Photocatalytic degradation of textile dye commonly used in cotton fabrics

T.S.R. Prasada Rao and G. Murali Dhar (Editors) Recent Advances in Basic and Applied Aspects of Industrial Catalysis Studies in Surface Science and Catalysis, Vol. 113 9 1998 Elsevier Science B.V. All rights reserved

329

PHOTOCATALYTIC DEGRADATION OF TEXTILE DYE COMMONLY USED IN COTTON FABRICS B. Neppolian, S. Sakthivel, Banumathi Arabindoo, M. Palanichamy and V. Murugesan* Department of Chemistry, Anna University, Chennai - 600 025, India.

ABSTRACT: UV light induced degradation of textile dye, reactive red M5B has been

carried out on TiO2 and ZnO semiconductor particles. Spectrophotometer and COD techniques have been used to elucidate the details of dye decolourisation and degradation. The experiments have been carried out with different amount of catalyst, various concentration of dye solution, different irradiation time and in the presence of air. The reactive red M5B has been degraded to colourless end products. The results suggest that the photocatalytic degradation of textile dye may be a viable method for the safe disposal of wastewater. 1. INTRODUCTION The textile industry is one of those industries that consume considerable amount of water in the manufacturing process. Water is primarily employed in the dyeing and finishing operations in which the fabrics are dyed and processed to finished products. Synthetic dyes are extensively used for textile dyeing, paper printing, colour photography and as additives in petroleum products. An estimated 10-15% of these dyes is lost in effluents during dyeing processes [1]. Azo dyes constitute the largest class of dyes used and their mutagenic, carcinogenic and toxic potential have been studied extensively [2]. The reactive dyes are commercially important class of textile dyes for which losses through processing operations are significant and treatment is problematic[3].Textile dyeing process frequently changes the dyestuff which cause considerable variation in the wastewater characteristics, particularly pH, colour and wastewater COD concentration [4]. Combination of strong colour, large pH swing, high dissolved solid content and high COD values of textile waste effluents are the main reasons for the difficulty in treating textile waste effluents. The dye molecules are highly structured polymers, hence they are very difficult to break down biologically and cannot be treated efficiently by an activated sludge process. Some physical and chemical methods are available for the treatment of textile effluents including coagulation and sedimentation, adsorption, bleaching, ion-exchange on synthetic adsorbent resins and reverse osmosis. Unfortunately, such processes have high operating costs and are of limited applicability [5]. Strong colour is another important component of textile wastewater, if not removed, would cause disturbance to the ecological

330 system of the receiving waters. Colour removal by activated carbon, hydrogen peroxide, sodium hypochlorite and other chemical agents has been widely practised [4,6-10]. Ozonation is a new technique that has been suggested in the recent literature as a potential alternative for decolourisation purpose [6,11].Scrutiny of the previous investigations has indicated that there is a lack of more systematic decolourisation and degradation studies of the textile waste effluents. Adsorption and coagulation do not result in dye degradation, but merely in physical removal of the dye material from the effluent, which still creates waste disposal problem. Chlorination and ozonation involve decolourisation by chemical reaction. Both chlorine and ozone degrade dyes when added to the textile effluent. However, discharge of chlorinated organics into the environment is becoming increasingly undesirable. Ozonation appears to be a much cleaner process but the chemical instability of ozone necessitates its generation on-site which requires significant electrical power and capita! costs. Over the past few years, photoassisted catalytic degradation of a variety of coloured organic and inorganic compounds in aqueous solutions has been reported in the literature [12]. The reason for this interest stems from the ability of these sensitizers to extend the photoresponse of large bandgap semiconductors into the visible region. The photocatalytic reactions of organic species in aqueous titania slurries can result in complete oxidation of carbon dioxide and water or mineral acids [ 13]. A simple monoazo dye such as Acid Orange 7 is oxdised to a colourless carboxylic acid end product by photosensitization on a TiO2 semiconductor surface [ 14]. The photocatalytic destruction of several classes of organic dyes in aqueous TiO2 suspensions utilizing highly concentrated solar energy is reported [15]. Visible light induced degradation of the textile diazo dye Naphthol Blue Black (NBB) has been carried out on TiO2 semiconductor nanoparticles [12]. Davis et al [16] investigated the use of photocatalytic process to decolourise wastewater contains a high proportion of dyeing and finishing wastewater. The photocatalytic process utilizes ultraviolet irradiation of an aerated wastewater containing TiO2 catalyst. ReCent studies have also shown that oxidation of organic compounds by way of the Fenton reagent is useful in the degradation of textile dyes[ 17]. Thus photocatalytic process is proved to be effective for dye decolourisation and degradation when irradiated with UV / visible light in the presence of air. However, the photocatalytic degradation of larger textile dyes commonly employed in cotton fabrics has not been reported so far. Hence an attempt has been made to study the photocatalytic degradation of the textile dye, reactive red M5B using TiO2 and ZnO in the form of slurries employing UV irradiation. The results of the photocatalytic degradation of the reactive red M5B dye have been presented here. 2. EXPERIMENTAL The photocatalyst (TiO2) used in this work was Degussa P25. Based on the manufacturer's information , the titania particles are a mixture of anatase and rutile crystalline phases (mostly anatase). TiO2 has an average particle size of 30nm and surface area 50 m2/g. The other photocatalyst used was ZnO powder (Merck), about 99% pure, possessing BET surface area 4.5 mE/g. The reactive red MSB dye (Chika Ltd) was used as such in the present study.

331 The study was carried out in a batch reactor. The reaction vessel consisted of double walled cylinder of 75 ml capacity with ports at the top for air sparger. Water was circulated in between the two walls of the reactor to arrest the heat produced during the reaction. This reactor assembly was placed on a magnetic stirring plate to further enhance the agitation. Quartz tube containing UV lamp, having an emission peak at about 254 nm and average intensity of 2000 gw/cm 2 was placed in the reactor. The slurry composed of dye solution and catalyst placed in the reactor was stirred magnetically and the samples were withdrawn from the reactor vessel periodically for the analysis of decolourisation and degradation. The decolourisation was observed by measuring the absorbance using spectrophotometer (model Systronics 106) and the degradation of the dye was measured using a COD digester. The COD tests were performed according to the standard methods [18]. The percent photodegradation efficiency for each sample was calculated from the following expression [191. P.P = (M/M0) x l 0 0 where P. P = percent photodegradation efficiency M = amount of COD in mg/1 removed during 't' h irradiation. M0 = initial COD in mg/1 of the dye solution before irradiation. 3. RESULTS AND DISCUSSION Textile dyes are usually not reactive under visible light because of their short-lived excited states. Deactivation of the excited singlet state usually occurs via a non-radiative internal conversion. Reactive red M5B has a strong absorption in the visible region with a broad maximum at 512 nm in water. The molecule does not fluoresce in aqueous solution or on TiO2 / ZnO surface. The photocatalytic degradation of reactive red M5B dye was studied using TiO2 and ZnO catalysts in the form of slurry employing UV irradiation under various experimental conditions and the results are discussed below.

3.1. Photocatalytic nature of TiO2 and ZnO in the destruction of organic dyes Fig. 1 illustrates the effect of light and the presence of the photocatalyst on the rate of Light Only

I Catalyst Only

Light+ Catalyst

500-

E >, 200

-

o r

._o o r~ t-

1001

O

a) 0 Irradiation time (h) figure I. Amount of TiO2 and 7nO: 150 rag. Volume of sample: 50mi. (a) Ti02 (b) ZnO

332 decomposition of reactive red M5B. While it is' obivious that intense light alone causes a slow decolourisation of the dye, the rate is much faster in the presence of TiO2 as well as ZnO. Similarly the dye solution slurried with catalyst in the absence of light, the decolourisation is found to be negligible. Thus degradation is observed only in the presence of light and catalyst. When SiO2 is substituted for TiO2 / ZnO, no photocatal~ic destruction of the dye is observed. These observations support the hypothesis that this is a photocatalytic oxidation process[ 15]. 3.2. Effect of amount of TiO2 and ZnO The photocatalytic degradation of reactive red M5B was carried out using slurries containing different amounts of TiO2 and ZnO for a constant irradiation time of 3 h and an initial dye concentration of 100 mg/1 (COD 45 mg/1).The percent photodegradation efficiency increases rapidly with increase in the amount of TiO2 and ZnO upto 25mg/50ml and then remains constant for further addition of the catalysts (Fig.2). It has been further observed that the amount TiO2 and ZnO has drastically increased when the initial concentration of the dye was kept at 200 mg/l (COD 90 mg/1) as shown in the Fig.3. It is seen that maximum percent photodegradation efficiency is only 70 % for 400 mg ZnO catalyst, beyond which there is no further removal even if the catalyst amount is increased whereas 200mg TiO2 could achieve 100 % photodegradation efficiency for the same concentration of the dye. Hence TiO2 is an effective catalyst for the degradation of reactive red dye M5B at higher concentration. This is due to the large surface area of TiO2 (50 m 2 / g) in comparison to ZnO ( 4.5 m z / g ) which facilitates large absorption light by TiO2 particles. Further the wastewater containing high concentration of this dye may be suitably diluted before the photocatalytic irradiation in order to reduce the amount of catalyst and to effect complete mineralisation of the dye into harmless end products. 3.3. Effect of initial concentration of dye The effect of concentration of dye on the photodegradation efficiency was studied by varying the initial dye concentration from 100 to 600 mg/l for a constant irradiation time of 4 h and 100 mg of the catalysts. The results are presented in Table 1. It can be seen that the photodegradation efficiency decreases with increasing initial concentration of the dye. With 100 mg/l of initial concentration of the dye, the photodegradation efficiency is 100 % for both TiO2 and ZnO and with 400 mg/1 of the dye the photodegradation efficiency is only 35 % and 20 % for TiO2 and ZnO respectively. The reason for this behaviour is that as the initial concentration increases, more and more dye molecules are adsorbed on the surface of the catalysts, but the intensity of light and irradiation time are constant and the "OH and 022.. radicals formed on the surface of the catalysts are also constant, so that the relative number of "OH and 022-. attacking the dye molecules decreases and the photodegradation efficiency also decreases. At high dye concentration (600 mg/1) the photodegradation efficiency was almost negligible in the case of ZnO catalyst and only 5% in the case of TiO2. The very poor photodegradation efficiency at high dye concentration is due to the fact that incident light would largely be used for dye excitation rather than the catalyst excitation. Hence in order to produce high photodegradation efficiency, the wastewater containing the dye should be suitably diluted.

333

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I0 2'5 Amount of catalyst ( m g )

5

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Figure 2. Concentration of dye'lO0 mg/I. Initial COD :45 mg/I. Irradiation Time : 3h.Volume of Sample 9,50 ml. Decolourisation

[]

Degradation

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Amount of Catalyst (rag) Figure 3. Concentration of dye: 2 0 0 m g / I Irradiation time 94 h. Volume of sample:50 ml ( a ) TiO 2 ( b ) ZnO

334 Table 1. Effect of concentration of the dye Amount of TiO2 and ZnO 9 100 mg / 50ml Conc.of dye

% Decolourisation

(mg / 1 )

Irradiation time" 4h COD ( mg / 1 )

Initial TiO2

ZnO

100

100

100

200

100

300

Photodegradation efficiency ( % )

Final TiO2

ZnO

TiO2

45

0

0

100

100

51

90

8

48

91

47

96

37

135

51

90

62

33

400

63

28

180

117

144

35

20

500

39

19

225

181

200

20

11

600

10

0

270

257

270

5

0

Table 2. Effect of irradiation time Concentration of dye 9 200 mg/1 Initial OD 9 90 mg / 1 Irradiation time ( h )

% Decolourisation TiO2

ZnO

Amount of TiO2 9 100 mg / 50 ml Amount of ZnO 9 300 ml~ / 50 ml Final COD (mg / 1)

Photodegradation efficiency (%)

ZnO

TiO2

ZnO

TiO2

ZnO

1.

63

52

38.00

58.00

58

36

2.

82

76

27.00

46.00

70

49

3.

100

100

17.80

36.00

80

60

4.

100

100

8.00

31.00

91

66

5.

100

100

4.80

14.50

95

84

6.

100

100

1.31

8.32

99

91

7.

100

100

0.00

4.80

100

95

335 3.4. Effect of illumination time

The relationship between photodegradation efficiency of the dye and the irradiation time is shown in the Table 2. It is clear that the photodegradation efficiency increases steadily with increasing irradiation time for both the catalysts. With an initial concentration of 200 mg/1 of the dye, the photodegradation efficiencies are about 90 % in the case of TiO2 after 4 h and 6 h in the case of ZnO. From this it is clear that the photodegradation efficiency is not only related to the irradiation time but also on the nature of the catalyst. When the intensity of light is constant, the number of "OH and 022.. radicals increase with increasing irradiation time [19]. So that as long as the irradiation time is long enough, the dye molecules can be photodegradated to smaller fragments. 4. CONCLUSION Photocatalytic oxidation of wastewater containing dissolved textile dyes is feasible for colour removal. In addition to remove the colour from the wastewater, the photocatalytic reaction simultaneously reduced the COD which suggests that the dissolved organics can be oxidised. These oxidation reactions require air, water, photocatalyst and UV light. The results suggest that the photocatalytic degradation of textile dye may be a viable method for decolourisation and oxidation of organics in wastewater. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15. 16. 17. 18. 19.

A.A. Vaidya and K.V. Dateye, Colourage., 14 ( 1982 ) 3. F. Joachim, A. Burrel and J. Anderson, Mutation Res., 156 ( 1985 ) 131. S. Abeta, T. Yoshida and K. Imada, Am. Dyest. Rep., 73 ( 1984 ) 20. C.F. Gurnham ( ed. ), Industrial waste control, Academic press, New York., 1965. P. Cooper, JSDC., 109 (1993 ) 97. E.H. Snider and J.J. Porter, J. Wat. Pollut.' Control Fed., 46 (1974) 886. G. McKay, Am. Dyes Rep., 69 (1990) 38. G. McKay, Chem. Engng. J., 28 (1984) 95. W.C. Kuo, Wat. Res., 26 (1992) 881. S.H. Lin and C.F. Peng, J. Environ. Sci. Hlth., A30 (1995) 89. S.H. Lin and Chi.M. Lin, War. Res., 27 (1993) 1743. Chouhaid Nasr, K. Vinodgopal, Luke Fisher, Surat Hotchandani, A.K. Chattopadhyay and Prashant V. Kamat, J. Phys. Chem., 100 (1996) 8442 and other references cited therein. S.F. Ollis, E. Pelizzetti and N. Serpone, Environ. Sci. Technol., 25 (1991) 1522. K. Vinodgopal, D. Wynkoop, P.V. Kamat, Environ. Sci. Technol., 29 (1995) 1163. P. Reeves, R. Ohlhausen, D. Sloan, K. Pamplin, T. Scoggins, C. Clark, B. Hutchinson and D. Green, Solar energy, 45 (1992) 413. R . J . Davis, J.L. Gainer, Gilbert O' Neal and I-Wen Wu, Water Environ. Res., 66 (1991) 50. E.G. Solozhenko, N. M. Soboleva and V.V Gonchanfl~, Wat. Res., 29 (1995) 2206 and other references cited therein. Standard methods for the examination of water and wastewater, 17th Edition, APHA, Washington D.C. 1989. Zhao Mengyue, Chen Shifu and Tao Yao~u, J. Chem. Tech. Biotechnol., 64 (1995) 339.