A reaction kinetic model for ozone decomposition in aqueous media valid for neutral and acidic pH

Abstract

A kinetic scheme for ozone decomposition in aqueous media has been developed. It can be applied for an extended range of pHs from acidic to neutral operating conditions. All experiments were made in a homogeneous medium under an assured kinetic controlling regime. Under no circumstances, a headspace existed in the reactor volume. The starting point of the reaction was always under the prerequisite of a true state of initial equilibrium conditions for the mixture water–ozone–oxygen. The model, that is not intended to be a true reaction mechanism, was derived from the 18 reaction steps mechanism proposed by Bühler et al. [R.E. Bühler, J. Staehelin, J. Hoigné, Ozone decomposition in water studied by pulse radiolysis. 1. HO2/O2− and HO3/O3− as intermediates, J. Phys. Chem. 88 (1984) 2560–2564] and Staehelin et al. [J. Staehelin, R.E. Bühler, J. Hoigné, Ozone decomposition in water studied by pulse radiolysis. 2. OH and HO4 as chain intermediates, J. Phys. Chem. 88 (1984) 5999–6004]. Most of the kinetic constants are known, but unfortunately they have not been obtained at the same pH (variations from 0.9 to almost 13 exist) and in one particular case was the result of a parametric estimation resorting to assumptions about the value of four other unknown constants in the proposed reaction sequence. With an accurate phenomenological modification represented by an analytical expression, a function of pH was introduced in what was found to be the most critical constant of the previously mentioned mechanism. The resulting set of reactions steps reproduces with very good agreement experiments made at pH 3, 4.8 and 6.3. These results should be useful to be applied as background information to analyze the use of ozone to degrading chlorinated organic compounds that render reaction by products (HCl), which reduce the pH of the reacting medium along the reaction evolution. Thus, the ultimate objective of this work is to derive a working and practical reaction sequence valid under these variable operating conditions.