Oxidation of acetamide herbicides in natural waters by ozone and by the combination of ozone/hydrogen peroxide: Kinetic study and process modeling

The oxidation by ozone and by the combination O-3/H2O2 of four herbicides included in the acetamide group (acetochlor, metolachlor, propachlor, and butachlor) was studied. In, a first step, the rate constants were determined for the reactions of the selected herbicides with ozone (k(o3)) and OH radicals (k(OH)). The sequence of reactivities for solutions in ultrapure water were as follows: for ozonation, metolachlor < propachlor < acetochlor < butachlor; for oxidation by hydroxyl radicals, propachlor < acetochlor < metolachlor < butachlor. The rate constants for the reaction with ozone ranged from 1.1 to 5.3 L/(mol(.)s), indicating that direct reactions with ozone will play a minor role during ozonation processes. Values for kOH ranged from 4.6 x 10(9) to 7.4 x 10(9) L/(mol(.)s). Therefore, reactions with OH radicals will be the major pathway for the oxidative transformation of these four herbicides, even when conventional ozonation is applied. In a second step, the oxidation of the investigated herbicides during ozonation processes in some natural and mineral waters was studied, including the influence of the operating variables, and the herbicide oxidation levels were determined. Next, the oxidation process was characterized in terms of oxidant concentrations (ozone and OH radicals), and finally the evolution of the herbicide concentration in an ozonation process was modeled and predicted by applying a kinetics approach based on the rate constants for the reactions with ozone and OH radicals and the calculated concentrations of the active oxidants.