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.     

Biohydrogen production from algal biomass (Anabaena sp. PCC 7120) cultivated in airlift photobioreactor

The present study deals with the optimization of pretreatment conditions followed by thermophilic dark fermentative hydrogen production using Anabaena PCC 7120 as substrate by mixed microflora. Different airlift photobioreactors with ratio of area of downcomer and riser (A_d/A_r) in range of 0.4–3.2 were considered. Maximum biomass concentration of 1.63 g L⁻¹ in 9 d under light intensity of 120 μE m⁻² s⁻¹ was observed at A_d/A_r of 1.6. The mixing time of the reactors was inversely proportional to A_d/A_r. Maximal H₂ production was found to be 1600 mL L⁻¹ upon pretreatment with amylase followed by thermophilic fermentation for 24 h compared to other methods like sonication (200 mL L⁻¹), autoclave (600 mL L⁻¹) and HCl treatment (1230 mL L⁻¹). The decrease of pH from 6.5 to 5.0 during fermentation was due to the accumulation of volatile fatty acids. Amylase pretreatment gave higher reducible sugar content of 7.6 g L⁻¹ as compare to other pretreatments. Thermophilic fermentation of pretreated Anabaena biomass by mixed bacterial culture was found suitable for H₂ production.     

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.     

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.     

Sustained hydrogen photoproduction by phosphorus-deprived Chlamydomonas reinhardtii cultures

This study demonstrates that, besides sulfur deprivation, sustained H₂ photoproduction in Chlamydomonas reinhardtii cultures can be generated by incubating algae under phosphorus-deprived (−P) conditions. However, phosphorus deficiency in algal cells could not be obtained by resuspension of algae in −P medium, evidently due to a significant reserve of phosphorus in cells. In this study, phosphorus deficiency was accomplished by inoculating the washed algae into the −P medium at low initial cell densities (below 2 mg Chl l⁻¹). After the initial growth period, where cells utilize intracellular phosphorus, algae established anaerobic environment followed by the period of H₂ photoproduction. The maximum H₂ output (∼70 ml l⁻¹) was obtained in cultures with the initial Chl content ∼1 mg l⁻¹. Cultures with Chl above 2 mg l⁻¹ did not produce H₂ gas. The physiological response of algal cultures to phosphorus deprivation demonstrated significant similarities with the response of algae to sulfur depletion.     

Maximizing the hydrogen photoproduction yields in Chlamydomonas reinhardtii cultures: The effect of the H2 partial pressure

Photoproduction of H₂ gas has been examined in sulfur/phosphorus-deprived Chalmydomonas reinhardtii cultures, placed in photobioreactors (PhBRs) with different gas phase to liquid phase ratios (V_g.p./V_l.p.). The results demonstrate that an increase in the ratio stimulates H₂ photoproduction activity in both algal suspension cultures and in algae entrapped in thin alginate films. In suspension cultures, a 4× increase (from ∼0.5 to ∼2) in V_g.p./V_l.p results in a 2× increase (from 10.8 to 23.1 mmol l⁻¹ or 264–565 ml l⁻¹) in the total yield of H₂ gas. Remarkably, 565 ml of H₂ gas per liter of the suspension culture is the highest yield ever reported for a wild-type strain in a time period of less than 190 h. In immobilized algae, where diffusion of H₂ from the medium to the PhBR gas phase is not affected by mixing, the maximum rate and yield of H₂ photoproduction occur in PhBRs with V_g.p./V_l.p above 7 or in a PhBR with smaller headspace, if the H₂ is effectively removed from the medium by continuous flushing of the headspace with argon. These experiments in combination with studies of the direct inhibitory effect of high H₂ concentrations in the PhBR headspace on H₂ photoproduction activity in algal cultures clearly show that H₂ photoproduction in algae depends significantly on the partial pressure of H₂ (not O₂ as previously thought) in the PhBR gas phase.     

Hydrogen permeation behavior of Nb30Ti35Ni35−xCox (x = 0…35) alloys containing high fractions of eutectic

The hydrogen permeability of cast Nb₃₀Ti₃₅Ni_35−xCo_x (x = 0…35) alloys is found to increase with the Co content, induced by the tailored microstructures containing high fractions of eutectic and less primary phase. Among the cast alloys, Nb₃₀Ti₃₅Co₃₅ consisting entirely of eutectic exhibits the highest permeability, particularly 2.65 × 10⁻⁸ mol H₂ m⁻¹ s⁻¹ Pa^−0.5 at 673 K. This permeability is further increased to 3.58 × 10⁻⁸ mol H₂ m⁻¹ s⁻¹ Pa^−0.5 by aligning eutectic grains perpendicularly to the membrane surface using directional solidification. The high permeability is attributed to the high hydrogen solubility and diffusivity in alloys. Our work demonstrates that hydrogen permeable alloys containing high fractions of eutectic may exhibit high permeability by adequately optimizing morphology, volume fraction and alignment of the bcc-Nb phase in the eutectic as the pathways for hydrogen permeation.     

Increased hydrogen photoproduction by means of a sulfur-deprived Chlamydomonas reinhardtii D1 protein mutant

The photoproduction of H₂ was studied in a sulfur-deprived Chlamydomonas reinhardtii D1 mutant that carried a double amino acid substitution. The leucine residue L159 was replaced by isoleucine, and the asparagine N230 was replaced by tyrosine (L159I-N230Y). Phenotypic characterization of the mutant showed some interesting features compared to its wild type, namely: (1) a lower chlorophyll content; (2) a higher photosynthetic capacity and higher relative quantum yield of photosynthesis; (3) a higher respiration rate; (4) a very high conversion of violaxanthin to zeaxanthin during H₂ production; (5) a prolonged period of H₂ production. In standard conditions, the mutant produced more than 500 ml of H₂, that is, more than one order of magnitude greater than its wild type, and about 5-times greater than the CC124 strain that was used for comparison. The better performance of the mutant was mainly the result of a longer production period. Biogas produced contained up to 99.5% H₂.     

New tolerant strains of purple nonsulfur bacteria for hydrogen production in a two-stage integrated system

In this study we described the isolation of eight new strains of purple non-sulfur bacteria resistant to salinity ≥30 g L⁻¹ and high concentration of VFAs (200 mM). These strains were characterized by their general physiological properties and the occurrence of hupSL genes. Some correlation was observed between the rate of H₂ photoproduction, the absence of hupSL genes and hydrogenase activity. Two fast-growing strains without hupSL genes showed high nitrogenase activity and hydrogen accumulation during growth on Ormerod medium. These strains were capable of H₂ photoproduction using non-treated dark culture (75% in water) after dark fermentation of starch at 30 g L⁻¹, unlike control strains, Rhodobacter capsulatus B10 and Rb. sphaeroides GL. New N7 and 13 strains identified as Rb. sphaeroides can be recommended for application in a two-stage H₂ production system.     

Effect of synthesis conditions on performance of a hydrogen selective nano-composite ceramic membrane

A hydrogen-selective nano-composite ceramic membrane was prepared by depositing a dense layer composed of SiO₂ and Al₂O₃ on top of a graded multilayer substrate using co-current chemical vapor deposition (CVD) method. The multilayer substrate was made by dip-coating a macroporous α-alumina tubular support by a series of boehmite solutions to get a graded structure. Using DLS analysis, it was concluded that decreasing hydrolysis time and increasing acid concentration lead to smaller particle size of boehmite sols. XRD analysis was carried out to investigate the structure of intermediate layer and an optimized calcination temperature of 973 K was obtained. SEM images indicated the formation of a graded membrane with a porous intermediate layer having a thickness of about 2 μm and a dense top selective layer with a thickness of 80–100 nm. Permeation tests showed that H₂ permeance flux decreased from 5 × 10⁻⁵ mol m⁻² s⁻¹ Pa⁻¹ for a fresh substrate to 6.30 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ after 6 h of deposition, but H₂ selectivity over N₂ increased considerably from 5.6 to 203.     

Equilibrium shift of methylcyclohexane dehydrogenation in a thermally stable organosilica membrane reactor for high-purity hydrogen production

A high-performance organosilica membrane was prepared via sol–gel processing for use in methylcyclohexane (MCH) dehydrogenation to produce high-purity hydrogen. The membrane showed a high H₂ permeance of 1.29 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹, with extremely high H₂/C₃H₈ and H₂/SF₆ selectivities of 6680 and 48,900, respectively, at 200 °C. The extraction of hydrogen from the membrane reactor led to the MCH conversion higher than the thermodynamic equilibrium, with almost pure hydrogen obtained in the permeate stream without considering the effect of carrier gas and sweep gas in the membrane reactor, and the organosilica membrane reactor was very stable under the reaction conditions employed.     

Photochemical characteristics of Chlamydomonas mutant hpm91 lacking proton gradient regulation 5 (PGR5) during sustained H2 photoproduction under sulfur deprivation

Renewable H₂ photoproduction by Chlamydomonas reinhardtii offers a desirable bio-system for solar fuels. However, its large-scale application is hindered mainly due to lack of ideal strains. We previously isolated a mutant hpm91 which lacks PGR5 and sustains H₂ photoproduction for 25 days. To understand the photosynthetic basis for this remarkable phenotype, we hereby investigated its photochemical characteristics during sulfur-deprived H₂ photoproduction using in vivo chlorophyll fluorescence spectroscopy. Compared to wild type, effective quantum yield of PSII and PSI of hpm91 increased upto 78.9% and 147.6%, respectively. Electron transport rate of each photosystem is closely correlated with the increase of quantum yield, suggesting overall enhanced photochemistry of hpm91 under such condition. Moreover, ATP synthase activity decays slower and remains higher in this mutant. These are in vivo evidence demonstrating increased photosynthetic efficiency of hpm91 promotes its H₂ photoproduction. Together with its competent photoheterotrophic growth in a larger photobioreactor, we propose that hpm91 is a valuable strain for re-engineering Chlamydomonas towards improving light energy efficiency in a large-scale system.     

Response of H2 production and Hox-hydrogenase activity to external factors in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803

The effects of external factors on both H₂ production and bidirectional Hox-hydrogenase activity were examined in the non-N₂-fixing cyanobacterium Synechocystis PCC 6803. Exogenous glucose and increased osmolality both enhanced H₂ production with optimal production observed at 0.4% and 20 mosmol kg⁻¹, respectively. Anaerobic condition for 24 h induced significant higher H₂ase activity with cells in BG11₀ showing highest activities. Increasing the pH resulted in an increased Hox-hydrogenase activity with an optimum at pH 7.5. The Hox-hydrogenase activity gradually increased with increasing temperature from 30 ^○C to 60 ^○C with the highest activity observed at 70 ^○C. A low concentration at 100 μM of either DTT or β-mercaptoethanol resulted in a minor stimulation of H₂ production. β-Mercaptoethanol added to nitrogen- and sulfur-deprived cells stimulated H₂ production significantly. The highest Hox-hydrogenase activity was observed in cells in BG11₀-S-deprived condition and 750 μM β-mercaptoethanol measured at a temperature of 70 °C; 14.32 μmol H₂ mg chl a⁻¹ min⁻¹.     

Preparation and performance of thin-layered PdAu/ceramic composite membranes

Preparation of 3–5 μm thick, hydrogen-selective PdAu layers via sequential electroless plating of Pd and Au onto ceramic microfiltration membranes was investigated employing a cyanide-free Au plating bath. The Au deposition rate was strongly dependent on bath temperature and alkalinity reaching an optimum at 333 K and pH 10. Homogenous alloying of the separate metal layers under atmospheric H₂ proved to be a protracted process and required approximately a week at 873 K for a PdAu layer as thin as 3 μm. After 300 h annealing at 823 K the 5 μm thick PdAu layer of a composite membrane still exhibited a Au gradient declining from 7.4 at.% at the top surface to 5.5 at.% at the support interface despite that the H₂ permeation rate had become stable. Nonetheless, the membrane exhibited a very high H₂ permeability of e.g. 1.3 × 10⁻⁸ mol m m⁻² s⁻¹ Pa^−0.5 at 673 K, but it decreased much faster with temperature below 573 K than above, likely due to a change from bulk H diffusion-controlled to H₂ adsorption or desorption-limited transport. The composite membrane withstood cycling between 523 and 723 K in H₂ well showing that differing thermal expansion of the joined metallic and ceramic materials stayed within the tolerance range up to 723 K.     

H2 production via water gas shift in a composite Pd membrane reactor prepared by the pore-plating method

In this work, H₂ production via catalytic water gas shift reaction in a composite Pd membrane reactor prepared by the ELP “pore-plating” method has been carried out. A completely dense membrane with a Pd thickness of about 10.2 μm over oxidized porous stainless steel support has been prepared. Firstly, permeation measurements with pure gases (H₂ and N₂) and mixtures (H₂ with N_2, CO or CO₂) at four different temperatures (ranging from 350 to 450 °C) and trans-membrane pressure differences up to 2.5 bar have been carried out. The hydrogen permeance when feeding pure hydrogen is within the range 2.68–3.96·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5, while it decreases until 0.66–1.35·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5 for gas mixtures. Furthermore, the membrane has been also tested in a WGS membrane reactor packed with a commercial oxide Fe–Cr catalyst by using a typical methane reformer outlet (dry basis: 70%H₂–18%CO–12%CO₂) and a stoichiometric H₂O/CO ratio. The performance of the reactor was evaluated in terms of CO conversion at different temperatures (ranging from 350 °C to 400 °C) and trans-membrane pressures (from 2.0 to 3.0 bar), at fixed gas hourly space velocity (GHSV) of 5000 h⁻¹. At these conditions, the membrane maintained its integrity and the membrane reactor was able to achieve up to the 59% of CO conversion as compared with 32% of CO conversion reached with conventional packed-bed reactor at the same operating conditions.     

Microstructure and hydrogen permeability of Nb40Hf30Ni30 ternary alloy

The microstructure and hydrogen permeability of the Nb₄₀Hf₃₀Ni₃₀ ternary alloys were investigated in particular. The as-cast Nb₄₀Hf₃₀Ni₃₀ alloy consists of the bcc- (Nb,Hf) solid solution, the B_f-HfNi compound and the fine eutectic phase of {(Nb,Hf) + HfNi}. The annealed Nb₄₀Hf₃₀Ni₃₀ alloy consists of the bcc-(Nb, Hf) solid solution and the fine eutectic phase of {(Nb,Hf) + HfNi}. The as-cast Nb₄₀Hf₃₀Ni₃₀ alloy shows a high Φ value of 4.3 × 10⁻⁸ [mol H₂ m⁻¹ s⁻¹ Pa^−0.5] at 673 K, and for the annealed Nb₄₀Hf₃₀Ni₃₀ alloy, the Φ value is 3.8 × 10⁻⁸ [mol H₂ m⁻¹ s⁻¹ Pa^−0.5] at 673 K. Annealing can’t enhance the hydrogen permeability of the Nb₄₀Hf₃₀Ni₃₀ ternary alloy.     

Purification and characterization of a highly thermostable, oxygen-resistant, respiratory [NiFe]-hydrogenase from a marine, aerobic hydrogen-oxidizing bacterium Hydrogenovibrio marinus

The membrane-bound [NiFe]-hydrogenase from Hydrogenovibrio marinus (HmMBH) was purified homogeneously under anaerobic conditions. Its molecular weight was estimated as 110 kDa, consisting of a heterodimeric structure of 66 kDa and 37 kDa subunits. The purified enzyme exhibited high activity in a wide temperature range: 185 U/mg at 30 °C and 615 U/mg at 85 °C (the optimum temperature). The K_m and k_cat/K_m values for H₂ were, respectively, 12 μM and 8.58 × 10⁷ M⁻¹ s⁻¹. The optimum reaction pH was 7.8, but its stability was particularly high at pH 4.0-7.0. Results show that HmMBH was remarkably thermostable and oxygen-resistant: its half-life was 75 h at 80 °C under H₂, and more than 72 h at 4 °C under air. The air-oxidized HmMBH for 72 h showed only weak EPR signals of Ni-B, suggesting a structural feature in which the active center is not easily oxidized.