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.

Kinetics of iron(III)-catalyzed autoxidation of sulfur(IV) in acetate buffered medium

“…The rate law (6) for the inhibition is same as reported by Alyea and Backstrom [27], Pasiuk-Bronikowska et al [9], Manoj et al [26] and Mudgal et al [29] for inhibition of sulfur(IV) autoxidation with different types of inhibitors. To explain the inhibition, the operation of a radical mechanism [3,11,13] involving oxysulfur radicals, viz., SO 3 -, SO 5 -, SO 4 -has been proposed.…”

“…The reaction has been studied [1][2][3][4][5][6][7][8][9][10] repeatedly and reviewed extensively [11][12][13][14][15][16][17][18]. However, most of the detailed studies pertain to low pH region (0-3) in which the kinetics obeys the rate law (1) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. For this reaction both radical and non-radical mechanisms have been proposed [1][2][3][4][5][6][7][8][9][10].…”

Inhibition of Atmospheric Aqueous Phase Autoxidation of Sulphur Dioxide by Volatile Organic Compounds: Mono-, di- and Tri-Substituted Benzenes and Benzoic Acids

“…In a recent and a more detailed study on the inhibition of S(IV) autoxidation by VOCs, most of which comprise aliphatic compounds, Dhayal et al [31] found the following relationship between k inh and B. www.prkm.co.uk B = (9 ± 2) × 10 −4 k inh (17) For uncatalysed S(IV) autoxidation, in general, the ratio k inh /B is about 10 3 . Accordingly, based on k inh values of 4.9 × 10 9 -7.2 × 10 9 L mol -1 s -1 for methoxybenzenes, the B values are expected to be about 10 6 L mol −1 .…”

“…Simultaneously, reaction (8) acts as a sink for the oxidation and removal of VOCs from the atmosphere. There are many studies on the chemistry of the reaction VOCs → sulfate radical anion and its inhibition kinetics [6,[12][13][14][15][16][17][18][21][22][23][24][25][30][31], which follows the rate law…”

Kinetics of the Oxidation of Hydrogen Sulfide by Atmospheric Oxygen in an Aqueous Medium

“…This led us to look afresh into the atmospheric chemistry of H 2 S oxidation and to examine the effect of several parameters such as reactant concentrations, pH and importantly the effect of some anthropogenic volatile organic compounds(VOCs) particularly those which are reported to strongly inhibit the oxidation of aqueous SO 2 by oxygen in laboratory water. The oxidation kinetics of aqueous SO 2 and effect of VOCs has been studied by several groups [3,8,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] .…”