Low O2 Concentrations Research Articles

Numerical study of inlet air conditions on methane flameless combustion characteristics

The current study investigates the influence of inlet air momentum and temperature for flameless characteristics. Three-dimensional computations were implemented by ANSYS FLUENT. A small-scale experimental combustor geometry was used for validation study. Turbulence modelling was performed by RNG k- ε model and the GRI-Eddy Dissipation Concept (GRI-EDC) model was used for the combustion modelling. The influence of inlet air velocity was investigated through using different air jet diameters at constant flow rate. Five different air jet diameters (4,6,8,10 and 12 mm) were studied. It was found that decreasing air jet diameter leads to an increase in the exhaust gas recirculation which reserve in mixture dilution (low O2 content mixture) subsequently, the reaction rate decreases which support the flameless mode creation. Also, this recirculated gas participates in heating up of fresh reactants which make the mixture tends to complete combustion. The influence of combustion air preheating temperature on NO and CO emissions were studied. CO and NO concentration were decreased with reducing the temperature of inlet preheated combustion air.

Assessment of MILD combustion in co/counter-swirl configuration using syngas as a fuel

Moderate or intense low oxygen dilution (MILD) combustion is investigated in a co/counter-swirl using OH∗-chemiluminescence, species measurements, OH planar laser-induced fluorescence (OH-PLIF), two-dimensional particle image velocimetry (2D-PIV), and microphone measurements. Experiments are performed in a two-stage combustor, where the first stage of combustor is catalytic stage and the second stage is the swirl stage (co/counter-swirl configuration). A fuel-rich syngas (20 % H2, 20 % CO, 12 % CO2, 2 % CH4, and 46 % N2) reacts with air in the catalytic stage. Then, the mixture of unconsumed syngas and products of catalytic combustion is supplied to the swirl stage where the mixture is burnt with oxidizer with varied oxygen concentration. The flame in the swirl stage is established by two concentric swirling streams where the inner stream is the hot gases from the catalytic stage, and the outer stream is the oxidizer. Co/counter-swirl flames are generated by changing the swirl direction of the inner stream. After achieving a stable flame, the macrostructure, flame steadiness, CO and NOx emissions, reaction zone distribution, and sound pressure level (SPL) are investigated at several oxygen concentrations (O2 = 13.13 %–21 %) with an aim to assess the MILD combustion mode in co/counter-swirl configuration.As the oxygen percentage decreases, the flame luminosity decreases for both co/counter-configuration. However, the reduction in luminosity is profound for co-swirl configuration. Clear distinctions between co and counter-swirl configurations are observed regarding flame height and stand-off height. Two-dimensional particle image velocimetry (2D-PIV) is utilized to understand these trends. The steadiness of the flame is investigated using standard deviations (SD) of OH∗-chemiluminescence images and global luminosity (I(t)). The flame steadiness is found to be improved as the oxygen concentration decreases. The OH-PLIF indicates the distributed nature of combustion. The NOx emission is found to be extremely low in all studied cases; however, the CO emission shows an increasing trend when O2 reduces. Finally, the sound pressure level and the dynamics stability are investigated using microphone measurements. The SPL decreases by ∼3 dB and ∼7 dB for the counter-swirl and co-swirl configuration, respectively. Furthermore, the frequency domain analysis suggests that the fundamental axial mode of the combustor is excited at high oxygen concentration. However, the unsteady combustion and chamber acoustics become decoupled at lower O2 concentrations. Thus, the present paper, for the first time, confirms that MILD combustion can be achieved in co/counter-swirl configuration, provided the oxygen concentration is low (∼13 %). The present study also establishes that the co-swirl configuration is more suitable than the counter-swirl for achieving the MILD combustion mode.

Amine Adsorbents Stability for Post-Combustion CO2 Capture: Determination and Validation of Laboratory Degradation Rates in a Multi-staged Fluidized Bed Pilot Plant.

Alternative to current liquid amine technologies for post-combustion CO2 capture, new technologies such as adsorbent-based processes are developed, wherein material lifetime and degradation is important. Herein a robust method to determine degradation rates in a laboratory setup is developed, which was validated with a continuous multi-staged fluidized bed pilot plant designed to capture 1 ton CO2 per day. An amine functionalized polystyrene adsorbent showed very good agreement between the experimental 1000-hour laboratory degradation rates and 2200 hours of degradation in a pilot plant. This validates how laboratory experiments can be extrapolated for sorbent screening and for scale-up. Resulting, the oxidative degradation in the desorber at high temperatures (120 °C) and low O2 concentrations (150 ppmv) is 3 times higher compared to the adsorber at low temperatures and high O2 (56 °C, 7 vol %). Laboratory degradation experiments can hence be used to further optimize process operations to limit degradation or screen for potential new adsorbents.

Protective facemask-induced facial thermal stress and breathing burden during exercise in gyms

During the COVID-19 pandemic, more than 90% of infections occurred in indoor environments such as restaurants and gyms. In particular, gym attendees are facing challenges of both infection risks and extra heat and breathing burdens with the use of protective facemasks (PFMs) during intensive exercise. To evaluate the PFM-induced thermal stress and breathing discomfort during high-intensity exercise, this study investigated the mechanisms of heat and air exchange between environment of inside and outside PFM via experiments and computational fluid dynamics simulations. In the experimental phase, the effects of the activity intensity and type of PFM on thermal and breathing comfort were tested. An uncomfortable microenvironment of the face covered by a PFM was found when the air temperature and absolute humidity increased by 4.1% and 23.1%, respectively, during high-intensity exercise, relative to the sitting scenario (30.8 °C, 0.026 kg/m3), together with a higher CO2 concentration (31.0% increase) and lower O2 concentration (8.3% decrease). Moreover, the masks newly designed provided better thermal and breathing conditions than traditional masks due to the larger surface area and lower thickness. In simulation phase, the model sensitivity analyses were conducted to assess the effects of eight factors on thermal and breathing comfort. The mask surface area is found to most highly affect not only the thermal comfort indices (temperature, absolute humidity) but also breathing comfort indices (the concentration of CO2 and O2). The study results provide an important reference for government policymaking, mask design, and public health.

Aminoethoxyvinylglycine and low oxygen controlled atmosphere storage shift functional microbiomes of ‘Gala’ apples

The complexity and interconnected dynamics of postharvest microbial communities require whole-microbiome characterization to fully understand relationships among pathogens, biocontrol, and fruit quality. This study investigated the effects of the preharvest plant growth regulator aminoethoxyvinylglycine (AVG, 0.25 g L−1), which inhibits ethylene production, and low O2 (0.5 % and 2 %) controlled atmosphere (CA) storage compared with air storage, on the surface microbiome of ‘Gala’ apples stored at 0.5 °C. AVG and low O2 concentrations affected bacterial but not fungal communities. Bacterial but not fungal diversity decreased after 9 months of CA storage. Microbial genera with potential biocontrol functions such as Pseudomonas and Aureobasidium were found across all time points and treatments. The microbiomes detected in CA stored fruit were the most different from those in air stored fruit late in storage. At this time, metagenome prediction methods found that low O2 fruit may harbor bacteria with fewer gene pathways using O2 as a reactant compared to fruit in air storage. The results indicate that storage conditions influence the composition, diversity, and function of microbiomes inhabiting fruit surfaces, which provide a greater basis for understanding biocontrol and pathogen population dynamics in a more holistic manner.

Benthic foraminifera and gromiids from oxygen-depleted environments – survival strategies, biogeochemistry and trophic interactions

Abstract. The oceans are losing oxygen (O2), and oxygen minimum zones are expanding due to climate warming (lower O2 solubility) and eutrophication related to agriculture. This trend is challenging for most marine taxa that are not well adapted to O2 depletion. For other taxa this trend might be advantageous because they can withstand low O2 concentrations or thrive under O2-depleted or even anoxic conditions. Benthic foraminifera are a group of protists that include taxa with adaptations to partly extreme environmental conditions. Several species possess adaptations to O2 depletion that are rare amongst eukaryotes, and these species might benefit from ongoing ocean deoxygenation. In addition, since some foraminifera can calcify even under anoxic conditions, they are important archives for paleoceanographic reconstruction in O2-depleted environments. This paper reviews the current state of knowledge about foraminifera from low-O2 environments. Recent advances in our understanding of specific survival strategies of foraminifera to withstand O2 depletion are summarized and discussed. These adaptations include an anaerobic metabolism, heterotrophic denitrification, symbiosis with bacteria, kleptoplasty and dormancy and have a strong impact on their preferred microhabitat in the sediments, especially the ability of some benthic foraminiferal species to denitrify. Benthic foraminifera also differ regarding their trophic strategies, which has an additional impact on the selection of their microhabitat. For example, some species are strict herbivores that feed exclusively on fresh phytodetritus and live close to the sediment surface, while some species are non-selective detrivores that occupy intermediate to deep infaunal habitats. There is evidence that foraminifers have the capacity to undergo phagocytosis, even under anoxia, and some foraminiferal species which can withstand low-O2 conditions seem to prey on meiofauna. Also, due to their high abundances in O2-depleted environments and their metabolic adaptations, benthic foraminifera are key players in marine nutrient cycling, especially within the marine N and P cycles. This review summarizes the denitrification rates for the species that are known to denitrify and the intracellular nitrate concentrations of the species that are known to intracellularly store nitrate. Finally, equations are provided that can be used to estimate the intracellular nutrient storage and denitrification rates of foraminifera and might be integrated into biogeochemical models.

The delaying effect of a novel polyethylene nanopackaging on postharvest stipe elongation of Flammulina velutipes and its mechanism analysis

The stipe elongation is a typical characteristic of quality deterioration of postharvest Flammulina velutipes. We explored the delaying effect of a novel polyethylene nanopackaging (PENP) on stipe elongation in F. velutipes during storage and the underlying mechanism. The changes in quality, physical and chemical properties, and gene expression of F. velutipes in PENP during 18-day storage at low temperature were analyzed by morphological observation and physiological, biochemical, and transcriptomic methods. The results showed that PENP treatment could reduce the respiration of F. velutipes by maintaining higher CO2 and lower O2 content, decrease the stipe elongation rate and keep the postharvest storage quality of F. velutipes. PENP also inhibited the increase in chitin and β-glucan content, the main components of the cell wall of F. velutipes, maintaining sufficient ATP content and energy charge (EC) values inside the stipe. Transcriptome analysis showed that PENP inhibited cell wall regeneration by down-regulating genes related to chitin degradation (Unigene0001917, Unigene0018811 and Unigene0007053) and β-glucan degradation (Unigene0010562 and Unigene0010561), thus delaying stipe elongation. During the early stage of storage, PENP also inhibited energy metabolism by down-regulating succinate dehydrogenase (SDH), cytochrome c oxidase (CCO), and F1F0-ATPase-related genes involved in the oxidative phosphorylation pathway. In summary, PENP delayed postharvest stipe elongation of F. velutipes by inhibiting the synthesis of chitin and glucan in the cell wall and maintaining low energy metabolism in the stipe cells during early storage.

Effect of dopants on soot oxidation over doped Ag/ZrO2 catalysts for catalyzed gasoline particulate filter

Soot oxidation was studied using silver-zirconia catalysts doped with Y, La, Ce and Pr under a low concentration of O2, typical for gasoline direct injection engine conditions, in view of practical gasoline particulate filter application. The Ag/La-ZrO2 catalyst revealed the best catalyst performance for soot oxidation exhibiting a Tmax of 405 °C for a fresh sample and 420 °C after aging at 700 °C for 6 h. Additionally, the doped catalysts exhibited improved activity for co-oxidation of CO and C3H6. The Ag/ZrO2 catalyst benefited from doping via the synergy between silver particles and doped zirconia.

The effect of coupled combustion reaction on conjugate heat transfer in boundary layer of a segregated oxidizer/fuel solid motor

Combustion reaction plays an important role in heat transfer of boundary layer of solid rocket motor. In the present study, a transient numerical model is developed to investigate the influence of coupled combustion reaction on heat transfer in boundary layer of the segregated oxidizer/fuel solid motor (SOFSM). The results show that conjugate heat transfer characteristics of non-reacting flow are distinct from those of the reacting flow with coupled combustion. The reacting flow with coupled combustion renders higher and more homogeneous temperature and velocity compared with the non-reacting flow. Due to the effect of combustion heat release on thermophysical properties of the gas phase species, the heat transfer coefficient and effective thermal conductivity of the reacting flow are much higher than those of the non-reacting flow. When the combustion time increases, the heat transfer coefficients decrease in the reacting and non-reacting flows due to reduced flow velocity and lower O2 concentration. The Nusselt number (Nu) of the reacting flow is also higher than that of the non-reacting flow. Significant decrease of Nu is found in the non-reacting flow with combustion time. However, the Nu varies very slightly along the flow direction in the reacting flow. As pressure increases from 1.5 MPa to 5.0 MPa, the high-temperature area becomes narrower and the flow velocity decreases in the combustion channel. The diffusion and mixing of the oxidizer and fuel gas along the radial direction are thus weakened in the coupled combustion flow. The total heat transfer coefficient decreases from 295.88 W m−2 K−1 to 46.52 W m−2 K−1 and Nu decreases from 36.27 to 16.56 with increase of pressure. The distribution of total heat transfer coefficient is found to be highly synchronous to the distribution of regression rate of coupled combustion process. The distribution of heat transfer coefficient directly affects parallel regression and coupled combustion of the TAGN propellant. The fitting relationship of Nu, i.e., Nu = 0.058Re0.6Pr0.424 is obtained for the coupled combustion flow in the motor, which provides better insight into the heat transfer in the SOFSM system.

Soil pore structure mediates the effects of soil oxygen on the dynamics of greenhouse gases during wetting–drying phases

The timing and magnitude of greenhouse gas (GHG) production depend strongly on soil oxygen (O2) availability, and the soil pore geometry characteristics largely regulate O2 and moisture conditions relating to GHG biochemical processes. However, the interactions between O2 dynamics and the concentration and flux of GHGs during the soil moisture transitions under various soil pore conditions have not yet been clarified. In this study, a soil-column experiment was conducted under wetting–drying phases using three pore-structure treatments, FINE, MEDIUM, and COARSE, with 0 %, 30 %, and 50 % coarse quartz sand applied to soil, respectively. The concentrations of soil gases (O2, nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4)) were monitored at a depth of 15 cm hourly, and their surface fluxes were measured daily. Soil porosity, pore size distribution, and pore connectivity were quantified using X-ray computed microtomography. The soil O2 concentrations were found to decline sharply as soil moisture increased to the water holding capacities of 0.46, 0.41, and 0.32 cm cm−3 in the FINE, MEDIUM, and COARSE, respectively. The dynamic patterns of the O2 concentrations varied across the soil pore structures, decreasing to anaerobic in FINE (<0.01 %) and MEDIUM (0.02 %), and to hypoxic (4.42 %) in COARSE. Correspondingly, the soil N2O concentration was the highest in FINE (101 μL L−1) and the lowest in COARSE (10 μL L−1), whereas the highest surface N2O flux was observed in MEDIUM (131 μg N m−2 h−1). As soil CO2 concentrations declined, CO2 fluxes increased from FINE to MEDIUM to COARSE. Most pores of FINE, MEDIUM, and COARSE were 15–80 μm, 85–100 μm, and 105–125 μm, respectively, in terms of diameter. The X-ray CT visible (>15 μm) porosity in FINE, MEDIUM and COARSE were 0.09, 0.17, and 0.28 mm3 mm−3, respectively. The corresponding Euler-Poincaré numbers were 180,280, 76,705, and −10,604, respectively, indicating higher connectivity in COARSE than in MEDIUM or FINE. In soil dominated by small air-filled porosity which limits gas diffusion and result in low soil O2 concentration, N2O concentration was increased and CO2 flux was inhibited as the moisture content increased. The turning point in the sharp decrease in O2 concentration was found to correspond with a moisture content, and a pore diameter of 95–110 μm was associated with the critical turning point between holding water and O2 depletion in soil. These findings suggest that O2-regulated biochemical processes are key to the production and flux of GHGs, which in turn are dependent on the soil pore structure and a coupling relationship between N2O and CO2. Improved understanding of the intense effect of soil physical properties provided an empirical foundation for the future development of mechanistic prediction models for how pore-space scale processes with high temporal (hourly) resolution up to GHGs fluxes at larger spatial and temporal scales.

The D-amino acid oxidase-carbon nanotubes: evaluation of cytotoxicity and biocompatibility of a potential anticancer nanosystem

The ‘enzyme prodrug therapy’ represents a promising strategy to overcome limitations of current cancer treatments by the systemic administration of prodrugs, converted by a foreign enzyme into an active anticancer compound directly in tumor sites. One example is D-amino acid oxidase (DAAO), a dimeric flavoenzyme able to catalyze the oxidative deamination of D-amino acids with production of hydrogen peroxide, a reactive oxygen species (ROS), able to favor cancer cells death. A DAAO variant containing five aminoacidic substitutions (mDAAO) was demonstrated to possess a better therapeutic efficacy under low O2 concentration than wild-type DAAO (wtDAAO). Recently, aiming to design promising nanocarriers for DAAO, multi-walled carbon nanotubes (MWCNTs) were functionalized with polyethylene glycol (PEG) to reduce their tendency to aggregation and to improve their biocompatibility. Here, wtDAAO and mDAAO were adsorbed on PEGylated MWCNTs and their activity and cytotoxicity were tested. While PEG-MWCNTs-DAAOs have shown a higher activity than pristine MWCNTs-DAAO (independently on the DAAO variant used), PEG-MWCNTs-mDAAO showed a higher cytotoxicity than PEG-MWCNTs-wtDAAO at low O2 concentration. In order to evaluate the nanocarriers’ biocompatibility, PEG-MWCNTs-DAAOs were incubated in human serum and the composition of protein corona was investigated via nLC-MS/MS, aiming to characterize both soft and hard coronas. The mDAAO variant has influenced the bio-corona composition in both number of proteins and presence of opsonins and dysopsonins: notably, the soft corona of PEG-MWCNTs-mDAAO contained less proteins and was more enriched in proteins able to inhibit the immune response than PEG-MWCNTs-wtDAAO. Considering the obtained results, the PEGylated MWCNTs conjugated with the mDAAO variant seems a promising candidate for a selective antitumor oxidative therapy: under anoxic-like conditions, this novel drug delivery system showed a remarkable cytotoxic effect controlled by the substrate addition, against different tumor cell lines, and a bio-corona composition devoted to prolong its blood circulation time, thus improving the drug’s biodistribution.

Redox control of tumor cell apoptosis during hypoxia

Currently, close attention is paid to studies aimed at searching for redox-sensitive targets for the regulation of tumor cell death. Tumor growth is characterized by impaired cell proliferation, differentiation, and apoptosis against the background of oxidative stress. Hypoxia contributes to the formation of mitochondrial dysfunction and acts as an additional factor that exacerbates oxidative stress in the tumor cell. Reactive oxygen species are general damaging factors, however, they can act as modulators of processes such as reception, intracellular signaling, proliferation, apoptosis, while taking part in the functioning of the cell redox system and contributing to the oxidative modification of macromolecules. One of the possible reasons for the activation of the production of reactive oxygen species is the low content of O2 in the cell, the final electron acceptor to ensure the functioning of the enzymes of the mitochondrial respiratory chain. The glutathione system makes a significant contribution to maintaining the balance between prooxidants and antioxidants in the cell. The role of this system is justified by the reduction potential of glutathione, which, acting as an acceptor of hydroxyl ions and singlet oxygen, significantly reduces the cytotoxic and damaging effects of reactive oxygen species. At the same time, it serves as a coenzyme for glutathione-dependent enzymes, which play a leading role not only in providing antioxidant processes, but also in maintaining the thiol disulfide balance. Hypoxia, which acts as a factor in the activation of free radical oxidation against the background of proliferation and apoptosis dysregulation, contributes to the formation of resistance of tumor cells to chemotherapeutic effects. In light of this, the importance of studying the redox-dependent mechanisms involved in the regulation and implementation of tumor cell death under insufficient oxygen supply becomes obvious, which is necessary for the development of personalized antitumor therapy. The article presents a review of modern literature, including the results of our own research, on the role of the thiol disulfide system and oxidatively modified proteins in the redox regulation of proliferation and apoptotic death of tumor cells, including under hypoxic conditions.

Photosynthetic capacity in seagrass seeds and early-stage seedlings of Zostera marina.

In many terrestrial seeds, photosynthetic activity supplies O2 to the developing plant embryo to sustain aerobic metabolism and enhance biosynthetic activity. However, whether seagrass seeds possess similar photosynthetic capacity to alleviate intra-seed hypoxic stress conditions is unknown. We used a novel combination of microscale variable chlorophyll fluorescence imaging, a custom-made O2 optode microrespirometry system and planar optode O2 imaging, to determine the O2 microenvironment and photosynthetic activity in developing seeds and seedlings of seagrass (Zostera marina). Developing, sheath-covered seeds exhibited high O2 concentrations in the photosynthetic active seed sheath and low O2 concentrations in the centre of the seed at the position of the embryo. In light, photosynthesis in the seed sheath increased O2 availability in central parts of the seed enabling enhanced respiratory energy generation for biosynthetic activity. Early-stage seedlings also displayed photosynthetic capacity in hypocotyl and cotyledonary tissues, which may be beneficial for seedling establishment. Sheath O2 production is important for alleviating intra-seed hypoxic stress, which might increase endosperm storage activity, improving the conditions for successful seed maturation and germination.

Low O2 and high CO2 atmospheres have no impact on efficacy of phytosanitary irradiation doses for Drosophila suzukii larvae

Use of irradiation as a phytosanitary treatment has been restricted in atmospheres with low O2 because extreme low O2 exposure can potentially induce radioprotective responses and enhance survival after irradiation. In addition to being low in O2, many controlled atmosphere treatments and modified atmosphere packaging materials maintain high CO2 levels along with low O2. Few studies have combinatorically tested the combined effects of high CO2 and low O2 on radiation sensitivity, and none have done so in a fully factorial design. Here we exposed larvae of the fly Drosophila suzukii to a range of concentrations of low O2 and high CO2 at a series of radiation doses up to 60 Gy. We found that low levels of O2 (≤ 5 kPa) induced a radioprotective response, with the radioprotective response greatest at the lowest O2 concentrations (2.5 & 0 kPa). Adding CO2 significantly decreased this protective response of low O2 on healthy adult emergence when O2 was 2.5 & 5 kPa, but this effect was only apparent at lower radiation doses, well below those used for phytosanitary treatments for this pest. Our study suggests that either controlled atmospheres or modified atmosphere packaging will not challenge the efficiency of phytosanitary irradiation for D. suzukii, and that high CO2 may act as an additional mitigation measure to disinfest insect pests from fresh commodities.

In situ 15 N-N2 O site preference and O2 concentration dynamics disclose the complexity of N2 O production processes in agricultural soil.

Arable soil continues to be the dominant anthropogenic source of nitrous oxide (N2 O) emissions owing to application of nitrogen (N) fertilizers and manures across the world. Using laboratory and in situ studies to elucidate the key factors controlling soil N2 O emissions remains challenging due to the potential importance of multiple complex processes. We examined soil surface N2 O fluxes in an arable soil, combined with in situ high-frequency measurements of soil matrix oxygen (O2 ) and N2 O concentrations, in situ 15 N labeling, and N2 O 15 N site preference (SP). The in situ O2 concentration and further microcosm visualized spatiotemporal distribution of O2 both suggested that O2 dynamics were the proximal determining factor to matrix N2 O concentration and fluxes due to quick O2 depletion after N fertilization. Further SP analysis and in situ 15 N labeling experiment revealed that the main source for N2 O emissions was bacterial denitrification during the hot-wet summer with lower soil O2 concentration, while nitrification or fungal denitrification contributed about 50.0% to total emissions during the cold-dry winter with higher soil O2 concentration. The robust positive correlation between O2 concentration and SP values underpinned that the O2 dynamics were the key factor to differentiate the composite processes of N2 O production in in situ structured soil. Our findings deciphered the complexity of N2 O production processes in real field conditions, and suggest that O2 dynamics rather than stimulation of functional gene abundances play a key role in controlling soil N2 O production processes in undisturbed structure soils. Our results help to develop targeted N2 O mitigation measures and to improve process models for constraining global N2 O budget.

Oxygen Depletion in Arctic Lakes: Circumpolar Trends, Biogeochemical Processes, and Implications of Climate Change

AbstractPolar amplification of climate change has the potential to cause large‐scale shifts in the dissolved oxygen (DO) dynamics of Arctic lakes, with implications for fish survival, greenhouse gas production, and drinking water quality. While DO is also a sentinel of environmental changes of physical, chemical, and biological nature (e.g., ice cover, temperature, dissolved organic carbon, photosynthesis, and respiration), no synthesis exists of current knowledge of DO dynamics across the diverse freshwater systems of the Arctic. We thus conducted a systematic review of the literature that yielded DO data from 167 sites north of the Subarctic limit (based on vegetation zones), spanning 76 years and including 40 sites with time series. The compilation revealed insufficient observations for adequate representativeness of oxygen dynamics over Arctic ecosystem gradients. We described the main processes controlling DO budgets of Arctic lakes and tested relationships of summer oxygen depletion with maximum depth and latitude. The meta‐analysis showed that most sites with low O2 concentrations were shallow (&lt;10 m) and situated toward the southern end of the latitudinal gradient. Permanently stratified lakes with deep, perennially anoxic basins were located toward the northern end of the gradient. By way of a conceptual model, we identified the direct and indirect drivers and mechanisms that lead to changes in oxygen budgets in the context of the warming Arctic. This comprehensive update on available data allowed us to suggest future research directions and recommend the use of moored instruments for continuous all‐season observations, combined with modeling, remote sensing, and paleo‐reconstructions.

Chitosan-coated iron oxide nanoparticles obtained by laser pyrolysis

The structural and magnetic properties of iron oxide nanoparticles (NPs) synthesized by laser pyrolysis, as well as their chitosan-stabilized aqueous suspensions were studied in regards to the ratio between O2 and Ar. During the synthesis, the flow of Fe(CO)5 precursor vapors and C2H4 sensitizer molecules was kept constant, while the ratio between O2 and Ar in the reactive mixture was increased (1:4, 1:2, 1:1, and 2:1). The formation of small particles (under 4 nm in size) was observed at lower O2 concentrations, whereas their mean crystallite size increased to ∼14 nm for those formed from the richest O2 content, which also induced the formation of particles with the highest magnetization saturation (101.4 emu/g). Maghemite and/or magnetite phases were identified as the main components in all samples, while a small amount of α iron/iron carbides presence was detected with the exception of sample obtained at 1:1 O2 to Ar ratio. The formation of these minor phases reveals the interplay between oxidative and reductive processes which depend on O2 content and C2H4 reactivity at different temperatures. After chitosan loading, all aqueous suspensions presented excellent stability (zeta potential values over 76 mV). Moreover, the samples stabilized using low polymer concentration (0.05 g/l) displayed relatively low hydrodynamic sizes (110–125 nm).

Investigation on the performance of fine iron ore particles for energy storage applications in a novel CLC reactor

This study highlights the use of iron ore in a new chemical looping fixed-bed reactor with high energy density for energy storage and back-up power applications. The reactor is designed to enable a slow diffusion-controlled oxidation of a large packed bed of iron that supplies the energy needed to heat up a high-pressure air flow, whilst avoiding large temperature profiles and hot spots. Thermogravimetric tests were carried out to assess the performance of iron ore as oxygen carrier at reactor conditions, i.e. oxidation at extremely low O2 concentrations surrounding the particles and long reaction times. The effect of particle size on reactivity and maximum conversion was analysed using dp50 = 4–150 μm solids. Higher conversions were observed as particle size decreases, achieving up to 93% at the end of the fast oxidation stage for dp50 = 4 μm solids at 980 °C. Reversible performance of the fine material was confirmed for >30 oxidation-reduction cycles under expected reactor conditions. These tests show that fine particles are the preferred choice to maximise the reactor energy storage density. Further analyses demonstrated the feasibility of a diffusion-controlled oxidation of the reduced fine iron ore for >100 min, thus showing it is a promising candidate material for the investigated reactor.

Clarifying Microbial Nitrous Oxide Reduction under Aerobic Conditions: Tolerant, Intolerant, and Sensitive.

One of the major challenges for the bioremediation application of microbial nitrous oxide (N2O) reduction is its oxygen sensitivity. While a few strains were reported capable of reducing N2O under aerobic conditions, the N2O reduction kinetics of phylogenetically diverse N2O reducers are not well understood. Here, we analyzed and compared the kinetics of clade I and clade II N2O-reducing bacteria in the presence or absence of oxygen (O2) by using a whole-cell assay with N2O and O2 microsensors. Among the seven strains tested, N2O reduction of Stutzerimonas stutzeri TR2 and ZoBell was not inhibited by oxygen (i.e., oxygen tolerant). Paracoccus denitrificans, Azospirillum brasilense, and Gemmatimonas aurantiaca reduced N2O in the presence of O2 but slower than in the absence of O2 (i.e., oxygen sensitive). N2O reduction of Pseudomonas aeruginosa and Dechloromonas aromatica did not occur when O2 was present (i.e., oxygen intolerant). Amino acid sequences and predicted structures of NosZ were highly similar among these strains, whereas oxygen-tolerant N2O reducers had higher oxygen consumption rates. The results suggest that the mechanism of O2 tolerance is not directly related to NosZ structure but is rather related to the scavenging of O2 in the cells and/or accessory proteins encoded by the nos cluster. IMPORTANCE Some bacteria can reduce N2O in the presence of O2, whereas others cannot. It is unclear whether this trait of aerobic N2O reduction is related to the phylogeny and structure of N2O reductase. The understanding of aerobic N2O reduction is critical for guiding emission control, due to the common concurrence of N2O and O2 in natural and engineered systems. This study provided the N2O reduction kinetics of various bacteria under aerobic and anaerobic conditions and classified the bacteria into oxygen-tolerant, -sensitive, and -intolerant N2O reducers. Oxygen-tolerant N2O reducers rapidly consumed O2, which could help maintain the low O2 concentration in the cells and keep their N2O reductase active. These findings are important and useful when selecting N2O reducers for bioremediation applications.

Static supply of different simulated flue gases for native microalgae cultivation in diluted cow manure digestate

The application of microalgae to sequester CO2 from flue gases can be an interesting process since it can contribute to mitigate CO2 emission into the atmosphere. One obstacle of such application is the high CO2 concentration in the flue gases, which can lead to low pH in the cultivation medium and hence process failure. This study aims to investigate static CO2 gas supply for microalgae cultivation as a potential alternative that might allow applying different flue gases with different compositions and higher CO2 concentrations. Two sets of experiments were performed. First, the effect of increasing the amount of supplied carbon was tested. In the second experiment, the applicability of such system for different flue gases regarding their oxygen and carbon content was tested. In all experiments, 50 times diluted cow manure digestate was used as a culture medium. By increasing CO2 concentration up to 10% in the supplied air, microalgae growth productivity of 48.7 mg/L/d was achieved. A further improvement of microalgae growth was shown with increasing the gas/culture volume ratio. Microalgae productivity rate increased form 48.7 mg/L/d to 73.5 mg/L/d when the volume of gas increased from 47% to 81% of total volume. Applying CO2 in air (O2 content around 20%) or in N2 (O2 content less than 2%) didn't show any difference regarding inorganic carbon dissolution, pH, ammonium nitrogen removal, CO2 fixation or biomass productivity. Generally, it can be concluded that static gas supply for microalgae cultivation can allow the application of different flue gases from different industries with low or high O2 content and with CO2 concentration as high as 20%. According to our results, a microalgae cultivation system with continuous static gas supply was proposed.