Renewable algal photo H2 production without S control using acetate enriched fermenter effluents

Renewable energy production using microorganisms is one of the challenging issues for environmental sustainability. Algal hydrogen (H₂) production has often been achieved by sulfur (S) and chloride ion (Cl^?) deprivation in a growth medium; however, it may not be realistic to control S or Cl^? concentrations in natural sources (e.g., wastewater). In this study, two different green algal species, Chlamydomonas reinhardtii and Chlorella sorokiniana were selected and their photosynthetic activities were compared with different acetate/Cl^? ratios both in batch and continuous modes. At 150 of acetate/Cl^? ratio, the H₂ production rates were 0.25–0.33 μmol L^?1 min^?1 for C. sorokiniana and 0.20–0.38 μmol L^?1 min^?1 for C. reinhardtii, respectively. The hydrogenase (HydA) reactivation and photosystem II (PSII) inhibitor test revealed that biohydrogen production by algae is due to photosynthetic activity. It was found that maintaining acetate/Cl^? ratios greater than 60–100 leads to continuous O₂ depletion and thus renewable H₂ production for both algal species. Molecular dynamics (MD) simulations of hydrogen bonding between Y_z and His₁₉₀ in PSII supported O₂ inhibition using acetate. Using fermenter effluents, C. sorokiniana and C. reinhardtii showed a successful continuous H₂ production of ~80 μmol L^?1 and ~95 μmol L^?1, respectively, for 15 days.     

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.     

Effect of co-cultivation of Chlamydomonas reinhardtii with Azotobacter chroococcum on hydrogen production

Chlamydomonas reinhardtii cc124 and Azotobacter chroococcum bacteria were co-cultured with a series of volume ratios and under a variety of light densities to determine the optimal culture conditions and to investigate the mechanism by which co-cultivation improves H₂ yield. The results demonstrated that the optimal culture conditions for the highest H₂ production of the combined system were a 1:40 vol ratio of bacterial cultures to algal cultures under 200 μE m^?2 s^?1. Under these conditions, the maximal H₂ yield was 255 μmol mg^?1 Chl, which was approximately 15.9-fold of the control. The reasons for the improvement in H₂ yield included decreased O₂ content, enhanced algal growth, and increased H₂ase activity and starch content of the combined system.     

Hydrogen production from butyrate by a marine mixed phototrophic bacterial consort

A newly enriched marine phototrophic bacterial consort was studied for its capability of hydrogen production in batch cultivations using butyrate as the sole carbon source. Analyses of denaturing gradient gel electrophoresis (DGGE) profiles showed that the mixed bacterial consort consisted mainly of Ectothiorhodospira, Sporolactobacillus, and Rhodovulum. Important parameters investigated include temperature, light intensity, initial pH, and butyrate concentration. The pH of the culture medium significantly increased as fermentation proceeded. Optimal cell growth was observed at temperature of 25–35 °C, light intensity of 80–120 μmol photons/m² s, initial pH of 8, butyrate concentration of 20–40 mmol/l. Optimal conditions for hydrogen production were 30 °C, light intensity of 80 μmol photons/m² s, initial pH 8. The increase of butyrate concentration (10–50 mmol/l) resulted in higher hydrogen production, but the yield of hydrogen production (mol H₂/mol butyrate) gradually decreased with increasing butyrate concentration. The maximal hydrogen yield and hydrogen production rate were estimated to be 2.52 ± 0.12 mol H₂/mol butyrate and 19.40 ± 2.32 ml/l h, respectively. These results indicate that optimization of the culture conditions resulted in a simultaneous increase in biohydrogen production and cell growth.     

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.     

A truncated antenna mutant of Chlamydomonas reinhardtii can produce more hydrogen than the parental strain

Photoproduction of H₂ gas was examined in the Chlamydomonas reinhardtii tla1 strain, CC-4169, containing a truncated light-harvesting antenna, along with its parental CC-425 strain. Although enhanced photosynthetic performance of truncated antenna algae has been demonstrated previously (Polle et al. Planta 2003; 217:49-59), improved H₂ photoproduction has yet to be reported. Preliminary experiments showed that sulfur-deprived, suspension cultures of the tla1 mutant could not establish anaerobiosis in a photobioreactor, and thus, could not photoproduce H₂ gas under conditions typical for the sulfur-deprived wild-type cells (Kosourov et al. Biotech Bioeng 2002; 78:731-40). However, they did produce H₂ gas when deprived of sulfur and phosphorus after immobilization within thin (∼300 μm) alginate films. These films were monitored for long-term H₂ photoproduction activity under light intensities ranging from 19 to 350 μE m⁻² s⁻¹ PAR. Both the tla1 mutant and the CC-425 parental strain produced H₂ gas for over 250 h under all light conditions tested. Relative to the parental strain, the CC-4169 mutant had lower maximum specific rates of H₂ production at low and medium light intensities (19 and 184 μE m⁻² s⁻¹), but it exhibited a 4-times higher maximum specific rate at 285 μE m⁻² s⁻¹ and an 8.5-times higher rate at 350 μE m⁻² s⁻¹ when immobilized at approximately the same cell density as the parental strain. As a result, the CC-4169 strain accumulated almost 4-times more H₂ than CC-425 at 285 μE m⁻² s⁻¹ and over 6-times more at 350 μE m⁻² s⁻¹ during 250-h experiments. These results are the first demonstration that truncating light-harvesting antennae in algal cells can increase the efficiency of H₂ photoproduction in mass culture at high light intensity.     

Biohydrogen production by a novel integration of dark fermentation and mixotrophic microalgae cultivation

Biohydrogen is usually produced via dark fermentation, which generates CO₂ emissions and produces soluble metabolites (e.g., volatile fatty acids) with high chemical oxygen demand (COD) as the by-products, which require further treatments. In this study, mixotrophic culture of an isolated microalga (Chlorella vulgaris ESP6) was utilized to simultaneously consume CO₂ and COD by-products from dark fermentation, converting them to valuable microalgae biomass. Light intensity and food to microorganism (F/M) ratio were adjusted to 150 μmol m⁻² s⁻¹ and F/M ratio, 4.5, respectively, to improve the efficiency of assimilating the soluble metabolites. The mixotrophic microalgae culture could reduce the CO₂ content of dark fermentation effluent from 34% to 5% with nearly 100% consumption of soluble metabolites (mainly butyrate and acetate) in 9 days. The obtained microalgal biomass was hydrolyzed with 1.5% HCl and subsequently used as the substrate for bioH₂ production with Clostridium butyricum CGS5, giving a cumulative H₂ production of 1276 ml/L, a H₂ production rate of 240 ml/L/h, and a H₂ yield of 0.94 mol/mol sugar.     

Energy efficient growth control of microalgae using photobiological methods

Microalgae have garnered interest for the production of valuable molecules ranging from therapeutic proteins to biofuels. However, microalgae also are associated with the considerable problem of phytoplankton bloom. In this study, we demonstrated algal growth using Isochrysis galbana as a model can be controlled photobiologically. Long dark period (24-h light: 24-h dark) unlike common photoperiod resulted in biomass loss and slower growth rates, but were unlikely to cause fatal damage. Algal cell growth can be significantly recovered with the onset of light. Also, it was confirmed that blue light-emitting diode (LED) illumination was able to effectively support cell growth of I. galbana as the sole light source. The blue LED intensity with 200% (580 l×, 18.52 μmol m⁻² s⁻¹) based on 8000 l× (98.4 μmol m⁻² s⁻¹) fluorescent lamp provided the best support for growth of I. galbana. We verified excessive light intensities lead to inhibition of algal growth, whereas low light intensities also did not promote algal growth. Further, I. galbana cell growth can be controlled using blue LED with extremely high LED intensities. These results may provide means to control algal population for either goal of growth or inhibition through proper use of such illumination.     

Elaboration and characterization of the brownmillerite Ca2Fe2O5 synthesized by citrate sol-gel method. Application for photocatalytic H2-Production under visible light over Ca2Fe2O5/ZnO hetero-system

The present work is devoted to the synthesis of the ferrite Ca₂Fe₂O₅ as photocatalyst crystallizing in the brownmillerite structure. The ternary oxide is prepared by sol-gel auto combustion and characterized by physical and electrochemical methods. The thermal analysis (TG/DSC) shows that, the formation of the brownmillerite is observed above 660 °C. The X-ray diffraction and BET analysis show respectively a single phase with an active surface area of ～6 m² g^?1. The SEM micrographs exhibit an inhomogeneous structure formed by agglomeration of irregular shaped grains, confirmed by the laser granulometry analysis. The forbidden band (～2.3 eV) determined from the diffuse reflectance, permits to explore ～ 30% of the sun spectrum into chemical energy. The p-type comportment of Ca₂Fe₂O₅ is demonstrated by the capacitance-potential (C^?2 - E) graph with a flat band potential (E_fb = 0.93 V_SCE), due to oxygen over-stoichiometry. The negative potential of the conduction band (?1.06 V_SCE) predicts the feasibility of the H₂ generation. Indeed, Ca₂Fe₂O₅ is chemical stable in a wide pH domain and is positively experimented as photocatalyst for the H₂-production under visible light. The best performance is obtained in alkaline medium (NaOH, 0.1 M) with a mean evolution rate of 18 μmol g^?1 min^?1. However, Ca₂Fe₂O₅ coupled to ZnO sol-gel (ZnO-SG) improves the catalytic performance. The H₂ evolution rate over (Ca₂Fe₂O₅/ZnO-SG) reached 24 μmol g^?1 min^?1 after 60 min. It has also been shown that ZnO–P, prepared by precipitation, is more efficient than that synthetized by sol-gel method (ZnO-SG) and TiO₂–P25.     

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.     

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.     

Investigation of the combined effects of acetate and photobioreactor illuminated fraction in the induction of anoxia for hydrogen production by Chlamydomonas reinhardtii

In the context of hydrogen production by microalgae, the growth of Chlamydomonas reinhardtii was characterized under autotrophic and mixotrophic conditions in a fully controlled photobioreactor (PBR). The combined effect of light transfer conditions, as represented by the illuminated fraction γ, with acetate consumption was observed upon establishment of anoxia. Anoxia was reached in batch cultures when γ was close to 1 (almost fully illuminated culture) in mixotrophic conditions while a value of γ ≈ 0.46 in autotrophic conditions was not sufficient. Based on these results, continuous hydrogen production was established in a cylindrical PBR operated in luminostat with constant illumination and in mixotrophic conditions. Maximum hydrogen gas production was equal to 1.4 ± 0.1 ml_H2 l⁻¹ h⁻¹ for photon flux density of 110 μmol m⁻² s⁻¹ and reactor illuminated fraction of γ = 0.5. Carbon mass balance was realized, emphasizing the necessity to work in strictly autotrophic conditions for hydrogen production with no concomitant CO₂ release.     

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.     

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.     

Screening and optimisation of hydrogen production by newly isolated nitrogen-fixing cyanobacterial strains

Recently, there has been a propensity to postpone dealing with the world's climate concerns until later, resulting in a 1.5 °C rise in temperature over the last century. Therefore, interest in biologically derived, inexhaustible energy sources based on solar energy is growing. Cyanobacteria have the potential to produce clean, renewable fuels in the form of hydrogen (H₂) gas, derived from solar energy and water. The current study reports the screening 11 cyanobacterial strains isolated from rice paddies and hotsprings for efficient H₂ producers. According to our findings, H₂ concentrations in the species ranged from 3.6 to 48.9 μmol mg⁻¹ Chl a h⁻¹. H₂ production by isolated species was shown to have a 2% positive influence on oxygen (O₂) and carbon dioxide (CO₂) concentrations and a 2% negative effect on all nitrogen gas (N₂) concentrations. It was discovered that at high CO₂ concentrations, photosynthesis is enhanced but H₂ production is suppressed. Anabaena variabilis BTA-1047 was found to be the most active H₂-producing species, with an H₂ production activity of 21.3 μmol mg⁻¹ Chl a h⁻¹. Moreover, a 1% O₂: 2% CO₂ gas mixture doubled the strain activity of H₂ production. The findings of the study called into the question the notion that only an anaerobic environment is required for H₂ production by N₂-fixing cyanobacterial species and explored whether H₂ productivity can be increased by stimulating the micro-anaerobic environment with a carbon source.     

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.     

Characterization of cell growth and photobiological H2 production of Chlamydomonas reinhardtii in ASSF industry wastewater

Photobiological H₂ production in microalgae is a promising approach for the development of alternative clean and renewable energy. As a unicellular green alga, Chlamydomonas reinhardtii is regarded as an ideal candidate for sustainable photo-H₂ production. However, growth and photo-H₂ producing are still expensive and energy extensive. Wastewater has been suggested as an economical resource for microalgae growth and biofuels production. In this study, we characterized the cell growth and photo-H₂ production of C. reinhardtii CC503 cultured in waste water from pressing process of fermented sweet sorghum stalks during Advanced Solid State Fermentation (ASSF). The maximal cells concentration reached 8.9 × 10⁶ cells/mL in ASSF wastewater medium (AWM) with the fastest growth rate of 0.19 × 10⁶ cells/h, compared to 18.2 × 10⁶ cells/mL and 0.36 × 10⁶ cells/h in TAP medium and to 1.3 × 10⁶ cells/mL and 0.02 × 10⁶ cells/h in BGII medium respectively. The optimized concentration of wastewater for algae cells growth was determined to be 13.3% (7.5 folds dilution), under which, surprisingly the photosynthetic H₂ evolution was increased by more than 700% compared to the cells grown in TAP medium. This system appears to be a good strategy for the development of an economical microalgal photobiological H₂ production scheme. Finally, the possible mechanism for such an H₂ enhancement was identified as the reduction of PSII activity in AWM grown cells.