More Toxic and Photoresistant Products from Photodegradation of Fenoxaprop-p-ethyl

The photodegradation pathway of the commonly used herbicide fenoxaprop-p-ethyl (FE) was elucidated, and the effects of the photodegradation on its toxicity evolution were investigated. Under solar irradiation, FE could undergo photodegradation, and acetone enhanced the photolysis rates significantly. The same photoproducts formed under the irradiation of λ > 200 nm and λ > 290 nm through rearrangement, loss of ethanol after rearrangement, de-esterification, dechlorination, photohydrolysis, and the breakdown of the ether linkages. One of the main transformation products, 4-[(6-chloro-2-benzoxazolyl)oxy] phenol (CBOP), was resistant to photodegradation under the irradiation of λ > 290 nm, and its photolysis rate was seven times slower than the parent under the irradiation of λ > 200 nm. Among the metabolites, CBOP (48 h EC₅₀ of 1.49−1.64 mg/L) and hydroquinone (48 h EC₅₀ of 0.25−0.28 mg/L) were more toxic to Daphnia magna than the parent FE (48 h EC₅₀ of 4.2−6.9 mg/L). Thus, more toxic and photoresistant products were generated from photolysis of the herbicide. Ecotoxicological effects of phototransformed products from pesticides should be emphasized for the ecological risk assessment of these anthropogenic pollutants.