The role of indirect photochemical degradation in the environmental fate of pesticides: a review

Abstract

Photochemical degradation contributes to the environmental fate of many pesticides in surface waters. A better understanding of the role of direct and indirect photochemical degradation of pesticides is necessary in order to predict their environmental fate and persistence. This review includes all major pesticide classes and focuses on the importance of dissolved organic matter (DOM) as a sensitizer in indirect photodegradation within aquatic systems. Photochemical studies conducted under environmentally relevant conditions (i.e., aqueous solutions with irradiation wavelengths >290 nm) are included. Comparisons are made between observed photodegradation rates in pure or buffered water and in water containing DOM to assess the extent of pesticide susceptibility to DOM-sensitized indirect photolysis. When data is available, the role of specific reactive species in indirect photodegradation is described. While it is possible to assess the relative importance of direct and indirect photodegradation on a pesticide-by-pesticide basis in many cases, it is often difficult to make generalizations based on compound class. Knowledge gaps and inconstancies in the current body of literature are discussed and areas that require additional research are described.

Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter

“…3 For example, DOM contributes to the indirect photodegradation of aquatic pollutants, including pesticides 4 and pharmaceuticals, 5 through the production of excited triplet states ( 3 DOM) and reactive intermediates, such as hydroxyl radicals. 6,7 3 DOM are long-lived species resulting from photon absorption by DOM to form excited, singlet DOM and subsequent partial relaxation via intersystem crossing.…”

Direct photolysis mechanism of pesticides in water

“…[5][6][7] The above experimental conditions, different from the natural aquatic environment, may result in different photochemical profiles, that is photolysis rates and photoproducts. For example, mecoprop-P mainly underwent either photo-induced substitution of 4-Cl by OH at a pH of 5.5 or photo-Claisen rearrangement at a pH of 2.2 by UV irradiation at 254 nm, while the cleavage of ether to form 4-chloro-2-methylphenol was the sole photoreaction in pure water under sunlight.…”

“…4) Direct photolysis of a pesticide proceeds via excited state(s) in the environment when it has a UV-visible absorption at >290 nm, its threshold wavelength is that of sunlight at the earth's surface, and it is possible for a photoproduct with a structure very different from the original pesticide to be formed. [5][6][7] In contrast, the reaction of a pesticide with a photogenerated reactive oxygen species (ROS) in indirect photolysis takes place especially in natural water containing dissolved organic matter (DOM) and/or NO 3 − , irrespective of its absorption profile. [5][6][7] The most reactive hydroxyl radical (•OH) oxidizes an alkyl group and aromatic ring in a relatively nonselective manner.…”

“…[5][6][7] In contrast, the reaction of a pesticide with a photogenerated reactive oxygen species (ROS) in indirect photolysis takes place especially in natural water containing dissolved organic matter (DOM) and/or NO 3 − , irrespective of its absorption profile. [5][6][7] The most reactive hydroxyl radical (•OH) oxidizes an alkyl group and aromatic ring in a relatively nonselective manner. 6,7) The singlet oxygen ( 1 O 2 ) reacts with an electron-rich unsaturated moiety to give hydroperoxide, 1,2-dioxetane or endoperoxide, and it also participates in the sulfur oxidation and oxygenation of heterocycles.…”

“…[5][6][7] The most reactive hydroxyl radical (•OH) oxidizes an alkyl group and aromatic ring in a relatively nonselective manner. 6,7) The singlet oxygen ( 1 O 2 ) reacts with an electron-rich unsaturated moiety to give hydroperoxide, 1,2-dioxetane or endoperoxide, and it also participates in the sulfur oxidation and oxygenation of heterocycles. 8,9) The excited states of pesticides via direct absorption of light may be quenched by DOM abundant in natural waters, and at the same time, DOM may generate ROS, causing the oxidation of pesticides.…”