Mechanism of Galvanic Current Generation in Semiconducting Magnetite during Interspecies Electron Transfer

Magnetite (Fe3O4) is an electrically conducting iron oxide mineral (FeOx) that serves as an electron transporting agent during direct interspecies electron transfer (DIET) for microbial species involved in syntropic reduction–oxidation electrochemical reactions. Although other metal oxides and FeOx phases possess good electrical conductivity, if not better, the reason for the exclusive efficient use of magnetite minerals by the microbial community is not understood. This work studied the possible origin of galvanic current transport in magnetite relative to other FeOx phases by studying the correlation between the electronic structure of the oxide and the electrochemical response of the oxide under oxidizing and reducing polarization conditions without the presence of any microbes. The combined results show that the unique ability of magnetite for DIET is due to its very low band gap (0.65 eV), which enables the oxide to easily switch its electrical conductivity between the n-type and p-type with electron injection and ejection. Consequently, our results suggest that DIET occurs through the generation of a biogalvanic current between the n-type anodic (higher electron chemical potential) and p-type cathodic (low electron chemical potential) sites of magnetite as a result of microbial electrochemical oxidation and reduction reactions, respectively. In contrast, other FeOx minerals exhibit a lower DIET as a result of the larger electronic gap and misalignment of band edges with respect to the redox potential of the electrochemical reactions.