Showing papers on "Ammonia published in 1994"

Activity and selectivity of pure manganese oxides in the selective catalytic reduction of nitric oxide with ammonia

Abstract: Manganese oxides of different crystallinity, oxidation state and specific surface area have been used in the selective catalytic reduction (SCR) of nitric oxide with ammonia between 385 and 575 K. MnO2 appears to exhibit the highest activity per unit surface area, followed by Mn5O8, Mn2O3, Mn3O4 and MnO, in that order. This SCR activity correlates with the onset of reduction in temperature-programmed reduction (TPR) experiments, indicating a relation between the SCR process and active surface oxygen. Mn2O3 is preferred in SCR since its selectivity towards nitrogen formation during this process is the highest. In all cases the selectivity decreases with increasing temperature. The oxidation state of the manganese, the crystallinity and the specific surface area are decisive for the performance of the oxides. The specific surface area correlates well with the nitric oxide reduction activity. The nitrous oxide originates from a reaction between nitric oxide and ammonia below 475 K and from oxidation of ammonia at higher temperatures, proven by using 15NH3. Participation of the bulk oxygen of the manganese oxides can be excluded, since TPR reveals that the bulk oxidation state remains unchanged during SCR, except for MnO, which is transformed into Mn3O4 under the applied conditions. In the oxidation of ammonia the degree of oxidation of the nitrogen containing products (N2, N2O, NO) increases with increasing temperature and with increasing oxidation state of the manganese. A reaction model is proposed to account for the observed phenomena.

Mechanistic analysis of ammonium inhibition of atmospheric methane consumption in forest soils.

Abstract: Methane consumption by forest soil was studied in situ and in vitro with respect to responses to nitrogen additions at atmospheric and elevated methane concentrations. Methane concentrations in intact soil decreased continuously from atmospheric levels at the surface to 0.5 ppm at a depth of 14 cm. The consumption rate of atmospheric methane in soils, however, was highest in the 4- to 8-cm depth interval (2.9 nmol per g of dry soil per day), with much lower activities below and above this zone. In contrast, extractable ammonium and nitrate concentrations were highest in the surface layer (0 to 2 cm; 22 and 1.6 μmol per g of dry soil, respectively), as was potential ammonium-oxidizing activity (19 nmol per g of dry soil per day). The difference in zonation between ammonium oxidation and methane consumption suggested that ammonia-oxidizing bacteria did not contribute significantly to atmospheric methane consumption. Exogenous ammonium inhibited methane consumption in situ and in vitro, but the pattern of inhibition did not conform to expectations based on simple competition between ammonia and methane for methane monooxygenase. The extent of ammonium inhibition increased with increasing methane concentration. Inhibition by a single ammonium addition remained constant over a period of 39 days. In addition, nitrite, the end product of methanotrophic ammonia oxidation, was a more effective inhibitor of methane consumption than ammonium. Factors that stimulated ammonium oxidation in soil, e.g., elevated methane concentrations and the availability of cosubstrates such as formate, methanol, or β-hydroxybutyrate, enhanced ammonium inhibition of methane oxidation, probably as a result of enhanced nitrite production.

Nitrogenase: A Nucleotide-Dependent Molecular Switch

Abstract: In the simplest terms, the biological nitrogen cycle is the reduction of atmospheric dinitrogen (N2) to ammonia with the subsequent reoxidation ammonia to dinitrogen (1). At the reduction level of ammonia, nitrogen incorporated into precursors for biological macromolecules such as proteins and nucleic acids. Reoxidation of ammonia to dinitrogen ("denitrification") by a variety of microbes (by way of nitrite and other oxidation levels of nitrogen) leads to the depletion of the "fixed," biologically usable, nitrogen pool. Besides the relatively small contribution from commercial ammonical fertilizer production, replenishing of the nitrogen pool falls mainly to a limited number of physiologically diverse microbes (e.g. eubacteria and archaebacteria; free-living and symbiotic; aerobic and anaerobic) that contain the nitrogenase enzyme system.

Posttranslational Regulation of Nitrate Reductase in Higher Plants.

Abstract: In many plants, nitrogen represents 2 to 6% of dry matter, most of it being present in the form of amino acids, proteins, or nucleic acids. In these organic compounds, nitrogen exists in its most reduced state (oxidation state -3). It is taken up from the soil primarily in the form of nitrate (oxidation state +5). Thus, nitrate has to be reduced by plants at the expense of reductants such as NADH or NADPH, requiring 8 mol of electrons [or 4 mol of NAD(P)H] per mol of nitrate. Reduction is a two-step mechanism. The first step, reduction of nitrate to nitrite (+3 oxidation state), is catalyzed by assimilatory NR, which is an NAD(P)H-dependent cytosolic enzyme. Nitrite is further reduced to ammonia (-3 oxidation state) in the plastids of leaves or roots by NiR. There are at least two important reasons why plants must exert control on the velocity of nitrate reduction. First, it is an energy-consuming process. As shown above, 8 electrons are required to reduce one nitrate to ammonium, but only 4 electrons are needed to reduce CO2 to the carbohydrate level. Accordingly, a C/N ratio of 10 in the plant biomass (a value found in many herbaceous plants) indicates that about 20% of the photosynthetically produced electrons are consumed for nitrate reduction. Plants have to avoid 'luxury' consumption of nitrate and energy. Second, and perhaps more important, the primary product of nitrate reduction, nitrite (NO2-), is cytotoxic and regarded to be mutagenic as a result of the ability to diazotize amino groups. HNO2 is also a weak acid that, in its undissociated form, can easily penetrate biomembranes, thereby leading to acidification of cells or subcellular compartments. Thus, it makes sense if nitrate reduction is controlled in such a way that it does not exceed nitrite and ammonium consumption. In fact, even under rapidly fluctuating environmental conditions, nitrite levels in plant tissues remain low (below 0.1 mM). Apparently, the overall rates of nitrate and nitrite reduction always match the availability of energy and of carbohydrate, perhaps with the exception of some extreme conditions to be discussed below. Synchronization of nitrate reduction and carbon metabolism may occur at the level of transcription or translation of participating enzymes. The expression of NR genes at the transcription level is highly affected by nitrate, but also by light, plant hormones, and other factors (for review, see Solomonson and Barber, 1990; Lillo, 1994). The enzyme protein is rather short-lived, being degraded with a half-time of a few hours (Li and Oaks, 1993). This high turnover rate permits control of nitrate reduction, e.g. in response to nitrate availability (Li and Oaks, 1993). However, over the years, a number of situations have been described in which the level of extractable NRA did not match NR protein or the rate of nitrate reduction in vivo, indicating that yet other regulatory mechanisms might exist that modulate the catalytic activity of the protein (Lillo, 1994, and refs. cited therein). Such a newly discovered type of NR modulation, a reversible protein phosphorylation, will be described below.

Performance of a high‐solids anaerobic digestion process under various ammonia concentrations

Abstract: Ammonia is produced during anaerobic digestion of protein-containing materials. While ammonia can be utilized by some members of the anaerobic population, excess ammonia can inhibit methanogenesis. High-solids anaerobic digestion may be especially sensitive to the effects of ammonia overproduction. This study explored the performance of a pilot scale high-solids anaerobic digestion process under various total ammonia nitrogen (TAN) concentrations. In general the high-solids digester could operate up to 1000 mg dm -3 without any inhibitory effect. The digester performed best when it was operated at TAN concentrations in the narrow range of 600 to 800 mg dm -3

The Effect of Nitrogen Deficiency and Sulfur-Containing Amino Acids on the Reduction of Sulfate to Hydrogen Sulfide by Wine Yeasts

Abstract: The formation of hydrogen sulfide in wine has been shown to arise from fermentations of grape musts with nitrogenous deficiencies. This work was instigated to explore this formation with the use of a qualitative, but simple, rapid, and explicit method for hydrogen sulfide detection. Two yeast strains were used that showed representative high and low formations of sulfide from sulfite. In synthetic medium, without added sulfite, the formation of hydrogen sulfide was shown to come exclusively from reduction of sulfate and was dependent upon the limiting concentration of ammonia. The amount of n -propanol formed was also found to be dependent upon the yeast strain and upon the ammonia present. Under ammonia limitations, the addition of non-sulfur containing amino acids tended to inhibit the formation of hydrogen sulfide from sulfate but increased formation of sulfite. Added cysteine brought about increased sulfide but inhibited sulfite formation. Added methionine inhibited both sulfide and sulfite formation, as expected. These results, while preliminary, show that sulfate can be the main substrate for hydrogen sulfide production. The amount of n -propanol formed was explained by metabolic control by methionine.

Removal of NO from flue gases by absorption to an iron(ii) thiochelate complex and subsequent reduction to ammonia

Abstract: THE combustion of fossil fuels generates SO2 and NOX pollutants which cause acid rain and urban smog1 Existing flue-gas desulphurization scrubbers involve wet limestone processes which are efficient for controlling SO2 emissions but are incapable of removing water-insoluble nitric oxide The current technique for postcombustion control of nitrogen oxide emissions, ammonia-based selective catalytic reduction, suffers from various problems2,3, including poisoning of the catalysts by fly ash rich in arsenic or alkali, disposal of spent toxic catalysts and the effects of ammonia by-products on plant components downstream from the reactor To circumvent the need for separate schemes to control SO2 and NOX, we have developed an iron(ii) thiochelate complex that enhances the solubility of NO in aqueous solution by rapidly and efficiently absorbing NO to form iron nitrosyl complexes The bound NO is then converted to ammonia by electrochemical reduction, regenerating the active iron(ii) catalyst for continued NO capture Our results suggest that this process can be readily integrated into existing wet limestone scrubbers for the simultaneous removal of SO2 and NOX

Ammonium Nitrate, Nitric Acid, and Ammonia Equilibrium in Wintertime Phoenix, Arizona

Abstract: A secondary aerosol equilibrium model, SEQUILIB, is applied to evaluate the effects of emissions reductions from precursor species on ambient concentrations during the winter in Phoenix, Arizona. The model partitions total nitrate and total ammonia to gas-phase nitric acid and ammonia and to particle-phase ammonium nitrate. Agreement between these partitions and ambient measures of these species was found to be satisfactory. Equilibrium isopleths were generated for various ammonium nitrate concentrations corresponding to high and low humidity periods which occurred during sampling. These diagrams show that ammonia is so abundant in Phoenix that massive reductions in its ambient concentrations would be needed before significant reductions in particulate ammonium nitrate would be observed. When total nitrate is reduced by reductions in its nitrogen oxides precursor, proportional reductions in particulate nitrate are expected. Many of the complex reactions in SEQUILIB do not apply to Phoenix, and its ability...

Microcalorimetric study of the acidity of tungstic heteropolyanions

Abstract: The number and strength of the acid centres of tungstic heteropolyacids have been determined by absorption calorimetry of ammonia. The initial heats are in the order H3PW12O40>H4SiW12O40>H6P2W21O71(H2O)3>H6P2W18O62, varying from 200 to 155kJ mol–1. An increase in the number of protons in Keggin heteropolyanions decreases the acidic strength. Moreover, the influence of the activation temperature on the acidity has been studied and confirms that there is a drastic modification of the solid at ca. 350 °C. The heat capacities of the heteropolyanions and of the corresponding ammonium salts, the thermokinetic parameters of ammonia absorption and the heat capacities of the acids and ammonium salts have been related to the porosity of the various samples.

Process for the recovery of fluorinated carboxylic acids

Abstract: Fluorinated carboxylic acids can be recovered from materials which contain them by releasing, if necessary, the carboxylic acids with sufficiently strong acids, esterifying the released acids and distilling off the esters. The esters can be advantageously directly hydrolyzed with aqueous ammonia solution to give the corresponding ammonium salts.

Selective Oxidation of Ammonia to Hydroxylamine with Hydrogen Peroxide on Titanium Based Catalysts

Abstract: The synthesis of hydroxylamine by oxidation of ammonia with hydrogen peroxide on titanium based catalysts is reported. Titanium silicalite is the best catalyst for the reaction. The influence of some reaction parameters on the main and side reactions is discussed and the reaction network is proposed. The role of titanium is pointed out by results of spectroscopic studies.

Process for the simultaneous absorption of sulfur oxides and production of ammonium sulfate

Abstract: Process for the removal of sulfur oxides from sulfur oxide-containing gas with simultaneous production of ammonium sulfate. The process is carried out by first passing hot sulfur oxide-containing gas through a prescrubber wherein the gas contacts saturated aqueous ammonium sulfate liquor which is recycled in the prescrubber, followed by passing the prescrubbed gas through an absorber wherein the prescrubbed gas contacts dilute aqueous ammonium sulfate liquor. The sulfur oxide in the sulfur oxide-containing gas is absorbed by the dilute aqueous ammonium sulfate liquor in the absorber, and scrubbed gas is removed from the absorber. The dilute aqueous ammonium sulfate liquor is treated with ammonia and air and the absorbed sulfur dioxide is converted to ammonium sulfate in the liquor. The dilute ammonium sulfate liquor is recycled into contact with the prescrubbed gas in the absorber. Dilute aqueous ammonium sulfate liquor is removed from the absorber and added to the saturated aqueous ammonium sulfate liquor in the prescrubber where it becomes saturated due to evaporation caused by the hot gas. Ammonium sulfate crystals form in the saturated aqueous ammonium sulfate liquor in the prescrubber are recovered as product from saturated aqueous ammonium sulfate withdrawn from the prescrubber.

Ammonia volatilization from urea and ammoniacal fertilizers surface applied to winter wheat and grassland

Abstract: Ammonia volatilization from urea, diammonium phosphate, ammonium sulphate and calcium ammonium nitrate surface applied to winter wheat and grassland was determined with windtunnels. The fertilizers were applied at a rate of 8–12 g N m−2 to plots on a non-calcareous sandy loam. Five experiments were carried out during March to June 1992, each experiment including 2 to 4 treatments with two or three replications. The daily ammonia loss rate was measured during 15 to 20 days. Cumulated daily loss of ammonia from urea followed a sigmoidal expression, while the cumulated ammonia loss from diammonium phosphate showed a logarithmic relationship with time from application. For ammonium sulphate and calcium ammonium nitrate no significant loss could be determined, because daily loss of ammonia were at the detection limit of the wind tunnels. Mean cumulated ammonia loss from plots receiving urea, diammonium phosphate, ammonium sulphate and calcium ammonium nitrate were 25%, 14%, <5% and <2%, respectively, during a 15–20 day measuring period.

Induction of ornithine-urea cycle in a freshwater teleost, Heteropneustes fossilis, exposed to high concentrations of ammonium chloride

Abstract: An ammoniotelic freshwater teleost, Heteropneustes fossilis , tolerated ambient ammonium chloride concentration up to 75 mM. Ammonia accumulated significantly in all the tissues within 7 days of treatment and the concentration remained high throughout the 4-week period of treatment. The activity of enzymes of the ornithine-urea (o-u) cycle were induced within 7 days, and thereafter remained high in both the liver and kidney of the fish. Urea accumulated significantly in various tissues simultaneous with the induction of o-u cycle enzymes. Accumulated ammonia induced the activity of the enzymes of the o-u cycle for its metabolic conversion to urea. This helped the freshwater fish to avoid toxaemia and to tolerate high concentrations of ammonia in the ambient medium.