Ligand-limited oxidation of ciprofloxacin by Mn(III)

Mn(III) species play critical roles in determining the environmental fate of antibiotics released into natural systems. However, their reactivity is strongly influenced by complexation reactions with (in)organic ligands. This study investigates the impact of Mn(III) complexation with pyrophosphate (PP), a model environmental ligand, on the redox-driven degradation of ciprofloxacin (CIP), a widely used antibiotic and environmental contaminant. Spectroscopic analysis and thermodynamic modeling revealed that Mn(III)-PP complexes initially dissociate into MnOH²⁺ species, which can then disproportionate and form MnO₂ colloids. Both dissociation and disproportionation reactions had comparable trends at pH 4 and 7, with reactivities that were strongly dependent on Mn(III):PP ratios. The progress of CIP oxidation following direct coordination with Mn compounds over time was sigmoidal, with an initial lag phase attributed to Mn(III)-PP complex dissociation and disproportionation. CIP degradation was predominantly governed by pH, with maximal rate constants decreasing from k = 0.390 h^-1 at pH 3 to k = 0.065 h^-1 at pH 5, and no CIP removal under circumneutral to alkaline conditions. Cyclic voltammetry also confirmed that the strongly pH-dependent redox potential of the Mn(III)/Mn(II) couple aligned with facile CIP oxidation under acidic conditions. These collective findings indicated that ligand complexation, such as with PP, enhanced Mn(III) stability and mitigated dissociation and disproportionation reactions. The new insight provided by this work on the speciation and redox activity of Mn(III) should thereby be considered for understanding ciprofloxacin degradation in contaminated water systems.