Insights into the influence of Fe2NiO4 NPs on biomethanation of H2 and CO2 to CH4: From the standpoint of electron transfer capacity, microbial communities and metabolism

For hydrogen (H2) storage and carbon dioxide (CO2) emission reduction, biomethanation under thermophilic condition (55 °C) was considered. To enhance the efficiency of biomethanation, current studies have focused on the contribution of iron-containing metal nanoparticles, neglecting the contribution of other key trace elements. In order to fill this gap, this study evaluates the role of Fe2NiO4 NPs in biomethanation, as Ni is a key trace element essential for microbial growth. Results indicate that the addition of Fe2NiO4 NPs improved the utilization and conversion rates of H2 and CO2, boosting methane (CH4) yield by 37.12 %. Its participation induced microbial secretion of abundant extracellular polymeric substances (EPS) rich in fulvic acids (FAs) as electron mediators, which increased the electron transfer rate by 21.60 %. Microbial community analysis reveals that the addition of Fe2NiO4 NPs reduced competition among autotrophic microbes for carbon sources, reinforcing carbon conversion to CH4. Lastly, the addition of Fe2NiO4 NPs increased the level of microbial metabolism and also elevated the abundance/enzymatic activity of enzymes associated with the hydrogenotrophic methane production pathways, thus facilitating the efficiency of biomethanation.