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The effects of phenol (C6H5OH), catechin [C6H4(OH)2], and gallic acid [HOOCC6H2(OH)3] on the kinetics of aqueous SO2 oxidation by molecular oxygen have been investigated for dilute systems, as pertain to the chemistry of troposphere and surface waters. In experiments performed under batch (homogeneous oxidation) or semibatch (heterogeneous oxidation) conditions, we measured the decay of oxygen with a Yellow Spring Instruments probe or the increase in S(VI) concentration with an Orion conductivity cell. All of the studied phenolic compounds caused inhibition of the uncatalyzed autoxidation of S(IV), but the inhibiting effectiveness of phenol was much lower than that of catechin and gallic acid. The dual role of phenol in not only inhibiting but also promoting action was demonstrated in experiments carried out in the presence of Co or Mn catalyst when the synergy effect was evident. Another extraordinary behavior was observed in the case of gallic acid at prolonged experiments leading to oscillations in the rate of S(VI) formation. The results are discussed in terms of a radical chain reaction with termination by a step first order in SO4 •- radicals and in a phenolic compound, with initiation supplemented by a step first order in phenoxyl radicals and in aqueous S(IV) species. From such a model of the inhibited autoxidation of S(IV), we determined the following rate constants for SO4 • scavenging:  4.3 × 109 M-1 s-1 for catechin and 2.6 × 109 M-1 s-1 for gallic acid. The consequences of this inhibition in the environment are discussed.

The kinetics of Co-catalyzed autoxidation of calcium sulfite was studied to deepen knowledge of the mechanism of this chain reaction. Laboratory experiments were performed under heterogeneous conditions using two different reactors: a stirred tank with a plane gas-liquid (slurry) interface and an impinger. The reaction course was followed by monitoring the conductivity of the reacting solution and by quenching with iodine solution, respectively. Mechanistic judgments were derived from the influence of sulfite and catalyst concentrations, and the solid CaS03 load on the kinetics of oxygen absorption. Appropriate reaction orders and rate constants were determined. Phenomena related to the solubility product law that imposed autoxidation rate limitations were analyzed. A soluble sulfate was shown to be a dual-action additive markedly accelerating the autoxidation due to increased sulfite and catalyst solubilities. The results are useful for designers of air pollution control processes.

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