Continuous photosynthetic biohydrogen production from acetate-rich wastewater: Influence of light intensity

Photo-biohydrogen by microalgae is attractive sustainable energy caused by the utilization of solar energy and water. However, due to oxygen (O₂) sensitive hydrogenase (HydA) activity, effective control of O₂ and light intensity is critical for achieving sustainable photosynthetic hydrogen (H₂) production. Here we demonstrate continuous algal H₂ production using acetate-enriched fermenter effluent, achieving the complete O₂ cessation without sulfur depletion. Average H₂ production of 108 ± 4 μmol L^?1 for Chlamydomonas reinhardtii and 88 ± 7 μmol L^?1 for Chlorella sorokiniana at 100 μmol m^?2 s^?1 were observed for 15 days, respectively. The highest light energy to H₂ energy conversion efficiency (LHCE) of 1.61% for C. reinhardtii and 1.06% for C. Sorokiniana was obtained under low light intensity (50 μmol m^?2 s^?1) but the LHCE decreased with the increase of light intensity followed by photoinhibition, which led to a decrease of HydA activity and H₂ production. Low H₂ production was observed at 50 μmol m^?2 s^?1 under the highest LHCE, in which microalgae exhibited photoinhibition biomass growth kinetics to produce chlorophyll a (Chl a) for electron generation. These results demonstrate that light is a feasible strategy for producing electron for H₂ production under anoxygenic photosynthesis.     

Photoautotrophic hydrogen production by nitrogen-deprived Chlamydomonas reinhardtii cultures

In this study we have demonstrated the possibility of phototrophic hydrogen production in C. reinhardtii under N-deprived conditions. When tested under air + CO₂, and Ar + CO₂ N-deprived C. reinhardtii demonstrated decrease in PSII activity mainly due to over reduction of PQ, in addition no ascorbate accumulation was observed in cells. Under air + CO₂ atmosphere cells accumulated excessive amounts of starch. When incubated under Ar + CO₂ atmosphere cells accumulated starch as nitrogen replete cultures and no hydrogen production was observed. Hydrogen production (86 ml H₂ per one l of culture) occurred under Ar + CO₂ atmosphere when particular two-step illumination protocol was implicated. In oxygen producing and early oxygen consuming stage cells were illuminated under light intensity 169 μE m^?2 s^?1. When light was switched to 30 μE m^?2 s^?1, cultures quickly respired all oxygen and transient to anaerobic conditions with subsequent hydrogen production 2 h later. Actual quantum yield of C. reinhardtii cultures was measured in photobioreactor and maximal quantum efficiency of PSII of dark adapted cells together with JIP test were studied.     

Evaluation of continuous biohydrogen production from enzymatically treated cornstalk hydrolysate

Enzymatically treated cornstalk hydrolysate was tested as substrate for H₂ production by Thermoanaerobacterium thermosaccharolyticum W16 in a continuous stirred tank reactor. The performance of strain W16 to ferment the main components of hydrolysate, mixture of glucose and xylose, in continuous culture was conducted at first, and then T. thermosaccharolyticum W16 was evaluated to ferment fully enzymatically hydrolysed cornstalk to produce H₂ in continuous operation mode. At the dilution rate of 0.020 h⁻¹, the H₂ yield and production rate reached a maximum of 1.9 mol H₂ mol⁻¹ sugars and 8.4 mmol H₂ L⁻¹ h⁻¹, respectively, accompanied with the maximum glucose and xylose utilizations of 86.3% and 77.6%. Continuous H₂ production from enzymatically treated cornstalk hydrolysate in this research provides a new direction for economic, efficient, and harmless H₂ production.     

An anaerobic sequential batch reactor for enhanced continuous hydrogen production from fungal pretreated cornstalk hydrolysate

Anaerobic sequencing batch reactor (ASBR) process offers great potential for H₂ production from wastewaters. In this study, an ASBR was used at first time for enhanced continuous H₂ production from fungal pretreated cornstalk hydrolysate by Thermoanaerobacterium thermosaccharolyticum W16. The reactor was operated at different hydraulic retention times (HRTs) of 6, 12, 18, and 24 h by keeping the influent hydrolysate constant at 65 mmol sugars L⁻¹. Results showed that increasing the HRT from 6 to 12 h led to the H₂ production rate increased from 6.7 to the maximum of 9.6 mmol H₂ L⁻¹ h⁻¹ and the substrate conversion reached 90.3%, although the H₂ yield remained at the same level of 1.7 mol H₂ mol⁻¹ substrate. Taking into account both H₂ production and substrate utilization efficiencies, the optimum HRT for continuous H₂ production via an ASBR was determined at 12 h. Compared with other continuous H₂ production processes, ASBR yield higher H₂ production at relatively lower HRT. ASBR is shown to be another promising process for continuous fermentative H₂ production from lignocellulosic biomass.     

Hydrogen production from C1 compounds by a novel marine hyperthermophilic archaeon Thermococcus onnurineus NA1

A novel marine hyperthermophile, Thermococcus onnurineus NA1, was found to grow on C1 carbon compounds, such as formate and carbon monoxide (CO), and produce hydrogen (H₂). In the present study, the growth and H₂ production of NA1 were examined to determine its potential as H₂ producer. NA1 showed relatively high specific growth rates, 0.48 h⁻¹ and 0.40 h⁻¹ with CO (20%, v/v) and formate (100 mM), respectively, when cultivated in batch mode in a minimal salt medium fortified with 1.0 g L⁻¹ yeast extract. On the other hand, cell growth in both cases stopped at approximately 6 h and the final cell densities were extremely low at 18.2 and 12.1 mg protein L⁻¹ with CO and formate, respectively. The maximum final cell density could be improved greatly to 36.0 mg protein L⁻¹ by optimizing CO content (50%, v/v) and yeast extract concentration (4.0 g L⁻¹), but it was still very low. During the cell growth, formate and CO were used as energy source rather than carbon source. In the resting cell experiments, NA1 exhibited remarkably high H₂ production activities as 385.0 and 207.5 μmol mg protein⁻¹ h⁻¹ for CO and formate, respectively. When formate (100 mM) or CO (100%, v/v) was added repeatedly at 30–35 h intervals, NA1 showed consistent H₂ production for 3 cycles with a yield of approximately 1.0 mol H₂ mol⁻¹ for both CO and formate. This study suggests that T. onnurineus NA1 has a high H₂ production potential from formate or CO but a method for achieving a high cell density culture is needed.     

Novel NiO/RP composite with remarkably enhanced photocatalytic activity for H2 evolution from water

The low photocatalytic activity of red phosphorus (RP) for H₂ production seriously restricts its wide application. In this work, we reported a facile synthesis and remarkable activity improvement of NiO/RP composite for water splitting. As a result, the 3% NiO/RP composite exhibited the highest photocatalytic activity for H₂ production (57.27 μmol g^?1 h^?1), which is 68.56 times higher than that of pure RP (0.82 μmol g^?1 h^?1) under visible light (λ ≥ 420 nm) irradiation. The investigation of photocarriers separation mechanism indicated NiO/RP composite applied a Z-scheme mechanism to promote the photocarriers separation. This is a potential strategy to dramatically enhance the photocatalytic activity of RP for H₂ evolution using transition metal oxide to efficiently separate the photocarriers under light irradiation.     

Morphology and defects design in g-C3N4 for efficient and simultaneous visible-light photocatalytic hydrogen production and selective oxidation of benzyl alcohol

Small surface area, deficient reaction sites, and poor visible-light harvest ability of the original graphitic carbon nitride (g-C₃N₄) severely restrict its photocatalytic H₂ production activity. Here, an ultrathin porous and N vacancies rich g-C₃N₄ (V_N-UP-CN) was fabricated by thermal oxidation exfoliation and high-temperature calcination under the Ar atmosphere. The ultrathin porous morphology increases the surface area and reaction sites of original g-C₃N₄, moreover, the produced N vacancies greatly broaden the light harvest ability of ultrathin porous g-C₃N₄ (UP–CN). Therefore, V_N-UP-CN displays the maximal H₂ production rate of 2856.7 μmol g^?1 h^?1 in triethanolamine solution under visible-light, and adding 0.5 M of K₂HPO₄ can further improve its H₂ production rate to 4043.9 μmol g^?1 h^?1. Importantly, V_N-UP-CN also shows good performance in simultaneous photocatalytic H₂ production and benzyl alcohol oxidation to benzaldehyde with the activities of 196.08 and 198.28 μmol g^?1 h^?1, respectively, which avoids the waste of sacrificial agent and photogenerated holes. This work affords an achievable way to design the efficient g-C₃N₄ photocatalyst by morphology and defect regulation, which can effectively utilize both photogenerated electrons and holes for H₂ and value-added chemical production.     

Efficient photocatalytic H2 evolution over Cu and Cu3P co-modified TiO2 nanosheet

Schottky junction and p-n heterojunction are widely employed to enhance the charge transfer during the photocatalysis process. Herein, Cu and Cu₃P co-modified TiO₂ nanosheet hybrid (Cu–Cu₃P/TiO₂) was fabricated using an in situ hydrothermal method. The ternary composite achieved the superior H₂ evolution rate of 6915.7 μmol g^?1 h^?1 under simulated sunlight, which was higher than that of Cu/TiO₂ (4643.4 μmol g^?1 h^?1) and Cu₃P/TiO₂ (6315.8 μmol g^?1 h^?1) and pure TiO₂ (415.7 μmol g^?1 h^?1). The enhanced activity can be attributed to the collaboration effect of Schottky junction and p-n heterojunction among Cu/TiO₂ and Cu₃P/TiO₂, which can harvest the visible light, reduce the recombination of charge carriers and lower the overpotential of H₂ evolution, leading to a fast H₂ evolution kinetics. This work develops a feasible method for the exploration of H₂ evolution photocatalyst with outstanding charge separation properties.     

Producing hydrogen from the fermentation of cheese whey and glycerol as cosubstrates in an anaerobic fluidized bed reactor

This study aimed to evaluate the effect of the organic loading rate (OLR) (60, 90, and 120 g Chemical Oxygen Demand (COD). L^?1. d^?1) on hydrogen production from cheese whey and glycerol fermentation as cosubstrates (50% cheese whey and 50% glycerol on a COD basis) in a thermophilic fluidized bed reactor (55 °C). The increase in the OLR to 90 gCOD.L^?1. d^?1 favored the hydrogen production rate (HPR) (3.9 L H₂. L^?1. d^?1) and hydrogen yield (HY) (1.7 mmol H₂. gCOD^?1_app) concomitant with the production of butyric and acetic acids. Employing 16S rRNA gene sequencing, the highest hydrogen production was related to the detection of Thermoanaerobacterium (34.9%), Pseudomonas (14.5%), and Clostridium (4.7%). Conversely, at 120 gCOD.L^?1. d^?1, HPR and HY decreased to 2.5 L H₂. L^?1. d^?1 and 0.8 mmol H₂. gCOD^?1_app, respectively, due to lactic acid production that was related to the genera Thermoanaerobacterium (50.91%) and Tumebacillus (23.56%). Cofermentation favored hydrogen production at higher OLRs than cheese whey single fermentation.     

Morphologies dependence of hydrogen evolution over CeO2 via ultrasonic triggering

Ultrasonic field can lead to cavitation bubbles explosion, which rises a high-frequency oscillation and generates a high-frequency current in semiconductor nanoparticles in suspension. However, the effect of nanoparticle morphology on ultrasonic-triggered H₂ production is still unclear. To this end, herein, nanorods CeO₂ (nrCeO₂), CeO₂ nanocubes (ncCeO₂), and CeO₂ nanospheres (nsCeO₂) were successfully synthesized. Among them, one-dimensional nrCeO₂ had the most abundant O-vacancies. As revealed by the COMSOL simulation, nanoparticle deformation was easier in nanorods compared with nanocubes and nanospheres, resulting in more efficient charge separation and facilitating H₂ production reaction in nrCeO₂. In detail, within a 5 h’ period, nrCeO₂ presented the highest H₂ production activity of 983.1 μmol g^?1 h^?1 with the positive charge (q+) trapping agent of CH₃OH, and that of 278.1 μmol g^?1 h^?1 in pure water. This work presents a new understanding about the relationship between nanoparticle morphology and H₂ production activity, and provides a promising, efficient, and clean H₂ production approach, which can be further extended to multi-field coupling reactor.     

Active-center-enriched Ni0.85Se/g-C3N4 S-scheme heterojunction for efficient photocatalytic H2 generation

Photocatalysts with abundant active sites are essential for photocatalytic H₂ evolution from water. Herein, Ni_0.85Se-deposited g-C₃N₄ was obtained by a physical solvent evaporation method. The investigation shows that Ni_0.85Se with unsaturated active Se atoms can significantly improve the photocatalytic activity of g-C₃N₄, and the H₂ production rate of Ni_0.85Se/g-C₃N₄ can reach 8780.3 μmol g^?1 h^?1, which is 3.5 and 92.9 times higher than that of Ni_0.85+xSe/g-C₃N₄ (2497.9 μmol g^?1 h^?1) and pure g-C₃N₄ (94.5 μmol g^?1 h^?1), respectively. This improvement can be attributed to the quick charge transfer between Ni_0.85Se and g-C₃N₄ with S-scheme heterojunction feature based on a series of trapping experiments and photoelectrochemical analysis. Moreover, abundant unsaturated Se atoms could provide more H₂ evolution active sites. This work sheds light on the construction of heterojunctions with abundant active sites for H₂ production.     

Sustained photobiological hydrogen production by Chlorella vulgaris without nutrient starvation

This article describes the ability of the Chlorella vulgaris BEIJ strain G-120 to produce hydrogen (H₂) via both direct and indirect pathways without the use of nutrient starvation. Photobiological H₂ production reached a maximum rate of 12 mL H₂ L^?1 h^?1, corresponding to a light conversion efficiency (light to H₂) of 7.7% (average 3.2%, over the 8-day period) of PAR, (photosynthetically active irradiance). Cells presented a maximum in vivo hydrogenase activity of 25.5 ± 0.2 nmoles H₂ μgChl^?1 h^?1 and the calculated in vitro hydrogenase activity was 830 ± 61 nmoles H₂ μgChl^?1 h^?1. The strain is able to grow either heterotrophically or photo autotrophically. The total output of 896 mL of H₂ was attained for illuminated culture and 405 mL for dark cultures. The average H₂ production rate was 4.98 mL L^?1 h^?1 for the illuminated culture and 2.08 mL L^?1 h^?1 for the one maintained in the dark.     

Fabrication of the SnS2/ZnIn2S4 heterojunction for highly efficient visible light photocatalytic H2 evolution

A series of SnS₂/ZnIn₂S₄ (x-SS/ZIS) photocatalysts with different mass ratios of SnS₂ were prepared by a hydrothermal method. The resulted composites were used for photocatalytic hydrogen evolution under visible light excitation. All the SS/ZIS composites exhibited significantly enhanced photocatalytic activity for H₂ evolution. Obviously, the highest H₂ evolution rate of 769 μmol g^?1 h^?1 was observed over 2.5-SS/ZIS, which was approximately 10.5 times that of the ZnIn₂S₄ (73 μmol g^?1 h^?1). The enhanced photocatalytic performance was attributed to the successful construction of SnS₂/ZnIn₂S₄ heterojunctions, leading to rapid charge separation and fast transfer of the photo-generated electrons and holes under light irradiation. On the basis of PL, electrochemical impedance spectroscopy (EIS), photocurrent measurements and the H₂ evolution tests, a plausible photocatalytic mechanism was proposed.     

Relating biomass composition and the distribution of metabolic functions in the co-fermentation of sugarcane vinasse and glycerol

This study aimed to determine the effect of increasing the organic loading rate (OLR) from 60 to 90 and 120 kg COD m^?3 d^?1 in the co-fermentation of glycerol and sugarcane vinasse (50%: 50% proportion on a COD basis) in a thermophilic anaerobic fluidized bed reactor (55 °C) at the fixed hydraulic retention time of 4 h. The highest values of hydrogen production rate (1851 mL H₂ d^?1 L^?1_bed) and yield (0.29 mmol H₂ g^?1 COD_added) were found at 120 kg COD m^?3 d^?1 and coincided with butanoate as a major liquid metabolite (2620 mg L^?1). The reverse β-oxidation of lactate into butanoate contributed to its synthesis and was linked to synergism between Clostridium (relative abundance of 77.8%) and Lacticaseibacillus (7.2%) in the reactor. The identification of the butyryl-CoA/acetate-CoA transferase gene, which may have catalyzed the conversion of butyryl-CoA into butanoate using acetate as an acceptor, also supported this.     

An efficient ternary photocatalyst via anchoring nickel complex and nickel oxides onto carbon nitride for visible light driven H2 evolution

Developing earth abundant, active and stable photocatalysts for water splitting is a critical but challenging procedure for efficient conversion and storage of sustainable energy. Here, a ternary photocatalyst was rationally prepared for efficient H₂ production by covalently anchoring a nickel molecule cocatalyst (NiL) onto graphitic carbon nitride nanosheets (CN) and introducing nickel oxides (NiO_x) as hole-transport materials. The lower H₂ overpotential by NiL and the faster separation of photoinduced carriers by NiO_x nanoparticles account for the efficient H₂ generation of CN without the help of noble metals. Eventually, the prepared NiL/NiO_x/CN catalyst exhibited excellent performance for H₂ evolution (289 μmol g^?1 h^?1) in TEOA solution under visible light irradiation, which is superior to 3NiL/CN (161 μmol g^?1 h^?1) and CN (Null). Furthermore, a possible mechanism of photocatalytic H₂ production for NiL/NiO_x/CN is proposed based on a series of electrochemical measurements. The noble-metal-free photocatalyst developed in this work will pave a new way to synthesize low-cost multicomponent photocatalysts for solar conversion.     

Inhibition of photosystem 2 in starch-enriched Chlamydomonas reinhardtii cells prevents the efficient induction of H2 production in sulfur-depleted cultures

In sulfur-deprived Chlamydomonas reinhardtii cells the activity of photosystem 2 (PSII) has been shown to have a crucial role in the photosynthetic production of H₂, since it allows the synthesis of internal reserves such as starch. In the present investigation, the PSII inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) was added in starch-enriched and sulfur-depleted C. reinhardtii cultures 1) at the very end of the aerobic phase, and 2) soon after the culture started to evolve H₂. In the former case, production of H₂ on a volumetric basis was completely down-regulated, although starch mobilization was enhanced. In vitro tests showed that the hydrogenase enzyme was active, although its efficiency of utilization in vivo was lowered very soon in the experiment. When an inhibitor of Rubisco such as glycolaldehyde was added under the same conditions, no substantial improvement in H₂ production rates was noted. These findings indicate that, aside starch storage, PSII plays an active role in the induction of the H₂ production process.     

Continuous biohydrogen production by a degenerated strain of Clostridium acetobutylicum ATCC 824

Degenerated strains of Clostridium acetobutylicum lack the ability to produce solvents and to sporulate, allowing the continuous production of hydrogen and organic acids. A degenerated strain of Clostridium acetobutylicum was obtained through successive batch cultures. Its kinetic characterization showed a similar specific growth rate than the wild type (0.25 h^?1), a higher butyric acid production of 6.8 g·L^?1 and no solvents production. A steady state was reached in a continuous culture at a dilution rate of 0.1 h^?1, with a constant hydrogen production of 507 mL·h^?1, corresponding to a volumetric rate of 6.10 L·L^?1 d^?1, and a yield of 2.39 mol of H₂ per mole of glucose which represents 60% of the theoretical maximum yield. These results suggest that the degeneration is an interesting alternative for hydrogen production with this strain, obtaining a high hydrogen production in a continuous culture with cells in a permanent acidogenic state.     

Optimization of photosynthetic hydrogen yield from platinized photosystem I complexes using response surface methodology

Light-dependent hydrogen production by platinized Photosystem I isolated from the cyanobacterium Thermosynechococcus elongatus BP-1 was optimized using response surface methodology (RSM). The process parameters studied included temperature, light intensity and wavelength, and platinum salt concentration. Application of RSM generated a model that agrees well with the data for H₂ yield (R² = 0.99 and p < 0.001). Significant effects on the total H₂ yield were seen when the platinum salt concentration and temperature were varied during platinization. However, light intensity during platinization had a minimal effect on the total H₂ yield within the region studied. The values of the parameters used during the platinization that optimized the production of H₂ were light intensity of 240 μE m⁻² s⁻¹, platinum salt concentration of 636 μM and temperature of 31 °C. A subsequent validation experiment at the predicted conditions for optimal process yield gave the maximum H₂ yield measured in the study, which was 8.02 μmol H₂ per mg chlorophyll.     

Synergistic enhancement of photocatalytic H2 production by Ni decorated 2D bubble-like carbon nitride

In this paper, a novel 2D bubble-like g-C₃N₄ (B–CN) with a highly porous and crosslinked structure is successfully synthesized via a cost-effective bottom-up process. The as-prepared B–CN photocatalyst delivers a considerably expanded specific surface area and increased active sites. Moreover, the 2D bubble-like structure can afford shortened diffusion paths for both photogenerated charge carriers and reactants. As a result, the photocatalytic H₂ evolution rate of B–CN reached 268.9 μmol g^?1 h^?1, over 5 times more than that of bulk C₃N₄. The Ni ions were further deposited on B–CN as a cocatalyst to enhance the photocatalytic activity. Benefit from the synergy of 2D bubble-like structure and Ni species cocatalyst, recombination of photoinduced charges was greatly inhibited and the hydrogen evolution reaction (HER) was significantly accelerated. The resulted catalyst achieved a dramatically high H₂ evolution rate of 1291 μmol g^?1 h^?1. This work provides an alternative way to synthesize novel porous carbon nitride together with non-noble metal cocatalysts toward enhanced photocatalytic activity for H₂ production.