| Author: | Lau, Tim-kwan |
| Title: | Endocrine disruptors elimination by UV aided oxidation processes in water via kinetics study and reaction mechanisms |
| Degree: | Ph.D. |
| Year: | 2007 |
| Subject: | Hong Kong Polytechnic University -- Dissertations. Endocrine toxicology. Water pollution -- Health aspects. Sewage -- Purification. Ultraviolet radiation. |
| Department: | Department of Civil and Structural Engineering |
| Pages: | xxiii, 233 leaves : ill. ; 30 cm. |
| Language: | English |
| Abstract: | Endocrine disruptor compounds (EDC) are agents that interfere with the production, transport, binding or elimination of natural hormones in vertebrate organisms, such as fishes and further to human body. It can be naturally occurring or anthropogenic. Numerous studies have shown that EDCs are environmentally persistent and significant amounts of which exist in sewage effluents, rivers receiving municipal and industrial effluents, and even groundwater sources infiltrated by sewage effluents. At present, little is known about the degradability and reaction products of three selected EDCs by various chemical treatment processes. Therefore, the aqueous degradation of the important EDCs (i.e. Di-n-butyl phthalate (DBF), Carbofuran (CBF), and Butylated Hydroxyanisole (BHA)) has been investigated under ultraviolet (UV) irradiation treatment, ozonation and other advance oxidation processes. The investigation was carried out under idealized conditions and has considered both reaction kinetics and degradation mechanisms. In the UV photolysis study, DBF is being irradiated under monochromatic UV at 254 nm over a wide pH range (3-11). It was found that more than 90% of 4 uM DBF can be degraded within an hour of irradiation in water. A simple model has been developed and used to predict the initial DBF photolysis rate constant at different pH values and initial DBF concentrations. The use of 254 nm UV to photodegrade DBF was found to be a relatively fast and clean process, especially in neutral to basic conditions. For the second probed EDC compound, CBF, has shown the efficiency of advanced oxidation process (UV/O3) is higher than those of the direct UV photolysis and ozonation processes. The pH-dependency of CBF has also been shown in both ozonation and UV/Os processes. Linear relationship could be found for the latter process in all pH, while for the former process, two stages of reactions (steady and accelerating) were found in the acidic and alkaline pH conditions, respectively. Other than pesticide pollutants, industrial wastewater containing BHA, a suspected EDC, was also investigated by different treatment processes including UV-irradiation, ozonation, UV/O3, and UV/S2O8 2-. O-demethylation, dimerization, and oxidation have been found to be the main degradation mechanisms. A systematic decay pathway was proposed based on ten identified intermediates in the studied processes, including a unique pathway leading to the formation of precipitates in the ozonation process. An unconventional minimum-type variation of BHA decay rate constants from acidic to caustic range has been found for both ozonation and UV/O3 processes. Furthermore, the degradation of BHA in water has been studied, with and without the aid of a green oxidant - potassium peroxydisulfate (S2O8 2-) in the presence of UV. Three distinctive phases of BHA reactivity towards UV/S2O8 2- at acidic, neutral and basic pH range were examined, where 80-100% mineralization has been observed within an hour of irradiation under 254 nm. A reduction in solution pH during the reaction was observed mainly due to the complete conversion of S2O8 2- to sulfate ions together with proton generation. Seven measurable intermediates were found via an oxidation and dimerization process at all tested pH levels. The BHA decay mechanisms are quite different in acid condition and at other pH levels. There are three unique intermediates that are only detectable at pH 3 via two additional pathways. This is due to the generation of weaker oxidants and/or radicals, which results in a slower degradation of the BHA and therefore the accumulation of these intermediates to detectable levels. The rate of BHA decay generally increases from low to high pH levels; however, the corresponding mineralization at higher pH is retarded due to the futile process of recombining radicals and involvement of intermediates. Therefore, neutral pH was suggested to be the optimum condition in terms of mineralization and moderate efficiency in removing BHA. Additionally, three major process variables have been selected for detailed investigation: (i) UV wavelength effects; (ii) pH effects; and (iii) S2O8 2- dosage effects. It was found that UV at 254 nm demonstrated the best removal efficiency in both direct photolysis and photo-oxidation (UV/S2O8 2-) processes. The reaction rate constant can be improved by increasing either the initial pH levels and/or the S2O8 2- dosage. When the S2O8 2- dosage is sufficiently provided in the UV system, the reaction kinetics can be simply characterized by pseudo first-order decay. However, when S2O8 2- dosage is deficient, though the decay of BHA is fast initially but the process will be retarded at a later stage, and a two-stage pattern is observed. An atypical model has been used to describe such a condition, this is specially useful for predicting the process performance if a shock loading during wastewater treatment is present. Since the UV/S2O8 2- process at acidic pH condition cannot effectively mineralize BHA, therefore metal-mediated oxidation processes were investigated in this study as alternatives. The removal and mineralization of BHA using UV/S2O8 2- with the assist of silver in both homogeneous and heterogeneous systems were investigated. Three different silver sources including two silver salts (Ag2SO4 and AgNO3) and silver oxide (Ag2O) were compared. In a homogeneous system, silver nitrate is selected for detail study due to lesser interference from its counter anion. The degradation rates of five different treatment processes (UV only, UV/Ag+, Ag+/S2O8 2-, UV/S2O8 2-, and UV/Ag+/S2O8 2-) were studied in order to fully comprehend the details of the UV/Ag+/S2O8 2- process. This process by generating the robust sulphate radicals showed the best performance over BHA degradation and mineralization by removing 100% of 0.1 mM BHA and more than 99% of TOC in 3 and 20 min, respectively (at its optimal condition with a silver to peroxydisulfate ratio of 7.5:1). For a heterogeneous system the solid form of silver oxide (Ag2O) was used to replace the Ag+ for better recycling. The same dosages of silver in both homogeneous and heterogeneous systems give similar performance in BHA removal. However, it is interesting to find that the heterogeneous system induces faster mineralization likely due to the selective and non-selective properties of SO4*- and *OH to the BHA and resulted intermediates, respectively. |
| Rights: | All rights reserved |
| Access: | open access |
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