- Email: [email protected]
ZnO-mediated photocatalytic degradation of reactive red 22 using thin film flat bed flow photoreactor
Pergamon
PII:
Solar Energy Vol. 73, No. 4, pp. 281–285, 2002 2003 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 2 ) 0 0 0 6 5 – 8 All rights reserved. Printed in Great Britain 0038-092X / 02 / $ - see front matter
www.elsevier.com / locate / solener
SUNLIGHT / ZnO-MEDIATED PHOTOCATALYTIC DEGRADATION OF REACTIVE RED 22 USING THIN FILM FLAT BED FLOW PHOTOREACTOR L. SELVA ROSELIN † , G. R. RAJARAJESWARI, ROSILDA SELVIN, V. SADASIVAM, B. SIVASANKAR and K. RENGARAJ Department of Chemistry, Anna University, Chennai 600 025, India Accepted 2 August 2002
Abstract—Photocatalytic degradation of one of the most widely used cotton dyes, namely reactive red 22 (RR 22), was investigated in the presence of a thin film of ZnO photocatalyst using a thin film flat bed flow photoreactor under solar radiation. The effects of reaction parameters such as pH, amount of ZnO coating, flow rate and concentration of the dye solution on the percentage removal of dye were examined. In a single pass mode at 30 ml / min flow rate, 52.7% decrease in concentration was achieved for 200mM dye solution (pH 10). It has been demonstrated that in continuous circulation mode the time required to decompose half the concentration of the dye in 500 ml of 200 mM was 15.8 min. Complete removal of 200 mM dye solution (pH 10) was achieved at about 100 min. 2003 Elsevier Science Ltd. All rights reserved.
the ground level, solar irradiation starts at about 300 nm. In addition, the | 5% UV component of sunlight is also useful in the excitation process. Most of the previous reports dealt with photocatalytic reactions induced by irradiation of semiconductor particles suspended in aqueous solution (Muszkat et al., 1992, 1995; Reeves et al., 1992; Li et al., 2001). The powder catalysts, however, suffer from two drawbacks. In the first place the turbidity created by the catalyst when kept as a suspension will considerably decrease the amount of solar light interacting with the catalyst. Secondly, the problem associated with the separation of the finely divided photocatalyst from the treated water. Fixing the photocatalyst onto the stationary inert support is preferable to avoid these complications. The catalyst can be immobilized onto the surface of various polymeric materials in different reactor configurations. Krysova et al. (1998) have studied the photocatalytic degradation of diuron in aqueous solution on a TiO 2 layer of a batch mode plate reactor irradiated with UV sun-bed tubes. It was estimated that the total power required for diuron to degrade from 1 3 10 24 M to 1 3 10 26 M is 271 Wh / dm 3 . Therefore the artificial UV light source tends to be somewhat expensive. When the sun is exploited as a source of light, capital cost and recurring expenses for artificial light sources can be saved in the detoxification plant. Muradov (1994) has suggested immobilized titania on a plate for the degradation of nitroglycerine and Rhodamine B under solar irradiation. In the present investigation, solar energy-
1. INTRODUCTION
Effluent streams, especially those from textile dyeing industries, are highly coloured and toxic. Conventional treatment methods are ineffective in completely removing the pollutants, thereby stimulating further research and development activity in the field. Recent studies have shown that the photocatalytic method resulted in complete degradation of organics (Ahmed and Ollis, 1984; Poulios et al., 1998). In this technique, a semiconductor on irradiation with a photon of sufficient energy, greater than or equal to band gap energy, promotes an electron from the valence band to conduction band. Once promoted the electron leaves a vacancy behind in the valence band called a ‘hole’. The positive holes can directly oxidize the organic material by removal of an electron or react with water or OH 2 forming a powerful oxidant such as a hydroxyl radical. The conduction band electrons take part in the reduction reaction with dissolved oxygen, producing superoxide anion radical. Superoxide anion radical is highly reactive and can oxidize any organic compound. The minimum energy required for excitation of an electron for the commonly used photocatalysts such as TiO 2 and ZnO is 3.2 eV, which corresponds to 387.5 nm. These semiconductors can be activated by sunlight, since at †
Author to whom correspondence should be addressed. Tel.: 191-442-351-126x3142; fax: 191-442-350-397; e-mail: [email protected] 281
282
L. S. Roselin et al.
mediated photocatalytic degradation of one of the widely used cotton dyes, namely reactive red 22 (RR 22), was studied by using a simple plate photoreactor in a batch mode in which the liquid film was allowed to flow over the thin layer of ZnO fixed on a glass plate. The reaction parameters such as flow rate, amount of catalyst used for coating, pH and concentration of the dye were optimized. 2. EXPERIMENTAL
2.1. Materials The dye used in the present study is Remazol red B (C.I. Reactive red 22 (RR 22), 14282), 2-(3-amino-4-methoxy phenyl sulphonyl) ethanol sulphate ester → 1-naphthyl-5-sulphonic acid, widely used in dyeing cotton and was obtained from Vanavil Dye House, India. The dye was used as such without any further purification. The dye shows an absorption maximum at 530 nm (´ 5 9800 l mol 21 cm 21 ). Zinc oxide (99.8% purity) used as photocatalyst was obtained from Merck. The surface area and particle size for zinc oxide are 10 m 2 / g and 110 nm, respectively. All other chemicals and reagents used for analysis and experimental work were of analytical grade obtained from Central Drug House (CDH) and used as such.
2.2. Photoreactor The thin film flat bed flow reactor consisted of a glass plate (27.3 3 22.7 cm 2 ) fixed with an inclination to facilitate the flow of solution. The glass plate was coated with a thin film (1–2 mm) of the photocatalyst using ZnO slurry and a thin layer chromatography applicator. The dye solution was pumped through a perforated glass tube so as to allow the flow as a thin film over the entire surface of the coated glass plate along the long axis of the support plate and collected into a receiving flask. The flow rate of the solution was maintained with the help of a peristaltic pump. The dye solution was exposed to direct sunlight as it flowed over the photocatalyst-coated glass plate. The intensity of the sunlight during the reaction time was in the range 808–1070 W/ m 2 . The progress of the reaction was followed by withdrawing 3 ml of the photolysed water from the receiving flask after each cycle and measuring the absorbance at 530 nm in a Systronics spectrophotometer (Model 118). Total mineralization of the dye was confirmed by estimation of inorganic minerals such as sulphate, nitrate and
ammonium ions by standard methods of analysis (APHA, 1989). 3. RESULTS AND DISCUSSION
3.1. Effect of flow rate In order to optimize the flow rate of the dye solution on the ZnO-coated glass plate for the degradation of the dye, a series of experiments were carried out at different flow rates in the range from 15 to 35 ml / min using the same concentration of the dye solution (75 mM) and the same amount of catalyst coating (1 g). The thickness of the liquid film and the residence time of the solution on the catalyst layer at all flow rates were theoretically calculated as 0.11, 0.14, 0.18, 0.22, 0.26 mm and 4.0, 3.0, 2.4, 2.0, 1.7 s / cm for flow rates 15, 20, 25, 30 and 35 ml / min, respectively. The % removal of the dye in single-pass mode was observed for varying flow rate. The results demonstrate that the % removal of dye decreases with increasing flow rate and the decrease slows down after a flow rate of about 30 ml / min (Fig. 1). Therefore, a flow rate of 30 ml / min has been fixed for further studies.
3.2. Effect of the amount of ZnO coating The minimum amount of ZnO required to form a uniform thin film coating to cover the entire surface of the chosen glass plate was found to be about 1 g. The thickness of the ZnO coating can be optimized for the photocatalytic removal of RR 22. Photocatalytic experiments were performed
Fig. 1. Effect of flow rate on the % removal of the dye ([RR 22], 75 mM; volume, 500 ml; pH 6.8; weight of the catalyst, 1 g (1.57 mg / cm 2 ); solar radiation intensity, 808–1070 W/ m 2 ).
Sunlight / ZnO-mediated photocatalytic degradation of reactive red 22 using thin film flat bed flow photoreactor
with various amounts of ZnO coating in the range 1–5 g. The % removal of the dye in single-pass mode was observed for varying amount of catalyst coating. It is seen from Fig. 2 that the % removal of the dye increases as the ZnO coating thickness increases. With increasing amount of ZnO the availability of semiconductor particles for absorption of photons increases, thereby producing a greater number of oxidizing sites, which consequently increase the rate of the reaction. The increase is not pronounced beyond 3 g ZnO coating. This may be due to the number of photons that are absorbed by the active site of the catalyst which is exposed to sunlight becomes constant beyond 3 g of ZnO coating since additional photocatalyst is not illuminated. Also the light intensity profile within the layer is, essentially, decreasing above the saturation level (Corboz et al., 2000). Therefore the optimum amount of ZnO for the experimental set-up is to be fixed as 3 g (i.e. 4.7 mg / cm 2 ) having a thickness of 1.4 mm.
3.3. Effect of pH The pH of the solution can be one of the most important parameters for the photocatalytic process, since the particle size, surface charge and band edge positions of the semiconductor oxide are strongly influenced by pH (Fox and Dulay, 1993). The photocatalytic experiment was carried out at various pH values in the range 4–12 to find out the optimum pH. As can be seen from Fig. 3, when the initial pH of the dye solution varied from 4 to 12, the % removal of the dye reached a maximum at 10 followed by a decrease on reaching pH 12. The pH effect on the photocatalytic degradation of the dye with ZnO was
283
Fig. 3. Effect of pH on the % removal of dye ([RR 22], 75 mM; volume, 500 ml; weight of the catalyst, 3 g (4.7 mg / cm 2 ); flow rate, 30 ml / min; solar radiation intensity, 808– 1070 W/ m 2 ).
explained on the basis of adsorption of the dye on the catalyst surface. It was found that with increasing pH the amount of dye adsorbed is decreased. The strong adsorption leads to a drastic decrease in the active centre on the catalyst surface which results in decrease in the absorption of the irradiation surface and consequently to a lowering of the reaction rate. A similar influence of pH on the photocatalytic degradation rate of the reactive black 5 dye was reported by Poulios and Tsachinis (1999). Furthermore, the increase in the removal of dye with pH may be explained on the basis that hydroxyl radicals are involved in the photocatalytic reaction. With increasing pH, the hydroxyl ion concentration increases and so the hydroxyl radical formation increases. In alkaline solution the formation of the OHE radical is much easier than in neutral and acidic solution (Shourong et al., 1997). In a high pH value (i.e. pH 12), a drastic decrease in the reaction rate could be due to the formation of Zn(OH) 2 . In addition, ZnO undergoes photo-corrosion as given in Eq. (1) (Domenech and Prieto, 1986) 1 ZnO 1 2h 1 → Zn 21 1 ] O 2 2
(1)
3.4. Effect of initial dye concentration
Fig. 2. Effect of catalyst weight on the % removal of the dye ([RR 22], 75 mM; volume, 500 ml; pH 6.8; flow rate, 30 ml / min; solar radiation intensity, 808–1070 W/ m 2 ).
The effect of concentration of RR 22 on the % removal of the dye was investigated in the concentration range 25–400 mM. It is seen from Fig. 4 that the % removal of the dye increases steeply as the concentration of the dye increases to 100 mM and thereafter there is only a marginal increase and at 400 mM it decreases. The increase was due to the increase in the availability of dye
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L. S. Roselin et al.
was achieved at about 100 min. In single pass mode at 30 ml / min flow rate, 52.7% decrease in concentration was achieved for 200 mM dye solution (pH 10). The finally treated water was tested for complete mineralization by estimation of inorganic ions such as sulphate, nitrate and ammonium ions. Results showed that the solar photolysis in the presence of immobilized ZnO has destroyed the RR 22 dye effectively (destruction efficiency reached 98.3%).
4. CONCLUSIONS
Fig. 4. Effect of initial dye concentration on the % removal of dye (volume, 500 ml; pH 10.0; weight of the catalyst, 3 g (4.7 mg / cm 2 ); flow rate, 30 ml / min; solar radiation intensity, 808–1070 W/ m 2 ).
molecules for oxidation. The decrease at higher concentration may be explained as follows: as the dye solution becomes dense it obstructs the penetration of light and hence a smaller number of oxidative species are generated. In addition, at higher concentrations, a greater number of dye molecules are adsorbed virtually masking the surface of the catalyst particle and preventing the photons interacting with the catalyst. Mengyue et al. (1995) have observed a similar decrease in the rate for the degradation of organophosphorus pesticides using thin films of TiO 2 under UV radiation.
3.5. Total removal of the dye Under the optimized conditions using 3 g ZnO coating, 500 ml of 200 mM RR 22 dye solution (pH 10) were allowed to flow on the thin film coated with ZnO at a flow rate of 30 ml / min. The dye solution was recirculated for complete decolourisation. Table 1 shows the % removal of dye with number of circulations and time. Complete removal of 200 mM dye solution (pH 10) Table 1. Photocatalytic removal of RR 22 dye a No. of circulations
Time (min)
% Removal of RR 22
0 1 2 3 4 5 6
0.0 16.7 33.3 50.0 66.6 83.1 99.5
0.0 52.7 67.3 78.9 89.0 93.8 98.3
a [RR 22], 200 mM; volume, 500 ml; flow rate, 30 ml / min; pH 10; catalyst weight, 3 g.
The feasibility of the photocatalytic decomposition of RR 22 dye in aqueous solution using a simple and inexpensive bench-scale photoreactor with immobilized ZnO under solar radiation has been shown. The experimental results demonstrated that the RR 22 dye in the textile effluents could be completely removed by using solar energy in conjunction with semiconductor photocatalysts.
Acknowledgements—L. Selva Roselin wishes to thank the Council of Scientific and Industrial Research (CSIR), New Delhi, India for financial support through a Senior Research Fellowship.
REFERENCES Ahmed S. and Ollis D. F. (1984) Solar photoassisted catalytic decomposition of the chlorinated hydrocarbons, trichloroethylene and dichloromethane. Solar Energy 33, 597–601. APHA (1989). Standard Methods for the Examination of Water and Wastewater, 17th American Public Health Association, Washington, DC. Corboz M., Alxneit S., Stoll G. and Tschudi H. R. (2000) On the determination of quantum efficiencies in heterogeneous photocatalysis. J. Phys. Chem. B 104, 10569–10577. Domenech X. and Prieto A. (1986) Stability of ZnO particles in aqueous suspensions under UV illumination. J. Phys. Chem. 90, 1123–1126. Fox M. A. and Dulay M. T. (1993) Heterogeneous photocatalysis. Chem. Rev. 93, 341–357. Krysova H., Krysa J., Macounova K. and Jirkovsky J. (1998) Photocatalytic degradation of diuron [3-(3,4-dichlorophenyl)1,1-dimethylurea] on the layer of TiO 2 particles in the batch mode plate film reactor. J. Chem. Technol. Biotechnol. 72, 169–175. Li F., Gu G., Huang G., Gu Y. and Wan H. (2001) TiO 2 assisted photo-catalysis degradation process of dye chemicals. J. Environ. Sci. 13, 64–68. Mengyue Z., Shifu C. and Yaowu T. (1995) Photocatalytic degradation of organophosphorus pesticides using thin films of TiO 2 . J. Chem. Technol. Biotechnol. 64, 339–344. Muradov N. Z. (1994) Solar detoxification of nitroglycerine contaminated water using immobilized titania. Solar Energy 52, 283–288. Muszkat L., Bir L. and Feigelson L. (1995) Solar photocatalytic mineralization of pesticides in polluted waters. J. Photochem. Photobiol. A: Chem. 87, 85–88.
Sunlight / ZnO-mediated photocatalytic degradation of reactive red 22 using thin film flat bed flow photoreactor Muszkat L., Hallmann M., Raucher D. and Bir L. (1992) Solar photodegradation of xenobiotic contaminants in polluted well water. J. Photochem. Photobiol. A: Chem. 65, 409– 417. Poulios I., Kositzi M. and Kouras A. (1998) Photocatalytic decomposition of triclopyr over aqueous semiconductor suspensions. J. Photochem. Photobiol. A: Chem. 115, 175– 183. Poulios I. and Tsachinis I. (1999) Photodegradation of the textile dye reactive black 5 in the presence of semiconducting oxides. J. Chem. Technol. Biotechnol. 74, 349–357.
285
Reeves P., Ohlhausen R., Sloan D., Pamplin K., Scoggins T., Clark C., Hutchinson B. and Green D. (1992) Photocatalytic destruction of organic dyes in aqueous TiO 2 suspensions using concentrated simulated and natural solar energy. Solar Energy 48, 413–420. Shourong Z., Qingguo H., Jun Z. and Bingkun W. (1997) The study on dye photoremoval in TiO 2 suspensions solution. J. Photochem. Photobiol. A: Chem. 108, 235–238.
PII:
Solar Energy Vol. 73, No. 4, pp. 281–285, 2002 2003 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 2 ) 0 0 0 6 5 – 8 All rights reserved. Printed in Great Britain 0038-092X / 02 / $ - see front matter
www.elsevier.com / locate / solener
SUNLIGHT / ZnO-MEDIATED PHOTOCATALYTIC DEGRADATION OF REACTIVE RED 22 USING THIN FILM FLAT BED FLOW PHOTOREACTOR L. SELVA ROSELIN † , G. R. RAJARAJESWARI, ROSILDA SELVIN, V. SADASIVAM, B. SIVASANKAR and K. RENGARAJ Department of Chemistry, Anna University, Chennai 600 025, India Accepted 2 August 2002
Abstract—Photocatalytic degradation of one of the most widely used cotton dyes, namely reactive red 22 (RR 22), was investigated in the presence of a thin film of ZnO photocatalyst using a thin film flat bed flow photoreactor under solar radiation. The effects of reaction parameters such as pH, amount of ZnO coating, flow rate and concentration of the dye solution on the percentage removal of dye were examined. In a single pass mode at 30 ml / min flow rate, 52.7% decrease in concentration was achieved for 200mM dye solution (pH 10). It has been demonstrated that in continuous circulation mode the time required to decompose half the concentration of the dye in 500 ml of 200 mM was 15.8 min. Complete removal of 200 mM dye solution (pH 10) was achieved at about 100 min. 2003 Elsevier Science Ltd. All rights reserved.
the ground level, solar irradiation starts at about 300 nm. In addition, the | 5% UV component of sunlight is also useful in the excitation process. Most of the previous reports dealt with photocatalytic reactions induced by irradiation of semiconductor particles suspended in aqueous solution (Muszkat et al., 1992, 1995; Reeves et al., 1992; Li et al., 2001). The powder catalysts, however, suffer from two drawbacks. In the first place the turbidity created by the catalyst when kept as a suspension will considerably decrease the amount of solar light interacting with the catalyst. Secondly, the problem associated with the separation of the finely divided photocatalyst from the treated water. Fixing the photocatalyst onto the stationary inert support is preferable to avoid these complications. The catalyst can be immobilized onto the surface of various polymeric materials in different reactor configurations. Krysova et al. (1998) have studied the photocatalytic degradation of diuron in aqueous solution on a TiO 2 layer of a batch mode plate reactor irradiated with UV sun-bed tubes. It was estimated that the total power required for diuron to degrade from 1 3 10 24 M to 1 3 10 26 M is 271 Wh / dm 3 . Therefore the artificial UV light source tends to be somewhat expensive. When the sun is exploited as a source of light, capital cost and recurring expenses for artificial light sources can be saved in the detoxification plant. Muradov (1994) has suggested immobilized titania on a plate for the degradation of nitroglycerine and Rhodamine B under solar irradiation. In the present investigation, solar energy-
1. INTRODUCTION
Effluent streams, especially those from textile dyeing industries, are highly coloured and toxic. Conventional treatment methods are ineffective in completely removing the pollutants, thereby stimulating further research and development activity in the field. Recent studies have shown that the photocatalytic method resulted in complete degradation of organics (Ahmed and Ollis, 1984; Poulios et al., 1998). In this technique, a semiconductor on irradiation with a photon of sufficient energy, greater than or equal to band gap energy, promotes an electron from the valence band to conduction band. Once promoted the electron leaves a vacancy behind in the valence band called a ‘hole’. The positive holes can directly oxidize the organic material by removal of an electron or react with water or OH 2 forming a powerful oxidant such as a hydroxyl radical. The conduction band electrons take part in the reduction reaction with dissolved oxygen, producing superoxide anion radical. Superoxide anion radical is highly reactive and can oxidize any organic compound. The minimum energy required for excitation of an electron for the commonly used photocatalysts such as TiO 2 and ZnO is 3.2 eV, which corresponds to 387.5 nm. These semiconductors can be activated by sunlight, since at †
Author to whom correspondence should be addressed. Tel.: 191-442-351-126x3142; fax: 191-442-350-397; e-mail: [email protected] 281
282
L. S. Roselin et al.
mediated photocatalytic degradation of one of the widely used cotton dyes, namely reactive red 22 (RR 22), was studied by using a simple plate photoreactor in a batch mode in which the liquid film was allowed to flow over the thin layer of ZnO fixed on a glass plate. The reaction parameters such as flow rate, amount of catalyst used for coating, pH and concentration of the dye were optimized. 2. EXPERIMENTAL
2.1. Materials The dye used in the present study is Remazol red B (C.I. Reactive red 22 (RR 22), 14282), 2-(3-amino-4-methoxy phenyl sulphonyl) ethanol sulphate ester → 1-naphthyl-5-sulphonic acid, widely used in dyeing cotton and was obtained from Vanavil Dye House, India. The dye was used as such without any further purification. The dye shows an absorption maximum at 530 nm (´ 5 9800 l mol 21 cm 21 ). Zinc oxide (99.8% purity) used as photocatalyst was obtained from Merck. The surface area and particle size for zinc oxide are 10 m 2 / g and 110 nm, respectively. All other chemicals and reagents used for analysis and experimental work were of analytical grade obtained from Central Drug House (CDH) and used as such.
2.2. Photoreactor The thin film flat bed flow reactor consisted of a glass plate (27.3 3 22.7 cm 2 ) fixed with an inclination to facilitate the flow of solution. The glass plate was coated with a thin film (1–2 mm) of the photocatalyst using ZnO slurry and a thin layer chromatography applicator. The dye solution was pumped through a perforated glass tube so as to allow the flow as a thin film over the entire surface of the coated glass plate along the long axis of the support plate and collected into a receiving flask. The flow rate of the solution was maintained with the help of a peristaltic pump. The dye solution was exposed to direct sunlight as it flowed over the photocatalyst-coated glass plate. The intensity of the sunlight during the reaction time was in the range 808–1070 W/ m 2 . The progress of the reaction was followed by withdrawing 3 ml of the photolysed water from the receiving flask after each cycle and measuring the absorbance at 530 nm in a Systronics spectrophotometer (Model 118). Total mineralization of the dye was confirmed by estimation of inorganic minerals such as sulphate, nitrate and
ammonium ions by standard methods of analysis (APHA, 1989). 3. RESULTS AND DISCUSSION
3.1. Effect of flow rate In order to optimize the flow rate of the dye solution on the ZnO-coated glass plate for the degradation of the dye, a series of experiments were carried out at different flow rates in the range from 15 to 35 ml / min using the same concentration of the dye solution (75 mM) and the same amount of catalyst coating (1 g). The thickness of the liquid film and the residence time of the solution on the catalyst layer at all flow rates were theoretically calculated as 0.11, 0.14, 0.18, 0.22, 0.26 mm and 4.0, 3.0, 2.4, 2.0, 1.7 s / cm for flow rates 15, 20, 25, 30 and 35 ml / min, respectively. The % removal of the dye in single-pass mode was observed for varying flow rate. The results demonstrate that the % removal of dye decreases with increasing flow rate and the decrease slows down after a flow rate of about 30 ml / min (Fig. 1). Therefore, a flow rate of 30 ml / min has been fixed for further studies.
3.2. Effect of the amount of ZnO coating The minimum amount of ZnO required to form a uniform thin film coating to cover the entire surface of the chosen glass plate was found to be about 1 g. The thickness of the ZnO coating can be optimized for the photocatalytic removal of RR 22. Photocatalytic experiments were performed
Fig. 1. Effect of flow rate on the % removal of the dye ([RR 22], 75 mM; volume, 500 ml; pH 6.8; weight of the catalyst, 1 g (1.57 mg / cm 2 ); solar radiation intensity, 808–1070 W/ m 2 ).
Sunlight / ZnO-mediated photocatalytic degradation of reactive red 22 using thin film flat bed flow photoreactor
with various amounts of ZnO coating in the range 1–5 g. The % removal of the dye in single-pass mode was observed for varying amount of catalyst coating. It is seen from Fig. 2 that the % removal of the dye increases as the ZnO coating thickness increases. With increasing amount of ZnO the availability of semiconductor particles for absorption of photons increases, thereby producing a greater number of oxidizing sites, which consequently increase the rate of the reaction. The increase is not pronounced beyond 3 g ZnO coating. This may be due to the number of photons that are absorbed by the active site of the catalyst which is exposed to sunlight becomes constant beyond 3 g of ZnO coating since additional photocatalyst is not illuminated. Also the light intensity profile within the layer is, essentially, decreasing above the saturation level (Corboz et al., 2000). Therefore the optimum amount of ZnO for the experimental set-up is to be fixed as 3 g (i.e. 4.7 mg / cm 2 ) having a thickness of 1.4 mm.
3.3. Effect of pH The pH of the solution can be one of the most important parameters for the photocatalytic process, since the particle size, surface charge and band edge positions of the semiconductor oxide are strongly influenced by pH (Fox and Dulay, 1993). The photocatalytic experiment was carried out at various pH values in the range 4–12 to find out the optimum pH. As can be seen from Fig. 3, when the initial pH of the dye solution varied from 4 to 12, the % removal of the dye reached a maximum at 10 followed by a decrease on reaching pH 12. The pH effect on the photocatalytic degradation of the dye with ZnO was
283
Fig. 3. Effect of pH on the % removal of dye ([RR 22], 75 mM; volume, 500 ml; weight of the catalyst, 3 g (4.7 mg / cm 2 ); flow rate, 30 ml / min; solar radiation intensity, 808– 1070 W/ m 2 ).
explained on the basis of adsorption of the dye on the catalyst surface. It was found that with increasing pH the amount of dye adsorbed is decreased. The strong adsorption leads to a drastic decrease in the active centre on the catalyst surface which results in decrease in the absorption of the irradiation surface and consequently to a lowering of the reaction rate. A similar influence of pH on the photocatalytic degradation rate of the reactive black 5 dye was reported by Poulios and Tsachinis (1999). Furthermore, the increase in the removal of dye with pH may be explained on the basis that hydroxyl radicals are involved in the photocatalytic reaction. With increasing pH, the hydroxyl ion concentration increases and so the hydroxyl radical formation increases. In alkaline solution the formation of the OHE radical is much easier than in neutral and acidic solution (Shourong et al., 1997). In a high pH value (i.e. pH 12), a drastic decrease in the reaction rate could be due to the formation of Zn(OH) 2 . In addition, ZnO undergoes photo-corrosion as given in Eq. (1) (Domenech and Prieto, 1986) 1 ZnO 1 2h 1 → Zn 21 1 ] O 2 2
(1)
3.4. Effect of initial dye concentration
Fig. 2. Effect of catalyst weight on the % removal of the dye ([RR 22], 75 mM; volume, 500 ml; pH 6.8; flow rate, 30 ml / min; solar radiation intensity, 808–1070 W/ m 2 ).
The effect of concentration of RR 22 on the % removal of the dye was investigated in the concentration range 25–400 mM. It is seen from Fig. 4 that the % removal of the dye increases steeply as the concentration of the dye increases to 100 mM and thereafter there is only a marginal increase and at 400 mM it decreases. The increase was due to the increase in the availability of dye
284
L. S. Roselin et al.
was achieved at about 100 min. In single pass mode at 30 ml / min flow rate, 52.7% decrease in concentration was achieved for 200 mM dye solution (pH 10). The finally treated water was tested for complete mineralization by estimation of inorganic ions such as sulphate, nitrate and ammonium ions. Results showed that the solar photolysis in the presence of immobilized ZnO has destroyed the RR 22 dye effectively (destruction efficiency reached 98.3%).
4. CONCLUSIONS
Fig. 4. Effect of initial dye concentration on the % removal of dye (volume, 500 ml; pH 10.0; weight of the catalyst, 3 g (4.7 mg / cm 2 ); flow rate, 30 ml / min; solar radiation intensity, 808–1070 W/ m 2 ).
molecules for oxidation. The decrease at higher concentration may be explained as follows: as the dye solution becomes dense it obstructs the penetration of light and hence a smaller number of oxidative species are generated. In addition, at higher concentrations, a greater number of dye molecules are adsorbed virtually masking the surface of the catalyst particle and preventing the photons interacting with the catalyst. Mengyue et al. (1995) have observed a similar decrease in the rate for the degradation of organophosphorus pesticides using thin films of TiO 2 under UV radiation.
3.5. Total removal of the dye Under the optimized conditions using 3 g ZnO coating, 500 ml of 200 mM RR 22 dye solution (pH 10) were allowed to flow on the thin film coated with ZnO at a flow rate of 30 ml / min. The dye solution was recirculated for complete decolourisation. Table 1 shows the % removal of dye with number of circulations and time. Complete removal of 200 mM dye solution (pH 10) Table 1. Photocatalytic removal of RR 22 dye a No. of circulations
Time (min)
% Removal of RR 22
0 1 2 3 4 5 6
0.0 16.7 33.3 50.0 66.6 83.1 99.5
0.0 52.7 67.3 78.9 89.0 93.8 98.3
a [RR 22], 200 mM; volume, 500 ml; flow rate, 30 ml / min; pH 10; catalyst weight, 3 g.
The feasibility of the photocatalytic decomposition of RR 22 dye in aqueous solution using a simple and inexpensive bench-scale photoreactor with immobilized ZnO under solar radiation has been shown. The experimental results demonstrated that the RR 22 dye in the textile effluents could be completely removed by using solar energy in conjunction with semiconductor photocatalysts.
Acknowledgements—L. Selva Roselin wishes to thank the Council of Scientific and Industrial Research (CSIR), New Delhi, India for financial support through a Senior Research Fellowship.
REFERENCES Ahmed S. and Ollis D. F. (1984) Solar photoassisted catalytic decomposition of the chlorinated hydrocarbons, trichloroethylene and dichloromethane. Solar Energy 33, 597–601. APHA (1989). Standard Methods for the Examination of Water and Wastewater, 17th American Public Health Association, Washington, DC. Corboz M., Alxneit S., Stoll G. and Tschudi H. R. (2000) On the determination of quantum efficiencies in heterogeneous photocatalysis. J. Phys. Chem. B 104, 10569–10577. Domenech X. and Prieto A. (1986) Stability of ZnO particles in aqueous suspensions under UV illumination. J. Phys. Chem. 90, 1123–1126. Fox M. A. and Dulay M. T. (1993) Heterogeneous photocatalysis. Chem. Rev. 93, 341–357. Krysova H., Krysa J., Macounova K. and Jirkovsky J. (1998) Photocatalytic degradation of diuron [3-(3,4-dichlorophenyl)1,1-dimethylurea] on the layer of TiO 2 particles in the batch mode plate film reactor. J. Chem. Technol. Biotechnol. 72, 169–175. Li F., Gu G., Huang G., Gu Y. and Wan H. (2001) TiO 2 assisted photo-catalysis degradation process of dye chemicals. J. Environ. Sci. 13, 64–68. Mengyue Z., Shifu C. and Yaowu T. (1995) Photocatalytic degradation of organophosphorus pesticides using thin films of TiO 2 . J. Chem. Technol. Biotechnol. 64, 339–344. Muradov N. Z. (1994) Solar detoxification of nitroglycerine contaminated water using immobilized titania. Solar Energy 52, 283–288. Muszkat L., Bir L. and Feigelson L. (1995) Solar photocatalytic mineralization of pesticides in polluted waters. J. Photochem. Photobiol. A: Chem. 87, 85–88.
Sunlight / ZnO-mediated photocatalytic degradation of reactive red 22 using thin film flat bed flow photoreactor Muszkat L., Hallmann M., Raucher D. and Bir L. (1992) Solar photodegradation of xenobiotic contaminants in polluted well water. J. Photochem. Photobiol. A: Chem. 65, 409– 417. Poulios I., Kositzi M. and Kouras A. (1998) Photocatalytic decomposition of triclopyr over aqueous semiconductor suspensions. J. Photochem. Photobiol. A: Chem. 115, 175– 183. Poulios I. and Tsachinis I. (1999) Photodegradation of the textile dye reactive black 5 in the presence of semiconducting oxides. J. Chem. Technol. Biotechnol. 74, 349–357.
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Reeves P., Ohlhausen R., Sloan D., Pamplin K., Scoggins T., Clark C., Hutchinson B. and Green D. (1992) Photocatalytic destruction of organic dyes in aqueous TiO 2 suspensions using concentrated simulated and natural solar energy. Solar Energy 48, 413–420. Shourong Z., Qingguo H., Jun Z. and Bingkun W. (1997) The study on dye photoremoval in TiO 2 suspensions solution. J. Photochem. Photobiol. A: Chem. 108, 235–238.