INTRODUCTION:

The textile industries use large volumes of water in their operations and therefore discharge large amount waste water into the environment, most of which is untreated. The waste water contains a various chemicals which include dyes.^{1, 2}The dye containing hazardous substances which can damage aquatic and vegetal life are discharged via effluent into the environment.³The removal of the non biodegradable organic chemical is a crucial ecological problem. Due to stability of modern dyes, conventional biological treatment methods for the industrial waste water are ineffective. Heterogeneous photocatalysis by semiconductor particles is a promising technology for the reduction of global environmental pollution.

Literature survey has revealed that a lot of work has been carried out in this field. The solar photochemical detoxification and disinfection for water with TiO₂ has been studied by Cooper et al.^4,5 Decolorization of textile industry waste water by photocatalytic degradation reported by Hachem et al.⁶ Oxidation process with TiO₂ semiconductor has been shown by Malato et al.^7,8 Semiconductor mediated photocatalysed degradation of an anthraquinone dye, remazol brilliant blue was reported by Saquib et al.⁹ The photocatalytic degradation of methyl orange by TiO₂ in aqueous phase is studied by Rashed and Amin.¹⁰ Photocatalytic reduction of methyl yellow on CdS nanoparticles mediated in reverse micelles was reported by Zang et al.¹¹

The photocatalytic reduction of azo dyes like naphthol blue back was done by Nasr et al.¹² Many other attempts has been done to study the photocatalytic activity of different semiconductors such as SnO2, ZrO, CdO, ZnO.^13-15 The present work deals with photocatalytic degradation of aqueous solution containing solochrome blue dye using CuS-CdS catalyst.

EXPERIMENTAL:

The stock solution of dye was prepared in distilled water and diluted when required. Sodium hydroxide (dilute) and hydrochloric acid (dilute) was used to maintain pH of the dye solution The pH of dye was recorded using pH meter (Elico - LI 120). Solution of dye was taken in a beaker then amount of CuS – CdS catalyst was added and the beaker was covered with water filter to avoid the thermal reaction. The dye solution was irradiated with sunlight for the degradation of dye and absorbance was recorded spectrophotometrically (Elico SL 159).

The CuS – CdS catalyst was prepared by taking known weight of salts of Cu and Cd and precipitating them by passing H₂S and hydrochloric acid (dilute). The precipitate was allowed to settle down. The precipitate was washed with distilled water and then it was allowed to dry at room temperature. The powder was calcinized at 350ºC temperature in muffle furnace for 4 hrs. The powder obtained was used as semiconductor for present work.

RESULTS AND DISCUSSION:

The IR spectrum of CuS-CdS consists of bands at 482, 542, 636, 1151, 1631 and 2015 cm^-1 it confirms the presence of CuS-CdS. The photocatalytic degradation of solochrome blue was observed at λmax 520 nm. The results for a typical run are given in Table -1. It was observed that absorbance of solution decreases with increasing time intervals which that the dye is photochemically degraded. A plot (Fig. 1) of 2 + log O.D was found to be straight line suggesting that degradation of dye by CuS – CdS follows pseudo first order rate law. Rate constant were calculated using following equation.

K = - 2.303 X slope

Table – 1. Typical run

Solochrome blue = 5 X 10^-3 M

CuS – CdS = 0.100 gm. pH = 6.0

Time (min)	O.D	2 + log O.D
0	0.804	1.905
15	0.617	1.790
30	0.437	1.640
45	0.220	1.342
60	0.080	0.903
75	0.030	0.477
90	0.012	0.079
105	0.010	0.041

K = 5.06 X 10^-3 sec^-1

Figure 1: Typical run

i) Effect of pH:

The degradation of dye was studied at various pH. It was found that as pH of the dye solution was raised, the rate of photocatalytic degradation increases. At pH 8.0 degradation attains maximum value. After this, further rate of degradation decreases. Increase in pH will increase the number of OH^- ions. A hole is generated in semiconductor photocatalyst, which abstracts an electron from OH^- ions, converting into OH^. free radical. This free radical is responsible for the degradation of dye. The effect of pH variation on rate is given in Table-2, Fig.-2.

Table -2 Effect of pH on dye degradation

Solochrome blue = 5 X 10 ^-3 M, CuS – CdS = 0.100 gm.

pH	K ( x 10^-3 sec^-1)
4.0	1.01
4.5	1.89
5.0	2.61
5.5	3.43
6.0	4.42
6.5	5.23
7.0	5.56
7.5	5.87
8.0	6.01
8.5	6.34
9.0	5.87
9.5	5.03
10.0	4.36

Figure 2: Effect of pH on dye degradation

ii) Effect of concentration of solochrome blue dye:

The concentration of solochrome blue dye was changed, keeping all other factors constant. The data summarized in Table 3 and also represented as Figure-3.The rate of photocatalytic degradation was found to be increase with increase in the concentration of solochrome blue. After optimum concentration (5 X 10^-3 M) rate of degradation of dye again decreases. At higher concentration of dye, dark colour of solution acts as a filter to the incident light and due to these rates of photocatalytic degradation of dye decreases.

Table -3 Effect of concentration of solochrome blue dye on degradation

CuS – CdS = 0.100 mg, pH = 8.0

Concentration ( x 10 ^-3 M)	K ( x 10^-3 sec^-1)
1	5.36
2	5.57
3	5.78
4	5.98
5	6.32
6	5.71
7	5.43
8	5.12

Figure 3: Effect of concentration

iii) Effect of amount of CuS-CdS catalyst:

The dependent of degradation of dye on the amount of CuS –CdS catalyst was studied and reported in Table - 4 and represented in Figure - 4. At 0.100 gm the degradation of dye was maximum. It may be due to more surface area of photocatalyst available to absorb light generating the excited states. After this the rate of degradation decreases due to interference of molecules of CuS – CdS. The abundance of molecules interferes in the pathway of other molecules gaining the excited state, thus resulting into decrease in the degradation of dye.

Table -4 Effect of amount of CuS-CdS catalyst on dye degradation

Solochrome blue = 1 x 10 ^-3 M, pH = 8.0

Amount of catalyst (gm)	K ( x 10^-3 sec^-1)
0.050	5.63
0.075	5.98
0.100	6.45
0.125	5.21
0.150	5.02
0.175	4.77

Figure 4: Effect of amount of CuS-CdS catalyst

On the basis of above studies, a tentative mechanism has been proposed for the degradation of dye by CuS – CdS catalyst.

ACKNOWLEDGEMENT:

The authors are thankful to Principal, P.V.P College, Pravaranagar for providing necessary facilities to carry out this work. The authors are also thankful to Prof.A.T.Kanade, Department of Electronic Science, P.V.P College, Pravaranagar for editing help.

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Received on 20.10.2011 Modified on 09.11.2011

Asian J. Research Chem. 4(12): Dec., 2011; Page 1892-1894