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In this study, the kinetics of Fe(II) oxidation in the presence of the iron oxyhydroxides ferrihydrite, Si-ferrihydrite, schwertmannite, lepidocrocite and goethite are investigated over the pH range 4–5.5. Despite limited sorption of Fe(II), the rate of Fe(II) oxidation is up to 70-fold faster than in the absence of any Fe oxyhydroxide phase over pH 4.5–5.5. Enhanced Fe(II) oxidation was minor or negligible at pH 4 with undetectable amounts of Fe(II) adsorbed to the iron oxyhydroxides at this pH. Heterogeneous rate constants derived from kinetic modeling were normalized to the concentration of adsorbed Fe(II) and deviated by no more than 13.8% at pH 4.5, 5 and 5.5, indicating that oxidation is proportional to the concentration of adsorbed Fe(II). Average rate constants were found to be: 2.12 ± 0.20, 1.30 ± 0.09, 1.69 ± 0.22, 1.20 ± 0.08 and 0.68 ± 0.09 M −1 s −1 for ferrihydrite, goethite, lepidocrocite, schwertmannite and Si-ferrihydrite, respectively. The role of reactive oxygen species, such as hydrogen peroxide, the hydroxyl radical and superoxide, towards the overall oxidation of Fe(II) was examined but found to have only a minor impact on Fe(II) oxidation when compared to the effect of heterogeneous oxidation. [ABSTRACT FROM AUTHOR]

Abstract: The rate at which iron– and aluminium–natural organic matter (NOM) complexes dissociate plays a critical role in the transport of these elements given the readiness with which they hydrolyse and precipitate. Despite this, there have only been a few reliable studies on the dissociation kinetics of these complexes suggesting half-times of some hours for the dissociation of Fe(III) and Al(III) from a strongly binding component of NOM. First-order dissociation rate constants are re-evaluated here at pH 6.0 and 8.0 and 25 °C using both cation exchange resin and competing ligand methods for Fe(III) and a cation exchange resin method only for Al(III) complexes. Both methods provide similar results at a particular pH with a two-ligand model accounting satisfactorily for the dissociation kinetics results obtained. For Fe(III), half-times on the order of 6–7h were obtained for dissociation of the strong component and 4-5min for dissociation of the weak component. For aluminium, the half-times were on the order of 1.5h and 1-2min for the strong and weak components, respectively. Overall, Fe(III) complexes with NOM are more stable than analogous complexes with Al(III), implying Fe(III) may be transported further from its source upon dilution and dispersion. [Copyright &y& Elsevier]