Effect of Chemical Properties of Humic Substances on Oxidation Rate of Ferrous Iron in Natural Waters

Abstract

 The oxidation kinetics of ferrous iron (Fe(II)) in the presence of humic substances, which widely occur in natural water, was investigated by determining the oxidation rate constants of Fe(II) by dissolved oxygen (O₂) and hydrogen peroxide(H₂O₂) (k_O2 and k_H2O2, respectively) in a bicarbonate buffer at pH 8. The Fe(II) oxidation experiments indicated that both k_O2 and k_H2O2 varied widely depending on the type of the co-existing humic substance, while the oxidation rate constant for O₂ was found to be greater in the presence of humic substance. The oxidation rate constant for H₂O₂ significantly correlated with the contents of carboxyl group and aromatic carbon in humic substance. The finding from our study indicates that the oxidation rate of Fe(II) is likely altered by the complexation of Fe(II) with humic substances with the rate dependent on the types and quantities of Fe-binding sites in humic substance in addition to the charge characteristics of coordinating ligand.