The current understanding of Cr(III)-Fe(III) hydroxide (Cr_1-xFe_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-xFe_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-xFe_x(OH)₃ oxidation with increasing pH was primarily due to the decreased ΔG value (i.e., the Gibbs free energy change) at higher pH. The decreased redox potentials (E_h) of Cr_1-xFe_x(OH)₃ suspensions with increasing Fe/Cr ratio accounted for the enhanced Cr(III) oxidation of iron-rich Cr_1-xFe_x(OH)₃. The siderophore greatly accelerated the Cr_1-xFe_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-xFe_x(OH)₃, which should be taken into consideration in assessing the long-term stability of Cr(III)-Fe(III) hydroxides.

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