Kinetics of oxidative degradation/mineralization pathways of the phenylurea herbicides diuron, monuron and fenuron in water during application of the electro-Fenton process

The electrochemical advanced oxidation method “electro-Fenton process” has been applied to three phenylurea herbicides, namely diuron, monuron and fenuron. The reactivity of these phenylurea herbicides toward hydroxyl radicals has been found to depend on the number of chlorine substituents of the aromatic cycle. The degradation reaction rate constants ranged from 4.8 × 109 to 12 × 109 M−1 s−1, and increased in the following order: diuron (2 Cl) < monuron (1 Cl) < fenuron (no Cl). In this work, we have also investigated the mineralization of aqueous solutions of these herbicides in term of chemical oxygen demand (COD) abatement and mineralization degree larger than 90% for a 3 h treatment time was found. Several aromatic by-products, mainly formed by oxidation of the N-terminal group, accompanied by hydroxylation of the aromatic cycle and/or side methyl group, and aromatic dehalogenation, have been identified. Further steps involved the oxidative opening of the aromatic ring, leading to carboxylic acids and inorganic ions.