Photocatalytic Degradation of Trifluralin, Clodinafop-Propargyl, and 1,2-Dichloro-4-Nitrobenzene As Determined by Gas Chromatography Coupled with Mass Spectrometry
Phototransformation is considered one of the most key factors affecting the fate of pesticides. Therefore, our study focused on photocatalytic degradation of three selected pesticide derivatives: trifluralin (1), clodinafop-propargyl (2), and 1,2-dichloro-4-nitrobenzene (3). The degradation was carried out in acetonitrile/water medium in the presence of titanium dioxide (TiO2) under continuous purging of atmospheric air. The course of degradation was followed by thin-layer chromatography and gas chromatography-mass spectrometry techniques. Electron ionization mass spectrometry was used to identify the degradation species. GC-MS analysis indicates the formation of several intermediate products which have been characterized on the basis of molecular ion, mass fragmentation pattern, and comparison with NIST library. The photocatalytic degradation of pesticides of different chemical structures manifested distinctly different degradation mechanism. The major routes for the degradation of pesticides were found to be (a) dealkylation, dehalogenation, and decarboxylation, (b) hydroxylation, (c) oxidation of side chain, if present, (d) isomerization and cyclization, (e) cleavage of alkoxy bond, and (f) reduction of triple bond to double bond and nitro group to amino. 1. Introduction The contamination of water bodies due to the presence of pesticides constitutes a pervasive problem and therefore advanced methods are in demand for the effective treatment of these pesticide polluted ground and surface waters. Advanced oxidation processes have proven effective for the removal of organic pollutants. During the last two decades, photocatalytic processes involving semiconductor particles under UV light illumination have been shown to be potentially advantageous and useful in the degradation of organic pollutants [1–3]. The process occurs as a result of the interaction of a semiconductor photocatalyst and UV radiation that yields highly reactive hydroxyl and superoxide radical anions, which are believed to be the main species responsible for the oxidation of organic substrates. The most commonly used photocatalyst is TiO2, which is inexpensive, abundant, photostable, and nontoxic [4]. The mechanism of photocatalysis is well documented in the literature [4, 5]. Briefly, when a semiconductor such as TiO2 absorbs a photon of energy equal to or greater than its band gap energy, an electron may be promoted from the valence band to the conduction band (e？) leaving behind an electron vacancy or “hole” in the valence band (h+), as shown in (1). If charge separation is
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