Photochemical degradation of imazamox in aqueous solution: influence of metal ions and anionic species on the ultraviolet photolysis

Behavior of the Chiral Herbicide Imazamox in Soils: Enantiomer Composition Differentiates between Biodegradation and Photodegradation

Imazamox is a chiral herbicide that, in laboratory experiments in the dark, exhibits pronounced enantioselective biodegradation in certain soils. Imazamox also shows rapid photodegradation. However, which processes are predominant in the field is not clear. We conducted a set of soil incubation experiments under natural sunlight (and corresponding dark controls), using enantioselective LC-MS/MS analysis as a probe to distinguish biodegradation and photodegradation. Under dark conditions, imazamox was degraded enantioselectively. In contrast, degradation was nonenantioselective and 2× faster when the soil was exposed to sunlight, suggesting that biodegradation (in the dark) and photodegradation (under sunlight) were the predominant degradation processes. We also investigated the effectiveness of strategies that were proposed to exclude photodegradation in field studies, covering of soil with sand or irrigation after herbicide application. The sand cover did not prevent photodegradation. On the contrary, degradation was 10× faster than in the dark and nonenantioselective. Computer simulations supported the explanation that imazamox was transported upward by capillary flow due to evaporation onto the sand surface, where it was rapidly photodegraded. Irrigation postponed but not completely prevented photodegradation. For mobile substances susceptible to photodegradation, upward transport to the soil surface thus needs to be considered when deriving rates for biodegradation from field studies.

Modelling the photochemistry of imazethapyr in rice paddy water

In this work the photochemistry of imazethapyr, an imidazolinone herbicide used in rice crops, was modelled in rice paddy water. The photochemical half-life time of the herbicide was assessed by means of the APEX software (Aqueous Photochemistry of Environmentally occurring Xenobiotics) taking into account the direct photolysis, the reactions with hydroxyl radicals (HO) and, in some cases, the reactions with the excited triplet states of chromophoric dissolved organic matter (³CDOM*). We found that direct photolysis and HO reaction can account for a half-life time ranging between 8 and 11 days in May, which is in quite good agreement with the half-life times measured in the field and reported in the literature. These findings suggest that direct photolysis and reaction with HO are important degradation pathways for imazethapyr in paddy water. Dissolved organic matter (DOM) has been reported in the literature to decrease the imazethapyr photodegradation rate. Our model computations confirm this finding but, upon comparison of model predictions with experimental data from the literature, we provide evidence of a non-negligible role of DOM-photosensitised processes in imazethapyr degradation, particularly in DOM-rich waters. We also assess an upper limit (10⁸ L mol^-1 s^-1) for the second-order rate constant of the reaction between imazethapyr and ³CDOM*. Furthermore, on the basis of literature-reported photodegradation pathways and by using both APEX and the US-EPA ECOSAR V2.0 software, we assess that the direct photolysis by-products of imazethapyr could pose a potential ecotoxicological threat to aquatic systems.

Palladium-Catalyzed Cross-Coupling Reactions: A Powerful Tool for the Synthesis of Agrochemicals

Pd-catalyzed cross-coupling reactions have become essential tools for the construction of carbon-carbon and carbon-heteroatom bonds. Over the last three decades, great efforts have been made with cross-coupling chemistry in the discovery, development, and commercialization of innovative new pharmaceuticals and agrochemicals (mainly herbicides, fungicides, and insecticides). In view of the growing interest in both modern crop protection and cross-coupling chemistry, this review gives a comprehensive overview of the successful applications of various Pd-catalyzed cross-coupling methodologies, which have been implemented as key steps in the synthesis of agrochemicals (on R&D and pilot-plant scales) such as the Heck, Suzuki, Sonogashira, Stille, and Negishi reactions, as well as decarboxylative, carbonylative, α-arylative, and carbon-nitrogen bond bond-forming cross-coupling reactions. Some perspectives and challenges for these catalytic coupling processes in the discovery of agrochemicals are briefly discussed in the final section. The examples chosen demonstrate that cross-coupling chemistry approaches open-up new, low-cost, and more efficient industrial routes to existing agrochemicals, and such methods also have the capability to lead the new generation of pesticides with novel modes of action for sustainable crop protection.

Direct photolysis mechanism of pesticides in water

Photodegradation is one of the most important abiotic transformations for pesticides in the aquatic environment, and the high energy of sunlight causes characteristic reactions such as bond scission, cyclization, and rearrangement, which are scarcely observed in hydrolysis and microbial degradation. This review deals with direct photolysis via excitation of a pesticide by absorbing natural or artificial sunlight in order to know its basic photochemistry, and indirect photolysis meaning either sensitization by dissolved organic matters or oxidation by reactive oxygen species is basically excluded. Several experimental approaches including spectroscopic techniques together with theoretical calculations are first discussed from the viewpoint of the reaction mechanisms in direct photolysis. Then, the typical photoreactions of pesticides are summarized by chemical classes and/or functional groups and discussed as far as possible in relation to their mechanisms.

Photodegradation behaviour of sethoxydim and its comercial formulation Poast® under environmentally-relevant conditions in aqueous media. Study of photoproducts and their toxicity

Photolysis is an important route for the abiotic degradation of many pesticides. However, the knowledge of the photolytic behaviour of these compounds and their commercial formulations under environmentally-relevant conditions are limited. The present study investigated the importance of photochemical processes on the persistence and fate of the herbicide sethoxydim and its commercial formulation Poast^® in aqueous media. Moreover, the effect of important natural water substances (nitrate, calcium, and ferric ions) on the photolysis of the herbicide was also studied. The results showed that additives existing in the commercial formulation Poast^® accelerated the rate of photolysis of sethoxydim by a factor of 3. On the contrary, the presence of nitrate and calcium ions had no effect on the photodegradation rate while ferric ions resulted in an important decrease in the half-life of sethoxydim possibly due to the formation of a complex. Different transformation products were identified in the course of sethoxydim irradiation and the effect of experimental conditions on their concentrations was investigated. Finally, Microtox^® test revealed that aqueous solutions of sethoxydim photoproducts increased the toxicity to the bacteria Vibrio fischeri.

The effect of Cu(2+) chelation on the direct photolysis of oxytetracycline: A study assisted by spectroscopy analysis and DFT calculation

The extensive usage of OTC and Cu(2+) in livestock and poultry industry caused high residues in natural environment. Co-contamination of OTC and Cu(2+) was a considerable environmental problem in surface waters. In this study, Cu(2+) mediated direct photolysis of OTC was studied. Cu(2+) chelating with OTC was found to greatly inhibit OTC photodegradation. To reveal the chelation mechanism of OTC-Cu complexes, multiple methods including UV-Vis absorption spectra, Infrared (IR) spectra, mass spectroscopy, and density functional theoretical (DFT) modeling were performed. Four OTC-Cu complexes were proposed. Cu(2+) preferably bond to O11O12 site with the binding constants logK = 8.19 and 7.86 for CuHL+ and CuL±, respectively. The second chelating site was suggested to be O2O3 with the binding constants of logK = 4.41 and 4.62 for Cu2HL3+ and Cu2L2+, respectively. The suppressed quantum yield of OTC by Cu2+ chelation was accused for their intra-/inter-molecular electron transfer, by which the energy in activated states was distributed. The occurrence of electron transfer between BCD ring and A ring also from BCD ring to Cu was evidenced by the TD-DFT result only for the OTC-Cu complexes. Besides, the cyclic voltammetry measurement also suggested one OTC-Cu(II)/OTC-Cu(I) redox couple. These results suggested that the persistence of OTC in environmental surface waters will probably be underestimated for neglecting the chelating effect of Cu2+. The photolysis quantum yield of OTC-Cu complexes, as well as the specific molar absorption constants, the equilibrium binding constants of Cu2+ with OTC could contribute to more accurate kinetic models of OTC.

Analysis of the Photodegradation of the Imidazolinone Herbicides Imazamox, Imazapic, Imazaquin, and Imazamethabenz-methyl in Aqueous Solution

The photodegradation of the imidazolinone herbicides imazamox, imazapic, imazaquin, and imazamethabenz-methyl has been investigated in phosphate-buffered solutions and buffered solutions containing natural organic matter (NOM). The hydrolysis of imazamethabenz-methyl, the only imidazolinone herbicide susceptible to hydrolysis, was also examined. The rate of hydrolysis of imazamethabenz-methyl increased with increasing pH, with the para isomer degrading more rapidly than the meta isomer. All photodegradation rate constants increased with pH and plateaued after pH 5.2. All imidzaolinones degraded more quickly under 253.7 nm lamps as compared to degradation under 310 nm lamps. Imazamox and imazapic degraded more rapidly than imazaquin at all pH values and had higher quantum yields. In addition, imazamox and imazapic quantum yields increased as a function of pH, whereas imazaquin quantum yields showed no trend as a function of pH. Photodegradation reaction rate constants decreased as the concentration of NOM was increased in the solutions due to the effect of light screening. Formulas for the proposed photoproducts for imazamox, imazapic, and imazaquin in pH 7 phosphate buffers were identified, and structures for the photoproducts are proposed.

Photochemical behavior of sethoxydim in the presence of vegetable oils

The photodecomposition of herbicides may be affected by adding vegetable oils to the spray tank. In this study nine vegetable oils were compared to assess the photodecomposition of sethoxydim under natural light conditions. The experiment was conducted as completely randomized factorial design with three replicates at the College of Agriculture, Ferdowsi University of Mashhad, Iran, in 2013. Each herbicidal solution (with and without vegetable oil) was exposed to sunshine with time intervals of 0, 5, 10, 20, 30, 60, 120, and 240 min. The results revealed that the half-life value was increased by adding castor bean and cottonseed oils to 1.39- and 1.18-fold, respectively, compared to nonvegetable oil. These values for turnip, olive, corn, soybean, sunflower, canola, and sesame oils were decreased down to 4.74-, 2.38-, 1.81-, 1.75-, 1.52-, 1.28-, and 1.11-fold, respectively. A positive relationship existed between the half-life of sethoxydim in the presence of vegetable oils and their viscosity. However, a negative relationship was monitored between unsaturated/saturated fatty acids ratio and the monounsaturated value with half-life. A positive relationship also existed between saturated fatty acids, polyunsaturated fatty acids, palmitic acid, and linoleic acid with half-life. This study revealed that the amount of fatty acids in vegetable oils is a determining factor in preventing or facilitating the photodecomposition of sethoxydim.

Leaching of three imidazolinone herbicides during sprinkler irrigation

Some imidazolinone herbicides have been shown to be mobile in soil, raising concern about their possible movement to ground water. Three imidazolinone herbicides (imazamethabenz-methyl, 497 g ha(-1); imazethapyr, 14.7 g ha(-1); and imazamox, 14.7 g ha(-1)) commonly used in crop production on the Canadian prairies were applied to a tile-drained field to assess their susceptibility to leach when subjected to sprinkler irrigation using a center pivot. Tile-drain flow began when the water table rose above tile-drain depth, and peak flow rates corresponded to the greatest depths of ground water above the tile drains. Interception of irrigation water by the tile drains in each quadrant of the field varied from ∼11 to 20% of the water applied. Under a worst-case scenario in which irrigation began the day after herbicide application and irrigation water was applied at 25 mm d(-1) for 12 d, there was evidence of preferential flow of all three herbicides and hydrolysis of imazamethabenz-methyl to imazamethabenz in the initial samples of tile-drain effluent. In subsequent samples, concentrations (analysis by LC-MS-MS) of the summation of imazamethabenz-methyl (25-24,000 ng L(-1)) plus its hydrolysis product imazamethabenz (63-26,500 ng L(-1)) greatly exceeded those of imazethapyr (<13-1260 ng L) and imazamox (19-599 ng L(-1)), thus reflecting relative application rates. In contrast, estimates of total transport of each herbicide from the root zone, which varied in each quadrant and ranged from 0.06 to 2.3% for imazamethabenz-methyl plus imazamethabenz, 0.71 to 3.1% for imazethapyr, and 0.61 to 2.8% for imazamox, did not reflect application rates. In shallow ground water (piezometer samples), there was inconsistent and infrequent detection all four compounds. With the frequency and amount of rainfall typically encountered in the prairie region of Canada, contamination of shallow ground water with detectable concentrations of the three imidazolinone herbicides would be unlikely.

Photodegradation of the herbicide imazethapyr in aqueous solution: effects of wavelength, pH, and natural organic matter (NOM) and analysis of photoproducts

The photodegradation of imazethapyr, 5-ethyl-2-(4-isopropyl-4-methyl-5-oxo-4,5-dihydroimidazol-1H-3-yl)nicotinic acid, has been investigated in phosphate buffers and in buffered solutions containing natural organic matter (NOM). Imazethapyr degrades most quickly under 253.7 nm light and at pH values >4. The presence of NOM in solution caused the reaction rate constants for the photodegradation to decrease, with higher concentrations of NOM having a larger effect. Calculations suggest light screening is the major effect of the NOM. Seven photoproducts have been identified, and a photodegradation mechanism is proposed.

Indirect photodegradation of clethodim in aqueous media. byproduct identification by quadrupole time-of-flight mass spectrometry

Aqueous photolysis of clethodim herbicide in the presence of natural substances such as humic acids (HA), nitrate, and Fe(III) ions has been investigated. The photodegradation rate of clethodim was retarded in the presence of HA compared to ultrapure water, while nitrate ions had no effect. On the other hand, water containing different concentrations of Fe(III) ions enhanced degradation of this herbicide. Clethodim transformation gave rise to the formation of nine byproducts, some of them, to the best of our knowledge, described for the first time in this work. The identification of these photoproducts has been accomplished by coupling liquid chromatography to quadrupole time-of-flight mass spectrometry. The main transformation reactions observed for clethodim were photoisomerization to the Z-isomer, S-oxidation of E- and Z-clethodim isomers giving rise to sulfoxide diastereoisomers, reduction of the oxime moiety to yield clethodim imine, oxidative cleavage of the C-S bond, and S-oxidation of clethodim imine leading to the formation of imine ketone and imine sulfoxide.

Efeitos de oxidantes e sais inorgânicos na degradação fotocatalítica do herbicida imazetapir mediada por dióxido de titânio

A degradação fotocatalítica de imazetapir, um herbicida da família das imidazolinonas, em suspensão aquosa de dióxido titânio foi investigada na presença de dois receptores de elétrons como persulfato de potássio e periodato de sódio, assim como na presença de dois sais inorgânicos como sulfato de sódio e dihidrogenofosfato de sódio a 30ºC. O modelo cinético de pseudo-primeira ordem foi empregado para discutir os resultados. O desaparecimento de imazetapir em função do tempo de irradiação foi analisado por cromatografia líquida de alta eficiência. Entre os dois oxidantes estudados a degradação foi mais rápida quando se adicionou o periodato de sódio, enquanto a presença do sulfato de sódio retarda a degradação de imazetapir mais que o dihidrogenofosfato na suspensão de TiO2.