Denitrification nitrogen oxides

K. A. Smith and J. R. M. Arah, Losses of Nitrogen by Denitrification and Emissions of Nitrogen Oxides from Soils, The Fertiliser Soeiety, 1990, Proeeedings No. 299. 

Nitrogen oxides are formed at various stages of the biological denitrification process. This process starts with nitrate as the nitrate is reduced through various steps, NO2, NO, N2O, and N2 can be formed and, depending on the conditions, released to the atmosphere. 

Table 19-1 demonstrates that with the exception of water vapor, all of these cycles have been severely perturbed by human activity. Of course, all of these cycles are also linked in many ways. For example, the combustion of fossil fuel has increased the fluxes of carbon, sulfur, and nitrogen oxides to the atmosphere. Denitrification, the production of N2O, is linked with the production of CO2 during respiration and decay. And of course, other important cycles are involved which are not depicted here. Look back at Fig. 17-8, which sums up the climate forcings by the key agents. 

Bioprocesses for the removal of nitrogen oxides from polluted air are an interesting alternative [58], but current reaction rates are still too low for large-scale applications. Advanced biological processes for the removal of NO from flue gases are based on the catalytic activity of either eukaryotes or prokaryotes, e.g. nitrification, denitrification, the use of microalgae and a combined physicochemical and biological process (BioDeNO ). 

Tolman W.B. (1995) Synthetic Modeling of the Interactions of Nitrogen Oxides with Copper Proteins Copper Nitrosyl Complexes Relevant to Putative Denitrification Intermediates, Adv. Chem. Ser., 246, 195. 

Denitrification, a dissimilatory pathway of nitrate reduction (see Section 3.3 also) into nitrogen oxides, N2O, and dinitrogen, N2, is performed by a wide variety of microorganisms in the forest ecosystems. Measurable rates of N20 production have been observed in many forest soils. The values from 2.1 to 4.0 kg/ha/yr are typical for forest soils in various places of Boreal and Sub-Boreal Forest ecosystems. All in situ studies (field monitoring) of denitrification in forest soils have shown large spatial and temporal variability in response to varying soils characteristics such as acidity, temperature, moisture, oxygen, ambient nitrate and available carbon. 

Nitrogen oxides and nitrogen gas from denitrification will also be present. 

Figure 4.1 shows that NOs" is the stable form of nitrogen over the usual range of pe + pH in aerobic environments. The fact that most of the N2 in the atmosphere has not been converted to NO3 therefore indicates that the biological mediation of this conversion in both directions is inefficient. Hence NO3 reduction to N2 occurs by indirect mechanisms involving intermediaries. Dissimilatory reduction of N03 (i.e. where the nitrogen oxide serves as an electron acceptor for the cell s metabolism but the N reduced is not used by the microbes involved) potentially occurs by two processes denitrification. 

The inherent ability of selective catalytic reduction (SCR) catalysts for stack gas denitrification to store ammonia adsorptively can be exploited with appropriate control algorithms to damp out the influence of fluctuations in the amount of gas and level of nitrogen oxides being treated. Moreover, it also forms the basis of the adsorptive reactor concept for the total denitrification of flue gases without ammonia... 

This chapter focuses on the chemistry ofbiomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NOf of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. 

Table 1 Redox conversions of nitrogenous oxides during denitrification...

Pathways and Controls of Nitrogen Oxide Reduction and Denitrification... 

Canonical denitrification is carried out by heterotrophic bacteria during which nitrate (or nitrite) serves as the terminal electron acceptor for organic matter oxidation and the nitrogen oxides are reduced mainly to nitrogen (some nitrous oxide may be formed). The characteristic feature of canonical denitrification is that the reduction of N-oxides is coupled to electron transport phosphorylation (Knowles, 1982, 1996 Koike and Hattori, 1975). The capacity for respiratory denitrification is widespread among bacteria and is distributed across various taxonomic subclasses, mainly within the Proteobacteria (Zumft, 1997). 

Figure 12 Major reduction-oxidation reactions involving nitrogen. The reactions are numbered as follows (1) mineralization, (2) ammonium assimilation, (3) nitrification, (4) assimilatory or dissimilatory nitrate reduction, (5) ammonium oxidation, (6) nitrite oxidation, (7) assimilatory or dissimilatory nitrate reduction, (8) assimilatory or dissimilatory nitrite reduction, (9) denitrification, (10) chemodenitrification, (11) anaerobic ammonium oxidation, and (12) dinitrogen fixation (after Capone, 1991) (reproduced by permission of ASM Press from Microbial Production and Consumption of Greenhouse Gases Methane, Nitrogen Oxides, and Halomethanes, 1991).

Clarens M., Bernet N., Delgenes J. P., and Moletta R. (1998) Effects of nitrogen oxides and denitrification by Pseudomonas stutzeri on acetotrophic methanogenesis by Metha-nosarcina mazei. FEMS Microbiol. Ecol. 25, 271-276. 

Some of the important reactions catalyzed by the P450 monooxygenase system include aliphatic hydroxylation, aromatic hydroxylation, epoxidation, heteroatom (N-, 0-, and S-)dealkylation, nitrogen oxidation, oxidative deamination, oxidative dehalo-genation, oxidative denitrification, and oxidative desulfuration. Most of these reactions result from the initial oxidation of a carbon atom, another reason that P450 is so important in the oxidative biotrans-formation of lipophilic chemicals. Some P450-cataly-zed oxidation reactions are illustrated in Table 2. 

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