Impact of zero-valent iron on carbon dioxide biomethanation under mesophilic and thermophilic conditions

Although zero-valent iron (ZVI) can generate hydrogen (H₂) for carbon dioxide (CO₂) biomethanation, its practical effectiveness is determined by mineral passivation and microbial adaptation. This study compared mesophilic (37°C) and thermophilic (55°C) conditions to clarify the coupling between abiotic H₂ generation and biological CH₄ production. Abiotic tests demonstrated that ZVI reacted with CO₂ to form siderite (FeCO₃) and H₂, while progressive FeCO₃ formation limited the sustained H₂ release due to surface passivation. Under biotic conditions, temperature controlled the fate of FeCO₃ and microbial succession. Thermophilic conditions promoted sludge-associated FeCO₃ aggregation rather than direct surface coverage, allowing continuous ZVI corrosion and a stable H₂ supply. Consequently, hydrogenotrophic methanogens rapidly dominated, increasing methane (CH₄) production, without an initial lag phase. In contrast, mesophilic conditions favored FeCO₃ adhesion on ZVI surfaces, suppressing H₂ evolution and delaying methanogenic pathway transition. A carbon balance analysis indicated that most CO₂ was chemically converted through ZVI corrosion, while CH₄ formation depended on the microbial utilization of the generated H₂. These results demonstrate that thermophilic biomethanation improves the synchronization between abiotic H₂ generation and microbial consumption, providing a more effective strategy for ZVI-mediated CO₂-to-CH₄ conversion.