Linuron degradation by photo-aided oxidation processes in aqueous phase-kinetic study and reaction mecahnisms

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Linuron degradation by photo-aided oxidation processes in aqueous phase-kinetic study and reaction mecahnisms


Author: Rao, Yongfang
Title: Linuron degradation by photo-aided oxidation processes in aqueous phase-kinetic study and reaction mecahnisms
Degree: Ph.D.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Decomposition (Chemistry)
Water -- Purification -- Photocatalysis
Water -- Purification -- Ozonization
Department: Dept. of Civil and Structural Engineering
Pages: xvii, 158, [40] leaves : ill. ; 31 cm.
InnoPac Record:
Abstract: The widespread application of herbicides as a routine practice to control weed growth has led to increasing environmental concerns in the past decades because of their low biodegradability and long-term persistence in soil, which makes them ubiquitous. Most herbicides are diffused into aquatic environment via agricultural runoff or leaching. Linuron (N-(3, 4-dichlorophenyl)-N'-methoxy-N'-methylurea) (LNR), one of the most commercialized phenylurea herbicides, has received particular attention in recent years due to the toxicity, being frequently detected in the surface and underground waters, and possible endocrine disrupting properties of LNR and/or its metabolites. Till now, little is known about the degradability and reaction products of LNR by Vis (visible light)-induced photocatalysis. The knowledge regarding the LNR decomposition by UV, ozonation, and UV/ozone processes is far from complete although UV and ozonation has shown good performance in terms of LNR decay in some studies. In particular, the information about the intermediates and end products remains limited. Therefore, the aqueous degradation of LNR has been investigated by UV, ozonation, UV/ozone and TiO₂/H₂O₂/Vis processes. The investigation was conducted under idealized conditions and has taken into account both degradation kinetics and reaction mechanisms. It has been found that ozonation and UV/O₃ are pH-dependent while UV is pH-independent in terms of LNR decomposition. Experimental results also indicated overall rate constants increased exponentially with pH above 9.0 while the increase of rate constants with pH below 9 is insignificant in sole-O₃ system. In UV photolysis study, the LNR decay rate constant is linearly increased with the intensity of light. Linear models were proposed on the basis of reaction kinetics of LNR decay by these three processes. All dominant parameters such as quantum yield (ΦLNR), kOOH (rate constant for the formation of free radical HOO・- from ozone decomposition at high pH), rate constant of linuron with ozone (k₀₃,LNR), rate constant of linuron with hydroxyl radical (kOH,LNR), and α (the ratio of the production rate of HO・and the decay rate of ozone in UV/O₃ system), involved in the three processes were determined with the aid of proposed linear models. The effect of various anions on the performance of ozonation has also been examined.
Eight intermediates escaped from previous studies were detected in the sole-UV system in this study. N-terminus demethoxylation, photohydrolysis with dechlorination, hydroxylation on the benzene ring and N-terminus demethylation were found to be the major mechanisms in the linuron decay under the irradiation of UV at 254 nm while N-terminus demethoxylation, dechlorination and hydroxylation on the benzene ring were observed to be involved in the ozonation process. Different decay pathways were proposed based on the identified intermediates in the studied three processes. The release of chlorine and nitrogen as well as mineralization has also been quantified. UV/O₃ has demonstrated the best performance among these three processes in terms of LNR decay, mineralization, dechlorination and denitrogenation. Furthermore, the degradation of LNR in TiO₂ suspension has been studied, with and without the aid of H₂O₂ under the irradiation of visible light. The removal of LNR in TiO₂-P25 suspension can be increased from 10% to nearly 100% after 3 hr of reaction by simply adding H₂O₂ to the process. Various types of TiO₂ including anatase, rutile and TiO₂-P25 exhibited different photocatalytic activities on LNR decay, while their performances were strongly dependent on the presence and/or absence of H₂O₂. The H₂O₂-assisted TiO₂ photocatalysis using visible light could be optimized by adjusting TiO₂ dosage, initial concentration of H₂O₂ and the initial pH of the system. The LNR decay rate, generally, increased with the increase of TiO₂ dosage, but too high the TiO₂ dosage was not cost-effective due to the light attenuation. The initial H₂O₂ concentration in the tested range did not show a significant influence on the reaction rate. A neutral initial pH level was found to be favorable for the H₂O₂-assisted photocatalysis under visible light, which made the proposed process more attractive for real application. The reaction mechanism of LNR degradation by the TiO2/H₂O₂/Vis process has been also examined through the investigation on the effects of various radical scavengers, monitoring the generation of photocurrent, examining the performance of other metal oxides in place of TiO₂ in this system, and comparing the intermediates and decay pathways of LNR by UV-TiO₂ and TiO₂/H₂O₂/Vis processes with 16 and 17 intermediates identified, respectively. The generation of electrons was first confirmed by monitoring photocurrent with a TiO₂-coated ITO electrode immersed in H₂O₂ solution under the irradiation of visible light. It has been revealed that demethoxylation and demethylation through alkylic-oxidation is the major mechanism of LNR degradation while dechlorination (hydroxylation at the chlorine site) and direct hydroxylation on the benzene ring is minor in both processes. The mineralization and the release of chlorine and nitrogen have been also studied.

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