Photodecomposition of 2,4-dichlorophenoxyacetic acid

Abstract

Three types of reactor operation, flow-differential, flow-integral, and batch-recycle were employed to study the kinetics of the photolysis and oxidation of 2,4-dichlorophenoxyacetic acid (2,4-D). The flow-through annular reactor was irradiated with a polychromatic lamp located at the axis of the annulus. Using aqueous solutions of pollutant-level concentrations (5-50 ppm), the initial rate was found proportional to the light abosrbed and otherwise independent of 2,4-D and dissolved oxygen concentrations. Using thin-layer chromatographic and spectroscopic analytical methods, 2,4-dichlorophenol (2,4-DCP) was identified as one of the photolysis products. Measurements were also made of the initial rate and rate at finite conversions for 2,4-DCP. The results were consistent with a rate equation based upon a photo-activation process followed by formation of peroxides. Of particular significance was the autocatalytic effect of intermediates on the rate of disappearance of 2,4-D. One such catalytic agent was 2,4-DCP. Carbon dioxide was produced as the final oxidation product when either 2,4-D or 2,4-DCP was the reactant. The amount of CO 2 formed proved that the aromatic ring was ruptured. This suggests the possibility of employing photochemical oxidation for reducing refractory, aromatic pollutants into fragments more susceptible to biological oxidation.