Photochemical transformation of phenylurea herbicides in surface waters: A model assessment of persistence, and implications for the possible generation of hazardous intermediates

• Phototransformation of PUHs is induced by 3CDOM*, OH, CO3- and direct photolysis.
• Photochemical persistence would be in the order IPT ∼ MTX < CTL < FEN < DIU.
• Toxic intermediates would be of some concern for MTX and DIU photolysis.
• Formation of toxic formylated derivatives upon OH reaction could be important.
• Reactivity shift from OH to 3CDOM* at high DOC could decrease toxicity.

This work models the phototransformation kinetics in surface waters of five phenylurea herbicides (diuron, fenuron, isoproturon, metoxuron and chlortoluron), for which important photochemical parameters are available in the literature (direct photolysis quantum yields and reaction rate constants with OH, CO3- and the triplet states of chromophoric dissolved organic matter, 3CDOM*). Model calculations suggest that isoproturon and metoxuron would be the least photochemically persistent and diuron the most persistent compound. Reactions with OH and 3CDOM* would be the main phototransformation pathways for all compounds in the majority of environmental conditions. Reaction with CO3- could be important in waters with low dissolved organic carbon (DOC), while direct photolysis would be negligible for fenuron, quite important for chlortoluron, and somewhat significant for the other compounds. The direct photolysis of metoxuron and diuron is known to increase toxicity, and such a photoreaction pathway would be enhanced at intermediate DOC values (1–4 mg C L1). The reaction between phenylureas and OH is known to produce toxic intermediates, differently from 3CDOM*. Therefore, the shift of reactivity from OH to 3CDOM* with increasing DOC could reduce the environmental impact of photochemical transformation.

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