Jointly published by React.Kinet.Catal.Lett. Akadémiai Kiadó, Budapest Vol. 91, No. 1, 161−168 (2007)
and Springer, Dordrecht 10.1007/s11144-007-5126-0
0133-1736/2007/US$ 20.00. © Akadémiai Kiadó, Budapest. All rights reserved.
RKCL5126
DECOLORIZATION OF AZO DYES USING CATALYTIC OZONATION
Chung-Hsin Wua*, Chao-Yin Kuob and Chung-Liang Changc a
Department of Environmental Engineering, Da-Yeh University, 112, Shan-Jiau Road, Da-Tsuen, Chang-Hua, Taiwan, R.O.C.
b
Department of Safety Health and Environmental Engineering, National Yunlin University of Science and Technology, Touliu, Yunlin, Taiwan, R.O.C.
c
Department of Environmental Engineering and Health, Yuanpei University of Science and Technology, Hsinchu, Taiwan, R.O.C.
Received March 6, 2007, accepted April 30, 2007
Abstract
Decolorization of C.I. Reactive Red 2 (RR2) and C.I. Acid Orange 6 (AO6) using a catalytic ozonation system was evaluated. The decolorization rates for RR2 and AO6 were accelerated by at least 30% by adding MnO2 to the O3 system; additionally, the decolorization rate increased as the MnO2 dosage and ozone power consumption increased. Enhanced MnO2 catalytic ozonation was more apparent for AO6 than for RR2.
Keywords: O3, MnO2, C.I. Reactive Red 2, C.I. Acid Orange 6
INTRODUCTION
The toxicity of numerous dyes renders them environmentally hazardous. Azo dyes are very common pollutants in dye effluents. Removing color from wastewater is typically more important than removing other colorless organic compounds,
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162 CHUNG-HSIN WU et al.: DECOLORIZATION
because already small amounts of dye are clearly visible, and decrease the water quality markedly. Numerous physical/chemical methods, such as coagulation, flocculation, adsorption and membrane filtration have been utilized to decolorize the textile effluents. However, in these approaches problems associated with sludge generation, adsorbent regeneration and membrane fouling appear. Advanced oxidation processes (AOPs) are alternative techniques for decolorizing and reducing recalcitrant wastewater loads from textile companies. Ozonation has an excellent potential for decolorization for the following reasons: (i) no sludge remains; (ii) danger to humans is minimum; (iii) decolorization and degradation occur in one step; (iv) ozonation is easily performed; (v) little space is required; and, (vi) all residual ozone can be decomposed easily into oxygen and water [1].
Several studies have provided evidence that ozone can decolorize water-soluble synthetic dyes [2-5]. Oxidation utilizing ozone has two primary reaction pathways: direct attack of the ozone molecule on the pollutants; and, generation of free radicals due to decomposition of ozone and subsequent attack of these free radicals on the pollutants [3]. The efficiency of the oxidation process can be improved by employing ozone combined with UV light [2-5], H2O2 [3], Mn2+ [6, 7], MnOx/GAC [8] and MnO2 [5, 9-14]. The
UV/ozone process is more efficient than the ozone process alone, because the UV radiation promotes ozone decomposition, yielding additional hydroxyl radicals enhancing thereby the decolorization rate. The principal mechanism driving the catalytic ozonation when using Mn2+ is the formation of Mn4+ and its subsequent catalytic action in producing free radicals [6]. Using Mn4+ directly can provide enhanced decolorization results. Moreover, Oyama [15] indicated that MnO2 is the most effective metal oxide for ozone decomposition
in gases. Therefore, this study utilized MnO2 catalytic ozonation to decolorize
azo dyes C.I. Reactive Red 2 (RR2) and C.I. Acid Orange 6 (AO6). The effects of MnO2 dosage and ozone power consumption in the catalytic ozonation
process were assessed.
MATERIALS AND METHOD
Parent compounds RR2 and AO6 were obtained from Aldrich and used without further purification. The formula, molecular weight, and maximum light absorption wavelength (λmax) was C19H10Cl2N6Na2O7S2, 615 g/mol and 538
nm for RR2, and C12H9N2NaO5S, 316 g/mol and 490 nm for AO6, respectively.
The MnO2 was purchased from Merck. All experimental chemicals were of
analytical grade. A dielectric barrier discharge (DBD) reactor was utilized to generate ozone. The DBD reactor, which consumed 4, 6 and 8 W of power at a
CHUNG-HSIN WU et al.: DECOLORIZATION 163
gas flow rate of 500 mL/min, was used to examine the effects of ozone power consumption on the catalytic ozonation process. The schematic diagram for the ozone generator, and the photoreactor presented herein is the same as those in Wu et al. [4] and Wu and Chang [5].
The water utilized was deionized and distilled twice with a MINIQ. Catalytic ozonation experiments were performed in a 3 L hollow, cylindrical glass reactor. The reaction system was stirred constantly at 300 rpm, and aerated with ozone at a flow rate of 500 mL/min to keep the MnO2 suspended.
The dye concentration was 40 ppm in each experiment. To assess the effects of MnO2 dose, the dosage was maintained at 1, 2 and 3 g under 4 W of ozone
power consumption. A 15-mL aliquot was withdrawn from the photoreactor at pre-specified intervals. The MnO2 suspension was separated by centrifugation
at 5000 rpm for 10 min, and then filtered through a 0.22 µm filter. Decolorization of RR2 and AO6 was measured using a spectrophotometer (HACH DR/4000U) at 538 and 490 nm, respectively. Vaporization and adsorption reactions were also conducted to compare the decolorization efficiency for dyes with that associated with ozonation reactions.
RESULTS AND DISCUSSION
The degradation of RR2 and AO6 was attributed to ozonation-based reactions, because no significant disappearance occurred during vaporization and adsorption reactions. Figures 1(a) and 1(b) present the efficiency of decolorization for RR2 and AO6 at various MnO2 dosages under 4 W of power.
After 120 min, the decolorization efficiency for RR2 in O3, O3/MnO2 (1 g),
O3/MnO2 (2 g) and O3/MnO2 (3 g) was 83%, 92%, 95% and 95%, respectively,
and that for AO6 was 67%, 91%, 90% and 93%, respectively (Fig. 1). Both RR2 and AO6 were decolorized by > 90% in catalytic ozonation systems after 120 min of treatment. Plotting ln(Co/C) vs. time yielded the pseudo-first order
decolorization rate constants (k). Table 1 shows a summary of the k values and the correlation coefficients for the various ozone-based systems. As all correlation coefficients for ozone-based systems exceed 0.95, the k values of ozone-based systems satisfy the pseudo-first order kinetics. Under 4 W, the k values for RR2 for 0, 1, 2 and 3 g MnO2 added to the O3 system were 0.80,
1.04, 1.21 and 1.24 h–1, respectively, and those for AO6 were 0.55, 1.13, 1.20 and 1.34 h–1, respectively (Table 1). For both RR2 and AO6, the decolorization rate was accelerated by at least 30% by adding MnO2 to the O3 system; the
differences between various MnO2 dosages for decolorization were very small.
When small amounts of MnO2 were added (50-200 mg), the influence of
168 CHUNG-HSIN WU et al.: DECOLORIZATION
CONCLUSIONS
This study investigated how the MnO2 dosage and ozone power
consumption affect the ozonation process. The decolorization rate constants are consistent with a pseudo-first-order kinetics. Under 4 W of ozone power consumption, the k values for RR2 with 0, 1, 2 and 3 g MnO2 added to the O3
system were 0.80, 1.04, 1.21 and 1.24 h–1, respectively, and those for AO6 were 0.55, 1.13, 1.20 and 1.34 h–1, respectively. Furthermore, the increase in decolorization rate percentages for RR2 under 4, 6 and 8 W of ozone power consumption in O3/MnO2 (2g) were 51%, 24% and 11%, respectively, and
those for AO6 were 118%, 70% and 152%, respectively. This study concluded that catalytic ozonation by MnO2 was more efficient than ozonation alone for
azo dye decolorization.
Acknowledgements. The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this
research under Contract No. NSC 95-2221-E-212-022.
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