Appearance of Sulphatoferric Complexes in the Oxidation of Ferrous Sulphate Solutions: A Study by Pulse Radiolysis

Abstract

The rate of the reaction between perhydroxyl radicals and ferrous ions, like that between hydroxyl radicals and ferrous ions, is little affected when the ferrous ions are associated with sulphate. However, sulphate ions displace HO–2 from the ferric hydroperoxide complex by an outer-sphere ion-pair formation mechanism (Id): SO2–4+ Fe3+(H2O)6,HO–2⇌ Fe3+(H2O)6,HO–2SO2–4, Fe3+(H2O)6,HO–2SO2–4→ Fe3+(H2O)5SO2–4+ HO–2+ H2O. The equilibrium constant for the first of these reactions is 90 ± 20 M–1, and the rate constant for the second is approximately 104 s–1. Sulphate reacts with the ferric-hydroperoxide-ferrous bridge compound similarly, with an equilibrium constant of 100 ± 20 M–1 and a rate constant of the same order of magnitude as for the ferric hydroperoxide complex. Additional reactions take place when the concentration of sulphate ions is high enough.

New determinations have been made of the rate constants and activation parameters for the reaction of hydrated ferric ions with sulphate. A detailed mechanism is given for reactions occurring during irradiation of the Fricke dosimeter in the absence of sodium chloride.