Aqueous photodegradation of sethoxydim herbicide: Qtof elucidation of its by-products, mechanism and degradation pathway

Direct photolysis mechanism of pesticides in water

“…4) The homolytic N-O cleavage to form an iminyl radical was suggested by the LFP study of cycloxydim at 266 nm 41) and the increased reaction rate of sethoxydim in acetonitrile over that in water. 120) In the case of aldoxime ethers such as fenpyroximate 4,121) and oxamyl, 122) the corresponding cyanides were formed via N-O cleavage. Incidentally, pyraclostrobin underwent N-demethoxylation, 4) probably via a homolytic N-O bond scission from S 1 .…”

“…4) The photoisomerization from anti (E) to syn (Z) of the oxime ether clethodim 186) and fenpyroximate 4,121) proceeded rapidly, while it was of minor importance for alloxydim 119) and sethoxydim 120) due to a more favorable N-O bond cleavage. Incidentally, pyrethroids that have a cyclopropyl moiety are known to undergo cis-trans isomerization via the homolytic cleavage of the C 1 -C 3 bond, followed by the recombination of the resulting biradical.…”

“…Incidentally, oxazole derivatives were formed in the aqueous photolysis of cycloxydim 4,41) and sethoxydim; 120) the Beckmann rearrangement, followed by the intramolecular reaction of carbocation with the enol oxygen, is one possible mechanism. Alternatively, in comparison to the photolysis of salicylaldoxime, 200) the photo-induced attack of the enol oxygen at the oxime nitrogen followed by rearrangement via azirine may be also a candidate mechanism.…”

Identification of sethoxydim degradation products in natural waters under different light sources by HPLC-QTOF-MS

“…This was described elsewhere [17]. Basically, the method was as follows: 10-50% (B) in 26 min, 50-60% (B) in 13 min, held at 60% (B) during 2 min.…”

“…In a previous work, we studied the direct photolysis of sethoxydim in ultrapure water under laboratory conditions giving rise to the formation of different identified by-products [17]. Nevertheless, it is known that the presence of natural substances such as humic acids, nitrate and carbonate ions in aqueous systems can exert a great influence on the photodegradation rates of parent herbicide as well as in the nature and the quantities of the by-products formed [6,18,19].…”