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

Abstract

This works shows that the addition of phenol and 2-propanol as model organic compounds significantly decreases the direct photolysis quantum yield of 4-chloro-2-methylphenoxyacetic acid (MCPA) upon UVB irradiation in aqueous solution. Laser flash photolysis data suggest that 2-propanol is able to decrease the formation of the MCPA excited states under irradiation. A decrease from 0.54 to 0.34 of the photolysis quantum yield of the anionic form of MCPA (which prevails over the undissociated one in surface waters) could have a considerable impact on the MCPA lifetime in ecosystems where the direct photolysis is the main phototransformation pathway. In surface water bodies where the direct photolysis has comparable kinetics as the reaction with • OH, a decrease of the quantum yield would enhance the relative importance of the • OH pathway, which yields considerably less toxic intermediates than the direct photolysis.

A model approach to assess the long-term trends of indirect photochemistry in lake water. The case of Lake Maggiore (NW Italy)

“…Indeed, the significant decrease of A further photochemistry issue is that the decrease of DOC is expected to enhance as well the direct photolysis of organic compounds. Indeed, the chromophoric portion of DOM competes with other dissolved molecules for radiation absorption (Lam et al, 2003;Latch et al, 2003) and, in some cases, DOM can decrease the direct photolysis quantum yields of other substrates (Vione et al, 2010b). Figure 7 shows the PCA analysis (PC1 vs. PC2) of the data, grouped by season.…”

Direct and indirect photolysis of triclocarban in the presence of dissolved organic matter

“…Ding et al (2014) also reported that isopropanol is capable of quenching the direct photolysis of TCC even though OH• was not identified in their ESR data. Like Vione et al (2010) they attribute this anomaly to the scavenging of 3 TCC by the alcohol. Thus, it is conceivable that the complete quenching of the indirect pathway is caused by reaction between the excited state DOM and methanol.…”

“…Recent research has demonstrated that simple alcohols are capable of quenching both OH• and reactive transients. Vione et al (2010) showed that isopropanol is capable of quenching 3 DOM involved in the indirect oxidation of 4-chloro-2-methylphenoxy-acetic acid (MCPA). They demonstrated by laser flash photolysis that the absorbance of the second transient species (presumably the triplet) is quenched significantly by approximately a factor of 3 at alcohol concentrations (25 mM) used in this and our previous study (Guerard et al, 2009) at pH = 7.8.…”

Phototransformation Processes of Emerging Contaminants in Surface Water

“…Reactions of T 1 . Most processes of direct photolysis follow this route, which is favoured by the relatively long life of T 1 [3,4,6]. The conceptually simplest ways for triplet-state reactivity would be bond-breaking, rearrangement or reaction with the solvent, but less straightforward pathways are also possible.…”