| Author: | So, Hiu Lam |
| Title: | Homogeneous and heterogeneous photodegradation of naphthalene and 1-naphthol in water by iron-catalysts |
| Advisors: | Chu, Wei (CEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Sewage -- Purification -- Biological treatment Iron catalysts Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xvii, 205 pages : color illustrations |
| Language: | English |
| Abstract: | The emergence of polycyclic aromatic hydrocarbons in wastewater is receiving concerns due to the carcinogenic effects and frequent detection in water bodies. In this study, the degradations of Naphthalene (C10H8) and its homologous of phenol, 1-Naphthol (C10H8O) using different iron catalysts were explored for the removal of such contaminants in wastewater. To extrapolate the removal kinetics to all other major reactor types, three common types of reactors were utilized to examine the degradation performance, using various light sources including UV-A and UV-C. The techniques included homogeneous, heterogeneous photocatalysis, and simultaneous physical and chemical processes with detail exploration on various parameters. Different models were used to represent the experimental data. To obtain a precise prediction, multiple regressions were conducted using the experimental data and the optimization of the results were visualized by Response Surface Methodology. Firstly, the removal of Naphthalene (Nap) in aqueous using peroxymonosulfate (PMS), Fe2+ and UV-A was studied. This study examined the role of different parameters, and the process is optimized with more than 90% removal in 20 minutes reaction (optimal conditions: ([Fe2+]0 = 0.250 mM, [PMS]0 = 0.250 mM, wavelength = 350 nm and pH = 2.8). The two-staged reaction kinetics were described by a proposed mathematical model. A stepwise addition of oxidants was conducted to mitigate the impacts of radical surge during the initial stage and a radical deficit at the later stage, and it also achieved a higher better performance. Besides, the decay pathways of Nap under the process were proposed by using LC-ESI/MS analysis. The Total Organic Carbon (TOC) content was discovered to increase first, followed by a decline after 2 h reaction. It is explained that the increase in TOC it was attributed to the partially degraded intermediates rather than the persistent parent compound Nap, since the latter was not completely combustible in the TOC analyzer, suggesting that the process is capable in degrading Nap into more degradable products such naphthoic acids and aldehydes. Naphthalene (Nap) degradation by heterogeneous Fe3O4/PMS/UV was then investigated. The reaction demonstrated radical and non-radical pathways simultaneously for Nap degradation. The system achieved total removal in 30 minutes, and a mixed order kinetic model was developed to describe the decay pattern, with rate constants representing adsorption (non-radical) and decay (radical) dominant reactions, respectively. The reaction is dominated by catalysis in acidic pH as HSO5- anions are adsorbed onto the protonated Fe3O4 surface for activation; In alkaline solution, the main removal pathway is via Nap adsorption onto the deprotonated Fe3O4 surface. The catalyst was reused for 5 times and the performance was even better after reuse. XPS study confirmed that the surface Fe content of Fe3O4 remained stable after reusing due to the effective UV-assisted ≡FeII—≡FeIII catalytic cycle. An additional C-O bond on Fe3O4 surface was identified after reuse, which is determined to be an additional OH bond on catalyst for extra adsorption site and for radical generation. As 1-Naphthol are identified as a primary intermediate product from the degradation process, a unique column reactor is designed to explore the performance of the degradation, involving a simultaneous photocatalysis and adsorption by PMS, FeOOH and UV-C in a novel column reactor was proposed and studied, The reactor consists of a submersible UV-C lamp, mixed quartz bead-FeOOH media bed and a continuous supply of oxidants. At optimal condition, the system achieved around 80% 1-Naphthol removal. The reaction was found to be highly pH dependent. At lower pH, photocatalysis and adsorption are favored as the catalyst surface is protonated (≡FeOH2+) to promote PMS activation and 1-Naphthol adsorption, and the dissolution of Fe3+ enhanced homogeneous activation of PMS. At pH level higher than 8, the reaction becomes retarded due to the deprotonation of catalyst surface and dissociation of 1-Naphthol into its conjugate base. Three radical types, superoxide radicals, singlet oxygen and sulfate radicals were confirmed contribute significantly to 1-Naphthol oxidation. The reactor design was optimized at 15 minutes retention time. Due to the regeneration of FeIII/FeII sites on FeOOH, the performance of reused catalyst is maintained at 90% after 5 cycles of reuses. A quadratic regression with R2 = 0.929 was used to model the concentration gradient of 1-Naphthol along the column at different time intervals. Accurate prediction is vital in designing reactors for real application. Hence, the experimental data of the column reactor was analyzed statistically for the prediction of the reactor performance. The proposed model includes four variables: concentration of 1Naphthol, concentration of PMS, FeOOH dosage and pH, along with their second-order effects and interactions, summing up to nine regressors. Statistical analyses were performed to verify the statistical significance of the regressors, and extra experimental sets were conducted to validate the proposed model. The mathematical model proposed exhibited good reproducibility with no significant statistical difference between the experimental and predicted values. Response surface methodology was used to explore the relationship between the four variables and the response. The process is optimized at pH 2 - 2.5; [PMS] = 1.2 - 1.4 mM; and FeOOH 2.75 - 3 g respectively, according to the overlaid contour plots. |
| Rights: | All rights reserved |
| Access: | open access |
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