Nitrates, nitrites natural sources

The nitrate-nitrite intake from natural sources is much higher than that from processed foods. Fassett (1977) estimated that the nitrate intake from 100 g of processed meat might be 50 mg and from 100 g of high-nitrate spinach, 200 mg. Wagner and Tannenbaum (1985) reported that nitrate in cured meats is insignificant compared to nitrite produced endogenously. Nitrate is produced in the body and recirculated to the oral cavity, where it is reduced to nitrite by bacterial action. 

The potential of NSR catalysts in the removal of NO from mobile sources has motivated in the last few years extensive investigations from both the academic and the industrial world, and several studies have been published in the open literature dealing with fundamental and practical aspects of LNT catalysts [4-53], However, the mechanisms that operate the NO adsorption and the respective subsequent reduction have not been completely clarified so far. It has been shown that under oxidizing conditions, NO are stored on the surface of a Ba-containing catalyst in various forms (surface nitrites/nitrates), whose precise nature is, however, still a matter of debate [9-29], Even less clear are the mechanisms, which are responsible for the reduction of stored NO when the A/F ratio is set to rich and the stored NO species are reduced over Pt to N2, ammonia, N20 or back to NO [11],... 

The potentially harmful effects of exposure to high concentrations of nitrates in drinking water result from reduction to nitrites, which combine with haemoglobin to form methaemoglobin (blue baby disease). Additionally, nitrosamine formation can cause cancer and hypertension. In nature, high levels of nutrients, such as nitrates, lead to eutrophication of water sources, which in, severe cases, lead to the extermination of the other aquatic life due the decreased levels of oxygen and luminosity. 

Based on irradiation experiments in which OH was quantified with the reaction transforming benzene into phenol, we have been able to model the steady-state [ OH] in the surface layer of natural waters under summertime irradiation conditions (22 Wm"2 intensity in the UV). Source data were the direct assessment of OH photoproduction by nitrate and nitrite, the literature rate constants for the reactions between OH and bicarbonate, carbonate and nitrite [109], and the correlation found between DOM (NPOC) and the OH sources and sinks [103]. The equation can be expressed as follows as a function of water composition [110] ... 

Among other sources of hydroxyl radicals, especially in natural aqueous systems, of special interest are the anions nitrite (NO7) and nitrate (NO3). They are... 

Many species of bacteria also have an assimilatory nitrite reductase which is located in the cytoplasm. There is relatively little known about such enzymes but the electron donor is throught to be NADPH and the active site again has siroheme (Cole, 1988). The assimilatory nitrite reductases of both plants and bacteria use nitrite that is provided as the product of the assimilatory nitrate reductases. Nitrate is a very common natural N source for plant and bacterial growth. 

Figure 1 Schematic summary of the sources and removal pathways of ROS in natural waters including singlet oxygen ( 02), superoxide (02 ), hydrogen peroxide (H Oj) and the hydroxyl radical ( OH). The main ROS are indicated by squares. Notation FW, freshwater SW, seawater Me"+ or Me " +, metal in the n-l- or (n-l)-l- oxidation state NO, the nitrate or nitrite anion and , unknown pathway.

Nitrate (NO.D in natural waters is derived from sources such as fertilizers and undertreated animal and human waste. U.S. environmental regulations set a maximum of 10 ppm NO3 nitrogen in drinking water. Nitrate is commonly analyzed by reduction to nitrite (NO2), followed by a colorimetric assay of NO2. Metallic Cd has been the most common reducing agent for NO3. However, the use of toxic Cd should be curtailed to protect the environment. 

Nitrates and nitrites are present in all tap and bottled waters. They are produced by erosion of natural deposits and during the natural decay of vegetable material in soil. Rainfall washes these salts from the sub-soil into groundwater. Nitrogenous fertihsers used on arable farmland and leaks from septic tanks can be significant sources of nitrates in groundwater and surface water. 

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