Degradation of the herbicide 2,4-DP by catalyzed ozonation using the O3/Fe2+/UVA system

Abstract

Acid solutions of the herbicide 2-(2,4-dichlorophenoxy)propionic acid (2,4-DP) of pH 3.0 at 25.0°C have been treated with ozone and ozonation catalyzed with Fe2+, Cu2+ and/or UVA light. This herbicide is slowly degraded by ozonation alone, while its destruction is enhanced under UVA irradiation. In the presence of Fe2+, the initial mineralization rate is accelerated due to the generation of oxidizing hydroxyl radical (OH), but a large proportion of stable products are formed. These species are partially removed when Fe2+ and UVA light are combined, since greater amounts of OH are produced and Fe3+ complexes are photodecomposed. Addition of Cu2+ to this system does not significantly improve its oxidizing ability, since the Cu2+/Cu+ pair gives a low additional OH concentration. The herbicide decay always follows a pseudo first-order reaction. 2,4-Dichlorophenol, chlorohydroquinone and chloro-p-benzoquinone are detected as aromatic intermediates by reversed-phase chromatography. The initial chlorine is always transformed into chloride ion. Ion-exclusion chromatography allows the quantification of generated carboxylic acids such as lactic, pyruvic, maleic, fumaric, oxalic and acetic. These acids are completely removed, except the two latter ones. Acetic acid remains stable in all cases. Oxalic acid is stable in the O3 system, being partially mineralized to CO2 by the O3/UVA one. It also yields stable Fe3+-oxalato complexes in the O3/Fe2+ system, which are rapidly photodecarboxylated in the O3/Fe2+/UVA and O3/Fe2+ + Cu2+/UVA methods. Cu2+-oxalato complexes also formed in the latter procedure are slowly mineralized with OH. A possible reaction sequence for 2,4-DP degradation involving all intermediates detected is proposed.