Author: Ma, Chi-wai
Title: Advanced chemical oxidation treatment of dye wastewater and remediation of chlorinated aromatic dyes (CADs) polluted soil by surfactant micellar/UV system
Degree: Ph.D.
Year: 2001
Subject: Sewage -- Purification
Dyes and dyeing -- Waste disposal
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Structural Engineering
Pages: xv, 177 leaves : ill. ; 30 cm
Language: English
Abstract: A quantitative estimation of direct ozonation and indirect free radical oxidation of dyes with assorted chromophores was studied through the examination of reaction kinetics in the ozonation process. The reaction kinetics of dye ozonation under different conditions was determined by adjusting the ozone doses, dye concentration, and reaction pH. The ozonation of dyes was found dominant by pseudo first-order reaction, and the rate constants decreased as the dye/ozone ratio increased. For all selected azo dyes, the dye decay rates increased as the initial pH of the solution increased, yet the decay rates of anthraquinone dyes would decrease in the same situation. The ozonation of dyes at a high pH contributed by hydroxyl free radicals was qualitatively verified by the use of a free radical scavenger. A proposed model, hi another way, quantitatively determines the fraction of contribution for dye decomposition between free radical oxidation and direct ozonation. Besides the ozonation reaction, the six selected dyes were UV-irradiated under 253.7 nm of UV light. Experimental results indicated that photo-degradation of all selected dyes following pseudo first order decay and the reaction mechanism was dominated by free radical reactions where acidic photo-products were formed. Moreover, the solubility of the dye molecules was found very important to determine the efficiency of photo-degradation. Dyes with higher water solubility were much easier to be decayed than that with lower water solubility. However, photo-sensitizer such as acetone could greatly enhance the photo-degradation rates for all dyes even for the ones with lower water solubility. In general, the higher the initial pH, the higher the reaction rates constants. Surfactant soil washing of hydrophobia dye contaminated soil was believed to be a useful technique to extract the contaminants to the aqueous solution in order to facilitate the wastewater treatment. Therefore, a typical soil contaminant, insoluble Chlorinated Aromatic Dye (CAD) - Disperse Red 13 (DR), was used to explore the reaction mechanism and kinetics of photodegradation in non-ionic surfactant solutions. Additional hydrogen source and photosensitizer could improve the decay rates. The decay rate of dye in surfactants depended on its micelle-water partition coefficient (Km) within surfactants. Basically, the photodegradation of CAD could be divided into three stages: initial lag stage, fast degradation stage and final retardation stage. However, no lag stage and higher degradation rate were observed after adding hydrogen source (NaBH4) or photosensitizer (acetone) to the surfactant micellar solution. Additional hydrogen source or photosensitizer had dosage limitations in such applications. The photoreduction of DR was the main reaction mechanism, in which photodechlorination was observed first with the generation of HCl as the final product, then followed by photodecolourization via the breaking of the azo bond of the chromophore. After surfactant soil washing, the humic materials will be extracted and present within micelle solution. Experimental results indicated that low dose of humics mainly acted as an additional hydrogen source to improve the photodegradation rate. However, high dose of humics would embed the characteristics of being a hydrogen source, but acted more like a quencher. A mathematical model (Stern-Volmer plot) based on possible reaction mechanisms in this system was proposed and successfully described the quantum yield's improving and quenching processes.
Rights: All rights reserved
Access: open access

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