Lemi Türker

Aspects of the metabolism of hydrogen production by Rhodobacter sphaeroides

Abstract Photosynthetic bacteria are favorable candidates for biological hydrogen production due to their high conversion efficiency and versatility in the substrates they can utilize. For large-scale hydrogen production, an integrated view of the overall metabolism is necessary in order to interpret results properly and facilitate experimental design. In this study, a summary of the hydrogen production metabolism of the photosynthetic purple non-sulfur (PNS) bacteria will be presented. Practically all hydrogen production by PNS bacteria occurs under a photoheterotrophic mode of metabolism. Yet results show that under certain conditions, alternative modes of metabolism—e.g. fermentation under light deficiency—are also possible and should be considered in experimental design. Two enzymes are especially critical for hydrogen production. Nitrogenase promotes hydrogen production and uptake hydrogenase consumes hydrogen. Though a wide variety of substrates can be used for growth, only a portion of these is suitable for hydrogen production. The efficiency of a certain substrate depends on factors such as the activity of the TCA cycle, the carbon-to-nitrogen ratio, the reduction-state of that material and the conversion potential of the substrate into alternative metabolites such as PHB. All these individual components of the hydrogen production interact and are subject to strict regulatory controls. An overall scheme for the hydrogen production metabolism is presented.

Kinetics of biological hydrogen production by the photosynthetic bacterium Rhodobacter sphaeroides O.U. 001

The kinetics and the effects of various parameters on hydrogen production by Rhodobacter sphaeroides O.U. 001 were investigated in a batch column photobioreactor. In particular, the effect of the inoculum age and the implementation of a light–dark cycle illumination scheme for emulating natural sunlight have been investigated in detail. The possibility of using yeast extract to replace the rather expensive vitamin mixture in the medium was also studied. The results show that hydrogen production is decreased when the initially inoculated bacteria have a high culture age. Exposure of the bacterial culture to light–dark cycles increased the amount of hydrogen compared to continuous illumination, all other parameters remaining the same. Similarly, the use of yeast extract to replace the vitamins increased the growth and hydrogen production rates, however, with a slight reduction in the total amount of gas produced and the hydrogen fraction in the evolved gas.

Substrate consumption rates for hydrogen production by Rhodobacter sphaeroides in a column photobioreactor

The effect of l-malic acid and sodium glutamate, which serve as the carbon and nitrogen source, respectively, on hydrogen production by Rhodobacter sphaeroides O.U.001 has been investigated in a batch water jacketed glass column photobioreactor (PBR), which has an inner volume of 400 ml. The PBR was operated at different carbon to nitrogen ratios at 32°C with a tungsten lamp at a light intensity of 200 W m−2. Carbon to nitrogen ratio was found to be an important parameter for bio-hydrogen production. Moreover, hydrogen gas production was dependent on certain threshold concentrations of sodium glutamate. l-malic acid consumption was found to be first order with respect to l-malic acid concentration, whereas sodium glutamate consumption was found to be second order with respect to glutamate concentration. It was concluded that there is a close relationship between the hydrogen production rate and substrate consumption rates. A kinetic model is developed, which relates hydrogen gas production per amount of biomass, l-malic acid, and sodium glutamate concentrations.

Photobiological hydrogen production by using olive mill wastewater as a sole substrate source

Abstract In the present work olive mill wastewater (OMW) collected from West Anatolia—Turkey during 2001, containing 36.02 g carbon, 5.26 g hydrogen, and 0.96 g nitrogen in 100 g suspended solid was used as a sole substrate for the production of hydrogen gas by Rhodobacter sphaeroides O.U.001 in 400 ml glass, column-photobioreactors. Hydrogen production studies on diluted-OMW were investigated in the range of 20% (v/v) and 1% (v/v) OMW containing media. Below 5% OMW containing media, bacterial growth rate fitted well to the logistic model where hydrogen production was observed for the ones below 4% OMW. A maximum hydrogen production potential (HPP) of 13.9 l H 2 / l OMW was obtained at 2% OMW. During the biological hydrogen production process, chemical oxygen demand (COD) of the diluted wastewater decreased from 1100 to 720 mg / l ; biochemical oxygen demand (BOD) decreased from 475 to 200 mg / l , and the total recoverable phenol content (ortho- and meta-substitutions) decreased from 2.32 to 0.93 mg / l . In addition, valuable by-products such as carotenoid (40 mg / l OMW ) and polyhydroxybutyrate (PHB) (60 mg / l OMW ) were obtained. According to these results, OMW was concluded to be a very promising substrate source for biohydrogen production process, with additional benefits of its utilization with regard to environmental and economical aspects.

Photoproduction of hydrogen from sugar refinery wastewater by Rhodobacter sphaeroides O.U. 001

Pretreated sugar refinery wastewater (SRWW) was used for the production of hydrogen by Rhodobacter sphaeroides O.U.001 in a 0.4 l column photobioreactor. Hydrogen was produced at a rate of 0.001 l hydrogen/h/l culture in 20% dilution of SRWW. To adjust the carbon concentration to 70 mM and nitrogen concentration to 2 mM, sucrose or L-malic acid was added as carbon source and sodium glutamate was added as nitrogen source to the 20% dilution of SRWW. By these adjustments, hydrogen production rate was increased to 0.005 l hydrogen/h/l culture. Continuous hydrogen production was achieved in the same medium for 100 days at three different dilution rates. The maximum hydrogen produced was 2.67 l (in 100 days) at a dilution rate of 0.0013 h−1.

Identification of by-products in hydrogen producing bacteria; Rhodobacter sphaeroides O.U. 001 grown in the waste water of a sugar refinery

Rhodobacter sphaeroides O.U. 001 is able to produce hydrogen anaerobically upon illumination. The cells were screened for the presence of valuable by-products such as poly-β-hydroxy (PHB) butyric acid aiming to improve the feasibility of the system. Also waste water from a sugar refinery was used for bacterial growth to further increase the feasibility. Under aerobic conditions the standard growth media containing l-malic acid and sodium glutamate in 7.5/10 and 15/2 molar ratios and a medium containing 30% waste water from sugar refinery were used. In this case the maximum concentration of PHB produced were approximately 0.2 g l− 1 in both of the standard media whereas it was 0.3 g l− 1 in medium containing 30% waste water. By using the medium containing 30% waste water, PHB and hydrogen productions were determined under anaerobic conditions. The maximum concentration of PHB produced was around 0.5 g l− 1 and the amount of gas collected was 35 ml in 108 h. From these results it can be concluded that PHB can be collected during hydrogen production. The use of waste water from sugar refinery increased the yield.

A DFT study on nitrotriazines

In this study, all possible mono-, di- and tri-nitro-substituted triazine compounds have been considered as potential candidates for high-energy density materials (HEDMs) by using quantum chemical treatment. Geometric and electronic structures, thermodynamic properties and detonation performances of these nitro-substituted triazines have been systematically studied using density functional theory (DFT, B3LYP) at the level of 6-31G(d,p), 6-31+G(d,p), 6-311G(d,p), 6-311+G(d,p) and cc-pVDZ basis sets. Moreover, thermal stabilities have been evaluated from the homolytic bond dissociation energies (BDEs). Detailed molecular orbital (MO) investigation has been performed on these potential HEDMs. According to the results of the calculations, mono-, di- and tri-nitro-substituted derivatives of symmetric 1,3,5-triazine have been found to be more stable than their 1,2,3 and 1,2,4 counterparts.

AM1 treatment of endohedrally hydrogen doped fullerene, nH2@C60

Abstract Endohedrally hydrogen doped C60 systems, nH2@C60 (n: 9,12,15,19,21,24) have been theoretically investigated at the level of AM1 (RHF) type quantum chemical treatment. It has been found that n:24 is the maximum number of hydrogen molecules which should result a stable composite system. The calculations indicate that all these structures are stable but highly endothermic. Also some geometrical and physicochemical properties of these structures are reported.

Cryptoannulenic Behavior of Cyclacenes

Abstract Cryptoannulenic behavior of cyclacenes has been investigated within the framework of Huckel molecular orbital theory. It has been found that the angle of total π-electron energy for cyclacenes, as it is the case for annulenes, alternatingly fluctuates within the series depending on the size and type of the peripheral circuits (4m or 4m + 2 type) of the cyclacene considered.

Gaussian-based computations in molecular science

A review article of over 300 hundred references describing the background and recent advances in the development and application of Gaussian based methods is presented.