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Removal of disperse blue 2BLN from aqueous solution by combination of ultrasound and exfoliated graphite
Ultrasonics Sonochemistry 14 (2007) 62–66 www.elsevier.com/locate/ultsonch
Removal of disperse blue 2BLN from aqueous solution by combination of ultrasound and exfoliated graphite Ji-Tai Li a
a,*
, Mei Li
a,b
, Ji-Hui Li b, Han-Wen Sun
a
College of Chemistry and Environmental Science, Hebei University, Key Laboratory of Analytical Science and Technology of Hebei Province, Baoding 071002, PR China b College of Chemistry and Material Science, Hebei Normal University, Shi Jiazhuang 050091, PR China Received 28 May 2005; accepted 29 January 2006 Available online 24 March 2006
Abstract This paper reports an efficient and convenient removal of disperse blue 2BLN from aqueous solution by the combination of ultrasound and exfoliated graphite. The various affecting factors were studied. The removal ratio of disperse blue 2BLN is 96.9% for the initial concentration of 200 mg/L using 600 mg/L exfoliated graphite (exfoliation volume of 300 mL/g) at 45 °C within 120 min under ultrasound. The combination method was more effective than sonolysis or exfoliated graphite treatment individually. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Ultrasound; Exfoliated graphite; Disperse blue 2BLN; Removal
1. Introduction Wastewater containing dyes is one of the major industrial pollutions. Removal of these pollutants from aqueous solution is an important practical problem. Physical, chemical and biological methods have been used to treat these dye wastewaters, such as the method of photocatalytic degradation in the presence of anatase TiO2 powder [1], the adsorption by fresh MnO2 [2] or by exfoliated graphite [3]. Recently, increasing attention has been focused on ultrasonic treatment. This is due to its greater efficiency in decomposing refractory compounds [4]. Ultrasonic irradiation differs from traditional energy (such as heat, light, or ionizing radiation) in duration, pressure and energy per molecule. Because of the immense temperatures and
* Corresponding author. Address: College of Chemistry and Environmental Science, Hebei University, Hezuo Road, No. 88, Baoding 071002, PR China. Tel.: +86 312 507 9361; fax: +86 312 507 9628. E-mail address: [email protected] (J.-T. Li).
1350-4177/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2006.01.006
pressures, extraordinary heating and cooling rates generated by cavitation bubble collapse, ultrasound provides an unusual mechanism for generating high-energy chemistry. The application of ultrasound has already been reported for the removal of organic compounds from aqueous solution such as methylorange [5], saccharin [6] and 2-chlorophenol [7]. Exfoliated graphite is a new kind of porous carbonbased adsorption material, which has many characteristics including low density, no pollution of the environment and easy disposal. Toxicological experiments on small and large mice illustrated low acute toxicity of exfoliated graphite, weak allergic and irritating effects. It has been reported that exfoliated graphite is used as a medical dressing for absorption drainage of a wound and used to remove crude oil, lubricating oil, diesel oil, petrol [8], heavy oil, biomedical liquids [9,10] and organic dyes [3]. Continuing our investigations in this area, we wish to report an efficient procedure for removal of disperse blue 2BLN from aqueous solution by the combination of ultrasound and exfoliated graphite.
J.-T. Li et al. / Ultrasonics Sonochemistry 14 (2007) 62–66
2. Experimental 2.1. Materials Disperse blue 2BLN (C14H9N2O4Br; MW = 349.14 g/ mole, Hebei Runkai Co., Ltd.) was applied as a model compound. The water used for preparation of the disperse blue solution was distilled. The preparation of exfoliated graphite was performed as follows: 10 g of natural flake graphite, 14 g of perchloric acid, 10 g of acetic anhydride and 5 g of potassium permanganate were placed into a 250 mL dry beaker. After this mixture in the reactor was stirred and reacted for 60 min at 30 °C, the graphite intercalation compound was prepared. It was then washed in water to neutrality, dehydrated and dried at 60 °C. Then, the graphite was rapidly heated to 1000 °C to form exfoliated graphite with exfoliation volume of 300 mL/g. It should be pointed out that there are at least three kinds of pores in a lump of exfoliated graphite. There are large pores among the worm-like particles, crevice-like pores on the surface of particles and pores inside the particles [11]. Representative SEM images revealing these characteristic pores in exfoliated graphite are shown in Fig. 1.
63
10–300 mg/L disperse blue 2BLN acidic solution at 542 nm obeys the Lambert–Beer law and the calibration curve of standard disperse blue 2BLN acidic solutions are used to estimate the [Disperse blue 2BLN]t and to calculate the removal ratio of disperse blue 2BLN. Sonication was performed on a Beijing DTL–500 ultrasonic cleaner (with a frequency of 28 kHz and a nominal power 500 W, Beijing Detailong Co., Ltd.). The reaction flask was located in the maximum energy area, where the surface of reactants is slightly lower than the level of the water in the cleaning bath, and addition or removal of water was used to control the temperature of the water bath. In order to compare the removal ratio of disperse blue 2BLN in the presence of exfoliated graphite with and without ultrasonic treatment, a thermostatic shaker was employed to agitate the disperse blue 2BLN solution containing exfoliated graphite. 2.3. General procedure
UV-2501PC (SHIMADZU, Japan) UV-Vis spectrometer was used to measure the removal ratio of disperse blue 2BLN. The disperse blue 2BLN removal ratio is calculated using the following equation:
A 100 mL beaker was charged with disperse blue 2BLN solution (50 mL) and exfoliated graphite. The mixture was irradiated in the ultrasonic cleaning bath at a set temperature for a period. Experimental conditions involved the exfoliated graphite (600 mg/L) with an initial disperse blue 2BLN concentration of 200 mg/L, pH = 0.5, ultrasonic frequency of 28 kHz (nominal power of 500 W), exfoliation volume of exfoliated graphite of 300 mL/g, temperature of 25 °C. A reaction time of 120 min was used throughout the course of the investigations except for some special experiments.
Removal ratio ¼ ð½Disperse blue 2BLNi
3. Results and discussion
2.2. Apparatus and analysis
½Disperse blue2BLNt Þ =½Disperse blue 2BLNi ; where [Disperse blue 2BLN]i is the disperse blue 2BLN initial concentration and [Disperse blue 2BLN]t is the concentration at measurable time t. The maximal absorbency of
Fig. 1. SEM images showing different pores in exfoliated graphite: (a) crevice-like pores on the surface of particles, (b) pores inside the worm-like particles.
3.1. Effect of ultrasound irradiation The UV–Visible spectra of the original disperse blue 2BLN solution and disperse blue 2BLN solution treated in three cases were determined by a UV–Vis spectrophotometer in the wavelength range from 200 to 800 nm as shown in Fig. 2. It can be seen that the maximal absorbency which appears at 542 nm of the disperse blue 2BLN solution in the presence of exfoliated graphite with ultrasound irradiation decreased markedly, but the maximal absorption peak (542 nm) of the disperse blue 2BLN solution under only ultrasound irradiation was not changed. The disperse blue 2BLN solution maximal absorption peak (542 nm) decreased a little in the presence of exfoliated graphite without ultrasound. As shown in Fig. 2 (curve 4), the chromophore of disperse blue 2BLN has disappeared. The results showed that the combination of ultrasound irradiation and exfoliated graphite is a more efficient removal of disperse blue 2BLN from aqueous solution. The reason may be that ultrasonic irradiation can provide a more efficient method of distributing and absorbing dye on the surface of exfoliated graphite, and can also enhance mass transfer to the surface rapidly.
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J.-T. Li et al. / Ultrasonics Sonochemistry 14 (2007) 62–66 0.6
1
Absorbance (A)
2
0.4
3
0.2
4
0.0 200
400
600
800
wavelength (nm) Fig. 2. UV–Visible spectra of original and treated disperse blue 2BLN solutions: (1) only ultrasound, (2) original, (3) only exfoliated graphite, (4) exfoliated graphite + ultrasound.
under ultrasonication for 120 min in the presence of exfoliated graphite. However, the removal ratio was only 18.9% by exfoliated graphite treatment individually. While using ultrasound alone, the disperse blue 2BLN could not be removed. These results also indicated that the removal efficiency of disperse blue 2BLN is more obvious in the presence of exfoliated graphite combined with ultrasound than that of using exfoliated graphite or ultrasound irradiation individually. There are probably two explanations to account for these facts. Firstly, the cavitation hot spots can destroy the dyes via sonochemistry [13]. Secondly, hydroxyl radicals generated in the cavitation bubbles oxidize the polar dye compounds on the surface of exfoliated graphite [5]. Perhaps, the special pore structure and large surface of exfoliated graphite particles enable the production of a large number of OH radicals under ultrasonic irradiation. 3.3. Effect of pH
The UV–Visible spectrum of the disperse blue 2BLN solution shows a maximal peak at 226 nm. The specific ultraviolet light absorption is a parameter which indicates the quantity of unsaturated bonds contained in organic material [1]; that is, the more unsaturated bonds the higher is the A226 (the absorption at the wavelength of 226 nm). The reduction of A226 in Fig. 2 (curve 4) implies that unsaturated bonds in disperse blue 2BLN are continuously decomposed in the combination method. But the procedure could not completely mineralize disperse blue 2BLN to CO2 and H2O [12]. 3.2. Effect of reaction time The effect of reaction time on the removal ratios of disperse blue 2BLN was shown in Fig. 3. This figure showed that the removal ratio of two courses in the presence of exfoliated graphite both increased with increasing reaction time. The removal ratio of disperse blue 2BLN was 93.4%
100
The effect of the pH of the solution on removal efficiency of disperse blue 2BLN was shown in Fig. 4. It was observed that the removal ratios of disperse blue 2BLN in the presence of exfoliated graphite with and without ultrasound all decreased gradually with increasing pH. At pH = 0.5, the highest removal ratio was achieved. The remarkable removal ratio of disperse blue 2BLN (curve 1) indicated that the treatments of some organic pollutants like disperse blue 2BLN could be performed in highly acidic medium in order to obtain the best removal ratio. One reason may be that sonication increases the number of OH radicals on the surface of exfoliated graphite at low pH values. The other reason may be that the surface of exfoliated graphite particles is positively charged in highly acidic solution and is easily capable of adsorbing the dye ions which have a negative charge [5]. This indicates that the pH of the solution is one of the most important parameters affecting the removal ratio.
100
1
1
80
Removal ratio (%)
Removal ratio (%)
80
60
40 2
20
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40
2
20 3
0
0 0
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Time (min) Fig. 3. Effect of time on removal ratio: (1) ultrasound + exfoliated graphite, (2) only exfoliated graphite, (3) only ultrasound.
0
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3
4
5
pH Fig. 4. Effect of pH on removal ratio: (1) ultrasound + exfoliated graphite, (2) only exfoliated graphite.
J.-T. Li et al. / Ultrasonics Sonochemistry 14 (2007) 62–66
3.4. Effect of temperature The effect of temperature on the removal ratio of disperse blue 2BLN was investigated in the range of 25– 45 °C. The results are shown in Figs. 5 and 6. Based on Figs. 5 and 6, the removal ratio of disperse blue 2BLN increased with temperature in the presence of exfoliated graphite with and without ultrasound irradiation. The reason may be that when the temperature is increased the dye molecules diffuse to the surface of exfoliated graphite more rapidly. In the combined method, the removal ratio of disperse blue 2BLN was about 93.4% at 25 °C, the removal ratio of disperse blue 2BLN increased to 96.9% at 45 °C. However the removal ratio was only 18.9% at 25 °C and 22.2% at 45 °C, respectively using only exfoliated graphite in the absence o´f ultrasound. Fig. 6 shows the time-dependent variations of the removal ratio of disperse blue 2BLN at different reaction temperatures. It was observed that the effect of temperature
1
90
Removal ratio (%)
80 70 60 50 40 30 2
65
fades as the reaction time increases. It may be that the time required for reaching a state of equilibrium in the adsorption system is different at different temperatures. The higher the reaction temperature, the shorter the time required for reaching a state of equilibrium in the system in the combination method. 3.5. Effect of exfoliation volume The effect of the exfoliation volume of exfoliated graphite on the removal ratio of disperse blue 2BLN was studied in the range of 100–300 mL/g exfoliation volume. It can be seen in Fig. 7 that the removal ratio of disperse blue 2BLN increases rapidly with the increase in exfoliation volume. The removal ratio of disperse blue 2BLN was 55.5% in the presence of exfoliated graphite with 100 mL/g exfoliation volume under ultrasound irradiation, and the removal ratio was 9.6% without ultrasound irradiation. When 300 mL/g of exfoliated graphite was used, the removal ratio of disperse blue 2BLN was increased to 93.4% under sonication and the removal ratio was increased to 18.9% without ultrasound. It is apparent that exfoliated graphite with larger exfoliation volumes improved the effect. This behavior could be explained by the effect of the pore structure on the adsorption behavior of organic adsorbates onto the surface. Exfoliation volume of exfoliated graphite increased with growth of pores inside the exfoliated graphite particles. Therefore, the larger the exfoliation volume, the higher the sorption capacity of exfoliated graphite for disperse blue 2BLN under ultrasound irradiation. On the other hand, sonication can cause the rupture of exfoliated graphite particles, with a consequent decrease in particle size and increase in surface area available for reaction [14].
20 25
30
35
40
45
3.6. Effect of initial disperse blue 2BLN concentration
o
Temperature ( C) Fig. 5. Effect of temperature on removal ratio: (1) ultrasound + exfoliated graphite, (2) only exfoliated graphite.
100
The effect of initial disperse blue 2BLN concentration on the removal ratio was shown in Fig. 8. Different initial
1 2
Removal ratio (%)
Removal ratio (%)
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60
40
20
0 0
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40
60
80
100
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Time (min) Fig. 6. Effect of temperature on removal ratio: (1) ultrasound + exfoliated graphite (45 °C), (2) ultrasound + exfoliated graphite (25 °C).
95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10
1
2
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300
Expoliation volume (mL/g) Fig. 7. Effect of exfoliation volume on removal ratio: (1) ultrasound + exfoliated graphite, (2) only exfoliated graphite.
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J.-T. Li et al. / Ultrasonics Sonochemistry 14 (2007) 62–66
blue 2BLN concentration was 200 mg/L, the optimum amount of exfoliated graphite was about 600 mg/L giving a removal ratio of 93.4%. When the initial disperse blue 2BLN concentration was 400 mg/L, the optimum added amount of exfoliated graphite was about 800 mg/L and the removal ratio was 92%. Larger quantities did not increase the removal ratios of disperse blue 2BLN.
1
Removal ratio (%)
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60
40
4. Conclusion 2
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0 100
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Initial concentration (mg/L) Fig. 8. Effect of initial disperse blue 2BLN concentration on removal ratio: (1) ultrasound + exfoliated graphite, (2) only exfoliated graphite.
Removal ratio (%)
90
1
80
We have provided an efficient and convenient procedure for the removal of disperse blue 2BLN from aqueous solution using the combination method of ultrasound irradiation and exfoliated graphite. At 25 °C disperse blue 2BLN could not be removed using sonication alone within 120 min and only 18.9% disperse blue 2BLN was removed when using exfoliated graphite alone. In the combined method (ultrasound/exfoliated graphite), the removal ratio of disperse blue 2BLN was 93.4%. It is hoped that this combination method can be extended and applied in the treatment of dye wastewaters that are difficult to degrade using chemical and biological methods. Acknowledgements We thank the Educational Ministry of China and Natural Science Foundation of Hebei Province (B2006000969), China, for financial support.
70
2
60
References 50 200
400
600
800
1000
Adding amount (mg/L) Fig. 9. Effect of amount of exfoliated graphite on removal ratio: (1) ultrasound + exfoliation graphite (initial disperse blue 2BLN concentration of 200 mg/L), (2) ultrasound + exfoliation graphite (initial disperse blue 2BLN concentration of 400 mg/L).
disperse blue 2BLN concentrations resulted in different removal ratios. The removal ratio decreased with increasing initial concentration in the presence of exfoliated graphite, with or without ultrasound. When the initial concentration was 200 mg/L, the removal ratio of disperse blue 2BLN was 93.4%. When the concentration was 400 mg/L, it was 87% using the combination method. As shown in Fig. 8, the removal efficiency of the combination method was better than that of exfoliated graphite alone in all experimental concentration ranges. 3.7. Effect of the amount of exfoliated graphite The effect of the added amount of exfoliated graphite in solution was investigated in the range of 200 mg/L1000 mg/L. As shown in Fig. 9, when the initial disperse
[1] C.N. Chang, Y.S. Ma, A.C. Chao, M.C. Tsai, H.F. Sung, Chemosphere 56 (2004) 1011. [2] Z.C. Ma, L.L. Dong, Y.H. Kang, L.H. Jiang, Techniques and Equipment for Environmental Pollution Control 1 (2002) 30. [3] L.N. Wang, X. Cheng, Y.P. Zheng, F.Y. Kang, J.F. Ceng, W.C. Sheng, Zhong Gou Fei Jin Shu Kuang Gong Ye Dao Kan 5 (2004) 59. [4] A. Rehorek, M. Tauber, G. Gu¨bitz, Ultrasonics Sonochemistry 11 (2004) 177. [5] J. Wang, B.D. Guo, X.D. Zhang, Z.H. Zhang, J.T. Han, J. Wu, Ultrasonics Sonochemistry 12 (2005) 331. [6] E.C. Bernardo, T. Fukuta, T. Fujita, E.P. Ona, Ultrasonics Sonochemistry 13 (2006) 13. [7] M.H. Entezari, M. Mostafai, A. Sarafraz-yazdi, Ultrasonics Sonochemistry 13 (2006) 37. [8] W.C. Shen, S.Z. Wen, N.Z. Cao, L. Zheng, W. Zhou, Y.J. Liu, J.L. Gu, Carbon 37 (1999) 351. [9] M. Toyoda, H. Umemura, M. Inagaki, New Carbon Materials 2 (2002) 1. [10] F.Y. Kang, Y.P. Zheng, H. Zhao, H.N. Wang, New Carbon Materials 3 (2003) 161. [11] M. Inagaki, M. Toyoda, F.Y. Kang, W.C. Shen, New Carbon Materials 4 (2003) 241. [12] Y.T. Shi, D.S. Xia, H.Y. Li, Q.F. Zeng, Environmental Protection of Chemical Industry 24 (2004) 20. [13] T.J. Mason, Practical Sonochemistry, Ellis Horwood, New York, London, 1991, pp. 21. [14] T.J. Mason, Practical Sonochemistry, Ellis Horwood, New York, London, 1991, pp. 27.
Removal of disperse blue 2BLN from aqueous solution by combination of ultrasound and exfoliated graphite Ji-Tai Li a
a,*
, Mei Li
a,b
, Ji-Hui Li b, Han-Wen Sun
a
College of Chemistry and Environmental Science, Hebei University, Key Laboratory of Analytical Science and Technology of Hebei Province, Baoding 071002, PR China b College of Chemistry and Material Science, Hebei Normal University, Shi Jiazhuang 050091, PR China Received 28 May 2005; accepted 29 January 2006 Available online 24 March 2006
Abstract This paper reports an efficient and convenient removal of disperse blue 2BLN from aqueous solution by the combination of ultrasound and exfoliated graphite. The various affecting factors were studied. The removal ratio of disperse blue 2BLN is 96.9% for the initial concentration of 200 mg/L using 600 mg/L exfoliated graphite (exfoliation volume of 300 mL/g) at 45 °C within 120 min under ultrasound. The combination method was more effective than sonolysis or exfoliated graphite treatment individually. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Ultrasound; Exfoliated graphite; Disperse blue 2BLN; Removal
1. Introduction Wastewater containing dyes is one of the major industrial pollutions. Removal of these pollutants from aqueous solution is an important practical problem. Physical, chemical and biological methods have been used to treat these dye wastewaters, such as the method of photocatalytic degradation in the presence of anatase TiO2 powder [1], the adsorption by fresh MnO2 [2] or by exfoliated graphite [3]. Recently, increasing attention has been focused on ultrasonic treatment. This is due to its greater efficiency in decomposing refractory compounds [4]. Ultrasonic irradiation differs from traditional energy (such as heat, light, or ionizing radiation) in duration, pressure and energy per molecule. Because of the immense temperatures and
* Corresponding author. Address: College of Chemistry and Environmental Science, Hebei University, Hezuo Road, No. 88, Baoding 071002, PR China. Tel.: +86 312 507 9361; fax: +86 312 507 9628. E-mail address: [email protected] (J.-T. Li).
1350-4177/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2006.01.006
pressures, extraordinary heating and cooling rates generated by cavitation bubble collapse, ultrasound provides an unusual mechanism for generating high-energy chemistry. The application of ultrasound has already been reported for the removal of organic compounds from aqueous solution such as methylorange [5], saccharin [6] and 2-chlorophenol [7]. Exfoliated graphite is a new kind of porous carbonbased adsorption material, which has many characteristics including low density, no pollution of the environment and easy disposal. Toxicological experiments on small and large mice illustrated low acute toxicity of exfoliated graphite, weak allergic and irritating effects. It has been reported that exfoliated graphite is used as a medical dressing for absorption drainage of a wound and used to remove crude oil, lubricating oil, diesel oil, petrol [8], heavy oil, biomedical liquids [9,10] and organic dyes [3]. Continuing our investigations in this area, we wish to report an efficient procedure for removal of disperse blue 2BLN from aqueous solution by the combination of ultrasound and exfoliated graphite.
J.-T. Li et al. / Ultrasonics Sonochemistry 14 (2007) 62–66
2. Experimental 2.1. Materials Disperse blue 2BLN (C14H9N2O4Br; MW = 349.14 g/ mole, Hebei Runkai Co., Ltd.) was applied as a model compound. The water used for preparation of the disperse blue solution was distilled. The preparation of exfoliated graphite was performed as follows: 10 g of natural flake graphite, 14 g of perchloric acid, 10 g of acetic anhydride and 5 g of potassium permanganate were placed into a 250 mL dry beaker. After this mixture in the reactor was stirred and reacted for 60 min at 30 °C, the graphite intercalation compound was prepared. It was then washed in water to neutrality, dehydrated and dried at 60 °C. Then, the graphite was rapidly heated to 1000 °C to form exfoliated graphite with exfoliation volume of 300 mL/g. It should be pointed out that there are at least three kinds of pores in a lump of exfoliated graphite. There are large pores among the worm-like particles, crevice-like pores on the surface of particles and pores inside the particles [11]. Representative SEM images revealing these characteristic pores in exfoliated graphite are shown in Fig. 1.
63
10–300 mg/L disperse blue 2BLN acidic solution at 542 nm obeys the Lambert–Beer law and the calibration curve of standard disperse blue 2BLN acidic solutions are used to estimate the [Disperse blue 2BLN]t and to calculate the removal ratio of disperse blue 2BLN. Sonication was performed on a Beijing DTL–500 ultrasonic cleaner (with a frequency of 28 kHz and a nominal power 500 W, Beijing Detailong Co., Ltd.). The reaction flask was located in the maximum energy area, where the surface of reactants is slightly lower than the level of the water in the cleaning bath, and addition or removal of water was used to control the temperature of the water bath. In order to compare the removal ratio of disperse blue 2BLN in the presence of exfoliated graphite with and without ultrasonic treatment, a thermostatic shaker was employed to agitate the disperse blue 2BLN solution containing exfoliated graphite. 2.3. General procedure
UV-2501PC (SHIMADZU, Japan) UV-Vis spectrometer was used to measure the removal ratio of disperse blue 2BLN. The disperse blue 2BLN removal ratio is calculated using the following equation:
A 100 mL beaker was charged with disperse blue 2BLN solution (50 mL) and exfoliated graphite. The mixture was irradiated in the ultrasonic cleaning bath at a set temperature for a period. Experimental conditions involved the exfoliated graphite (600 mg/L) with an initial disperse blue 2BLN concentration of 200 mg/L, pH = 0.5, ultrasonic frequency of 28 kHz (nominal power of 500 W), exfoliation volume of exfoliated graphite of 300 mL/g, temperature of 25 °C. A reaction time of 120 min was used throughout the course of the investigations except for some special experiments.
Removal ratio ¼ ð½Disperse blue 2BLNi
3. Results and discussion
2.2. Apparatus and analysis
½Disperse blue2BLNt Þ =½Disperse blue 2BLNi ; where [Disperse blue 2BLN]i is the disperse blue 2BLN initial concentration and [Disperse blue 2BLN]t is the concentration at measurable time t. The maximal absorbency of
Fig. 1. SEM images showing different pores in exfoliated graphite: (a) crevice-like pores on the surface of particles, (b) pores inside the worm-like particles.
3.1. Effect of ultrasound irradiation The UV–Visible spectra of the original disperse blue 2BLN solution and disperse blue 2BLN solution treated in three cases were determined by a UV–Vis spectrophotometer in the wavelength range from 200 to 800 nm as shown in Fig. 2. It can be seen that the maximal absorbency which appears at 542 nm of the disperse blue 2BLN solution in the presence of exfoliated graphite with ultrasound irradiation decreased markedly, but the maximal absorption peak (542 nm) of the disperse blue 2BLN solution under only ultrasound irradiation was not changed. The disperse blue 2BLN solution maximal absorption peak (542 nm) decreased a little in the presence of exfoliated graphite without ultrasound. As shown in Fig. 2 (curve 4), the chromophore of disperse blue 2BLN has disappeared. The results showed that the combination of ultrasound irradiation and exfoliated graphite is a more efficient removal of disperse blue 2BLN from aqueous solution. The reason may be that ultrasonic irradiation can provide a more efficient method of distributing and absorbing dye on the surface of exfoliated graphite, and can also enhance mass transfer to the surface rapidly.
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J.-T. Li et al. / Ultrasonics Sonochemistry 14 (2007) 62–66 0.6
1
Absorbance (A)
2
0.4
3
0.2
4
0.0 200
400
600
800
wavelength (nm) Fig. 2. UV–Visible spectra of original and treated disperse blue 2BLN solutions: (1) only ultrasound, (2) original, (3) only exfoliated graphite, (4) exfoliated graphite + ultrasound.
under ultrasonication for 120 min in the presence of exfoliated graphite. However, the removal ratio was only 18.9% by exfoliated graphite treatment individually. While using ultrasound alone, the disperse blue 2BLN could not be removed. These results also indicated that the removal efficiency of disperse blue 2BLN is more obvious in the presence of exfoliated graphite combined with ultrasound than that of using exfoliated graphite or ultrasound irradiation individually. There are probably two explanations to account for these facts. Firstly, the cavitation hot spots can destroy the dyes via sonochemistry [13]. Secondly, hydroxyl radicals generated in the cavitation bubbles oxidize the polar dye compounds on the surface of exfoliated graphite [5]. Perhaps, the special pore structure and large surface of exfoliated graphite particles enable the production of a large number of OH radicals under ultrasonic irradiation. 3.3. Effect of pH
The UV–Visible spectrum of the disperse blue 2BLN solution shows a maximal peak at 226 nm. The specific ultraviolet light absorption is a parameter which indicates the quantity of unsaturated bonds contained in organic material [1]; that is, the more unsaturated bonds the higher is the A226 (the absorption at the wavelength of 226 nm). The reduction of A226 in Fig. 2 (curve 4) implies that unsaturated bonds in disperse blue 2BLN are continuously decomposed in the combination method. But the procedure could not completely mineralize disperse blue 2BLN to CO2 and H2O [12]. 3.2. Effect of reaction time The effect of reaction time on the removal ratios of disperse blue 2BLN was shown in Fig. 3. This figure showed that the removal ratio of two courses in the presence of exfoliated graphite both increased with increasing reaction time. The removal ratio of disperse blue 2BLN was 93.4%
100
The effect of the pH of the solution on removal efficiency of disperse blue 2BLN was shown in Fig. 4. It was observed that the removal ratios of disperse blue 2BLN in the presence of exfoliated graphite with and without ultrasound all decreased gradually with increasing pH. At pH = 0.5, the highest removal ratio was achieved. The remarkable removal ratio of disperse blue 2BLN (curve 1) indicated that the treatments of some organic pollutants like disperse blue 2BLN could be performed in highly acidic medium in order to obtain the best removal ratio. One reason may be that sonication increases the number of OH radicals on the surface of exfoliated graphite at low pH values. The other reason may be that the surface of exfoliated graphite particles is positively charged in highly acidic solution and is easily capable of adsorbing the dye ions which have a negative charge [5]. This indicates that the pH of the solution is one of the most important parameters affecting the removal ratio.
100
1
1
80
Removal ratio (%)
Removal ratio (%)
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40 2
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20 3
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Time (min) Fig. 3. Effect of time on removal ratio: (1) ultrasound + exfoliated graphite, (2) only exfoliated graphite, (3) only ultrasound.
0
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5
pH Fig. 4. Effect of pH on removal ratio: (1) ultrasound + exfoliated graphite, (2) only exfoliated graphite.
J.-T. Li et al. / Ultrasonics Sonochemistry 14 (2007) 62–66
3.4. Effect of temperature The effect of temperature on the removal ratio of disperse blue 2BLN was investigated in the range of 25– 45 °C. The results are shown in Figs. 5 and 6. Based on Figs. 5 and 6, the removal ratio of disperse blue 2BLN increased with temperature in the presence of exfoliated graphite with and without ultrasound irradiation. The reason may be that when the temperature is increased the dye molecules diffuse to the surface of exfoliated graphite more rapidly. In the combined method, the removal ratio of disperse blue 2BLN was about 93.4% at 25 °C, the removal ratio of disperse blue 2BLN increased to 96.9% at 45 °C. However the removal ratio was only 18.9% at 25 °C and 22.2% at 45 °C, respectively using only exfoliated graphite in the absence o´f ultrasound. Fig. 6 shows the time-dependent variations of the removal ratio of disperse blue 2BLN at different reaction temperatures. It was observed that the effect of temperature
1
90
Removal ratio (%)
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fades as the reaction time increases. It may be that the time required for reaching a state of equilibrium in the adsorption system is different at different temperatures. The higher the reaction temperature, the shorter the time required for reaching a state of equilibrium in the system in the combination method. 3.5. Effect of exfoliation volume The effect of the exfoliation volume of exfoliated graphite on the removal ratio of disperse blue 2BLN was studied in the range of 100–300 mL/g exfoliation volume. It can be seen in Fig. 7 that the removal ratio of disperse blue 2BLN increases rapidly with the increase in exfoliation volume. The removal ratio of disperse blue 2BLN was 55.5% in the presence of exfoliated graphite with 100 mL/g exfoliation volume under ultrasound irradiation, and the removal ratio was 9.6% without ultrasound irradiation. When 300 mL/g of exfoliated graphite was used, the removal ratio of disperse blue 2BLN was increased to 93.4% under sonication and the removal ratio was increased to 18.9% without ultrasound. It is apparent that exfoliated graphite with larger exfoliation volumes improved the effect. This behavior could be explained by the effect of the pore structure on the adsorption behavior of organic adsorbates onto the surface. Exfoliation volume of exfoliated graphite increased with growth of pores inside the exfoliated graphite particles. Therefore, the larger the exfoliation volume, the higher the sorption capacity of exfoliated graphite for disperse blue 2BLN under ultrasound irradiation. On the other hand, sonication can cause the rupture of exfoliated graphite particles, with a consequent decrease in particle size and increase in surface area available for reaction [14].
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The effect of initial disperse blue 2BLN concentration on the removal ratio was shown in Fig. 8. Different initial
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Expoliation volume (mL/g) Fig. 7. Effect of exfoliation volume on removal ratio: (1) ultrasound + exfoliated graphite, (2) only exfoliated graphite.
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blue 2BLN concentration was 200 mg/L, the optimum amount of exfoliated graphite was about 600 mg/L giving a removal ratio of 93.4%. When the initial disperse blue 2BLN concentration was 400 mg/L, the optimum added amount of exfoliated graphite was about 800 mg/L and the removal ratio was 92%. Larger quantities did not increase the removal ratios of disperse blue 2BLN.
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Initial concentration (mg/L) Fig. 8. Effect of initial disperse blue 2BLN concentration on removal ratio: (1) ultrasound + exfoliated graphite, (2) only exfoliated graphite.
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We have provided an efficient and convenient procedure for the removal of disperse blue 2BLN from aqueous solution using the combination method of ultrasound irradiation and exfoliated graphite. At 25 °C disperse blue 2BLN could not be removed using sonication alone within 120 min and only 18.9% disperse blue 2BLN was removed when using exfoliated graphite alone. In the combined method (ultrasound/exfoliated graphite), the removal ratio of disperse blue 2BLN was 93.4%. It is hoped that this combination method can be extended and applied in the treatment of dye wastewaters that are difficult to degrade using chemical and biological methods. Acknowledgements We thank the Educational Ministry of China and Natural Science Foundation of Hebei Province (B2006000969), China, for financial support.
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Adding amount (mg/L) Fig. 9. Effect of amount of exfoliated graphite on removal ratio: (1) ultrasound + exfoliation graphite (initial disperse blue 2BLN concentration of 200 mg/L), (2) ultrasound + exfoliation graphite (initial disperse blue 2BLN concentration of 400 mg/L).
disperse blue 2BLN concentrations resulted in different removal ratios. The removal ratio decreased with increasing initial concentration in the presence of exfoliated graphite, with or without ultrasound. When the initial concentration was 200 mg/L, the removal ratio of disperse blue 2BLN was 93.4%. When the concentration was 400 mg/L, it was 87% using the combination method. As shown in Fig. 8, the removal efficiency of the combination method was better than that of exfoliated graphite alone in all experimental concentration ranges. 3.7. Effect of the amount of exfoliated graphite The effect of the added amount of exfoliated graphite in solution was investigated in the range of 200 mg/L1000 mg/L. As shown in Fig. 9, when the initial disperse
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