Methane Production from Propionate Enhanced by Met@Fe3O4 via Increasing Microbe–Material Attachment in a Direct Interspecies Electron-Transfer Process

Abstract

Propionic acid is an important metabolic intermediate of anaerobic digestion, but its biotransformation to methane is difficult. Although Fe₃O₄ was reported to benefit the direct interspecies electron transfer (DIET) in propionate digestion, the poor attachment of Fe₃O₄ to microbes limited further improvement of methane yield. In this study, the use of methionine-modified Fe₃O₄ (Met@Fe₃O₄) aiming at increasing the attachment of Fe₃O₄ to microbes and enhancing the electron transfer to boost methane production from propionate was reported, and its role was explored. It was observed that in the presence of Met@Fe₃O₄, the methane yield and the maximal methane production rate were, respectively, 81.2 and 152.1% greater than those of the control and 37.6 and 82.7% higher than those of Fe₃O₄. The investigation of the mechanism indicated that the lower electrochemical impedance of Met@Fe₃O₄ made it more effective for promoting not only the intracellular electron transfer of acetoclastic methanogenesis but also extracellular DIET mainly rather than Fe₃O₄, resulting in the improvement of enzyme and microbial activities involved in anaerobic propionate degradation. Further studies demonstrated that the protein on the cell surface was more likely to bind to Met@Fe₃O₄ because of the enhanced hydrophilic nature, and the attachment of Met@Fe₃O₄ to microbes was increased, leading to a faster extracellular electron-transfer efficiency. This study provided an alternative method to improve propionate biodegradation for efficient energy recovery.