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

The current understanding of Cr(III)-Fe(III) hydroxide (Cr 1 -x Fe x (OH) 3 ) oxidation in the dark is primarily focused on strong oxidants, yet the role of oxygen has generally been overlooked. Meanwhile, the effects of organic ligands on the Cr(III) oxidation are poorly known. Herein, we determined the kinetics of Cr 1 -x Fe x (OH) 3 oxidation by oxygen in the dark as a function of pH and Fe/Cr ratio with/without the presence of a representative organic ligand-siderophore. Results showed that the Cr(III) oxidation rate increased linearly with increasing pH and Fe/Cr ratio. Thermodynamic calculations suggested that the enhanced Cr 1 -x Fe x (OH) 3 oxidation with increasing pH was primarily due to the decreased Δ G value (i.e., the Gibbs free energy change) at higher pH. The decreased redox potentials ( E h ) of Cr 1 -x Fe x (OH) 3 suspensions with increasing Fe/Cr ratio accounted for the enhanced Cr(III) oxidation of iron-rich Cr 1 -x Fe x (OH) 3 . The siderophore greatly accelerated the Cr 1 -x Fe x (OH) 3 oxidation at alkaline pH by promoting the formation of soluble organically complexed Cr(III), which can be oxidized readily by oxygen via mineral-surface catalyzed oxidation. Overall, this study highlights the specific role of oxygen and its synergistic role with the siderophore in the oxidation of solid Cr 1 -x Fe x (OH) 3 , which should be taken into consideration in assessing the long-term stability of Cr(III)-Fe(III) hydroxides.