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Ultrasonics Sonochemistry 15 (2008) 37–42

Sonochemical decolorization of acid black 210 in the presenceof exfoliated graphite

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,

Hezuo Road, No. 88, Baoding 071002, PR Chinab College of Chemistry and Material Science, Hebei Normal University, ShiJiazhuang 050091, PR China

Received 27 August 2006; received in revised form 17 January 2007; accepted 19 January 2007Available online 3 February 2007

Abstract

The decolorization of acid black 210 by ultrasonic irradiation in the presence of exfoliated graphite was investigated. Low pH valueand large exfoliation volume of exfoliated graphite favored the ultrasonic decolorization of acid black 210. The combination method ofultrasonic waves and exfoliated graphite achieved better results than either exfoliated graphite or ultrasound alone. Improved decolor-ization efficiency has been observed in the present system compared to the process using a combination of ultrasound and activated car-bon. In the combination method of ultrasonic waves and exfoliated graphite, approximately 99.5% of acid black 210 was removed using0.8 g/l exfoliated graphite at a pH of 1 within 120 min at 51 �C.� 2007 Elsevier B.V. All rights reserved.

Keywords: Ultrasound; Exfoliated graphite; Decolorization; Acid black 210

1. Introduction

Dyes and pigments are one of the problematic groups ofpollutants. They are discharged from various industries suchas dyestuff manufacturing, dyeing, printing, and textile fin-ishing. These waste materials usually require especial treat-ment before being released to the environmental. In recentyears, the combined use of ultrasonic technique and othertechniques for the treatment organic wastewater has beenwidely investigated [1–11]. Ultrasonic irradiation differsfrom traditional energy sources (such as heat, light, or ioniz-ing radiation) in duration, pressure, and energy per mole-cule. Because of the immense temperatures and pressures,extraordinary heating and cooling rates generated by cavita-tion bubble collapse, ultrasound provides an unusual mech-anism for generating high-energy chemistry [12]. Organicpollutants can be degraded via pyrolysis in the cavitationbubble and via the reaction with hydroxyl radicals formed

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

doi:10.1016/j.ultsonch.2007.01.004

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

from water pyrolysis in the cavitation bubble. Althoughultrasound can be used alone for degradation of the organicpollutions, the degradation efficiency is still low. In com-bined methods, ultrasound irradiation coupled with othertechniques can increase the degradation efficiency andreduce the time required to remove the organic pollutants.

Exfoliated graphite is an excellent inorganic carbonmaterial with a porous structure. It is reported that exfoli-ated graphite is able to adsorb heavy oils and biomedicalmolecules because of its large pores structure [13–15].Recently, we have reported that decolorizing of direct scar-let 4BS was carried out to give a good results using exfoli-ated graphite under ultrasound irradiation [16]. Here, wewould like to report our observations on the decolorizationof acid black 210 in water by a combination of ultrasoundirradiation and exfoliated graphite. We compared theresults with that of treatment by the combined use of ultra-sound and activated carbon. It shows that the ultrasonicdecolorization effects of acid black 210 are more obviousin the presence of exfoliated graphite than in the presenceof activated carbon.

38 M. Li et al. / Ultrasonics Sonochemistry 15 (2008) 37–42

2. Experimental

2.1. Materials

Acid black 210 was employed as a model compound andthe chemical structure of the dye is shown in Scheme 1. Theacid black 210 wastewater was prepared in laboratory bydissolving a commercial acid black 210 in distilled water.The exfoliated graphite with exfoliation volume of300 ml/g (bulk density: 3.33 · 10�3 g/cm3) was preparedaccording to the method in literature [16]. Activated car-bon was analytical grade reagent purchased from Tianjin,China, and its density and size are about 0.41 g/cm3 and14–30 mesh, respectively.

2

2.2. Apparatus and analysis

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 volumeof water inside the ultrasonic bath (with a stainless steeltank of 20.6 l) was 17 l. The total acoustic power injectedinto the sample solution was found to be 1.51 W by calo-rimetry [17]. In order to compare the decolorization effi-ciency of acid black 210 in the presence of exfoliatedgraphite with and without ultrasonic treatment, a thermo-stat shaker was employed to agitate the acid black 210solution containing exfoliated graphite.

The quantification of acid black 210 was performed by aUV–vis spectrometer (U-3010, Hitachi, Japan). The per-centage of acid black 210 decolorization efficiencies werecalculated by using the following equation:

decoloring % ¼ ð1� Ct=C0Þ � 100

where, C0 is the initial dye concentration, Ct is the dye con-centration at measurable time t. The value of Ct calculatedfrom absorption peak of 465 nm.

200 300 400 500 600 700 8000

1

f

ed

c

ba

Ab

sorb

ance

Wavelength (nm)

2.3. General procedure

A 150 ml beaker was charged with acid black 210 aque-ous solutions (50 ml) and exfoliated graphite with exfolia-tion volume of 300 ml/g. The initial acid black 210concentration was 60 mg/l. The reaction beaker waslocated in the ultrasonic bath (observation of the surfaceof the reaction solution during vertical adjustment of vesseldepth will show the optimum position by the point atwhich maximum surface disturbance occurs), where thesurface of reactants is slightly lower than the level of thewater in the cleaning bath. The acid black 210 aqueous

H2N N

NH2

N SO2NH

NaO3S

N N NO2

OHH2N

SO3Na

N N

Scheme 1. Structure of acid black 210.

solution containing exfoliated graphite was irradiated for120 min, the temperature in the water bath of theultrasonic cleaner during sonication rose from 22 to51 �C. A digital beckmann thermometer (SWC-II, Nanjing,China) was used to monitor the temperature of the waterbath. Agitated experiments without ultrasound wereconducted simultaneously at 22–51 �C in a thermostat sha-ker with a digital thermometer and a heater (SHA-B,Jiangsu, China).

3. Results and discussion

3.1. Effect of ultrasound coupled with exfoliated graphite

The decolorization of acid black 210 in ultrasound,exfoliated graphite (agitated without ultrasound), andultrasound coupled with exfoliated graphite systems wereinvestigated. Fig. 1 shows the changes in the UV–visabsorption spectra of the acid black 210 solutions treatedusing the above different systems, and the time-dependentvariations of decolorization efficiencies are plotted inFig. 2. As shown in Figs. 1 (curves a, d, and f) and 2 (fullcircles, and full squares), no decoloration of acid black 210can be done only by ultrasonic irradiation, while approxi-mately 57%, 98.3% of acid black 210 was decolorized for120 min in exfoliated graphite (agitated without ultra-sound) and ultrasound coupled with exfoliated graphitesystems. The results confirm that the combination method(ultrasound/exfoliated graphite) is more effective thanusing sonolysis or exfoliated graphite treatmentindividually. It is possible that ultrasound increased the

Fig. 1. UV–visible spectra of acid black 210 solutions: (a) onlyultrasound, (b) original, (c) only activated carbon (agitated withoutultrasound), (d) only exfoliated graphite (agitated without ultrasound), (e)ultrasound + activated carbon, (f) ultrasound + exfoliated graphite. Solu-tion pH: 1, contact time: 120 min, temperature: 22–51 �C, exfoliatedgraphite dosage: 0.8 g/l, exfoliation volume: 300 ml/g, activated carbondosage: 0.8 g/l.

0 20 40 60 80 100 120

0

20

40

60

80

100

deco

lorin

g %

t/min

Fig. 2. The decolorization of acid black 210 in different systems: (j)ultrasound + exfoliated graphite, (d) only exfoliated graphite (agitatedwithout ultrasound), (s) ultrasound + activated carbon, (h) only acti-vated carbon (agitated without ultrasound). Solution pH: 1, temperature:22–51 �C, exfoliated graphite dosage: 0.8 g/l, exfoliation volume: 300 ml/g,activated carbon dosage: 0.8 g/l.

M. Li et al. / Ultrasonics Sonochemistry 15 (2008) 37–42 39

surface area of the exfoliated graphite, and also improvedthe mass transfer from the solution to the exfoliated graph-ite surface by cavitation process, which in turn increasedadsorption the dye on exfoliated graphite.

Based on Fig. 1 (curve f), the two visible absorptionpeaks, which appear at 465 and 606 nm, have disappearedin the combined method after 120 min sonication. Theresult indicates that the chromophores of acid black 210are removed. In addition, the ultraviolet light absorptionpeak of 316 nm, which indicates the quantity of unsatu-rated bonds of organic compounds, is also removed byultrasound coupled with exfoliated graphite in parallel withthe decolorization of acid black 210.

Fig. 3. SEM micrographs of exfoliated graphite and activated

3.2. Comparison of sonochemical decolorization using

exfoliated graphite and activated carbon

The decolorization of acid black 210 using commercialactivated carbon (agitated without ultrasound) and ultra-sound coupled with activated carbon were also investigated.Based on Figs. 1 (curves c and e) and 2 (open squares andopen circles), the acid black 210 decolorization efficiencyincreased in the coupled ultrasound/ activated carbon pro-cess compared with that in activated carbon (agitated with-out ultrasound). Figs. 1 (curves e and f) and 2 (open circlesand full squares) also show a comparison between the cou-pled ultrasound/activated carbon process and the coupledultrasound/exfoliated graphite process under the same con-ditions. The acid black 210 was almost completely decolor-ized at 120 min in ultrasound/exfoliated graphite process,but the decolorization efficiency was only 58.4% at the samemoment in ultrasound/activated carbon process. Thisshows that the decolorization efficiency is obviously higherin ultrasound/exfoliated graphite process than in ultra-sound/activated carbon process. This is relevant with thepore structure of exfoliated graphite and activated carbon.Representative SEM (scanning electron microscope) imagesrevealing the characteristic pores in exfoliated graphite andactivated carbon are shown in Fig. 3. From Fig. 3, we cansee that exfoliated graphite exhibits patterns of large cre-vice-like pores, but activated carbon shows patterns ofsmall pores on the surface of particles. Exfoliated graphitewith a majority of macro pores makes it adsorb large mol-ecules more readily, such as acid black 210 molecules. But,most commercial activated carbons have high adsorptioncapacity for low molecular weigh compounds because ofits mainly micro porous structure [18].

3.3. Effect of pH on decolorization of acid black 210

The effect of the initial pH values on the ultrasonic decol-orization of acid black 210 in the presence of exfoliated

carbon: (a) exfoliated graphite, and (b) activated carbon.

0 20 40 60 80 100 1200

20

40

60

80

100

deco

lorin

g %

t/min

Fig. 4. Effect of pH on ultrasonic decolorization of acid black 210 in thepresence of exfoliated graphite: (d) pH = 1, (j) pH = 2, (m) pH = 3, (s)pH = 7, (w) pH = 11. Exfoliated graphite dosage: 0.8 g/l, temperature:22–51 �C, exfoliation volume: 300 ml/g.

0 20 40 60 80 100 120

0

20

40

60

80

100

deco

lorin

g %

t/min

Fig. 5. Effect of exfoliated graphite dosage on ultrasonic decolorization ofacid black 210: (d) 1.0 g/l, (n) 0.8 g/l, (j) 0.6 g/l, (s) 0.4 g/l, (w) 0.2 g/l.Solution pH: 1, temperature: 22–51 �C, exfoliation volume: 300 ml/g.

10

20

30

40

50

60

70

80

90

100

deco

lorin

g %

40 M. Li et al. / Ultrasonics Sonochemistry 15 (2008) 37–42

graphite was investigated in the pH range 1.0 and 11.0. Asshown in Fig. 4, the ultrasonic decolorization of acid black210 in the presence of exfoliated graphite is influenced bythe solution initial pH value. The lower the solution pHbecomes, the higher decolorization efficiency of acid black210 is. It indicates that low pH value favors the decoloriza-tion of the dye through acoustic cavitation. In the previousreports [7,9,19], a similar trend was observed for the effectof pH on decomposition of organic compounds in ultra-sound coupled with another technique. Perhaps, thechange in the solution pH value results in the change ofhydrophobic characteristic of the dye, which affects theultrasonic decolorization efficiency.

0 20 40 60 80 100 1200

t/min

Fig. 6. Effect of exfoliation volume on decolorization of acid black 210:(full symbols) ultrasound + exfoliated graphite, (open symbols) onlyexfoliated graphite (agitated without ultrasound). (squares) 300 ml/g,(circles) 200 ml/g, (triangles) 100 ml/g. Solution pH: 1, temperature: 22–51 �C, exfoliated graphite dosage: 0.8 g/l.

3.4. Effect of exfoliated graphite dosage on decolorization of

acid black 210

The effect of exfoliated graphite dosage on the ultrasonicdecolorization of acid black 210 was investigated and theresults are shown in Fig. 5. As shown in Fig. 5, the ultra-sonic decolorization efficiencies increased with the exfoli-ated graphite dosage, but the ultrasonic decolorizationefficiency did not increase when the dosage exceeds 0.8 g/l.Therefore, we can determine the proper exfoliated graphitedosage to be 0.8 g/l.

3.5. Effect of exfoliation volume on decolorization of acid

black 210

The effect of the exfoliation volume of the exfoliatedgraphite on the decolorization of acid black 210 was studiedusing three exfoliated graphite samples with the differentexfoliation volumes: 100 ml/g, 200 ml/g and 300 ml/g. Asshown in Fig. 6, the exfoliation volume of the graphite

affects strongly the decolorization efficiency, the decoloriza-tion increases with the exfoliation volume increasing.Because the pore structure and adsorption properties ofexfoliated graphite depend strongly on the exfoliation vol-ume [13,14,20], large exfoliation volume of exfoliated graph-ite favors the ultrasonic decolorization of acid black 210.

3.6. Effect of the addition of H2O2 in ultrasound/exfoliated

graphite system on the decolorization of acid black 210

To enhance the decolorization efficiency, the effect of theaddition of H2O2 in ultrasound/exfoliated graphite system

0 20 40 60 80 100 1200

20

40

60

80

100

deco

lorin

g %

time/min

Fig. 7. Effect of the addition of H2O2 in ultrasound/exfoliated graphitesystem on the decolorization of acid black 210: (j) ultrasound + exfoli-ated graphite + H2O2 pH = 1, (s) ultrasound + exfoliated graphitepH = 1, (m) ultrasound + exfoliated graphite + H2O2 pH = 7, (n) ultra-sound + exfoliated graphite pH = 7. Exfoliated graphite dosage: 0.8 g/l,temperature: 22–51 �C, exfoliation volume: 300 ml/g, concentration ofH2O2: 5.44 g/l.

0 20 40 60 80 100 1200

20

40

60

80

100

deco

lorin

g %

t/min

Fig. 8. Effect of temperature on the decolorization of acid black 210: (fullsymbols) 51 �C, (open symbols) 30 �C, (circles) ultrasound + exfoliatedgraphite, (pentagrams) ultrasound + activated carbon, (squares) onlyexfoliated graphite (agitated without ultrasound), (triangles) only acti-vated carbon (agitated without ultrasound). Solution pH: 1, exfoliatedgraphite dosage: 0.8 g/l, exfoliation volume: 300 ml/g, activated carbondosage: 0.8 g/l.

M. Li et al. / Ultrasonics Sonochemistry 15 (2008) 37–42 41

on the ultrasonic decolorization of acid black 210 wasinvestigated at two different pH values: 1.0 and 7.0. Theresults are given in Fig. 7. Because of the acid black 210solution was decolorized almost completely only by ultra-sound/exfoliated graphite system at a pH of 1.0, the addi-tion of H2O2 in ultrasound/exfoliated graphite system hada very slightly influence on the decolorization of acid black210. But, it should be noticed that the decolorization effi-ciency of acid black 210 was obvious higher in ultra-sound/exfoliated graphite–H2O2 system compared toultrasound/exfoliated graphite system at pH 7. Fig. 7shows a color removal of about 53.4% and 38.6% after120 min sonication in ultrasound/exfoliated graphite–H2O2 and ultrasound/exfoliated graphite system, respec-tively. It is possible that hydrogen peroxide in aqueoussolution can be cleaved into hydroxyl radicals during thecollapse of the cavitation bubbles, which could enhancethe reaction with hydroxyl radicals. Hence, the decoloriza-tion efficiency was enhanced by the add H2O2.

3.7. Effect of temperature on the decolorization of

acid black 210

The effect of temperature on the decolorization of acidblack 210 in different systems (exfoliated graphite, activatedcarbon, ultrasound/activated carbon and ultrasound/exfoliated graphite) were examined within 120 min at theconstant temperature of 30 �C and 51 �C. As shown inFig. 8, with increasing temperature, the decolorization effi-ciencies of acid black 210 in all four systems were increased.Among of these systems, the combination of ultrasoundand exfoliated graphite is found to be an excellent method

in terms of decolorization efficiency of acid black 210 fromaqueous solution.

4. Conclusion

The major contribution of this work is to emphasize thatthere is a high potential for increasing the decolorizationefficiency of acid black 210 using a combination of ultra-sound and exfoliated graphite. Approximately 99.5% ofacid black 210 was removed using 0.8 g/l exfoliated graph-ite at a pH of 1 within 120 min at 51 �C. Low pH value andlarge exfoliation volume of exfoliated graphite favored theultrasonic decolorization of acid black 210. The presentmethod was more effective than using ultrasound combinedwith activated carbon for the removal of acid black 210.

Acknowledgement

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

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