Oxidation of MCPA and 2,4-d by UV Radiation, Ozone, and the Combinations UV/H₂O₂ and O₃/H₂O₂

Abstract

The phenoxyalkyl acid derivative herbicides MCPA (4-chloro 2-methylphenoxyacetic acid) and 2,4-d (2,4-dichlorophenoxyacetic acid) were oxidized in ultrapure water by means of a monochromatic UV irradiation and by ozone, as well as by the combinations UV/H₂O₂ and O₃/H₂O₂. In the direct photolysis of MCPA, the quantum yield at 20°C was directly evaluated and a value of 0.150 mol Eins⁻¹ was obtained in the pH range 5–9, while a lower value of 0.41 × 10⁻² mol Eins⁻¹ was determined at pH = 3. Similarly, for 2,4-d a value of 0.81 × 10⁻² mol Eins⁻¹ was deduced, independent of the pH of work. The influence of the additional presence of hydrogen peroxide was established in the combined process UV/H₂O₂, and the specific contribution of the radical pathway to the global photo-degradation was evaluated. The oxidation by ozone and by the combination O₃/H₂O₂ was also studied, with the determination of the rate constants for the reactions of both herbicides with ozone and hydroxyl radicals at 20°C. These rate constants for the direct reactions with ozone were 47.7 and 21.9 M⁻¹ s⁻¹ for MCPA and 2,4-d respectively, while the found values for the rate constants corresponding to the radical reactions were 6.6 × 10⁹ and 5.1 × 10⁹ M⁻¹ s⁻¹.

Keywords:

• Phenoxyalkyl acid derivative herbicides
• MCPA
• 2,4-d
• UV radiation
• Ozone
• Hydroxyl radicals
• Quantum yields
• Rate constants

Acknowledgment

The authors wish to gratefully acknowledge financial support from the Junta de Extremadura through Project 2PR01A004.