A kinetic study of enhancing effect by phenolic compounds on the hydroxyl radical generation during ozonation

Ozone decomposition in aqueous solution proceeds through a radical type chain mechanism. These reactions involve the very reactive and catalytic intermediates O₂^- radical, OH radical, HO₂ radical, OH^-, H₂O₂, etc. OH radical is proposed as an important factor in the ozonation of water among them. In the present study, the enhancing effects of several phenolic compounds; phenol, 2-, 3-, 4-monochloro, 2,4-dichloro, 2,4,6-trichlorophenol on OH radical generation were mathematically evaluated using the electron spin resonance (ESR)/spin-trapping technique. OH radical was trapped with a 5,5-dimethyl-1 -pyrroline-N-oxide (DMPO) as a stable adduct, DMPO-OH. The initial velocities of DMPO-OH generation in ozonated water containing phenolic compounds were quantitatively measured using a combined system of ESR spectroscopy with stopped-flow apparatus, which was controlled by homemade software. The initial velocities of DMPO-OH generation increased as a function of the ozone concentration. The relation among ozone concentration, amount of phenolic compounds and the initial velocity (ν₀) of DMPO-OH generation was mathematically analyzed and the following equation was obtained, ν ₀ (10^-6 M/s) = (A′ × [PhOHs (10^-9 M)] + 0.0005) exp (60 × [ozone (10^-9 M)]). The equation fitted very well with the experimental results, and the correlation coefficient was larger than 0.98.