Photochemical behaviour of 4-chloro-2-methylphenoxyacetic acid: Influence of pH and irradiation wavelength

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• 作者：Zertal ; Abdennour ; Sehili ; Tahar ; Boule ; Pierre
• 关键词：MCPA ; Photolysis ; Photocatalysis ; Photorearrangement ; Wavelength effect ; Induced phototransformation ; Toxicity
• 刊名：Journal of Photochemistry and Photobiology A: Chemistry
• 出版年：2001
• 出版时间：December 13, 2001
• 年：2001
• 卷：146
• 期：1-2
• 页码：37-48
• 全文大小：263 K

文摘

The kinetics of phototransformation of MCPA is studied under various irradiation conditions. The photocatalytic transformation on TiO₂ in aqueous suspension is almost specific and leads to 4-chloro-2-methylphenol (P₇) as the main photoproduct. The same product is obtained when MCPA is irradiated on silica in the absence of water. The direct phototransformation is more complex: the reaction is not influenced by oxygen but it depends on the pH of the solution and on the irradiation wavelength. With the anionic form irradiated between 254 and 350nm, photohydrolysis of C–Cl bond is almost quantitative (yield>86 % ). It leads to the hydroxylated photoproduct P₂. With the molecular form the main product P₅ results from a photochemical rearrangement of the molecule. With both forms, some other photoproducts are also identified and quantified, particularly methylhydroquinone (P₁) and P₇. 2-Methylphenol (P₆) is only obtained with the anionic form as a minor product. However, irradiation of solutions in sunlight or with lamps emitting mainly at 365nm (about 2 and 6 % of the light is emitted at 334 and 313nm, respectively) yields P₇ as the main photoproduct. Its formation is self-accelerated. This wavelength effect is attributed to reactions induced by quinonic compounds formed as intermediates since the disappearance of MCPA is more efficient in presence of quinonic products. P₇ is also the major photoproduct when phototransformation is induced by nitrite ions or Fe(III) perchlorate. Besides, it appears from Microtox^® test that photoproducts formed at wavelength shorter than 350nm are more toxic to the marine bacterium Vibrio fischeri than the initial compound.