Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO2

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Abstract

The photocatalytic activity of commercial ZnO powder has been investigated and compared with that of Degussa P25 TiO2. Laboratory experiments with acid brown 14 as the model pollutant have been carried out to evaluate the performance of both ZnO and TiO2 catalysts. Solar light was used as the energy source for the photocatalytic experiments. These catalysts were examined for surface area, particle size and crystallinity. The effect of initial dye concentration, catalyst loading, irradiation time, pH, adsorption of acid brown 14 on ZnO and TiO2, intensity of light and comparison of photocatalytic activity with different commercial catalysts were studied. The progress of photocatalytic degradation of the acid brown 14 has been observed by monitoring the change in substrate concentration of the model compound employing HPLC and measuring the absorbance in UV–Visible spectrophotometer for decolourisation. The photodegradation rate was determined for each experiment and the highest values were observed for ZnO suggesting that it absorbs large fraction of the solar spectrum and absorption of more light quanta than TiO2. The complete mineralisation was confirmed by total organic carbon (TOC) analysis, COD measurement and estimation of the formation of inorganic ions such as NH4+, NO3, Cl and SO42−.

Introduction

The removal of organic pollutants in wastewater is an important measure in environmental protection. Dyestuffs and other commercial colourants have emerged as a focus of environmental remediation efforts [1], [2], [3]. These efforts have largely been targeted at removing colourants from the effluents of tannery, textile mills and other colourant manufacturing units. These dyes eventually get deposited as sludges on landfills. Within the overall category of dyestuffs, azo dyes constitute a significant portion probably due to least desirable consequences on the surrounding ecosystem. In recent review articles it has been suggested that photosensitized degradation on semiconductor surfaces can be a remediation for coloured organic pollutants [1], [2], [3], [4], [5], [6]. In our earlier work [5], [6], [7], [8] we have demonstrated that acid green 16, a commercial leather dye, can be oxidized to colourless end products using TiO2 and ZnO catalysts in the presence of sunlight/UV light as the energy source. Among the various semiconductors employed, TiO2 and ZnO are known good photocatalysts for the degradation of several environmental contaminants [5], [6], [9], [10], [11] due to their high photosensitivity, stability and large band gap. When illuminated with an appropriate light source, the photocatalyst generates electron/hole pairs with free electrons produced in the empty conduction band leaving positive holes in the valence band. These electron/hole pairs are capable of initiating a series of chemical reactions that eventually mineralize the pollutants. Many toxic chemicals can be degraded by this process. Moreover the formation of harmless eco-friendly end products represents another attractive feature of this process. The extent of minerlisation is the most important criterion for assessing the feasibility of any oxidation process as incomplete oxidation may lead to an intermediate, which may be more toxic than the parent pollutant. To the best of our knowledge, the photooxidation of diazo dye such as Acid brown 14 which is widely used in tannery and textile industries has not been reported. Oxidative decolourisation of these compounds by conventional methods such as ozonation is expected to be more difficult than the simple monoazo dyes [6]. In the present studies, the photocatalytic decomposition of acid brown 14 was studied with ZnO and the results were compared with TiO2 (Degussa P-25).

Section snippets

Materials

Acid brown 14 (Fig. 1) obtained from Clariant Chemical Company, India was used as such without any further purification. The commercially available photocatalysts ZnO (E Merck), TiO2 (Degussa P25), SnO2, ZrO2, CdS and WO3 (S.D. Fine Chemicals, India) were used as received. All other chemicals were of reagent grade quality. α-Fe2O3 catalyst was prepared by hydrolysis of Fe(NO3)3. 9H2O at 60°C for 24 h. The colloidal solution was boiled and allowed to settle. The precipitate obtained was filtered,

Characterisation studies

Fig. 2 illustrates the absorption spectra of TiO2 and ZnO. The spectrum of ZnO consists of a single, broad intense absorption from ca. 470 nm to lower wavelengths, usually ascribed to charge-transfer process from the valence band (mainly formed by 2p orbitals of the oxide anions) to the conduction band (mainly formed by 3d orbitals of the Zn2+ cations) [12]. TiO2 shows intense absorption from ca. 420 nm until band gap characteristic absorption maximum at 400 nm [13]. The absorbance of ZnO is more

Conclusions

Solar light induced degradation of a complex diazo dye, acid brown 14 has been completely degraded by both ZnO and TiO2 catalysts. Complete mineralisation was attained with the formation of carbon dioxide, water, nitrate, ammonia, sulphate and chloride. Acid brown 14 was converted completely into colourless intermediates within 120 and 300 min by ZnO and TiO2, respectively. However complete degradation was observed within 360 min with ZnO and 420 min while using TiO2. Comparison of photocatalytic

Acknowledgements

The authors gratefully acknowledge the financial support by the Ministry of Environment and Forests, Govt. of India, New Delhi, for this Project.

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