Kinetics of the Ozone-p-Chlorobenzoic Acid Reaction

Abstract

The kinetics of p-chlorobenzoic acid ozonation was investigated. From experiments carried out at acidic pH it was suggested that the direct rate constant between the ozone molecule and p-chlorobenzoic acid is higher than the reported value of 0.15 M⁻¹s⁻¹. Runs completed in homogeneous contact pattern allowed for the calculation of the direct rate constant with a value of approximately 4.5 M⁻¹s⁻¹. Additionally, some competitive experiments were carried out in the presence of a reference compound (atrazine or simazine). From these competitive runs conducted at pH 1, values of 3.1 and 6.7 M⁻¹s⁻¹ were obtained by considering the presence of hydroxyl radicals. If the occurrence of these radicals is ruled out, the new values obtained are 3.6 and 2.2 M⁻¹s⁻¹. In any case, apparently the ozonation of p-chlorobenzoic acid seems to be faster than stated in the literature. However, the addition of tert-butyl alcohol either in homogeneous or heterogeneous experiments completely inhibited the p-chlorobenzoic acid oxidation, suggesting the generation of radical species even at strongly acidic conditions.

Keywords:

• ozone
• chlorobenzoic acid
• kinetics
• water treatment
• rate constant

Acknowledgements

The authors thank the CICYT of Spain and European FEDER Funds for the economic support through project PPQ2000/0412