Kinetics of the reactions of the SO –4 radical with SO –4, S2O 2–8, H2O and Fe2+

Abstract

The sulphate radical anion, SO^–₄, is known to be a key intermediate in the aqueous aerobic oxidation of SO₂ initiated by free radicals. The absorption spectrum of SO^–₄ has been obtained following photolysis of K₂S₂O₈ solutions at 248 nm. Kinetic spectrophotometric techniques have been employed to investigate the decay of SO^–₄ in the presence of S₂O^2–₈ and Fe^II. Several rate constants have been determined at 293 K: 2k(SO^–₄+ SO^–₄)=(8.9 ± 0.3)× 10⁸ dm³ mol^–1 s^–1(at infinite dilution), k(SO^–₄+ S₂O^2–₈)=(6.1 ± 0.6)× 10⁸ dm³ mol^–1 s^–1(at infinite dilution) and k(SO^–₄+ H₂O)=(500 ± 60) s^–1. The reaction of SO^–₄ with Fe^II proceeds via a rapid equilibration step followed by electron transfer, which is rate-determining SO^–₄+ Fe²⁺ [graphic omitted] [Fe^II(SO₄)]⁺ [graphic omitted] [Fe^III(SO₄)]⁺ [graphic omitted] Fe³⁺+ SO^2–₄. Values of k_1,k_–1 and k₂ at 293 K are (3.0 ± 1.9)× 10⁸ dm³ mol^–1 s^–1, (4.8 ± 3.0)× 10⁴ s^–1 and (6.5 ± 2.1)× 10³ s^–1 respectively. The implications of these results for current cloud and precipitation chemistry models are discussed.