Direct and induced phototransformation of mecoprop [2-(4-chloro-2-methylphenoxy)propionic acid] in aqueous solution

Abstract

Mecoprop was irradiated under various conditions of pH, oxygenation and wavelengths in order to study the reactions involved in the phototransformation. Four main photoproducts were identi®ed: 2-(4-hydroxy-2-methylphenoxy)propionic acid (I), o-cresol (II), 2-(5-chloro-2-hydroxy-3-methylphenyl)propionic acid (III) and 4-chloro-o-cresol (IV). When the anionic form of mecoprop was irradiated between 254 nm and 310 nm (UV-C or UV-B), I was the main photoproduct. At 254 nm its formation initially accounted for more than 80% of the transformation. It has not previously been reported in the literature. The reaction results from a heterolytic photohydrolysis. Product II accounted for only a low percentage of the transformation. The stoichiometry was different with the molecular form: the main photoproduct, III, resulted from a rearrangement after a homolytic scission. Products I, II and IV were also formed as minor photoproducts. Some other minor photoproducts were also identi®ed. In contrast, IV was the main photoproduct under sunlight irradiation or when solutions were irradiated in near-UV light (UV-A). This wavelength effect is attributed to the involvement of an induced phototransformation; IV is also the main photoproduct when the phototransformation is induced by Fe(III) perchlorate or nitrite ions. In usual environmental conditions the excitation of the molecular form is negligible and the phototransformation is mainly due to induced photoreactions.

Photochemical behaviour of dichlorprop [(±)‐2‐(2,4‐dichlorophenoxy)propanoic acid] in aqueous solution

“…The quantum yields for the two forms do not differ significantly, and only a small enhancing effect of oxygen on the transformation rate was observed with the neutral form, which can be explained by the involvement of photooxidation reactions. Quantum yields are significantly lower than with mecoprop 6. This effect of the introduction of a second chlorine atom on the ring has previously been reported in the case of chlorobenzenes and chlorophenols 12.…”

“…The devices used to irradiate solutions at different wavelengths have been described elsewhere 6. Monochromatic irradiations in parallel beam at 283 nm (maximum of absorption of DCPP) were used for the measurement of quantum yields.…”

“…No pH effect was reported. The failure to detect 2,4‐dichlorophenol in sunlight irradiation of 2,4‐D seems to be inconsistent with the wavelength effect observed on the degradation behaviour of MCPA [4‐chloro‐2‐methylphenoxyacetic acid]5 and mecoprop [2‐(4‐chloro‐2‐methylphenoxy)propanoic acid] 6. It was recently reported that 4‐chloro‐2‐methylphenol accumulates in higher amounts when solutions of MCPA or mecoprop are exposed to sunlight than when they are irradiated at 254 nm or in the range 290–350 nm.…”

Nitrite and nitrate induced photodegradation of monolinuron in aqueous solution

“…By excitation of HNO 2 form (pKa 3.37; Eq. 3) the quantum yield becomes even higher, about 0.45 (Meunier and Boule 2000).…”

Asulam in Aqueous Solutions: Fate and Removal Under Solar Irradiation

“…When the pesticides does not absorb solar light, its transformation can be photoinduced by different absorbing species, present or added in the aquatic medium (Nitrate [1], Humic substances [2], Fe(III) aquacomplexes [3], . .…”