OXIDATION OF SULFUR DIOXIDE IN AQUEOUS SYSTEMS WITH PARTICULAR REFERENCE TO THE ATMOSPHERE

Abstract

Laboratory studies of the uncatalyzed liquid phase oxidation of SO2 by oxygen are reviewed; significant discrepancies exist between the derived pseudo first order rate coefficients. The production rate is independent of the concentration of oxygen. For pH < 7 the activation energy for this reaction is 7.1 × 103 J mol–1, and for 7 ≲ pH ≲ 9.5 it is 8.9 × 104 J mol–1. The mechanism of oxidation is probably via a free radical chain involving SO3– and SO5–. Sulfates may also be produced in solution by the oxidation of SO2 by ozone. In this case, the rate of production of sulfates varies linearly with the concentration of ozone and bisulfite and the activation energy is 1.6 × 104 J mol–1. Laboratory studies of the catalyzed liquid phase oxidation of SO2 suggest that d[SO2-4]/dt = K[M+]-[H+][SO2-3], where [M+] is the concentration of the metal catalyst (e.g. Fe3+, Mn2+, Cu2+). There is some evidence, which needs confirmation, that catalysis by certain ‘mixed salts’ produces an oxidation rate about ten times greater than with either salt alone. The oxidation of SO2 in solution is inhibited by a large number of compounds present in the atmosphere. Consequently, the net effect of positive and negative catalysts on the oxidation rate of SO2 in cloud and rain water could be small. Extrapolation of laboratory results to the atmosphere suggests that in ‘clean’ (rural) air, the uncatalyzed oxidation of SO2 by O3 and possibly O2 and catalyzed ‘mixed salt’ oxidation should be competitive. For ‘dirty’ (urban) air, uncatalyzed oxidation and iron catalyzed oxidation should be competitive, but, if manganese is present in solutions in concentration above 10–6 M, the manganese catalyzed oxidation may dominate. However, extrapolation of laboratory results to the atmosphere is fraught with danger. Some field observations of the concentrations of cloud condensation nuclei and sulfates in air entering and leaving clouds suggest that the production of sulfates by the oxidation of SO2 in clouds is important. Also, preliminary calculations suggest that the liquid phase oxidations of SO2 in clouds may be the dominant world-wide source of sulfates in the atmosphere. Suggestions are made for further laboratory, field and modeling studies which should improve our understanding of liquid phase SO2 oxidation in the atmosphere.