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J. Environ. Eng. Manage., 19(5), 277-282 (2009) ELECTRO-FENTON DEGRADATION OF SYNTHETIC DYE MIXTURE: INFLUENCE
OF INTERMEDIATES
Vahid Vatanpour,* Nezamaddin Daneshvar and Mohammad Hossein Rasoulifard
Water and Wastewater Treatment Research Laboratory Key Words: Electrochemical advanced oxidation processes (EAOPs), malachite green, orange II,
hydroquinone-like intermediates, wastewater treatment ABSTRACT
Electro-Fenton process is a potentially useful oxidation process for destroying toxic organic compounds in aqueous medium. In this study, the electro-Fenton degradation of a solution mixture of Malachite Green (MG) and Orange II catalyzed by ferric ions was examined. Results showed that this system could degrade and mineralize the dye mixture. It was shown that absorbance decrease in MG was accelerated in the presence of Orange II, whereas absorbance decrease of Orange II at the same conditions was depressed. This behavior was attributed to generation of hydroquinone-like intermediates from degradation of Orange II that can accelerate Fenton reaction by reduction of Fe3+ to Fe2+ ions. GC-MS detection of the products formed in the Orange II electro-Fenton degradation showed the generation of dihydroxynaphthalene compounds that are probably responsible for acceleration of MG degradation. INTRODUCTION
15% of dyes of the total world production are lost dur- ing synthesis and processing with wastewater [4]. Rapid industrialization and urbanization result in Thus, there is an urgent need for textile industries to the discharge of large amount of waste to the envi- develop effective methods of water processing. ronment, which in turn creates more pollution. The Many studies have reported decoloration of solu- majority of colored effluents are caused by the release tions containing only one dye by different methods. of dyes to the environment from textile, dyestuff, and However, industrial effluents usually contain mixture dyeing industries. Color is usually the first contami- of dyes. As a result, investigation of treatment of real nant to be recognized in wastewater. A very small wastewater or mixture of dyes is important. amount of dye in water (10-20 mg L-1) is highly visi- Development of the appropriate techniques for ble and affects water transparency and gas solubility treatment of mixture of dyes wastewater is important for the protection of natural waters. To eliminate dyes Many industrial and agricultural activities use from aqueous colored effluents and reduce their eco- water in an excessive way. However, it is now well logical consequences, several biological, chemical and known that the fresh water resources are limited, so electrochemical techniques have been proposed: an- they must be protected. The availability of clean water aerobic/aerobic degradation [5,6], Fenton’s reagent [7], for various human needs in the next decades seems to TiO2 photocatalyst [8], electro-coagulation/electro- become a challenge to take up. There are many classes flocculation [9] and anodic oxidation [10]. Other of dyes such as acidic, basic, neutral, azo, disperse, di- physical/chemical techniques including adsorption [11] rect, reactive, etc. Out of these dyes, azo dyes are and flotation [12] have also been employed. Physi- most frequently used. These dyes contain one or more cal/chemical methods do not degrade the pollutants azo bonds (–N=N–) in their structure [2]. Triphenyl- but they only transfer them from the liquid phase to methane dyes are the next in the list. Some of these the solid phase, thus causing secondary pollution. dyes are toxic and potentially carcinogenic [3]. About Conventional wastewater treatment based on biologi- *Corresponding author Email: vahidvatanpoor@yahoo.com J. Environ. Eng. Manage., 19(5), 277-282 (2009) cal process is not efficient enough to remove recalci- trant dyestuffs from effluents, because high molecular weight compounds are not easily degraded by bacteria MATERIALS AND METHODS
[13]. Despite the high oxidative efficiency of Fenton’s reagent, its application is limited by the storage and 1. Chemicals
shipment of concentrated H2O2 solutions and the gen- MG and Orange II were purchased from Merck Therefore, it is necessary to find an effective (Germany) and used without further purification. wastewater treatment capable of removing color and HClO4 (70%), HNO3 (65%), NaClO4·H2O and degrading toxic organic compounds from industrial Fe(NO3)3·9H2O were obtained from Merck. NaOH effluents. As an alternative, an indirect electrochemi- cal process for decoloration of wastewater containing dyes is proposed. The electric current induces redox 2. Instruments
reactions upon the electrodes surface resulting in the formation of reactive intermediates that could destruct Electrolyses were performed with a DC power the organic compounds. It makes the treatment of liq- supply. The cell voltage was determined with a UNI-T uids, gases and solids possible; it is compatible with (UT2002) digital multimeter. The solution pH was the environment because the main reagent, the elec- measured with a Metrohm 654 pH-meter. The dyes tron, is a clean one [14]. In situ electrochemical pro- spectra were obtained by using a Lightwave S 2000 UV-Vis spectrophotometer in wavelength of 484 nm Fe(II) by the simultaneous reduction of oxygen and for Orange II and 619 nm for MG. For GC-MS analy- Fe(III) on a cathode surface can solve the problems of sis, a GC system (Agilent 6890) with a 30 m × 0.25 Fenton’s reagent. In electro-Fenton process, Fe2+ or mm HP-5 capillary column coupled with a HP 5989A Fe3+ ions are added to the solution and hydrogen per- mass spectrometer operating in electron ionization oxide is electrogenerated from the two-electron reduc- mode at 70 eV was used. TOC values were deter- mined by catalytic oxidation with a Skala-Formics 2 on the cathode of an undivided electrolytic TOC analyzer. All samples were filtered (0.22 µm) and acidified with HNO3, (1% HNO3, 2 mM). 3. Electrolytic System
Fenton reaction involves several sequential reaction steps according to which hydroxyl (•OH) and hydrop- The experiments were conducted at room tem- HO ) free radicals are the key intermediates perature in an open, undivided and cylindrical glass in the reaction. The free radical mechanism consists of cell of 400 mL capacity and performed at constant po- tential. The commercial graphite felt (thickness = 0.4 cm) with 9.5 cm2 surface area was selected as cathode. Carbon is widely used as a cathode material for hy-drogen peroxide generation because it exhibits a range of electrochemical activities towards oxygen reduction, high overpotential for hydrogen evolution and low catalytic activity for hydrogen peroxide decomposi-tion [26]: The Pt sheet of 1 cm2 area was used as anode and Electro-Fenton process has been used for decoloration the reference electrode was a saturated calomel elec- of single-dye solutions such as Acid Red 14 [18], in- trode (SCE). In all experiments, solutions were stirred digo carmine [19], direct Orange 61 [20], Malachite magnetically at 600 rpm. Prior to the electrolysis, pure Green [21] and other dyes [22,23]. Note that only two O2 was bubbled for 10 min through the solution. Dur- recent studies reported removal of mixture of dyes by ing electrolysis, O2 was sparged at 20 mL min-1. Solu- tions of 200 mL containing one or two dyes (C0 = 10-5 The present work studied the decoloration and M) in 0.05 M NaClO4 at initial pH 3.0 were per- mineralization of mixture of two dyes Malachite fromed by applying a constant potential of -0.5 V. The Green (MG) and Orange II from two different group conditions for this work were optimized from the pre- of triphenylmethane and azo, respectively by electro- vious work [27]. The value of pH 3.0 was chosen be- Fenton process. Effect of presence of one dye on re- cause several studies [15,16] have shown that the op- moval of another dye and influence of initial concen- timum pH for Fenton’s reaction and production of tration of counter dye on decoloration of another dye H2O2 is in the range 2.8-3.0. Before the study of elec- Vatanpour et al.: MG and Orange II Mixture Treatment by EF tro-Fenton process, a catalytic amount of 10-4 M Fe3+ Results show that degradation of MG is faster than The oxidizing power of the electro-Fenton sys- 4. Analysis Procedure
tem to mineralize dye solutions was evaluated from their TOC decay. Figure 1b shows selected TOC-time Samples were withdrawn from the reactor at plot for the degradation of the mixture of MG and Or- regular time intervals and the removal of color was ange II solution using 9.5 cm2 graphite felt cathode. A evaluated by simultaneous spectrophotometeric de- continuous TOC abatement was observed attaining termination using mean centering of ratio spectra [28]. 79% of mineralization after 180 min of electrolysis. This method has been inspired from successive ratio TOC decay shows that this electro-Fenton system can derivatives of ratio spectra in two steps. The mean degrade and mineralize organic pollutants. centering method uses mean centering of ratio spectra instead of derivatives of them. By eliminating deriva- 2. Electro-Fenton Degradation of MG and Orange
tive steps, signal-to-noise ratio is enhanced dramati- II in Separate and Mixed Solution
Figures 2a and 2b show the absorbance decrease RESULTS AND DISCUSSION
at 619 nm (MG) and 484 nm (Orange II) alone or in mixed solution. The removal efficiency of both MG 1. Decoloration and Mineralization of a Mixture
and Orange II was altered in the mixed solution com- Containing MG and Orange II
pared with the separate case. In the presence of Or- ange II, decoloration of MG was accelerated whereas A mixture of MG and Orange II at initial con- decoloration of Orange II was reduced to some extent centration of 10-5 M for each one was treated by elec- when MG was present. The depression of the absorb- tro-Fenton process at -0.5 V. Electrochemically gen- ance decrease at 484 nm can be attributed to competi- erated hydroxyl radicals react with dye molecules tive trap of hydroxyl radicals by MG [29] and the ef- leading to their oxidation. Figure 1a shows a rapid de- fect of high concentration of pollutant. However, the cay of each dye present in the synthetic dye mixture as absorbance decrease at 619 nm was surprisingly ac- a function of time during the electro-Fenton process. celerated in the presence of Orange II. It was men- tioned that some degradation intermediates obtained from degradation of aromatic compounds such as quinone-like compounds could expedite the reduction of ferric ions to ferrous ions (Eqs. 10 and 11) [29-32]. As a result, rate of Fenton reaction was accelerated. Fig. 1. Removal of an aqueous mixture of dyes MG and Orange II (C0 = 10-5 M of each dye) during electro-Fenton treatment in the presence of Fe3+ ions as catalyst. [Fe3+] = 10-4 M, [NaClO4] = 0.05 M, pH = 3 and E = -0.5 V vs. SCE. (a) decoloration; (b) mineralization. J. Environ. Eng. Manage., 19(5), 277-282 (2009) Destruction of dyes by electro-Fenton Fig. 3. Effect of one dye on the other: [NaClO4] = 0.05 degradation of: (a) MG, alone or mixed, (b) M, pH = 3.0, [Fe3+] = 10-4 M and E = -0.5 V. (a) Orange II, alone or mixed. [MG] = 10-5 M, Influence of Orange II concentration on MG (619 nm) at [MG] = 10-5 M; (b) Influence of MG concentration on Orange II (484 nm) at [Orange It can be hypothesized, based on what observed for Fenton systems, that Orange II or some intermedi- ates generated in the degradation of Orange II could accelerate the removal of MG by accelerating of hy- 3. Influence of Initial Orange II Concentration on
MG Degradation
In order to show whether hydroquinone-like in- termediates generated in degradation of Orange II or not, electro-Fenton degradation intermediates of Or- With regard to these findings, that Orange II is ange II alone was determined after 60 min electrolysis source of hydroquinone-like intermediates, investiga- by GC-MS analysis. Results were reported in previous tion of the effect of the Orange II initial concentration work [27]. It is observed that one of the identified in- on MG degradation is examined. Figure 3a shows that termediates is 1,2-dihydroxynaphthalene. It was re- removal efficiency of MG increased by increasing Or- ported that ortho-dihydroxy aromatic compounds such ange II concentration up to 2 × 10-5 M. This effect can as catechol and 2,3-dihydroxybenzoic acid can reduce be attributed to more conversion of Fe3+ to Fe2+ by in- ferric ions to ferrous ions [32,33]. Probably generated creasing of produced intermediates from degradation 1,2-dihydroxynaphthalene from degradation of Or- of Orange II. However, at higher Orange II concentra- ange II can reduce Fe3+ to Fe2+ and therefore, lead to tion, scavenging of hydroxyl radicals overcome in ac- increasing of absorbance decrease rate at 619 nm in celerating effect and therefore, removal efficiency of Generally speaking, in electro-Fenton system 4. Influence of MG Concentration on Orange II
Fe2+ is regenerated through the reduction of Fe3+ in Degradation
cathode surface (Eq. 13) and then reduced to Fe2+ by H 2O2 (Eq. 14). However, at the same time in the pres- ence of hydroquinone-like intermediates, regeneration In previous sections, it was observed that ab- of Fe2+ is prompted and causing a rapid degradation of sorbance decrease at 484 nm (Orange II) in the pres- ence of MG was depressed. Figure 3b shows that by increasing MG concentration, removal efficiency of Orange II is lower. It can be attributed to high scav- Vatanpour et al.: MG and Orange II Mixture Treatment by EF enging of •OH by high concentration of MG, which reacts quicker with OH, as shown in Fig. 1. 6. Mohanty, S., N. Dafale and N.N. Rao, Microbial decolorization of reactive Black-5 in a two-stage CONCLUSIONS
anaerobic-aerobic Reactor using acclimatized activated textile sludge. Biodegradation, 17(5), This paper has considered the electro-Fenton treatment of mixture of two dyes using in situ hydro- 7. Meriç, S., H. Selcuk, M. Gallo and V. Belgiorno, gen peroxide produced by oxygen reduction on graph-ite-felt cathode. Experimental results showed that: Decolourisation and detoxifying of Remazol Red ‧Removal efficiency of dyes is attributed to their dye and its mixture using Fenton’s reagent. structure. At the same conditions, degradation of 8. Gupta, A.K., A. Pal and C. Sahoo, Photocatalytic ‧Aromatic compounds can catalyze the Fenton reac- degradation of a mixture of Crystal Violet (Basic tion when they are transformed into hydroquinone- like intermediates by hydroxyl radicals. Hydro- quinone-like compounds promote the Fenton reac- tion by accelerating the regeneration of ferrous ions, 9. Szpyrkowicz, L., Hydrodynamic effects on the which is the slow step (Eq. 9) in the mechanism of performance of electro-coagulation/electro- flotation for the removal of dyes from textile GC-MS analysis and degradation rate of MG in the presence of Orange II show that one of the gener- wastewater. Ind. Eng. Chem. Res., 44(20), 7844- ated intermediates of Orange II degradation proba- bly can accelerate the regeneration of Fe2+ from re- 10. Panizza, M. and G. Cerisola, Electrochemical duction of Fe3+ and therefore, lead to acceleration oxidation as a final treatment of synthetic tannery wastewater. Environ. Sci. Technol., 38(20), 5470- ACKNOWLEDGMENTS
Chakraborty, S., J.K. Basu, S. De and S. DasGupta, Adsorption of reactive dyes from a The authors would like to express their gratitude textile effluent using sawdust as the adsorbent. Ind. to the University of Tabriz, Iran for the financial sup- Eng. Chem. Res., 45(13), 4732-4741 (2006). port and assistance and thank Mr. Mahmoud Zarei for 12. Dafnopatidou, E.K., G.P. Gallios, E.G. Tsatsaroni running the GC-MS experiments and Mr. Jafarizad for TOC analysis. and N.K. Lazaridis, Reactive dyestuffs removal from aqueous solutions by flotation, possibility of REFERENCES
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Revision Received: June 18, 2008
26. Panizza, M. and G. Cerisola, Removal of organic and Accepted: June 18, 2008

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