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peroxide. FeOOH was selected as iron oxide. The effect of FeOOH concentration, H2O2 concentration, the presence of radical scavenger, and pH on the dye removal and the decomposition of hydrogen peroxide was investigated. The rate of H2O2 decomposition was obtained as a pseudo-first-order kinetics relative to FeOOH concentration. However, despite of the increased H2O2 decomposition rate, the dye removal rate was not proportional to FeOOH concentration because FeOOH surface plays a role of scavenging OH radical. The H2O2 decomposition by FeOOH at pH 7 was more significant than that at pH 3, suggesting the possibility for overcoming limitations of homogeneous Fenton reaction which occurs only in acidic condition. The mechanism for the dye removal under the iron oxide catalysed decomposition of hydrogen peroxide was suggested, based on the experimental results obtained in this study.
Decolorization of Dye with Iron Oxide Catalysed Decomposition of Hydrogen Peroxide Joonseon Jeong, Jeyong Yoon School of Chemical Engineering, College of Engineering, Seoul National University, San 56-1, Shilim-Dong, Kwanak-Goo, Seoul, Korea (151-742) ABSTRACT This study describes the dye removal under the iron oxide(FeOOH) catalysed decomposition of hydrogen peroxide. FeOOH was selected as iron oxide. The effect of FeOOH concentration, H 2 O 2 concentration, the presence of radical scavenger, and pH on the dye removal and the decomposition of hydrogen peroxide was investigated. The rate of H 2 O 2 decomposition was obtained as a pseudo-first-order kinetics relative to FeOOH concentration. However, despite of the increased H 2 O 2 decomposition rate, the dye removal rate was not proportional to FeOOH concentration because FeOOH surface plays a role of scavenging OH radical. The H 2 O 2 decomposition by FeOOH at pH 7 was more significant than that at pH 3, suggesting the possibility for overcoming limitations of homogeneous Fenton reaction which occurs only in acidic condition. The mechanism for the dye removal under the iron oxide catalysed decomposition of hydrogen peroxide was suggested, based on the experimental results obtained in this study. KEYWORDS FeOOH, hydrogen peroxide, dye, hydroxyl radical, decolorization INTRODUCTION AOPs (Advanced Oxidation Processes) which generate hydroxyl radical (OH•) have been introduced to water treatments recently. AOPs comprise various types such as O 3 /H 2 O 2 , Fe 2+ /Fe 3+ /H 2 O 2 , UV/H 2 O 2 , and UV/TiO 2 , depending on the how to produce OH•. Among these types, Fenton’s reagent, a mixture of ferrous iron and hydrogen peroxide, has been applied to treat a various wastewater and contaminated soils (Venkatadri and Peters, 1993). But it is effective only in acidic pH. In addition, the process generates a lot of iron sludge which need further treatment. Alternatively, a number of researchers investigated about hydrogen peroxide decomposition and contaminant degradation by iron oxide catalysed reaction (Valentine et al., 1998; Watts et al., 1993; Abbot et al., 1990). Iron oxides are abundant in natural water and known to effectively decompose hydrogen peroxide for generating hydroxyl radicals. Especially, it has the advantage which can operate in neutral pH condition. We selected dye as a model compound in this study. The treatment of dye wastewater is one of the most urgent subjects in pollution control because of its resistance to biodegradation (Ganesh et al., 1994). This study reports the mechanisms for the oxidation of commercial dye and H 2 O 2 decomposition in H 2 O 2 /iron oxide system. MATERIALS AND METHODS All solutions were prepared with deionised/distilled water, treated with a Barnstead NANO pure system, and analytical reagent grade chemicals. Hydrogen peroxide (30.0-35.5%) was obtained from the Junsei Chemical Co., Ltd. The α-FeOOH (30-50 mesh), Fe 3 O 4 , Fe 2 O 3 and tert-butanol (99.5+%) were purchased from Aldrich Chemical Company. Reactive Red 6 was obtained from Imperial Chemical Industries. All solutions used in experiment were prepared from dilution of the stock solution. The experiments were conducted in open batch reactor with mechanical mixing. The initial pH was controlled with solutions of 0.1N NaOH or 0.1N HClO 4 and pH was not further adjusted. The pH variation during the reaction was <±0.1 pH unit. The reactions were initiated by injecting FeOOH to the mixed solution of H 2 O 2 (20-21 mM) and Dye (100-110 mg/L). Hydrogen - 31 - peroxide concentration was determined spectrophotometrically using Ti Ⅳ method (Eisenberg, 1943). The aqueous concentration of dye was determined by measuring the absorbance intensity at the maximum absorbance wavelength(528nm), the calibration curve was established by standard solution before every experiments. UV/VIS spectrophotometric measurements were performed on the Hewlett-Packard Diode Array Spectrophotometer(HP 8452). The pH was measured by pH-meter (Orion 710A). RESULT AND DISCUSSION In preliminary test, three types of iron oxide FeOOH(Goethite), Fe 3 O 4 (Magnetite) and Fe 2 O 3 (Hematite)) were considered. FeOOH(Goethite) was found to be the most reactive with hydrogen peroxide among three iron oxides at neutral pH condition(data not shown). This fact has also been reported by Lin et al. (1998). Hence, we investigated more precisely FeOOH /H 2 O 2 system and most of the experiments were performed in pH 7. In separate test, it was confirmed that the dye removal by adsorption on iron oxide surface was negligible and also any significant photochemical reaction was not observed by natural light under our experimental condition. Hydrogen peroxide decomposition kinetics -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0 5 10 15 20 25 time (hr) ln([H 2 O 2 ]/[H 2 O 2 ] 0 ) iron oxide 0.1 g/L iron oxide 0.3 g/L iron oxide 0.5 g/L iron oxide 1 g/L 0 0.0005 0.001 0.0015 0.002 0.0025 0 0.2 0.4 0.6 0.8 1 1.2 iron oxide concentration (g/L) Observed first-order rate constant (min -1 ) k FeOOH+H2O2 = 0.002 min -1 (g/L) -1 R 2 = 0.9945 Figure 1. Observed first-order fit of H 2 O 2 Figure 2. Observed first-order rate constant decomposition for different concentrations of iron oxide. as a function of iron oxide concentration. ([H 2 O 2 ] 0 = 20mM , pH 7 ) ([H 2 O 2 ] 0 = 20mM , pH 7 ) Figure 1 present the H 2 O 2 decomposition as a function of FeOOH concentration. Any organic compound was not added in this reaction. Experimental data was represented with the expressions of ln ([H 2 O 2 ]/[H 2 O 2 ] 0 ) as a function of the reaction time. Good linear plot in Figure 1 indicates that the H 2 O 2 decomposition is the first order reaction with H 2 O 2 . This results agree well with the previous study (Lin et al., 1998). A plot of the observed first-order rate constant for H 2 O 2 decomposition k obs (][ 22 FeOOHk OHFeOOH + ) is presented in Figure 2. The good linear relationship (R 2 =0.99) between the rate constant and the concentration of FeOOH indicates that the H 2 O 2 decomposition is the first order reaction with FeOOH. The second order rate constant obtained from the result of Figure 2 is 0.002 min -1 (g/L as FeOOH) -1 . - 32 - Effect of FeOOH concentrations on dye removal 0 0.001 0.002 0.003 0.004 00.511.5 FeOOH concentration (g/L) rate constant k''(min -1 ) 0 0.01 0.02 0.03 0.04 0 0.2 0.4 0.6 0.8 1 1.2 FeOOH concentration (g/L) Stoichiometric efficiency, E (mM dye / mM H 2 O 2 ) Figure 3. Observed first-order rate constant for Figure 4. Relationship between dye removal dye removal as a function of FeOOH concentration efficiency and FeOOH concentration ([H 2 O 2 ] 0 = 20mM, [dye] 0 = 100 mg/L, pH 7) ([H 2 O 2 ] 0 = 20mM, [dye] 0 = 100 mg/L, pH 7) Hydroxyl radical can be generated by the reaction between hydrogen peroxide and metal surface (Weiss et al., 1934). Dye removal can occur as a result of the reaction between dye and hydroxyl radicals. The dye removal was investigated as a function of FeOOH concentration. Under the assumption that OH radical is the only reactive species on dye removal in this system, dye removal can be described by ][ ][ dyek dt dyed obs =− (1) ssOHdyeobs OHkk ][ •= •+ A plot of observed first order rate constant (min -1 ) for dye removal as a function of FeOOH concentration is shown in Figure 3. All R 2 for the fitness of observed first order reaction were more than 0.99 at each FeOOH concentration. In contrast with the result of Figure 2, dye removal was not proportional to the FeOOH concentration. Rather, the observed first order rate constant for dye removal was reduced in the presence of excess FeOOH (1.0 g/L). This phenomena can be rationalized from the assumption that some portions of OH radical which is generated as a result of the reaction between hydrogen peroxide and FeOOH is wasted, not to be used for dye removal. That is, produced OH radical can be scavenged by FeOOH itself. The stoichiometric efficiency (E) which Valentine et al.(1998) defined was presented as a function of FeOOH (equation (2)). Figure 4 show that E for dye removal is inversely proportional to the FeOOH concentration. This fact confirms again that FeOOH can act as a OH radical scavenger. ][ ][ 22 OH compound E ∆ ∆ = (2) Effect of radical scavenger on H 2 O 2 decomposition and dye removal In the presence of tert-butanol which is a well-known OH radical scavenger (k = 7.6x10 8 M -1 s -1 , Buxton et al. (1988)), hydrogen peroxide decomposition and dye removal were investigated to examine the dye removal mechanism. Figure 5 & 6 show the different effect of tert-butanol on hydrogen peroxide decomposition and dye removal. The presence of tert-butanol caused only slight change of hydrogen peroxide decomposition, whereas 10 mM of tert-butanol completely stopped the dye removal. These observations support the following explanations for hydrogen peroxide decomposition and dye removal. Figure 5 indicates that the portion of OH radical reacts with H 2 O 2 is minimal in this experimental condition since the presence of OH radical scavenger has no significant effect on hydrogen peroxide decomposition. One - 33 - possible explanation is that most of the H 2 O 2 decomposition only takes place by FeOOH surface catalysed reaction. However, dye removal occurs mainly by the reaction with OH radicals since the dye removal is greatly affected by the presence of tert-butanol (Figure 6). The hydroxyl radicals produced by H 2 O 2 /FeOOH system face four different competitive reactions with FeOOH surface, dye, H 2 O 2 and tert-butanol. Hence, dye removal is characterized by each relative rate constants with OH radical and the concentration of its competing compounds for OH radical. It is known that OH radical is very reactive with iron oxide surface (k ≥ 10 11 M -1 s -1 , Miller et al. (1999)), dye, and tert-butanol. The rate constant of reaction between dye and hydroxyl radical (k ≈ 10 10 M -1 s -1 )is obtained from the dye removal experiment under the pseudo steady state condition of OH radical(equation (1), data not shown). In comparison, the reaction rate of H 2 O 2 with hydroxyl radical is relatively slow (k= 2.7x10 7 M -1 s -1 , Buxton et al. (1988)). The relative difference of the respective rate constant of reaction make a decision of OH radical reaction pathway. 0 5 10 15 20 25 0 5 10 15 20 25 time (hr) H 2 O 2 Conc. (mM) 10 mM 5 mM 1 mM 0.1 mM no t-BuOH 0 20 40 60 80 100 120 0 5 10 15 20 25 time (hr) Dye Conc. (mg/L) 10 mM 5 mM 1 mM 0.1 mM no t-BuOH Figure 5. Effect of tert-butanol on H 2 O 2 Figure 6. Effect of tert-butanol on dye removal decomposition ([H 2 O 2 ] 0 = 20 mM, [dye] 0 = 108 mg/L, pH 7) ([H 2 O 2 ] 0 = 20 mM, [dye] 0 = 108 mg/L, pH 7) Effect of pH on H 2 O 2 decomposition and dye removal 0 5 10 15 20 25 0 5 10 15 20 25 time (hr) H 2 O 2 Conc (mM) pH3 pH7 pH10 0 20 40 60 80 100 120 0 5 10 15 20 25 time (hr) Dye Conc (mg/L) pH3 pH7 pH10 Figure 7. The effect of pH on the H 2 O 2 Figure 8. The effect of pH on the dye decomposition removal ([H 2 O 2 ] 0 = 20 mM, FeOOH = 0.5 g/L, [dye] 0 ≈ 100 mg/L) The effect of pH on the H 2 O 2 decomposition and dye removal are presented in Figure 7 & 8. The H 2 O 2 - 34 - decomposition and dye removal are enhanced as pH increases. The H 2 O 2 decomposition and dye removal were significantly hindered at pH of 3 favorable for homogeneous Fenton reaction. This is a clear evidence for indicating that the reaction mechanism of iron oxide catalysed H 2 O 2 decomposition differs from that of homogeneous Fenton reaction mechanism. Reaction Mechanism Based on the results from Figure 1-8, following mechanisms of the catalytic decomposition of H 2 O 2 and dye removal were suggested in Table 1 and Figure 9. Similar mechanism for surface catalysed H 2 O 2 decomposition in the presence of sand was suggested by Miller et al. (1999). No Reaction 1 ≡ Fe( Ⅲ ) + H 2 O 2 → ≡ Fe( Ⅱ ) + H + + •HO 2 2 ≡ Fe( Ⅱ ) + H 2 O 2 → ≡ Fe( Ⅲ ) + OH - + OH• 3 ≡ Fe( Ⅲ ) + •O 2 - → ≡ Fe( Ⅱ ) + O 2 4 ≡ Fe( Ⅱ ) + •HO 2 → ≡ Fe( Ⅲ ) + HO 2 - 5 ≡ Fe( Ⅱ ) + OH• → ≡ Fe( Ⅲ ) + OH - 6 Dye + OH• → products 7 tert-butanol + OH• → scavenging Table 1. Mechanism of FeOOH catalysed H 2 O 2 decomposition and dye removal H 2 O 2 + Dye Products Scavenged H 2 O 2 /HO 2 - t-BuOH Surface ⋅ OH H 2 O 2 ⋅ HO 2 /O 2 - Surface Scavenging ≡ Fe(II) ≡ Fe(III) reduced state oxidized state Figure 9. The reaction scheme for H 2 O 2 decomposition and dye removal The reaction 1, which generates a reduced state of FeOOH surface by the reaction between H 2 O 2 and FeOOH surface, is the rate-limiting step of H 2 O 2 decomposition. Therefore, the increase of FeOOH enhances H 2 O 2 decomposition rate proportionally. OH radicals are produced by reaction 2 and scavenged mainly by two reactions in this experimental condition (FeOOH reactions and tert-butanol reaction). Dye removal (reaction 6) is competitive reaction with reaction 5 & 7. CONCLUSION The removal of dye (Reactive Red 6) in hydrogen peroxide/iron oxide (FeOOH) system was investigated. We have following conclusions The FeOOH had the best catalytic effect on hydrogen peroxide decomposition among three iron oxides (FeOOH, Fe 3 O 4 and Fe 2 O 3 ). - 35 - The hydrogen peroxide decomposition by FeOOH followed a first order reaction with the H 2 O 2 and FeOOH concentration. The rate of dye removal under the presence of H 2 O 2 was not proportional to the FeOOH concentration. This is because the FeOOH surface not only generates the hydroxyl radicals, but also act as OH radical scavenger. The presence of tert-butanol reduced the rate of dye removal significantly with no or slight effect on hydrogen peroxide decomposition. The rate H 2 O 2 decomposition and dye removal showed the significant pH dependence. ACKNOWLEDGEMENT Financial aid from the Brain Korea 21 Program (the Ministry of Education) is gratefully acknowledged. REFERENCES Abbot J. and Brown D.G. (1990). Kinetics of Iron-Catalysed Decomposition of Hydrogen Peroxide in Alkaline Solution, Int. J. Chem. Kinetics, 22, 963-974 Buxton G. V., Greenstock C. L., Helman W. P. and Ross A. B. (1988). Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms, and hydroxyl radicals in aqueous solution. J. Phys. Chem. Ref. Data., 17, 513-886 Eisenberg G.M. (1943). Colorimetric determination of hydrogen peroxide. Ind. Eng. Chem. Anal., 15 (5), 327–328. Ganesh R. and Boardman G.D. and Michelsen D. (1994). Fate of Azo Dyes in Sludge. Wat. Res., 28, 1367-1376 Lin S. and Gurol M.D. (1998). Catalytic Decomposition of Hydrogen Peroxide on Iron Oxide: Kinetics, Mechanism, and Implications. Environ. Sci. Technol., 32, 1417-1423 Miller C.M. and Valentine R.L. (1999). Mechanistic Studies of Surface Catalysed H 2 O 2 Decomposition and Contaminant Degradation in the Presence of Sand, Wat. Res., 33(12), 2805-2816. Valentine R.L. and Ann Wang H.C. (1998). Iron Oxide Surface Catalysed Oxidation of Quinoline by Hydrogen Peroxide. J. Envir. Engrg., 124(1), 31-38 Venkatadri R. and Peters R.W. (1993). Chemical Oxidation Technologies: Ultraviolet Light/Hydrogen Peroxide, Fenton’s Reagent, and Titanium Dioxide-Assisted Photocatalysis. Hazard. Waste. Mater., 10, 107-149 Watts R.J. and Udell M.D. and Monsen R.M. (1993). Use of Iron Minerals in Optimizing the Peroxide Treatment of Contaminated Soils. Water Environ. Res., 65(7), 839-844 Weiss J. J. and Haber F. (1934). The catalytic decomposition of hydrogen peroxide by iron salts. Proc. Roy. Soc. London, Ser. A, 147, 332 - 36 - . reaction pathway. 0 5 10 15 20 25 0 5 10 15 20 25 time (hr) H 2 O 2 Conc. (mM) 10 mM 5 mM 1 mM 0 .1 mM no t-BuOH 0 20 40 60 80 10 0 12 0 0 5 10 15 20 25 time (hr). ≥ 10 11 M -1 s -1 , Miller et al. (19 99)), dye, and tert-butanol. The rate constant of reaction between dye and hydroxyl radical (k ≈ 10 10 M -1 s -1 )is