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

Abstract

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 _0.8Fe _2.2O 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.

Abstract

An enhancement strategy for the bio-reduction of Cr ⁶⁺ in the presence of manganese ferrite nanostructures using Shewanella oneidensis.