Indian Journal of Chemical Technology Vol. 8, January 200 1 , pp. 36-40

ZnO - photoassisted degradation of textile dye using solar energy

B Neppolian, S Sakthivel, Banumathi Arabindoo, M Palanichamy & V Murugesan* Department of Chemistry, Anna University, Chennai 600 025, India

Received 19 July 1999; accepted 8 November 2000

This paper describes the efficiency of photocatalytic degradation of textile dye, reactive blue 4, using ZnO in the form of slurry. The results showed that the dye molecules can be completely degraded to CO2• SO/', NH/, N03' and H20 under solar irradiation. The effects of various parameters viz., ZnO loading, initial concentration of dye, persulphate ion, pH and sodium carbonate on the photocatalytic degradation were investigated. The degradation of dye was found to be effective in alkaline solution. This work envisages the great potential that sunlight mediated photocatalyst has in the removal of textile dyes from wastewater.

Over the past decade, dyes have been posing a potential environmental hazard as their manufacturing involves a variety of organic chemicals, some of which are even carcinogenic I . 7x 1 05 Tons and 1 0,000 different types of dyes and pigments are produced annually worldwide. Of these, an alarming 10% are lost in industrial effluents2 • Effluents especially those from textile manufacturing facilities using dyeing processes are highly coloured. This is due to the stability of modem dyes such as reactive, acidic, basic, naphthol, vat dyes etc. , which will have deleterious effect in wastewater for a long time.

The present study focusses the ability of solar energy to destroy organic dyes through photoassisted catalytic degradation. This heterogeneous oxidation process involves the use of photocatalytic semiconductor, ZnO illuminated with solar light. A redox environment is produced which can oxidise organic compounds in an oxygenated aqueous suspension. The primary oxidant responsible for the oxidation of organic compounds is the highly reactive hydroxyl radical ("OH). When a photon reacts with a semiconductor particle, an electron will be excited from the valence band (vb) to the conduction band (cb) and leaves an electronic vacancy called a hole (H+). The activated electron reduces the adsorbed oxygen and H+ to produce hydroxide ion or H2• The hole oxidises the surface-adsorbed water or hydroxide ion to produce hydroxyl radical . The overall redox reaction of photocatalytic oxidation process is the indirect oxidation of organic compounds into carbon dioxide, water and mineral acids3. This process may

*For correspondence

be more effective than the conventional treatment methods such as chemical oxidation, biological method, ozonation, coagulation and incineration4. Several reports have been published on the effect of photocatalysts in the degradation of organic compounds. Auguliaro et al 5. indicated that the removal of organic compounds increases linearly with catalyst loading. However, the presence of excess photocatalyst in the aqueous solution could cause a shielding effect in the penetration of light. The photocatalytic degradation of wastewater containing textile dye was studied by Kiwi et at. However the photocatalytic degradation of larger textile dyes which are commonly employed in Indian textile industries has not been reported so far. Hence an attempt has been made for photocatalytic degradation of textile dye, reactive blue 4, using ZnO in the form of slurry employing sunlight.

Experimental Procedure

The photocatalyst, ZnO used in this study was obtained from Merck, about 99% pure, possessing BET surface area of 10 m2/g. The textile dye, reactive blue 4, obtained from Chika Ltd., was used as such in the present study. Solutions were prepared by dissolving the dye in double distilled water.

The study was carried out in a batch reactor. The reaction vessel consists of double walled cylindrical glass vessel of 200 mL capacity with ports at the top for air sparger. Water was circulated in between the walls of the reactor to arrest the heat produced during the reaction. This reactor assembly was placed on a magnetic stirrer and stirred to prevent settling of the

NEPPOLIAN et at.: DEGRADATION OF TEXTILE DYE 37

0.8 . 0.7

J 0.6

-6- lIght Cllon. ____ CClloIy.t ObM � UfOIhtt Cotoly.t

.... U O.� 0.4 0.3 0.2 0. 1

0 �-'r-�r--'r---.---.-����-4 o 4 6 8

Irradialian timo ( h )

Fig. l-Effect of photocatalytic nature of ZnO in the degradation of textile dye

�oo c: OJ :il � 80 . � 15 60 e C> OJ "0 40 � z: Il. 20

IOOrn� NozC03 200"'11 NozC03 �O m9 No2C03

O�--r---.--.r--.---.---r---r--� 0 2 3 4 5 6 7 e

Irradiation time ( h )

Fig. 2-Effect of carbonate ion in the degradation of textile dye

catalyst. At periodic intervals, samples were drawn from the reactor vessel for measurement of

Fig. 1 illustrates the effect of light both in the absence and presence of photocatalyst. It is obvious that light without photocatalyst causes no decolourisation of the dye. Similarly for the dye solution slurried with ZnO in the absence of sunlight, the decolourisation was found to be negligible. Thus degradation was observed only in the presence of light and photocatalyst. These observations support the hypothesis that this is a photocatalytic oxidation process. Chemical oxygen demand was measured in all the experiments in order to estimate the extent of degradation of dissolved organic compounds.

Effect of illumination time The relationship between the photo degradation

efficiency and the i llumination time is shown in Table 1 . It is clear that the photodegradation

Table l-Effect of illumination time SI. Time of No irradiation

(h)

Decolouri­sation

(%)

Final COD

(mg /L)

Photo­degradation Efficiency

(%) 1 2 49 62.0 23 2 4 77 53.0 35 3 6 1 00 36.5 55 4 8 1 00 1 1 .0 86 5 10 1 00 8 .0 90 6 1 2 100 2.0 98 7 1 4 100 0.0 100

Initial COD : 81 mgIL Dye concentration : 3xlO-4 M pH : 3.8 Catalyst amount : 400 mg/l00 mL

decolourisation and degradation of dye molecules. The decolourisation was observed by measuring the absorbance using spectrophotometer (Model Systronics 1 06) and the degradation of the dye using a COD (chemical oxygen demand) digester. The COD tests were performed according to the standard methods7 •

The photodegradation efficiency for each sample was calculated from the following expression.

� ; [M;.;.M ]x 100

where 11 = photodegradation efficiency, Mo = initial COD in mg/L of the dye solution

before irradiation. M = amount of COD in mg/L after time t (h)

irradiation (final COD).

Results and Discussion efficiency increases with increasing illumination time. With an initial concentration of 3x l0-4 M dye and 400 mg of ZnOIl 00 mL the photodegradation efficiency was about 86 % in 8 h of illumination. The dye could be completely degraded into carbon dioxide and water in about 1 4 h of illumination. This is due to the fact that when the intensity of light is constant, the number of hydroxyl radicals and 0/- ' radicals increases with increase in illumination time8. So as long as the illumination time is long enough, the dye molecules can be photodegraded to smaller fragments like CO2 and H20.

Effect of initial concentration

From Table 2, it could be seen that after 8 h of illumination with 200 mgll OO mL catalyst, the photodegradation efficiency is high, almost 1 00 % at

INDIAN J CHEM. TECHNOL., JANUARY 200 1

Table 2-Effect of initial concentration

SI . No. Conc. of dye (x l O-4M ) Decolourisation Initial COD Final COD Photodegradation

(%) (mg/L) (mg/L) efficiency (%)

I I 1 00 27 0 1 00 2 2 1 00 54 1 6 70 3 3 52 8 1 47 42 4 4 27 1 08 74 32 5 5 1 8 1 35 1 30 4

Solar irradiation time : 8 h pH : 3.8 Catalyst amount : 200 mg / 1 00 mL

Table 3--Effect of ZnO loading

SI. No. Amount of Decolourisation ZnO (%) (mg)

I 1 00 32 2 200 52 3 300 8 1 4 400 1 00 5 500 1 00 6 600 1 00 Initial COD : 8 1 mg/L pH : 3.8 Irradiation time : 8 h Dye concentration : 3x I 0 -4 M

low initial concentration (l x 1 0-4 M) but is only 4 % with 5 x 10-4 M. The possible explanation for this behaviour is that as the initial concentration of the dye increases, more dye molecules are adsorbed on the surface of ZnO, but the intensity of light and illumination. time are constant and 0/- . and 'OH radicals formed on the surface of ZnO are also constant. So the reactive 'OH and 0/- . radicals attacking the dye molecules decrease and simultaneously the photodegradation efficiency decreases. Alternately the solar light photons entering the reactor can be absorbed by both ZnO catalyst and the dye in the solution. When the concentration of dye is increased, the path length of photons entering the solution is decreased, which decreases the photon absorption by the catalyst. This, in turn, decreases the degradation efficiency.

Effect of Z1l0 loading

Experiments were conducted using different amounts of ZnO suspension for a constant illumination time of 8 h and the results are presented in Table 3. Photodegradation efficiency increases rapidly with increase in the amount of ZnO up to 400 mg/J OO mL. When the amount exceeds 400 mg, the photodegradation efficiency increases very slowly. The increase in the amount of catalyst increases the

Final COD Photodegradation (mg / L) efficiency (%)

64 2 1 47 42 28 65 I I 86 8 90 1 0 88

number of active sites on the ZnO surface which in turn increases the number of 'OH and 02

2- • radicals. When the concentration of ZnO is above 500 mg/JOO mL, the degradation rate decreases due to the retardation of the light penetration and also increases in the rate of deactivation of activated molecules by collision with ground state ZnO, according to the reaction,

ZnO* + ZnO -t ZnO# + ZnO

ZnO* : zinc oxide with active species adsorbed on its surface. ZnO# : deactivated form of ZnO.

Effect of per sulphate ion

Some of the additives enhance the photodegradation efficienc/. However the additives should dissociate into harmless by-products and lead to the formation of 'OH and other oxidising agents. Persulphate ions are quite useful in this endeavour9 and it is also cheap. It enhances the degradation of organic compounds due to fact that the persulphate ions avoid electronlhole recombination (reactions 2 and 3) and also produces a powerful oxidant (SO/ - ) which generates more 'OH (reaction 4). Hence the effect of persulphate ion (oxidising agent) in the rate of photodegradation of the

/

NEPPOLIAN et al.: DEGRADATION OF TEXTILE DYE 39

Table 4-Effect of persulphate ion

SI. No.

Conc.of dye (x 10-4 M)

Decolourisation (%)

1 3 100 2 4 100 3 5 100 4 6 94 5 7 83 Catalyst amount : 400 mg/l00 mL pH : 3.8 Amount of persulphate : 1 00 mg / 100 mL

Time (h)

4 6 6 8 8

Initial COD

(mg/L) 8 1 108 1 35 1 62 1 89

Final COD

(mg/L) 0.0 0.0 0.0

40.0 140.0

Photodegradation efficiency (%)

100 100 100 75 26

Table 5-Effect of pH

SI. Conc.of dye

No. (M) 1 3x l O-4

2 3x l O-4

3 3x l O-4

4 5xlO-4

5 I x l O·3

Catalyst amount : 400 mgll OO mL

pH

3.0 3.8 9.0 9.0 9.0

Irradiation time (h)

8 8 2 8 8

dye has been studied. From Table 4, it is clear that complete degradation of dye could be achieved in 4 h solar irradiation in the presence of 1 ()() mg K2S20g/ 100 mL. When the same concentration of dye solution was irradiated (400 mg ZnO/l OO mL) without adding persulphate ion for 8 h, the degradation was found to be only 86 %. Hence the use of small amount of oxidant like persulphate ion enhances the photodegradation efficiency, if the dye concentration is relatively high.

. . . ( 1 )

. . . (2)

. . . (3)

. . . (4)

S04- 0+ dye molecules �sol- + n CO2 + other inorganics . . . (5)

Effect of pH

The pH of slurry containing 3x l O-4 M dye solution and 400 mg of ZnO/l OO mL was 3 .8 . At this pH, when it was irradiated in sunlight for 8 h, the degradation was found to be 86 % and decolourisation was 100 % (Table 5). When the pH of the slurry was adjusted to 3.0 with dilute H2S04 the decolourisation and degradation decreased to 80 % and 72 % respectively for the same irradiation time. When the

Decolourisation Photodegradation efficiency

(%) (%)

80 72 1 00 86 1 00 1 00 1 00 100 1 00 93

pH was raised to 9 with 6N NaOH, 100 % decolourisation and degradation was observed even at the end of 2 h irradiation. At the same pH, the concentration of dye was increased from 1x 10-4 M to 5x l O·4 M and then to 1 x l O·3 M. Decolourisation was 1 00 % in both concentrations but degradation was 100 % and 93% respectively at the end of 8 h irradiation. It is clear that alkaline pH range is more advantageous than acidic one for solar light assisted catalytic decolourisation and degradation of dyes as it would aid in the formation of highly oxidising hydroxide radicals by oxidising hydroxide ions. Increase of concentration of dye above 5x l O·4 M in the alkaline pH values is not advantageous as light photons would be largely absorbed by dye itself preventing them in reaching the catalyst.

Effect of carbonate ion The effect of carbonate ion in the degradation of

dye was studied by adding 50, 100 and 200 mg of sodium carbonate. In all the experiments, the slurry used is composed of 400 mg of ZnO and 3x l O·4 M dye solution. The initial volume of slurry was maintained at 1 00 mL. The results are depicted in Fig. 2. When the amount of sodium carbonate was increased from 50 to 1 00 mg, the irradiation time decreased from 7 to 3 h for the complete degradation of dye. Hence there may be large formation of hydroxide radicals with higher amount of sodium carbonate for the degradation of dye. However when the amount of sodium carbonate was increased to 200

INDIAN J CHEM. TECHNOL., JANUARY 2001

mg, the degradation time was found to be 5 h. This increase of 2 h irradiation time may be attributed to the conversion of ZnO catalyst produ,eing zinc hydroxide precipitate. Conclusion

The photocatalytic degradation with ZnO is effective in the removal of colour from wastewater. In addition to the removal of colour from the wastewater, the reaction simultaneously reduces the COD, suggesting that the dissolved organics are oxidised. The photocatalytic degradation efficiency of textile dye has been found to be high in alkaline medium than in acidic medium. The oxidation reaction requires air, water, photocatalyst and solar irradiation. It is concluded that the photocatalytic degradation of textile dyes may be a versatile method for decolourising and demineralising organics in wastewater. Acknowledgement

The authors gratefully acknowledge the financial support from the Ministry of Environment and

Forests, Govt. of India, New Delhi for this major sponsored project. The authors are thankful to Prof. A. Kalanidhi, Vice-chancellor, Anna University, for his constant encouragement and providing all the facilities to carry out the work.

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