Coupled effects of ferroferric oxide supplement and ethanol co-metabolism on the methanogenic oxidation of propionate

Abstract Direct interspecies electron transfer (DIET) is a new electron-transfer strategy for enhanced propionate degradation. Ethanol can enrich the DIET species of Geobacter and conductive ferroferric oxide (Fe3O4) can promote DIET. Therefore, coupled effects of ethanol and Fe3O4 on propionate degradation were investigated. The maximum CH4 production rate was increased by 81.4% by adding Fe3O4 when simultaneously fed with ethanol and propionate, while the improvement could not be observed without ethanol. The sludge conductivity and the electron transfer system activity by adding Fe3O4 were increased by 2.66 and 2.73 times, respectively. Besides, the relative abundance of functional microbes such as Geobacter, Syntrophobacter, Smithella, and Methanosaeta, and their functional genes were increased by the supplement of Fe3O4. The improvement of propionate degradation by adding Fe3O4 was largely attributed to the co-existence of ethanol degradation. The DIET between Geobacter and Methanosaeta might provide more energies or rapidly consume the oxidation products to promote the propionate degradation.