Modulating interspecies electron and proton transfer by conductive materials toward enhanced anaerobic digestion: A review

Abstract

Transfer of electron and proton plays a pivotal role in regulating methanogenic metabolism during anaerobic digestion (AD) of organic wastes. Conductive materials (CMs), including electro-conductive materials (ECM) and proton-conductive materials (PCM), have been employed to modulate interspecies electron transfer (ET) and proton transfer (PT) respectively to facilitate AD. However, systematic reviews have not overviewed how CMs modulate interspecies ET and PT and their roles in enhancing AD. This paper provides comprehensive insights into the roles of CMs-driven interspecies ET and PT in enhancing the methanogenic conversion, including the contributions of ECM-driven interspecies ET to enhancing electron utilization efficiency and inducing microbial interactions and the contributions of PCM-driven interspecies PT to enhancing proton and electron utilization efficiency and reinforcing energy harvesting. Furthermore, the responses of AD performance to CMs-driven interspecies ET/PT have been summarized. The engineering application potential of ECM- and PCM-assisted AD is discussed. Finally, the uncoupled electron and proton transport and the unclear multiple mechanism by which CMs enhance AD are identified as the main knowledge gaps in CMs-assisted AD, and the future efforts, which can focus on developing novel CMs with mixed electron–proton conductivity and establishing the network of materials properties to their effect on AD, are proposed. This review offers a fundamental understanding of the roles of CMs-driven interspecies ET/PT in enhancing AD and provides critical guidance for boosting CMs-assisted AD.