Degradation of octylphenol and nonylphenol by ozone – Part II: Indirect reaction

The indirect reaction of octylphenol (OP) and nonylphenol (NP) with hydroxyl radicals (OH) during ozonation was investigated at pH values ranging from 6 to 9. A parameter Rct, representing the ratio of the OH-exposure to the ozone-exposure, was measured using a method involving a low concentration of p-chlorobenzoic acid as a OH-probe compound during the ozonation. By assuming that Rct is a constant value at a given pH, the second order rate constants of the alkylphenol reaction with hydroxyl radicals were determined as 1.4(±0.2) × 1010 and 1.1(±0.2) × 1010 M−1 s−1 for OP and NP, respectively. The proportions of each alkylphenol degraded by direct molecular ozone reaction and indirect hydroxyl radical reaction were predicted at different pH values. The contribution of indirect OH reactions with each AP was found to represent over 50% of the total degradation for pH ≈ 7, and the contribution increases substantially with pH > 7.