A comparative study on the stability of Fe(III) minerals formed by the nitrate/nitrite-reducing Fe(II) oxidation processes

The nitrate-reducing Fe(II)-oxidizing (NRFO) process plays a crucial role in geochemical Fe N coupled cycling under anoxic conditions. Both nitrate and nitrite can act as electron acceptors for NRFO bacteria, resulting in the formation of Fe(III) minerals. Nevertheless, the stability of Fe(III) minerals generated during the NRFO process remains unclear. In this study, the Fe(III) minerals were obtained using the model NRFO bacteria strain Acidovorax sp. BoFeN1 in the presence of nitrate or nitrite. The stability of the Fe(III) minerals was examined in the presence of the model Fe(III) reduction bacteria Shewanella oneidensis MR-1 (strain MR-1), the chemical reducing reagent sodium dithionite (Na 2 S 2 O 4 ), and hydrochloric acid (HCl), respectively. The main Fe(III) minerals were goethite in the nitrate treatments (named M nitrate ) and tightly attached to the surface of cells. Meanwhile, the main Fe(III) minerals were lepidocrocite in the nitrite treatments (named M nitrite ), showing slight adherence to the cell surface. The reductive dissolution kinetics of M nitrate by strain MR-1 revealed a gradual increase in Fe(II). Little Fe(II) was formed when reduced M nitrite , which was attributed to the residual nitrite acted as a competitive electron acceptor to Fe(III), inhibiting the microbial Fe(III) reduction processes. Furthermore, the Fe(III) minerals were harvested by centrifugation and then reacted with sodium dithionite and hydrochloric acid (HCl), respectively. The incorporation of sodium dithionite and hydrochloric acid facilitated the dissolution of Fe(III) minerals. In the M nitrite treatments, the Fe(III) minerals exhibit enhanced stability due to their inherent higher crystallinity and the protective influence of cell surface and microbial carbon. This study broadens our understanding of the formation and transformation of microbial Fe(III) minerals, thereby improving our knowledge of biogeochemical iron cycling under anoxic conditions.