N2 Production Research Articles

Effective Fixation of Carbon in g-C3 N4 Enabled by Mg-Induced Selective Reconstruction.

The methodology of metal-involved preparation for carbon materials is favored by researchers and has attracted tremendous attention. Decoupling this process and the underlying mechanism in detail are highly required. Herein, the intrinsic mechanism of carbon fixation in graphitic carbon nitride (g-C3 N4 ) via the magnesium-involved carbonization process is reported and clarified. Magnesium can induce the displacement reaction with the small carbon nitride molecule generated by the pyrolysis of g-C3 N4 , thus efficiently fixing the carbon onto the in situ template of Mg3 N2 product to avoid the direct volatilization. As a result, the N-doped carbon nanosheet frameworks with interconnected porous structure and suitable N content are constructed by reconstruction of carbon and nitrogen species, which exhibit a comparable photoelectric conversion efficiency (8.59%) and electrocatalytic performances to that of Pt (8.40%) for dye-sensitized solar cells.

Optical and mass spectrometric measurements of dissociation in low frequency, high density, remote source O2/Ar and NF3/Ar plasmas

Remote plasma sources are widely used in applications such as chamber cleaning and flowable chemical vapor deposition. In these processes, it is desirable that the dissociation rate of feed gases be as high as possible and stable. Here, the authors present results on radical densities and gas dissociation fractions for a 400 kHz toroidal transformer-coupled plasma source (MKS Instruments), operating at a power density of 5–50 W/cm3 with feed gas mixtures of O2 or NF3 in Ar and pressures of 0.4 or 2.0 Torr. Radical densities and feed gas dissociation percentages in the plasma were measured by optical emission spectroscopy combined with Ar actinometry. In the plasma, O2 was about 60% dissociated in dilute O2 mixtures (10%–20%). Dissociation decreased with the increasing addition of O2, dropping to 10% dissociation for 90% O2 in the feed gas. NF3 was >95% dissociated for all NF3/Ar mixtures. Little or no dependence on the flow rate was found. Plasma products flow into an anodized Al downstream chamber that is probed by vacuum ultraviolet (VUV) absorption spectroscopy and line-of-sight molecular beam mass spectrometry. In the downstream chamber, O recombined on the walls to form O2 (detected by VUV O2 absorption). The measured downstream O/O2 ratio was a strongly increasing function of an increasing flow rate reproduced by a downstream global model with O wall recombination probability of γO between 0.001 and 0.002. NF3 does not reform in the downstream chamber, as verified by VUV absorption and mass spectrometry. No NF or NF2 was detected, and F mostly recombined to form F2 at the back of the downstream chamber, along with N2. The F2, F, and N2 product absolute number densities were consistent with the 3:1 F:N mass balance of the NF3 feed gas.

Denitrification Process and N2O Production Characteristics of Heterotrophic Nitrifying Bacterium Pseudomonas aeruginosa YL

Because of the massive discharge of nitrogenous wastewater, the eutrophication of a water body is becoming increasingly serious, and how to effectively remove nitrogen from this wastewater remains an urgent problem to be solved. In this study, due to disadvantages in the traditional biological nitrogen removal process, such as complex and long procedures, high energy consumption, weak impact resistance, and N2O release, the nitrogen removal theory by heterotrophic nitrification was further analyzed by discussing the physiological-biochemical, heterotrophic nitrification-aerobic denitrification, and N2O production characteristics of a high-efficiency heterotrophic nitrifying bacteria Pseudomonas aeruginosa YL. Results show that the strain YL had an eminent heterotrophic nitrification and aerobic denitrification ability, and NH4+-N, NO2--N, and NO3--N with concentration of 100 mg·L-1 could be completely removed during the 24-hour incubation period. There was almost no intermediate product in the process of heterotrophic nitrification, however when NO3--N was used as nitrogen source, the accumulation of NO2--N reached 25.55 mg·L-1. Meanwhile, the successful expression of denitrification genes napA, nirK, and nosZ further confirmed the aerobic denitrification ability of strain YL. Gaseous nitrogen products accounted for about 30%-40% of removed TN in the heterotrophic nitrification-aerobic denitrification process by strain YL, and N2 was the main denitrification product. When NH4+-N, NO2--N, and NO3--N were used as the sole nitrogen source, N2 production amounted to 3.46, 3.49, and 3.36 mg, respectively. In contrast, only small amounts of N2O were produced in the denitrification process by strain YL, and the total amount was 6.63 μg when NH4+-N was the nitrogen source, which was much lower than in the cases of NO2--N and NO3--N as the sole nitrogen source. In addition, high C/N, low pH, high temperature, high NH4+-N, and high NO2--N conditions could result in more N2O generation. Nevertheless, most environmental factors had little effect on N2O production of strain YL, and the maximum N2O production was significantly lower than that of short-cut nitrification system and autotrophic nitrification system. These results demonstrated that strain YL exhibited excellent abilities of nitrogen removal, N2O emission control, and tolerance to environmental conditions, and could be an effective candidate for treating wastewater without secondary air pollution.

The effects of trace narasin on the biogeochemical N-cycle in a cultivated sandy loam.

Narasin is an antibiotic administered to broiler chickens to prevent coccidiosis. After storage, excreta containing parent narasin compounds is commonly spread as nitrogen fertilizer, yielding initial soil concentrations in the low μg·kg-1 range. In soil, antibiotics have been found to modify one or more pathways in the biogeochemical nitrogen cycle. The concentrations tested are often too high to be considered environmentally relevant, despite evidence that sub-therapeutic doses may also be disruptive. We conducted soil mesocosm experiments to determine the overall impact of trace narasin on major nitrogen pools and fluxes in soils treated with 0, 1, 10, 100, or 1000ng·kg-1 narasin. Water content in the mesocosms varied from 40% to 80% water-filled pore space (WFPS), simulating a range of different redox conditions. Under aerobic conditions (40% WFPS), exposure to narasin inhibited nitrification, yielding increases in soil ammonium by up to 76%, perhaps by targeting nitrifying fungi. Under the same conditions, narasin caused soil nitrate concentrations to decline 17-39%. When the soil was near saturation (80% WFPS), nitrate increased by an average of 30%. Mass balances and isotopic enrichment of N2O indicate that NAR may also affect anammox and the rate of nitrifier nitrification/denitrification. In aerobic soils, N2O flux increased with antibiotic dose and the rise in flux strongly correlates to the N2O:N2 product ratio from dentification. This relationship suggests that N2O flux may increase in soils exposed to narasin even when total denitrification is modestly inhibited. We conclude that trace concentrations of narasin can significantly modify biogeochemical activities in soil on short timescales. Our results indicate the potential for extremely low concentrations of antibiotics to impact agricultural productivity, terrestrial N2O flux, and non-point source nitrogen pollution.

Hotspot of dissimilatory nitrate reduction to ammonium (DNRA) process in freshwater sediments of riparian zones

Dissimilatory nitrate reduction to ammonium (DNRA), an important intermediate process in the N-cycle, links N-compound oxidation and reduction processes. Hence, the oxic-anoxic interface would be the hotspot of the DNRA process. In freshwater ecosystems, the riparian zone is the most typical carrier of the oxic-anoxic interface. Here we report spatio-temporal evidence of a higher abundance and rate of DNRA in the riparian zone than in the open water sediments based on molecular and 15N isotopic-tracing technologies, hence signifying a hotspot for the DNRA process. These abudance and rates were significantly higher than those in open water sediments. 15N isotopic paring technology revealed that the DNRA hotspot promoted higher rates of N-compound oxidation (NO2−), reduction (NO3− and DNRA), and N2 production (anammox and denitrification) in the riparian zone than those in open water sediment. However, high-through sequencing analysis showed that the DNRA bacteria in the riparian zone and openwater sediments were insignificantly different. Network and correlation analysis showed that the DNRA abundance and rates were significantly positively correlated with TOM, TC/NH4+, and TC/NO2−, but not with the dominant genera (Anaeromyxobacter, Lacunisphaera, and Sorangium), which played different roles on the connection in the respective community networks. The DNRA process in the riparian zone could be driven mainly by the related environmental biogeochemical characteristics induced by anthropogenic changes, followed by microbial processes. This result provides valuable information for the management of riparian zones because anthropogenic changes in the riparian water table are expected to increase, inducing consequent changes in the reduction from NO3− to NH4+.

Evaluation and improvement of phosphate solubilization by an isolated bacterium Pantoea agglomerans ZB.

A phosphate solubilizing bacterium ZB was isolated from the rhizosphere soil of Araucaria, which falls into the species Pantoea agglomerans. Optimization for phosphate solubilization by strain ZB was performed. At optimum culture conditions, the isolate showed great ability of solubilizing different insoluble inorganic phosphate sources viz. Ca3(PO4)2 (TCP), Hydroxyapatite (HP), CaHPO4, AlPO4, FePO4 along with rock phosphates (RPs). Inoculation with planktonic cells was found to enhance dissolved phosphorous as compared to that achieved by symplasma inoculation. Besides inoculation with different status of cells, pre-incubation could also exert a great effect on phosphate solubilization ability of P. agglomerans. When isolate ZB was cultured with glucose as carbon sources, phosphorous was more efficiently dissolved from HP and RP without pre-incubation in comparison to that obtained with pre-cultivation. Pre-cultivation, however, was more suitable for P solubilization than no pre-cultivation when bacteria were grown with xylose. A positive correlation was detected between the production of organic acids and phosphate solubilization. P. agglomerans ZB possessed many plant growth promotion traits such as N2 fixation and production of indole 3-acetic acid, phytase, alkaline phosphatase. Pot experiment showed inoculation with single isolate ZB or biofertilizer prepared from semi-solid fermentation of isolate ZB with spent mushroom substrate (SMS) compost could enhance plant growth with respect to number of leaves, plant leave area, stem diameter, root length, root dry mass, shoot dry mass and biomass when compared to the abiotic control, revealing strain ZB could be a promising environmental-friendly biofertilizer to apply for agricultural field.

Effects of silver nanoparticles on coupled nitrification–denitrification in suspended sediments

The effects of varying concentrations of Ag NPs on coupled nitrification and denitrification (CND) in two suspended sediments (SPSs) sizes were investigated using isotopic tracer method. In general, 0.5 and 5 mg/L Ag NPs had less effect on CND, while 2 and 10 mg/L Ag NPs exhibited the improvement and inhibition effect, respectively. The CND improvement by 2 mg/L NPs was mainly due to the enhanced nitrifying and denitrifying enzyme activity. However, 10 mg/L Ag NPs inhibited NH4+ oxidation by directly reducing the AMO activity and AOB abundance. The inhibition on NAR and NIR activity and their encoding narG and nirK gene abundance further inhibited NO3− and NO2− reduction, leading to a dramatic decrease in the 15N–N2 production. The above inhibition effects were attributed to the nano-effects of Ag NPs, which led to the excessive ROS amount and the decreased T-AOC level in microbial systems. But the connection between nitrification and denitrification was not broken after Ag NPs exposure. Moreover, the results indicated that N-cycling in clay and silt-type SPS systems could be more sensitive than sand-type SPS systems to NP exposure. The findings provide a basis for evaluating the environmental risks of Ag NPs in water–sediment systems.

Denitrification and Anaerobic Ammonium Oxidation in Soil Nitrogen Migration Process in a Farmland of Wanshandang Lake

To explore the rate variation and contribution to N loss of denitrification and anaerobic ammonia oxidation (ANAMMOX) in the nitrogen migration process of farmland soils in southern China, we assess the physicochemical characteristics soil samples of different soil layers from farmland and different land use types (farmland, river channel, riparian zone, and lake sediment) in a wheat-rice rotation area of Wanshandang Lake. Illumina MiSeq sequencing and quantitative real-time polymerase chain reaction (qPCR) are used to investigate the microbial community composition and functional gene abundances of the samples. The potential denitrification and ANAMMOX rate (calculated by N2) of each sample was determined by an isotope culture experiment. It was demonstrated that the potential denitrification rate was significantly positively correlated with TOC, NH4+-N, and NO3--N (P<0.05), and with the abundances of nirS, nirK, and nosZ (P<0.05). The denitrification rate of surface soils was (11.51±1.04) nmol·(g·h)-1, which was significantly higher than other soil layers and other land use types (P<0.05). While the ANAMMOX rate in farmland soils was the highest in the 20-30 cm layer and reached (0.48±0.07) nmol·(g·h)-1. In addition, denitrification was the main cause of N loss in surface soils of the studied farmland, accounting for 91.9%-99.7% of overall loss, and ANAMMOX played an important role in the production of N2 in deep soils.

A New in Situ Method for Tracing Denitrification in Riparian Groundwater

The spatiotemporal dynamics of denitrification in groundwater are still not well-understood because of a lack of efficient methods to quantify this biogeochemical reaction pathway. Previous research used the ratio of N2 to argon (Ar) to quantify net production of N2 via denitrification by separating the biologically generated N2 component from the atmospheric-generated components. However, this method does not allow the quantification of the atmospheric components accurately because the differences in gas partitioning between N2 and Ar are being neglected. Moreover, conventional (noble) gas analysis in water is both expensive and labor-intensive. We overcome these limitations by using a portable mass spectrometer system, which enables a fast and efficient in situ analysis of dissolved (noble) gases in groundwater. By analyzing a larger set of (noble) gases (N2, He, Ar, and Kr) combined with a physically meaningful excess air model, we quantified N2 originating from denitrification. Consequently, we were able to study the spatiotemporal dynamics of N2 production due to denitrification in riparian groundwater over a six-month period. Our results show that denitrification is highly variable in space and time, emphasizing the need for spatially and temporally resolved data to accurately account for denitrification dynamics in groundwater.

N2(A) in the Terrestrial Thermosphere

AbstractRecent work has indicated the presence of a nitric oxide (NO) product channel in the reaction between the higher vibrational levels of the first electronically excited state of molecular nitrogen, N2(A ), and atomic oxygen. A steady‐state model for the N2(A) vibrational distribution in the terrestrial thermosphere is here described and validated by comparison with N2 A‐X, Vegard‐Kaplan dayglow spectra from the Ionospheric Spectroscopy and Atmospheric Chemistry spectrograph. A computationally cheaper method is needed for implementation of the N2(A) chemistry into time‐dependent thermospheric models. It is shown that by scaling the photoelectron impact production of ionized N2 by a Gaussian centered near 100 km, the level‐specific N2(A) production rates between 100 and 200 km can be reproduced to within an average of 5%. This scaling, and thus the N2 electron impact ionization/excitation ratio, is nearly independent of existing uncertainties in the 2–20 nm solar soft X‐ray irradiance. To investigate this independence, the N2 electron‐impact excitation cross sections in the GLOW photoelectron model are replaced with the results of Johnson et al. (2005, https://doi.org/10.1029/2005JA011295) and the multipart work of Malone et al. (2009 https://doi.org/10.1103/PhysRevA.79.032704) (Malone, Johnson, Young, et al., 2009, https://doi.org/10.1088/0953-4075/42/22/225202; Malone, Johnson, Kanik, et al., 2009, https://doi.org/10.1103/PhysRevA.79.032705; Malone et al., 2009, https://doi.org/10.1103/PhysRevA.79.032704), together denoted J05M09. Upon updating these cross sections it is found that (1) the total N2 triplet excitation rate remains nearly constant; (2) the steady state N2(A) vibrational distribution is shifted to higher levels; (3) the total N2 singlet excitation rate responsible for the Lyman‐Birge‐Hopfield emission is reduced by 33%. It is argued that adopting the J05M09 cross sections supports (1) the larger X‐ray fluxes measured by the Student Nitric Oxide Explorer (SNOE) and (2) a temperature‐independent N2(A)+O reaction rate coefficient.

Nitrogen transformation processes and gaseous emissions from a humic gley soil at two water filled pore spaces

The artificial drainage of heavy textured gley soils is prevalent on pasture. Drainage of a soil profile reduces the water filled pore space (WFPS) in the upper soil horizons with consequences for N2 and N2O emissions, the fate of nitrogen (N), transformational processes and microbial and bacterial communities. The present intact soil column study with isotopically enriched fertiliser investigates all these aspects simultaneously under two WFPS treatments (80% (HS) and 55% (LS) saturation). Results showed significant differences in nitrous oxyde (N2O) emissions, in both pattern and amount, with maxima at 11.97 mg N2O-N/m2h for HS and at 1.64 for LS. Isotopic enrichment data showed a significant predominance (74.8–97.2%) of nitrification in LS, with a possible reduction in NH4+ but a higher concentration of nitrate (NO3−) in N losses. Denitrification dominated in HS (72.5–73.4%), possibly leading to high ammonium (NH4+) losses. Enrichment values showed differential apportionment patterns. A high component of N2O emission derived from denitrification in HS (6.0% HS; 0.4% LS) with a significant amount of N2O (62.9%) transformed to N2 (3.7% LS). A higher percentage of 15N was retained in LS soil. HS showed a lower amount of unaccounted N highlighting lower losses. Differences in gene copy concentrations (GCC) were found across most analysed genes (16S, nirS, nirK, nosZ1, nosZ2, amoA and nrfA). Both HS and LS treatments showed similar potentials for N2O production and its reduction to N2, but a reduced potential for nitrification and dissimilatory nitrate reduction to ammonium (DNRA) in HS. This study explained the effect of drainage and rewetting on gaseous emissions providing an explanation in terms of community switches. On the current soil type, structures to manage watertable heights would push the system towards complete denitrification with only N2 production but may present risks in terms of ammonia (NH3) and NH4+ losses.

Looking back to look ahead: a vision for soil denitrification research.

Denitrification plays a critical role in regulating ecosystem nutrient availability and anthropogenic reactive nitrogen (N) production. Its importance has inspired an increasing number of studies, yet it remains the most poorly constrained term in terrestrial ecosystem N budgets. We censused the peer-reviewed soil denitrification literature (1975-2015) to identify opportunities for future studies to advance our understanding despite the inherent challenges in studying the process. We found that only one-third of studies reported estimates of both nitrous oxide (N2 O) and dinitrogen (N2 ) production fluxes, often the dominant end products of denitrification, while the majority of studies reported only net N2 O fluxes or denitrification potential. Of the 236 studies that measured complete denitrification to N2 , 49% used the acetylene inhibition method, 84% were conducted in the laboratory, 81% were performed on surface soils (0-20cm depth), 75% were located in North America and Europe, and 78% performed treatment manipulations, mostly of N, carbon, or water. To improve understanding of soil denitrification, we recommend broadening access to technologies for new methodologies to measure soil N2 production rates, conducting more studies in the tropics and on subsoils, performing standardized experiments on unmanipulated soils, and using more precise terminology to refer to measured process rates (e.g., net N2 O flux or denitrification potential). To overcome the greater challenges in studying soil denitrification, we envision coordinated research efforts based on standard reporting of metadata for all soil denitrification studies, standard protocols for studies contributing to a Global Denitrification Research Network, and a global consortium of denitrification researchers to facilitate sharing ideas, resources, and to provide mentorship for researchers new to the field.

N-loss stoichiometry in a Peru ODZ eddy

Assuming heterotrophic denitrification as the dominant microbial process, Richards (1965) formulated a stoichiometry governing nitrogen loss in open-ocean oxygen deficient zones (ODZs). It prescribes the quantitative coupling between the oxidation of organic matter by NO–3 in the absence of O2 and the corresponding production of CO2, N2, and PO–34. Applied globally, this relationship defines key linkages between the C, N, and P cycles. However, the validity of Richards's stoichiometry is challenged by recognition of complex microbial N processing in ODZs including anammox as an important pathway and nitrite reoxidation. Whereas Richards's stoichiometry would result in N2-N production to NO–3 removal rates of 1.17, dominance by anammox with respect to biogenic N2 production could in theory result in a ratio as high as 2. Ratios with PO–34 production provide an additional constraint on the quantity and composition of respired organic matter. Here we use a mesoscale eddy with extreme N-loss in the Peru ODZ as a "natural laboratory" to examine N-loss stoichiometry. Its intense biogeochemical signatures, relatively well-defined timescales, and simplified hydrography allowed for the development of strong co-occurring gradients in NO–3, NO–2, biogenic N2, and PO–34. The production of biogenic N2 as compared with the removal of NO–3 (analyzed either directly or as N deficits) was slightly less than predicted by Richards's stoichiometry and did not at all support any "excess" biogenic N2. PO–34 production, however, was twice the expectation from Richards's stoichiometry suggesting that respired organic matter was P-rich as compared with C:N:P Redfield composition. These results suggest major gaps remain between current understanding of microbial N pathways in ODZs and their net biogeochemical output.

Structure, Dynamics, and Reactivity for Light Alkane Oxidation of Fe(II) Sites Situated in the Nodes of a Metal-Organic Framework.

Metal organic frameworks (MOFs), with their crystalline, porous structures, can be synthesized to incorporate a wide range of catalytically active metals in tailored surroundings. These materials have potential as catalysts for conversion of light alkanes, feedstocks available in large quantities from shale gas that are changing the economics of manufacturing commodity chemicals. Mononuclear high-spin (S = 2) Fe(II) sites situated in the nodes of the MOF MIL-100(Fe) convert propane via dehydrogenation, hydroxylation, and overoxidation pathways in reactions with the atomic oxidant N2O. Pair distribution function analysis, N2 adsorption isotherms, X-ray diffraction patterns, and infrared and Raman spectra confirm the single-phase crystallinity and stability of MIL-100(Fe) under reaction conditions (523 K in vacuo, 378-408 K C3H8 + N2O). Density functional theory (DFT) calculations illustrate a reaction mechanism for the formation of 2-propanol, propylene, and 1-propanol involving the oxidation of Fe(II) to Fe(III) via a high-spin Fe(IV)═O intermediate. The speciation of Fe(II) and Fe(III) in the nodes and their dynamic interchange was characterized by in situ X-ray absorption spectroscopy and ex situ Mössbauer spectroscopy. The catalytic relevance of Fe(II) sites and the number of such sites were determined using in situ chemical titrations with NO. N2 and C3H6 production rates were found to be first-order in N2O partial pressure and zero-order in C3H8 partial pressure, consistent with DFT calculations that predict the reaction of Fe(II) with N2O to be rate determining. DFT calculations using a broken symmetry method show that Fe-trimer nodes affecting reaction contain antiferromagnetically coupled iron species, andhighlight the importance of stabilizing high-spin (S = 2) Fe(II) species for effecting alkane oxidation at low temperatures (<408 K).

Denitrification responses to increasing cadmium exposure in Baltic Sea sediments

Benthic ecosystems have come under intense pressure, due to eutrophication-driven oxygen decline and industrial metal contamination. One of the most toxic metals is Cadmium (Cd), which is lethal to many aquatic organisms already at low concentrations. Denitrification by facultative anaerobic microorganisms is an essential process to transform, but also to remove, excess nitrate in eutrophied systems. Cd has been shown to decrease denitrification and sequester free sulfide, which is available when oxygen is scarce and generally inhibits complete denitrification (i.e. N2O to N2). In polluted sediments, an interaction between oxygen and Cd may influence denitrification and this relationship has not been studied. For example, in the Baltic Sea some sediments are double exposed to both Cd and hypoxia. In this study, we examined how the double exposure of Cd and fluctuations in oxygen affects denitrification in Baltic Sea sediment. Results show that oxygen largely regulated N2O and N2 production after 21 days of exposure to Cd (ranging from 0 to 500 μg/L, 5 different treatments, measured by the isotope pairing technique (IPT)). In the high Cd treatment (500 μg/L) the variation in N2 production increased compared to the other treatments. Increases in N2 production are suggested to be an effect of 1) enhanced nitrification that increases NO3− availability thus stimulating denitrification, and 2) Cd successfully sequestrating sulfide (yielding CdS), which allows for full denitrification to N2. The in situ field sediment contained initially high Cd concentrations in the pore water (∼10 μg/L) and microbial communities might already have been adapted to metal stress, making the effect of low Cd levels negligible. Here we show that high levels of cadmium pollution might increase N2 production and influence nitrogen cycling in marine sediments.

Theoretical study on the reduction reactions from solid char(N): The effect of the nearby group and the high-spin state

A fundamental understanding of the reduction reactions from solid char(N) is proposed with the aim of providing useful information to aid in minimising NOx emission. The direct reduction with (a) nearby radical; (b) nearby oxygen and (c) nearby vacancy is calculated. The results show that the nearby oxygen will increase the activation energy by 51.8 kJ/mol and the nearby vacancy will reduce the exothermicity by 269.7 kJ/mol. This leads us to the firm conclusion that the nearby radical site will provide a favorable route. The indirect reduction reactions from surface migration are also determined. According to the calculated results, the high-spin singlet reactions are more complicated than the low-spin triplet reactions. The barrier height involved in the singlet reactions is much lower than that encountered in the triplet reactions, indicating that the reaction with singlets holds potential of being an important channel for N2 production. Our qualitative analysis of the density functional theory (DFT) results confirms that the high-spin excited-state changing the electronic properties of the carbonaceous surface is much more favorable for the reduction than the low-spin ground-state. Much more emphasis therefore should be placed on the high-spin states during the mechanism study of the heterogeneous reactions.

Quantifying N2O reduction to N2 during denitrification in soils via isotopic mapping approach: Model evaluation and uncertainty analysis

The last step of denitrification, i.e. the reduction of N2O to N2, has been intensively studied in the laboratory to understand the denitrification process, predict nitrogen fertiliser losses, and to establish mitigation strategies for N2O. However, assessing N2 production via denitrification at large spatial scales is still not possible due to lack of reliable quantitative approaches. Here, we present a novel numerical “mapping approach” model using the δ15Nsp/δ18O slope that has been proposed to potentially be used to indirectly quantify N2O reduction to N2 at field or larger spatial scales. We evaluate the model using data obtained from seven independent soil incubation studies conducted under a He–O2 atmosphere. Furthermore, we analyse the contribution of different parameters to the uncertainty of the model. The model performance strongly differed between studies and incubation conditions. Re-evaluation of the previous data set demonstrated that using soils-specific instead of default endmember values could largely improve model performance. Since the uncertainty of modelled N2O reduction was relatively high, further improvements to estimate model parameters to obtain more precise estimations remain an on-going matter, e.g. by determination of soil-specific isotope fractionation factors and isotopocule endmember values of N2O production processes using controlled laboratory incubations. The applicability of the mapping approach model is promising with an increasing availability of real-time and field based analysis of N2O isotope signatures.

Source of organic detritus and bivalve biomass influences nitrogen cycling and extracellular enzyme activity in estuary sediments

In aquatic ecosystems, natural processes that remove nitrogen from the biologically available pool (e.g. denitrification) have been intensively studied as an ecosystem function that reduces eutrophication. The quantity of sediment organic matter is a key driver of denitrification with percent organic content positively related to rates of nitrogen removal; however, few studies have investigated the influence of the quality of organic matter on nitrogen cycling in estuarine sediments despite shifts in primary producers with eutrophication. This laboratory study using intact benthic communities investigates the influence of various organic detritus sources, which vary in their C:N ratio, on nitrogen gas (N2) and solute fluxes and extracellular enzyme activity in estuarine sediments. A custom-built tank with a removable front plate was used with a planar optode film to image sediment oxygenation. Mangrove leaf detritus significantly increased the net N2 production in sediments, while the deposition of other detrital sources and control sediments produced net N2 consumption. Sulfatase activity was significantly reduced in the mangrove leaves and seagrass treatments, suggesting alteration of heterotrophic microbial activity with reducing oxygen conditions. Leucine aminopeptidase activity, indicating nitrogen cycling, was reduced in all treatments, suggesting the organic detritus provided a nitrogen supplement or reduced the activity of extracellular enzymes producing microbes. Bivalve biomass increased net nitrogen gas fluxes in some treatments. Our results indicate different detrital sources may have varying impacts on the removal of bioavailable nitrogen through denitrification and show that feedbacks in biogeochemical cycles may occur with changes in organic detrital source pools.

CO2-Argon-Steam Oxy-Fuel Production for (CARSOXY) Gas Turbines

Due to growing concerns about carbon emissions, Carbon Capture and Storage (CCS) techniques have become an interesting alternative to overcome this problem. CO2-Argon-Steam-Oxy (CARSOXY)-fuel gas turbines are an innovative example that integrates CCS with gas turbine powergen improvement. Replacing air-fuel combustion by CARSOXY combustion has been theoretically proven to increase gas turbine efficiency. Therefore, this paper provides a novel approach to continuously supply a gas turbine with a CARSOXY blend within required molar fractions. The approach involves H2 and N2 production, therefore having the potential of also producing ammonia. Thus, the concept allows CARSOXY cycles to be used to support production of ammonia whilst increasing power efficiency. An ASPEN PLUS model has been developed to demonstrate the approach. The model involves the integrations of an air separation unit (ASU), a steam methane reformer (SMR), water gas shift (WGS) reactors, pressure swing adsorption (PSA) units and heat exchanged gas turbines (HXGT) with a CCS unit. Sensitivity analyses were conducted on the ASU-SMR-WGS-PSA-CCS-HXGT model. The results provide a baseline to calibrate the model in order to produce the required CARSOXY molar fraction. A MATLAB code has also been developed to study CO2 compression effects on the CARSOXY gas turbine compressor. Thus, this paper provides a detailed flowsheet of the WGS-PSA-CCS-HXGT model. The paper provides the conditions in which the sensitivity analyses have been conducted to determine the best operable regime for CARSOXY production with other high valuable gases (i.e., hydrogen). Under these specifications, the sensitivity analyses on the (SMR) sub-model spots the H2O mass flow rates, which provides the maximum hydrogen level, the threshold which produces significant CO2 levels. Moreover, splitting the main CH4 supply to sub-supply a SMR reactor and a furnace reactor correlates to best practices for CARSOXY. The sensitivity analysis has also been performed on the (ASU) sub-model to characterise its response with respect to the variation of air flow rate, distillation/boiling rates, product/feed stage locations and the number of stages of the distillation columns. The sensitivity analyses have featured the response of the ASU-SMR-WGS-PSA-CCS-HXGT model. In return, the model has been qualified to be calibrated to produce CARSOXY within two operability modes, with hydrogen and nitrogen or with ammonia as by-products.

Rate/Concentration Kinetic Petals: A Transient Method to Examine the Interplay of Surface Reaction Processes.

Transient pulse response experiments are used to construct rate/concentration kinetic dependencies, RC Petals and provide a new method to distinguish the timing and interplay of adsorption, surface reaction, and product formation on complex (industrial) materials. A petal shape arises as the dynamic "reaction-diffusion" experiment forces the concentration and reaction rate to return to zero. In contrast to the typical steady-state "Langmuir-type" RC dependence, RC petals have two branches, which arise as a result of decoupled gas and surface concentrations in the non-steady-state regime. To demonstrate this approach, the characteristics of petal shapes using ammonia decomposition as a probe reaction are presented. Ammonia, hydrogen, and nitrogen transformation rates are compared on three simple materials: iron, cobalt, and a bimetallic CoFe preparation when ammonia is pulsed at 550 °C in a low-pressure diffusion reactor. All materials demonstrate a two-branch kinetic RC dependence for ammonia adsorption, and rate constants are quantified in the low-coverage regime. We found that H2 and N2 product formation was dependent on the concentration of surface intermediates for all materials with one exception: for cobalt, an additional fast hydrogen generation process was observed; the rate of which coincided with ammonia adsorption. Nitrogen generation was only significant for CoFe and cobalt and on the CoFe catalyst, a self-inhibition property was observed. A method for estimating the number of active sites based on the RC petals is presented and was applied to the iron and CoFe samples. The surface coverage and rate of formation/conversion of surface intermediates are interpreted from the examination of shape characteristics of the RC petals for each material.