Aromatic nitroderivatives are compounds of considerable environmental concern, because some of them are phytotoxic (especially the nitrophenols, and particularly 2,4-dinitrophenol), others are mutagenic and potentially carcinogenic (e.g., the nitroderivatives of polycyclic aromatic hydrocarbons, such as 1-nitropyrene), and all of them absorb sunlight as components of the brown carbon. The latter has the potential to affect the climatic feedback of atmospheric aerosols. Most nitroderivatives are secondarily formed in the environment and, among their possible formation processes, photonitration upon irradiation of nitrate or nitrite is an important pathway that has periodically gained considerable attention.
The photonitration process is the formation of nitroderivatives from suitable substrates under irradiation conditions . Irradiation is usually essential to the formation of the reactive species (nitrating agents), but in some cases it may also play a key role in activating the substrate to the nitration reaction, which would otherwise not take place in the dark with the same nitrating agent (vide infra for the case of the nitration of nitrophenols) .
Starting from earlier works in the early 1980s , studies on photonitration in aqueous solution have focused on processes that are triggered by the photochemistry of nitrate and nitrite ions. The fact that the reaction takes place in the aqueous phase, the wide environmental occurrence of both nitrate and nitrite, as well as the ability of both ions to absorb environmental UV radiation from sunlight , explain why the photonitration reactions have attracted interest, in an attempt to account for the formation of nitroderivatives in surface waters and, most notably, in atmospheric waters and aerosols.
In particular, there has been interest towards photonitration in three successive periods—(i) from the early 1980s to the mid-1990s, when the main goal was to account for the formation of phytotoxic nitrophenols that, together with acid rains, play a role in forest decline ; (ii) the 2000s, when the process attracted attention to account for the occurrence (and secondary formation) of toxic, mutagenic and potentially carcinogenic nitroaromatic compounds in atmospheric aerosols ; and (iii) the present time, when most interest is accounted for by the fact that the –NO2 group of nitroderivatives enables them to absorb sunlight (visible radiation in addition to the UV, because most nitroderivatives are yellow both as solids and in aqueous solution). As part of the so-called “brown carbon”, nitroaromatic compounds play a role in the absorption of sunlight by airborne aerosols and, by so doing, they affect to a potentially significant extent the still-debated and insufficiently defined climate feedback of the aerosols . Another reason for the recent interest towards photonitration is the acknowledgement of the important role it may play in photochemical processes for water treatment, which are gaining increasing attention and where the occurrence of nitrate and nitrite under technical UV irradiation might favour the formation of harmful compounds .
Whatever the reason for the focus on the photonitration reactions, most of the relevant studies had/have to address the issue of the photonitration pathway(s). On the one side, this task is apparently made easy by the fact that both nitrate and nitrite are able to produce the nitrating agent •NO2 (nitrogen dioxide) under irradiation . However, the photonitration process has several features (effect of pH, because photonitration is usually enhanced under acidic conditions, and effect of the addition of scavengers of the hydroxyl radical, •OH, which are very common in the natural environment) that often have to be explained as well. Actually, the attempts to account for the whole features of the photonitration process, based on the role of •NO2 as the sole nitrating agent, and on the pH effect as only affecting the production of •NO2 by nitrate and nitrite have proven, on the whole, largely unsatisfactory . To make things worse, photonitration appeared to induce the formation of nitroderivatives from substrates that were too electron-poor to react with •NO2 . Indeed, more detailed studies have shown that irradiation of both nitrate and nitrite produces a plethora of potential nitrating agents , in different amounts (also depending on the conditions, e.g., the pH of the solution) and with different reactivity, which interact with each substrate to finally produce the observed effects. Therefore, the photonitration process is not amenable to a simple explanation, based on the role of a single nitrating agent.
Unfortunately, essential information about the photonitration pathway(s) has been fragmented in several different publications, and there is currently not a single paper where all the mechanistic findings are summarised and compared. This review paper is intended to fill this gap, also in view of the recent interest towards the photochemical formation of nitroderivatives as brown-carbon components, and as potential contaminants of treated water and wastewater. Therefore, researchers working in the photonitration field could find here a reference and a guide through an environmentally interesting but, at the same time, very tricky process. This review paper deals with the photonitration of aromatic compounds, which has been the main research focus so far. However, it is important to point out that the formation of nitro- and nitrosoderivatives under irradiation has also been observed in the presence of aliphatic precursors such as primary amines , which have the potential to yield toxic and carcinogenic compounds.
Evidence of the possible occurrence of photonitration processes in the natural environment has been obtained in the case of natural surface waters, although biological nitration pathways cannot be ruled out. In particular, it appears that the environmental conditions of the Rhône delta lagoons (shallow and sunlit waters, elevated concentrations of nitrate and nitrite) are particularly favourable to the transformation of phenolic compounds, arising from the environmental dissipation of phenoxyacid herbicides, into the corresponding nitroderivatives :
It should be remarked that natural waters are indeed much more complex compared to simplified laboratory systems, thus interaction between different components is highly likely. For instance, nitrate and nitrite occur together in the Rhône-delta lagoon water, and the processes they may trigger differ from the mere sum of the separate photoreactions. In particular, the oxidation of NO2− to •NO2 can be carried out by the •OH radicals produced by both nitrate and nitrite photolysis . Moreover, surface waters contain additional components that can affect the process. For example, iron oxides such as hematite (α-Fe2O3) have been shown to remarkably favour the photonitration both of phenol  and of the chlorinated phenols that undergo nitration in the Rhône delta lagoons . Further, the oxidation of NO2− to •NO2 by •OH can be inhibited by a number of •OH scavengers that occur in natural waters, such as natural organic matter and inorganic carbon. That said, Scheme 10 shows a summary of the possible reaction pathways that might account for the photonitration of 2,4-dichlorophenol into 2,4-dichloro-6-nitrophenol in lagoon waters. Similar reaction pathways are likely to be operational for the other compounds.
Scheme 10. Environmental reaction pathways that could account for the photochemical nitration of 2,4-dichlorophenol into 2,4-dichloro-6-nitrophenol.