Rika Kodaka

Degradation of Flumioxazin in Illuminated Water–Sediment Systems

The aerobic aquatic metabolism of flumioxazin was studied in two water-sediment systems under illumination and in darkness to investigate its degradation profiles. (14)C-Flumioxazin separately labeled at the 1- and 2-positions of the tetrahydrophthalimide moiety or uniformly labeled at the phenyl ring was applied to a overlying water at a rate equivalent to 600 g ai/ha by assuming uniform distribution in the water layer to a depth of 100 cm. Flumioxazin was rapidly degraded at 20 °C in the overlying waters irrespective of irradiation with half-lives of 0.1-0.4 day. Both various modes of liquid chromatography-mass spectrometry (LC-MS) and nuclear magnetic resonance (NMR) spectroscopy analyses showed four major degradates under irradiation. Two of them were formed via successive hydrolysis of the cyclic imide ring, and the others were 2-arizidinone derivatives via photoinduced rearrangement. The presence of sediment under illumination greatly reduced the formation of these degradates and accelerated their degradation. The partitions of flumioxazin and its degradates to the bottom sediment not only reduced their fractions in the water layer subjected to hydrolysis and photolysis but also enhanced their microbial degradation in the sediment. The illuminated water-sediment systems were considered to more adequately represent the behavior of flumioxazin and its degradates in the environment than the corresponding studies of aqueous photolysis and water-sediment in darkness.

Behavior of cyphenothrin in aquatic environment

The behavior of cyphenothrin (1) [(RS)-α-cyano-3-phenoxybenzyl (1RS)-cis-trans-2,2-dimethyl-3-(2-methylprop-1-enyl)cyclopropanecarboxylate] in an aquatic environment was investigated by using the 14C-labeled trans and cis isomers. In parallel with the rapid partition from water phase to bottom sediment, 1 was degraded with the first-order half-lives of 2.0 (trans-1) and 7.3 days (cis-1) in the water-sediment system under dark conditions. 1 underwent extensive microbial degradation via ester cleavage to form 3-phenoxybenzoic acid, finally forming bound residues and mineralizing to CO2. Aqueous photolysis significantly accelerated the degradation of 1 with a half-life of <1 day, mainly via photo-induced oxidation at the 2-methylprop-1-enyl group and ester cleavage without cis-trans isomerization. These results strongly suggest that 1 is unlikely to persist in the actual aquatic environment due to its rapid photolysis and extensive microbial degradation.

Photodegradation of Strobilurin Fungicide Mandestrobin in Water

Photodegradation of a new strobilurin fungicide, mandestrobin, was investigated in buffered aqueous solution and synthetic humic water (SHW) under continuous irradiation with artificial sunlight (λ > 290 nm). In both aquatic media, the direct photolysis preferentially proceeded via homolytic bond cleavage at the benzyl phenyl ether, and the subsequent recombination of geminate radicals in a solvent cage gave the photo-Claisen rearrangement products. A radical mechanism in the photochemical rearrangement was strongly supported by a radical-trapping technique using a novel nitroxide spin label combined with electron spin resonance (ESR) and liquid chromatography-mass spectrometry (LC-MS) analyses. Photosensitized generation of hydroxyl radical in SHW might significantly contribute to enhancing the formation of a benzyl alcohol derivative. The series of photolysis products steadily degraded and finally mineralized to carbon dioxide.