Mechanistic insights into metal-mediated anaerobic digestion: From material function to process enhancement

• Metals drive direct interspecies electron transfer (DIET) in AD. • Trace metals act as essential enzyme cofactors for methanogenesis. • Redox-active metals shuttle electrons, stabilizing microbial pathways • Zero-valent metals generate H 2 , fueling hydrogenotrophic methanogens. • Metal oxides and sulfides detoxify inhibitors, enhancing process stability. Anaerobic digestion (AD) is an established waste-to-energy technology, yet its efficiency is often hindered by slow hydrolysis, process instability, and limited biogas yields. Recent advances highlight the potential of metal-based additives, including trace metals, oxides, zero-valent metals, nanoparticles, and composites, as multifunctional enhancers that overcome these limitations. This review synthesizes mechanistic insights into how these materials improve AD performance through diverse pathways such as direct interspecies electron transfer (DIET), enzyme cofactor supplementation, redox cycling, in-situ hydrogen generation, sulfide precipitation, Fenton-like reactions, and adsorption of inhibitory compounds. Special emphasis is placed on linking material properties with microbial and biochemical responses, including shifts in community structure and enzymatic activity that collectively enhance methane production and process stability. Comparative evaluation reveals that zero-valent iron and iron oxides stand out for their multifunctionality and cost-effectiveness, while metal-based nanoparticles and composites offer unique advantages in conductivity, nutrient delivery, and synergistic inhibition control. Environmental risks, recovery strategies, and future directions, such as smart, recyclable, and hybrid materials, are also discussed. By establishing a mechanistic foundation, this review supports the design of sustainable, high-performance metal-assisted AD systems that advance the goals of the circular economy and renewable energy.