Enhanced Oxidation of Cr(III)-Fe(III) Hydroxides by Oxygen in Dark and Alkaline Environments: Roles of Fe/Cr Ratio and Siderophore

The air oxidation rate of Cr1-x Fe x (OH)(3) in dark andalkaline environments was greatly affected by pH, the Fe/Cr ratio,and the presence of siderophores. The current understanding of Cr(III)-Fe(III) hydroxide(Cr1-x Fe x (OH)(3)) oxidation in the dark is primarily focusedon strong oxidants,yet the role of oxygen has generally been overlooked. Meanwhile, theeffects of organic ligands on the Cr(III) oxidation are poorly known.Herein, we determined the kinetics of Cr1-x Fe x (OH)(3) oxidation byoxygen in the dark as a function of pH and Fe/Cr ratio with/withoutthe presence of a representative organic ligand-siderophore.Results showed that the Cr(III) oxidation rate increased linearlywith increasing pH and Fe/Cr ratio. Thermodynamic calculations suggestedthat the enhanced Cr1-x Fe x (OH)(3) oxidation with increasing pH wasprimarily due to the decreased & UDelta;G value (i.e.,the Gibbs free energy change) at higher pH. The decreased redox potentials(E (h)) of Cr1-x Fe x (OH)(3) suspensionswith increasing Fe/Cr ratio accounted for the enhanced Cr(III) oxidationof iron-rich Cr1-x Fe x (OH)(3). The siderophore greatly accelerated the Cr1-x Fe x (OH)(3) oxidation atalkaline pH by promoting the formation of soluble organically complexedCr(III), which can be oxidized readily by oxygen via mineral-surfacecatalyzed oxidation. Overall, this study highlights the specific roleof oxygen and its synergistic role with the siderophore in the oxidationof solid Cr1-x Fe x (OH)(3), which should be taken into considerationin assessing the long-term stability of Cr(III)-Fe(III) hydroxides.

Automated summary

The air oxidation rate of Cr1-x Fe x (OH)(3) in dark and alkaline environments is influenced by pH, the Fe/Cr ratio, and the presence of siderophores. The role of oxygen in the dark and the effects of organic ligands on Cr(III) oxidation are often overlooked. This study investigates the kinetics of Cr1-x Fe x (OH)(3) oxidation by oxygen in the dark, considering pH, Fe/Cr ratio, and the presence of siderophores. Results show that pH and Fe/Cr ratio linearly affect the Cr(III) oxidation rate, while the siderophore promotes the formation of soluble organically complexed Cr(III) and enhances its oxidation by oxygen. This study emphasizes the importance of oxygen and its interaction with siderophores in the oxidation of Cr(III)-Fe(III) hydroxides.