Photochemical behaviour of the systemic fungicide carboxin

Irradiation of carboxin (1) with a 500 W UV lamp (filter Pyrex) in CH3CN leads to the products 3, 5-8, 12-14 depending upon the reaction conditions. All photo-products were isolated and characterized. Photooxidation occurred even if unsensitized, while photoalteration was very slow in the absence of oxygen. The main oxygenated-products 3 and 13 were also recovered under biomimetic conditions by exposure of an aqueous solution of 1 to sunlight.     

Photochemical behaviour of musk tibetene

BACKGROUND: Synthetic musk compounds are widely used as additives in personal care and household products. The photochemical degradation of musk tibetene in aqueous solutions or in acetonitrile/water mixtures under different conditions was studied in order to assess its environmental fate. METHODS: Musk tibetene dissolved (or suspended) in water and/or acetonitrile/water mixtures was irradiated at different times by UV-light and by solar light. The irradiation mixtures were analyzed by NMR and TLC. The photoproducts formed were identified by GC-MS and NMR data. RESULTS: The experimental results indicated that musk tibetene was photodegradable in water or acetonitrile/water mixtures with half-life reaction times close to 20 minutes. The irradiation mixtures were separated by chromatographic techniques yielding three photoproducts (3,3,5,6,7-pentamethyl-4-nitro-3H-indole, 3,3,5,6,7-pentamethyl-4-nitro-1H-indoline and 3,3,5,6,7-pentamethyl-4-nitro-3H-indolinone) identified by means of spectroscopic analysis. DISCUSSION: The numerical modelling of the photodegradation concentration-time profiles gave (8.13 +/- 0.15) x 10(-2) and (1.34 +/- 0.04) x 10(-2) mol/E for the overall primary quantum yield of direct photolysis for musk tibetene and the major intermediate (3,3,5,6,7-pentamethyl-4-nitro-3H-indolinone), respectively, in the wavelength range 305-366 nm. The half-life times of photodegradation of the both substances varied from 1-1.5 hours at 20 degrees N during the summer season to 6-10 hours for highest latitudes in winter. CONCLUSIONS: Under solar light, musk tibetene was photolabile in acetonitrile and acetonitrile/water 1/1, while it was slowly degraded when suspended in water. In all media, musk tibetene was photodegraded into three photoproducts. By using a kinetic model, the overall primary quantum yields of direct photolysis of musk tibetene and its main photoproduct, in the wavelength range 305-366 nm, were estimated, indicating that the photodegradation rate for musk tibetene is faster than the photolysis rate of the major by-product. RECOMMENDATIONS AND PERSPECTIVES: The results indicate that, in order to assess the environmental impact of musk tibetene on the aquatic ecosystem, great attention should be focused on the major photoproduct which is proved to be more persistent than the parent compound under light irradiation. The predicted half-life times of direct photolysis for both substances ranged from 1-1.5 hours at 20 degrees N during the summer season to about 6-10 hours for highest latitudes in winter, indicating that, from a photochemical point of view, the environmental persistence of these substances increases by increasing the latitudes and during the cold seasons, making more realistic an intake of these xenobiotic molecules into the food chain of aquatic living organisms. Tanabe reports in his Editorial (Tanabe 2005) that "It is necessary to have knowledge of the global picture of synthetic musk pathways. So, it is conceivable that now is the time to study the transport, persistency, distribution, bioaccumulation and toxic potential of this new environmental menace on a global scale, especially in developing countries". Therefore, the future environmental analysis and investigations on the eco-toxicity of nitro musk compounds should take into account not only the presence of the parent compounds but also their photochemical intermediates or end-by-products.     

Effects of sensitizers on the photodegradation of the systemic fungicide triadimenol

The photochemical behaviour of triadimenol (1) under various conditions has been examined. Significant degradation is obtained only in the presence of electron-acceptor sensitizers as 9,10-dicyanoanthracene or 2,4,6-triphenylpyrylium tetrafluoroborate, and long irradiation times are required. 1H-1,2,4-Triazole (2), 4-chlorophenyl formate (3), 4-chlorophenol (4), 1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H-1,2,4-triazol-1-yl)butan-2-one (5), 4-chlorophenyl 2,2-dimethylpropanoate (6) and 4-chlorobenzoic acid (7) were identified as photoproducts by NMR and GC-MS.     

Photochemical behaviour of carbendazim in aqueous solution

To elucidate the photochemical behaviour of carbendazim (or MBC) in superficial waters, photolysis studies have been carried out in aqueous solutions at several pH using a UV light source (high pressure mercury arc lamp) or a solar light simulator (xenon arc lamp). The kinetics of photodecomposition of carbendazim was determined using HPLC-DAD and the identification of photoproducts was carried out with HPLC-MS (ESI negative and positive mode). According to the experimental results carbendazim is a rather stable molecule in the dark or in environmental conditions. The pH influence of the environmental medium on the photodegradation rate has been confirmed. The photochemical process can be considerably accelerated in alkaline solutions using HPK-quartz irradiation (quantum efficiency at pH 9 phi = 3.1 x 10(-3) degraded molecule per absorbed photon) while the photodegradation is not as efficient under a simulated sun irradiation (quantum efficiency in the suntest phi = 10(-4) at pH 7). Three photoproducts have been tentatively identified in pure water: 2-aminobenzimidazole, benzimidazole isocyanate and monocarbomethoxy-guanidine (issued from the cleavage of the benzimidazole ring). The last one seems very stable and could be accumulated in the environment.     

Influence of the amendment of corn straw on the degradation behaviour of the fungicide dithianon in soil

Degradation studies were conducted with the fungicide (14)C-dithianon under standard conditions for 64 days in soil. The compound is characterized by mineralization losses of approx. 33% and the formation of non-extractable (bound) residues of approx. 63% in 64 days. The microbial activity of the soil was stimulated by an amendment of corn straw simulating post-harvest conditions. This addition of straw decreased the mineralization of the compound initially. At the end of the incubation period, however, the mineralization rate was higher in the straw amended soil compared to the control. The addition of straw increased the amount of radiocarbon in the desorption solutions. Thus higher amounts of incorporated radiocarbon could be found in the biomass of the amended soil. Model calculations show that the straw amendment has a sustained influence on the mineralization of the compound. Potential mechanisms of the effect of dissolved organic matter on the sorption/desorption equilibrium are discussed.     

Photochemistry and environment IV- Photochemical behaviour of monochlorophenols in dilute aqueous solution

The photochemical behaviour of chlorophenols is different to that of non-halogenated phenols. In the former, the first step is a C-Cl bond scission, which is not influenced by oxygen. Chlorine is converted into hydrochloric acid. For monochlorophenols, the position of the chlorine on the ring strongly influences the transformation. In the molecular form, 2-chlorophenol is converted into pyrocatechol. In the anionic form however, it is reduced in a cyclopentadienic acid which dimerizes according to a Diels-Alder reaction. The irradiation of 3-chlorophenol leads to resorcinol whatever the pH. This would appear to suggest a photohydrolysis mechanism. With 4-chlorophenol, the photochemical conversion is not so specific. Hydroquinone is formed (mainly in aerated solution), along with polyphenolic oligomers. A radical mechanism is proposed.     

Sensitized photooxygenation of the fungicide furalaxyl

BACKGROUND: The photolysis of pesticides is of high current interest since light is one of the most important abiotic factors which are responsible for the environmental fate of these substances and may induce their conversion into noxious products. The action of light can also be mediated by oxygen and synthetic or naturally occurring substances which act as sensitizers. Our objective in this study was to investigate the photochemical behaviour of the systemic fungicide furalaxyl in the presence of oxygen and various sensitizers, and to compare the toxicity of the main photoproduct(s) to that of the parent compound. Previous reports on the direct photolysis of the pesticide demonstrated a very slow degradation and the only identified photoproducts were N-2,6-xylyl-D,L-alaninare and 2,6-dimethylaniline. METHODS: Solutions of furalaxyl in CH3CN were photooxygenate using a 500W high-pressure mercury lamp (through a Pyrex glass filter, lambda>300 nm) or a 650W halogen lamp or sunlight and the proper sensitizer. When sunlight was used, aqueous solutions were employed. The photodegradation was checked by NMR and/or GC-MS. The photoproducts were spectroscopically evidenced and, when possible, isolated chromatographically. Acute toxicity tests were performed on the rotifer Brachionus calyciflorus, the crustacean cladoceran Daphnia magna and the anostracan Thamnocephalus platyurus, while chronic toxicity tests (sublethal endpoints) comprised a producer, the alga Pseudokirchneriella subcapitata and the crustacean Ceriodaphnia dubia, as a consumer. RESULTS AND DISCUSSION: In the presence of both oxygen and sensitizer, furalaxyl underwent rapid photochemical transformations mainly to N-disubstituted formamide, maleic anhydride and a 2(5H)-furanone derivative. The formation of these products was rationalized in terms of a furan endoperoxide intermediate derived from the reaction of furalaxyl with active dioxygenated species (singlet oxygen, superoxide anion or ground state oxygen). The 2(5H)-furanone exhibited a higher toxicity than the parent compound. CONCLUSION: This work reports the first data on the photosensitized oxygenation of furalaxyl with evidence of the high tendency of the pesticide to undergo photodegradation under these conditions leading, among other things, to a 2(5H)-furanone, which is more toxic than the starting furalaxyl towards aquatic organisms. RECOMMENDATIONS AND OUTLOOK: Investigation highlights that the photolytic fate of a pesticide, although quite stable to direct photoreaction due to its low absorption of solar radiation at ground level, can be significantly influenced in the environment by the presence of substances with energy or electron-transfer properties as natural dyes, e.g. chlorophyll, or synthetic pollutants, e.g. polycyclic aromatic hydrocarbons (PAH).     

Spectrophotometric determination of the fungicide captan

A spectrophotometric method for the determination of the fungicide captan has been developed based on its reaction with thiosemicarbazide. The absorbance was measured at 315 nm. The effects of the proportion of water, thiosemicarbazide concentration, pH and temperature on this reaction was studied to select the best chemical conditions. The calibration graph was obtained between 1.2 and 30 ppm and the detection limit is 0.5 ppm. The RSD (n = 10) was 1.34%. Possible interference of various pesticides was examined. The method was applied to the determination of captan in agrochemical formulations. Results agree well with the labelled values and also with results that were obtained by a polarographic method.     

Review of strobilurin fungicide chemicals

Strobilurins are natural products isolated and identified from specific fungi. Natural strobilurins were named in the order of their discovery as strobilurin-A followed by strobilurin-B, C, D etc. Their discovery opened the door for new chemistry of synthetic fungicides. Applying Quantitative Structural Activity Relationship (QSAR) on the structures of the natural strobilurins, many pesticide companies were able to discover many synthetic analogues that are more efficacious and more stable fungicides. At present there are about eight synthetic strobilurins in the fungicides worldwide market. Some of these products are worldwide registered for use as agrochemical and some are in the process of registration. This class of fungicides is relatively new, as crop protection products and information about them is still fairly scarce. In this review, syntheses and chemistry of natural and synthetic strobilurins are discussed. Also, the mode of action, efficacy, biotic/abiotic degradation, analytical methods, and agricultural uses are discussed.     

Review of strobilurin fungicide chemicals

Strobilurins are natural products isolated and identified from specific fungi. Natural strobilurins were named in the order of their discovery as strobilurin-A followed by strobilurin-B, C, D etc. Their discovery opened the door for new chemistry of synthetic fungicides. Applying Quantitative Structural Activity Relationship (QSAR) on the structures of the natural strobilurins, many pesticide companies were able to discover many synthetic analogues that are more efficacious and more stable fungicides. At present there are about eight synthetic strobilurins in the fungicides worldwide market. Some of these products are worldwide registered for use as agrochemical and some are in the process of registration. This class of fungicides is relatively new, as crop protection products and information about them is still fairly scarce. In this review, syntheses and chemistry of natural and synthetic strobilurins are discussed. Also, the mode of action, efficacy, biotic/abiotic degradation, analytical methods, and agricultural uses are discussed.     

Microbial conversion of fungicide vinclozolin

An ecological safety study of using vinclozolin in field and laboratory experiments showed that the effect of the preparation led to a decrease in the abundance of actinomycetes and mycelial fungi and an enhancement of nitrification. The residual amounts of vinclozolin in soil after 12 months were 6-12% of the dose introduced. The persistent chlorinated derivatives of the toxicant were found. Microbial strains pertaining to the genera Pseudomonas and Bacillus were isolated that utilized vinclozolin as the sole source of carbon and energy.     

Dye-sensitized photodegradation of the fungicide carbendazim and related benzimidazoles

The present work studies the visible-light-promoted photodegradation of the colorless fungicide carbendazim (methyl 2-benzimidazolecarbamate) and several 2-substituted benzimidazoles (SBZ's), in water or water-methanol solution, in the presence of air and, as a photosensitizer, the synthetic xanthene dye Rose Bengal (RB) or the natural pigment riboflavin (Rf). The results indicate that the degradation of each particular SBZ depends on its chemical structure and on the sensitizer employed. In the presence of RB, the degradation always operates via a singlet molecular oxygen (O(2)((1)Delta(g)))-mediated mechanism, through a highly efficient process, as deduced from the comparison of the rate constants for physical and chemical quenching of O(2)((1)Delta(g)). In the presence of Rf, the visible-light irradiation of any of the studied SBZ's produces a series of competitive processes that depend on the relative concentrations of Rf and SBZ. These processes include the quenching of excited singlet and triplet Rf states by the SBZ and the generation of both O(2)((1)Delta(g)) and superoxide radical anion (O(2)(-)), the latter generated by electron transfer from excited Rf species to the dissolved oxygen. The overall result is the photodegradation of the SBZ and the photoprotection of the sensitizer.     

Photochemical oxidation of halocarbons in the troposphere

Measurements of the reactivity of hydroxyl radicals with some simple halocarbons are reported, together with some new measurements of the air concentration of halocarbons in the N-and S hemisphere. The OH reactivities are used to derive life-times of the halocarbons with respect to photooxidation in the troposphere. It is found that the chlorohydrocarbons containing H atoms are moderately reactive with lifetimes of 1 y or less but the fully halogenated compounds carbon tetrachloride and Freons 11 and 12 are essentially inert towards photo-oxidation.The air concentration and reactivity data are combined to provide an estimate of the magnitude of the tropospheric sink for the halocarbons due to oxidation by OH radicals. For some of the more reactive halocarbons this sink exceeds the estimated anthropogenic emissions and an additional, possibly natural, source is indicated.     

Photochemical oxidants: state of the science

Atmospheric photochemical processes resulting in the production of tropospheric ozone (O(3)) and other oxidants are described. The spatial and temporal variabilities in the occurrence of surface level oxidants and their relationships to air pollution meteorology are discussed. Models of photooxidant formation are reviewed in the context of control strategies and comparisons are provided of the air concentrations of O(3) at select geographic locations around the world. This overall oxidant (O(3)) climatology is coupled to human health and ecological effects. The discussion of the effects includes both acute and chronic responses, mechanisms of action, human epidemiological and plant population studies and briefly, efforts to establish cause-effect relationships through numerical modeling. A short synopsis is provided of the interactive effects of O(3) with other abiotic and biotic factors. The overall emphasis of the paper is on identifying the current uncertainties and gaps in our understanding of the state of the science and some suggestions as to how they may be addressed.     

Photochemical ozone formation in the Sydney airshed

The theoretical photostationary state relationship, k₃[O₃][NO]/k₁[NO₂] = 1, was tested in the free atmosphere using data obtained from a series of air mass trajectory experiments which were performed as part of the Sydney Oxidant Study. These data show that in general, the relationship was valid to within a factor of two for the Sydney airshed when three minute averaged concentration measurements were used. The effects of various averaging times upon the experimental photostationary state relationship arc discussed.     

The Inhibition of Photochemical Smog

Additional inhibitors for the conversion of NO to NO₂ in C3H6—NO—0₂ irradiated mixtures have been tested at 25°C. These mixtures initially contained 16 mTorr C₃H₆, 8 mTorr NO, 0.012 mTorr NO₂, additive, and enough O₂ to bring the total pressure to 100 Torr. The NO₂ pressure was monitored photometrically. In the absence of additive, the NO₂ pressure first increases with irradiation time reaching a maximum conversion at about 15 minutes. As the irradiation time increases beyond 15 min, the NO₂ pressure drops. Before adding the inhibitors, runs were done with 10 Torr of CO added, and in these runs the conversion was speeded so that the maximum in NO₂ pressure occurred at 10 min. This enhancement in conversion rate is considered to be diagnostic for the presence of HO radicals. Next 10-min irradiations were done with various amounts of hexafluorobenzene (C₆F₆), nitrobenzene (C₆H₅NO₂), or naphtha lene (C₁₀H₈) added. The NO₂ pressure was reduced to one-half its value in the absence of inhibitor with 270 mTorr C₆F₆’, 220 mTorr C₆H₅N0₂, or 4 mTorr C₁₀Hg. The C₁₀H₈ is a very efficient inhibitor, but additions of up to 1 8.5 mTorr C₁₀H₈ did not reduce the N0₂ pressure to zero. Studies of the percent conversion of NO to NO₂ vs. irradiation time were done with either 4.2 mTorr C₁₀H₈ or 40 mTorr 2,6-di-ferf-butyl-4-methylphenol (Ph) added. In the former case the peak conversion was delayed from 15 to 22 min, while in the latter case no delay occurred. However, with the Ph added, there appeared to be some reduction in the maximum value of percent conversion.     

Fate of the fungicide dodemorph during anaerobic digestion of biological waste

The aim of this study was to examine the stability and distribution of the fungicide dodemorph during anaerobic digestion of biological waste, including a short subsequent aerobic treatment. Three reactor experiments were conducted. The influences of pH and dissolved organic carbon (DOC) were further studied in batch experiments. The reactor experiments showed no decrease in the concentration of dodemorph, indicating stability of the fungicide. In connection with the results of the batch experiments, a clear pH-dependency of the partitioning of dodemorph between solid and water phase could be shown. At pH values below the pK(a) value of 7.8, protonated dodemorph molecules predominate. However, a release into the water phase was only observed at pH values of 5 and below. It can be assumed that in the pH range between 5 and 7.8 dodemorph forms bonds with carboxyl groups of organic matter. Dodemorph was not completely extractable from the solid waste matrix due to strong bonds with the organic matter. Solubilising effects by water ingredients of the process water were not apparent.