Phototransformation of 2,4-D Ethyl Ester on Different Soil Surfaces: Identification of Photoproducts

Abstract

The application of 2,4-dichlorophenoxyaceticacid (2,4-D), one of the most widely used halogenated agrochemicals, is an inexpensive tool to manage broad-leaf weeds in many crops. As its dose is very high (450–1800 g/ha) and as its persistence in soil is moderate, 2,4-D carries risk to contaminate water and the food chain. Recently, it has been labeled as an endocrine disrupter. So, we need a revisit to its fate in the environment, particularly the pattern of degradation. The objective of the investigation was to describe the photochemical behavior of 2,4-D on different soil surfaces. The irradiation of 2,4-D was conducted on the surfaces of alluvial, black and red soil and inert glass under UV light (254 nm) and sunlight. The half-life values of 2,4-D on glass surface under UV light was 23.60 ± 4.31 minutes. But on soil surfaces the rate of photolysis slowed down with the half-life values of 40.72 ± 5.08, 104.78 ± 11.32 and 159.55 ± 17.52 minutes under UV light, and 6.07 ± 0.35, 3.04 ± 0.32 and 12.37 ± 2.63 days under sunlight on alluvial, black and red soil, respectively. The slower reaction rate on soil surface was due to the quenching effect imparted by humic substances, which were absent on glass surface. Major photoproducts formed from the irradiation of 2,4-D in soil surface were 2,4-dichlophenoxyaceticacid (I), 2,4-dichlophenoxyacetylchloride (II), 2,4-dichlorophenol (III), chlorophenoxyaceticacid-2-chloroethyl ester (IV), chlorophenoxyaceticacid-ethyl ester (V), and phenoxyaceticacid-ethyl ester (VI). Albeit the photoreaction is slow on soil surfaces, 2,4-D is transformed into less toxic components under sunlight.