The Effect of Humic acid on the degradation of organic pesticides byFenton reaction in acidic environment
| Autor: | Shih-Jian Li, 李世堅 |
|---|---|
| Rok vydání: | 2014 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 102 Pesticides, preventing plants from the attack of insects, are used to increase the growth of crops its yield. However, extensive uses of pesticides have drawn a great deal of environmental concerns. Their residuals in the environment may become airborne, water-borne, and detrimental to the ecology as well as the public health, causing severe threats to the environment. In the study, degradation of imidacloprid and parathion by Fenton process was investigated at the acidic environment. Additionally, the effect of humic acid presence in the Fenton degradation was evaluated. The parameters of pH, ORP, target compound concentrations, TOC and COD were measured. The pH was found decreasing due to H+ production during Fenton degradation. The ORP increased significantly during the first 2 minutes of the Fenton process, and exhibited a slightly-increasing trend after the first two minutes due ․OH depletion. In this study, the parathion removal was found most effective (75.78%) when the ratio of hydrogen peroxide to ferrous sulfate concentrations was 4mM to 5mM. With the presence of humic acid, the best removal efficiency of parathion increased up to 89.49%. For imidacloprid, the ratio of hydrogen peroxide to ferrous sulfate concentrations was found 4mM to 5mM, at which, the best removal efficiency of 84.20% was observed. With the presence of humic acid the best removal efficiency decreased to 74.2%. Apparently, the humic acid had different effects on the two investigated pollutants. The humic acid in the system played two roles. One is the facilitating agent to enhance Fe3+ reduction to Fe2+, which subsequently generated more ․OH. The second is another contaminants which competed with the target contaminant for ․OH. However, in the system containing IX parathion, the humic acid enhanced more ․OH generation than consumption. In the system containing imidacloprid, because of the halogenated structure, the electron in the hydroxyl radical may be transferred to the ring structure, causing less efficiency in ring cleavage, leaving the ․OH and H+ to form water. When the removal of parathion was 75.8%, the TOC and COD mineralization rates were 61.4% and 63%, respectively. Imidacloprid removal efficiency of 84.2%, the TOC and COD mineralization rates were 50.12% and 40.18%, respectively. Although the pesticide concentration decreased, the mineralization rate is slightly impaired, because the target pollutants are broken into intermediates rather than being mineralized completely. The initial Fe2+ concentration was about 280mg / L, and the consumption of Fe2+ was 21.4% in the degradation of parathion. For imidacloprid degradation, the Fe2+ was consumed 28.6% and 34.3% for imidacloprid and imidacloprid humic mixture, respectively. In the investigated system, H2O2 is the major consuming chemical. Its initial concentration was about 2000 mg / L, and the H2O2 concentration dropped to 200 mg / L, after the reaction started. At the end of the reaction, only 80~130 mg / L H2O2 was observed. The average molecular weight of the used humic acid was 2566 ± 17 DA. After the ferrous sulfate was added, the average molecular weight of reduced to 727 ± 74 DA and 698 ± 119 DA, respectively, for the humic acid mixture containing parathion and imidacloprid. After 30 minutes of Fenton reaction, the average molecular weights were 440 ± 38 DA and 411 ± 78 DA, respectively. The addition of Fe2+ initiated a series of redox reactions causing the breakdown of humic acid structure, and the generated ․ OH further oxidize the organicfractions to form even smaller organic structures. X The observed degradation trend exhibited two different patterns of chemical oxidation. In the first part, the degradation showed the first degradation pattern, while the second part exhibited the second order degradation. In order to understand the impact of humic acid molecular size on Fenton degradation, the humic acid was polymerized using laccase to increase the humic acid molecular weight, and the resulting humic acid was employed for subsequent Fenton degradation. The polymerized humic acid has the average molecular weight of 7498 ± 144 DA. It was found that the increase in humic acid molecular weight did not enhance the contaminant removal. The may be due to the fact that the residual of polymerizing agent of catechol became another organic contaminant competing with ․OH. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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