Inhibition vs. enhancement of the nitrate-induced phototransformation of organic substrates by the •OH scavengers bicarbonate and carbonate

Abstract

Contrary to common expectations, the hydroxyl scavengers, carbonate and bicarbonate, are able to enhance the phototransformation by nitrate of a number of substituted phenols. Carbonate and bicarbonate, in addition to modifying the solution pH, are also able to induce a considerable formation of the carbonate radicals upon nitrate photolysis. The higher availability of less-reactive species than the hydroxyl radical would contribute to substantially enhance the photodegradation of the phenols/phenolates that are sufficiently reactive toward the carbonate radical. This phenomenon has a potentially important impact on the fate of the relevant compounds in surface waters. In contrast, the degradation of compounds that are not sufficiently reactive toward CO(3)(-*) is inhibited by carbonate and bicarbonate because of the scavenging of *OH.

Laboratory and field evidence of the photonitration of 4-chlorophenol to 2-nitro-4-chlorophenol and of the associated bicarbonate effect

“…Direct photolysis, reaction with • OH and other transient species, and nitration are possible transformation pathways in paddyfield water (Armbrust 1999;Chiron et al 2009). Nitroderivatives like 2N4CP absorb a larger fraction of sunlight compared with 4CP, but they have lower quantum yields for the direct photolysis (Vione et al 2009). Additionally, the nitro group is expected to decrease the reaction rate constants with • OH and other transient species (Neta et al 1988;Minero et al 2006), and would inhibit further nitration processes (Machado and Boule 1995;Dzengel et al 1999).…”

“…They modify the pH of the solution, consume the • OH radicals produced upon irradiation of nitrate and nitrite, and enhance the generation of the radicals CO 3 −• (Chiron et al 2009a). The overall effect is strongly substrate-dependent (Vione et al 2009). The formation of 2N4CP as a secondary pollutant is of concern because this compound is refractory to biological degradation, and it is toxic to organisms (The Merck Index 2006).…”

“…4CP could be an important compound in this respect because of its fast photonitration kinetics, most notably in bicarbonate-poor waters. Previous studies have shown that bicarbonate can have a major influence on photonitration processes and on the transformation of aromatic compounds upon nitrate photolysis (Chiron et al 2009a;Vione et al 2009). This is the first work to our knowledge that provides field data to demonstrate the importance of the bicarbonate effect on photonitration.…”

Effect of dissolved organic compounds on the photodegradation of the herbicide MCPA in aqueous solution

“…Table 1. Data concerning lagoon water in the Rhône delta (Chiron et al, 2007) and water in Lake Candia (Vione et al, 2009d Captions to the Figures Figure 1. Absorption spectra (molar absorption coefficients ε) of MCPA at pH 1.8 and 7.7, adjusted by addition of HClO 4 and NaOH, respectively.…”

Phototransformation of the sunlight filter benzophenone-3 (2-hydroxy-4-methoxybenzophenone) under conditions relevant to surface waters

“…The assessment of the reactivity between organic compounds and CO 3 −• can be carried out with a semi-quantitative screening method, which makes use of nitrate and bicarbonate under UVB irradiation (Vione et al, 2009). The application of this method (see SM) suggested that the reaction between BP-3 and CO 3 −• might possibly be important, which prompted us to use the laser flash photolysis technique to get insight into reaction kinetics.…”