Nitrate Reduction Research Articles

Interaction between nitrate and trichloroethene bioreduction in mixed anaerobic cultures

Bioremediation of trichloroethene (TCE)-contaminated sites often leads to groundwater acidification, while nitrate-polluted sites tend to generate alkalization. TCE and nitrate often coexist at contaminated sites; however, the pH variation caused by nitrate self-alkalization and TCE self-acidification and how these processes affect nitrate reduction and reductive dichlorination, have not been studied. This study investigated the interaction between nitrate and TCE, two common groundwater co-contaminants, during bioreduction in serum bottles containing synthetic mineral salt media and microbial consortia. Our results showed that TCE concentrations up to 0.3 mM stimulated nitrate reduction, while the effect of nitrate on TCE reductive dechlorination was more complex. Nitrate primarily inhibited the reduction of TCE to dichloroethene (DCE) but enhanced the reduction of vinyl chloride (VC) to ethene. Mechanistic analysis suggested that this inhibition was due to the thermodynamic favorability of nitrate reduction over TCE reduction, while the promotion of VC reduction was linked to pH stabilization via self-alkalization. As the initial nitrate concentration increased from 0 to 3 mM, the relative abundance of putatively denitrifying genera, such as Petrimonas and Trichlorobacter, increased. However, the abundance of fermentative Clostridium sharply declined from 31.11 to 1.51%, indicating strong nitrate inhibition. Additionally, the relative abundance of Dehalococcoides, a genus capable of reducing TCE to ethene, slightly increased from 23.91 to 24.26% at nitrate concentrations up to 0.3 mM but decreased to 18.65% as the nitrate concentration increased to 3 mM, suggesting that Dehalococcoides exhibits a degree of tolerance to high nitrate concentrations under specific conditions. Overall, our findings highlight the potential for simultaneous reduction of TCE and nitrate, even at elevated concentrations, facilitated by self-regulating pH control in anaerobic mixed dechlorinating consortia. This study provides novel insights into bioremediation strategies for addressing co-contaminated sites.

In Situ Spectroscopic Probing of the Hydroxylamine Pathway of Electrocatalytic Nitrate Reduction on Iron-Oxy-Hydroxide.

Crystalline γ-FeO(OH) dominantly possessing ─OH terminals (𝛾-FeO(OH)c), polycrystalline γ-FeO(OH) containing multiple ─O, ─OH, and Fe terminals (𝛾-FeO(OH)pc), and α-Fe2O3 majorly containing ─O surface terminals are used as electrocatalysts to study the effect of surface terminals on electrocatalytic nitrate reduction reaction (eNO3RR) selectivity and stabilization of reaction intermediates. Brunauer-Emmett-Teller analysis and electrochemically determined surface area suggest a high active surface area of 117.79 m2 g-1 (ECSA: 0.211 cm2) for 𝛾-FeO(OH)c maximizing the surface accessibility for nitrate adsorption and exhibiting selective eNO3RR to NH3 at pH 7 with a yield rate 18.326mg h-1 cm-2, >85% Faradaic efficiency (FE), and at least nine-times catalyst-recyclability. 15N- and D-labeling combined with in situ IR and Raman studies validate the adsorption of nitrate ions on the ─OH terminals of 𝛾-FeO(OH)c and the generation of nitrite and hydroxyl amine as eNO3RR intermediates. A kinetic isotope effect (KIE) value of 2.1 indicates H2O as the proton source and proton-coupled electron transfer as the rate-limiting step. The rotating-ring disk electrochemical (RRDE) study and subsequent Koutecký-Levich analysis reveal the electron-transfer rate constant (k) for the 2e- reduction of nitrate to nitrite is 5.7 × 10-6cm s-1. This study provides direct evidence of the hydroxyl amine formation as the dominant pathway of eNO3RR on γ-FeO(OH).

Specific Enrichment of arsM-Carrying Microorganisms with Nitrogen Fixation and Dissimilatory Nitrate Reduction Function Enhances Arsenic Methylation in Plant Rhizosphere Soil.

Plants can recruit microorganisms to enhance soil arsenic (As) removal and nitrogen (N) turnover, but how microbial As methylation in the rhizosphere is affected by N biotransformation is not well understood. Here, we used acetylene reduction assay, arsM gene amplicon, and metagenome sequencing to evaluate the influence of N biotransformation on As methylation in the rhizosphere of Vetiveria zizanioides, a potential As hyperaccumulator. V. zizanioides was grown in mining soils (MS) and artificial As-contaminated soils (AS) over two generations in a controlled pot experiment. Results showed that the content of dimethylarsinic acid in the rhizosphere was significantly positively correlated with the rate of N fixation and the activity of nitrite reductase. The As-methylating species (e.g., Flavisolibacter and Paraflavitalea) were significantly enriched in the root-associated compartments in the second generation of MS and AS. Notably, higher abundance of genes involved in N fixation (nifD, nifK) and dissimilatory nitrate reduction to ammonium (narG/H, nirB/D/K/S) was detected in the second generation of MS than in the first generation. The metabolic pathway analysis further demonstrated that N fixing-stimulative and DNRA-stimulative As-methylating species could provide ammonium to enhance the synthesis of S-adenosyl-l-methionine, serving as methyl donors for soil As methylation. This study highlights two important N conversion-stimulative As-methylating pathways and has important implications for enhancing phytoremediation in As-contaminated soils.

Dynamic restructuring of electrocatalysts in the activation of small molecules: challenges and opportunities.

Electrochemical activation of small molecules plays an essential role in sustainable electrosynthesis, environmental technologies, energy storage and conversion. The dynamic structural changes of catalysts during the course of electrochemical reactions pose challenges in the study of reaction kinetics and the design of potent catalysts. This short review aims to provide a balanced view of in situ restructuring of electrocatalysts, including its fundamental thermodynamic origins and how these compare to those in thermal and photocatalysis, and highlighting both the positive and negative impacts of in situ restructuring on the electrocatalyst performance. To this end, examples of in situ electrocatalyst restructuring within a focused scope of reactions (i.e. electrochemical CO2 reduction, hydrogen evolution, oxygen reduction and evolution, and dinitrogen and nitrate reduction) are used to demonstrate how restructuring can benefit or adversely affect the desired process outcome. Prospects of manipulating in situ restructuring towards an energy-efficient and durable electrocatalytic process are discussed. The practicality of pulse electrolysis on an industrial scale is questioned, and the need for genius schemes, such as self-healing catalysis, is emphasized.

Boosting Amino Acid Synthesis with WOx Sub-Nanoclusters.

The conversion of nitrate-rich wastewater and biomass-derived blocks into high-value products using renewably generated electricity is a promising approach to modulate the artificial carbon and nitrogen cycle. Here, a new synthetic strategy of WOx sub-nanoclusters is reported and supported on carbon materials as novel efficient electrocatalysts for nitrate reduction and its coupling with α-keto acids. In acidic solutions, the NH3-NH2OH selectivity can also optimized by adjusting the potential, with the total FE exceeding 80% over a wide potential range. After introducing α-keto acids, the WOx/D-CB electrode achieves remarkable activity and selectivity toward C2-C6 amino acids. For glycine and alanine, impressive FEs of 49.34% and 38.22% based on transitional metal oxides can be obtained, surpassing those of WOx nanoclusters with larger size. In situ analysis and mechanistic studies reveal the critical role of WOx sub-nanoclusters in reducing the energy barriers of key steps in alanine synthesis. This work opens up new insights into the rational design of cluster catalysts to promote electrochemical amino acid synthesis.

Nonprecious Triple-Atom Catalysts with Ultrahigh Activity for Electrochemical Reduction of Nitrate to Ammonia: A DFT Screening.

Electrochemical nitrate reduction to ammonia (NO3RR) is promising to not only tackle environmental issues caused by nitrate but also produce ammonia at room temperatures. However, two critical challenges are the lack of effective electrocatalysts and the understanding of related reaction mechanisms. To overcome these challenges, we employed first-principles calculations to thoroughly study the performance and mechanisms of triple-atom catalysts (TACs) composed of transition metals (including 27 homonuclear TACs and 4 non-noble bimetallic TACs) anchored on N-doped carbon (NC). We found five promising candidates possessing not only thermodynamic and electrochemical stability, but also high activity and selectivity for ammonia production. Among them, non-noble homonuclear Ni3@NC TAC show high activity with low theoretical limiting potential of -0.31 VRHE. Surprisingly, bimetallic Co2Ni@NC, Co2Cu@NC, and Fe2Ni@NC TACs show ultrahigh activity with theoretical limiting potentials of 0.00 VRHE, without a potential determining step in the whole reaction pathways, representing the best theoretical activity been reported up to date. These promising candidates are facilitated by circumventing the limit of scaling relationships, a well-known obstacle for single-atom catalysts. This study indicates that designing suitable TACs can be a promising strategy for efficiently electro-catalyzing NO3RR and breaking the limit of the scaling relationship.

The blood pressure lowering effect of beetroot juice is impaired in periodontitis and recovered after periodontal treatment

We have previously demonstrated that subgingival levels of nitrate-reducing bacteria, as well as the in vitro salivary nitrate reduction capacity (NRC), were diminished in periodontitis patients, increasing after periodontal treatment. However, it remains unclear if an impaired NRC in periodontitis can affect systemic health. To determine this, the effect of nitrate-rich beetroot juice (BRJ) on blood pressure was determined in 15 periodontitis patients before and 70 days after periodontal treatment (i.e., professional mechanical plaque removal, oral hygiene instruction, and subgingival instrumentation), as well as in a healthy control group of 15 individuals. Additionally, subgingival and tongue samples were taken to analyse the bacterial composition with Illumina sequencing of the 16S rRNA gene. In healthy individuals, the systolic and diastolic blood pressure (SBP and DPB) decreased significantly (both P < 0.01) 90 min after BRJ intake, but not in periodontitis patients. However, after periodontal treatment, this blood pressure-lowering effect was recovered (P < 0.05 for SBP; P < 0.01 for DBP). Lower levels of salivary nitrate after identical doses of BRJ intake indicated a potentially higher NRC in healthy individuals (P < 0.05). Periodontitis-associated bacteria decreased in tongue and subgingival samples after periodontal treatment (P < 0.01). In contrast, nitrate-reducing bacteria were associated with health in both habitats, but increased only in subgingival plaque after periodontal treatment (P < 0.001). This is the first study showing that periodontitis could limit the blood-pressure lowering effects of nitrate reduction by the oral microbiota. We propose that an impaired NRC represents a potential link between periodontitis and systemic conditions, which should be confirmed in future randomized controlled trials. Future work should also aim to determine if nitrate prebiotic supplementation and/or tongue cleaning could improve the treatment of periodontitis and its associated comorbidities.

Microbial Nitrogen Removal in South San Francisco Bay: Does It Play a Role in Eutrophication Resistance?

The ecosystem response to anthropogenic nitrogen (N) loading in estuarine systems is determined by hydrodynamics, biogeochemical transformation rates, and other system-specific characteristics. Historically, San Francisco (SF) Bay has been an outlier from other estuaries with an unusual resistance to eutrophication, despite having extremely high rates of nitrogen loading. Recent increases in phytoplankton biomass and an unprecedented harmful algal bloom, however, have increased the urgency to understand rates and drivers of nitrogen removal in the system. To assess benthic N cycling rates, we conducted seasonal measurements across nine sites in South and Lower South SF Bay, the two sub-embayments with the highest rates of area-normalized N loading, to determine the rates and potential drivers of denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Denitrification rates averaged 60.6 ± 8.1 µmol m−2 h−1 and were primarily coupled to nitrification. Denitrification rates were positively correlated with DNRA rates and % clay. DNRA rates ranged from 0 to 20 µmol m−2 h−1 and on an annual basis averaged ~ 10% of total benthic nitrate reduction, with a negative correlation to % clay content in the surface sediment. The measured denitrification rates account for the removal of, on average, 14% of N loaded annually to South SF Bay, leaving a sizeable portion for alternate fates (e.g., recycling, export, or burial) and potential for substantial temporal and spatial variability (1–79%). This identifies the relative importance of sediment denitrification in ecosystems characterized by high nutrients and low productivity.

Photo-Catalytic Reduction of Nitrate by Ag-TiO2/Formic Acid Under Visible Light: Selectivity of Nitrogen and Mechanism

Photo-Catalytic Reduction of Nitrate by Ag-TiO2/Formic Acid Under Visible Light: Selectivity of Nitrogen and Mechanism

Nitrogen Assimilation Plays a Role in Balancing the Chloroplastic Glutathione Redox Potential Under High Light Conditions.

Nitrate reduction requires reducing equivalents produced by the photosynthetic electron transport chain. Therefore, it has been suggested that nitrate assimilation provides a sink for electrons under high light conditions. We tested this hypothesis by monitoring photosynthetic efficiency and the chloroplastic glutathione redox potential (chl-EGSH) of plant lines with mutated glutamine synthetase 2 (GS2) and ferredoxin-dependent glutamate synthase 1 (GOGAT1). Mutant lines incorporated significantly less isotopically-labelled nitrate into amino acids than wild-type plants, demonstrating impaired nitrogen assimilation. When nitrate assimilation was compromised, photosystem II (PSII) proved more vulnerable to photodamage. The effect of the nitrate assimilation pathway on the chl- EGSH was monitored using the chloroplast-targeted roGFP2 biosensor (chl-roGFP2). Remarkably, while oxidation followed by reduction of chl-roGFP2 was detected in WT plants in response to high light, oxidation values were stable in the mutant lines, suggesting that chl-EGSH relaxation after high light-induced oxidation is achieved by diverting excess electrons to the nitrogen assimilation pathway. Importantly, similar ΦPSII and chl-roGFP2 patterns were observed at elevated CO2, suggesting that mutant phenotypes are not associated with photorespiration activity. Together, these findings indicate that the nitrogen assimilation pathway serves as a sustainable energy dissipation route, ensuring efficient photosynthetic activity and fine-tuning redox metabolism under light-saturated conditions.

Tuning Nitrogen Configurations in Nitrogen-Doped Graphene Encapsulating Fe3C Nanoparticles for Enhanced Nitrate Electroreduction.

Electrochemical nitrate reduction reaction (NO3RR) offers a promising technology for the synthesis of ammonia (NH3) and removal of nitrate in wastewater. Herin, we fabricate a series of Fe3C nanoparticles in controllable pyridinic-N doped graphene (Fe3C@NG-X) by a self-sacrificing template method for the NO3RR. Fe3C@NG-10 exhibits high catalytic performance with a Faradaic efficiency (FE) of 94.03% toward NH3 production at -0.5 V vs. Reversible hydrogen electrode (RHE) and an NH3 yield rate of 477.73 mmol h-1 gcat-1. Additionally, the catalyst also has a FE above 90% across a broad potential range and NO3- concentration range (12.5-500 mM). During the electrocatalytic process, the material experienced structural reconstruction, forming Fe/Fe3C@NG-X heterojunction. Experimental investigations demonstrate that the remarkable electrocatalytic activity is attributed to the high proportion of pyridinic-N content, and the reconstruction further enhances the overall reduction process.

Intensifying Interfacial Reverse Hydrogen Spillover for Boosted Electrocatalytic Nitrate Reduction to Ammonia.

Rational regulation of active hydrogen (*H) behavior is crucial for advancing electrocatalytic nitrate reduction reaction (NO3RR) to ammonia (NH3), yet in-depth understanding of the *H generation, transfer, and utilization remains ambiguous, and explorations for *H dynamic optimization are urgently needed. Herein we engineer a Ni3N nanosheet array intimately decorated with Cu nanoclusters (NF/Ni3N-Cu) for remarkably boosted NO3RR. From comprehensive experimental and theoretical investigations, the Ni3N moieties favors water dissociation to generate *H, and then *H can rapidly transfer to the Cu via unique reverse hydrogen spillover mediating interfacial Ni-N-Cu bridge bond, thus increasing *H coverage on the Cu site for subsequent deoxygenation/hydrogenation. More impressively, such intriguing reverse hydrogen spillover effect can be further strengthened via elegant engineering of the Ni3N/Cu heterointerface with more intimate contact. Consequently, the NF/Ni3N-Cu with Cu nanoclusters intimate anchoring presents record NH3 yield rate of 1.19 mmol h-1 cm-2 and Faradaic efficiency of 98.7 % at -0.3 V vs. RHE, being on par with the state-of-the-art ones. Additionally, with NF/Ni3N-Cu as the cathode, a high-performing Zn-NO3 - battery can be assembled. This contribution illuminates a novel pathway to optimize *H behavior via distinct reverse hydrogen spillover for promoted NO3RR and other hydrogenation reactions.

Coupling Nitrate‐to‐Ammonia Conversion and Sulfion Oxidation Reaction Over Hierarchical Porous Spinel MFe2O4 (M═Ni, Co, Fe, Mn) in Wastewater

AbstractThe construction of coupled electrolysis systems utilizing renewable energy sources for electrocatalytic nitrate reduction and sulfion oxidation reactions (NO3RR and SOR), is considered a promising approach for environmental remediation, ammonia production, and sulfur recovery. Here, a simple chemical dealloying method is reported to fabricate a hierarchical porous multi‐metallic spinel MFe2O4 (M═Ni, Co, Fe, Mn) dual‐functional electrocatalysts consisting of Mn‐doped porous NiFe2O4/CoFe2O4 heterostructure networks and Ni/Co/Mn co‐doped Fe3O4 nanosheet networks. The excellent NO3RR with high NH3 Faradaic efficiency of 95.2% at ‐0.80 V versus reversible hydrogen electrode (vs RHE) and NH3 yield rate of 608.9 µmol h−1 cm−2 at −1.60 V vs RHE, and impressive SOR performance (100 mA cm−2@0.98 V vs RHE) is achieved for MFe2O4. Key intermediates such as *NO, *NH2, and NH3 are identified in the NO3RR process by in situ Fourier transform infrared spectroscopy (in situ FTIR). The MFe2O4‐assembled two‐electrode coupling system (NO3RR||SOR) shows an ultra‐low cell voltage of 1.14 V at 10 mA cm−2, much lower than the NO3RR||OER (oxygen evolution reaction, 10 mA cm−2@2.62 V), simultaneously achieving two expected targets of value‐added ammonia generation and sulfur recovery, and also demonstrating high durability of 18 h. This work also demonstrates the great potential of spinel ferrite‐based catalysts for environmental remediation.

Nitrogen source type modulates heat stress response in coral symbiont (Cladocopium goreaui).

Ocean warming due to climate change endangers coral reefs, and regional nitrogen overloading exacerbates the vulnerability of reef-building corals as the dual stress disrupts coral-Symbiodiniaceae mutualism. Different forms of nitrogen may create different interactive effects with thermal stress, but the underlying mechanisms remain elusive. To address the gap, we measured and compared the physiological and transcriptional responses of the Symbiodiniaceae Cladocopium goreaui to heat stress (31°C) when supplied with different types of nitrogen (nitrate, ammonium, or urea). Under heat stress (HS), cell proliferation and photosynthesis of C. goreaui declined, while cell size, lipid storage, and total antioxidant capacity increased, both to varied extents depending on the nitrogen type. Nitrate-cultured cells exhibited the most robust acclimation to HS, as evidenced by the fewest differentially expressed genes (DEGs) and less ROS accumulation, possibly due to activated nitrate reduction and enhanced ascorbate biogenesis. Ammonium-grown cultures exhibited higher algal proliferation and ROS scavenging capacity due to enhanced carotenoid and ascorbate quenching, but potentially reduced host recognizability due to the downregulation of N-glycan biosynthesis genes. Urea utilization led to the greatest ROS accumulation as genes involved in photorespiration, plant respiratory burst oxidase (RBOH), and protein refolding were markedly upregulated, but the greatest cutdown in photosynthate potentially available to corals as evidenced by photoinhibition and selfish lipid storage, indicating detrimental effects of urea overloading. The differential warming nitrogen-type interactive effects documented here has significant implication in coral-Symbiodiniaceae mutualism, which requires further research.IMPORTANCERegional nitrogen pollution exacerbates coral vulnerability to globally rising sea-surface temperature, with different nitrogen types exerting different interactive effects. How this occurs is poorly understood and understudied. This study explored the underlying mechanism by comparing physiological and transcriptional responses of a coral symbiont to heat stress under different nitrogen supplies (nitrate, ammonium, and urea). The results showed some common, significant responses to heat stress as well as some unique, N-source dependent responses. These findings underscore that nitrogen eutrophication is not all the same, the form of nitrogen pollution should be considered in coral conservation, and special attention should be given to urea pollution.

Tandem Active Sites in Cu/Mo‐WO3 Electrocatalysts for Efficient Electrocatalytic Nitrate Reduction to Ammonia

AbstractElectrocatalytic NO3− reduction to NH3 is a promising technique for both ammonia synthesis and nitrate wastewater treatment. However, this conversion involves tandem processes of H2O dissociation and NO3− hydrogenation, leading to inferior NH3 Faraday efficiency (FE) and yield rate. Herein, a tandem catalyst by anchoring atomically dispersed Cu species on Mo‐doped WO3 (Cu5/Mo0.6‐WO3) for the NO3RR is constructed, which achieves a superior FENH3 of 98.6% and a yield rate of 26.25 mg h−1 mgcat−1 at −0.7 V (vs RHE) in alkaline media, greatly exceeding the performance of Mo0.6‐WO3 and Cu5/WO3 counterparts. Systematic electrochemical measurement results reveal that the promoted activation of NO3− on Cu sites, accompanying accelerated water dissociation producing active hydrogens on Mo sites, are responsible for this superior performance. In situ infrared spectroscopy and theoretical calculation further demonstrate that atomically dispersed Cu sites accelerate the conversion of NO3− to NO2−, and the Mo dopant activates adjacent Cu sites, resulting in the decreased energy barrier of *NO2 to *NO and the stepwise hydrogenation processes, making the synthesis of NH3 thermodynamically favorable. This work demonstrates the critical role of tandem active sites at atomic level in enhancing the electrocatalytic NO3− reduction to NH3, paving a feasible avenue for developing high‐performance electrocatalysts.

Residual Nitrite, Nitrate, and Volatile N-Nitrosamines in Organic and Conventional Ham and Salami Products.

Nitrite and nitrate in meat products may be perceived negatively by consumers. These compounds can react to form carcinogenic volatile N-nitrosamines. "Nitrite-free" (i.e., uncured) organic meat products may contain nitrate from natural sources (e.g., spices and water). We studied the quality of ham and salami (conventional cured; organic cured; organic uncured). Residual nitrite and nitrate, volatile N-nitrosamines, microbial load, surface color, water activity, and pH were determined, considering one week of refrigerated storage in open or unopened packages. Residual nitrite and nitrate in organic, uncured salami were similar to cured salami, presumably from the addition of herbs and spices and nitrate reduction by nitrate reductase from microorganisms. For cooked ham, residual nitrite was significantly lower in the organic, uncured sample, while residual nitrate was not detected. N-nitrosodiphenylamine was detected in all samples at day 0, exceeding, in three out of five cured and both uncured products, the US legal limit of 10 µg/kg of volatile N-nitrosamines in foods. This finding warrants further investigation. The microbial load in salami products was dominated by bacteria from starter cultures. In ham, a slight increase in total aerobic count and lactic acid bacteria during storage was noted. Overall, the microbial quality of the products was as expected for the respective product types.

Unlocking the potential of simultaneous organics oxidation and nitrate reduction over an S-scheme magnetic CoFe2O4@g-C3N4 heterojunction under visible light irradiation: Performance and mechanism

In this study, a novel S-scheme CoFe2O4@g-C3N4 (CF@GCN) heterojunction was prepared via a simple hydrothermal method and its visible-light-driven catalytic potential was evaluated in terms of tetracycline (TC) oxidation and nitrate reduction simultaneously. A series of characterizations validated the effective fabrication of CF@GCN, exhibiting remarkable photocatalytic potentials. In the separated system of TC and nitrate, a nearly 100% removal of TC was observed in 60 min under the optimal conditions (pH = 5.0, Catalyst dosage = 0.4 g/L, TC concentration = 5.0 mg/L). The quenching experiments emphasized the concurrent participation of both non-radical species (h+) and radical species (O2•− and HO•) in TC degradation. Meanwhile, more than 96% of nitrate removal was observed in 90 min under the optimal conditions (pH = 3.0, Catalyst dosage = 0.4 g/L, nitrate concentration = 100 mg/L, in the presence of formic acid). In the combined system, both TC oxidation and nitrate reduction occurred simultaneously and the efficiency of TC oxidation was lower compared to the individual system. The novel S-scheme composite has demonstrated exceptional potential in enhancing the reusability and stability of the catalyst, even after five cycles of trials. A detailed mechanistic pathway for TC oxidation and nitrate reduction was proposed, relying on the identification of reactive species and intermediates. In general, our findings propose a promising method with synergistic properties for the simultaneous oxidation of organics and reduction of nitrate in wastewater over an S-scheme heterojunction.

Tetrapodal iron complexes invoke observable intermediates in nitrate and nitrite reduction.

This study investigates the mechanistic pathways of nitrate and nitrite reduction by the tetrapodal iron complex [Py2Py(afamcyp)2Fe]OTf2, revealing key intermediates to elucidate the reaction process. Using UV-Vis, IR, mass and NMR spectroscopies, stable binding of oxyanions to the iron centre was observed, supporting the formation of the iron(iii)-hydroxide intermediate [Py2Py(afamcyp)2Fe(OH)]OTf2. This intermediate is less stable than in previous systems, providing insights into the behaviour of metalloenzymes. A bimetallic mechanism is proposed for nitrogen oxyanion reduction where additional iron is required to drive the complete reaction, resulting in the formation of the final nitrosyl complex, Py2Py(pimcyp)2Fe(NO), and water. Our findings enhance the understanding of iron-based reduction processes and contribute to the broader knowledge of oxyanion reduction mechanisms.

Diversity and physiology of abundant Rhodoferax species in global wastewater treatment systems.

Wastewater treatment plants rely on complex microbial communities for bioconversion and removal of pollutants, but many process-critical species are still poorly investigated. One of these genera is Rhodoferax, an abundant core genus in wastewater treatment plants across the world. The genus has been associated with many metabolic traits such as iron reduction and oxidation and denitrification. We used 16S rRNA gene amplicon data to uncover the diversity and abundance of Rhodoferax species in Danish and global treatment plants. Publicly available metagenome-assembled genomes were analyzed based on phylogenomics to delineate species and assign taxonomies based on the SeqCode. The phylogenetic analysis of "Rhodoferax" revealed that species previously assigned to Rhodoferax in wastewater treatment plants should be considered as at least eight different genera, with five representing previously undescribed genera. Genome annotation showed potential for several key-bioconversions in wastewater treatment, such as nitrate reduction, carbohydrate degradation, and accumulations of various storage compounds. Iron oxidation and reduction capabilities were not predicted for abundant species. Species-resolved FISH-Raman was performed to gain an overview of the morphology and ecophysiology of selected taxa to clarify their potential role in global wastewater treatment systems. Our study provides a first insight into the functional and ecological characteristics of several novel genera abundant in global wastewater treatment plants, previously assigned to the Rhodoferax genus.

Ammonia synthesis from nitrate reduction by the modulation of a built-in electric field and external stimuli

Introducing a built-in electric field/external stimuli is an efficient strategy to promote the nitrate reduction reaction for ammonia production.