J. Environ. Eng. Manage., 19(5), 277-282 (2009) 277

ELECTRO-FENTON DEGRADATION OF SYNTHETIC DYE MIXTURE: INFLUENCE OF INTERMEDIATES

Vahid Vatanpour,* Nezamaddin Daneshvar and Mohammad Hossein Rasoulifard Water and Wastewater Treatment Research Laboratory

Department of Applied Chemistry University of Tabriz

Tabriz, Iran

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.

*Corresponding author Email: vahidvatanpoor@yahoo.com

INTRODUCTION Rapid industrialization and urbanization result in

the discharge of large amount of waste to the envi-ronment, which in turn creates more pollution. The majority of colored effluents are caused by the release of dyes to the environment from textile, dyestuff, and dyeing industries. Color is usually the first contami-nant to be recognized in wastewater. A very small amount of dye in water (10-20 mg L-1) is highly visi-ble and affects water transparency and gas solubility of waterbodies [1].

Many industrial and agricultural activities use water in an excessive way. However, it is now well known that the fresh water resources are limited, so they must be protected. The availability of clean water for various human needs in the next decades seems to become a challenge to take up. There are many classes of dyes such as acidic, basic, neutral, azo, disperse, di-rect, reactive, etc. Out of these dyes, azo dyes are most frequently used. These dyes contain one or more azo bonds (–N=N–) in their structure [2]. Triphenyl-methane dyes are the next in the list. Some of these dyes are toxic and potentially carcinogenic [3]. About

15% of dyes of the total world production are lost dur-ing synthesis and processing with wastewater [4]. Thus, there is an urgent need for textile industries to develop effective methods of water processing.

Many studies have reported decoloration of solu-tions containing only one dye by different methods. However, industrial effluents usually contain mixture of dyes. As a result, investigation of treatment of real wastewater or mixture of dyes is important.

Development of the appropriate techniques for treatment of mixture of dyes wastewater is important for the protection of natural waters. To eliminate dyes from aqueous colored effluents and reduce their eco-logical consequences, several biological, chemical and electrochemical techniques have been proposed: an-aerobic/aerobic degradation [5,6], Fenton’s reagent [7], TiO2 photocatalyst [8], electro-coagulation/electro-flocculation [9] and anodic oxidation [10]. Other physical/chemical techniques including adsorption [11] and flotation [12] have also been employed. Physi-cal/chemical methods do not degrade the pollutants but they only transfer them from the liquid phase to the solid phase, thus causing secondary pollution. Conventional wastewater treatment based on biologi-

278 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 [13]. Despite the high oxidative efficiency of Fenton’s reagent, its application is limited by the storage and shipment of concentrated H2O2 solutions and the gen-eration of Fe(III) sludge.

Therefore, it is necessary to find an effective wastewater treatment capable of removing color and degrading toxic organic compounds from industrial effluents. As an alternative, an indirect electrochemi-cal process for decoloration of wastewater containing dyes is proposed. The electric current induces redox reactions upon the electrodes surface resulting in the formation of reactive intermediates that could destruct the organic compounds. It makes the treatment of liq-uids, gases and solids possible; it is compatible with the environment because the main reagent, the elec-tron, is a clean one [14]. In situ electrochemical pro-duction of H2O2 in acidic media and regeneration of Fe(II) by the simultaneous reduction of oxygen and Fe(III) on a cathode surface can solve the problems of Fenton’s reagent. In electro-Fenton process, Fe2+ or Fe3+ ions are added to the solution and hydrogen per-oxide is electrogenerated from the two-electron reduc-tion of O2 on the cathode of an undivided electrolytic cell [15,16]:

O2 + 2H+ + 2e− → H2O2 (1)

Fenton reaction involves several sequential reaction steps according to which hydroxyl (•OH) and hydrop-eroxyl ( •

2HO ) free radicals are the key intermediates in the reaction. The free radical mechanism consists of the following steps [17]:

Fe2+ + H2O2 → Fe3+ + •OH + OH− (2)

•OH + H2O2 → H2O + •2HO (3)

Fe3+ + •2HO → Fe2+ + H+ + O2 (4)

Fe2+ + •2HO → Fe3+ + HO2

− (5)

Fe2+ + •OH → Fe3+ + OH− (6)

Electro-Fenton process has been used for decoloration of single-dye solutions such as Acid Red 14 [18], in-digo carmine [19], direct Orange 61 [20], Malachite Green [21] and other dyes [22,23]. Note that only two recent studies reported removal of mixture of dyes by electro-Fenton [24,25].

The present work studied the decoloration and mineralization of mixture of two dyes Malachite Green (MG) and Orange II from two different group of triphenylmethane and azo, respectively by electro-Fenton process. Effect of presence of one dye on re-moval of another dye and influence of initial concen-tration of counter dye on decoloration of another dye

were examined.

MATERIALS AND METHODS

1. Chemicals MG and Orange II were purchased from Merck

(Germany) and used without further purification. HClO4 (70%), HNO3 (65%), NaClO4·H2O and Fe(NO3)3·9H2O were obtained from Merck. NaOH was purchased from Fluka.

2. Instruments

Electrolyses were performed with a DC power

supply. The cell voltage was determined with a UNI-T (UT2002) digital multimeter. The solution pH was measured with a Metrohm 654 pH-meter. The dyes spectra were obtained by using a Lightwave S 2000 UV-Vis spectrophotometer in wavelength of 484 nm for Orange II and 619 nm for MG. For GC-MS analy-sis, a GC system (Agilent 6890) with a 30 m × 0.25 mm HP-5 capillary column coupled with a HP 5989A mass spectrometer operating in electron ionization mode at 70 eV was used. TOC values were deter-mined by catalytic oxidation with a Skala-Formics TOC analyzer. All samples were filtered (0.22 µm) and acidified with HNO3, (1% HNO3, 2 mM).

3. Electrolytic System

The experiments were conducted at room tem-

perature in an open, undivided and cylindrical glass 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]:

H2O2 + 2H+ + 2e− → 2H2O (7)

The Pt sheet of 1 cm2 area was used as anode and the reference electrode was a saturated calomel elec-trode (SCE). In all experiments, solutions were stirred magnetically at 600 rpm. Prior to the electrolysis, pure O2 was bubbled for 10 min through the solution. Dur-ing electrolysis, O2 was sparged at 20 mL min-1. Solu-tions of 200 mL containing one or two dyes (C0 = 10-5 M) in 0.05 M NaClO4 at initial pH 3.0 were per-fromed by applying a constant potential of -0.5 V. The conditions for this work were optimized from the pre-vious work [27]. The value of pH 3.0 was chosen be-cause several studies [15,16] have shown that the op-timum pH for Fenton’s reaction and production of 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 279

0

0.2

0.4

0.6

0.8

1

Time (min)

C/C

0

MG

Orange II

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200

TO

C/T

OC

0

(a)

(b)

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.

tro-Fenton process, a catalytic amount of 10-4 M Fe3+ was added to the solution.

4. Analysis Procedure

Samples were withdrawn from the reactor at

regular time intervals and the removal of color was evaluated by simultaneous spectrophotometeric de-termination using mean centering of ratio spectra [28]. This method has been inspired from successive ratio derivatives of ratio spectra in two steps. The mean centering method uses mean centering of ratio spectra instead of derivatives of them. By eliminating deriva-tive steps, signal-to-noise ratio is enhanced dramati-cally.

RESULTS AND DISCUSSION

1. Decoloration and Mineralization of a Mixture

Containing MG and Orange II A mixture of MG and Orange II at initial con-

centration of 10-5 M for each one was treated by elec-tro-Fenton process at -0.5 V. Electrochemically gen-erated hydroxyl radicals react with dye molecules leading to their oxidation. Figure 1a shows a rapid de-cay of each dye present in the synthetic dye mixture as a function of time during the electro-Fenton process.

Results show that degradation of MG is faster than Orange II.

The oxidizing power of the electro-Fenton sys-tem to mineralize dye solutions was evaluated from their TOC decay. Figure 1b shows selected TOC-time plot for the degradation of the mixture of MG and Or-ange II solution using 9.5 cm2 graphite felt cathode. A continuous TOC abatement was observed attaining 79% of mineralization after 180 min of electrolysis. TOC decay shows that this electro-Fenton system can degrade and mineralize organic pollutants.

2. Electro-Fenton Degradation of MG and Orange

II in Separate and Mixed Solution Figures 2a and 2b show the absorbance decrease

at 619 nm (MG) and 484 nm (Orange II) alone or in mixed solution. The removal efficiency of both MG and Orange II was altered in the mixed solution com-pared with the separate case. In the presence of Or-ange II, decoloration of MG was accelerated whereas decoloration of Orange II was reduced to some extent when MG was present. The depression of the absorb-ance decrease at 484 nm can be attributed to competi-tive trap of hydroxyl radicals by MG [29] and the ef-fect of high concentration of pollutant. However, the absorbance decrease at 619 nm was surprisingly ac-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.

k = 1×109 M-1 s-1 (8)

k = 1×109 M-1 s-1 (9)

k = 4.4×102 M-1 s-1 (10)

k = 4.4×104 M-1 s-1 (11)

k = 1×109 M-1 s-1 (12)

280 J. Environ. Eng. Manage., 19(5), 277-282 (2009)

102030405060708090

100

Rem

oval

Eff

icie

ncy

(%)

0102030405060708090

100

0 20 40 60 80 100

Time (min)

only MGmixed

(a)

(b)

only Orange IImixed

Fig. 2. Destruction of dyes by electro-Fenton

degradation of: (a) MG, alone or mixed, (b) Orange II, alone or mixed. [MG] = 10-5 M, [Orange II] = 10-5 M, [NaClO4] = 0.05 M, pH = 3.0, [Fe3+] = 10-4 M and E = -0.5 V.

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-droxyl radical production.

In order to show whether hydroquinone-like in-termediates generated in degradation of Orange II or not, electro-Fenton degradation intermediates of Or-ange II alone was determined after 60 min electrolysis by GC-MS analysis. Results were reported in previous work [27]. It is observed that one of the identified in-termediates is 1,2-dihydroxynaphthalene. It was re-ported that ortho-dihydroxy aromatic compounds such as catechol and 2,3-dihydroxybenzoic acid can reduce ferric ions to ferrous ions [32,33]. Probably generated 1,2-dihydroxynaphthalene from degradation of Or-ange II can reduce Fe3+ to Fe2+ and therefore, lead to increasing of absorbance decrease rate at 619 nm in electro-Fenton process.

Generally speaking, in electro-Fenton system Fe2+ is regenerated through the reduction of Fe3+ in cathode surface (Eq. 13) and then reduced to Fe2+ by H2O2 (Eq. 14). However, at the same time in the pres-ence of hydroquinone-like intermediates, regeneration of Fe2+ is prompted and causing a rapid degradation of contaminant.

Fe3+ + e− → Fe2+ (13)

0

1020

30

40

5060

70

8090

100

01020

30

40

5060

708090

100

0 20 40 60 80 100 120

Time (min)

Rem

oval

Eff

icie

ncy

(%)

(a)

(b)

Fig. 3. Effect of one dye on the other: [NaClO4] = 0.05

M, pH = 3.0, [Fe3+] = 10-4 M and E = -0.5 V. (a) 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 II] = 2 × 10- 5 M.

Fe3+ + H2O2 → Fe2+ + •2HO + H+ (14)

3. Influence of Initial Orange II Concentration on

MG Degradation With regard to these findings, that Orange II is

source of hydroquinone-like intermediates, investiga-tion of the effect of the Orange II initial concentration on MG degradation is examined. Figure 3a shows that removal efficiency of MG increased by increasing Or-ange II concentration up to 2 × 10-5 M. This effect can be attributed to more conversion of Fe3+ to Fe2+ by in-creasing of produced intermediates from degradation of Orange II. However, at higher Orange II concentra-tion, scavenging of hydroxyl radicals overcome in ac-celerating effect and therefore, removal efficiency of MG is reduced.

4. Influence of MG Concentration on Orange II

Degradation In previous sections, it was observed that ab-

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-

□ : No Orange II ∆ : 10-5 M ◊ : 2 × 10-5 M ○ : 3 × 10-5 M

◊ : No MG □ : 10-5 M ∆ : 2 × 10-5 M ○ : 3 × 10-5 M

Vatanpour et al.: MG and Orange II Mixture Treatment by EF 281

enging of •OH by high concentration of MG, which reacts quicker with OH, as shown in Fig. 1.

CONCLUSIONS

This paper has considered the electro-Fenton

treatment of mixture of two dyes using in situ hydro-gen peroxide produced by oxygen reduction on graph-ite-felt cathode. Experimental results showed that: ‧Removal efficiency of dyes is attributed to their

structure. At the same conditions, degradation of MG is faster than Orange II.

‧Aromatic compounds can catalyze the Fenton reac-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, which is the slow step (Eq. 9) in the mechanism of the simple Fenton reaction.

‧GC-MS analysis and degradation rate of MG in the presence of Orange II show that one of the gener-ated intermediates of Orange II degradation proba-bly can accelerate the regeneration of Fe2+ from re-duction of Fe3+ and therefore, lead to acceleration of MG degradation.

ACKNOWLEDGMENTS

The authors would like to express their gratitude

to the University of Tabriz, Iran for the financial sup-port and assistance and thank Mr. Mahmoud Zarei for running the GC-MS experiments and Mr. Jafarizad for TOC analysis.

REFERENCES

1. Chung, K.T. and S.E. Stevens, Degradation of azo

dyes by environmental microorganisms and helminths. Environ. Toxicol. Chem., 12(11), 2121-2132 (1993).

2. Stylidi, M., D.I. Kondarides and X.E. Verykios, Visible light-induced photocatalytic degradation of Acid Orange 7 in aqueous TiO2 suspensions. Appl. Catal. B-Environ. 47(3), 189-201 (2004).

3. Cisneros, R.L., A.G. Espinoza and M.I. Litter, Photodegradation of an azo dye of the textile industry. Chemosphere, 48(4), 393-399 (2002).

4. Chen, Y.X., K. Wang and L.P. Lou, Photodegradation of dye pollutants on silica gel supported TiO2 particles under visible light irradiation. J. Photoch. Photobio. A., 163(1-2), 281-287 (2004).

5. Senan, R.C. and T.E. Abraham, Bioremediation of textile azo dyes by aerobic bacterial consortium - Aerobic degradation of selected azo dyes by bacterial consortium. Biodegradation, 15(4), 275-

280 (2004). 6. Mohanty, S., N. Dafale and N.N. Rao, Microbial

decolorization of reactive Black-5 in a two-stage anaerobic-aerobic Reactor using acclimatized activated textile sludge. Biodegradation, 17(5), 403-413 (2006).

7. Meriç, S., H. Selcuk, M. Gallo and V. Belgiorno, Decolourisation and detoxifying of Remazol Red dye and its mixture using Fenton’s reagent. Desalination, 173(3), 239-248 (2005).

8. Gupta, A.K., A. Pal and C. Sahoo, Photocatalytic degradation of a mixture of Crystal Violet (Basic Violet 3) and Methyl Red dye in aqueous suspensions using Ag+ doped TiO2. Dyes Pigments, 69(3), 224-232 (2006).

9. Szpyrkowicz, L., Hydrodynamic effects on the performance of electro-coagulation/electro-flotation for the removal of dyes from textile wastewater. Ind. Eng. Chem. Res., 44(20), 7844-7853 (2005).

10. Panizza, M. and G. Cerisola, Electrochemical oxidation as a final treatment of synthetic tannery wastewater. Environ. Sci. Technol., 38(20), 5470-5475 (2004).

11. Chakraborty, S., J.K. Basu, S. De and S. DasGupta, Adsorption of reactive dyes from a textile effluent using sawdust as the adsorbent. Ind. Eng. Chem. Res., 45(13), 4732-4741 (2006).

12. Dafnopatidou, E.K., G.P. Gallios, E.G. Tsatsaroni and N.K. Lazaridis, Reactive dyestuffs removal from aqueous solutions by flotation, possibility of water reuse, and dyestuff degradation. Ind. Eng. Chem. Res., 46(7), 2125-2132 (2007).

13. Shaul, G.M., T.J. Holdsworth, C.R. Dempsey and K.A. Dostal, Fate of water-soluble azo dyes in the activated sludge process. Chemosphere, 22(1-2), 107-119 (1991).

14. Jüttner, K., U. Galla and H. Schmieder, Electrochemical approaches to environmental problems in the process industry. Electrochim. Acta, 45(15-16), 2575-2594 (2000).

15. Brillas, E., E. Mur, R. Sauleda, L. Sanchez, J. Peral, X. Domènech and J. Casado, Aniline mineralization by AOPs: Anodic oxidation, photocatalysis, electro-Fenton and photoelectro-Fenton processes. Appl. Catal. B-Environ., 16(1), 31-42 (1998).

16. Brillas, E., J.C. Calpe and J. Casado, Mineralization of 2,4-D by advanced oxidation electrochemical oxidation processes. Water Res., 34(8), 2253-2262 (2000).

17. Barb, W.G., J.H. Baxendale, P. George and K.R. Hargrave, Reactions of ferrous and ferric ions

282 J. Environ. Eng. Manage., 19(5), 277-282 (2009)

with hydrogen peroxide. 2. The ferric ion reaction. T. Faraday Soc., 47(6), 591-616 (1951).

18. Wang, A.M., J.H. Qu, J. Ru, H.J. Liu and J.T. Ge, Mineralization of an azo dye Acid Red 14 by electro- Fenton’s reagent using an activated carbon fiber cathode. Dyes Pigments, 65(3), 227-233 (2005).

19. Flox, C., S. Ammar, C. Arias, E. Brillas, A.V. Vargas-Zavala and R. Abdelhedi, Electro-Fenton and photoelectro-Fenton degradation of indigo carmine in acidic aqueous medium. Appl. Catal. B-Environ., 67(1-2), 93-104 (2006).

20. Hammami, S., N. Oturan, N. Bellakhal, M. Dachraoui and M.A. Oturan, Oxidative degradation of direct orange 61 by electro-Fenton process using a carbon felt electrode: Application of the experimental design methodology. J. Electroanal. Chem., 610(1), 75-84 (2007).

21. Oturan, M.A., E. Guivarch, N. Oturan, I. Sirés, Oxidation pathways of malachite green by Fe3+-catalyzed electro-Fenton process. Appl. Catal. B-Environ., 82(3-4), 244-254 (2008).

22. Xie, Y.B. and X.Z. Li, Interactive oxidation of photoelectrocatalysis and electro-Fenton for azo dye degradation using TiO2-Ti mesh and reticulated vitreous carbon electrodes. Mater. Chem. Phys., 95(1), 39-50 (2006).

23. Guivarch, E., S. Trevin, C. Lahitte and M.A. Oturan, Degradation of azo dyes in water by Electro-Fenton process. Environ. Chem. Lett., 1(1), 38-44 (2003).

24. Lahkimi, A., M.A. Oturan, N. Oturan and M. Chaouch, Removal of textile dyes from water by the electro-Fenton process. Environ. Chem. Lett., 5(1), 35-39 (2007).

25. Sirés, I., E. Guivarch, N. Oturan, M.A. Oturan, Efficient removal of triphenylmethane dyes from aqueous medium by in situ electrogenerated Fenton’s reagent at carbon-felt cathode, Chemosphere, 72(4), 592-600 (2008).

26. Panizza, M. and G. Cerisola, Removal of organic

pollutants from industrial wastewater by electrogenerated Fenton’s reagent. Water Res., 35(16), 3987-3992 (2001).

27. Daneshvar, N., S. Aber, V. Vatanpour and M.H. Rasoulifard, Electro-Fenton treatment of dye solution containing Orange II: Influence of operational parameters. J. Electroanal. Chem., 615(2), 165-174 (2008).

28. Bahram, M., T. Madrakian, E. Bozorgzadeh and A. Afkhami, Micelle-mediated extraction for simultaneous spectrophotometeric determination of aluminum and beryllium using mean centering of ratio spectra. Talanta, 72(2), 408-414 (2007).

29. Chen, F., W. Ma, J. He and J. Zhao, Fenton degradation of Malachite Green catalyzed by aromatic additives. J. Phys. Chem. A, 106(41), 9485-9490 (2002).

30. Chen, F., J. He, J. Zhao and J.C. Yu, Photo-Fenton degradation of Malachite Green catalyzed by aromatic compounds under visible light irradiation. New J. Chem., 26(3), 336-341 (2002).

31. Chen, R. and J.J. Pignatello, Role of quinone intermediates as electron shuttles in Fenton and photoassisted Fenton oxidations of aromatic compounds. Environ. Sci. Technol., 31(8), 2399-2406 (1997).

32. Du, Y., M. Zhou and L. Lei, Role of the intermediates in the degradation of phenolic compounds by Fenton-like process. J. Hazard. Mater., 136(3), 859-865 (2006).

33. Xu, J. and R.B. Jordan, Kinetics and mechanism of the oxidation of 2,3-dihydroxybenzoic acid by iron(III). Inorg. Chem., 27(25), 4563-4566 (1988).

Discussions of this paper may appear in the discus-sion section of a future issue. All discussions should be submitted to the Editor-in-Chief within six months of publication.

Manuscript Received: March 28, 2008 Revision Received: June 18, 2008

and Accepted: June 18, 2008