Toxicity denitrification

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. 

Biological denitrification is more complex to operate because it relies on the capabilities of active bacteria, but effectively convert nitrate to not toxic chemicals (N2). It can be subject to transient states generating malfunctions and requires thus greater care. Biological denitrification produces bacteria sludge, requires chemical additives (carbon source and other nutrients for bacteria) and especially requires large installations. 

The lack of CH4 production in the presence of O2 in situ may be due to a combination of factors, of which O2 toxicity is just one. For example, methanogens were more sensitive to desiccation than O2 exposure in a paddy soil (Fetzer et al., 1993). The oxidized products of denitrification, NO and N2O, have a toxic efiect on methanogens similar to that of O2 (Kliiber and Conrad, 1998). A negative correlation between redox potential and CH4 production is often observed in the absence of O2, but this probably reflects competition between methanogens and their competitors for reductants, not a physiological requirement for a certain redox potential. 

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

NO(g), N02(g) n, IV Combustion of fossil fuels, automobiles, denitrification in soils Atmosphere Assists in production of ozone in troposphere, toxic effects on plants... 

Selective TNT toxicity is readily evident in the nitrogen cycle (Figure 3.2). Soils contaminated with TNT often contain elevated concentrations of ammonia and nitrate [7,13] as a result of TNT degradation pathways described in Chapter 2. However, Fuller and Manning [7] found that ammonia oxidation is relatively insensitive to TNT concentrations with regression coefficients (r2) between 0.15 and 0.39 (p > 0.087). In contrast, these authors hypothesized that the denitrification portion of the nitrogen cycle was easily disrupted by TNT, resulting in the accumulation of denitrification... 

Typical assessments of denitrification activity only assess the activity of the first three enzymes and do not consider the sensitivity of nitrous oxide reductase to toxic compounds. However, as hypothesized by others [14,15], it appears that nitrous oxide reductase is much more sensitive to TNT than the other three enzymes based on corresponding EC50 values of 400 mg kg-1 for the first three enzymes and 26 mg kg-1 for nitrous oxide reductase [10],... 

Levels of [N03] in waste water are controlled by legislation, limits being recommended by the World Health Organization, the Environmental Protection Agency (in the US) and the European Community. Nitrites, because of their toxicity, must also be removed. Methods of nitrate removal include anion exchange, reverse osmosis (see Box 15.3), and denitrification. The last process is a biological one in which certain anaerobic bacteria reduce [N03] and [N02] to N2 ... 

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. 

There are other such examples. Many unsaturated soils are known to convert to N2(g> via NO3 (i.e., nitrification then denitrification). They achieve this because there are local oxic and anoxic environments in the soil waters that, respectively, allow nitrification and denitrification to proceed. In stratified lakes, nitrification may occur in the oxygenated epilimnion (upper layer) and denitrification in the hypolimnion (bottom water) and in the sediment pore water where dissolved oxygen concentrations fall to zero. The nitrification and denitrification process is important in preserving the fishery in Indian Creek Reservoir in the Sierra Nevada mountains. This reservoir is fed by the tertiary effluent from the City of South Lake Tahoe sewage treatment plant. The effluent has at times contained 15 to 20 mg NH4 N/liter. Levels of ammonia of this magnitude are toxic to fish, yet in the reservoir there is a thriving fishery. This is achieved because the top waters of the lake nitrify the ammonia to nitrate and this is reduced to N2(g) by the anoxic bottom waters. The summer concentrations of nitrogen species of the reservoir are approximately 4 mg NOa -N/Iiter and 4 mg NH4-N/liter. 

In recent literature some authors have thrown doubt on the value of nitrification. Alexander (1965) states that it is a mixed blessing and, possibly, a frequent evil. He bases his conclusion chiefly on the fact that nitrates are easily lost through leaching or as gases by denitrification and nitrites are chemically unstable. In addition, the acids formed during nitrification may, if not neutralized, form soluble K, Ca, Mg, Mn, P and Al. In extreme cases aluminum toxicity may be encountered. Presumably since most plants can utihze ammonia nitrogen, and some prefer it, nitrate formation is not considered especially essential. 

Simultaneous removal of heavy metals and nitrate. In addition to denitrification, the above series of columns were also used for the removal of heavy metals. A decrease in the denitrification capacity of the column was used as an index of the critical heavy metal concentrations causing toxicity to the P. aeruginosa. The toxic effect of metals (lead, chromium, copper, cadmium and zinc) was examined in a separate study (unpublished data). Of these metals copper proved to have the highest toxicity as it fully prevented denitrification in a very short time at a concentration of 10 ppm. Regarding their toxic effect the next in order was lead, followed by cadmium, zinc, and chromium. 

The removal of heavy metals on fixed-bed columns was performed primarily with lead and zinc in a concentration of 5 ppm as this proved to be most critical in the practical realization of large scale operations. The metal content of input and output waste waters was determined by atomic absorption spectrophotometry. In the series of experiments the appropriately treated columns were kept in operation only for a certain period of time (3-400 hours), when denitrification capacity suddenly dropped in a few hours from 90-95% to 0, indicating that the microbes accumulated a lethal dose of the toxic heavy metal (pH = 7.5, at 25 C, with continuous feedings at the 5 ppm level). Cell death was also verified by the lack of TTC reduction. 

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