First stages of photodegradation of the urea herbicides Fenuron, Monuron and Diuron (original) (raw)
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Study of the photodegradation of urea-type herbicides by capillary gas chromatography
Chromatographia, 2003
The objective of this study was to assess the characteristics of the in vitro phototransformation of the urea-type herbicides diuron, fenuron, chloroxuron, and methabenzthiazuron to obtain information about their possible fate, and about their derivatives, in the environment. Loss and oxidation of the alkyl chains are the dominant processes occurring during degradation of the ureas; substitution of halogen atoms by hydroxyl groups and hydroxylation of the aromatic ring are secondary processes. The large amounts of dimers suggest that radical processes dominate in the photodegradation of ureas. The well known carcinogenicity of azo compounds makes this of outstanding importance.
Photochemical oxidation processes for the elimination of phenyl-urea herbicides in waters
Journal of Hazardous Materials, 2006
Four phenyl-urea herbicides (linuron, chlorotoluron, diuron, and isoproturon) were individually photooxidized by monochromatic UV radiation in ultra-pure aqueous solutions. The influence of pH and temperature on the photodegradation process was established, and the first-order rate constants and quantum yields were evaluated. The sequence of photodecomposition rates was: linuron > chlorotoluron > diuron > isoproturon. The simultaneous photooxidation of mixtures of the selected herbicides in several types of waters was then performed by means of UV radiation alone, and by UV radiation combined with hydrogen peroxide. The types of waters used were: ultra-pure water, a commercial mineral water, a groundwater, and a lake water. The influence of the independent variables in these processes – the presence or absence of tert-butyl alcohol, types of herbicide and waters, and concentration of hydrogen peroxide – were established and discussed. A kinetic study was performed using a competitive kinetic model that allowed various rate constants to be evaluated for each herbicide. This kinetic model allows one to predict the elimination of these phenyl-urea herbicides in contaminated waters by the oxidation systems used (UV alone and combined UV/H2O2). The herbicide concentrations predicted by this model agree well with the experimental results that were obtained.
Microchemical journal, 2005
Various biotic or abiotic factors influence the fate of pesticides in the natural environment, of which ultraviolet (UV) component in the sunlight is one of the most powerful forces. Studies on the photodegradation of pesticides have not only significance from the point of view of environmental science, but also very important in researching and developing new, safer, and effective pesticides. Previously we have studied the photodegradation of triazine- and urea-type herbicides in details. This paper is intended to outline the photodegradation pattern of some commonly used N-containing herbicides that belong to the groups of the mentioned triazines (atrazine, cyanazine, terbuthylazine, terbutryn) and ureas (chloroxuron, methabenzthiazuron, diuron, fenuron), as well as thiolcarbamates (butylate, cycloate, EPTC, molinate, vernolate), in order to gain information about their possible fate and derivatives in the environment.The most significant processes of photodegradation of triazines are the partial or complete loss of side-chains, or rather the substitution of the heteroatom-containing side-chain to hydroxyl-group. The chemical characteristics of the side chains determines basically the speed of the degradation. It can be concluded that the C–S bond breaks down much easier than C–Cl bond, and the chlorine atom remaining on the triazine-ring promotes the loss of alkyl-chains to a higher extent than that the hydroxy-group. In the case of ureas, the chemical properties of groups in both the N and N′ position can influence the degradation process. It could be stated the proximity of large aryl-substituent and methyl-group in the N position is unfavorable and instabilizing, promoting the further degradation of compound, at the same time, loss of these groups is the preferred degradation route. In the case of thiolcarbamates, the most frequent processes are the α- and β-oxidation of alkyl-groups connecting to the nitrogen atom. In the most cases, the N-formyl and N-dealkylated products were identified in the degradation mixture. The thioalkyl-group showed fair stability under the circumstances of photodegradation. Its partly or completely degraded products cannot be detected, and represent only very small part of the mixture.
Rapid Communications in Mass Spectrometry, 2006
The first stages of the photocatalytic degradation of the compounds chlortoluron [3-(3-chloro-4methylphenyl)-1,1-dimethylurea] and chloroxuron {3-[4-(4-chlorophenoxy)phenyl]-1,1-dimethy-lurea}, belonging to the class of phenyl-urea herbicides, were investigated using high-performance liquid chromatography (HPLC) coupled to electrospray ionization ion trap tandem mass spectrometry (ESI-IT-MS/MS). Degradation was accomplished under solar radiation, using TiO 2 embedded into a polyvinylidene fluoride (PVDF) transparent matrix as a heterogeneous photocatalyst. Aliquots of the chlorinated herbicide solutions were withdrawn at different times and subjected to gradient elution, reversed-phase HPLC separations, specifically optimized to obtain the highest resolution between peaks related to the herbicide degradation by-products. The latter were then investigated using MS detection; in particular, MS/MS measurements were made and structural information was obtained from the interpretation of fragmentation data. Several by-products were identified; the most important ones are hydroxylated compounds arising from the interaction between the two chlorinated herbicides and OH radicals generated at the TiO 2 surface under irradiation. Other by-products were generated by slightly different processes, namely demethylation, dearylation and dechlorination, eventually followed by interaction with OH radicals.
Applied Catalysis B: Environmental, 2011
The role of hydroxyl radical in different solar photocatalytic processes, namely photo-Fenton, titanium dioxide-based oxidation and organic photocatalysis (triphenylpyrylium) has been investigated. Using a methodology based on the flash photolytic generation of OH • from N-hydroxypyridine-2(1H)-thione, followed by t-stilbene trapping, the reaction rate constants of different pesticides (dimethoate, methidathion, alachlor and pyrimethanyl) with this reactive oxygen species in deaerated acetonitrile have been determined. The results obtained under photo-Fenton conditions are in reasonable agreement with the estimated rate constants; hence the assumption that the photo-Fenton reaction mainly involves participation of the hydroxyl radical seems plausible. The oxidation pattern using titanium dioxide was also investigated; however, under these conditions no clear correlation could be found due to participation of an alternative electron transfer mechanism.
Photodegradation study of some triazine-type herbicides
Microchemical journal, 2003
Triazines are amongst the most widely used herbicides. Since triazines can be found in many environmental compartments, their fate in ecosystems and the characterization of their degradation pathways in the environment are to be determined. In this paper we report on a study intended to investigate the photodegradation of some triazine-type herbicides: atrazine, cyanazine, terbuthylazine and terbutryn. The rate of photodegradation process was determined, and degradation schemes were outlined for the compounds studied. Moreover, experiments with different degrading energies were carried out in order to gain information about the effect of total degrading energy on the photodegradation process. The most significant processes of photodegradation of triazines are the partial or complete loss of side-chains, or rather the substitution of the heteroatom-containing side-chain to hydroxyl-group. Besides consecutive processes, loss of the different side-chains takes place parallely also, thus, different metabolites will be formed having mixed side-chains, until the cyanuric acid and 2-amino-4,6-dihydroxy-1,3,5-s-triazine are formed by losing all the side-chains. The presence of the dimer products could be detected during the degradation of all triazines. This proves the radical character of processes occurring during the photodegradation. Increasing the degradation energy (15 to 125 W) has raised the degradation rate by 2–5, and the chlorine containing metabolite—which was still present in the completely degraded mixture during the low-energy experiments—has completely disappeared from the mixture, thus, the increased degrading energy is favorable to the formation of less dangerous, nature identical metabolites.
Applied Catalysis B: Environmental, 2008
The degradation pathways exhibited by three phenyl-urea herbicides: isoproturon (ISO), chlortoluron (CHLT) and chloroxuron (CHLOX), during photocatalytic (on supported TiO 2 under intense solar radiation) and electro-Fenton (EF) treatment were investigated by HPLC coupled to electrospray ionization single and tandem mass spectrometry (HPLC-ESI-MS and MS/MS). In particular, the dependence of degradation efficiency on the initial concentration ratio between substrate and Fe(III) ion was assessed for the EF treatment and a 1:1 ratio was found to be optimal. A comparison between photocatalytic and EF degradation, in terms of structures, number and evolution on a similar time scale (up to 5 h) of by-products, was then performed. Similar reactivities were found in the two cases, hydroxylation (single and multiple, with H, alkyl groups or Cl substitution, depending on the herbicide) and demethylation on the dimethylamino moiety (eventually followed by hydroxylation) being the most relevant processes in by-products generation. The scale of EF degradation efficiency for the three herbicides was almost identical to the photocatalytic one (ISO > CHLT % CHLOX), yet electro-Fenton proved to be a more efficient process, generally leading to a faster further degradation of by-products. #
Electronic structure calculations have been performed to determine the thermochemistry and kinetics of the reaction between OH and the radicals of the S enantiomer of the herbicide Metolachlor, 2-chloro-N-(2-methyl-6ethylphenyl)-N(2-methoxy-1-methylethyl) acetamide (MC), produced by photoinduced breaking of the C-Cl bond. Both density functional and ab initio composite methods were employed to calculate the structure of reactants, intermediates, transition states and products. The expected relative abundance of each product was calculated. and compared to the experimentally observed concentrations. It is shown that a combination of thermodynamic and kinetic characteristics interplay to produce the expected theoretical abundances, which turn out to be in agreement with the experimentally observed distribution of products.
Reaction pathways and mechanisms of photodegradation of pesticides
Journal of Photochemistry and Photobiology B-biology, 2002
The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processes involved, the nature of reactive intermediates and final products. Potential use of photochemical processes in advanced oxidation methods for water treatment is also discussed. Processes considered include direct photolysis leading to homolysis or heterolysis of the pesticide, photosensitized photodegradation by singlet oxygen and a variety of metal complexes, photolysis in heterogeneous media and degradation by reaction with intermediates generated by photolytic or radiolytic means.