Dissolved H2 Concentrations Research Articles

Control of dissolved H2 concentration enhances electron generation, transport and TCE reduction by indigenous microbial community

Electrokinetic enhanced bioremediation (EK-Bio) is practical for trichloroethene (TCE) dechlorination because the cathode can produce a wide range of dissolved H2 (DH) concentrations of 1.3–0 mg/L from the electrode to the aquifer. In this study, TCE dechlorination was investigated under different DH concentrations. The mechanisms were discussed by analyzing the microbial community structure and abundance of organohalide-respiring bacteria (OHRB) using 16S rRNA, and the gene abundances of key enzymes in the TCE electron transport chain using metagenomic analysis. The results showed that the moderate DH concentration of 0.19–0.53 mg/L exhibited the most pronounced TCE dechlorination, even better than the higher DH concentrations, due to the optimal redox environment, the enrichments of OHRB, reductive dehalogenase (rdhA) genes and key enzyme genes in the electron generation and transport chain. More electrons were obtained from H2 metabolism by Dehalobacter by promoting the formation of [NiFe] hydrogenase (HupS/L/C) or from glycolysis by versatile OHRB by stimulating the formation of formate and enriching formate dehydrogenase (FDH) under moderate DH conditions. In addition, the enhanced amino acid metabolism improved the vitamin K cycle for electron transport and enriched the reductive dechlorinating enzyme (RDase) genes. This study identifies the optimal DH concentration that facilitates bioremediation efficiency, provides insights into microbial community shifts and key enzymatic pathways in EK-Bio remediation.

Effect of cathode ink formulation on the hydrogen crossover and cell performance of proton exchange membrane water electrolyzers

The permeation of H2 through the membranes of proton exchange membrane water electrolyzers (PEMWEs) is a critical safety concern because of the risk of explosion when H2 mixes with O2 at the anode and increases in concentration. In this study, we investigated the modification of the cathode catalyst layer in the membrane electrode assembly as a strategy for achieving the safe operation of PEMWEs. The effects of the polytetrafluoroethylene (PTFE) content and type of ionomer in the cathode catalyst layer on the dissolved H2 concentration, H2 crossover, and electrochemical performance were investigated. The lowest dissolved H2 concentration and H2 permeation rate were achieved when 8 wt% PTFE was used. Consequently, the H2 volume fraction in O2 at the anode was less than 0.88 %. Additionally, using the Nafion ionomer (D520, ion exchange capacity: 1 mmol g−1), H2 volume fractions of 1.27 % and 1.34 % were obtained at 0.08 and 5 A cm−2, respectively. These values are below the lower explosion limit of H2 in O2 (4 %), implying that the PEMWE can be safely operated in the low-to-high current density range under ambient pressure. These results provide key guidelines for the design of high-safety cathode catalyst layers for PEMWEs.

Considerations on the use of microsensors to profile dissolved H2 concentrations in microbial electrochemical reactors.

Measuring the distribution and dynamics of H2 in microbial electrochemical reactors is valuable to gain insights into the processes behind novel bioelectrochemical technologies, such as microbial electrosynthesis. Here, a microsensor method to measure and profile dissolved H2 concentrations in standard H-cell reactors is described. Graphite cathodes were oriented horizontally to enable the use of a motorized microprofiling system and a stereomicroscope was used to place the H2 microsensor precisely on the cathode surface. Profiling was performed towards the gas-liquid interface, while preserving the electric connections and flushing the headspace (to maintain anoxic conditions) and under strict temperature control (to overcome the temperature sensitivity of the microsensors). This method was tested by profiling six reactors, with and without inoculation of the acetogen Sporomusa ovata, at three different time points. H2 accumulated over time in the abiotic controls, while S. ovata maintained low H2 concentrations throughout the liquid phase (< 4 μM) during the whole experimental period. These results demonstrate that this setup generated insightful H2 profiles. However, various limitations of this microsensor method were identified, as headspace flushing lowered the dissolved H2 concentrations over time. Moreover, microsensors can likely not accurately measure H2 in the immediate vicinity of the solid cathode, because the solids cathode surface obstructs H2 diffusion into the microsensor. Finally, the reactors had to be discarded after microsensor profiling. Interested users should bear these considerations in mind when applying microsensors to characterize microbial electrochemical reactors.

Microbial taxa related to natural hydrogen and methane emissions in serpentinite-hosted hyperalkaline springs of New Caledonia.

The southeastern part of New Caledonia main island (Grande Terre) is the location of a large ophiolitic formation that hosts several hyperalkaline springs discharging high pH (∼11) and warm (<40°C) fluids enriched in methane (CH4) and hydrogen (H2). These waters are produced by the serpentinization of the ultrabasic rock formations. Molecular surveys had previously revealed the prokaryotic diversity of some of these New Caledonian springs, especially from the submarine chimneys of Prony Bay hydrothermal field. Here we investigate the microbial community of hyperalkaline waters from on-land springs and their relationships with elevated concentrations of dissolved H2 (21.1-721.3 μmol/L) and CH4 (153.0-376.6 μmol/L). 16S rRNA gene analyses (metabarcoding and qPCR) provided evidence of abundant and diverse prokaryotic communities inhabiting hyperalkaline fluids at all the collected springs. The abundance of prokaryotes was positively correlated to the H2/CH4 ratio. Prokaryotes consisted mainly of bacteria that use H2 as an energy source, such as microaerophilic Hydrogenophaga/Serpentinimonas (detected in all sources on land) or anaerobic sulfate-reducing Desulfonatronum, which were exclusively found in the most reducing (Eh ref H2 ∼ -700 mV) and the most H2-enriched waters discharging at the intertidal spring of the Bain des Japonais. The relative abundance of a specific group of uncultured Methanosarcinales that thrive in serpentinization-driven ecosystems emitting H2, considered potential H2-consuming methanogens, was positively correlated with CH4 concentrations, and negatively correlated to the relative abundance of methylotrophic Gammaproteobacteria. Firmicutes were also numerous in hyperalkaline waters, and their relative abundance (e.g., Gracilibacter or Dethiobacter) was proportional to the dissolved H2 concentrations, but their role in the H2 budget remains to be assessed. The prokaryotic communities thriving in New Caledonia hyperalkaline waters are similar to those found in other serpentinite-hosted high-pH waters worldwide, such as Lost City (North Atlantic) and The Cedars (California).

The key factor in High Pressure Hydrogenotrophic Denitrification: Hydrogen partial pressure

In the envisaged hydrogen economy, H2 could be an interesting alternative electron donor for the denitrification of drinking water or wastewater. The main obstacle to engineering the hydrogenotrophic denitrification process is the low solubility of H2 in water under atmospheric pressure, which limits denitrification rate and nitrogen removal efficiency. In this paper, we demonstrated a novel configuration of hydrogenotrophic denitrification, namely High Pressure Hydrogenotrophic Denitrification (HPHD). Elevated H2 partial pressure (pH2) was employed to increase dissolved H2 concentration and concomitantly enhance denitrification rate. Our results showed that the specific denitrification rate increased from 9.6 mg N/(gVSS·h) at 0.5 bars to 51.0 mg N/(gVSS·h) at 9 bars in HPHD. The denitrification effect could be retained with elevated pH2 at a low temperature. The specific denitrification rate at 3 bars and 15 °C was 20.5 mg N/(gVSS·h), approximately 1.5 times that at 1 bar and 30 °C, which was quite beneficial for hydrogenotrophic denitrification under cold conditions. Different from nitrite reduction, less impact was observed on nitrate reduction by low temperature, which explained high nitrite accumulation in HPHD at 15 °C. Overall, our investigations shed light on the role of pH2 in the promising solution for nitrogen removal in HPHD.

Combined carbon, hydrogen, and clumped isotope fractionations reveal differential reversibility of hydrogenotrophic methanogenesis in laboratory cultures

Stable isotope analysis has been widely used to aid the source identification of methane. However, the isotopic (13C/12C and D/H) and isotopologue (13CH3D and 12CH2D2) signatures of microbial methane in natural environments are often different from those in laboratory cultures in which methanogens are typically grown under optimal conditions. Growth phase and hydrogen (H2) concentration have been proposed as factors controlling the isotopic compositions of methane produced via hydrogenotrophic methanogenesis, but their effects on the relationship among carbon, hydrogen and doubly-substituted “clumped” isotopologue systems have not been assessed in a quantitative framework. Here we experimentally investigate the bulk (δ13C and δD) and clumped (Δ13CH3D) isotopologue compositions of methane produced by hyperthermophilic hydrogenotrophic (CO2-reducing) methanogens using batch and fed-batch systems at different growth phases and H2 mixing ratios (Methanocaldococcus bathoardescens at 82 or 60 °C and on 80 or 25 % H2; Methanothermobacter thermautotrophicus str. ΔH at 65 °C and on 20, 5 or 1.6 % H2). We observed a large range (from 18 to 63 ‰) of carbon isotope fractionations, with larger values observed during later growth phase, consistent with previous observations. In contrast, hydrogen isotope fractionations remained relatively constant at –317 ± 25 ‰. Linear growth was observed for experiments with M. bathoardescens, suggesting that dissolution of gaseous H2 into liquid media became the rate limit as cell density increased. Accordingly, the low (and undersaturated) dissolved H2 concentrations can explain the increased carbon isotope fractionations during the later growth phase. The δD and Δ13CH3D values indicated departure from equilibrium throughout experiments. As the cell density increased and dissolved H2 decreased, Δ13CH3D decreased (further departure from equilibrium), contrary to expectations from previous models. Our isotopologue flow network model reproduced the observed trends when the last H-addition step is less reversible relative to the first three H-addition steps (up to CH3-CoM). In this differential reversibility model, carbon, hydrogen and clumped isotopologue fractionations are largely controlled by the reversibility of the first three H-addition steps under high H2 concentrations; the last H-addition step becomes important under low H2. The magnitude of depletion and decreasing trend in Δ13CH3D values were reproduced when a large (≥6 ‰) secondary clumped kinetic isotope effect was considered in the model. This study highlights the advantage of combined bulk and clumped isotope analyses and the importance of physiological factors (growth phase) and energy availability (dissolved H2 concentration) when using isotope analyses to better understand methanogenic metabolisms and methane cycling processes.

Investigating Abiotic and Biotic Mechanisms of Pyrite Reduction.

Pyrite (FeS2) has a very low solubility and therefore has historically been considered a sink for iron (Fe) and sulfur (S) and unavailable to biology in the absence of oxygen and oxidative weathering. Anaerobic methanogens were recently shown to reduce FeS2 and assimilate Fe and S reduction products to meet nutrient demands. However, the mechanism of FeS2 mineral reduction and the forms of Fe and S assimilated by methanogens remained unclear. Thermodynamic calculations described herein indicate that H2 at aqueous concentrations as low as 10–10 M favors the reduction of FeS2, with sulfide (HS–) and pyrrhotite (Fe1–xS) as products; abiotic laboratory experiments confirmed the reduction of FeS2 with dissolved H2 concentrations greater than 1.98 × 10–4 M H2. Growth studies of Methanosarcina barkeri provided with FeS2 as the sole source of Fe and S resulted in H2 production but at concentrations too low to drive abiotic FeS2 reduction, based on abiotic laboratory experimental data. A strain of M. barkeri with deletions in all [NiFe]-hydrogenases maintained the ability to reduce FeS2 during growth, providing further evidence that extracellular electron transport (EET) to FeS2 does not involve H2 or [NiFe]-hydrogenases. Physical contact between cells and FeS2 was required for mineral reduction but was not required to obtain Fe and S from dissolution products. The addition of a synthetic electron shuttle, anthraquinone-2,6-disulfonate, allowed for biological reduction of FeS2 when physical contact between cells and FeS2 was prohibited, indicating that exogenous electron shuttles can mediate FeS2 reduction. Transcriptomics experiments revealed upregulation of several cytoplasmic oxidoreductases during growth of M. barkeri on FeS2, which may indicate involvement in provisioning low potential electrons for EET to FeS2. Collectively, the data presented herein indicate that reduction of insoluble FeS2 by M. barkeri occurred via electron transfer from the cell surface to the mineral surface resulting in the generation of soluble HS– and mineral-associated Fe1–xS. Solubilized Fe(II), but not HS–, from mineral-associated Fe1–xS reacts with aqueous HS– yielding aqueous iron sulfur clusters (FeSaq) that likely serve as the Fe and S source for methanogen growth and activity. FeSaq nucleation and subsequent precipitation on the surface of cells may result in accelerated EET to FeS2, resulting in positive feedback between cell activity and FeS2 reduction.

Lithogenic hydrogen supports microbial primary production in subglacial and proglacial environments

Life in environments devoid of photosynthesis, such as on early Earth or in contemporary dark subsurface ecosystems, is supported by chemical energy. How, when, and where chemical nutrients released from the geosphere fuel chemosynthetic biospheres is fundamental to understanding the distribution and diversity of life, both today and in the geologic past. Hydrogen (H2) is a potent reductant that can be generated when water interacts with reactive components of mineral surfaces such as silicate radicals and ferrous iron. Such reactive mineral surfaces are continually generated by physical comminution of bedrock by glaciers. Here, we show that dissolved H2 concentrations in meltwaters from an iron and silicate mineral-rich basaltic glacial catchment were an order of magnitude higher than those from a carbonate-dominated catchment. Consistent with higher H2 abundance, sediment microbial communities from the basaltic catchment exhibited significantly shorter lag times and faster rates of net H2 oxidation and dark carbon dioxide (CO2) fixation than those from the carbonate catchment, indicating adaptation to use H2 as a reductant in basaltic catchments. An enrichment culture of basaltic sediments provided with H2, CO2, and ferric iron produced a chemolithoautotrophic population related to Rhodoferax ferrireducens with a metabolism previously thought to be restricted to (hyper)thermophiles and acidophiles. These findings point to the importance of physical and chemical weathering processes in generating nutrients that support chemosynthetic primary production. Furthermore, they show that differences in bedrock mineral composition can influence the supplies of nutrients like H2 and, in turn, the diversity, abundance, and activity of microbial inhabitants.

Jet-nozzle based improvement of dissolved H2 concentration for efficient in-situ biogas upgrading in an up-flow anaerobic sludge blanket (UASB) reactor

Improvement of dissolved concentration and mass transfer coefficient (KLa) of H2 is a main strategy for efficient biogas upgrading process. In this study, Jet nozzle reactor (JNR) was applied to improve dissolved H2 (DH2) concentration and KLa for successful in-situ biogas upgrading in an up-flow anaerobic sludge blanket (UASB) reactor. JNR could improve the DH2 concentration and KLa to 1.71 and 1.72 times higher than those of control reactor, respectively. Increased DH2 concentration and KLa contributed to increase CH4 content to 96.1% in UASB combined with JNR (JN-UASB). JN-UASB achieved maximum H2 conversion efficiency of 97.9% and minimum H2 content of 3.9% in biogas. Community of hydrogenotrophic methanogens and efficiency of H2 conversion in JN-UASB were 1.1 and 1.3 times higher than those in control UASB (C-UASB), respectively. This study confirmed that DH2 concentration and KLa increased by JNR affected the improvement of H2 methanation efficiency via rapid hydrogenotrophic methanogenesis. The findings of this study provide technical strategy and further research direction to upgrade biogas.

3-Nitrooxypropanol supplementation had little effect on fiber degradation and microbial colonization of forage particles when evaluated using the in situ ruminal incubation technique

3-Nitrooxypropanol (3-NOP) is an investigational compound that acts as an enzyme inhibitor to decrease ruminal methanogenesis. We hypothesized that when feeding 3-NOP to cattle fed a high-forage diet, H2 would accumulate in the rumen, which could suppress microbial colonization of feed particles and fiber degradation. Therefore, the study investigated the effects of supplementing a high-forage diet with 3-NOP on ruminal fiber degradability and microbial colonization of feed particles using the in situ technique. Eight ruminally cannulated beef cattle were allocated to 2 groups (4 cattle/group) in a crossover design with 2 periods and 2 dietary treatments. The treatments were control (basal diet) and 3-NOP (basal diet supplemented with 3-NOP, 150 mg/kg of dry matter). The basal diet consisted of 45% barley silage, 45% chopped grass hay, and 10% concentrate (dry matter basis). Samples of dried, ground barley silage and grass hay were incubated in the rumen of each animal for 0, 4, 12, 24, 36, 48, 96, 120, 216, and 288 h to determine neutral detergent fiber (NDF) degradation kinetics. An additional 2 bags were incubated for 4 and 48 h to evaluate the bacterial community attached to the incubated forages. Dietary supplementation of 3-NOP decreased (-53%) the dissolved methane concentration and increased (+780%) the dissolved H2 concentration in ruminal fluid, but did not substantially alter in situ NDF degradation. The addition of 3-NOP resulted in a decrease in the α-diversity of the microbial community with colonizing communities showing reduced numbers of amplicon sequence variants and phylogenetic diversity compared with control diets. Principal coordinate analysis plots indicated that forages incubated in animals fed 3-NOP resulted in highly specific changes to targeted microbes compared with control diets based on unweighted analysis (considering only absence and presence of taxa), but did not alter the overall composition of the colonizing community based on weighted UniFrac distances; unchanged relative abundances of major taxa included phyla Bacteroidetes, Firmicutes, and Fibrobacteres. The effect of 3-NOP on colonizing methanogenic microbes differed depending upon the forage incubated, as abundance of genus Methanobrevibacter was decreased for barley silage but not for grass hay. In conclusion, 3-NOP supplementation of a high-forage diet decreased ruminal methanogenesis and increased dissolved H2 concentration, but had no negative effects on ruminal fiber degradation and only minor effects on relative abundances of the major taxa of bacteria adhered to forage substrates incubated in the rumen.

Enhancement of mass transfer conditions to increase the productivity and efficiency of dark fermentation in continuous reactors

Hydrogen (H2) produced by dark fermentation is an alternative to fulfill the requirements of the transportation sector and to be a complementary source in the forthcoming electricity grid. However, the dark fermentative H2 production is limited by the accumulation of H2 in the fermentation broth. In continuous stirred-tank reactors (CSTR), such phenomenon is associated with poor mass transfer conditions. Nevertheless, this parameter has been scarcely considered to enhance H2 production. In this research, the effect of the H2 mass transfer conditions on the productivity and efficiency of continuous H2 production was evaluated using a series of CSTR operated at H2 mass transfer coefficients (kLa) ranging from 1.04 to 4.23 1/h. The results showed that volumetric H2 production rate (VHPR) and H2 yield increased 74 and 78%, respectively, due to enhanced mass transfer conditions. This behavior was driven by 53% decrease of the dissolved H2 concentration. More specifically, the maximum VHPR of 7.66 L/L-d with a H2 yield of 1.1 mol H2/mol hexose was obtained at a kLa = 4.23 1/h. Furthermore, 16S-DGGE analysis and sequencing revealed that Clostridium and Lactobacillus were the dominant bacterial genera in continuous operation. In particular, Clostridium increased its occurrence at kLa of 2.72–4.23 1/h as a response to lower dissolved H2 concentrations. The novelty of this work relies on the demonstration that mass transfer conditions control not only the H2 accumulation and reactor performance (VHPR and H2 yield), but they also influence the metabolic pathways and the composition of the microbial community.

High-Sensitivity Fiber-Optic Sensor for Hydrogen Detection in Gas and Transformer Oil

This paper reports on the characterization of a palladium (Pd)-based fiber-optic hydrogen (H2) sensor for health monitoring of distribution and power transformers in the electrical grid. The sensor consists of a Pd foil, which expands due to H2 absorption, and a fiber Bragg grating that measures this expansion. Fifteen sensors were manufactured and characterized in gas and oil environments at various H2 concentrations and temperatures. In gas, the sensors were evaluated at 60 °C, 75 °C, 90 °C, 105 °C, and 120 °C and H2 concentrations from 0.01% to 5%. In oil, the same sensors were evaluated at 90 °C and dissolved H2 concentrations from 5 to 2700 ppm. Furthermore, the influence of carbon monoxide (CO), which is often present in transformers and can impact H2 response of the sensor, was investigated. At 90 °C in gas, the response to 0.01% and 5% H2 was on average 28 and 719 pm, respectively. At 90 °C in oil, the response to 5 and 2700 ppm dissolved H2 was on average 11 and 763 pm, respectively. The average, relative accuracy is better than 20% over the whole measurement range in gas and for measurements above 100 ppm in oil. These specifications make the sensor a promising candidate for online monitoring of H2 in transformers.

Hydrogen-based syntrophy in an electrically conductive biofilm anode

We experimentally and theoretically investigated implications of H2 and a rate-limiting step in a mixed-culture biofilm anode fed with n-butyrate, one of the poorest substrates to exoelectrogens. Acetate and i-butyrate were formed as intermediates during anaerobic degradation of n-butyrate, which suggested oxidative acetogenesis of n-butyrate in syntrophy with H2 scavengers in the biofilm anode. Methane was not detected in an anode chamber, and no current was generated in the biofilm anode using H2 as the electron donor. These results indicated that acetogens would be a main H2 consumer in the biofilm. Pyrosequencing data showed dominance of Geobacter in the biofilm anode (83.6% of total sequences), along with Sphaerochaeta and Treponema, which supports the syntrophy between exoelectrogens and acetogens. Electrical conductivity of the butyrate-fed biofilm anode was as high as 0.67 mS/cm, demonstrating that EET does not limit current density in the biofilm. In-situ monitoring of dissolved H2 concentration proved H2 production (up to 12.4 µM) and consumption during current generation in the biofilm, which supports significance of H2–based syntrophy in the electrically conductive biofilm using n-butyrate as the primary electron donor.

Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming

Protected from harmful radiation, subfreezing temperatures, and low pressures, subsurface rock-hosted habitats provide potentially sustainable refugia for microbial ecosystems inside small rocky planets, such as Mars. For many chemolithotrophic communities on Earth, water–rock alteration reactions have been shown to produce the key electron donors and acceptors necessary to sustain microbial life on geologic timescales. Here we quantitatively demonstrate that radiolysis likely generated concentrations of dissolved H2 capable of sustaining microbial communities in the subsurface of Noachian Mars (3.7–4.1 Gyr ago). When considering an environment with H2O groundwater, dissolved H2 concentrations reach up to ∼55 mM in a cold early Mars climate scenario and ∼35 mM in a warm early Mars climate scenario; whereas when considering an environment with eutectic NaCl brine groundwater, dissolved H2 concentrations reach up to ∼85 mM in a cold early Mars climate scenario and ∼45 mM in a warm early Mars climate scenario. Specifically within the subsurface habitable zone, dissolved H2 concentrations range from ∼50–55 mM for a cold climate scenario with H2O groundwater. For a warm climate scenario with H2O groundwater, dissolved H2 concentrations within the subsurface habitable zone range from ∼1–30 mM. For a cold climate scenario with eutectic NaCl brine groundwater, dissolved H2 concentrations within the subsurface habitable zone range from ∼65–85 mM. For a warm climate scenario with eutectic NaCl brine groundwater, dissolved H2 concentrations within the subsurface habitable zone range from ∼1-40 mM. Radiolysis likely produced [1.3–4.8] × 1010 moles H2 per year globally during the Noachian depending on the assumed porosity and groundwater composition. Radiolytic H2, and CH4 derived from radiolytic H2, can be locked in hybrid clathrate hydrates within the cryosphere and released by large impacts, volcanism, or obliquity variations. This process could warm the Noachian climate to above-freezing temperatures, and we predict that ∼1–8 warming events would be possible during the Noachian and Hesperian solely from radiolytically produced H2. We demonstrate that the region immediately beneath the cryosphere, termed the subcryospheric highly-fractured zone (SHZ), likely contained dissolved H2 concentrations and temperatures suitable for life regardless of the background climate scenario, making it the most consistently habitable environment on ancient Mars in terms of reductant availability. Material from this zone can be exposed by faulting and in the ejecta and uplifts of impacts, making the SHZ a crucial astrobiological target for testing the subsurface biosphere hypothesis.

Effects of low-strength ultrasonication on dark fermentative hydrogen production: Start-up performance and microbial community analysis

Our previous studies have demonstrated the positive impacts of low-strength (LS)-ultrasonication on biological methane production via anaerobic digestion. In this study, the effects of LS-ultrasonication and effluent recirculation on dark fermentative hydrogen production using upflow anaerobic sludge blanket reactor were investigated. Significant increase in hydrogen (H2) yield (up to 0.89 mol H2/mol hexose) was observed under LS-ultrasonication conditions. This enhanced H2 yield seems to be associated with the decrease in dissolved H2 concentration via a degassing effect, as evident from increased H2 content (up to 20%). Effluent recirculation alone led to positive effects. However, applying both LS-ultrasonication and effluent recirculation decreased H2 yield compared to the trial with only LS-ultrasonication (0.89–0.72 mol H2/mol hexose), possibly due to the cumulative shear stress and sludge bed expansion above the ultrasonication zone. Microbial community analysis revealed that the presence/absence of LS-ultrasonication and/or effluent recirculation significantly affected the microbial communities. Ethanoligenes cloacae, Aeromonas hydrophila and Clostridium pasteurianum were identified as principal H2 producers under the LS-ultrasonic conditions, while C. butyricum, the most commonly reported H2 producer, was more abundant in the controls. The energy balance of the LS-ultrasonication system was evaluated and future research directions were suggested.

Anaerobic biodegradation of dissolved ethanol in a pilot-scale sand aquifer: Variability in plume (redox) biogeochemistry

The use of ethanol in alternative fuels has led to contamination of groundwater with high concentrations of this easily biodegradable organic compound. Previous laboratory and field studies have shown vigorous biodegradation of ethanol plumes, with prevalence of reducing conditions and methanogenesis. The objective of this study was to further our understanding of the dynamic biogeochemistry processes, especially dissolved gas production, that may occur in developing and aging plume cores at sites with ethanol or other organic contamination of groundwater. The experiment performed involved highly-detailed spatial and temporal monitoring of ethanol biodegradation in a 2-dimensional (175cm high×525cm long) sand aquifer tank for 330days, with a vertical shift in plume position and increased nutrient inputs occurring at ~Day 100. Rapid onset of fermentation, denitrification, sulphate-reduction and iron(III)-reduction occurred following dissolved ethanol addition, with the eventual widespread development of methanogenesis. The detailed observations also demonstrate a redox zonation that supports the plume fringe concept, secondary reactions resulting from a changing/moving plume, and time lags for the various biodegradation processes. Additional highlights include: i) the highest dissolved H2 concentrations yet reported for groundwater, possibly linked to vigorous fermentation in the absence of common terminal electron-acceptors (i.e., dissolved oxygen, nitrate, and sulphate, and iron(III)-minerals) and methanogenesis; ii) evidence of phosphorus nutrient limitation, which stalled ethanol biodegradation and perhaps delayed the onset of methanogenesis; and iii) the occurrence of dissimilatory nitrate reduction to ammonium, which has not been reported for ethanol biodegradation to date.

Redirection of Metabolic Hydrogen by Inhibiting Methanogenesis in the Rumen Simulation Technique (RUSITEC).

A decrease in methanogenesis is expected to improve ruminant performance by allocating rumen metabolic hydrogen ([2H]) to more energy-rendering fermentation pathways for the animal. However, decreases in methane (CH4) emissions of up to 30% are not always linked with greater performance. Therefore, the aim of this study was to understand the fate of [2H] when CH4 production in the rumen is inhibited by known methanogenesis inhibitors (nitrate, NIT; 3-nitrooxypropanol, NOP; anthraquinone, AQ) in comparison with a control treatment (CON) with the Rumen Simulation Technique (RUSITEC). Measurements started after 1 week adaptation. Substrate disappearance was not modified by methanogenesis inhibitors. Nitrate mostly seemed to decrease [2H] availability by acting as an electron acceptor competing with methanogenesis. As a consequence, NIT decreased CH4 production (−75%), dissolved dihydrogen (H2) concentration (−30%) and the percentages of reduced volatile fatty acids (butyrate, isobutyrate, valerate, isovalerate, caproate and heptanoate) except propionate, but increased acetate molar percentage, ethanol concentration and the efficiency of microbial nitrogen synthesis (+14%) without affecting gaseous H2. Nitrooxypropanol decreased methanogenesis (−75%) while increasing both gaseous and dissolved H2 concentrations (+81% and +24%, respectively). Moreover, NOP decreased acetate and isovalerate molar percentages and increased butyrate, valerate, caproate and heptanoate molar percentages as well as n-propanol and ammonium concentrations. Methanogenesis inhibition with AQ (−26%) was associated with higher gaseous H2 production (+70%) but lower dissolved H2 concentration (−76%), evidencing a lack of relationship between the two H2 forms. Anthraquinone increased ammonium concentration, caproate and heptanoate molar percentages but decreased acetate and isobutyrate molar percentages, total microbial nitrogen production and efficiency of microbial protein synthesis (−16%). Overall, NOP and AQ increased the amount of reduced volatile fatty acids, but part of [2H] spared from methanogenesis was lost as gaseous H2. Finally, [2H] recovery was similar among CON, NOP and AQ but was largely lower than 100%. Consequently, further studies are required to discover other so far unidentified [2H] sinks for a better understanding of the metabolic pathways involved in [2H] production and utilization.

Interplay of the Open Circuit Potential-Relaxation and the Dissolution Behavior of a Single H2 Bubble Generated at a Pt Microelectrode

The dissolution behavior of a single H2 bubble electrochemically generated at a Pt microelectrode in 1 M H2SO4 was studied. The open circuit potential (OCP) relaxation after the polarization end was recorded and correlated with the dissolved H2 concentration at the interface electrode/electrolyte/gas. Simultaneously, the shrinking of the bubble was followed optically by means of a high speed camera. In addition, analytical modeling and numerical simulations for the bubble dissolution were performed. Three characteristic regions are identified in the OCP and the bubble radius transients: (i) slow relaxation and shrinking, (ii) transition region, and (iii) a long-term slowed down dissolution process. The high supersaturation after polarization remains longer than theoretically predicted and feeds the bubble in region (i). This reduces the dissolution rate of the bubble which differs significantly from that of nonelectrochemically produced bubbles. Numerical multispecies simulations prove that oxygen and nit...

水中スパークおよびレーザー誘起気泡崩壊後の残留気泡に関する考察

To clarify a relationship between non-condensable gases and generation of residual microbubble after the collapse of a single bubble generated by laser or spark, the single bubble dynamics and concentration of the dissolved hydrogen gas were analyzed in ultrapure water. The projected cross-sectional area of the residual microbubbles per potential energy of the bubble was 0.86 mm2 J-1 in the laser case and 3.5 mm2 J-1 in the spark case. The dissolved H2 concentrations per pulse were 0.23 µgL-1J-1 in case of the laser and 0.69 µgL-1J-1 in case of the spark. The amount of the generated residual microbubbles showed a strong relationship with the dissolved H2 concentration. It is possible to conclude that part of the non-condensable gases such as hydrogen generated by the laser and spark form residual microbubbles.

Additive methane-mitigating effect between linseed oil and nitrate fed to cattle.

The objective of this study was to test the effect of linseed oil and nitrate fed alone or in combination on methane (CH4) emissions and diet digestibility in cows. The experiment was conducted as a 2 × 2 factorial design using 4 multiparous nonlactating Holstein cows (initial BW 656 ± 31 kg). Each experimental period lasted 5 wk, with measures performed in the final 3 wk (wk 3 to 5). Diets given on a DM basis were 1) control (CON; 50% natural grassland hay and 50% concentrate), 2) CON with 4% linseed oil (LIN), 3) CON with 3% calcium nitrate (NIT), and 4) CON with 4% linseed oil plus 3% calcium nitrate (LIN+NIT). Diets were offered twice daily and were formulated to deliver similar amounts (DM basis) of CP (12.2%), starch (25.5%), and NDF (39.5%). Feed offer was restricted to 90% of voluntary intake (12.4 kg DMI/d). Total tract digestibility and N balance were determined from total feces and urine collected separately for 6 d during wk 4. Daily CH4 emissions were quantified using open chambers for 4 d during wk 5. Rumen fermentation and microbial parameters were analyzed from samples taken before and 3 h after the morning feeding. Rumen concentrations of dissolved hydrogen (H2) were measured continuously up to 6 h after feeding using a H2 sensor. Compared with the CON diet linseed oil and nitrate decreased (P < 0.01) CH4 emissions (g/kg DMI) by 17 and 22%, respectively, when fed alone and by 32% when combined. The LIN diet reduced CH4 production throughout the day, increased (P = 0.02) propionate proportion, and decreased (P = 0.03) ruminal protozoa concentration compared with CON diet. The NIT diet strongly reduced CH4 production 3 h after feeding, with a simultaneous increase in rumen dissolved H2 concentration, suggesting that nitrate does not act only as an electron acceptor. As a combined effect, linseed plus nitrate also increased H2 concentrations in the rumen. Diets had no effect (P > 0.05) on total tract digestibility of nutrients, except linseed oil, which tended to reduce (P < 0.10) fiber digestibility. Nitrogen balance (% of N intake) was positive for all diets but retention was less (P = 0.03) with linseed oil. This study demonstrates an additive effect between nitrate and linseed oil for reducing methanogenesis in cows without altering diet digestibility.