SOx Radical Monoanions--Reactions in Solution and in the Gas Phase
Robert E. Huie, L W. Sieck
Sulfur dioxide, SO2, is a major air pollutant and a natural trace atmospheric constituent. The major anthropogenic source of SO2 is the combustion of fossil fuels, although much also arises from scrubbing natural gas and from smelting sulfide ores (1). The major natural sources are the oxidation of naturally emitted reduced sulfur compounds and volcanic emissions, with the latter being particularly important as an episodic source. SO2 is a strong nucleophile and can react with many molecules by substitution at electrophilic positions (2). Although it can react with an oxygen-atom donor to generate SO3, these reactions are generally very slow. An important set of reactions involving SO2 are reactions leading to the radical monoanions .SO2-,SO3.-,SO4.-, and So5.- and subsequent reactions of these species. These radical anions play particularly important roles in various techniques for controlling SO2 emissions. In aqueous flue-gas scrubbers, the autoxidation of SO2 is a chain reaction involving these radical anions; electron beam-based scrubbers may involve the generation and reaction of these radicals in both the gas phase and the liquid phase and they are also involved in the free radical oxidation of SO2 in atmospheric water droplets. In this review, we address some of the basic chemistry involving these radical anions in both the gas and liquid phases. We will not attempt to list all of the measured rate constants, but will discuss representative examples. Rate constants for the liquid phase reactions of these radicals have been complied (3) and are available in a computer-searchable database (4).Studies of gas phase ion chemistry and of liquid phase free radical chemistry are carried out by independent scientific communities, each with its own language and preferred units. In this review, we have adhered to the traditions of the respective communities and have used particle density units (cm-3) for the gass phase and molar units (mol L-1) for the solution phase. The conversion between the two is: k(cm3s-1)x(6.023X10^20^Lmol-1s-1).
S-Centered Radicals (The Chemistry of Free Radicals)