OXIDATION OF CHLOROPHENOLS WITH HYDROGEN PEROXIDE CATALYZED BY SUPPORTED IRON OXIDE

• Main Authors: Hsu-Hui Huang, 黃旭暉
• Other Authors: Jong-Nan Chen
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/87730386612551966887

博士 === 國立交通大學 === 環境工程所 === 91 === The application of Fenton-like reaction in soil remediation and wastewater treatment has been investigated in last decade. Main limitation of this type of reaction is the production of a large amount of hydroxide precipitate which requires further separation. To overcome this problem, the combination of hydrogen peroxide (H2O2) with the heterogeneous catalysts such as iron oxides and supported iron oxides for the catalytic oxidation of organics in water system has attracted more attentions recently and deserves further investigation. This research was to explore the catalytic oxidation process with the combination of H2O2 and supported iron oxide for the treatment of target pollutant, monosubstituted - chlorophenols (MCP), in water system. Prior to the MCP oxidation catalyzed by supported iron oxide, the interaction of H2O2 and MCP with different kind of iron oxide and carrier including the catalytic activity toward H2O2 and MCP decomposition, transport phenomena on H2O2 decay kinetics, and oxidation mechanism for MCP removal were investigated. In addition, the catalysts and carriers prepared in this study were characterized. The synthesized and commercial iron oxides used in this study were identified by X-ray Diffraction as goethite, hematite and amorphous ferrihydrite, respectively. The surface properties of iron oxide including pHpzc, specific surface area, and iron content were determined. From the observation of SEM, he surface morphology of carrier, e.g. granular activated carbon (GAC), was modified significantly by different chemical treatments. From the EDS analysis, the higher weight percentage of oxygen was observed for the GAC modified by concentrated nitric acid. In the absence of pollutant, the catalytic activity toward H2O2 decomposition was the highest for granular ferrihydrite, less for goethite, and much less for hematite no matter the rate constant was modified on mass or surface area basis. Factors including H2O2 concentrations, catalyst dosage, and flow disturbance affecting the H2O2 decomposition were examined. The reaction rate of H2O2 was proportional to both H2O2 and iron oxide dosage. The activation energy of goethite toward H2O2 decomposition was determined as 66.9kJ/mol, indicating that the apparent rate for goethite was dominated by the rate of the intrinsic chemical reaction. The catalytic activity of granular ferrihydrite was affected significantly by the mixing speed and particle size for its larger Thiele modulus (φ) and Damkohler number (Da). In the part of oxidizing 2-CP by iron oxide, the catalytic activity toward 2-CP removal at neutral pH condition exhibited a converse series of those toward H2O2 decomposition for the iron oxides described above. The oxidation efficiency of 2-CP was also corresponding to the inverse sequence of surface specific area and solution pH, suggesting a guideline on selecting materials as catalyst in application. At acidic condition, 2-CP could be effectively degraded by Fenton-like reaction in comparison with the 2-CP oxidation rate catalyzed at neutral pH. A linear relationship between the rate of 2-CP oxidation and ferrous ions dissolution has been obtained. Since the dissolved rate was much less than that of 2-CP oxidation, the detachment of ferrous ions from surface of granular ferrihydrite could be proposed as the rate-determining step for the overall oxidation rate of 2-CP. A proposed reaction mechanism was suggested to describe the behavior of 2-CP oxidation and iron dissolution. The mechanism indicated that the pollutant intermediate species may affect significantly the oxidation rate as well. In the study of H2O2 catalysis by supported iron oxide without the addition of organics, the role of GAC acted not only a carrier but also a catalyst. The interaction between the coated active phase, iron oxide, and carrier, GAC, was found to promote both the apparent and the intrinsic catalytic activity on H2O2 decomposition. The H2O2 decomposition kinetics can be well described with the Langmuir- Hinshelwood model while the surface activity site of the catalyst was not limited. The effect of reaction product oxygen was found to influence either on the interphase or intraphase transport phenomenon. From the theoretical inference, the H2O2 decomposition rate was dominated by the intrinsic surface reaction rate rather than the mass transport rate of H2O2 to catalyst surface when the oxygen gas film was removed by the strong sheer stress. Moreover, the catalytic activity toward H2O2 followed an increased sequence of pHpzc for the three modified GAC. MCP removal was conducted with considering the factor including types of GAC and MCP, pH condition, and dosage of H2O2 and catalyst. The reduction efficiency of MCP for the modified activated carbon was comparable to those of the other oxidation systems using metal oxides as catalyst. The release of chloride ions revealed that the heterogeneous catalytic oxidation of MCP by GAC did occur. Furthermore, the ratio of dechlorination to DOC loss for the 4-CP removal was much higher than that for 2-CP removal, implying that the reaction pathway and oxidative intermediates for various MCP in this catalytic process would be different. The Eley-Rideal mechanism has be proposed to be dominant in this catalytic oxidation process, which could explain why the efficiency relating 4-CP loss and H2O2 decay is independent of 4-CP concentration. The 4-CP reaction rate could be described as first order in both the GAC3 and H2O2 concentration. Finally, the feasibility study about 4-CP removal showed that the combination of coated iron oxide and GAC could promote the overall reduction on 4-CP removal. The advanced removal of MCP catalyzed by supported iron oxide suggested that the combination of both methods (adsorption plus catalytic oxidation) into a single process could offer an attractive alternative for wastewater treatment.