Enhanced detoxification of Cr(6+) by Shewanella oneidensis via adsorption on spherical and flower-like manganese ferrite nanostructures

Maximizing the safe removal of hexavalent chromium (Cr(6+)) 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 Cr(6+) and its detoxification. This work describes a synthetic method for achieving the best chemical composition of spherical and flower-like manganese ferrite (Mn(x)Fe(3−x)O(4)) nanostructures (NS) for Cr(6+) adsorption. We selected NS with the highest adsorption performance to study its efficiency in the extracellular reduction of Cr(6+) into a trivalent state (Cr(3+)) by Shewanella oneidensis (S. oneidensis) MR-1. Mn(x)Fe(3−x)O(4) NS 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 Mn(x)Fe(3−x)O(4) was evaluated by inductively coupled plasma atomic emission spectroscopy. Our results reveal that the oxidation state of the manganese precursor significantly affects the Cr(6+) adsorption efficiency of Mn(x)Fe(3−x)O(4) NS. The best adsorption capacity for Cr(6+) is 16.8 ± 1.6 mg Cr(6+)/g by the spherical Mn(0.2)(2+)Fe(2.8)(3+)O(4) nanoparticles at pH 7, which is 1.4 times higher than that of Mn(0.8)Fe(2.2)O(4) nanoflowers. This was attributed to the relative excess of divalent manganese in Mn(0.2)(2+)Fe(2.8)(3+)O(4) based on our XPS analysis. The lethal concentration of Cr(6+) for S. oneidensis MR-1 was 60 mg L(−1) (determined by flow cytometry). The addition of Mn(0.2)(2+)Fe(2.8)(3+)O(4) nanoparticles to S. oneidensis MR-1 enhanced the bio-reduction of Cr(6+) 2.66 times compared to the presence of the bacteria alone. This work provides a cost-effective method for the removal of Cr(6+) with a minimum amount of sludge production.