Photoreactivity of the fungicide chlorothalonil in aqueous medium

Abstract

The photoreactivity of chlorothalonil was studied by means of steady-state irradiation and laser-flash photolysis. Experiments were conducted in water containing acetonitrile as a co-solvent. This fungicide undergoes very slow phototransformation in the first stages of the reaction, but the consumption profile is auto-accelerated. To understand the reaction mechanism, we undertook a detailed study of the rates, products and transient species. The rates and photoproduct distribution vary greatly with the oxygen concentration. Concerning the transient species, we measured the absorption of the triplet, its yield of formation, and its reactivity with oxygen in various water-acetonitrile mixtures and with isopropanol. The reduced radical, CTH˙, could be produced and its transient spectrum was recorded. Combining all the experimental data, it is hypothesized that in the first step of the reaction CT is excited to the triplet state. The triplet has several possible fates including reduction by organic constituents to form the radical which gives photoproducts. Another characteristic of the CT triplet is its capacity to generate singlet oxygen. The production of this species was measured by phosphorescence and compared to the percentage of the triplet trapped by oxygen in air-saturated solutions. The yield varies from 0.88 in pure acetonitrile to 0.48 in water-acetonitrile (95 : 5, v/v). Therefore, in surface waters, chlorothalonil is expected to sensitize the photooxidation of micropollutants, and to be competitively phototransformed through reaction with any H donor or electron donor water constituents.

Humic Substances Enhance Chlorothalonil Phototransformation via Photoreduction and Energy Transfer

“…This is in line with the involvement of 3 CT* in the reaction. 32,33 Solution deoxygenation also drastically increases the rate of CT phototransformation in the presence of HSs. The ratio (r CT ) HS argon /(r CT ) HS aerated , where (r CT ) HS argon is the rate of CT phototransformation in the presence of HS and in the absence of oxygen, is between 52 and 88 (Table 1).…”

“…It shows the same m/z value in HPLC/MS but with a shorter HPLC retention time than 4-hydroxychorothalonil and is identified to 2hydroxychorothalonil. 33 In polychromatic light, HSs reduces the rate of formation of this compound. At 406 nm, this compound is only detected in the irradiated solutions containing HSs.…”

“…CT disappears via reactions 1−5 and (r CT ) aerated can be written as follows: An accelerating effect is observed in the course of the reaction due to photoproduct accumulation. 33 However, this effect is small at 10% conversion, and for simplification, we neglect it. From the relationship:…”

Direct photolysis mechanism of pesticides in water

“…142) The LFP study with quenching by 3 O 2 showed T* as a key intermediate in the photolysis of chlorothalonil, and reductive dechlorination was the main reaction in water. 27,28) Photosubstitution seems more favorable in the aqueous photolysis of pesticides that have a chlorinated aryloxy moiety. Phenoxyacetic herbicides, such as 2,4-D, 143) 2,4,5-T, 113) and triclopyr, 144) generally produced the corresponding 2-OH derivatives; however, reductive dechlorination at the 4-position was more favorable for dichlorprop.…”

“…68) Not only electronic transitions with E S and E T values relevant to direct photolysis 50,[69][70][71] but also threedimensional structures in excited states and of reactive intermediates 41,[70][71][72] can be examined using TD-DFT calculations. Since the calculations generally suppose an isolated molecule in a vacuum, a solvation effect has been taken into account in some cases, indirectly by using a polarized continuum 28,33,50) or Onsager's cavity model, 70,73) or directly by introducing water molecules into the calculations. 49) A bond liable to be photocleaved 53,73,74) together with the preference of a homolytic or heterolytic mechanism 33,75) can be conveniently evaluated by its dissociation energy.…”