Modeling of photooxidation of acetamide herbicides in natural waters by UV radiation and the combinations UV/H₂O₂ and UV/O₃

Abstract

The photooxidation of several herbicides belonging to the acetamides group (specifically propachlor, metolachlor and butachlor) by means of a monochromatic UV irradiation in ultrapure water was studied. In the case of propachlor, the quantum yield for its photolysis was evaluated directly, and a value 0.127 ± 0.01 mol Eins −1 was obtained independent of the pH. The quantum yields for the photolysis of metolachlor and butachlor were determined by using a competition kinetic model which used propachlor as reference compound, and values of 0.56 ± 0.05 and 0.78 ± 0.04 mol Eins −1 were deduced respectively for metolachlor and butachlor. In a second step, the elimination of the selected herbicides in some natural and mineral waters was studied by means of several oxidation systems: UV radiation alone, and UV radiation combined with hydrogen peroxide and ozone. The influence of the operating variables in these processes (acetamide type, presence or absence of tert-butyl alcohol and types of waters) are discussed as a consequence of the amounts of herbicides removed. Two different kinetic studies were carried out for the oxidation of the herbicides: the first one for UV radiation alone and UV radiation combined with H 2 O 2 ; the second one for the combination UV radiation plus ozone. In these studies, the specific rate constants were evaluated in every process. Finally, kinetic models for the prediction of the elimination of these herbicides in the natural waters selected by the different oxidation systems are proposed. The values of concentrations predicted by these models agree well with the experimental results obtained.