Toxicity nitrification

Although O2 leakage compromises the root s internal aeration, some leakage is desirable for a number of purposes. These include oxidation of toxic products of anaerobic metabolism in submerged soil such as ferrous iron (van Raalte, 1944 Bouldin, 1966 van Mensvoort et al., 1985) nitrification of ammonium to nitrate, there being benefits in mixed nitrate-ammonium nutrition (Kronzucker et al., 1999, 2000) and mobilization of sparingly soluble nutrients such as P (Saleque and Kirk, 1995) and Zn (Kirk and Bajita, 1995) as a result of acidification due to iron oxidation and cation-anion intake imbalance. 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

Beg, S.A. et al. Effect of toxicants on biological nitrification for treatment of fertilizer industry wastewater. In Proceedings of 35th Industrial Waste Conference, Purdue University, Lafayette, BSf, 1980 Vol. 35, 826-834. 

Nitrification is limited in most soils by the supply rate of NH4+ (40, 41). Competition exists between nitrifiers and vegetation, which may both be limited by the availability of NH4 +. This microbial demand for NH4 +, coupled with the high cation-exchange capacity of most temperate forest soils, leads to surface-water NH4+ concentrations that are usually undetectable. Nitrification rates may also be limited by inadequate microbial populations, lack of water, allelopathic effects (toxic effects produced by inhibitors manufactured by vegetation), or by low soil pH. 

Svenson, A., Sanden, B., Dalhammar, G., Remberger, M. and Kaj, L. (2000) Toxicity identification and evaluation of nitrification inhibitors in wastewaters, Environmental Toxicology 15 (5), 527-532. 

In some cases, the effects of complex environmental mixtures could be accounted for in terms of concentration-additive effects of a few chemicals. In sediments of the German river Spittelwasser, which were contaminated by chemical industries in its vicinity, around 10 chemicals of a cocktail of several hundred compounds were found to explain the toxicity of the complex mixture to different aquatic organisms (Brack et al. 1999). The complex mixture of chemicals contained in motorway runoff proved toxic to a crustacean species (Gammarus pulex). Boxall and Maltby (1997) identified 3 polycyclic aromatic hydrocarbons (PAHs) as the cause of this toxicity. Subsequent laboratory experiments with reconstituted mixtures revealed that the toxicity of motorway runoff could indeed be traced to the combined concentration-additive effects of the 3 PAHs. Svenson et al. (2000) identified 4 fatty acids and 2 monoterpenes to be responsible for the inhibitory effects on the nitrification activity of the bacteria Nitrobacter in wastewater from a plant for drying wood-derived fuel. The toxicity of the synthetic mixture composed of 6 dominant toxicants agreed well with the toxicity of the original sample. 

Numerous organic chemicals have been identified as nitrification inhibitors. The inhibition is due to the toxicity of these chemicals to organisms that convert ammonium nitrogen to nitrite, which is the first step of the nitrification process. 

NOj m Intermediate in nitrification, denitrification, and NOf reduction Waters Toxic for fish... 

NH3(g), NRT -III Fertilizer, animal feed lots Atmosphere, soil Waters Nitrification of NH4 (from precipitates) leads to acidification of soils Toxicity of NH3 to fish, increased chlorine demand in chlorination of drinking water... 

The MTC values for the various effluents samples tested in the trial ranged from 5 to 48%. Effluents can exhibit a wide range of toxicities. Comparison of the results with nitrification and respiration inhibition tests where applicable showed that the MARA detected toxicity. 

Nitrification converts ammonia to the nitrate form, thus eliminating toxicity to fish and other aquatic life and reducing the nitrogenous oxygen demand. Ammonia is first oxidized to nitrite and then to nitrate by autotrophic bacteria. The reactions are... 

FIGURE 3.2 The toxicity of TNT to the nitrogen and carbon soil biogeochemical cycles. Values are concentrations of TNT shown to inhibit the specified process by 50%. (For nitrification, the entire nitrification process is indicated but the assay only measures the production of nitrite and not nitrate.)... 

Reaction inhibitors slow reaction rates. Nitrogen mineralization and nitrification (conversion of organic nitrogen and ammonium to nitrate) rates in soils, for example, can be slowed temporarily by chemicals that specifically slow or stop the microorganisms involved. Toxic metals can also operate as enzyme inhibitors, by replacing the metal coenzyme portion of an enzyme and thereby inactivating it. 

Nitrification, followed by denitrification, is an important process in the removal of nitrogen from wastewaters. Removal of the nitrogenous constituents helps to minimize the toxicity of the water and to reduce its oxygen demand. The removal of nitrogenous material commences when the sewage is formed and almost all the urea is decomposed to ammonia and carbon dioxide. The toxic ammonia in a waste is first nitrified to nitrite and nitrate by aerobic biological processes and then denitrified anaerobically to molecular nitrogen. 

On account of the high toxicity and considerable stability of phenols, these compounds lead to important problems in wastewater treatment. The biological treatment of water flows was inhibited by phenol at a concentration of 75 mg hydroquinone at a concentration of 15 mg 1 inhibited the biochemical treatment of wastewaters and the threshold concentration of the effect of 2,4-dinitrophenol on the biological treatment was found to be 20 mg 1 - m-aminophenol at a concentration of 1 mg 1 inhibited the BOD (biochemical oxygen demand) and nitrification of wastewaters. 

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