PHOTOCHEMICAL-REACTIONS INVOLVED IN THE TOTAL MINERALIZATION OF 2,4-D BY FE-3+/H2O2/UV

The photoenhancement of Fenton-type (Fe3+ + H2O2) oxidation of 2,4-dichlorophenoxyacetic acid in the near-UV is due to several Fe(III)-sensitized photochemical reactions. Fe3+/H2O2/UV systems were studied for potential use in waste treatment. Dechlorination and conversion of the first approximately 40% of ring and carboxy carbon of 2,4-D to CO2 are due mostly to hydroxyl radical (OH.) reactions. Dark Fenton-generated OH. is supplemented by the following photoreactions that give OH. directly and/or provide in situ Fe2+ for the Fenton reaction: (i) photolysis of ferric ion, FeOH2+ --> Fe2+ + OH.; (ii) photodecarboxylation of a ferric-2,4-D complex, [RCO2-Fe]2+ --> Fe2+ + CO2 + R.; and (iii) additional photochemical reaction(s) that may involve photolysis of a ferric-peroxy complex, Fe-O2H2+. The relative importance of each depends on [H2O2], except for photodecarboxylation, which is always minor. The remaining approximately 60% of carbon mineralization occurs almost exclusively by photolysis/decarboxylation of Fe(III) complexes of degradation intermediates, probably ring-opened products coordinated via carboxyl and hydroxyl groups; OH' plays no significant role during this stage. Oxalic acid was identified as one intermediate. Dioxygen is consumed in the overall reaction and accelerates carbon mineralization.