|Title:||The photocatalytic degradation of 2, 4-Dichlorophenoxyacetic acid herbicide by bismuth tungstate|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Civil and Structural Engineering|
|Pages:||xi, 83 leaves : ill. ; 30 cm.|
|Abstract:||The common herbicide 2, 4-Dichlorophenoxyacetic acid is widely used in agriculture and gardening. Because of its high solubility in water, it is a concerned environmental pollutant. After its application to crops, the unused portions can leach below the root zone or be washed out during precipitation and contaminate nearby water sources. The biodegradability of this pollutant in natural environment is very low. Various amounts of 2, 4-D are detected in surface water and ground water not only during application of the herbicide but also after a long period of use. There are many remediation methods for the removal of 2, 4-D in aqueous solutions. The aim of this project is to investigate the photodegradation of 2, 4-D by bismuth tungstate under visible light. The photocatalytical reactions were studied under various conditions including the examination of the effects of different initial pH, 2, 4-D concentrations, light intensities, and hydrogen peroxide. Photocatalytic reactions were slightly enhanced due to the increasing of hydroxide ions and photocatalytic rates of 2, 4-D were increased with pH at low pH range, but the reactions were gradually retarded due to effect of charge repulsion. The photocatalytic degradation rates of 2, 4-D were found to be retarded when the concentration of 2, 4-D increased. The decrease in the rates is likely due to the decrease in the total surface area (or number of active sites) available for photocatalytic reaction as the dosage of Bi₂WO₆ correspondingly decreased. The photocatalytic degradation rates of 2, 4-D increased when the light intensity increased due to the increase in the concentration of hydrogen ion (H⁺) or hydroxide ion (OH⁻) in aqueous solution. The reaction rate was found to increase with H₂O₂ dosages at low H₂O₂ dosages. The optimal H₂O₂ dosage is 0.03 mM. At maximum, 2.04 times rate increments was achieved respectively at the optimal H₂O₂ dosage of 0.03 mM compared to that of sole photocatalysis without H₂O₂. Above this optimal dosage, however, the overdose of H₂O₂ retards the photocatalytic rates. In the reutilization of Bi₂WO₆, the photodegradation rates of 2, 4-D was slightly decreased as the times of the re-use of Bi₂WO₆ increased. After the fourth time of the reutilization of Bi₂WO₆, the rate of the degradation of 2, 4-D can still remained at 45% of that of the first time, which indicated the Bi₂WO₆ can satisfy the needs in the actual industry.|
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