The OH Radical Yield in the H_2O_2 + O_3 (Peroxone) Reaction

摘要

The peroxone process is one of the AOPs that lead to ~*OH. Hitherto, it has been generally assumed that the ~*OH 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 μM) has been followed as a function of the O_3 concentration in a solution containing H_2O_2 (1 mM) and tertiary butanol (tBuOH, 0.5 mM) in excess over the trace compounds. With authentic ~*OH generated by γ-radiolysis such a competition can be adequately fitted by known ~*OH rate constants. Fitting the peroxone data, however, the consumption of the trace indicators can only be rationalized if the ~*OH yield is near 0.5 (4-CBA: 0.56, 4-NBA: 0.49, atrazine: 0.6). Additional information for an ~*OH yield much below unity has been obtained by a product analysis of the reactions of tBuOH with ~*OH and dimethyl sulfoxide with ~*OH. The mechanistic interpretation for the low ~*OH yield is as follows (Merenyi et al. Environ. Sci. Technol. 2010, 44, 3505-3507). In the reaction of O_3 with HO_2~- an adduct (HO_5~-) is formed that decomposes into O_3~(*-) and HO_2~* in competition with 2 O_2 + OH~-. The latter process reduces the free-radical yield.