Photochemical transformation of acifluorfen under laboratory and natural conditions.

Acifluorfen was irradiated in pure water at various excitation wavelengths and pH values. Numerous photoproducts were obtained which were identified by [1H]NMR and/or HPLC-MS/MS. The main reaction pathways were photo-decarboxylation, photo-cleavage of the ether bonding with formation of phenolic compounds, photo-dechlorination and photo-Claisen type rearrangement. Decarboxylation was observed in acidic and neutral media whereas cleavage of the ether bonding dominated in basic media. The photo-Claisen type rearrangement only occurred on excitation at short wavelengths. The quantum yield of photolysis was significantly lower at 313 nm (6.1 x 10(-5)) than at 254 nm (2.0 x 10(-3)). The photoreactivity of acifluorfen was then studied in conditions approaching environmental conditions. Acifluorfen was dissolved in pure water, in water containing humic substances or in a natural water, and exposed to solar light in June at Clermont-Ferrand (latitude 46 degrees N). In pure water, the half-life was estimated at 10 days and photo-decarboxylation accounted for 30% of the conversion. The presence of humic substances (10 mg litre-1) did not affect the rate of photo-transformation. However, the half-life of acifluorfen dissolved in the natural water was only 6.8 days.