Photochemical transformation of atrazine and formation of photointermediates under conditions relevant to sunlit surface waters: Laboratory measures and modelling

•Atrazine is photodegraded in surface waters by OH, 3CDOM* and direct photolysis.•Photodegradation is fastest in shallow, NO3−(NO2−)-rich and DOM-poor waters.•Desethylatrazine is the main transformation intermediate, highest yield with OH.•Overlap between intermediates of atrazine photo- and biodegradation.•Good agreement between model predictions and field data.

By combination of laboratory experiments and modelling, we show here that the main photochemical pathways leading to the transformation of atrazine (ATZ, 2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) in surface waters would be direct photolysis, reaction with OH and with the triplet states of chromophoric dissolved organic matter (3CDOM*). Reaction with 3CDOM* would be favoured by elevated water depth and dissolved organic carbon content, while opposite conditions would favour direct photolysis and OH reaction. Desethylatrazine (DEA, 4-amino-2-chloro-6-isopropylamino-1,3,5-triazine) was the main detected intermediate of ATZ phototransformation. Its formation yield from ATZ (ratio of DEA formation to ATZ transformation rate) would be 0.93 ± 0.14 for OH, 0.55 ± 0.05 for 3CDOM*, and 0.20 ± 0.02 for direct photolysis. Direct photolysis and OH reaction also yielded 4-amino-2-hydroxy-6-isopropylamino-1,3,5-triazine (DEAOH) and 6-amino-2-chloro-4-ethylamino-1,3,5-triazine (DIA). Reaction with excited triplet states also produced 2-hydroxy-4,6-diamino-1,3,5-triazine (AN) and 2-chloro-4,6-diamino-1,3,5-triazine (CAAT). Therefore, if biological processes can be neglected and if the low formation yields do not prevent detection, DEAOH and DIA could be used as markers of ATZ direct photolysis and OH reaction, while AN and CAAT could be markers of ATZ reaction with 3CDOM*. Model predictions concerning ATZ phototransformation were compared with available field data from the literature. When sufficiently detailed field information was provided, good agreement was found with the model.

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