Role of carbonate speciation on the oxidation of Fe(II) by H2O2

Abstract

A unified model for Fe(II) speciation and oxidation by hydrogen peroxide is described. This model uses ion–ion-specific interactions with classic ion pair formation theory to describe ferrous iron speciation under conditions typical of natural waters. The speciation data were then utilized to evaluate the species-specific rates of Fe(II) oxidation by hydrogen peroxide for a range of media compositions. At a pH below 5.0, the oxidation rate of Fe(II) is well described in terms of the Fe2+. However, for a pH above 5, the Fe(CO3) complex is the most kinetically active species. Combining these new Fe(II)–H2O2 rates with rates for Fe(II) oxidation by molecular oxygen, a correlation between one-electron transfer rates and the ΔG of electron transfer was established. This correlation is consistent with the Marcus theory of electron transfer. The model for Fe(II) oxidation by molecular oxygen and hydrogen peroxide allows the prediction of steady-state hydrogen peroxide concentrations that will result from the oxidation of micromolar levels of Fe(II). The greatest hydrogen peroxide production by Fe(II) oxidation is predicted in freshwater systems with low total alkalinity.