Abstract
The humic and fulvic acids extracted from a Ranker type soil sensitize the transformation of monuron. When monuron is irradiated at 365 nm in the presence of the fulvic acid, its degradation is faster in deoxygenated medium than in air-saturated solution. Chloride ions are released, and the para-hydroxylated derivative is formed as upon direct photolysis. It is deduced that the consumption of monuron observed in the absence of oxygen is due to an energy transfer from reactive triplet states of the fulvic acid to monuron. Energy transfer reactions also take place when hydroquinone or acetophenone are used as sensitizers, showing that the energy level of the triplet state of monuron is lower than 311 kJ mol −1. The rate of monuron disappearance in the presence of the humic acid is lower in deoxygenated than in aerated medium. This oxygen effect indicates that there is no energy transfer. A hydrogen atom transfer from monuron to triplet states of the humic acid probably takes place.
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