N2 Production Research Articles

A comprehensive root cause analysis of anammox bioreactor performance decline

The cultivation of anaerobic ammonia oxidizing bacteria (anammox) has gained enormous awareness over the last few decades. Although numerous studies focus massively on successfully growing these anammox to different enrichment environments, in reality, the failure rates are somewhat comparable to the reported success rates. This study combines a variety of measurement techniques to observe and monitor the sequence of a bioreactor performance decline following elevated influent substrate concentration. After attaining stable substrate removal throughout a nitrogen loading rate (NLR) range of 0.691 to 1.669 kg-N·m−3·d−1, the performance of the lab-scale anammox-sequencing batch reactor (SBR) abruptly broke down as the NLR reached 2.01 kg-N·m−3·d−1. The gathered information showed that the increased NLR firstly caused a significant and unfavorable change in the free ammonia (FA) and free nitrous acid (FNA) concentration in the bioreactor. A subsequent drop in N2 production and a decline from a peak high of 0.381 to a low of 0.012 kg-N·kg-VSS−3·d−1 of the specific nitrogen removal rate (SNRR) led to an 82% absurd decline in microbial cellular energy production. Prior to these anammox switching to survival mode and secreting larger quantities (32% higher) of extracellular polymeric substances (EPS), the activity of syntrophic decomposers increased substantially leading to the internal production of excess CO2 in the bioreactor and thereby diverging the bioreactor pH to lower levels. The purposes of this study are to understand the reason an anammox process shows different signals during a decline phase and to enable immediate response to performance deterioration.

Anisotropy Measurements from the Near-Threshold Photodissociation of the N2-NO Complex.

We have used velocity map ion imaging to measure the angular anisotropy of the NO (A) products from the photodissociation of the N2-NO complex. Our experiment ranged from 108 to 758 cm-1 above the threshold energy to form NO (A) + N2 (X) products, and these measurements reveal, for the first time, a strong angular anisotropy from photodissociation. At 108 cm-1 above the photodissociation threshold, we observed NO (A) photoproducts recoil preferentially perpendicular to the laser polarization axis with an average anisotropy parameter, β = -0.25; however, as the available energy was increased, the anisotropy increased, and at 758 cm-1 above the threshold energy, we found an average β = +0.28. The observed changes in the angular anisotropy of the NO (A) photoproduct are qualitatively similar to those observed for the photodissociation of the Ar-NO complex and likely result from changes in the region of the excited state potential energy surface accessed during the electronic excitation. At the lowest available energy, we also noted a large contribution from hotband excitation; however, this contribution decreased as the available energy increased. The outsized contribution at the lowest available energy may result from hotbands having better Franck-Condon overlap with the excited electronic state near threshold. Finally, we contrast the experimental center of mass translational energy distribution with a statistical energy distribution determined from phase space theory. The experimental and statistical distributions show pronounced disagreement, particularly at low kinetic energies, with the experimental one showing less dissociation resulting in high rotational levels of the fragments.

An Underestimated Contribution of Deltaic Denitrification in Reducing Nitrate Export to the Coastal Zone (Po River–Adriatic Sea, Northern Italy)

In transitional environments, the role of sediments biogeochemistry and denitrification is crucial for establishing their buffer potential against nitrate (NO3−) pollution. The Po River (Northern Italy) is a worldwide hotspot of eutrophication. However, benthic N dynamics and the relevance of denitrification in its delta have not yet been described. The aim of the present study was to quantify the contribution of denitrification in attenuating the NO3− loading transported to the sea during summer. Benthic fluxes of dissolved inorganic nitrogen (N) and denitrification rates were measured in laboratory incubations of intact sediment cores collected, along a salinity gradient, at three sections of the Po di Goro, the southernmost arm of the Po Delta. The correlation between NO3− consumption and N2 production rates demonstrated that denitrification was the main process responsible for reactive N removal. Denitrification was stimulated by both NO3− availability in the Po River water and organic enrichment of sediment likely determined by salinity-induced flocculation of particulate organic load, and inhibited by increasing salinity, along the river–sea gradient. Overall, denitrification represented a sink of approximately 30% of the daily N loading transported in middle summer, highlighting a previously underestimated role of the Po River Delta.

Electrocatalytic nitrate reduction on bimetallic Palladium-Copper Nanowires: Key surface structure for selective dinitrogen formation

Selective electrocatalytic nitrate reduction reaction (NO3RR) to dinitrogen (N2) is essential to nitrogen-cycle management and environmental sustainability. Palladium-based nanomaterials were extensively studied as NO3RR catalysts, but only delivered limited N2 selectivity (<50%, NH3 dominated the product). Here we report that, by tuning bimetallic composition of CuPd alloy nanowires, the Cu28Pd72 nanowires deliver a near-unit conversion of NO3–-N and an unprecedented N2 product selectivity of 75.9 ± 5.2% in treating 22.5 mg L−1 NO3–-N solution at −0.80 V vs. Ag/AgCl. Combining in-situ spectrometric measurements and theoretical calculations, we demonstrate that the contiguous Pd ensemble with a specific size at catalyst surface is essential to the N2 formation via a kinetically-favored pathway of NO2* → NO* → N* → N2*. Alloying Pd with Cu is indispensable as Cu contributes to the N*-N* pairing by initiating the NO3––NO2* conversion and lowering the Hads coverage on catalyst. This work discloses the key surface atomic structure for N2 formation, and provides a guideline for the design of efficient NO3RR catalyst that enables selective NO3––N2 conversion.

Bioaugmentation potential of a cold-adapted and nitrate-reducing fungus to enhance nitrate removal in woodchip bioreactors

Woodchip bioreactors are a promising approach to remove nitrate from agricultural drainage systems; however, their low removal efficiency at cold temperatures remains a significant challenge. Here we analyzed the efficacy of fungal inoculation to enhance nitrate removal in woodchip bioreactor microcosms under cold conditions. Among the cold-adapted, nitrate-reducing, and cellulose-degrading fungi previously isolated from soil, we identified Linnemannia hyaline strain SCG-10 as the fastest nitrate reducer. Inoculation of this fungal strain enhanced the rates of nitrate reduction, N2 production, and dissolved organic carbon production in woodchip bioreactor microcosms at 5 °C. Production of N2O was minor (<1%) in this study most likely due to the enhanced activity of N2O reducers by labile carbon. Quantitative PCR for nitrogen cycle-related genes showed the active transcription of both fungal and bacterial denitrification genes. These results suggest that inoculating cold-adapted, nitrate-reducing, and cellulose-degrading fungi is a promising approach to remove nitrate from agricultural fields under cold conditions.

CuSn Double‐Metal Hydroxides for Direct Electrochemical Ammonia Oxidation to Dinitrogen

AbstractDirect electrochemical ammonia oxidation reaction (dAOR) to form N2 is of great significance to process ammonia‐containing wastewater or to make efficient ammonia fuel cells. Herein, CuSn double‐metal hydroxides are synthesized via a co‐precipitation method and used in dAOR with 10 mM −N Among the synthesized hydroxides, CuSn(OH)6 shows a decent −N partial current density (1.2 mA cm−2) and a substantially higher −N Faradaic efficiency (84.5 %). In situ infrared spectroscopy confirms that the intermediacy of dimeric species facilitates the formation of desired N2 product during the dAOR process. The synergy between Cu and Sn is critical for the enhanced dAOR activity, as Cu serves as the catalytic center while Sn adsorbs −N intensively.

An analysis of protists in Pacific oxygen deficient zones: implications for Prochlorococcus and N2 -producing bacteria.

Ocean oxygen deficient zones (ODZs) host 30%-50% of marine N2 production. Cyanobacteria photosynthesizing in the ODZ create a secondary chlorophyll maximum and provide organic matter to N2 -producing bacteria. This chlorophyll maximum is thought to occur due to reduced grazing in anoxic waters. We first examine ODZ protists with long amplicon reads. We then use non-primer-based methods to examine the composition and relative abundance of protists in metagenomes from the Eastern Tropical North and South Pacific ODZs and compare these data to the oxic Hawaii Ocean Time-series (HOT) in the North Pacific. We identify and quantify protists in proportion to the total microbial community. From metagenomic data, we see a large drop in abundance of fungi and protists such as choanoflagellates, radiolarians, cercozoa and ciliates in the ODZs but not in the oxic mesopelagic at HOT. Diplonemid euglenozoa were the only protists that increased in the ODZ. Dinoflagellates and foraminifera reads were also present in the ODZ though less abundant compared to oxic waters. Denitrification has been found in foraminifera but not yet in dinoflagellates. DNA techniques cannot separate dinoflagellate cells and cysts. Metagenomic analysis found taxonomic groups missed by amplicon sequencing and identified trends in abundance.

Process Optimization of Electrochemical Treatment of COD and Total Nitrogen Containing Wastewater.

In this work, an electrochemical method for chemical oxygen demand (COD) and total nitrogen (TN, including ammonia, nitrate, and nitrite) removal from wastewater using a divided electrolysis cell was developed, and its process optimization was investigated. This process could effectively relieve the common issue of NO3−/NO2− over-reduction or NH4+ over-oxidation by combining cathodic NO3−/NO2− reduction with anodic COD/NH4+ oxidation. The activity and selectivity performances toward pollutant removal of the electrode materials were investigated by electrochemical measurements and constant potential electrolysis, suggesting that Ti electrode exhibited the best NO3−/NO2− reduction and N2 production efficiencies. In-situ Fourier transform infrared spectroscopy was used to study the in-situ electrochemical information of pollutants conversion on electrode surfaces and propose their reaction pathways. The effects of main operating parameters (i.e., initial pH value, Cl− concentration, and current density) on the removal efficiencies of COD and TN were studied. Under optimal conditions, COD and TN removal efficiencies from simulated wastewater reached 92.7% and 82.0%, respectively. Additionally, reaction kinetics were investigated to describe the COD and TN removal. Results indicated that COD removal followed pseudo-first-order model; meanwhile, TN removal followed zero-order kinetics with a presence of NH4+ and then followed pseudo-first-order kinetics when NH4+ was completely removed. For actual pharmaceutical wastewater treatment, 79.1% COD and 87.0% TN were removed after 120 min electrolysis; and no NH4+ or NO2− was detected.

Community patch dynamics governs direct and indirect nutrient recycling by aggregated animals across spatial scales

AbstractAnimals can have pervasive effects on ecosystems as they modify their biogeochemical and physical environments. In particular, when animals occur in high densities these effects can result in dramatic changes in the physical environment and biogeochemical hotspots or hot moments. While most research to date has focused on the direct role of animals in biogeochemical cycles, few have examined how animals indirectly influence biogeochemical cycles across scales.Freshwater mussels occur as spatially heterogeneous, dense and species‐rich aggregations in many river ecosystems world‐wide. Here we examined how mussel communities (a) directly influence the flux of particulate and dissolved nutrients and (b) indirectly effect the flux of N2production, via denitrification, across a gradient of mussel biomass and differences in community composition at the patch‐ (0.25 m2) and stream reach‐scales (60–80 m).We combined measurements of ammonia (N) and soluble reactive phosphorous (P) excretion and C, N and P biodeposition rates for 10 species with biomass and distribution estimates for seven mixed‐species aggregations to quantify direct mussel contributions to biogeochemical cycling and the spatial heterogeneity of their impact. Additionally, we sampled sediments at a fine spatial scale to determine how mussel biomass and richness influence potential denitrification (indirect flux) rates at the patch‐ and reach‐scales.We predicted that increasing mussel biomass would lead to greater direct and indirect fluxes of nutrients, manifesting in heterogeneous nutrient redistribution within and among stream reaches. We also predicted that variation in community composition would result in differential nutrient excretion and egestion stoichiometries.Our results indicate that mussel aggregations directly influence soluble and particulate nutrient fluxes with community composition, particularly phylogenetic tribe composition, controlling the stoichiometry. Mussel aggregations also indirectly influenced nutrient fluxes as greater mussel biomass and species richness resulted in higher denitrification rates as mediated by their interactions with the sediments and enhancement of nutrient availability. Our results underscore the importance of patchy communities in acting as biogeochemical control points.A freePlain Language Summarycan be found within the Supporting Information of this article.

Electrochemical Reduction of Gaseous Nitrogen Oxides on Transition Metals at Ambient Conditions.

Mitigating nitrogen oxide (NOx) emissions is critical to tackle global warming and improve air quality. Conventional NOx abatement technologies for emission control suffer from a low efficiency at near ambient temperatures. Herein, we show an electrochemical pathway to reduce gaseous NOx that can be conducted at high reaction rates (400 mA cm-2) under ambient conditions. Various transition metals are evaluated for electrochemical reduction of NO and N2O to reveal the role of electrocatalyst in determining the product selectivity. Specifically, Cu is highly selective toward NH3 formation with >80% Faradaic efficiency in NO electroreduction. Furthermore, the partial pressure study of NO electroreduction revealed that a high NO coverage facilitates the N-N coupling reaction. In acidic electrolytes, the formation of NH3 is greatly favored, whereas the N2 production is suppressed. Additional mechanistic studies were conducted by using flow electrochemical mass spectrometry to gain further insights into reaction pathways. This work provides a promising avenue toward abating gaseous NOx emissions at ambient conditions by using renewable electricity.

Distinct membrane fouling characteristics of anammox MBR with low NO2−-N/NH4+-N ratio

The bacterial growth and death, and extracellular polymeric substances (EPS) and soluble microbial products (SMP) in aerobic membrane bioreactor (MBR) cause severe membrane fouling. Anammox bacteria grow slowly but produce much EPS and SMP. Therefore, the membrane fouling characteristic of anammox MBR is still indistinct. A NO2−-N/NH4+-N < 1.0 into in the influent of an anammox MBR applies to investigate: 1) the slowest growing anammox bacteria (Candidatus Jettenia) could be enriched or not; 2) its membrane fouling characteristic. Results showed that Candidatus Jettenia successfully accumulated from 0.01% to 26.19%. The fouling characteristic of anammox MBR was entirely different from other MBRs. Firstly, obvious low transmembrane pressure (<4 KPa, 125 days) and low amount of foulants (0.22 gVSS/m2) might result from N2 production and the slow-growing Candidatus Jettenia. Secondly, the analysis of the components of membrane foulants indicated that polysaccharides of SMP in the gel layer and pore foulants were the key factors affecting membrane fouling. Finally, the large particle size of foulants (200 μm) might be caused by anammox bacteria living inside the foulants under anaerobic conditions. This study provides systematic insights into membrane characteristics of anammox MBR and a basis for the enrichment of anammox bacteria by MBR.

Chronically stressed benthic macroinvertebrate communities exhibit limited effects on ecosystem function in a microtidal estuary

Anthropogenically driven alterations to coastal sediments and their benthic macroinvertebrate communities impair ecosystem function. However, this paradigm is yet to be tested in ecosystems that typically harbour underdeveloped communities lacking larger bioturbating species. Here, we investigated the effects of sediment condition and macroinvertebrate communities on benthic metabolism, nutrient exchange and denitrification (N2 production), and assessed the relative importance of taxon richness, abundance, biomass and community bioturbation potential in influencing these processes in 2 regions of the highly modified, microtidal Peel-Harvey Estuary in temperate Western Australia. Sediment condition influenced benthic metabolism more than the macroinvertebrate community, whereas the reverse was true for nutrient exchange. Denitrification was driven by sediment condition and the community in the upper and lower estuary, respectively, highlighting the change in controls of this nitrogen-removal process within estuaries. Overall, benthic macroinvertebrates had little to no effect on many ecosystem processes, exhibiting the limited functional role played by these chronically stressed biota in this estuary. There was also no interaction between sediment condition and the community, suggesting a functional decoupling between these 2 ecosystem components. Where significant macroinvertebrate effects were detected, community biomass was the most frequently selected predictor, demonstrating its fundamental role in ecosystem function. This study reveals pressing implications of what might be expected when benthic environments become particularly degraded and the highly limited potential of the resultant benthic macroinvertebrate communities to provide key ecosystem services such as nutrient processing.

Systematic decision-making framework for evaluation of process alternatives for sustainable ammonia (NH3) production

Ammonia (NH3) is the second highest produced chemical commodity worldwide, which commonly used in manufacturing of fertilisers, refrigerants, dyes and other chemicals. Haber Bosch process is the most established process to produce NH3 from hydrogen (H2) and nitrogen (N2). However, this process releases large amount of greenhouse gases (GHG) annually as this process consumes H2 which produces from steam methane reforming process that utilises methane (CH4) from natural gas. In addition, such process also consumes large quantity of energy to support the production of NH3. In order to enhance the sustainability of NH3 production, renewable energy (RE) such as solar can be used to replace the traditional energy system that consumes fossil fuels. Besides, production of H2 and N2 via water electrolysis and air separation technology which powered by RE can further enhance the sustainability of NH3 production. Such process is known as green NH3 production. One of the challenges of utilising RE is to have a stable supply of power. Therefore, concentrated solar power (CSP) is a good option to overcome this challenge, because it has capability of providing stable power supply. Based on literature review and market survey, a superstructure which consists of all possible and available technologies for production of green NH3 using CSP as an energy supply is presented. Next, the advantages and disadvantages of each alternative are evaluated and summarised in a comparison table. Ranking matrices are developed to evaluate and rank the alternatives quantitatively to determine the optimum pathway for green NH3 production. Based on the analysis, a conceptual integrated green NH3 production which involves a CSP tower plant with a two-tank molten salt thermal energy storage (TES) system is determined as the optimum pathway. Such process fulfils the electricity demand of a pressure swing adsorption (PSA) air separator, polymer electrolyte membrane (PEM) and NH3 synthesis plant. The results of this work are useful for feasibility studies on the commercialisation of green NH3 production.

Pathways and Microbes Responsible for N2 Production in Soils Under Oxic Conditions

Pathways and Microbes Responsible for N2 Production in Soils Under Oxic Conditions

Conception and optimization of an ammonia synthesis superstructure for energy storage

Ammonia has a potential as carbon-free and high-energy density compound for chemical storage of renewable energies and its synthesis from green H2 requires to be as energetically efficient as possible. In this work, a superstructure optimization methodology for process synthesis is proposed and applied to an ammonia production process. The approach covers three different scales: process, equipment, and molecules. Process scale refers to finding the optimal process structure, while the equipment scale is related to the best set of operating conditions, and the molecular scale studies two catalysts (Fe and Ru) with their respective kinetic rates. The optimization intends to minimize the Levelized Cost of Ammonia (LCOA) and maximize the energy efficiency. The best trade-off is found with the Ru catalyst, with an LCOA of 766€/tNH3 and 57.2% of energy efficiency. In comparison with reference cases, the LCOA decreases 0.6% and the energy efficiency increases 1.5%. The main improvement is found in the pressure, 75bar, reduced in 25%. The production of 11.6tNH3/day equals to 2.5MW of stored power from 3MW supplied as H2 to the process, with a total energy consumption of 10.67kWh/kgNH3, including prior H2 and N2 production processes.

Biotic and Abiotic Controls on Dinitrogen Production in Coastal Sediments

AbstractDinitrogen gas (N2) production removes fixed N from the marine biosphere; however, questions remain over the relative influence of microbial processes in determining N2 efflux from coastal sediments. Here, we quantify N2 production processes and controlling factors along a ∼2,500 km continental shelf sediment transect of China via combined molecular and isotopic approaches. We show that denitrification is the dominant pathway of sedimentary N2 production, which is greatly higher than anaerobic ammonium oxidation. N2 production rates by denitrification increase with nosZ gene abundance and decrease with bacterial diversity. The effects of temperature and dissolved oxygen on denitrification are mainly through their influence on biotic factors including functional genes, microbial biomass, and bacterial diversity. The rate of anaerobic ammonium oxidation increases with hzsB gene abundance. Temperature, dissolved oxygen, and dissolved organic carbon appear to indirectly affect anaerobic ammonium oxidation rate by regulating hzsB gene abundance. When extrapolated to the entire sampling region, the annual flux of sedimentary denitrification is estimated at ∼9.2 Tg N yr−1. These results improve our understanding of sediment N removal processes and the mechanistic factors that regulate denitrification fluxes.

Anammox bacteria drive fixed nitrogen loss in hadal trench sediments

Benthic N2 production by microbial denitrification and anammox is the largest sink for fixed nitrogen in the oceans. Most N2 production occurs on the continental shelves, where a high flux of reactive organic matter fuels the depletion of nitrate close to the sediment surface. By contrast, N2 production rates in abyssal sediments are low due to low inputs of reactive organics, and nitrogen transformations are dominated by aerobic nitrification and the release of nitrate to the bottom water. Here, we demonstrate that this trend is reversed in the deepest parts of the oceans, the hadal trenches, where focusing of reactive organic matter enhances benthic microbial activity. Thus, at ∼8-km depth in the Atacama Trench, underlying productive surface waters, nitrate is depleted within a few centimeters of the sediment surface, N2 production rates reach those reported from some continental margin sites, and fixed nitrogen loss is mainly conveyed by anammox bacteria. These bacteria are closely related to those known from shallow oxygen minimum zone waters, and comparison of activities measured in the laboratory and in situ suggest they are piezotolerant. Even the Kermadec Trench, underlying oligotrophic surface waters, exhibits substantial fixed N removal. Our results underline the role of hadal sediments as hot spots of deep-sea biological activity, revealing a fully functional benthic nitrogen cycle at high hydrostatic pressure and pointing to hadal sediments as a previously unexplored niche for anaerobic microbial ecology and diagenesis.

Gas processing with intrinsically porous 2D membranes

The importance of membranes for the processing of gases with significant technological as well as environmental impact is rapidly increasing given their advantages in operation, low energy consumption, and efficiencies. In this work we employ density functional theory calculations to investigate the performance of a number of intrinsically porous two-dimensional (2D) membranes for gas separation applications. Our results indicate that gases with small kinetic diameters such as He, H2O, Ne, H2, and CO2 present low energy barriers and high permeability through these membranes, in contrast to other components of natural and atmospheric gases. Additionally, we show that some of the studied membranes present high selectivities for a number of permeating gases. Our results indicate that these materials are promising candidates for applications in gas processing such as natural gas dehydration and sweetening, He recovery, O2 and N2 production from air, H2 purification, and CO2 capturing.

Inside Back Cover: Deciphering and Suppressing Over‐Oxidized Nitrogen in Nickel‐Catalyzed Urea Electrolysis (Angew. Chem. Int. Ed. 51/2021)

Over-oxidation in alkaline urea oxidation to dominate NO2− formation over N2 has been discovered by Xuejing Yang, Yefei Li, Ming Gong, and co-workers in their Research Article on page 26656. The identified nitrogen conversion network and reaction mechanism has important implications for sustainable chemistry and inspires new rationales for novel catalyst designs for the electrochemical urea oxidation into N2 products.

Deciphering and Suppressing Over-Oxidized Nitrogen in Nickel-Catalyzed Urea Electrolysis.

Urea electrolysis is a prospective technology for simultaneous H2 production and nitrogen suppression in the process of water being used for energy production. Its sustainability is currently founded on innocuous N2 products; however, we discovered that prevalent nickel-based catalysts could generally over-oxidize urea into NO2 - products with ≈80 % Faradaic efficiencies, posing potential secondary hazards to the environment. Trace amounts of over-oxidized NO3 - and N2 O were also detected. Using 15 N isotopes and urea analogues, we derived a nitrogen-fate network involving a NO2 - -formation pathway via OH- -assisted C-N cleavage and two N2 -formation pathways via intra- and intermolecular coupling. DFT calculations confirmed that C-N cleavage is energetically more favorable. Inspired by the mechanism, a polyaniline-coating strategy was developed to locally enrich urea for increasing N2 production by a factor of two. These findings provide complementary insights into the nitrogen fate in water-energy nexus systems.