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 x 101 x [O3 (M)]2.25. The rate constant of DMPO-OH generation, 8.7 x 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.
All Science Journal Classification (ASJC) codes
- Ecological Modelling
- Water Science and Technology
- Waste Management and Disposal