Kinetics of paracetamol oxidation by ozone and hydroxyl radicals, formation of transformation products and toxicity

• We investigated the reaction of paracetamol with ozone and hydroxyl radicals at pH 7.2 and 5 respectively.
• Rate constants were determined and reaction kinetic was discussed.
• The degradation of paracetamol by ozone and hydroxyl radicals was modelled in real water conditions.
• Ozonation transformation products were studied and reaction pathways were proposed.
• Acute toxicity of the ozonated solutions was monitored by means of Vibrio fisheri assay.

Paracetamol oxidation by ozonation and H2O2/UV processes was investigated. The second-order rate constant for the reaction of paracetamol with ozone was determined at pH 7.2 (kO3/PRC=2.57×106M-1s-1). The rate constant of the elementary reaction of ozone with the ionized form of PRC was then calculated and the pH dependence of the ozonation reaction of PRC was estimated. The second-order rate constant of the reaction of paracetamol with HO radicals was also determined at pH 5 through the H2O2/UV oxidation system (kHO/PRC=4.94×109M-1s-1). In the light of the high rate constants obtained, two conclusions on the kinetics of paracetamol ozonation in real water were drawn: (i) hydroxyl radicals do not have any impact on the removal of paracetamol during ozonation of real waters; (ii) a half-life of 0.03 s can be estimated for paracetamol, for a residual ozone concentration of 0.4 mg L−1. Hydroquinone and two other ozonation transformation products were identified by LC/UV, LC/MS and MS/MS analyses. In parallel, for the first time, toxicity was measured in ozonated paracetamol solutions with the luminescent bacteria Vibrio fisheri test. The results showed an increase in toxicity as paracetamol degraded. This toxicity could not be assigned to hydroquinone formation only. One or several other transformation products more toxic than paracetamol might be formed.