Maximizing the safe removal of hexavalent chromium (Cr6+) from waste streams is an increasing demand due to the environmental, economic and health benefits. The integrated adsorption and bio-reduction method can be applied for the elimination of the highly toxic Cr6+and its detoxification. This work describes a synthetic method for achieving the best chemical composition of spherical and flower-like manganese ferrite (MnxFe3−xO4) nanostructures (NS) for Cr6+adsorption. We selected NS with the highest adsorption performance to study its efficiency in the extracellular reduction of Cr6+into a trivalent state (Cr3+) byShewanella oneidensis(S. oneidensis) MR-1. MnxFe3−xO4NS were prepared by a polyol solvothermal synthesis process. They were characterised by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectrometry (XPS), dynamic light scattering (DLS) and Fourier transform-infrared (FTIR) spectroscopy. The elemental composition of MnxFe3−xO4was evaluated by inductively coupled plasma atomic emission spectroscopy. Our results reveal that the oxidation state of the manganese precursor significantly affects the Cr6+adsorption efficiency of MnxFe3−xO4NS. The best adsorption capacity for Cr6+is 16.8 ± 1.6 mg Cr6+/g by the spherical Mn0.22+Fe2.83+O4nanoparticles at pH 7, which is 1.4 times higher than that of Mn0.8Fe2.2O4nanoflowers. This was attributed to the relative excess of divalent manganese in Mn0.22+Fe2.83+O4based on our XPS analysis. The lethal concentration of Cr6+forS. oneidensisMR-1 was 60 mg L−1(determined by flow cytometry). The addition of Mn0.22+Fe2.83+O4nanoparticles toS. oneidensisMR-1 enhanced the bio-reduction of Cr6+2.66 times compared to the presence of the bacteria alone. This work provides a cost-effective method for the removal of Cr6+with a minimum amount of sludge production.