The radiation chemistry of aqueous ferrous sulfate solutions at natural pH

Abstract

The yield of Fe^III species has been determined in FeSO₄ solutions irradiated at natural pH with ⁶⁰Co γ-rays for a wide range of solute conditions. The stoichiometry of the hydrogen peroxide reaction was investigated and found to be abnormal if H₂O₂ is present in excess. If, however, FeS0₄ is present in excess, as it is in the irradiation of FeSO₄ solutions, deviations from the normal stoichiometry are trivial. In nitrous oxide saturated solutions initial G(Fe^III) values were found to be from 6.90 to 7.72 for FeSO₄ concentrations in the range (0.5-2.0) × 10^-3 M. Non-linearity in the yield of ferric ion concentration against dose plots was attributed to competition between hydrogen atom reduction of ferric species with hydrogen atom oxidation of ferrous species. The reaction between hydrogen atom and the ferric species was estimated to be close to diffusion controlled. In aerated solutions, the Fe^III yield was, within experimental error, a linear function of dose for FeSO₄ concentrations in the range (0.5 - 2.0) × 10^.-3 M with G(Fe^III) values of 12.27 ± 0.06, 12.62 ± 0.07 and 12.77 ± 0.10 for 0.5, 1.0 and 2.0 × M respectively. At lower FeSO₄ concentrations. deviation from linearity was noticeable. The deviation was attributed to competition between Fe^II and Fe^III species for the O₂^- radical anion. k(O₂^-· + Fe^II)/k(O₂^-·+ Fe^III) was determined to be about 4.0. In deaerated solu- tion, the Fe^III yield against absorbed dose for 1 × M FeSO₄ solution was followed to extended doses. A limiting Fe^III yield of about 0.75 times the aerated solution yield was observed in the closed system. The data were quite well described by values calculated from the reaction mechanism, published rate constants and the rate ratio of 0.016 for k(H^· + Fe^II)/k(H^· + Fe^III) determined from the N₂O experiments, and demonstrate the importance of reducing radical reactions with insoluble ferric species.