Photodegradation of Diquat and Paraquat in aqueous solutions by titanium dioxide: evolution of degradation reactions and characterisation of intermediates☆
Introduction
The use of titanium dioxide, TiO2, in the photocatalytic degradation of organic pollutants has attracted much attention as a promising method of depolluting contaminated waters and soils. Diquat [(C12H12N2)2+ · 2Br−] and Paraquat [(C12H14N2)2+ · 2Cl−] are commonly used herbicides with a variety of applications owing to their physical properties, such as high solubility in water, low vapour pressure, high binding potential, which make them suitable for many agriculture uses. Due to their recognized toxicity, ease of handling and fast effect, their degradation has been the subject of a number of photocatalytic studies (Funderburk and Bozarth, 1967; Kinkennon et al., 1995; Tennakone and Kottegoda, 1996; Moctezuma et al., 1999; Vohra and Tanaka, 2001, Vohra and Tanaka, 2003). Nevertheless, none of these studies resort to mass spectrometric analysis for monitoring the degradation processes and for characterising the intermediates that are formed during photocatalytic degradation. Since mass spectrometric techniques based on electrospray ionisation and ion trap mass spectrometry is a very powerful tool for the identification and characterisation of unknown compounds even at trace level, we decided to use this methodology for a detailed investigation on the photocatalytic degradation of Diquat and Paraquat aqueous solutions. It is well known that the photoactivity efficiency depends on the crystal structure type, morphology and pore size of the catalyst (Shengping et al., 2001). We therefore compared the decomposition of both herbicides by commercial TiO2 and ‘home prepared’ Ti1−xFexO2 (x=0% and 4%), using an adequate UV artificial light as well as natural solar radiation. UV spectroscopy was used to investigate the stability of the herbicides solutions and to follow and optimise the photodegradation reactions. The characterisation of degradation intermediates up to complete degradation has been achieved by means of electrospray ionisation (ESI) mass spectrometry combined with collision-induced dissociation (CID) and tandem mass spectrometry, MS/MS. The effect of several reaction parameters and experimental conditions on the overall degradation process of the herbicides has also been studied. The very low detection limit of mass spectrometry shows that this technique is far superior to UV spectroscopy to evaluate total degradation of the herbicides solutions.
Section snippets
Chemicals
All the chemicals used were of reagent grade. Diquat dibromide monohydrate and Paraquat dichloride were purchased from Sigma-Aldrich (Milwaukee, WI, USA) and used without further purification. Potassium bromide with 99.5% purity was purchased from Merck (Darmstadt, Germany).
Two different types of titanium dioxide, Degussa P25, a known mixture of approximately 70% anatase and 30% rutile, kindly supplied by Degussa (Frankfurt am Main, Germany), and ‘home prepared’ anatase phase TiO2 or Ti1−xFexO2
Stability of herbicides solutions
The stability of Diquat and Paraquat aqueous solutions was investigated by means of UV spectroscopy, considering the following experimental parameters: pH, temperature and standing time. With respect to pH, a significant decrease in absorbance for Diquat as well as for Paraquat solutions was observed for very alkaline media, as reported for Paraquat (Roberts et al., 2002), while in very acidic media just a slight decrease is observed, although more pronounced for Diquat. In the 3–8 pH range no
Acknowledgements
The authors are grateful to Professor Ana Lobo for helpful discussions and Professor Fatima Piedade for Paraquat and Diquat X-ray library search.
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Part of this work has been presented for the first time at the “XVIII Meeting of the Portuguese Society of Chemistry”, Aveiro, Portugal, 2002.