Influence of humic substances on the riboflavin photosensitized transformation of 2,4,6-trimethylphenol.

International audienceHumic substances are known to affect the fate of organic chemicals in the environment. While their capacity to produce reactive species upon irradiation has been intensively studied, their inhibiting properties have been much less investigated. In the present work, we studied the influence of various humic substances (humic acids, fulvic acids, natural organic matter) on the riboflavin photosensitized transformation of 2,4,6-trimethylphenol which takes place via oxidation of the phenol by the triplet excited state of riboflavin. Between 2 and 20 mg L−1 humic acids show an inhibiting effect on this reaction while below 2 mg L−1, a small accelerating effect is generally observed. At 25 μM 2,6-dimethyl-1,4-benzoquinone also inhibits the photoreaction significantly. It is proposed that the quinone traps the superoxide anion produced in the course of the reaction yielding semiquinone radicals. The reduction potential of the quinone is low enough for making possible a subsequent reduction of 2,4,6-trimethylphenoxyl radical by semiquinone and a regeneration of 2,4,6-trimethylphenol. In the case of humic substances, the trapping of superoxide anion might be achieved by humic quinone moieties. In accordance, soil fulvic acids and aquatic natural organic matter which show a much lower electron accepting capacity than soil extracted humic acids do not show any inhibiting effect

Influence of humic substances on the riboflavin photosensitized transformation of 2,4,6-trimethylphenol.

International audienceHumic substances are known to affect the fate of organic chemicals in the environment. While their capacity to produce reactive species upon irradiation has been intensively studied, their inhibiting properties have been much less investigated. In the present work, we studied the influence of various humic substances (humic acids, fulvic acids, natural organic matter) on the riboflavin photosensitized transformation of 2,4,6-trimethylphenol which takes place via oxidation of the phenol by the triplet excited state of riboflavin. Between 2 and 20 mg L−1 humic acids show an inhibiting effect on this reaction while below 2 mg L−1, a small accelerating effect is generally observed. At 25 μM 2,6-dimethyl-1,4-benzoquinone also inhibits the photoreaction significantly. It is proposed that the quinone traps the superoxide anion produced in the course of the reaction yielding semiquinone radicals. The reduction potential of the quinone is low enough for making possible a subsequent reduction of 2,4,6-trimethylphenoxyl radical by semiquinone and a regeneration of 2,4,6-trimethylphenol. In the case of humic substances, the trapping of superoxide anion might be achieved by humic quinone moieties. In accordance, soil fulvic acids and aquatic natural organic matter which show a much lower electron accepting capacity than soil extracted humic acids do not show any inhibiting effect

The role of triplet state keto - Enol tautomerism in the photodeamination of metamitron

International audienceSubstituted 4-amino-1,2,4-triazin-5-ones undergo photodeamination through cleavage of the N - NH 2 bond in the presence of oxygen and water. To elucidate the mechanism of this reaction, we investigated the photolysis of metamitron (4-amino-6-phenyl-3-methyl-1,2,4-triazin-5-one) by nanosecond laser flash photolysis, steady-state irradiation, and ab initio calculations. Upon pulsed laser excitation of deoxygenated aqueous metamitron, two transient species are clearly detected. The predictions of ab initio results are consistent with experimental results: (i) it is proposed here that the transient species are, respectively, the keto and diradical forms of the metamitron keto - enol tautomerism in the triplet state, and (ii) in water, the activation free energy barrier of enolization is drastically decreased. Thus, the formation of the diradical triplet is enabled in aqueous solvent. A detailed analysis of the intermediate structures that lead to the final products (HNO 2 and deaminometamitron) is provided

Light induced degradation of the fungicide Thiophanate-methyl in water: Formation of a sensitizing photoproduct

International audienceThis work demonstrates that the fungicide thiophanate-methyl generates a sensitizing photoproduct upon irradiation in solar light and explains how the reaction proceeds. The degradation profileof aqueous thiophanate-methyl in simulated solar light showed an autoaccelerated shape indicating the formation of some photodegrading compounds. Among the detected photoproducts, I (λmax = 340 nm), a quinoxaline derivative generated in the very beginning of the reaction seemed to be the molecule responsible for this autoacceleration. Further experiments were conducted to confirm this hypothesis. The laser flash photolysis (λexc = 355 nm) of I generated a transient species reminiscent to the triplet excited state of unsubstituted quinoxaline. The two triplets reacted with the phenolic probe, 2,4,6-trimethylphenol, with k = 2 × 109 M−1 s−1. Steady-state irradiations showed that thiophanate-methyl drastically accelerated the photolysis of the probe in an auto-accelerated reaction confirming the production of a sensitizing photoproduct. Moreover, unsubstituted quinoxaline sensitized the degradation of thiophanate-methyl showing that this latter contains labile H atoms. All these data confirmed that I was the sensitizer responsible for the auto-accelerated light induced transformation of thiophanate-methyl. The mechanism of formation of I was studied by quantum calculations. It proceeds through photocyclisation to form a SS bridge, followed by intra and intermolecular H abstractions and finally departure of HSS.The sensitizing properties of micropollutants photoproducts open a large array of investigations. Our study provides a reliable approach to investigate the overlooked photosensitizing effect of photoproducts during photochemical reactions in water and opens the way to a vast array of research

Photochemical fate of carbamazepine in surface freshwaters: Laboratory measures and modeling

International audienceIt is shown here that carbamazepine (CBZ) would undergo direct photolysis and reaction with *OH as the main phototransformation pathways in surface waters. Environmental lifetimes are expected to vary from a few weeks to several months, and predictions are in good agreement with available field data. Acridine (I) and 10,11- dihydro-10,11-trans-dihydroxy-CBZ (V) are the main quantified phototransformation intermediates upon direct photolysis and *OH reaction, respectively. The photochemical yield of mutagenic I from CBZ is in the 3−3.5% range, and it is similar for both direct photolysis and *OH reaction: it would undergo limited variation with environmental conditions. In contrast, the yield of V would vary in the 4−8.5% range depending on the conditions, because V is formed from CBZ by *OH (9.0% yield) more effectively than upon direct photolysis (1.4% yield). Other important photointermediates, mostly formed from CBZ upon *OH reaction, are an aromatic-ringdihydroxylated CBZ (VI) and N,N-bis(2-carboxyphenyl)urea (VII). Compounds VI and VII are formed by photochemistry and are not reported as human metabolites; thus, they could be used as tracers of CBZ phototransformation in surface waters. Interestingly, VI has recently been detected in river water

Modelling the photochemical fate of ibuprofen in surface waters

International audienceWe show that the main photochemical processes involved in the phototransformation of anionic ibuprofen (IBP) in surface waters are the reaction with *OH, the direct photolysis and possibly the reaction with the triplet states of chromophoric dissolved organic matter ( 3CDOM*). These conclusions were derived by use of a model of surface water photochemistry, which adopted measured parameters of photochemical reactivity as input data. The relevant parameters are the polychromatic UVB photolysis quantum yield (Φ IBP = 0.33 ± 0.05, μ±σ), the reaction rate constant with *OH (k IBP,*OH=(1.0 ± 0.3){dot operator}10 10 M -1 s -1), the 1O 2 rate constant (kIBP,1O2=(6.0 ± 0.6){dot operator}10 4 M -1 s -1), while the reaction with CO3-* can be neglected. We adopted anthraquinone-2-sulphonate (AQ2S) and riboflavin (Ri) as CDOM proxies and the reaction rate constants with the respective triplet states were k IBP,3AQ2S*=(9.7 ± 0.2){dot operator}10 9 M -1 s -1 and k IBP,3Ri* = 4.5{dot operator}10 7 M -1 s -1. The reaction with 3CDOM* can be an important IBP sink if its rate constant is comparable to that of 3AQ2S*, while it is unimportant if the rate constant is similar to the 3Ri* one. The photochemical pathways mainly lead to the transformation (oxidation and/or shortening) of the propanoic lateral chain of IBP, which appears to be significantly more reactive than the isobutyl one. Interestingly, none of the detected intermediates was produced by substitution on the aromatic ring

Modelling the photochemical fate of ibuprofen in surface waters

International audienceWe show that the main photochemical processes involved in the phototransformation of anionic ibuprofen (IBP) in surface waters are the reaction with *OH, the direct photolysis and possibly the reaction with the triplet states of chromophoric dissolved organic matter ( 3CDOM*). These conclusions were derived by use of a model of surface water photochemistry, which adopted measured parameters of photochemical reactivity as input data. The relevant parameters are the polychromatic UVB photolysis quantum yield (Φ IBP = 0.33 ± 0.05, μ±σ), the reaction rate constant with *OH (k IBP,*OH=(1.0 ± 0.3){dot operator}10 10 M -1 s -1), the 1O 2 rate constant (kIBP,1O2=(6.0 ± 0.6){dot operator}10 4 M -1 s -1), while the reaction with CO3-* can be neglected. We adopted anthraquinone-2-sulphonate (AQ2S) and riboflavin (Ri) as CDOM proxies and the reaction rate constants with the respective triplet states were k IBP,3AQ2S*=(9.7 ± 0.2){dot operator}10 9 M -1 s -1 and k IBP,3Ri* = 4.5{dot operator}10 7 M -1 s -1. The reaction with 3CDOM* can be an important IBP sink if its rate constant is comparable to that of 3AQ2S*, while it is unimportant if the rate constant is similar to the 3Ri* one. The photochemical pathways mainly lead to the transformation (oxidation and/or shortening) of the propanoic lateral chain of IBP, which appears to be significantly more reactive than the isobutyl one. Interestingly, none of the detected intermediates was produced by substitution on the aromatic ring

Photochemical Fate of Carbamazepine in Surface Freshwaters: Laboratory Measures and Modeling

It is shown here that carbamazepine (CBZ) would undergo direct photolysis and reaction with ^•OH as the main phototransformation pathways in surface waters. Environmental lifetimes are expected to vary from a few weeks to several months, and predictions are in good agreement with available field data. Acridine (<b>I</b>) and 10,11-dihydro-10,11-<i>trans</i>-dihydroxy-CBZ (<b>V</b>) are the main quantified phototransformation intermediates upon direct photolysis and ^•OH reaction, respectively. The photochemical yield of mutagenic <b>I</b> from CBZ is in the 3–3.5% range, and it is similar for both direct photolysis and ^•OH reaction: it would undergo limited variation with environmental conditions. In contrast, the yield of <b>V</b> would vary in the 4–8.5% range depending on the conditions, because <b>V</b> is formed from CBZ by ^•OH (9.0% yield) more effectively than upon direct photolysis (1.4% yield). Other important photointermediates, mostly formed from CBZ upon ^•OH reaction, are an aromatic-ring-dihydroxylated CBZ (<b>VI</b>) and <i>N</i>,<i>N</i>-bis­(2-carboxyphenyl)­urea (<b>VII</b>). Compounds <b>VI</b> and <b>VII</b> are formed by photochemistry and are not reported as human metabolites; thus, they could be used as tracers of CBZ phototransformation in surface waters. Interestingly, <b>VI</b> has recently been detected in river water