- Email: [email protected]
Photocatalytic degradation of azo-dye Acid Red 18
Desalination 185 (2005) 449–456
Photocatalytic degradation of azo-dye Acid Red 18 Sylwia Mozia, Maria Tomaszewska*, Antoni W. Morawski Technical University of Szczecin, Institute of Chemical and Environmental Engineering, Department of Water and Environmental Engineering, Pulaskiego 10, 70–322 Szczecin, Poland email: [email protected] Received 20 February 2005; accepted 20 March 2005
Abstract The presented studies have focused on application of photocatalysis in degradation of azo-dye Acid Red 18 (C20H11N2Na3O10S3) under UV irradiation. The effect of parameters such as initial dye concentration and catalyst dosage on the photocatalytic degradation of model dye was investigated. Another important factor affecting the photodegradation process performance was the temperature of the reaction mixture. The reaction temperatures were equal to 293, 323, 333 and 343 K. The initial dye concentrations amounted to 10 and 30 mg/ dm3 and the photocatalyst (TiO2 Aeroxide1 P25, Degussa, Germany) dosage was in the range of 0.1–0.5 g/ dm3. The effectiveness of model dye photodecomposition was evaluated on the basis of changes of Acid Red 18 and total organic carbon (TOC) concentration and total dissolved substances (TDS) content. The effectiveness of photodecomposition of Acid Red 18 increased with increasing the catalyst concentration. However, an unfavourable effect of light scattering and reduction of light penetration through the solution in case of photocatalyst loading of 0.5 g/dm3 was observed. It was found that a linear correlation between the apparent rate constant and the reaction temperature exists in the range of 293–333 K. At the temperature of 343 K a decrease in the reaction rate was observed. Keywords: Photocatalysis; Azo-dye; TiO2; Effect of reaction temperature; Effect of initial dye concentration; Effect of photocatalyst concentration
1. Introduction Textile industries generate wastewaters that contain considerable amounts of non-fixed dyes, especially of azo-dyes, and a huge amount of inorganic salts. The most common
treatment methods, including adsorption, biological degradation, chlorination or ozonation, are not efficient enough to remove these compounds from the treated water streams. In view of these, ‘‘Advanced Oxidation Processes’’ (AOP) seem to be a very promising way of treatment of wastewaters from textile
*Corresponding author. Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2005.04.050
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S. Mozia et al. / Desalination 185 (2005) 449–456
industry. Among these processes, heterogeneous photocatalysis was found as an emerging destructive technology leading to the total mineralization of most of organic pollutants. There are several studies concerning photocatalytic degradation of azo-dyes under UV irradiation [1–12]. In brief, the mechanism of this process can be presented as follows [3]: þ TiO2 þ hvðUVÞ ! TiO2 ðe CB þ hVB Þ
ð1Þ
þ TiO2 ðhþ VB Þ þ H2 O ! TiO2 þ H þ OH
ð2Þ
TiO2 ðhþ VB Þ þ OH ! TiO2 þ OH
ð3Þ
TiO2 ðe CB Þ
þ O2 ! TiO2 þ
O 2
solution, light intensity and irradiation time, reaction temperature, addition of oxidants and the presence of other organic and inorganic compounds. The presented studies have focused on application of photocatalysis for degradation of azo-dye Acid Red 18 (C20H11N2Na3O10S3) under UV irradiation. The effect of parameters such as initial dye concentration, catalyst dosage and reaction temperature on the photocatalytic degradation of model dye was investigated. 2. Materials and methods
ð4Þ
þ O 2 þ H ! HO2
ð5Þ
Dye þ OH ! degradation products
ð6Þ
When a semiconductor, such as TiO2 absorbs a photon of energy that is equal to or greater than its band gap width (Eg in case of TiO2 amounts to 3.2 eV), an electron (e) may be promoted from the valence band (VB) to the conduction band (CB) thus generating an electron vacancy – ‘‘hole’’ (hþ). The electron and the hole can migrate to the catalyst surface where they participate in redox reactions with different species adsorbed on catalyst surface. Holes can react with surface-bond H2O or OH to produce the hydroxyl radical OH whereas electrons during reaction with oxygen can generate superoxide radical anion O2. The hydroxyl radicals (OH) and superoxide radical anions (O2) are supposed to be the primary oxidizing species in the photocatalytic oxidation processes. These oxidative reactions would result in the bleaching of a dye [4]. The main factors influencing the photocatalytic degradation of azo-dyes are initial dye concentration, catalyst loading, pH of the
2.1. Materials Titanium dioxide Aeroxide1 P25 (Degussa, Germany) was used as a photocatalyst. The commercially available azo-dye Acid Red 18 (C20H11N2Na3O10S3) produced by Chemical Factory Boruta – Kolor Sp. z o.o. (Poland) was used as a model compound. A chemical structure of the model compound is presented in Fig. 1. 2.2. Determination of the photocatalytic activity The photodegradation reaction was carried out in glass reactors containing 700 ml of a model solution of Acid Red 18 and a defined amount of a photocatalyst. To limit the evaporation of water the upper side of the
HO Na SO 3
N
N
NaSO 3 SO 3Na Fig. 1. Chemical structure of Acid Red 18.
S. Mozia et al. / Desalination 185 (2005) 449–456
reactor was covered with a plate from polymethyl metacrylate (PMMA) that was transparent to UV radiation. The concentration of model dye was equal to 10 or 30 mg/dm3 and the catalyst concentrations ranged from 0.1 to 0.5 g TiO2/dm3. The solutions in the reactors were continuously stirred during the experiment. After 15 min in the dark, the reaction solution was illuminated with a mercury lamp Philips Cleo, emitting UV-A light (max = 355 nm). The UV lamp was positioned above the reactor. The illumination intensity at the irradiation plate was 52 W/m2 for UV range. The illumination intensities were measured with an LB 901 radiometer equipped with the PD204AB (Macam Photometrics Ltd) and CM3 (Kipp & Zonen) external sensors. No cooling of the UV lamp was applied. The reaction temperatures were equal to 293, 323, 333 and 343 K. A schematic diagram of the experimental setup is presented in Fig. 2. After a defined time of irradiation the samples of solution were filtered through a 0.45 mm membrane filter and analyzed. The decomposition rate of Acid Red 18 was estimated on the basis of changes in UV/VIS
Fig. 2. Schematic diagram of the experimental setup: (1) UV lamp; (2) magnetic stirrer; (3) stirrer; (4) thermometer; (5) heater; (6) water bath; (7) glass reactor.
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spectra (Jasco V530 spectrometer, Japan), total organic carbon (TOC) concentration (‘‘Multi N/C 2000’’ analyzer, Analytik Jena, Germany), total dissolved substances (TDS) content (UltrameterTM 6P, MYRON L COMPANY USA) and pH of the solution. 3. Results and discussion At the beginning of the investigations a photolysis experiment (i.e. without addition of TiO2) was performed in order to determine the photostability of Acid Red 18 under the conditions applied. The solution was placed in the same batch reactor that was applied in the photocatalysis experiments. The parameters monitored were: dye concentration, TOC content, conductivity and TDS value. It was found that after 5 h of illumination of dye solution the measured parameters were practically constant, what means that no degradation of model compound took place. A negligible change of reaction volume was observed due to evaporation of water during heating. Therefore, it can be stated that in the presence of TiO2 a true heterogenous catalytic regime takes place. In order to determine the effect of the initial dye concentration, catalyst loading and reaction temperature on the effectiveness of photodegradation of Acid Red 18 the photodecomposition of model dye was performed. The initial concentration of azo-dye was equal to 10 or 30 mg/dm3 and the catalyst loading was in the range of 0.1–0.3 mg/ dm3. The reaction temperatures amounted to 293, 323, 333 and 343 K. Fig. 3 presents, as an example, the results of decolorization of Acid Red 18 in time for the reaction temperatures equal to 293 and 333 K. It can be seen that at low dye concentration (10 mg/dm3) the effect of TiO2 dose on the effectiveness of fading of the solution
S. Mozia et al. / Desalination 185 (2005) 449–456
100 90 80 70 60 50 40
293 K
10 mg/dm3, 0.1gTiO2/dm3 10 mg/dm3, 0.3gTiO2/dm3 10 mg/dm3, 0.5gTiO2/dm3 30 mg/dm3, 0.1gTiO2/dm3 30 mg/dm3, 0.3gTiO2/dm3 30 mg/dm3, 0.5gTiO2/dm3
30 20 10 0 0
50
100
150
200
250
300
350
dye decomposition [%]
time [min]
100 90 80 70 60 50 40 30 20 10 0
333 K
10 mg/dm3, 0.1gTiO2/dm3 10 mg/dm3, 0.3gTiO2/dm3 10 mg/dm3, 0.5gTiO2/dm3 30 mg/dm3, 0.1gTiO2/dm3 30 mg/dm3, 0.3gTiO2/dm3 30 mg/dm3, 0.5gTiO2/dm3 0
50
100
150
200
250
300
350
time [min]
Fig. 3. Photocatalytic decomposition of Acid Red 18 in time for different initial dye concentrations and photocatalyst dosages. Reaction temperature: 293 K (upper graph) and 333 K (bottom graph).
is practically negligible. In this case a complete decolorization was obtained after 2 h of illumination conducted at 293 K and after 1 h for the other temperatures, regardless of photocatalyst dose applied. When a solution containing 30 mg/dm3 was applied, the lowest rate of decolorization was obtained with the TiO2 concentration of 0.1 g/dm3. The solution was decolorized after 5 h of irradiation only in case of photodegradation conducted at 333 K. At the temperature of 293 K the model dye was decomposed after 4 h for catalyst dosages of 0.3 and 0.5 g/ dm3. For the other temperatures applied the solution was faded after 4 and 3 h of illumination with catalyst concentration equal to 0.3 and 0.5 g /dm3, respectively. The fading
of the solution was associated with cleavage of azo linkage in dye molecule. Azo-dyes are characterized by nitrogen to nitrogen double bonds (N = N) that are usually attached to two radicals of which at least one but usually both are aromatic groups (benzene or naphthalene rings). The color of azo-dyes is determined by the azo bonds and their associated chromophores and auxochromes. Azo bonds are the most active bonds in azo-dye molecules and can be oxidized by positive hole or hydroxyl radical or reduced by electron in the conduction band [12]. The cleavage of N = N bonds leads to the decolorization of dyes, what was observed in the discussed experiment (Fig. 3). The UV-VIS spectra recorded during photodegradation of model dye are presented in Fig. 4. The spectrum of dye before photodegradation (t = 0 h) exhibits three main peaks at a wavelengths of 507, 330 and 215 nm. The absorption in the visible region can be attributed to chromophore containing azo linkage, whereas the bands observed in the UV region can be assigned to aromatic (naphthalene) rings present in Acid Red 18 molecule (Fig. 1). It can be observed that
t=0h
Absorbance [a. u.]
dye decomposition [%]
452
t=4h
t=1h t=2h
t=5h
t=3h
200
300
400 500 Wavelength[nm]
600
Fig. 4. Changes in the UV-VIS spectra of Acid Red 18 during photodegradation; initial dye concentration: 30 mg/dm3; reaction temperature 333 K; catalyst loading: 0.3 g TiO2/dm3.
S. Mozia et al. / Desalination 185 (2005) 449–456
during irradiation not only a rapid decolorization of the dye, but also significant degradation of the aromatic structure proceeds. It must be emphasized that decolorization of the solution does not provide a complete data on the azo-dye degradation. Therefore, monitoring of other parameters such as TOC, TDS, conductivity or pH should be conducted. Fig. 5 presents the effectiveness of photodegradation of Acid Red 18 determined on the basis of changes of TOC concentration. It can be observed that the effect of catalyst loading on the degradation rate is noticeable at both dye concentration applied. The lowest degradation rate was obtained at 30 mg/dm3, 0,1gTiO2/dm3 30 mg/dm3, 0,3gTiO2/dm3 30 mg/dm3, 0,5gTiO2/dm
TOC degradation [%]
10 mg/dm3, 0,1gTiO2/dm3 10 mg/dm3, 0,3gTiO2/dm3 10 mg/dm3, 0,5gTiO2/dm3
100 90 293 K 80 70 60 50 40 30 20 10 0 0 50
100
150
200
250
300
350
time [min]
30 mg/dm3, 0,1gTiO2/dm3 30 mg/dm3, 0,3gTiO2/dm3 30 mg/dm3, 0,5gTiO2/dm3
TOC degradation [%]
10 mg/dm3, 0,1gTiO2/dm3 10 mg/dm3, 0,3gTiO2/dm3 10 mg/dm3, 0,5gTiO2/dm3
100 90 80 70 60 50 40 30 20 10 0
333 K
0
50
100
150
200
250
300
350
time [min]
Fig. 5. Changes of TOC concentration in time for different initial dye concentrations and photocatalyst dosages. Reaction temperature: 293 K (upper graph) and 333 K (bottom graph).
453
the catalyst concentration equal to 0.1 g/dm3. The greatest effect of the TiO2 concentration was observed especially in case of photodegradation conducted at 293 K. For the initial dye concentration equal to 10 mg/dm3 TOC degradation after 2 h of irradiation was ca. 40% lower than in case of higher catalyst dosages. When the initial concentration of Acid Red 18 was equal to 30 mg/dm3 the effectiveness of TOC photodegradation after 5 h of illumination amounted to ca. 32, 80 and 87% for catalyst dosages of 0.1. 0.3 and 0.5 g TiO2/dm3, respectively. The degradation rate at the temperature of 333 K was higher. After 2 h of process performance with initial dye concentration of 10 mg/dm3 TOC was removed completely. In case of initial Acid Red 18 concentration equal to 30 mg/dm3 the effectiveness of TOC degradation after 5 h of irradiation was in the range of 49–87%, depending on catalyst loading. Another parameter indicating the rate of the dye photodecomposition could be TDS content. The TDS parameter includes all the dissolved species present in water, i.e. inorganic ions (e.g. sulfates, nitrates etc.) and all manner of organic compounds. Fig. 6 presents changes of TDS concentration in time for photocatalytic decomposition of Acid Red 18 conducted at 293 and 333 K. It can be observed that for initial dye concentration equal to 10 mg/dm3 the discussed parameter increased during the first hour of illumination and after that time remained practically constant. This could suggest that the dye was almost completely decomposed into CO2, H2O and inorganic species (SO4 2 and NO3 ) and the measured value was mainly due to the presence of the latter compounds. This conclusion is supported by TOC measurements, which revealed that concentration of organic carbon after 2 h of illumination was equal to 0. When the initial dye concentration amounted to 30 mg/dm3 the value of TDS increased continuously during
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S. Mozia et al. / Desalination 185 (2005) 449–456 30 mg/dm3, 0,1gTiO2/dm3 30 mg/dm3, 0,3gTiO2/dm3 30 mg/dm3, 0,5gTiO2/dm3
TDS [ppm]
10 mg/dm3, 0,1gTiO2/dm3 10 mg/dm3, 0,3gTiO2/dm3 10 mg/dm3, 0,5gTiO2/dm3
100 90 293 K 80 70 60 50 40 30 20 10 0 0 50
100
150
200
250
300
350
time [min] 30 mg/dm3, 0,1gTiO2/dm3 30 mg/dm3, 0,3gTiO2/dm3 30 mg/dm3, 0,5gTiO2/dm3
TDS [ppm]
10 mg/dm3, 0,1gTiO2/dm3 10 mg/dm3, 0,3gTiO2/dm3 10 mg/dm3, 0,5gTiO2/dm3
100 90 333 K 80 70 60 50 40 30 20 10 0 0 50
100
150
200
250
300
350
time [min]
Fig. 6. Changes of TDS concentration in time for different initial dye concentrations and photocatalyst dosages. Reaction temperature: 293 K (upper graph) and 333 K (bottom graph).
the whole process performance. This indicates that the process of Acid Red 18 photodegradation was not finished during 5 h of experiment. The obtained results indicated that the effectiveness of photodecomposition of Acid Red 18 increased with increasing the catalyst concentration. However, the difference between the photodegradation rate at 0.3 and 0.5 g TiO2/dm3 was not so substantial as between the rate at 0.1 and 0.3 g TiO2/dm3. It is generally known that as the catalyst loading is increased, there is an increase in the surface area of the catalyst available for adsorption and degradation. On the other hand, an increase in the catalyst loading increases the solution opacity leading to
decrease in the penetration of the photon flux in the reactor and thereby decreasing the photocatalytic degradation rate [13]. Moreover, the loss in surface area by agglomeration (particle–particle interactions) at high solid concentration is observed [14]. A much smaller effect of an increase of catalyst concentration from 0.3 to 0.5 g TiO2/ dm3 than from 0.1 to 0.3 g TiO2/dm3 on the effectiveness of photodecomposition of Acid Red 18 is in agreement with these statements. Light scattering and reduction of light penetration through the solution in case of photocatalyst loading of 0.5 g/dm3 is a limiting factor of the photodegradation of the dye. Therefore, it seems to be much beneficial to perform the photodegradation at a concentration of 0.3 g TiO2/dm3. In order to compare the course of the photodecomposition of Acid Red 18 at different temperatures the apparent rate constants were calculated. It is known from literature [15] that the photocatalytic oxidation of organic pollutants follows LangmuirHinshelwood kinetics. This kind of the reaction can be represented as follows:
dC ¼ kC dt
ð7Þ
and after integration: C ¼ C0 expðktÞ
ð8Þ
where C0 is the initial concentration of dye, C is the concentration of dye after time t of photocatalytic decomposition and k is a rate constant related to the reaction properties of the solute which depends on the reaction conditions, such as reaction temperature, the pH of the solution and the photocatalytic activity increases with increasing this value [15]. The rate constants k were obtained from the linear transforms ln(C0/C) = f(t).
S. Mozia et al. / Desalination 185 (2005) 449–456 0,009 0,008 0,007
-1
k [min ]
0,006 0,005 0,004 0,003 0,002 0,001 0 290
300
310
320
330
340
350
temperature [K]
Fig. 7. The dependence of the apparent first-order constant (k) of photocatalytic decomposition of Acid Red 18 on the reaction temperature. Initial dye concentration: 30 mg/dm3. Catalyst dose: 0.1 g TiO2/ dm3.
Fig. 7 presents the dependence of the apparent first-order constant (k) of photocatalytic decomposition of Acid Red 18 on the reaction mixture temperature calculated for catalyst dose of 0.1 g/dm3 and initial solution concentration equal to 30 mg/dm3. It can be observed that a linear correlation between the apparent rate constant and the reaction temperature exists in the range of 293–333 K. At the temperature of 343 K a decrease in the reaction rate can be observed. An increase of the solution temperature from 293 to 323 K resulted in 17.5% increase of the rate constant (from 0.0066–0.0080 min1). Rising the temperature up to 333 K resulted in an insignificant increase of k value (up to 0.0082 min1). When the temperature increased up to 343 K the rate constant decreased and reached the value of 0.0076 min1. Similar relationship between the apparent first-order constant (k) and reaction temperature was obtained by Reuterga˚rdh et al. [2]. They found a linear dependence fitting the graph of the Arrhenius equation for the temperatures from 303 to 333 K. Chen et al. [16] attributed the increase of the rate constant k with rising the
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temperature from 283 to 323 K to the increasing collision frequency of molecules in solution. They stated that for TiO2 photocatalysis irradiation is the primary source of electron–hole pair generation at ambient temperature as the band gap energy is too high to overcome by thermal activation. The obtained results showed that further increasing of the reaction temperature (up to 343 K) resulted in a decrease of the rate constant k (Fig. 7). According to Herrmann [17] when the reaction temperature tends to the boiling point of water, the exothermic adsorption of the degraded compound becomes disfavoured and tends to become the rate limiting step. Similarly, Zhou et al. [18] supposed that the decrease of the rate constant k at high temperatures can be due to reduction of the adsorptive capacity associated with the organics and dissolved oxygen. 4. Conclusions
Photocatalysis is a very effective method of degradation of azo-dyes. A complete mineralization of Acid Red 18 was obtained in case of initial dye concentration of 10 mg/dm3. In case of initial Acid Red 18 concentration equal to 30 mg/dm3 the rate of TOC degradation after 5 h of irradiation was in the range of 32–87%, depending on catalyst loading and reaction temperature. The effectiveness of photodecomposition of Acid Red 18 increased with increasing the catalyst concentration. However, an unfavourable effect of light scattering and reduction of light penetration through the solution in case of photocatalyst loading of 0.5 g/dm3 was observed. Therefore, the optimum TiO2 concentration was found to be 0.3 g/dm3. A linear correlation between the apparent rate constant and the reaction temperature
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exists in the range of 293–333 K. At the temperature of 343 K a decrease in the reaction rate was observed. References [1] M. Muruganandham and M. Swaminathan, Sol. Energ. Mat. Sol. C, 81 (2004) 439–457. [2] L.B. Reuterga˚rdh and M. Iangphasuk, Chemosphere, 35 (1997) 585–596. [3] I.K. Konstantinou and T.A. Albanis, Appl. Catal. B. Environ., 49 (2004) 1–14. [4] M. Qamar, M. Saquib and M. Muneer, Dyes Pigments, 65 (2005) 1–9. [5] M. Karkmaz, E. Puzenat, C. Guillard and J.M. Herrmann, Appl. Catal. B. Environ., 51 (2004) 183–194. [6] W. Feng, D. Nansheng and H. Helin, Chemosphere, 41 (2000) 1233–1238. [7] M. Sameiro, T. Gonc¸alves, E.M.S. Pinto, P. Nkeonye and A.M.F. Oliveira-Campos, Dyes Pigments, 64 (2005) 135–139. [8] C. Hachem, F. Bocquillon, O. Zahraa and M. Bouchy, Dyes Pigments, 49 (2001) 117–125.
[9] B. Zielin˜ska, J. Grzechulska, B. Grzmil and A.W. Morawski, Appl. Catal. B. Environ., 35 (2001) L1–L7. [10] J. Grzechulska and A.W. Morawski, Appl. Catal. B. Environ., 36 (2002) 45–51. [11] B. Zielin˜ska, J. Grzechulska and A.W. Morawski, J. Photochem. Photobiology. A: Chemistry, 157 (2003) 65–70. [12] Ch Zhu, L. Wang, L. Kong, X. Yang, L. Wang, S. Zheng, F. Chen, F. MaiZhi and H. Zong, Chemosphere, 41 (2000) 303–309. [13] S.P. Kamble, S.B. Sawant and V.G. Pangarkar, Ind. Eng. Chem. Res., 42 (2003) 6705–6713. [14] S. Kaneco, M. Arifur Rahmana, T. Suzuki, H. Katsumata, K. Ohta, J. Photochem. Photobiology. A: Chemistry, 163 (2004) 419–424. [15] S.-S. Hong, M.S. Lee, H.-S. Hwang, K.-T. Lim, S.S.Park, Ch.S. Ju and G.-D. Lee, Sol. Energ. Mat. Sol. C, 80 (2003) 273–282. [16] D. Chen and A.K. Ray, Water Res., 32 (1998) 3223–3234. [17] J.-M. Herrmann, Catal. Today, 53 (1999) 115– 129. [18] S. Zhou and A.K. Ray, Ind. Eng. Chem. Res., 42 (2003) 6020–6033.
Photocatalytic degradation of azo-dye Acid Red 18 Sylwia Mozia, Maria Tomaszewska*, Antoni W. Morawski Technical University of Szczecin, Institute of Chemical and Environmental Engineering, Department of Water and Environmental Engineering, Pulaskiego 10, 70–322 Szczecin, Poland email: [email protected] Received 20 February 2005; accepted 20 March 2005
Abstract The presented studies have focused on application of photocatalysis in degradation of azo-dye Acid Red 18 (C20H11N2Na3O10S3) under UV irradiation. The effect of parameters such as initial dye concentration and catalyst dosage on the photocatalytic degradation of model dye was investigated. Another important factor affecting the photodegradation process performance was the temperature of the reaction mixture. The reaction temperatures were equal to 293, 323, 333 and 343 K. The initial dye concentrations amounted to 10 and 30 mg/ dm3 and the photocatalyst (TiO2 Aeroxide1 P25, Degussa, Germany) dosage was in the range of 0.1–0.5 g/ dm3. The effectiveness of model dye photodecomposition was evaluated on the basis of changes of Acid Red 18 and total organic carbon (TOC) concentration and total dissolved substances (TDS) content. The effectiveness of photodecomposition of Acid Red 18 increased with increasing the catalyst concentration. However, an unfavourable effect of light scattering and reduction of light penetration through the solution in case of photocatalyst loading of 0.5 g/dm3 was observed. It was found that a linear correlation between the apparent rate constant and the reaction temperature exists in the range of 293–333 K. At the temperature of 343 K a decrease in the reaction rate was observed. Keywords: Photocatalysis; Azo-dye; TiO2; Effect of reaction temperature; Effect of initial dye concentration; Effect of photocatalyst concentration
1. Introduction Textile industries generate wastewaters that contain considerable amounts of non-fixed dyes, especially of azo-dyes, and a huge amount of inorganic salts. The most common
treatment methods, including adsorption, biological degradation, chlorination or ozonation, are not efficient enough to remove these compounds from the treated water streams. In view of these, ‘‘Advanced Oxidation Processes’’ (AOP) seem to be a very promising way of treatment of wastewaters from textile
*Corresponding author. Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2005.04.050
450
S. Mozia et al. / Desalination 185 (2005) 449–456
industry. Among these processes, heterogeneous photocatalysis was found as an emerging destructive technology leading to the total mineralization of most of organic pollutants. There are several studies concerning photocatalytic degradation of azo-dyes under UV irradiation [1–12]. In brief, the mechanism of this process can be presented as follows [3]: þ TiO2 þ hvðUVÞ ! TiO2 ðe CB þ hVB Þ
ð1Þ
þ TiO2 ðhþ VB Þ þ H2 O ! TiO2 þ H þ OH
ð2Þ
TiO2 ðhþ VB Þ þ OH ! TiO2 þ OH
ð3Þ
TiO2 ðe CB Þ
þ O2 ! TiO2 þ
O 2
solution, light intensity and irradiation time, reaction temperature, addition of oxidants and the presence of other organic and inorganic compounds. The presented studies have focused on application of photocatalysis for degradation of azo-dye Acid Red 18 (C20H11N2Na3O10S3) under UV irradiation. The effect of parameters such as initial dye concentration, catalyst dosage and reaction temperature on the photocatalytic degradation of model dye was investigated. 2. Materials and methods
ð4Þ
þ O 2 þ H ! HO2
ð5Þ
Dye þ OH ! degradation products
ð6Þ
When a semiconductor, such as TiO2 absorbs a photon of energy that is equal to or greater than its band gap width (Eg in case of TiO2 amounts to 3.2 eV), an electron (e) may be promoted from the valence band (VB) to the conduction band (CB) thus generating an electron vacancy – ‘‘hole’’ (hþ). The electron and the hole can migrate to the catalyst surface where they participate in redox reactions with different species adsorbed on catalyst surface. Holes can react with surface-bond H2O or OH to produce the hydroxyl radical OH whereas electrons during reaction with oxygen can generate superoxide radical anion O2. The hydroxyl radicals (OH) and superoxide radical anions (O2) are supposed to be the primary oxidizing species in the photocatalytic oxidation processes. These oxidative reactions would result in the bleaching of a dye [4]. The main factors influencing the photocatalytic degradation of azo-dyes are initial dye concentration, catalyst loading, pH of the
2.1. Materials Titanium dioxide Aeroxide1 P25 (Degussa, Germany) was used as a photocatalyst. The commercially available azo-dye Acid Red 18 (C20H11N2Na3O10S3) produced by Chemical Factory Boruta – Kolor Sp. z o.o. (Poland) was used as a model compound. A chemical structure of the model compound is presented in Fig. 1. 2.2. Determination of the photocatalytic activity The photodegradation reaction was carried out in glass reactors containing 700 ml of a model solution of Acid Red 18 and a defined amount of a photocatalyst. To limit the evaporation of water the upper side of the
HO Na SO 3
N
N
NaSO 3 SO 3Na Fig. 1. Chemical structure of Acid Red 18.
S. Mozia et al. / Desalination 185 (2005) 449–456
reactor was covered with a plate from polymethyl metacrylate (PMMA) that was transparent to UV radiation. The concentration of model dye was equal to 10 or 30 mg/dm3 and the catalyst concentrations ranged from 0.1 to 0.5 g TiO2/dm3. The solutions in the reactors were continuously stirred during the experiment. After 15 min in the dark, the reaction solution was illuminated with a mercury lamp Philips Cleo, emitting UV-A light (max = 355 nm). The UV lamp was positioned above the reactor. The illumination intensity at the irradiation plate was 52 W/m2 for UV range. The illumination intensities were measured with an LB 901 radiometer equipped with the PD204AB (Macam Photometrics Ltd) and CM3 (Kipp & Zonen) external sensors. No cooling of the UV lamp was applied. The reaction temperatures were equal to 293, 323, 333 and 343 K. A schematic diagram of the experimental setup is presented in Fig. 2. After a defined time of irradiation the samples of solution were filtered through a 0.45 mm membrane filter and analyzed. The decomposition rate of Acid Red 18 was estimated on the basis of changes in UV/VIS
Fig. 2. Schematic diagram of the experimental setup: (1) UV lamp; (2) magnetic stirrer; (3) stirrer; (4) thermometer; (5) heater; (6) water bath; (7) glass reactor.
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spectra (Jasco V530 spectrometer, Japan), total organic carbon (TOC) concentration (‘‘Multi N/C 2000’’ analyzer, Analytik Jena, Germany), total dissolved substances (TDS) content (UltrameterTM 6P, MYRON L COMPANY USA) and pH of the solution. 3. Results and discussion At the beginning of the investigations a photolysis experiment (i.e. without addition of TiO2) was performed in order to determine the photostability of Acid Red 18 under the conditions applied. The solution was placed in the same batch reactor that was applied in the photocatalysis experiments. The parameters monitored were: dye concentration, TOC content, conductivity and TDS value. It was found that after 5 h of illumination of dye solution the measured parameters were practically constant, what means that no degradation of model compound took place. A negligible change of reaction volume was observed due to evaporation of water during heating. Therefore, it can be stated that in the presence of TiO2 a true heterogenous catalytic regime takes place. In order to determine the effect of the initial dye concentration, catalyst loading and reaction temperature on the effectiveness of photodegradation of Acid Red 18 the photodecomposition of model dye was performed. The initial concentration of azo-dye was equal to 10 or 30 mg/dm3 and the catalyst loading was in the range of 0.1–0.3 mg/ dm3. The reaction temperatures amounted to 293, 323, 333 and 343 K. Fig. 3 presents, as an example, the results of decolorization of Acid Red 18 in time for the reaction temperatures equal to 293 and 333 K. It can be seen that at low dye concentration (10 mg/dm3) the effect of TiO2 dose on the effectiveness of fading of the solution
S. Mozia et al. / Desalination 185 (2005) 449–456
100 90 80 70 60 50 40
293 K
10 mg/dm3, 0.1gTiO2/dm3 10 mg/dm3, 0.3gTiO2/dm3 10 mg/dm3, 0.5gTiO2/dm3 30 mg/dm3, 0.1gTiO2/dm3 30 mg/dm3, 0.3gTiO2/dm3 30 mg/dm3, 0.5gTiO2/dm3
30 20 10 0 0
50
100
150
200
250
300
350
dye decomposition [%]
time [min]
100 90 80 70 60 50 40 30 20 10 0
333 K
10 mg/dm3, 0.1gTiO2/dm3 10 mg/dm3, 0.3gTiO2/dm3 10 mg/dm3, 0.5gTiO2/dm3 30 mg/dm3, 0.1gTiO2/dm3 30 mg/dm3, 0.3gTiO2/dm3 30 mg/dm3, 0.5gTiO2/dm3 0
50
100
150
200
250
300
350
time [min]
Fig. 3. Photocatalytic decomposition of Acid Red 18 in time for different initial dye concentrations and photocatalyst dosages. Reaction temperature: 293 K (upper graph) and 333 K (bottom graph).
is practically negligible. In this case a complete decolorization was obtained after 2 h of illumination conducted at 293 K and after 1 h for the other temperatures, regardless of photocatalyst dose applied. When a solution containing 30 mg/dm3 was applied, the lowest rate of decolorization was obtained with the TiO2 concentration of 0.1 g/dm3. The solution was decolorized after 5 h of irradiation only in case of photodegradation conducted at 333 K. At the temperature of 293 K the model dye was decomposed after 4 h for catalyst dosages of 0.3 and 0.5 g/ dm3. For the other temperatures applied the solution was faded after 4 and 3 h of illumination with catalyst concentration equal to 0.3 and 0.5 g /dm3, respectively. The fading
of the solution was associated with cleavage of azo linkage in dye molecule. Azo-dyes are characterized by nitrogen to nitrogen double bonds (N = N) that are usually attached to two radicals of which at least one but usually both are aromatic groups (benzene or naphthalene rings). The color of azo-dyes is determined by the azo bonds and their associated chromophores and auxochromes. Azo bonds are the most active bonds in azo-dye molecules and can be oxidized by positive hole or hydroxyl radical or reduced by electron in the conduction band [12]. The cleavage of N = N bonds leads to the decolorization of dyes, what was observed in the discussed experiment (Fig. 3). The UV-VIS spectra recorded during photodegradation of model dye are presented in Fig. 4. The spectrum of dye before photodegradation (t = 0 h) exhibits three main peaks at a wavelengths of 507, 330 and 215 nm. The absorption in the visible region can be attributed to chromophore containing azo linkage, whereas the bands observed in the UV region can be assigned to aromatic (naphthalene) rings present in Acid Red 18 molecule (Fig. 1). It can be observed that
t=0h
Absorbance [a. u.]
dye decomposition [%]
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t=4h
t=1h t=2h
t=5h
t=3h
200
300
400 500 Wavelength[nm]
600
Fig. 4. Changes in the UV-VIS spectra of Acid Red 18 during photodegradation; initial dye concentration: 30 mg/dm3; reaction temperature 333 K; catalyst loading: 0.3 g TiO2/dm3.
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during irradiation not only a rapid decolorization of the dye, but also significant degradation of the aromatic structure proceeds. It must be emphasized that decolorization of the solution does not provide a complete data on the azo-dye degradation. Therefore, monitoring of other parameters such as TOC, TDS, conductivity or pH should be conducted. Fig. 5 presents the effectiveness of photodegradation of Acid Red 18 determined on the basis of changes of TOC concentration. It can be observed that the effect of catalyst loading on the degradation rate is noticeable at both dye concentration applied. The lowest degradation rate was obtained at 30 mg/dm3, 0,1gTiO2/dm3 30 mg/dm3, 0,3gTiO2/dm3 30 mg/dm3, 0,5gTiO2/dm
TOC degradation [%]
10 mg/dm3, 0,1gTiO2/dm3 10 mg/dm3, 0,3gTiO2/dm3 10 mg/dm3, 0,5gTiO2/dm3
100 90 293 K 80 70 60 50 40 30 20 10 0 0 50
100
150
200
250
300
350
time [min]
30 mg/dm3, 0,1gTiO2/dm3 30 mg/dm3, 0,3gTiO2/dm3 30 mg/dm3, 0,5gTiO2/dm3
TOC degradation [%]
10 mg/dm3, 0,1gTiO2/dm3 10 mg/dm3, 0,3gTiO2/dm3 10 mg/dm3, 0,5gTiO2/dm3
100 90 80 70 60 50 40 30 20 10 0
333 K
0
50
100
150
200
250
300
350
time [min]
Fig. 5. Changes of TOC concentration in time for different initial dye concentrations and photocatalyst dosages. Reaction temperature: 293 K (upper graph) and 333 K (bottom graph).
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the catalyst concentration equal to 0.1 g/dm3. The greatest effect of the TiO2 concentration was observed especially in case of photodegradation conducted at 293 K. For the initial dye concentration equal to 10 mg/dm3 TOC degradation after 2 h of irradiation was ca. 40% lower than in case of higher catalyst dosages. When the initial concentration of Acid Red 18 was equal to 30 mg/dm3 the effectiveness of TOC photodegradation after 5 h of illumination amounted to ca. 32, 80 and 87% for catalyst dosages of 0.1. 0.3 and 0.5 g TiO2/dm3, respectively. The degradation rate at the temperature of 333 K was higher. After 2 h of process performance with initial dye concentration of 10 mg/dm3 TOC was removed completely. In case of initial Acid Red 18 concentration equal to 30 mg/dm3 the effectiveness of TOC degradation after 5 h of irradiation was in the range of 49–87%, depending on catalyst loading. Another parameter indicating the rate of the dye photodecomposition could be TDS content. The TDS parameter includes all the dissolved species present in water, i.e. inorganic ions (e.g. sulfates, nitrates etc.) and all manner of organic compounds. Fig. 6 presents changes of TDS concentration in time for photocatalytic decomposition of Acid Red 18 conducted at 293 and 333 K. It can be observed that for initial dye concentration equal to 10 mg/dm3 the discussed parameter increased during the first hour of illumination and after that time remained practically constant. This could suggest that the dye was almost completely decomposed into CO2, H2O and inorganic species (SO4 2 and NO3 ) and the measured value was mainly due to the presence of the latter compounds. This conclusion is supported by TOC measurements, which revealed that concentration of organic carbon after 2 h of illumination was equal to 0. When the initial dye concentration amounted to 30 mg/dm3 the value of TDS increased continuously during
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S. Mozia et al. / Desalination 185 (2005) 449–456 30 mg/dm3, 0,1gTiO2/dm3 30 mg/dm3, 0,3gTiO2/dm3 30 mg/dm3, 0,5gTiO2/dm3
TDS [ppm]
10 mg/dm3, 0,1gTiO2/dm3 10 mg/dm3, 0,3gTiO2/dm3 10 mg/dm3, 0,5gTiO2/dm3
100 90 293 K 80 70 60 50 40 30 20 10 0 0 50
100
150
200
250
300
350
time [min] 30 mg/dm3, 0,1gTiO2/dm3 30 mg/dm3, 0,3gTiO2/dm3 30 mg/dm3, 0,5gTiO2/dm3
TDS [ppm]
10 mg/dm3, 0,1gTiO2/dm3 10 mg/dm3, 0,3gTiO2/dm3 10 mg/dm3, 0,5gTiO2/dm3
100 90 333 K 80 70 60 50 40 30 20 10 0 0 50
100
150
200
250
300
350
time [min]
Fig. 6. Changes of TDS concentration in time for different initial dye concentrations and photocatalyst dosages. Reaction temperature: 293 K (upper graph) and 333 K (bottom graph).
the whole process performance. This indicates that the process of Acid Red 18 photodegradation was not finished during 5 h of experiment. The obtained results indicated that the effectiveness of photodecomposition of Acid Red 18 increased with increasing the catalyst concentration. However, the difference between the photodegradation rate at 0.3 and 0.5 g TiO2/dm3 was not so substantial as between the rate at 0.1 and 0.3 g TiO2/dm3. It is generally known that as the catalyst loading is increased, there is an increase in the surface area of the catalyst available for adsorption and degradation. On the other hand, an increase in the catalyst loading increases the solution opacity leading to
decrease in the penetration of the photon flux in the reactor and thereby decreasing the photocatalytic degradation rate [13]. Moreover, the loss in surface area by agglomeration (particle–particle interactions) at high solid concentration is observed [14]. A much smaller effect of an increase of catalyst concentration from 0.3 to 0.5 g TiO2/ dm3 than from 0.1 to 0.3 g TiO2/dm3 on the effectiveness of photodecomposition of Acid Red 18 is in agreement with these statements. Light scattering and reduction of light penetration through the solution in case of photocatalyst loading of 0.5 g/dm3 is a limiting factor of the photodegradation of the dye. Therefore, it seems to be much beneficial to perform the photodegradation at a concentration of 0.3 g TiO2/dm3. In order to compare the course of the photodecomposition of Acid Red 18 at different temperatures the apparent rate constants were calculated. It is known from literature [15] that the photocatalytic oxidation of organic pollutants follows LangmuirHinshelwood kinetics. This kind of the reaction can be represented as follows:
dC ¼ kC dt
ð7Þ
and after integration: C ¼ C0 expðktÞ
ð8Þ
where C0 is the initial concentration of dye, C is the concentration of dye after time t of photocatalytic decomposition and k is a rate constant related to the reaction properties of the solute which depends on the reaction conditions, such as reaction temperature, the pH of the solution and the photocatalytic activity increases with increasing this value [15]. The rate constants k were obtained from the linear transforms ln(C0/C) = f(t).
S. Mozia et al. / Desalination 185 (2005) 449–456 0,009 0,008 0,007
-1
k [min ]
0,006 0,005 0,004 0,003 0,002 0,001 0 290
300
310
320
330
340
350
temperature [K]
Fig. 7. The dependence of the apparent first-order constant (k) of photocatalytic decomposition of Acid Red 18 on the reaction temperature. Initial dye concentration: 30 mg/dm3. Catalyst dose: 0.1 g TiO2/ dm3.
Fig. 7 presents the dependence of the apparent first-order constant (k) of photocatalytic decomposition of Acid Red 18 on the reaction mixture temperature calculated for catalyst dose of 0.1 g/dm3 and initial solution concentration equal to 30 mg/dm3. It can be observed that a linear correlation between the apparent rate constant and the reaction temperature exists in the range of 293–333 K. At the temperature of 343 K a decrease in the reaction rate can be observed. An increase of the solution temperature from 293 to 323 K resulted in 17.5% increase of the rate constant (from 0.0066–0.0080 min1). Rising the temperature up to 333 K resulted in an insignificant increase of k value (up to 0.0082 min1). When the temperature increased up to 343 K the rate constant decreased and reached the value of 0.0076 min1. Similar relationship between the apparent first-order constant (k) and reaction temperature was obtained by Reuterga˚rdh et al. [2]. They found a linear dependence fitting the graph of the Arrhenius equation for the temperatures from 303 to 333 K. Chen et al. [16] attributed the increase of the rate constant k with rising the
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temperature from 283 to 323 K to the increasing collision frequency of molecules in solution. They stated that for TiO2 photocatalysis irradiation is the primary source of electron–hole pair generation at ambient temperature as the band gap energy is too high to overcome by thermal activation. The obtained results showed that further increasing of the reaction temperature (up to 343 K) resulted in a decrease of the rate constant k (Fig. 7). According to Herrmann [17] when the reaction temperature tends to the boiling point of water, the exothermic adsorption of the degraded compound becomes disfavoured and tends to become the rate limiting step. Similarly, Zhou et al. [18] supposed that the decrease of the rate constant k at high temperatures can be due to reduction of the adsorptive capacity associated with the organics and dissolved oxygen. 4. Conclusions
Photocatalysis is a very effective method of degradation of azo-dyes. A complete mineralization of Acid Red 18 was obtained in case of initial dye concentration of 10 mg/dm3. In case of initial Acid Red 18 concentration equal to 30 mg/dm3 the rate of TOC degradation after 5 h of irradiation was in the range of 32–87%, depending on catalyst loading and reaction temperature. The effectiveness of photodecomposition of Acid Red 18 increased with increasing the catalyst concentration. However, an unfavourable effect of light scattering and reduction of light penetration through the solution in case of photocatalyst loading of 0.5 g/dm3 was observed. Therefore, the optimum TiO2 concentration was found to be 0.3 g/dm3. A linear correlation between the apparent rate constant and the reaction temperature
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