Abstract
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. However, photonitration triggered by nitrate and nitrite is a very complex process, because the two ionic species under irradiation produce a wide range of nitrating agents (such as •NO2, HNO2, HOONO, and H2OONO+), which are affected by pH and the presence of organic compounds and, in turn, deeply affect the nitration of aromatic precursors. Moreover, aromatic substrates can highly differ in their reactivity towards the various photogenerated species, thereby providing different behaviours towards photonitration. Despite the high complexity, it is possible to rationalise the different photonitration pathways in a coherent framework. In this context, this review paper has the goal of providing the reader with a guide on what to expect from the photonitration process under different conditions, how to study it, and how to determine which pathway(s) are prevailing in the formation of the observed nitroderivatives.
Highlights
The photonitration process is the formation of nitroderivatives from suitable substrates under irradiation conditions [1]
Starting from earlier works in the early 1980s [3,4], studies on photonitration in aqueous solution have focused on processes that are triggered by the photochemistry of nitrate and nitrite ions
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 [6,7]; (ii) the 2000s, when the process attracted attention to account for the occurrence of toxic, mutagenic and potentially carcinogenic nitroaromatic compounds in atmospheric aerosols [8,9]; 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
Summary
The photonitration process is the formation of nitroderivatives from suitable substrates under irradiation conditions [1]. 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 [10,11,12,13,14,15,16] 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 [17,18,19,20,21].
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