Nitrate, photolysis

Finally, our recent research work has demonstrated that hazardous compounds can be formed in the presence of the nitrating agent N02, arising from nitrate photolysis or nitrite oxidation [88-91], and the chlorinating Cl2 " [92], Formation of the latter from OH and CD can only take place in acidic solution, but chloride oxidation is, for instance, possible upon charge-transfer processes in the presence of irradiated Fe(III) oxide colloids (represented as =Fe3+—OH in Equation 17.32) [93],... 

The nitrating agent nitrogen dioxide can be produced in surface waters by nitrate photolysis (Equation 17.16) and nitrite oxidation (Equation 17.19). It is unstable in aqueous solution and can either undergo dimerization and hydrolysis (Equations 17.44 and 17.45), or react with dissolved organic compounds ... 

L 1 and 0.6 jag L, respectively. Due to photolysis, nitrate present in the water at a concentration of 55 mg L 1 was reduced to nitrite generating an additional problem (see section II.D.l). This is because nitrite is thought to be involved in the formation of nitrosamines. Experimental conditions in this plant have been investigated to provide the optimum operating conditions for the removal of herbicides and to minimize the impact of nitrate photolysis. The results obtained in this plant led to concentrations of atrazine and nitrite in the effluent lower than 0.1 and 100 pg L 1, respectively. It must be said, for safe operation of the plant, that an equivalent ozone dose higher than 3 kW energy and a radiant power for the UV lamp between 20 and 35 kW were needed. 

Bopp L, Boucher O, Aumont O, Belviso S, Dufresne JL, Pham M, Monfray P (2004) Will marine dimethylsulfide emissions amplify or alleviate global warming A model study. Can J Fish Aquat Sci 61 826-835 Bouillon RC, Miller WL (2004) Determination of apparent quantum yield spectra of DMS photo-degradation in an in situ iron-induced Northeast Pacific Ocean bloom. Geophys. Res. Lett. 31 Article no. L06310 Bouillon RC, Miller WL (2005) Photodegradation of dimethyl sulfide (DMS) in natural waters Laboratory assessment of the nitrate-photolysis-induced DMS oxidation. Environ Sci Technol 39 9471-9477... 

Hoffmann and co-workers [178,179] have studied the photolysis of nitrate and nitrite in both water and water ice over broad temperature ranges. In the case of nitrite formation from nitrate photolysis, the photochemical reaction reaches a steady state condition that can be described by the following rate expression ... 

Nitrate photolysis is a relevant source of hydroxyl in natural waters. A study carried out on the Greifensee Lake water indicates that nitrate photolysis is a much more important source of hydroxyl when compared with the photolysis of hydrogen peroxide or the Fenton reaction [12]. Nitrate photochemistry can thus lead to a steady-state hydroxyl concentration around 5 x 10 16 M [8]. 

Phenol can be nitrated upon nitrate irradiation, yielding 2- and 4-nitrophenol [54,58,79,99,100]. The generation of OH + NO2 upon nitrate photolysis (reactions 1 and 2) would suggest the possibility that phenol nitration might follow an OH-mediated pathway as in the gas phase [80,81]. Furthermore, hydroxyl-mediated nitration in aqueous solution has been described in the case of benzene [107]. However, the addition of hydroxyl scavengers to the system (formate [79], 2-propanol [58]) favours the formation of ni-trophenols, while an OH-mediated nitration would be inhibited by hydroxyl consumption. The positive effect of the scavengers can be accounted for in the hypothesis that phenol nitration takes place upon reaction with nitrogen... 

The pH trend of nitrophenol formation is interesting also because with this datum, it is possible to discuss the role of peroxynitrous acid in a transformation process induced by nitrate photolysis, thus giving a first answer to the observation made by Mack and Bolton [24], HOONO forms upon irradiation of nitrate (reactions 9 and 10), although its formation at k > 280 nm is uncertain. In the presence of HOONO phenol undergoes various transformation reactions, among which nitration [57]. The pH trend of nitrophenol formation in the presence of HOONO is of the kind Rate a [H+], as discussed in... 

Apart from nitrophenols, 4-nitrocatechol and nitrobenzoquinone have also been detected as nitro derivatives [54,79,100]. They are secondary photoproducts and are thought to originate from the nitration of catechol and hydroquinone, in the latter case followed by the oxidation of nitrohy-droquinone [54,100]. 4-Nitrocatechol might in principle derive from catechol nitration or from 4-nitrophenol hydroxylation. However, the conversion of 4-nitrophenol into 4-nitrocatechol upon nitrate photolysis is rather limited [109] and cannot account for the observed time evolution starting from phenol [54]. 

Finally, 4-nitrosophenol has also been detected as a transformation intermediate of phenol upon nitrate photolysis [54,58,79,99,100]. The occurrence of 4-nitrosophenol might be accounted for by the irradiation of photoformed nitrite and by the thermal reactions induced by nitrous acid. Indeed, the formation of 4-nitrosophenol upon nitrate irradiation is favoured at acidic pH [58] and 4-nitrosophenol forms in relevant amount in the presence of both nitrite under irradiation [44,59,110] and nitrous acid in the dark [55, 65-67]. 

The irradiation of hydroquinone/nitrate mixtures yields benzoquinone and hydroxybenzoquinone plus a very small amount of nitrohydroquinone. The interpretation of the results is, however, complicated by the fact that the absorption spectra of hydroquinone and nitrate overlap. The oxidation intermediates mainly originate upon direct photolysis of hydroquinone [79], but the minor formation of nitrohydroquinone requires the contribution of nitrate photolysis [78]. 

Last but not least, the transformation of 4-hydroxybenzoate in the presence of nitrate under irradiation is quite interesting from a mechanistic point of view. The substrate undergoes nitration to 4-hydroxy-3-nitrobenzoate, and nitration is inhibited by addition of hydroxyl scavengers [143]. This marks a difference from all the other cases reported so far. Whenever the effect of OH scavengers on nitration upon nitrate photolysis has been studied, the typical result was an enhancement effect consistent with nitration by... 

The best conditions for photolysis in natural aqueous bodies involve low turbidity, low concentrations of dissolved organic compounds, bright sunny days, and quiescent waters or slow-moving streams. For example, the rate of photoproduction of OH radicals from nitrate photolysis (see below) is lower at higher latitudes. For example, at a latitude 0 N (like in Lake Victoria, in Tanzania) this rate is more than twice that for latitude 60 N (like in lake Tyrifjor-den, in Norway). Likewise, at this last latitude the rate can vary by a factor of 40 from mid-summer to mid-winter. In deep-water bodies (i.e., lakes) the photoprocesses are generally much slower. Some... 

These OH radicals produce several environmentally important oxidation reactions. They can be scavenged primarily by dissolved organic matter, DOM (in waters with high dissolved organic carbon concentration, DOC), or by carbonate and hydrogen carbonate ions (in waters with high alkalinity). The bromide ion is their key scavenger in seawater. Nitrate photolysis is likely to provide a key pathway for the oxidation of different types of DOM. 

Chang, J.S., R.R. Barker, J.E. Davenport, and D.M. Goldan, Chlorine nitrate photolysis by a new technique Very low pressure photolysis. Chem Phys Lett 60, 385, 1979. 

The very simple ultraviolet absorption spectrum of PAN, first observed by Stephens for samples obtained from ethyl nitrate photolysis with gas chromatographic separation, was confirmed in the early 1980s by using high purity samples.Interestingly, PAN has no strong structural features in its ultraviolet spectrum and does not absorb above 290 nm. The PAN molecule is not readily photolyzed in the atmosphere at altitudes below 5 to 7 km. " Photolytic lifetimes for PAN are calculated to be on the order of 20 X 10 sec The reaction of PAN with hydroxyl radical... 

Zafiriou, O.C. and True, M.B., 1979b. Nitrate photolysis in seawater by sunlight. Mar. Chem., 8 33-42. 

Hydroxyl radicals can also be produced by the photolysis of hydrogen peroxide, however, this process is not efficient since the absorption of H2O2 > 300 nm is very small. Thus nitrate photolysis would appear to be the major source of hydroxyl radicals in surface waters. 

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