Pathway Analysis of Phenol Degradation by UV/TiO2 Photocatalysis Using the Carbon-13 Isotopic Labelling Technique PDF Download
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Author: Lindelwa Jay
Publisher:
ISBN:
Category : Water
Languages : en
Pages : 0
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Book Description
The United States Environmental Protection Agency (USEPA) has enlisted phenolic compounds as pollutants of priority concern, as they tend to persist in the environment over a long period, accumulate and exert toxic effects on humans and animals. The entrance of phenolic compounds into the aquatic environment results from natural, industrial, domestic and agricultural activities. Their presence may be due to the degradation or decomposition of natural organic matter present in the water, through the disposal of industrial and domestic wastes into water bodies and through runoffs from agricultural land. Several specific new technologies, called Advanced Oxidation Processes (AOP), have been developed to eliminate dangerous organic chemicals such as phenol from polluted waters. The photocatalytic process, based on UV irradiated semiconductor (TiO2), represents one of AOP that provide an interesting route to the destruction of many organic substances to CO2, H2O and corresponding mineral acids. TiO2 is usually used as a photocatalyst in two crystal structures: Rutile and Anatase. The photoreactivity of P-25 Degussa, consisting of anatase and rutile (4/1 w/w), exceeds that of pure anatase and rutile in several reaction systems. This study characterised the three TiO2 powder forms by BET, XRD, XRF and SEM analyses to contribute to a better understanding of their physical properties. Anatase was revealed to have 98.4% purity x-ray fluorescence analysis, Degussa at 96.7% and rutile at 75.7% was reported to have the most impurities (~25%). This study investigated the UV/TiO2 photocatalytic degradation of phenol, with emphasis on the effects of; solution pH, catalyst load, initial phenol concentration, dissolved oxygen and UV radiation intensity. Degradation studies were conducted in a batch reactor with photons for catalyst (TiO2) activation supplied by a medium pressure 400 W UV lamp immersed in the pollutant solution housed in a double jacket quartz sleeve, which served as the cooling system for the lamp. Temperature control in the reactor was achieved by circulating cold water through the outer cavity of the quartz sleeve. The reactor contents in all batches were aerated at a flow rate of 10 mL/min. Thorough mixing of the contents of the reactor was achieved by continuous agitation with a magnetic stirrer. The experimental reaction time was set at 60-100minutes. Samples were monitored by GC-MS analysis. Results showed that UV/TiO2 photocatalysis is an effective method for the removal of phenol from wastewaters. The efficiency of the process depends strongly on the experimental conditions. Degradation of organic compounds in water is often accompanied by the formation of several intermediate compounds, some of which may be more toxic than the original pollutant. The carbon-13 isotopic labelling technique was employed to track the degradation mechanism of phenol to better understand the degradation pathway. Carbon-13 tracking results revealed that hydroquinone, catechol, benzoquinone, resorcinol and maleic acid were the main intermediates containing the C-13 isotope. A reaction pathway was postulated based on these findings. Kinetic isotope effect (KIE) experiments were conducted and a primary isotope effect was observed. This confirmed that the C-13 isotope position on the labelled phenol was the site of bond breaking in the rate-limiting step.
Author: Lindelwa Jay
Publisher:
ISBN:
Category : Water
Languages : en
Pages : 0
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Book Description
The United States Environmental Protection Agency (USEPA) has enlisted phenolic compounds as pollutants of priority concern, as they tend to persist in the environment over a long period, accumulate and exert toxic effects on humans and animals. The entrance of phenolic compounds into the aquatic environment results from natural, industrial, domestic and agricultural activities. Their presence may be due to the degradation or decomposition of natural organic matter present in the water, through the disposal of industrial and domestic wastes into water bodies and through runoffs from agricultural land. Several specific new technologies, called Advanced Oxidation Processes (AOP), have been developed to eliminate dangerous organic chemicals such as phenol from polluted waters. The photocatalytic process, based on UV irradiated semiconductor (TiO2), represents one of AOP that provide an interesting route to the destruction of many organic substances to CO2, H2O and corresponding mineral acids. TiO2 is usually used as a photocatalyst in two crystal structures: Rutile and Anatase. The photoreactivity of P-25 Degussa, consisting of anatase and rutile (4/1 w/w), exceeds that of pure anatase and rutile in several reaction systems. This study characterised the three TiO2 powder forms by BET, XRD, XRF and SEM analyses to contribute to a better understanding of their physical properties. Anatase was revealed to have 98.4% purity x-ray fluorescence analysis, Degussa at 96.7% and rutile at 75.7% was reported to have the most impurities (~25%). This study investigated the UV/TiO2 photocatalytic degradation of phenol, with emphasis on the effects of; solution pH, catalyst load, initial phenol concentration, dissolved oxygen and UV radiation intensity. Degradation studies were conducted in a batch reactor with photons for catalyst (TiO2) activation supplied by a medium pressure 400 W UV lamp immersed in the pollutant solution housed in a double jacket quartz sleeve, which served as the cooling system for the lamp. Temperature control in the reactor was achieved by circulating cold water through the outer cavity of the quartz sleeve. The reactor contents in all batches were aerated at a flow rate of 10 mL/min. Thorough mixing of the contents of the reactor was achieved by continuous agitation with a magnetic stirrer. The experimental reaction time was set at 60-100minutes. Samples were monitored by GC-MS analysis. Results showed that UV/TiO2 photocatalysis is an effective method for the removal of phenol from wastewaters. The efficiency of the process depends strongly on the experimental conditions. Degradation of organic compounds in water is often accompanied by the formation of several intermediate compounds, some of which may be more toxic than the original pollutant. The carbon-13 isotopic labelling technique was employed to track the degradation mechanism of phenol to better understand the degradation pathway. Carbon-13 tracking results revealed that hydroquinone, catechol, benzoquinone, resorcinol and maleic acid were the main intermediates containing the C-13 isotope. A reaction pathway was postulated based on these findings. Kinetic isotope effect (KIE) experiments were conducted and a primary isotope effect was observed. This confirmed that the C-13 isotope position on the labelled phenol was the site of bond breaking in the rate-limiting step.
Author: Jesus Moreira del Rio
Publisher:
ISBN:
Category :
Languages : en
Pages : 434
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Book Description
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: Cheela V R Sankar
Publisher: LAP Lambert Academic Publishing
ISBN: 9783659789137
Category :
Languages : en
Pages : 80
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Book Description
The three-fold increase in the population along with the geometric growth in the necessities of the homosapiens created many industries which generate a wide variety of highly toxic wastes. The effluents of these industries often contain aromatic compounds that are resistant to natural degradation and therefore persist in the environment. In fact the prediction of the removal of individual xenobiotic chemicals still remains a very difficult task, with disparities in performance between laboratory-scale and full-scale systems. One of the major organic pollutants found in waste waters is phenol. Phenol besides being toxic, it is also a potential carcinogenic and mutagenic compound. To meet the effluent standards among the various treatment techniques that are available biological treatment was found to be feasible which may either result in incomplete mineralization or otherwise transforms the pollutant from one form to the other and photocatalytic degradation through solar irradiation using titanium dioxide as catalyst was found to be an alternative.
Author: Emomotimi Emily Bamuza-Pemu
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Algal infestation in water bodies causes the release of soluble organic compounds that impact negatively on the taste and odour of the water. With increasing pollution in water bodies and increasing nutrient loading from agricultural activities, most water reservoirs in South Africa and around the world have become affected by this problem. In this study, an advanced oxidation process (AOP), namely, photocatalysis was evaluated for its potential to degrade aromatic compounds: and taste and odour causing bi-cyclic compounds originating from algae. Semiconductor photocatalysis is an environmentally friendly technology requiring no chemical inputs which is capable of completely mineralising organic pollutants to CO2 and H2O thereby eliminating production of unwanted by-products. Although processes involved in the photo-degradation have been reported for a wide range of pollutants, the degradative pathway in this process has not been fully established. In this study, compounds including phenol, 2-chlorophenol, 4-chlorophenol and nitrophenol were successfully eliminated from simulated wastewater. Degradation of geosmin at an environmentally significant initial concentration of 220 ng/L to levels below the lowest detectable concentration was achieved with an optimum catalyst concentration of 60 mg/L at a rate of 14.78 ng/L/min. Higher catalysts loading above 60 mg/L resulted in a decrease in degradation rates. An increase in initial geosmin concentration resulted in a decrease in rates. Ionic species commonly found in surface waters (HCO3 -, and SO4 2- ) significantly reduced the efficiency of geosmin degradation. Degradation of geosmin produced acyclic intermediates from ring fission tentatively identified as 3,5-dimethylhex-1-ene, 2,4-dimethylpentan-3-one, 2-methylethylpropanoate and 2-heptanal. The results obtained indicate that the degradation of organic pollutants in aqueous solution is as a result of synergic action from hydroxyl radicals, positive holes and direct photolysis by UV radiation, though the predominant pathway of degradation is via hydroxyl radicals in solution. Major aromatic intermediates of phenol degradation include catechol, resorcinol and hydroquinone produced in the order catechol> resorcinol> hydroquinone. All three are produced within 2 minutes of photocatalytic reaction of phenol and remain in solution until all phenol is degraded in aerated systems. Production of resorcinol in non-aerated systems is transient, further supporting the hydroxyl radical dominant reaction pathway.
Author: Chih-Chang Wang
Publisher:
ISBN:
Category : Chlorophenols
Languages : en
Pages : 230
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Author: M. Schiavello
Publisher:
ISBN:
Category : Science
Languages : en
Pages : 218
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Book Description
Photocatalysis is a reaction which is accelerated by light while a heterogeneous reaction consists of two phases ( a solid and a liquid for example). Heterogeneous Photocatalysis is a fast developing science which to date has not been fully detailed in a monograph. This title discusses the basic principles of heterogeneous photocatalysis and describes the bulk and surface properties of semiconductors. Applications of various types of photoreactions are described and the problems related to the modeling and design of photoreactors are covered.
Author: Emomotimi Emily Bamuza-Pemu
Publisher:
ISBN:
Category : Drinking water
Languages : en
Pages : 312
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Book Description
Algal infestation in water bodies causes the release of soluble organic compounds that impact negatively on the taste and odour of the water. With increasing pollution in water bodies and increasing nutrient loading from agricultural activities, most water reservoirs in South Africa and around the world have become affected by this problem. In this study, an advanced oxidation process (AOP), namely, photocatalysis was evaluated for its potential to degrade aromatic compounds; and taste and odour causing bi-cyclic compounds originating from algae. Semiconductor photocatalysis is an environmentally friendly technology requiring no chemical inputs which is capable of completely mineralising organic pollutants to CO2 and H2O thereby eliminating production of unwanted by-products. Although processes involved in the photo-degradation have been reported for a wide range of pollutants, the degradative pathway in this process has not been fully established. In this study, compounds including phenol, 2-chlorophenol, 4-chlorophenol and nitrophenol were successfully eliminated from simulated wastewater. Degradation of geosmin at an environmentally significant initial concentration of 220 ng/L to levels below the lowest detectable concentration was achieved with an optimum catalyst concentration of 60 mg/L at a rate of 14.78 ng/L/min. Higher catalysts loading above 60 mg/L resulted in a decrease in degradation rates. An increase in initial geosmin concentration resulted in a decrease in rates. Ionic species commonly found in surface waters (HCO3 -, and SO4 2-) significantly reduced the efficiency of geosmin degradation. Degradation of geosmin produced acyclic intermediates from ring fission tentatively identified as 3,5-dimethylhex-1-ene, 2,4-dimethylpentan-3-one, 2-methylethylpropanoate and 2-heptanal. The results obtained indicate that the degradation of organic pollutants in aqueous solution is as a result of synergic action from hydroxyl radicals, positive holes and direct photolysis by UV radiation, though the predominant pathway of degradation is via hydroxyl radicals in solution. Major aromatic intermediates of phenol degradation include catechol, resorcinol and hydroquinone produced in the order catechol > resorcinol > hydroquinone. All three are produced within 2 minutes of photocatalytic reaction of phenol and remain in solution until all phenol is degraded in aerated systems. Production of resorcinol in non-aerated systems is transient, further supporting the hydroxyl radical dominant reaction pathway.Description:
Author: 沈運祥
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Author: Ha Son Ngo
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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The objective of the thesis is to better understand the mechanisms of photocatalytic degradation occurring under air or under nitrogen by studying the disappearance, mineralization and intermediate products of a simple molecule acetic acid. The reactions are carried out under these two atmospheres in gas phase and dynamic mode in order to place themselves under conditions of depollution or of energy generation. Firstly, we studied the degradation of acetic acid using the reference photocatalyst, TiO2 P25. Regardless of the atmosphere, air or N2, we have shown that the decarboxylation reaction is the first step in the disappearance of acetic acid. However, the fate of the methyl group depends on the carrier gas and the molar flow rate (in other words, the concentration of the pollutant in the gas phase). The mechanism of degradation occurring on the surface of the photocatalyst is then represented to explain the importance of this mechanism compared to that involving the hydroxyl radicals. The schematization of the mechanism includes the regeneration of the photocatalyst and the possible formation of H2O2, which has been observed in the literature. The study of the degradation products of acetic acid labelled with carbon 13 on the carbonyl group (CH313COOH) suggests that acetone and acetaldehyde do not result from the reduction of the carboxylic group. In a second step, the impact of photonic flux and moisture in the presence of TiO2 P25 as well as the effect of different commercial TiO2 on the conversion and more particularly the distribution of the intermediate products have been studied. Comparison of the effectiveness of different commercial TiO2s was discussed by considering the presence of rutile phase, the nature of the active species, the specific surface area of TiO2, the number of OH groups on the surface of the catalysts, the presence of impurities and the porosity of the materials.
Author: Radwa Elsalamony
Publisher: LAP Lambert Academic Publishing
ISBN: 9783659549106
Category :
Languages : en
Pages : 264
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There we will focus on the water pollution which is an important problem. This work aims to study the photocatalytic degradation of chlorophenols. They belong to a notable group of pollutants because of their high toxicity, and hardly biodegradable, and are difficult to remove from the environment. Photocatalytic degradation of chlorophenols using UV light was investigated over mesoporous catalysts. Ti-MCM-41(20), Ti-MCM-41(10), TiO2/MCM-41, Fe-Ti-MCM-41, TiO2/Fe-MCM-41 and TiO2/SBA-15 catalysts were prepared from the corresponding metal salts by the sol-gel technique. The catalyst were investigated using X-ray diffraction (XRD), FT-IR spectroscopes, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and BET surface areas of the samples were determined using the nitrogen adsorption and desorption isotherms. 100 ppm was used as model pollutants. High purity 2,4,6-trichlorophenol and 4-chlorophenol solutions individually and 0.1 g/L of each catalyst was tested at different irradiation times. At each interval time 10 ml of irradiated solution was taken and analyzed by High Performance Liquid Chromatography (HPLC), Ion Chromatography (IC). Aromatic interme
