Comparison of mesophilic and thermophilic dark fermentation with nickel ferrite nanoparticles supplementation for biohydrogen production

• BioH 2 production was enhanced by supplementing with NiFe 2 O 4 NPs. • NiFe 2 O 4 NPs promoted mesophilic fermentation more than thermophilic process. • Bacterial activity was improved by Fe and Ni ions from corrosion of NiFe 2 O 4 NPs. • Clostridium butyricum were selectively enriched in the presence of NiFe 2 O 4 NPs. • Butyrate-type fermentation dominated bioH 2 production added with NiFe 2 O 4 NPs. In this work, nickel ferrite nanoparticles (NiFe 2 O 4 NPs) was prepared to improve hydrogen (H 2 ) production by dark fermentation. Moderate amounts (50–200 mg/L) promoted H 2 generation, while excess NiFe 2 O 4 NPs (over 400 mg/L) lowered H 2 productivity. The highest H 2 yields of 222 and 130 mL/g glucose were obtained in the 100 mg/L (37 °C) and 200 mg/L NiFe 2 O 4 NPs (55 °C) groups, respectively, and the values were 38.6% and 28.3% higher than those in the control groups (37 °C and 55 °C). Soluble metabolites showed that NiFe 2 O 4 NPs enhanced the butyrate pathway, corresponding to the increased abundance of Clostridium butyricum in mesophilic fermentation. The endocytosis of NiFe 2 O 4 NPs indicated that the released iron and nickel favored ferredoxin and hydrogenase synthesis and activity and that NiFe 2 O 4 NPs could act as carriers in intracellular electron transfer. The NPs also optimized microbial community structure and increased the levels of extracellular polymeric substances, leading to increased H 2 production.