Degradation and mineralization of sulcotrione and mesotrione in aqueous medium by the electro-Fenton process: a kinetic study
Résumé
Introduction: The degradation and mineralization of two triketone (TRK) herbicides, including sulcotrione and mesotrione, by the electro-Fenton process (electro-Fenton using Pt anode (EF-Pt), electro-Fenton with BDD anode (EF-BDD) and anodic oxidation with BDD anode) were investigated in acidic aqueous medium. Methods: The reactivity of both herbicides toward hydroxyl radicals was found to depend on the electron-withdrawing effect of the aromatic chlorine or nitro substituents. The degradation of sulcotrione and mesotrione obeyed apparent first-order reaction kinetics, and their absolute rate constants with hydroxyl radicals at pH 3. 0 were determined by the competitive kinetics method. Results and discussion: The hydroxylation absolute rate constant (kabs) values of both TRK herbicides ranged from 8. 20 × 108 (sulcotrione) to 1. 01 × 109 (mesotrione) L mol-1 s-1, whereas those of the TRK main cyclic or aromatic by-products, namely cyclohexane 1,3-dione, (2-chloro-4-methylsulphonyl) benzoic acid and 4-(methylsulphonyl)-2-nitrobenzoic acid, comprised between 5. 90 × 108 and 3. 29 × 109 L mol-1 s-1. The efficiency of mineralization of aqueous solutions of both TRK herbicides was evaluated in terms of total organic carbon removal. Mineralization yields of about 97-98% were reached in optimal conditions for a 6-h electro-Fenton treatment time. Conclusions: The mineralization process steps involved the oxidative opening of the aromatic or cyclic TRK by-products, leading to the formation of short-chain carboxylic acids, and, then, of carbon dioxide and inorganic ions.