Reaction Pathways for the Photodegradation of the Organophosphorus Cyanophos in Aqueous Solutions

Abstract

Photodegradation in aqueous solutions is an important pathway for many agrochemicals such as pesticides. In the present work, the photochemical transformation of cyanophos (CYA) was investigated in aqueous solutions using UV light within the 254-313 nm range as well as solar light. The study was performed in order to have a deep insight into the mechanistic pathways for the photochemical disappearance of CYA. Upon UV irradiation of an aerated solution of CYA, the degradation quantum yield was found equal to 1.8 x 10(-2). It is independent of the excitation wavelength but varies with oxygen concentration. It increased by a factor of 2 from oxygen-saturated to oxygen-free solution. Photosensitized experiments were performed using acrylamide and hydroquinone as energy acceptor and energy donor substrates, respectively. They show that both singlet and triplet excited states were involved in the photochemical behavior of CYA. The laser flash photolysis experiments clearly showed the involvement of the triplet excited state which was efficiently quenched by molecular oxygen and acrylamide with the rate constants 1.97 x 10(9) and 2.71 x 10(9) mol(-1) L s(-1), respectively. The photoproducts structures were proposed according to the mass spectral data using the LC/MS technique. The analytical study shows that various processes such as hydrolysis, homolytic bond dissociations and Photo-Fries process occur.

Photosensitization Reactions In Vitro and In Vivo

“…An M+16 nucleoside oxidation product was formed from the photosensitizing dG 1 Py moiety in which the cyclohexenyl ring opened with a carbonyl group at the rupture site. 18 O labeling studies showed that the oxygen atom originated from H 2 18 O and not from 18 O 2 .…”

Assessment of Herbicide Transport and Distribution in Subsurface Environments of an Orange Field

“…In recent years, particular scientific interest has been focused on the pesticide nonpoint source (NPS) pollution associated to the water environmental safety. The increasing use of pesticides and other man-made chemicals represents the primary cause of contamination of soils, ground, and surface waters (Liebezeit et al, 2011;Matula, 2009Matula, , 2011Menager et al, 2010). These compounds show a relatively high persistence, widespread and rapid dispersion, and they may cause serious health and ecoenvironmental problems.…”

Direct photolysis mechanism of pesticides in water

“…The T 1 absorption of cyanophos at 400 nm after the end of a pulse decayed to form a long-lived species appearing after 10 μsec, which was supposed to be the protonated amide intermediate from the product analysis. 42) The benzyl radicals formed via the homolytic cleavage of the C-S bond were detected in 20% aqueous acetonitrile for ethiofencarb and thiobencarb after pulse end. 35) Long-lived species insensitive to oxygen successively appeared at 320 nm and might be assigned from the product analysis to the corresponding carbanion (ethiofencarb) and carbocation (thiobencarb) via heterolytic cleavage of the C-S bond.…”

“…125) The involvement of S* has been proposed for cyanophos, 42) and the weakening of the P-O-aryl bond by excitation may account for this reaction, as theoretically estimated for fenitrothion. 21) In the case of coumaphos upon exposure to UV light at 334 nm, either dimerization at the C 3 -C 4 position of the coumarin moiety or oxidative ring cleavage was the main reaction instead of ester hydrolysis.…”

“…125) LFP and/or quenching studies have shown T* being involved in the P=S oxidation of cyanophos 42) and parathion. 125) Dye-sensitized photolysis in aqueous alcohol under oxygen did not give concrete evidence of 1 O 2 as a key reactant for OP pesticides.…”