On the discrepancy between reported studies of the uncaialysed aqueous oxidation of SO₂by O₂

Abstract

The uncatalysed aqueous oxidation of S0 2 is a reaction with diverse applications, and it has long been the subject of considerable controversy. The extensive literature on this reaction is critically reviewed and the results of kinetic studies are presented in tabular form. A number of factors which may be responsible for the discrepancy between reported experiments are discussed, and of these the pH and the purity of experimental solutions appear to be the most important. The following rate equation is proposed for predicting atmospheric oxidation rates via this reaction:The mechanism appears to be of little consequence in the conversion of atmospheric SO 2 .

Quantification of SO2 Oxidation on Interfacial Surfaces of Acidic Micro-Droplets: Implication for Ambient Sulfate Formation

“…7 The interfacial SO 2 oxidation pathway by O 2 involves the formation of SO ][H + ] 0.5 . [9][10][11][12] Radojevic concluded that the non-catalyzed oxidation by O 2 was not significant for ambient SO 2 conversion as compared to other aqueous-phase oxidation pathways. 11 However, the lower reported activation energy for HSO 3 -+ O 2 under acidic conditions of 1.7 kcal mol -1 for pH < 7 vs. 4 21.3 kcal mol -1 for the pH range of 7 ≤ pH ≤ 9.5, 8,9 suggests that an efficient oxidation pathway under acidic conditions may still be important.…”

“…9−12 Radojevic concluded that the noncatalyzed oxidation by O 2 was not significant for ambient SO 2 conversion as compared to other aqueous-phase oxidation pathways. 11 However, the lower reported activation energy for HSO 3 − + O 2 under acidic conditions of 1.7 kcal mol −1 for pH < 7 vs 21.3 kcal mol −1 for the pH range of 7 ≤ pH ≤ 9.5, 8,9 suggests that an efficient oxidation pathway under acidic conditions may still be important. Furthermore, the experimental setup of purging oxygen gas into solution may limit the available surface area for interfacial contact.…”

“…The noncatalyzed oxidation of S­(IV) by O 2 was mainly carried out by purging air or O 2 into a salt solution containing HSO 3 – /SO 3 2– or by exposing either air or O 2 to droplets containing of S­(IV) ions. The S­(VI) formation rate was determined to be proportional to [SO 3 2– ]­[H + ] 0.5 . − Radojevic concluded that the noncatalyzed oxidation by O 2 was not significant for ambient SO 2 conversion as compared to other aqueous-phase oxidation pathways . However, the lower reported activation energy for HSO 3 – + O 2 under acidic conditions of 1.7 kcal mol –1 for pH < 7 vs 21.3 kcal mol –1 for the pH range of 7 ≤ pH ≤ 9.5, , suggests that an efficient oxidation pathway under acidic conditions may still be important.…”

Fast-Atom Bombardment Mass Spectrometry in Heterogeneous Atmospheric Chemistry

“…From the beginning of the 20th century, it has been established that sulfite oxidation does not occur at all in the absence of transition metal ions, primarily iron ions, 41 emitted into the atmosphere in a large quantity from natural and anthropogenic sources. Such a unique role of iron is due to hydrolytic instability [42][43][44] of Fe(III) and the closeness of the redox potentials 45,46 TMI, primarily manganese(II) ions, can enhance the iron catalytic effect result in a synergism 47 that produces a greater rate of S(IV) depletion 48,49 than one would expect from a sum of the rates determined separately as attributable to those metals 50 and for residual iron.…”

“…To control the progress of the reaction under field conditions where the drops are exposed to a combination of sunlight and bombardment by atmospheric radicals (OH • , HO 2 • , NO 3 • ), one needs to stop the process by a procedure that keeps the composition of the system unchanged. The addition of easily oxidized organic substances such as pyrogallol or hydroquinone as inhibitors of sulfite oxidation 50 is a traditionally used approach 41,53 in chemical kinetics. The cessation of S(IV) oxidation reached by addition of these organic compounds is due to fast scavenging of oxysulfur radicals, SO 3-5 -• , as chain carriers 54 and redistribution of iron ions; 55 ferric ions are reduced to ferrous ions, which are catalytically inactive, 42 when concentrations of the chain carriers are sustained at low levels by addition of inhibitors.…”