Quantitative analysis for the enhancement of hydroxyl radical generation by phenols during ozonation of water

Generation of hydroxy] radical, one of the major reactive species in the ozonation of water, was directly and quantitatively measured with the combination technique of spin-trapping, stopped-flow and electron spin resonance (ESR) spectroscopy. Hydroxyl radical in ozonated water was trapped with 5,5-dimethyl-pyrrolidine-1-oxyl (DMPO) as a stable radical, DMPO-OH, and the amount of DMPO-OH was quantitatively measured with an ESR spectrometer. The ESR signal of DMPO-OH increased linearly with increasing ozone and 1 M ozone produced 0.011 M DMPO-OH, if one molecule of ozone produces one molecule of (OH)-O-.. The kinetics of DMPO-OH generation was determined with a stopped-flow/ESR method and analyzed by two different methods. The logarithmic plot of the initial velocity of DMPO-OH generation vs the ozone concentration gave a linear relationship, which was expressed as v(o) (M s(-1)) = 8.7 x 10(1) x[O-3 (M)](2.25). The rate constant of DMPO-OH generation, 8.7 x 10(1) M-1.25 s(-1) is close to the reaction constant of ozone with an hydroxide ion. The decomposition rate of ozone to (OH)-O-. was estimated to be 0.026 s(-1) at 40 mu M aqueous ozone from the semilogarithmic plot of the amount of DMPO-OH against reaction time. The presence of phenol derivatives increased both the generation rate and the final amount of DMPO-OH in a dose dependent manner. The enhancing effect of phenols on (OH)-O-. generation was also analyzed with the above two methods the decomposition of ozone and the generation of DMPO-OH. The effect depended on the kind, position and number of substituents of phenol. The chlorine substitution of phenol had a more enhancing effect than that of methyl substitution and the ortho-substitution by chlorine showed a stronger enhancement. 2,4-Dichlorophenol showed the strongest enhancing effect on (OH)-O-. generation among phenols examined, followed by 2-chlorophenol. (C) 1998 Elsevier Science Ltd. All rights reserved.