Ozone decomposition in water studied by pulse radiolysis. 1. Perhydroxyl (HO2)/hyperoxide (O2-) and HO3/O3- as intermediates

Abstract

Pulse radiolysis allows one to selectively enter the chain mechanism for ozone decomposition in water by OH or O2- and to study the elementary reactions involved. Details of the reaction mechanism of the first chain propagation step (O2- + O3) in the Weiss model have been resolved to include the two additional transients O3- (λmax = 430 ± 10 nm, εmax = 2000 M-1 cm-1) and HO3 (λmax = 350 ± 20 nm, (εmax = 300 ± 30 M-1 cm-1). The elementary steps are as follows: O2- + O3 → O3- + O2, k = (1.6 ± 0.2) × 109 M-1 s-1; O3- + H+ ⇌ HO3 (a, b), ka = (5.2 ± 0.6) × 1010 M-1 s-1, kb = (3.7 ± 0.3) × 104 s-1; HO3 → OH + O2, k = (1.1 ± 0.1) × 105 s-1. A pK(HO3/O3-) = 6.15 ± 0.05 is derived. Phosphate buffer also takes part in the protonation/deprotonation of HO3/O3-: O3- + H2PO4- ⇌ HO3 + HPO42- (a, b), ka = (2.1 ± 0.2) × 108 M-1 s-1, kb = (2.0 ± 0.3) × 107 M-1 s-1. Consequences of these high rate constants on the possible chain termination processes and on the possibility of interactions of organic solutes with the chain reactions are discussed.