Ozone decomposition in water studied by pulse radiolysis. 2. Hydroxyl and hydrogen tetroxide (HO4) as chain intermediates

Abstract

Ozone decomposition in pure water involves a chain mechanism, initiated by the reaction OH^- + O₃ and propagated by O₂ ^- and OH. In the present studies this chain is initiated by pulse radiolysis of aqueous solutions of ozone. The chain propagation steps were studied in two parts: (I) O₂ ^- → O₃ ^- → HO₃ → OH and (II) OH + O₃ → ... → HO₂ → O₂ ^-. By computer simulation of the rate curves, it is shown that from OH + O₃ an intermediate HO₄ must be formed, most likely a charge-transfer complex (HO·O₃) which eventually decays into HO₂. The derived rate constants are <I>k</I>(OH+O₃) = (2.0 ± 0.5) × 10⁹ M^-1 s^-1, <I>k</I>(HO₄→HO₂+O₂) = (2.8 ± 0.3) × 10⁴ s^-1, and <I>k</I>(HO₄→OH+O₃) < <I>k</I>(HO₄→HO₂+O₂). The spectrum of HO₄ is derived. It is similar to the one of ozone, but the absorption coefficients are about 50% larger. In the presence of high ozone concentration, the dominant chain termination reactions are HO₄ + HO₄ and HO₄ + HO₃. The effect on chain length, dose, overall rate, and pH and of added scavengers is described. The implications for the natural ozone decay mechanism are discussed.