DARK AND PHOTOASSISTED FE3+-CATALYZED DEGRADATION OF CHLOROPHENOXY HERBICIDES BY HYDROGEN-PEROXIDE

The herbicides 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) were degraded in acidic aerated solutions of H2O2 and Fe2+ or Fe3+. Conditions leading to complete mineralization could be achieved using Fe3+/H2O2, which thereby became the focus of the study. Herbicide transformation by Fe3+ /H2O2 was sensitive to pH (optimum, 2.7-2.8) and was inhibited by methanol or chloride due to scavenging of the active oxidant, and by sulfate due to complexation of Fe3+. The corresponding polychlorophenol was a transient, low-yield intermediate. Dechlorination of 0.1 mM herbicide was rapid and quantitative. Conversion to CO2 ranged from about 40 to 70%, depending on [H2O2] (10-500 mM) and was independent of Fe oxidation state. Degradation was markedly accelerated by irradiation with visible light containing a small UV component. Photoassisted conversion of herbicides to CO2 was quantitative in less than 2 h using H2O2 to herbicide molar ratios as low as 5. The stoichiometry indicated dioxygen consumption.