The •OH Radical Yield in the H2O2 + O3 (Peroxone) Reaction

The peroxone process is one of the AOPs that lead to (OH)-O-center dot. Hitherto, it has been generally assumed that the (OH)-O-center dot yield is unity with respect to O-3 consumption. Here, experimental data are presented that suggest that it must be near 0.5. The first evidence is derived from competition experiments. The consumption of 4-chlorobenzoic acid (4-CBA), 4-nitrobenzoic acid (4-NBA) and atrazine present in trace amounts (1 mu M) has been followed as a function of O-3 concentration in a solution containing H2O2 (1 mM) and tertiary butanol (tBuOH, 0.5 mM) in excess over the trace compounds. With authentic (OH)-O-center dot generated by gamma-radiolysis such a competition can be adequately fitted by known (OH)-O-center dot rate constants. Fitting the peroxone data, however, the consumption of the trace indicators can only be rationalized if the (OH)-O-center dot yield is near 0.5 (4-CBA: 0.56, 4-NBA: 0.49, atrazine: 0.6). Additional information for an (OH)-O-center dot yield much below unity has been obtained by a product analysis of the reactions of tBuOH with (OH)-O-center dot and dimethyl sulfoxide with (OH)-O-center dot. The mechanistic interpretation for the low (OH)-O-center dot yield is as follows (Merenyi et al. Environ. Sci. Technol. 2010, 44, 3505-3507). In the reaction of O-3 with HO2- an adduct (HO5-) is formed that decomposes into O-3(center dot-) and HO2 center dot in competition with 2 O-2 + OH-. The latter process reduces the free-radical yield.