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Study of the sonophotocatalytic degradation of basic blue 9 industrial textile dye over slurry titanium dioxide and influencing factors
Ultrasonics Sonochemistry 15 (2008) 1038–1042
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Study of the sonophotocatalytic degradation of basic blue 9 industrial textile dye over slurry titanium dioxide and influencing factors Antonia Sandoval González, Susana Silva Martínez * Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Avenve Universidad 1001, Col Chamilpa, Cuernavaca, Mor. C.P. 62209, Mexico
a r t i c l e
i n f o
Article history: Received 2 September 2007 Received in revised form 25 January 2008 Accepted 22 March 2008 Available online 29 March 2008 Keywords: Basic blue 9 Sonophotocatalysis Photocatalysis Sonolysis Methylene blue TiO2
a b s t r a c t The sonophotocatalytic degradation of basic blue 9 industrial textile dye has been studied in the presence of ultrasound (20 kHz) over a TiO2 slurry employing an UV lamp (15 W, 352 nm). It was observed that the color removal efficiency was influenced by the pH of the solution, initial dye concentration and TiO2 amount. It was found that the dye degradation followed apparent first order kinetics. The rate constant increased by decreasing dye concentration and was affected by the pH of the solution with the highest degradation obtained at pH 7. The first order rate constants obtained with sonophotocatalysis were twofold and tenfold than those obtained under photocatalysis and sonolysis, respectively. The chemical oxygen demand was abated over 80%. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Water pollution due to the release of dye compounds during the dyeing and finishing operations into textile effluents has recently become a major concern [1,2]. Colored wastewaters impose serious aesthetic and environmental problems because of their color and their high chemical oxygen demand. Furthermore, direct discharge of such effluents can cause the formation of toxic aromatic amines under anaerobic conditions in waters, and contaminate the soil and groundwater. An estimation of about 15% of the total world production of dyes is lost in wastewater stream during such operations [3]. Recently, several advanced oxidation processes have been developed and applied specifically to wastewater treatment. In order to increase the extent of reduction toxicity, hybrid methods constituting two or more individual processes have been developed [4]. Some of these include ultrasound/hydrogen peroxide (H2O2) or ultrasound/ozone, UV light/H2O2 or UV/ozone, sonophotocatalytic oxidation (the simultaneous use of ultrasound and photocatalysis), photo-Fenton processes and photocatalytic oxidation processes [5–10]. The use of sonophotocatalysis on semiconductors has recently attracted the attention of different research groups [9–14]. It is important to have simultaneous irradiation of ultrasound and UV light rather than the sequential operation because continuous cleaning of the catalyst plays a substantial role, * Corresponding author. Tel./fax: +52 777 329 70 84. E-mail address: [email protected] (S.S. Martínez). 1350-4177/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2008.03.008
as demonstrated by the work of Gogate and coworkers using formic acid degradation [14]. Selli [10] reported that the combination of photocatalysis and ultrasound showed synergistic effects on the degradation of acid orange 8 (azo dye) involving the aqueous phase due to the formation of H2O2 by ultrasound and not the photocatalyst–water interface. Mrowetz and collaborators [13] reported an enhancement in the rate constants of acid orange 8 and acid red 1 dye compounds during the sonophotocatalytic degradation where the synergism was more pronounced for the later. The purposes of this work were to study the degradation of the basic blue 9 dye (BB9), used by the local textile industry, by sonophotocatalysis in the presence of TiO2, and compare its degradation under sonolysis and photocatalysis. The effects of catalyst amount, dye concentration and pH on the sonophotocatalytic degradation were investigated. The experimental results were assessed in terms of chemical oxygen demand (COD) and color reduction to determine the overall treatment efficiency of the degradation processes. COD is an index of water pollution by organics and it is a parameter used for quality discharge. COD differences along the time are exclusively related to the degree of oxidation of the organic matter as a whole. The BB9 dye compound is known as methylene blue (C16H18ClN3 S) and its molecular structure is shown in the Scheme 1. This organic compound with large molecular structure is a water soluble cationic dye found in wastewaters and it is potentially carcinogenic. The photocatalytic degradation of MB dye compound has been studied by other research groups. Hasnat and coworkers [15]
A.S. González, S.S. Martínez / Ultrasonics Sonochemistry 15 (2008) 1038–1042
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Scheme 1.
investigated the photocatalytic degradation of methylene blue (MB) and procion red in TiO2 dispersion under visible light. The rate of degradation followed first-order kinetics based on the Langmuir– Hinshelwood model and reported that the MB photocatalytic degradation rate increased by increasing the pH. Nagaveni and collaborators [16] studied the photocatalytic degradation of MB employing synthesized nano TiO2 by combustion. A comparison of its degradation with commercial Degussa P-25 TiO2 was performed and higher degradation was reported under the combustion synthesized nano TiO2. Akbal [17] investigated the degradation of MB and methyl orange. This author reported that color removal efficiency was affected by the concentration of the dye, amount of TiO2 added and the pH of the solution. Other research groups have achieved the MB removal from aqueous solution under adsorption onto different materials. Crini and Peindy [18] used material containing carboxylic groups and reported that the adsorption kinetics followed an apparent-second order model and the best fit of the experimental data was for the Freundlich isotherm. Jesionowski [19] applied aminosilane-functionalised silica as an adsorbent and found an increased efficiency of MB adsorption as compared to the unmodified silica. The Activated carbon has also been employed for the removal of MB [20–22]. Finally, the photoelectrochemical degradation of MB has also been investigated with nano TiO2 under high potential bias [23]. High decolorization efficiency and chemical oxygen demand removal efficiency were reported. The majority of these studies have been carried out employing the MB dye compound as analytical grade. 2. Experimental 2.1. Chemicals All solutions were prepared with distilled water using sulfuric acid or sodium hydroxide (analytical grade) to adjust the pH of the solution. BB9 dye was provided by the local textile industry (industrial grade, Ciba Specialty Chemicals) and the titanium dioxide used was commercial (Degussa P25, analytical grade). All chemicals were used as received without further purification and purchased from Sigma–Aldrich. 2.2. Procedures All degradation tests were performed in a sonophotoreactor with volume of 300 ml and recirculation that permitted us to investigate the effects of sonolysis, photolysis and photocatalysis either separately or simultaneously. A cylindrical ultrasound cell with volume of 150 ml was jacketed with cooling water circulation to maintain the reaction temperature at 25 ± 3 °C (Fig. 1). An ultrasound probe (13 mm diameter) was dipped into the cell solution and was powered by an ultrasonic processor (Cole Parmer) set at an amplitude of 80% which corresponds to a power input of 76 W at a frequency of 20 kHz. A fast stream of oxygen was feed to the test solution trough the ultrasound cell. A 15 W UV lamp (cut-off wavelength 352 nm, 44 cm long with 3 cm diameter. Cole Parmer) hosted in a Pyrex jacket illuminated the cylindrical photoreactor. The test solution was recirculated inside the jacket of the photoreactor and the irradiation UV light was 5 mm away from it. This de-
Fig. 1. Schematic representation of the sonophotoreactor employed to carry out the experiments of sonolysis, photocatalysis and sonophotocatalysis under different experimental conditions.
sign allowed a complete penetration of the UV light in the test solution. The TiO2 slurry was uniformly transported by the recirculation of the test solution throughout the sonophotoreactor in the photocatalytic and sonophotocatalytic experiments. Samples (2 ml) were withdrawn from the sonophotoreactor at different irradiation (light and/or sound) time intervals to measure both the absorption spectra in the wavelength interval of 1 nm from 300 to 800 nm with a spectrophotometer (DR/4000 U HACH) and the chemical oxygen demand (COD). The maximum absorption wavelength of BB9 was found at 663 nm. The absorbance measurements at 663 nm were employed to calculate the dye concentration from the calibration curve (absorbance vs. BB9 concentration) built at the corresponding pH of the aqueous solution. During sampling, care was taken to withdraw a volume of less than 10% of the initial volume. TiO2 was separated from the samples by centrifugation followed by filtration through a Millipore membrane as collected prior the analysis. The COD was analyzed using standard method and standard tubes [24] inside the concentration range of 0– 40 mg l1 COD. The COD method involves a reaction between the organic matter and the dichromate ion in a 50% sulfuric acid solution. The results are reported as an average value obtained from two or three analyses of the samples during degradation. 3. Results and discussion 3.1. Effect of catalyst addition In order to determine the effect of catalyst addition on color removal efficiency, experiments were performed with different catalyst amounts using an initial concentration of 0.06 mM of textile dye BB9. The amount of catalyst is an important parameter that can affect the degradation rate of organic compounds. The optimal catalyst concentration reported in the literature for TiO2 Degussa P25 ranges from 0.1 to 5.0 g l1, depending on the nature of the compounds and the photoreactor geometry [25]. Fig. 2 shows the effect of catalyst amount on the rate of sonophotocatalytic degradation as a function of TiO2 amount at pH 3. It can be observed that the degradation rate increases with increasing the amount of the catalyst until it reaches a plateau at 100 mg l1 of TiO2 and becomes independent of the amount of catalyst. Accordingly, the concentration of 100 mg l1 of catalyst was selected to perform further the degradation of BB9 dye at different operational parameters. The rate constants were calculated considering the concentration values obtained from the maxima absorbance measured at 663 nm as a function of degradation time.
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A.S. González, S.S. Martínez / Ultrasonics Sonochemistry 15 (2008) 1038–1042
3.2. Effect of initial dye concentration and pH solution
Fig. 2. Rate constants of 0.06 mM BB9 for the sonophotocatalytic degradation as a function of TiO2 amount, at pH 3 and 25 °C. The inset shows the percentage of dye color removal.
The color removal (CR, %) or decolorization was calculated using the following equation: " !, # 700 nm 700 nm 700 nm X X X AbsBt AbsAt AbsBt 100 CR;% ¼ ð1Þ 400 nm
400 nm
400 nm
where AbsBt and AbsAt correspond to the absorbance of the BB9 aqueous solutions recorded before and after treatment, respectively, inside the wavelength range of 400–700 nm. The color removal efficiency increased from 89 to 97% at 50 min of irradiation time when the catalyst concentration increased from 50 to 100 mg l1, as a result of an increase in the number of photons and TiO2 particles that increase the number of dye molecules adsorbed onto the catalyst. The color removal was not increased further with catalyst concentration over 100 mg l1 (inset of Fig. 2).
The effect of different initial dye concentration at constant pH and different initial pH on the degradation rate was studied in 100 mg l1 TiO2 suspensions. Fig. 3, reports the apparent first order rate constants obtained under different experimental conditions. It is observed that the reaction rate decreased with increasing the initial BB9 dye concentration, although this is much less so for sonolysis. The degradation of BB9 was much faster under both sonophotocatalysis and photocatalysis, having higher rate constant values for the former. These results clearly demonstrated that the combination of photocatalysis and sonolysis produced an additive effect on the degradation rates as shown by those obtained by sonophotocatalysis. This additive effect was observed in all the investigated range of initial dye concentrations. Additional experiments revealed that the sonication of the catalyst previous to the addition of BB9 up to 45 min did not lead to a significant increase of the degradation rate with the sequential application of photocatalysis; similar results have also been reported by other research groups [10,26]. The enhancement in the BB9 degradation by the combined effect of photocatalysis and sonolysis (sonophotocatalysis) can be explained by the increased of mass transport of chemical species between the solution phase and the catalyst surface and, the additional yields of OH radicals by acoustic cavitation [27,28]. Also, the continuous cleaning of the TiO2 surface by acoustic cavitation [4] might also have some role in modifying the photocatalytic rate. Additionally, the initial pH of the solution (plot B) influences the rate of the degradation with the process of sonophotocatalysis. High rate constants were obtained at pH 7 and pH 3 with higher rate constant values attained with the former, while the degradation rate decreased at pH 9. Lower rate constant values were progressively obtained with increasing the BB9 dye concentration at pH 3 and 7. However, very small decrease of the rate constant values obtained at pH 9 was observed with increasing the dye concentration. These results can be related to the adsorption of BB9 onto the TiO2 surface that is affected by the pH. The pollutant and thus the rates of degradation will be maximum near the point of zero charge (pzc) of the catalyst [4,29]. The point of zero charge (pzc) of TiO2 surface is at pHpzc = 7.1 [30]. This is in agreement with the higher degradation rates observed for the BB9 under sonophotocatalysis at pH 7. Shimizu et al. [31], observed an U-shaped change in the concentration of methylene blue in the presence of
Fig. 3. Rate constants of BB9 obtained at different (A) degradation processes and (B) pH solutions in the sonophotocatalysis process as a function of the initial dye concentration. 100 mg l1 TiO2.
A.S. González, S.S. Martínez / Ultrasonics Sonochemistry 15 (2008) 1038–1042
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Fig. 4. (A) Concentration profile plot and (B) First order kinetic plot obtained with 100 mg l1 TiO2 at pH 3 and 25 °C under . TiO2 adsorption, d Sonolysis, j Photocatalysis and Sonophotocatalysis, as a function of time during the degradation of 0.06 mM BB9. The solid lines (A) were obtained using the kinetic parameters obtained from the linear fit of plot (B).
Table 1 Kinetics for the degradation of 0.06 mM BB9 at pH 3 and 100 mg l1 TiO2 Degradation process
k (min1)
Linear correlation coefficient (R2)
TiO2 adsorption Sonolysis Photocatalysis Sonophotocatalysis
0.0019 ± 0.0002 0.00967 ± 0.0005 0.04135 ± 0.0019 0.0894 ± 0.0027
0.9704 0.9736 0.9762 0.9904
TiO2 along with the change in pH values (pH 3–12), and the highest degradation ratio was observed at around pH 7, in agreement with the results stated in this work. These authors studied the degradation of MB by the irradiation of ultrasound onto TiO2 in aqueous solution. 3.3. Degradation of BB9 under different experimental conditions Fig. 4 shows the concentration decay profile (plot A) and the kinetics (plot B) of a solution initially containing 0.06 mM BB9 as
a function of time under different experimental conditions in the presence of TiO2 (100 mg l1) at pH 3. A negligible degradation of the BB9 dye can be observed under dark conditions (.) with a color removal efficiency of 5% at 50 min of reaction time. The color removal efficiency increased up to 43% under sonolysis, 85% under photocatalysis and 97% under sonophotocatalysis at 50 min of irradiation (sound and/or light) time. The degradation of BB9 followed an apparent first order kinetic reaction. The rate constant obtained with sonophotocatalysis increased twofold and tenfold with respect to that obtained under photocatalysis and sonolysis, respectively, under similar conditions as shown in Table 1. Table 1 reports the kinetic parameters obtained for the degradation of BB9 under different treatment conditions. Chemical oxidation or degradation aims at the mineralization of the contaminants to carbon dioxide, water and inorganics or, at least, their transformation into harmless products. The degradation of the BB9 dye compound involves color removal and mineralization. The degree of mineralization was followed by measuring the COD during the BB9 sonophotocatalytic degradation. Fig. 5
Fig. 5. (A) Plot of Chemical Oxygen Demand abatement and (B) Plot of the absorption spectra as a function of irradiation (light and sound) time for a solution initially containing 0.06 mM BB9 and 100 mg l1 TiO2 suspension at pH 3.
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shows two plots that represent the COD and the variation of the absorption spectra of the 0.06 mM BB9 dye compound in a solution containing 100 mg l1 TiO2 suspension at different irradiation (light and sound) time at pH 3 during the sonophotocatalytic degradation. The COD was abated over 80% after 70 min of irradiation (light and sound) from its initial value and the absorption spectra of the dye chanced from 0 to 50 min. The shoulder at 612 nm disappeared, indicating a change in the molecular structure or composition of the dye. 4. Conclusions This study demonstrated that the color of basic blue 9 industrial textile dye was removed efficiently by sonophotocatalytic degradation and the COD was decreased over 80%. It was found that the sonophotocatalytic degradation reaction followed apparent first-order kinetics and was dependent on the pH solution with the highest degradation at pH 7. Acknowledgements The authors thank Secretaría de Educación Pública through PROMEP (Programa de Mejoramiento a Profesores) program for sponsorship this project and Baxter S.A., for providing the distilled water free of charge at any time it was needed. References [1] Y. Chen, K. Wang, L. Lou, J. Photochem. Photobiol. A: Chem. 163 (2004) 281. [2] J. Photochem. Photobiol. A: Chem. 159 (2003) 241. [3] H. Zollinger, Color Chemistry, Synthesis, Properties and Applications of Organic Dye and Pigments, VCH Publishers, NY, 1983.
[4] P.R. Gogate, A.B. Pandit, AIChE J. 50 (2004) 1051. [5] M.R. Dhananjeyan, J. Kiwi, P. Albers, O. Enea, Helv. Chim. Acta 84 (2000) 3433. [6] A.O. Martins, V.M. Canalli, C.M.N. Azevedo, M. Pires, Dye Pigment 68 (2006) 227. [7] B. Neppolian, H.C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, Chemosphere 46 (2002) 1173. [8] P.C. Fung, K.M. Sin, S.M. Tsui, Color. Technol. 116 (2000) 170. [9] S. Kaur, V. Singh, Ultrason. Sonochem. 14 (2007) 531. [10] E. Selli, Phys. Chem. Chem. Phys. 4 (2002) 6123. [11] L. Davydov, E.P. Reddy, P. France, P.G. Smirniotis, Appl. Catal. B: Environ. 32 (2001) 95. [12] S. Matsuzawa, J. Tanaka, S. Sato, T. Ibusuki, J. Photochem. Photobiol. A: Chem. 149 (2002) 183. [13] M. Mrowetz, C. Pirola, E. Selli, Ultrason. Sonochem. 10 (2003) 247. [14] P.R. Gogate, S. Mujumdar, A.B. Pandit, Ind. Eng. Chem. Res. 41 (2002) 3370. [15] M.A. Hasnat, I.A. Siddiquey, A. Nuruddin, Dye Pigment 66 (2005) 185. [16] K. Nagaveni, G. Sivalingam, M.S. Hegde, G. Madras, Appl. Catal. B: Environ. 48 (2004) 83. [17] F. Akbal, Environ. Prog. 24 (2005) 317. [18] G. Crini, H.N. Peindy, Dye Pigment 70 (2006) 204. [19] T. Jesionowski, Dye Pigment 67 (2005) 81. [20] S. Senthilkumaar, P.R. Varadarajan, K. Porkodi, C.V. Subbhuraam, J. Colloid Interface Sci. 284 (2005) 78. [21] H. Hameed, A.L. Ahmad, K.N.A. Latiff, Dye Pigment 75 (2007) 143. [22] E.N. El Qada, S.J. Allen, G.M. Walker, Chem. Eng. J. 124 (2006) 103. [23] T. An, G. Li, Y. Xiong, X. Zhu, H. Xing, G. Liu, Mater. Phys. Mech. 4 (2001) 101. [24] Water Analysis Handbook, 1992, Hach Company, Lovel and CO. [25] S. Parra, S.E. Stanca, I. Guanaquillo, K.R. Thampi, Appl. Catal. B: Environ. 51 (2004) 107. [26] P. Theron, P. Pichat, C. Petrier, C. Guillard, Water Sci. Technol. 44 (2001) 263. [27] K.S. Suslick, Sci. Am. 260 (1989) 80. [28] Y.Ch. Chen, A.V. Vorontsov, P.G. Smirniotis, Photochem. Photobiol. Sci. 2 (2003) 69. [29] V. Subramanian, V.G. Pangarkar, A.A.C.M. beenackers, Clean Products Process 2 (2000) 149. [30] A. Di-Paola, S. Ikeda, G. Marcì, B. Ohtani, L. Palmesano, Int. J. Photoenergy 3 (2001) 171. [31] N. Shimizu, C. Ogino, M.F. Dadjour, T. Murata, Ultrason. Sonochem. 14 (2007) 184.
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Study of the sonophotocatalytic degradation of basic blue 9 industrial textile dye over slurry titanium dioxide and influencing factors Antonia Sandoval González, Susana Silva Martínez * Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Avenve Universidad 1001, Col Chamilpa, Cuernavaca, Mor. C.P. 62209, Mexico
a r t i c l e
i n f o
Article history: Received 2 September 2007 Received in revised form 25 January 2008 Accepted 22 March 2008 Available online 29 March 2008 Keywords: Basic blue 9 Sonophotocatalysis Photocatalysis Sonolysis Methylene blue TiO2
a b s t r a c t The sonophotocatalytic degradation of basic blue 9 industrial textile dye has been studied in the presence of ultrasound (20 kHz) over a TiO2 slurry employing an UV lamp (15 W, 352 nm). It was observed that the color removal efficiency was influenced by the pH of the solution, initial dye concentration and TiO2 amount. It was found that the dye degradation followed apparent first order kinetics. The rate constant increased by decreasing dye concentration and was affected by the pH of the solution with the highest degradation obtained at pH 7. The first order rate constants obtained with sonophotocatalysis were twofold and tenfold than those obtained under photocatalysis and sonolysis, respectively. The chemical oxygen demand was abated over 80%. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Water pollution due to the release of dye compounds during the dyeing and finishing operations into textile effluents has recently become a major concern [1,2]. Colored wastewaters impose serious aesthetic and environmental problems because of their color and their high chemical oxygen demand. Furthermore, direct discharge of such effluents can cause the formation of toxic aromatic amines under anaerobic conditions in waters, and contaminate the soil and groundwater. An estimation of about 15% of the total world production of dyes is lost in wastewater stream during such operations [3]. Recently, several advanced oxidation processes have been developed and applied specifically to wastewater treatment. In order to increase the extent of reduction toxicity, hybrid methods constituting two or more individual processes have been developed [4]. Some of these include ultrasound/hydrogen peroxide (H2O2) or ultrasound/ozone, UV light/H2O2 or UV/ozone, sonophotocatalytic oxidation (the simultaneous use of ultrasound and photocatalysis), photo-Fenton processes and photocatalytic oxidation processes [5–10]. The use of sonophotocatalysis on semiconductors has recently attracted the attention of different research groups [9–14]. It is important to have simultaneous irradiation of ultrasound and UV light rather than the sequential operation because continuous cleaning of the catalyst plays a substantial role, * Corresponding author. Tel./fax: +52 777 329 70 84. E-mail address: [email protected] (S.S. Martínez). 1350-4177/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2008.03.008
as demonstrated by the work of Gogate and coworkers using formic acid degradation [14]. Selli [10] reported that the combination of photocatalysis and ultrasound showed synergistic effects on the degradation of acid orange 8 (azo dye) involving the aqueous phase due to the formation of H2O2 by ultrasound and not the photocatalyst–water interface. Mrowetz and collaborators [13] reported an enhancement in the rate constants of acid orange 8 and acid red 1 dye compounds during the sonophotocatalytic degradation where the synergism was more pronounced for the later. The purposes of this work were to study the degradation of the basic blue 9 dye (BB9), used by the local textile industry, by sonophotocatalysis in the presence of TiO2, and compare its degradation under sonolysis and photocatalysis. The effects of catalyst amount, dye concentration and pH on the sonophotocatalytic degradation were investigated. The experimental results were assessed in terms of chemical oxygen demand (COD) and color reduction to determine the overall treatment efficiency of the degradation processes. COD is an index of water pollution by organics and it is a parameter used for quality discharge. COD differences along the time are exclusively related to the degree of oxidation of the organic matter as a whole. The BB9 dye compound is known as methylene blue (C16H18ClN3 S) and its molecular structure is shown in the Scheme 1. This organic compound with large molecular structure is a water soluble cationic dye found in wastewaters and it is potentially carcinogenic. The photocatalytic degradation of MB dye compound has been studied by other research groups. Hasnat and coworkers [15]
A.S. González, S.S. Martínez / Ultrasonics Sonochemistry 15 (2008) 1038–1042
1039
Scheme 1.
investigated the photocatalytic degradation of methylene blue (MB) and procion red in TiO2 dispersion under visible light. The rate of degradation followed first-order kinetics based on the Langmuir– Hinshelwood model and reported that the MB photocatalytic degradation rate increased by increasing the pH. Nagaveni and collaborators [16] studied the photocatalytic degradation of MB employing synthesized nano TiO2 by combustion. A comparison of its degradation with commercial Degussa P-25 TiO2 was performed and higher degradation was reported under the combustion synthesized nano TiO2. Akbal [17] investigated the degradation of MB and methyl orange. This author reported that color removal efficiency was affected by the concentration of the dye, amount of TiO2 added and the pH of the solution. Other research groups have achieved the MB removal from aqueous solution under adsorption onto different materials. Crini and Peindy [18] used material containing carboxylic groups and reported that the adsorption kinetics followed an apparent-second order model and the best fit of the experimental data was for the Freundlich isotherm. Jesionowski [19] applied aminosilane-functionalised silica as an adsorbent and found an increased efficiency of MB adsorption as compared to the unmodified silica. The Activated carbon has also been employed for the removal of MB [20–22]. Finally, the photoelectrochemical degradation of MB has also been investigated with nano TiO2 under high potential bias [23]. High decolorization efficiency and chemical oxygen demand removal efficiency were reported. The majority of these studies have been carried out employing the MB dye compound as analytical grade. 2. Experimental 2.1. Chemicals All solutions were prepared with distilled water using sulfuric acid or sodium hydroxide (analytical grade) to adjust the pH of the solution. BB9 dye was provided by the local textile industry (industrial grade, Ciba Specialty Chemicals) and the titanium dioxide used was commercial (Degussa P25, analytical grade). All chemicals were used as received without further purification and purchased from Sigma–Aldrich. 2.2. Procedures All degradation tests were performed in a sonophotoreactor with volume of 300 ml and recirculation that permitted us to investigate the effects of sonolysis, photolysis and photocatalysis either separately or simultaneously. A cylindrical ultrasound cell with volume of 150 ml was jacketed with cooling water circulation to maintain the reaction temperature at 25 ± 3 °C (Fig. 1). An ultrasound probe (13 mm diameter) was dipped into the cell solution and was powered by an ultrasonic processor (Cole Parmer) set at an amplitude of 80% which corresponds to a power input of 76 W at a frequency of 20 kHz. A fast stream of oxygen was feed to the test solution trough the ultrasound cell. A 15 W UV lamp (cut-off wavelength 352 nm, 44 cm long with 3 cm diameter. Cole Parmer) hosted in a Pyrex jacket illuminated the cylindrical photoreactor. The test solution was recirculated inside the jacket of the photoreactor and the irradiation UV light was 5 mm away from it. This de-
Fig. 1. Schematic representation of the sonophotoreactor employed to carry out the experiments of sonolysis, photocatalysis and sonophotocatalysis under different experimental conditions.
sign allowed a complete penetration of the UV light in the test solution. The TiO2 slurry was uniformly transported by the recirculation of the test solution throughout the sonophotoreactor in the photocatalytic and sonophotocatalytic experiments. Samples (2 ml) were withdrawn from the sonophotoreactor at different irradiation (light and/or sound) time intervals to measure both the absorption spectra in the wavelength interval of 1 nm from 300 to 800 nm with a spectrophotometer (DR/4000 U HACH) and the chemical oxygen demand (COD). The maximum absorption wavelength of BB9 was found at 663 nm. The absorbance measurements at 663 nm were employed to calculate the dye concentration from the calibration curve (absorbance vs. BB9 concentration) built at the corresponding pH of the aqueous solution. During sampling, care was taken to withdraw a volume of less than 10% of the initial volume. TiO2 was separated from the samples by centrifugation followed by filtration through a Millipore membrane as collected prior the analysis. The COD was analyzed using standard method and standard tubes [24] inside the concentration range of 0– 40 mg l1 COD. The COD method involves a reaction between the organic matter and the dichromate ion in a 50% sulfuric acid solution. The results are reported as an average value obtained from two or three analyses of the samples during degradation. 3. Results and discussion 3.1. Effect of catalyst addition In order to determine the effect of catalyst addition on color removal efficiency, experiments were performed with different catalyst amounts using an initial concentration of 0.06 mM of textile dye BB9. The amount of catalyst is an important parameter that can affect the degradation rate of organic compounds. The optimal catalyst concentration reported in the literature for TiO2 Degussa P25 ranges from 0.1 to 5.0 g l1, depending on the nature of the compounds and the photoreactor geometry [25]. Fig. 2 shows the effect of catalyst amount on the rate of sonophotocatalytic degradation as a function of TiO2 amount at pH 3. It can be observed that the degradation rate increases with increasing the amount of the catalyst until it reaches a plateau at 100 mg l1 of TiO2 and becomes independent of the amount of catalyst. Accordingly, the concentration of 100 mg l1 of catalyst was selected to perform further the degradation of BB9 dye at different operational parameters. The rate constants were calculated considering the concentration values obtained from the maxima absorbance measured at 663 nm as a function of degradation time.
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3.2. Effect of initial dye concentration and pH solution
Fig. 2. Rate constants of 0.06 mM BB9 for the sonophotocatalytic degradation as a function of TiO2 amount, at pH 3 and 25 °C. The inset shows the percentage of dye color removal.
The color removal (CR, %) or decolorization was calculated using the following equation: " !, # 700 nm 700 nm 700 nm X X X AbsBt AbsAt AbsBt 100 CR;% ¼ ð1Þ 400 nm
400 nm
400 nm
where AbsBt and AbsAt correspond to the absorbance of the BB9 aqueous solutions recorded before and after treatment, respectively, inside the wavelength range of 400–700 nm. The color removal efficiency increased from 89 to 97% at 50 min of irradiation time when the catalyst concentration increased from 50 to 100 mg l1, as a result of an increase in the number of photons and TiO2 particles that increase the number of dye molecules adsorbed onto the catalyst. The color removal was not increased further with catalyst concentration over 100 mg l1 (inset of Fig. 2).
The effect of different initial dye concentration at constant pH and different initial pH on the degradation rate was studied in 100 mg l1 TiO2 suspensions. Fig. 3, reports the apparent first order rate constants obtained under different experimental conditions. It is observed that the reaction rate decreased with increasing the initial BB9 dye concentration, although this is much less so for sonolysis. The degradation of BB9 was much faster under both sonophotocatalysis and photocatalysis, having higher rate constant values for the former. These results clearly demonstrated that the combination of photocatalysis and sonolysis produced an additive effect on the degradation rates as shown by those obtained by sonophotocatalysis. This additive effect was observed in all the investigated range of initial dye concentrations. Additional experiments revealed that the sonication of the catalyst previous to the addition of BB9 up to 45 min did not lead to a significant increase of the degradation rate with the sequential application of photocatalysis; similar results have also been reported by other research groups [10,26]. The enhancement in the BB9 degradation by the combined effect of photocatalysis and sonolysis (sonophotocatalysis) can be explained by the increased of mass transport of chemical species between the solution phase and the catalyst surface and, the additional yields of OH radicals by acoustic cavitation [27,28]. Also, the continuous cleaning of the TiO2 surface by acoustic cavitation [4] might also have some role in modifying the photocatalytic rate. Additionally, the initial pH of the solution (plot B) influences the rate of the degradation with the process of sonophotocatalysis. High rate constants were obtained at pH 7 and pH 3 with higher rate constant values attained with the former, while the degradation rate decreased at pH 9. Lower rate constant values were progressively obtained with increasing the BB9 dye concentration at pH 3 and 7. However, very small decrease of the rate constant values obtained at pH 9 was observed with increasing the dye concentration. These results can be related to the adsorption of BB9 onto the TiO2 surface that is affected by the pH. The pollutant and thus the rates of degradation will be maximum near the point of zero charge (pzc) of the catalyst [4,29]. The point of zero charge (pzc) of TiO2 surface is at pHpzc = 7.1 [30]. This is in agreement with the higher degradation rates observed for the BB9 under sonophotocatalysis at pH 7. Shimizu et al. [31], observed an U-shaped change in the concentration of methylene blue in the presence of
Fig. 3. Rate constants of BB9 obtained at different (A) degradation processes and (B) pH solutions in the sonophotocatalysis process as a function of the initial dye concentration. 100 mg l1 TiO2.
A.S. González, S.S. Martínez / Ultrasonics Sonochemistry 15 (2008) 1038–1042
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Fig. 4. (A) Concentration profile plot and (B) First order kinetic plot obtained with 100 mg l1 TiO2 at pH 3 and 25 °C under . TiO2 adsorption, d Sonolysis, j Photocatalysis and Sonophotocatalysis, as a function of time during the degradation of 0.06 mM BB9. The solid lines (A) were obtained using the kinetic parameters obtained from the linear fit of plot (B).
Table 1 Kinetics for the degradation of 0.06 mM BB9 at pH 3 and 100 mg l1 TiO2 Degradation process
k (min1)
Linear correlation coefficient (R2)
TiO2 adsorption Sonolysis Photocatalysis Sonophotocatalysis
0.0019 ± 0.0002 0.00967 ± 0.0005 0.04135 ± 0.0019 0.0894 ± 0.0027
0.9704 0.9736 0.9762 0.9904
TiO2 along with the change in pH values (pH 3–12), and the highest degradation ratio was observed at around pH 7, in agreement with the results stated in this work. These authors studied the degradation of MB by the irradiation of ultrasound onto TiO2 in aqueous solution. 3.3. Degradation of BB9 under different experimental conditions Fig. 4 shows the concentration decay profile (plot A) and the kinetics (plot B) of a solution initially containing 0.06 mM BB9 as
a function of time under different experimental conditions in the presence of TiO2 (100 mg l1) at pH 3. A negligible degradation of the BB9 dye can be observed under dark conditions (.) with a color removal efficiency of 5% at 50 min of reaction time. The color removal efficiency increased up to 43% under sonolysis, 85% under photocatalysis and 97% under sonophotocatalysis at 50 min of irradiation (sound and/or light) time. The degradation of BB9 followed an apparent first order kinetic reaction. The rate constant obtained with sonophotocatalysis increased twofold and tenfold with respect to that obtained under photocatalysis and sonolysis, respectively, under similar conditions as shown in Table 1. Table 1 reports the kinetic parameters obtained for the degradation of BB9 under different treatment conditions. Chemical oxidation or degradation aims at the mineralization of the contaminants to carbon dioxide, water and inorganics or, at least, their transformation into harmless products. The degradation of the BB9 dye compound involves color removal and mineralization. The degree of mineralization was followed by measuring the COD during the BB9 sonophotocatalytic degradation. Fig. 5
Fig. 5. (A) Plot of Chemical Oxygen Demand abatement and (B) Plot of the absorption spectra as a function of irradiation (light and sound) time for a solution initially containing 0.06 mM BB9 and 100 mg l1 TiO2 suspension at pH 3.
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shows two plots that represent the COD and the variation of the absorption spectra of the 0.06 mM BB9 dye compound in a solution containing 100 mg l1 TiO2 suspension at different irradiation (light and sound) time at pH 3 during the sonophotocatalytic degradation. The COD was abated over 80% after 70 min of irradiation (light and sound) from its initial value and the absorption spectra of the dye chanced from 0 to 50 min. The shoulder at 612 nm disappeared, indicating a change in the molecular structure or composition of the dye. 4. Conclusions This study demonstrated that the color of basic blue 9 industrial textile dye was removed efficiently by sonophotocatalytic degradation and the COD was decreased over 80%. It was found that the sonophotocatalytic degradation reaction followed apparent first-order kinetics and was dependent on the pH solution with the highest degradation at pH 7. Acknowledgements The authors thank Secretaría de Educación Pública through PROMEP (Programa de Mejoramiento a Profesores) program for sponsorship this project and Baxter S.A., for providing the distilled water free of charge at any time it was needed. References [1] Y. Chen, K. Wang, L. Lou, J. Photochem. Photobiol. A: Chem. 163 (2004) 281. [2] J. Photochem. Photobiol. A: Chem. 159 (2003) 241. [3] H. Zollinger, Color Chemistry, Synthesis, Properties and Applications of Organic Dye and Pigments, VCH Publishers, NY, 1983.
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