Nitrate denitrification with nitrite or nitrous oxide as intermediate products: Stoichiometry, kinetics and dynamics of stable isotope signatures

Mechanism of denitrification in subsurface-dammed Ryukyu limestone aquifer, southern Okinawa Island, Japan

Denitrification crucially regulates the attenuation of groundwater nitrate and is unlikely to occur in a fast-flowing aquifer such as the Ryukyu limestone aquifer in southern Okinawa Island, Japan. However, evidences of denitrification have been observed in several wells within this region. This study analyzed environmental isotopes (δ15NNO3 and ẟ18ONO3) to derive the rationale for denitrification at this site. Additionally, the presence of two subsurface dams in the study area may influence the processes involved in nitrate attenuation. Herein, we analyzed 150 groundwater samples collected spatially and seasonally to characterize the variations in the groundwater chemistry and stable isotopes during denitrification. The values of δ15NNO3 and δ18ONO3 displayed a progressive trend up to +59.7 ‰ and + 21 ‰, respectively, whereas the concentrations of NO3--N decreased to 0.1 mg L-1. In several wells, the enrichment factors of δ15NNO3 ranged from -6.6 to -2.1, indicating rapid denitrification, and the δ15NNO3 to δ18ONO3 ratios varied from 1.3:1 to 2:1, confirming the occurrence of denitrification. Denitrification intensively proceeds under conditions of depleted dissolved oxygen concentrations (<2 mg L-1), sluggish groundwater flow with longer residence times, high concentrations of dissolved organic carbon (>1.2 mg L-1), and low groundwater levels during the dry season with precipitation rates of <100 mm per month (Jun-Sep). SF6 analysis indicated the exclusive occurrence of denitrification in specific wells with groundwater residence times exceeding 30 years. These wells are located in close proximity to the major NE-SW fault system in the Komesu area, where the hydraulic gradient was below 0.005. Detailed geological and lithological investigations based on borehole data revealed that subsurface dams did not cause denitrification while the major NE-SW fault system uplifted the impermeable basement rock of the Shimajiri Group, creating a lithological gap at an equivalent depth that ultimately formed a sluggish groundwater area, promoting denitrification.

Combining multi-isotope technology, hydrochemical information, and MixSIAR model to identify and quantify nitrate sources of groundwater and surface water in a multi-land use region

Accurate identification of nitrate (NO3-) sources is the premise of non-point source pollution control in watersheds. The multiple isotope techniques (δ15N-NO3-, δ18O-NO3-, δ2H-H2O, δ18O-H2O), combined with hydrochemistry characteristics, land use information, and Bayesian stable isotope mixing model (MixSIAR), were used to identify the sources and contributions of NO3- in the agricultural watershed of the upper Zihe River, China. A total of 43 groundwater (GW) and 7 surface water (SFW) samples were collected. The results showed that NO3- concentrations of 30.23% GW samples exceeded the WHO maximum permissible limit level, whereas SFW samples did not exceed the standard. The NO3- content of GW varied significantly among different land uses. The averaged GW NO3- content in livestock farms (LF) was the highest, followed by vegetable plots (VP), kiwifruit orchards (KF), croplands (CL), and woodlands (WL). Nitrification was the main transformation process of nitrogen, while denitrification was not significant. Hydrochemical analysis results combined with NO isotopes biplot showed that manure and sewage (M&S), NH4+ fertilizers (NHF), and soil organic nitrogen (SON) were the mixed sources of NO3-. The MixSIAR model summarized that M&S was the main NO3- contributor for the entire watershed, SFW, and GW. For contribution rates of sources in GW of different land use patterns, the main contributor in KF was M&S (contributing 59.00% on average), while M&S (46.70%) and SON (33.50%) contributed significantly to NO3- in CL. Combined with the traceability results and the situation that land use patterns are changing from CL to KF in this area, improving fertilization patterns and increasing manure use efficiency are necessary to reduce NO3- input. These research results will serve as a theoretical foundation for controlling NO3- pollution in the watershed and adjusting agricultural planting structures.

Nitrate removal by combining chemical and biostimulation approaches using micro-zero valent iron and lactic acid

The occurrence of nitrate is the most significant type of pollution affecting groundwater globally, being a major contributor to the poor condition of water bodies. This pollution is related to livestock-agricultural and urban activities, and the nitrate presence in drinking water has a clear impact on human health. For example, it causes the blue child syndrome. Moreover, the high nitrate content in aquifers and surface waters significantly affects aquatic ecosystems since it is responsible for the eutrophication of surface water bodies. A treatability test was performed in the laboratory to study the decrease of nitrate in the capture zone of water supply wells. For this purpose, two boreholes were drilled from which groundwater and sediments were collected to conduct the test. The goal was to demonstrate that nitrate in groundwater can be decreased much more efficiently using combined abiotic and biotic methods with micro-zero valent iron and biostimulation with lactic acid, respectively, than when both strategies are used separately. The broader implications of this goal derive from the fact that the separate use of these reagents decreases the efficiency of nitrate removal. Thus, while nitrate is removed using micro-valent iron, high concentrations of harmful ammonium are also generated. Furthermore, biostimulation alone leads to overgrowth of other microorganisms that do not result in denitrification, therefore complete denitrification requires more time to occur. In contrast, the combined strategy couples abiotic denitrification of nitrate with biostimulation of microorganisms capable of biotically transforming the abiotically generated harmful ammonium. The treatability test shows that the remediation strategy combining in situ chemical reduction using micro-zero valent iron and biostimulation with lactic acid could be a viable strategy for the creation of a reactive zone around supply wells located in regions where groundwater and porewater in low permeability layers are affected by diffuse nitrate contamination.

Iron-Activated Carbon Systems to Enhance Aboriginal Aerobic Denitrifying Bacterial Consortium for Improved Treatment of Micro-Polluted Reservoir Water: Performances, Mechanisms, and Implications

Although many source waterbodies face nitrogen pollution problems, the lack of organic electron donors causes difficulties when aerobic denitrifying bacteria are used to treat micro-polluted water. Different forms of iron with granular activated carbon (AC) as carriers were used to stimulate aboriginal microorganisms for the purification of micro-polluted source water. Compared with the iron-absent AC system, targeted pollutants were significantly removed (75.76% for nitrate nitrogen, 95.90% for total phosphorus, and 80.59% for chemical oxygen demand) in the sponge-iron-modified AC system, which indicated that iron promoted the physical and chemical removal of pollutants. In addition, high-throughput sequencing showed that bacterial distribution and interaction were changed by ion dosage, which was beneficial for pollutant transformation and reduction. Microbial functions, such as pollutant removal and expression of functional enzymes that were responsible for the transformation of nitrate nitrogen to ammonia, were highly efficient in iron-applied systems. This study provides an innovative strategy to strengthen in situ remediation of micro-pollution in waterbodies.

Effective and selective electroreduction of aqueous nitrate catalyzed by copper particles on multi-walled carbon nanotubes

Drinking-water contamination with nitrate ions is inevitable and wide spreading, which demands feasible removal. Water de-nitration by potentiostatic electroreduction is described here. A novel electrocatalyst based on nano-copper particles, supported onto multi-walled carbon nanotubes (MWCNTs), and spray-deposited onto fluorine doped tin oxide-glass substrates, is described. The Cu/MWCNT/FTO electrode has been characterized by several methods and assessed as a working electrode in aqueous nitrate ion electroreduction, in comparison with MWCNT sprayed on FTO (MWCNT/FTO) with no copper. Comparison with earlier reported electrodes is also described. XRD patterns confirm the presence of nano-copper crystallites, in the electrode, with average size ⁓45 nm. Within 2 h of electrolysis, Cu/MWVNT/FTO exhibits more than 65% removal of nitrate at -1.80 V (vs. SCE). In longer time (7 h) the electrode completely converts the nitrate into N₂ (∼65%) and (NH₄⁺) ∼35% with no NO₂^- ions. The kinetics show 0.76 order with respect to nitrate, and a rate constant 4.53 × 10^-2 min^-1 higher than earlier counterparts. The new electrode functions under various conditions of temperature, pH, electrolyte type and concentration and inter-electrode spacing, only at ambient applied potential. Moreover, the electrode exhibits stability under nitrate electroreduction conditions, and can be recovered and reused for multiple times without efficiency loss. XRD and EDS results also confirm the electrode stability after multiple reuse. Compared to earlier systems, the Cu/MWCNT/FTO is environmentally stable, safe, non-costly with high nitrate removal efficiency and selectivity.

Enhanced Denitrification of Integrated Sewage Treatment System by Supplementing Denitrifying Carbon Source

Integrated sewage treatment system (ISTY) is a new technology for rural domestic sewage treatment. In the ISTY, the carbon source in the denitrification stage is often insufficient, affecting the denitrification efficiency. In order to improve the denitrification efficiency, several commonly available agricultural wastes, peanut shell (PS), sawdust (SD), peat (PT), and their mixtures (MT), were selected as supplementary carbon sources in the denitrification stage of ISTY to study the denitrification efficiency. Results show that PS exhibited a high carbon release capacity. PS released an enormous amount of carbon in 144 h, and the cumulative total organic carbon was 41.99 ± 0.7 mg/(g·L). The optimum carbon source dosage was 3 g/L, the nitrate removal rates of PS exceeded 95% after 48 h, and the denitrification rates were 9.35 mg/(g·L), which were 63.92% higher than that of the control group. After running the ISTY for 120 h, and with PS as supplementary carbon sources, the removal rate of TN increased from 29.76% to 83.86%. At the genus level, the dominant denitrifying bacteria in ISTY, after adding PS, were Pseudomonas and Cupriavidus, accounting for 78.68%, an increase of 72.90% compared with the control group. This evidence suggested that PS can obviously enhance the denitrification efficiency of the ISTY as a supplementary carbon source.

Enhanced nitrogen imbalances in agroecosystems driven by changing cropping systems in a coastal area of eastern China: from field to watershed scale

Agricultural activities exacerbate nitrogen (N) imbalances in the agroecosystem by disturbing the N inputs and outputs, yet the influence of changes in cropping systems on the N balance of agroecosystems remains unclear. In this study, at the field scale, we calculated the N balance of four cropping systems, (1) traditional crops with traditional crop rotation (G-G), (2) vegetables with traditional crop rotation (V-G), (3) vegetables with vegetable rotation (V-V), and (4) greenhouse vegetables (GHV); then analyzed the influence of changes in cropping systems from 1995 to 2015 on the N balances in the agroecosystems in sub-watersheds of the Dagu River. The results indicate that N balances were higher in GHV, V-V, and V-G than G-G, due to significantly higher inputs of N fertilizers and lower N use efficiency (NUE) in vegetable cultivation compared to traditional crops. Driven by economic benefits between 1995 and 2015, V-G, V-V, and GHV replaced G-G in a considerable number of cultivation areas in the sub-watersheds. These changes resulted in an increase of 109.9-170.1% in the N balance in the agroecosystem in the sub-watersheds between 1995 and 2015. In the entire watershed, the total N surplus contribution by V-V, V-G, and GHV increased from 39.3% to 79.1% between 1995 and 2015. These findings suggest that increased vegetable cultivation contributed to the increased risk of N pollution in agricultural production. Thus, there should be a focus on the management of cropping systems to control N loss from agricultural lands.

Direct measurement of dissolved dinitrogen to refine reactive modelling of denitrification in agricultural soils

Nitrogen fertilizers used in agriculture often cause nitrate leaching towards shallow groundwater, especially in lowland areas where soil permeability, ploughing, clay content, and the flat topography minimizes surface runoff. The introduction of good agricultural practices to reduce the nitrate amount entering the groundwater system is crucial to ameliorate the kinetic control on nitrate denitrification capacity. With this aim, a series of anaerobic mesocosms, consisting of loamy and clay soils and nitrate rich water, were modelled using acetate and natural organic matter as electron donors. Acetate was chosen because it is the main intermediate in many biodegradation pathways of organic compounds, and hence it is a suitable carbon source for denitrification. To account for the spatial variability of soil parameters, the experiments were performed in triplicates. The geochemical code PHREEQC(3) was used to simulate kinetic denitrification, and equilibrium reactions of gas and mineral phases. The reactive modelling results highlighted a rapid acetate and nitrate degradation rate, a rapid production of dissolved inorganic carbon and dinitrogen, and a steady concentration of dissolved iron and sulphate, suggesting that the main pathway of nitrate attenuation is through denitrification; concomitantly excluding the occurrence of other processes leading to nitrate consumption. In the absence of acetate, the loamy soil, poor of natural organic matter, did not allow to complete the denitrification process. This modelling study investigates in detail the relationship between the denitrification process in natural soils, with excess and in limitation of organic substrates, and the occurrence and fate of dissolved dinitrogen analysed with a high precision membrane inlet mass spectrometer. Results demonstrate that modelling nitrate degradation processes as a whole, using geochemical datasets and codes, will improve the estimates of agricultural landscapes denitrification and support better nitrogen management, especially in lowland environments.

Underrecognized niche of spore-forming bacilli as a nitrite-producer isolated from the processing lines and end-products of powdered infant formula

Although nitrite in powdered milk formula (PIF) is a recognized health risk for infants, the presence of nitrite in PIF has only been investigated as a chemical contaminant during the inspection of end-products. The risk posed by microbial sources of nitrite during the PIF manufacturing process has not been considered. This is the first study to report the taxonomy and physiological characteristics of nitrite-producing bacteria isolated from PIF processing environments. All isolates identified as nitrite-producers (133 out of 501 strains collected over four years) from work-in-process and end-products of PIF were spore-forming bacilli. Nitrite-producing metabolism under PIF processing conditions was found in not only thermophilic isolates (3 Bacillus, 60 Geobacillus from 63 strains; 100%) but also in mesophilic isolates (65 Bacillus, 1 Anoxybacillus from 70 strains; 65.7%). Geobacillus was the only highly heat-resistant sporeformer and vigorous nitrite-producer exhibiting dramatic increases in nitrite over short periods of incubation (a maximum value within 3 h). High conversions of nitrate to nitrite (up to 88.8%) was also observed, highlighting bacteria as a key source of nitrite in PIF processing lines. Further research into the diversity of metabolic activity observed in this study can facilitate specialized management of nitrite-producers in PIF processing lines.

Assessing the fate of explosives derived nitrate in mine waste rock dumps using the stable isotopes of oxygen and nitrogen

Ammonium nitrate (NH₄NO₃) mixed with fuel oil is a common blasting agent used to fragment rock into workable size fractions at mines throughout the world. The decomposition and oxidation of undetonated explosives can result in high NO₃^- concentrations in waters emanating from waste rock dumps. We used the stable isotopic composition of NO₃^- (δ¹⁵N- and δ¹⁸O-NO₃^-) to define and quantify the controls on NO₃^- composition in waste rock dumps by studying water-unsaturated and saturated conditions at nine coal waste rock dumps located in the Elk Valley, British Columbia, Canada. Estimates of the extent of nitrification of NH₄NO₃ in oxic zones in the dumps, initial NO₃^- concentrations prior to denitrification, and the extent of NO₃^- removal by denitrification in sub-oxic to anoxic zones are provided. δ¹⁵N data from unsaturated waste rock dumps confirm NO₃^- is derived from blasting. δ¹⁵N- and δ¹⁸O-NO₃^- data show extensive denitrification can occur in saturated waste rock and in localized zones of elevated water saturation and low oxygen concentrations in unsaturated waste rock. At the mine dump scale, the extent of denitrification in the unsaturated waste rock was inferred from water samples collected from underlying rock drains.

Denitrification behavior and microbial community spatial distribution inside woodchip-based solid-phase denitrification (W-SPD) bioreactor for nitrate-contaminated water treatment

The NO₃^- removal pathway and microorganisms change along with the height of an up-flow W-SPD bioreactor was investigated in this study. Modeling and microbial community analysis were used to analyze the denitrification behavior in W-SPD bioreactor. The results showed that NO₃^- removal rate matched for zero-order (R² > 0.97) and first-order (R² > 0.94) combination Michaelis-Menten kinetics, whereas microbial reaction rate suited for modified logistic model (R² > 0.99). The excellent denitrification performance (92.5%-96.4%) and microorganisms' quantity occurred in the middle of W-SPD bioreactor. Moreover, high-throughput sequencing analysis revealed that dominant denitrifiers, carbonaceous compound degrading bacteria and fermentative bacteria co-existed in W-SPD system, which was vital for efficiently sustainable NO₃^- removal. Hence, aerobic degradation, heterotrophic denitrification and dissimilatory nitrate reduction to ammonium (DNRA) occurred successively along the water direction in the bioreactor, offering reasonable references for W-SPD bioreactor study and application.

Dynamic changes of carbon isotope apparent fractionation factor to describe transition to syntrophic acetate oxidation during cellulose and acetate methanization

To identify predominant metabolic pathway for cellulose methanization new equations that take into account dynamics of 13C are added to the basic model of cellulose methanization. The correct stoichiometry of hydrolysis, acidogenesis, acetogenesis and methanogenesis steps including biomass is considered. Using experimental data by Laukenmann et al. [Identification of methanogenic pathway in anaerobic digesters using stable carbon isotopes. Eng. Life Sci. 2010;10:1-6], who reported about the importance of ace`tate oxidation during mesophilic cellulose methanization, the model confirmed that, at high biomass concentration of acetate oxidizers, the carbon isotope fractionation factor amounts to about 1.085. The same model, suggested firstly for cellulose degradation, was used to describe, secondly, changes in, and in methane and carbon dioxide during mesophylic acetate methanization measured by Grossin-Debattista [Fractionnements isotopiques (13C/12C) engendres par la methanogenese: apports pour la comprehension des processus de biodegradation lors de la digestion anaerobie [doctoral thesis]. 2011. Bordeaux: Universite Bordeaux-1;2011. Available from: http://ori-oai.u-bordeaux1.fr/pdf/2011/GROSSIN-DEBATTISTA_JULIEN_2011.pdf . French].The model showed that under various ammonium concentrations, at dominating acetoclastic methanogenesis, the value decreases over time to a low level (1.016), while at dominating syntrophic acetate oxidation, coupled with hydrogenotrophic methanogenesis, slightly increases, reaching 1.060 at the end of incubation.

A stable isotope approach and its application for identifying nitrate source and transformation process in water

Nitrate contamination of water is a worldwide environmental problem. Recent studies have demonstrated that the nitrogen (N) and oxygen (O) isotopes of nitrate (NO3(-)) can be used to trace nitrogen dynamics including identifying nitrate sources and nitrogen transformation processes. This paper analyzes the current state of identifying nitrate sources and nitrogen transformation processes using N and O isotopes of nitrate. With regard to nitrate sources, δ(15)N-NO3(-) and δ(18)O-NO3(-) values typically vary between sources, allowing the sources to be isotopically fingerprinted. δ(15)N-NO3(-) is often effective at tracing NO(-)3 sources from areas with different land use. δ(18)O-NO3(-) is more useful to identify NO3(-) from atmospheric sources. Isotopic data can be combined with statistical mixing models to quantify the relative contributions of NO3(-) from multiple delineated sources. With regard to N transformation processes, N and O isotopes of nitrate can be used to decipher the degree of nitrogen transformation by such processes as nitrification, assimilation, and denitrification. In some cases, however, isotopic fractionation may alter the isotopic fingerprint associated with the delineated NO3(-) source(s). This problem may be addressed by combining the N and O isotopic data with other types of, including the concentration of selected conservative elements, e.g., chloride (Cl(-)), boron isotope (δ(11)B), and sulfur isotope (δ(35)S) data. Future studies should focus on improving stable isotope mixing models and furthering our understanding of isotopic fractionation by conducting laboratory and field experiments in different environments.

Fe0 and Fe0 fully covered with Cu0 (Fe0 + Fe/Cu) in a fixed bed reactor for nitrate removal

To develop a cost-effective, feasible and robust technology for nitrate removal by chemical degradation, a Fe⁰ and Fe⁰ fully covered with Cu⁰ (i.e., Fe⁰ + Fe/Cu) fixed reactor was set up in this study.