Fate Of NO Research Articles

Fate of NO and Ammonia in Chemical Looping Combustion─Investigation in a 300 W Chemical Looping Combustion Reactor System

Chemical looping combustion (CLC) is a novel combustion concept that transfers oxygen from air to fuel using an oxygen carrier that circulates between an air reactor and a fuel reactor. Thus, the combustion products, H2O and CO2, are obtained in a separate flow, and ideally, a pure CO2 gas stream is obtained after condensation of H2O. Consequently, CLC has a unique potential for avoiding the high costs and energy penalties of CO2 capture. Further, NO emissions can potentially be avoided. CLC is flameless, and the temperature is too low for the formation of thermal NOx. Moreover, fuel NOx and prompt NOx do not form in the air reactor in the absence of fuel. In the fuel reactor, the absence of oxygen prevents normal NOx formation. However, when using fuels containing nitrogen, NO may form in the fuel reactor because the oxygen carrier can oxidize fuel nitrogen compounds. To achieve a CO2 stream suited for storage, NO must be removed. Dependent upon how NO is removed, the process could be free from any NO emissions. NO formation and NO reduction were investigated in a 300 W CLC reactor by adding either NH3 or NO. The work involved two different oxygen carriers, Linz–Donawitz (LD) slag and ilmenite, two temperatures, 850 and 900°C, two circulation rates, and different flows of syngas fuel. Further, operation without fuel with a fully and partially oxidized oxygen carrier was studied. For LD slag, lower fuel flow promoted the formation of NO and decreased the reduction of NO. Likewise, higher temperatures raised NO formation and lowered NO reduction. Ilmenite, however, was by far more superior with respect to NO. Thus, NO formation only occurred in the absence of fuel and with a fully oxidized oxygen carrier. Likewise, NO was fully reduced to N2 for all conditions, except in the absence of fuel and with fully oxidized ilmenite.

Tracing the Sources and Fate of NO3- in the Vadose Zone-Groundwater System of a Thousand-Year-Cultivated Region.

Excess nitrate (NO3-) loading in terrestrial and aquatic ecosystems can result in critical environmental and health issues. NO3--rich groundwater has been recorded in the Guanzhong Plain in the Yellow River Basin of China for over 1000 years. To assess the sources and fate of NO3- in the vadose zone and groundwater, numerous samples were collected via borehole drilling and field surveys, followed by analysis and stable NO3- isotope quantification. The results demonstrated that the NO3- concentration in 38% of the groundwater samples exceeded the limit set by the World Health Organization. The total NO3- stock in the 0-10 m soil profile of the orchards was 3.7 times higher than that of the croplands, suggesting that the cropland-to-orchard transition aggravated NO3- accumulation in the deep vadose zone. Based on a Bayesian mixing model applied to stable NO3- isotopes (δ15N and δ18O), NO3- accumulation in the vadose zone was predominantly from manure and sewage N (MN, 27-54%), soil N (SN, 0-64%), and chemical N fertilizer (FN, 4-46%). MN was, by far, the greatest contributor to groundwater NO3- (58-82%). The results also indicated that groundwater NO3- was mainly associated with the soil and hydrogeochemical characteristics, whereas no relationship with modern agricultural activities was observed, likely due to the time delay in the thick vadose zone. The estimated residence time of NO3- in the vadose zone varied from decades to centuries; however, NO3- might reach the aquifer in the near future in areas with recent FN loading, especially those under cropland-to-orchard transition or where the vadose zone is relatively thin. This study suggests that future agricultural land-use transitions from croplands to orchards should be promoted with caution in areas with shallow vadose zones and coarse soil texture.

A study of nitric oxide dynamics in a growing biofilm using a density dependent reaction-diffusion model

One of a number of critical roles played by NO· as a chemical weapon (generated by the immune system) is to neutralize pathogens. However, the virulence of pathogens depends on the production activity of reductants to detoxify NO·. Broad reactivity of NO· makes it complicated to predict the fate of NO· inside bacteria and its effects on the treatment of any infection. Here, we present a mathematical model of biofilm response to NO·, as a stressor. The model is comprised of a PDE system of highly nonlinear reaction-diffusion equations that we study in computer simulations to determine the positive and negative effects of key parameters on bacterial defenses against NO·. From the reported results, we conjecture that the oscillatory behavior of NO· under a microaerobic regime is a temporal phenomenon and does not give rise to a spatial pattern. It is also shown computationally that decreasing the initial size of the biofilm colony negatively impacts the functionality of reducing agents that deactivate NO·. Whereas nutrient deprivation results in the development of biofilms with heterogeneous structure, its effect on the activity of NO· reductants depends on the oxygen availability, biofilm size, and the amount of NO·.

Evaluating the Sources and Fate of Nitrate in the Alluvial Aquifers in the Shijiazhuang Rural and Suburban Area, China: Hydrochemical and Multi-Isotopic Approaches

The identification of the sources and fate of NO\(_{3}^{-}\) contaminants is important to protect the water quality of aquifer systems. In this study, NO\(_{3}^{-}\) contaminated groundwater from the drinking water wells in the Shijiazhuang area, China, was chemically (NO\(_{3}^{-}\)/Cl\(^{\rm{{-}}}\) ratio) and isotopically (\(δ\)\(^{\rm{15}}\)N\(_{\rm{NO3}}\), \(\delta\)\(^{\rm{18}}\)O\(_{\rm{NO3}}\) and \(\delta\)\(^{\rm{13}}\)C\(_{\rm{DOC}}\); \(\delta\)\(^{\rm{2}}\)H\(_{\rm{H2O}}\), \(\delta\)\(^{\rm{18}}\)O\(_{\rm{H2O}}\)) characterized to identify the sources of NO\(_{3}^{-}\) and address subsequent biogeochemical processes. The positive correlations between dominant anions and cations suggested that the dissolution of calcium carbonate and gypsum minerals was the most effective process in the groundwater. Elevated concentrations of NO\(_{3}^{-}\), Cl\(^{\rm{{-}}}\) and Mg\(^{\rm{2+}}\) could be related to the wastewater irrigation and usage of fertilizers. The natural water in the study area originated primarily from precipitation and experienced a limited extent of evaporation, as demonstrated by measurements of \(\delta\)\(^{\rm{2}}\)H\(_{\rm{H2O}}\) and \(\delta\)\(^{\rm{18}}\)O\(_{\rm{H2O}}\). A cross-plot of \(\delta\)\(^{\rm{15}}\)N\(_{\rm{NO3}}\) vs. \(\delta\)\(^{\rm{18}}\)O\(_{\rm{NO3}}\) gave an enrichment of the 15N isotope relative to the 18O isotope by a factor of 2. A further insight into the denitrification process was obtained by the synergistic changes in \(\delta\)\(^{\rm{13}}\)C\(_{\rm{DOC}}\) and \(\delta\)\(^{\rm{15}}\)N\(_{\rm{NO3}}\) values, confirming that a low extent of denitrification occurred. Nitrification processes were evaluated by means of \(\delta\)\(^{\rm{18}}\)O\(_{\rm{NO3}}\) and \(\delta\)\(^{\rm{18}}\)O\(_{\rm{H2O}}\). The initial \(\delta\)\(^{\rm{15}}\)N\(_{\rm{NO3}}\) value(s) of the NO\(_{3}^{-}\) source(s) were roughly estimated between 2‰ and 5‰. Based on the level of natural NO\(_{3}^{-}\), anthropogenic activities were considered the main reason for the elevated NO\(_{3}^{-}\) concentration of the shallow groundwater. NH\(_{4}^{+}\) fertilizers were the major source of NO\(_{3}^{-}\) in the non-wastewater irrigated area, while wastewater was regarded as the primary source of NO\(_{3}^{-}\) in the wastewater-irrigated area. A low content of NO\(_{3}^{-}\) in deep groundwater might mainly be influenced by precipitation and soil organic N that was involved in denitrification reactions. Some of the deep groundwater samples could have been contaminated by wastewater. The mixing process of multiple NO\(_{3}^{-}\) sources was identified as another important factor affecting the NO\(_{3}^{-}\) concentration of the groundwater in the study area. The combined use of \(\delta\)\(^{\rm{15}}\)N\(_{\rm{NO3}}\), \(\delta\)\(^{\rm{18}}\)O\(_{\rm{NO3}}\) and \(\delta\)\(^{\rm{13}}\)C\(_{\rm{DOC}}\) results and hydrochemical data (NO\(_{3}^{-}\)/Cl\(^{\rm{{-}}}\) ratios) gives an insight into the mixing effect of different NO\(_{3}^{-}\) sources and processes affecting NO\(_{3}^{-}\) concentration under conditions of intensive land-use activities.

Sources and fate of nitrate in a groundwater‐fed estuary elucidated using stable isotope ratios of nitrogen and oxygen

We used δ15N‐nitrate (NO−3) and δ18O‐NO−3 to unravel the provenance and fate of NO−3 in a groundwater‐fed estuary. A total of 13 monthly and two time series surveys were undertaken in the Werribee River estuary near Melbourne, Australia. The different survey timescales provided a comprehensive evaluation of the hydrological effects on the biogeochemistry of NO−3, which accounted for tidal variability and episodic runoff events. The distribution of δ15N and δ18O of the estuarine NO−3 along a mixing line between shallow and deep groundwater provided strong evidence for the predominance of groundwater‐derived NO−3 in the estuary. During dry periods when water residence time in the estuary was extended, shallow groundwater contributed 60% to 76% of the NO−3 (calculated from δ15N‐NO−3 and δ18O‐NO−3) to the estuary, and assimilation removed ∼ 70% of this groundwater‐derived NO−3. During wet periods, deep groundwater provided more (62%) NO−3 than shallow groundwater, and there was no indication of NO−3 consumption. Occasional sources of NO−3, which were also reflected by their δ15N‐NO−3 and δ18O‐NO−3 values, included NO−3 from nitrification and sewage‐derived NO−3 particularly at the bay entrance to the estuary. Greater emphasis should be placed upon the role of groundwater as a substantial NO−3 end member when assessing NO−3 biogeochemistry using methods relying on the dual isotopic composition of NO−3.

The fate of NO<sub>x</sub> emissions due to nocturnal oxidation at high latitudes: 1-D simulations and sensitivity experiments

Abstract. The fate of nitrogen oxide pollution during high-latitude winter is controlled by reactions of dinitrogen pentoxide (N2O5) and is highly affected by the competition between heterogeneous atmospheric reactions and deposition to the snowpack. MISTRA (MIcrophysical STRAtus), a 1-D photochemical model, simulated an urban pollution plume from Fairbanks, Alaska to investigate this competition of N2O5 reactions and explore sensitivity to model parameters. It was found that dry deposition of N2O5 made up a significant fraction of N2O5 loss near the snowpack, but reactions on aerosol particles dominated loss of N2O5 over the integrated atmospheric column. Sensitivity experiments found the fate of NOx emissions were most sensitive to NO emission flux, photolysis rates, and ambient temperature. The results indicate a strong sensitivity to urban area density, season and clouds, and temperature, implying a strong sensitivity of the results to urban planning and climate change. Results suggest that secondary formation of particulate (PM2.5) nitrate in the Fairbanks downtown area does not contribute significant mass to the total PM2.5 concentration, but appreciable amounts are formed downwind of downtown due to nocturnal NOx oxidation and subsequent reaction with ammonia on aerosol particles.

A Kinetic Platform to Determine the Fate of Nitric Oxide in Escherichia coli

Nitric oxide (NO•) is generated by the innate immune response to neutralize pathogens. NO• and its autoxidation products have an extensive biochemical reaction network that includes reactions with iron-sulfur clusters, DNA, and thiols. The fate of NO• inside a pathogen depends on a kinetic competition among its many targets, and is of critical importance to infection outcomes. Due to the complexity of the NO• biochemical network, where many intermediates are short-lived and at extremely low concentrations, several species can be measured, but stable products are non-unique, and damaged biomolecules are continually repaired or regenerated, kinetic models are required to understand and predict the outcome of NO• treatment. Here, we have constructed a comprehensive kinetic model that encompasses the broad reactivity of NO• in Escherichia coli. The incorporation of spontaneous and enzymatic reactions, as well as damage and repair of biomolecules, allowed for a detailed analysis of how NO• distributes in E. coli cultures. The model was informed with experimental measurements of NO• dynamics, and used to identify control parameters of the NO• distribution. Simulations predicted that NO• dioxygenase (Hmp) functions as a dominant NO• consumption pathway at O2 concentrations as low as 35 µM (microaerobic), and interestingly, loses utility as the NO• delivery rate increases. We confirmed these predictions experimentally by measuring NO• dynamics in wild-type and mutant cultures at different NO• delivery rates and O2 concentrations. These data suggest that the kinetics of NO• metabolism must be considered when assessing the importance of cellular components to NO• tolerance, and that models such as the one described here are necessary to rigorously investigate NO• stress in microbes. This model provides a platform to identify novel strategies to potentiate the effects of NO•, and will serve as a template from which analogous models can be generated for other organisms.

Photochemical Renoxification of Nitric Acid on Real Urban Grime

The fate of NO(x) (=NO + NO(2)) is important to understand because NO(x) is a significant player in air quality determination through its role in O(3) formation. Here we show that renoxification of the urban atmosphere may occur through the photolysis of HNO(3) deposited onto urban grime. The photolysis occurs 4 orders of magnitude faster than in water with J values at noon on July 1 in Toronto of 1.2 × 10(-3) s(-1) for nitrate on urban grime and 1.0 × 10(-7) s(-1) for aqueous nitrate. Photolysis of nitrate present on urban grime probably follows the same mechanism as aqueous nitrate photolysis, involving the formation of NO(2), OH, and possibly HONO. Thus NO(x) may be rapidly returned to the atmosphere rather than being ultimately removed from the atmosphere through film wash off.

Hypoxic events stimulate nitrogen recycling in a shallow salt‐wedge estuary: The Yarra River estuary, Australia

The Yarra River estuary is a salt‐wedge estuary prone to periods of stratification‐induced anoxia and hypoxia (O2 < 100 µmol L−1) during low‐flow events. Nitrate reduction pathways were examined using the 15N isotope pairing technique in intact sediment cores, emulating in situ conditions, to evaluate the fate of NO during changing oxygen conditions. Water‐column concentrations of dissolved inorganic carbon (DIC), O2, NH, and NOx (NO + NO) were also measured to examine any deviation from conservative behavior (denoted Δ) in response to oxygen variability within the estuary. The estuary was a source of NH in the anoxic bottom waters. Whole‐system estimates using deviations from conservative behavior and core incubations were in good agreement and showed that NH was regenerated more efficiently relative to DIC under hypoxic conditions. For the whole system, mean ΔDIC : ΔNH ratios under oxic (85 ± 33) and hypoxic (20 ± 3) conditions were significantly different. The more‐efficient NH regeneration during hypoxia was attributed to rapid mineralization rates and cessation of nitrification; dissimilatory nitrate reduction to ammonium (DNRA) was not a significant contributor. Unexpectedly, the denitrification : DNRA ratio was significantly higher under hypoxic conditions, with denitrification contributing 99.1% ± 0.3% of total nitrate reduction. DNRA rates were significantly higher during oxic conditions (123.5 ± 30.7 µmol m−2 h−1) when compared with rates during hypoxia (0.6 ± 0.1 µmol m−2 h−1). The increase in DNRA in the presence of oxygen was attributed to the alleviation of NO limitation during these conditions.

Characteristics of nitrate in major rivers and aquifers of the Sanjiang Plain, China

The characteristics of nitrate (NO(3)(-)) in major rivers and aquifers of the Sanjiang Plain, China were investigated by hydrogeochemical conditions, nitrogen isotope technique and CFCs trace. An overall understanding on the sources and fate of NO(3)(-) in the surface water and the groundwater was obtained. The NO(3)(-) concentrations in the surface water were low and no samples exceeds the WTO standards. However, 11.4% of the groundwater samples exceeded the WTO standards, indicating local NO(3)(-) pollution in rural areas. Redox condition analysis revealed that most of the surface water had oxic condition, while for the shallow groundwater (mean well depth smaller than 30 m), the redox condition began to change into anoxic zone, and the deep groundwater (mean well depth larger than 50 m) showed strong anoxic condition. The δ(15)N-NO(3) data indicated soil N and fertilizer contributed the major sources in the surface water, and NO(3)(-) in the groundwater mainly showed a manure origin. In the Songhua-Heilong River, dilution effect was dominating, while for the Wusuli River, it showed that mix with water contained excess of NO(3)(-) resulted in the NO(3)(-) concentration increased along the river. Additionally, the NO(3)(-) transportation in the groundwater was analyzed by groundwater ages derived from environmental tracer (CFCs) data. The relation between the groundwater ages and the NO(3)(-) concentrations showed that the young groundwater with the age less than 60 years had higher NO(3)(-) concentrations than the old groundwater over the age of 60 years because anthropogenic activities began to boom from 1950s in the Sanjiang Plain.

Tracing sources of soil nitrate using the dual isotopic composition of nitrate in 2 M KCl-extracts

Soils comprise a critical interface between the atmosphere, lithosphere, hydrosphere and biosphere, and play a major role in the cycling of nitrogen (N), an element crucial to plant growth. Isotope techniques constitute a powerful tool to study the origin and fate of N compounds (e.g. NO 3 −) within the environment including soils. The objective of our study was to test the usefulness of the isotope composition of soil NO 3 − extracted with 2 M KCl (soil NO 3) as a tool to investigate the origin and fate of NO 3 − in the environment. Specifically issues related to repeat extractions, crop type, length of fertilization, and soil depth were addressed. Soils from four contrasting agricultural management regimes were sampled. Within the relatively confined study area (4 ha), the isotopic compositions of soil NO 3 differed markedly due to management treatments (up to 6 and 17‰ for δ 15N and δ 18O, respectively), but were repeatable among replicate plots (±1‰). Differences in both δ 15N and δ 18O values were observed between legume and non-legume treatments, as well as fertilized versus non-fertilized treatments, which were larger than the variability observed between replicate plots. Differences in the isotopic composition of extractable soil nitrate were not limited to the surface layer, but also occurred within deeper soil layers. This study indicates that the analysis of the natural abundance stable isotope composition of soil NO 3 may provide a promising additional tool for tracing the origin and fate of NO 3 − in the soil zone.

An integrated approach to assessing nitroso-redox balance in systemic inflammation

Most studies examining the metabolic fate of NO during systemic inflammation have focused on measuring the quantitatively predominating, stable anions nitrite and nitrate within the circulation. However, these are not necessarily the NO-related products that govern NO metabolism and signaling in tissues. We assessed all major NO derivatives temporally in blood and vital organs during inflammation and explored their relationship to insult severity and redox status. Male rats receiving intraperitoneal endotoxin or vehicle were sacrificed for organ and blood sampling between 0 and 24 h. Endotoxin induced transient and organ-specific changes in a variety of NO metabolites. Nitrite and nitrate increased, peaking at 8 and 12 h, respectively. S- and N-nitrosation and heme-nitrosylation products also peaked at 8 h; these posttranslational protein modifications were associated with decreased myocardial function (echocardiography). Evidence of oxidative stress and systemic inflammation was also obtained. The rise in most NO derivatives was proportional to insult severity. All metabolite levels normalized within 24 h, despite evidence of persisting myocardial dysfunction and clinical unwellness. Our findings point to a complex interplay between NO production, antioxidant defense, and redox status. Although the precise (patho)physiologic roles of specific NO derivatives and their diagnostic/prognostic utility await further investigation, nitroso species in erythrocytes are the most sensitive markers of NO in systemic inflammation, detectable before clinical symptoms manifest.

Intrusion of CO2 and impurities in a freshwater aquifer — Impact evaluation by reactive transport modelling

Abstract Contamination of fresh groundwater is one of the environmental issues of Carbon Capture and Storage. We simulate with the reactive transport code TOUGHREACT the intrusion of CO 2 + impurities from oxycombustion capture in a freshwater aquifer model based on a glauconitic sandstone aquifer. We choose an unfavorable scenario where the contaminant gas presents maximal contents in SO x , NO x and O 2 . Water quality changes after a 10 years intrusion are exposed. French quality standards in pH, Fe, Mn and sulphates concentrations for drinking water are reminded in the results for reference. Acidification and carbonation of water induces Fe- and Mn-bearing minerals dissolution that increases the concentration of these metals in water. Contamination by sulphates is due to the SO 2 oxidation by O 2 , both of these components being carried by the contaminant gas. The vertical extent of contamination is enhanced by both the upward migration of gaseous CO 2 and downward migration of dense carbonated water. The presence of SO 2 and NO in the contaminant gas, even in very low concentrations, induces a negative local pH shift of 1 unit by comparison with a pure- CO 2 contaminant gas. This pH shift has consequences on Fe and Mn release amplitude. The code does not allow realistic modelling of the fate of NO x in the aquifer. But we hypothesize this component to be oxidized by O 2 into nitrite or nitrates (health-significant substances) that persist into the aquifer. Perspectives are addressed.

Tracing the Origin and Fate of NO x in the Arctic Atmosphere Using Stable Isotopes in Nitrate

Atmospheric nitrogen oxides (NOx =NO+ NO2) play a pivotal role in the cycling of reactive nitrogen (ultimately deposited as nitrate) and the oxidative capacity of the atmosphere. Combined measurements of nitrogen and oxygen stable isotope ratios of nitrate collected in the Arctic atmosphere were used to infer the origin and fate of NOx and nitrate on a seasonal basis. In spring, photochemically driven emissions of reactive nitrogen from the snowpack into the atmosphere make local oxidation of NOx by bromine oxide the major contributor to the nitrate budget. The comprehensive isotopic composition of nitrate provides strong constraints on the relative importance of the key atmospheric oxidants in the present atmosphere, with the potential for extension into the past using ice cores.

Fate of nitrate in the capillary fringe and shallow groundwater in a drained sandy soil

It is commonly assumed that nitrate (NO 3 −) and other anions entering the soil move downward through the vadose zone, and then move horizontally in the groundwater. Recent laboratory studies, however, indicate that water movement and transport of pollutants can also take place in the capillary fringe (CF) above the water table (WT). This field study evaluated the fate of NO 3 − in the CF and shallow groundwater (SGW) for a sandy soil (Aeric Alaquod) with shallow water table. Ten L of a solution containing approximately 18 mmol L − 1 nitrate [2.66 g L − 1 Mg(NO 3) 2] and 77 mmol L − 1 bromide (9.12 g L − 1 KBr) were applied to the soil above the CF. The movement of both NO 3 − and Br − was monitored for 84 days by using tension lysimeters installed at depths between 45 and 105 cm at radial distances of 20, 60, 120, 220 and 320 cm from the application point. Nitrate and Br − plumes that entered the CF from the unsaturated zone moved horizontally in the CF until both species were partially carried into the groundwater by the fluctuating WT following rain events. Normalized concentrations of NO 3 −N and Br − remained comparable as they moved horizontally in the CF up to 320 cm from the tracer application spot. However, below the WT the detected normalized concentration of Br − was higher than that for NO 3 − indicating nitrate loss, perhaps due to denitrification. When monitoring subsurface NO 3 −, solely relying on collection of groundwater samples may lead to an underestimation of the extent of NO 3 − contamination and transport in the subsurface.

Transport and Fate of Nitrate at the Ground‐Water/Surface‐Water Interface

Although numerous studies of hyporheic exchange and denitrification have been conducted in pristine, high-gradient streams, few studies of this type have been conducted in nutrient-rich, low-gradient streams. This is a particularly important subject given the interest in nitrogen (N) inputs to the Gulf of Mexico and other eutrophic aquatic systems. A combination of hydrologic, mineralogical, chemical, dissolved gas, and isotopic data were used to determine the processes controlling transport and fate of NO(3)(-) in streambeds at five sites across the USA. Water samples were collected from streambeds at depths ranging from 0.3 to 3 m at three to five points across the stream and in two to five separate transects. Residence times of water ranging from 0.28 to 34.7 d m(-1) in the streambeds of N-rich watersheds played an important role in allowing denitrification to decrease NO(3)(-) concentrations. Where potential electron donors were limited and residence times were short, denitrification was limited. Consequently, in spite of reducing conditions at some sites, NO(3)(-) was transported into the stream. At two of the five study sites, NO(3)(-) in surface water infiltrated the streambeds and concentrations decreased, supporting current models that NO(3)(-) would be retained in N-rich streams. At the other three study sites, hydrogeologic controls limited or prevented infiltration of surface water into the streambed, and ground-water discharge contributed to NO(3)(-) loads. Our results also show that in these low hydrologic-gradient systems, storm and other high-flow events can be important factors for increasing surface-water movement into streambeds.

Use of stable nitrogen isotope fractionation to estimate denitrification in small constructed wetlands treating agricultural runoff

Constructed wetlands (CWs) in the agricultural landscape reduce non-point source pollution through removal of nutrients and particles. The objective of this study was to evaluate if measurements of natural abundance of 15NO 3 − can be used to determine the fate of NO 3 − in different types of small CWs treating agricultural runoff. Nitrogen removal was studied in wetland trenches filled with different filter materials (T1 — sand and gravel; T3 — mixture of peat, shell sand and light-weight aggregates; T8 — barley straw) and a trench formed as a shallow pond (T4). The removal was highest during summer and lowest during autumn and winter. Trench T8 had the highest N removal during summer. Measurements of the natural abundance of 15N in NO 3 − showed that denitrification was not significant during autumn/winter, while it was present in all trenches during summer, but only important for nitrogen removal in trench T8. The 15N enrichment factors of NO 3 − in this study ranged from − 2.5 to − 5.9‰ (T3 and T8, summer), thus smaller than enrichment factors found in laboratory tests of isotope discrimination in denitrification, but similar to factors found for denitrification in groundwater and a large CW. The low enrichment factors compared to laboratory studies was attributed to assimilation in plants/microbes as well as diffusion effect. Based on a modified version of the method presented by Lund et al. [Lund LJ, Horne AJ, Williams AE, Estimating denitrification in a large constructed wetland using stable nitrogen isotope ratios. Ecol Engineer 2000; 14: 67–76], denitrification and assimilation were estimated to account for 53 to 99 and 1 to 47%, respectively, of the total N removal during summer. This method is, however, based on a number of assumptions, and there is thus a need for a better knowledge of the effect of plant uptake, microbial assimilation as well as nitrification on N isotopic fractionation before this method can be used to evaluate the contribution of dinitrification in CWs.

Inflammatory Modulation of Hepatocyte Apoptosis by Nitric Oxide: In Vivo, In Vitro, and In Silico Studies

Nitric oxide (NO*) and its reaction products are key players in the physiology and pathophysiology of inflammatory settings such as sepsis and shock. The consequences of the expression of inducible NO* synthase (iNOS, NOS-2) can be either protective or damaging to the liver. We have delineated two distinct hepatoprotective actions of NO*: the stimulation of cyclic guanosine monophosphate and the inhibition of caspases by S-nitrosation. In contrast, iNOS/NO* promotes hepatocyte death under conditions of severe redox stress, such as hemorrhagic shock or ischemia/reperfusion. Redox stress activates an unknown molecular switch that transforms NO*, which is hepatoprotective under resting conditions, into an agent that induces hepatocyte death. We hypothesize that the magnitude of the redox stress is a major determinant for the effects of NO* on cell survival by controlling the chemical fate of NO*. To address this hypothesis, we have carried out studies in relevant in vivo and in vitro settings. Moreover, we have constructed an initial mathematical model of caspase activation and coupled it to a model describing some of the reactions of NO* in hepatocytes. Our studies suggest that modulation of iron, oxygen, and superoxide may dictate whether NO* is hepatoprotective or hepatotoxic.

Prediction of Groundwater Nitrate Contamination after Closure of an Unlined Sheep Feedlot

Nitrate contamination of groundwater by a sheep feedlot in Hawke9s Bay, New Zealand led to closure of the feedlot in 1998. However, knowledge of the processes controlling how long the contamination will remain and an analysis of whether the current land use (vineyard) will also impact groundwater quality are required to assess long-term groundwater quality issues after feedlot operations cease. To determine the fate of NO 3 following land use changes and the rate of reduction of contamination that may be expected under natural conditions following such changes, we compared the chemical concentrations of NO 3 –N, Cl, and alkalinity (HCO 3 ) in the groundwater and rivers from surveys conducted in 1994 and 1995 with sampling conducted in 2001. Profile sampling of total N and C of the 3 –N concentrations were under the feedlot (>140 g m −3 NO 3 –N) and down gradient. In 2001, 3 yr after the feedlot closed, the Cl concentrations had increased in down-gradient wells but remained similar to the 1994 survey in other wells. There has been a decrease in NO 3 –N concentrations in most wells, compared with the peak NO 3 –N concentrations recorded in the 1995 survey, but an increase compared with 1994. Alkalinity concentrations in wells located within the influence of the feedlot are approximately 150 g m −3 lower than in surrounding wells. This indicates that nitrification reactions are affecting the HCO 3 concentrations in the feedlot-influenced wells. However, the HCO 3 concentrations of some of these wells are increasing, indicating that nitrification could be slowing down and the aquifer is beginning to recover. SPASMO modeling indicates that NO 3 contamination from the site will continue for the next 3 to 5 yr. The impact of NO 3 leaching due to current land use practices is likely to be much less than the feedlot. The model predicts there will be an improvement in groundwater quality in the next 3 to 5 yr as NO 3 from the feedlot eventually leaves the vadose zone profile and mixes into the unconfined aquifer.

Biogeochemistry and natural attenuation of nitrate in groundwater at an explosives test facility

An interdisciplinary study was conducted to characterize the distribution and fate of NO 3 − in groundwater at Lawrence Livermore National Laboratory (LLNL) Site 300, a high-explosives test facility in the semi-arid Altamont Hills of California. Site 300 groundwater contains NO 3 − concentrations ranging from <0.5 to >200 mg NO 3 −/L. Several lines of evidence strongly suggest that denitrification is naturally attenuating NO 3 − in the confined, O 2-depleted region of the bedrock aquifer under study (Tnbs 2): (a) both NO 3 − and dissolved O 2(DO) concentrations in groundwater decrease dramatically as groundwater flows from unconfined to confined aquifer conditions, (b) stable isotope signatures (i.e., δ 15N and δ 18O) of groundwater NO 3 − indicate a trend of isotopic enrichment that is characteristic of denitrification, and (c) dissolved N 2 gas, the product of denitrification, was highly elevated in NO 3 −-depleted groundwater in the confined region of the Tnbs 2 aquifer. Long-term NO 3 − concentrations were relatively high and constant in recharge-area monitoring wells (typically 70–100 mg NO 3 −/L) and relatively low and constant in the downgradient confined region (typically <0.1–3 mg NO 3 −/L), suggesting a balance between rates of NO 3 − loading and removal by denitrification. Chemolithoautotrophic denitrification with pyrite as the electron donor is plausible in the Tnbs 2 aquifer, based on the low dissolved organic C concentrations (<1.5 mg/L) that could not support heterotrophic denitrification, the common occurrence of disseminated pyrite in the aquifer, and the trend of increasing SO 2− 4 as groundwater flows from aerobic, unconfined to anoxic, confined aquifer conditions. Nitrate sources were investigated by experimentally determining the δ 15N and δ 18O signatures of NO 3 − from three potential anthropogenic sources of NO 3 − at Site 300: Ba(NO 3) 2 (mock explosive), HNO 3, and photolysis of the explosive RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine). The isotopic signatures of these potential NO 3 − sources were markedly different than those of NO 3 − in Tnbs 2 groundwater samples, suggesting that other sources must contribute significantly to the NO 3 − loading at Site 300. In particular, NO 3 − and NO 2 − resulting from RDX photolysis reflected dramatically depleted δ 15N (ca. −7.4‰) and δ 18O (ca. −25.7‰) values.