INTRODUCTION:
Water is a basic requirement in all industrial processes, domestic and commercial activities. Industrial textile dyes are present in their effluents in different concentrations. The waste water generated from these various processes contain different contaminants depending upon processes, mainly pharmaceutical, textile, acrylic fiber, pesticides and other organic chemicals manufacturing industries etc. These effluents are intensely colored and are contaminated with high concentration of organic compounds such as suspended and dissolved salts and many other recalcitrant compounds. Even small concentration of these compounds present in effluent causes toxicity and foul odors to water. If these effluents are improperly treated, they will pose to serious threat to all species on earth because hydrolysis of the pollutants in waste water can produce a great deal of toxic products. Degradation of these non-biodegradable organic compounds is not possible by conventional biological treatment processes. Lately, there has been a lot of interest in application of the advanced oxidation processes (AOP’s) for the removal of these organic compounds. Many processes such as photolysis, photocatalytic oxidation, ozonation, Fenton oxidation, wet air oxidation and membrane separation has been proposed for the degradation of these compounds even at low concentration.
The photocatalytic bleaching was found to be the most promising and efficient process in controlling the environmental pollution, waste water treatment etc, in which semiconductor particles act as photocatalysts or short-circuited microelectrodes on excitation. On excitation, semiconductor generates electron-hole pair which may be used either for reduction or oxidation of the dye molecules.
Exhaustive researches in the field of photocatalysis have shown various fascinating applications of photocatalytic reactions based on the use of semiconductors1, 2. The photocatalytic degradation of methylene blue, rhodamine-B and methyl orange in presence of CdS as photocatalyst has been reported3, 4. Photoreduction of fluorescent dye 2’,7’-dichlorofluorescein was reported by Marchesi et al.5 Punjabi et al.6 studied the photoreduction of congo red by ascorbic acid and EDTA as reductants and cadmium sulfide as photocatalyst. Kim et al.7 used ZnO coated TiO2 nanoparticles for the flexible dye-sensitized solar cells. Use of semiconducting iron (II) oxide in photocatalytic bleaching of some dyes (malachite green, crystal violet and methylene blue) has been reported by Ameta et al.8 Photocatalytic degradation of brilliant red dye and textile waste water has been suggested by Martins et al.9 Photocatalytic degradation of acid blue-62 over CuO-SnO2 nanocomposite photocatalyst under simulated sunlight reported by Xia10.
Chen et al.11 showed the activities of different metal oxides as photocatalysts. The photodegradation of arylmethane and azo dyes over TiO2/In2O3 nano composite films was reported by Skorb et al.12. Sun et al.13 reported the photocatalytic activity of titanium cobalt oxides in the degradation of methyl orange. Photo induced transformation of some organophosphorous pesticides over TiO2 was investigated by Calza et al.14 Reddy et al.15 showed the photocatalytic activity of Bi2O3 for the treatment of phenolic wastes. Photocatalytic degradation of brilliant red dye and textile waste water has been suggested by Martins et al. 16. The synthesis, characterization and photocatalytic activity of lanthanum cerium oxide (LaCeO3) catalyst was reported by Jose et al. 17. Photocatalytic degradation of cetylpyridinium chloride over TiO2 has been reported by Singhal et al. 18. Photoreduction of Congo red by ascorbic acid and EDTA over cadmium sulphide as photocatalyst was carried out by Kothari et al. 19. Photocatalytic activity of antimony (III) sulphide in bleaching of Azure-B was carried out by Ameta et al. 20.
Experimental procedure:
Cationic dye (malachite green) and semiconducting bismuth oxide powder were used in the present investigation. All the solutions were prepared in doubly distilled water. The photocatalytic bleaching of the dye was observed by taking dye solution and bismuth oxide together. Irradiation was carried out by keeping the whole assembly exposed to a 200W Tungsten lamp (Philips; light intensity = 50.0 mW cm–2). The intensity of light at various distances from the lamp was measured with the help of a solarimeter. A water filter was used to cut out thermal radiations. The pH of the solutions was measured with the help of digital pH-meter.
0.03640 g of malachite green was dissolved in 100.0 mL of doubly distilled water to prepare its 1.0 × 10-3 M solution, which was used as stock solution. The stock solution was further diluted as and when required. The absorbance of the dye solutions were determined with the help of a spectrophotometer at λmax = 615 nm for malachite green. The solutions of the dyes were divided into four parts; the first beaker containing only dye solution was kept in dark; the second beaker containing only dye solution was kept in light; in the third beaker dye solution and 0.10 g of semiconductor bismuth oxide was kept in dark and in the fourth beaker dye solution with 0.10 g of semiconductor bismuth oxide was exposed to light.
These beakers were kept for 4 hours and then the absorbance of solution in each beaker was measured. It was observed that the solutions in the first three beakers had the almost same initial absorbance while the solution in the fourth beaker had a decrease in its initial value of absorbance. Thus, by performing blank experiment it was confirmed that the reaction between malachite green and semiconductor powder is neither thermal nor photochemical but it is a photocatalytic reaction. The progress of the reaction was monitored spectrophotometrically by taking absorbance of the reaction mixture at different time interval. From these results, it is clear that reaction requires both light and semiconductor to degrade dye, hence showing the photocatalytic nature of the reaction.
Results and discussion:
Photocatalytic bleaching of malachite green is observed at lmax = 615 nm. Degradation of the dyes in absence of semiconductor (Bi2O3) is negligible. Thus, photocatalytic bleaching is favorably affected by semiconductor. A plot of optical density (2 + log OD) versus time is linear and hence, the reactions follow pseudo first-order kinetics (Table 1 and Figure 1). The rate constants are determined with the help of the curves.
Table -1: Typical Run
|
Time(min) |
Malachite green |
|
|
Optical Density (OD) |
2 + log O.D. |
|
|
0 |
0.508 |
1.7059 |
|
10 |
0.447 |
1.6503 |
|
20 |
0.400 |
1.6021 |
|
30 |
0.355 |
1.5502 |
|
40 |
0.318 |
1.5024 |
|
50 |
0.282 |
1.4502 |
|
60 |
0.253 |
1.4031 |
|
70 |
0.224 |
1.3502 |
|
80 |
0.205 |
1.3118 |
|
90 |
0.181 |
1.2577 |
|
100 |
0.162 |
1.2095 |
|
110 |
0.144 |
1.1584 |
|
120 |
0.135 |
1.1303 |
|
130 |
0.120 |
1.0792 |
|
140 |
0.107 |
1.0294 |
|
150 |
0.094 |
0.9731 |
|
160 |
0.086 |
0.9345 |
|
170 |
0.077 |
0.8865 |
|
180 |
0.067 |
0.8261 |
|
190 |
0.061 |
0.7853 |
|
200 |
0.053 |
0.7243 |
|
210 |
0.046 |
0.6628 |
|
220 |
0.041 |
0.6128 |
|
230 |
0.036 |
0.5563 |
|
240 |
0.033 |
0.5185 |
Figure: - 1. A Typical Run
(® - Bi2O3 = 0.10g, [Malachite green] x 10-5 = 2.00 x 10-5 M, pH = 7.50, k = 19.19 x 10-5 s-1, light intensity 50 mWcm-2)
Effect of pH:-
The pH of the solution is likely to affect the bleaching of the Malachite green. The effect of pH on the rate of bleaching of Malachite green was investigated in the pH range 7.00 – 10.00. The results are reported in Table 2 and Figure 2. It is evident from the observed data that the rate of photocatalytic bleaching of malachite green is optimum at pH 7.50 after that the rate constant decreases on further increasing the pH of the solution.
The increase in the rate of photocatalytic bleaching with increase in pH may be due to more generations of •OH radicals, which are produced from the interaction of OH- and hole (h+) of the semiconductor. These •OH oxidize the dye molecules in their leuco forms, which ultimately degrade in the non-hazardous products. But after pH 7.50 for malachite green, the dye molecules becomes neutral and feel less attraction to OH- and hence the rate of the reaction decreases on further increasing pH of the solutions.
Table-2: EFFECT OF pH
|
pH |
k ´ 104 (sec-1) |
|
7.00 |
1.16 |
|
7.25 |
1.59 |
|
7.50 |
1.92 |
|
7.75 |
1.82 |
|
8.00 |
1.73 |
|
8.25 |
1.72 |
|
8.50 |
1.70 |
|
8.75 |
1.69 |
|
9.00 |
1.66 |
|
9.25 |
1.63 |
|
9.50 |
1.59 |
|
9.75 |
1.58 |
|
10.00 |
1.55 |
Figure: - 2. Effect of pH
(® - Bi2O3 = 0.10g, [Malachite green] = 2.00 x 10-5 M, k = 1.92 x 10-4 s-1, Light intensity = 50 mWcm-2)
Effect of dYE CONCENTRATION:-
Effect of concentration of malachite green was studied by taking different concentrations of this dye. The results are tabulated in Table 3 and Figure 3. It was observed that the rate of photocatalytic bleaching increases with an increase in the concentration of the dyes.
It may be due to the fact that as the concentration of dye increases more dye molecules are available for excitation and energy transfer and hence, an increase in the rate of photocatalytic bleaching of the dye was observed. The rate of photocatalytic bleaching was found to decrease with further increase in the concentration of the dye i.e. above 2.00 × 10-5 M. This may be attributed to the fact that after certain concentration, the dye itself will start acting as a filter for the incident light and it will not permit the desired light intensity to reach the semiconductor particles; thus, decreasing the rate of photocatalytic bleaching of dye.
Table – 3: EFFECT OF DYES CONCENTRATION
|
Dye ´ 105 M |
k ´ 104 (sec-1) |
|
1.00 |
1.46 |
|
1.20 |
1.54 |
|
1.40 |
1.59 |
|
1.60 |
1.68 |
|
1.80 |
1.79 |
|
2.00 |
1.92 |
|
2.20 |
1.84 |
|
2.40 |
1.77 |
|
2.60 |
1.68 |
Figure: - 3 Effect of dyes concentration
(® - Bi2O3 = 0.10g, pH = 7.50, k = 1.92 x 10-4 s-1, Light intensity = 50 mWcm-2)
Effect of amount of SAMICONDUCTOR:-
The amount of semiconductor is also likely to affect
the rate of photocatalytic bleaching of malachite green hence; different
amounts of photocatalyst were used. The results are reported in Table 4 and
Figure 4. It was observed that the rate of photocatalytic bleaching of
malachite green increases with an increase in the amount of semiconductor but
ultimately, it became almost constant after a certain amount i.e. 0.10 g.
This may be attributed to the fact that as the amount of semiconductor was increased, the exposed surface area increased, which absorb more number of photons and as a result the rate of photocatalytic bleaching of the dye was increased, but after a certain limit, if the amount of semiconductor was further increased, then there will be no increase in the exposed surface area of the photocatalyst. It may be considered like a saturation point; above which any increase in the amount of semiconductor has negligible or no effect on the rate of photocatalytic bleaching of the dye, as any increase in the amount of semiconductor after this saturation point will only increase the thickness of the layer at the bottom of the reaction vessel. This was confirmed by taking reaction vessels of different dimensions. The saturation point shifts to higher range for larger vessels, while reverse was true for smaller vessels.
Table – 4: EFFECT OF AMOUNT OF SEMICONDUCTOR
|
Amount of Semiconductor (g) |
k ´ 104 (sec-1) |
|
0.02 |
1.62 |
|
0.04 |
1.73 |
|
0.06 |
1.75 |
|
0.08 |
1.83 |
|
0.10 |
1.92 |
|
0.12 |
1.92 |
|
0.14 |
1.90 |
|
0.16 |
1.88 |
|
0.18 |
1.87 |
Figure: - 4. Effect of amount of semiconductor
(® - [Malachite green] = 2.00 x 10-5 M, pH = 7.50, k = 1.92 x 10-4 s-1, Light intensity = 50 mWcm-2)
EFFECT OF LIGHT INTENSITY:-
To observe the effect of intensity of light on the photocatalytic bleaching of the dye, the light intensity was varied. The results obtained are reported in Table 5 and Figure 5.
The data indicate that an increase in the light intensity increases the rate of reaction and the optimum value was found at 50 mWcm–2. It may be explained on the basis that as the light intensity was increased, the number of photons striking per unit area also increased, resulting into a higher rate of degradation. Further increase in the intensity beyond the maximum limits result in decrease in the rate of reaction. It may be probably due to thermal side reactions.
Malachite green (MG) absorbs radiations of suitable wavelength and gives rise to its excited singlet state. Then it undergoes intersystem crossing (ISC) to give the triplet state of the dyes. The involvement of triplet state was confirmed by using triplet state scavengers, where the reaction rate was almost negligible. On the other hand, the semi-conducting bismuth oxide (SC) also utilizes the radiant energy to excite its electron from valence band to the conduction band; thus, leaving behind a hole. This hole abstracts an electron from OH– ions to generate ·OH radicals. These radicals will oxidize the dye to its leuco form, which may ultimately degrade to harmless products. The participation of ·OH radicals as an active oxidizing species was confirmed by using hydroxyl radical scavenger isopropanol, where the rate of bleaching was drastically reduced.
Table – 5: EFFECT OF LIGHT INTENSITY
Intensity of Light (mWcm2) |
k ´ 104 (sec-1) |
|
10.0 |
1.60 |
|
20.0 |
1.66 |
|
30.0 |
1.73 |
|
40.0 |
1.88 |
|
50.0 |
1.92 |
|
60.0 |
1.89 |
CONCLUSIONS:
Thus, malachite green dye can be photocatalytically bleached by using Bi2O3 as visible light photocatalyst.
ACKNOWLEDGEMENT:
We are thankful to Prof. Suresh C. Ameta, Former Professor & Head, Department of Chemistry, M. L. Sukhadia University, Udaipur for his valuable suggestions and discussion.
Figure: - 5. Effect of light intensity
(® - Bi2O3 = 0.10g, [Malachite green] = 2.00 x 10-5 M, pH = 7.50, k = 1.92 x 10-4 s-1)
REFERENCES:
1. Ameta, S. C.; Chaudhary, R.; Ameta, R.; Vardia, J.; J. Ind. Chem. Soc. 2003, 80, 257.
2. Scheavello, M.; Heterogeneous Catalysis, Wiley: New York, 1997.
3. Takazawa, T.; Watarable, T.; Honda, K.; J. Phys. Chem. 1978, 82, 1391.
4. Zang, L.; Shen, J.; J. Chem. Soc., Chem. Commun. 1996, 472.
5. Marchesi, E.; Rota, C.; Fann, Y. C.; Chignell, C. F.; Mason, R. P.; Free Radic. Biol. Med. 1999, 26, 148.
6. Punjabi, P. B.; Ameta, R.; Vyas, R.; Kothari, S.; Ind. J. Chem. 2005, 44A, 2266.
7. S. S. Kim, J. H. Yun, Y. E. Sung, J. Photochem. Photobiol., 171A (2005) 275.
8. R. Ameta, J. Vardia, P. B. Punjabi, S. C. Ameta, Indian J. Chem. Tech., 13 (2006) 114.
9. A. F. Martins, M. L. Wilde, C. Dasilveria, Indian J. Chem. Tech., 13 (2006) 14.
10. H. L. Xia, H. S. Zhuang, T. Zhang, D. C. Xia, J. Environ. Sci. (China), 19 (2007) 1141.
11. JC Chen; GC Fang and SS ShiInt. J. Environ. Pollut., 2009, 37, 86.
12. EV Skorb; EA Vrtinovich and AL Kula. J. Photochem. Photobiol., 2008, 193A, 97.
13. CG Sun; L Tao and ML Fan, Catal. Lett., 2009,129, 26.
14. P Calza; C Massolino and E Pelizzetti. J. Photchem. Photobiol., 2008, 199A,42.
15. JK Reddy; K Lalitha and VD Kumari. Catal. Lett, 2008, 121,131.
16. A.F. Martins, M.L. Wilde, C. Dasilveria, Indian. J. Chem. Tech. 13 (2006) 14.
17. J. Jose, J. Ameta, P.B. Punjabi, V.K. Sharma, S.C. Ameta, Bull. Catal. Soc. India 6 (2007) 110.
18. B.Singhal, A. Porwal, A. Sharma, R. Ameta, S. C. Ameta (1997) J. Photochem. Photobiol. A, 108, 85–88.
19. S. Kothari, R. Vyas, R. Ameta, P. B. Punjabi, Indian J. Chem., 44A, 2266-2269 (2005).
20. R. Ameta, A. Pandey, P. B. Punjabi, S. C. Ameta, Chem. Environ. Res., 14, 255-260 (2005).
Received on 23.12.2010 Modified on 02.01.2011
Accepted on 23.01.2011 © AJRC All right reserved
Asian J. Research Chem. 4(8): August, 2011; Page 1393-1296