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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]

Abstract: Hydrogen production via anaerobic fermentation may suffer from instability or inhibition due to volatile fatty acids concentrations and inorganic elements deviations from optimality. A slight addition of white mud from ammonia–soda process (WMA) to the fermentation may show a feasible implement approach to use as buffer and inorganic nutrient. Batch tests were conducted to study the effect of the WMA addition on the hydrogen fermentation of food waste (FW) for the purpose of the maximum hydrogen yield (MHY) while achieving process stability and microbial germination through the endogenous alkalinity and inorganic elements of WMA. This was attributed to the existence of calcium, sodium, magnesium, potassium and alkali-rich materials, like calcium carbonate. 3 g of WMA addition to 200 g FW and fermentation time of 36 h resulted in obtaining the MHY of 145.4 mL H2/g VS, with a corresponding lag-phase time of 3.0 h. Under these operating conditions, the fermentation process demonstrated one of the highest measured stability with final pH 5.3. [Copyright &y& Elsevier]

Abstract: The effect of lime mud from paper-making process (LMP) addition on the H2 fermentation of food waste (FW) was investigated. It was found that a slight addition of LMP (1.0–4.0 g in 200 g FW) significantly enhanced the H2 fermentation performance, not only increasing the total amount of H2 produced but also accelerating the whole reaction, shortening the lag period, and increasing the H2 production rate. Fermentation stability and microbial germination were also facilitated by LMP addition. This was attributed to the existence of Ca, Fe, Mn and alkaline substances such as CaCO3 and NaOH. The batch process treating a mixture of FW and LMP was showed that the highest hydrogen production of 137.6 mL H2/g VS was achieved at final pH 5.0, adding 3 g LMP (in 200 g FW) to the fermentation process, which lag-phase time was about 2.5 h. [Copyright &y& Elsevier]

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] • 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. [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] • 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]