Enhanced detoxification of Cr6+ by Shewanella oneidensis via adsorption on spherical and flower-like manganese ferrite nanostructures

Abstract

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 (Mn_xFe_3-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⁺) by Shewanella oneidensis (S. oneidensis) MR-1. Mn_xFe_3-xO₄ 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_xFe_3-xO₄ was 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 Mn_xFe_3-xO₄ NS. The best adsorption capacity for Cr⁶⁺ is 16.8 ± 1.6 mg Cr⁶⁺/g by the spherical Mn_0.2²⁺Fe_2.8³⁺O₄ nanoparticles at pH 7, which is 1.4 times higher than that of Mn_0.8Fe_2.2O₄ nanoflowers. This was attributed to the relative excess of divalent manganese in Mn_0.2²⁺Fe_2.8³⁺O₄ based on our XPS analysis. The lethal concentration of Cr⁶⁺ for S. oneidensis MR-1 was 60 mg L^-1 (determined by flow cytometry). The addition of Mn_0.2²⁺Fe_2.8³⁺O₄ nanoparticles to S. oneidensis MR-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.

Fig. 1. TEM images for the spherical (A) undoped Fe₃O₄ NPs, (B) Mn_xFe_3−xO₄ of precursor ratio [Mn(acac)₂]/[Fe(acac)₃] = 0.33, (C) [Mn(acac)₃]/[Fe(acac)₃] = 0.33 prepared at 200 °C as reaction temperatures, (D) undoped Fe₃O₄ NPs and (E) and (F) Mn_xFe_3−xO₄ NPs prepared with the same precursor ratio but at 250 °C. (G) Impact of reaction temperatures on D_TEM of Mn_xFe_3−xO₄ NPs prepared from [Mn(acac)₂]/[Fe(acac)₃] = 0.33. *P < 0.05.

Fig. 2. XRD patterns for fcc lattice of Fe₃O₄ (A) and Mn_xFe_3−xO₄ NPs where [Mn(acac)₂]/[Fe(acac)₃] were 0.14 (B), 0.33 (C), 0.6 (D), 1 (E), 1.66 (F), 3 (G). The horizontal arrow pointed out the shifting in the peak of 311 from the reference Fe₃O₄ (PDF card no. 01-089-0688) towards the lower diffraction angle of MnFe₂O₄ (PDF card no. 00-010-0319) in response to the increase in [Mn(acac)₂]/[Fe(acac)₃]. A secondary phase of MnCO₃ (Reference ICDD PDF card no. 00-044-1472) was found for NPs prepared from (1 ≤ [Mn(acac)_{2 or 3}]/[Fe(acac)₃] ≤ 3) (E)–(G). The synthesis temperature for all NPs was 250 °C. The vertical arrow indicated the gradual increase in [Mn(acac)₂]/[Fe(acac)₃] from (A)–(G) Mn₃O₄ (PDF card no. 01-080-0382).

Fig. 3. TEM images and histograms for D_TEM of NFs prepared from [Mn(acac)₃]/[Fe(acac)₃] = (A) and (B) 1 & 3, respectively at 200 °C, (C) ratio = 7 at 250 °C.

Fig. 4. XRD patterns of Mn_xFe_3−xO₄ NFs prepared from [Mn(acac)₃]/[Fe(acac)₃] ratio equal to (A) 1 and (B) 3 at 200 °C, (C) 7 at 250 °C. A secondary phase matched MnCO₃ (Reference ICDD PDF card no. 00-044-1472) was found for NFs prepared from a precursor ratio equal to 7 at 250 °C. No detected peaks matched Mn₃O₄ (PDF card no. 01-080-0382).

Fig. 5. (A) Elemental analysis of Mn_xFe_3−xO₄ NS that were prepared at 200 °C and 250 °C. *P < 0.05 and **P < 0.01 in comparison to Mn_x³⁺Fe_3−x³⁺O₄ NPs of similar precursor ratios; (B) adsorption capacity of NPs for Cr⁶⁺, *P < 0.05 and **P < 0.01 in comparison to Fe₃O₄ NPs; (C)–(E) adsorption isotherm of Cr⁶⁺ by Fe₃O₄, Mn_0.2²⁺Fe_2.8³⁺O₄ and Mn₃O₄ NPs respectively.

Fig. 6. High-resolution XPS spectra of Mn 2p in Mn_0.2²⁺Fe_2.8³⁺O₄ (A); Mn 2p in Mn_x³⁺Fe_3−x³⁺O₄ NPs (B); Fe 2p in Mn_0.2²⁺Fe_2.8³⁺O₄ (C) and Fe 2p in Mn_x³⁺Fe_3−x³⁺O₄ NPs (D).

Fig. 7. In the presence of the tested agents (A) viability of tested Shewanella under sublethal dose of Cr⁶⁺ (B) removal of Cr⁶⁺ by Shewanella strains. *P < 0.05 and **P < 0.01 in relation to the impact of Mn_0.2²⁺Fe_2.8³⁺O₄ NPs in each data set separately.