Total Oxygen Flux Research Articles

Effects of baffle position in serpentine flow channel on the performance of proton exchange membrane fuel cells

This study used a three-dimensional numerical model of a proton exchange membrane fuel cell with five types of channels: a smooth channel (Case 1); eight rectangular baffles were arranged in the upstream (Case 2), midstream (Case 3), downstream (Case 4), and entire cathode flow channel (Case 5) to study the effects of baffle position on mass transport, power density, net power, etc. Moreover, the effects of back pressure and humidity on the voltage were investigated. Results showed that compared to smooth channels, the oxygen and water transport facilitation at the GDL-Channel interface were added 11.53%−20.60% and 7.81%−9.80% at 1.68 A·cm−2 by adding baffles. The closer the baffles were to upstream, the higher the total oxygen flux, but the lower the flux uniformity and the worse the water removal. The oxygen flux of upstream baffles was 8.14% higher than that of downstream baffles, but oxygen flux uniformity decreased by 18.96% at 1.68 A·cm−2. The order of water removal and voltage improvement was Case 4 > Case 5 > Case 3 > Case 2 > Case 1. Net power of Case 4 was 9.87% higher than smooth channel. To the Case 4, when the cell worked under low back pressure or high humidity, the voltage increments were higher. The potential increment for the backpressure of 0 atm was 0.9% higher than that of 2 atm. The potential increment for the humidity of 100% was 7.89% higher than that of 50%.

Measuring the through-plane and in-plane oxygen apparent diffusion coefficients in the gas diffusion layer

Accurately predicting oxygen mass transport resistance and current distribution in fuel cells requires significant knowledge of oxygen diffusion coefficients in both in-plane and through-plane directions. However, there are few methods for measuring in-plane oxygen diffusion coefficients, which are important parameters for simulating oxygen flux distribution in gas diffusion layers (GDLs). In this study, we establish measurement methods and calculations for both in-plane and through-plane oxygen diffusion coefficients. Using in-house designed cells, we measure the in-plane and through-plane oxygen apparent diffusion coefficients of a commercial carbon paper (AvCarb EP40) at various torques and gas flow rates. We also simulate oxygen flux distributions in the GDL under each torque and gas flow rate. Our results show that the channel part is the major contributor to total oxygen flux at high torque conditions and that an increase in torque leads to a decrease in the contribution from the land part. Simulation results also suggest that a higher gas flow rate and lower torque contribute to a more uniform distribution of oxygen flux in the GDL.

Quantifying Importance of Macrobenthos for Benthic‐Pelagic Coupling in a Temperate Coastal Shelf Sea

AbstractBenthic oxygen fluxes consist mostly of advective and diffusive terms. Both terms in the German Bight exhibit a prominent annual cycle but with opposite variation patterns. To understand the driving mechanisms quantitatively, a novel 3‐D benthic‐pelagic coupled model resolving interactions among macrobenthos, bioturbation, oxygen consumption, and carbon early diagenesis was applied to reconstruct the benthic states. Simulation results show a satisfactory agreement with field data and reveal that the benthic oxygen flux is determined by not only pelagic drivers but also by internal dynamics associated with the interaction between organic carbon and macrobenthos, and bedform morphodynamics. Variation of advective flux, characterized by summer‐low and winter‐high, is mainly driven by hydrodynamics and bedform morphodynamics, while variation of diffusive flux, featured by summer‐high and winter‐low, is a compound effect of pelagic and benthic drivers with a dominant control by macrobenthos through bioturbation. The role of bioturbation in benthic oxygen consumption is twofold: (a) on the one hand, it alters the particulate organic carbon (POC) distribution in surface sediments, thereby changing the availability of POC to oxygen consumption; (b) on the other hand, it mixes oxygen down into sediments, thereby facilitating oxygen consumption. Our results indicate that the first role prevails in sandy seafloor characterized by energetic hydrodynamics, while the second role becomes increasingly important along with a weakening of bottom currents. We found that bioturbation contributes up to 87% 4% and 55% 8% of the total benthic oxygen fluxes in muddy seabed and at a regional scale (the German Bight), respectively.

Bottom Boundary Layer Oxygen Fluxes During Winter on the Oregon Shelf

AbstractIn this study, lander‐based eddy covariance measurements made 30 cm above a sandy seafloor are combined with ocean‐observing and ship‐based measurements to evaluate magnitudes of oxygen fluxes within the bottom boundary layer (BBL) during winter on the Oregon shelf. The total oxygen fluxes observed at a mid‐shelf station were −12.3 ± 7.6 and −18.6 ± 16.9 mmol m−2d−1 during February 2018 and January 2019 deployments, respectively, and −51.8 ± 34.9 mmol m−2d−1 during a deployment on the inner shelf in January 2019. These mean (±SD) flux derivations are greater than fluxes determined previously at nearly the same locations during the summer upwelling season, which points to the activation of benthic respiration by winter wave‐dominated BBL dynamics. To determine the direct flux contributions (or potential bias) at wave frequencies, a phase‐based method of spectral decomposition was applied to separate wave‐induced and turbulence‐induced components of velocity and dissolved oxygen time‐series. This approach identified 25.4, 18.6, and 23.3% of the average total EC oxygen fluxes from the three deployments as carried by waves or possibly turbulence stretched by waves. Additionally, we observed evidence of large waves regularly resuspending sediments and transporting entrained particles past the EC sensors. We apply this evidence to infer that wave frequencies must also contribute significantly to the real spectral domain of benthic oxygen fluxes and that the total fluxes of this study reflect elevated oxygen consumption rates without artifacts. This assessment may not be true for all data sets and may be dependent on wave properties and the quality and treatment of the measurements.

Керамические композиционные мембраны на основе Bi3Ru3O11 – Bi1,6Er0,4O3 для получения кислорода

Using hot uniaxial pressing in an argon atmosphere with a stress of 35 MPa and with a holding at 800 °C for 1 hour, ceramic composites of Bi3Ru3O11 – 50, 65 wt % Bi1,6Er0,4O3 were obtained. It was found that phase composition of the composites does not change during gas chromatographic testing at 800 °C and well corresponds to the specified one. Microstructure of the obtained composites was tested and the formation of dense composites with a total porosity of less than 1% and with a uniform distribution of the Bi3Ru3O11 and Bi1,6Er0,4O3 components in bulk of material was demonstrated. Transport properties (total conductivity, oxygen fluxes and selectivity of separating oxygen over nitrogen) of the obtained composites at 600 – 800 °C had been investigated. Thus, at 800 °C the electrical conductivity of Bi3Ru3O11 – 50, 65 wt % Bi1,6Er0,4O3 was about 200 and 50 Ohm–1∙cm–1, respectively, while the metallic nature of their temperature dependence of conductivity is correlated to that for the Bi3Ru3O11. The value of oxygen permeability for the obtained ceramic composites of about 7∙10–9 mol·cm–1·s–1 at 800 °C, which is compared to other membrane materials based on bismuth oxide, demonstrated the potential of their further use in the tasks for obtaining of pure oxygen from air.

Estimating the fate of oxygen ion outflow from the high-altitude cusp

Abstract. We have investigated the oxygen escape-to-capture ratio from the high-altitude cusp regions for various geomagnetic activity levels by combining EDI and CODIF measurements from the Cluster spacecraft. Using a magnetic field model, we traced the observed oxygen ions to one of three regions: plasma sheet, solar wind beyond a distant X-line or dayside magnetosheath. Our results indicate that 69 % of high-altitude oxygen escapes the magnetosphere, from which most escapes beyond the distant X-line (50 % of total oxygen flux). Convection of oxygen to the plasma sheet shows a strong dependence on geomagnetic activity. We used the Dst index as a proxy for geomagnetic storms and separated data into quiet conditions (Dst>0 nT), moderate conditions (0>Dst>-20 nT), and active conditions (Dst<-20 nT). For quiet magnetospheric conditions we found increased escape due to low convection. For active magnetospheric conditions we found an increase in both parallel velocities and convection velocities, but the increase in convection velocities is higher, and thus most of the oxygen gets convected into the plasma sheet (73 %). The convected oxygen ions reach the plasma sheet in the distant tail, mostly beyond 50 RE.

Oxygen in the Southern Ocean From Argo Floats: Determination of Processes Driving Air‐Sea Fluxes

AbstractThe Southern Ocean is of outsized significance to the global oxygen and carbon cycles with relatively poor measurement coverage due to harsh winters and seasonal ice cover. In this study, we use recent advances in the parameterization of air‐sea oxygen fluxes to analyze 9 years of oxygen data from a recalibrated Argo oxygen data set and from air‐calibrated oxygen floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project. From this combined data set of 150 floats, we find a total Southern Ocean oxygen sink of −183 ± 80 Tmol yr−1 (positive to the atmosphere), greater than prior estimates. The uptake occurs primarily in the Polar‐Frontal Antarctic Zone (PAZ, −94 ± 30 Tmol O2 yr−1) and Seasonal Ice Zone (SIZ, −111 ± 9.3 Tmol O2 yr−1). This flux is driven by wintertime ventilation, with a large portion of the flux in the SIZ passing through regions with fractional sea ice. The Subtropical Zone (STZ) is seasonally driven by thermal fluxes and exhibits a net outgassing of 47 ± 29 Tmol O2 yr−1 that is likely driven by biological production. The Subantarctic Zone (SAZ) uptake is −25 ± 12 Tmol O2 yr−1. Total oxygen fluxes were separated into a thermal and nonthermal component. The nonthermal flux is correlated with net primary production and mixed layer depth in the STZ, SAZ, and PAZ, but not in the SIZ where seasonal sea ice slows the air‐sea gas flux response to the entrainment of deep, low‐oxygen waters.

Oxygen transport in periodically ventilated polychaete burrows

Burrowing organisms play a critical role for the functioning of coastal marine sediments, in part due to their pumping of oxygenated water through the burrow. In cohesive sediments, oxygenated burrow water allows for the diffusive flux of oxygen across the burrow wall and into the sediment, where it is consumed. In this study, we quantified the burrow excurrent velocities, volume of water ventilated and oxygenation patterns within the burrow of the polychaete Alitta succinea. We determined that periodic ventilation of the burrow results in oscillations of the flux of oxygen across the burrow wall and oxygen concentration within the sediment near the burrow wall. Additionally, we investigated the effects of temperature changes on oxygen dynamics in the burrow. The volumetric flow rate and frequency of burrow ventilation increased with temperature. Correspondingly, the frequency of the oscillations in oxygen flux across the burrow walls also increased with temperature. However, the time-averaged flux of oxygen across the burrow wall did not change with temperature (1.5 ± 0.3ol m−2 d−1), and the distance of oxygen penetration into the burrow wall decreased with temperature (from 3.4 ± 0.5 at 6 °C to 1.6 ± 0.1 at 33 °C). Thus, seasonal changes in the volume of oxygenated sediment, as well as the pattern of oxygenation that sediment experiences, are expected to be significant while the total oxygen flux is expected to remain relatively uniform. We show that burrower ventilation behavior mediates the effects of temperature on sediment oxygen uptake.

Determination of Sediment Oxygen Demand in the Ziya River Watershed, China: Based on Laboratory Core Incubation and Microelectrode Measurements.

A study coupling sedimentcore incubation and microelectrode measurementwas performed to explore the sediment oxygen demand (SOD) at 16 stations in the Ziya River Watershed, a severely polluted and anoxic river system in the north of China. Total oxygen flux values in the range 0.19–1.41 g/(m2·d) with an average of 0.62 g/(m2·d) were obtained by core incubations, and diffusive oxygen flux values in the range 0.15–1.38 g/(m2·d) with an average of 0.51 g/(m2·d) were determined by microelectrodes. Total oxygen flux obviously correlated with diffusive oxygen flux (R2 = 0.842). The microelectrode method produced smaller results than the incubation method in 15 of 16 sites, and the diffusive oxygen flux was smaller than the total oxygen flux. Although the two sets of SOD values had significant difference accepted by the two methods via the Wilcoxon signed-rank test (p < 0.05), the microelectrode method was shown to produce results that were similar to those from the core incubation method. The microelectrode method, therefore, could be used as an alternative method for traditional core incubation method, or as a method to verify SOD rates measured by other methods. We consider that high potential sediment oxygen demand would occur in the Ziya River Watershed when the dissolved oxygen (DO) recovered in the overlying water.

Numerical analysis of convective and diffusive fuel transports in high-temperature proton-exchange membrane fuel cells

The fuel transports in high-temperature proton-exchange membrane fuel cells have been numerically examined. Both convective and diffusive fuel transports are analyzed in detail. The former is often neglected in straight flow channel configurations while it has been reported to become important for serpentine or interdigitated flow channel configurations. By using a two-dimensional isothermal model, we have performed numerical simulations of a high-temperature proton-exchange membrane fuel cell with a straight flow channel configuration. The present results show that even in a straight flow channel configuration, the convection can play a significant role in fuel transports for the anode side. Examination of the flow field data reveals that the anode gas mixture is transported toward the catalyst layer (CL) whereas the gas mixture in the cathode channel moves away from the reaction site. It is also observed that as the flow moves downstream, the flow rate decreases in the anode channel but increases in the cathode channel. Species transport data are examined in detail by splitting the total flux of fuel transport into convective and diffusive flux components. For oxygen transport in the cathode gas diffusion layer (GDL), diffusion is dominant; in addition, the convective flux has a negative contribution to the total oxygen flux and is negligible compared to the diffusion flux. However, for hydrogen transport to the reaction site, both convection and diffusion are shown to be important processes in the anode GDL. At high cell voltages (i.e., low current densities), it is even observed that the convective contribution to the total hydrogen flux is larger than the diffusive one.

Myoglobin's old and new clothes: from molecular structure to function in living cells.

Myoglobin, a mobile carrier of oxygen, is without a doubt an important player central to the physiological function of heart and skeletal muscle. Recently, researchers have surmounted technical challenges to measure Mb diffusion in the living cell. Their observations have stimulated a discussion about the relative contribution made by Mb-facilitated diffusion to the total oxygen flux. The calculation of the relative contribution, however, depends upon assumptions, the cell model and cell architecture, cell bioenergetics, oxygen supply and demand. The analysis suggests that important differences can be observed whether steady-state or transient conditions are considered. This article reviews the current evidence underlying the evaluation of the biophysical parameters of myoglobin-facilitated oxygen diffusion in cells, specifically the intracellular concentration of myoglobin, the intracellular diffusion coefficient of myoglobin and the intracellular myoglobin oxygen saturation. The review considers the role of myoglobin in oxygen transport in vertebrate heart and skeletal muscle, in the diving seal during apnea as well as the role of the analogous leghemoglobin of plants. The possible role of myoglobin in intracellular fatty acid transport is addressed. Finally, the recent measurements of myoglobin diffusion inside muscle cells are discussed in terms of their implications for cytoarchitecture and microviscosity in these cells and the identification of intracellular impediments to the diffusion of proteins inside cells. The recent experimental data then help to refine our understanding of Mb function and establish a basis for future investigation.

Seasonal and spatial variability of sediment oxygen fluxes in the Beobsan intertidal flat of Taean Bay, mid-western Korean Peninsula

Core incubation and oxygen microelectrode measurements were performed at two stations in the Beobsan intertidal flat of Taean Bay on the mid-western Korean Peninsula. Total and diffusive fluxes were determined for four different seasons (August and October 2006 and February and May 2007). Diffusive oxygen fluxes ranged from 7.62 to 11.4 mmol m−2 d−1 at station S1 and 8.85 to 14.3 mmol m−2 d-1 at station S2. Total oxygen fluxes varied from 20.2 to 27.6 mmol m−2 d−1 at station S1 and from 18.7 to 31.7 mmol m−2 d−1 at station S2. Total oxygen fluxes were 2∶3 times higher than diffusive oxygen fluxes, a result that agrees well with previous studies of the coastal and continental shelf area. The difference between the total and diffusive fluxes is caused by the bio-irrigation activities of macrofauna living in the sediments, which enhance the ventilation of the sediment and introduce oxygenated water to the deeper, anoxic sediment layer. The total oxygen flux at station S1 exhibited small seasonal variation while that at station 2 showed large seasonal variation. Organic carbon contents in the surface sediments correlated well with total oxygen flux, suggesting that the input flux of organic matter is a major factor controlling the seasonal variation in total oxygen flux. Annual benthic aerobic respiration rates were determined to be 8.8 and 9.1 mol C m−2 yr−1 at stations S1 and S2, respectively; these values are very similar to estimations for Aarhus Bay, Denmark and the southwestern lagoon of New Caledonia.

Advective and diffusive contributions to reactive gas transport during pyrite oxidation in the unsaturated zone

Pyrite oxidation in unsaturated mine waste rock dumps and soils is limited by the supply of oxygen from the atmosphere. In models, oxygen transport through the subsurface is often assumed to be driven by diffusion. However, oxygen comprises 23.2% by mass of dry air, and when oxygen is consumed at depth in the unsaturated zone, a pressure gradient is created between the reactive zone and the ground surface, causing a substantial advective air flow into the subsurface. To determine the balance between advective and diffusive transport, a one‐dimensional multicomponent unsaturated zone gas transport model is developed. Both advection‐diffusion and Maxwell‐Stefan model formulations are presented. A steady state analytical solution is derived that provides insight into solution behavior and which can be used to test numerical models. A numerical solution is obtained for both the steady and transient cases. At steady state, advection comprises approximately 23% of the total oxygen flux, with the contribution of advection being almost entirely determined by the composition of the atmosphere. Other parameters, such as the permeability, have a negligible effect on the proportion of advective flows at steady state. However, greater pressure gradients are found in low‐permeability soils. In transient cases, advective fluxes depend on the initial conditions and can be far greater than diffusive fluxes. In contrast to steady state conditions the transient case is sensitive to other model parameters; for example, the time to approach steady state depends exponentially on the distance between the soil surface and the subsurface reactive zone.

Oxygen exchange flux between sediment and waterin an intertidal sandflat, measured in situ by the eddy-correlation method

High temporal-resolution fluctuations in oxygen concentration and vertical velocity were measured over an intertidal sandflat (water depth 5 Hz) observed in the vertical velocity and oxygen concentration data made little contribution to the total oxygen flux (0 to 7%). However, trends (steady change over a longer time scale) caused significant artifacts in the estimated fluxes for several cases. Thus, removal of trends from raw time-series data is recommended. The co-spectrum of the fluctuating components of vertical velocity and oxygen con- centration revealed that the oxygen flux at a frequency band between 0.3 and 1.4 Hz (at a period from 0.7 to 3.3 s) was a major contributor to the total oxygen flux. This frequency was consistent with the dominant frequency of vertical velocity, indicating that transport and exchange of porewater and overlying water by wind-induced waves may be crucial processes to dissolved oxygen flux between permeable sandy sediments and shallow waters.

Slow steady transport with loading and bulk reactions: The mixed ionic conductorLa2CuO4+δ

We consider slow, steady transport for the normal state of the superconductor La$_2$CuO$_{4+\delta}$ in a one-dimensional geometry, with surface fluxes sufficiently general to permit oxygen to be driven into the sample (``loaded'') either by electrochemical means or by high oxygen partial pressure. We include the bulk reaction O$\to$O$^{2-}+2h$, where neutral atoms ($a$) go into ions ($i$) and holes ($h$). For slow, steady transport, the transport equations simplify because the bulk reaction rate density $r$ and the bulk loading rates $\partial_t n$ then are uniform in space and time. All three fluxes $j$ must be specified at each surface, which for a uniform current density $J$ corresponds to five independent fluxes. These fluxes generate two types of static modes at each surface and a bulk response with a voltage profile that varies quadratically in space, characterized by $J$ and the total oxygen flux $j_O$ (neutral plus ion) at each surface. One type of surface mode is associated with electrical screening; the other type is associated both with diffusion and drift, and with chemical reaction (the {\it diffusion-reaction mode}). The diffusion-reaction mode is accompanied by changes in the chemical potentials $\mu$, and by reactions and fluxes, but it neither carries current (J=0) nor loads the system chemically ($j_O=0$). Generation of the diffusion-reaction mode may explain the phenomenon of ``turbulence in the voltage'' often observed near the electrodes of other mixed ionic electronic conductors (MIECs).

Contributions of advective and diffusive oxygen transport through multilayer composite caps over mine waste

The relative contributions of four mechanisms of oxygen transport in multilayer composite (MLC) caps placed over oxygen-consuming mine waste were evaluated using numerical and analytical methods. MLC caps are defined here as caps consisting of earthen and geosynthetic (polymeric) components where a composite barrier layer consisting of a geomembrane (1–2 mm thick polymeric sheet) overlying a clay layer is the primary barrier to transport. The transport mechanisms that were considered are gas-phase advective transport, gas-phase diffusive transport, liquid-phase advective transport via infiltrating precipitation and liquid-phase diffusive transport. A numerical model was developed to simulate gas-phase advective–diffusive transport of oxygen through a multilayer cap containing seven layers. This model was also used to simulate oxygen diffusion in the liquid phase. An approximate analytical method was used to compute the advective flux of oxygen in the liquid phase. The numerical model was verified for limiting cases using an analytical solution. Comparisons were also made between model predictions and field data for earthen caps reported by others. Results of the analysis show that the dominant mechanism for oxygen transport through MLC caps is gas-phase diffusion. For the cases that were considered, the gas-phase diffusive flux typically comprises at least 99% of the total oxygen flux. Thus, designers of MLC caps should focus on design elements and features that will limit diffusion of gas-phase oxygen.

Oxygen demand in coastal marine sediments: comparing in situ microelectrodes and laboratory core incubations

A study coupling core incubation and a large number of in situ microelectrode measurements ( n=18) was performed at a single station in the Golfe de Fos, a semi-enclosed estuarine system in the South of France on the Mediterranean Sea. Total and diffusive oxygen fluxes were determined for two different seasons (October 2000 and May 2001). The average total and diffusive oxygen fluxes were 21.5 and 12.7 mmol m −2 day −1, respectively, in October and 18.5 and 16.5 mmol m −2 day −1, respectively, in May. The average total oxygen demand does not vary significantly between the two periods (21.5±5 to 18.5±4 mmol m −2 day −1). The spread of the values obtained by microelectrode measurements reveals the variability at the decimeter scale indicating a control by small-scale processes. Little difference was found in May between the total and diffusive oxygen fluxes when the number of microelectrode profiles ( n=18) and core incubation ( n=9) was sufficient to catch larger fluxes linked to “organic hot spots”. Although present at a population density of 5700 individuals for 0.1 m 2, macrofauna is mostly composed of surface deposit feeders. The little difference found between total and diffusive oxygen demand points towards not only the population density but also the faunal assemblage as a determining factor for sediment irrigation.

High phosphorylation efficiency and depression of uncoupled respiration in mitochondria under hypoxia.

Mitochondria are confronted with low oxygen levels in the microenvironment within tissues; yet, isolated mitochondria are routinely studied under air-saturated conditions that are effectively hyperoxic, increase oxidative stress, and may impair mitochondrial function. Under hypoxia, on the other hand, respiration and ATP supply are restricted. Under these conditions of oxygen limitation, any compromise in the coupling of oxidative phosphorylation to oxygen consumption could accentuate ATP depletion, leading to metabolic failure. To address this issue, we have developed the approach of oxygen-injection microcalorimetry and ADP-injection respirometry for evaluating mitochondrial function at limiting oxygen supply. Whereas phosphorylation efficiency drops during ADP limitation at high oxygen levels, we show here that oxidative phosphorylation is more efficient at low oxygen than at air saturation, as indicated by higher ratios of ADP flux to total oxygen flux at identical submaximal rates of ATP synthesis. At low oxygen, the proton leak and uncoupled respiration are depressed, thus reducing maintenance energy expenditure. This indicates the importance of low intracellular oxygen levels in avoiding oxidative stress and protecting bioenergetic efficiency.

Interactions between metal oxides and species of nitrogen and iodine in bioturbated marine sediments

By using a gold amalgam (Au/Hg) voltammetric microelectrode, we have measured simultaneously and with millimeter resolution the distributions of O 2, Mn(II), Fe(II), I(−I), and HS(−I) in bioturbated sediment cores from the Laurentian Trough. We also measured nitrate and ammonia in the pore water, total I and ascorbate- and HCl-extractable Fe and Mn in the solid-phase sediment, and fluxes of O 2, NO 3 −, and NH 4 + across the sediment–water interface. The concentrations of O 2 and Mn(II) were below their respective detection limits of 3 and 5 μM between 4 and 12 mm depth, but a sharp iodide maximum occurred at the depth where upward diffusing Mn(II) was being removed. We propose that the iodide peak is maintained through the reduction of IO 3 − by Mn(II), reoxidation of I(−I) to IO 3 − in the oxic zone above the peak and oxidation to I 2 below where it is ultimately trapped by reaction with organic matter. The iodide production rate is sufficient to account for the oxidation of all of the upward diffusing Mn(II) by IO 3 −. Nitrate plus nitrite (ΣNO 3) decreased to a minimum within 10 mm of the sediment–water interface, in agreement with flux measurements which showed ΣNO 3 uptake by the sediment. Below the minimum, ΣNO 3 rebounded, and reached a maximum at 40- to 50-mm depth. This rebound is attributed to the anaerobic oxidation of ammonia by manganese oxides. Fe(II) was always first detected below the anoxic ΣNO 3 maximum, and was accompanied by colloidal or complexed Fe(III). A sharp upward-directed ammonia gradient was recorded near the sediment–water interface, but no ammonia was released during the first 48 h of the incubations. If the ammonia removal were due to coupled bacterial nitrification–denitrification, more than one half of the total measured oxygen uptake (6.7 to 7.3 mmol/m 2/d) would be required, and more organic carbon would be oxidized by nitrate than by oxygen. This scenario is not supported by nitrate flux calculations. Alternatively, the oxidation of ammonia to N 2 by manganese oxides is a potential removal mechanism. It would require one quarter of the total oxygen flux. The high-resolution profiles of redox species support the conceptualization of bioturbated sediments as a spatially and temporally changing mosaic of redox reactions. They show evidence for a multitude of reactions whose relative importance will vary over time, and for reaction pathways complementing those usually considered in diagenetic studies.

Extraction of oxygen from CO2 using glow-discharge and permeation techniques

A promising method to extract oxygen from CO2, which constitutes more than 95% of the Mars atmosphere, is by using glow-discharge dissociation of CO2 combined with the permeation of the generated oxygen through a Ag membrane. Quadrupole mass spectrometry was utilized to measure the oxygen flux through a 0.35-mm-thick, 2.01 cm2 area membrane separating a two-chamber system. On the upstream side of the membrane, a dc glow discharge was established with 350 V and 5 mA current. It was found that as much as 75% of the CO2 was dissociated to form CO and O (part of which recombined to form O2). The atomic and molecular oxygen generated were adsorbed on the membrane surface, subsequently dissolved into the Ag, and diffused through the membrane thickness to the downstream surface where it desorbed as molecular oxygen. The resulting oxygen flux was studied as a function of upstream CO2 pressure, discharge current, discharge probe to membrane distance, and membrane temperature. It was found that the atomic oxygen generated a much higher concentration gradient across the membrane than did the molecular oxygen which resulted in a correspondingly higher oxygen flux through the membrane. The total oxygen flux through the membrane with a CO2 glow-discharge pressure of 5 Torr and a membrane temperature of 450 °C was found to be greater than 1014 cm−2 s−1. This method can be used to continuously and efficiently supply oxygen for astronauts in a future manned mission to Mars.