Anodic oxidation and electro-Fenton treatment of rotenone

Rotenone, a widely used botanical insecticide submitted to strong restrictions regarding its environmental hazards, was studied as a target compound for electro-Fenton (EF) treatment in aqueous–acetonitrile mixture (70:30) of pH 3.0. In this system, the degradation of organic pollutants occurs by attack of hydroxyl radicals (OH) which are produced from the reaction of added ferrous catalyst (Fe2+) and hydrogen peroxide (H2O2) electrogenerated by oxygen reduction at carbon felt cathode. The degradative efficiency of EF system was comparatively studied versus anodic oxidation method (AO) in absence and presence of H2O2. It was found that only EF is sufficiently powerful to induce fast and efficient mineralization of rotenone and its degradation intermediates.The mineralization of rotenone was found to depend largely on organic solvent type, metal ion catalyst, applied current and initial rotenone concentration. The best operative conditions are achieved using aqueous–acetonitrile mixture of pH 3.0 in the presence of 0.2 mM Fe2+ catalyst with a current intensity of 100 mA. Under these optimized conditions, 30 min were sufficient to completely degrade rotenone in 100 mL of a 20 mg L−1 solution. A nearly complete mineralization (∼96% of COD removal) was achieved after 8 h treatment.Rotenone removal kinetic was found to obey the pseudo-first order model and the absolute second order rate constant (kRot = 2.49 × 109 M−1 s−1) for the reaction between the substrate and OH was derived.HPLC–MS and HPLC–DAD analysis were applied to identify and follow the evolution of rotenone oxidation products. Three stable aromatic intermediates were observed and two of these were identified as 12aβ-hydroxyrotenone and hydroquinone. Subsequent attack of these intermediates by OH radicals leads to the formation of aliphatic carboxylic acids such as succinic, acetic, oxalic and formic, quantified by ion-exclusion chromatography.