N2 Production Research Articles

Rates and microbial communities of denitrification and anammox across coastal tidal flat lands and inland paddy soils in East China

Denitrification and anammox are the main pathways of N loss in wet habitats while their relative contribution to N2 production and underlying microbial mechanisms across marine-terrestrial ecotone remain unclear. Here we investigated the rates of anammox and denitrification, and the distribution patterns of anammox bacteria and nosZ clade I and II denitrifiers across coastal tidal flat to inland paddy soils. Results showed that denitrification dominated the N2 production over anammox in all samples, accounting for 87.1%–100% of total N2 production. Coincident with the rate, the abundance of nosZ clade I and II genes were 1–2 orders of magnitude higher than those of hzsB gene. The community of anammox bacteria was mainly driven by salinity while nosZ clade I and II denitrifiers communities were mainly determined by both salinity and pH. Alphaproteobacteria dominated in the nosZ clade I community in all samples while Betaproteobacteria and Gammaproteobacteria were mainly present in low salinity wetland and paddy soils. The nosZ clade II community was composed of more phyla, in which Bacteoidetes and Chloroflexi were ubiquitous while Gemmatimonadetes was present only in low salinity wetland and paddy soils. These findings have direct implications for explicitly incorporating both nosZ clade I and II into future N loss estimation and N2O mitigation in marine-terrestrial ecotone.

Using modern low-oxygen marine ecosystems to understand the nitrogen cycle of the Paleo- and Mesoproterozoic oceans.

During the productive Paleoproterozoic (2.4-1.8Ga) and less productive Mesoproterozoic (1.8-1.0Ga), the ocean was suboxic to anoxic and multicellular organisms had not yet evolved. Here, we link geologic information about the Proterozoic ocean to microbial processes in modern low-oxygen systems. High iron concentrations and rates of Fe cycling in the Proterozoic are the largest differences from modern oxygen-deficient zones. In anoxic waters, which composed most of the Paleoproterozoic and~40% of the Mesoproterozoic ocean, nitrogen cycling dominated. Rates of N2 production by denitrification and anammox were likely linked to sinking organic matter fluxes and in situ primary productivity under anoxic conditions. Additionally autotrophic denitrifiers could have used reduced iron or methane. 50% of the Mesoproterozoic ocean may have been suboxic, promoting nitrification and metal oxidation in the suboxic water and N2 O and N2 production by partial and complete denitrification in anoxic zones in organic aggregates. Sulfidic conditions may have composed ~10% of the Mesoproterozoic ocean focused along continental margins. Due to low nitrate concentrations in offshore regions, anammox bacteria likely dominated N2 production immediately above sulfidic zones, but in coastal regions, higher nitrate concentrations probably promoted complete S-oxidizing autotrophic denitrification at the sulfide interface.

Individual amounts of Lewis and Brønsted acid sites on metal oxides from NH3 adsorption equilibrium: Case of TiO2 based solids

The present study is dedicated to the development of an original method for the measurement of the individual amounts QASi of the acid sites ASi (acidity of Lewis and Brønsted) present on three TiO2 based solids of increasing composition complexity: TiO2-P25, 6% WO3/TiO2-P25 and a sulfated 0.7 % V2O5/9% WO3/TiO2 NH3-SCR catalyst. The method is based on quantitative characterizations of the NH3 adsorption equilibrium (adsorption temperature Ta and pressure Pa) by the association of (a) the AEIR method providing the individual coverage θASi(Ta, Pa) of the adsorbed NH3 species on the ASi sites and (b) the total amount (in μmol/g) of the adsorbed NH3 species: QTNH3(Ta, Pa) by using a mass spectrometer. For a solid having n types of ASi sites, the QASi values are obtained from the numerical solution of linear equation systems (with at least n equations) obtained considering that QTNH3(Ta, Pa) is equal to the sum of the contribution of each adsorbed NH3 species: QASi θASi(Ta, Pa).This imposes the measurement of at least n QTNH3(Ta, Pa) in a Ta range preventing the contribution of parallel surface processes (i.e NH3 oxidation). On TiO2-P25, n = 3 (two Lewis and one weak Brønsted acid sites) and the QASi amounts are obtained from three QTNH3(Ta, Pa) values. The others solids having two Lewis (L1 and L2) and two Brønsted (B1 and B2) acid sites impose a series of m measurements of QTNH3(Ta, Pa) with m>> n: the QASi are obtained by optimization between theoretical and experimental QTNH3(Ta, Pa) curves such as 109, 202, 70 and 130 μmol/g for QL2, QL1, QB2 and QB1 respectively of 0.7 % V2O5/9% WO3/TiO2. It is shown that these four amounts of sites permit to conclude that the L2 Lewis acid site is the one forming the pivotal NH3ads-L2 species of the reaction by using nitrogen mass balances between the amounts of adsorbed NH3 species and the N2 production in the presence of NO.

Earthworm gut: An overlooked niche for anaerobic ammonium oxidation in agricultural soil

Soil fauna takes an active part in accelerating turnover of nutrients in terrestrial ecosystems. Anaerobic ammonium oxidation (anammox) has been widely characterized, however, whether anammox is active in earthworm gut and the effect of earthworm on anammox in soil remain unknown. In this study, the activity, abundance and community of anammox bacteria in earthworm guts and soils from microcosms were determined using a 15N-tracing technique, quantitative PCR, and anammox bacterial 16S rRNA gene amplicon sequencing. Results showed that anammox rates in guts ranged between 5.81 and 14.19 nmol N g−1 dw gut content h−1, which were significantly (P < 0.01) higher than that in their surrounding soils during 30 day incubation. On the contrary, abundances of hzsB genes encoding subunit B hydrazine synthase in guts were significantly (P < 0.05) lower than those in their surrounding soils. Anammox rates, denitrification N2 production rates and hzsB genes in soils with earthworms were significantly (P < 0.05) lower than those in control soils. Anammox bacterial compositions differed significantly (P < 0.05) between gut and soil, and earthworm altered anammox bacterial communities in soils. Brocadia, Kuenenia and abundant unclassified anammox bacteria were detected in collected soils and gut contents, in which Brocadia was only detected in guts. These results suggested that microbes in earthworm gut increase, but present of earthworm reduces anammox and denitrification associated N loss by altering the anammox bacterial community compositions in soils.

Both microbial abundance and community composition mattered for N2 production rates of the overlying water in one high-elevation river

Overlying water is another potential hotspot of nitrogen removal through anammox and denitrification reactions in river systems. However, N2 production and the controlling factors have rarely been investigated in the overlying water of high-elevation rivers. This study analyzed the abundance and community of denitrifying and anammox bacteria as well as their effects on N2 production rates in the overlying water of the Yellow River source region (elevation range: 2687−4223 m). Higher suspended particle concentrations remarkably promoted functional gene abundances of both denitrifying and anammox bacteria (r > 0.9, p < 0.01). N2 production rates in overlying water samples ranged from 0.25 to 4.22 μmol N2 L−1 d−1. The overlying water was estimated to contribute to 36.8% (on average) of riverine N2 emission flux. Higher temperatures markedly accelerated N2 production rates (p = 0.051). Moreover, N2 production rates were positively related to both anammox and denitrifying bacterial abundances (p < 0.05), and such relationships were markedly affected by corresponding community compositions. The explanatory power of denitrifier abundance (R2 = 0.56) for N2 production rate variations was greatly elevated when it was integrated with community composition (R2 = 0.92). This study highlights the significance of overlying water nitrogen removal in the Yellow River source region; moreover, the effects of both microbial abundance and community composition on riverine N2 production rates should be considered in future research.

Impacts of human disturbance on the biogeochemical nitrogen cycle in a subtropical river system revealed by nitrifier and denitrifier genes

Human activities have largely modified nitrogen (N) sources supply, cycling and export from land to ocean. Nitrification and denitrification are vital processes alleviating N pollution in aquatic ecosystems but the diverse responses and niche of microbial N retention to human disturbance are still understudied. Here we investigated the changes in N species and functional genes in the urban, agriculture and reservoir river sections of the Jiulong River (southeast China). Our results show that ammonia-oxidizing bacteria (AOB) (Nitrosomonas) were dominant in the urban river section receiving ammonium-rich sewage that enhanced nitrification and subsequent denitrification, while ammonia-oxidizing archaea (AOA) was more abundant than AOB in the river section flowing through areas of pomelo (Citrus maxima) agriculture with low pH, low ammonium and very high nitrate input. Warm temperatures and large total suspended matter (TSM) in the wet season promoted growth of nitrifiers and denitrifiers, which were mostly particle-attached. The potential river N retention through gaseous N removal (PRN2O and PRN2) in the agriculture section with huge N loading was among the lowest. Strong nitrification and denitrification were suspected to occur in the agricultural acid soil system rather than in the river network. In addition, the decreased TSM and N concentration promoted free-living microbes in the reservoir. The highest PRN2 and N2 production observed in the reservoir in the dry season implied that denitrification and anammox occurring in sediments was likely to increase N retention. This study suggests the diverse factors involved in processing of N pollution among diverse landscapes.

Role of algal accumulations on the partitioning between N2 production and dissimilatory nitrate reduction to ammonium in eutrophic lakes

Cyanobacterial blooms change benthic nitrogen (N) cycling in eutrophic lake ecosystems by affecting organic carbon (OC) delivery and changing in nutrients availability. Denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) are critical dissimilatory nitrate reduction pathways that determine N removal and N recycling in aquatic environments. A mechanistic understanding of the influence of algal accumulations on partitioning among these pathways is currently lacking. In the present study, a manipulative experiment in aquarium tanks was conducted to determine the response of dissimilatory nitrate reduction pathways to changes in algal biomass, and the interactive effects of OC and nitrate. Potential dinitrogen (N2) production and DNRA rates, and related functional gene abundances were determined during incubation of 3–4 weeks. The results indicated that high algal biomass promoted DNRA but not N2 production. The concentrations of dissolved organic carbon were the primary factor affecting DNRA rates. Low nitrate availability limited N2 production rates in treatments with algal pellets and without nitrate addition. Meanwhile, the AOAamoA gene abundance was significantly correlated with the nrfA and nirS gene abundances, suggesting that coupled nitrification-denitrification/DNRA was prevalent. Partitioning between N2 production and DNRA was positively correlated with the ratios of dissolved organic carbon to nitrate. Correspondingly, in Lake Taihu during summer to fall, the relatively high organic carbon/nitrate might favorably facilitate DNRA over denitrification, subsequently sustaining cyanobacterial blooms.

Turnover of Urea in a Soil from the North China Plain as Affected by the Urease Inhibitor NBPT and Wheat Straw

In the North China Plain (NCP), urea is applied to predominantly high-pH soils and high gaseous losses of N are observed in the forms of ammonia (NH3) and nitrous oxide (N2O). The production of N2O and N2 is further stimulated by the addition of fresh organic materials. The urease inhibitor (UI) N-(n-butyl) thiophosphoric triamide (NBPT) restricts the loss of N by reducing the rate of urea hydrolysis and limiting the substrates for NH3 and N2O emission. We investigated the turnover of N—as well as the emissions of NH3, N2O and CO2—as affected by the addition of wheat straw and urea +NBPT in an incubation experiment using soil from a farmer’s field, typical for the NCP. Added wheat straw prolonged the persistence of NH4+ after the application of both urea and urea +NBPT, while reducing the formation of nitrate. Wheat straw may therefore either act as a stimulant of hydrolysis or as an inhibitor of nitrification. Urea increased soil respiration and the emission of N2O, possibly acting as a primer for microbial activity as described in earlier studies. In combination with the application of organic C sources, this effect of urea may be a main driver of gaseous N loss.

Spatial variations in sedimentary N-transformation rates in the North Sea (German Bight)

Abstract. In this study, we investigate the role of sedimentary N cycling in the southern North Sea. We present a budget of ammonification, nitrification and sedimentary NO3- consumption and denitrification in contrasting sediment types of the German Bight (southern North Sea), including novel net ammonification rates. We incubated sediment cores from four representative locations in the German Bight (permeable, semi-permeable and impermeable sediments) with labeled nitrate and ammonium to calculate benthic fluxes of nitrate and ammonium and gross rates of ammonification and nitrification. Ammonium fluxes generally suggest oxic degradation of organic matter, but elevated fluxes at one sampling site point towards the importance of bioirrigation or short-term accumulation of organic matter. Sedimentary fluxes of dissolved inorganic nitrogen are an important source for primary producers in the water column, supporting ∼7 % to 59 % of the average annual primary production, depending on water depth. We find that ammonification and oxygen penetration depth are the main drivers of sedimentary nitrification, but this nitrification is closely linked to denitrification. One-third of freshly produced nitrate in impermeable sediment and two-thirds in permeable sediment were reduced to N2. The semi-permeable and permeable sediments are responsible for ∼68 % of the total benthic N2 production rates, which, based solely on our data, amounts to ∼1030 t N d−1 in the southern North Sea. Thus, we conclude that semi-permeable and permeable sediments are the main sinks of reactive N, counteracting eutrophication in the southern North Sea (German Bight).

Differences in labile soil organic matter explain potential denitrification and denitrifying communities in a long-term fertilization experiment

Content and quality of organic matter (OM) may strongly affect the denitrification potential of soils. In particular, the impact of soil OM fractions of differing bioavailability (soluble, particulate, and mineral-associated OM) on denitrification remains unresolved. We determined the potential N2O and N2 as well as CO2 production for samples of a Haplic Chernozem from six treatment plots (control, mineral N and NP, farmyard manure - FYM, and FYM + mineral N or NP) of the Static Fertilization Experiment Bad Lauchstädt (Germany) as related to OM properties and denitrifier gene abundances. Soil OM was analyzed for bulk chemical composition (13C-CPMAS NMR spectroscopy) as well as water-extractable, particulate, and mineral-associated fractions. Soils receiving FYM had more total OM and larger portions of labile fractions such as particulate and water-extractable OM. Incubations were run under anoxic conditions without nitrate limitation for seven days at 25 °C in the dark to determine the denitrification potential (N2O and N2) using the acetylene inhibition technique. Abundances of nirS, nirK, and nosZ (I + II) genes were analyzed before and after incubation. The denitrification potential, defined as the combined amount of N released as N2O + N2 over the experimental period, was larger for plots receiving FYM (25.9–27.2 mg N kg−1) than pure mineral fertilization (17.1–19.2 mg N kg−1) or no fertilization (12.6 mg N kg−1). The CO2 and N2O production were well related and up to three-fold larger for FYM-receiving soils than under pure mineral fertilization. The N2 production differed significantly only between all manured and non-manured soils. Nitrogenous gas emissions related most closely to water-extractable organic carbon (WEOC), which again related well to free particulate OM. The larger contribution of N2 production in soils without FYM application, and thus, with less readily decomposable OM, coincided with decreasing abundances of nirS genes (NO2− reductase) and increasing abundances of genes indicating complete denitrifying organisms (nosZ I) during anoxic conditions. Limited OM sources, thus, favored a microbial community more efficient in resource use. This study suggests that WEOC, representing readily bioavailable OM, is a straightforward indicator of the denitrification potential of soils.

Nitrite oxidation exceeds reduction and fixed nitrogen loss in anoxic Pacific waters

The diversity of nitrogen-based dissimilatory metabolisms in anoxic waters continues to increase with additional studies to the marine oxygen deficient zones (ODZs). Although the microbial oxidation of nitrite (NO2–) has been known for over a century, studies of the pathways and microbes involved have generally proceeded under the assumption that nitrite oxidation to nitrate requires dioxygen (O2). Anaerobic NO2– oxidation until now has been conclusively shown only for anammox bacteria, albeit only as a limited sink for NO2– in their metabolism compared to the NO2– reduced to N2. Here, using direct experimental techniques optimized for replicating in situ anoxic conditions, we show that NO2– oxidation is substantial, widespread, and consistent across the ODZs of the eastern tropical Pacific Ocean. Regardless of the specific oxidant, NO2– oxidation rates are up to an order of magnitude larger than simultaneous N2 production rates for which these zones are known, and cannot be explained by anammox rates alone. Higher rates of NO2– oxidation over reduction in anoxic waters are paradoxical but help to explain how anammox rates can be enhanced over denitrification in shallow anoxic waters (σθ < 26.4) at the edge of the ODZs but not within the ODZ core. Furthermore, nitrite oxidation may be the key to reconciliation of the perceived imbalance of the global fixed nitrogen loss budget.

Manipulating the crystalline morphology and facet orientation of copper and copper-palladium nanocatalysts supported on stainless steel mesh with the aid of cationic surfactant to improve the electrochemical reduction of nitrate and N2 selectivity

Surfactants, namely, cetyltrimethylammonium chloride, polydiallyldimethylammonium chloride, and benzethonium chloride were used to control the crystal growth of metallic Cu nanoparticles supported on stainless steel mesh electrodes as to improve the selective electrochemical conversion of NO3− to non-toxic N2. Results showed that the Cu(200)/Cu(111) ratio controlled the selectivity of N2, NO2−, and NH4+. The Kd value increased from 10% to 30% when the Cu(200)/Cu(111) intensity ratio was decreased 60% to 30%, meaning increase Cu(111) increased N2 production. Furthermore, the presence of a second metal, namely, Pd increased the N2 selectivity. The best N2 yield (XN2 = 22%), occurring on monometallic Cu, synthesized with BZT at 1-time CMC was further increased to XN2 = 65% over bimetallic Pd0.27Cu0.73/SS. The selectivity of nitrite (SNO2-) and ammonium (SNH4+) on Cu/SS were 33.1 and 43.5%, respectively, which were decreased to 0.30 and 34.0%, respectively, on bimetallic Pd0.27Cu0.73/SS.

The improvement on total nitrogen removal in nitrate reduction by using a prepared CuO–Co3O4/Ti cathode

In this work, a CuO–Co3O4/Ti composite was prepared via the coating-calcination method and employed as a cathode for the NO3−-N reduction to increase the removal efficiency of total nitrogen (TN). SEM, EDS, and XRD characterization results indicated that CuO and Co3O4 were successfully introduced to the surface of Ti. The CuO–Co3O4/Ti electrode eventually removed NO3−-N with the main products of N2, NH4+-N and NO2−-N. In comparison to the Co3O4/Ti electrode, the better hydrogen evolution properties of the CuO–Co3O4/Ti electrode resulted in pH increase and NH3 gas release, so the TN removal for CuO–Co3O4/Ti electrode was improved approximately 20%. The presence of Cl− with the concentration up to 1000 mg L−1 greatly promoted the removal of TN from 40.1% to 94.0%, as a result of NH4+-N oxidation with free chlorine produced from the anode. Furthermore, the CuO–Co3O4/Ti electrode was applied to conduct three types of actual wastewater (biological effluent of municipal wastewater and industrial wastewater, and a regeneration concentrate from an anion exchange process) for nitrate removal. The highest TN removal efficiency (78.5%) and current efficiency (54.5%), and the lowest energy consumption (2 × 10−4 kWh mg−1 TN) were obtained for the regeneration concentrate, suggesting the feasibility of the CuO–Co3O4/Ti electrode to the water with high conductivity and high Cl− concentration for removing TN by the reduction of nitrate.

Abundance, diversity, and distribution patterns along with the salinity of four nitrogen transformation-related microbes in the Yangtze Estuary

PurposeThe abundance and composition of nitrogen transformation-related microbes with certain environmental parameters for living conditions provide information about the nitrogen cycle in the Yangtze Estuary. The aim of this study was to explore the impacts of salinity on four N-related microbes and reveal the phylogenetic characteristics of microorganisms in the Yangtze Estuary ecosystem. A molecular biology method was used for the quantitation and identification of four microbes in the Yangtze River: ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), denitrifying microbes (nirS-type), and anaerobic ammonia-oxidizing (anammox) bacteria. Sequence identification was performed on the levels of phylum, class, order, family, and genus, and the sequences were then matched to species.ResultThe results showed that the dominant species of AOA were crenarchaeote enrichment cultures, thaumarchaeote enrichment cultures, and Nitrosopumilus maritimus cultures, and the dominant AOB species were betaproteobacterium enrichment cultures and Nitrosomona sp. The denitrifying microbes were identified as the phylum Proteobacteria, classes Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria, and the species Thauera selenatis. The dominant species of the anammox bacteria was Candidatus Brocadia sp. In the estuarine sediments of the Yangtze River, the nirS gene abundance (1.31 × 107–9.50 × 108 copies g−1 sediments) was the highest among all the detected genes, and the abundance of bacterial amoA, archaeal amoA, and nirS was significantly correlated. Closely correlated with the abundance of the bacterial amoA gene, salinity was an important factor in promoting the abundance and restraining the community diversity of AOB. Moreover, the distribution of the AOB species exhibited regional patterns in the estuarine zone.ConclusionsThe results indicated that salinity might promote abundance while limiting the diversity of AOB and that salinity might have reverse impacts on AOA. Denitrifying microbes, which showed a significant correlation with the other genes, were thought to interact with the other genes during nitrogen migration. The results also implied that AOA has a lower potential nitrification rate than AOB and that both the anammox and denitrification processes (defined by nirS gene) account for N2 production.

Recognizing both denitrification and nitrogen consumption improves performance of stream diel N2 flux models

AbstractThere is keen interest to enhance denitrification within intervening aquatic habitats between agricultural areas and downstream aquatic ecosystems to reduce nitrogen (N) loading impacts to receiving ecosystems. We conducted a series of measurements to examine whole system in situ diel denitrification estimates in experimental ditch and stream environments using a Bayesian one‐station diel N2 flux model. Model estimates revealed complex patterns that indicate fluxes may be controlled by the balance of both N2 production via denitrification and consumption driven by physical or biological processes associated with strong diel patterns in environmental conditions. We investigated potential improvements in model fits to observed data associated with the addition of a N2 consumption term to represent biological (N2 fixation) or physical (bubble formation and N2 scavenging) mechanisms associated with daytime photosynthesis. We also expanded the current one‐station diel flux model to a two‐station model to estimate denitrification in discrete reaches. Our modified diel N2 flux models improved model fit significantly across three metrics (Nash–Sutcliffe efficiency, root mean square ratio, and percent bias) increasing their utility in shallow, open canopy, lotic systems. While more studies are needed to understand specific mechanisms associated with N2 consumption processes in small agricultural drainages as well as environmental conditions affecting their relative importance, these results improve estimates of N2 flux where dynamic conditions and heterogeneity of habitats create severe diel patterns in factors controlling dissolved gas concentrations and prohibit accurate estimates of N2 flux using existing models.

(Invited) Electrocatalytic Ammonia Oxidation

Much of the global effort toward producing solar fuels has involved splitting water to obtain molecular hydrogen. Unfortunately, hydrogen gas has a relatively low energy-per-volume, and extreme cryogenic temperature and pressure is needed to bring hydrogen to a condensed state. An interesting choice for a hydrogen storage medium that circumvents these setbacks is ammonia. Synthesis of ammonia (largely via the Haber-Bosch process) is already the second-largest chemical industrial process in the world, and as a result the infrastructure for ammonia storage and transport already exists. Ammonia can be easily liquefied under 10 bar of pressure which leads to a three-fold higher energy density than hydrogen gas in state-of-the-art carbon fiber cylinders. One significant bottleneck limiting the use of ammonia for hydrogen storage is the inability to efficiently extract hydrogen from ammonia on-demand.Electrolysis offers a convenient route to decompose ammonia into nitrogen and hydrogen, which potentially only requires 0.1 V. However, there is a lack of fundamental understanding of the reactions involved in the electrolysis. In this talk, the electrolysis of liquid ammonia with heterogeneous catalysts, as well as ammonia dissolved in non-aqueous solvents, using two and three electrode measurements, will be presented.1,2 The possible mechanisms of the anodic and cathodic reactions will be discussed, as well as the cause and magnitude of the related overpotentials which limit the efficiency. Further, while there have been multiple reports on heterogeneous catalysts for electrocatalytic oxidation of NH3, with limited success, homogenous molecular catalysts have been non-existent until now.3 A transition metal complex, [Ru(tpy)(dmabpy)NH3]2+ (tpy = 2,2′:6′,2′′-terpyridine, dmabpy = 4,4'-dimethylamino-2,2'-bipyridine), will be presented which shows the first example of electrocatalytic activity in presence of ammonia at room temperature and ambient pressure. Gas chromatography measurements reveals production of N2 and H2 in a 1:3 ratio following bulk electrolysis of ammonia. Absorption measurements following reaction of [Ru(tpy)(dmabpy)NH3]3+ with one equivalent of ammonia clearly show regeneration of [Ru(tpy)(dmabpy)NH3]2+ which provides evidence of redox disproportionation to produce a Ru(IV) imido species as an intermediate in formation of the N-N bond. Nitrogen isotope labeling experiments reveal incorporation of the coordinated amine with free ammonia in solution to produce dinitrogen. Results from cyclic voltammetry, stopped-flow, NMR and rotating ring-disc measurements will also be presented. From these combined results, a tentative mechanism will be presented and the implications discussed.1) Little, D.J., Smith, M.R., Hamann, T.W.; “Electrolysis of Liquid Ammonia for Hydrogen Generation” Energy & Environmental Science, 2015, 8, 2775 – 27812) Little, D.J., Edwards, D., Smith, M.R., Hamann, T.W.; “As Precious as Platinum: Iron Nitride for Electrocatalytic Oxidation of Liquid Ammonia” ACS Applied Materials & Interfaces 2017, 9 (19), 16228 – 162353) Habib-Zadeh, F., Miller, S.L., Hamann, T.W., Smith, M.R.; “Homogenous Electro-Catalytic Oxidation of Ammonia to N2 Under Mild Conditions” Proceedings of the National Academy of Science 2019, 116 (8) 2849 – 2853

Nitrogen loss by nirS-type denitrifying bacterial communities in eutrophic coastal sediments

Denitrification plays a dominant role in the removal of reactive inorganic nitrogen species in different ecosystems. In this study, we investigated the activity, abundance, and community composition of nirS-encoding denitrifying bacteria from sediments of Jiaozhou Bay (JZB). The rates of denitrification, measured by 15N amendment and incubation experiments, varied from 0.18 to 9.97 nmol N g−1 h−1 and contributed to 81.45–99.93% of the total N2 production, while the estimated flux of nitrogen removal was about 5.66 × 104 t N y−1 in surface sediments. Additionally, the abundance of nirS gene ranged from 5.20 × 105 to 3.01 × 108 copies per gram. High-throughput sequencing technique was used to target the nirS gene, which revealed 19 dominant OTUs at 93% similarity that were assigned to 7 clusters. Cluster 2 was mainly affiliated to Thiothrix, while Cluster 4 was mainly affiliated to Woeseia and the other clusters were similar to uncultured denitrifiers obtained from sediments of several coastal estuaries and bays. NH4+ and NO3− contents were identified as the two main factors affecting community composition and distribution of denitrifiers among the surface sediments of JZB. Overall, our results showed that the community of nirS-type denitrifying bacteria had a diverse composition and could highly contribute to nitrogen loss in eutrophic systems.

Comparing performances, costs and energy balance of ex situ remediation processes for PAH-contaminated marine sediments.

This study proposes a comparison of different ex situ technologies aimed at the removal of polycyclic aromatic hydrocarbons from marine sediments in terms of performances, costs and energy balance. In accordance with the principles of water-energy nexus, anaerobic bioremediation, soil washing and thermal desorption were investigated under low liquid phase and temperature conditions using phenanthrene (PHE) as model compound. After 42days of anaerobic bioremediation, the highest PHE biodegradation of 68 and 64% was observed under denitrifying and methanogenic conditions, respectively, accompanied by N2 and CH4 production and volatile fatty acid accumulation. During soil washing, more than 97% of PHE was removed after 60min using a solid-to-liquid ratio of 1:3. Along the same treatment time, low-temperature thermal desorption (LTTD) allowed a PHE removal of 88% at 200°C. The economic analysis indicated that LTTD resulted in a higher cost (i.e. 1782 €m-3) than bioremediation and soil washing (228 and 371 €m-3, respectively). The energy balance also suggested that bioremediation and soil washing are more sustainable technologies as a lower required energy (i.e. 16 and 14kWhm-3, respectively) than LTTD (i.e. 417kWhm-3) is needed.

Reduction of nitrate on tin-modified palladium-platinum electrodes

The Sn-modified PdPt (Sn/PdPt) electrodes, prepared on Au film substrates by sputtering the PdPt alloy followed by Sn-adsorption, were characterized by cyclic voltammetry and X-ray photoelectron spectroscopy and examined for the reduction of nitrate. The unmodified PdPt (Pd/Pt = 31/69) electrodes did not show any catalytic activity for the nitrate reduction. Tin-modification of the electrodes significantly increased the reduction activity, and the maximum activity was obtained at the Sn surface coverage (θSn) of 0.8. The main reduction product was N2 (44%). Based on the comparison of the observed electrocatalytic activity and product distribution on the Sn/PdPt electrodes and that expected for a physical mixture of Sn/Pt and Sn/Pd, it became clear that there exist synergetic effects between the Pd and Pt on both the activity and N2 production. Further analysis of the product distribution based on the number of bond formations and cleavage clarified that the N2 production capability was enhanced as a result of the enhanced NO bond cleavage, NN bond formation, and the reduced NH bond formation.

Modelling of an Atmospheric–Pressure Air Glow Discharge Operating in High–Gas Temperature Regimes: The Role of the Associative Ionization Reactions Involving Excited Atoms

A model of a stationary glow-type discharge in atmospheric-pressure air operated in high-gas-temperature regimes (1000 K &lt; Tg &lt; 6000 K), with a focus on the role of associative ionization reactions involving N(2D,2P)-excited atoms, is developed. Thermal dissociation of vibrationally excited nitrogen molecules, as well as electronic excitation from all the vibrational levels of the nitrogen molecules, is also accounted for. The calculations show that the near-threshold associative ionization reaction, N(2D) + O(3P) → NO+ + e, is the major ionization mechanism in air at 2500 K &lt; Tg &lt; 4500 K while the ionization of NO molecules by electron impact is the dominant mechanism at lower gas temperatures and the high-threshold associative ionization reaction involving ground-state atoms dominates at higher temperatures. The exoergic associative ionization reaction, N(2P) + O(3P) → NO+ + e, also speeds up the ionization at the highest temperature values. The vibrational excitation of the gas significantly accelerates the production of N2(A3∑u+) molecules, which in turn increases the densities of excited N(2D,2P) atoms. Because the electron energy required for the excitation of the N2(A3∑u+) state from N2(X1∑g+, v) molecules (e.g., 6.2 eV for v = 0) is considerably lower than the ionization energy (9.27 eV) of the NO molecules, the reduced electric field begins to noticeably fall at Tg &gt; 2500 K. The calculated plasma parameters agree with the available experimental data.