| Summary: | 碩士 === 國立聯合大學 === 環境與安全衛生工程學系碩士班 === 101 === Photo-Fenton process, especially using natural solar light as light source, has attracted great interest for the treatment of dye wastewater in recent years. However, the inductive mechanisms and effect of solar photo-enhancement involving reduction of ferric ion to ferrous ion on degradation of dye wastewater are still not clear nowadays. The possible inductive Fe(III) reduction i.e. Fe(II) regeneration mechanisms may include direct photoreduction, photodecarboxylation, photosensitization, and quinone-like cycle reduction. Therefore, the objective of this study is mainly to investigate the effect of various inductive Fe(III) reduction mechanisms involved in solar photo-Fenton (like) process for treatment of dye wastewater. Moreover, the ANOVA and ANN analysis were applied to examine the qualitative and quantitative effect of three main factor, namely, gH2O2/gCOD, [H2O2]0/[Fe3+]0, and reaction time, on the mineralization efficiency of dye wastewater. Three organic dyes wastewater with different structure and characteristics including Acid Orange 10 (AO10), Alizarin Violet 3B (AV), and Methylene Blue (MB) were investigated in this study.
Results showed that the Fe3+ reduction pathways could be divided into three stages in solar photo-Fenton-like degradation of AO10 and MB wastewater. In the initial reaction stage (0-0.01 min), the direct photoreduction accounted for a 28-39% of Fe3+ reduction and the Fenton-like mechanism account for a 54-64% of Fe3+ reduction while Fe3+ reduction was only 4-6% through photosensitization. In the second reaction stage (1-10 min), because the intermediate - oxalic acid was generated, leading to the major Fe(III) reduction mechanism transferred to photodecarboxylation, which accounted for a 91-97% of Fe3+ reduction in this stage. In addition, the reduction rate of Fe(III) was more than tens of times and hundreds of times of direct photoreduction and Fenton-like reaction mechanism, respectively. In the third reaction stage (15 min later), H2O2 and the generated oxalic acid were decomposed gradually, resulting in the Fe3+ reduction through the photodecarboxylation and Fenton-like mechanisms were consequently reduced and the Fe3+ reduction would be majorly by way of direct photoreduction. On the other hand, because the molecular structure of AV has a quinone-like structure, the Fe3+ reduction was mainly through the pathway of quinone-like cycle reduction (48.6%) and photodecarboxylation (47%) in the initial reaction. Therefore, the direct photoreduction and Fenton-like Fe3+ reduction mechanisms only accounted for 1.73% and 1.93% of total Fe3+reduction, respectively.
Photo-Fenton (like) process with the radiation of various light wave bands would affect the degradation efficiency of dye wastewater as well as the Fe3+ reduction mechanisms. It was found that because the Fe3+ reduction mechanisms relied primarily on the direct photoreduction and Fenton-like pathway in the initial degradation of AO10 and MB wastewater, the degradation efficiency of AO10 and MB wastewater was not noticeable with the irradiation of visible light band. On the other hand, more Fe2+ could be generated through the quinone-like cycle reduction pathway in AV wastewater so that the impact of irradiation of light wave band was less. In contrast, photo-Fenton (like) process with the radiation of UV light band showed much higher degradation efficiency for AO10 and MB, even AV wastewater since the photodecarboxylation accompanying with Fe3+ reduction was more effective.
The use of the analysis of variance (ANOVA) and the connection weights of the artificial neural networks (ANN) allow to perform an analysis of the relative relevance of each factor with respect to others. Results showed that the quantitative saliency of three main factor (gH2O2/gCOD, [H2O2]0/[Fe3+]0, and reaction time) on the mineralization efficiency of dye wastewater were 43.63%, 10.91%, and 45.44% for AO10, respectively, 11.71%, 30.08%, and 58.20% for MB, respectively, and 25.16%, 20.71%, and 54.13% for AV, respectively. It can be concluded that the reaction time is the most significant factor affecting dye mineralization. The dosage of [H2O2]0/[Fe3+]0 have a greater impact weights in MB and AV while an increasing in dosage of gH2O2/gCOD is more beneficial for the mineralization of AO10.
Based on the results obtained in this study, it was revealed that solar energy had a significant promotion effect on the Fenton like reaction mainly through various and incessant mechanism of reduction of Fe3+ ions and regeneration of Fe2+ ions, especially for dye wastewater. The use of ANOVA and ANN analysis provided meaningful information for controlling major process factors efficiently, leading to improve reaction efficiency in practical application.
Keywords: solar photo-Fenton (like) process, dye wastewater, Fe3+ reduction mechanism, ANOVA, artificial neural network , direct photoreduction, photosensitization reduction, photodecarboxylation, quinone-like cycle reduction
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