Degradation of the herbicide 2,4-dichlorophenoxyacetic acid by ozonation catalyzed with Fe2+ and UVA light

Abstract

Solutions of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) with concentrations up to near saturation at pH 3.0 and at 25°C have been treated with ozone and ozonation catalyzed with Fe2+ and/or UVA light. Direct ozonation yields a slow depollution, while all contaminants are completely removed under UVA irradiation. The highest oxidizing power is achieved when Fe2+ and UVA light are combined, since greater amounts of oxidizing hydroxyl radical are generated and Fe3+ complexes are photodecomposed. The initial mineralization rate is enhanced when herbicide concentration increases and more hydroxyl radicals are produced by the catalyzed ozonation processes. The herbicide decay always follows a pseudo first-order reaction. Reverse-phase chromatography allows the detection and quantification of aromatic intermediates such as 2,4-dichlorophenol, 4,6-dichlororesorcinol and chlorohydroquinone. In all treatments, fast dechlorination reactions take place leading to chloride ion accumulation in the medium. The evolution of generated carboxylic acids such as glycolic, glyoxylic, maleic, fumaric and oxalic has been followed by ion-exclusion chromatography. Only oxalic acid remains stable in the O3 system, being quickly mineralized to CO2 by hydroxyl radicals formed in the O3/UVA one. A high stability of oxalic acid in the O3/Fe2+ system has also been found, since it yields Fe3+-oxalato complexes. These species are photodecarboxylated under UVA irradiation in the O3/Fe2+/UVA system. A possible reaction pathway for 2,4-D mineralization involving all intermediates detected is proposed.