Etude comparative de la réactivité de chloro-2, méthoxy-2, méthylthio-2 s-triazines vis-à-vis des radicaux hydroxyles

Reactivity of chloro-2, methoxy 2 and methylthio-2 s-triazines towards hydroxyl radicals

Laboratoire de Chimie de l'Eau et des Nuisances - URA CNRS 1468. Ecole Supérieure d'Ingénieurs de Poitiers, 40, avenue du Recteur-Pineau 86022 Poitiers Cedex

Résumé

Les constantes cinétiques de réaction des radicaux hydroxyles sur quelques s-triazines (terbutylazine, cyanazine, atratone, simétone, amétryne, simétryne) en milieu aqueux ont été déterminées à partir d'expériences de cinétique compétitive. La décomposition de l'ozone dissous par les ions hydroxyde et hydroperoxyde a été utilisée comme source de production de radicaux hydroxyles. Les résultats obtenus ont permis de montrer que les méthylthio-2 s-triazines (amétryne et simétryne, k_M > 2 10¹⁰ l mol^-1 s^-1) réagissent beaucoup plus rapidement avec les radicaux hydroxyles que les méthoxy-2 s-triazines (atratone et simétone, k_M = (3 à 5) 10⁹ l mol^-1 s^-1) et que les chloro-2 s-triazines (cyanazine, terbutylazine, et atrazine, k_M = (1 à 3) 10⁹ l mol^-1 s^-1). Par ailleurs, le remplacement d'un groupement éthylamino par un groupement isopropylamino diminue la réactivité de la s-triazine vis-à-vis des radicaux hydroxyles.

Abstract

Rate constants for the reaction of hydroxyl radicals with some s-triazines (table 2) have been measured using competitive oxidation experiments. Aqueous solutions containing two organic solutes have been oxidized in a semi-batch reactor (figure 1) by ozone in the absence or in the presence of hydrogen peroxide. The solutions of organic compounds (< 2 µmol l^-1) were prepared in phosphate-buffered ultra-pure water (pH= 7.5) or in tap water. The temperature of the solution was 20.0 ± 0.5°C. In some experiments, atrazine or propazine (k_Atrazine= 2.4 10⁹l mol^-1 s^-1, k_Propazine= 1.8 10⁹ l mol^-1 s^-1, table 1) or sodium paranitrobenzoate (k_PNB= 2.6 10⁹ l mol^-1 s^-1) was added as competitor in the solution. The concentrations of organic solutes during oxidation were determined by HPLC.

Assuming that the rate of degradation of the organic solutes M, and M₂ (equation I and II) is only due to hydroxyl radicals produced from the decomposition of ozone, the relative rate constants (k_M1/k_M2) and the absolute rate constants for the reaction of hydroxyl radicals with M₁ and M₂ were evaluated from the competitive kinetic expression (equation III).

The results showed that the relative concentrations of organic compounds could be well described by equation III (figures 2,3, 5-7). The reaction rate constants indicated that methylthio-2 s-triazines (ametryne and simetryne) are more reactive towards hydroxyl radicals than methoxy-2 s-triazines (atraton and simeton) and than chloro-2 s-triazines (cyanazine, terbutylazine, atrazine) (table 3 and 4, figure 4). The second-order rate constants ranged from 1.8 10⁹ l mol^-1 s^-1 to 4.7 10⁹ l mol^-1 s^-1 excepted for methylthio-2 s-triazines (> 2 10¹⁰ l mol^-1 s^-1).