Evaluation of nano-scaled zero valent iron (nZVI) effects on continuous syngas biomethanation under the thermophilic condition

•nZVI can enhance the continuous thermophilic syngas biomethanation.•Iron migration and toxicity are both evaluated.•DIET established by nZVI plays a more important role than biocorrosion.•Potential dominating syntrophic microorganisms are enriched.•Strengthening interspecific association is feasible to improve AD performance.