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For low yield in dark fermentation (DF), in this study, the carbonized humic acid (CHA) was produced and added to DF for enhancing biohydrogen (bioH 2 ) yield at mesophilic condition. The highest bioH 2 yield was 151.08 mL/g glucose with the addition of CHA at 80 mg/L, which was 35.27% and 16.53% higher than those of 0 mg/L CHA and 80 mg/L mineral humic acid (MHA) groups, respectively. Electrons preferentially conducted via the butyrate pathway due to CHA amendments, which corresponded to the prediction of relevant functional genes. Furthermore, CHA possessed distinctive advantages over MHA, which acted as an electron shuttle to facilitate electron transfer, released metal ions as an essential signal mediator and favored the reduction of ferredoxin, obtaining more H 2 . The use of CHA in the field of H 2 -DF depicted the high-value utilization and industrial chain extension of MHA., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Low biological hydrogen (bioH 2 ) production due to non-optimal metabolic pathways occurs frequently. In this work, magnetic nitrogen-doped activated carbon (MNAC) was prepared and added into the inoculated sludge with glucose as substrate to enhance hydrogen (H 2 ) yield by mesophilic dark fermentation (DF). The highest H 2 yield appeared in 400 mg/L AC (252.8 mL/g glucose) and 600 mg/L MNAC group (304.8 mL/g glucose), which were 26.02% and 51.94% higher than that of 0 mg/L MNAC group (200.6 mL/g glucose). The addition of MNAC allowed for efficient enrichment of Firmicutes and Clostridium-sensu-stricto-1, accelerating the metabolic pathway shifted towards butyrate type. The Fe ions released by MNAC facilitated electron transfer and favored the reduction of ferredoxin (Fd), thereby obtaining more bioH 2 . Finally, the generation of [Fe-Fe] hydrogenase and cellular components of H 2 -producing microbes (HPM) during homeostasis was discussed to understand on the use of MNAC in DF system., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Dark fermentation (DF) is a green hydrogen (H 2 ) production process, but it is far below the theoretical H 2 yield. In this study, calcium ferrite nanoparticles (CaFe 2 O 4 NPs) were produced to augment H 2 yield via DF. The highest H 2 yield of 250.1 ± 6.5 mL/g glucose was achieved at 100 mg/L CaFe 2 O 4 NPs. Furtherincreasein CaFe 2 O 4 NPs above 100 mg/L, such as 600 mg/L, would slightly lower H 2 yield to 208.6 ± 2.6 mL/g glucose. The CaFe 2 O 4 NPs in DF system released calcium and iron ions, promoting granular sludge formation andDF microbial activity. Soluble metabolites revealed that butyric acid was raised by CaFe 2 O 4 NPs, which indicated the improved metabolic pathway for more H 2 . Microbial structure composition further illustrated that CaFe 2 O 4 NPs could increase the abundance of dominant microbial populations, with the supremacy of Firmicutes up to 71.22 % in the bioH 2 evolution group augmented with 100 mg/L CaFe 2 O 4 NPs., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Low hydrogen (H 2 ) yield via dark fermentation often occurs, being mainly due to H 2 generation pathway shift. In this study, lanthanum-iron oxide nanoparticles (LaFeO 3 NPs) were prepared to investigate their effects on bioH 2 production. The highest H 2 yield of 289.8 mL/g glucose was found at 100 mg/L of LaFeO 3 , being 47.6% higher than that from the control (196.3 mL/g glucose). The relative abundance of Firmicutes increased from 54.2% to 67.5%. The large specific surface area of LaFeO 3 provided sufficient sites for the colonization of Firmicutes and increased the bacterial access to nutrients. Additionally, the La 3+ gradually released from LaFeO 3 NPs raised microbial transmembrane transport capacity, promoting glycolytic efficiency and Fe availability, thereby increasing hydrogenase content, and shifting the bioH 2 evolution to butyrate pathway for more H 2 . This provides the novelty for biochemical utilization of La and new insights into the improved H 2 yield amended with LaFeO 3 ., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Effects of biochar (BC) and ferrous iron (Fe 2+ ) additions on hydrogen (H 2 ) production from glucose were investigated using batch experiment. The glucose with both BC and Fe 2+ additions were incubated at 37°C for H 2 production. As compared with the control group (without BC and Fe 2+ additions), the synergic effects of BC and Fe 2+ make the lag phase time decease from 4.25 to 2.12h, and H 2 yield increase from 158.0 to 234.4ml/g glucose. Moreover, suitable concentrations of BC and Fe 2+ serve to enhance volatile fatty acid generation during H 2 evolution. These results indicate that H 2 production is improved by BC and Fe 2+ regulations, where synergic mechanisms are described as follows: BC acts as support carriers of anaerobes and system pH buffers, which promotes the biofilm formation and maintains suitable pH environment; Appropriate Fe 2+ concentration can improve hydrogenase activity in H 2 production., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

This paper investigated the utilization of calcined-red mud (CRM) pretreatment to enhance fermentative hydrogen yields from brewers' spent grain (BSG). The BSG samples were treated with different concentrations (0.0-20g/L) of CRM at 55°C for 48h, before the biohydrogen process with heat-treated anaerobic sludge inoculum. The highest specific hydrogen production of 198.62ml/g-VS was obtained from the BSG treated with 10g/L CRM, with the corresponding lag time of 10.60h. Hydrogen yield increments increased by 67.74%, compared to the control tests without CRM. The results demonstrated that the CRM could hydrolyze more cellulose and further provided adequate broth and suitable pH value for efficient fermentative hydrogen. The model-based analysis showed that the modified Gompertz model presented a better fit for the experimental data than the first-order model., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

This study investigated the use of calcined-lime mud from papermaking process (CLMP) pretreatment to improve fermentative hydrogen yields from corn-bran residue (CBR). CBR samples were pretreated with different concentrations (0-15 g/L) of CLMP at 55°C for 48 h, prior to the thermophilic fermentation with heat-treated anaerobic sludge inoculum. The maximum hydrogen yield (MHY) of 338.91 ml/g-VS was produced from the CBR pretreated with 10 g/L CLMP, with the corresponding lag-phase time of 8.24h. Hydrogen yield increments increased from 27.76% to 48.07%, compared to the control. The CLMP hydrolyzed more cellulose, which provided adequate substrates for hydrogen production., (Copyright © 2015 Elsevier Ltd. All rights reserved.)