| Summary: | 博士 === 國立成功大學 === 化學工程學系碩博士班 === 95 === Hydroxyl radical (HO•) is very reactive, underlying the chemistry of advanced oxation processes (AOPs) for degrading organic compounds in water. Among various AOPs, Fenton’s reagent (H2O2/Fe2+) has been known to be an effective and simple oxidant. The major drawback of Fenton’s reaction is the production of substantial amount of Fe(OH)3 sludge that requires further separation and disposal. To solve this problem, using the low concentration of iron ions in conventional homogeneous Fenton and Fenton-like reaction was attempted. Furthermore, the application of iron oxide as the heterogeneous photoassisted Fenton catalyst in oxidizing organic contaminants deserves an in-depth investigation.
This study investigated Fenton and Fenton-like reactions at low iron concentration (≦10 mg l-1) to oxidize three commercial azo dyes, namely Red MX-5B, Reactive Black 5 (RB5) and Orange G. This investigation reveals that both of these methods can remove the color of these dyes completely after 120 min of reaction in the presence of only 1 mg l-1 iron concentration. However, only 37%, 28% and 31% TOC of 0.1 mM Red MX-5B, Reactive Black 5 and Orange G can be eliminated after 480 min of reaction in the presence of 1 mg l-1 Fe3+, and 100 mg l-1 H2O2 at pH 2.5. This result is also found that it is easily to decolorize but hardly to mineralize the azo dyes in Fenton-like system at low iron concentration. In addition, this system has two major shortcomings. The first is that the system must be operated at low pH value. The second is that the TOC removal of this system is low. Therefore, heterogeneous catalysts of the photoassisted Fenton reaction instead of conventional homogeneous Fenton and Fenton-like reaction.
A novel activated alumina-supported iron oxide (denoted as FeAA-25), prepared using a fluidized-bed reactor (FBR), and was utilized as a catalyst of the heterogeneous photoassisted Fenton degradation of azo-dye RB5. The major components coated on the surface were identified as amorphous iron oxide and α-FeOOH. The characteristics of supported FeAA-25 including the particle morphology, surface atomic compositions, total and oxalate-soluble Fe contents and the specific surface area were determined. Photooxidation of RB5 by H2O2 was performed with FeAA-25 in a batch reactor in the range of pH 2.5–6.0. The degradation of RB5 comes from two perspectives. One is the catalysis from the iron oxide (hetergeneous reaction). The other is the catalysis from the Fe ions leaching form the catalyst to solution (homogeneous reaction). Conclusively, although the homogeneous catalysis is primarily below pH 3.5, heterogeneous catalysis contributes increasing importance to the oxidation of RB5 at pH 4.0–6.0. It has been found that the presence of UVA light, both for homogeneous and heterogeneous reactions shows that the efficiency of degradation of RB5 could be raised about 20%.
Furthermore, a simplified mechanism of RB5 decomposition that is consistent with the experimental findings for a solution with a pH of up to 7.0 is proposed. About 70% decolorization was measured and 45% of the total organic carbon was eliminated on the surface of the iron oxide at pH 7.0 after 240 min in the presence of 0.1 mM RB5, 5.0 g FeAA-25/L, 29.4 mM H2O2, under 15W UVA. In this mechanism, the adsorption and desorption behaviours of RB5 on FeAA-25 were also studied. The advantage of the use of the FeAA-25 as the heterogeneous photoassisted Fenton catalyst is that the degradation up to pH 7.0. However, the degradation rate of the system is slow.
Finally, we hope to combine the homogeneous photo-Fenton with heterogeneous photoassisted Fenton for degradation of RB5. For reducing the amount of Fe leaching from the FeAA-25 catalyst in an acidic environment, we modify the structure of the FeAA-25 catalyst surface by calcining. After calcining, the FeAA-25 became another novel photo-catalyst (denoted as FeAA-500). The photo-catalytic activity of the FeAA-500 was evaluated in the photooxidative degradation of 0.1 mM RB5 in the presence of H2O2 and UVA light (λ = 365 nm) in a solution with a pH of 2.5. Complete decolorization of the model pollutant RB5 was achieved; the total organic carbon (TOC) removal ratio was 95% after 120 min of reaction.
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