Ozonation of 2,6-, 3,4-, and 3,5-dichlorophenol isomers within aqueous solutions

The kinetics and mechanisms associated with the ozonation of 2,6-, 3,4-, and 3,5-dichlorophenols (DCPs) in aqueous solutions were studied. It was found that 2 moles of ozone are required for the initial degradation of 1 mole of DCP. The overall reaction between ozone and a DCP was found to be second order (first order per reactant). The overall reaction rate constants of all three DCPs increased with pH because of increased dissociation of the dichlorophenol molecules to dichlorophenoxide ions. Due to its high dissociation constant, 2,6-DCP is most reactive of the three with ozone with the overall reaction rate constants varying from 1.31 x 10(5) to 3.36X 10(5) M-1 s(-1) within the pH range of 2.0-4.0. Various chlorobenzenediols, chlorocyclohexadienediones, ketones, esters, and carboxylic acids were identified as intermediate and final ozonation products using gas chromatography/mass spectrometry analysis. The initial electrophilic addition of ozone at the ortho and para positions of the molecular and ionic forms is proposed as the likely mechanism for DCP ozonation.