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[Display omitted] • CHA was produced to enhance HBu/HAc-DF pathway for more H 2. • H 2 evolution via dark fermentation (DF) raised by 35.27% with CHA. • CHA acted as an electron shuttle to facilitate electron transfer. • Ca2+ and Fe2+ ions were released from CHA to stimulate microbial activity. • CHA could promote the expression of functional genes from DF. 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrothermal carbon was used as an additive to enhance dark fermentation. • 600 mg/L Fe-HCM addition enhanced biohydrogen yield by 59%. • HCM and Fe-HCM enhanced the butyric acid-based fermentation pathway. • HCM and Fe-HCM raised the abundance level of Firmicutes from 35% to 41% and 56%. Dark fermentation (DF) for hydrogen (H 2) evolution is often limited to industrial application due to its low H 2 yield. In this work, hydrothermal carbon microspheres (HCM) and iron modified HCM (Fe-HCM) were prepared by hydrothermal process using waste corn cob. Subsequently, HCM and Fe-HCM were used in DF for more H 2. The highest H 2 yields amended with HCM and Fe-HCM at 600 mg/L were achieved to be 119 and 154 mL/g glucose (0.87 and 1.2 mol H 2 /mol glucose), respectively, being 24% and 59% higher than that of control yield. Soluble metabolites revealed HCM and Fe-HCM promoted butyric acid-based DF. Microbial composition depicted that HCM and Fe-HCM improved the abundance level of Firmicutes from 35% to 41% and 56%, while the abundance level of Clostridium_sensu_stricto_1 rose from 25% to 38% and 51%, respectively. This provides valuable guidance for hydrothermal carbon used in biofuel production. [ABSTRACT FROM AUTHOR]

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 2.12 h, and H 2 yield increase from 158.0 to 234.4 ml/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. [ABSTRACT FROM AUTHOR]

[Display omitted] • CoAl 0.2 Fe 1.8 O 4 and CoCu 0.2 Fe 1.8 O 4 NPs were synthesized to raise bioH 2 yield. • Humic acids were reduced by addition of CoAl 0.2 Fe 1.8 O 4 or CoCu 0.2 Fe 1.8 O 4 NPs. • CoAl 0.2 Fe 1.8 O 4 acted on cell membrane while CoCu 0.2 Fe 1.8 O 4 acted on EPS. • CoCu 0.2 Fe 1.8 O 4 obtained Clostridium_sensu_stricto_1 more than CoAl 0.2 Fe 1.8 O 4. • CoCu 0.2 Fe 1.8 O 4 NPs improved H 2 yield more than CoAl 0.2 Fe 1.8 O 4 NPs. Two various materials, copper and aluminum doped cobalt ferrite nanoparticles (NPs) were fabricated for investigating their effects of addition amounts on hydrogen (H 2) synthesis and process stability. CoCu 0.2 Fe 1.8 O 4 NPs enhanced H 2 production more than CoAl 0.2 Fe 1.8 O 4 NPs under same condition. The highest H 2 yield of 212.25 ml/g glucose was found at optimal dosage of 300 mg/L CoCu 0.2 Fe 1.8 O 4 NPs, revealing the increases of 43.17% and 6.67% compared with the control without NPs and 300 mg/L CoAl 0.2 Fe 1.8 O 4 NPs groups, respectively. NPs level of more than 400 mg/L inhibited H 2 generation. Further investigations illustrated that CoCu 0.2 Fe 1.8 O 4 NPs were mainly distributed on extracellular polymer substance while CoAl 0.2 Fe 1.8 O 4 NPs were mostly enriched on cell membrane, which facilitated electron transfer behavior. Community structure composition demonstrated that CoCu 0.2 Fe 1.8 O 4 and CoAl 0.2 Fe 1.8 O 4 separately caused a 9.67% and 9.03% increase in Clostridium sensu stricto 1 compared with the control reactor without NPs exposure. [ABSTRACT FROM AUTHOR]

In this work, the cobalt ferrate nanoparticles were prepared by a sol-gel method for investigating their influence on hydrogen production from dark fermentation. A moderate amounts (0.1–0.4 g/L) of these nanoparticles effectively enhanced the fermentation process. The highest hydrogen yield of 205.24 mL/g glucose was achieved at a dosing level of 0.4 g/L, which was a 31.8% enhancement compared to the control, while excess (0.5 g/L) nanoparticles decreased the hydrogen production by 9.7% compared to the control group. The mechanism analysis exhibited that the nanoparticles could either attach to the microbial surface or enter the cell interior without destroying the cell integrity, which promoted the electron transfer to anaerobes and stimulated the activities of hydrogen-producing enzymes such as enzymes and coenzymes. The microbial community revealed that a moderate amount of cobalt ferrate nanoparticles increased the abundance of Clostridium sensu stricto 1 from 14.49% to 18.84%, which was dominant and favored the sustainable conversion of glucose wastewater into clean hydrogen. [Display omitted] • Cobalt ferrate nanoparticles (CoFe 2 O 4 NPs) were synthesized to increase H 2 yield. • BioH 2 production was enhanced by 31.8% with 0.4 g/L CoFe 2 O 4 NPs. • CoFe 2 O 4 NPs changed the type of fermentation to butyric acid fermentation. • CoFe 2 O 4 NPs promoted microbial activity by acting on the microbial interface. • CoFe 2 O 4 NPs improved the abundance of Clostridium_sensu_stricto_1. [ABSTRACT FROM AUTHOR]

[Display omitted] • CaFe 2 O 4 NPs were produced to enhance butyrate-DF pathway for more H 2. • H 2 evolution via dark fermentation (DF) raised by 32.30% with CaFe 2 O 4 NPs. • Fe and Ca ions were released from CaFe 2 O 4 NPs to stimulate microbial activity. • Firmicutes in DF was selectively enriched in the presence of CaFe 2 O 4 NPs. • CaFe 2 O 4 NPs could promoted the expression of functional genes from DF. 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. Further increase in 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 and DF 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. [ABSTRACT FROM AUTHOR]

The accelerated chemical-industry has caused a rapid increase of calcium-containing alkali wastes, containing a large amount of calcium, magnesium, aluminum, and iron ions and caused a great risk to environment. Anaerobic digestion or dark fermentation is one of the most promising technologies to recover biogas, such as methane and hydrogen. Nevertheless, the hydrolysis processes of lignocellulosic biomass and waste activated sludge were the rate-limiting step of the biochemical reactions, which focused on pretreatment to improve the biodegradability of substrate. In addition, when some easily acidified wastes, such as kitchen residue, fruit and vegetable waste, and high concentration organic wastewater, are used as substrate to produce hydrogen and methane, volatile fatty acid accumulation often occurs, causing the process instability. Thus, this paper reviewed the main roles of calcium-based alkali materials such as calcium oxide, calcium peroxide and calcium hydroxide on the substrate pretreatment for obtaining high biodegradability, while others (e.g. calcium carbonate, lime and red muds) used as additives for maintaining process stability, thereby increasing biogas yield from anaerobic digestion and dark fermentation. [Display omitted] • Ca-alkali materials are used as pretreatment agents or additives for AD and DF. • Ca affects H 2 and CH 4 metabolisms as well as their syntrophic partners. • Alkalinity, mineral nutrient and microbial carrier are crucial factors. • Lime mud provides trace elements and improves process buffering effect. • Red mud promotes direct interspecies electron transfer. [ABSTRACT FROM AUTHOR]

[Display omitted] • MgFe 2 O 4 and Ca 0.5 Mg 0.5 Fe 2 O 4 NPs were synthesized to improve bioH 2 yield. • MgFe 2 O 4 and Ca 0.5 Mg 0.5 Fe 2 O 4 NPs lowered the negative effects of humic acids. • MgFe 2 O 4 NPs enriched Clostridium_sensu_stricto_1 more than Ca 0.5 Mg 0.5 Fe 2 O 4 NPs. • MgFe 2 O 4 NPs improved bioH 2 yield higher than Ca 0.5 Mg 0.5 Fe 2 O 4 NPs. Dark fermentation (DF) is an eco-friendly process that simultaneously achieves organic matter degradation and obtains hydrogen (H 2). Nonetheless, low H 2 yield mainly caused by poor activity of key microbes, is still a problem that requires being resolved. In this work, MgFe 2 O 4 and Ca 0.5 Mg 0.5 Fe 2 O 4 nanoparticles (NPs) were synthetized and served as additives to boost H 2 form from DF. H 2 productivity gradually increased with the rise of NPs, and declined when NPs exceeded their optimal dosages. The highest H 2 yield was 183.6 ± 3.2 mL/g glucose at 100 mg/L of MgFe 2 O 4 NPs, being 35.2 % higher than that of the control yield (135.8 ± 3.1 mL/g glucose). However, the highest H 2 yield of 171.9 ± 2.5 mL/g glucose occurred at 400 mg/L of Ca 0.5 Mg 0.5 Fe 2 O 4 NPs, increasing by 26.6 % over the control. Interestingly, the two NPs favored the butyric acid pathway for H 2 synthesis. This provides guidance for multi-element oxide NPs used in DF. [ABSTRACT FROM AUTHOR]

[Display omitted] • BioH 2 yield via dark fermentation increased by 47% with LaFeO 3 NPs. • Butyrate fermentation pathway was strengthened using LaFeO 3 NPs. • Gene copy number of hydrogenase was elevated adding LaFeO 3 NPs. • Firmicutes was selectively enriched and colonized by LaFeO 3 supplementation. • Glycolysis was promoted with LaFeO 3 addition by PICRUSt2 analysis. 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 La3+ 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. [ABSTRACT FROM AUTHOR]