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

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 × 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 × 10⁹ and 2.71 × 10⁹ 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.