Application of the electro-Fenton process to mesotrione aqueous solutions: Kinetics, degradation pathways, mineralization and evolution of the toxicity

Abstract

<p>The kinetics and the mechanism of degradation of the mesotrione herbicide by three electrochemical advanced oxidation processes (EAOPs) systems, namely electro-Fenton (EF) with Pt anode (EF-Pt), anodic oxidation with BDD anode (AO) and EF-boron doped diamond anode (EF-BDD), were investigated in acidic aqueous solutions. The degradation of mesotrione obeyed apparent first-order reaction kinetics, and its absolute rate constant value with hydroxyl radicals at pH 3.0, determined by the competitive kinetics method, was found to be 8.20 × 10<sup>8</sup> L mol<sup>–1</sup> s<sup>–1</sup>. Fourteen different mesotrione degradation products were separated and/or identified using HPLC / MS<sup>n</sup> analyses. A rationalized scheme was proposed for the reaction pathways of mesotrione degradation in the EF process. The mesotrione mineralization yield values ranged between about 58 and 97%<strong> </strong>for a 6-h electrolysis time, according to the type of EAOP system and the electrolysis current intensity value. The evolution of the toxicity of mesotrione aqueous solutions with electrolysis time (<em>t</em>) was investigated during treatment by the EF-Pt and EF-BDD systems. Toxicity was measured with the Microtox® method, based on the luminescence inhibition of marine bacteria <em>Vibrio fischeri</em>. The curves % inhibition <em>vs.</em> time were found to depend on the toxicity of formed mesotrione degradation products.</p>