High H2 Partial Pressure Research Articles

Reversibility of hydrolysis inhibition at high hydrogen partial pressure in dry anaerobic digestion processes fed with wheat straw and inoculated with anaerobic granular sludge

In dry anaerobic digestion (AD), methanogenic performances are lowered by high solid contents. Low performances are often caused by a decrease of the gas-liquid transfer kinetics leading to local accumulation of inhibitory by-products. Hydrogen was previously identified as an inhibitor of hydrolytic and acetogenic microbial activities in dry AD. CO2 is also generated but its impact on the microbial activity remains unknown. In this study, the reversibility of dry AD inhibition at high H2 partial pressure (PH2 of 1 bar) was investigated by adding CO2 (400 mbars) after 11 and 18 days of methanogenesis inhibition, in an AD process operated at 25% TS, using wheat straw as substrate and inoculated with anaerobic granular sludge. As soon as CO2 was added, the methanogenic activity rapidly recovered within 3 days, from 0.41 ± 0.1 to 3.77 ± 0.8 and then 2.25 ± 0.3, likely through the hydrogenotrophic pathway followed by the acetoclastic pathway, respectively. This result was confirmed by the high abundance of Methanomicrobiales (83%) and the emergence of Methanosarcinales sp (up to 17%) within the methanogenic community. Furthermore, the recovery kinetics were impacted by the duration of the inhibition period suggesting a different impact of the high PH2 on hydrogenotrophic and acetoclastic methanogens.

Hydrogen production from a thermophilic alkaline waste activated sludge fermenter: Effects of solid retention time (SRT)

This study aims to investigate the effects of solid retention times (SRTs) on hydrogen production via thermophilic alkaline fermentation of waste activated sludge. The reactor was subjected to a SRT from 10 to 6 days during approximately 82 days of operation. The results revealed that SRT had minor influence on hydrolysis and hydrolysis efficiency in different phases were from 48.11% to 50.55%. Nevertheless, the efficiency of acidogenesis process was highly related to SRT and longer SRT could enhance the acidogenesis. On the other hand, acidogenesis efficiency was also related to H2 partial pressure and high H2 partial pressure negatively affected the acidogenesis. Thus, the maximum acidification was achieved in phase 1 (21.29%) resulting in the maximum H2 yield in phase 1 (95.94 mL/g VSS; SRT = 10 days; H2 partial pressure = 0–18%). Phyla Actinobacteria and Proteobacteria, who are highly related to hydrolytic microbial population, were abundant in all phases that resulted in high hydrolysis extent. H2 production was attributed to the relative high abundance of Clostridia. Thus, this study suggested that longer SRT and lower H2 partial pressure was necessary to improve the H2 yield under alkaline pH condition.

Synergetic promotion of syntrophic methane production from anaerobic digestion of complex organic wastes by biochar: Performance and associated mechanisms

Biochar was added to a mesophilic anaerobic digester to promote syntrophic volatile fatty acids (VFAs) oxidation and methane production from complex organic wastes. Compared with conventional operation, biochar addition effectively shortened the lag time by 27.5–64.4% and increased the maximum methane production rate by 22.4%–40.3%. With a biochar dosage of 15 g/L, the system performed well under an organic loading rate as high as 3 g substrate/g inoculums. Biochar showed a remarkable buffering capacity to alleviate pH decrease caused by VFAs accumulation. In order to gain knowledge on associated mechanisms, a specific experiment was conducted using butyrate as substrate. It was identified that syntrophic degradation of butyrate to acetate occurred under high H2 partial pressure. By microbial community analysis, it was further revealed that biochar addition brought about the enrichment of Anaerolineaceae and Methanosaeta, typical microorganisms for direct interspecies electron transfer.

Nucleation and growth of copper selective-area atomic layer deposition on palladium nanostructures

The nucleation and growth of copper atomic layer deposition (ALD) on palladium have been investigated for applications in nanoscale devices. Palladium nanostructures were fabricated by electron beam lithography and range in size from 250 nm to 5 μm, prepared on oxidized silicon wafers. Copper ALD using Cu(thd)2(s) and H2(g) as reactants was carried out to selectively deposit copper on palladium seeded regions to the exclusion of surrounding oxide surfaces. Nuclei sizes and densities have been quantified by scanning electron microscopy for different growth conditions. It is found that growth occurs via island growth at temperatures of 150-190 °C and alloy growth at temperatures above 210 °C. In the lower temperature window, nucleation density increases with decreasing temperature, reaching a maximum of 4.8 ± 0.2 × 109/cm2 at 150 °C, but growth is too slow for significant deposition at the lowest temperatures. At higher temperatures, individual nuclei cannot be quantified due to extensive mixing of copper and palladium layers. For the lower temperatures where nuclei can be quantified, rates of nucleation and growth are enhanced at high H2 partial pressures. At the smallest length scales, conformality of the deposited over-layers is limited by a finite nuclei density and evolving grain structure that cause distortion of the original nanostructure shape during growth.

Potentially shifting from interspecies hydrogen transfer to direct interspecies electron transfer for syntrophic metabolism to resist acidic impact with conductive carbon cloth

Anaerobic digesters usually become sour during the operation and inhibit methanogenesis. This effect occurs due to the interspecies electron exchange for the syntrophic metabolism of alcohols and volatile fatty acids (VFAs) via interspecies H2 transfer (IHT). Conductive materials can promote direct interspecies electron transfer (DIET) between the syntrophs and methanogens, providing an alternative to IHT. However, the cooperative mechanism of these two working modes during syntrophic metabolism is unknown, especially during acidic impacts. The results of this study demonstrated that anaerobic digesters supplemented with conductive carbon cloth had a higher capacity to resist the acidic impacts. Conversely, the digesters supplemented with non-conductive cotton cloth (control) were nearly stagnant when the H2 partial pressure of the anaerobic systems increased. Further experiments at high H2 partial pressure to inhibit IHT exhibited almost no effect on the syntrophic metabolism in the carbon cloth-supplemented group, in which the methane production approached the stoichiometric production (350mLCH4/gCODremoval), while the methane production in the control group ceased. The microbial community analysis revealed that the surface sludge attached to the carbon cloth had the highest abundance of Geobacter and Methanosaeata species known to participate in DIET. These results suggested that the predominant working mode for the interspecies electron exchange might have shifted from IHT to DIET in the presence of the conductive carbon cloth during acidic impacts. DIET primarily occurred on the surface sludge of the carbon cloth and replaced IHT to proceed the syntrophic metabolism and maintained stable anaerobic methanogenesis.

The thermodynamics analysis and experimental validation for complicated systems in CO2 hydrogenation process

Catalytic conversion of CO2 into chemicals and fuels is an alternative to alleviate climate change and ocean acidification. The catalytic reduction of CO2 by H2 can lead to the formation of various products: carbon monoxide, carboxylic acids, aldehydes, alcohols and hydrocarbons. In this paper, a comprehensive thermodynamics analysis of CO2 hydrogenation is conducted using the Gibbs free energy minimization method. The results show that CO2 reduction to CO needs a high temperature and H2/CO2 ratio to achieve a high CO2conversion. However, synthesis of methanol from CO2 needs a relatively high pressure and low temperature to minimize the reverse water–gas shift reaction. Direct CO2 hydrogenation to formic acid or formaldehyde is thermodynamically limited. On the contrary, production of CH4 from CO2 hydrogenation is the thermodynamically easiest reaction with nearly 100% CH4 yield at moderate conditions. In addition, complex reactions with more than one product are also calculated in this work. Among the considered carboxylic acids (HCOOH, CH3COOH and C2H5COOH), propionic acid dominates in the product stream (selectivity above 90%). The same trend can also be found in the hydrogenation of CO2 to aldehydes and alcohols with the major product of propionaldehyde and butanol, respectively. In the process of CO2 hydrogenation to alkenes, low temperature, high pressure, and high H2 partial pressure favor the CO2 conversion. C4H6 is the most thermodynamically favorable among all considered alkynes under different temperatures and pressures. The thermodynamic calculations are validated with experimental results, suggesting that the Gibbs free energy minimization method is effective for thermodynamically understanding the reaction network involved in the CO2 hydrogenation process, which is helpful for the development of high-performance catalysts.

Mechanisms of Mo2C(101)-Catalyzed Furfural Selective Hydrodeoxygenation to 2-Methylfuran from Computation

The selective formation of 2-methylfuran (F-CH3) and furan from furfural (F-CHO) hydrogenation and hydrodeoxygenation on clean and 4H precovered Mo2C(101) surfaces has been systematically computed on the basis of periodic density functional theory including dispersion correction (PBE-D3). The clean Mo2C(101) surface has two distinct surface sites: unsaturated C and Mo sites for the adsorption of H and furfural, respectively. The selectivity comes from the different preference of furfural hydrogenation and dissociation (F-CHO + H = F-CH2O vs F-CHO = F-CO + H) under the variation of H2 partial pressure. On the basis of the computed minimum energy path on the clean surface, microkinetics shows that high H2 partial pressure can promote 2-methylfuran formation and suppress furan formation. To verify this proposed selectivity trend of 2-methylfuran at high H2 partial pressure, the 4H precovered Mo2C(101) surface (0.25 monolayer hydrogen coverage), which provides neighboring hydrogens for promoting furfural hydr...

Communities stimulated with ethanol to perform direct interspecies electron transfer for syntrophic metabolism of propionate and butyrate

Direct interspecies electron transfer (DIET) has been considered as an alternative to interspecies H2 transfer (IHT) for syntrophic metabolism, but the microorganisms capable of metabolizing the key intermediates, such as propionate and butyrate, via DIET have yet to be described. A strategy of culturing the enrichments with ethanol as a DIET substrate to stimulate the communities for the syntrophic metabolism of propionate and/or butyrate was proposed in this study. The results showed that the syntrophic propionate and/or butyrate degradation was significantly improved in the ethanol-stimulated reactor when propionate/butyrate was the sole carbon source. The conductivity of the ethanol-stimulated enrichments was as 5 folds (for propionate)/76 folds (for butyrate) as that of the traditional enrichments (never ethanol fed). Microbial community analysis revealed that Geobacter species known to proceed DIET were only detected in the ethanol-stimulated enrichments. Together with the significant increase of Methanosaeta and Methanosarcina species in these enrichments, the potential DIET between Geobacter and Methanosaeta or Methanosarcina species might be established to improve the syntrophic propionate and/or butyrate degradation. Further experiments demonstrated that granular activated carbon (GAC) could improve the syntrophic metabolism of propionate and/or butyrate of the ethanol-stimulated enrichments, while almost no effects on the traditional enrichments. Also, the high H2 partial pressure could inhibit the syntrophic propionate and/or butyrate degradation of the traditional enrichments, but its effect on that of the ethanol-stimulated enrichments was negligible.

Anaerobic granule-based biofilms formation reduces propionate accumulation under high H2 partial pressure using conductive carbon felt particles

Syngas based co-digestion is not only more economically attractive than separate syngas methanation but also able to upgrade biogas and increase overall CH4 amount simultaneously. However, high H2 concentration in the syngas could inhibit syntrophic degradation of propionate, resulting in propionate accumulation and even failure of the co-digestion system. In an attempt to reduce propionate accumulation via enhancing both H2 interspecies transfer (HIT) and direct interspecies electron transfer (DIET) pathways, layered granule-based biofilms induced by conductive carbon felt particles (CCFP) was employed. The results showed that propionate accumulation was effectively reduced with influent COD load up to 7gL−1d−1.Two types of granule-based biofilms, namely biofilm adhered to CCFP (B-CCFP) and granules formed by self-immobilization (B-SI) were formed in the reactor. Clostridium, Syntrophobacter, Methanospirillum were possibly involved in HIT and Clostridium, Geobacter, Anaerolineaceae, Methanosaeta in DIET, both of which might be responsible for the high-rate propionate degradation.

Hydrogen formation and its regulation in Ruminococcus albus: involvement of an electron-bifurcating [FeFe]-hydrogenase, of a non-electron-bifurcating [FeFe]-hydrogenase, and of a putative hydrogen-sensing [FeFe]-hydrogenase.

Ruminococcus albus 7 has played a key role in the development of the concept of interspecies hydrogen transfer. The rumen bacterium ferments glucose to 1.3 acetate, 0.7 ethanol, 2 CO2, and 2.6 H2 when growing in batch culture and to 2 acetate, 2 CO2, and 4 H2 when growing in continuous culture in syntrophic association with H2-consuming microorganisms that keep the H2 partial pressure low. The organism uses NAD(+) and ferredoxin for glucose oxidation to acetyl coenzyme A (acetyl-CoA) and CO2, NADH for the reduction of acetyl-CoA to ethanol, and NADH and reduced ferredoxin for the reduction of protons to H2. Of all the enzymes involved, only the enzyme catalyzing the formation of H2 from NADH remained unknown. Here, we report that R. albus 7 grown in batch culture on glucose contained, besides a ferredoxin-dependent [FeFe]-hydrogenase (HydA2), a ferredoxin- and NAD-dependent electron-bifurcating [FeFe]-hydrogenase (HydABC) that couples the endergonic formation of H2 from NADH to the exergonic formation of H2 from reduced ferredoxin. Interestingly, hydA2 is adjacent to the hydS gene, which is predicted to encode an [FeFe]-hydrogenase with a C-terminal PAS domain. We showed that hydS and hydA2 are part of a larger transcriptional unit also harboring putative genes for a bifunctional acetaldehyde/ethanol dehydrogenase (Aad), serine/threonine protein kinase, serine/threonine protein phosphatase, and a redox-sensing transcriptional repressor. Since HydA2 and Aad are required only when R. albus grows at high H2 partial pressures, HydS could be a H2-sensing [FeFe]-hydrogenase involved in the regulation of their biosynthesis.

(Invited) Ge1-xSnx Optical Devices: Growth and Applications

Ge has been considered as a promising candidate for the active region in optical interconnect applications such as lasers and photodetectors, since its band gap corresponds to a wavelength with less loss in SiO2 waveguides. However, to increase the efficiency of these applications, a direct band gap material is required.Incorporation of Sn into Ge has been considered to obtain a direct band gap material [1,2]. According to theoretical calculations [3], the critical Sn content to achieve the direct transition depends on the strain. When the GeSn layer is fully relaxed, around 11% of Sn is required. On the other hand, when the layer experiences compressive strain, the critical Sn content increases. One option to obtain highly strain-relaxed GeSn with high Sn content is to increase the thickness of the GeSn layer. In addition, the active region of the optical devices has to be thick to obtain a high efficiency. However, it is well known that Sn segregation to the surface easily occurs [4], which results in Sn precipitation/agglomeration on the surface when a thick layer is grown.The Sn agglomeration is the result of the collision among Sn atoms on the surface. Therefore, we focused on the freezing of Sn atoms during GeSn growth. GeSn layers were grown with Ge2H6 and SnCl4 precursors on Ge(001) substrates at 320°C by atmospheric CVD in an Epsilon like equipment from ASM. The target thickness of the GeSn layers is 200 nm. N2 and H2 with different flows ranging from 10 to 40 slm were used as carrier gases to study the impact of the partial pressure of the precursors on Sn agglomeration. It is expected that using low carrier gas flows enhances the Sn freezing due to the higher Ge2H6 and SnCl4 partial pressures. In addition, it is expected that switching the carrier gas from N2 to H2suppresses the Sn migration on the surface due to the resulting surface H-termination.We found opposite tendencies for the surface Sn agglomeration as a function of the H2 and N2 carrier gas flow. A lower N2 suppresses the surface Sn agglomeration. As we expected, a high partial pressure of the precursors enhances the freezing of Sn atoms into the subsurface. On the other hand, when H2 is used as a carrier gas, a higher H2 flow suppresses the surface Sn agglomeration. The growth temperature (320°C) is close to the temperature at which adsorbed H starts to desorb thermally from the Ge surface (340°C) [5, 6]. However, a high H2 partial pressure may suppress the desorption of surface H which is provided by the deposition by Ge2H6. The Sn segregation should be suppressed by the surfactant effect of surface H [6]. The different behavior of these two carrier gases will be discussed in detail.[1] D. W. Jenkins and J. D. Dow, Phys. Rev. B, 36, 7994 (1987).[2] M. R. Bauer, et al., Solid State Commun. 127, 355 (2003).[3] Y. Shimura, et al., J. Appl. Phys. submitted.[4] E. Kasper, et al., Thin Solid Films, 520, 3195 (2012).[5] L. Surnev and M. Tikhov, Surf. Sci. 138, 40 (1984).[6] A. Sakai and T. Tatsumi, Appl. Phys. Lett. 64, 52 (1994).

Catalyst deactivation in on-board H2 production by fuel dehydrogenation

The production of H2 for on-board application is a very interesting challenge for industrial and academic researchers. The aim is the application of on-board hydrogen production on the airplanes using kerosene as H2 source. In this work an in depth study into the partial dehydrogenation (PDH) of two hydrocarbons blends and desulfurized JetA1 fuel has been performed by using 1 wt.%Pt–1 wt.%Sn/γ-Al2O3 and 1 wt.%Pt–1 wt.%Sn–0.5%K/γ-Al2O3 to find a way to produce H2 “on-board” for the feeding of the fuel-cell apparatus. The mechanism of deactivation by coke was studied in depth combining Raman spectroscopy and Temperature-programmed oxidation (TPO) analyses. Microstructure analysis of metallic particles in fresh and deactivated catalysts was investigated by HRTEM. Relatively high H2 partial pressure increases catalyst life by controlling full dehydrogenation coke-forming reaction. By feeding model organic molecules, it was possible to identify the contribution of each class of compounds to the H2 production as well as the amount and type of coke formed. A relatively complex reaction pathway, which is able to evidence the role of different sites and reactions involved in PDH processes, was proposed.

Pressuring CO2 to React

Chemistry One sustainable approach for converting CO2 from large-scale industrial production processes into chemicals and fuels is by reacting it with hydrogen generated from solar or wind energy. The catalytic hydrogenation of CO2 into methanol offers a potential route for conversion, but most commercial catalysts, which use copper and zinc oxide supported on alumina and operate at pressures of 50 to 100 atm, have low single-pass conversions and require product separation and reactant recycling. Bansode and Urakawa, noting that high pressures thermodynamically favor the methanol product, studied this reaction on such catalysts at higher pressures. They could boost single-pass conversions to >95% at 355 atm and 260°C and at high H2 partial pressures (H2/CO2 ratios > 10:1). These conditions should suppress formation of CO as a final product, but the increasing conversion for methanol as temperature was increased to 260°C suggests that the reaction proceeds initially through reduction to CO. This reaction is endothermic and competes with exothermic methanol synthesis. They also show that addition of a solid acid catalyst, H-ZSM-5, as a physical mixture led to the single-pass production of dimethyl ether with selectivities of >80%; when this catalyst was added in a second series reactor, light alkanes or propylene could be produced as well. J. Catal. 309 , 66 (2014).

Growth and electrical properties of Al-catalyzed Si nanowires

We report the growth and electrical characterization of small-diameter, Al-catalyzed silicon nanowires. Large quantities of Si nanowires can be grown with a fast growth rate at a low temperature of 520 °C via a vapor-liquid-solid process using Al as catalyst. The use of high H2 partial pressure was found to effectively mitigate the Al catalyst oxidation problem, and higher SiH4 partial pressure was found to produce smaller diameter nanowires. Electrical measurements show that small diameter (d<∼25 nm) Al-catalyzed nanowires behave as p-type semiconductors with good gate response, while larger diameter Al-catalyzed nanowires behave as degenerately Al-doped semiconductors.

Deposition of controllable preferred orientation silicon films on glass by inductively coupled plasma chemical vapor deposition

An inductively coupled plasma (ICP) system with the adjustable distance between the inductance coil and substrates was designed to effectively utilize the spatial confinement of ICP discharge, and then control the gas-phase transport process. The effects of the gas phase processes on the crystallinity and preferred orientation of silicon films deposited on glass were systematically investigated. The investigation was conducted in the ICP-chemical vapor deposition process with the precursor gas of a SiH4∕H2 mixture at a substrate temperature of 350°C. Highly crystallized silicon films with different preferred orientations, (111) or (220), could be selectively deposited by adjusting the SiH4 dilution ratio [R=[SiH4]∕([SiH4]+[H2])] or total working pressure. When the total working pressure is 20Pa, the crystallinity of the silicon films increases with the increase of the SiH4 dilution ratio, while the preferred orientation was changed from (111) to (220). In the case of the fixed SiH4 dilution (10%), the silicon film with I(220)∕I(111) of about 3.5 and Raman crystalline fraction of about 89.6% has been deposited at 29.7nm∕min when the total working pressure was increased to 40Pa. At the fixed SiH4 partial pressure of 2Pa, the film crystallinity decreases and the preferred orientation is always (111) with increasing the H2 partial pressure from 18to58Pa. Atomic force microscope reveals that the film deposited at a relatively high H2 partial pressure has a very rough surface caused by the devastating etching of H atoms to the silicon network.

Density Functional Theory and Kinetic Studies of Methanation on Iron Surface

The production of methane from CO and H2 on a clean Fe(100) surface was studied using periodic DFT calculations in conjunction with kinetic modeling. It was found that the adsorption and subsequent dissociation of CO and H2 at a finite temperature are significantly influenced by the presence of coadsorbed CO and H. The surface coverage of CO is controlled by entropy, and a maximum of 50% coverage can be attained. The kinetic modeling adopting the computed activation barriers for the elementary steps involved in methanation revealed that CH is a major C1 species on the surface. The production of CH4 was found to be more favored at a high reaction temperature and H2 partial pressure but suppressed by a high pressure of CO.

Kinetics of Aqueous-Phase Reforming of Oxygenated Hydrocarbons: Pt/Al2O3 and Sn-Modified Ni Catalysts

Reaction kinetics studies were conducted on the aqueous-phase reforming of ethylene glycol to produce hydrogen at temperatures near 500 K over Pt/Al2O3 and Raney NiSn catalysts. Ethylene glycol reforming proceeds through similar mechanisms over Pt and NiSn catalysts, involving initial dehydrogenation of ethylene glycol, followed by C−C bond cleavage and water−gas shift. The initial dehydrogenation of ethylene glycol appears to be kinetically significant over Pt/Al2O3, whereas the subsequent rate of C−C cleavage appears to be kinetically significant over R−Ni14Sn. The reforming reaction is fractional order in the ethylene glycol concentration, because of the strong adsorption of the oxygenated reactant, and negative order in the system pressure, through product inhibition by adsorption of H2 and/or CO at high pressures. High selectivity for hydrogen production is achieved for gas-phase products over Pt/Al2O3, whereas the addition of Sn is necessary to avoid alkane formation by methanation over Ni-based catalysts. The rate of methane formation increases at high system pressures, suggesting that the R−Ni14Sn catalyst is still vulnerable to methanation at high H2 and CO2 partial pressures. The reforming of ethylene glycol is accompanied by significant production of acetic acid through bifunctional dehydrogenation/isomerization and dehydration/hydrogenation routes over the metal and support. These bifunctional routes can be used to produce long-chain alkanes or partially reduced chemical intermediates by appropriate use of catalyst−support combinations, catalyst modifiers, and process conditions.

Nanostructure and infrared photoluminescence of nanocrystalline Ge formed by reduction of Si0.75Ge0.25O2/Si0.75Ge0.25 using various H2 pressures

Nanocrystalline Ge in SiO2 was synthesized by the reduction of Si0.75Ge0.25O2 with H2, at various annealing temperatures (ranging from 700 to 900 °C), with various H2 partial pressures (100% N2, 6% H2:94% N2, and 100% H2), and for a range of times. Cross-sectional transmission electron microscopy (XTEM) reveals a strong dependence of nanocrystal distribution and structure on annealing temperature and H2 partial pressure. The oxide/semiconductor interface served as a seed layer for the growth of large Ge crystallites, whereas nanocrystals formed in the bulk of the oxide for high H2 partial pressures or low temperatures. The mechanisms involved in nanocrystal formation are discussed in the context of XTEM and Raman spectroscopy results. Select nanocrystalline Ge samples were characterized optically with infrared photoluminescence (PL) measurements at 4 K, using a 488 nm Ar laser excitation. Nanocrystalline Ge samples with small nanocrystals (<15 nm) exhibited a broad infrared PL background ranging from about 1.0–1.7 μm. The broad background is attributed to quantum confinement in the Ge nanocrystals, consistent with a wide distribution of nanocrystal sizes and hence a broad range of confinement energies.

Hydrogen profiles and localization of methanogenic activities in the highly compartmentalized hindgut of soil-feeding higher termites (Cubitermes spp.).

It has been shown that the coexistence of methanogenesis and reductive acetogenesis in the hindgut of the wood-feeding termite Reticulitermes flavipes is based largely on the radial distribution of the respective microbial populations and relatively high hydrogen partial pressures in the gut lumen. Using Clark-type microelectrodes, we showed that the situation in Cubitermes orthognathus and other soil-feeding members of the subfamily Termitinae is different and much more complex. All major compartments of agarose-embedded hindguts were anoxic at the gut center, and high H(2) partial pressures (1 to 10 kPa) in the alkaline anterior region rendered the mixed segment and the third proctodeal segment (P3) significant sources of H(2). Posterior to the P3 segment, however, H(2) concentrations were generally below the detection limit (<100 Pa). All hindgut compartments turned into efficient hydrogen sinks when external H(2) was supplied, but methane was formed mainly in the P3/4a and P4b compartments, and in the latter only when H(2) or formate was added. Addition of H(2) to the gas headspace stimulated CH(4) emission of living termites, indicating that endogenous H(2) production limits methanogenesis also in vivo. At the low H(2) partial pressures in the posterior hindgut, methanogens would most likely outcompete homoacetogens for this electron donor. This might explain the apparent predominance of methanogenesis over reductive acetogenesis in the hindgut of soil-feeding termites, although the presence of homoacetogens in the anterior, highly alkaline region cannot yet be excluded. In addition, the direct contact of anterior and posterior hindgut compartments in situ permits a cross-epithelial transfer of H(2) or formate, which would not only fuel methanogenesis in these compartments, but would also create favorable microniches for reductive acetogenesis. In situ rates and spatial distribution of H(2)-dependent acetogenic activities are addressed in a companion paper (A. Tholen and A. Brune, Appl. Environ. Microbiol. 65:4497-4505, 1999).

Enrichment of High-Rate PCE Dechlorination and Comparative Study of Lactate, Methanol, and Hydrogen as Electron Donors To Sustain Activity

The influence of three electron donors on the ability to sustain rapid tetrachloroethene (PCE) dechlorination in mixed cultures was investigated. Experiments were performed in recycle columns containing either a high-rate PCE dechlorinating culture developed from granular sludge or a (1:1) mixture of this culture with an enrichment culture derived from soil cores taken from a chlorinated ethene contaminated site. Columns were fed PCE (aqueous concentration of 5 mg/L) and either lactate, methanol, or H2 as their electron donor. Comparisons between electron donors were formulated using the observed rates of PCE disappearance and the extent of dechlorination achieved in each column. Over a period exceeding 1 year, the observed PCE half-life in each column was found to decrease from approximately 13 h to slightly less than 2 h, regardless of electron donor fed. Although initial mass balance studies demonstrated that the extent of dechlorination varied between inoculum and electron donor, over time, the recycle columns behaved similarly, and PCE was recovered as approximately 80% vinyl chloride and 20% ethene in all systems. Rates and extents of PCE dechlorination were improved over time in systems containing high H2 partial pressures (0.8 atm) in the presence of an active methanogenic community.