Impact of Fe(II) oxidation in the presence of iron-reducing bacteria on subsequent Fe(III) bio-reduction

Highlights: • Fe(II) oxidation by O₂ inactivated up to 1.71 log of Fe(III) reducing bacteria. • Fe(II) oxidation enhanced the subsequent Fe(III) bio-reduction. • The enhancement was mainly due to the increase in Fe(III) bio-availability. • Impact of Fe(II) oxidation on Fe(III) bio-reduction depends on the net effect. • The impact should be considered for Fe cycling in redox-dynamic systems. The interplay of Fe(II) oxidation and Fe(III) bio-reduction occurs widely in both natural and engineered redox-dynamic systems. This study aimed to unravel the impact of Fe(II) oxidation by O₂ in the presence of iron-reducing bacteria on subsequent Fe(III) bio-reduction. Mixed solutions of Fe²⁺ (0.1–0.5 mM) and Shewanella oneidensis strain MR-1 (MR-1, 2.0 × 10⁷ CFU/mL) at neutral pH were first exposed to laboratory air for Fe(II) oxidation and bacterial inactivation, and then the resultant Fe(III) suspensions were switched to anoxic conditions for bio-reduction by the surviving bacteria. In the oxidation step, the coexisting MR-1 was inactivated by 0.8–1.71 orders of magnitude within 60 min. In the subsequent bio-reduction step, the resultant Fe(III) was bio-reduced by the surviving MR-1. Bio-reduction of the resultant Fe(III) by the surviving MR-1 was 1.8–2.5 times faster than that of the Fe(III) that was produced from Fe²⁺ oxidation without MR-1 by fresh MR-1 cells at 2.0 × 10⁷ CFU/mL. Although MR-1 inactivation during Fe(II) oxidation may inhibit Fe(III) bio-reduction, the increase in bio-availability of the resultant Fe(III) and the residual reactivity of dead cells led to net enhancement of bio-reduction under the tested conditions. Lepidocrocite was the sole Fe(III) mineral that was produced from Fe²⁺ oxidation without MR-1, while 19% ferrihydrite was produced from Fe²⁺ oxidation in the presence of MR-1. The formation of low-crystallinity ferrihydrite accounts for the increase in bio-availability of the Fe(III) minerals. The findings of this study highlight an important but overlooked impact underlying the interplay of Fe(II) oxidation and Fe(III) bio-reduction.