Perspectives on Potential Applications of Nanometal Derivatives in Gaseous Bioenergy Pathways: Mechanisms, Life Cycle, and Toxicity

Nanosized metal derivatives (NMDs), referring to metals and their oxides, are extensively utilized as additives for anaerobic digestion (AD) and dark fermentation (DF) processes, for enhancing the production of methane (CH4) and hydrogen (H2), respectively. NMDs-derived positive impacts were widely confirmed in many previous studies; however, no consensus exists about how these have been acquired. Undoubtedly, NMDs affect extracellular electron transfer (EET). Consequently, we explore how biotic–biotic interactions, referring to direct interspecies electron transfer (DIET) among AD partners, and biotic–abiotic exchanges, which are mediated by redox reactions with metals, are affected. In this perspective, the mechanisms behind all those effects are reviewed and explained in detail, considering the specific properties of each NMD, e.g., size and type. We discuss previous studies that offer contradicting interpretations about which process dominates metal oxidation, metal reduction, or DIET. In addition, the fate of NMDs residues in the digestate after the treatment process is discussed, focusing on NMDs toxicity. From previous literature, the environmental impacts are evaluated for the production process of NMDs that are utilized in AD and DF processes via life-cycle assessment. This review provides a comprehensive understanding of NMDs–microbes interactions, which are mandatory for (i) building clear scientific knowledge about processes in play and (ii) engineering favorable conditions to achieve optimum yields in AD and DF processes.