Mn(II) inhibits rather than promotes As(III) oxidation during co-oxidation of As(III) and Fe(II) by oxygen

As(III) and Fe(II) co-oxidation by oxygen is an important process in arsenic migration, transformation, and pollution remediation in various aqueous environments. Fe(II) frequently co-exists with Mn(II) in natural settings, and their synergistic oxidation process is typically regarded as the supporting factor for As(III) oxidation. Herein, we found that Mn(II) significantly inhibited As(III) oxidation during As(III) and Fe(II) co-oxidation under near-neutral pH (6.0-8.0) conditions. Fe(II) was mainly oxidized by oxygen to lepidocrocite and ferrihydrite, leading to the production of active intermediates (H₂O₂, O₂^•-, and ^•OH). As(III) oxidation mechanism primarily involved the interaction of H₂O₂ with Fe(II)-As(III) complexes. ^•OH contributed much to Mn(II) oxidation, which promoted Fe(II) oxidation. The enhanced Fe(II) oxidation facilitated ferrihydrite formation, which provided microporous adsorption sites for As(III) rather than As(V). This process impeded the oxidation of H₂O₂ and Mn(III/IV) species for As(III). As(III) oxidation efficiency initially decreased and then reached equilibrium with increasing Mn/Fe from 0 to 2.00. As pH rising from 6.0 to 8.0, the simultaneous increases in H₂AsO₃^- proportion and Fe(II) oxidation rate initially improved and then declined As(III) oxidation efficiency. An elevated oxygen level can promote Fe(II) oxidation to ferrihydrite, thereby reducing As(III) oxidation efficiency. Mn(II) also inhibits As(III) oxidation during As(III) and Fe(II) co-oxidation in the presence of heavy metal ions such as Ni(II), Cu(II), and Zn(II). This research expands our understanding of the behavior of arsenic in aqueous environments, and also offers theoretical insights for addressing arsenic pollution in these settings.