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.