Comparative photocatalytic study of two selected pesticide derivatives, indole-3-acetic acid and indole-3-butyric acid in aqueous suspensions of titanium dioxide

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Abstract

Heterogeneous photocatalysed degradation of two selected pesticide derivatives such as indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) has been investigated in aqueous suspensions of titanium dioxide by monitoring the change in substrate concentration employing UV spectroscopic analysis technique and depletion in total organic carbon (TOC) content as a function of irradiation time. The degradation kinetics was studied under different conditions such as pH, types of TiO2, substrate and catalyst concentration, and in the presence of electron acceptor such as hydrogen peroxide (H2O2) besides molecular oxygen. The degradation rates were found to be strongly influenced by all the above parameters. The photocatalyst Degussa P25 showed comparatively highest photocatalytics. The pesticide derivative, indole-3-acetic acid was found to degrade slightly faster than indole-3-butyric acid.

Introduction

A wide variety of organic pollutants especially pesticides are introduced into the water system from various sources such as industrial effluents, agricultural runoff and chemical spills [1], [2]. Their toxicity, stability to natural decomposition and persistence in the environment has been the cause of much concern to the societies and regulation authorities around the world [3].

The control of organic pollutants in water is an important measure in environmental protection. Among many processes proposed and/or being developed for the destruction of the organic contaminants, biodegradation has received the greatest attention. However, many organic chemicals, especially those that are toxic or refractory, are not amendable to microbial degradation. Recently considerable attention has been focussed on the use of semiconductor as a means to oxidize toxic organic chemicals [4], [5], [6], [7], [8], [9], [10], [11], [12]. The mechanism constituting heterogeneous photocatalytic oxidation processes has been discussed extensively in the literature [13], [14]. Briefly, when a semiconductor such as TiO2 absorbs a photon of energy equal to or greater than its band gap width, an electron may be promoted from the valence band to the conduction band (ecb) leaving behind an electron vacancy or “hole” in the valence band (hvb+). If charge separation is maintained, the electron and hole may migrate to the catalyst surface where they participate in redox reactions with sorbed species. Specially, hvb+ may react with surface-bound H2O or OH to produce the hydroxyl radical and ecb is picked up by oxygen to generate superoxide radical anion (O2radical dot) as indicated in the following Eqs. (1), (2), (3), (4).TiO2 + hν  ecb + hvb+O2 + ecb  O2radical dotO2radical dot + H2O  OHradical dot + OH +O2 + HO2H2O + hvb+  OHradical dot + H+

It has been suggested that the hydroxyl radicals (OHradical dot) and superoxide radical anions (O2radical dot) are the primary oxidizing species in the photocatalytic oxidation processes. In many of these studies, although the initial disappearance of the pollutant is rapid, a number of by-products are formed, which can potentially be harmful to the environment.

The pesticide derivative, indole-3-acetic acid (IAA) is a plant growth regulator that affects cell division and cell elongation and it is used to stimulate rooting and cuttings of herbaceous and woody ornamental plants. The pesticide derivative, indole-3-butyric acid (IBA) enhances the growth and development of food crops and ornamentals plants when applied to soil, cuttings or leaves. As pesticides, these chemical substances attract, kill and repel insects. They also help to keep cats and dogs away from places where they are not wanted.

To the best of our knowledge, no efforts have been made to study the photocatalysed degradation of these two systems. We have, therefore, made an attempt to study the degradation of IAA and IBA in aqueous suspension of titanium dioxide under a variety of condition.

Section snippets

Reagent and chemicals

Indole-3-acetic acid and indole-3-butyric acid were obtained from Otto Chemika Biochemika Reagents, Mumbai, India and used as such without any further purification. The water employed in all the studies was double distilled. The photocatalyst, titanium dioxide Degussa P25 was used in most of the experiments, whereas other catalyst powders, namely, Hombikat UV100 (Sachtleben Chemie GmbH), PC500 (Millennium Inorganics) and TTP (Travancore Titanium Products, India) were used for comparative

Photolysis of TiO2 suspensions containing pesticide derivatives

Irradiation of an aqueous solution of pesticide derivative IAA and IBA in the presence of TiO2 samples, led to decrease in the absorption intensity and depletion in TOC as a function of time. The change in absorption intensity and depletion in TOC as a function of irradiation time for the pesticide derivatives is shown in Fig. 2, Fig. 3, Fig. 4, Fig. 5. It was observed that 95% decomposition and 76% mineralization of IAA takes place. Whereas, in case of IBA, 93% decomposition and 74.5%

Conclusion

TiO2 can efficiently photocatalyse the pesticide derivative indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) in presence of light and oxygen. The indole-3-acetic acid was found to degrade slightly faster as compared to the indole-3-butyric acid. The photocatalyst Degussa P25 was found to be more efficient as compared with other photocatalyst TiO2 powders. The addition of electron acceptor enhanced the degradation rate of the pollutants. The observations of these investigations clearly

Acknowledgements

Financial support from Department of Science and Technology, New Delhi, India and Department of Chemistry, Aligarh Muslim University, Aligarh, India, is gratefully acknowledged. Total organic carbon (TOC) analyzer used for the analysis of the samples was a gift instrument from the Alexander von Humboldt Foundation, Bonn, Germany.

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