Ph.D. Theses
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Browsing Ph.D. Theses by Author "Bekbölet, Miray."
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Item 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.Item Evaluation of natural organic matter-metal oxide adsorption isotherms under influential structural concepts(Thesis (Ph.D.)-Bogazici University. Institute of Environmental Sciences, 2006., 2006.) Süphandağ, Şerif Altan.; Bekbölet, Miray.The adsorption of molecules onto a surface is a necessary prerequisite to any surface mediated chemical process. Therefore, the mechanism of binding of humic acids to TiO2 surface has to be addressed in order to improve the understanding of photocatalytic degradation. From the fundamental point of view, natural organic matter (NOM) for water scientists can be perceived as very complex entities both in terms of chemistry and comprehension. Especially the fulvic (FA) and humic acids (HA), the soluble portions of NOM, are often considered as oligomeric and polymeric materials. In literature, there are ongoing discussions regarding the high molecular mass is due to aggregation of small units, as in micelles, or due to covalently bound units, as in polymers. The essence of this thesis is to evaluate the adsorption under the light of these two debatable approaches and bring reasonable interpretations of the observed isotherm patterns through the use of proposed structures. In this dissertation the isotherms of humic acid (HA) on titanium dioxide are analyzed at acidic, neutral, and basic pH. Combined effects of pH and increasing ionic strength were evaluated in order to asses the effect of changing solution matrix on the molecular structure of humic acid.|Keywords: humic acid, titanium dioxide, adsorption, natural organic matter, metal oxidesItem Investigating the role of the Turkish straits system as a phylogeographical break in the mediterranean - black sea transition(Thesis (Ph.D.) - Bogazici University. Institute of Environmental Sciences, 2013., 2013.) Kalkan, Evrim.; Bilgin, İbrahim Raşit.; Bekbölet, Miray.Understanding the allopatric isolation and evolutionary processes in the marine realm can be difficult due to high dispersal potential of pelagic larvae. In addition, the role of barriers in shaping gene flow patterns between the populations of marine species can be less pronounced than their terrestrial counterparts. Straits are such potential barriers to gene flow in the marine environment, resulting in the isolation of populations on either side. The Turkish Straits System, comprising two straits (the Dardanelles and the Bosphorus Strait) and the Sea of Marmara forms the only connection between the Black Sea and the Mediterranean, and is a unique ecosystem with a well-defined two-layered stratification and current regime. The role of Turkish Straits System on gene flow, as a barrier and/or corridor has been proposed, but not extensively tested using genetics. Here, using three regions of mitochondrial DNA (CO1, COIII and 16S) and five microsatellite markers I tried to understand the effect of the system on gene flow in populations of the Mediterranean Mussel, Mytilus galloprovincialis (Lamarck, 1819), the common European prawn, Palaemon elegans Rathke, 1837 and the marbled crab, Pachygrapsus marmoratus (Fabricius, 1787). With this study, individuals belonging to three species were collected from 42 sampling sites, encompassing the Black Sea, the Turkish Straits System and the Mediterranean Sea. The results of the mtDNA analyses of Mytilus galloprovincialis showed that the Black Sea populations were isolated and differentiated from those in the Aegean during the last ice age and subsequently were able to colonize the Sea of Marmara and Aegean twice, with larval transport via the surface currents of the Turkish Straits System. However, individuals from the Aegean population were not able to migrate into the Turkish Straits System and the Black Sea in the reverse direction due to the lower-layer currents of the system. Microsatellite analyses did not support the mtDNA differentiation observed among the Black Sea and the Aegean mussel populations, suggesting that they did not correspond to different species. Two different haplogroups were detected in Palaemon elegans as a result of the mtDNA analyses, though with a lower degree of differentiation than previously recorded in the literature. The results of CO1 analysis for Pachygrapsus marmoratus also indicated a weak restriction of gene flow from the Mediterranean to the Black Sea. For all three species, the Turkish Straits System played semi-permeable barrier role to gene flow and dispersal. This semi-permeable characteristic of the Turkish Straits System, simultaneously acting as a barrier and corridor to gene flow is relatively uncommon, and has been documented in the Turkish Straits System for the first time, using genetics methods.Item Investigation of troposphric aeosols by multiwavelength Mie-Raman lidar(Thesis (Ph.D.)-Bogazici University. Institute of Environmental Sciences, 2010., 2010.) Hüseyinoğlu, Mehmet Fatih.; Bekbölet, Miray.; Allahverdi, Kerim.Atmospheric aerosols affect the Earth’s radiation balance by absorbing or scattering the fluxes of solar or terrestrial radiation. This change leads to radiative forcing of the atmosphere. Scattering of the solar radiation by aerosols tends to cool the atmosphere and this is called the "aerosol direct effect". Aerosols modify the cloud properties effecting rainfall patterns and this is called the "aerosol indirect effect". Thus, aerosols cause global cooling. Aerosols also have negative effect on public health; inhaling aerosols results with chemicals entering the human body and deposited into lungs causing lung cancer, asthma, premature death and cardiovascular problems. Lidar is the acronym of "light detection and ranging", with a laser transmitter and a telescope receiver supplied with electronic and optical instruments. Aerosol optical and physical parameters and important atmospheric data about the clouds and water vapor can be calculated by analyzing aerosol extinction and backscattering coefficients which can be obtained by the remote sensing technology of lidars. For the purpose of investigating the aerosols, a Multiwavelength Mie-Raman lidar has been designed, developed and operated between 2009 and 2010. By this lidar, extinction and backscattering at multiple wavelengths were measured and converted to microphysical properties of aerosols by mathematical methods. The retrieved data contains size distribution, effective radius, volume, surface area and number density, complex refractive index, volume and particle polarization, water vapor mixing ratio. The ash plums from the Iceland volcanic eruption were detected and characterized.