Abiotic Process for Fe(II) Oxidation and Green Rust Mineralization Driven by a Heterotrophic Nitrate Reducing Bacteria (Klebsiella mobilis)

Green rusts (GRs) are mixed Fe­(II)–Fe­(III) hydroxides with a high reactivity toward organic and inorganic pollutants. GRs can be produced from ferric reducing or ferrous oxidizing bacterial activities. In this study, we investigated the capability of Klebsiella mobilis to produce iron minerals in the presence of nitrate and ferrous iron. This bacterium is well-known to reduce nitrate using an organic carbon source as electron donor but is unable to enzymatically oxidize Fe­(II) species. During incubation, GR formation occurred as a secondary iron mineral precipitating on cell surfaces, resulting from Fe­(II) oxidation by nitrite produced via bacterial respiration of nitrate. For the first time, we demonstrate GR formation by indirect microbial oxidation of Fe­(II) (i.e., a combination of biotic/abiotic processes). These results therefore suggest that nitrate-reducing bacteria can potentially contribute to the formation of GR in natural environments. In addition, the chemical reduction of nitrite to ammonium by GR is observed, which gradually turns the GR into the end-product goethite. The nitrogen mass-balance clearly demonstrates that the total amount of ammonium produced corresponds to the quantity of bioreduced nitrate. These findings demonstrate how the activity of nitrate-reducing bacteria in ferrous environments may provide a direct link between the biogeochemical cycles of nitrogen and iron.