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Ultrasonics Sonochemistry 15 (2008) 717–723

Decolorizing of azo dye Reactive red 24 aqueous solutionusing exfoliated graphite and H2O2 under ultrasound irradiation

Mei Li a,b, Ji-Tai Li a,*, Han-Wen Sun a

a College of Chemistry and Environmental Science, Hebei University, Key Laboratory of Analytical Science and Technology of Hebei Province,

Baoding 071002, PR Chinab College of Chemistry and Material Science, Hebei Normal University, Shi Jiazhuang 050091, PR China

Received 8 February 2007; received in revised form 16 June 2007; accepted 2 October 2007Available online 9 October 2007

Abstract

At its natural pH (6.95), the decolorization of Reactive red 24 in ultrasound, ultrasound/H2O2, exfoliated graphite, ultrasound/exfoliated graphite, exfoliated graphite/H2O2 and ultrasound/exfoliated graphite/H2O2 systems were compared. An enhancement wasobserved for the decolorization in ultrasound/exfoliated graphite/H2O2 system. The effect of solution pH, H2O2 and exfoliated graphitedosages, and temperature on the decolorization of Reactive red 24 was investigated. The sonochemical treatment in combination withexfoliated graphite/H2O2 showed a synergistic effect for the decolorization of Reactive red 24. The results indicated that under properconditions, there was a possibility to remove Reactive red 24 very efficient from aqueous solution.

The decolorization of other azo dyes (Reactive red 2, Methyl orange, Acid red 1, Acid red 73, Acid red 249, Acid orange 7, Acid blue113, Acid brown 75, Acid green 20, Acid yellow 42, Acid mordant brown 33, Acid mordant yellow 10 and Direct green 1) was alsoinvestigated, at their natural pH.� 2007 Elsevier B.V. All rights reserved.

PACS: 43.35.Vz; 81.65.Mq

Keywords: Ultrasound; Exfoliated graphite; H2O2; Decolorization; Azo dye; Wastewater

1. Introduction

Azo dyes, which are difficult to be degraded biologically,are commonly found in the dye, textile and paper manufac-turing wastewater. If these are not removed from the trea-ted wastewater, a number of synthetic azo dyes cause aformidable contamination of water, as color tends to per-sist even after the conventional treatment given to thewastewater. Color removal from industrial effluents hasbeen a major concern in wastewater treatment [1]. Overthe last few years, advanced oxidation processes (AOP)such as sonolysis, photocatalysis and sonocatalysis havebeen proposed to treat dye wastewater and showed greatlyobvious effects. The application of acoustic cavitation bub-

1350-4177/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.ultsonch.2007.10.001

* Corresponding author. Tel.: +86 312 5079361; fax: +86 312 5079628.E-mail address: lijitai@mail.hbu.edu.cn (J.-T. Li).

bles in wastewater treatment is one of the most importantsubjects. Therefore, it has attracted great attention fromthe academic researchers. Rehorek et al. studied the degra-dation of azo dye using power ultrasound [2]. Wang et al.investigated the sonocatalytic degradation of methylorange in the presence of TiO2 catalysts [3]. Okitsu et al.observed that the ultrasonic technique was effective in thedecolorization and decomposition of azo dyes [4].

The effect of ultrasonic irradiation on removal organiccontaminants from aqueous solutions can be explained phys-ically or chemically. Physically, the ultrasonic waves canclean surface of solid particles, reduce the particle size andincrease mass transfer [5]. Chemically, when ultrasonic wavesare irradiated into an aqueous solution, cavitation occurs.The violent collapse of the cavitation bubbles results in ahot spot, which has an extremely high temperature and pres-sure [6,7]. The hot spot enables the generation of OH radicals.

Table 1The decolorization efficiency of dyes by ultrasound/exfoliated graphite-H2O2

Dyes kmax (nm) Time (min) Decolorization efficiencies (%)

Reactive red 2

NH

SO3NaNaO3S

HO

NN

NN

Cl

Cl

N

(615.33 g/mol)

539 180 90.00

Methyl orange

NaO3S NCH3

CH3

NN

(327.37 g/mol)

467 180 70.23

Acid red 1

HO

N

SO3NaNaO3S

NHCOCH3

N

(509.42 g/mol)

536 180 44.84

Acid red 73

HO

N

NaO3S

N

SO3Na

NN

(556.48 g/mol)

534 150 65.68

Acid red 249

OH NH

SO3Na

N

Cl

O O2S CH3

NaO3S

N

(748.11 g/mol)

550 180 85.69

Acid orange 7

OH

NaO3S N N

(350.33 g/mol)

477 150 79.52

Acid blue 113

NaO3S

NH

SO3Na

NN NN

(681.65 g/mol)

562 120 63.71

Acid brown 75

N OH

OH

OHNaO3S

NaO3S

N

N N

N N

NO2

NO2

NO2

OH

(843.58 g/mol)

430 180 42.20

Acid green 20

NH2

H2N

SO3Na SO3Na

N NN

OH

N

(586.51 g/mol)

604 120 77.06

718 M. Li et al. / Ultrasonics Sonochemistry 15 (2008) 717–723

Table 1 (continued)

Dyes kmax (nm) Time (min) Decolorization efficiencies (%)

Acid yellow 42

CH3N

SO3Na

NaO3SOHN

N

NCH3

OHN

N

NN

(758.69 g/mol)

418 180 49.04

Acid mordant brown 33

OH

SO3Na

N

NH2

NH2

O2N

N

(375.29 g/mol)

442 120 82.19

Acid mordant yellow 10

NaO3S N N

COONa

OH

(366.28 g/mol)

354 180 35.11

Direct green 1

NH2OH

HO NNN

SO3NaNaO3S

NNN

(767.70 g/mol)

604 180 72.33

Reactive red 24

NH

SO3NaNaO3S

HO

NN

NN

Cl

NH

ClNaO3S

N

(808.48 g/mol)

535 120 89.09

0 20 40 60 80 100 120

0

20

40

60

80

100

Rem

oval

effi

cien

cy (

%)

Time (min)

Fig. 1. The decolorization efficiency of Reactive red 24 in differentreaction systems: (•) exfoliated graphite + H2O2 + ultrasound, (�) exfoli-ated graphite + ultrasound, (j) exfoliated graphite + H2O2 + stirredwithout ultrasound, (h) exfoliated graphite + stirred without ultrasound,(q) H2O2 + ultrasound and (.) only ultrasound.

M. Li et al. / Ultrasonics Sonochemistry 15 (2008) 717–723 719

During the sonochemical treatments of many organiccompounds in water, the oxidative decomposition generallyproceeds via the reaction with OH radical [4]. In addition,organic compounds in the collapsing hot bubbles and at thebubble/water interface can be thermally decomposed [8].

Exfoliated graphite is an excellent inorganic materialextensively used in chemical, mechanical, and atomic energyindustries because of their excellent properties, e.g. com-pressibility, resilience, thermal stability, corrosion resis-tance, etc. Recently, exfoliated graphite attracted attentionof scientists and engineers as an absorbent with a high sorp-tion capacity especially for macromolecular organic materi-als, such as heavy oils and biomedical molecules, because ofits large pores structure [9–11]. In this paper, the decoloriza-tion of azo dye aqueous solution using sonication in the pres-ence of exfoliated graphite-H2O2 was investigated.

2. Experimental

2.1. Materials

Fourteen different azo dyes have been investigated:Reactive red 24 (Tianjin, China), Reactive red 2 (Tianjin,

0 20 40 60 80 100 120-10

0

10

20

30

40

50

60

70

80

90

100

Rem

oval

effi

cien

cy (%

)

Time (min)

Fig. 3. Effect of hydrogen peroxide on Reactive red 24 decolorization: (q)6.80 g/l, (�) 5.44 g/l, (•) 4.08 g/l and (j) 2.72 g/l.

0 20 40 60 80 100 120

0

20

40

60

80

100R

emov

al e

ffici

ency

(%)

Time (min)

Fig. 2. Effect of solution pH on Reactive red 24 decolorization: (h)pH = 1, (•) pH = 2, (�) pH = 4, (.) pH = 6.95 and (j) pH = 10.

720 M. Li et al. / Ultrasonics Sonochemistry 15 (2008) 717–723

China), Methyl orange (Tianjin, China), Acid red 1(Hebei,China), Acid red 73(Hebei, China), Acid red 249(Hebei,China), Acid orange 7(Hebei, China), Acid blue 113(Hebei,China), Acid brown 75(Hebei, China), Acid green20(Hebei, China), Acid yellow 42(Hebei, China), Acidmordant brown 33(Hebei, China), Acid mordant yellow10(Hebei, China) and Direct green 1(Hebei, China). Thestructures of the above azo dyes are listed in Table 1. Syn-thetic azo dye wastewater is prepared in laboratory by dis-solving a commercial azo dye in distilled water.

2.2. Preparation of the exfoliated graphite

Natural flake graphite was used as the host material.A mixture solution of acetic anhydride (10 g) and per-chloric acid (14 g) was maintained at 20 �C in a waterbath. Graphite (10 g) and potassium permanganate (5 g)were added to this solution, and then the mixture wasstirred for 60 min at 30 �C. During this time, the graphiteintercalation compound was formed. The graphite inter-calation compound was washed to neutrality with water,dried at 60 �C, and then abrupt heated to 1000 �C toform exfoliated graphite with exfoliation volume of300 ml/g.

2.3. Apparatus and analysis

UV-2501PC (Shimadzu, Japan) UV–vis spectrometerwas used to determine the dye concentrations. The percent-age of azo dye decolorization efficiencies were calculated byusing the following equation:

Percentage decolorization efficiency ¼ ð1� Ct=C0Þ � 100

where, C0 is the initial dye concentration, Ct is the dye con-centration at measurable time t.

The exfoliated graphite solid particles before and afterremoving Reactive red 24 from aqueous solution usingultrasound/exfoliated graphite-H2O2 were photographedby scanning electron microscope (SEM) (S-570, Hitachi,Japan) to demonstrate the different pore structure charac-teristics. The IR spectrum of the exfoliated graphite parti-cles (after treating Reactive red 24 from aqueous solutionusing ultrasound/exfoliated graphite-H2O2) was recordedon FTIR spectrometer (M-1370, Perkin-Elmer Company,USA).

Sonication was performed on a Beijing DTL-500 ultra-sonic cleaner (with a frequency of 28 kHZ and a nominalpower 500 W, Beijing Detailong Co., Ltd.). The reactionflask was located in the ultrasonic bath (observation ofthe surface of the reaction solution during vertical adjust-ment of vessel depth will show the optimum position bythe point at which maximum surface disturbance occurs),where the surface of reactants is slightly lower than thelevel of the water in the cleaning bath. The reaction temper-ature was controlled by addition or removal of water fromultrasonic bath.

2.4. General procedure

In a 150 mL beaker, aqueous solution of dye (50 mL),hydrogen peroxide, and exfoliated graphite were added.The mixture was irradiated in the ultrasonic cleaning bathat a set temperature for a period.

Experimental conditions involved the exfoliated graph-ite (0.8 g/l) with an initial dye concentration of7.414 · 10�5 mol/l, hydrogen peroxide concentration of4.08 g/l, natural pH of dye, ultrasonic frequency of28 kHz (nominal power of 500 W), exfoliation volumeof exfoliated graphite of 300 ml/g, temperature of 50 �C.A reaction time of 120 min was used throughout thecourse of the investigations except for some specialexperiments.

M. Li et al. / Ultrasonics Sonochemistry 15 (2008) 717–723 721

3. Results and discussion

3.1. Effect of ultrasound coupled with exfoliated graphite-

H2O2

The decolorization of azo dye Reactive red 24(kmax = 535 nm) in ultrasound, ultrasound/H2O2, exfoli-

Fig. 4. IR spectra of exfoliated graphite and dye. (1) Exfoliated graphite afteReactive red 24 and (3) Exfoliated graphite.

ated graphite, ultrasound/exfoliated graphite, exfoliatedgraphite/H2O2 and ultrasound/exfoliated graphite/H2O2

systems at natural pH for 120 min were compared inFig. 1. According to the Fig. 1, no decolorization occurredin ultrasound individually or ultrasound/H2O2 systems. Inultrasound/exfoliated graphite, exfoliated graphite (stirredwithout ultrasound), exfoliated graphite-H2O2 (stirred

r treating Reactive red 24 using ultrasound/exfoliated graphite-H2O2, (2)

0 20 40 60 80 100 120

0

10

20

30

40

50

60

70

80

90

Rem

oval

effi

cien

cy (

%)

Time (min)

Fig. 5. Effect of exfoliated graphite dosage on Reactive red 24 decolor-ization: (•) 1.0 g/l, (h) 0.8 g/l, (.) 0.6 g/l, (�) 0.4 g/l and (j) 0.2 g/l.

Fig. 6. SEM micrographs of exfoliated graphite particles: (1) Exfoliatedgraphite before ultrasound, (2) Exfoliated graphite after treating Reactivered 24 aqueous solution by ultrasound/exfoliated graphite-H2O2.

722 M. Li et al. / Ultrasonics Sonochemistry 15 (2008) 717–723

without ultrasound) and ultrasound/exfoliated graphite-H2O2 systems, the removal efficiency was 41.54%,46.36%, 45.66% and 89.09%, respectively. It is clear thatthe sonochemical treatment in combination with exfoliatedgraphite-H2O2 showed a synergistic effect for azo dyedecolorization.

So, we performed a series of experiments using ultra-sound coupled with exfoliated graphite-H2O2.

3.2. Effect of pH on decolorization of Reactive red 24

The ultrasonic decolorization of Reactive red 24 in thepresence of exfoliated graphite-H2O2 was investigated atpH = 1–10, the results are shown in Fig. 2. When the reac-tion time was 120 min, the color removal efficiencies are98.44% (pH = 1), 96.88% (pH = 2), 92.60% (pH = 4),89.09% (Reactive red 24 natural pH = 6.95) and 33.90%(pH = 10), respectively. From the results we can find thatReactive red 24 is readily decomposed by the ultrasound/exfoliated graphite-H2O2 process under strong acidic con-ditions. The decolorization efficiency of Reactive red 24at low pH is better than that at higher pH. It is possiblethat the change in the solution pH results in the changeof hydrophobic property of the dye, which affects the ultra-sonic decolorization.

3.3. Effect of hydrogen peroxide dosage on decolorization of

Reactive red 24

The effect of the initial concentration of H2O2 on sono-chemical decolorization of Reactive red 24 was investi-gated.The results are shown in Fig. 3. With increasing theH2O2 initial concentration, the oxidative ability of the sys-tem was promoted. The decolorization efficiencies of dyesolutions were increased. When hydrogen peroxide dosageexceeds 4.08 g/l, the decolorization efficiency increased veryslowly with further increase of the hydrogen peroxidedosage.

From the IR spectra (Fig. 4) of exfoliated graphite aftertreating Reactive red 24 using ultrasound/exfoliated graph-ite-H2O2, we can see that the carbonyl group (quinone) ispresent in product molecule, because the carbon–oxygendouble bond stretching frequency gives a strong peak at1698 cm�1, it is characteristic IR absorption of carbonylgroup in quinone. This indicates that the hydroxyl groupin Reactive red 24 has been oxidized to quinonic carbonylgroup in ultrasound/exfoliated graphite-H2O2 process, butthe procedure could not mineralize Reactive red 24 to CO2

and H2O.

3.4. Effect of exfoliated graphite dosage on decolorization of

Reactive red 24

The effect of exfoliated graphite dosage on the sono-chemical decolorization of Reactive red 24 was investi-gated. The results are shown in Fig. 5. From this we cansee that keeping H2O2 concentration constant, the decolor-

ization efficiencies of dye solutions increased with the exfo-liated graphite dosage, but as the exfoliated graphitedosage exceeds 0.6 g/l, the color removal ratio increasedvery slowly.

Sonication can cause exfoliated graphite particle rup-ture, with a consequent decrease in particle size andincrease in surface area available for reaction. This wouldbe sufficient to explain any enhanced removal efficienciesof Reactive red 24 aqueous solutions [12]. We examinedby scanning electro microscope surfaces of exfoliatedgraphite particles which were obtained after treating Reac-tive red 24 aqueous solution using ultrasound/exfoliatedgraphite-H2O2 (Fig. 6). It reveals that the surfaces exhibitspatterns of deep ellipsoidal pores to increase the effectivearea available for reaction, but the surfaces of exfoliatedgraphite without ultrasound irradiation show patterns ofcrevice-like pores.

0 20 40 60 80 100 120

0

10

20

30

40

50

60

70

80

90

Rem

oval

effi

cien

cy (%

)

Time (min)

Fig. 7. Effect of temperature on Reactive red 24 decolorization: (�) 50 �C,(•) 37 �C and (j) 25 �C.

M. Li et al. / Ultrasonics Sonochemistry 15 (2008) 717–723 723

3.5. Effect of temperature on decolorization of Reactive red

24

The ultrasonic irradiation of Reactive red 24 aqueoussolution in the presence of exfoliated graphite-H2O2 wascarried out under different temperatures. The decoloriza-tion efficiency curves under different temperatures are plot-ted in Fig. 7.

It is revealed that Reactive red 24 removal efficiencyincreased at 50 �C compared with that at 25 �C and37 �C. The decolorization efficiency increases by about38.66% when the temperature rises from 25 to 50 �C. Withthe temperature heightening, the mass transfer of differentspecies increased, which could improve the decolorizationefficiency.

3.6. Decolorization of other azo dyes in ultrasound/exfoliated

graphite-H2O2 system

Using ultrasound/exfoliated graphite-H2O2 system, atnatural pH of dye, we did a series of experiments for decol-orization of some azo dyes. The data in Table 1 indicated

that the combination method is an efficient for removalof some azo dyes from aqueous solution. According to pre-vious reports [13], decolorization of the dye was related tothe size of the molecule and the type or position of substit-uents about azo bonds. In experiment, we try to observethe relationship between dyes structure and ultrasonicdecolorization, but the regular result was not found.

4. Conclusion

The couple ultrasound/exfoliated graphite-H2O2 processappears to have a positive synergistic effect on the decolor-ization of some azo dyes. The present method was an effi-cient and convenient procedure for treating some dyewastewaters.

Acknowledgement

We thank Natural Science Foundation of Hebei Prov-ince (B2006000969), China, for financial support.

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