Photolysis of fluometuron in the presence of natural water constituents

International audiencePhototransformation of the herbicide fluometuron (1 ΜM) in natural sunlight was investigated in neutral Milli-Q water and in synthetic waters containing either fulvic acids, nitrate ions or both in order to mimic reactions taking place in aquatic environments. Fluometuron degradation followed a pseudo-first order kinetics. The reaction was faster in synthetic than in Milli-Q water. Fulvic acids (10 mg L-1) increased the rate of fluometuron photolysis by a factor 2.5 and nitrates (25 mg L-1) by a factor 15. Identification of major photoproducts was conducted under laboratory conditions using LC-ESI-MS. Numerous photoproducts were detected and tentatively characterized. In the presence of nitrates, hydroxylation of the aromatic ring with or without hydrolysis of CF3 into CO2H and oxidation of the urea chain leading to demethylation were observed. In the presence of fulvic acids, hydroxylation of the aromatic ring was the major reaction route

Humic Substances Enhance Chlorothalonil Phototransformation via Photoreduction and Energy Transfer

ABSTRACT: The photodegradation of chlorothalonil, a polychlorinated aromatic fungicide widely used in agriculture, was investigated under ultraviolet–visible irradiation in the presence and absence of different humic substances that significantly enhance the chlorothalonil phototransformation. On the basis of a kinetic model, an analytical study, the effect of scavengers, the chlorothalonil phosphorescence measurement, and varying irradiation conditions, it was possible to demonstrate that this accelerating effect is due to their capacity to reduce the chlorothalonil triplet state via H-donor reaction and to energy transfer from the triplet humic to ground state chlorothalonil. Energy transfer occurs at wavelengths below 450 nm and accounts for up to 30% of the reaction in deoxygenated medium upon irradiation with polychromatic light (300–450 nm). This process is more important with Elliott humic and fulvic acids and with humic acids extracted from natural carbonaceous material than with Nordic NOM and Pahokee peat humic acids. The obtained results are of high relevance to understanding the processes involved in chlorothalonil phototransformation and the photoreactivity of humic substances. Chlorothalonil is one of the rare molecules shown to react by energy transfer from excited humic substances

Phototransformation du fluométuron et du mécoprop dans le compartiment aquatique (Rôle des états excités triplets des substances humiques)

Ce travail porte sur l'étude du comportement photochimique de contaminants organiques dans les eaux naturelles reconstituées. Nous avons établi que les états excités triplets (EET) des substances humiques (SH) réagissent de façon directe avec le 2,4,6-triméthylphénol (TMP). Ce résultat consacre TMP comme la première sonde moléculaire, apte à démontrer une éventuelle implication des EET dans la transformation photosensibilisée des composés organiques. La lumière solaire transforme lentement le fluorométuron (FM) et le mécoprop (MCPP). Les réactions sont nettement accélérées par les constituants chromophores des eaux naturelles (nitrates ou SH). L'identification des photoproduits et des espèces transitoires a permis de clarifier les mécanismes respectifs des phototransformations directes ou induites par SH ou NO3- des deux polluants. L'étude sur l'influence de plusieurs paramètres (protonation, ajout de chlorures) a livré une vue plus complète de la photoréactivité de ces contaminantsCLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocSudocFranceF

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

Auto-remediation of surface waters by solar-light: Photolysis of 1-naphthol, and two herbicides in pure and synthetic waters

International audienceSolar-light reaching the earth surface is able to induce chemical reactions in absorbing chemicals. For many organic pollutants dispersed in surface waters, photolysis is an important transformation process. It results from absorption of light by the chemical itself or it is mediated by sensitizing or photoinducing chromophores contained in surface waters. Coloured dissolved organic matter (cDOM) and nitrate ions are among these. This paper gives a brief description of major photochemical processes that may occur in surface waters. It is illustrated by three examples of light induced pollutant transformation. The high volume chemical 1-naphthol and the herbicides methabenzthiazuron and mesotrione were chosen as substrates. Analytical and kinetic aspects of the reactions are reported

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

Photosensitizing and antioxidant activities of humic substances: A case study of beta(2)-adrenoceptor agonist terbutaline

SSCI-VIDE+ATARI+CFE:JMCInternational audienceHumic substance

Inhibition of humic substances mediated photooxygenation of furfuryl alcohol by 2,4,6-trimethylphenol: an evidence for reactivity of the phenol with humic triplet excited states .

International audienceTo probe the reactivity of 2,4,6-trimethylphenol with humic substances triplet excited states, we investigated its influence on the humic substances-mediated photo-oxygenation of furfuryl alcohol. Elliott soil humic and fulvic acids were employed for these experiments. Added in the concentration range 10-4-10-3 M, 2,4,6-trimethylphenol inhibited furfuryl alcohol photooxygenation to an extent depending on its concentration. The inhibiting effect decreased as oxygen concentration was increased. By postulating that 2,4,6-trimethylphenol competes with oxygen for reaction with humic substances triplet excited states and with furfuryl alcohol for reaction with singlet oxygen, we obtained kinetic laws describing the consumption profiles of furfuryl alcohol and 2,4,6-trimethylphenol. Experimental rates of 2,4,6-trimethylphenol and furfuryl alcohol loss could be satisfactorily fitted taking 1.09-1.16 for the ratio k2/k3, where k2 and k3 are the reaction rate constants of humic triplets excited states with oxygen and 2,4,6-trimethylphenol, respectively. These types of experiments could be extended to a variety of substrates in order to measure their reaction rate constant with humic triplet excited states

Fulvic acid-mediated phototransformation of mecoprop. A pH-dependent reaction

International audienceElliott soil fulvic acid sensitized the phototransformation of mecoprop in aq. medium. The reaction was selective, leading to the main formation of 4-chloro-2-methylphenol in aerated and in deoxygenated neutral soln. In turn, 4-chloro-2-methylphenol underwent a fulvic acid-mediated phototransformation. A pH decrease from 8 to 2.2 led to an increase of the rate of mecoprop loss by a factor of 10. The roles of hydroxyl radicals, fulvic triplets and singlet oxygen were investigated using the scavenging technique. At pH 6.5 where mecoprop was in the anionic form, triplet excited states and hydroxyl radicals were the main oxidant species contributing to 40 ± 10% and 20 ± 5% of the reaction, resp. At pH 2.2 where mecoprop was protonated, fulvic triplets and hydroxyl radicals contributed to 75 ± 10% and 16 ± 5%, resp. The enhancement of the reaction upon acidification might result from two phenomena tending to the same effect: (i) the capacity of protonated MCP to reach intra-humic microdomains and there interact with reactive species, and (ii) a greater photosensitizing property of FA in acidic mediu

Relationship between Photosensitizing and Emission Properties of Peat Humic Acid Fractions Obtained by Tangential Ultrafiltration

Peat humic acid was fractionated by tangential ultrafiltration intosixnominalmolecularweight(NMW)fractions,HA5-10,HA10-20, HA20-50, HA50-100, HA100-300 and HA>300, which were purified by dialysis usinga0.5kDamembrane.Theabsorbingandemission properties of the separated fractions were compared and their ability to generate singlet oxygen under light excitation was evaluated, using furfuryl alcohol (FFA) as a singlet oxygen scavenger. The absorbance, the emission intensity, and the apparent first order rate constants of FFA loss were normalized per mole of organic carbon (a*, IF*, and k*, respectively). The fraction absorbance decreased with NMW, except for HA>300 which was less absorbing than HA100-300. The low NMW fractions and the HA>300 fraction generally showed lower k* and IF* values compared to the HA50-100 and HA100-300 fractions. A plot of k* versus IF* indicates that the first order rate constant of FFA photo-oxygenation increased with the intensity of fluorescence at 380, 430, and 500 nm (R2 ) 0.77-0.84). This shows that the distribution of fluorescent centers among fractions paralleled that of photosensitizing centers. Plotting k* or IF* versus a* at 365nmreveals the apparent relativequantum efficiency of the different fractions. Higher values for low NMW fractions and HA50-100 are either due higher percentages of absorbing centers able to produce singlet oxygen or exhibit fluorescence or to lower quenching processes