Reactivity and Role of SO5•- Radical in Aqueous Medium Chain Oxidation of Sulfite to Sulfate and Atmospheric Sulfuric Acid Generation

Abstract

This study reevaluates the role of peroxymonosulfate anion radical (-O3SOO• or SO5•-) intermediate during radical-induced chain oxidation of HSO3-/SO32- in oxygenated aqueous solution. The SO5•- radical absorption band in the UV is weak: e = 1065 ± 80 M-1 cm-1 at λmax (260−265 nm). The SO5•- radical takes part in two radical−radical and four radical−solute reactions, partially producing the other chain carrier, the SO4•- radical, in either case. In this study, employing the pulse-radiolysis technique but adopting a new approach, these two types of reactions of the SO5•- radical have been separately quantified (at room temperature). For example, over pH 3.5−12, the branching ratio of (SO5•- + SO5•-) reactions giving rise to either the SO4•- radical or S2O82- is found to remain ∼1. The respective reaction rate constants for I → 0 are (2.2 ± 0.3) and (2.1 ± 0.3) × 108 M-1 s-1. The (SO5•- + HSO3-) reactions in acid pH follow two paths, forming the SO4•- radical in one and regenerating the SO3•- radical in th...