Sustainable biohydrogen production by dark fermentation using carbon monoxide as the sole carbon and energy source

This study evaluated the kinetics of biomass growth, biohydrogen production and substrate utilization using carbon monoxide as the sole carbon and energy source. Experiments were conducted at different initial CO concentration in the range 1.8–5.12 mmol/L over a period of 144 h in order to assess the effect of CO concentration on biomass growth, substrate utilization and H₂ production. Complete utilization (100%) of CO was achieved up to an initial concentration of 3.8 mmol/L and it gradually decreased to 84.5% for 4.4 mmol/L and 83.7% for 5.12 mmol/L. The experimental results of CO utilization were fitted to substrate utilization kinetic models reported in the literature, and it followed a modified Gompertz model. A maximum yield of H₂ on CO was found to be 70.8% and a maximum H₂ production of 29.9 mmol/L was obtained for an initial CO concentration of 5.12 mmol/L. The experimental results on biohydrogen production matched well with the values predicted using the modified Gompertz model. Furthermore, the experimental data on specific growth rate of the ananerobic biomass at different H₂ concentration was fitted to different product inhibition models and the best fit was obtained with Aiba model. This study showed product inhibition on both specific growth rate of biomass and H₂ production due to H₂ accumulation in the gas phase. A very good correlation between the experimental specific growth rate and the Han-Levenspiel model predicted values were obtained with a high determination coefficient (R²) value of more than 0.96.     

Prolongation of H2 production during mixed carbon sources fermentation in E. coli batch cultures: New findings and role of different hydrogenases

Hydrogen (H₂) gas production in batch cultures was studied upon utilization of the mixture of glucose, glycerol and formic acid by Escherichia coli BW25113 wild type (wt) at pH of 5.5–7.5. At pH 7.5H₂ was continuously produced during 240 h but at pH 6.5 and 5.5 it was detected till 168 h and 120 h, respectively. Specific growth rate (μ) of wt was the highest (1.05 h^?1) at pH 6.5. Moreover, at pH 5.5 in hycE μ decreased by ～4.14 fold compared to wt, suggesting major role of Hyd-3 in cell growth. H₂ yield (8.8 mmol H₂ L^?1) was the highest at pH 7.5. In hybC H₂ yield was increased ～1.62 fold than in wt. These data might be applied for biomass and biohydrogen production from various organic wastes where mixtures of carbon sources are present.     

Biohydrogen production from crude glycerol by immobilized Klebsiella sp. TR17 in a UASB reactor and bacterial quantification under non-sterile conditions

Biohydrogen production from crude glycerol by immobilized Klebsiella sp. TR17 was investigated in an up-flow anaerobic sludge blanket (UASB) reactor. The reactor was operated under non-sterile conditions at 40^○C and initial pH 8.0 at different hydraulic retention times (HRTs) (2–12 h) and glycerol concentrations (10–30 g/L). Decreasing the HRT led to an increase in hydrogen production rate (HPR) and hydrogen yield (HY). The highest HPR of 242.15 mmol H₂/L/d and HY of 44.27 mmol H₂/g glycerol consumed were achieved at 4 h HRT and glycerol concentrations of 30 and 10 g/L, respectively. The main soluble metabolite was 1,3-propanediol, which implies that Klebsiella sp. was dominant among other microorganisms. Fluorescence in situ hybridization (FISH) revealed that the microbial community was dominated by Klebsiella sp. with 56.96, 59.45, and 63.47% of total DAPI binding cells, at glycerol concentrations of 10, 20, and 30 g/L, respectively.     

Evaluation of hydrogen production by Rhodobacter sphaeroides O.U.001 and its hupSL deficient mutant using acetate and malate as carbon sources

Rhodobacter sphaeroides O.U.001 is one of the candidates for photobiological hydrogen production among purple non-sulfur bacteria. Hydrogen is produced by Mo-nitrogenase from organic acids such as malate or lactate. A hupSL in frame deletion mutant strain was constructed without using any antibiotic resistance gene. The hydrogen production potential of the R. sphaeroides O.U.001 and its newly constructed hupSL deleted mutant strain in acetate media was evaluated and compared with malate containing media. The hupSL⁻R. sphaeroides produced 2.42 l H₂/l culture and 0.25 l H₂/l culture in 15 mM malate and 30 mM acetate containing media, respectively, as compared to the wild type cells which evolved 1.97 l H₂/l culture and 0.21 l H₂/l culture in malate and acetate containing media, correspondingly. According to the results, hupSL⁻R. sphaeroides is a better hydrogen producer but acetate alone does not seem to be an efficient carbon source for photoheterotrophic H₂ production by R. sphaeroides.     

Effect of coexistence of H2S on production of hydrogen and solid carbon by methane decomposition using Fe catalyst

Biogas derived from livestock manure and food residue contains CO₂ and H₂S as well as methane. The effect of CO₂ and H₂S coexistence on the production of hydrogen and solid carbon by methane decomposition over iron oxide catalysts was investigated. The catalytic activity for methane decomposition was decreased by the coexistence of H₂S. Moreover, the activity decrease was aggravated by the coexistence of CO₂ as well as H₂S, and higher temperature was required to mitigate the activity decrease by the coexistence of CO₂. By increasing the amount of catalyst, the upstream catalyst was preferentially poisoned, but the downstream catalyst developed catalytic activity thanks to its sacrifice. With 2 g of catalyst, the maximum conversion of pure methane was about 85% at 840 °C, but it was slightly less than 80% in the presence of H₂S or H₂S + CO₂. When the catalyst amount was increased to 4 g, the conversion of pure methane was about 90% at 800 °C, but 84% in the presence of H₂S and 80% in the presence of H₂S + CO₂. The poisoning by H₂S was irreversible at low temperatures but became reversible at higher temperatures. Since H₂S is adsorbed by the deposited carbon, the procedure for further removal of H₂S may be omitted. The coexistence of H₂S also affected the shape of the deposited carbon. Although carbon-based catalysts are known to be effective for methane decomposition in the presence of H₂S, iron oxide catalysts have the advantage of superior methane conversion at low temperatures. By flowing methane with CO₂ and H₂S from the downstream side after the reaction flowing from the upstream side for a certain period of time, the catalytic lifetime was drastically extended and the amount of hydrogen and solid carbon produced was dramatically increased, compared to the case of flowing from upstream all the way.     

Defining the roles of the hydrogenase 3 and 4 subunits in hydrogen production during glucose fermentation: A new model of a H2-producing hydrogenase complex

Hydrogen (H₂)-producing hydrogenase (Hyd) activity of E. coli wild type and mutants with defects in subunits of Hyd-3 or Hyd-4 during fermentation at different glucose concentrations and pHs was studied. Hyd-3 was mainly responsible for H₂ production but a significant contribution by Hyd-4 to total H₂ production depended on the glucose concentration and pH. Surprisingly, not all Hyd-3 or Hyd-4 subunits contributed towards H₂ production. Hyd-4 mainly exhibited H₂-oxidizing activity in cells growing on 0.2% glucose at pH 7.5, while at pH 5.5 it had a significant impact on H₂ production. Importantly, a hyfG mutant (lacking the large subunit of Hyd-4) had a ~2.2 fold decrease in H₂ production when cells were grown with 0.2% glucose. A similar role of Hyd-4 was shown at pH 6.5 grown with 0.8% glucose. This study provides new information to allow improvements in H₂ production yield and in our general understanding of H₂ metabolism.     

Biohydrogen production by Anabaena sp. PCC 7120 wild-type and mutants under different conditions: Light, nickel, propane, carbon dioxide and nitrogen

Increasing awareness of environmental problems caused by the current use of fossil fuel-based energy, has led to the search for alternatives. Hydrogen is a good alternative and the cyanobacterium Anabaena sp. PCC 7120 is naturally able to produce molecular hydrogen, photosynthetically from water and light. However, this H₂ is rapidly consumed by the uptake hydrogenase.This study evaluated the hydrogen production of Anabaena sp. PCC 7120 wild-type and mutants: hupL⁻ (deficient in the uptake hydrogenase), hoxH⁻ (deficient in the bidirectional hydrogenase) and hupL⁻/hoxH⁻ (deficient in both hydrogenases) on several experimental conditions, such as gas atmosphere (argon and propane with or without N₂ and/or CO₂ addition), light intensity (54 and 152 ??Em⁻²s⁻¹), light regime (continuous and light/dark cycles 16 h/8 h) and nickel concentrations in the culture medium.In every assay, the hupL⁻ and hupL⁻/hoxH⁻ mutants stood out over wild-type cells and the hoxH⁻ mutant. Nevertheless, the hupL⁻ mutant showed the best hydrogen production except in an argon atmosphere under 16 h light/8 h dark cycles at 54 ??Em⁻²s⁻¹ in the light period, with 1 ??M of NiCl₂ supplementation in the culture medium, and under a propane atmosphere.In all strains, higher light intensity leads to higher hydrogen production and if there is a daily 1% of CO₂ addition in the gas atmosphere, hydrogen production could increase 5.8 times, related to the great increase in heterocysts differentiation (5 times more, approximately), whereas nickel supplementation in the culture medium was not shown to increase hydrogen production. The daily incorporation of 1% of CO₂ plus 1% of N₂ did not affect positively hydrogen production rate.     

Treatment of dark fermentative H2 production effluents by microbial fuel cells: A tutorial review on promising operational strategies and practices

Deriving biohydrogen from dark fermentation is a practically suitable pathway for scaling-up and envisaged mass production. However, a common issue with these systems is the incomplete conversion of feedstock as a result of which, a process effluent with notable organic strength is left behind. The main components of dark fermentation effluents are volatile fatty acids that can be utilized by integrated applications involving bioelectrochemical systems, particularly microbial fuel cells (MFCs) to generate electrical energy. In this work, MFCs deployed to treat dark fermentative H₂ production effluents are assessed to take a look into the current standing of this specific research area and address what MFC design and operating features (reactor configuration, mode of operation, anode surface and reactor size) seem favorable towards improved working efficiency (e.g. power density, Coulombic efficiency, COD removal). Furthermore, promising technological implementations are outlined and suggestions, conclusions for future studies for this field are given.     

Enhancement of Escherichia coli bacterial biomass and hydrogen production by some heavy metal ions and their mixtures during glycerol vs glucose fermentation at a relatively wide range of pH

Escherichia coli growth and H₂ production were followed in the presence of heavy metal ions and their mixtures during glycerol or glucose fermentation at pH 5.5–7.5. Ni²⁺ (50 μM) with Fe²⁺ (50 μM) but not sole metals stimulated bacterial biomass during glycerol fermentation at pH 6.5. Ni²⁺+Fe³⁺ (50 μM), Ni² +Fe³⁺+Mo⁶⁺ (20 μM) and Fe³⁺+Mo⁶⁺ (20 μM) but not sole metals enhanced up to 3-fold H₂ yield but Cu⁺ or Cu²⁺ (100 μM) inhibited it. At pH 7.5 stimulating effect on biomass was observed by Ni²⁺+Fe²⁺+Mo⁶⁺. H₂ production was enhanced 2.7 fold particularly by Ni²⁺+Fe³⁺+Mo⁶⁺ at the late stationary growth phase. Whereas at pH 5.5 increased biomass was when Fe²⁺+Mo⁶⁺ or Mo⁶⁺ were added. H₂ yield was decreased compared with that at pH 6.5, but metal ions again enhanced it. During glucose fermentation at pH 6.5 biomass was increased by the mixtures of metal ions, and 1.2 fold increased H₂ yield was observed. At pH 7.5 Ni²⁺+Fe²⁺ increased biomass but Cu⁺ or Cu²⁺ had suppressing effect; Fe³⁺+Mo⁶⁺ stimulated H₂ production. At pH 5.5 biomass also was raised by Ni²⁺+Fe²⁺+Mo⁶⁺; H₂ yield was increased upon Mo⁶⁺ and Mo⁶⁺+Fe²⁺ or Mo⁶⁺+Fe³⁺ additions. The results point out the importance of Ni²⁺, Fe²⁺, Fe³⁺ and Mo⁶⁺ and some of their combinations for E. coli bacterial growth and H₂ production mostly during glycerol but not glucose fermentation and at acidic conditions (pH 5.5 and 6.5). They can be used for optimizing fermentation processes on glycerol, controlling bacterial biomass and developing H₂ production biotechnology.     

Continuous H2 production during fermentation of the carbon components of lignocellulose hydrolysates: Insight into the influence of pH conditions on the conversion efficiency of individual sugars

Sugars released from lignocellulose biomass are a promising substrate for biohydrogen production. This study evaluates the effect of pH controlled between 4.0 and 7.5 on continuous dark-fermentative H₂ production from the mixture of cellobiose, xylose and arabinose. High hydrogen production rate was obtained for pH values between 6.0 and 7.0 with a maximum of 7.41 ± 0.16 L/L-d at pH 7.0. On the other hand, the highest H₂ yields of around 1.74 ± 0.02 mol/mol_consumed were obtained at pH 4.5, 5.0 and 6.0. Cellobiose was completely utilized in nearly the entire pH range, while the highest consumption of xylose and arabinose was obtained at pH 6.0 and 7.0, respectively. It shows the challenges in selecting optimum pH for fermentation of mixed sugars. Significant impact of pH conditions on the microbial structure was observed. Between pH 4.0 and 7.0 Clostridium genus dominated the consortium, while above pH 7.0 relative abundance of Bacillus genus increased significantly.     

Development of hydrogen production by liquid phase plasma process of water with NiTiO2/carbon nanotube photocatalysts

Hydrogen evolution by water photocatalysis using liquid phase plasma system was disserted over metal-loaded TiO₂ photocatalysts. Carbon nanotube was applied as a support for the metal-loaded TiO₂ nanocrystallites. Photocatalytic activities of the photocatalysts were estimated for hydrogen production from water. Hydrogen was produced from the photodecomposition of water by liquid phase plasma irradiation. The rate of hydrogen evolution was improved by the metal loading on the TiO₂ surface. TiO₂ nanocrystallites were incorporated above 40 wt% onto the carbon nanotube support. The carbon nanotubes could be applied as a useful photocatalytic support for the fixation of TiO₂. Hydrogen evolution was enhanced by the Ni loading on the TiO₂ nanocrystallites supported on the carbon nanotube. Hydrogen evolution was increased apparently with addition of the alcohols which contributes as a kind of sacrificial reagent promoting the photocatalysis.     

Enhanced thermal diffusivity and dehydrogenation of 2LiNH2MgH2 by doping with super activated carbon

Doping with the additives in metal-N–H system has been regarded as one of the most effective approaches to improve its hydrogen storage properties. Herein, we prepared super activated carbon (SuperC) through the activation of commercial activated carbon by KOH and evaluated its effect on dehydrogenation properties of 2LiNH₂MgH₂. Our studies show that doping with SuperC could effectively lower its dehydrogenation temperatures. For instance, 2LiNH₂MgH₂–10 wt% SuperC can release 4.86 wt% of hydrogen upon heating up to 300 °C with the onset and peak dehydrogenation temperatures of 71 °C and 168 °C, respectively. Moreover, the release of byproduct NH₃ was successfully suppressed. Measurement of thermal diffusivity suggests that the enhanced dehydrogenation properties may be ascribed to the improved heat transfer for solid-solid reaction resulting from doping with SuperC.