A test of the potentialities of the APEX software (Aqueous Photochemistry of Environmentally occurring Xenobiotics). Modelling the photochemical persistence of the herbicide cycloxydim in surface waters, based on literature kinetic data

• The APEX software allowed the assessment of CXD phototransformation kinetics.
• The photochemical half-life time of CXD is in the day–month range.
• Phototransformation is faster in shallow waters, poor in organic matter.
• Direct photolysis is by far the main CXD phototransformation pathway.

Based on literature kinetic data, it is shown that the photochemical transformation of cycloxydim (CXD) in surface waters would mainly occur by direct photolysis. Reaction of CXD with 1O2 and with the triplet states of chromophoric dissolved organic matter would only account for a small fraction (⩽2%) of total phototransformation. The second-order reaction rate constant between CXD and OH is not known. However, estimates based on data analysis with the APEX software and on diffusion-control kinetics in water indicate that OH would account for less than 10% of CXD phototransformation. The photochemical half-life time of CXD in mid-latitude summertime would vary in the day–month range, depending on water chemistry and depth. Transformation would be favoured in shallow waters with low dissolved organic carbon.

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