Isoproturon Reappearance after Photosensitized Degradation in the Presence of Triplet Ketones or Fulvic Acids

Isoproturon (IPU) is a phenylurea herbicide used to control broad-leaf grasses on grain fields. Photosensitized transformation induced by excited triplet states of dissolved organic matter ((DOM)-D-3*) has been identified as an important degradation pathway for IPU in sunlit waters, but the reappearance of IPU in the absence of light is observed after the initial photolysis. In this study, we elucidate the kinetics of this photodegradation and dark-reappearance cycling of IPU in the presence of DOM proxies (aromatic ketones and reference fulvic acids). Using mass spectrometry and nuclear magnetic resonance spectroscopic techniques, a semi-stable intermediate (IPUint) was found to be responsible for IPU reversion and was identified as a hydroperoxyl derivative of IPU. IPUint is photogenerated from incorporation of diatomic oxygen to IPU and is subjected to thermolysis whose rate depends on temperature, pH, the presence of DOM, and inorganic ions. These results are important to understand the overall aquatic fate of IPU and structurally similar compounds under diurnal conditions.