Review on Mechanisms of Iron Accelerants and Their Effects on Anaerobic Digestion

Anaerobic digestion is an important technology for energy recovery from organic waste. However, methanogenesis is restricted by some barriers, such as the low-speed bottleneck of interspecies electron transfer (IET), the low hydrogen partial pressure limitation, trace element deficiency, etc., resulting in poor system stability and low methane production. Recently, multiple iron accelerants have been employed to overcome the above challenges and have been proven effective in enhancing methanogenesis. This study reviews the effects of iron accelerants (Fe 0 , Fe 3 O 4 and magnetite, Fe 2 O 3 and hematite, iron salts and other iron accelerants) on anaerobic digestion in terms of methane production, process stability and the microbial community and elaborates the mechanisms of iron accelerants in mediating the direct interspecies electron transfer (DIET) of the syntrophic methanogenic community, strong reducibility promoting methanogenesis, provision of nutrient elements for microorganisms, etc. The potential engineering application of iron accelerants in anaerobic digestion and the current research advances regarding the environmental impacts and the recovery of iron accelerants are also summarized. Although iron accelerants exhibit positive effects on anaerobic digestion, most of the current research focuses on laboratory and small-scale investigations, and its large-scale engineering application should be further verified. Future research should focus on elucidating the mechanisms of iron accelerants for enhancing anaerobic digestion, developing diverse application methods for different types of anaerobic systems, optimizing large-scale engineering applications, and exploring the environmental impacts and high-efficiency recovery strategies of iron accelerants.