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

Generation of hydroxyl 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. 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 v0 (M s−1)=8.7×101×[O3 (M)]2.25. The rate constant of DMPO-OH generation, 8.7×101 M−1.25 s−1, is close to the reaction constant of ozone with an hydroxide ion. The decomposition rate of ozone to OH was estimated to be 0.026 s−1 at 40 μM 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 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 generation among phenols examined, followed by 2-chlorophenol.