Soil mineral-mediated controls on Mn(III) oxidative reactivity

Dissolved Mn(III) plays a critical role in controlling the fate of environmental contaminants. Using ciprofloxacin (CIP) as a model organic pollutant, this study examines how goethite and gibbsite, common soil minerals, affect the oxidative reactivity of Mn(III) stabilized by ligands of varying complexation strength. Among the four ligands tested, pyrophosphate (PP), a redox-inert but strongly complexing ligand, exhibited contrasting behavior depending on whether minerals were absent or present. In mineral-free systems, Mn(III)-PP complexes were generally unreactive toward CIP oxidation. In contrast, mineral surfaces enhanced CIP oxidation by promoting pH-dependent dissociation of Mn(III)-PP complexes. At pH 4, Mn(III)-PP preferentially adsorbed onto minerals, destabilizing Mn(III) complexes in solution and favoring its conversion to hydrolyzed MnOH²⁺ species. These intermediates underwent disproportionation, generating MnO₂ colloids that subsequently promoted Mn(III) dissociation/disproportionation. Spectroscopy (UV-Vis, vibrational, and X-ray photoelectron) revealed a progressive decline in aqueous Mn(III)-PP complexes, and a corresponding accumulation of Mn(III)/Mn(IV) in solid phases. These results highlight Mn(III)-ligand coordination at mineral surfaces as a key regulator of Mn(III) reactivity. By elucidating the oxidation mechanisms of organic pollutants mediated by dissolved Mn(III) on minerals, this study enhances our understanding of Mn(III) dynamics in aquatic environments.