Reaction Mechanism of Photoreduction of 2,4-Dichlorophenoxyacetic Acid in Surfactant Micelles

The direct photolysis of 2,4-dichlorophenoxyacetic acid (2,4-D) in water is slow and ineffective, but surfactant micelles can greatly enhance the photochemical reaction of organic contaminants. The effect of an anionic (sodium dodecyl sulfate), a cationic (cetyltrimethylammonium bromide), and two nonionic (Brij 35, Tween 80) surfactants on the phototransformation of 2,4-D at 253.7 nm was examined. Except for the anionic surfactant, the other micelles were significant facilitators in both removal efficiencies and decay rates of 2,4-D. This herbicide was then irradiated in the presence of Brij 35 micelles at five different initial pH levels. 2,4-D is an organic acid, and its speciation (molecular or ionic form) is controlled by pH. It was found that the photoreduction of 2,4-D was more effective at lower initial pH than that in basic conditions. The transformation of 2,4-D in micelles occurred predominantly through reductive chlorination, resulting in generation of less chlorinated products. The presence of hydroxylated products in low yields indicated photohydrolysis was a minor competitive reaction pathway. The measurable intermediates were 2-chlorophenoxyacetic acid, 4-chlorophenoxyacetic, chlorohydroxyphenoxyacetic acid, phenoxyacetic acid, hydroxyphenyoxyacetic acid, and phenol. The transformed products were identified by comparing the HPLC retention times of the standards, and confirmed by LC-MS.