Enhancement of hydroxyl radical generation by phenols and their reaction intermediates during ozonation

Generation of hydroxyl radicals, one of the major active species in ozonati on of water was directly observed with a spin-trapping/electron spin resona nce (ESR) technique using 5,5-dimethyl-1-pyrrolineN-oxide (DMPO) as a spin- trapping reagent. Hydroxyl radical were trapped with DMPO as a stable radic al, DMPO-OH. Eighty mu M of ozone produced 1.08 X 10-6M of DMPO-OH, indicat ing that 1.4% of . OH is trapped with DMPO. Generation rate of DMPO-OH was determined by ESR/stopped-flow measurement. Phenol derivatives increased th e amount and generation rate of DMPO-OH, indicating that phenol derivatives enhance . OH generation during ozonation of water. Ozonation of 2,3-, 2,5-, 2,6-dichlorophenol gave an ESR spectra of triplet tines whose peak height ratio were 1:2:1. ESR parameters of the triplet lin es agreed with those of the corresponding dichloro-psemiquinone radical. Oz onation of 2,4,5- and 2,4,6-trichlorophenol gave the same spectra as those of 2,5- and 2,6-dichlorophenol, respectively, indicating that a chlorine gr oup in p-position is substituted with a hydroxy group during ozonation. Amo unts of the radical increased in an ozone-concentration dependent manner an d were inhibited by addition of hydroxyl radical scavengers. These results suggest that p-semiquinone radicals are generated from the chiorophenols by hydroxyl radicals during ozonation. The p-semiquinone radicals were at lea st partly responsible for enhancements of DMPO-OH generation. (C) 1998 Publ ished by Elsevier Science Ltd. All rights reserved.