As(III) oxidation kinetics of biogenic manganese oxides formed by Acremonium strictum strain KR21-2

- As(III) oxidation kinetics by biogenic manganese oxides varies during their formation.
- Immature biogenic manganese oxides exhibit very slow As(III) oxidation rates.
- The As(III) oxidation rate increases as biogenic manganese oxides mature.
- A Mn(II) oxidizing enzyme prevents release of Mn(II) and surface passivation.
- Self-regeneration of biogenic manganese oxides enhances As remediation efficiency.

We investigated As(III) oxidation kinetics of biogenic manganese oxides (BMOs) harvested at various stages of formation by a manganese-oxidizing fungus, Acremonium strictum strain KR21-2. Immature BMOs harvested at an early stage of BMO formation were rich in lower-valence Mn(II) and Mn(III) and oxidized As(III) very slowly. As the BMOs matured, the Mn(IV) content in the BMOs increased and had higher As(III) oxidation rates. The slower As(III) oxidation rates of the immature BMOs may be due to site-blocking by larger amounts of sorbed Mn(II) and/or due to increased structural Mn(III), which is less reactive than Mn(IV). Freshly mature BMOs demonstrated varying As(III) oxidation kinetics depending on dissolved oxygen. In deaerated solutions, the As(III) oxidation rate was slower and Mn(II) was released as As(III) oxidation progressed, indicating surface passivation with reduction of the manganese oxide phase producing Mn(II) and possibly Mn(III). In contrast, in air-equilibrated solutions, As(III) oxidation followed pseudo-first-order rate kinetics throughout the reaction without apparent passivation or significant release of Mn(II). We hypothesize that the active Mn(II) oxidase present in BMOs reoxidized Mn(II) and Mn(III) as autoclaved BMOs exhibited surface passivation even under air-equilibrated conditions. This self-regeneration of BMO minimizes passivation and is hence an important pathway for continuous As(III) oxidation. The data presented provide new insights into the potential applicability of BMOs for As remediation as well as for the oxidative treatments of chemicals other than As(III).