Synergy in the Autoxidation of S(IV) Inhibited by Phenolic Compounds

Abstract

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.