The application of ozone along with hydrogen peroxide, commonly referred to as peroxone oxidation, is a widely investigated technique for wastewater treatment. Degradation of ozone in water is a key step in the pollutant degradation mechanism, particularly in peroxone oxidation. However, the degradation of ozone in water is not understood at a low pH (<6). This study reveals that current ozone degradation models overestimate degradation at a low pH because the rate constants involved in the dissociation equilibrium of the hydroperoxyl radical are inaccurate. Here, the rate constants of forward and backward reactions were calculated with ab initio quantum chemical calculations computed from the CCSD (T) theory to be 1.45 × 103 s–1 and 8.6 × 107 m3 kmol –1 s–1, respectively. After modifying the current kinetic model by using the calculated rate constants, the predictions of ozone half-lives at a low pH (<6) are improved by 1–2 orders of magnitude in pure water (without organic matter and carbonate species) in comparison with the available experimental results. The ozone decomposition kinetic model was used to develop a comprehensive kinetic model for peroxone oxidation of toluene. The results demonstrate that the new rate constants considerably improve the peroxone oxidation process as well.

An ab initio study, using the coupled cluster calculations (CCSD) method was conducted to investigate the kinetics of the ozone degradation in gas and aqueous phases considering the reaction of ozone with the hydroperoxyl radical. Two potential transition state paths, oxygen and hydrogen transfer, are studied and compared. It was revealed by the ab initio quantum chemical calculations that the calculated overall rate constant in the gas phase differs by approximately an order of magnitude from measured values. However, the calculated selectivity (branching fraction), which was measured directly with isotope studies of hydrogen atom transfer, is almost exactly equal to the experimental value at 298.15 K. The sensitivity analysis showed that adding the reaction between ozone and hydroperoxyl radical to the kinetic model accelerates the decomposition process by more than four times in the aqueous phase (pH = 7–8.5), and for an order of magnitude change in the rate constant of this reaction, the decomposition half‐life changes by 20–45 %. This result might affect our understanding of atmospheric ozone chemistry.

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An ab initio study was conducted to investigate the kinetics of the ozone degradation in gas and aqueous phases considering the reaction of ozone with hydroperoxyl radical using coupled cluster calculations (CCSD) method. Oxygen atom transfer and hydrogen atom transfer are studied and compared as two potential transition state paths. The ab initio quantum chemical calculations showed that the calculated overall rate constant in the gas phase differs by almost an order of magnitude from the measured values. However, the calculated selectivity (branching fraction) of hydrogen atom transfer is almost exactly the experimental value at room temperature, which was measured directly with isotope studies. This result may affect our understanding of atmospheric ozone chemistry.