation of new fluorophores, shift of the spectra to lowerwavelength region and decline of intensity after long photocatalytic irradiation times), decrease of TOC content during degradation, the spectral changes in FTIR and NMRindicated the oxidative degradation of humic substances.Considering the complexity and polydispersity of the humic macromolecules, theywere fractionated into well defined subcomponents of known molecular sizes usingultrafiltration through membranes in the range of 100-1 kDa. The effect of photocatalyticoxidation on the molecular size fractions of humic substances were also evaluated on acomparative basis by UV-vis and fluorescence spectroscopy. As confirmed by the spectroscopic evaluation of the molecular size distributiondata, photocatalytic degradation of humic acid leads to the formation of lower molecularsize (small fractions) and higher UV-absorbing compounds. For fractions less than 10 kDa,UV254 absorbing moieties in treated humic acid samples become higher than that of rawhumic substances designating the generation of new species during photocatalysis.Based on the comparison between the synchronous scan fluorescence spectra ofthe molecular size fractions of raw and oxidized humic substances, it could be concludedthat oxidative cleavage of the molecule leads to a blue shift of the spectra which isindicative to the formation of new fluorophores in each size fraction. It is assumed thatoxidative degradation of the macromolecule occurs through an unselective pathway, by thereaction of .OH radicals with the various moieties within each fraction.
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    Mechanistic investigation on solar light initiated destruction of indicator bacteria
    (Thesis (M.S.) - Bogazici University. Institute of Environmental Sciences, 2021., 2021.) Lale, Ezgi.; Bekbölet, Miray.; Uyguner, Ceyda Senem.
    Due to the increasing shortage of clean drinking water around the world, some point of use disinfection techniques has been improved to obtain purified drinking water from pathogen microorganisms. Water disinfection using natural or artificial sunlight has been widely studied for the inactivation of microorganisms. Pathogens can be destructed with the synergistic effects of the solar light and thermal heat. Recently, application of solar photocatalysis has gained attraction for the effective inactivation of microorganisms. In this study, solar light initiated destruction of bacteria typically Escherichia coli as the model indicator organism and characterization of the released organic matter were evaluated in the presence/absence of photocatalysts, namely TiO2 and its second generation type Fe-doped TiO2. Via destruction of cell membrane, contents of released proteins, carbohydrates and potassium were determined. Solar photolytic/photocatalytic inactivation of E. coli was conducted in the presence/absence of water matrix that provides the ionic characteristics of natural waters and in the presence/absence of humic acid that is a dissolved organic matter analogue in natural waters, hence the degradation of humic acid by solar light initiated processes in the presence of E. coli was also investigated. Characterization of organic matter in respect of E. coli inactivation was evaluated by specified and specific ultraviolet-visible and fluorescence parameters, dissolved organic carbon contents, and excitation emission matrix fluorescence contour plots. E. coli inactivation was followed by bacteria reduction kinetic parameters, and detection of organic (protein and carbohydrate) and inorganic (K⁺ ion leakage) contents under specified conditions in relation with the spectroscopic parameters.
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    Trace-level metals and natural organic matter interactions: |oxidative/adsorptive removal pathways
    (Thesis (M.S.) - Bogazici University. Institute of Environmental Sciences, 1999., 1999.) Uyguner, Ceyda Senem.; Bekbölet, Miray.
    The destructive removal of humic acids in natural waters has gained increased attention, because these substances are known to be major precursors of carcinogenic disinfection byproducts which are produced during chlorination. Since the conventional treatment methods are expensive and complicated, heterogeneous photocatalytic degradation processes using Ti02 have been applied as an alternative treatment for the destruction of humic acid. This study was conducted to investigate the effects of chromium and manganese ions on the photocatalytic degradation of humic acid (10 mg L-1 ) in aqueous medium at neutral pH conditions (pH=6.7). Bench scale experiments were carried out using titanium dioxide (0.25 mg mL-1 ) as the photocatalyst and Black Light Fluorescent Lamp as the irradiation source. For comparison purposes, the photocatalytic degradation of humic acid was performed both in the presence and absence of metal ions. The degradation was followed by pseudo first order and Langmuir-Hinshelwood kinetics in terms ofColor436, Color4oo, UV280 and UV 254. The presence of chromium and manganese ions resulted in decreased removal rates on the photocatalytic degradation of humic acid compared to the baseline rate of humic acid alone. Pseudo first order reaction rate constants for Color436 and UV254 were found to be 3.86x10-2 min-1 and 2.73x10-2 min-1 in the absence of metal ions. Color436 removal rate constant decreased to 3.13x10-2 min-1 in the presence of 0.10 mg L-1 chromium ion and in the presence of 0.05 mg L-I manganese ion, a value of 3.37xlO-2 min-1 was attained. On the other hand, a similar trend was observed for UV254, the removal rate constant in the presence of chromium was calculated to be the lowest with a value of 2.06x10-2 min-1 whereas, in the presence of manganese it was found to be 2.31xlO-2 min-I. The decrease in the photocatalytic oxidation rate might be explained by the formation of a weak complex between humic acid and the metal ion which might have blocked the active sites on the Ti02 surface. Complimentary adsorption experiments were also performed to evaluate the effect of adsorption intensity on the photocatalytic degradation rates. The results indicated that no significant difference was observed with the adsorption capacity (KF) values, but compared to the other two, the humic acid + manganese system exhibited the highest values.