Generation of p-semiquinone radicals from chlorophenols in water during ozonation

The formation of radical species during ozonation of phenols in water was investigated with ESR spectroscopy. Ozonation of 2,3-, 2,5- and 2,6-dichlorophenol gave ESR spectra with a triplet of lines whose peaks have a height ratio of 1:2:1. The ESR parameters of the triplet were similar to those of the corresponding dichloro-p-semiquinone radical. Ozonation of 2,4,5- and 2,4,6-trichlorophenol gave the same spectra that resulted from the ozonation of 2,5- and 2,6-dichlorophenol, respectively, indicating the replacement of the chlorine in the p-position by a hydroxyl group during ozonation. The amount of the radical increased with increasing ozone concentration and was inhibited by two hydroxyl radical scavengers, mannitol and uric acid. The Fenton system, however, produced no radicals from phenols. These results indicate that an active species in addition to the hydroxyl radical participates in the formation of the p-semiquinone radical from chlorophenols during ozonation.