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 S2O2–8 and FeII. 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+ S2O2–8)=(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 FeII proceeds via a rapid equilibration step followed by electron transfer, which is rate-determining SO–4+ Fe2+ [graphic omitted] [FeII(SO4)]+ [graphic omitted] [FeIII(SO4)]+ [graphic omitted] Fe3++ SO2–4. Values of k1,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.