Investigating the role of MnFe₂O₄ Nanoparticles and Bacteria on Cr(VI) remediation

Effect of Structural Features of MnFe₂O₄ Nanoparticles on Electron Transfer Performance from Bacteria to the Electrode of Microbial Fuel Cells

Recycling and managing energy resources with environmentally benign technologies is of utmost societal interest. Using traditional bio-based techniques for sewage 6 treatment, 2000 KJ m-3 of energy is consumed. Comparatively, the possible maximum amount of energy produced by the complete combustion of organic materials in wastewater in the UK is 9500 kJ m-3. Hence, the internal power content in wastewater is approximately five times more than the energy required to treat wastewater. Therefore, understanding and optimising the current know-how in the operation of this modern cost-effective treatment approach using microbial fuel cells will help to bring this technology closer to scale-up and commercialisation.

The aim of this research is to improve the efficiency of electricity production by microbial fuel cells, a process dependent on the electron transfer performance from bacteria to the anode electrode. In the absence of natural electron shuttles (redox mediators), bacteria biosorb external redox mediators which are reduced inside the cells by electrons, they then move out of the cells towards the anode surface to be re-oxidized. The size and shape of the nanostructures which are used as external mediators have a significant impact on their redox behaviour and biosorption.

Microbial reduction of Cr6+ has been regarded as a suitable Cr remediation approach because of being more eco-friendly than the conventional physico-chemical strategies, which are often costly. Recently, many types of bacteria have been reported to detoxify Cr6+ to less toxic Cr3+, including dissimilatory metal-reducing bacteria such as Shewanella oneidensis MR-1. Enhancing the bacterial tolerance to Cr6+ is an effective way to improve the reduction of Cr6+. Zero-valent iron nanoparticles (ZVI NPs) can easily be oxidised to ferric oxides and hydroxides in water. The active surface of ZVI NPs can be decreased due to the attached layers of iron oxides and hydroxides. Shewanella, as iron-reducing bacteria, can reduce the adsorbed Fe3+ to Fe2+, which reverses the oxidation of ZVI NPs. Hematite (α-Fe2O3) particles enhance the bio-reduction of Cr6+ by S. oneidensis MR-1, but they cause cytotoxicity to such kind of bacteria. Among other candidate materials that could possibly act as mediators, manganese ferrite nanoparticles (MnFe2O4 NPs) were chosen in this study. MnFe2O4 is a binary metal oxide with a modified electronic structure resulting from the introduction of foreign metal elements within a single metal oxide lattice. Such a structure enables better electric conductivity and chemical stability. Since MnFe2O4 NPs have electrochemical properties, they can link S. oneidensis MR-1 with Cr6+ as an electron mediator from the cell to Cr6+, a terminal electron acceptor. The theoretical capacity of manganese ferrite is 917 mA h g−1, being higher than transition metal oxides (≥ 500 mA h g−1). Furthermore, both ions of manganese (Mn4+ and Mn2+) and iron (Fe2+, and Fe3+) have been reported to allow electron transfer from bacteria. Additionally, MnFe2O4 NPs display anti-microbial activity at high concentration.

The project aims to find the optimal size and shape features for the best performance nanomediators in order to achieve the maximum efficiency of electron transfer. Additionally, the project seeks to improve energy recovery from wastewater using microbial fuel cell technology. In order to achieve this, the researchers investigated the effect of the structural characteristics of manganese-ferrite nanoparticles on the electron transfer from bacteria to the anode of microbial fuel cells.