Modelling the Photocatalytic Degradation Kinetics of Organochloride Chemicals in Aqueous Solutions PDF Download
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Languages : en
Pages : 172
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Author:
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Category :
Languages : en
Pages : 172
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Author: Zakhele Siyanda Prince Khuzwayo
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ISBN:
Category :
Languages : en
Pages :
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Polychlorinated endocrine disrupting chemicals are environmental pollutants that are increasingly found in water sources. As a result of their hydrophobic properties, they generally accumulate in adipocytes of humans and wildlife when ingested. In this study, the feasibility of the advanced oxidation processes (AOPs) such as heterogeneous photocatalysis technology is investigated for the treatment of organochlorides in water systems. Titanium dioxide (TiO2) is the semiconductor catalyst of interest. The literature suggests that the most prominent organochlorides in the region are organochloride pesticides (OCPs). A group of 5 compounds were identified for the investigation: DDT, DDE, heptachlor, chlordane, and a polychlorinated biphenyl compound named 2,3,4-trichlorobiphenyl. Reverse phase solid phase extracted (RP-SPE) surface water organic analytes analysis was conducted using gas chromatography mass spectroscopy (GC-MS). Results from most sampling sites showed high concentration levels of the organochlorides in the environment. Heterogeneous photocatalysed mineralisation processes of organochlorides in aqueous systems were conducted in a batch reactor. Organochloride spiked solutions of differing catalyst concentrations are irradiated using a UV lamp for a period of 30 minutes. The reaction kinetics are determined and weighed against conventional photolysis. Results showed improved photo degradation of organochlorides under photocatalytic imposed conditions in comparison to photolysis. Results also suggest that photocatalytic degradation of organochlorides is less favoured with increased TiO2 catalyst concentrations. Enhanced catalyst performance studies through TiO2 surface property modification were conducted using copper nitrate (CuNO3) as a catalyst dopant. The doped catalyst showed slightly improved degradation of organochlorides at particular catalyst concentrations. Simulated photocatalytic rate of reaction kinetics results are in correlation with the modeled experimental results. They show variability of the degradation constant in the rate of reaction with varied catalyst concentrations. Catalyst concentration efficiency of each compound and the rate of the reactions were determined.
Author: Jesus Moreira del Rio
Publisher:
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Category :
Languages : en
Pages : 434
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Abstract Scaling up a photoreactor requires both knowledge of optical properties of the slurry medium and an established kinetic model. Measuring the scattering and absorption coefficients of particles suspended in water involves the use of specialized optical equipment, as well as the partial solution of the radiative transfer equation (RTE). In addition, modeling of the radiation field in photoreactors with complicated geometries offers special challenges. On the other hand, most of the kinetic models (KM) for phenol photodegradation reported in the literature were obtained for a single organic chemical species only. In fact, neglecting all the intermediate species generated during the photoreaction, is a common oversimplification that limits the KM application. As a result, once the radiation and kinetic models fully established, energy efficiencies can be obtained. In this PhD dissertation, the photocatalytic degradation of phenol over four different TiO2 catalysts is studied. It is proven that phenol yields hydroquinone, catechol, benzoquinone, and acetic and formic acids as main intermediate species. The radiation field inside photocatalytic reactors is predicted by solving the RTE. From the solution of the RTE, the local volumetric rate of energy absorption (LVREA) is also calculated. The radiation field inside an annular photoreactor is simulated using the Monte Carlo (MC) method for different TiO2 suspensions in water. All simulations are performed by using both the spectral distribution, and the wavelength-averaged scattering and absorption coefficients. The Henyey-Greenstein phase function is adopted to represent forward, isotropic and backward scattering modes. It is assumed that the UV lamp reflects the back-scattered photons by the slurried medium. It is proven, photo-absorption rates, using MC simulations and spectral distribution of the optical coefficients, agree closely with experimental observations from a macroscopic balance (MB). It is also found that the scattering mode of the probability density function, is not a critical factor for a consistent representation of the radiation field. iv When solving the RTE, two optical parameters are needed: (1) the absorption and scattering coefficients, and (2) the phase function. In this research work, the MC method, along with an optimization technique, is shown to be effective in predicting the wavelength-averaged absorption and scattering coefficients for different TiO2 powders. To accomplish this, the LVREA and the transmitted radiation (Pt) in the photoreactor are determined by using a MB. The optimized coefficients are calculated ensuring that they comply with a number of physical constrains, falling in between bounds established via independent criteria. The optimization technique is demonstrated by finding the absorption and scattering coefficients for different semiconductors that best fit the experimental values from the MB. The objective function in this optimization is given by the least-squared error for the LVREA. A photocatalyst is synthesized and its optical properties determined by the proposed method. This approach is a general and promising one; not being restricted to reactors of concentric geometry, specific semiconductors and/or particular photocatalytic reactor unit scale. Based on the proposed intermediate reactions, a phenomenological based unified kinetic model is proposed for describing the obtained experimental observations in phenol photodegradation. This Langmuir-Hinshelwood (L-H) kinetic model is based on a Series- Parallel reaction network. This reaction model is found to be applicable to the various TiO2 photocatalyst in the present study. This unified kinetic network is based on the identified and quantified chemical species in the photoconversion of phenol and its intermediates. In order to minimize the number of optimized parameters, the adsorption constants of the different intermediate species on the different catalysts configuration, are obtained experimentally. It is shown that the unified kinetic model requires a number of significant assumptions to be effective; avoiding overparametization. As a result, the unified kinetic model is adapted for each specific TiO2 photocatalyst under study. These different models adequately describe the experimental results. It is shown that this approach results in good and objective parameter estimates in the L-H kinetic model, which is typically applied to photocatalytic reactors. Finally, two efficiency factors, the quantum yield and the photochemical and thermodynamic efficiency factor, are obtained, in this PhD dissertation. These factors are based on the kinetic model proposed and the radiation being absorbed by the photocatalyst particles. The v efficiency calculations consider stoichiometric relationships involving observable chemical species and OH- groups. The obtained efficiency factors point toward a high degree of photon utilization and, as a result, the value of photocatalysis and Photo-CREC-Water reactors for the conversion of organic pollutants in water is confirmed.
Author: Khaled M. Mezughi
Publisher:
ISBN:
Category :
Languages : en
Pages : 212
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Organic contaminants from industrial and/or domestic effluents may be harmful to humans directly or indirectly by degrading the quality of the aquatic environment. Consequently these contaminants must be reduced to levels that are not harmful to humans and the environment before disposal. Chemical, physical and biological methods exist for the removal of these pollutants from effluents. Among the available chemical methods, heterogeneous photocatalytic oxidation has been found particularly effective in removing a large number of persistent organics in water. In this study, photocatalytic degradation was explored for the removal of reactive azo-dye (textile dye), triclocarban (disinfectant), clopyralid (herbicide) and three endocrine disrupting compounds (EDCs) (estrone, 17?-estradiol and 17?-ethinylestradiol) from synthetic effluents. The major factors affecting the photocatalytic processes including the initial concentration of the target compounds, the amount of catalyst, the light intensity, the type of catalyst, the electron acceptor, the irradiation time and the pH were studied. Other oxidation techniques including (O3, H2O2, UV) were also studied. Generally UV light is used in combination with titanium dioxide, as photocatalyst, to generate photoinduced charge separation leading to the creation of electron-hole pairs. The holes act as electron acceptors hence the oxidation of organics occur at these sites. These holes can also lead to the formation of hydroxyl radicals which are also effective oxidants capable of degrading the organics. The results obtained in this study indicated that photolysis (i.e. UV only) was found to have no effect on the degradation of reactive azo-dye (RO16). However, complete photocatalytic degradation of 20 mg/L (3.24?10-2 mM) RO16 was achieved in 20 minutes in the presence of 1g/L TiO2 Degussa P25 at pH 5.5. Comparison between various types of catalysts (i.e. Degussa P25, VP Aeroperl, Hombifine N) gave varied results but Degussa P25 was the most effective photocatalyst hence it was selected for this study. For RO16 the optimum catalyst concentration was 0.5 g/L TiO2 with initial concentration of 20 mg/L RO16. It was found that the disappearance of RO16 satisfactorily followed the pseudo first-order kinetics according to Langmuir-Hinshelwood (L-H) model. The rate constant was k= 0.0928 mol/min. Photodegradation of TCC was studied in 70%v acetonitrile: 30%v water solutions. UV light degraded TCC effectively and the reaction rates increased with decreasing initial concentration of TCC. UV/TiO2 gave unsatisfactory degradation of triclocarban (TCC) since only 36% were removed in 60 minutes with initial concentration of TCC 20 mg/L. The degradation of clopyralid and the EDCs was studied using three oxidation systems UV/TiO2, UV/H2O2 and O3. Complete degradation of clopyralid (3,6-DCP) was achieved with UV/TiO2 in about 90 minutes at an optimum catalyst concentration of 1g/L. Zero-order kinetics was found to describe the first stage of the photocatalytic reaction in the concentration range 0.078-0.521 mM. At pH 5 the rate constant was 2.09?10-6? 4.32?10-7 M.s-1.Complete degradation of all the three EDCs was achieved with UV/H2O2 in 60 minutes at catalyst concentration of (2.94?10-2 M). On the other hand complete degradation of the EDCs was achieved in just 2 minutes with ozonation. For high concentration EDCs, TiO2/UV gave low efficiency of degradation as compared with ozone and H2O2/UV. First-order kinetics was found to describe the photocatalytic reaction of the EDCs.
Author:
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Category : Hydrology
Languages : en
Pages : 962
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Author: Jose Ricardo Alvarez Corena
Publisher:
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Category :
Languages : en
Pages : 252
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Abstract: The simultaneous degradation of five organic contaminants: 1,4 dioxane, n-nitrosodimethylamine, tris-2-chloroethyl phosphate, gemfibrozil, and 17[beta] estradiol, was investigated using a 1 L batch water-jacketed UV photoreactor utilizing titanium dioxide (TiO2) nanoparticles (Degussa P-25) as a photocatalyst. The primary objectives of this research were: (1) to experimentally assess the feasibility of heterogeneous photocatalysis as a promising alternative for the degradation of organic compounds in water; and (2) to model the chemical reactions by the application of two different approaches based on adsorption -- surface reactions (Langmuir-Hinshelwood) and its simplification to a first order rate reaction. These objectives were motivated by the lack of information regarding simultaneous degradation of organic compounds in different categories as found in real aqueous matrices, and generation of specific intermediates that could eventually represent a potential risk to the environment. Contaminants were chosen based on their occurrence in water sources, their representativeness of individual sub-categories, and their importance as part of the CCL3 as potential contaminants to be regulated. Contaminant degradation was evaluated over time, and the TiO2 concentration and solution pH were varied under constant UV irradiation, oxygen delivery rate, mixing gradient, and temperature. Specific accomplishments of this study were: (1) reaction kinetics data were obtained from the UV/TiO2 experiments and showed the potential that this UV/TiO2 process has for effectively removing different types of organic compounds from water; (2) a good fit was obtained between photocatalytic reaction kinetics models and the contaminant data using pseudo first-order and Langmuir-Hinshelwood (L-H) models; (3) results of the analytical methods developed in this study were validated by measurements performed by a certified laboratory; (4) the reaction kinetic parameters obtained in this study were normalized to electrical energy per order, reactor volume and surface area of the photocatalyst in order to provide rate constants with wider applicability for scale-up to more complex systems; and (5) degradation intermediates from the oxidation process and from interaction among compounds were identified and possible pathways for their formation suggested. This research has provided a better understanding of the photocatalytic process for the removal of organic contaminants from complex aqueous matrices.
Author: York R. Smith
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Category : Thesis
Languages : en
Pages : 136
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This thesis is written as a series of two technical papers that have been published in technical journals. Each paper has its own abstract, introduction, experimental methods, results and discussion as well as figures and tables. Chapter 1 gives a brief overview of photocatalytic materials. Chapter 2 contains the first paper, which examines the physicochemical aspects that influences the photo oxidation kinetics of a model dye compound. Chapter 3 contains the second paper in which titania nanotubes are grown over a wire substrate as opposed to a foil substrate. A wire substrate demonstrates better photo oxidation kinetics than the foil configuration.
Author: Hamir S. Rathore
Publisher: CRC Press
ISBN: 1439836248
Category : Science
Languages : en
Pages : 663
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Pesticides play an important role in controlling pests that carry diseases and threaten crop production. In recent years, however, there has been increased concern about the adverse impacts of pesticides and their degradation products on public health and the environment. A considerable amount of work is being done to develop nonchemical methods of pest control, but it is not yet feasible to dispense with the use of chemical pesticides. Pesticides: Evaluation of Environmental Pollution brings together, in a single volume, current knowledge on environmental pollution caused by pesticides. It helps readers evaluate the effects that pesticide residues have in all compartments of the environment. Featuring contributions by eminent scientists from around the world, the book gives an overview of the fate and transport of pesticides and their degradation in the environment. Detailing the sources, concentration, and hazards of residues, it examines their effects in humans, birds and mammals, fish, soil invertebrates, soil microflora, aquatic invertebrates, water, milk products, and more. The book also addresses endocrine-disrupting pesticides and explores biopesticides as alternatives to chemical pesticides. A review of data on the potential hazards of pesticides, this reference will be of interest to readers working in the areas of chemical crop protection and pollution management. It adds a balanced perspective to the debate between those who think that pesticides should be banned and those who consider the continued use of large quantities to be necessary for the survival of humanity. See also Handbook of Pesticides: Methods of Pesticide Residues Analysis (CRC Press, 2009).
Author:
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Category : Air
Languages : en
Pages : 698
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Indexes material from conference proceedings and hard-to-find documents, in addition to journal articles. Over 1,000 journals are indexed and literature published from 1981 to the present is covered. Topics in pollution and its management are extensively covered from the standpoints of atmosphere, emissions, mathematical models, effects on people and animals, and environmental action. Major areas of coverage include: air pollution, marine pollution, freshwater pollution, sewage and wastewater treatment, waste management, land pollution, toxicology and health, noise, and radiation.
Author:
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Category : Medicine
Languages : en
Pages : 2432
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Vols. for 1963- include as pt. 2 of the Jan. issue: Medical subject headings.
