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Photocatalytic decolorization of Remazol Red RR in aqueous ZnO suspensions
Applied Catalysis B: Environmental 54 (2004) 19–24
Photocatalytic decolorization of Remazol Red RR in aqueous ZnO suspensions A. Akyol a , H.C. Yatmaz a,∗ , M. Bayramoglu b a
Environmental Engineering Department, Gebze Institute of Technology, 41400 Gebze, Turkey b Chemical Engineering Department, Istanbul University, 34850, Avcilar, Istanbul, Turkey Received 10 August 2003; received in revised form 25 April 2004; accepted 31 May 2004 Available online 22 July 2004
Abstract The photocatalytic decolorization of aqueous solutions of Remazol Red RR, a commercial azo-reactive textile dye, in the presence of various semiconductor powder suspensions has been investigated in a quartz batch reactor with the use of artificial light sources (UV-C). ZnO and TiO2 have been found the most active photocatalysts; however ZnO indicated slightly higher efficiency. The effects of various process variables on decolorization performance of the process have been investigated. The results showed that the decolorization efficiency increases with increase in pH, attaining maximum value at pH 10 for ZnO. The zero-point charge for ZnO is 9.0 above which ZnO surface is negatively charged by adsorbed OH− ions, favoring the formation of strong oxidant OH• radicals. The efficiency is inversely related to the dye concentration; increasing dye concentration enhances dye adsorption on the active sites of the catalyst surface, and consequently hinders OH− adsorption on the same sites, this results with a decreasing OH• formation rate. © 2004 Elsevier B.V. All rights reserved. Keywords: Photocatalysis; Decolorization; Azo-reactive dye; ZnO; TiO2 ; Textile wastewater
1. Introduction Large quantities of dyes used in the textile industry are lost to the effluents during manufacturing and processing operations [1]. These colored dye effluents create severe environmental pollution problems by releasing toxic and potential carcinogenic substances into the aquasphere. Since the increased public concern with these pollutants, international environmental standards are becoming more stringent; therefore new treatment methods are required for the removal of persistent dye organic chemicals or converting them to harmless compounds in water. Several studies have been carried out for biological, physical and chemical treatment of dye containing effluents [2–5]. Among these, biodegradation, adsorption, chlorination and ozonation are the most commonly used conventional methods. Dyes are usually resistant to aerobic degradation and carcinogenic compounds may be generated during the anaerobic treatment, for example aromatic amines from azo dyes; in these respects, ∗ Corresponding author. Tel.: +90 262 754 2360; fax: +90 262 653 8490. E-mail address: [email protected] (H.C. Yatmaz).
0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.05.021
bio-treatment alone has been found to be ineffective for the treatment of dye effluents [4]. On the other hand, physical methods such as flocculation, reverse osmosis and adsorption are not destructive and mainly create pollutant concentrated phases. Furthermore, chemical treatment using chlorine or ozone has led to more successful results, but since the required high dosages are not found economically feasible [2]. Recent developments of advanced oxidation processes (AOPs), have led to new improvements of the oxidative degradation of the organic compounds. UV radiation in the presence of H2 O2 has yielded encouraging results of color removal from azo-reactive dye containing waters [5]. Heterogeneous photocatalysis has emerged an important destructive technology leading to the total mineralization of most of the organic pollutants including organic reactive dyes [6–13]. The reason photocatalysis attracts increased interest is that the process may use atmospheric oxygen as the oxidant and can be carried out under ambient conditions. Moreover, the process utilizes semiconductor catalysts such as TiO2 , ZnO which are largely available, inexpensive, nontoxic and leads to total mineralization of organic chemicals to CO2 , water and mineral acids. Electrochemically assisted
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photocatalytic process is also investigated to degrade reactive azo dyes [14,15]. TiO2 is the most commonly used effective photocatalyst for a wide range of organic chemical degradation. ZnO is another semiconductor, which is investigated in recent years, as a potential photocatalyst. In comparative studies for the treatment of cellulose bleaching effluents and textile mill effluents, TiO2 was found to be more efficient for cellulose bleaching effluents and for some organic dyes [16–20], meanwhile, ZnO has emerged as a more efficient catalyst for the destruction of azo-reactive dyes [21–24]. Moreover, visible light-induced degradation of an azo dye on coupled TiO2 /CdO-ZnO nanoporous films was also investigated to show higher photocatalytic activity [25]. Studies performed with ZnO powders with different morphologies indicated that the particle crystallinity, rather than the surface area, significantly affects its photocatalytic activity [26]. In this study, various semiconductor photocatalysts (ZnO, TiO2 , SnO2 and SnO) were compared for the decolorization efficiency of aqueous solution of a commercial textile dye, Remazol Red RR. After the selection of the most active catalyst, subsequent experiments were conducted to investigate the effects of various process variables on the process performance.
2.2. Apparatus Schematic representation of the experimental setup is shown in Fig. 1. The laboratory-scale quartz photoreactor was designed in column shaped in 25 cm height and 4.5 cm diameter. Air was blown into the reaction medium by an air pump at a flow rate of 150 mL/min, to maintain the solution saturated with oxygen during the course of the reaction. Stirring is applied at 600 rpm to ensure a complete suspension of catalyst particles. The reactor was surrounded by six UV lamps, which predominantly emit at 254 nm (6 W, Philips TUV G6T5), positioned so as to ensure homogenous radiation field inside the reactor. Blowing cooled air between the lamps and the quartz reactor eliminated heat effect of the lamps, thus the temperature of the reaction medium was maintained constant at 25 ◦ C within ±0.2 ◦ C. The intensity of irradiation entering the quartz reactor was measured by a chemical actinometric method using potassium ferrioxalate (K3 Fe(C2 O4 )3 ) [27]. The actinometer solution was irradiated under conditions similar to those used for photoreaction. Light intensity of all six lamps measured using 300 mL of the actinometer solution was found as 1.6 × 10−6 Einstein/L s. 2.3. Experimental procedure and analysis
2. Experimental 2.1. Materials Azo-reactive dye, Remazol red RR, was obtained from DyStar (Germany). The characteristics of the dye were provided as monoazo type and reactive groups were given as vinylsulphonyl (VS) and monohalogentriazine (MHT) by DyStar [5]. The photocatalysts were obtained from different sources and were used as received. TiO2 is from Degussa (P-25) as average primary particle size 21 nm, specific surface area (BET) 50 ± 15 m2 /g, ZnO (BET, <5 m2 /g) and SnO2 from Merck and SnO from Riedel-de Haen.
The batch experiments were carried out with 300 mL dye solutions prepared in appropriate concentrations using deionized water. Dye solutions were stirred in the dark for 30 min after the addition of the catalyst. 5 mL samples of suspension were withdrawn at regular intervals and were immediately centrifuged at 3500 rpm for 10 min to completely remove catalyst particles. The progress of photocatalytic decolorization was monitored by measuring the absorbance of the solution samples with UV–vis spectrophotometer (Shimadzu UV 2101 PC), at λmax = 525 nm. For exploring the effect of the pH, the solution’s pH was adjusted initially by adding 0.01N NaOH or 0.01N H2 SO4 , otherwise the experiments were carried at the original pH of the solution.
Fig. 1. Experimental setup in the photocatalytic experiments: (A) lamp assembly, (B) quartz batch reactor, (C) UV lamps, (D, E) air inlets, (F) flowmeter, (G) stirrer, (H) sampling point, (I) ice bath, (J) switches for power supply.
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3. Results and discussion Catalyst type, catalyst loading, initial pH and initial dye concentration were investigated for their effects on the efficiency of the decolorization process, defined as Efficiency = 100 ×
(C0 − C) C0
(1)
where C0 is the initial dye concentration. 3.1. Catalyst type Initially, blank experiments were performed under UV radiation without addition of any catalyst and negligible decolorization efficiency was observed. To enhance the efficiency, different catalysts were tried under various process conditions. Typical results are shown in Fig. 2 at dye concentration = 50 mg/L, initial solution pH = 6, catalyst loading = 1 g/L, reaction time = 45 min. The results showed that ZnO exhibits higher photocatalytic activity than the others, especially TiO2 , the same trend was also obtained in other studies with azo-reactive dyes [23,24], and this was explained as ZnO having a greater quantum efficiency than TiO2 . The low cost is another important advantage of ZnO. On the other hand, SnO2 and SnO exhibit less activity, because the light energy is not sufficient to excite these catalysts due to their wide band gap energies. Beside the band gap energy, the charge carrier density, as well as the crystal structure and the crystallinity may have also important impacts on the photocatalyst activity. Similar runs carried out for comparative purpose with two different ZnO samples, from Merck and Riedel-de Haen, revealed that approximately 5% higher efficiency was obtained with ZnO Merck sample. Thus, subsequent experiments were carried out with Merck ZnO.
Fig. 3. (a) Efficiency–time curves for various catalyst loadings. (b) Efficiency as a function of catalyst loading, at 25 min.
3.2. Catalyst loading To determine the effect of the catalyst loading, a series of experiments were carried out by varying the amount of catalyst from 0.5 to 3.5 g/L (dye concentration = 150 mg/L, solution pH = 6.5). The decolorization efficiencies for various catalyst loadings are depicted in Fig. 3a, as a function of time. As seen, initial slopes of the curves increase greatly by increasing catalyst loading from 0.5 to 1.5 g/L, above which initial slopes are nearly equal. As shown also in Fig. 3b, for a constant reaction time, e.g. 25 min, the decolorization efficiency exhibits a sharp increase by increasing loading up to 1.5–2 g/L, and the slope of the curve decrease with further increase in catalyst loading up to 3.5 g/L. At lower loading levels, such as 0.5 g/L, the catalyst surface and absorption of light by the catalyst surface are the limiting factors, and an increase in catalyst loading greatly enhances the process efficiency. On the other hand, at higher loading levels, irradiation field inside the reaction medium is reduced due to the light scattering by catalyst particles. 3.3. Initial pH
Fig. 2. The effect of the catalyst type on the decolorization efficiency.
The role of pH on the decolorization efficiency was studied in the pH range 5–11 at 100 mg/L dye concentration and
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Fig. 4. Effect of initial pH.
Fig. 5. Effect of initial dye concentration.
1.5 g/L catalyst loading. The pH of the solution is adjusted before irradiation and it is not controlled during the course of the reaction. A slight decrease in the solution pH, approximately 0.5 pH units, is observed at the end of 30 min. The results in Fig. 4 show that the rate of decolorization increases with increase in pH, exhibiting maximum rate at pH 10. Using azo dye of acid brown 14, Sakthivel et al. [23] have observed similar trend with ZnO, commenting that acid–base property of the metal oxide surfaces can have considerable implications upon their photocatalytic activity. The zero-point charge for ZnO is 9.0 and above this value, ZnO surface is negatively charged by means of adsorbed OH− ions. The presence of large quantities of OH− ions on the particle surfaces as well as in the reaction medium favors the formation of OH• radical, which is now widely accepted as the principal oxidizing specie responsible for the dye decolorization process. Lizama et al. [22] also found out that pH 11 was the optimum value for RB-19 dye removal. They commented that textile processes using reactive dyes produce effluents with high pH, suitable for the use of ZnO as a catalyst.
entering the solution decreases, which result in lower photon absorption on catalyst particles, and consequently lower photocatalytic reaction rates. The progress in the absorbance spectrums of the dye solution during the reaction was also monitored with initial 100 mg/L dye concentration and 1.5 g/L catalyst loading at pH 6. As seen in Fig. 6, the reductions in three absorbance peaks at 280, 375 and 525 nm, indicate the degradation of the dye molecule to smaller intermediates. On the other hand, no new absorption peaks appear during the reaction; these support the hypothesis that intermediate products formed during the dye degradation are also successfully degraded towards to a complete mineralization; approximately 75% decrease in the absorbance under 300 nm, is also a strong indication of the degradation of the intermediates. The COD and TOC of the solution must also be monitored during the reaction for a complete elucidation of the degradation mechanism. Meanwhile, it is clear that a total mineralization needs a longer reaction time than 30 min, which may be sufficient for a successful decolorization process.
3.4. Initial dye concentration The effect of initial dye concentration on the decolorization efficiency was investigated by varying the initial concentration from 50 to 200 mg/L at constant catalyst loading (1.5 g/L and at solution pH between 6 and 6.5) and results are shown in Fig. 5. As seen in the figure, decolorization efficiency is inversely affected by the dye concentration. This negative effect can be commented as follows; as the dye concentration is increased, the equilibrium adsorption of dye on the catalyst surface active sites increases, hence competitive adsorption of OH− on the same sites decreases, which means a lower formation rate of OH• radical which is the principal oxidant indispensable for a high degradation efficiency. On the other hand, considering Beer–Lambert law, as the initial dye concentration increases, the path length of photons
Fig. 6. Typical absorbance spectrums of the dye solution during the course of the reaction.
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tively charged by means of adsorbed OH− ions; this favors the formation of OH• radical, the principal oxidizing species responsible for the dye decolorization process. 3. Decolorization efficiency is inversely related to the dye concentration. By increasing dye concentration, the equilibrium adsorption of dye on the catalyst surface sites increases, and consequently OH• formation rate decreases due to the hindered OH− adsorption on the same sites. Furthermore, the path length of photons entering the solution decreases also according to Beer–Lambert law, which result in lower photon absorption on catalyst particles.
Fig. 7. Effect of light power on the decolorization efficiency.
3.5. Effect of light intensity The influence of light intensity on the degradation efficiency has been examined at constant dye concentration (150 mg/L, pH = 6.5) and catalyst loading (1.5 g/L). The light intensity is simply altered by varying nominal light power (12 W–24 W–36 W). As seen in Fig. 7, the efficiency increases monotonically with increasing light power; meanwhile, this dependency is not linear as revealed by a regression analysis. It is hoped that kinetic modeling and mechanistic studies will elucidate the effect of the light intensity, as well as the other ones, on the process efficiency.
4. Conclusion In this study, photocatalytic decolorization of an azo-reactive textile dye, Remazol red RR, has been investigated using various semiconductor metal oxide catalysts. ZnO powder has been found as the most active catalyst, exhibiting slightly higher activity than TiO2 . Subsequent experiments are conducted with ZnO to investigate its photocatalytic activity under various process conditions. It has been found that: 1. At lower catalyst loadings, such as 0.5 g/L, the catalyst surface and absorption of light on catalyst surface are the limiting factors, thus, an increase in catalyst loading greatly enhances the process efficiency. At high loadings, on the other hand, irradiation field inside the reaction medium is reduced due to the light scattering by catalyst particles. The impact of catalyst loading on the process efficiency is stronger at low dye concentrations. 2. The decolorization efficiency increases with increase in pH, attaining maximum value at pH 10. The zero-point charge for ZnO is 9.0 above which ZnO surface is nega-
Acknowledgements This study is part of research projects supported by Gebze Institute of Technology Research Fund (Nos. 01-A-03-05-17 and 02-A-03-03-01).
References [1] A. Reife, H.S. Fremann, Environmental Chemistry of Dyes and Pigments, Wiley, 1996. [2] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Bioresource Technol. 77 (2001) 247. [3] P.P. Zamora, A. Kunz, S.G. Moraes, R. Pelegrini, P.C. Moleiro, J. Reyes, N. Duran, Chemosphere 38 (1999) 835. [4] S. Ledakowicz, M. Solecka, R. Zylla, J. Biotechnol. 89 (2001) 175. [5] D. Georgiou, P. Melidis, A. Aivasidis, K. Gimouhopoulos, Dyes Pigments 52 (2002) 69. [6] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard, J.M. Herrmann, Appl. Catal. B: Environ. 39 (2002) 75. [7] M. Saquib, M. Muneer, Dyes Pigments 53 (2002) 237. [8] M. Stylidi, D.I. Kondarides, X.E. Verykios, Appl. Catal. B: Environ. 40 (2003) 271. [9] K. Tanaka, K. Padermpole, T. Hisanaga, Water Res. 34 (2000) 327. [10] M. Kaneko, I. Okura (Eds.), Photocatalysis: Science and Technology, Springer-Verlag, Japan, 2002. [11] C. Hu, J.C. Yu, Z. Hao, P.K. Wong, Appl. Catal. B: Environ. 42 (2003) 47. [12] C.G. Silva, J.L. Faria, J. Photochem. Photobiol. A: Chem. 155 (2003) 133. [13] C. Hachem, F. Bocquillon, O. Zahraa, M. Bouchy, Dyes Pigments 49 (2001) 117. [14] M.V.B. Zanoni, J.J. Sene, M.A. Anderson, J. Photochem. Photobiol. A: Chem. 157 (2003) 55. [15] R. Pelegrini, P.P. Zamora, A.R. Andrade, J. Reyes, N. Duran, Appl. Catal. B: Environ. 22 (1999) 83. [16] P.P. Zamora, S.G. Moraes, R. Pelegrini, M. Freire Jr., J. Reyes, H. Manslla, N. Duran, Chemosphere 36 (1998) 2119. [17] M.C. Yeber, J. Rodriguez, J. Freer, N. Duran, H.D. Mansilla, Chemosphere 41 (2000) 1193. [18] B. Neppolian, H.C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, J. Hazard. Mater. B 89 (2002) 303. [19] I. Poulios, E. Micropoulou, R. Panou, E. Kostopoulou, Appl. Catal. B: Environ. 41 (2003) 345. [20] M.L. Curri, R. Comparelli, P.D. Cozzoli, G. Mascolo, A. Agostiano, Mater. Sci. Eng. C 23 (2003) 285. [21] C.A.K. Gouvea, F. Wypych, S.G. Moraes, N. Duran, N. Nagata, P.P. Zamora, Chemosphere 40 (2000) 433.
24
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[22] C. Lizama, J. Freer, J. Baeza, H.D. Mansilla, Catal. Today 76 (2002) 235. [23] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energy Mater. Sol. C 77 (2003) 65. [24] N. Daneshvar, D. Salari, A.R. Khataee, J. Photochem. Photobiol. A: Chem. 162 (2004) 317.
[25] R.S. Parra, I.H. Perez, M.E. Rincon, S.L. Ayala, M.C.R. Ahumada, Sol. Energy Mater. Sol. C 76 (2003) 189. [26] D. Li, H. Haneda, Chemosphere 51 (2003) 129. [27] J.G. Calvert, J.N. Pits, Photochemistry, Wiley, New York, USA, 1966.
Photocatalytic decolorization of Remazol Red RR in aqueous ZnO suspensions A. Akyol a , H.C. Yatmaz a,∗ , M. Bayramoglu b a
Environmental Engineering Department, Gebze Institute of Technology, 41400 Gebze, Turkey b Chemical Engineering Department, Istanbul University, 34850, Avcilar, Istanbul, Turkey Received 10 August 2003; received in revised form 25 April 2004; accepted 31 May 2004 Available online 22 July 2004
Abstract The photocatalytic decolorization of aqueous solutions of Remazol Red RR, a commercial azo-reactive textile dye, in the presence of various semiconductor powder suspensions has been investigated in a quartz batch reactor with the use of artificial light sources (UV-C). ZnO and TiO2 have been found the most active photocatalysts; however ZnO indicated slightly higher efficiency. The effects of various process variables on decolorization performance of the process have been investigated. The results showed that the decolorization efficiency increases with increase in pH, attaining maximum value at pH 10 for ZnO. The zero-point charge for ZnO is 9.0 above which ZnO surface is negatively charged by adsorbed OH− ions, favoring the formation of strong oxidant OH• radicals. The efficiency is inversely related to the dye concentration; increasing dye concentration enhances dye adsorption on the active sites of the catalyst surface, and consequently hinders OH− adsorption on the same sites, this results with a decreasing OH• formation rate. © 2004 Elsevier B.V. All rights reserved. Keywords: Photocatalysis; Decolorization; Azo-reactive dye; ZnO; TiO2 ; Textile wastewater
1. Introduction Large quantities of dyes used in the textile industry are lost to the effluents during manufacturing and processing operations [1]. These colored dye effluents create severe environmental pollution problems by releasing toxic and potential carcinogenic substances into the aquasphere. Since the increased public concern with these pollutants, international environmental standards are becoming more stringent; therefore new treatment methods are required for the removal of persistent dye organic chemicals or converting them to harmless compounds in water. Several studies have been carried out for biological, physical and chemical treatment of dye containing effluents [2–5]. Among these, biodegradation, adsorption, chlorination and ozonation are the most commonly used conventional methods. Dyes are usually resistant to aerobic degradation and carcinogenic compounds may be generated during the anaerobic treatment, for example aromatic amines from azo dyes; in these respects, ∗ Corresponding author. Tel.: +90 262 754 2360; fax: +90 262 653 8490. E-mail address: [email protected] (H.C. Yatmaz).
0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.05.021
bio-treatment alone has been found to be ineffective for the treatment of dye effluents [4]. On the other hand, physical methods such as flocculation, reverse osmosis and adsorption are not destructive and mainly create pollutant concentrated phases. Furthermore, chemical treatment using chlorine or ozone has led to more successful results, but since the required high dosages are not found economically feasible [2]. Recent developments of advanced oxidation processes (AOPs), have led to new improvements of the oxidative degradation of the organic compounds. UV radiation in the presence of H2 O2 has yielded encouraging results of color removal from azo-reactive dye containing waters [5]. Heterogeneous photocatalysis has emerged an important destructive technology leading to the total mineralization of most of the organic pollutants including organic reactive dyes [6–13]. The reason photocatalysis attracts increased interest is that the process may use atmospheric oxygen as the oxidant and can be carried out under ambient conditions. Moreover, the process utilizes semiconductor catalysts such as TiO2 , ZnO which are largely available, inexpensive, nontoxic and leads to total mineralization of organic chemicals to CO2 , water and mineral acids. Electrochemically assisted
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A. Akyol et al. / Applied Catalysis B: Environmental 54 (2004) 19–24
photocatalytic process is also investigated to degrade reactive azo dyes [14,15]. TiO2 is the most commonly used effective photocatalyst for a wide range of organic chemical degradation. ZnO is another semiconductor, which is investigated in recent years, as a potential photocatalyst. In comparative studies for the treatment of cellulose bleaching effluents and textile mill effluents, TiO2 was found to be more efficient for cellulose bleaching effluents and for some organic dyes [16–20], meanwhile, ZnO has emerged as a more efficient catalyst for the destruction of azo-reactive dyes [21–24]. Moreover, visible light-induced degradation of an azo dye on coupled TiO2 /CdO-ZnO nanoporous films was also investigated to show higher photocatalytic activity [25]. Studies performed with ZnO powders with different morphologies indicated that the particle crystallinity, rather than the surface area, significantly affects its photocatalytic activity [26]. In this study, various semiconductor photocatalysts (ZnO, TiO2 , SnO2 and SnO) were compared for the decolorization efficiency of aqueous solution of a commercial textile dye, Remazol Red RR. After the selection of the most active catalyst, subsequent experiments were conducted to investigate the effects of various process variables on the process performance.
2.2. Apparatus Schematic representation of the experimental setup is shown in Fig. 1. The laboratory-scale quartz photoreactor was designed in column shaped in 25 cm height and 4.5 cm diameter. Air was blown into the reaction medium by an air pump at a flow rate of 150 mL/min, to maintain the solution saturated with oxygen during the course of the reaction. Stirring is applied at 600 rpm to ensure a complete suspension of catalyst particles. The reactor was surrounded by six UV lamps, which predominantly emit at 254 nm (6 W, Philips TUV G6T5), positioned so as to ensure homogenous radiation field inside the reactor. Blowing cooled air between the lamps and the quartz reactor eliminated heat effect of the lamps, thus the temperature of the reaction medium was maintained constant at 25 ◦ C within ±0.2 ◦ C. The intensity of irradiation entering the quartz reactor was measured by a chemical actinometric method using potassium ferrioxalate (K3 Fe(C2 O4 )3 ) [27]. The actinometer solution was irradiated under conditions similar to those used for photoreaction. Light intensity of all six lamps measured using 300 mL of the actinometer solution was found as 1.6 × 10−6 Einstein/L s. 2.3. Experimental procedure and analysis
2. Experimental 2.1. Materials Azo-reactive dye, Remazol red RR, was obtained from DyStar (Germany). The characteristics of the dye were provided as monoazo type and reactive groups were given as vinylsulphonyl (VS) and monohalogentriazine (MHT) by DyStar [5]. The photocatalysts were obtained from different sources and were used as received. TiO2 is from Degussa (P-25) as average primary particle size 21 nm, specific surface area (BET) 50 ± 15 m2 /g, ZnO (BET, <5 m2 /g) and SnO2 from Merck and SnO from Riedel-de Haen.
The batch experiments were carried out with 300 mL dye solutions prepared in appropriate concentrations using deionized water. Dye solutions were stirred in the dark for 30 min after the addition of the catalyst. 5 mL samples of suspension were withdrawn at regular intervals and were immediately centrifuged at 3500 rpm for 10 min to completely remove catalyst particles. The progress of photocatalytic decolorization was monitored by measuring the absorbance of the solution samples with UV–vis spectrophotometer (Shimadzu UV 2101 PC), at λmax = 525 nm. For exploring the effect of the pH, the solution’s pH was adjusted initially by adding 0.01N NaOH or 0.01N H2 SO4 , otherwise the experiments were carried at the original pH of the solution.
Fig. 1. Experimental setup in the photocatalytic experiments: (A) lamp assembly, (B) quartz batch reactor, (C) UV lamps, (D, E) air inlets, (F) flowmeter, (G) stirrer, (H) sampling point, (I) ice bath, (J) switches for power supply.
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3. Results and discussion Catalyst type, catalyst loading, initial pH and initial dye concentration were investigated for their effects on the efficiency of the decolorization process, defined as Efficiency = 100 ×
(C0 − C) C0
(1)
where C0 is the initial dye concentration. 3.1. Catalyst type Initially, blank experiments were performed under UV radiation without addition of any catalyst and negligible decolorization efficiency was observed. To enhance the efficiency, different catalysts were tried under various process conditions. Typical results are shown in Fig. 2 at dye concentration = 50 mg/L, initial solution pH = 6, catalyst loading = 1 g/L, reaction time = 45 min. The results showed that ZnO exhibits higher photocatalytic activity than the others, especially TiO2 , the same trend was also obtained in other studies with azo-reactive dyes [23,24], and this was explained as ZnO having a greater quantum efficiency than TiO2 . The low cost is another important advantage of ZnO. On the other hand, SnO2 and SnO exhibit less activity, because the light energy is not sufficient to excite these catalysts due to their wide band gap energies. Beside the band gap energy, the charge carrier density, as well as the crystal structure and the crystallinity may have also important impacts on the photocatalyst activity. Similar runs carried out for comparative purpose with two different ZnO samples, from Merck and Riedel-de Haen, revealed that approximately 5% higher efficiency was obtained with ZnO Merck sample. Thus, subsequent experiments were carried out with Merck ZnO.
Fig. 3. (a) Efficiency–time curves for various catalyst loadings. (b) Efficiency as a function of catalyst loading, at 25 min.
3.2. Catalyst loading To determine the effect of the catalyst loading, a series of experiments were carried out by varying the amount of catalyst from 0.5 to 3.5 g/L (dye concentration = 150 mg/L, solution pH = 6.5). The decolorization efficiencies for various catalyst loadings are depicted in Fig. 3a, as a function of time. As seen, initial slopes of the curves increase greatly by increasing catalyst loading from 0.5 to 1.5 g/L, above which initial slopes are nearly equal. As shown also in Fig. 3b, for a constant reaction time, e.g. 25 min, the decolorization efficiency exhibits a sharp increase by increasing loading up to 1.5–2 g/L, and the slope of the curve decrease with further increase in catalyst loading up to 3.5 g/L. At lower loading levels, such as 0.5 g/L, the catalyst surface and absorption of light by the catalyst surface are the limiting factors, and an increase in catalyst loading greatly enhances the process efficiency. On the other hand, at higher loading levels, irradiation field inside the reaction medium is reduced due to the light scattering by catalyst particles. 3.3. Initial pH
Fig. 2. The effect of the catalyst type on the decolorization efficiency.
The role of pH on the decolorization efficiency was studied in the pH range 5–11 at 100 mg/L dye concentration and
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Fig. 4. Effect of initial pH.
Fig. 5. Effect of initial dye concentration.
1.5 g/L catalyst loading. The pH of the solution is adjusted before irradiation and it is not controlled during the course of the reaction. A slight decrease in the solution pH, approximately 0.5 pH units, is observed at the end of 30 min. The results in Fig. 4 show that the rate of decolorization increases with increase in pH, exhibiting maximum rate at pH 10. Using azo dye of acid brown 14, Sakthivel et al. [23] have observed similar trend with ZnO, commenting that acid–base property of the metal oxide surfaces can have considerable implications upon their photocatalytic activity. The zero-point charge for ZnO is 9.0 and above this value, ZnO surface is negatively charged by means of adsorbed OH− ions. The presence of large quantities of OH− ions on the particle surfaces as well as in the reaction medium favors the formation of OH• radical, which is now widely accepted as the principal oxidizing specie responsible for the dye decolorization process. Lizama et al. [22] also found out that pH 11 was the optimum value for RB-19 dye removal. They commented that textile processes using reactive dyes produce effluents with high pH, suitable for the use of ZnO as a catalyst.
entering the solution decreases, which result in lower photon absorption on catalyst particles, and consequently lower photocatalytic reaction rates. The progress in the absorbance spectrums of the dye solution during the reaction was also monitored with initial 100 mg/L dye concentration and 1.5 g/L catalyst loading at pH 6. As seen in Fig. 6, the reductions in three absorbance peaks at 280, 375 and 525 nm, indicate the degradation of the dye molecule to smaller intermediates. On the other hand, no new absorption peaks appear during the reaction; these support the hypothesis that intermediate products formed during the dye degradation are also successfully degraded towards to a complete mineralization; approximately 75% decrease in the absorbance under 300 nm, is also a strong indication of the degradation of the intermediates. The COD and TOC of the solution must also be monitored during the reaction for a complete elucidation of the degradation mechanism. Meanwhile, it is clear that a total mineralization needs a longer reaction time than 30 min, which may be sufficient for a successful decolorization process.
3.4. Initial dye concentration The effect of initial dye concentration on the decolorization efficiency was investigated by varying the initial concentration from 50 to 200 mg/L at constant catalyst loading (1.5 g/L and at solution pH between 6 and 6.5) and results are shown in Fig. 5. As seen in the figure, decolorization efficiency is inversely affected by the dye concentration. This negative effect can be commented as follows; as the dye concentration is increased, the equilibrium adsorption of dye on the catalyst surface active sites increases, hence competitive adsorption of OH− on the same sites decreases, which means a lower formation rate of OH• radical which is the principal oxidant indispensable for a high degradation efficiency. On the other hand, considering Beer–Lambert law, as the initial dye concentration increases, the path length of photons
Fig. 6. Typical absorbance spectrums of the dye solution during the course of the reaction.
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tively charged by means of adsorbed OH− ions; this favors the formation of OH• radical, the principal oxidizing species responsible for the dye decolorization process. 3. Decolorization efficiency is inversely related to the dye concentration. By increasing dye concentration, the equilibrium adsorption of dye on the catalyst surface sites increases, and consequently OH• formation rate decreases due to the hindered OH− adsorption on the same sites. Furthermore, the path length of photons entering the solution decreases also according to Beer–Lambert law, which result in lower photon absorption on catalyst particles.
Fig. 7. Effect of light power on the decolorization efficiency.
3.5. Effect of light intensity The influence of light intensity on the degradation efficiency has been examined at constant dye concentration (150 mg/L, pH = 6.5) and catalyst loading (1.5 g/L). The light intensity is simply altered by varying nominal light power (12 W–24 W–36 W). As seen in Fig. 7, the efficiency increases monotonically with increasing light power; meanwhile, this dependency is not linear as revealed by a regression analysis. It is hoped that kinetic modeling and mechanistic studies will elucidate the effect of the light intensity, as well as the other ones, on the process efficiency.
4. Conclusion In this study, photocatalytic decolorization of an azo-reactive textile dye, Remazol red RR, has been investigated using various semiconductor metal oxide catalysts. ZnO powder has been found as the most active catalyst, exhibiting slightly higher activity than TiO2 . Subsequent experiments are conducted with ZnO to investigate its photocatalytic activity under various process conditions. It has been found that: 1. At lower catalyst loadings, such as 0.5 g/L, the catalyst surface and absorption of light on catalyst surface are the limiting factors, thus, an increase in catalyst loading greatly enhances the process efficiency. At high loadings, on the other hand, irradiation field inside the reaction medium is reduced due to the light scattering by catalyst particles. The impact of catalyst loading on the process efficiency is stronger at low dye concentrations. 2. The decolorization efficiency increases with increase in pH, attaining maximum value at pH 10. The zero-point charge for ZnO is 9.0 above which ZnO surface is nega-
Acknowledgements This study is part of research projects supported by Gebze Institute of Technology Research Fund (Nos. 01-A-03-05-17 and 02-A-03-03-01).
References [1] A. Reife, H.S. Fremann, Environmental Chemistry of Dyes and Pigments, Wiley, 1996. [2] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Bioresource Technol. 77 (2001) 247. [3] P.P. Zamora, A. Kunz, S.G. Moraes, R. Pelegrini, P.C. Moleiro, J. Reyes, N. Duran, Chemosphere 38 (1999) 835. [4] S. Ledakowicz, M. Solecka, R. Zylla, J. Biotechnol. 89 (2001) 175. [5] D. Georgiou, P. Melidis, A. Aivasidis, K. Gimouhopoulos, Dyes Pigments 52 (2002) 69. [6] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard, J.M. Herrmann, Appl. Catal. B: Environ. 39 (2002) 75. [7] M. Saquib, M. Muneer, Dyes Pigments 53 (2002) 237. [8] M. Stylidi, D.I. Kondarides, X.E. Verykios, Appl. Catal. B: Environ. 40 (2003) 271. [9] K. Tanaka, K. Padermpole, T. Hisanaga, Water Res. 34 (2000) 327. [10] M. Kaneko, I. Okura (Eds.), Photocatalysis: Science and Technology, Springer-Verlag, Japan, 2002. [11] C. Hu, J.C. Yu, Z. Hao, P.K. Wong, Appl. Catal. B: Environ. 42 (2003) 47. [12] C.G. Silva, J.L. Faria, J. Photochem. Photobiol. A: Chem. 155 (2003) 133. [13] C. Hachem, F. Bocquillon, O. Zahraa, M. Bouchy, Dyes Pigments 49 (2001) 117. [14] M.V.B. Zanoni, J.J. Sene, M.A. Anderson, J. Photochem. Photobiol. A: Chem. 157 (2003) 55. [15] R. Pelegrini, P.P. Zamora, A.R. Andrade, J. Reyes, N. Duran, Appl. Catal. B: Environ. 22 (1999) 83. [16] P.P. Zamora, S.G. Moraes, R. Pelegrini, M. Freire Jr., J. Reyes, H. Manslla, N. Duran, Chemosphere 36 (1998) 2119. [17] M.C. Yeber, J. Rodriguez, J. Freer, N. Duran, H.D. Mansilla, Chemosphere 41 (2000) 1193. [18] B. Neppolian, H.C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, J. Hazard. Mater. B 89 (2002) 303. [19] I. Poulios, E. Micropoulou, R. Panou, E. Kostopoulou, Appl. Catal. B: Environ. 41 (2003) 345. [20] M.L. Curri, R. Comparelli, P.D. Cozzoli, G. Mascolo, A. Agostiano, Mater. Sci. Eng. C 23 (2003) 285. [21] C.A.K. Gouvea, F. Wypych, S.G. Moraes, N. Duran, N. Nagata, P.P. Zamora, Chemosphere 40 (2000) 433.
24
A. Akyol et al. / Applied Catalysis B: Environmental 54 (2004) 19–24
[22] C. Lizama, J. Freer, J. Baeza, H.D. Mansilla, Catal. Today 76 (2002) 235. [23] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energy Mater. Sol. C 77 (2003) 65. [24] N. Daneshvar, D. Salari, A.R. Khataee, J. Photochem. Photobiol. A: Chem. 162 (2004) 317.
[25] R.S. Parra, I.H. Perez, M.E. Rincon, S.L. Ayala, M.C.R. Ahumada, Sol. Energy Mater. Sol. C 76 (2003) 189. [26] D. Li, H. Haneda, Chemosphere 51 (2003) 129. [27] J.G. Calvert, J.N. Pits, Photochemistry, Wiley, New York, USA, 1966.