Aqueous oxidation of phenylurea herbicides by triplet aromatic ketones

Excited triplet states of dissolved natural organic matter (DOM) are important players for the transformation of organic chemical contaminants in sunlit natural waters. The present study focuses on kinetics and mechanistic aspects of the transformation of phenylurea herbicides induced by well-defined excited triplet states, which have been chosen to model DOM triplet states having oxidative character. The aromatic ketones benzophenone, 3'-methoxyacetophenone, and 2-acetonaphthone were used to photogenerate their triplet states and oxidize a series of eleven substituted phenylureas. Quenching of the excited triplet states by the phenylureas was measured using laser flash photolysis in the microsecond time domain, while the oxidation kinetics of the phenylureas was followed under steady-state irradiation. Second-order rate constants for quenching and oxidation were largely identical for a given pair of ketone and phenylurea. They reached the diffusion-controlled limit (approximately 4 x 10(9) M(-1) s(-1)) and decreased with increasing free energy of electron transfer from the phenylurea to the ketone triplet. These results confirm those already obtained using phenols as the substrates to be oxidized and suggest that oxidation rates are mainly determined by the bimolecular rate constant for electron transfer, a rule that can possibly be extended to various organic contaminants. A refined estimate of the effective reduction potential of DOM excited triplet states was also obtained.