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

The kinetics of phototransformation of MCPA is studied under various irradi ation conditions. The photocatalytic transformation on TiO2 in aqueous susp ension is almost specific and leads to 4-chloro-2-methylphenol (P-7) 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 com plex: 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 irrad iated between 254 and 350 mn, photohydrolysis of C-Cl bond is almost quanti tative (yield > 86%). It leads to the hydroxylated photoproduct P-2. With t he molecular form the main product P-5 results from a photochemical rearran gement of the molecule. With both forms, some other photoproducts are also identified and quantified, particularly methylhydroquinone (P-1) and P-7. 2 -Methylphenol (P-6) is only obtained with the anionic form as a minor produ ct. However, irradiation of solutions in sunlight or with lamps emitting, m ainly at 365 nm (about 2 and 6% of the light is emitted at 334 and 313 nm, respectively) yields P7 as the main photoproduct. Its formation is self-acc elerated. This wavelength effect is attributed to reactions induced by quin onic compounds formed as intermediates since the disappearance of MCPA is m ore efficient in presence of quinonic products. P-7 is also the major photo product when phototransformation is induced by nitrite ions or Fe(III) perc hlorate. Besides, it appears from Microtox(R) test that photoproducts forme d at wavelength shorter than 350 run are more toxic to the marine bacterium Vibrio fischeri than the initial compound. (C) 2001 Elsevier Science B.V. All rights reserved.