Protection of Shewanella oneidensis MR-1 by manganese ferrite nanoparticles during chromate bio-reduction†

Shewanella oneidensis (S. oneidensis) MR-1 is a metal-reducing bacterium that can bio-reduce the carcinogenic hexavalent chromium (Cr⁶⁺) to a less toxic trivalent form (Cr³⁺). The bacteriocidal effect of Cr⁶⁺ challenges the above bio-reduction process. This work aims to illustrate the protective role of manganese ferrite nanoparticles (Mn_0.2Fe_2.8O₄ NPs) to S. oneidensis MR-1 bacteria during the bio-reduction of Cr⁶⁺. Nanostructures were characterised by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The interaction between S. oneidensis MR-1, Cr⁶⁺ and Mn_0.2Fe_2.8O₄ NPs was monitored by X-ray photoelectron spectroscopy (XPS), which helped to unravel the oxidation states of Cr. The XPS analysis provided key insights into the oxidation states of Mn and Fe, confirming the redox interactions facilitating Cr⁶⁺ reduction. Mn_0.2Fe_2.8O₄ NPs boosted the detoxification of the removed Cr⁶⁺ by 2.1 and 1.4 times compared to using S. oneidensis MR-1 alone and NPs alone, respectively. Scanning electron microscopy (SEM) imaging evaluated the changes in the morphology of bacterial cells. After exposure to Cr⁶⁺, S. oneidensis MR-1 cells revealed their inability to produce nanofibers, which are electrically conductive bacterial appendages. Yet, Mn_0.2Fe_2.8O₄ NPs provoked the formation of bacterial nanofibers. These findings highlight the potential of Mn_0.2Fe_2.8O₄ NPs for enhancing the bioremediation of Cr⁶⁺ contaminated environments.