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Solar photocatalytic decolorization of methylene blue in water
Chemosphere 45 (2001) 77±83
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Solar photocatalytic decolorization of methylene blue in water W.S. Kuo *, P.H. Ho Department of Safety, Health and Environmental Engineering, National Lien-Ho Institute of Technology, Miao-Li 360, Taiwan, ROC Received 15 July 2000; accepted 7 December 2000
Abstract In this study, a photocatalytic decolorization system equipped with immobilized TiO2 and illuminated by solar light was used to remove the color of wastewater. To examine the decoloring eciency of this system, photocatalytic decolorization of an organic dye such as methylene blue was studied as an example. The eects of light source, pH, as well as the initial concentration of dye were also investigated. It was observed that the solution of methylene blue could be almost completely decolorized by the solar light/TiO2 ®lm process while there was about 50% color remaining with solar irradiation only. In addition, it was found that the decoloring eciency of solution was higher with solar light irradiation than with arti®cial UV light irradiation, even though the arti®cial UV light source supplied higher UV intensity at 254 nm. The color removal rate of methylene blue with solar light irradiation was almost twice that of arti®cial UV light irradiation. This phenomena was mainly attributed to that some visible light range of solar light was useful for exciting the methylene blue molecules adsorbed on TiO2 ®lm, leading to a photosensitization process undergoing and decoloring eciency promoted. This solar-assisted photocatalytic device showed potential application for decoloring organic dyes in wastewater. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Photocatalytic decolorization; Solar light; Methylene blue; TiO2
1. Introduction The widespread presence of organic dyes in industrial wastewater results in a potentially serious environmental problem. Due to the nature of synthetic dyes, conventional biological treatment methods are ineective for decoloring such wastewaters (Arslan and Balcioglu, 1999). In conventional industrial wastewater treatment practices, organic dyes were usually removed with adsorbents or coagulation. However, new environmental laws may consider the spent adsorbents or sludge as hazardous wastes and require further treatment. Consequently, novel technologies with more eciency and less energy used have been stimulated intensive research. An alternative to conventional methods are advanced *
Corresponding author. Fax: +886-37-333187. E-mail address: [email protected] (W.S. Kuo).
oxidation processes (AOPs) based on the generation of very reactive species such as hydroxyl radicals (OH) that oxidize a broad range of organic pollutants quickly and nonselectively. Photocatalytic oxidation using a semiconductor such as TiO2 as photocatalyst is one of AOPs. As TiO2 is illuminated by light rays with wavelength below 380 nm, the photons excite valence band electrons across the band gap into the conduction band, leaving holes behind in the valence band. The holes in TiO2 will react with water molecules or hydroxide ions (OH ) and then produce hydroxyl radicals (OH) (Crittenden et al., 1996). Oxygen is usually supplied as electron acceptor to prolong the recombination of electron±hole pairs during photocatalytic oxidation. Several investigations (Davis et al., 1994; Matthews, 1997; Wu and Dong, 1997; Wang and Fu, 1998; Zhan et al., 1998; Li and Zhao, 1999; Poulios and Tsachpinis, 1999; Xu et al., 1999) have reported that TiO2 photocatalysis was an eective
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 1 ) 0 0 0 0 8 - X
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W.S. Kuo, P.H. Ho / Chemosphere 45 (2001) 77±83
method for decoloring and oxidizing organic dyes in wastewater. However, the photocatalyst TiO2 was usually used as powder in solution, which required the separation of TiO2 particles from the puri®ed water after the decoloring reaction. In addition, the process was generally equipped with an arti®cial light source, leading to a considerable increasing of operation cost. These drawbacks may be an obstacle to the TiO2 photocatalysis for practical application. In this study, a photocatalytic decolorization system, in which solar light could be used as irradiation source and Degussa P-25 TiO2 as a stationary phase were on the SS304 blades of agitator and the inner surface of a SS304 reactor vessel, was used to remove the color of wastewater. In this system, separation of photocatalyst from the puri®ed water after decolorization reaction was not necessary and no other aeration equipment was required to supply oxygen. To examine the decoloring eciency of this system, photocatalytic decolorization of an organic dye such as methylene blue was studied as an example. The eects of light source, pH, as well as the initial concentration of dye were also investigated.
2. Materials and methods 2.1. Materials Methylene blue (MB) (C16 H18 ClN3 S) was provided by Koch-Light Lab., England, and used without further puri®cation. The TiO2 powder P-25 (mainly anatase form, mean particle size: 30 nm, BET surface area: 50 15 m2 =g) from Degussa (Frankfurt, Germany) were immobilized on the surface of agitator's blades and inner surface of reactor by using PTFE (TEFLON) resin-bonded technique (Kuo, 2000). 2.2. Apparatus The experimental setup of this study was shown in Fig. 1. The eective volume of photoreactor (36 cm L 27.5 cm W 8 cm H) was 8.0 L. Three 15 W lowpressure mercury vapor lamps (illuminated wavelength: 254 nm, light intensity at 9 cm: 3.0 mW/cm2 ) or solar light were used to supply irradiation. The rotating speed of motor was kept at 70 rpm during the experiment. The size of the SS304 blades was 3 cm W 15 cm L with a thickness of 1 mm. The total area of TiO2 ®lm was approximately 2500 cm2 , i.e., 0.8 g TiO2 coated. The light intensity of UV lamps was measured by a UVP radiometer (UVP, USA) at 254 nm. The light intensity of sunshine was measured by a UVP radiometer (UVP, USA) at 254 and 365 nm.
Fig. 1. Schematic diagram of photocatalytic decolorization reactor.
2.3. Procedures 2.3.1. UV/TiO2 ®lm photocatalysis decolorization experiments Three 15 W low-pressure mercury vapor lamps (illuminated wavelength: 254 nm, light intensity at 9 cm: 3.0 mW/cm2 ) were used to supply UV irradiation. Solutions containing methylene blue with the concentrations of 5 and 10 lM were prepared at pH 4, 7, and 10. The prepared solutions were placed into the photoreactor and irradiated by arti®cial UV light. Sampling was carried out for 6 h at preset intervals. To investigate the eect of TiO2 ®lm, a similar photoreactor without TiO2 ®lm was also performed under the same conditions. 2.3.2. Solar light/TiO2 ®lm photocatalysis decolorization experiments Solar light was used as light source. Solutions containing methylene blue with the concentrations of 0.5, 5, 10, and 30 lM were prepared at pH 4 and 7. The prepared solutions were placed into the photoreactor and irradiated by solar light. Sampling was carried out for 6 h at preset intervals. To investigate the eect of TiO2 ®lm, a similar photoreactor without TiO2 ®lm was also performed under the same conditions. 2.4. Analyses Analytical measurements of methylene blue in water was determined with a spectrophotometer (UV160, Shimadu, Japan) for absorbance measurement and a DR/4000 spectrophotometer (Hach, USA) for ADMI measurement. The eciency of decolorization was calculated on the basis of ADMI reduction of solution.
W.S. Kuo, P.H. Ho / Chemosphere 45 (2001) 77±83
79
Fig. 2. Eect of pH and initial concentration of methylene blue on decoloring eciency with arti®cial UV light irradiation (UV light intensity at 254 nm: 3.0 mW/cm2 , (a) C0 5 lM, (b) C0 10 lM).
3. Results and discussion 3.1. Photocatalytic decolorization of methylene blue with arti®cial light source (UV lamps) Fig. 2 shows the color removal eciency of methylene blue solution as a function of pH and the initial concentration of dye. It was found that the better color removal eciency was generally reached with the pH level of 4.0, especially for the UV/TiO2 ®lm system, which was qualitatively consistent with the ®nding of Lee et al. (1999). This could be due to more methylene blue ions (C16 H18 N3 S ) adsorbed on the surface of TiO2 ®lm and reacted with free radicals at the level of pH. Also, since the natural electricity of TiO2 surface was signi®cantly aected by the pH of solution, the eect of pH on decoloring eciency was found to be enlarged for the UV/TiO2 ®lm system. On the other hand, it was
observed that the color removal eciency decreased with the initial concentration of methylene blue. To more quantitatively evaluate the eect of pH and initial concentration of methylene blue, the pseudo-®rst-order model as expressed by Eq. (1), which was generally used for photocatalytic oxidation process if the initial concentration of pollutant was low (Herrmann et al., 1997), was applied to obtain the rate constants, dC kC; dt
1
in which C is the concentration of methylene blue at time t, and k is the pseudo-®rst-order rate constant. The rate constants obtained from the curves in Fig. 2 were given in Table 1. It was shown that the color removal rate increased signi®cantly with TiO2 photocatalysis compared to UV only system, especially at the pH level of 4.
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Table 1 The pseudo-®rst-order rate constants of methylene blue with arti®cial UV lighta irradiation Run no.
pH
C0 (lM)b
k1 (h 1 )c
k2 (h 1 )d
k2=k1
1 2 3 4 5 6
4 7 10 4 7 10
5 5 5 10 10 10
0.095 0.067 0.171 0.049 0.023 0.058
0.999 0.456 0.514 0.573 0.307 0.238
10.52 6.81 3.01 11.69 13.35 4.10
a
UV light intensity at 254 nm: 3.0 mW/cm2 . C0 : initial concentration of methylene blue. c k1: the pseudo-®rst-order rate constant under arti®cial UV light irradiation. d k2: the pseudo-®rst-order rate constant with arti®cial UV light/TiO2 ®lm system. b
3.2. Photocatalytic decolorization of methylene blue with a natural light source (solar light) Fig. 3 shows the color removal eciency of methylene blue as a function of solar light intensity. The decoloring eciency increased as solar light intensity increased. It was also found that the eect of solar light intensity on decoloring eciency was more signi®cant in the solar light irradiation only system than in the solar light/TiO2 ®lm system. This could be attributed to the self-photofading of methylene blue (which is also a photocatalyst) (Jain et al., 1999) with irradiation of some visible region of solar light. In addition, the electrons in TiO2 could be excited eectively even at very low solar light irradiation with a level of 50.2 lW/cm2 , showing that reduction of decoloring eciency was not
signi®cant for the solar light/TiO2 ®lm system as solar light intensity decreased. The eect of solar light intensity could also be revealed on the basis of pseudo-®rstorder rate constants. The rate constants obtained from the curves in Fig. 3 were listed in Table 2. The k2=k1 value was increased as weaker solar light illuminated, suggesting that the promotion of color removal with TiO2 photocatalysis was more prominent at lower solar light intensity. Fig. 4 showed that the color removal eciency of methylene blue as a function of the initial concentration of methylene blue under solar light irradiation. The increased concentration of methylene blue always decreased the decoloring eciency in both the solar light system and the solar light/TiO2 ®lm system. The adverse eect of increasing initial concentration of dye was less
Fig. 3. Eect of solar light intensity on the decoloring eciency of wastewater (C0 5 lM, pH: 7.0).
W.S. Kuo, P.H. Ho / Chemosphere 45 (2001) 77±83
81
Table 2 The pseudo-®rst-order rate constants of methylene blue with solar light irradiation Run no.
I1 (mW=cm2 )a
I2 (mW=cm2 )b
C0 (lM)c
k1 (h 1 )d
k2 (h 1 )e
k2=k1
1 2 3 4 5
0.055 0.050 0.113 0.085 0.095
0.73 0.66 1.40 1.12 1.27
0.5 5 5 10 30
0.365 0.069 0.149 0.025 0.024
0.954 0.535 0.912 0.302 0.163
2.61 7.72 6.14 11.85 6.83
a
I1 : solar light intensity at 254 nm. I2 : solar light intensity at 365 nm. c C0 : the initial concentration of methylene blue. d k1: the pseudo-®rst-order rate constant under solar light irradiation. e k2: the pseudo-®rst-order rate constant with solar light/TiO2 ®lm system. b
Fig. 4. Eect of initial concentration of methylene blue on the decoloring eciency of wastewater (solar light intensity at 254 nm: 85 113 lW=cm2 , pH: 7.0).
critical for the solar light/TiO2 ®lm system than the solar light system. This could be due to the fact that the TiO2 photocatalyst played an important role in depreciating the eect of the apparent reduction of light penetration into solution as increasing the concentration of dye. On the basis of k2=k1 values listed in Table 2, the promotion of color removal with TiO2 photocatalysis was actually ecient no matter which concentration of dye was treated. 3.3. Comparisons of the decoloring eciency of methylene blue with various light sources Fig. 5 shows the comparisons of color removal ef®ciency of methylene blue with various light sources.
The decoloring eciency of methylene blue with solar light irradiation was more ecient than that with arti®cial UV light irradiation. It was found that the color removal rate of methylene blue with solar light irradiation was almost twice that of arti®cial UV light irradiation as comparing the data of Run No. 3 in Table 2 to the data of Run No. 2 in Table 1. This phenomena was partially attributed to a higher temperature of dye solution generally found under solar light irradiation. Previous work (Lee et al., 1999) reported that the activation energy of UV/TiO2 photocatalysis for methylene blue was 14.45 kJ/mole. According to this reported data and Arrhenius's equation, the color removal rate of methylene blue could be increased 20% from the operation temperature of 25±35°C. In addition, though
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Fig. 5. Comparisons of the color removal eciency of methylene blue with various light sources (C0 5 lM).
the intensity of solar light @ 254 nm (0.1 mW/cm2 ) was much lower than that of arti®cial UV light @ 254 nm (3.0 mW/cm2 ), the measured intensity of solar light @ 365 nm was with a mean value of 1.4 mW/cm2 during the experimental period, which was able to allow the methylene blue molecules adsorbed on TiO2 ®lm (MB ads ) undergoing a photosensitization mechanisms (Eqs. (2)±(5)) or forming electronically excited oxygen atoms ± the singlet oxygen atom (O(1 D)) (Eq. (6)) and then promoted decoloring eciency. Accordingly, the photocatalytic device used in this study showed potential application for decoloring wastewater with solar light irradiation. The general drawback of consuming energy for a UV/TiO2 photocatalysis process could be minimized. MB ads
solar light
! MB ads
2
MB ads Ti O2 ! MB ads h e
3
e O2 ! O2
4
4. Conclusions This study revealed the prospect of solar photocatalytic decolorization of organic dyes in water. The photocatalytic decoloring eciency of methylene blue solution was higher with solar light irradiation than with arti®cial UV light irradiation, even though the arti®cial UV light source supplied higher UV intensity at 254 nm. This phenomena was mainly attributed to a photosensitization process involving the TiO2 semiconductor and the adsorbed dye itself under solar light irradiation, leading to more OH generated and decoloring eciency promoted. In addition, wastewater irradiated by solar irradiation was found to be warmer than that irradiated by UV lamp under the imposed conditions, which generally resulted in an increased color removal rate. Moreover, an acidic pH level and a lower initial concentration of dye solution were found to be bene®cial for color removal. Accordingly, this solar-assisted photocatalytic device showed potential application for decoloring organic dyes in wastewater.
Acknowledgements
h OH ! OH
5
MBads O2 ! MBads 2O 1 D
6
The authors are grateful to Lein-Ho Educational Foundation and National Science Council, Taiwan for ®nancial support (NSC 89-2211-E-239-001).
W.S. Kuo, P.H. Ho / Chemosphere 45 (2001) 77±83
References Arslan, I., Balcioglu, I.A., 1999. Degradation of commercial reactive dyestus by heterogeneous and homogeneous advanced oxidation processes: a comparative study. Dyes Pigments 43, 95±108. Crittenden, J.C., Zhang, Y., Hand, D.W., Perram, D.L., Marchand, E.G., 1996. Solar detoxi®cation of fuel-contaminated groundwater using ®xed-bed photocatalysts. Water Environ. Res. 68, 270±278. Davis, R.J., Gainer, J.L., O'Neal, G., Wu, I.W., 1994. Photocatalytic decolorization of wastewater dyes. Water Environ. Res. 66, 50±53. Herrmann, J.M., Ahiri, H., Ait-Ichou, Y., Lassaletta, G., Gonzalez-Elipe, A.R., Fernandez, A., 1997. Characterization and photocatalytic activity in aqueous medium of TiO2 and Ag±TiO2 coating on quartz. Appl. Catal. B 13, 219±228. Jain, S., Dangi, G., Vardia, J., Ameta, S.C., 1999. Photocatalytic reduction of some alkali carbonates in the presence of methylene blue. Int. J. Energy Res. 23, 71±77. Kuo, W.S., 2000. Preparation and photocatalytic activity of metal-supported resin-bonded titania. J. Environ. Sci. Health A 35, 419±433.
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Lee, B.N., Liaw, W.D., Lou, J.C., 1999. Photocatalytic decolorization of methylene blue in aqueous TiO2 suspension. Environ. Eng. Sci. 16, 165±175. Li, X.Z., Zhao, Y.G., 1999. Advanced treatment of dyeing wastewater for reuse. Water Sci. Technol. 39, 249±255. Matthews, L.R., 1997. The incorporation of titania into modi®ed silicates for solar photodegradation of aqueous species. J. Sol-Gel Sci. Technol. 8, 619±623. Poulios, I., Tsachpinis, I., 1999. Photodegradation of the textile dye reactive Black 5 in the presence of semiconducting oxides. J. Chem. Technol. Biotechnol. 74, 349±357. Wang, Y., Fu, Y., 1998. Solar photocatalytic degradation of dierent types of dyes. Chin. J. Biomed. Eng. 17, 117±126. Wu, H., Dong, X., 1997. Study on solar photocatalytic decolorization of dye euent. Acta Energiae Solaris Sinica 18, 380±384. Xu, N., Shi, Z., Fan, Y., Dong, J., Shi, J., Hu, M.Z.C., 1999. Eects of particle size of TiO2 on photocatalytic degradation of methylene blue in aqueous suspensions. Ind. Eng. Chem. Res. 38, 373±379. Zhan, H., Chen, K., Tian, H., 1998. Photocatalytic degradation of acid azo dyes in aqueous TiO2 suspension. II. The eect of pH values. Dyes Pigments 37, 241±247.
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Solar photocatalytic decolorization of methylene blue in water W.S. Kuo *, P.H. Ho Department of Safety, Health and Environmental Engineering, National Lien-Ho Institute of Technology, Miao-Li 360, Taiwan, ROC Received 15 July 2000; accepted 7 December 2000
Abstract In this study, a photocatalytic decolorization system equipped with immobilized TiO2 and illuminated by solar light was used to remove the color of wastewater. To examine the decoloring eciency of this system, photocatalytic decolorization of an organic dye such as methylene blue was studied as an example. The eects of light source, pH, as well as the initial concentration of dye were also investigated. It was observed that the solution of methylene blue could be almost completely decolorized by the solar light/TiO2 ®lm process while there was about 50% color remaining with solar irradiation only. In addition, it was found that the decoloring eciency of solution was higher with solar light irradiation than with arti®cial UV light irradiation, even though the arti®cial UV light source supplied higher UV intensity at 254 nm. The color removal rate of methylene blue with solar light irradiation was almost twice that of arti®cial UV light irradiation. This phenomena was mainly attributed to that some visible light range of solar light was useful for exciting the methylene blue molecules adsorbed on TiO2 ®lm, leading to a photosensitization process undergoing and decoloring eciency promoted. This solar-assisted photocatalytic device showed potential application for decoloring organic dyes in wastewater. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Photocatalytic decolorization; Solar light; Methylene blue; TiO2
1. Introduction The widespread presence of organic dyes in industrial wastewater results in a potentially serious environmental problem. Due to the nature of synthetic dyes, conventional biological treatment methods are ineective for decoloring such wastewaters (Arslan and Balcioglu, 1999). In conventional industrial wastewater treatment practices, organic dyes were usually removed with adsorbents or coagulation. However, new environmental laws may consider the spent adsorbents or sludge as hazardous wastes and require further treatment. Consequently, novel technologies with more eciency and less energy used have been stimulated intensive research. An alternative to conventional methods are advanced *
Corresponding author. Fax: +886-37-333187. E-mail address: [email protected] (W.S. Kuo).
oxidation processes (AOPs) based on the generation of very reactive species such as hydroxyl radicals (OH) that oxidize a broad range of organic pollutants quickly and nonselectively. Photocatalytic oxidation using a semiconductor such as TiO2 as photocatalyst is one of AOPs. As TiO2 is illuminated by light rays with wavelength below 380 nm, the photons excite valence band electrons across the band gap into the conduction band, leaving holes behind in the valence band. The holes in TiO2 will react with water molecules or hydroxide ions (OH ) and then produce hydroxyl radicals (OH) (Crittenden et al., 1996). Oxygen is usually supplied as electron acceptor to prolong the recombination of electron±hole pairs during photocatalytic oxidation. Several investigations (Davis et al., 1994; Matthews, 1997; Wu and Dong, 1997; Wang and Fu, 1998; Zhan et al., 1998; Li and Zhao, 1999; Poulios and Tsachpinis, 1999; Xu et al., 1999) have reported that TiO2 photocatalysis was an eective
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 1 ) 0 0 0 0 8 - X
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W.S. Kuo, P.H. Ho / Chemosphere 45 (2001) 77±83
method for decoloring and oxidizing organic dyes in wastewater. However, the photocatalyst TiO2 was usually used as powder in solution, which required the separation of TiO2 particles from the puri®ed water after the decoloring reaction. In addition, the process was generally equipped with an arti®cial light source, leading to a considerable increasing of operation cost. These drawbacks may be an obstacle to the TiO2 photocatalysis for practical application. In this study, a photocatalytic decolorization system, in which solar light could be used as irradiation source and Degussa P-25 TiO2 as a stationary phase were on the SS304 blades of agitator and the inner surface of a SS304 reactor vessel, was used to remove the color of wastewater. In this system, separation of photocatalyst from the puri®ed water after decolorization reaction was not necessary and no other aeration equipment was required to supply oxygen. To examine the decoloring eciency of this system, photocatalytic decolorization of an organic dye such as methylene blue was studied as an example. The eects of light source, pH, as well as the initial concentration of dye were also investigated.
2. Materials and methods 2.1. Materials Methylene blue (MB) (C16 H18 ClN3 S) was provided by Koch-Light Lab., England, and used without further puri®cation. The TiO2 powder P-25 (mainly anatase form, mean particle size: 30 nm, BET surface area: 50 15 m2 =g) from Degussa (Frankfurt, Germany) were immobilized on the surface of agitator's blades and inner surface of reactor by using PTFE (TEFLON) resin-bonded technique (Kuo, 2000). 2.2. Apparatus The experimental setup of this study was shown in Fig. 1. The eective volume of photoreactor (36 cm L 27.5 cm W 8 cm H) was 8.0 L. Three 15 W lowpressure mercury vapor lamps (illuminated wavelength: 254 nm, light intensity at 9 cm: 3.0 mW/cm2 ) or solar light were used to supply irradiation. The rotating speed of motor was kept at 70 rpm during the experiment. The size of the SS304 blades was 3 cm W 15 cm L with a thickness of 1 mm. The total area of TiO2 ®lm was approximately 2500 cm2 , i.e., 0.8 g TiO2 coated. The light intensity of UV lamps was measured by a UVP radiometer (UVP, USA) at 254 nm. The light intensity of sunshine was measured by a UVP radiometer (UVP, USA) at 254 and 365 nm.
Fig. 1. Schematic diagram of photocatalytic decolorization reactor.
2.3. Procedures 2.3.1. UV/TiO2 ®lm photocatalysis decolorization experiments Three 15 W low-pressure mercury vapor lamps (illuminated wavelength: 254 nm, light intensity at 9 cm: 3.0 mW/cm2 ) were used to supply UV irradiation. Solutions containing methylene blue with the concentrations of 5 and 10 lM were prepared at pH 4, 7, and 10. The prepared solutions were placed into the photoreactor and irradiated by arti®cial UV light. Sampling was carried out for 6 h at preset intervals. To investigate the eect of TiO2 ®lm, a similar photoreactor without TiO2 ®lm was also performed under the same conditions. 2.3.2. Solar light/TiO2 ®lm photocatalysis decolorization experiments Solar light was used as light source. Solutions containing methylene blue with the concentrations of 0.5, 5, 10, and 30 lM were prepared at pH 4 and 7. The prepared solutions were placed into the photoreactor and irradiated by solar light. Sampling was carried out for 6 h at preset intervals. To investigate the eect of TiO2 ®lm, a similar photoreactor without TiO2 ®lm was also performed under the same conditions. 2.4. Analyses Analytical measurements of methylene blue in water was determined with a spectrophotometer (UV160, Shimadu, Japan) for absorbance measurement and a DR/4000 spectrophotometer (Hach, USA) for ADMI measurement. The eciency of decolorization was calculated on the basis of ADMI reduction of solution.
W.S. Kuo, P.H. Ho / Chemosphere 45 (2001) 77±83
79
Fig. 2. Eect of pH and initial concentration of methylene blue on decoloring eciency with arti®cial UV light irradiation (UV light intensity at 254 nm: 3.0 mW/cm2 , (a) C0 5 lM, (b) C0 10 lM).
3. Results and discussion 3.1. Photocatalytic decolorization of methylene blue with arti®cial light source (UV lamps) Fig. 2 shows the color removal eciency of methylene blue solution as a function of pH and the initial concentration of dye. It was found that the better color removal eciency was generally reached with the pH level of 4.0, especially for the UV/TiO2 ®lm system, which was qualitatively consistent with the ®nding of Lee et al. (1999). This could be due to more methylene blue ions (C16 H18 N3 S ) adsorbed on the surface of TiO2 ®lm and reacted with free radicals at the level of pH. Also, since the natural electricity of TiO2 surface was signi®cantly aected by the pH of solution, the eect of pH on decoloring eciency was found to be enlarged for the UV/TiO2 ®lm system. On the other hand, it was
observed that the color removal eciency decreased with the initial concentration of methylene blue. To more quantitatively evaluate the eect of pH and initial concentration of methylene blue, the pseudo-®rst-order model as expressed by Eq. (1), which was generally used for photocatalytic oxidation process if the initial concentration of pollutant was low (Herrmann et al., 1997), was applied to obtain the rate constants, dC kC; dt
1
in which C is the concentration of methylene blue at time t, and k is the pseudo-®rst-order rate constant. The rate constants obtained from the curves in Fig. 2 were given in Table 1. It was shown that the color removal rate increased signi®cantly with TiO2 photocatalysis compared to UV only system, especially at the pH level of 4.
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Table 1 The pseudo-®rst-order rate constants of methylene blue with arti®cial UV lighta irradiation Run no.
pH
C0 (lM)b
k1 (h 1 )c
k2 (h 1 )d
k2=k1
1 2 3 4 5 6
4 7 10 4 7 10
5 5 5 10 10 10
0.095 0.067 0.171 0.049 0.023 0.058
0.999 0.456 0.514 0.573 0.307 0.238
10.52 6.81 3.01 11.69 13.35 4.10
a
UV light intensity at 254 nm: 3.0 mW/cm2 . C0 : initial concentration of methylene blue. c k1: the pseudo-®rst-order rate constant under arti®cial UV light irradiation. d k2: the pseudo-®rst-order rate constant with arti®cial UV light/TiO2 ®lm system. b
3.2. Photocatalytic decolorization of methylene blue with a natural light source (solar light) Fig. 3 shows the color removal eciency of methylene blue as a function of solar light intensity. The decoloring eciency increased as solar light intensity increased. It was also found that the eect of solar light intensity on decoloring eciency was more signi®cant in the solar light irradiation only system than in the solar light/TiO2 ®lm system. This could be attributed to the self-photofading of methylene blue (which is also a photocatalyst) (Jain et al., 1999) with irradiation of some visible region of solar light. In addition, the electrons in TiO2 could be excited eectively even at very low solar light irradiation with a level of 50.2 lW/cm2 , showing that reduction of decoloring eciency was not
signi®cant for the solar light/TiO2 ®lm system as solar light intensity decreased. The eect of solar light intensity could also be revealed on the basis of pseudo-®rstorder rate constants. The rate constants obtained from the curves in Fig. 3 were listed in Table 2. The k2=k1 value was increased as weaker solar light illuminated, suggesting that the promotion of color removal with TiO2 photocatalysis was more prominent at lower solar light intensity. Fig. 4 showed that the color removal eciency of methylene blue as a function of the initial concentration of methylene blue under solar light irradiation. The increased concentration of methylene blue always decreased the decoloring eciency in both the solar light system and the solar light/TiO2 ®lm system. The adverse eect of increasing initial concentration of dye was less
Fig. 3. Eect of solar light intensity on the decoloring eciency of wastewater (C0 5 lM, pH: 7.0).
W.S. Kuo, P.H. Ho / Chemosphere 45 (2001) 77±83
81
Table 2 The pseudo-®rst-order rate constants of methylene blue with solar light irradiation Run no.
I1 (mW=cm2 )a
I2 (mW=cm2 )b
C0 (lM)c
k1 (h 1 )d
k2 (h 1 )e
k2=k1
1 2 3 4 5
0.055 0.050 0.113 0.085 0.095
0.73 0.66 1.40 1.12 1.27
0.5 5 5 10 30
0.365 0.069 0.149 0.025 0.024
0.954 0.535 0.912 0.302 0.163
2.61 7.72 6.14 11.85 6.83
a
I1 : solar light intensity at 254 nm. I2 : solar light intensity at 365 nm. c C0 : the initial concentration of methylene blue. d k1: the pseudo-®rst-order rate constant under solar light irradiation. e k2: the pseudo-®rst-order rate constant with solar light/TiO2 ®lm system. b
Fig. 4. Eect of initial concentration of methylene blue on the decoloring eciency of wastewater (solar light intensity at 254 nm: 85 113 lW=cm2 , pH: 7.0).
critical for the solar light/TiO2 ®lm system than the solar light system. This could be due to the fact that the TiO2 photocatalyst played an important role in depreciating the eect of the apparent reduction of light penetration into solution as increasing the concentration of dye. On the basis of k2=k1 values listed in Table 2, the promotion of color removal with TiO2 photocatalysis was actually ecient no matter which concentration of dye was treated. 3.3. Comparisons of the decoloring eciency of methylene blue with various light sources Fig. 5 shows the comparisons of color removal ef®ciency of methylene blue with various light sources.
The decoloring eciency of methylene blue with solar light irradiation was more ecient than that with arti®cial UV light irradiation. It was found that the color removal rate of methylene blue with solar light irradiation was almost twice that of arti®cial UV light irradiation as comparing the data of Run No. 3 in Table 2 to the data of Run No. 2 in Table 1. This phenomena was partially attributed to a higher temperature of dye solution generally found under solar light irradiation. Previous work (Lee et al., 1999) reported that the activation energy of UV/TiO2 photocatalysis for methylene blue was 14.45 kJ/mole. According to this reported data and Arrhenius's equation, the color removal rate of methylene blue could be increased 20% from the operation temperature of 25±35°C. In addition, though
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Fig. 5. Comparisons of the color removal eciency of methylene blue with various light sources (C0 5 lM).
the intensity of solar light @ 254 nm (0.1 mW/cm2 ) was much lower than that of arti®cial UV light @ 254 nm (3.0 mW/cm2 ), the measured intensity of solar light @ 365 nm was with a mean value of 1.4 mW/cm2 during the experimental period, which was able to allow the methylene blue molecules adsorbed on TiO2 ®lm (MB ads ) undergoing a photosensitization mechanisms (Eqs. (2)±(5)) or forming electronically excited oxygen atoms ± the singlet oxygen atom (O(1 D)) (Eq. (6)) and then promoted decoloring eciency. Accordingly, the photocatalytic device used in this study showed potential application for decoloring wastewater with solar light irradiation. The general drawback of consuming energy for a UV/TiO2 photocatalysis process could be minimized. MB ads
solar light
! MB ads
2
MB ads Ti O2 ! MB ads h e
3
e O2 ! O2
4
4. Conclusions This study revealed the prospect of solar photocatalytic decolorization of organic dyes in water. The photocatalytic decoloring eciency of methylene blue solution was higher with solar light irradiation than with arti®cial UV light irradiation, even though the arti®cial UV light source supplied higher UV intensity at 254 nm. This phenomena was mainly attributed to a photosensitization process involving the TiO2 semiconductor and the adsorbed dye itself under solar light irradiation, leading to more OH generated and decoloring eciency promoted. In addition, wastewater irradiated by solar irradiation was found to be warmer than that irradiated by UV lamp under the imposed conditions, which generally resulted in an increased color removal rate. Moreover, an acidic pH level and a lower initial concentration of dye solution were found to be bene®cial for color removal. Accordingly, this solar-assisted photocatalytic device showed potential application for decoloring organic dyes in wastewater.
Acknowledgements
h OH ! OH
5
MBads O2 ! MBads 2O 1 D
6
The authors are grateful to Lein-Ho Educational Foundation and National Science Council, Taiwan for ®nancial support (NSC 89-2211-E-239-001).
W.S. Kuo, P.H. Ho / Chemosphere 45 (2001) 77±83
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